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ArticleMay 23, 2023

Carbon Storage in Coal Fly Ash by Reaction with Oxalic Acid
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Hao Wu
Hao Wu
Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
School of Energy Resources, China University of Geosciences, Beijing 100083, China
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https://orcid.org/0000-0003-2746-5337
Sean Aruch
Sean Aruch
Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
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Roni Grayevsky
Roni Grayevsky
Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
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Yanbin Yao
Yanbin Yao
School of Energy Resources, China University of Geosciences, Beijing 100083, China
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https://orcid.org/0000-0003-3838-4305
Simon Emmanuel*
Simon Emmanuel
Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
*Email: [email protected]
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https://orcid.org/0000-0002-6773-5015

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ACS ES&T Engineering

Cite this: ACS EST Engg. 2023, 3, 9, 1227–1235

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https://doi.org/10.1021/acsestengg.3c00063

Published May 23, 2023

CC-BY 4.0 .

Abstract

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Coal combustion is one of the leading sources of CO₂ emissions, and it is predicted to remain so for the foreseeable future. The environmental effects of coal burning can be partially offset by utilizing fly ash, which is a combustion byproduct, to mineralize and store carbon. Our study tests a novel method for carbon storage through the reaction of fly ash with oxalic acid (H₂C₂O₄), creating durable solid oxalate phases. Our results show that whewellite (CaC₂O₄·H₂O) and weddellite (CaC₂O₄·(2 + x)H₂O, x ≤ 0.5) are formed when fly ash reacts with H₂C₂O₄ at ambient temperature and pressure. We examined 2 types of ash and found that the reaction occurs relatively rapidly, reaching completion within 4 days. Moreover, the reacted material comprised ∼18% Ca oxalate. During the reaction, portlandite, the primary calcium-bearing mineral in the ash, was dissolved entirely, although mass balance calculations indicate that amorphous phases also serve as an important source of Ca for the oxalate minerals. Reaction modeling suggests that Ca is released by two phases that dissolve at different rates, with the rapidly dissolving phase releasing Ca at a rate 40 times faster than that of the slow phase. Based on our calculations, 1 tonne of reacted coal fly ash could store over 34 kg of carbon, and the method has the potential to store more than 35 Mt of carbon per year on a global scale. Thus, our findings indicate that reacting fly ash with oxalic acid could reduce the environmental impact of coal burning, and adapting the technique for use with other alkaline solid wastes may represent a critical green technology.

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1. Introduction

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Coal combustion currently generates 35% of electricity worldwide (1) and is expected to remain an essential component of the global energy market for the foreseeable future. (2,3) However, coal is also one of the leading sources of CO₂ emissions, and reducing its carbon footprint is critical to mitigating its environmental impact. (1,3,4) Coal combustion produces a waste product called coal fly ash (CFA), which is a combination of crystalline and amorphous phases. (5−7) Although the precise composition varies with the type of coal being burned and the combustion conditions, (8,9) 1 tonne of coal typically generates 50–250 kg of ash, (10,11) and more than 1 Gt of coal ash are produced globally every year. (12) By 2032, this value is predicted to reach 2.1 billion tonnes. (13)

CFA often has high CaO and MgO contents, making it chemically reactive. (14,15) Mineralization of these elements can stabilize fly ash, (16) allowing it to be re-utilized as a substitute for clinker or in concrete aggregate, thereby reducing CO₂ emissions from cement production. (17−19) Moreover, the mineralization of fly ash has been recognized as a promising technology to help achieve global environmental targets and solve waste disposal issues. (19)

Several methods have been proposed to convert Ca in CFA into CaCO₃, (19−24) and the effect of various physical parameters, such as temperature, liquid-to-solid ratio, bicarbonate concentration, and CO₂ pressure have been explored in a number of studies. (21,25,26) However, the direct conversion of the Ca in fly ash to calcium carbonate requires high temperatures and/or high CO₂ pressures due to the low solubility of CO₂ and slow reaction kinetics at ambient conditions. (21,25,26) Although regenerable solvents have been proposed as a way to convert Ca into carbonates at standard pressure and temperatures below 80 °C, (27,28) these methods have yet to be adopted at scale.

In this study, we tested an alternative method that reacts CFA directly with oxalic acid (H₂C₂O₄) at ambient temperature and pressure to store carbon through mineralization. Crucially, oxalic acid can be synthesized from CO₂ by electrochemical reduction or biological process, (29−34) and it can react with calcium-bearing phases to create stable Ca oxalate minerals called whewellite (CaC₂O₄·H₂O) and weddellite (CaC₂O₄·(2 + x)H₂O, x < 0.5). (35) Notably, the C/Ca ratio in Ca oxalate minerals is twice that of calcium carbonate, (29,35) so that fly ash containing oxalate phases could store more carbon than fly ash containing carbonates.

The reaction between fly ash and oxalic acid can be represented schematically as:

\underset{lime}{CaO (s)} + \underset{oxalic acid}{H_{2} C_{2} O_{4} (aq)} \to \underset{whewellite}{{CaC}_{2} O_{4} \cdot H_{2} O (s)}

(1)

Recently, a similar reaction between olivine and oxalic acid at ambient temperature and pressure was found to form stable Mg and Fe oxalates in a matter of days, and it was suggested that this could be a relatively rapid pathway to mineralizing carbon. (36) However, the feasibility of utilizing fly ash in a similar way has yet to be tested.

In our study, we use a combination of experimental methods to determine the efficacy of storing carbon in CFA by reaction with oxalic acid. We characterize the products of the reaction between two kinds of CFA, and we propose a model to describe the reaction rate. Furthermore, we discuss the potential of the reaction for reducing global carbon emissions.

2. Materials and Methods

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2.1. CFA Samples and Experimental Protocol

CFA was obtained from the sample repository at the Israel Geological Survey, which contains ash from the Hadera Power Station and Rutenberg Power Plant, operated by the Israel Electricity Company. The coal used at the plant is purchased from three primary suppliers (Russia, South Africa, and Colombia), and the CFA is a product of mixing between these sources. Two different types of fly ash, referred to as KLP and Zibulo, were used in the study. These fly ash samples have an average particle size in the range of 13.1–20.2 μm and a BET surface area of 1.6–2.4 m² g^–1. (37)

In the experiments, 1 g of dried CFA was reacted in a 250 mL solution with 0.1 mol/L oxalic acid (99.8%; SD Fine Chem Limited) and agitated continuously at room temperature and pressure. The effect of exposure time was tested by terminating the reaction after 0.5 h, 1 h, 2 h, 4 h, 8 h, 16 h, 1 d, 2 d, 4 d, 8 d, and 16 d. After termination, the fluid was filtered with 0.22 μm filters in a vacuum filter system. The solid filtrate was rinsed with 250 mL of deionized water to remove residual oxalic acid, dried at 40 °C, and then weighed.

2.2. Mineralogical and Chemical Characterization

The solid phase was characterized using a combination of methods. Scanning electron microscopy (SEM; Extra-High-Resolution Scanning Electron Microscope Magellan 400 L analyzer) provided high-resolution images of the samples and facilitated the identification of oxalate phases. To characterize the crystalline phases, both before and after the reaction, samples were also analyzed using X-ray powder diffraction (XRD; X-Ray Powder Diffractometer D8 Advance, Bruker AXS) with a CuKα source (λ = 1.5406 Å) at room temperature. Diffraction measurements were collected in the range of 5–80°. Measurements showed that the primary minerals in the unreacted fly ash are mullite (Al(Al_0.83Si_1.08O_4.85)) and quartz (SiO₂) (Table S1). Together they comprise 95% and 93% of the crystalline phases in KLP and Zibulo, respectively. Additional minerals, primarily hematite (Fe₂O₃) and portlandite (Ca(OH)₂), make up the remaining mineral phases. CFA often contains a high proportion of amorphous phases, sometimes exceeding 60%, (38) so that the XRD analyses represent only a partial characterization of the solid phases.

X-ray fluorescence (XRF; Micro X-ray Fluorescence spectrometer M4 Tornado, Bruke) was applied to the unreacted and reacted fly ash samples to determine the overall chemical composition of the major elements. Before measurement with XRF, the powdered CFA samples were pressed into pellets. Based on the CaO content of the KLP and Zibulo samples (6.0 and 5.7%, respectively; Table 1), these two fly ash samples were classified as Class F. (38) This classification reflects a relatively low Ca content, which is similar to the results obtained for Israeli fly ash samples reported in previous studies. (5)

Table 1. Chemical Composition (Major Elements) of the Fly Ash Samples Based on XRF

	KLP-unreacted^a (mass %)	KLP-reacted^a (mass %)	Zibulo-unreacted^a (mass %)	Zibulo-reacted^a (mass %)
SiO₂	39.56	36.95	36.11	36.01
Al₂O₃	26.81	22.98	23.67	19
Fe₂O₃	2.07	1.55	4.09	3.05
CaO	6.01	7.32	5.73	7.94
TiO₂	1.65	1.48	1.60	1.35
K₂O	0.50	0.44	0.71	0.59
Na₂O	<0.01	<0.01	1.09	0.66
MgO	0.54	0.17	1.38	0.30
P₂O₅	0.79	0.04	0.58	<0.01
SO₃	1.11	0.06	1.20	0.06
LOI	5.81	4.52	3.45	3.83

^{^a}

Percentages for oxides are absolute values and not normalized to 100%. Values for LOI include both water and organic carbon.

At the end of each experiment, the filtered fluid phase was analyzed using inductively coupled plasma mass spectrometry (ICP-MS; Analytik Jena Plasma Quant MS Elite quadrupole ICP-MS analyzer). This provided the concentrations of major elements in the reacted fluids.

2.3. Thermal Gravimetric Analyses and Ca Oxalate Content

We used thermal gravimetric analysis (TGA; STA Jupiter 449F3 analyzer) to precisely determine the proportion of oxalate minerals in the reacted fly ash samples. The TGA measurements were based on the protocol reported in previous studies, (39) with 29–31 mg of sample being heated from 50 to 950 °C in a nitrogen environment at a rate of 10 °C min^–1. For samples containing calcium.

Oxalate minerals, three stages of mass loss were observed with increasing temperature: (40,41) (1) dehydration of hydrated Ca oxalate to form anhydrous calcium oxalate (CaC₂O₄) in the range 120–350 °C; (2) decomposition of CaC₂O₄ to CaCO₃ between 350 and 550 °C; and (3) decomposition of CaCO₃ to CaO from 550 to 800 °C. Loss on ignition (LOI) was determined using a similar method: samples with a mass of 49–51 mg were heated from 50 to 950 °C in air at a heating rate of 20 °C min^–1; the temperature was held constant for 15 min at 105, 750, and 950 °C.

To calculate the maximum proportion of anhydrous calcium oxalate in the reacted fly ash (P_CaC₂O₄^max) from the TGA data, we used the following equation: (25,35,41)

P_{Ca C_{2} O_{4}}^{\max} = \frac{M_{Ca C_{2} O_{4}} \times ({Δ m}_{s 2} + {Δ m}_{s 3})}{M_{Ca C_{2} O_{4}} - M_{CaO}}

(2)

where M_CaC₂O₄ is the molar mass of anhydrous calcium oxalate (CaC₂O₄), M_CaO is the molar mass of calcium oxide (CaO), Δm_s2 is the proportional mass loss in Step 2, and Δm_s3 is the proportional mass loss in Step 3.

To calculate the minimum amount of calcium oxalate, we made a minor modification to eq 2. Thermal gravimetry analyses of the unreacted samples (Figure 1) revealed a small but significant loss of mass (0.96% and 0.64% of KLP and Zibulo, respectively) in the 350–800 °C temperature range. Assuming that this mass loss also contributes to the mass change in the samples after the reaction, the minimal Ca oxalate content (P_CaC₂O₄^min) is given by:

P_{Ca C_{2} O_{4}}^{\min} = \frac{M_{Ca C_{2} O_{4}} \times ({Δ m}_{s 2} + {Δ m}_{s 3} - {Δ m}_{u})}{M_{Ca C_{2} O_{4}} - M_{CaO}}

(3)

where Δm_u is the proportional mass loss of the unreacted sample at 350–800 °C. Although we report both values, the actual oxalate content is likely to be closer to the maximal estimate calculated in eq 2. This is because the mass change in the unreacted phases in the 350–800 °C temperature range is probably due to the decomposition of Ca-bearing phases, such as calcite and portlandite. (42,43) On exposure to oxalic acid, these minerals are readily converted into Ca oxalate phases.

Figure 1. Thermal gravimetric analysis of unreacted coal fly ash samples. Three steps were observed in the unreacted samples: (1) loss of water; (2) decomposition of Ca-bearing phases, such as calcite and portlandite; (3) reaction between unoxidized carbon and iron oxide.

To estimate the maximal mass proportion of carbon stored in the reacted fly ash (P_C), we substituted the minimum and maximum values for the Ca oxalate content into the following expression:

P_{C} = P_{Ca C_{2} O_{4}}^{\max} \frac{2 M_{C}}{M_{Ca C_{2} O_{4}}}

(4)

where M_C is the molar mass of carbon. These values can be compared with the potential carbon storage of reacted fly ash (S_C; i.e., the mass proportion of carbon that could be stored in reacted fly ash if all the Ca was bound to oxalate), which is calculated by:

S_{C} = F_{CaO} \frac{2 M_{C}}{M_{CaO}}

(5)

where F_CaO is the mass proportion of CaO in the reacted sample, estimated from XRF.

Here, we also define the effective carbon storage capacity per unit mass of unreacted fly ash (ECSC; i.e., the maximal mass of carbon per unit mass of unreacted CFA that is eventually stored as Ca oxalate) as:

ECSC = {ξ P}_{CaO} \frac{2 M_{C}}{M_{CaO}}

(6)

where P_CaO is the mass proportion of CaO in the unreacted sample, and ξ is the overall conversion efficiency of Ca to Ca oxalate.

2.4. Reaction Rate Model

The rate of Ca dissolution during the reaction was evaluated with a modified shrinking-particle model. (44,45) The model assumes that the reactive Ca-bearing particles are spherical and of uniform size, and predicts the proportion of the remaining mass of unreacted Ca at any given time. The mass of an individual particle, m_p, is given by:

m_{p} = \frac{4}{3} π r^{3} \times \frac{M_{w}}{V_{m}}

(7)

where r is the grain radius (assumed to be the average grain size of the Ca-bearing particles), M_w is the molecular mass of CaO (56 g mol^–1), and V_m is the molar volume of CaO (16.76 × 10^–6 m³ mol^–1). It then follows that the remaining mass proportion of unreacted particles (P_rem) in the sample during the experiment is:

P_{rem} = \frac{r_{t}^{3}}{r_{0}^{3}}

(8)

where r_t is the grain radius at time t. The grain radius at time t will depend on the grain dissolution rate (J), and is given by the expression:

r_{t} = r_{0} - V_{m} J t

(9)

Substituting eq 9 into eq 8 gives the following analytical expression:

P_{rem} = \frac{{(r_{0} - V_{m} J t)}^{3}}{r_{0}^{3}}

(10)

Fly ash contains both amorphous or crystalline Ca-bearing phases that could have very different dissolution rates. (38,46,47) Therefore, CaO release may require a multirate approach, and in our model, we consider the simplest such scenario in which fly ash dissolves at two distinct rates (J₁ and J₂). In this case, the remaining proportion of unreacted Ca becomes:

P_{rem} = f \frac{{(r_{0} - V_{m} J_{1} t)}^{3}}{r_{0}^{3}} + (1 - f) \frac{{(r_{0} - V_{m} J_{2} t)}^{3}}{r_{0}^{3}}

(11)

where f is the proportion of total Ca initially associated with phase 1.

For each experimental data set, a MATLAB code was used to identify optimal values for J₁, J₂, and f by comparing the model (eq 11) with the proportion of remaining reactive Ca (P_rem) given by:

P_{rem} = 1 - P_{react}

(12)

where P_react is the mass proportion of reacted CaO in the solid phase at time t, estimated from the TGA results. We note that by calculating P_rem from the TGA data, Ca precipitation is assumed to be directly coupled to Ca release.

In the simulations, we assumed an initial partical radius of 15 μm. Optimal parameters were found by using a MATLAB error minimization algorithm based on the simplex search method, and the norm function was used to evaluate the error. To assess the uncertainties of the model parameters, we used a bootstrapping method in which we removed a single datum and then determined the best-fit parameters. This operation was performed for each point, and the relative standard deviations (RSD) for J₁, J₂, and f were calculated.

3. Results and Discussion

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3.1. Textural Changes to Fly Ash during Reaction with Oxalic Acid

The SEM images revealed that many of the fly ash particles in the unreacted samples (both KLP and Zibulo) were spherical in shape, ranging in size from ∼100 nm to tens of micrometers (Figure 2 and Supplementary Figure S1). Many of the particles also appeared in aggregate form. The spherical shape of the particles reflects the combined processes of coagulation, condensation, and nucleation of evaporated materials following combustion. (48) Particles with a defined crystal habit were not observed in the unreacted samples (Figure 2a,b). In previous studies, spherical particles in fly ash were reported to exhibit a wide range of compositions, comprising carbonaceous, silica, aluminosilicate, and Ca and Fe bearing phases. (6)

Figure 2. Electron micrographs of the two types of coal fly ash: (a) unreacted KLP fly ash; (b) unreacted Zibulo fly ash; (c) KLP fly ash after 4 days of exposure to 0.1 M oxalic acid; (d) Zibulo fly ash, again after 4 days of reaction in 0.1 M oxalic acid. After the reaction, the presence of euhedral crystals─most likely Ca oxalate─are observed. Additional SEM images are provided in Supplementary Figure S1.

After the reaction with oxalic acid, both types of CFA showed significant textural changes (Figure 2 and Supplementary Figure S1). Although many spherical particles were still present, distinct new phases with an apparent crystalline habit had formed. The size of these crystals was in the range of ∼0.5 to 10 μm. Chemical analysis with EDS indicated that the dominant cation in the crystals was Ca, consistent with the presence of Ca oxalate. In both the KLP and Zibulo samples, the composition of the unreacted spherical particles was predominantly aluminosilicate.

3.2. Fly Ash Mineralogy and Chemical Composition

X-ray diffraction confirmed that a significant level of Ca oxalate was present in the reacted fly ash (Figure 3). As early as 0.5 h after exposure to oxalic acid, both whewellite (CaC₂O₄·H₂O) and weddellite (CaC₂O₄·(2 + x)H₂O, x < 0.5) appeared (Figure 4), although other Ca-bearing minerals were no longer detected. The whewellite content increased steadily during the first 4 days of the reaction, reaching proportions of 32.6% (KLP) and 41.5% (Zibulo) of the crystalline phases. By contrast, weddellite showed a different pattern: peak values were observed after 4 h, followed by a steady decrease at longer reaction times, with values eventually falling below the detection limit. This result indicates that whewellite may have been formed by the conversion of weddellite, which is consistent with the greater thermodynamic stability of whewellite at the experimental conditions applied in the study. (49) Additionally, the near-constant values for whewellite after 4 days are likely to indicate that the conversion of the reactive Ca into oxalate phases was complete.

