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ArticleSeptember 23, 2016

Aggregation, Adsorption, and Morphological Transformation of Graphene Oxide in Aqueous Solutions Containing Different Metal Cations
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Environmental Science & Technology

Cite this: Environ. Sci. Technol. 2016, 50, 20, 11066–11075

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https://doi.org/10.1021/acs.est.6b04235

Published September 23, 2016

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Abstract

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The colloidal behavior of graphene oxide (GO) has been extensively studied in the presence of common environmental cations, but the aggregation, adsorption, and morphological transformation of GO under heavy metal ions have not been investigated. We observed that heavy metal cations (Cr³⁺, Pb²⁺, Cu²⁺, Cd²⁺, Ag⁺) destabilized GO suspension more aggressively than common cations (Ca²⁺, Mg²⁺, Na⁺, K⁺). In addition to electric double-layer (EDL) suppression, heavy metal cations can easily cross the EDL, bind to GO surface, and then change the surface potential, which is a more efficient pathway for GO aggregation. According to aggregation kinetics, the destabilizing ability of cations follows the order of Cr³⁺ ≫ Pb²⁺ > Cu²⁺ > Cd²⁺ > Ca²⁺ > Mg²⁺ ≫ Ag⁺ > K⁺ > Na⁺. The destabilizing capability of metal cations is consistent with their adsorption affinity with GO, which is determined by their electronegativity and hydration shell thickness. GO nanosheets can be transformed to 1D tube-like carbon material, 2D multiple overlapped GO plane, and 3D sphere-like particles during aggregation, thereby combined to form a sphere-like aggregated GO, which is for the first time observed by TEM and AFM images. Therefore, the aggregation of GO 2D nanosheets follows the Schulze-Hardy rule, which is usually used for spherical particles. An integrative process of adsorption-transformation-aggregation is proposed to better understand the nanomaterial (e.g., GO) colloidal behavior, environmental risk, self-assembly process, and application as a novel adsorbent.

Subjects

Introduction

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The discovery of graphene (1-3) has sparked an interest in scientific communities because of its special electrical, (2, 3) optical, (4) and mechanical properties (5) and unique 2D surface. (6) Graphene oxide (GO) is a 2D carbon nanomaterial with abundant oxygen-containing groups (OCGs) at the edge and surface defects. (1, 7) Compared with graphene, GO maintains the excellent characteristics of the 2D structure, aromatic plane, and large surface area and is also much easier to be synthesized, functionalized, and chemically assembled. (7, 8) GO has been applied in various fields, including energy storage, (9, 10) advanced catalysis, (11, 12) sensitive detectors, (13, 14) and environmental pollutant management, (15-23) because of its fantastic characteristics as a precursory material. Due to its various applications and large-scale production, GO release into the environment is inevitable. GO exhibited acute toxicity to microbial communities and harm to human cells. (24-27) GO toxicity is closely related to its size and structure. (26, 27) As a typical amphiphilic material, GO possesses both a hydrophobic aromatic plane and hydrophilic OCGs. GO can be well-dispersed in water and forms a colloid suspension because of the strong electrostatic repulsion originating from the OCG deprotonation. (1, 28) Therefore, it is necessary to investigate the colloidal stability and environmental transport of GO to accurately evaluate its fate and health risk.

As novel carbon nanomaterials, the GO colloid properties have aroused great interest (29-33) and have been extensively studied with common environmental cations (Ca²⁺, Mg²⁺, K⁺, Na⁺). Chowdhury et al. (29) reported that common cations can induce GO aggregation and that natural organic matter (NOM) can improve GO dispersion. Wu et al. (30) indicated that the pH also influenced the stability of GO and that the aggregation results followed the Schulze-Hardy rule. Though GO is a typical 2D material, its aggregation is well accorded with DLVO sphere–sphere mode, (30, 31) which has not been well explained. The pH, divalent cations, and NOM play complex roles in the fate of GO and reduced GO (rGO). (31) For example, some cations have the same valence, but their destabilizing ability is distinct for GO solution. The critical coagulation concentration (CCC) of Ca²⁺ for GO suspension is always less than Mg²⁺. (29, 30) However, a relevant interaction mechanism is still unclear for GO suspension.

In addition to its amphiphilic colloidal properties, GO is also considered an efficient adsorbent for heavy metal ions. (15, 34) Zhao et al. (15) found that few-layered GO has super adsorption capacity to metal ions. Metal cation adsorption on GO is an endothermic and spontaneous process, (15) and cations can be bound to the surface through OCG complexation. (35) For the GO-heavy metal system, the aggregation process is always accompanied by surface adsorption, but the aggregation kinetics and surface adsorption of GO were separately investigated. The linkage between aggregation and surface adsorption needs to be explored to elucidate intrinsic mechanisms of cation-GO interaction.

In contrast to common hard carbon materials, such as carbon nanotubes and fullerene, GO is a special 2D soft plane that is thermodynamically unstable. (1, 3, 36, 37) Its surface morphology can be easily distorted. (22, 38, 39) GO’s microstructure and morphology will further determine its toxicity. Although some studies have focused on the aggregation process of GO, the microstructure transformation during aggregation has not been investigated. The knowledge on microstructure of aggregated GO will be very useful in understanding the GO aggregation process and interaction mechanisms.

In this study, heavy metal cations (Cr³⁺, Pb²⁺, Cu²⁺, Cd²⁺, and Ag⁺) and common environmental cations (H⁺, K⁺, Na⁺, Ca²⁺, and Mg²⁺) are introduced to systematically investigate the aggregation behavior of GO colloids under a wide range of concentrations. The difference in the cation destabilizing capability is discussed according to their intrinsic electronegativity and hydration shell thickness beyond cation valence. The adsorption behavior of heavy metal onto GO was concurrently monitored with the aggregation process of GO colloids for the first time. The microstructure of the aggregated residues was characterized by TEM and AFM to investigate the aggregation processes.

Materials and Methods

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GO Synthesis and Characterization

GO was fabricated according to a modified Hummers method described previously. (18, 36) The detailed processes and reagents used are presented in the Supporting Information. The electrophoretic mobility (EPM) and hydrodynamic diameter (D_h) of GO were determined in various solutions, including different cation types (Cr³⁺, Pb²⁺, Cu²⁺, Cd²⁺, Ca²⁺, Ma²⁺, Ag⁺, K⁺, Na⁺) and a wide range of cation concentrations. EPM and D_h were measured by a ZetaSizer Nano ZS following well-established procedures. (29, 30, 40, 41) The GO surface morphology and microstructure transformation during aggregation is examined by transmission electron microscopy (TEM), following standard sample preparation. Atomic force microscope (AFM) is applied to investigate GO physical dimensions and thickness change before and after metal ions interaction. The surface functionalities and the interaction between amphiphilic GO and cations were detected by Fourier transform infrared spectroscopy (FT-IR). The detailed processes of the structural characterization and the EPM and D_h variation determination are summarized in the SI.

Aggregation Kinetics of GO

The variation of D_h as a function of cation type and concentration was measured by time-resolved dynamic light scattering (TR-DLS) to identify the destabilization capability and the CCC values of various cations, including common environmental cations and heavy metals. The detailed processes are summarized in the SI.

Adsorption Behavior of Heavy Metals onto GO during Aggregation

Divalent Pb²⁺, Cu²⁺, and Cd²⁺ were chosen as typical heavy metal cations that may have strong affinity to GO and can interact with GO’s surface plane across surrounding EDL. The adsorption conditions were all kept the same as the aggregation experiment to concurrently monitor the aggregation process and adsorption behavior. The processes are detailed in the SI.

Results and Discussion

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Structural Characterization of GO

AFM image analysis (Figure 1A) revealed that GO is an ideal 2D material with a thickness of 1 nm and irregular polygon surrounding. The size of GO is not uniform, and the greatest length in the lateral dimension varies from 200 nm to 1 μm. The ratio of the length to the thickness ranges from 200 to 1000, indicating that GO is far from spherical. According to the TEM image in Figure 1B, GO seems to be a well dispersed monolayer rather than aggregate because of the strong electrostatic repulsion interaction between GO nanoparticles. (28) At the edge of GO plane, several wrinkles are found which are considered as high surface energy sites for pollutant adsorption. (18, 37) FTIR (Figure 1C) revealed that GO possesses abundant functional groups, including −OH, −COOH, and −O–. The large peak at approximately 3400 cm^–1 is assigned to −OH, which originates from water, hydroxide, and carboxyl groups. The sharp peaks at 1731, 1621, 1384, 1054, and 833 cm^–1 are assigned to C═O, C═C, and C–O–C asymmetric stretching and C–O–H and C–O–C bending motion, respectively. (38, 39)

Figure 1. AFM (A), TEM (B), and FTIR (C) characterization of GO.

Electrokinetic and Hydrodynamic Properties of GO

As presented in Figure 2A₁, GO was negatively charged from pH 1 to 12, and the isoelectric point was below pH 1. GO’s EPM and D_h are insensitive at high pH range (pH 3 to 12). The EPM of GO changed gently, and D_h remained relatively unchanged around ∼280 nm from pH 3 to 12 (Figure 2A₂); but once H⁺ concentrations are in excess of 1 mmol/L (pH < 3), EPM and D_h all change sharply. The EPM of GO increases sharply from −3.13 × 10^–8 to −0.13 × 10^–8 m² V^–1 S^–1 when pH decreased from 3 to 1. The D_h change is followed by size variation from 280 to 7360 nm with pH 2.4 to 1.12. The followed variation of D_h behind EPM indicates GO aggregation with H⁺ is mainly an electrostatic repulsive force reduction process. Similar electrokinetic behavior of carbon-based nanomaterials, i.e., carbon nanotubes, (40) fullerenes, (41) graphene oxide, (29) and reduced graphene oxide, (31) with solution pH has also been observed. Because the pH in aquatic environments usually ranges from 5 to 9, the released GO colloid should be stable and easily transported in the aquatic environment if not taking into account the influence of the coexisting metal cations.

Figure 2. Electrophoretic mobility (EPM) and hydrodynamic diameter of GO as a function of the cation concentration of H⁺ (A₁, A₂), monovalent metal cations (B₁, B₂), divalent metal cations (C₁, C₂), and trivalent metal cations (D₁, D₂).

The electrokinetic and hydrodynamic properties of GO as a function of the concentration of monovalent, divalent, and trivalent cations are shown in Figure 2. The surface zeta potential derived from EPM is presented in Figure S1. GO was highly negatively charged in deionized water with a zeta potential of −66 mV (Figure S1), which made it stable in water with D_h = ∼280 nm. Comparing the destabilizing ability of different valent cations, Cr³⁺ displayed the most aggressive influence on the EPM and the hydrodynamic size of GO in the aqueous suspension and then followed by divalent cations and then by monovalent cations. As predicted by the Schulze-Hardy rule, the higher valence the cations are, the higher charge screening capability the cations process.

Further analysis revealed that EPM and D_h curves depend not only mainly on cations’ valence but also highly on cations’ types though they possess the same valence. As presented in Figure 2B₁,C₁, GO EPM was constant, and the differences were negligible before the EPM inflection point for the same valent cations. The electrolyte concentrations at the EPM inflection point were 0.9, 0.08, and 0.002 mmol/L for monovalent, divalent, and trivalent cations, respectively; but once concentrations are in excess of their EPM inflection point, the GO EPM increased notably, and significant differences appear from the same valent cations comparison. The EPM variation intensity of GO is also distinct during low, middle, and high concentration ranges which is especially obvious for divalent and trivalent cations. The EPM variation intensity of GO at high concentration (>0.08 mmol/L) is weaker than at concentration ranges of 0.008–0.08 mmol/L for divalent cations, indicating multiple interactions are involved besides EDL suppression (Figure 2C₁). Comparing with EMP variation, an obvious D_h change is always detected at higher concentration followed by EPM. For monovalent cations, the cation concentration at the EPM inflection point (C_EPM) was 0.9 mmol/L and increased to 20, 12, and 8 mmol/L for Na⁺, K⁺, and Ag⁺ respectively, at the D_h inflection point (C_Dh). For divalent cations, C_EPM was 0.008 mmol/L, and C_Dh increased to 0.4, 0.3, 0.3, 0.2, and 0.08 mmol/L for Mg²⁺, Ca²⁺, Cd²⁺, Cu²⁺, and Pb²⁺, respectively. For Cr³⁺C_Dh = 0.008 mmol/L is also 4 times higher than C_EPM = 0.002 mmol/L. The ratio of C_Dh/C_EPM was 200, 120, and 80 for Na⁺, K⁺, and Ag⁺, 50, 38, 38, 25, and 10 for Mg²⁺, Ca²⁺, Cd²⁺, Cu²⁺, and Pb²⁺, and 4 for Cr³⁺. The C_Dh/C_EPM ratio decreased from 200 to 80 for monovalent cations to 50–10 for divalent cations and then to 4 for Cr³⁺. The reduced concentration gap between C_EPM and C_Dh with the increase in cation valence suggested that the charge screening effect of GO induced by higher valence cations contributed more to the GO aggregation, and the difference of C_Dh for the same valent cations reveals GO aggregation is also dependent on cation types which is beyond the traditional Schulze-Hardy rule. According to the D_h variation, GO will be stable and have strong migration potential in environmental water, because K⁺ and Na⁺ concentrations in most natural water are less than 10 mmol/L. Among the common environmental divalent cations, Ca²⁺ and Mg²⁺ are more aggressive in GO destabilization. The aggregation of GO in the presence of heavy metal cations was much more significant than the common cations.

To test real coagulation phenomena of GO, Figure 3 presents a direct visual demonstration of the GO flocculation situation after 12 h in the presence of different cation types and concentrations. The GO concentration was controlled at 0.04 mg/mL, which was consistent with the aggregation experiments. GO is well-dispersed in water and low concentration cation electrolyte solutions. When the cation concentration reached a critical concentration, i.e., 10 mmol/L for Ag⁺, 20 mmol/L for K⁺, 30 mmol/L for Na⁺, 0.2 mmol/L for Pb²⁺, 0.3 mmol/L for Pb²⁺, 0.4 mmol/L for Cu²⁺, 0.5 mmol/L for Cd²⁺, 0.4 mmol/L for Ca²⁺, 0.7 mmol/L for Mg²⁺, and 0.045 mmol/L for trivalent Cr³⁺, visible coagulation appeared which also indicates metal cations can be used as a flocculent for GO recovery. The bulk precipitation phenomena in Figure 3 are consistent with the corresponding changes of the EPM and D_h with cation concentration in Figure 2. Similarly, the destabilization capability of cations to GO followed the order of Cr³⁺ ≫ Pb²⁺ > Cu²⁺ > Cd²⁺ > Ca²⁺ > Mg²⁺ ≫ Ag⁺ > K⁺ > Na⁺. Based on the Schulze-Hardy rule, (42, 43) the same valent cations should produce similar charge screening effects. In our study, however, heavy metal cations displayed a more aggressive ability in GO aggregation: Pb²⁺, Cu²⁺, Cd²⁺ > Ca²⁺, Mg²⁺, and Ag ⁺ > K ⁺, Na⁺. The difference in the destabilizing ability for equivalent cations indicated that GO aggregation is a more complex process than just the EDL suppression mechanism.

Figure 3. Bulk flocculation of GO in aqueous solution containing monovalent cations (A), divalent cations (B), and trivalent cations (C) with different concentrations to determine their destabilization capability.

Aggregation Kinetics of GO

A typical GO aggregation kinetics profile as a function of K⁺ concentration is presented in Figure 4 (A) and as a function of Na⁺, Ag⁺, Mg²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Pb²⁺, and Cr³⁺ in Figure S2. The aggregation rate increases with the enhancement of EDL repulsion screening of the cation concentration. The GO attachment efficiency (α) was determined from the aggregation kinetic profiles for different nitrate salts. Once the solution reaches the critical coagulation concentration (CCC) (e.g., 28 mmol/L for K⁺), thermal diffusion is the main limitation for GO aggregation instead of energy-barrier repulsion. The reaction-limited and diffusion-limited regimes of GO in monovalent, divalent, and trivalent cations are presented in Figure 4 (B, C, D), indicating that GO aggregation is in general agreement with the DLVO theory.

Attachment efficiency (α) is used to normalize the aggregation kinetics. The α value increased quickly from 1% to 100% with a decrease in EDL repulsion in the reaction-limited regime. The CCC values for different cations are determined from the intersection of the reaction-limited and diffusion-limited fitting lines, which indicate the total suppression of EDL, and are summarized in Table 1. For the common environmental cations, the CCC values of GO were 36 mmol/L for Na⁺, 28 mmol/L for K⁺, 1.50 mmol/L for Mg²⁺, and 1.35 mmol/L for Ca²⁺ which is similar to previous reported CCC values (44 mmol/L Na⁺, 1.3 mmol/L Mg²⁺, and 0.9 mmol/L Ca²⁺). (29) For valence equivalent cations, the CCC values of the heavy metals were significantly lower than the corresponding common cations, i.e., CCC_Ag (19 mmol/L) < CCC_K (28 mmol/L) < CCC_Na (36 mmol/L), CCC_Pb (0.300 mmol/L) < CCC_Cu (0.725 mmol/L) < CCC_Cd (1.300 mmol/L) ≈ CCC_Ca (1.350 mmol/L) < CCC_Mg (1.500 mmol/L). The CCC values of nanomaterials such as GO for heavy metal ions are first reported in the current study. The reaction-limited and diffusion-limited regions are the concentration ranges before and after the CCC that represent the different flocculation processes. According to the Schulze-Hardy rule, the CCC ratio of contra-ion should be Z^–6 for well-dispersed colloids (Z is the valence of the ions). By normalizing the CCC to CCC_Na, we can see that CCC_Na:CCC_K:CCC_Ag:CCC_Mg:CCC_Ca:CCC_Cd:CCC_Cu:CCC_Pb:CCC_Cr = 1:0.778:0.528:2^–4.6:2^–4.7:2^–4.8:2^–5.6:2^–6.9:3^–5.5. The CCC ratio of divalent and trivalent cations to CCC_Na is almost proportional to Z^–6, indicating that the GO aggregation process is generally consistent with the Schulze-Hardy rule, although GO is not a spherical colloid. The Schulze-Hardy rule is an experimentally empirical formula that is only suitable to describe the aggregation originating from double-layer compression. Theoretically, the cations with the same valence should display the same destabilization capability. (42, 43) However, the CCC values of GO for equivalent cations are distinct, which indicates that the flocculation behavior of GO with the same valent cations is beyond just the EDL suppression and additional mechanisms are involved.

Table 1. Physicochemical Properties of the Tested Cations and Their Critical Coagulation Concentrations (CCC) for Graphene Oxide (GO)

metal cations	ionic radius (Å) (51, 52)	hydration shell thickness (Å) (51, 52)	electronegativity (53)	CCC (mmol/L)	ratio of CCC to CCC_Na
Na⁺	0.95	2.63	0.93	36	1
K⁺	1.33	1.98	0.82	28	0.778
Ag⁺	1.26	2.15	1.93	19	0.528
Mg²⁺	0.65	3.63	1.31	1.50	2^–4.585
Ca²⁺	0.99	3.13	1.01	1.350	2^–4.737
Cd²⁺	0.97	3.29	1.69	1.300	2^–4.791
Cu²⁺	0.72	3.47	1.90	0.725	2^–5.634
Pb²⁺	1.32	2.69	2.33	0.300	2^–6.907
Cr³⁺	0.64	3.48	1.66	0.085	3^–5.506

Adsorption of Heavy Metal Cations onto GO

In addition to double-layer compression, the adsorption (binding) of metal cations onto nanosheets was suggested as a potential mechanism for the aggregation of graphene oxide. For example, the more effective destabilization of Ca²⁺ compared to Mg²⁺ on GO in aquatic environments was attributed to the strong binding of Ca²⁺ ions with hydroxyl and carbonyl functional groups on GO surfaces in comparison with Mg²⁺. (29) However, along with aggregation, the adsorption behavior of metal cations has not been investigated quantitatively. The adsorption isotherms of Pb²⁺, Cu²⁺, and Cd²⁺ are plotted in Figure 5A. The relationship between EPM and the adsorption of GO with metal cation concentration is presented in Figure 5B.