Figure 3. X-ray diffraction patterns of the two types of coal fly ash: (a) unreacted KLP fly ash; (b) unreacted Zibulo fly ash; (c) KLP fly ash after 16 days of exposure to 0.1 M oxalic acid; (d) Zibulo fly ash, again after 16 days of reaction in 0.1 M oxalic acid. Note that in this figure, only the major mineral phases are indicated.

Figure 4. Proportion of whewellite (CaC₂O₄·H₂O) and weddellite (CaC₂O₄·(2 + x)H₂O, x < 0.5) during the reaction between coal fly ash samples and 0.1 M oxalic acid. Values are based on XRD measurements. Each point represents a separate experiment.

Following the complete removal of Ca-bearing crystalline phases at a very early stage in the experiments, the amount of Ca oxalate continued to increase. This suggests that much of the Ca was provided by the dissolution of an amorphous source. This is supported by considering the relatively low CaO content in the unreacted fly ash calculated from the XRD measurements (Figure 3 and Table S1); in the KLP and Zibulo samples, the predicted CaO associated with crystalline phases is only 1.5 and 1.1%, respectively (Figure 5). This contrasts sharply with the much higher concentrations measured by XRF (6 and 5.7% for KLP and Zibulo, respectively; Figure 5), which reflect the total CaO in the samples.

Figure 5. Comparison of the chemical compositions for whole samples (total) and crystalline components only in the unreacted fly ash. Total values are based on XRF measurements (Table 1), while the crystalline component is estimated from the XRD results (Supplementary Table S1). The difference between the total and crystalline values are attributed to the presence of amorphous phases. Error bars represent the variance (1σ) of measurements.

The XRF analysis of the samples after the reaction indicated that relatively minor compositional changes had occurred: Al₂O₃, Fe₂O₃, MgO, P₂O₅, and SO₃ decreased slightly, while CaO content increased by 1.3 and 2.2% for the KLP and Zibulo samples, respectively (Table 1). These changes suggest that labile phases containing Al, Fe, Mg, and Si in the unreacted fly ash were dissolved, and Ca was preferentially precipitated into insoluble oxalate phases during the reaction. The ICP-MS measurements of the reacted fluid revealed that the dissolved major elements (Table 2) represented less than 4.7% (KLP) and 8.1% (Zibulo) of the mass in the unreacted fly ash. Thus, overall, a relatively small proportion of the fly ash dissolved congruently during the reaction. With respect to Ca, mass balance calculations at the end of the experiments reveal that 92% and 86% is bound to solid phases for KLP and Zibulo, respectively.

Table 2. Comparison of Major Element Concentrations in the Unreacted Fly Ash and in the Fluid Phase at the End of the Experiments

	KLP fluid^a (mass %)	KLP fly ash^b (mass %)	Zibulo fluid^a (mass %)	Zibulo fly ash^b (mass %)
Si	1.46	18.49	1.86	16.88
Al	1.88	14.19	3.17	12.52
Fe	0.56	1.45	1.39	2.86
Ca	0.35	4.29	0.58	4.10
Mg	0.44	0.32	1.13	0.83

^{^a}

Percentages are calculated per unit mass of unreacted fly ash; values are based on ICP-MS measurements of the reacted fluid.

^{^b}

Based on XRF measurements. Values are not normalized to 100%.

3.3. Ca Oxalate Quantification with Thermal Gravimetry

The TGA analyses for the reacted samples (Figure 6) showed three distinct stages of mass loss that were consistent with the thermal decomposition of Ca oxalate. (40,41) The CaC₂O₄ contents estimated from the TGA measurements were in agreement with the XRD results, with a plateau being reached after 4 days of reaction for both types of CFA (Figure 7). The CaC₂O₄ contents after 16 days of reaction were found to be in the range of 14.7–16.4% for the KLP ash and 17.2–18.3% for Zibulo. Substituting these values into eq 4, we calculated that 1 tonne of reacted KLP fly ash would contain 27.6–30.8 kg of carbon, while for Zibulo ash the range was 32.3–34.3 kg. These values corresponded well with the theoretical carbon storage potential for reacted fly ash (S_C in eq 5), which is 31.4 kg t^–1 and 34 kg t^–1 for KLP and Zibulo, respectively. These results suggest that at the end of the experiments, 88–98% of the Ca in the solid phases is present as Ca oxalate in the KLP samples, while the corresponding range for the Zibulo fly ash is 95–100%. Combining these values with the proportion of Ca remaining in the solid phase after the reaction (Section 3.2), the overall conversion efficiencies (ξ in eq 6) of Ca to Ca oxalate were calculated to be 81–90% for KLP and 82–86% for Zibulo.

Figure 6. Thermal gravimetric analysis of coal fly ash samples after 16 days of reaction in 0.1 M oxalic acid. Three steps were observed in the reacted samples: (1) dehydration of hydrated Ca oxalate to form anhydrous calcium oxalate (CaC₂O₄) at 120–350 °C; (2) decomposition of CaC₂O₄ to CaCO₃ at 350–550 °C; and (3) decomposition of CaCO₃ to CaO at 550–800 °C.

Figure 7. Anhydrous calcium oxalate (CaC₂O₄) content as a function of time based on thermal gravimetry measurements. Both the maximum (green) and minimum (orange) calcium oxalate contents (calculated using eqs 2 and 3, respectively) are shown. Shaded regions represent the range. Each point represents a separate experiment.

Based on eq 6, the effective carbon storage capacities (ECSC) for unreacted fly ash were estimated to be 23.1 and 21.1 kg t^–1 for KLP and Zibulo ash, respectively. Compared to other carbon storage methods in fly ash that convert Ca to carbonate minerals, the ECSC values obtained using the oxalic acid method were significantly higher. For example, in a previous study, (26) a storage capacity of 10 kg carbon per tonne of fly ash was achieved at a higher temperature (40 °C versus 25 °C in our experiments) and with a fly ash sample possessing a higher proportion of CaO (16.4% versus 5.7–6% in our experiments). Our results demonstrate that using the oxalic acid method, up to 63 kg carbon per tonne of fly ash could be stored for samples with a similar level of CaO content.

3.4. Rate Modeling of Ca Release

We found that the dissolution rate model with 2 distinct rates (eq 11) fitted the experimental data well (Figure 8), suggesting that at least two phases dominate the release of Ca during the experiments. In both the KLP and Zibulo ashes, the model indicated that the more reactive phase released Ca about 40 times faster than the less reactive phase (Table 3). The rate constants for the KLP ash are slightly higher than those of the Zibulo ash, while the Zibulo ash has a greater proportion of the reactive phase (48.6%) than KLP (39.7%). These differences could stem from variations in particle size and Ca partitioning during the combustion process.

Figure 8. Remaining mass proportion of Ca-bearing phases in coal fly ash as a function of time for two different samples fitted by the dissolution model. Both the experimental data and optimized model fitting (described in Section 2.4) are shown in each figure. Error bars represent the variance (1σ) of the measurements.

Table 3. Fly Ash Dissolution Rates and Optimized Model Parameters

	J₁ (mol m^–2 s^–1)	J₁ RSD	J₂ (mol m^–2 s^–1)	J₂ RSD	f	f RSD	error^a
KLP	1.52 × 10^–4	4.4%	3.51 × 10^–6	3.8%	0.397	1.8%	0.081
Zibulo	1.00 × 10^–4	10.5%	2.55 × 10^–6	6.8%	0.486	4.0%	0.093

^{^a}

Calculated by the second-order norm function in MATLAB.

The two different rates probably reflect the initial rapid dissolution of portlandite under the low pH of our experiments, (50) coupled with the slower dissolution rate associated with the amorphous Ca-bearing phases comprising calcium aluminosilicate glasses. (46) If this is indeed the case, 51.4% of the Ca in Zibulo ash and 61.3% in KLP ash are associated with slow-dissolving amorphous material, and mass balance considerations dictate that CaO in the glass phases in both ash types are in the range of 2.9–3.6% of the total mass.

3.5. Evaluation of the Potential Applications

At present, 1 Gt of CFA is produced worldwide each year, (12) and CaO contents for CFA are typically in the range of 0.1–52%. (51) Assuming a similar level of efficiency to that found in our experiments, the effective carbon storage capacity for fly ash could vary in the range of 0.4–200 kg t^–1. Taking a conservative value for CaO content of 9% (corresponding to an ECSC value of 35 kg t^–1), the reaction between oxalic acid and fly ash can potentially store 35 Mt of carbon worldwide, which is equivalent to 128 Mt of CO₂. Currently, carbon capture, utilization, and storage (CCUS) technologies store only 40 Mt CO₂ each year, (52,53) although the projected scale of CCUS projects needs to be approximately 2.6 Gt each year to reach climate goals. (54) This indicates that carbon storage in CFA using Ca oxalate could represent ∼5% of the global environmental target.

Our experiments suggest that other forms of industrial waste containing high Ca, Mg, and Fe contents could also be effective materials for storing carbon using the oxalate method. Possible materials include cement and concrete in building debris, slag from steel manufacturing, mining, and mineral processing wastes, and phosphogypsum generated during fertilizer production. Prior research has indicated that alkaline wastes have great potential as carbon storage materials. (19) However, CFA represents only 12.3% of the total amount. If our method were to demonstrate a similar level of efficiency for other kinds of alkaline waste, as much as 1.04 Gt year^–1 of CO₂ could be stored through oxalate mineralization.

Because CFA is utilized in the construction industry, ash containing Ca oxalate could provide a way to enhance carbon storage in infrastructure projects. The compressive strength and durability of cement blended with fly ash mineralized with CaCO₃ are higher than that with fresh fly ash, (55) and this might also be the case for fly ash containing Ca oxalate. Additionally, Ca oxalate is relatively insoluble even under acidic conditions, which could help improve the long-term resistance of cement and concrete to weathering. (56) Moreover, toxic elements in fly ash are reported to coprecipitate with Ca carbonate, (57) and this might also be the case for fly ash containing Ca oxalate. However, further research is needed to explore these additional possible benefits.

4. Concluding Remarks

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Our study demonstrated the potential of the oxalic acid method for efficiently storing carbon in CFA. By converting a high percentage of calcium in fly ash into calcium oxalate at room temperature and pressure, we were able to achieve a storage capacity that would be equivalent to storing 128 Mt CO₂ year^–1 in fly ash through mineralization if applied on a global scale. However, we only explored the reaction under a very limited range of conditions, and further testing of the reaction could lead to even more efficient conversion of Ca and potential mineralization of additional cations, such as Fe and Mg. Additionally, our experiments suggest that other forms of industrial waste containing high Ca, Mg, and Fe contents could also be effective materials for storing carbon using the oxalate method.

Although our experiments reached a high level of Ca utilization within days, the eventual deployment of the method to store carbon will depend on several factors. First, the Ca content in fly ash can vary significantly depending on the coal source, so the method may not be viable for ash with extremely low Ca contents. Second, although Ca oxalate is insoluble at ambient temperature and pressure, its stability under field conditions is not well known, and assessing its environmental behavior is crucial. Third, the direct conversion of CO₂ to oxalic acid is feasible but is currently uneconomical; (30) in the future, the cost of forming oxalic acid could be reduced significantly by catalytic pathways or bioengineering processes. (36,58)

Data Availability

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Data will be made available on request.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.3c00063.

Electron micrographs of the two types of coal fly ash; mineralogical composition of the unreacted fly ash samples based on XRD measurements (PDF).

ee3c00063_si_001.pdf (241.96 kb)

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Author Information

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Corresponding Author
- Simon Emmanuel - Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel; https://orcid.org/0000-0002-6773-5015; Email: [email protected]
Authors
- Hao Wu - Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel; School of Energy Resources, China University of Geosciences, Beijing 100083, China; https://orcid.org/0000-0003-2746-5337
- Sean Aruch - Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
- Roni Grayevsky - Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
- Yanbin Yao - School of Energy Resources, China University of Geosciences, Beijing 100083, China; https://orcid.org/0000-0003-3838-4305
Notes
The authors declare no competing financial interest.

Acknowledgments

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Dr. Nadya Teutsch is thanked for providing the coal ash samples. The Israel Science Foundation and the Council for Higher Education of Israel are thanked for generous funding. We also thank the National Natural Science Foundation of China (42125205) and the China Scholarship Council.