Figure 5. Isotherms of Pb²⁺, Cu²⁺, and Cd²⁺ onto GO during the aggregation process (A) and relationship between EPM and the adsorption of GO (B).

The adsorption isotherm is an in situ reflection of the flocculation process. At the same equilibrium concentration of 0.06 mmol/L in pH = 4.5 solution, the magnitude of the adsorption capacity of metal ions followed the order of Pb²⁺ (0.710 mmol/g) > Cu²⁺ (0.655 mmol/g) > Cd²⁺ (0.566 mmol/g) (Table S1). Adsorption capacity of GO is much higher than other adsorbents, (44) such as oxidized carbon nanotubes, (45) activated carbon, (46) biochar, (47) and biosorbent. (48) According to the regression coefficient (R²) (Table S1), the isotherms of cations are well fitted by the Freundlich model, which means that the adsorption sites on the GO surface are heterogeneously distributed. The Freundlich N index followed the order of Pb²⁺ (0.174) < Cu²⁺ (0.225) < Cd²⁺ (0.254). The strongest interaction affinity between cation and GO is Pb²⁺, followed by Cu²⁺ and Cd²⁺, which is consistent with their destabilizing ability. The correlations indicated that GO aggregation which cannot be explained by EDL suppression may be related to surface adsorption. For the GO-cation system, the adsorption region was mainly located at the initial concentrations of 0.01–0.10 mmol/L (removal efficiency, r%, between 20 and 80%) (Figure 5 B) which precisely corresponds to the fast increase region of GO’s EPM with cation concentration. This phenomenon demonstrates that adsorption is more efficient with the EPM change and triggers the aggregation of GO.

Interaction Mechanisms of Metal Cations with GO

FTIR spectra can reflect the electronic contributions and functional group variations of GO through nonadiabatic interactions, (38) which can be further employed to analyze the interaction between cations and adsorption sites on the GO surface. The FTIR spectra of GO before and after the influence of different cations are presented in Figure S3A. H⁺ does not influence the GO groups, and the aggregation is induced by EDL compression only. With the presence of metal cations, the binding motion of epoxy groups (C–O–C) (Figure S3B) presented an obvious enhancement and wavelength shift. These variations indicate that the epoxy groups take part in metal cation capture. In addition to epoxy groups, peaks also appeared between 1800 and 1750 cm^–1 next to the carboxyl (C═O) (Figure S3C and D), which may originate from the protonation of carboxyl after cation combination. (49) As reported, heavy metal cations preferably combine with high energy sites such as OCGs. (15, 29, 30, 34) Moreover, cations can also interact with GO’s aromatic framework through cation-π interactions. The FTIR spectra revealed that the C═C stretching peak of GO shifted from 1621 cm^–1 to 1629, 1623, and 1620 cm^–1 in the presence of Na⁺, K⁺, and Ag⁺, respectively. The variation of the C═C stretching peak of GO is more obvious after interacting with divalent cations and shifted from 1621 cm^–1 for the original GO to 1634 (Mg²⁺), 1627 (Ca²⁺), 1617 (Cd²⁺), 1614 (Cu²⁺), and 1597 cm^–1 (Pb²⁺). Induced by the binding of heavy metals, the vibration energy level of C═C followed the order of Na⁺ > K⁺ > Ag⁺ for monovalent cations and Mg²⁺ > Ca²⁺ > Cd^2 +> Cu²⁺ > Pb²⁺ for divalent cations. These sequences are in line with their destabilization ability (hydration shell of the metal ions). The changes in C═C stretching of GO indicate that the adsorbed cations can pass through the EDL and directly interact with the GO plane.

The classic Schulze-Hardy rule can explain the discrepancy of aggregation between different valence cations but cannot explain the difference among equivalent cations. The flocculation ability of metal cations is also influenced by the corresponding adsorption capability of the GO nanosheets via surface binding mechanisms. As evidenced in the FTIR spectra (Figure S3), metal cations can be bonded to the hydrophobic aromatic surface through cation-π interactions in addition to binding to OCGs and electrostatic attractions. In previous studies, OCG bonding and cation-π bonding are considered as a donor–acceptor or charge-transfer interaction, which is also an electrostatic interaction that has a dominant effect for most reaction systems. (50) The cation-π interaction energy is mainly dependent on the electronegativity of metal cations over the center of the aromatic plane. (50) The physicochemical characteristics (51-53) of the tested cations are listed in Table 1. The electronegativity is an index used to evaluate their electron attraction capability, which is determined by the atom’s intrinsic electronic density and distribution. The electronegativity of monovalent cations follows the order of Ag⁺ > Na⁺ > K⁺ and is Pb²⁺ > Cu²⁺ > Cd²⁺ > Mg²⁺ > Ca²⁺ for divalent cations, which is consistent with their destabilization ability for GO suspensions, except Na⁺ > K⁺ and Mg²⁺ > Ca²⁺. Therefore, both aggregation and adsorption of GO were influenced by the interaction between cations and GO, which was mainly determined by the electronegativity of the metal cations.

To elucidate the interface interaction between cations and GO in aqueous solution, it is important to consider the influence of water molecules. (50, 54, 55) Water is a dipole molecule, and water molecules will align themselves around ions to form a hydration shell due to polarity. (51) For example, metal cations are surrounded by H₂O via the negative end of oxygen. The primary inner hydration shell is well-ordered and bonded through tight hydrogen bonds. The outer shell is diffused with less bonding energy. The bonded water shell shields the electronegativity of the cation through dipole interactions and then further excludes the cation benzene interaction distance and further reduces the bonding affinity of heavy cations with OCGs and the aromatic plane. (50, 54, 55) The hydration radius is determined by the ion radii and its electrostatic potential. The hydration shell thickness of Na⁺ (2.62 Å) is higher than K⁺ (1.98 Å), and Mg²⁺ (3.63 Å) is also higher than Ca²⁺ (3.13 Å), which is why the flocculation ability follows the sequence of K⁺ > Na⁺ and Ca²⁺ > Mg²⁺, which is not consistent with their electronegativity. The hydration shell thickness weakens the affinity of metal cations with GO. Therefore, adsorption and aggregation potentials of GO are jointly determined by the electronegativity and hydration shell thickness of the metal cations.

The Microstructure of GO Flocculation

Few studies have focused on the microstructure change of GO under cation influence, which is critical in understanding the potential development of GO aggregates. In this study, two typical cations are selected. Cd²⁺ is selected as a typical heavy metal cation which has strong affinity to GO and can directly interact with GO surface via crossing EDL. Mg²⁺ is present as a common environmental cation that has weak affinity to GO and mainly destabilizes GO through EDL suppression instead of surface adsorption. The observed concentration of Cd²⁺ (0.40 mmol/L) and Mg²⁺ (0.70 mmol/L) is set at a critical aggregation point, at which the aggregation of GO initially occurs, and the structural transformation is easily monitored.

The microstructure of GO with Cd²⁺ is presented in Figure 6. Comparing with the well dispersed monolayer case, GO nanosheets are initiated to flocculate, and tight and sphere-like aggregates (diameter ≈ 1 μm) are formed with small, curled GO nanosheets accumulating around (Figure 6A). In addition to the tight 3D bulk GO, unique 1D carbon tube-like structures with open end-caps are found (length ≈ 1.5 μm and width ≈ 0.5 μm), and the dispersed GO displayed 1D needle-like structures with length ≈ 0.5 μm (Figure 6 B, C). The large tube-like structure seems to originate from multiple GO crispation, and the small size, needle-like structure curled to form a GO small fractal sheet. Comparing the size of 1D tube-like and needle-like structures, the final size of the aggregate is mainly determined by its initial dimensions of GO sheets. Surprisingly, well-formed 1D tube-like, 2D multiple folded, and 3D bulk GO microstructures are all found under TEM investigation (Figure 6 D, E, F). More detailed TEM images of the GO aggregate microstructure are presented in Figure S4 to verify the widespread structural transformation. (56-58) These TEM images confirmed that the aggregation of the GO suspension is not only an accumulative process via EDL suppression but also accompanied by structural transformation of GO into 1D, 2D, and 3D nanomaterials.

Figure 6. TEM and AFM images of the GO flocculate during aggregation with the presence of Cd²⁺. TEM images of aggregated GO (A), magnified version (B), with small size curled GO in the dispersed situation (C), with a typical 1D tube-like structure (D), with a 2D multiple folded structure (E), and a 3D tightly aggregated structure (F). AFM images of flocculated GO (G, H). G1 and H1 show the testing line height in parts G and H, respectively.

The microstructure of the GO aggregate under Cd²⁺ is further detected by AFM which is also consistent with the TEM investigation. A seriously aggregated flocculate of GO was formed with some dispersed tube-like GO attached (Figure 6G). The edge of 2D GO is more regular relative to the original GO nanosheets (Figure 6H). The piled 2D plane is 3–5 nm in height (Figure 6H1), which equals 3–5 overlapped layers. Different from the microstructure of GO induced by Cd²⁺, the GO structure under Mg²⁺ inducement is not multifarious (Figure S5) as it cannot cross EDL and adsorption on the GO plane. Part of GO initially comes into a formless gel-like aggregate (Figure S5A–C) followed by Mg²⁺ EDL suppression. Except for the gel-like structure, a large part of aggregates is 3D sphere-like bulk with different sizes (diameter from 0.4–0.8 μm) (Figure S5D–F) which may originate from the unit of gel-like structures. The small size aggregate will further accumulate with surrounding interaction (Figure S5G), and larger size sphere bulk agglomerates are formed beyond the small size (diameter around 2 μm) (Figure S5H,I). Comparing with Mg²⁺, Cd²⁺ can get across through EDL and directly interact with GO’s surface plane so the different and diverse microstructures under Cd²⁺ might be induced by direct cation’s surface interaction. The ideal 2D monolayer GO can be transformed to a 3D sphere-like structure under both heavy metal ions and common cations, which is why GO aggregation can well accord with the DLVO sphere–sphere model.

Compared with graphene, GO can be well-dispersed in water solution and remain more stable because of the interelectric repulsion and hydrogen bonding with water. Heavy metal cations can cross the EDL barrier and coordinate with OCGs to accept the unpaired electron to reduce the deformation barrier and neutralize the surface charge of GO. In addition to OCGs coordination, cations can further interact with the aromatic plane via a cation-π complex and supply a directional inducement for microstructure transformation. Our study is the first observation that the 2D GO nanosheets can be rolled into 1D carbon nanotube-like material, 2D multiplied structure, and 3D sphere bulk under the inducement of heavy metals which may be helpful in understanding the self-assembly and microstructure manipulation of GO.

We suggest the adsorption of heavy metals with GO nanosheets (process I), the formation of 1D-, 2D-, and 3D-like structures (process II), and the aggregation of sphere-like flocculate (process III) are three aspects of the interaction of GO with heavy metal cations, which are schematically demonstrated in Figure 7. One cation can interact with 1–3 aromatic planes of the graphene nanosheets through cation × 1π, cation × 2π, and cation × 3π. (50, 59-63) This electronic interaction is strong enough to deform the benzene ring. (59, 60) Strong cation-π interactions bend and roll the aromatic plane of GO and form a more thermodynamically favorable 1D tube-like structure, when cations interact with the GO nanoplane in a certain direction (Figure 7, process II). As a result, the hydrophilicity and negative charges of GO particles weaken. In addition to the common cation-π interaction model, cations can induce aromatic planes to form a π × cation × π structure (i.e., cation × 2π). (61, 62) Cation × 2π strengthens the π–π interactions and further governs the macromolecular structure. (63) Cations can link or induce aromatic planes to form face-to-face multiplied structures through cation × 2π (Figure 7, process II). Furthermore, strong cation clusters can attract aromatic planes in all directions to form a novel interaction of cation × 3π. As a result, multi- or monolayer GO nanosheets assemble around the cation cluster by forming a cation × 3π cage-like structure (Figure 7, process II). In contrast, except for the cation × 3π inducement, the 3D GO bulk aggregate can be formed with the presence of Mg²⁺ via EDL suppression because Mg²⁺ displayed weak affinity to the GO plane. The combined interactions of strong π–π interactions and cation × 3π between aromatic planes can activate 2D graphene oxide into a tight sphere-like 3D aggregation, as shown in the AFM images of Figures 6G and 6H. Following EDL suppression during the aggregation, 1D, 2D, and 3D structures accumulate and combine to form a sphere-like aggregated GO (Figure 7, process III). With the proposed adsorption-structural transformation-aggregation processes (Figure 7), the 2D GO nanosheets were converted into spherical colloids in the presence of heavy metals, which is a potential reason why aggregation of the GO nanosheets also follows the DLVO sphere–sphere mode. This study strongly suggests that the adsorption, structural transformation, and aggregation should be considered together to precisely elucidate the colloidal behavior and environmental transport of nanomaterials such as graphene oxide, carbon nanotubes, and fullerene.

Figure 7. Proposed interactions of heavy metal cations with GO nanosheets in three steps: (I) adsorption of heavy metal onto GO nanosheets; (II) formation of 1D-, 2D-, and 3D-like structures; and (III) aggregation of sphere-like flocculation.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b04235.

Material and methods, surface zeta potential (Figure S1), aggregation kinetics of GO (Figure S2), FT-IR spectra of GO and GO aggregate induced by different cations (Figure S3), and TEM images of GO (Figures S4 and S5) (PDF)

es6b04235_si_001.pdf (1.21 MB)

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Author Information

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Corresponding Author
- Baoliang Chen - Department of Environmental Science, Zhejiang University, Hangzhou 310058, China; Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China; Email: [email protected]
Authors
- Kaijie Yang - Department of Environmental Science, Zhejiang University, Hangzhou 310058, China; Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China
- Xiaoying Zhu - Department of Environmental Science, Zhejiang University, Hangzhou 310058, China; Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China
- Baoshan Xing - Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States
Notes
The authors declare no competing financial interest.

Acknowledgment

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This project was supported by the National Natural Science Foundation of China (Grant 21537005, 21425730, and 21277120) and the National Basic Research Program of China (Grant 2014CB441106).

References

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Shen, Yi; Chen, Baoliang
Environmental Science & Technology (2015), 49 (12), 7364-7372CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Graphene nanosheets, as a novel nanoadsorbent, can be further modified to optimize the adsorption capability for various pollutants. To overcome the structural limits of graphene (aggregation) and graphene oxide (hydrophilic surface) in water, sulfonated graphene (GS) was prepd. by diazotization reaction using sulfanilic acid. It was demonstrated that GS not only recovered a relatively complete sp2-hybridized plane with high affinity for arom. pollutants but also had sulfonic acid groups and partial original oxygen-contg. groups that powerfully attracted pos. charged pollutants. The satd. adsorption capacities of GS were 400 mg/g for phenanthrene, 906 mg/g for methylene blue and 58 mg/g for Cd2+, which were much higher than the corresponding values for reduced graphene oxide and graphene oxide. GS as a graphene-based adsorbent exhibits fast adsorption kinetic rate and superior adsorption capacity toward various pollutants, which mainly thanks to the multiple adsorption sites in GS including the conjugate π region sites and the functional group sites. Moreover, the sulfonic acid groups endow GS with the good dispersibility and single or few nanosheets which guarantee the adsorption processes. It is great potential to expose the adsorption sites of graphene nanosheets for pollutants in water by regulating their microstructures, surface properties and water dispersion.
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Ji, L.; Chen, W.; Xu, Z.; Zheng, S.; Zhu, D. Graphene nanosheets and graphite oxide as promising adsorbents for removal of organic contaminants from aqueous solution Journal of Environment Quality 2013, 42, 191– 198 DOI: 10.2134/jeq2012.0172
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Graphene nanosheets and graphite oxide as promising adsorbents for removal of organic contaminants from aqueous solution
Ji, Liangliang; Chen, Wei; Xu, Zhaoyi; Zheng, Shourong; Zhu, Dongqiang
Journal of Environmental Quality (2013), 42 (1), 191-198CODEN: JEVQAA; ISSN:1537-2537. (American Society of Agronomy)
Graphenes are an emerging class of carbon nanomaterials whose adsorption properties toward org. compds. have not been well understood. In the present study, graphene nanosheets were prepd. by reoxidn. and abrupt heating of graphite oxide, which was prepd. by sequential chem. oxidn. of com. nonporous graphite powder. Adsorption properties of three arom. compds. (naphthalene, 2-naphthol, and 1-naphthylamine) and one pharmaceutical compd. (tylosin) on graphene nanosheets and graphite oxide were examd. to explore the potential of these two adsorbents for the removal of org. contaminants from aq. solns. Compared with the literature data of adsorption on carbon nanotubes, adsorption of bulky, flexible tylosin on graphene nanosheets exhibited markedly faster adsorption kinetics, which can be attributed to their opened-up layer structure. Graphene nanosheets and graphite oxide showed similar sequences of adsorption affinity: 1-naphthylamine > 2-naphthol > tylosin > naphthalene (with much larger differences obsd. on graphite oxide). It was proposed that the strong adsorption of the three arom. compds. was mainly due to π-π electron donor-acceptor interactions with the graphitic surfaces of adsorbents. Addnl., Lewis acid-base interaction was likely an important factor contributing to the strong adsorption of 1-naphthylamine and tylosin, esp. for the O-functionality-abundant graphite oxide. After being normalized on the basis of adsorbent surface area, adsorption affinities of all four tested adsorbates on graphene nanosheets were very close to those on nonporous graphite powder, reflecting complete accessibility of the adsorbent surface area in adsorption.
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Wang, J.; Chen, Z.; Chen, B. Adsorption of polycyclic aromatic hydrocarbons by graphene and graphene oxide nanosheets Environ. Sci. Technol. 2014, 48, 4817– 4825 DOI: 10.1021/es405227u
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Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets
Wang, Jun; Chen, Zaiming; Chen, Baoliang
Environmental Science & Technology (2014), 48 (9), 4817-4825CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The adsorption of naphthalene, phenanthrene, and pyrene onto graphene (GNS) and graphene oxide (GO) nanosheets was investigated to probe the potential adsorptive sites and mol. mechanisms. The microstructure and morphol. of GNS and GO were characterized by elemental anal., XPS, FTIR, Raman, SEM, and TEM. Graphene displayed high affinity to the polycyclic arom. hydrocarbons (PAHs), whereas GO adsorption was significantly reduced after oxygen-contg. groups were attached to GNS surfaces. An unexpected peak was found in the curve of adsorption coeffs. (Kd) with the PAH equil. concns. The hydrophobic properties and mol. sizes of the PAHs affected the adsorption of G and GO. The high affinities of the PAHs to GNS are dominated by π-π interactions to the flat surface and the sieving effect of the powerful groove regions formed by wrinkles on GNS surfaces. In contrast, the adsorptive sites of GO changed to the carboxyl groups attaching to the edges of GO because the groove regions disappeared and the polar nanosheet surfaces limited the π-π interactions. The TEM and SEM images initially revealed that after loading with PAH, the conformation and aggregation of GNS and GO nanosheets dramatically changed, which explained the observations that the potential adsorption sites of GNS and GO were unusually altered during the adsorption process.
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Chen, X.; Chen, B. Macroscopic and spectroscopic investigations on adsorption of nitroaromatic compounds to graphene oxide, and reduced graphene oxide and graphene nanosheets Environ. Sci. Technol. 2015, 49, 6181– 6189 DOI: 10.1021/es5054946
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Bianco, A. Graphene: safe or toxic? The two faces of the medal Angew. Chem., Int. Ed. 2013, 52, 4986– 4997 DOI: 10.1002/anie.201209099
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Graphene: Safe or Toxic? The Two Faces of the Medal
Bianco, Alberto
Angewandte Chemie, International Edition (2013), 52 (19), 4986-4997CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Graphene is considered the future revolutionary material. For its development, it is of fundamental importance to evaluate the safety profile and the impact on health. Graphene is part of a bigger family which has been identified as the graphene family nanomaterials (GFNs). Clarifying the existence of multiple graphene forms allows better understanding the differences between the components and eventually correlating their biol. effects to the physicochem. characteristics of each structure. Some in vitro and in vivo studies clearly showed no particular risks, while others have indicated that GFNs might become health hazards. This Minireview critically discusses the recent studies on the toxicity of GFNs to provide some perspective on the possible risks to their future development in materials and biomedical sciences.
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Akhavan, O.; Ghaderi, E.; Emamy, H.; Akhavan, F. Genotoxicity of graphene nanoribbons in human mesenchymal stem cells Carbon 2013, 54, 419– 431 DOI: 10.1016/j.carbon.2012.11.058
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Genotoxicity of graphene nanoribbons in human mesenchymal stem cells
Akhavan, Omid; Ghaderi, Elham; Emamy, Hamed; Akhavan, Fatima
Carbon (2013), 54 (), 419-431CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
Single-layer reduced graphene oxide nanoribbons (rGONRs) were obtained through an oxidative unzipping of multi-walled carbon nanotubes and a subsequent deoxygenation by hydrazine and bovine serum albumin. Human mesenchymal stem cells (hMSCs) were isolated from umbilical cord blood and used for checking the concn.- and time-dependent cytotoxic and genotoxic effects of the rGONRs and reduced graphene oxide sheets (rGOSs). The cell viability assay indicated significant cytotoxic effects of 10 μg/mL rGONRs after 1 h exposure time, while the rGOSs exhibited the same cytotoxicity at concn. of 100 μg/mL after 96 h. The oxidative stress was found as the main mechanism involved in the cytotoxicity of the rGOSs which induced a slight cell membrane damage, while RNA efflux of the hMSCs indicated that neither generation of reactive oxygen species nor the significant membrane damage of the cells could explain the cell destructions induced by the rGONRs. Our results demonstrated that, the rGONRs could penetrate into the cells and cause DNA fragmentations as well as chromosomal aberrations, even at low concn. of 1.0 μg/mL after short exposure time of 1 h.
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Akhavan, O.; Ghaderi, E. Toxicity of graphene and graphene oxide nanowalls against bacteria ACS Nano 2010, 4, 5731– 5736 DOI: 10.1021/nn101390x
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Toxicity of graphene and graphene oxide nanowalls against bacteria
Akhavan, Omid; Ghaderi, Elham
ACS Nano (2010), 4 (10), 5731-5736CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Bacterial toxicity of graphene nanosheets in the form of graphene nanowalls deposited on stainless steel substrates was investigated for both Gram-pos. and Gram-neg. models of bacteria. The graphene oxide nanowalls were obtained by electrophoretic deposition of Mg2+-graphene oxide nanosheets synthesized by a chem. exfoliation method. On the basis of measuring the efflux of cytoplasmic materials of the bacteria, it was found that the cell membrane damage of the bacteria caused by direct contact of the bacteria with the extremely sharp edges of the nanowalls was the effective mechanism in the bacterial inactivation. In this regard, the Gram-neg. Escherichia coli with an outer membrane were more resistant to the cell membrane damage caused by the nanowalls than the Gram-pos. Staphylococcus aureus lacking the outer membrane. Moreover, the graphene oxide nanowalls reduced by hydrazine were more toxic to the bacteria than the unreduced graphene oxide nanowalls. The better antibacterial activity of the reduced nanowalls was assigned to the better charge transfer between the bacteria and the more sharpened edges of the reduced nanowalls, during the contact interaction.
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Akhavan, O.; Ghaderi, E.; Akhavan, A. Size-dependent genotoxicity of graphene nanoplatelets in human stem cells Biomaterials 2012, 33, 8017– 8025 DOI: 10.1016/j.biomaterials.2012.07.040
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Size-dependent genotoxicity of graphene nanoplatelets in human stem cells
Akhavan, Omid; Ghaderi, Elham; Akhavan, Alireza
Biomaterials (2012), 33 (32), 8017-8025CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
Reduced graphene oxide nanoplatelets (rGONPs) were synthesized by sonication of covalently PEGylated GO sheets followed by a chem. redn. using hydrazine and bovine serum albumin. Human mesenchymal stem cells (hMSCs), as a fundamental factor in tissue engineering, were isolated from umbilical cord blood (as a recently proposed source for extg. fresh hMSCs) to investigate, for the first time, the size-dependent cyto- and geno-toxic effects of the rGONPs on the cells. The cell viability test showed significant cell destructions by 1.0 μg/mL rGONPs with av. lateral dimensions (ALDs) of 11±4 nm, while the rGO sheets with ALDs of 3.8±0.4 μm could exhibit a significant cytotoxic effect only at highconcn. of 100 μg/mL after 1 h exposure time. Although oxidative stress and cell wall membrane damage were detd. as the main mechanism involved in the cytotoxicity of the rGO sheets, neither of them could completely describe the cell destructions induced by the rGONPs, esp. at the concns. \\≤\\ 1.0 μg/mL. In fact, the rGONPs showed genotoxic effects on the stem cells through DNA fragmentations and chromosomal aberrations, even at low concn. of 0.1 μg/mL. Our results present essential knowledge for more efficient and innocuous applications of graphene sheets and particularly nanoplatelets in upcoming nanotechnol.-based tissue engineering as, e.g., drug transporter and scaffolds.
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Li, D.; Müller, M. B.; Gilje, S. L.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets Nat. Nanotechnol. 2008, 3, 101– 105 DOI: 10.1038/nnano.2007.451
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Processable aqueous dispersions of graphene nanosheets
Li, Dan; Mueller, Marc B.; Gilje, Scott; Kaner, Richard B.; Wallace, Gordon G.
Nature Nanotechnology (2008), 3 (2), 101-105CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
A prerequisite for exploiting most applications for graphene is the availability of processable graphene sheets in large quantities. The direct dispersion of hydrophobic graphite or graphene sheets in water without the assistance of dispersing agents has generally been considered to be an insurmountable challenge. Here it is reported that chem. converted graphene sheets obtained from graphite can readily form stable aq. colloids through electrostatic stabilization. This discovery has enabled us to develop a facile approach to large-scale prodn. of aq. graphene dispersions without the need for polymeric or surfactant stabilizers. The findings make it possible to process graphene materials using low-cost soln. processing techniques, opening up enormous opportunities to use this unique carbon nanostructure for many technol. applications.
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Chowdhury, I.; Duch, M. C.; Mansukhani, N. D.; Hersam, M. C.; Bouchard, D. Colloidal properties and stability of graphene oxide nanomaterials in the aquatic environment Environ. Sci. Technol. 2013, 47, 6288– 6296 DOI: 10.1021/es400483k
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Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment
Chowdhury, Indranil; Duch, Matthew C.; Mansukhani, Nikhita D.; Hersam, Mark C.; Bouchard, Dermont
Environmental Science & Technology (2013), 47 (12), 6288-6296CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
While graphene oxide (GO) has been found the most toxic graphene-based nanomaterial, its environmental fate is still unexplored. The aggregation kinetics and stability of GO were studied using time-resolved dynamic light scattering over a wide range of aquatic chemistries (pH, salt types (NaCl, MgCl2, CaCl2), ionic strength) relevant to natural and engineered systems. Although pH did not have a notable effect on GO stability from pH 4 to 10, salt type and ionic strength had significant effects on GO stability due to elec. double layer compression, similar to other colloidal particles. The crit. coagulation concn. (CCC) values of GO were detd. to be 44mM NaCl, 0.9mM CaCl2, and 1.3mM MgCl2. Aggregation and stability of GO in the aquatic environment followed colloidal theory (DLVO and Schulze-Hardy rule), even though GO's shape is not spherical. CCC values of GO were lower than reported fullerene CCC values and higher than reported C nanotube CCC values. CaCl2 destabilized GO more aggressively than MgCl2 and NaCl due to the binding capacity of Ca2+ with hydroxyl and carbonyl functional groups of GO. Natural org. matter significantly improved the stability of GO in water primarily due to steric repulsion. Long-term stability studies demonstrated that GO was highly stable in both natural and synthetic surface waters, although it settled quickly in synthetic groundwater. While GO remained stable in synthetic influent wastewater, effluent wastewater collected from a treatment plant rapidly destabilized GO, indicating GO will settle out during the wastewater treatment process and likely accumulate in biosolids and sludge. Overall, our findings indicate that GO nanomaterials will be stable in the natural aquatic environment and that significant aq. transport of GO is possible.
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Wu, L.; Liu, L.; Gao, B.; Muňoz-Carpena, R.; Zhang, M.; Chen, H.; Zhou, Z. H.; Wang, H. Aggregation kinetics of graphene oxides in aqueous solutions: Experiments, mechanisms, and modeling Langmuir 2013, 29, 15174– 15181 DOI: 10.1021/la404134x
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Aggregation Kinetics of Graphene Oxides in Aqueous Solutions: Experiments, Mechanisms, and Modeling
Wu, Lei; Liu, Lin; Gao, Bin; Munoz-Carpena, Rafael; Zhang, Ming; Chen, Hao; Zhou, Zuhao; Wang, Hao
Langmuir (2013), 29 (49), 15174-15181CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
Although graphene oxide (GO) was used in many applications to improve human life quality, its environmental fate and behavior are still largely unknown. A novel approach that combines exptl. measurements and theor. calcns. was used to det. the aggregation kinetics of GO sheets in aq. solns. under different chem. conditions (e.g., cation valence and pH). Exptl. data showed that both cation valence and pH showed significant effect on the aggregation of GO sheets. The measured crit. coagulation concns. were in good agreement with the predictions of the extended Schulze-Hardy rule. Ca2+ and Mg2+ were more effective than Na+ in aggregating the GO sheets, which could be attributed to the crosslinking between GO sheets by the divalent cations through bridging the functional groups at the edges of the GO sheets. When soln. pH increases, deprotonation of carboxylic groups was found to play a key role in increasing GO sheet stability and surface charge development. Probably edge-to-edge and face-to-face interactions were the dominant modes of GO aggregation in the presence of divalent metal ions and H+, resp. A modified attachment efficiency (α) model was developed from the Maxwell approach with considerations of both primary and secondary min. The model predictions matched the exptl. measurements of the aggregation kinetics of GO sheets in aq. solns. under all of the tested exptl. conditions well.
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Chowdhury, I.; Mansukhani, N. D.; Guiney, L. M.; Hersam, M. C.; Bouchard, D. Aggregation and stability of reduced graphene oxide: Complex roles of divalent cations, pH, and natural organic matter Environ. Sci. Technol. 2015, 49, 10886– 10893 DOI: 10.1021/acs.est.5b01866
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Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter
Chowdhury, Indranil; Mansukhani, Nikhita D.; Guiney, Linda M.; Hersam, Mark C.; Bouchard, Dermont
Environmental Science & Technology (2015), 49 (18), 10886-10893CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The aggregation and stability of graphene oxide (GO) and 3 successively reduced GO (rGO) nanomaterials were studied. Reduced GO species were partially reduced GO (rGO-1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically, the effect of pH, ionic strength, ion valence, and presence of natural org. matter (NOM) were studied. Results show that stability of GO in water decreases with successive redn. of functional groups, with pH having the greatest influence on rGO stability. Stability is also dependent on ion valence and the concn. of surface functional groups. While pH did not noticeably affect stability of GO in the presence of 10mM NaCl, adding 0.1 mM CaCl2 reduced stability of GO with increased pH. This is due to adsorption of Ca2+ on the surface functional groups of GO which reduces the surface charge of GO. As the concn. of rGO functional groups decreased, so did the influence of Ca2+ on rGO stability. Crit. coagulation concns. (CCC) of GO, rGO-1h, and rGO-2h were ∼200, 35, and 30mM NaCl, resp. In the presence of CaCl2, CCC values of GO and rGO are quite similar, however. Long-term studies show that a significant amt. of rGO-1h and rGO-2h remain stable in Call's Creek surface water, while effluent wastewater readily destabilizes rGO. In the presence NOM and divalent cations (Ca2+, Mg2+), GO aggregates settle from suspension due to GO functional group bridging with NOM and divalent ions. However, rGO-1h and rGO-2h remain suspended due to their lower functional group concn. and resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations, and NOM can play complex roles in the fate of rGO and GO.