References

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Fuel (2022), 318 (), 123636CODEN: FUELAC; ISSN:0016-2361. (Elsevier Ltd.)
The accelerated mineralization of fly ash is a potential way to achieve CO2 emissions redn. However, the slow reaction at the diffusion control stage is the bottleneck. Due to the strong diffusion of supercrit. CO2, it has been to strengthen mineralization, but the degree of improvement in mineralization efficiency and the changes that take place in the transition from non-supercrit. to supercrit. CO2 are not clear. In the mineralization process, CaO in the ash reacts with CO2 to form CaCO3. In order to exclude the influence of other alk. oxides, powd. and block CaO were used to research the mechanism, and expts. were then carried out on samples of fly ash with different CaO content. Firstly, powd. CaO was used to conduct dry and wet mineralization expts. In dry mineralization, the mineralization efficiency of the process changes in two stages as the pressure increases, first undergoing a gentle-increase and then a rapid one. Diffusion depth expts. on block CaO show that the supercrit. diffusion depth is higher than the non-supercrit. diffusion depth by a factor of 1.22, indicating that supercrit. CO2 can improve the degree of mineralization in diffusion stage. The presence of water could promote mineralization. In wet mineralization, the efficiency of the process changes in three stages as the pressure increases, with first a gentle increase then a more rapid one and finally an attenuated rate of increase. The supercrit. mineralization efficiency at 8 MPa was 55.27%, a factor of 2.09 larger than for non-supercrit. mineralization at 3 MPa (26.39%). This is because that supercrit. CO2 has increased the diffusion and greatly improved soly. in water. The results of DFT calcns. show that H2O can promote the adsorption of CO2 by CaO, which is one of the reasons why water promotes mineralization. Finally, wet mineralization expts. on fly ash show that at 8 MPa, the mineralization efficiencies of HB, SD, SX and YN fly ashes are 1.94, 1.30, 2.03 and 1.62 times their values at 3 MPa. Although the difference in efficiency is affected by the CaO content, supercrit. CO2 can effectively improve the mineralization efficiency.
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Meng, J.; Liao, W.; Zhang, G. Emerging CO2-Mineralization Technologies for Co-Utilization of Industrial Solid Waste and Carbon Resources in China. Minerals 2021, 11, 274, DOI: 10.3390/MIN11030274
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Emerging CO2-mineralization technologies for co-utilization of industrial solid waste and carbon resources in China
Meng, Junlin; Liao, Wenjie; Zhang, Guoquan
Minerals (Basel, Switzerland) (2021), 11 (3), 274CODEN: MBSIBI; ISSN:2075-163X. (MDPI AG)
CO2 mineralization (aka mineral carbonation) is a promising method for the chem. sequestration of CO2 via reaction with oxides of alk. or alk.-earth metals to form carbonates. It has documented advantages over similar technol. solns. to climate change. The huge amt. of industrial solid waste, as a serious environmental issue confronted by China, can provide addnl. alky. sources for CO2 mineralization. In this study, we present an overview of the latest advances in the emerging technologies of CO2-mineralization via industrial solid waste in China, from the perspective of both theor. and practical considerations. We summarize the types of industrial solid waste that are used (mainly coal fly ash, steel slag, phosphogypsum, and blast furnace slag) and the technol. options available in the literature, with an emphasis on the discussion of the involved process-intensification methods and valuable chems. produced. Furthermore, we illustrate the current status of pertinent policies, and research and development activities in China. Finally, we identify the current knowledge gaps, particularly in understanding the overall sustainability performance of these CO2-mineralization technologies, and indicate that the tech., economic, and environmental challenges of promoting and commercializing these technologies for the co-utilization of industrial solid waste and carbon resources call for, amongst other things, more joint efforts by chemists, chem. engineers, and environmental scientists, and more feedback from the energy and industrial sectors.
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Karalis, K.; Kollias, K.; Bartzas, G.; Mystrioti, C.; Xenidis, A. CO2 Sequestration Using Fly Ash from Lignite Power Plants. Mater. Proc. 2022, 5, 131, DOI: 10.3390/MATERPROC2021005131
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Gadikota, G. Carbon Mineralization Pathways for Carbon Capture, Storage and Utilization. Commun. Chem. 2021, 4, 23, DOI: 10.1038/s42004-021-00461-x
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Carbon mineralization pathways for carbon capture, storage and utilization
Gadikota, Greeshma
Communications Chemistry (2021), 4 (1), 23CODEN: CCOHCT; ISSN:2399-3669. (Nature Research)
Carbon mineralization is a versatile and thermodynamically downhill process that can be harnessed for capturing, storing, and utilizing CO2 to synthesize products with enhanced properties. Here the author discusses the advances in and challenges of carbon mineralization, and concludes that tuning the chem. interactions involved will allow us to unlock its potential for advancing low carbon energy and resource conversion pathways.
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Ji, L.; Yu, H.; Wang, X.; Grigore, M.; French, D.; Gözükara, Y. M.; Yu, J.; Zeng, M. CO2 Sequestration by Direct Mineralisation Using Fly Ash from Chinese Shenfu Coal. Fuel Process. Technol. 2017, 156, 429– 437, DOI: 10.1016/J.FUPROC.2016.10.004
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CO2 sequestration by direct mineralisation using fly ash from Chinese Shenfu coal
Ji, Long; Yu, Hai; Wang, Xiaolong; Grigore, Mihaela; French, David; Gozukara, Yesim M.; Yu, Jianglong; Zeng, Ming
Fuel Processing Technology (2017), 156 (), 429-437CODEN: FPTEDY; ISSN:0378-3820. (Elsevier Ltd.)
Fly ash is a potential source of highly reactive feedstock for CO2 mineral carbonation. It does not require pre-treatment, but it has a low carbonation rate and efficiency. To address these issues, we studied the carbonation performance and mechanism of a fly ash from Shenfu coal of China. The effects of temp., solid to liq. ratio and gas flow rate on the carbonation efficiency of the fly ash were systematically investigated in a direct mineralization process. Our results indicated that calcium in lime and portlandite had a higher reactivity towards CO2 than that in other calcium bearing phases either cryst. or amorphous. Solely increasing the temp. did not improve carbonation efficiency. However, expts. in a batch reactor under elevated temp. (140, 180, and 220 °C) and pressure conditions (10 and 20 bar) using recyclable additives showed that a combination of high temp. and pressure significantly improved carbonation efficiency in the presence of 0.5 mol/L Na2CO3. Our multiple-cycle expts. showed that Na2CO3 facilitated the pptn. of calcium carbonate and was well regenerated in the process.
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Ji, L.; Yu, H.; Zhang, R.; French, D.; Grigore, M.; Yu, B.; Wang, X.; Yu, J.; Zhao, S. Effects of Fly Ash Properties on Carbonation Efficiency in CO2 Mineralisation. Fuel Process. Technol. 2019, 188, 79– 88, DOI: 10.1016/J.FUPROC.2019.01.015
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Effects of fly ash properties on carbonation efficiency in CO2 mineralisation
Ji, Long; Yu, Hai; Zhang, Ruijie; French, David; Grigore, Mihaela; Yu, Bing; Wang, Xiaolong; Yu, Jianglong; Zhao, Shuaifei
Fuel Processing Technology (2019), 188 (), 79-88CODEN: FPTEDY; ISSN:0378-3820. (Elsevier Ltd.)
CO2 mineralization by industrial wastes is a promising option for mitigating carbon emissions safely and permanently with low material cost. But there is still absence of a detailed understanding on how fly ash properties affect the carbonation reactions. To fill this knowledge gap, five coal combustion fly ashes, Beijing (BJ), Wuhai (WH), Hazelwood (HW), Yallourn (YA) and Loy Yang (LY) ashes, from China and Australia were selected for carbonation studies. Expts. were performed in a batch reactor at 40 and 140°C with 20 bar initial CO2 pressure, 200 g/L solid to liq. ratio, 450 rpm stirring rate to compare the carbonation performance of the five fly ashes and the effect of fly ash properties on carbonation reactions. Then BJ, YA and HW ashes were then selected for further study in a wide temp. range (40-220°C) because of their higher CO2 sequestration capacity than the other two ashes. SEM (SEM) with energy dispersive spectrometry (EDS) were used to characterize morphol. and elemental properties of fresh and carbonated fly ash samples. Compared to LY and WH ashes, BJ, YA and HW ashes displayed much higher CO2 sequestration capacity due to the higher fraction of reactive Ca/Mg-bearing cryst. phases, including lime (CaO) and portlandite (Ca(OH)2) in BJ ash, periclase (MgO) and srebrodolskite (Ca2Fe2O5) in YA ash, and periclase and brucite (Mg(OH)2) in HW ash.
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Yin, T.; Yin, S.; Srivastava, A.; Gadikota, G. Regenerable Solvents Mediate Accelerated Low Temperature CO2 Capture and Carbon Mineralization of Ash and Nano-Scale Calcium Carbonate Formation. Resour. Conserv. Recycl. 2022, 180, 106209 DOI: 10.1016/J.RESCONREC.2022.106209
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Regenerable solvents mediate accelerated low temperature CO2 capture and carbon mineralization of ash and nano-scale calcium carbonate formation
Yin, Tianhe; Yin, Shufan; Srivastava, Akanksha; Gadikota, Greeshma
Resources, Conservation and Recycling (2022), 180 (), 106209CODEN: RCREEW; ISSN:1879-0658. (Elsevier B.V.)
The dual need to remove CO2 from our emissions and treat alk. industrial residues such as ash materials motivate the design of innovative pathways to simultaneously capture and convert CO2 into mineralized carbonates. Direct carbon mineralization is one approach that addresses the need to simultaneously treat alk. industrial residues and mineralize CO2 emissions. Low CO2 soly. in water and slow kinetics at ambient temp. have challenged the direct carbon mineralization of alk. industrial residues. To address these challenges, the use of CO2 capture solvents that enhance CO2 soly. and facilitate accelerated carbon mineralization of fly ash at temps. below 90 °C is investigated. Calcium carbonate formation results in the inherent regeneration of the solvent. The carbon mineralization extents of non-calcium carbonate content in fly ash were 50% and 51% and in waste ash were 58% and 62% in 2.5 M sodium glycinate and 30 wt% MEA solns., resp. The expts. were performed at 50 °C for 3 h with CO2 partial pressure of 1 atm in a continuously stirred slurry environment with 15 wt.% solid. Furthermore, nanoscale CaCO3 is successfully synthesized from dissolved calcium using CO2-loaded sodium glycinate and surfactants such as CTAB (Cetyl Tri-Me Ammonium Bromide). Surfactants such as CTAB bind to the calcium carbonate surface and regulate the growth of calcium carbonate particles. These innovative approaches demonstrate the feasibility of directly storing CO2 in fly ash and waste ash as calcium carbonate and producing nanoscale calcium carbonate using regenerable CO2 capture solvents.
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Ji, L.; Yu, H.; Yu, B.; Jiang, K.; Grigore, M.; Wang, X.; Zhao, S.; Li, K. Integrated Absorption–Mineralisation for Energy-Efficient CO2 Sequestration: Reaction Mechanism and Feasibility of Using Fly Ash as a Feedstock. Chem. Eng. J. 2018, 352, 151– 162, DOI: 10.1016/J.CEJ.2018.07.014
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Integrated absorption-mineralisation for energy-efficient CO2 sequestration: Reaction mechanism and feasibility of using fly ash as a feedstock
Ji, Long; Yu, Hai; Yu, Bing; Jiang, Kaiqi; Grigore, Mihaela; Wang, Xiaolong; Zhao, Shuaifei; Li, Kangkang
Chemical Engineering Journal (Amsterdam, Netherlands) (2018), 352 (), 151-162CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
The most crit. challenge for the large-scale implementation of amine-based carbon dioxide (CO2) capture is the high energy consumption of absorbent thermal regeneration. To reduce the energy requirement, absorbent thermal regeneration can be replaced by a chem. method that integrates amine scrubbing, chem. regeneration and CO2 mineralization in one process. However, the mechanisms of the process and the application of industrial waste as feedstocks have not been fully investigated. In the present work, we studied the integrated CO2 absorption-mineralization process using the benchmark solvent monoethanolamine (MEA) as an amine absorbent and fly ash as a chem. regeneration agent. We investigated the mechanism involved in the mineralization in detail and studied the performance of MEA in regeneration by mineralization of calcium oxide (CaO) at various CO2-loadings. The performance stability of MEA was verified in multicycle CO2 absorption-mineralization expts. We also investigated the tech. feasibility of using fly ash as a feedstock for absorbent regeneration. Our results show that MEA can be regenerated after a carbonation reaction with both calcium oxide and fly ash at 40°C, and that the CO2 absorbed by MEA is pptd. as calcium carbonate. Compared with traditional thermal regeneration-based CO2 capture, the integrated CO2 absorption-mineralization process displays a similar cyclic CO2-loading (0.21mol/mol) but has great advantages in energy redn. and capital cost savings due to the smaller energy requirement of amine regeneration and the limitation of CO2 compression and pipeline transport. This technol. has great potential for industrial application, particularly with CaO-contg. wastes such as fly ash and carbide slag.
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Pastero, L.; Curetti, N.; Ortenzi, M. A.; Schiavoni, M.; Destefanis, E.; Pavese, A. CO2 Capture and Sequestration in Stable Ca-Oxalate, via Ca-Ascorbate Promoted Green Reaction. Sci. Total Environ. 2019, 666, 1232– 1244, DOI: 10.1016/J.SCITOTENV.2019.02.114
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CO2 capture and sequestration in stable Ca-oxalate, via Ca-ascorbate promoted green reaction
Pastero, Linda; Curetti, Nadia; Ortenzi, Marco Aldo; Schiavoni, Marco; Destefanis, Enrico; Pavese, Alessandro
Science of the Total Environment (2019), 666 (), 1232-1244CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
The increase in the amt. of CO2 (CO2) emissions related to many anthropic activities is a persistent and growing problem. During the last years, many solns. have been set out, none of them being the ultimate one. Investigators agree on the need of a synergic approach to the problem, in terms of many complementary methods of sequestration that, combined with the redn. of prodn., will be able to decrease the concn. of the CO2 in the atm. We explore the use of a green reaction to trap the CO2 into a stable cryst. phase (weddellite) resorting to a multidisciplinary approach. CO2 is reduced and pptd. as Ca oxalate through vitamin C as a sacrificial reductant. Ca oxalate crystals obtained show a startling good quality that increases their already great stability over a wide chem. and phys. conditions' range.
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Schuler, E.; Demetriou, M.; Shiju, N. R.; Gruter, G. J. M. Towards Sustainable Oxalic Acid from CO2 and Biomass. ChemSusChem 2021, 14, 3636– 3664, DOI: 10.1002/CSSC.202101272
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Towards Sustainable Oxalic Acid from CO2 and Biomass
Schuler, Eric; Demetriou, Marilena; Shiju, N. Raveendran; Gruter, Gert-Jan M.
ChemSusChem (2021), 14 (18), 3636-3664CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. To quickly and drastically reduce CO2 emissions and meet our ambitions of a circular future, we need to develop carbon capture and storage (CCS) and carbon capture and utilization (CCU) to deal with the CO2 that we produce. While we have many alternatives to replace fossil feedstocks for energy generation, for materials such as plastics we need carbon. The ultimate circular carbon feedstock would be CO2. A promising route is the electrochem. redn. of CO2 to formic acid derivs. that can subsequently be converted into oxalic acid. Oxalic acid is a potential new platform chem. for material prodn. as useful monomers such as glycolic acid can be derived from it. This work is part of the European Horizon 2020 project "Ocean" in which all these steps are developed. This Review aims to highlight new developments in oxalic acid prodn. processes with a focus on CO2-based routes. All available processes are critically assessed and compared on criteria including overall process efficiency and triple bottom line sustainability.
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Costa, R. S.; Aranha, B. S. R.; Ghosh, A.; Lobo, A. O.; da Silva, E. T. S. G.; Alves, D. C. B.; Viana, B. C. Production of Oxalic Acid by Electrochemical Reduction of CO2 Using Silver-Carbon Material from Babassu Coconut Mesocarp. J. Phys. Chem. Solids 2020, 147, 109678 DOI: 10.1016/J.JPCS.2020.109678
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Production of oxalic acid by electrochemical reduction of CO2 using silver-carbon material from babassu coconut mesocarp
Costa, Rubens S.; Aranha, Bruna S. R.; Ghosh, Anupama; Lobo, Anderson O.; da Silva, Everson T. S. G.; Alves, Diego C. B.; Viana, Bartolomeu C.
Journal of Physics and Chemistry of Solids (2020), 147 (), 109678CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Ltd.)
The present study provides an efficient carbon dioxide (CO2) redn. process to produce oxalic acid with approx. 29% faradaic efficiency (based on HPLC) using a carbon material from babassu biomass coupled with silver. A new carbon-silver hybrid material has been prepd. from babassu mesocarp by hydrothermal carbonization; this material initiates the electrochem. redn. of CO2 at a lower potential and catalyzes the conversion of CO2 into oxalic acid. To evaluate the catalytic potential of this reaction, other materials, such as glassy carbon and hydrothermal carbon, which generated a higher cathodic c.d., were tested at the same potential. The rate of CO2 redn. was found to be increased at low potentials in the presence of the hybrid carbon-silver material synthesized by a one-pot hydrothermal method from babassu biomass in 48 h (Ag@C-48).
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Yang, Y.; Gao, H.; Feng, J.; Zeng, S.; Liu, L.; Liu, L.; Ren, B.; Li, T.; Zhang, S.; Zhang, X.; Yang, Y.; Gao, H.; Feng, J.; Zeng, S.; Liu, L.; Zhang, S.; Zhang, X.; Ren, B.; Li, T. Aromatic Ester-Functionalized Ionic Liquid for Highly Efficient CO2 Electrochemical Reduction to Oxalic Acid. ChemSusChem 2020, 13, 4900– 4905, DOI: 10.1002/CSSC.202001194
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Aromatic Ester-Functionalized Ionic Liquid for Highly Efficient CO2 Electrochemical Reduction to Oxalic Acid
Yang, Yingliang; Gao, Hongshuai; Feng, Jiaqi; Zeng, Shaojuan; Liu, Lei; Liu, Licheng; Ren, Baozeng; Li, Tao; Zhang, Suojiang; Zhang, Xiangping
ChemSusChem (2020), 13 (18), 4900-4905CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
Electrochem. redn. of CO2 into valuable chems. is a significant route to use CO2 resources. Among various electroredn. products, oxalic acid (H2C2O4) is an important chem. for pharmaceuticals, rare earth extn., and metal processing. Here, an aprotic arom. ester-functionalized ionic liq. (IL), 4-(methoxycarbonyl) phenol Et4N+ ([TEA][4-MF-PhO]), was designed as an electrolyte for CO2 electroredn. into oxalic acid. It exhibited a large oxalic acid partial c.d. of 9.03 mA cm-2 with a faradaic efficiency (FE) of 86% at -2.6 V (vs. Ag/Ag+), and the oxalic acid formation rate was ≤168.4μmol cm-2 h-1, which is the highest reported value to date. Also, the results of d. functional theory calcns. demonstrated that CO2 was efficiently activated to a -COOH intermediate by bis-active sites of the arom. ester anion via the formation of a [4-MF-PhO-COOH]- adduct, which finally dimerized into oxalic acid.
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Rowley, M. C.; Estrada-Medina, H.; Tzec-Gamboa, M.; Rozin, A.; Cailleau, G.; Verrecchia, E. P.; Green, I. Moving Carbon between Spheres, the Potential Oxalate-Carbonate Pathway of Brosimum Alicastrum Sw; Moraceae. Plant Soil 2017, 412, 465– 479, DOI: 10.1007/s11104-016-3135-3
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Moving carbon between spheres, the potential oxalate-carbonate pathway of Brosimum alicastrum Sw.; Moraceae
Rowley, Mike C.; Estrada-Medina, Hector; Tzec-Gamboa, Magnolia; Rozin, Aviram; Cailleau, Guillaume; Verrecchia, Eric P.; Green, Iain
Plant and Soil (2017), 412 (1-2), 465-479CODEN: PLSOA2; ISSN:0032-079X. (Springer)
Aims: The Oxalate-Carbonate Pathway (OCP) is a biogeochem. process that transfers atm. CO2 into the geol. reservoir as CaCO3; however, until now all investigations on this process have focused on species with limited food benefits. This study evaluates a potential OCP assocd. with Brosimum alicastrum, a Neotropical species with agroforestry potential (ca. 70-200 kg-nuts yr-1), in the calcareous soils of Haiti and Mexico. Methods / results: Enzymic anal. demonstrated significant concns. of calcium oxalate (5.97 % D.W.) were assocd. with B. alicastrum tissue in all sample sites. The presence of oxalotrophism was also confirmed with microbiol. analyses in both countries. High concns. of total calcium (>7 g kg-1) and lithogenic carbonate obscured the localised alkalinisation and identification of secondary carbonate assocd. with the OCP at most sample sites, except Ma Rouge, Haiti. Soils adjacent to subjects in Ma Rouge demonstrated an increase in pH (0.63) and CaCO3 concn. (5.9 %) that, when coupled with root-like secondary carbonate deposits in Mexico, implies that the OCP does also occur in calcareous soils. Conclusions: Therefore this study confirms that the OCP also occurs in calcareous soils, adjacent to B. alicastrum, and could play a fundamental and un-accounted role in the global calcium-carbon coupled cycle.
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Gadd, G. M.; Bahri-Esfahani, J.; Li, Q.; Rhee, Y. J.; Wei, Z.; Fomina, M.; Liang, X. Oxalate Production by Fungi: Significance in Geomycology, Biodeterioration and Bioremediation. Fungal Biol. Rev. 2014, 28, 36– 55, DOI: 10.1016/J.FBR.2014.05.001
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Curetti, N.; Pastero, L.; Bernasconi, D.; Cotellucci, A.; Corazzari, I.; Archetti, M.; Pavese, A. Thermal Stability of Calcium Oxalates from CO2 Sequestration for Storage Purposes: An In-Situ HT-XRPD and TGA Combined Study. Minerals 2022, 12, 53, DOI: 10.3390/MIN12010053
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Thermal Stability of Calcium Oxalates from CO2 Sequestration for Storage Purposes: An In-Situ HT-XRPD and TGA Combined Study
Curetti, Nadia; Pastero, Linda; Bernasconi, Davide; Cotellucci, Andrea; Corazzari, Ingrid; Archetti, Maurizio; Pavese, Alessandro
Minerals (Basel, Switzerland) (2022), 12 (1), 53CODEN: MBSIBI; ISSN:2075-163X. (MDPI AG)
Calcium oxalates are naturally occurring biominerals and can be found as a byproduct of some industrial processes. Recently, a new and green method for carbon capture and sequestration in stable calcium oxalate from oxalic acid produced by carbon dioxide redn. was proposed. The reaction resulted in high-quality weddellite crystals. Assessing the stability of these weddellite crystals is crucial to forecast their reuse as solid-state reservoir of pure CO2 and CaO in a circular economy perspective or, eventually, their disposal. The thermal decompn. of weddellite obtained from the new method of carbon capture and storage was studied by coupling in-situ high-temp. X-ray powder diffraction and thermogravimetric anal., in order to evaluate the dehydration, decarbonation, and the possible prodn. of unwanted volatile species during heating. At low temp. (119-255 °C), structural water release was superimposed to an early CO2 feeble evolution, resulting in a water-carbon dioxide mixt. that should be sepd. for reuse. Furthermore, the storage temp. limit must be considered bearing in mind this CO2 release low-temp. event. In the range 390-550 °C, a two-component mixt. of carbon monoxide and dioxide is evolved, requiring oxidn. of the former or gas sepn. to reuse pure gases. Finally, the last decarbonation reaction produced pure CO2 starting from 550 °C.
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Grayevsky, R.; Reiss, A. G.; Emmanuel, S. Carbon Storage through Rapid Conversion of Forsterite into Solid Oxalate Phases. Energy Fuels 2023, 37, 509– 517, DOI: 10.1021/acs.energyfuels.2c03245
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Carbon Storage through Rapid Conversion of Forsterite into Solid Oxalate Phases
Grayevsky, Roni; Reiss, Amit G.; Emmanuel, Simon
Energy & Fuels (2023), 37 (1), 509-517CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
Carbon capture and storage are likely to be crit. components in lowering atm. CO2 levels. Mineralization is often proposed as a method to store carbon and typically involves reacting CO2 directly with silicate minerals, such as forsterite, to form carbonate minerals. However, this reaction is slow under std. conditions, so that sequestering significant amts. of carbon can take years or decades. Here, we demonstrate the feasibility of using a reaction between oxalic acid and forsterite to create stable carbon-bearing oxalate minerals. We performed a series of batch expts. at room temp. and pressure to quantify the forsterite dissoln. rate and the efficiency of Mg utilization. Our results show that conversion of forsterite to Mg and Fe oxalate is achieved rapidly: after 30 days, 52% of Mg was converted to Mg oxalate so that 1 t of forsterite can be used to store 177 kg of carbon. Our calcns. show that reacting ultramafic mine tailings with oxalic acid has the potential to make a significant contribution toward the global target for CO2 removal by carbon capture and storage.