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Hong, B. J.; Compton, O. C.; An, Z.; Eryazici, I.; Nguyen, S. T. Successful stabilization of graphene oxide in electrolyte solutions: Enhancement of biofunctionalization and cellular uptake ACS Nano 2012, 6, 63– 73 DOI: 10.1021/nn202355p
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Successful Stabilization of Graphene Oxide in Electrolyte Solutions: Enhancement of Biofunctionalization and Cellular Uptake
Hong, Bong Jin; Compton, Owen C.; An, Zhi; Eryazici, Ibrahim; Nguyen, SonBinh T.
ACS Nano (2012), 6 (1), 63-73CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Aq. dispersions of graphene oxide are inherently unstable in the presence of electrolytes, which screen the electrostatic surface charge on these nanosheets and induce irreversible aggregation. Two complementary strategies, utilizing either electrostatic or steric stabilization, have been developed to enhance the stability of graphene oxide in electrolyte solns., allowing it to stay dispersed in cell culture media and serum. The electrostatic stabilization approach entails further oxidn. of graphene oxide to low C/O ratio (∼1.1) and increases ionic tolerance of these nanosheets. The steric stabilization technique employs an amphiphilic block copolymer that serves as a noncovalently bound surfactant to minimize the aggregate-inducing nanosheet-nanosheet interactions. Both strategies can stabilize graphene oxide nanosheets with large dimensions (>300 nm) in biol. media, allowing for an enhancement of >250% in the bioconjugation efficiency of streptavidin in comparison to untreated nanosheets. Notably, both strategies allow the stabilized nanosheets to be readily taken up by cells, demonstrating their excellent performance as potential drug delivery vehicles.
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Ren, X. M.; Li, J. X.; Tan, X. L.; Shi, W. Q.; Chen, C. L.; Shao, D. D.; Wen, T.; Wang, L. F.; Zhao, G. X.; Sheng, G. P.; Wang, X. K. Impact of Al₂O₃ on the aggregation and deposition of grapheme oxide Environ. Sci. Technol. 2014, 48, 5493– 5500 DOI: 10.1021/es404996b
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There is no corresponding record for this reference.
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Yang, S. T.; Chang, Y. L.; Wang, H. F.; Liu, G. B.; Chen, S.; Wang, Y. W.; Liu, Y. F.; Cao, A. Folding/aggregation of graphene oxide and its application in Cu²⁺ removal J. Colloid Interface Sci. 2010, 351, 122– 127 DOI: 10.1016/j.jcis.2010.07.042
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Folding/aggregation of graphene oxide and its application in Cu2+ removal
Yang, Sheng-Tao; Chang, Yanli; Wang, Haifang; Liu, Gangbo; Chen, Sheng; Wang, Yanwen; Liu, Yuanfang; Cao, Aoneng
Journal of Colloid and Interface Science (2010), 351 (1), 122-127CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
Graphene oxide (GO) can be aggregated by Cu2+ in aq. soln. with a huge Cu2+ absorption capacity. The Cu2+ causes GO sheets to be folded and also to form large aggregates that were characterized by confocal microscopy and at. force microscopy. The folding/aggregation is most likely triggered by the coordination between GO and Cu2+. The equil. Cu2+ concns. and equil. absorption capacity of GO est. the max. absorption capacity of GO for Cu2+ and the absorption model. GO has a huge absorption capacity for Cu2+, which is ∼10 times of that of active carbon. Representative results are presented and the implication to Cu2+ removal is discussed.
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Wang, J.; Chen, B. Adsorption and coadsorption of organic pollutant and heavy metal by graphene oxide and reduced graphene materials Chem. Eng. J. 2015, 281 (1) 379– 388 DOI: 10.1016/j.cej.2015.06.102
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Hummers, W. S.; Offeman, R. E. Preparation of graphite oxide J. Am. Chem. Soc. 1958, 80, 1339 DOI: 10.1021/ja01539a017
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Preparation of graphitic oxide
Hummers, Wm. S., Jr.; Offeman, Richard E.
Journal of the American Chemical Society (1958), 80 (), 1339CODEN: JACSAT; ISSN:0002-7863.
See U.S. 2,798,878 (C.A. 51, 15080a).
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Pan, Z. H.; Liu, N.; Fu, L.; Liu, Z. F. Wrinkle engineering: A new approach to massive graphene nanoribbon arrays J. Am. Chem. Soc. 2011, 133, 17578– 17581 DOI: 10.1021/ja207517u
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Wrinkle Engineering: A New Approach to Massive Graphene Nanoribbon Arrays
Pan, Zhong-Huai; Liu, Nan; Fu, Lei; Liu, Zhong-Fan
Journal of the American Chemical Society (2011), 133 (44), 17578-17581CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Wrinkles are often formed on CVD-graphene in an uncontrollable way. By designing the surface morphol. of growth substrate together with a suitable transfer technique, the authors are able to engineer the dimension, d., and orientation of wrinkles on transferred CVD-graphene. Such kind of wrinkle engineering is employed to fabricate highly aligned graphene nanoribbon (GNR) arrays by self-masked plasma-etching. Strictly consistent with the designed wrinkles, the d. of GNR arrays varied from ∼0.5 to 5 GNRs/μm, and over 88% GNRs are <10 nm in width. Elec. transport measurements of these GNR-based FETs exhibit an on/off ratio of ∼30, suggesting an opened bandgap. The authors' wrinkle engineering approach allows very easily for a massive prodn. of GNR arrays with bandgap-required widths, which opens a practical pathway for large-scale integrated graphene devices.
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Acik, M.; Lee, G.; Mattevi, C.; Chhowalla, M.; Cho, K.; Chabal, Y. J. Unusual infrared-absorption mechanism in thermally reduced graphene oxide Nat. Mater. 2010, 9, 840– 845 DOI: 10.1038/nmat2858
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Unusual infrared-absorption mechanism in thermally reduced graphene oxide
Acik, M.; Lee, G.; Mattevi, C.; Chhowalla, M.; Cho, K.; Chabal, Y. J.
Nature Materials (2010), 9 (10), 840-845CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
IR absorption of at. and mol. vibrations in solids can be affected by electronic contributions through nonadiabatic interactions, such as the Fano effect. Typically, the IR-absorption lineshapes are modified, or IR-forbidden modes are detectable as a modulation of the electronic absorption. In contrast to such known phenomena, the authors report here the observation of a giant-IR-absorption band in reduced graphene oxide, arising from the coupling of electronic states to the asym. stretch mode of a yet-unreported structure, consisting of O atoms aggregated at the edges of defects. Free electrons are induced by the displacement of the O atoms, leading to a strong IR absorption that is in phase with the phonon mode. This new phenomenon is only possible when all other O-contg. chem. species, including hydroxyl, carboxyl, epoxide and ketonic functional groups, are removed from the region adjacent to the edges, i.e., clean graphene patches are present.
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Peng, L.; Xu, Z.; Liu, Z.; Wei, Y. Y.; Sun, H. Y.; Li, Z.; Zhao, X. L.; Gao, C. An iron-based green approach to 1-h production of single-layer graphene oxide Nat. Commun. 2015, 6, 5716 DOI: 10.1038/ncomms6716
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An iron-based green approach to 1-h production of single-layer graphene oxide
Peng, Li; Xu, Zhen; Liu, Zheng; Wei, Yangyang; Sun, Haiyan; Li, Zheng; Zhao, Xiaoli; Gao, Chao
Nature Communications (2015), 6 (), 5716CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
As a reliable and scalable precursor of graphene, graphene oxide (GO) is of great importance. However, the environmentally hazardous heavy metals and poisonous gases, explosion risk and long reaction times involved in the current synthesis methods of GO increase the prodn. costs and hinder its real applications. Here we report an iron-based green strategy for the prodn. of single-layer GO in 1 h. Using the strong oxidant K2FeO4, our approach not only avoids the introduction of polluting heavy metals and toxic gases in prepn. and products but also enables the recycling of sulfuric acid, eliminating pollution. Our dried GO powder is highly sol. in water, in which it forms liq. crystals capable of being processed into macroscopic graphene fibers, films and aerogels. This green, safe, highly efficient and ultralow-cost approach paves the way to large-scale com. applications of graphene.
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Saleh, N. B.; Pfefferle, L. D.; Elimelech, M. Aggregation kinetics of multiwalled carbon nanotubes in aquatic systems: Measurements and environmental implications Environ. Sci. Technol. 2008, 42 (21) 7963– 7969 DOI: 10.1021/es801251c
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Aggregation Kinetics of Multiwalled Carbon Nanotubes in Aquatic Systems: Measurements and Environmental Implications
Saleh, Navid B.; Pfefferle, Lisa D.; Elimelech, Menachem
Environmental Science & Technology (2008), 42 (21), 7963-7969CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The initial aggregation kinetics of multiwalled carbon nanotubes (MWNTs) were examd. through time-resolved dynamic light scattering. Aggregation of MWNTs was evaluated by varying soln. pH and the concn. of monovalent (NaCl) and divalent (CaCl2 and MgCl2) salts. Suwannee River humic acid (SRHA) was used to study the effect of background natural org. matter on MWNT aggregation kinetics. Increasing salt concn. and addn. of divalent calcium and magnesium ions induced MWNT aggregation by suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The crit. coagulation concn. (CCC) values for MWNTs were estd. as 25mM NaCl, 2.6mM CaCl2, and 1.5mM MgCl2. An increase in soln. pH from acidic (pH 3) to basic (pH 11) conditions resulted in a substantial (over 2 orders of magnitude) decrease in MWNT aggregation kinetics, suggesting the presence of ionizable functional groups on the MWNT carbon scaffold. The presence of humic acid in soln. markedly enhanced the colloidal stability of MWNTs, reducing the aggregation rate by nearly 2 orders of magnitude. The enhanced MWNT stability in the presence of humic acid is attributable to steric repulsion imparted by adsorbed humic acid macromols. The results suggest that MWNTs are relatively stable at soln. pH and electrolyte conditions typical of aquatic environments.
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Bouchard, D.; Ma, X.; Isaacson, C. Colloidal properties of aqueous fullerenes: Isoelectric points and aggregation kinetics of C60 and C60 derivatives Environ. Sci. Technol. 2009, 43 (17) 6597– 6603 DOI: 10.1021/es901354r
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Overbeek, J. T. G. The Rule of Schulze and Hardy Pure Appl. Chem. 1980, 52, 1151– 1161 DOI: 10.1351/pac198052051151
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The rule of Schulze and Hardy
Overbeek, J. Theodoor G.
Pure and Applied Chemistry (1980), 52 (5), 1151-61CODEN: PACHAS; ISSN:0033-4545.
A theory is developed to explain the rule of Schulze and Hardy dealing with the coagulation of colloids. A combination of van der Waals attraction and electrostatic repulsion can explain the rule, if adsorption of counterions in the Stern layer is taken into account. The adsorption potential must then increase fairly strongly with the charge of the counterion. Structural forces (due to the mol. structure of the solvent) may be important, but the quant. theory of these forces has not yet been sufficiently developed to incorporate them in the theory of coagulation.
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Sano, M.; Okamura, J.; Shinkai, S. Colloidal nature of single-walled carbon nanotubes in electrolyte solution: The Schulze-Hardy Rule Langmuir 2001, 17, 7172– 7173 DOI: 10.1021/la010698+
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There is no corresponding record for this reference.
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Deng, X. J.; Lu, L. L.; Li, H. W.; Luo, F. The adsorption properties of Pb(II) and Cd(II) on functionalized graphene prepared by electrolysis method J. Hazard. Mater. 2010, 183, 923– 930 DOI: 10.1016/j.jhazmat.2010.07.117
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The adsorption properties of Pb(II) and Cd(II) on functionalized graphene prepared by electrolysis method
Deng, Xiaojiao; Lue, Lili; Li, Hongwei; Luo, Fang
Journal of Hazardous Materials (2010), 183 (1-3), 923-930CODEN: JHMAD9; ISSN:0304-3894. (Elsevier B.V.)
The functionalized graphene (GNSPF6) was fabricated by simple and fast method of electrolysis with K hexafluorophosphate soln. as electrolyte under the static potential of 15 V. The characterization results of TEM, atom force microscopy, XPS, x-ray powder diffraction, Raman spectroscopy and thermogravimetric anal. indicate that graphite rod was completely exfoliated to graphene layer contg. 30 wt.% PF6[n.713] with the av. thickness ∼1.0 nm. Our sample of GNSPF6 was developed for the removal of Pb(II) or Cd(II) from water, and the detd. adsorption capacities are 406.6 mg/g (pH =5.1) for Pb(II) and 73.42 mg/g (pH =6.2) for Cd(II), which is much higher than that by our previous sample of GNSC8P and C nanotube. The adsorption processes reach equil. in just 40 min and the adsorption isotherms are described well by Langmuir and Freundlich classical isotherms models.
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Li, Y. H.; Ding, J.; Luan, Z. K.; Di, Z. C.; Zhu, Y. F.; Xu, C. L.; Wu, D. H.; Wei, B. Q. Competitive adsorption of Pb, Cu and Cd ions from aqueous solutions by multiwalled carbon nanotubes Carbon 2003, 41, 2787– 2792 DOI: 10.1016/S0008-6223(03)00392-0
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Competitive adsorption of Pb2+, Cu2+ and Cd2+ ions from aqueous solutions by multiwalled carbon nanotubes
Li, Yan-Hui; Ding, Jun; Luan, Zhaokun; Di, Zechao; Zhu, Yuefeng; Xu, Cailu; Wu, Dehai; Wei, Bingqing
Carbon (2003), 41 (14), 2787-2792CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Science Ltd.)
The individual and competitive adsorption capacities of Pb2+, Cu2+ and Cd2+ by HNO3-treated multiwalled carbon nanotubes (CNTs) were studied. The max. sorption capacities calcd. by applying the Langmuir equation to single ion adsorption isotherms were 97.08 for Pb2+, 24.49 for Cu2+, and 10.86 mg/g for Cd2+ at an equil. concn. of 10 mg/L. The competitive adsorption studies showed that the affinity order of 3 metal ions adsorbed by CNTs is Pb2+ > Cu2+ > Cd2+. The Langmuir adsorption model can represent exptl. data of Pb2+ and Cu2+ well, but does not provide a good fit for Cd2+ adsorption data. The effects of soln. pH, ionic strength, and CNT dosage on the competitive adsorption of Pb2+, Cu2+, and Cd2+ ions were investigated. The comparison of CNTs with other adsorbents suggests that CNTs have great potential applications in environmental protection regardless of their higher cost at present.
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Imamoglu, M.; Tekir, O. Removal of copper (II) and lead (II) ions from aqueous solutions by adsorption on activated carbon from a new precursor hazelnut husks Desalination 2008, 228, 108– 113 DOI: 10.1016/j.desal.2007.08.011
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Removal of copper(II) and lead(II) ions from aqueous solutions by adsorption on activated carbon from a new precursor hazelnut husks
Imamoglu, Mustafa; Tekir, Oktay
Desalination (2008), 228 (1-3), 108-113CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
Activated carbon was prepd. from hazelnut husks with ZnCl2 activation at 973 K in nitrogen atm. BET surface area of the activated carbon was found 1092 m2 g-1. The removal of Cu(II) and Pb(II) from aq. solns. was studied by batch method. The effects of initial pH, contact time, activated carbon dosage and initial metal concns. were investigated. The activated carbon exhibited good adsorption potential for Cu and Pb ions. The exptl. data were analyzed by both Freundlich and Langmuir isotherms. The max. adsorption capacity of the adsorbent for Cu(II) and Pb(II) ions was calcd. from the Langmuir isotherm and found to be 6.645 and 13.05 mg g-1, resp.
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Xu, Y.; Chen, B. Organic carbon and inorganic silicon speciation in rice-bran-derived biochars affect its capacity to adsorb cadmium in solution J. Soils Sediments 2015, 15, 60– 70 DOI: 10.1007/s11368-014-0969-2
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Organic carbon and inorganic silicon speciation in rice-bran-derived biochars affect its capacity to adsorb cadmium in solution
Xu, Yiliang; Chen, Baoliang
Journal of Soils and Sediments (2015), 15 (1), 60-70CODEN: JSSOAJ; ISSN:1439-0108. (Springer)
Purpose: Biochar enriched in oxygenated functional groups and minerals has a high adsorption capacity for heavy metals. The roles and contributions of the org. and mineral fractions in biochars to heavy metal adsorption are crit. to understanding the long-term effect of biochar application in contaminated soil. Batch adsorption expts. were performed on differently treated rice-bran-derived biochars to det. the contributions of the org. and mineral fractions to cadmium (Cd) adsorption in this study. Materials and methods: Rice-bran-derived biochars were produced at 300 and 700 °C (named R300 and R700). The Cd adsorption onto the original biochars and biochars treated with water, hydrochloric acid (HCl), and HCl + HF was compared on the basis of their adsorption isotherm curves. Biochar properties were characterized by CHN elemental anal., XRD, zeta potential, Fourier transform IR spectroscopy (FTIR), and SEM-EDX before and after Cd adsorption. Results and discussion: The Cd adsorption capacity gradually decreased as the water-washing times increased, and ten- and hundred-fold decreases were obsd. on acid-treated biochars. The relative contribution of water-sol. matter to the decrease of Cd in soln. was 87.4 % and 71.4 % for R300 and R700, resp. The contribution of HCl-sol. minerals was 11.5 % (R300) and 28.3 % (R700), while the role of insol. silicon oxide could be ignored. The adsorption mechanism between Cd and the org. fractions or minerals in biochars was complexation, as illustrated by the FTIR spectra. Conclusions: Both the org. and mineral fractions in biochars play an important role in Cd adsorption. The relative contribution of water-sol. matter was much higher than that of HCl-sol. ashes and insol. silicon oxide minerals. The results indicate that the adsorption capacity of biochars for Cd may greatly decrease in leached and acidic soils with the loss of sol. org. and mineral fractions.
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Reddad, Z.; Gerente, C.; Andres, Y.; Le Cloirec, P. Adsorption of several metal ions onto a low-cost biosorbent: Kinetic and equilibrium studies Environ. Sci. Technol. 2002, 36, 2067– 2073 DOI: 10.1021/es0102989
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Adsorption of Several Metal Ions onto a Low-Cost Biosorbent: Kinetic and Equilibrium Studies
Reddad, Zacaria; Gerente, Claire; Andres, Yves; Le Cloirec, Pierre
Environmental Science and Technology (2002), 36 (9), 2067-2073CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Sugar beet pulp generated by sugar-refining factories has been shown to be an effective adsorbent for the removal of heavy metals from aq. solns. The structural components related to the metallic adsorption having been detd., batch adsorption studies were performed for several metal ions, namely, Pb2+, Cu2+, Zn2+, Cd2+, and Ni2+ cations. Two simple kinetic models, i.e., pseudo-first- and pseudo-second-order, were tested to investigate the adsorption mechanisms. The kinetic parameters of the models were calcd. and discussed. For an 8 × 10-4M initial metal concn., the initial sorption rates (v0) ranged from 0.063 mmol/g-min for Pb2+ to 0.275 mmol/g-min for Ni2+ ions, in the order Ni2+ > Cd2+ > Zn2+ > Cu2+ > Pb2+. The equil. data fitted well with the Langmuir and Freundlich models and showed the following affinity order of the material: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+. The metal removal was strongly dependent on pH and, to a lesser extent, ionic strength. Ion exchange, with Ca2+ ions neutralizing the carboxyl groups of the polysaccharide, was found to be the predominant mechanism, along with complexation for Pb2+, Cu2+, and Zn2+ metals.
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Mimmo, T. J.; Marzadori, C.; Montecchio, D.; Gessa, C. Characterisation of Ca- and Al-pectate gels by thermal analysis and FT-IR spectroscopy Carbohydr. Res. 2005, 340, 2510– 2519 DOI: 10.1016/j.carres.2005.08.011
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Ma, J. C.; Dougherty, D. A. The cation-π interaction Chem. Rev. 1997, 97, 1303– 1324 DOI: 10.1021/cr9603744
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The Cation-π Interaction
Ma, Jennifer C.; Dougherty, Dennis A.
Chemical Reviews (Washington, D. C.) (1997), 97 (5), 1303-1324CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review with 293 refs. concerning the title interaction with emphasis on side chain interactions in proteins and protein-ligand interactions.
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Tansel, B.; Sager, J.; Rector, T.; Garland, J.; Strayer, R. F.; Levine, L. F.; Roberts, M.; Hummerick, M.; Bauer, J. Significance of hydrated radius and hydration shells on ionic permeability during nanofiltration in dead end and cross flow modes Sep. Purif. Technol. 2006, 51, 40– 47 DOI: 10.1016/j.seppur.2005.12.020
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Significance of hydrated radius and hydration shells on ionic permeability during nanofiltration in dead end and cross flow modes
Tansel, Berrin; Sager, John; Rector, Tony; Garland, Jay; Strayer, Richard F.; Levine, Lanfang; Roberts, Michael; Hummerick, Mary; Bauer, Jan
Separation and Purification Technology (2006), 51 (1), 40-47CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
The aerobic rotational membrane system (ARMS) is one of the water recovery and recycling processes considered for use during long space missions. The ARMS effluent contains high levels of dissolved inorg. salts to be removed by the closed loop water recovery and recycling system. The purpose of this study was to investigate the permeability characteristics of ions present in the ARMS effluent through a nanofiltration membrane. Expts. were conducted in stirred dead end and cross flow filtration modes to evaluate the effectiveness of nanofiltration process to remove dissolved solids. The rejection characteristics of the ions were evaluated in relation to ionic hydrated radius, hydration no. and ionic viscosity parameters. Permeability of ions through the nanofiltration membrane showed a strong correlation with their hydrated radii. The ions with relatively smaller crystal radii (i.e., Mg2+ and Ca2+) have higher hydration nos. and larger hydrated radii. These ions also hold their hydration shells more strongly, hence, they were removed by both filtration modes but more effectively by dead end filtration. On the other hand, the ions with larger crystal radii (i.e., K+ and Na+) have weaker hydration shells, hence, they may be able to detach from their hydration layer while passing through the nanofiltration membrane. In dead end mode, the ions are subjected to relatively larger shear forces to pass through the membrane in comparison to cross flow where the ions move parallel to the membrane surface, hence the shear force is reduced. The permeation of the ions was consistently higher in the dead end mode than the cross flow mode operated at the same pressure.
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Nightingale, E. R. Phenomenological theory of ion solvation. Effective radii of hydrated ions J. Phys. Chem. 1959, 63, 1381– 1387 DOI: 10.1021/j150579a011
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Phenomenological theory of ion solvation. Effective radii of hydrated ions
Nightingale, E. R., Jr.
Journal of Physical Chemistry (1959), 63 (), 1381-7CODEN: JPCHAX; ISSN:0022-3654.
The empirical correction to Stokes' law proposed by Robinson and Stokes (Electrolyte Solutions, 1955, 512 pp. (C.A. 49, 11388h)) was extended for small ions to provide a concordant set of radii for the hydrated ions. Ions with a crystal ionic radius of about 2 A. exhibited a min. hydrated radius of 3.3 A., corresponding to the max. in the equiv. cond. The internal consistency of the set of radii was demonstated by correlation with the temp. coeff. of equiv. cond., the viscosity B-coeff. in the Jones-Dole equation (C.A. 24, 3696), and the partial molar ionic entropy. Except for the small monat. ions with the min. hydrated radius, the hydrated ionic radius at 25° was a linear function of the viscosity B-coeff. The significance of this relation was discussed in terms of the structural modification rendered by the ions to water.
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Trivedi, P.; Axe, L.; Dyer, J. Adsorption of metal ions onto goethite: Single-adsorbate and competitive systems Colloids Surf., A 2001, 191, 107– 121 DOI: 10.1016/S0927-7757(01)00768-3
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Adsorption of metal ions onto goethite: single-adsorbate and competitive systems
Trivedi, P.; Axe, L.; Dyer, J.
Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2001), 191 (1-2), 107-121CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
Adsorption of anthropogenically released toxic metals such as Ni and Zn to goethite effects their mobility and bioavailability in aquatic environments. Sorption studies were conducted to understand competitive adsorption of environmentally important metals such as Ni, Zn, and Ca onto the goethite surface. Adsorption edges conducted as a function of ionic strength suggest that these metals are chemisorbed to the goethite surface. The adsorption affinity follows the order of the inverse of the hydrated radii multiplied by the no. of waters in the primary solvation shell: Zn>Ni>Ca. Isotherm studies revealed a linear relation between the amt. of metal adsorbed and the aq. bulk phase concns., where site satn. was obtained by reducing the goethite concn. to 0.1 g/L. Accordingly the single-site Langmuir model provided a good fit; equil. consts. were independent of pH indicative of one type of adsorption reaction. The equil. consts. for both transition metals (Ni and Zn) were greater than that of Ca, suggesting that transition metals have a greater affinity for the surface. Analyses of site densities revealed 2 types of sites on the surface of goethite: high affinity ones to which transition metals bind, and low affinity sites that comprise 100× that of the high affinity ones. From the isotherm studies, it appears that only the alk. earth metals such as Ca adsorb to this lower affinity site. The single-site Langmuir model was able to accurately describe adsorption competition between Ni and Zn for the goethite surface. In contrast, no competitive effects were obsd. in Ni-Ca and Zn-Ca binary systems.
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Sa, R. J.; Zhu, W. L.; Shen, J. H.; Gong, Z.; Cheng, J. G.; Chen, K. X.; Jiang, H. L. How does ammonium dynamically interact with benzene in aqueous media? A first principle study using the Car-Parrinello molecular dynamics Method J. Phys. Chem. B 2006, 110, 5094– 5098 DOI: 10.1021/jp051692m
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How Does Ammonium Dynamically Interact with Benzene in Aqueous Media? A First Principle Study Using the Car-Parrinello Molecular Dynamics Method
Sa, Rongjian; Zhu, Weiliang; Shen, Jianhua; Gong, Zhen; Cheng, Jiagao; Chen, Kaixian; Jiang, Hualiang
Journal of Physical Chemistry B (2006), 110 (10), 5094-5098CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The Car-Parrinello mol. dynamics (CPMD) method was used to study the dynamic characteristics of the cation-π interaction between ammonium and benzene in gaseous and aq. media. The results obtained from the CPMD calcn. on the cation-π complex in the gaseous state were very similar to those calcd. from the Gaussian98 program with DFT and MP2 algorithms, demonstrating that CPMD is a valid approach for studying this system. Unlike the interaction in the gaseous state, our 12-ps CPMD simulation showed that the geometry of the complex in aq. soln. changes frequently in terms of the interaction angles and distances. Furthermore, the simulation revealed that the ammonium is constantly oscillating above the benzene plane in an aq. environment and interacts with benzene mostly through three of its hydrogen atoms. In contrast, the interaction of the cation with the arom. mol. in the gaseous state involves two hydrogen atoms. In addn., the free energy profile in aq. soln. was studied using constrained CPMD simulations, resulting in a calcd. binding free energy of -5.75 kcal/mol at an optimum interaction distance of ∼3.25 Å, indicating that the cation-π interaction between ammonium and benzene is stable even in aq. soln. Thus, this CPMD study suggested that the cation-π interaction between an ammonium (group) and an arom. structure could take place even on surfaces of protein or nucleic acids in soln.
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Xu, Y. C.; Shen, J. H.; Zhu, W. L.; Luo, X. M.; Chen, K. X.; Jiang, H. L. Influence of the water molecule on cation-π Interaction: Ab initio second order Møller-Plesset Perturbation Theory (MP2) calculations J. Phys. Chem. B 2005, 109, 5945– 5949 DOI: 10.1021/jp044568w
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Influence of the Water Molecule on Cation-π Interaction: Ab Initio Second Order Moller-Plesset Perturbation Theory (MP2) Calculations
Xu, Yechun; Shen, Jianhua; Zhu, Weiliang; Luo, Xiaomin; Chen, Kaixian; Jiang, Hualiang
Journal of Physical Chemistry B (2005), 109 (12), 5945-5949CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The influence of introducing water mols. into a cation-π complex on the interaction between the cation and the π system was investigated using the MP2/6-311++G** method to explore how a cation-π complex changes in terms of both its geometry and its binding strength during the hydration. The calcn. on the methylammonium-benzene complex showed that the cation-π interaction is weakened by introducing H2O mols. into the system. For example, the optimized interaction distance between the cation and the benzene becomes longer and longer, the transferred charge between them becomes less and less, and the cation-π binding strength becomes weaker and weaker as the water mol. is introduced one by one. Furthermore, the introduction of the third water mol. leads to a dramatic change in both the complex geometry and the binding energy, resulting in the destruction of the cation-π interaction. The decompn. on the binding energy shows that the influence is mostly brought out through the electrostatic and induction interactions. This study also demonstrated that the basis set superposition error, thermal energy, and zero-point vibrational energy are significant and needed to be cor. for accurately predicting the binding strength in a hydrated cation-π complex at the MP2/6-311++G** level. Therefore, the results are helpful to better understand the role of water mols. in some biol. processes involving cation-π interactions.
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Geim, A. K.; Novoselov, K. S. The rise of graphene Nat. Mater. 2007, 6, 183– 191 DOI: 10.1038/nmat1849
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The rise of graphene
Geim, A. K.; Novoselov, K. S.
Nature Materials (2007), 6 (3), 183-191CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
A review. Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when com. products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top expts. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
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Quintana, M.; Grzelczak, M.; Spyrou, K.; Calvaresi, M.; Bals, S.; Kooi, B.; Van Tendeloo, G.; Rudolf, P.; Zerbetto, F.; Prato, M. A simple road for the transformation of few-layer graphene into MWNTs J. Am. Chem. Soc. 2012, 134, 13310– 13315 DOI: 10.1021/ja303131j
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Patra, N.; Wang, B. Y.; Král, P. Nanodroplet activated and guided folding of graphene nanostructures Nano Lett. 2009, 9, 3766– 3771 DOI: 10.1021/nl9019616
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Nanodroplet Activated and Guided Folding of Graphene Nanostructures
Patra, Niladri; Wang, Boyang; Kral, Petr
Nano Letters (2009), 9 (11), 3766-3771CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
We demonstrate by mol. dynamics simulations that water nanodroplets can activate and guide the folding of planar graphene nanostructures. Once the nanodroplets are deposited at selected spots on the planar nanostructure, they can act as catalytic elements that initiate conformational changes and help to overcome deformation barriers assocd. with them. Nanodroplets can induce rapid bending, folding, sliding, rolling, and zipping of the planar nanostructures, which can lead to the assembly of nanoscale sandwiches, capsules, knots, and rings.
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Gong, Z.; Shen, H. Y.; Zhu, W. L.; Luo, X. M.; Chen, K. X.; Jiang, H. L. A computational study on electron transfer mechanism between alkaline earth metal atoms and cyclooctatetraene to form cation-π bonded complexes Chem. Phys. Lett. 2006, 423, 339– 343 DOI: 10.1016/j.cplett.2006.03.096
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Zhu, W. L.; Luo, X. M.; Puah, C. M.; Tan, X. J.; Shen, J. H.; Gu, J. D.; Chen, K. X.; Jiang, H. L. The multiplicity, strength, and nature of the interaction of nucleobases with alkaline and alkaline earth metal cations: A density functional theory investigation J. Phys. Chem. A 2004, 108, 4008– 4018 DOI: 10.1021/jp036911n
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The Multiplicity, Strength, and Nature of the Interaction of Nucleobases with Alkaline and Alkaline Earth Metal Cations: A Density Functional Theory Investigation
Zhu, Weiliang; Luo, Xiaomin; Puah, Chum Mok; Tan, Xiaojian; Shen, Jianhua; Gu, Jiande; Chen, Kaixian; Jiang, Hualiang
Journal of Physical Chemistry A (2004), 108 (18), 4008-4018CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
D. functional theory (DFT) calcns. were performed at the B3LYP/6-311++G(d,p) level to systematically explore the geometrical multiplicity and binding strength for the complexes formed by alk. and alk. earth metal cations, viz. Li+, Na+, K+, Be2+, Mg2+, and Ca2+ (Mn+, hereinafter), with nucleobases, namely, adenine, cytosine, guanine, thymine, and uracil. Morokuma decompn. and orbital anal. were used to analyze the binding components. A total of 150 initial structures were designed and optimized, of which 93 optimized structures were found, which could be divided into two different types: cation-π complex and cation-heteroatom complex. In the former, a Mn+ is located above the nucleobase ring, while in the latter a Mn+ directly interacts in flank with the heteroatom(s) of a nucleobase. The strongest binding of -319.2 kcal/mol was found in the Be2+-guanine complex. Furthermore, the planar ring structures of the nucleobases in some cation-π complexes were deformed, destroying more or less the aromaticity of the corresponding nucleobases. In the cation-heteroatom complex, bidentate binding is generally stronger than unidentate binding, and of which the bidentate binding with five-membered ring structure has the strongest interaction. Moreover, the calcd. Mulliken charges showed that the transferred charge is linearly proportional to the binding strength. MO coeff. anal. indicated a significant orbital interaction in cation-π complex, but not in cation-heteroatom interaction. In addn., Morokuma decompn. revealed that electrostatic interaction is more important for cation-heteroatom binding. The majority of the calcd. ΔH values are in good agreement with the exptl. results. In those cases with significant differences, the exptl. results are proximate to an av. of the ΔH values of two isomers formed by the same nucleobase and cation.
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Kumpf, R. A.; Dougherty, D. A. A mechanism for ion selectivity in potassium channels: Computational studies of cation-π interactions Science 1993, 261, 1708– 1710 DOI: 10.1126/science.8378771
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A mechanism for ion selectivity in potassium channels: computational studies of cation-π interactions
Kumpf, Robert A.; Dougherty, Dennis A.
Science (Washington, DC, United States) (1993), 261 (5129), 1708-10CODEN: SCIEAS; ISSN:0036-8075.
A combination of computational methods has been used to evaluate the interaction between the π face of a benzene mol. and the monovalent cations of lithium, sodium, potassium, and rubidium. In the gas phase, the ions are strongly bound, and the affinity for benzene follows the expected electrostatic trend (lithium, largest; rubidium, smallest). However, in an aq. environment, a reordering occurs such that the potassium ion is preferred over all the other ions for 2:1 benzene:ion complexes. The selectivity sequence parallels that seen in voltage-gated potassium channels. Given that several conserved arom. residues are present in the pore region of such channels, these results suggest that the cation-π interaction may be responsible for the ion selectivity in potassium channels.
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Reddy, A. S.; Vijay, D.; Sastry, G. M.; Sastry, G. N. From subtle to substantial: Role of metal ions on π-π interactions J. Phys. Chem. B 2006, 110, 2479– 2481 DOI: 10.1021/jp060018h
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Duan, M. Y.; Song, B.; Shi, G. S.; Li, H. K.; Ji, G. F.; Hu, J.; Chen, X. G.; Fang, H. P. Cation × 3π: Cooperative interaction of a cation and three benzenes with an anomalous order in binding energy J. Am. Chem. Soc. 2012, 134, 12104– 12109 DOI: 10.1021/ja302918t
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Cite this: Environ. Sci. Technol. 2016, 50, 20, 11066–11075