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Lieberman, R. N.; Knop, Y.; Palmerola, N. M.; Muñoz, C.; Cohen, H.; Izquiredo, M.; Lorenzo, J. A.; Taboas, B.; Font, O.; Querol, X. Production of Environmentally Friendly Sand-like Products from Granitoid Waste Sludge and Coal Fly Ash for Civil Engineering. J. Clean. Prod. 2019, 238, 117880 DOI: 10.1016/J.JCLEPRO.2019.117880
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Chancey, R. T.; Stutzman, P.; Juenger, M. C. G.; Fowler, D. W. Comprehensive Phase Characterization of Crystalline and Amorphous Phases of a Class F Fly Ash. Cem. Concr. Res. 2010, 40, 146– 156, DOI: 10.1016/J.CEMCONRES.2009.08.029
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Comprehensive phase characterization of crystalline and amorphous phases of a Class F fly ash
Chancey, Ryan T.; Stutzman, Paul; Juenger, Maria C. G.; Fowler, David W.
Cement and Concrete Research (2010), 40 (1), 146-156CODEN: CCNRAI; ISSN:0008-8846. (Elsevier Ltd.)
A comprehensive approach to qual. and quant. characterization of cryst. and amorphous constituent phases of a largely heterogeneous Class F fly ash is presented. Traditionally, fly ash compn. is expressed as bulk elemental oxide content, generally detd. by X-ray fluorescence spectroscopy. However, such anal. does not discern between relatively inert cryst. phases and highly reactive amorphous phases of similar elemental compn. X-ray diffraction was used to identify the cryst. phases present in the fly ash, and the Rietveld quant. phase anal. method was applied to det. the relative proportion of each of these phases. A synergistic method of X-ray powder diffraction, SEM, energy dispersive spectroscopy, and multispectral image anal. was developed to identify and quantify the amorphous phases present in the fly ash.
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Li, L.; Liu, W.; Qin, Z.; Zhang, G.; Yue, H.; Liang, B.; Tang, S.; Luo, D. Research on Integrated CO2 Absorption-Mineralization and Regeneration of Absorbent Process. Energy 2021, 222, 120010 DOI: 10.1016/J.ENERGY.2021.120010
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Research on integrated CO2 absorption-mineralization and regeneration of absorbent process
Li, Long; Liu, Weizao; Qin, Zhifeng; Zhang, Guoquan; Yue, Hairong; Liang, Bin; Tang, Shengwei; Luo, Dongmei
Energy (Oxford, United Kingdom) (2021), 222 (), 120010CODEN: ENEYDS; ISSN:0360-5442. (Elsevier Ltd.)
The hot potassium-alkali method provides excellent performance for the absorption of CO2 from flue gas. However, the high energy consumption by absorbent regeneration poses a crit. barrier to the widespread industrialization of the hot potassium-alkali method. In this study, an integrated CO2 absorption-mineralization and regeneration of absorbent (IAMR) process was proposed using K2CO3 soln. as the absorbent and steel slag as the desorbent at normal temp. and pressure. This method greatly reduced the energy consumption and costs compared with the traditional thermal regeneration method. Under the optimal conditions, i.e. a K2CO3 concn. of 1.0 mol/L, reaction temp. of 60°C and liq.-solid (K2CO3 soln.-steel slag) ratio of 14 mL/g, the carbonation conversion of the steel slag reached 58.63% after 120min, corresponding to a CO2 storage capacity of 212 kg/t steel slag. The reaction process showed that the main component Ca2SiO4 in the steel slag had high soly. activity in K2CO3 soln. which significantly enhanced the rate and efficiency of CO2 sequestration. Moreover, the performance stability of K2CO3 soln. during CO2 absorption-desorption circulation was discussed. This research is of great significance for the simultaneous treatment of alk. waste slags (steel slag, fly ash, etc.) and mitigation of greenhouse gases.
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Price, D.; Dollimore, D.; Fatemi, N. S.; Whitehead, R. Mass Spectrometric Determination of Kinetic Parameters for Solid State Decomposition Reactions. Part 1. Method; Calcium Oxalate Decomposition. Thermochim. Acta 1980, 42, 323– 332, DOI: 10.1016/0040-6031(80)85093-3
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Mass spectrometric determination of kinetic parameters for solid state decomposition reactions. Part 1. Method; calcium oxalate decomposition
Price, D.; Dollimore, D.; Fatemi, N. S.; Whitehead, R.
Thermochimica Acta (1980), 42 (3), 323-32CODEN: THACAS; ISSN:0040-6031.
A description is given of an expt. in which a time-of-flight mass spectrometer is used to monitor the thermal decompn. of solid samples placed in the ion source region. Because the mass spectrometer can both identify and quantify the gaseous products, the technique yields direct insight into the decompn. mechanisms and values for kinetic parameters. An assessment of the technique is given and its application illustrated by a study of anhyd. Ca oxalate decompn. in vacuo.
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Hourlier, D. Thermal Decomposition of Calcium Oxalate: Beyond Appearances. J. Therm. Anal. Calorim. 2019, 136, 2221– 2229, DOI: 10.1007/s10973-018-7888-1
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Thermal decomposition of calcium oxalate: beyond appearances
Hourlier, Djamila
Journal of Thermal Analysis and Calorimetry (2019), 136 (6), 2221-2229CODEN: JTACF7; ISSN:1388-6150. (Springer)
The goal of this study is twofold: to take a fresh look at the decompn. of calcium oxalate and to warn users of thermogravimetric anal. against the hasty interpretation of results obtained. Since the pioneer work of Duval 70 years ago, the scientific community has agreed unanimously as to the decompn. of anhyd. calcium oxalate (CaC2O4) into calcium carbonate (CaCO3) and CO gas, and that of the calcium carbonate into calcium oxide (CaO), and CO2 gas. We will demonstrate how these reactions, simple in appearance, in fact result from a succession of reactive phenomena involving numerous constituents both solid (CaCO3, free carbon) and gaseous (CO2 and CO) produced by intermediary reactions. The mass losses evaluated in the two distinct domains correspond closely to the molar masses of CO and CO2, resp. The simple math. calcn. of that mass loss has simply concealed the existence of other reactions, and, most particularly the Boudouard reaction and that of solid phases between CaCO3 and C. It just goes to show that appearances can be deceiving.
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Payá, J.; Monzó, J.; Borrachero, M. V.; Perris, E.; Amahjour, F. Thermogravimetric Methods for Determining Carbon Content in Fly Ashes. Cem. Concr. Res. 1998, 28, 675– 686, DOI: 10.1016/S0008-8846(98)00030-1
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Thermogravimetric methods for determining carbon content in fly ashes
Paya, J.; Monzo, J.; Borrachero, M. V.; Perris, E.; Amahjour, F.
Cement and Concrete Research (1998), 28 (5), 675-686CODEN: CCNRAI; ISSN:0008-8846. (Elsevier Science Ltd.)
Unburnt carbon content in fly ashes were detd. using thermogravimetry. After being heated in inert atm., carbon reacts towards iron oxide components in fly ash, and after it is heated in an oxidizing environment, carbon oxidn. process overlaps carbonate decompn. A thermogravimetric method combining inert and oxidizing atmospheres was designed, in such a way that hydrated lime, calcium carbonate, and unburnt carbon contents for several fly ashes were detd. Carbon content in fly ash-sized fractions also was detd., with highest carbon content found in the coarsest fractions.
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Chindaprasirt, P.; Rattanasak, U.; Taebuanhuad, S. Resistance to Acid and Sulfate Solutions of Microwave-Assisted High Calcium Fly Ash Geopolymer. Mater. Struct. 2013, 46, 375– 381, DOI: 10.1617/s11527-012-9907-1
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Resistance to acid and sulfate solutions of microwave-assisted high calcium fly ash geopolymer
Chindaprasirt, Prinya; Rattanasak, Ubolluk; Taebuanhuad, Sompop
Materials and Structures (Dordrecht, Netherlands) (2013), 46 (3), 375-381CODEN: MASTED; ISSN:1359-5997. (Springer)
In this paper, 90-W microwave radiation for 5 min plus a shortened heat curing period was applied to cure the fresh geopolymer paste. Results showed that microwave radiation contributed to the dissoln. of fly ash in the alk. soln. Numerous gel formations were obsd. in microscopic scale. This resulted in a dense composite and strong bonding between the fly ash and the geopolymer matrix leading to high strength gain compared to those of the control pastes cured at 65 °C for 24 h. In addn., resistances to the sulfuric acid and sulfate attacks of the microwave geopolymer were superior to that of the control as indicated by the relatively low strength loss. The microwave radiation also helped the geopolymer attaining thermal stability as the dense matrixes were obtained.
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Sendula, E.; Lamadrid, H. M.; Rimstidt, J. D.; Steele-MacInnis, M.; Sublett, D. M.; Aradi, L. E.; Szabó, C.; Caddick, M. J.; Zajacz, Z.; Bodnar, R. J. Synthetic Fluid Inclusions XXIV. In Situ Monitoring of the Carbonation of Olivine Under Conditions Relevant to Carbon Capture and Storage Using Synthetic Fluid Inclusion Micro-Reactors: Determination of Reaction Rates. Front. Clim. 2021, 3, 722447 DOI: 10.3389/fclim.2021.722447
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Aughenbaugh, K. L.; Chancey, R. T.; Stutzman, P.; Juenger, M. C.; Fowler, D. W. An Examination of the Reactivity of Fly Ash in Cementitious Pore Solutions. Mater. Struct. 2013, 46, 869– 880, DOI: 10.1617/s11527-012-9939-6
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An examination of the reactivity of fly ash in cementitious pore solutions
Aughenbaugh, Katherine L.; Chancey, Ryan T.; Stutzman, Paul; Juenger, Maria C.; Fowler, David W.
Materials and Structures (Dordrecht, Netherlands) (2013), 46 (5), 869-880CODEN: MASTED; ISSN:1359-5997. (Springer)
Fly ash is frequently used to replace cement in concrete, but it is difficult to predict performance based only on the oxide compn., which is typically the only compositional information available. In order to better utilize fly ash in concrete, it is important to develop more meaningful characterization methods and correlate these with performance. The research presented here uses a combination of anal. methods, including x-ray powder diffraction, SEM coupled with multispectral image anal., and soln. anal. to det. the compns. of the glassy phases in a specific fly ash and to examine the fly ashes reactivity in late- and early-age cement pore solns., ultrapure water, and sodium hydroxide. The dissoln. of individual glassy phases in the fly ash was tracked over time and the pptn. of reaction products monitored. A high-calcium aluminosilicate glass was the most reactive, a low-calcium aluminosilicate glass was of intermediate reactivity and a medium-calcium aluminosilicate glass had the lowest reactivity in the solns. tested for a specific fly ash. This result suggests the glass compn. has a strong effect on reactivity, but that that there is not a strict correlation between calcium content and glass reactivity.
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Surface Chemistry of Calcium Aluminosilicate Glasses
Snellings, Ruben
Journal of the American Ceramic Society (2015), 98 (1), 303-314CODEN: JACTAW; ISSN:0002-7820. (Wiley-Blackwell)
The surface chem. of synthetic calcium aluminosilicate glasses exposed to aq. solns. of varying pH was described using zeta potential measurements and batch surface titrns. Element release and proton consumption were measured to characterize the reactions at the surface as a function of pH. It was found that proton-metal exchange or leaching was the dominant proton consumption process at low pH. The exchange reaction was obsd. to maintain charge balance, indicating that over the limited duration of the expts. no repolymn. of the silica-rich leached layer occurred. At high pH, dissoln. and hydrolysis of aq. ions controlled the proton mass balance, no evidence for the formation of leached layers was found. Silica-rich glasses were found to be more resistant to corrosion by either proton-metal exchange or dissoln. than CaO-rich glasses. Glass basicity was found to be reflected in the establishment of a higher pH when immersing the glass in neutral aq. solns. and in a reduced stability of the glass surface toward aq. solns. High pH, high surface charge and reduced glass network polymn. act together in enhancing the reaction of CaO-rich glasses with aq. solns.
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Particulate pollution case studies which illustrate uses of individual particle analysis by scanning electron microscopy
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Environmental geochemistry and health (1989), 11 (3-4), 157-62 ISSN:0269-4042.
Individual particle analysis by scanning electron microscopy is a high resolution analytical technique which can provide the detail necessary to solve many particulate pollution problems. In an electron microscope a beam of electrons is focused on a specimen resulting in a number of signals which are used to collect chemical and morphological information from individual features either manually or automatically. Case studies are presented which illustrate the applications of the technique to three paniculate pollution problems of differing complexity.
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Conti, C.; Brambilla, L.; Colombo, C.; Dellasega, D.; Gatta, G. D.; Realini, M.; Zerbi, G. Stability and Transformation Mechanism of Weddellite Nanocrystals Studied by X-Ray Diffraction and Infrared Spectroscopy. Phys. Chem. Chem. Phys. 2010, 12, 14560– 14566, DOI: 10.1039/C0CP00624F
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Stability and transformation mechanism of weddellite nanocrystals studied by X-ray diffraction and infrared spectroscopy
Conti, Claudia; Brambilla, Luigi; Colombo, Chiara; Dellasega, David; Gatta, G. Diego; Realini, Marco; Zerbi, Giuseppe
Physical Chemistry Chemical Physics (2010), 12 (43), 14560-14566CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
Stability of weddellite, the dihydrate phase of calcium oxalate [CaC2O4·(H2O)2+x], mainly detected in kidney stones and in oxalate films found on the surfaces of several ancient monuments, has been studied. Under ambient conditions, weddellite is unstable and quickly changes into whewellite, the monohydrate phase of calcium oxalate (CaC2O4·H2O). Crystal structure of weddellite was detd. by single crystal X-ray diffraction. Synthesized nanocrystals of weddellite were kept under different hygrometric conditions in order to study, by X-ray powder diffraction, the effect of humidity on their stability. The mechanism of transformation of weddellite nanocrystals has been investigated by IR spectroscopy using D2O as a structural probe.
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Luo, Q.; Chen, G.; Sun, Y.; Ye, Y.; Qiao, X.; Yu, J. Dissolution Kinetics of Aluminum, Calcium, and Iron from Circulating Fluidized Bed Combustion Fly Ash with Hydrochloric Acid. Ind. Eng. Chem. Res. 2013, 52, 18184– 18191, DOI: 10.1021/ie4026902
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Dissolution Kinetics of Aluminum, Calcium, and Iron from Circulating Fluidized Bed Combustion Fly Ash with Hydrochloric Acid
Luo, Qing; Chen, Guilan; Sun, Yuzhu; Ye, Yinmei; Qiao, Xiuchen; Yu, Jianguo
Industrial & Engineering Chemistry Research (2013), 52 (51), 18184-18191CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
The dissoln. of aluminum, calcium, and iron from calcined fly ash with hydrochloric acid was investigated using a batch reactor at temps. from 315 to 345 K and over the acid concn. range of 1.4-6.0 M. Calcium was preferentially released compared to aluminum though these two elements in the calcined fly ash were mainly incorporated into the same mineralogical phase of anorthite. The dissolved fractions of aluminum, calcium, and iron increased with leaching time, but it decreased for silicon, which indicated the continuous pptn. of silicon during the leaching process. The dissoln. kinetic data were successfully examd. according to a semiempirical Avrami-type equation. The activation energies for dissoln. of aluminum, calcium, and iron from the calcined fly ash with hydrochloric acid are 32, 28, and 19 kJ/mol, resp.
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Bhatt, A.; Priyadarshini, S.; Acharath Mohanakrishnan, A.; Abri, A.; Sattler, M.; Techapaphawit, S. Physical, Chemical, and Geotechnical Properties of Coal Fly Ash: A Global Review. Case Stud. Constr. Mater. 2019, 11, e00263 DOI: 10.1016/J.CSCM.2019.E00263
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Zhang, Y.; Jackson, C.; Krevor, S. An Estimate of the Amount of Geological CO2 Storage over the Period of 1996–2020. Environ. Sci. Technol. Lett. 2022, 9, 693– 698, DOI: 10.1021/ACS.ESTLETT.2C00296
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An Estimate of the Amount of Geological CO2 Storage over the Period of 1996-2020
Zhang, Yuting; Jackson, Christopher; Krevor, Samuel
Environmental Science & Technology Letters (2022), 9 (8), 693-698CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
The climate impact of carbon capture and storage depends on how much CO2 is stored underground, yet databases of industrial-scale projects report capture capacity as a measure of project size. We review publicly available sources to est. the amt. of CO2 that has been stored by facilities since 1996. We organize these sources into three categories corresponding to the assocd. degree of assurance: (1) legal assurance, (2) quality assurance through auditing, and (3) no assurance. Data were found for 20 facilities, with an aggregate capture capacity of 36 Mt of CO2 year-1. Combining data from all categories, we est. that 29 Mt of CO2 was geol. stored in 2019 and there was cumulative storage of 197 Mt over the period of 1996-2020. These are climate relevant scales commensurate with recent cumulative and ongoing emissions impacts of renewables in some markets, e.g., solar photovoltaics in the United States. The widely used capture capacity is in aggregate 19-30% higher than storage rates and is not a good proxy for estg. storage vols. However, the discrepancy is project-specific and not always a reflection of project performance. This work provides a snapshot of storage amts. and highlights the need for uniform reporting on capture and storage rates with quality assurance.
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Pan, S. Y.; Hung, C. H.; Chan, Y. W.; Kim, H.; Li, P.; Chiang, P. C. Integrated CO2 Fixation, Waste Stabilization, and Product Utilization via High-Gravity Carbonation Process Exemplified by Circular Fluidized Bed Fly Ash. ACS Sustainable Chem. Eng. 2016, 4, 3045– 3052, DOI: 10.1021/acssuschemeng.6b00014
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Integrated CO2 Fixation, Waste Stabilization, and Product Utilization via High-Gravity Carbonation Process Exemplified by Circular Fluidized Bed Fly Ash
Pan, Shu-Yuan; Hung, Chen-Hsiang; Chan, Yin-Wen; Kim, Hyunook; Li, Ping; Chiang, Pen-Chi
ACS Sustainable Chemistry & Engineering (2016), 4 (6), 3045-3052CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
The valorization of industrial solid wastes in civil engineering is one of the main routes for enhancing resource cycle toward environmental and social sustainability. In this study, an integrated approach to capturing CO2 in flue gas and stabilizing solid wastes for utilization as supplementary cementitious material via a high-gravity carbonation (HiGCarb) process was proposed. The fly ash (FA) generated from a circular fluidized bed boiler in the petrochem. industry was used. The effect of different operating parameters on the carbonation conversion was evaluated by the response surface methodol. The maximal carbonation conversion of FA was 77.2% at a rotation speed of 743 rpm and an L/S ratio of 18.9 at 57.3°. In addn., the workability, strength development, and durability of the blended cement with different substitution ratios (i.e., 10%, 15%, and 20%) of carbonated FA were evaluated. Cement with carbonated FA exhibited superior properties, e.g., initial compressive strength (3400 psi at 7 d in 10% substitution ratio) and durability (autoclave expansion <0.15%) compared to cement with fresh FA. After HiGCarb, the physico-chem. properties of FA were upgraded, e.g., lower heavy-metal leaching and stabilized vol. expansion, which were beneficial to usage as green materials in construction engineering.
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Goss, S. L.; Lemons, K. A.; Kerstetter, J. E.; Bogner, R. H. Determination of Calcium Salt Solubility with Changes in PH and PCO2, Simulating Varying Gastrointestinal Environments. J. Pharm. Pharmacol. 2007, 59, 1485– 1492, DOI: 10.1211/JPP.59.11.0004
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Determination of calcium salt solubility with changes in pH and PCO2, simulating varying gastrointestinal environments
Goss, Sandra L.; Lemons, Karen A.; Kerstetter, Jane E.; Bogner, Robin H.
Journal of Pharmacy and Pharmacology (2007), 59 (11), 1485-1492CODEN: JPPMAB; ISSN:0022-3573. (Pharmaceutical Press)
The amt. of calcium available for absorption is dependent, in part, on its sustained soly. in the gastrointestinal (GI) tract. Many calcium salts, which are the calcium sources in supplements and food, have pH-dependent soly. and may have limited availability in the small intestine, the major site of absorption. The equil. soly. of four calcium salts (calcium oxalate hydrate, calcium citrate tetrahydrate, calcium phosphate, calcium glycerophosphate) were detd. at controlled pH values (7.5, 6.0, 4.5 and ≤ 3.0) and in distd. water. The soly. of calcium carbonate was also measured at pH 7.5, 6.0 and 4.5 with two CO2 environments (0.3 and 152 mmHg) above the soln. The pptn. profile of CaCO3 was calcd. using in-vivo data for bicarbonate and pH from literature and equil. calcns. As pH increased, the soly. of each calcium salt increased. However, in distd. water each salt produced a different pH, affecting its soly. value. Although calcium citrate does have a higher soly. than CaCO3 in water, there is little difference when the pH is controlled at pH 7.5. The partial pressure of CO2 also played a role in calcium carbonate soly., depressing the soly. at pH 7.5. The calcns. of sol. calcium resulted in profiles of available calcium, which agreed with previously published in-vivo data on absorbed calcium. The exptl. data illustrate the impact of pH and CO2 on the soly. of many calcium salts in the presence of bicarbonate secretions in the intestine. Calcd. profiles using in-vivo calcium and bicarbonate concns. demonstrate that large calcium doses may not further increase intestinal calcium absorption once the calcium carbonate soly. product has been reached.
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Hunter, H. A.; Ling, F. T.; Peters, C. A. Coprecipitation of Heavy Metals in Calcium Carbonate from Coal Fly Ash Leachate. ACS EST Water 2021, 1, 339– 345, DOI: 10.1021/ACSESTWATER.0C00109
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Murcia Valderrama, M. A.; van Putten, R. J.; Gruter, G. J. M. The Potential of Oxalic – and Glycolic Acid Based Polyesters (Review). Towards CO2 as a Feedstock (Carbon Capture and Utilization – CCU). Eur. Polym. J. 2019, 119, 445– 468, DOI: 10.1016/J.EURPOLYMJ.2019.07.036
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The potential of oxalic - and glycolic acid based polyesters (review). Towards CO2 as a feedstock (Carbon Capture and Utilization - CCU)
Murcia Valderrama, Maria A.; van Putten, Robert-Jan; Gruter, Gert-Jan M.
European Polymer Journal (2019), 119 (), 445-468CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
A review. Plastic materials are indispensable in everyday life because of their versatility, high durability, lightness and cost-effectiveness. As a consequence, worldwide plastic consumption will continue to grow from around 350 million metric tons per annum today to an estd. 1 billion metric tons per annum in 2050. For applications where polymers are applied in the environment or for applications where polymers have a bigger chance of ending up in the environment, (bio)degradable polymers need to be developed to stop endless accumulation of non-degradable polymers irreversibly littering our planet. As monomers and polymers represent more than 80% of the chem. industry's total prodn. vol., a transition from fossil feedstock today (99% of the current feedstock for polymers is fossil-based) to a significantly larger percentage of renewable feedstock in the future (carbon that is already "above the ground") will be required to meet the greenhouse gas redn. targets of the Paris Agreement (>80% CO2 redn. target for the European Chem. Industry sector in 2050). The combination of the predicted polymer market growth and the emergence of new feedstocks creates a fantastic opportunity for novel sustainable polymers. To replace fossil based feedstock, there are only three sustainable alternative sources: biomass, CO2 and existing plastics (via recycling). The ultimate circular feedstock would be CO2: it can be electrochem. reduced to formic acid derivs. that can subsequently be converted into useful monomers such as glycolic acid and oxalic acid. In order to assess the future potential for these polyester building blocks, we will review the current field of polyesters based on these two monomers. Representative synthesis methods, general properties, general degrdn. mechanisms, and recent applications will be discussed in this review. The application potential of these polyesters for a wide range of purposes, as a function of prodn. cost, will also be assessed. It is important to note that polymers derived from CO2 do not necessarily always lead to lower net overall CO2 emissions (during prodn. of after use, e.g. degrdn. in landfills). This needs to be evaluated using robust LCA's and this information is currently not available for the materials discussed in this review.
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ACS ES&T Engineering