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Abstract
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Figure 1
Figure 1. AFM (A), TEM (B), and FTIR (C) characterization of GO.
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Figure 2
Figure 2. Electrophoretic mobility (EPM) and hydrodynamic diameter of GO as a function of the cation concentration of H⁺ (A₁, A₂), monovalent metal cations (B₁, B₂), divalent metal cations (C₁, C₂), and trivalent metal cations (D₁, D₂).
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Figure 3
Figure 3. Bulk flocculation of GO in aqueous solution containing monovalent cations (A), divalent cations (B), and trivalent cations (C) with different concentrations to determine their destabilization capability.
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Figure 4
Figure 4. Effects of cation type and cation concentration on GO aggregation. Typical aggregation kinetics of GO at different K⁺ concentrations (A). Attachment efficiency (α) of GO as a function of monovalent cation concentration (B), divalent cation concentration (C), and trivalent cation concentration (D).
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Figure 5
Figure 5. Isotherms of Pb²⁺, Cu²⁺, and Cd²⁺ onto GO during the aggregation process (A) and relationship between EPM and the adsorption of GO (B).
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Figure 6
Figure 6. TEM and AFM images of the GO flocculate during aggregation with the presence of Cd²⁺. TEM images of aggregated GO (A), magnified version (B), with small size curled GO in the dispersed situation (C), with a typical 1D tube-like structure (D), with a 2D multiple folded structure (E), and a 3D tightly aggregated structure (F). AFM images of flocculated GO (G, H). G1 and H1 show the testing line height in parts G and H, respectively.
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Figure 7
Figure 7. Proposed interactions of heavy metal cations with GO nanosheets in three steps: (I) adsorption of heavy metal onto GO nanosheets; (II) formation of 1D-, 2D-, and 3D-like structures; and (III) aggregation of sphere-like flocculation.
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References
This article references 63 other publications.
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  Zhu, Jixin; Zhu, Ting; Zhou, Xiaozhu; Zhang, Yanyan; Lou, Xiong Wen; Chen, Xiaodong; Zhang, Hua; Hng, Huey Hoon; Yan, Qingyu
  Nanoscale (2011), 3 (3), 1084-1089CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
  The authors report an environment-friendly approach to synthesize transition metal oxide nanoparticles (NPs)/reduced graphene oxide (rGO) sheets hybrids by combining the redn. of graphene oxide (GO) with the growth of metal oxide NPs in one step. Either Fe2O3 or CoO NPs were grown onto rGO sheets in EtOH soln. through a solvothermal process, during which GOs were reduced to rGO without the addn. of any strong reducing agent, e.g. hydrazine, or requiring any post-high-temp. annealing process. The GO or rGO during the pptn. of metal oxide NPs may act as heterogeneous nucleation seeds to facilitate the formation of small crystal grains. This may allow more efficient diffusion of Li ions and lead to high specific capacities. These metal oxide NPs-rGO hybrids were used as anodes for Li-ion batteries, which showed high capacities and excellent charge-discharge cycling stability in the voltage window between 0.01 and 3.0 V. For example, Fe2O3 NPs/rGO hybrids showed specific capacity of 881 mA h g-1 in the 90th cycle at a discharge c.d. of 302 mA g-1 (0.3 C), while CoO NPs/rGO hybrids showed a lower capacity of 600 mA h g-1 in the 90th cycle at a discharge c.d. of 215 mA g-1 (0.3 C). These nanohybrids also show excellent capacities at high C rate currents, e.g. 611 mA h g-1 for Fe2O3/rGO sample in the 300th cycle at 2014 mA g-1 (2 C). Such synthesis technique can be a promising route to produce advanced electrode materials for Li-ion batteries.
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  Using reduced graphene oxide (RGO) as a two-dimensional support, the authors have succeeded in selective anchoring of semiconductor and metal nanoparticles at sep. sites. Photogenerated electrons from UV-irradiated TiO2 are transported across RGO to reduce silver ions into silver nanoparticles at a location distinct from the TiO2 anchored site. The ability of RGO to store and shuttle electrons, as visualized via a stepwise electron transfer process, demonstrates its capability to serve as a catalyst nanomat and transfer electrons on demand to adsorbed species. These findings pave the way for the development of next generation catalyst systems and can spur advancements in graphene-based composites for chem. and biol. sensors.
12. 12
  Scheuermann, G. M.; Rumi, L. G.; Steurer, P.; Bannwarth, W.; Mülhaupt, R. Palladium Nanoparticles on graphite oxide and its functionalized graphene derivatives as highly active catalysts for the Suzuki-Miyaura coupling reaction J. Am. Chem. Soc. 2009, 131, 8262– 8270 DOI: 10.1021/ja901105a
  12
  Palladium Nanoparticles on Graphite Oxide and Its Functionalized Graphene Derivatives as Highly Active Catalysts for the Suzuki-Miyaura Coupling Reaction
  Scheuermann, Gil M.; Rumi, Luigi; Steurer, Peter; Bannwarth, Willi; Mulhaupt, Rolf
  Journal of the American Chemical Society (2009), 131 (23), 8262-8270CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Pd2+-exchanged graphite oxide and chem. derived graphenes therefrom were employed as supports for Pd nanoparticles. The influence of catalyst prepn., carbon functionalization, and catalyst morphol. on the catalytic activity in the Suzuki-Miyaura coupling reactions was investigated. The catalysts were characterized by means of spectroscopy (FT-IR, solid-state 13C NMR, AAS, XPS), X-ray scattering (WAXS), surface area anal. (BET, methylene blue adsorption), and electron microscopy (TEM, ESEM). In contrast to the conventional Pd/C catalyst, graphite oxide and graphene-based catalysts gave much higher activities with turnover frequencies exceeding 39 000 h-1, accompanied by very low palladium leaching (<1 ppm).
13. 13
  Song, H. J.; Zhang, L. C.; He, C. L.; Qu, Y.; Tian, Y. F.; Lv, Y. Graphene sheets decorated with SnO₂ nanoparticles: In situ synthesis and highly efficient materials for cataluminescence gas sensors J. Mater. Chem. 2011, 21, 5972– 5977 DOI: 10.1039/c0jm04331a
  13
  Graphene sheets decorated with SnO2 nanoparticles: in situ synthesis and highly efficient materials for cataluminescence gas sensors
  Song, Hongjie; Zhang, Lichun; He, Chunlan; Qu, Ying; Tian, Yunfei; Lv, Yi
  Journal of Materials Chemistry (2011), 21 (16), 5972-5977CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
  Graphene sheets decorated with SnO2 nanoparticles were prepd. through a facile hydrothermal-assisted in situ synthesis route. According to the XPS, XRD, FESEM and TEM anal., rutile SnO2 nanocrystals were exclusively deposited on graphene sheets with high d. and high uniformity to form layered composite sheets. Propanal, a common volatile org. compd., was selected as a model to study the cataluminescence (CTL) sensing properties of the SnO2/graphene composite. The strong CTL emission could be generated due to the catalyzing oxidization of propanal on the surface of SnO2/graphene composite and this composite was an efficient sensing material for propanal. The authors further studied the anal. characteristics of the CTL sensor based on SnO2/graphene composite sensing material for propanal under the optimal exptl. conditions. The linear range of the propanal gas sensor was 1.34-266.67 μg mL-1 (r = 0.9987), over two orders of magnitude, and the detection limit was 0.3 μg mL-1(S/N = 3).
14. 14
  Chitara, B.; Panchakarla, L. S.; Krupanidhi, S. B.; Rao, C. N. R. Infrared photodetectors based on reduced graphene oxide and graphene nanoribbons Adv. Mater. 2011, 23, 5419– 5424 DOI: 10.1002/adma.201101414
  14
  Infrared Photodetectors Based on Reduced Graphene Oxide and Graphene Nanoribbons
  Chitara, Basant; Panchakarla, L. S.; Krupanidhi, S. B.; Rao, C. N. R.
  Advanced Materials (Weinheim, Germany) (2011), 23 (45), 5419-5424CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  IR detection is demonstrated by both reduced graphene oxide (RGO) and graphene nanoribbons (GNRs) in terms of time-resolved photocurrent and photoresponse. The responsivity and external quantum efficiency of RGO are 4 mA W-1 and 0.3%, resp., whereas for GNR these values are significantly higher (∼1000 times), being 1 A W-1 and 80%, resp., for an incident wavelength of 1550 nm at 2 V. All these results clearly demonstrate that RGO and GNRs, but esp. the later, can be effectively used as high-selectivity, high-sensitivity, and high-speed nanometer-scale photodetectors and photoelectronic switches, suggesting a strong impact in many future applications of graphene, including in nanoelectronics.
15. 15
  Zhao, G. X.; Li, J. X.; Ren, X. M.; Chen, C. L.; Wang, X. K. Few-Layered Graphene Oxide nanosheets as superior sorbents for heavy metal ion pollution management Environ. Sci. Technol. 2011, 45, 10454– 10462 DOI: 10.1021/es203439v
  15
  Few-layered graphene oxide nanosheets as superior sorbents for heavy metal ion, water pollution control
  Zhao, Guixia; Li, Jiaxing; Ren, Xuemei; Chen, Changlun; Wang, Xiangke
  Environmental Science & Technology (2011), 45 (24), 10454-10462CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Graphene has attracted multidisciplinary study because of its unique physicochem. properties. Herein, few-layered graphene oxide nanosheets were synthesized from graphite using the modified Hummers method, and were used as sorbents for the removal of Cd(II) and Co(II) ions from large vols. of aq. solns. The effects of pH, ionic strength, and humic acid on Cd(II) and Co(II) sorption were investigated. The results indicated that Cd(II) and Co(II) sorption on graphene oxide nanosheets was strongly dependent on pH and weakly dependent on ionic strength. The abundant oxygen-contg. functional groups on the surfaces of graphene oxide nanosheets played an important role on Cd(II) and Co(II) sorption. The presence of humic acid reduced Cd(II) and Co(II) sorption on graphene oxide nanosheets at pH < 8. The max. sorption capacities (Csmax) of Cd(II) and Co(II) on graphene oxide nanosheets at pH 6.0 ± 0.1 and T = 303 K were about 106.3 and 68.2 mg/g, resp., higher than any currently reported. The thermodn. parameters calcd. from temp.-dependent sorption isotherms suggested that Cd(II) and Co(II) sorptions on graphene oxide nanosheets were endothermic and spontaneous processes. The graphene oxide nanosheets may be suitable materials in heavy metal ion pollution cleanup if they are synthesized in large scale and at low price in near future.
16. 16
  Zhao, G. X.; Jiang, L.; He, Y. D.; Li, J. X.; Dong, H. L.; Wang, X. K.; Hu, W. P. Sulfonated grapheme for persistent aromatic pollutant management Adv. Mater. 2011, 23, 3959– 3963 DOI: 10.1002/adma.201101007
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  Yang, K.; Chen, B.; Zhu, L. Graphene-coated materials using silica particles as a framework for highly efficient removal of aromatic pollutants in water Sci. Rep. 2015, 5, 11641 DOI: 10.1038/srep11641
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  Graphene-coated materials using silica particles as a framework for highly efficient removal of aromatic pollutants in water
  Yang, Kaijie; Chen, Baoliang; Zhu, Lizhong
  Scientific Reports (2015), 5 (), 11641CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
  The substantial aggregation of pristine graphene nanosheets decreases its powerful adsorption capacity and diminishes its practical applications. To overcome this shortcoming, graphene-coated materials (GCMs) were prepd. by loading graphene onto silica nanoparticles (SiO2). With the support of SiO2, the stacked interlamination of graphene was held open to expose the powerful adsorption sites in the interlayers. The adsorption of phenanthrene, a model arom. pollutant, onto the loaded graphene nanosheets increased up to 100 fold compared with pristine graphene at the same level. The adsorption of GCMs increased with the loading amt. of the graphene nanosheets and dramatically decreased with the introduction of oxygen-contg. groups in the graphene nanosheets. The highly hydrophobic effect and the strong π-π stacking interactions of the exposed graphene nanosheets contributed to their superior adsorption of GCMs. An unusual GCM peak adsorption coeff. (Kd) was obsd. with the increase in sorbate concn. The sorbate concn. at peak Kd shifted to lower values for the reduced graphene oxide and graphene relative to the graphene oxide. Therefore, the replacement of water nanodroplets attached to the graphene nanosheets through weak non-hydrogen bonding with phenanthrene mols. via strong π-π stacking interactions is hypothesized to be an addnl. adsorption mechanism for GCMs.
18. 18
  Yang, K.; Wang, J.; Chen, B. Facile fabrication of stable monolayer and few layer graphene nanosheets as superior sorbents for persistent aromatic pollutant management in water J. Mater. Chem. A 2014, 2, 18219– 18224 DOI: 10.1039/C4TA04300F
  18
  Facile fabrication of stable monolayer and few-layer graphene nanosheets as superior sorbents for persistent aromatic pollutant management in water
  Yang, Kaijie; Wang, Jun; Chen, Baoliang
  Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (43), 18219-18224CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
  A facile method was employed to prep. stable monolayer and few-layer graphene (ASG) nanosheets by loading on nanosilica substrates. The adsorption capability of ASG was enhanced by a factor of up to 102 for phenanthrene compared with pristine graphene, which was higher than that in previous reports for nanomaterials. Interaction mechanisms of monolayer graphene with org. solutes were discussed.
19. 19
  Shen, Y.; Fang, Q.; Chen, B. Environmental applications of three-dimensional graphene-based macrostructures: Adsorption, transformation and detection Environ. Sci. Technol. 2015, 49, 67– 84 DOI: 10.1021/es504421y
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20. 20
  Shen, Y.; Chen, B. Sulfonated graphene nanosheets as a superb adsorbent for various environmental pollutants in water Environ. Sci. Technol. 2015, 49, 7364– 7372 DOI: 10.1021/acs.est.5b01057
  20
  Sulfonated Graphene Nanosheets as a Superb Adsorbent for Various Environmental Pollutants in Water
  Shen, Yi; Chen, Baoliang
  Environmental Science & Technology (2015), 49 (12), 7364-7372CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Graphene nanosheets, as a novel nanoadsorbent, can be further modified to optimize the adsorption capability for various pollutants. To overcome the structural limits of graphene (aggregation) and graphene oxide (hydrophilic surface) in water, sulfonated graphene (GS) was prepd. by diazotization reaction using sulfanilic acid. It was demonstrated that GS not only recovered a relatively complete sp2-hybridized plane with high affinity for arom. pollutants but also had sulfonic acid groups and partial original oxygen-contg. groups that powerfully attracted pos. charged pollutants. The satd. adsorption capacities of GS were 400 mg/g for phenanthrene, 906 mg/g for methylene blue and 58 mg/g for Cd2+, which were much higher than the corresponding values for reduced graphene oxide and graphene oxide. GS as a graphene-based adsorbent exhibits fast adsorption kinetic rate and superior adsorption capacity toward various pollutants, which mainly thanks to the multiple adsorption sites in GS including the conjugate π region sites and the functional group sites. Moreover, the sulfonic acid groups endow GS with the good dispersibility and single or few nanosheets which guarantee the adsorption processes. It is great potential to expose the adsorption sites of graphene nanosheets for pollutants in water by regulating their microstructures, surface properties and water dispersion.
21. 21
  Ji, L.; Chen, W.; Xu, Z.; Zheng, S.; Zhu, D. Graphene nanosheets and graphite oxide as promising adsorbents for removal of organic contaminants from aqueous solution Journal of Environment Quality 2013, 42, 191– 198 DOI: 10.2134/jeq2012.0172
  21
  Graphene nanosheets and graphite oxide as promising adsorbents for removal of organic contaminants from aqueous solution
  Ji, Liangliang; Chen, Wei; Xu, Zhaoyi; Zheng, Shourong; Zhu, Dongqiang
  Journal of Environmental Quality (2013), 42 (1), 191-198CODEN: JEVQAA; ISSN:1537-2537. (American Society of Agronomy)
  Graphenes are an emerging class of carbon nanomaterials whose adsorption properties toward org. compds. have not been well understood. In the present study, graphene nanosheets were prepd. by reoxidn. and abrupt heating of graphite oxide, which was prepd. by sequential chem. oxidn. of com. nonporous graphite powder. Adsorption properties of three arom. compds. (naphthalene, 2-naphthol, and 1-naphthylamine) and one pharmaceutical compd. (tylosin) on graphene nanosheets and graphite oxide were examd. to explore the potential of these two adsorbents for the removal of org. contaminants from aq. solns. Compared with the literature data of adsorption on carbon nanotubes, adsorption of bulky, flexible tylosin on graphene nanosheets exhibited markedly faster adsorption kinetics, which can be attributed to their opened-up layer structure. Graphene nanosheets and graphite oxide showed similar sequences of adsorption affinity: 1-naphthylamine > 2-naphthol > tylosin > naphthalene (with much larger differences obsd. on graphite oxide). It was proposed that the strong adsorption of the three arom. compds. was mainly due to π-π electron donor-acceptor interactions with the graphitic surfaces of adsorbents. Addnl., Lewis acid-base interaction was likely an important factor contributing to the strong adsorption of 1-naphthylamine and tylosin, esp. for the O-functionality-abundant graphite oxide. After being normalized on the basis of adsorbent surface area, adsorption affinities of all four tested adsorbates on graphene nanosheets were very close to those on nonporous graphite powder, reflecting complete accessibility of the adsorbent surface area in adsorption.
22. 22
  Wang, J.; Chen, Z.; Chen, B. Adsorption of polycyclic aromatic hydrocarbons by graphene and graphene oxide nanosheets Environ. Sci. Technol. 2014, 48, 4817– 4825 DOI: 10.1021/es405227u
  22
  Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets
  Wang, Jun; Chen, Zaiming; Chen, Baoliang
  Environmental Science & Technology (2014), 48 (9), 4817-4825CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The adsorption of naphthalene, phenanthrene, and pyrene onto graphene (GNS) and graphene oxide (GO) nanosheets was investigated to probe the potential adsorptive sites and mol. mechanisms. The microstructure and morphol. of GNS and GO were characterized by elemental anal., XPS, FTIR, Raman, SEM, and TEM. Graphene displayed high affinity to the polycyclic arom. hydrocarbons (PAHs), whereas GO adsorption was significantly reduced after oxygen-contg. groups were attached to GNS surfaces. An unexpected peak was found in the curve of adsorption coeffs. (Kd) with the PAH equil. concns. The hydrophobic properties and mol. sizes of the PAHs affected the adsorption of G and GO. The high affinities of the PAHs to GNS are dominated by π-π interactions to the flat surface and the sieving effect of the powerful groove regions formed by wrinkles on GNS surfaces. In contrast, the adsorptive sites of GO changed to the carboxyl groups attaching to the edges of GO because the groove regions disappeared and the polar nanosheet surfaces limited the π-π interactions. The TEM and SEM images initially revealed that after loading with PAH, the conformation and aggregation of GNS and GO nanosheets dramatically changed, which explained the observations that the potential adsorption sites of GNS and GO were unusually altered during the adsorption process.
23. 23
  Chen, X.; Chen, B. Macroscopic and spectroscopic investigations on adsorption of nitroaromatic compounds to graphene oxide, and reduced graphene oxide and graphene nanosheets Environ. Sci. Technol. 2015, 49, 6181– 6189 DOI: 10.1021/es5054946
  There is no corresponding record for this reference.
24. 24
  Bianco, A. Graphene: safe or toxic? The two faces of the medal Angew. Chem., Int. Ed. 2013, 52, 4986– 4997 DOI: 10.1002/anie.201209099
  24
  Graphene: Safe or Toxic? The Two Faces of the Medal
  Bianco, Alberto
  Angewandte Chemie, International Edition (2013), 52 (19), 4986-4997CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Graphene is considered the future revolutionary material. For its development, it is of fundamental importance to evaluate the safety profile and the impact on health. Graphene is part of a bigger family which has been identified as the graphene family nanomaterials (GFNs). Clarifying the existence of multiple graphene forms allows better understanding the differences between the components and eventually correlating their biol. effects to the physicochem. characteristics of each structure. Some in vitro and in vivo studies clearly showed no particular risks, while others have indicated that GFNs might become health hazards. This Minireview critically discusses the recent studies on the toxicity of GFNs to provide some perspective on the possible risks to their future development in materials and biomedical sciences.
25. 25
  Akhavan, O.; Ghaderi, E.; Emamy, H.; Akhavan, F. Genotoxicity of graphene nanoribbons in human mesenchymal stem cells Carbon 2013, 54, 419– 431 DOI: 10.1016/j.carbon.2012.11.058
  25
  Genotoxicity of graphene nanoribbons in human mesenchymal stem cells
  Akhavan, Omid; Ghaderi, Elham; Emamy, Hamed; Akhavan, Fatima
  Carbon (2013), 54 (), 419-431CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
  Single-layer reduced graphene oxide nanoribbons (rGONRs) were obtained through an oxidative unzipping of multi-walled carbon nanotubes and a subsequent deoxygenation by hydrazine and bovine serum albumin. Human mesenchymal stem cells (hMSCs) were isolated from umbilical cord blood and used for checking the concn.- and time-dependent cytotoxic and genotoxic effects of the rGONRs and reduced graphene oxide sheets (rGOSs). The cell viability assay indicated significant cytotoxic effects of 10 μg/mL rGONRs after 1 h exposure time, while the rGOSs exhibited the same cytotoxicity at concn. of 100 μg/mL after 96 h. The oxidative stress was found as the main mechanism involved in the cytotoxicity of the rGOSs which induced a slight cell membrane damage, while RNA efflux of the hMSCs indicated that neither generation of reactive oxygen species nor the significant membrane damage of the cells could explain the cell destructions induced by the rGONRs. Our results demonstrated that, the rGONRs could penetrate into the cells and cause DNA fragmentations as well as chromosomal aberrations, even at low concn. of 1.0 μg/mL after short exposure time of 1 h.
26. 26
  Akhavan, O.; Ghaderi, E. Toxicity of graphene and graphene oxide nanowalls against bacteria ACS Nano 2010, 4, 5731– 5736 DOI: 10.1021/nn101390x
  26
  Toxicity of graphene and graphene oxide nanowalls against bacteria
  Akhavan, Omid; Ghaderi, Elham
  ACS Nano (2010), 4 (10), 5731-5736CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Bacterial toxicity of graphene nanosheets in the form of graphene nanowalls deposited on stainless steel substrates was investigated for both Gram-pos. and Gram-neg. models of bacteria. The graphene oxide nanowalls were obtained by electrophoretic deposition of Mg2+-graphene oxide nanosheets synthesized by a chem. exfoliation method. On the basis of measuring the efflux of cytoplasmic materials of the bacteria, it was found that the cell membrane damage of the bacteria caused by direct contact of the bacteria with the extremely sharp edges of the nanowalls was the effective mechanism in the bacterial inactivation. In this regard, the Gram-neg. Escherichia coli with an outer membrane were more resistant to the cell membrane damage caused by the nanowalls than the Gram-pos. Staphylococcus aureus lacking the outer membrane. Moreover, the graphene oxide nanowalls reduced by hydrazine were more toxic to the bacteria than the unreduced graphene oxide nanowalls. The better antibacterial activity of the reduced nanowalls was assigned to the better charge transfer between the bacteria and the more sharpened edges of the reduced nanowalls, during the contact interaction.
27. 27
  Akhavan, O.; Ghaderi, E.; Akhavan, A. Size-dependent genotoxicity of graphene nanoplatelets in human stem cells Biomaterials 2012, 33, 8017– 8025 DOI: 10.1016/j.biomaterials.2012.07.040
  27
  Size-dependent genotoxicity of graphene nanoplatelets in human stem cells
  Akhavan, Omid; Ghaderi, Elham; Akhavan, Alireza
  Biomaterials (2012), 33 (32), 8017-8025CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
  Reduced graphene oxide nanoplatelets (rGONPs) were synthesized by sonication of covalently PEGylated GO sheets followed by a chem. redn. using hydrazine and bovine serum albumin. Human mesenchymal stem cells (hMSCs), as a fundamental factor in tissue engineering, were isolated from umbilical cord blood (as a recently proposed source for extg. fresh hMSCs) to investigate, for the first time, the size-dependent cyto- and geno-toxic effects of the rGONPs on the cells. The cell viability test showed significant cell destructions by 1.0 μg/mL rGONPs with av. lateral dimensions (ALDs) of 11±4 nm, while the rGO sheets with ALDs of 3.8±0.4 μm could exhibit a significant cytotoxic effect only at highconcn. of 100 μg/mL after 1 h exposure time. Although oxidative stress and cell wall membrane damage were detd. as the main mechanism involved in the cytotoxicity of the rGO sheets, neither of them could completely describe the cell destructions induced by the rGONPs, esp. at the concns. \\≤\\ 1.0 μg/mL. In fact, the rGONPs showed genotoxic effects on the stem cells through DNA fragmentations and chromosomal aberrations, even at low concn. of 0.1 μg/mL. Our results present essential knowledge for more efficient and innocuous applications of graphene sheets and particularly nanoplatelets in upcoming nanotechnol.-based tissue engineering as, e.g., drug transporter and scaffolds.
28. 28
  Li, D.; Müller, M. B.; Gilje, S. L.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets Nat. Nanotechnol. 2008, 3, 101– 105 DOI: 10.1038/nnano.2007.451
  28
  Processable aqueous dispersions of graphene nanosheets
  Li, Dan; Mueller, Marc B.; Gilje, Scott; Kaner, Richard B.; Wallace, Gordon G.
  Nature Nanotechnology (2008), 3 (2), 101-105CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
  A prerequisite for exploiting most applications for graphene is the availability of processable graphene sheets in large quantities. The direct dispersion of hydrophobic graphite or graphene sheets in water without the assistance of dispersing agents has generally been considered to be an insurmountable challenge. Here it is reported that chem. converted graphene sheets obtained from graphite can readily form stable aq. colloids through electrostatic stabilization. This discovery has enabled us to develop a facile approach to large-scale prodn. of aq. graphene dispersions without the need for polymeric or surfactant stabilizers. The findings make it possible to process graphene materials using low-cost soln. processing techniques, opening up enormous opportunities to use this unique carbon nanostructure for many technol. applications.
29. 29
  Chowdhury, I.; Duch, M. C.; Mansukhani, N. D.; Hersam, M. C.; Bouchard, D. Colloidal properties and stability of graphene oxide nanomaterials in the aquatic environment Environ. Sci. Technol. 2013, 47, 6288– 6296 DOI: 10.1021/es400483k
  29
  Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment
  Chowdhury, Indranil; Duch, Matthew C.; Mansukhani, Nikhita D.; Hersam, Mark C.; Bouchard, Dermont
  Environmental Science & Technology (2013), 47 (12), 6288-6296CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  While graphene oxide (GO) has been found the most toxic graphene-based nanomaterial, its environmental fate is still unexplored. The aggregation kinetics and stability of GO were studied using time-resolved dynamic light scattering over a wide range of aquatic chemistries (pH, salt types (NaCl, MgCl2, CaCl2), ionic strength) relevant to natural and engineered systems. Although pH did not have a notable effect on GO stability from pH 4 to 10, salt type and ionic strength had significant effects on GO stability due to elec. double layer compression, similar to other colloidal particles. The crit. coagulation concn. (CCC) values of GO were detd. to be 44mM NaCl, 0.9mM CaCl2, and 1.3mM MgCl2. Aggregation and stability of GO in the aquatic environment followed colloidal theory (DLVO and Schulze-Hardy rule), even though GO's shape is not spherical. CCC values of GO were lower than reported fullerene CCC values and higher than reported C nanotube CCC values. CaCl2 destabilized GO more aggressively than MgCl2 and NaCl due to the binding capacity of Ca2+ with hydroxyl and carbonyl functional groups of GO. Natural org. matter significantly improved the stability of GO in water primarily due to steric repulsion. Long-term stability studies demonstrated that GO was highly stable in both natural and synthetic surface waters, although it settled quickly in synthetic groundwater. While GO remained stable in synthetic influent wastewater, effluent wastewater collected from a treatment plant rapidly destabilized GO, indicating GO will settle out during the wastewater treatment process and likely accumulate in biosolids and sludge. Overall, our findings indicate that GO nanomaterials will be stable in the natural aquatic environment and that significant aq. transport of GO is possible.
30. 30
  Wu, L.; Liu, L.; Gao, B.; Muňoz-Carpena, R.; Zhang, M.; Chen, H.; Zhou, Z. H.; Wang, H. Aggregation kinetics of graphene oxides in aqueous solutions: Experiments, mechanisms, and modeling Langmuir 2013, 29, 15174– 15181 DOI: 10.1021/la404134x
  30
  Aggregation Kinetics of Graphene Oxides in Aqueous Solutions: Experiments, Mechanisms, and Modeling
  Wu, Lei; Liu, Lin; Gao, Bin; Munoz-Carpena, Rafael; Zhang, Ming; Chen, Hao; Zhou, Zuhao; Wang, Hao
  Langmuir (2013), 29 (49), 15174-15181CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
  Although graphene oxide (GO) was used in many applications to improve human life quality, its environmental fate and behavior are still largely unknown. A novel approach that combines exptl. measurements and theor. calcns. was used to det. the aggregation kinetics of GO sheets in aq. solns. under different chem. conditions (e.g., cation valence and pH). Exptl. data showed that both cation valence and pH showed significant effect on the aggregation of GO sheets. The measured crit. coagulation concns. were in good agreement with the predictions of the extended Schulze-Hardy rule. Ca2+ and Mg2+ were more effective than Na+ in aggregating the GO sheets, which could be attributed to the crosslinking between GO sheets by the divalent cations through bridging the functional groups at the edges of the GO sheets. When soln. pH increases, deprotonation of carboxylic groups was found to play a key role in increasing GO sheet stability and surface charge development. Probably edge-to-edge and face-to-face interactions were the dominant modes of GO aggregation in the presence of divalent metal ions and H+, resp. A modified attachment efficiency (α) model was developed from the Maxwell approach with considerations of both primary and secondary min. The model predictions matched the exptl. measurements of the aggregation kinetics of GO sheets in aq. solns. under all of the tested exptl. conditions well.
31. 31
  Chowdhury, I.; Mansukhani, N. D.; Guiney, L. M.; Hersam, M. C.; Bouchard, D. Aggregation and stability of reduced graphene oxide: Complex roles of divalent cations, pH, and natural organic matter Environ. Sci. Technol. 2015, 49, 10886– 10893 DOI: 10.1021/acs.est.5b01866
  31
  Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter
  Chowdhury, Indranil; Mansukhani, Nikhita D.; Guiney, Linda M.; Hersam, Mark C.; Bouchard, Dermont
  Environmental Science & Technology (2015), 49 (18), 10886-10893CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The aggregation and stability of graphene oxide (GO) and 3 successively reduced GO (rGO) nanomaterials were studied. Reduced GO species were partially reduced GO (rGO-1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically, the effect of pH, ionic strength, ion valence, and presence of natural org. matter (NOM) were studied. Results show that stability of GO in water decreases with successive redn. of functional groups, with pH having the greatest influence on rGO stability. Stability is also dependent on ion valence and the concn. of surface functional groups. While pH did not noticeably affect stability of GO in the presence of 10mM NaCl, adding 0.1 mM CaCl2 reduced stability of GO with increased pH. This is due to adsorption of Ca2+ on the surface functional groups of GO which reduces the surface charge of GO. As the concn. of rGO functional groups decreased, so did the influence of Ca2+ on rGO stability. Crit. coagulation concns. (CCC) of GO, rGO-1h, and rGO-2h were ∼200, 35, and 30mM NaCl, resp. In the presence of CaCl2, CCC values of GO and rGO are quite similar, however. Long-term studies show that a significant amt. of rGO-1h and rGO-2h remain stable in Call's Creek surface water, while effluent wastewater readily destabilizes rGO. In the presence NOM and divalent cations (Ca2+, Mg2+), GO aggregates settle from suspension due to GO functional group bridging with NOM and divalent ions. However, rGO-1h and rGO-2h remain suspended due to their lower functional group concn. and resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations, and NOM can play complex roles in the fate of rGO and GO.
32. 32
  Hong, B. J.; Compton, O. C.; An, Z.; Eryazici, I.; Nguyen, S. T. Successful stabilization of graphene oxide in electrolyte solutions: Enhancement of biofunctionalization and cellular uptake ACS Nano 2012, 6, 63– 73 DOI: 10.1021/nn202355p
  32
  Successful Stabilization of Graphene Oxide in Electrolyte Solutions: Enhancement of Biofunctionalization and Cellular Uptake
  Hong, Bong Jin; Compton, Owen C.; An, Zhi; Eryazici, Ibrahim; Nguyen, SonBinh T.
  ACS Nano (2012), 6 (1), 63-73CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Aq. dispersions of graphene oxide are inherently unstable in the presence of electrolytes, which screen the electrostatic surface charge on these nanosheets and induce irreversible aggregation. Two complementary strategies, utilizing either electrostatic or steric stabilization, have been developed to enhance the stability of graphene oxide in electrolyte solns., allowing it to stay dispersed in cell culture media and serum. The electrostatic stabilization approach entails further oxidn. of graphene oxide to low C/O ratio (∼1.1) and increases ionic tolerance of these nanosheets. The steric stabilization technique employs an amphiphilic block copolymer that serves as a noncovalently bound surfactant to minimize the aggregate-inducing nanosheet-nanosheet interactions. Both strategies can stabilize graphene oxide nanosheets with large dimensions (>300 nm) in biol. media, allowing for an enhancement of >250% in the bioconjugation efficiency of streptavidin in comparison to untreated nanosheets. Notably, both strategies allow the stabilized nanosheets to be readily taken up by cells, demonstrating their excellent performance as potential drug delivery vehicles.
33. 33
  Ren, X. M.; Li, J. X.; Tan, X. L.; Shi, W. Q.; Chen, C. L.; Shao, D. D.; Wen, T.; Wang, L. F.; Zhao, G. X.; Sheng, G. P.; Wang, X. K. Impact of Al₂O₃ on the aggregation and deposition of grapheme oxide Environ. Sci. Technol. 2014, 48, 5493– 5500 DOI: 10.1021/es404996b
  There is no corresponding record for this reference.
34. 34
  Yang, S. T.; Chang, Y. L.; Wang, H. F.; Liu, G. B.; Chen, S.; Wang, Y. W.; Liu, Y. F.; Cao, A. Folding/aggregation of graphene oxide and its application in Cu²⁺ removal J. Colloid Interface Sci. 2010, 351, 122– 127 DOI: 10.1016/j.jcis.2010.07.042
  34
  Folding/aggregation of graphene oxide and its application in Cu2+ removal
  Yang, Sheng-Tao; Chang, Yanli; Wang, Haifang; Liu, Gangbo; Chen, Sheng; Wang, Yanwen; Liu, Yuanfang; Cao, Aoneng
  Journal of Colloid and Interface Science (2010), 351 (1), 122-127CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
  Graphene oxide (GO) can be aggregated by Cu2+ in aq. soln. with a huge Cu2+ absorption capacity. The Cu2+ causes GO sheets to be folded and also to form large aggregates that were characterized by confocal microscopy and at. force microscopy. The folding/aggregation is most likely triggered by the coordination between GO and Cu2+. The equil. Cu2+ concns. and equil. absorption capacity of GO est. the max. absorption capacity of GO for Cu2+ and the absorption model. GO has a huge absorption capacity for Cu2+, which is ∼10 times of that of active carbon. Representative results are presented and the implication to Cu2+ removal is discussed.
35. 35
  Wang, J.; Chen, B. Adsorption and coadsorption of organic pollutant and heavy metal by graphene oxide and reduced graphene materials Chem. Eng. J. 2015, 281 (1) 379– 388 DOI: 10.1016/j.cej.2015.06.102
  There is no corresponding record for this reference.
36. 36
  Hummers, W. S.; Offeman, R. E. Preparation of graphite oxide J. Am. Chem. Soc. 1958, 80, 1339 DOI: 10.1021/ja01539a017
  36
  Preparation of graphitic oxide
  Hummers, Wm. S., Jr.; Offeman, Richard E.
  Journal of the American Chemical Society (1958), 80 (), 1339CODEN: JACSAT; ISSN:0002-7863.
  See U.S. 2,798,878 (C.A. 51, 15080a).
37. 37
  Pan, Z. H.; Liu, N.; Fu, L.; Liu, Z. F. Wrinkle engineering: A new approach to massive graphene nanoribbon arrays J. Am. Chem. Soc. 2011, 133, 17578– 17581 DOI: 10.1021/ja207517u
  37
  Wrinkle Engineering: A New Approach to Massive Graphene Nanoribbon Arrays
  Pan, Zhong-Huai; Liu, Nan; Fu, Lei; Liu, Zhong-Fan
  Journal of the American Chemical Society (2011), 133 (44), 17578-17581CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Wrinkles are often formed on CVD-graphene in an uncontrollable way. By designing the surface morphol. of growth substrate together with a suitable transfer technique, the authors are able to engineer the dimension, d., and orientation of wrinkles on transferred CVD-graphene. Such kind of wrinkle engineering is employed to fabricate highly aligned graphene nanoribbon (GNR) arrays by self-masked plasma-etching. Strictly consistent with the designed wrinkles, the d. of GNR arrays varied from ∼0.5 to 5 GNRs/μm, and over 88% GNRs are <10 nm in width. Elec. transport measurements of these GNR-based FETs exhibit an on/off ratio of ∼30, suggesting an opened bandgap. The authors' wrinkle engineering approach allows very easily for a massive prodn. of GNR arrays with bandgap-required widths, which opens a practical pathway for large-scale integrated graphene devices.
38. 38
  Acik, M.; Lee, G.; Mattevi, C.; Chhowalla, M.; Cho, K.; Chabal, Y. J. Unusual infrared-absorption mechanism in thermally reduced graphene oxide Nat. Mater. 2010, 9, 840– 845 DOI: 10.1038/nmat2858
  38
  Unusual infrared-absorption mechanism in thermally reduced graphene oxide
  Acik, M.; Lee, G.; Mattevi, C.; Chhowalla, M.; Cho, K.; Chabal, Y. J.
  Nature Materials (2010), 9 (10), 840-845CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
  IR absorption of at. and mol. vibrations in solids can be affected by electronic contributions through nonadiabatic interactions, such as the Fano effect. Typically, the IR-absorption lineshapes are modified, or IR-forbidden modes are detectable as a modulation of the electronic absorption. In contrast to such known phenomena, the authors report here the observation of a giant-IR-absorption band in reduced graphene oxide, arising from the coupling of electronic states to the asym. stretch mode of a yet-unreported structure, consisting of O atoms aggregated at the edges of defects. Free electrons are induced by the displacement of the O atoms, leading to a strong IR absorption that is in phase with the phonon mode. This new phenomenon is only possible when all other O-contg. chem. species, including hydroxyl, carboxyl, epoxide and ketonic functional groups, are removed from the region adjacent to the edges, i.e., clean graphene patches are present.
39. 39
  Peng, L.; Xu, Z.; Liu, Z.; Wei, Y. Y.; Sun, H. Y.; Li, Z.; Zhao, X. L.; Gao, C. An iron-based green approach to 1-h production of single-layer graphene oxide Nat. Commun. 2015, 6, 5716 DOI: 10.1038/ncomms6716
  39
  An iron-based green approach to 1-h production of single-layer graphene oxide
  Peng, Li; Xu, Zhen; Liu, Zheng; Wei, Yangyang; Sun, Haiyan; Li, Zheng; Zhao, Xiaoli; Gao, Chao
  Nature Communications (2015), 6 (), 5716CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
  As a reliable and scalable precursor of graphene, graphene oxide (GO) is of great importance. However, the environmentally hazardous heavy metals and poisonous gases, explosion risk and long reaction times involved in the current synthesis methods of GO increase the prodn. costs and hinder its real applications. Here we report an iron-based green strategy for the prodn. of single-layer GO in 1 h. Using the strong oxidant K2FeO4, our approach not only avoids the introduction of polluting heavy metals and toxic gases in prepn. and products but also enables the recycling of sulfuric acid, eliminating pollution. Our dried GO powder is highly sol. in water, in which it forms liq. crystals capable of being processed into macroscopic graphene fibers, films and aerogels. This green, safe, highly efficient and ultralow-cost approach paves the way to large-scale com. applications of graphene.
40. 40
  Saleh, N. B.; Pfefferle, L. D.; Elimelech, M. Aggregation kinetics of multiwalled carbon nanotubes in aquatic systems: Measurements and environmental implications Environ. Sci. Technol. 2008, 42 (21) 7963– 7969 DOI: 10.1021/es801251c
  40
  Aggregation Kinetics of Multiwalled Carbon Nanotubes in Aquatic Systems: Measurements and Environmental Implications
  Saleh, Navid B.; Pfefferle, Lisa D.; Elimelech, Menachem
  Environmental Science & Technology (2008), 42 (21), 7963-7969CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The initial aggregation kinetics of multiwalled carbon nanotubes (MWNTs) were examd. through time-resolved dynamic light scattering. Aggregation of MWNTs was evaluated by varying soln. pH and the concn. of monovalent (NaCl) and divalent (CaCl2 and MgCl2) salts. Suwannee River humic acid (SRHA) was used to study the effect of background natural org. matter on MWNT aggregation kinetics. Increasing salt concn. and addn. of divalent calcium and magnesium ions induced MWNT aggregation by suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The crit. coagulation concn. (CCC) values for MWNTs were estd. as 25mM NaCl, 2.6mM CaCl2, and 1.5mM MgCl2. An increase in soln. pH from acidic (pH 3) to basic (pH 11) conditions resulted in a substantial (over 2 orders of magnitude) decrease in MWNT aggregation kinetics, suggesting the presence of ionizable functional groups on the MWNT carbon scaffold. The presence of humic acid in soln. markedly enhanced the colloidal stability of MWNTs, reducing the aggregation rate by nearly 2 orders of magnitude. The enhanced MWNT stability in the presence of humic acid is attributable to steric repulsion imparted by adsorbed humic acid macromols. The results suggest that MWNTs are relatively stable at soln. pH and electrolyte conditions typical of aquatic environments.
41. 41
  Bouchard, D.; Ma, X.; Isaacson, C. Colloidal properties of aqueous fullerenes: Isoelectric points and aggregation kinetics of C60 and C60 derivatives Environ. Sci. Technol. 2009, 43 (17) 6597– 6603 DOI: 10.1021/es901354r
  There is no corresponding record for this reference.
42. 42
  Overbeek, J. T. G. The Rule of Schulze and Hardy Pure Appl. Chem. 1980, 52, 1151– 1161 DOI: 10.1351/pac198052051151
  42
  The rule of Schulze and Hardy
  Overbeek, J. Theodoor G.
  Pure and Applied Chemistry (1980), 52 (5), 1151-61CODEN: PACHAS; ISSN:0033-4545.
  A theory is developed to explain the rule of Schulze and Hardy dealing with the coagulation of colloids. A combination of van der Waals attraction and electrostatic repulsion can explain the rule, if adsorption of counterions in the Stern layer is taken into account. The adsorption potential must then increase fairly strongly with the charge of the counterion. Structural forces (due to the mol. structure of the solvent) may be important, but the quant. theory of these forces has not yet been sufficiently developed to incorporate them in the theory of coagulation.
43. 43
  Sano, M.; Okamura, J.; Shinkai, S. Colloidal nature of single-walled carbon nanotubes in electrolyte solution: The Schulze-Hardy Rule Langmuir 2001, 17, 7172– 7173 DOI: 10.1021/la010698+
  There is no corresponding record for this reference.
44. 44
  Deng, X. J.; Lu, L. L.; Li, H. W.; Luo, F. The adsorption properties of Pb(II) and Cd(II) on functionalized graphene prepared by electrolysis method J. Hazard. Mater. 2010, 183, 923– 930 DOI: 10.1016/j.jhazmat.2010.07.117
  44
  The adsorption properties of Pb(II) and Cd(II) on functionalized graphene prepared by electrolysis method
  Deng, Xiaojiao; Lue, Lili; Li, Hongwei; Luo, Fang
  Journal of Hazardous Materials (2010), 183 (1-3), 923-930CODEN: JHMAD9; ISSN:0304-3894. (Elsevier B.V.)
  The functionalized graphene (GNSPF6) was fabricated by simple and fast method of electrolysis with K hexafluorophosphate soln. as electrolyte under the static potential of 15 V. The characterization results of TEM, atom force microscopy, XPS, x-ray powder diffraction, Raman spectroscopy and thermogravimetric anal. indicate that graphite rod was completely exfoliated to graphene layer contg. 30 wt.% PF6[n.713] with the av. thickness ∼1.0 nm. Our sample of GNSPF6 was developed for the removal of Pb(II) or Cd(II) from water, and the detd. adsorption capacities are 406.6 mg/g (pH =5.1) for Pb(II) and 73.42 mg/g (pH =6.2) for Cd(II), which is much higher than that by our previous sample of GNSC8P and C nanotube. The adsorption processes reach equil. in just 40 min and the adsorption isotherms are described well by Langmuir and Freundlich classical isotherms models.
45. 45
  Li, Y. H.; Ding, J.; Luan, Z. K.; Di, Z. C.; Zhu, Y. F.; Xu, C. L.; Wu, D. H.; Wei, B. Q. Competitive adsorption of Pb, Cu and Cd ions from aqueous solutions by multiwalled carbon nanotubes Carbon 2003, 41, 2787– 2792 DOI: 10.1016/S0008-6223(03)00392-0
  45
  Competitive adsorption of Pb2+, Cu2+ and Cd2+ ions from aqueous solutions by multiwalled carbon nanotubes
  Li, Yan-Hui; Ding, Jun; Luan, Zhaokun; Di, Zechao; Zhu, Yuefeng; Xu, Cailu; Wu, Dehai; Wei, Bingqing
  Carbon (2003), 41 (14), 2787-2792CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Science Ltd.)
  The individual and competitive adsorption capacities of Pb2+, Cu2+ and Cd2+ by HNO3-treated multiwalled carbon nanotubes (CNTs) were studied. The max. sorption capacities calcd. by applying the Langmuir equation to single ion adsorption isotherms were 97.08 for Pb2+, 24.49 for Cu2+, and 10.86 mg/g for Cd2+ at an equil. concn. of 10 mg/L. The competitive adsorption studies showed that the affinity order of 3 metal ions adsorbed by CNTs is Pb2+ > Cu2+ > Cd2+. The Langmuir adsorption model can represent exptl. data of Pb2+ and Cu2+ well, but does not provide a good fit for Cd2+ adsorption data. The effects of soln. pH, ionic strength, and CNT dosage on the competitive adsorption of Pb2+, Cu2+, and Cd2+ ions were investigated. The comparison of CNTs with other adsorbents suggests that CNTs have great potential applications in environmental protection regardless of their higher cost at present.