Cite this: ACS EST Engg. 2023, 3, 9, 1227–1235

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https://doi.org/10.1021/acsestengg.3c00063

Published May 23, 2023

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Abstract
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Figure 1
Figure 1. Thermal gravimetric analysis of unreacted coal fly ash samples. Three steps were observed in the unreacted samples: (1) loss of water; (2) decomposition of Ca-bearing phases, such as calcite and portlandite; (3) reaction between unoxidized carbon and iron oxide.
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Figure 2
Figure 2. Electron micrographs of the two types of coal fly ash: (a) unreacted KLP fly ash; (b) unreacted Zibulo fly ash; (c) KLP fly ash after 4 days of exposure to 0.1 M oxalic acid; (d) Zibulo fly ash, again after 4 days of reaction in 0.1 M oxalic acid. After the reaction, the presence of euhedral crystals─most likely Ca oxalate─are observed. Additional SEM images are provided in Supplementary Figure S1.
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Figure 3
Figure 3. X-ray diffraction patterns of the two types of coal fly ash: (a) unreacted KLP fly ash; (b) unreacted Zibulo fly ash; (c) KLP fly ash after 16 days of exposure to 0.1 M oxalic acid; (d) Zibulo fly ash, again after 16 days of reaction in 0.1 M oxalic acid. Note that in this figure, only the major mineral phases are indicated.
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Figure 4
Figure 4. Proportion of whewellite (CaC₂O₄·H₂O) and weddellite (CaC₂O₄·(2 + x)H₂O, x < 0.5) during the reaction between coal fly ash samples and 0.1 M oxalic acid. Values are based on XRD measurements. Each point represents a separate experiment.
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Figure 5
Figure 5. Comparison of the chemical compositions for whole samples (total) and crystalline components only in the unreacted fly ash. Total values are based on XRF measurements (Table 1), while the crystalline component is estimated from the XRD results (Supplementary Table S1). The difference between the total and crystalline values are attributed to the presence of amorphous phases. Error bars represent the variance (1σ) of measurements.
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Figure 6
Figure 6. Thermal gravimetric analysis of coal fly ash samples after 16 days of reaction in 0.1 M oxalic acid. Three steps were observed in the reacted samples: (1) dehydration of hydrated Ca oxalate to form anhydrous calcium oxalate (CaC₂O₄) at 120–350 °C; (2) decomposition of CaC₂O₄ to CaCO₃ at 350–550 °C; and (3) decomposition of CaCO₃ to CaO at 550–800 °C.
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Figure 7
Figure 7. Anhydrous calcium oxalate (CaC₂O₄) content as a function of time based on thermal gravimetry measurements. Both the maximum (green) and minimum (orange) calcium oxalate contents (calculated using eqs 2 and 3, respectively) are shown. Shaded regions represent the range. Each point represents a separate experiment.
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Figure 8
Figure 8. Remaining mass proportion of Ca-bearing phases in coal fly ash as a function of time for two different samples fitted by the dissolution model. Both the experimental data and optimized model fitting (described in Section 2.4) are shown in each figure. Error bars represent the variance (1σ) of the measurements.
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References
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  Surface and bulk characterization of an ultrafine South African coal fly ash with reference to polymer applications
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  Applied Surface Science (2014), 317 (), 73-83CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
  South African coal-fired power stations produce about 25 million tons of fly ash per annum, of which only approx. 5% is currently reused. A growing concern about pollution and increasing landfill costs stimulates research into new ways to utilize coal fly ash for economically beneficial applications. Fly ash particles may be used as inorg. filler in polymers, an application which generally requires the modification of their surface properties. In order to design expts. that will result in controlled changes in surface chem. and morphol., a detailed knowledge of the bulk chem. and mineralogical compns. of untreated fly ash particles, as well as their morphol. and surface properties, is needed. In this paper, a combination of complementary bulk and surface techniques was explored to assess the physicochem. properties of a classified, ultrafine coal fly ash sample, and the findings were discussed in the context of polymer application as fillers. The sample was categorized as a Class F fly ash (XRF). Sixty-two percent of the sample was an amorphous glass phase, with mullite and quartz being the main identified cryst. phases (XRD, FTIR). Quant. carbon and sulfur anal. reported a total bulk carbon and sulfur content of 0.37% and 0.16% resp. The spatial distribution of the phases was detd. by 2D mapping of Raman spectra, while TGA showed a very low wt. loss for temps. ranging between 25 and 1000 °C. Individual fly ash particles were characterized by a monomodal size distribution (PSD) of spherical particles with smooth surfaces (SEM, TEM, AFM), and a mean particle size of 4.6 μm (PSD). The BET active surface area of this sample was 1.52 m2/g and the chem. compn. of the fly ash surface (AES, XPS) was significantly different from the bulk compn. and varied considerably between spheres. Many properties of the sample (e.g. spherical morphol., small particle size, thermal stability) appeared to be suitable for its applicability as filler in polymers, although the wide variation in surface compn. between individual particles may challenge the development of a suitable surface modification technique. The observation that the bulk and surface compns. of the particles were so intrinsically different, strongly suggested that surface characterization is important when considering compatibility between matrixes when applying fly ash as filler in polymers.
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  Vassilev, S. V.; Menendez, R.; Alvarez, D.; Diaz-Somoano, M.; Martinez-Tarazona, M. R. Phase-Mineral and Chemical Composition of Coal Fly Ashes as a Basis for Their Multicomponent Utilization. 1. Characterization of Feed Coals and Fly Ashes☆. Fuel 2003, 82, 1793– 1811, DOI: 10.1016/S0016-2361(03)00123-6
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  Phase-mineral and chemical composition of coal fly ashes as a basis for their multicomponent utilization. 1. Characterization of feed coals and fly ashes
  Vassilev, Stanislav V.; Menendez, Rosa; Alvarez, Diego; Diaz-Somoano, Mercedes; Martinez-Tarazona, M. Rosa
  Fuel (2003), 82 (14), 1793-1811CODEN: FUELAC; ISSN:0016-2361. (Elsevier Science Ltd.)
  The phase-mineral and chem. compn. of feed coals and their fly ashes (FAs) produced in four large Spanish thermo-elec. power stations was characterized as a basis for multicomponent FA utilization. The feed fuels used are bituminous coals, semi-anthracites and anthracites with high detrital mineral abundance and mixed carbonate and sulfide-sulfate authigenic mineral tendency. Their mineral compn. includes quartz, kaolinite, illite-muscovite, pyrite, chlorite, plagioclase, K-feldspar, gypsum, siderite, calcite, dolomite, marcasite, montmorillonite, jarosite, and ankerite. The FAs studied have aluminosilicate compn. with higher concns. of alk. and alk.-earth oxides than Fe oxide. Elements such as Ag, As, Ba, Cr, Cs, Li, P, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr are relatively enriched in these FAs in comparison with the resp. mean values for bituminous coal ashes worldwide. The FAs consist basically of aluminosilicate glass, to a lesser extent of mineral matter (with high silicate abundance and dominant oxide tendency) and moderate char occurrence. The phase-mineral compn. (in decreasing order of significance) of these FAs is normally glass, mullite, quartz, char, kaolinite-metakaolinite, hematite, cristobalite, plagioclase, K-feldspar, melilite, anhydrite, wollastonite, magnetite and corundum plus 42 important accessory minerals or phases. A scheme of conventional sepn. procedures was applied to recover sequentially six initial and potentially useful and/or hazardous products from FAs, namely: (1) a ceramic cenosphere conc.; (2) a water-sol. salt conc.; (3) a magnetic conc.; (4) a char conc.; (5) a heavy conc.; and finally (6) an improved FA residue.
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  Akinyemi, S. A.; Gitari, W. M.; Petrik, L. F.; Nyakuma, B. B.; Hower, J. C.; Ward, C. R.; Oliveira, M. L. S.; Silva, L. F. O. Environmental Evaluation and Nano-Mineralogical Study of Fresh and Unsaturated Weathered Coal Fly Ashes. Sci. Total Environ. 2019, 663, 177– 188, DOI: 10.1016/J.SCITOTENV.2019.01.308
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  Environmental evaluation and nano-mineralogical study of fresh and unsaturated weathered coal fly ashes
  Akinyemi, Segun A.; Gitari, Wilson M.; Petrik, Leslie F.; Nyakuma, Bemgba B.; Hower, James C.; Ward, Colin R.; Oliveira, Marcos L. S.; Silva, Luis F. O.
  Science of the Total Environment (2019), 663 (), 177-188CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
  Coal combustion and the disposal of combustion wastes emit enormous quantities of nano-sized particles that pose significant health concerns on exposure, particularly in unindustrialized countries. Samples of fresh and weathered class F fly ash were analyzed through various techniques including X-ray fluorescence (XRF), X-ray diffraction (XRD), focused ion beam SEM (FIB-SEM), field-emission gun SEM (FE-SEM), high-resoln. transmission electron microscopy (HR-TEM) coupled with energy dispersive x-ray spectroscopy (EDS), and Raman Spectroscopy. The imaging techniques showed that the fresh and weathered coal fly ash nanoparticles (CFA-NPs) are mostly spherical shaped. The cryst. phases detected were quartz, mullite, ettringite, calcite, maghemite, hematite, gypsum, magnetite, clay residues, and sulfides. The most abundant cryst. phases were quartz mixed with Al-Fe-Si-K-Ti-O-amorphous phases whereas mullite was detected in several amorphous phases of Al, Fe, Ca, Si, O, K, Mg, Mn, and P. The analyses revealed that CFA-NPs are 5-500 nm in diam. and encapsulate several potentially hazardous elements (PHEs). The carbon species were detected as 5-50 nm carbon nanoballs of graphitic layers and massive fullerenes. Lastly, the aspects of health risks related to exposure to some detected ambient nanoparticles are also discussed.
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  Most of the people in this world are concerned about the environmental impacts of coal based power plants. Coalfired power plants are one of the sources of SOx, NOx and mercury emissions. These emissions have serious impact on the health of neighboring people such as increased rates of premature death, to the exacerbation of chronic respiratory diseases. The objectives of this review article are: i) To identify the issues of air pollution and the environmental impact of coal fired power plants, ii) To discuss techniques and equipment that can contribute to improve the environmental sustainability. The objectives were tackled by going through extensive literature review, field observations, and attending the applicable meetings and conferences. The research outcomes show that coal will continue to be the key energy source in some countries in the next 20 years, because the reserves of coal with them are relatively high. Such countries have to undertake special considerations to minimize the bad impact on the local and global environment. The most important problem to be solved is to limit the emissions (particulate matter, nitrogen oxides, sulfur oxides, carbon oxides, etc.). In order to achieve redn. in the emissions it is necessary to apply advanced and efficient technologies. This article presents functioning of a coal fired power plant, discuss environmental impacts and recommend technologies to make coal fired power plants environmentally sustainable.
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  A review of the multi-component utilization of coal fly ash
  Blissett, R. S.; Rowson, N. A.
  Fuel (2012), 97 (), 1-23CODEN: FUELAC; ISSN:0016-2361. (Elsevier Ltd.)
  A review. Coal fly ash is generated during the combustion of coal for energy prodn. Its use as an industrial byproduct has received a great deal of attention over the past 2 decades as more sustainable solns. to waste problems have been sought. The present paper reviews the potential applications for coal fly ash as a raw material: as a soil amelioration agent in agriculture, in the manuf. of glass and ceramics, in the prodn. of zeolites, in the formation of mesoporous materials, in the synthesis of geopolymers, for use as catalysts and catalyst supports, as an adsorbent for gases and waste water processes, and for the extn. of metals. The review then analyses the impact that a multi-stage process could have by examg. the technol. capable of sepns. to produce hollow microspheres, enriched C, magnetic spheres, fine ash product, and coarse ash product. The applications for these coal fly ash derived products were also reviewed. There is significant potential for the increased use of coal fly ash both in its raw and refined state. Probably by processing the coal fly ash, the scope for creating new industrial synergies is enhanced.
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  Golewski, G. L. Improvement of Fracture Toughness of Green Concrete as a Result of Addition of Coal Fly Ash. Characterization of Fly Ash Microstructure. Mater. Charact. 2017, 134, 335– 346, DOI: 10.1016/J.MATCHAR.2017.11.008
  15
  Improvement of fracture toughness of green concrete as a result of addition of coal fly ash. Characterization of fly ash microstructure
  Golewski, Grzegorz Ludwik
  Materials Characterization (2017), 134 (), 335-346CODEN: MACHEX; ISSN:1044-5803. (Elsevier)
  Composites with the addn. of coal fly ash (CFA) can be included in the sustainable and green concrete. Effective promotion of green concrete incorporating CFA is necessary to minimize the threat to the environment posed by CFA waste disposal and to reduce cement consumption thus cutting CO2 emissions. This study investigates the influence of the curing time on the fracture toughness of concrete produced with different concns. of CFA. Concns. of 20% - CFA-20 and 30% - CFA-30 of CFA were used and the results were compared with a ref. mixt. with 100% Ordinary Portland Cement (OPC) - CFA-00. Compressive strength - fcm and fracture toughness under mode I - KSIc (tension at bending), were detd. after: 3, 7, 28, 90, 180 and 365 days. The results obtained lead to the conclusion that, it is possible to make green concrete contg. CFA with high fracture toughness. Furthermore, this is one of the ways to reduce cement industry CO2 emissions.20% additive of CFA guarantees high fracture toughness in mature concretes, whereas concrete with 30% CFA additive is characterized by highest dynamic increase of the parameter KSIc. The exptl. program was completed by the anal. of microstructure of CFA by using SEM. These studies showed that the main morphol. forms in the CFA are single grains, such as: pyrospheres, cenospheres, plerospheres, multispheres, ferrospheres, grains of irregular shapes, and amorphous grains. Grains of CFA may also occur in larger quantities, as: jointed grains, clusters and agglomerates.
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  Climate change: A guide to CO2 sequestration
  Lackner, Klaus S.
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  Storage capacities and times for various CO2 sequestration methods are estd.
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17. 17
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  A review of mineral carbonation technologies to sequester CO2
  Sanna, A.; Uibu, M.; Caramanna, G.; Kuusik, R.; Maroto-Valer, M. M.
  Chemical Society Reviews (2014), 43 (23), 8049-8080CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  A review. Carbon dioxide (CO2) capture and sequestration includes a portfolio of technologies that can potentially sequester billions of tonnes of CO2 per yr. Mineral carbonation (MC) is emerging as a potential CCS technol. soln. to sequester CO2 from smaller/medium emitters, where geol. sequestration is not a viable option. In MC processes, CO2 is chem. reacted with calcium- and/or magnesium-contg. materials to form stable carbonates. This work investigates the current advancement in the proposed MC technologies and the role they can play in decreasing the overall cost of this CO2 sequestration route. In situ mineral carbonation is a very promising option in terms of resources available and enhanced security, but the technol. is still in its infancy and transport and storage costs are still higher than geol. storage in sedimentary basins ($17 instead of $8 per tCO2). Ex situ mineral carbonation has been demonstrated on pilot and demonstration scales. However, its application is currently limited by its high costs, which range from $50 to $300 per tCO2 sequestered. Energy use, the reaction rate and material handling are the key factors hindering the success of this technol. The value of the products seems central to render MC economically viable in the same way as conventional CCS seems profitable only when combined with EOR. Large scale projects such as the Skyonic process can help in reducing the knowledge gaps on MC fundamentals and provide accurate costing and data on processes integration and comparison. The literature to date indicates that in the coming decades MC can play an important role in decarbonising the power and industrial sector.
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18. 18
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  Materials and engineering: Rebuilding the world
  Bourzac, Katherine; Savage, Neil; Owens, Brian; Scott, Andrew R.
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  Scientists commercializing technologies that enable sturdier and more-sustainable structures.
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19. 19
  Pan, S. Y.; Chen, Y. H.; Fan, L. S.; Kim, H.; Gao, X.; Ling, T. C.; Chiang, P. C.; Pei, S. L.; Gu, G. CO2 Mineralization and Utilization by Alkaline Solid Wastes for Potential Carbon Reduction. Nat. Sustain. 2020, 3, 399– 405, DOI: 10.1038/s41893-020-0486-9
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20. 20
  Tamilselvi Dananjayan, R. R.; Kandasamy, P.; Andimuthu, R. Direct Mineral Carbonation of Coal Fly Ash for CO2 Sequestration. J. Clean. Prod. 2016, 112, 4173– 4182, DOI: 10.1016/J.JCLEPRO.2015.05.145
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21. 21
  Yuan, Q.; Zhang, Y.; Wang, T.; Wang, J.; Romero, C. E. Mineralization Characteristics of Coal Fly Ash in the Transition from Non-Supercritical CO2 to Supercritical CO2. Fuel 2022, 318, 123636 DOI: 10.1016/J.FUEL.2022.123636
  21
  Mineralization characteristics of coal fly ash in the transition from non-supercritical CO2 to supercritical CO2
  Yuan, Qixin; Zhang, Yongsheng; Wang, Tao; Wang, Jiawei; Romero, Carlos E.
  Fuel (2022), 318 (), 123636CODEN: FUELAC; ISSN:0016-2361. (Elsevier Ltd.)
  The accelerated mineralization of fly ash is a potential way to achieve CO2 emissions redn. However, the slow reaction at the diffusion control stage is the bottleneck. Due to the strong diffusion of supercrit. CO2, it has been to strengthen mineralization, but the degree of improvement in mineralization efficiency and the changes that take place in the transition from non-supercrit. to supercrit. CO2 are not clear. In the mineralization process, CaO in the ash reacts with CO2 to form CaCO3. In order to exclude the influence of other alk. oxides, powd. and block CaO were used to research the mechanism, and expts. were then carried out on samples of fly ash with different CaO content. Firstly, powd. CaO was used to conduct dry and wet mineralization expts. In dry mineralization, the mineralization efficiency of the process changes in two stages as the pressure increases, first undergoing a gentle-increase and then a rapid one. Diffusion depth expts. on block CaO show that the supercrit. diffusion depth is higher than the non-supercrit. diffusion depth by a factor of 1.22, indicating that supercrit. CO2 can improve the degree of mineralization in diffusion stage. The presence of water could promote mineralization. In wet mineralization, the efficiency of the process changes in three stages as the pressure increases, with first a gentle increase then a more rapid one and finally an attenuated rate of increase. The supercrit. mineralization efficiency at 8 MPa was 55.27%, a factor of 2.09 larger than for non-supercrit. mineralization at 3 MPa (26.39%). This is because that supercrit. CO2 has increased the diffusion and greatly improved soly. in water. The results of DFT calcns. show that H2O can promote the adsorption of CO2 by CaO, which is one of the reasons why water promotes mineralization. Finally, wet mineralization expts. on fly ash show that at 8 MPa, the mineralization efficiencies of HB, SD, SX and YN fly ashes are 1.94, 1.30, 2.03 and 1.62 times their values at 3 MPa. Although the difference in efficiency is affected by the CaO content, supercrit. CO2 can effectively improve the mineralization efficiency.
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22. 22
  Meng, J.; Liao, W.; Zhang, G. Emerging CO2-Mineralization Technologies for Co-Utilization of Industrial Solid Waste and Carbon Resources in China. Minerals 2021, 11, 274, DOI: 10.3390/MIN11030274
  22
  Emerging CO2-mineralization technologies for co-utilization of industrial solid waste and carbon resources in China
  Meng, Junlin; Liao, Wenjie; Zhang, Guoquan
  Minerals (Basel, Switzerland) (2021), 11 (3), 274CODEN: MBSIBI; ISSN:2075-163X. (MDPI AG)
  CO2 mineralization (aka mineral carbonation) is a promising method for the chem. sequestration of CO2 via reaction with oxides of alk. or alk.-earth metals to form carbonates. It has documented advantages over similar technol. solns. to climate change. The huge amt. of industrial solid waste, as a serious environmental issue confronted by China, can provide addnl. alky. sources for CO2 mineralization. In this study, we present an overview of the latest advances in the emerging technologies of CO2-mineralization via industrial solid waste in China, from the perspective of both theor. and practical considerations. We summarize the types of industrial solid waste that are used (mainly coal fly ash, steel slag, phosphogypsum, and blast furnace slag) and the technol. options available in the literature, with an emphasis on the discussion of the involved process-intensification methods and valuable chems. produced. Furthermore, we illustrate the current status of pertinent policies, and research and development activities in China. Finally, we identify the current knowledge gaps, particularly in understanding the overall sustainability performance of these CO2-mineralization technologies, and indicate that the tech., economic, and environmental challenges of promoting and commercializing these technologies for the co-utilization of industrial solid waste and carbon resources call for, amongst other things, more joint efforts by chemists, chem. engineers, and environmental scientists, and more feedback from the energy and industrial sectors.
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23. 23
  Karalis, K.; Kollias, K.; Bartzas, G.; Mystrioti, C.; Xenidis, A. CO2 Sequestration Using Fly Ash from Lignite Power Plants. Mater. Proc. 2022, 5, 131, DOI: 10.3390/MATERPROC2021005131
  There is no corresponding record for this reference.
24. 24
  Gadikota, G. Carbon Mineralization Pathways for Carbon Capture, Storage and Utilization. Commun. Chem. 2021, 4, 23, DOI: 10.1038/s42004-021-00461-x
  24
  Carbon mineralization pathways for carbon capture, storage and utilization
  Gadikota, Greeshma
  Communications Chemistry (2021), 4 (1), 23CODEN: CCOHCT; ISSN:2399-3669. (Nature Research)
  Carbon mineralization is a versatile and thermodynamically downhill process that can be harnessed for capturing, storing, and utilizing CO2 to synthesize products with enhanced properties. Here the author discusses the advances in and challenges of carbon mineralization, and concludes that tuning the chem. interactions involved will allow us to unlock its potential for advancing low carbon energy and resource conversion pathways.
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25. 25
  Ji, L.; Yu, H.; Wang, X.; Grigore, M.; French, D.; Gözükara, Y. M.; Yu, J.; Zeng, M. CO2 Sequestration by Direct Mineralisation Using Fly Ash from Chinese Shenfu Coal. Fuel Process. Technol. 2017, 156, 429– 437, DOI: 10.1016/J.FUPROC.2016.10.004
  25
  CO2 sequestration by direct mineralisation using fly ash from Chinese Shenfu coal
  Ji, Long; Yu, Hai; Wang, Xiaolong; Grigore, Mihaela; French, David; Gozukara, Yesim M.; Yu, Jianglong; Zeng, Ming
  Fuel Processing Technology (2017), 156 (), 429-437CODEN: FPTEDY; ISSN:0378-3820. (Elsevier Ltd.)
  Fly ash is a potential source of highly reactive feedstock for CO2 mineral carbonation. It does not require pre-treatment, but it has a low carbonation rate and efficiency. To address these issues, we studied the carbonation performance and mechanism of a fly ash from Shenfu coal of China. The effects of temp., solid to liq. ratio and gas flow rate on the carbonation efficiency of the fly ash were systematically investigated in a direct mineralization process. Our results indicated that calcium in lime and portlandite had a higher reactivity towards CO2 than that in other calcium bearing phases either cryst. or amorphous. Solely increasing the temp. did not improve carbonation efficiency. However, expts. in a batch reactor under elevated temp. (140, 180, and 220 °C) and pressure conditions (10 and 20 bar) using recyclable additives showed that a combination of high temp. and pressure significantly improved carbonation efficiency in the presence of 0.5 mol/L Na2CO3. Our multiple-cycle expts. showed that Na2CO3 facilitated the pptn. of calcium carbonate and was well regenerated in the process.
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26. 26
  Ji, L.; Yu, H.; Zhang, R.; French, D.; Grigore, M.; Yu, B.; Wang, X.; Yu, J.; Zhao, S. Effects of Fly Ash Properties on Carbonation Efficiency in CO2 Mineralisation. Fuel Process. Technol. 2019, 188, 79– 88, DOI: 10.1016/J.FUPROC.2019.01.015
  26
  Effects of fly ash properties on carbonation efficiency in CO2 mineralisation
  Ji, Long; Yu, Hai; Zhang, Ruijie; French, David; Grigore, Mihaela; Yu, Bing; Wang, Xiaolong; Yu, Jianglong; Zhao, Shuaifei
  Fuel Processing Technology (2019), 188 (), 79-88CODEN: FPTEDY; ISSN:0378-3820. (Elsevier Ltd.)
  CO2 mineralization by industrial wastes is a promising option for mitigating carbon emissions safely and permanently with low material cost. But there is still absence of a detailed understanding on how fly ash properties affect the carbonation reactions. To fill this knowledge gap, five coal combustion fly ashes, Beijing (BJ), Wuhai (WH), Hazelwood (HW), Yallourn (YA) and Loy Yang (LY) ashes, from China and Australia were selected for carbonation studies. Expts. were performed in a batch reactor at 40 and 140°C with 20 bar initial CO2 pressure, 200 g/L solid to liq. ratio, 450 rpm stirring rate to compare the carbonation performance of the five fly ashes and the effect of fly ash properties on carbonation reactions. Then BJ, YA and HW ashes were then selected for further study in a wide temp. range (40-220°C) because of their higher CO2 sequestration capacity than the other two ashes. SEM (SEM) with energy dispersive spectrometry (EDS) were used to characterize morphol. and elemental properties of fresh and carbonated fly ash samples. Compared to LY and WH ashes, BJ, YA and HW ashes displayed much higher CO2 sequestration capacity due to the higher fraction of reactive Ca/Mg-bearing cryst. phases, including lime (CaO) and portlandite (Ca(OH)2) in BJ ash, periclase (MgO) and srebrodolskite (Ca2Fe2O5) in YA ash, and periclase and brucite (Mg(OH)2) in HW ash.
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27. 27
  Yin, T.; Yin, S.; Srivastava, A.; Gadikota, G. Regenerable Solvents Mediate Accelerated Low Temperature CO2 Capture and Carbon Mineralization of Ash and Nano-Scale Calcium Carbonate Formation. Resour. Conserv. Recycl. 2022, 180, 106209 DOI: 10.1016/J.RESCONREC.2022.106209
  27
  Regenerable solvents mediate accelerated low temperature CO2 capture and carbon mineralization of ash and nano-scale calcium carbonate formation
  Yin, Tianhe; Yin, Shufan; Srivastava, Akanksha; Gadikota, Greeshma
  Resources, Conservation and Recycling (2022), 180 (), 106209CODEN: RCREEW; ISSN:1879-0658. (Elsevier B.V.)
  The dual need to remove CO2 from our emissions and treat alk. industrial residues such as ash materials motivate the design of innovative pathways to simultaneously capture and convert CO2 into mineralized carbonates. Direct carbon mineralization is one approach that addresses the need to simultaneously treat alk. industrial residues and mineralize CO2 emissions. Low CO2 soly. in water and slow kinetics at ambient temp. have challenged the direct carbon mineralization of alk. industrial residues. To address these challenges, the use of CO2 capture solvents that enhance CO2 soly. and facilitate accelerated carbon mineralization of fly ash at temps. below 90 °C is investigated. Calcium carbonate formation results in the inherent regeneration of the solvent. The carbon mineralization extents of non-calcium carbonate content in fly ash were 50% and 51% and in waste ash were 58% and 62% in 2.5 M sodium glycinate and 30 wt% MEA solns., resp. The expts. were performed at 50 °C for 3 h with CO2 partial pressure of 1 atm in a continuously stirred slurry environment with 15 wt.% solid. Furthermore, nanoscale CaCO3 is successfully synthesized from dissolved calcium using CO2-loaded sodium glycinate and surfactants such as CTAB (Cetyl Tri-Me Ammonium Bromide). Surfactants such as CTAB bind to the calcium carbonate surface and regulate the growth of calcium carbonate particles. These innovative approaches demonstrate the feasibility of directly storing CO2 in fly ash and waste ash as calcium carbonate and producing nanoscale calcium carbonate using regenerable CO2 capture solvents.
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28. 28
  Ji, L.; Yu, H.; Yu, B.; Jiang, K.; Grigore, M.; Wang, X.; Zhao, S.; Li, K. Integrated Absorption–Mineralisation for Energy-Efficient CO2 Sequestration: Reaction Mechanism and Feasibility of Using Fly Ash as a Feedstock. Chem. Eng. J. 2018, 352, 151– 162, DOI: 10.1016/J.CEJ.2018.07.014
  28
  Integrated absorption-mineralisation for energy-efficient CO2 sequestration: Reaction mechanism and feasibility of using fly ash as a feedstock
  Ji, Long; Yu, Hai; Yu, Bing; Jiang, Kaiqi; Grigore, Mihaela; Wang, Xiaolong; Zhao, Shuaifei; Li, Kangkang
  Chemical Engineering Journal (Amsterdam, Netherlands) (2018), 352 (), 151-162CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
  The most crit. challenge for the large-scale implementation of amine-based carbon dioxide (CO2) capture is the high energy consumption of absorbent thermal regeneration. To reduce the energy requirement, absorbent thermal regeneration can be replaced by a chem. method that integrates amine scrubbing, chem. regeneration and CO2 mineralization in one process. However, the mechanisms of the process and the application of industrial waste as feedstocks have not been fully investigated. In the present work, we studied the integrated CO2 absorption-mineralization process using the benchmark solvent monoethanolamine (MEA) as an amine absorbent and fly ash as a chem. regeneration agent. We investigated the mechanism involved in the mineralization in detail and studied the performance of MEA in regeneration by mineralization of calcium oxide (CaO) at various CO2-loadings. The performance stability of MEA was verified in multicycle CO2 absorption-mineralization expts. We also investigated the tech. feasibility of using fly ash as a feedstock for absorbent regeneration. Our results show that MEA can be regenerated after a carbonation reaction with both calcium oxide and fly ash at 40°C, and that the CO2 absorbed by MEA is pptd. as calcium carbonate. Compared with traditional thermal regeneration-based CO2 capture, the integrated CO2 absorption-mineralization process displays a similar cyclic CO2-loading (0.21mol/mol) but has great advantages in energy redn. and capital cost savings due to the smaller energy requirement of amine regeneration and the limitation of CO2 compression and pipeline transport. This technol. has great potential for industrial application, particularly with CaO-contg. wastes such as fly ash and carbide slag.
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29. 29
  Pastero, L.; Curetti, N.; Ortenzi, M. A.; Schiavoni, M.; Destefanis, E.; Pavese, A. CO2 Capture and Sequestration in Stable Ca-Oxalate, via Ca-Ascorbate Promoted Green Reaction. Sci. Total Environ. 2019, 666, 1232– 1244, DOI: 10.1016/J.SCITOTENV.2019.02.114
  29
  CO2 capture and sequestration in stable Ca-oxalate, via Ca-ascorbate promoted green reaction
  Pastero, Linda; Curetti, Nadia; Ortenzi, Marco Aldo; Schiavoni, Marco; Destefanis, Enrico; Pavese, Alessandro
  Science of the Total Environment (2019), 666 (), 1232-1244CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
  The increase in the amt. of CO2 (CO2) emissions related to many anthropic activities is a persistent and growing problem. During the last years, many solns. have been set out, none of them being the ultimate one. Investigators agree on the need of a synergic approach to the problem, in terms of many complementary methods of sequestration that, combined with the redn. of prodn., will be able to decrease the concn. of the CO2 in the atm. We explore the use of a green reaction to trap the CO2 into a stable cryst. phase (weddellite) resorting to a multidisciplinary approach. CO2 is reduced and pptd. as Ca oxalate through vitamin C as a sacrificial reductant. Ca oxalate crystals obtained show a startling good quality that increases their already great stability over a wide chem. and phys. conditions' range.
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30. 30
  Schuler, E.; Demetriou, M.; Shiju, N. R.; Gruter, G. J. M. Towards Sustainable Oxalic Acid from CO2 and Biomass. ChemSusChem 2021, 14, 3636– 3664, DOI: 10.1002/CSSC.202101272
  30
  Towards Sustainable Oxalic Acid from CO2 and Biomass
  Schuler, Eric; Demetriou, Marilena; Shiju, N. Raveendran; Gruter, Gert-Jan M.
  ChemSusChem (2021), 14 (18), 3636-3664CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. To quickly and drastically reduce CO2 emissions and meet our ambitions of a circular future, we need to develop carbon capture and storage (CCS) and carbon capture and utilization (CCU) to deal with the CO2 that we produce. While we have many alternatives to replace fossil feedstocks for energy generation, for materials such as plastics we need carbon. The ultimate circular carbon feedstock would be CO2. A promising route is the electrochem. redn. of CO2 to formic acid derivs. that can subsequently be converted into oxalic acid. Oxalic acid is a potential new platform chem. for material prodn. as useful monomers such as glycolic acid can be derived from it. This work is part of the European Horizon 2020 project "Ocean" in which all these steps are developed. This Review aims to highlight new developments in oxalic acid prodn. processes with a focus on CO2-based routes. All available processes are critically assessed and compared on criteria including overall process efficiency and triple bottom line sustainability.
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31. 31
  Costa, R. S.; Aranha, B. S. R.; Ghosh, A.; Lobo, A. O.; da Silva, E. T. S. G.; Alves, D. C. B.; Viana, B. C. Production of Oxalic Acid by Electrochemical Reduction of CO2 Using Silver-Carbon Material from Babassu Coconut Mesocarp. J. Phys. Chem. Solids 2020, 147, 109678 DOI: 10.1016/J.JPCS.2020.109678
  31
  Production of oxalic acid by electrochemical reduction of CO2 using silver-carbon material from babassu coconut mesocarp
  Costa, Rubens S.; Aranha, Bruna S. R.; Ghosh, Anupama; Lobo, Anderson O.; da Silva, Everson T. S. G.; Alves, Diego C. B.; Viana, Bartolomeu C.