46. 46
  Imamoglu, M.; Tekir, O. Removal of copper (II) and lead (II) ions from aqueous solutions by adsorption on activated carbon from a new precursor hazelnut husks Desalination 2008, 228, 108– 113 DOI: 10.1016/j.desal.2007.08.011
  46
  Removal of copper(II) and lead(II) ions from aqueous solutions by adsorption on activated carbon from a new precursor hazelnut husks
  Imamoglu, Mustafa; Tekir, Oktay
  Desalination (2008), 228 (1-3), 108-113CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
  Activated carbon was prepd. from hazelnut husks with ZnCl2 activation at 973 K in nitrogen atm. BET surface area of the activated carbon was found 1092 m2 g-1. The removal of Cu(II) and Pb(II) from aq. solns. was studied by batch method. The effects of initial pH, contact time, activated carbon dosage and initial metal concns. were investigated. The activated carbon exhibited good adsorption potential for Cu and Pb ions. The exptl. data were analyzed by both Freundlich and Langmuir isotherms. The max. adsorption capacity of the adsorbent for Cu(II) and Pb(II) ions was calcd. from the Langmuir isotherm and found to be 6.645 and 13.05 mg g-1, resp.
47. 47
  Xu, Y.; Chen, B. Organic carbon and inorganic silicon speciation in rice-bran-derived biochars affect its capacity to adsorb cadmium in solution J. Soils Sediments 2015, 15, 60– 70 DOI: 10.1007/s11368-014-0969-2
  47
  Organic carbon and inorganic silicon speciation in rice-bran-derived biochars affect its capacity to adsorb cadmium in solution
  Xu, Yiliang; Chen, Baoliang
  Journal of Soils and Sediments (2015), 15 (1), 60-70CODEN: JSSOAJ; ISSN:1439-0108. (Springer)
  Purpose: Biochar enriched in oxygenated functional groups and minerals has a high adsorption capacity for heavy metals. The roles and contributions of the org. and mineral fractions in biochars to heavy metal adsorption are crit. to understanding the long-term effect of biochar application in contaminated soil. Batch adsorption expts. were performed on differently treated rice-bran-derived biochars to det. the contributions of the org. and mineral fractions to cadmium (Cd) adsorption in this study. Materials and methods: Rice-bran-derived biochars were produced at 300 and 700 °C (named R300 and R700). The Cd adsorption onto the original biochars and biochars treated with water, hydrochloric acid (HCl), and HCl + HF was compared on the basis of their adsorption isotherm curves. Biochar properties were characterized by CHN elemental anal., XRD, zeta potential, Fourier transform IR spectroscopy (FTIR), and SEM-EDX before and after Cd adsorption. Results and discussion: The Cd adsorption capacity gradually decreased as the water-washing times increased, and ten- and hundred-fold decreases were obsd. on acid-treated biochars. The relative contribution of water-sol. matter to the decrease of Cd in soln. was 87.4 % and 71.4 % for R300 and R700, resp. The contribution of HCl-sol. minerals was 11.5 % (R300) and 28.3 % (R700), while the role of insol. silicon oxide could be ignored. The adsorption mechanism between Cd and the org. fractions or minerals in biochars was complexation, as illustrated by the FTIR spectra. Conclusions: Both the org. and mineral fractions in biochars play an important role in Cd adsorption. The relative contribution of water-sol. matter was much higher than that of HCl-sol. ashes and insol. silicon oxide minerals. The results indicate that the adsorption capacity of biochars for Cd may greatly decrease in leached and acidic soils with the loss of sol. org. and mineral fractions.
48. 48
  Reddad, Z.; Gerente, C.; Andres, Y.; Le Cloirec, P. Adsorption of several metal ions onto a low-cost biosorbent: Kinetic and equilibrium studies Environ. Sci. Technol. 2002, 36, 2067– 2073 DOI: 10.1021/es0102989
  48
  Adsorption of Several Metal Ions onto a Low-Cost Biosorbent: Kinetic and Equilibrium Studies
  Reddad, Zacaria; Gerente, Claire; Andres, Yves; Le Cloirec, Pierre
  Environmental Science and Technology (2002), 36 (9), 2067-2073CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Sugar beet pulp generated by sugar-refining factories has been shown to be an effective adsorbent for the removal of heavy metals from aq. solns. The structural components related to the metallic adsorption having been detd., batch adsorption studies were performed for several metal ions, namely, Pb2+, Cu2+, Zn2+, Cd2+, and Ni2+ cations. Two simple kinetic models, i.e., pseudo-first- and pseudo-second-order, were tested to investigate the adsorption mechanisms. The kinetic parameters of the models were calcd. and discussed. For an 8 × 10-4M initial metal concn., the initial sorption rates (v0) ranged from 0.063 mmol/g-min for Pb2+ to 0.275 mmol/g-min for Ni2+ ions, in the order Ni2+ > Cd2+ > Zn2+ > Cu2+ > Pb2+. The equil. data fitted well with the Langmuir and Freundlich models and showed the following affinity order of the material: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+. The metal removal was strongly dependent on pH and, to a lesser extent, ionic strength. Ion exchange, with Ca2+ ions neutralizing the carboxyl groups of the polysaccharide, was found to be the predominant mechanism, along with complexation for Pb2+, Cu2+, and Zn2+ metals.
49. 49
  Mimmo, T. J.; Marzadori, C.; Montecchio, D.; Gessa, C. Characterisation of Ca- and Al-pectate gels by thermal analysis and FT-IR spectroscopy Carbohydr. Res. 2005, 340, 2510– 2519 DOI: 10.1016/j.carres.2005.08.011
  There is no corresponding record for this reference.
50. 50
  Ma, J. C.; Dougherty, D. A. The cation-π interaction Chem. Rev. 1997, 97, 1303– 1324 DOI: 10.1021/cr9603744
  50
  The Cation-π Interaction
  Ma, Jennifer C.; Dougherty, Dennis A.
  Chemical Reviews (Washington, D. C.) (1997), 97 (5), 1303-1324CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review with 293 refs. concerning the title interaction with emphasis on side chain interactions in proteins and protein-ligand interactions.
51. 51
  Tansel, B.; Sager, J.; Rector, T.; Garland, J.; Strayer, R. F.; Levine, L. F.; Roberts, M.; Hummerick, M.; Bauer, J. Significance of hydrated radius and hydration shells on ionic permeability during nanofiltration in dead end and cross flow modes Sep. Purif. Technol. 2006, 51, 40– 47 DOI: 10.1016/j.seppur.2005.12.020
  51
  Significance of hydrated radius and hydration shells on ionic permeability during nanofiltration in dead end and cross flow modes
  Tansel, Berrin; Sager, John; Rector, Tony; Garland, Jay; Strayer, Richard F.; Levine, Lanfang; Roberts, Michael; Hummerick, Mary; Bauer, Jan
  Separation and Purification Technology (2006), 51 (1), 40-47CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
  The aerobic rotational membrane system (ARMS) is one of the water recovery and recycling processes considered for use during long space missions. The ARMS effluent contains high levels of dissolved inorg. salts to be removed by the closed loop water recovery and recycling system. The purpose of this study was to investigate the permeability characteristics of ions present in the ARMS effluent through a nanofiltration membrane. Expts. were conducted in stirred dead end and cross flow filtration modes to evaluate the effectiveness of nanofiltration process to remove dissolved solids. The rejection characteristics of the ions were evaluated in relation to ionic hydrated radius, hydration no. and ionic viscosity parameters. Permeability of ions through the nanofiltration membrane showed a strong correlation with their hydrated radii. The ions with relatively smaller crystal radii (i.e., Mg2+ and Ca2+) have higher hydration nos. and larger hydrated radii. These ions also hold their hydration shells more strongly, hence, they were removed by both filtration modes but more effectively by dead end filtration. On the other hand, the ions with larger crystal radii (i.e., K+ and Na+) have weaker hydration shells, hence, they may be able to detach from their hydration layer while passing through the nanofiltration membrane. In dead end mode, the ions are subjected to relatively larger shear forces to pass through the membrane in comparison to cross flow where the ions move parallel to the membrane surface, hence the shear force is reduced. The permeation of the ions was consistently higher in the dead end mode than the cross flow mode operated at the same pressure.
52. 52
  Nightingale, E. R. Phenomenological theory of ion solvation. Effective radii of hydrated ions J. Phys. Chem. 1959, 63, 1381– 1387 DOI: 10.1021/j150579a011
  52
  Phenomenological theory of ion solvation. Effective radii of hydrated ions
  Nightingale, E. R., Jr.
  Journal of Physical Chemistry (1959), 63 (), 1381-7CODEN: JPCHAX; ISSN:0022-3654.
  The empirical correction to Stokes' law proposed by Robinson and Stokes (Electrolyte Solutions, 1955, 512 pp. (C.A. 49, 11388h)) was extended for small ions to provide a concordant set of radii for the hydrated ions. Ions with a crystal ionic radius of about 2 A. exhibited a min. hydrated radius of 3.3 A., corresponding to the max. in the equiv. cond. The internal consistency of the set of radii was demonstated by correlation with the temp. coeff. of equiv. cond., the viscosity B-coeff. in the Jones-Dole equation (C.A. 24, 3696), and the partial molar ionic entropy. Except for the small monat. ions with the min. hydrated radius, the hydrated ionic radius at 25° was a linear function of the viscosity B-coeff. The significance of this relation was discussed in terms of the structural modification rendered by the ions to water.
53. 53
  Trivedi, P.; Axe, L.; Dyer, J. Adsorption of metal ions onto goethite: Single-adsorbate and competitive systems Colloids Surf., A 2001, 191, 107– 121 DOI: 10.1016/S0927-7757(01)00768-3
  53
  Adsorption of metal ions onto goethite: single-adsorbate and competitive systems
  Trivedi, P.; Axe, L.; Dyer, J.
  Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2001), 191 (1-2), 107-121CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
  Adsorption of anthropogenically released toxic metals such as Ni and Zn to goethite effects their mobility and bioavailability in aquatic environments. Sorption studies were conducted to understand competitive adsorption of environmentally important metals such as Ni, Zn, and Ca onto the goethite surface. Adsorption edges conducted as a function of ionic strength suggest that these metals are chemisorbed to the goethite surface. The adsorption affinity follows the order of the inverse of the hydrated radii multiplied by the no. of waters in the primary solvation shell: Zn>Ni>Ca. Isotherm studies revealed a linear relation between the amt. of metal adsorbed and the aq. bulk phase concns., where site satn. was obtained by reducing the goethite concn. to 0.1 g/L. Accordingly the single-site Langmuir model provided a good fit; equil. consts. were independent of pH indicative of one type of adsorption reaction. The equil. consts. for both transition metals (Ni and Zn) were greater than that of Ca, suggesting that transition metals have a greater affinity for the surface. Analyses of site densities revealed 2 types of sites on the surface of goethite: high affinity ones to which transition metals bind, and low affinity sites that comprise 100× that of the high affinity ones. From the isotherm studies, it appears that only the alk. earth metals such as Ca adsorb to this lower affinity site. The single-site Langmuir model was able to accurately describe adsorption competition between Ni and Zn for the goethite surface. In contrast, no competitive effects were obsd. in Ni-Ca and Zn-Ca binary systems.
54. 54
  Sa, R. J.; Zhu, W. L.; Shen, J. H.; Gong, Z.; Cheng, J. G.; Chen, K. X.; Jiang, H. L. How does ammonium dynamically interact with benzene in aqueous media? A first principle study using the Car-Parrinello molecular dynamics Method J. Phys. Chem. B 2006, 110, 5094– 5098 DOI: 10.1021/jp051692m
  54
  How Does Ammonium Dynamically Interact with Benzene in Aqueous Media? A First Principle Study Using the Car-Parrinello Molecular Dynamics Method
  Sa, Rongjian; Zhu, Weiliang; Shen, Jianhua; Gong, Zhen; Cheng, Jiagao; Chen, Kaixian; Jiang, Hualiang
  Journal of Physical Chemistry B (2006), 110 (10), 5094-5098CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The Car-Parrinello mol. dynamics (CPMD) method was used to study the dynamic characteristics of the cation-π interaction between ammonium and benzene in gaseous and aq. media. The results obtained from the CPMD calcn. on the cation-π complex in the gaseous state were very similar to those calcd. from the Gaussian98 program with DFT and MP2 algorithms, demonstrating that CPMD is a valid approach for studying this system. Unlike the interaction in the gaseous state, our 12-ps CPMD simulation showed that the geometry of the complex in aq. soln. changes frequently in terms of the interaction angles and distances. Furthermore, the simulation revealed that the ammonium is constantly oscillating above the benzene plane in an aq. environment and interacts with benzene mostly through three of its hydrogen atoms. In contrast, the interaction of the cation with the arom. mol. in the gaseous state involves two hydrogen atoms. In addn., the free energy profile in aq. soln. was studied using constrained CPMD simulations, resulting in a calcd. binding free energy of -5.75 kcal/mol at an optimum interaction distance of ∼3.25 Å, indicating that the cation-π interaction between ammonium and benzene is stable even in aq. soln. Thus, this CPMD study suggested that the cation-π interaction between an ammonium (group) and an arom. structure could take place even on surfaces of protein or nucleic acids in soln.
55. 55
  Xu, Y. C.; Shen, J. H.; Zhu, W. L.; Luo, X. M.; Chen, K. X.; Jiang, H. L. Influence of the water molecule on cation-π Interaction: Ab initio second order Møller-Plesset Perturbation Theory (MP2) calculations J. Phys. Chem. B 2005, 109, 5945– 5949 DOI: 10.1021/jp044568w
  55
  Influence of the Water Molecule on Cation-π Interaction: Ab Initio Second Order Moller-Plesset Perturbation Theory (MP2) Calculations
  Xu, Yechun; Shen, Jianhua; Zhu, Weiliang; Luo, Xiaomin; Chen, Kaixian; Jiang, Hualiang
  Journal of Physical Chemistry B (2005), 109 (12), 5945-5949CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The influence of introducing water mols. into a cation-π complex on the interaction between the cation and the π system was investigated using the MP2/6-311++G** method to explore how a cation-π complex changes in terms of both its geometry and its binding strength during the hydration. The calcn. on the methylammonium-benzene complex showed that the cation-π interaction is weakened by introducing H2O mols. into the system. For example, the optimized interaction distance between the cation and the benzene becomes longer and longer, the transferred charge between them becomes less and less, and the cation-π binding strength becomes weaker and weaker as the water mol. is introduced one by one. Furthermore, the introduction of the third water mol. leads to a dramatic change in both the complex geometry and the binding energy, resulting in the destruction of the cation-π interaction. The decompn. on the binding energy shows that the influence is mostly brought out through the electrostatic and induction interactions. This study also demonstrated that the basis set superposition error, thermal energy, and zero-point vibrational energy are significant and needed to be cor. for accurately predicting the binding strength in a hydrated cation-π complex at the MP2/6-311++G** level. Therefore, the results are helpful to better understand the role of water mols. in some biol. processes involving cation-π interactions.
56. 56
  Geim, A. K.; Novoselov, K. S. The rise of graphene Nat. Mater. 2007, 6, 183– 191 DOI: 10.1038/nmat1849
  56
  The rise of graphene
  Geim, A. K.; Novoselov, K. S.
  Nature Materials (2007), 6 (3), 183-191CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
  A review. Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when com. products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top expts. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
57. 57
  Quintana, M.; Grzelczak, M.; Spyrou, K.; Calvaresi, M.; Bals, S.; Kooi, B.; Van Tendeloo, G.; Rudolf, P.; Zerbetto, F.; Prato, M. A simple road for the transformation of few-layer graphene into MWNTs J. Am. Chem. Soc. 2012, 134, 13310– 13315 DOI: 10.1021/ja303131j
  There is no corresponding record for this reference.
58. 58
  Patra, N.; Wang, B. Y.; Král, P. Nanodroplet activated and guided folding of graphene nanostructures Nano Lett. 2009, 9, 3766– 3771 DOI: 10.1021/nl9019616
  58
  Nanodroplet Activated and Guided Folding of Graphene Nanostructures
  Patra, Niladri; Wang, Boyang; Kral, Petr
  Nano Letters (2009), 9 (11), 3766-3771CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  We demonstrate by mol. dynamics simulations that water nanodroplets can activate and guide the folding of planar graphene nanostructures. Once the nanodroplets are deposited at selected spots on the planar nanostructure, they can act as catalytic elements that initiate conformational changes and help to overcome deformation barriers assocd. with them. Nanodroplets can induce rapid bending, folding, sliding, rolling, and zipping of the planar nanostructures, which can lead to the assembly of nanoscale sandwiches, capsules, knots, and rings.
59. 59
  Gong, Z.; Shen, H. Y.; Zhu, W. L.; Luo, X. M.; Chen, K. X.; Jiang, H. L. A computational study on electron transfer mechanism between alkaline earth metal atoms and cyclooctatetraene to form cation-π bonded complexes Chem. Phys. Lett. 2006, 423, 339– 343 DOI: 10.1016/j.cplett.2006.03.096
  There is no corresponding record for this reference.
60. 60
  Zhu, W. L.; Luo, X. M.; Puah, C. M.; Tan, X. J.; Shen, J. H.; Gu, J. D.; Chen, K. X.; Jiang, H. L. The multiplicity, strength, and nature of the interaction of nucleobases with alkaline and alkaline earth metal cations: A density functional theory investigation J. Phys. Chem. A 2004, 108, 4008– 4018 DOI: 10.1021/jp036911n
  60
  The Multiplicity, Strength, and Nature of the Interaction of Nucleobases with Alkaline and Alkaline Earth Metal Cations: A Density Functional Theory Investigation
  Zhu, Weiliang; Luo, Xiaomin; Puah, Chum Mok; Tan, Xiaojian; Shen, Jianhua; Gu, Jiande; Chen, Kaixian; Jiang, Hualiang
  Journal of Physical Chemistry A (2004), 108 (18), 4008-4018CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  D. functional theory (DFT) calcns. were performed at the B3LYP/6-311++G(d,p) level to systematically explore the geometrical multiplicity and binding strength for the complexes formed by alk. and alk. earth metal cations, viz. Li+, Na+, K+, Be2+, Mg2+, and Ca2+ (Mn+, hereinafter), with nucleobases, namely, adenine, cytosine, guanine, thymine, and uracil. Morokuma decompn. and orbital anal. were used to analyze the binding components. A total of 150 initial structures were designed and optimized, of which 93 optimized structures were found, which could be divided into two different types: cation-π complex and cation-heteroatom complex. In the former, a Mn+ is located above the nucleobase ring, while in the latter a Mn+ directly interacts in flank with the heteroatom(s) of a nucleobase. The strongest binding of -319.2 kcal/mol was found in the Be2+-guanine complex. Furthermore, the planar ring structures of the nucleobases in some cation-π complexes were deformed, destroying more or less the aromaticity of the corresponding nucleobases. In the cation-heteroatom complex, bidentate binding is generally stronger than unidentate binding, and of which the bidentate binding with five-membered ring structure has the strongest interaction. Moreover, the calcd. Mulliken charges showed that the transferred charge is linearly proportional to the binding strength. MO coeff. anal. indicated a significant orbital interaction in cation-π complex, but not in cation-heteroatom interaction. In addn., Morokuma decompn. revealed that electrostatic interaction is more important for cation-heteroatom binding. The majority of the calcd. ΔH values are in good agreement with the exptl. results. In those cases with significant differences, the exptl. results are proximate to an av. of the ΔH values of two isomers formed by the same nucleobase and cation.
61. 61
  Kumpf, R. A.; Dougherty, D. A. A mechanism for ion selectivity in potassium channels: Computational studies of cation-π interactions Science 1993, 261, 1708– 1710 DOI: 10.1126/science.8378771
  61
  A mechanism for ion selectivity in potassium channels: computational studies of cation-π interactions
  Kumpf, Robert A.; Dougherty, Dennis A.
  Science (Washington, DC, United States) (1993), 261 (5129), 1708-10CODEN: SCIEAS; ISSN:0036-8075.
  A combination of computational methods has been used to evaluate the interaction between the π face of a benzene mol. and the monovalent cations of lithium, sodium, potassium, and rubidium. In the gas phase, the ions are strongly bound, and the affinity for benzene follows the expected electrostatic trend (lithium, largest; rubidium, smallest). However, in an aq. environment, a reordering occurs such that the potassium ion is preferred over all the other ions for 2:1 benzene:ion complexes. The selectivity sequence parallels that seen in voltage-gated potassium channels. Given that several conserved arom. residues are present in the pore region of such channels, these results suggest that the cation-π interaction may be responsible for the ion selectivity in potassium channels.
62. 62
  Reddy, A. S.; Vijay, D.; Sastry, G. M.; Sastry, G. N. From subtle to substantial: Role of metal ions on π-π interactions J. Phys. Chem. B 2006, 110, 2479– 2481 DOI: 10.1021/jp060018h
  There is no corresponding record for this reference.
63. 63
  Duan, M. Y.; Song, B.; Shi, G. S.; Li, H. K.; Ji, G. F.; Hu, J.; Chen, X. G.; Fang, H. P. Cation × 3π: Cooperative interaction of a cation and three benzenes with an anomalous order in binding energy J. Am. Chem. Soc. 2012, 134, 12104– 12109 DOI: 10.1021/ja302918t
  There is no corresponding record for this reference.
Supporting Information
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b04235.
- Material and methods, surface zeta potential (Figure S1), aggregation kinetics of GO (Figure S2), FT-IR spectra of GO and GO aggregate induced by different cations (Figure S3), and TEM images of GO (Figures S4 and S5) (PDF)
- es6b04235_si_001.pdf (1.21 MB)
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Aggregation, Adsorption, and Morphological Transformation of Graphene Oxide in Aqueous Solutions Containing Different Metal CationsClick to copy article linkArticle link copied!

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Abstract

Introduction

Materials and Methods

GO Synthesis and Characterization

Aggregation Kinetics of GO

Adsorption Behavior of Heavy Metals onto GO during Aggregation

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Structural Characterization of GO

Figure 1

Electrokinetic and Hydrodynamic Properties of GO

Figure 2

Figure 3

Aggregation Kinetics of GO

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Adsorption of Heavy Metal Cations onto GO

Figure 5

Interaction Mechanisms of Metal Cations with GO

The Microstructure of GO Flocculation

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Aggregation, Adsorption, and Morphological Transformation of Graphene Oxide in Aqueous Solutions Containing Different Metal Cations
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