  Journal of Physics and Chemistry of Solids (2020), 147 (), 109678CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Ltd.)
  The present study provides an efficient carbon dioxide (CO2) redn. process to produce oxalic acid with approx. 29% faradaic efficiency (based on HPLC) using a carbon material from babassu biomass coupled with silver. A new carbon-silver hybrid material has been prepd. from babassu mesocarp by hydrothermal carbonization; this material initiates the electrochem. redn. of CO2 at a lower potential and catalyzes the conversion of CO2 into oxalic acid. To evaluate the catalytic potential of this reaction, other materials, such as glassy carbon and hydrothermal carbon, which generated a higher cathodic c.d., were tested at the same potential. The rate of CO2 redn. was found to be increased at low potentials in the presence of the hybrid carbon-silver material synthesized by a one-pot hydrothermal method from babassu biomass in 48 h (Ag@C-48).
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32. 32
  Yang, Y.; Gao, H.; Feng, J.; Zeng, S.; Liu, L.; Liu, L.; Ren, B.; Li, T.; Zhang, S.; Zhang, X.; Yang, Y.; Gao, H.; Feng, J.; Zeng, S.; Liu, L.; Zhang, S.; Zhang, X.; Ren, B.; Li, T. Aromatic Ester-Functionalized Ionic Liquid for Highly Efficient CO2 Electrochemical Reduction to Oxalic Acid. ChemSusChem 2020, 13, 4900– 4905, DOI: 10.1002/CSSC.202001194
  32
  Aromatic Ester-Functionalized Ionic Liquid for Highly Efficient CO2 Electrochemical Reduction to Oxalic Acid
  Yang, Yingliang; Gao, Hongshuai; Feng, Jiaqi; Zeng, Shaojuan; Liu, Lei; Liu, Licheng; Ren, Baozeng; Li, Tao; Zhang, Suojiang; Zhang, Xiangping
  ChemSusChem (2020), 13 (18), 4900-4905CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Electrochem. redn. of CO2 into valuable chems. is a significant route to use CO2 resources. Among various electroredn. products, oxalic acid (H2C2O4) is an important chem. for pharmaceuticals, rare earth extn., and metal processing. Here, an aprotic arom. ester-functionalized ionic liq. (IL), 4-(methoxycarbonyl) phenol Et4N+ ([TEA][4-MF-PhO]), was designed as an electrolyte for CO2 electroredn. into oxalic acid. It exhibited a large oxalic acid partial c.d. of 9.03 mA cm-2 with a faradaic efficiency (FE) of 86% at -2.6 V (vs. Ag/Ag+), and the oxalic acid formation rate was ≤168.4μmol cm-2 h-1, which is the highest reported value to date. Also, the results of d. functional theory calcns. demonstrated that CO2 was efficiently activated to a -COOH intermediate by bis-active sites of the arom. ester anion via the formation of a [4-MF-PhO-COOH]- adduct, which finally dimerized into oxalic acid.
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33. 33
  Rowley, M. C.; Estrada-Medina, H.; Tzec-Gamboa, M.; Rozin, A.; Cailleau, G.; Verrecchia, E. P.; Green, I. Moving Carbon between Spheres, the Potential Oxalate-Carbonate Pathway of Brosimum Alicastrum Sw; Moraceae. Plant Soil 2017, 412, 465– 479, DOI: 10.1007/s11104-016-3135-3
  33
  Moving carbon between spheres, the potential oxalate-carbonate pathway of Brosimum alicastrum Sw.; Moraceae
  Rowley, Mike C.; Estrada-Medina, Hector; Tzec-Gamboa, Magnolia; Rozin, Aviram; Cailleau, Guillaume; Verrecchia, Eric P.; Green, Iain
  Plant and Soil (2017), 412 (1-2), 465-479CODEN: PLSOA2; ISSN:0032-079X. (Springer)
  Aims: The Oxalate-Carbonate Pathway (OCP) is a biogeochem. process that transfers atm. CO2 into the geol. reservoir as CaCO3; however, until now all investigations on this process have focused on species with limited food benefits. This study evaluates a potential OCP assocd. with Brosimum alicastrum, a Neotropical species with agroforestry potential (ca. 70-200 kg-nuts yr-1), in the calcareous soils of Haiti and Mexico. Methods / results: Enzymic anal. demonstrated significant concns. of calcium oxalate (5.97 % D.W.) were assocd. with B. alicastrum tissue in all sample sites. The presence of oxalotrophism was also confirmed with microbiol. analyses in both countries. High concns. of total calcium (>7 g kg-1) and lithogenic carbonate obscured the localised alkalinisation and identification of secondary carbonate assocd. with the OCP at most sample sites, except Ma Rouge, Haiti. Soils adjacent to subjects in Ma Rouge demonstrated an increase in pH (0.63) and CaCO3 concn. (5.9 %) that, when coupled with root-like secondary carbonate deposits in Mexico, implies that the OCP does also occur in calcareous soils. Conclusions: Therefore this study confirms that the OCP also occurs in calcareous soils, adjacent to B. alicastrum, and could play a fundamental and un-accounted role in the global calcium-carbon coupled cycle.
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34. 34
  Gadd, G. M.; Bahri-Esfahani, J.; Li, Q.; Rhee, Y. J.; Wei, Z.; Fomina, M.; Liang, X. Oxalate Production by Fungi: Significance in Geomycology, Biodeterioration and Bioremediation. Fungal Biol. Rev. 2014, 28, 36– 55, DOI: 10.1016/J.FBR.2014.05.001
  There is no corresponding record for this reference.
35. 35
  Curetti, N.; Pastero, L.; Bernasconi, D.; Cotellucci, A.; Corazzari, I.; Archetti, M.; Pavese, A. Thermal Stability of Calcium Oxalates from CO2 Sequestration for Storage Purposes: An In-Situ HT-XRPD and TGA Combined Study. Minerals 2022, 12, 53, DOI: 10.3390/MIN12010053
  35
  Thermal Stability of Calcium Oxalates from CO2 Sequestration for Storage Purposes: An In-Situ HT-XRPD and TGA Combined Study
  Curetti, Nadia; Pastero, Linda; Bernasconi, Davide; Cotellucci, Andrea; Corazzari, Ingrid; Archetti, Maurizio; Pavese, Alessandro
  Minerals (Basel, Switzerland) (2022), 12 (1), 53CODEN: MBSIBI; ISSN:2075-163X. (MDPI AG)
  Calcium oxalates are naturally occurring biominerals and can be found as a byproduct of some industrial processes. Recently, a new and green method for carbon capture and sequestration in stable calcium oxalate from oxalic acid produced by carbon dioxide redn. was proposed. The reaction resulted in high-quality weddellite crystals. Assessing the stability of these weddellite crystals is crucial to forecast their reuse as solid-state reservoir of pure CO2 and CaO in a circular economy perspective or, eventually, their disposal. The thermal decompn. of weddellite obtained from the new method of carbon capture and storage was studied by coupling in-situ high-temp. X-ray powder diffraction and thermogravimetric anal., in order to evaluate the dehydration, decarbonation, and the possible prodn. of unwanted volatile species during heating. At low temp. (119-255 °C), structural water release was superimposed to an early CO2 feeble evolution, resulting in a water-carbon dioxide mixt. that should be sepd. for reuse. Furthermore, the storage temp. limit must be considered bearing in mind this CO2 release low-temp. event. In the range 390-550 °C, a two-component mixt. of carbon monoxide and dioxide is evolved, requiring oxidn. of the former or gas sepn. to reuse pure gases. Finally, the last decarbonation reaction produced pure CO2 starting from 550 °C.
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36. 36
  Grayevsky, R.; Reiss, A. G.; Emmanuel, S. Carbon Storage through Rapid Conversion of Forsterite into Solid Oxalate Phases. Energy Fuels 2023, 37, 509– 517, DOI: 10.1021/acs.energyfuels.2c03245
  36
  Carbon Storage through Rapid Conversion of Forsterite into Solid Oxalate Phases
  Grayevsky, Roni; Reiss, Amit G.; Emmanuel, Simon
  Energy & Fuels (2023), 37 (1), 509-517CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
  Carbon capture and storage are likely to be crit. components in lowering atm. CO2 levels. Mineralization is often proposed as a method to store carbon and typically involves reacting CO2 directly with silicate minerals, such as forsterite, to form carbonate minerals. However, this reaction is slow under std. conditions, so that sequestering significant amts. of carbon can take years or decades. Here, we demonstrate the feasibility of using a reaction between oxalic acid and forsterite to create stable carbon-bearing oxalate minerals. We performed a series of batch expts. at room temp. and pressure to quantify the forsterite dissoln. rate and the efficiency of Mg utilization. Our results show that conversion of forsterite to Mg and Fe oxalate is achieved rapidly: after 30 days, 52% of Mg was converted to Mg oxalate so that 1 t of forsterite can be used to store 177 kg of carbon. Our calcns. show that reacting ultramafic mine tailings with oxalic acid has the potential to make a significant contribution toward the global target for CO2 removal by carbon capture and storage.
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  https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XjtVCgsbjP&md5=87cb80ee50f7c51610b18dc166b81ae2
37. 37
  Lieberman, R. N.; Knop, Y.; Palmerola, N. M.; Muñoz, C.; Cohen, H.; Izquiredo, M.; Lorenzo, J. A.; Taboas, B.; Font, O.; Querol, X. Production of Environmentally Friendly Sand-like Products from Granitoid Waste Sludge and Coal Fly Ash for Civil Engineering. J. Clean. Prod. 2019, 238, 117880 DOI: 10.1016/J.JCLEPRO.2019.117880
  There is no corresponding record for this reference.
38. 38
  Chancey, R. T.; Stutzman, P.; Juenger, M. C. G.; Fowler, D. W. Comprehensive Phase Characterization of Crystalline and Amorphous Phases of a Class F Fly Ash. Cem. Concr. Res. 2010, 40, 146– 156, DOI: 10.1016/J.CEMCONRES.2009.08.029
  38
  Comprehensive phase characterization of crystalline and amorphous phases of a Class F fly ash
  Chancey, Ryan T.; Stutzman, Paul; Juenger, Maria C. G.; Fowler, David W.
  Cement and Concrete Research (2010), 40 (1), 146-156CODEN: CCNRAI; ISSN:0008-8846. (Elsevier Ltd.)
  A comprehensive approach to qual. and quant. characterization of cryst. and amorphous constituent phases of a largely heterogeneous Class F fly ash is presented. Traditionally, fly ash compn. is expressed as bulk elemental oxide content, generally detd. by X-ray fluorescence spectroscopy. However, such anal. does not discern between relatively inert cryst. phases and highly reactive amorphous phases of similar elemental compn. X-ray diffraction was used to identify the cryst. phases present in the fly ash, and the Rietveld quant. phase anal. method was applied to det. the relative proportion of each of these phases. A synergistic method of X-ray powder diffraction, SEM, energy dispersive spectroscopy, and multispectral image anal. was developed to identify and quantify the amorphous phases present in the fly ash.
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39. 39
  Li, L.; Liu, W.; Qin, Z.; Zhang, G.; Yue, H.; Liang, B.; Tang, S.; Luo, D. Research on Integrated CO2 Absorption-Mineralization and Regeneration of Absorbent Process. Energy 2021, 222, 120010 DOI: 10.1016/J.ENERGY.2021.120010
  39
  Research on integrated CO2 absorption-mineralization and regeneration of absorbent process
  Li, Long; Liu, Weizao; Qin, Zhifeng; Zhang, Guoquan; Yue, Hairong; Liang, Bin; Tang, Shengwei; Luo, Dongmei
  Energy (Oxford, United Kingdom) (2021), 222 (), 120010CODEN: ENEYDS; ISSN:0360-5442. (Elsevier Ltd.)
  The hot potassium-alkali method provides excellent performance for the absorption of CO2 from flue gas. However, the high energy consumption by absorbent regeneration poses a crit. barrier to the widespread industrialization of the hot potassium-alkali method. In this study, an integrated CO2 absorption-mineralization and regeneration of absorbent (IAMR) process was proposed using K2CO3 soln. as the absorbent and steel slag as the desorbent at normal temp. and pressure. This method greatly reduced the energy consumption and costs compared with the traditional thermal regeneration method. Under the optimal conditions, i.e. a K2CO3 concn. of 1.0 mol/L, reaction temp. of 60°C and liq.-solid (K2CO3 soln.-steel slag) ratio of 14 mL/g, the carbonation conversion of the steel slag reached 58.63% after 120min, corresponding to a CO2 storage capacity of 212 kg/t steel slag. The reaction process showed that the main component Ca2SiO4 in the steel slag had high soly. activity in K2CO3 soln. which significantly enhanced the rate and efficiency of CO2 sequestration. Moreover, the performance stability of K2CO3 soln. during CO2 absorption-desorption circulation was discussed. This research is of great significance for the simultaneous treatment of alk. waste slags (steel slag, fly ash, etc.) and mitigation of greenhouse gases.
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40. 40
  Price, D.; Dollimore, D.; Fatemi, N. S.; Whitehead, R. Mass Spectrometric Determination of Kinetic Parameters for Solid State Decomposition Reactions. Part 1. Method; Calcium Oxalate Decomposition. Thermochim. Acta 1980, 42, 323– 332, DOI: 10.1016/0040-6031(80)85093-3
  40
  Mass spectrometric determination of kinetic parameters for solid state decomposition reactions. Part 1. Method; calcium oxalate decomposition
  Price, D.; Dollimore, D.; Fatemi, N. S.; Whitehead, R.
  Thermochimica Acta (1980), 42 (3), 323-32CODEN: THACAS; ISSN:0040-6031.
  A description is given of an expt. in which a time-of-flight mass spectrometer is used to monitor the thermal decompn. of solid samples placed in the ion source region. Because the mass spectrometer can both identify and quantify the gaseous products, the technique yields direct insight into the decompn. mechanisms and values for kinetic parameters. An assessment of the technique is given and its application illustrated by a study of anhyd. Ca oxalate decompn. in vacuo.
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41. 41
  Hourlier, D. Thermal Decomposition of Calcium Oxalate: Beyond Appearances. J. Therm. Anal. Calorim. 2019, 136, 2221– 2229, DOI: 10.1007/s10973-018-7888-1
  41
  Thermal decomposition of calcium oxalate: beyond appearances
  Hourlier, Djamila
  Journal of Thermal Analysis and Calorimetry (2019), 136 (6), 2221-2229CODEN: JTACF7; ISSN:1388-6150. (Springer)
  The goal of this study is twofold: to take a fresh look at the decompn. of calcium oxalate and to warn users of thermogravimetric anal. against the hasty interpretation of results obtained. Since the pioneer work of Duval 70 years ago, the scientific community has agreed unanimously as to the decompn. of anhyd. calcium oxalate (CaC2O4) into calcium carbonate (CaCO3) and CO gas, and that of the calcium carbonate into calcium oxide (CaO), and CO2 gas. We will demonstrate how these reactions, simple in appearance, in fact result from a succession of reactive phenomena involving numerous constituents both solid (CaCO3, free carbon) and gaseous (CO2 and CO) produced by intermediary reactions. The mass losses evaluated in the two distinct domains correspond closely to the molar masses of CO and CO2, resp. The simple math. calcn. of that mass loss has simply concealed the existence of other reactions, and, most particularly the Boudouard reaction and that of solid phases between CaCO3 and C. It just goes to show that appearances can be deceiving.
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42. 42
  Payá, J.; Monzó, J.; Borrachero, M. V.; Perris, E.; Amahjour, F. Thermogravimetric Methods for Determining Carbon Content in Fly Ashes. Cem. Concr. Res. 1998, 28, 675– 686, DOI: 10.1016/S0008-8846(98)00030-1
  42
  Thermogravimetric methods for determining carbon content in fly ashes
  Paya, J.; Monzo, J.; Borrachero, M. V.; Perris, E.; Amahjour, F.
  Cement and Concrete Research (1998), 28 (5), 675-686CODEN: CCNRAI; ISSN:0008-8846. (Elsevier Science Ltd.)
  Unburnt carbon content in fly ashes were detd. using thermogravimetry. After being heated in inert atm., carbon reacts towards iron oxide components in fly ash, and after it is heated in an oxidizing environment, carbon oxidn. process overlaps carbonate decompn. A thermogravimetric method combining inert and oxidizing atmospheres was designed, in such a way that hydrated lime, calcium carbonate, and unburnt carbon contents for several fly ashes were detd. Carbon content in fly ash-sized fractions also was detd., with highest carbon content found in the coarsest fractions.
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43. 43
  Chindaprasirt, P.; Rattanasak, U.; Taebuanhuad, S. Resistance to Acid and Sulfate Solutions of Microwave-Assisted High Calcium Fly Ash Geopolymer. Mater. Struct. 2013, 46, 375– 381, DOI: 10.1617/s11527-012-9907-1
  43
  Resistance to acid and sulfate solutions of microwave-assisted high calcium fly ash geopolymer
  Chindaprasirt, Prinya; Rattanasak, Ubolluk; Taebuanhuad, Sompop
  Materials and Structures (Dordrecht, Netherlands) (2013), 46 (3), 375-381CODEN: MASTED; ISSN:1359-5997. (Springer)
  In this paper, 90-W microwave radiation for 5 min plus a shortened heat curing period was applied to cure the fresh geopolymer paste. Results showed that microwave radiation contributed to the dissoln. of fly ash in the alk. soln. Numerous gel formations were obsd. in microscopic scale. This resulted in a dense composite and strong bonding between the fly ash and the geopolymer matrix leading to high strength gain compared to those of the control pastes cured at 65 °C for 24 h. In addn., resistances to the sulfuric acid and sulfate attacks of the microwave geopolymer were superior to that of the control as indicated by the relatively low strength loss. The microwave radiation also helped the geopolymer attaining thermal stability as the dense matrixes were obtained.
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44. 44
  Kimball, B. E. Biogeochemical Cycling of Copper in Acid Mine Drainage. Doctoral dissertation; The Pennsylvania State University, 2009. (accessed 2022-10-31).
  There is no corresponding record for this reference.
45. 45
  Sendula, E.; Lamadrid, H. M.; Rimstidt, J. D.; Steele-MacInnis, M.; Sublett, D. M.; Aradi, L. E.; Szabó, C.; Caddick, M. J.; Zajacz, Z.; Bodnar, R. J. Synthetic Fluid Inclusions XXIV. In Situ Monitoring of the Carbonation of Olivine Under Conditions Relevant to Carbon Capture and Storage Using Synthetic Fluid Inclusion Micro-Reactors: Determination of Reaction Rates. Front. Clim. 2021, 3, 722447 DOI: 10.3389/fclim.2021.722447
  There is no corresponding record for this reference.
46. 46
  Aughenbaugh, K. L.; Chancey, R. T.; Stutzman, P.; Juenger, M. C.; Fowler, D. W. An Examination of the Reactivity of Fly Ash in Cementitious Pore Solutions. Mater. Struct. 2013, 46, 869– 880, DOI: 10.1617/s11527-012-9939-6
  46
  An examination of the reactivity of fly ash in cementitious pore solutions
  Aughenbaugh, Katherine L.; Chancey, Ryan T.; Stutzman, Paul; Juenger, Maria C.; Fowler, David W.
  Materials and Structures (Dordrecht, Netherlands) (2013), 46 (5), 869-880CODEN: MASTED; ISSN:1359-5997. (Springer)
  Fly ash is frequently used to replace cement in concrete, but it is difficult to predict performance based only on the oxide compn., which is typically the only compositional information available. In order to better utilize fly ash in concrete, it is important to develop more meaningful characterization methods and correlate these with performance. The research presented here uses a combination of anal. methods, including x-ray powder diffraction, SEM coupled with multispectral image anal., and soln. anal. to det. the compns. of the glassy phases in a specific fly ash and to examine the fly ashes reactivity in late- and early-age cement pore solns., ultrapure water, and sodium hydroxide. The dissoln. of individual glassy phases in the fly ash was tracked over time and the pptn. of reaction products monitored. A high-calcium aluminosilicate glass was the most reactive, a low-calcium aluminosilicate glass was of intermediate reactivity and a medium-calcium aluminosilicate glass had the lowest reactivity in the solns. tested for a specific fly ash. This result suggests the glass compn. has a strong effect on reactivity, but that that there is not a strict correlation between calcium content and glass reactivity.
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47. 47
  Snellings, R. Surface Chemistry of Calcium Aluminosilicate Glasses. J. Am. Ceram. Soc. 2015, 98, 303– 314, DOI: 10.1111/JACE.13263
  47
  Surface Chemistry of Calcium Aluminosilicate Glasses
  Snellings, Ruben
  Journal of the American Ceramic Society (2015), 98 (1), 303-314CODEN: JACTAW; ISSN:0002-7820. (Wiley-Blackwell)
  The surface chem. of synthetic calcium aluminosilicate glasses exposed to aq. solns. of varying pH was described using zeta potential measurements and batch surface titrns. Element release and proton consumption were measured to characterize the reactions at the surface as a function of pH. It was found that proton-metal exchange or leaching was the dominant proton consumption process at low pH. The exchange reaction was obsd. to maintain charge balance, indicating that over the limited duration of the expts. no repolymn. of the silica-rich leached layer occurred. At high pH, dissoln. and hydrolysis of aq. ions controlled the proton mass balance, no evidence for the formation of leached layers was found. Silica-rich glasses were found to be more resistant to corrosion by either proton-metal exchange or dissoln. than CaO-rich glasses. Glass basicity was found to be reflected in the establishment of a higher pH when immersing the glass in neutral aq. solns. and in a reduced stability of the glass surface toward aq. solns. High pH, high surface charge and reduced glass network polymn. act together in enhancing the reaction of CaO-rich glasses with aq. solns.
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48. 48
  Heasman, I.; Watt, J. Particulate Pollution Case Studies Which Illustrate Uses of Individual Particle Analysis by Scanning Electron Microscopy. Environ. Geochem. Health 1989, 11, 157– 162, DOI: 10.1007/BF01758666
  48
  Particulate pollution case studies which illustrate uses of individual particle analysis by scanning electron microscopy
  Heasman I; Watt J
  Environmental geochemistry and health (1989), 11 (3-4), 157-62 ISSN:0269-4042.
  Individual particle analysis by scanning electron microscopy is a high resolution analytical technique which can provide the detail necessary to solve many particulate pollution problems. In an electron microscope a beam of electrons is focused on a specimen resulting in a number of signals which are used to collect chemical and morphological information from individual features either manually or automatically. Case studies are presented which illustrate the applications of the technique to three paniculate pollution problems of differing complexity.
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49. 49
  Conti, C.; Brambilla, L.; Colombo, C.; Dellasega, D.; Gatta, G. D.; Realini, M.; Zerbi, G. Stability and Transformation Mechanism of Weddellite Nanocrystals Studied by X-Ray Diffraction and Infrared Spectroscopy. Phys. Chem. Chem. Phys. 2010, 12, 14560– 14566, DOI: 10.1039/C0CP00624F
  49
  Stability and transformation mechanism of weddellite nanocrystals studied by X-ray diffraction and infrared spectroscopy
  Conti, Claudia; Brambilla, Luigi; Colombo, Chiara; Dellasega, David; Gatta, G. Diego; Realini, Marco; Zerbi, Giuseppe
  Physical Chemistry Chemical Physics (2010), 12 (43), 14560-14566CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  Stability of weddellite, the dihydrate phase of calcium oxalate [CaC2O4·(H2O)2+x], mainly detected in kidney stones and in oxalate films found on the surfaces of several ancient monuments, has been studied. Under ambient conditions, weddellite is unstable and quickly changes into whewellite, the monohydrate phase of calcium oxalate (CaC2O4·H2O). Crystal structure of weddellite was detd. by single crystal X-ray diffraction. Synthesized nanocrystals of weddellite were kept under different hygrometric conditions in order to study, by X-ray powder diffraction, the effect of humidity on their stability. The mechanism of transformation of weddellite nanocrystals has been investigated by IR spectroscopy using D2O as a structural probe.
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50. 50
  Luo, Q.; Chen, G.; Sun, Y.; Ye, Y.; Qiao, X.; Yu, J. Dissolution Kinetics of Aluminum, Calcium, and Iron from Circulating Fluidized Bed Combustion Fly Ash with Hydrochloric Acid. Ind. Eng. Chem. Res. 2013, 52, 18184– 18191, DOI: 10.1021/ie4026902
  50
  Dissolution Kinetics of Aluminum, Calcium, and Iron from Circulating Fluidized Bed Combustion Fly Ash with Hydrochloric Acid
  Luo, Qing; Chen, Guilan; Sun, Yuzhu; Ye, Yinmei; Qiao, Xiuchen; Yu, Jianguo
  Industrial & Engineering Chemistry Research (2013), 52 (51), 18184-18191CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
  The dissoln. of aluminum, calcium, and iron from calcined fly ash with hydrochloric acid was investigated using a batch reactor at temps. from 315 to 345 K and over the acid concn. range of 1.4-6.0 M. Calcium was preferentially released compared to aluminum though these two elements in the calcined fly ash were mainly incorporated into the same mineralogical phase of anorthite. The dissolved fractions of aluminum, calcium, and iron increased with leaching time, but it decreased for silicon, which indicated the continuous pptn. of silicon during the leaching process. The dissoln. kinetic data were successfully examd. according to a semiempirical Avrami-type equation. The activation energies for dissoln. of aluminum, calcium, and iron from the calcined fly ash with hydrochloric acid are 32, 28, and 19 kJ/mol, resp.
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51. 51
  Bhatt, A.; Priyadarshini, S.; Acharath Mohanakrishnan, A.; Abri, A.; Sattler, M.; Techapaphawit, S. Physical, Chemical, and Geotechnical Properties of Coal Fly Ash: A Global Review. Case Stud. Constr. Mater. 2019, 11, e00263 DOI: 10.1016/J.CSCM.2019.E00263
  There is no corresponding record for this reference.
52. 52
  About CCUS. (accessed May 9, 2023)
  There is no corresponding record for this reference.
53. 53
  Zhang, Y.; Jackson, C.; Krevor, S. An Estimate of the Amount of Geological CO2 Storage over the Period of 1996–2020. Environ. Sci. Technol. Lett. 2022, 9, 693– 698, DOI: 10.1021/ACS.ESTLETT.2C00296
  53
  An Estimate of the Amount of Geological CO2 Storage over the Period of 1996-2020
  Zhang, Yuting; Jackson, Christopher; Krevor, Samuel
  Environmental Science & Technology Letters (2022), 9 (8), 693-698CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
  The climate impact of carbon capture and storage depends on how much CO2 is stored underground, yet databases of industrial-scale projects report capture capacity as a measure of project size. We review publicly available sources to est. the amt. of CO2 that has been stored by facilities since 1996. We organize these sources into three categories corresponding to the assocd. degree of assurance: (1) legal assurance, (2) quality assurance through auditing, and (3) no assurance. Data were found for 20 facilities, with an aggregate capture capacity of 36 Mt of CO2 year-1. Combining data from all categories, we est. that 29 Mt of CO2 was geol. stored in 2019 and there was cumulative storage of 197 Mt over the period of 1996-2020. These are climate relevant scales commensurate with recent cumulative and ongoing emissions impacts of renewables in some markets, e.g., solar photovoltaics in the United States. The widely used capture capacity is in aggregate 19-30% higher than storage rates and is not a good proxy for estg. storage vols. However, the discrepancy is project-specific and not always a reflection of project performance. This work provides a snapshot of storage amts. and highlights the need for uniform reporting on capture and storage rates with quality assurance.
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54. 54
  The Role of CO₂ Storage. (accessed May 9, 2023).
  There is no corresponding record for this reference.
55. 55
  Pan, S. Y.; Hung, C. H.; Chan, Y. W.; Kim, H.; Li, P.; Chiang, P. C. Integrated CO2 Fixation, Waste Stabilization, and Product Utilization via High-Gravity Carbonation Process Exemplified by Circular Fluidized Bed Fly Ash. ACS Sustainable Chem. Eng. 2016, 4, 3045– 3052, DOI: 10.1021/acssuschemeng.6b00014
  55
  Integrated CO2 Fixation, Waste Stabilization, and Product Utilization via High-Gravity Carbonation Process Exemplified by Circular Fluidized Bed Fly Ash
  Pan, Shu-Yuan; Hung, Chen-Hsiang; Chan, Yin-Wen; Kim, Hyunook; Li, Ping; Chiang, Pen-Chi
  ACS Sustainable Chemistry & Engineering (2016), 4 (6), 3045-3052CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
  The valorization of industrial solid wastes in civil engineering is one of the main routes for enhancing resource cycle toward environmental and social sustainability. In this study, an integrated approach to capturing CO2 in flue gas and stabilizing solid wastes for utilization as supplementary cementitious material via a high-gravity carbonation (HiGCarb) process was proposed. The fly ash (FA) generated from a circular fluidized bed boiler in the petrochem. industry was used. The effect of different operating parameters on the carbonation conversion was evaluated by the response surface methodol. The maximal carbonation conversion of FA was 77.2% at a rotation speed of 743 rpm and an L/S ratio of 18.9 at 57.3°. In addn., the workability, strength development, and durability of the blended cement with different substitution ratios (i.e., 10%, 15%, and 20%) of carbonated FA were evaluated. Cement with carbonated FA exhibited superior properties, e.g., initial compressive strength (3400 psi at 7 d in 10% substitution ratio) and durability (autoclave expansion <0.15%) compared to cement with fresh FA. After HiGCarb, the physico-chem. properties of FA were upgraded, e.g., lower heavy-metal leaching and stabilized vol. expansion, which were beneficial to usage as green materials in construction engineering.
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56. 56
  Goss, S. L.; Lemons, K. A.; Kerstetter, J. E.; Bogner, R. H. Determination of Calcium Salt Solubility with Changes in PH and PCO2, Simulating Varying Gastrointestinal Environments. J. Pharm. Pharmacol. 2007, 59, 1485– 1492, DOI: 10.1211/JPP.59.11.0004
  56
  Determination of calcium salt solubility with changes in pH and PCO2, simulating varying gastrointestinal environments
  Goss, Sandra L.; Lemons, Karen A.; Kerstetter, Jane E.; Bogner, Robin H.
  Journal of Pharmacy and Pharmacology (2007), 59 (11), 1485-1492CODEN: JPPMAB; ISSN:0022-3573. (Pharmaceutical Press)
  The amt. of calcium available for absorption is dependent, in part, on its sustained soly. in the gastrointestinal (GI) tract. Many calcium salts, which are the calcium sources in supplements and food, have pH-dependent soly. and may have limited availability in the small intestine, the major site of absorption. The equil. soly. of four calcium salts (calcium oxalate hydrate, calcium citrate tetrahydrate, calcium phosphate, calcium glycerophosphate) were detd. at controlled pH values (7.5, 6.0, 4.5 and ≤ 3.0) and in distd. water. The soly. of calcium carbonate was also measured at pH 7.5, 6.0 and 4.5 with two CO2 environments (0.3 and 152 mmHg) above the soln. The pptn. profile of CaCO3 was calcd. using in-vivo data for bicarbonate and pH from literature and equil. calcns. As pH increased, the soly. of each calcium salt increased. However, in distd. water each salt produced a different pH, affecting its soly. value. Although calcium citrate does have a higher soly. than CaCO3 in water, there is little difference when the pH is controlled at pH 7.5. The partial pressure of CO2 also played a role in calcium carbonate soly., depressing the soly. at pH 7.5. The calcns. of sol. calcium resulted in profiles of available calcium, which agreed with previously published in-vivo data on absorbed calcium. The exptl. data illustrate the impact of pH and CO2 on the soly. of many calcium salts in the presence of bicarbonate secretions in the intestine. Calcd. profiles using in-vivo calcium and bicarbonate concns. demonstrate that large calcium doses may not further increase intestinal calcium absorption once the calcium carbonate soly. product has been reached.
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57. 57
  Hunter, H. A.; Ling, F. T.; Peters, C. A. Coprecipitation of Heavy Metals in Calcium Carbonate from Coal Fly Ash Leachate. ACS EST Water 2021, 1, 339– 345, DOI: 10.1021/ACSESTWATER.0C00109
  There is no corresponding record for this reference.
58. 58
  Murcia Valderrama, M. A.; van Putten, R. J.; Gruter, G. J. M. The Potential of Oxalic – and Glycolic Acid Based Polyesters (Review). Towards CO2 as a Feedstock (Carbon Capture and Utilization – CCU). Eur. Polym. J. 2019, 119, 445– 468, DOI: 10.1016/J.EURPOLYMJ.2019.07.036
  58
  The potential of oxalic - and glycolic acid based polyesters (review). Towards CO2 as a feedstock (Carbon Capture and Utilization - CCU)
  Murcia Valderrama, Maria A.; van Putten, Robert-Jan; Gruter, Gert-Jan M.
  European Polymer Journal (2019), 119 (), 445-468CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
  A review. Plastic materials are indispensable in everyday life because of their versatility, high durability, lightness and cost-effectiveness. As a consequence, worldwide plastic consumption will continue to grow from around 350 million metric tons per annum today to an estd. 1 billion metric tons per annum in 2050. For applications where polymers are applied in the environment or for applications where polymers have a bigger chance of ending up in the environment, (bio)degradable polymers need to be developed to stop endless accumulation of non-degradable polymers irreversibly littering our planet. As monomers and polymers represent more than 80% of the chem. industry's total prodn. vol., a transition from fossil feedstock today (99% of the current feedstock for polymers is fossil-based) to a significantly larger percentage of renewable feedstock in the future (carbon that is already "above the ground") will be required to meet the greenhouse gas redn. targets of the Paris Agreement (>80% CO2 redn. target for the European Chem. Industry sector in 2050). The combination of the predicted polymer market growth and the emergence of new feedstocks creates a fantastic opportunity for novel sustainable polymers. To replace fossil based feedstock, there are only three sustainable alternative sources: biomass, CO2 and existing plastics (via recycling). The ultimate circular feedstock would be CO2: it can be electrochem. reduced to formic acid derivs. that can subsequently be converted into useful monomers such as glycolic acid and oxalic acid. In order to assess the future potential for these polyester building blocks, we will review the current field of polyesters based on these two monomers. Representative synthesis methods, general properties, general degrdn. mechanisms, and recent applications will be discussed in this review. The application potential of these polyesters for a wide range of purposes, as a function of prodn. cost, will also be assessed. It is important to note that polymers derived from CO2 do not necessarily always lead to lower net overall CO2 emissions (during prodn. of after use, e.g. degrdn. in landfills). This needs to be evaluated using robust LCA's and this information is currently not available for the materials discussed in this review.
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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.3c00063.
- Electron micrographs of the two types of coal fly ash; mineralogical composition of the unreacted fly ash samples based on XRD measurements (PDF).
- ee3c00063_si_001.pdf (241.96 kb)
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Carbon Storage in Coal Fly Ash by Reaction with Oxalic AcidClick to copy article linkArticle link copied!

ACS ES&T Engineering

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1. Introduction

2. Materials and Methods

2.1. CFA Samples and Experimental Protocol

2.2. Mineralogical and Chemical Characterization

2.3. Thermal Gravimetric Analyses and Ca Oxalate Content

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2.4. Reaction Rate Model

3. Results and Discussion

3.1. Textural Changes to Fly Ash during Reaction with Oxalic Acid

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3.2. Fly Ash Mineralogy and Chemical Composition

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3.3. Ca Oxalate Quantification with Thermal Gravimetry

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3.4. Rate Modeling of Ca Release

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3.5. Evaluation of the Potential Applications

4. Concluding Remarks

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Carbon Storage in Coal Fly Ash by Reaction with Oxalic Acid
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