Particle-Scale Understanding of Arsenic Interactions with Sulfidized Nanoscale Zerovalent Iron and Their Impacts on Dehalogenation Reactivity

Co-occurrence of organic contaminants and arsenic oxoanions occurs often at polluted groundwater sites, but the effect of arsenite on the reactivity of sulfidized nanoscale zerovalent iron (SNZVI) used to remediate groundwater has not been evaluated. Here, we study the interaction of arsenite [As(III)] with SNZVI at the individual-particle scale to better understand the impacts on the SNZVI properties and reactivity. Surface and intraparticle accumulation of As was observed on hydrophilic FeS–Fe0 and hydrophobic FeS2–Fe0 particles, respectively. X-ray absorption spectroscopy indicated the presence of realgar-like As–S and elemental As0 species at low and high As/Fe concentration ratios, respectively. Single-particle inductively coupled plasma time-of-flight mass spectrometry analysis identified As-containing particles both with and without Fe. The probability of finding As-containing particles without Fe increased with the S-induced hydrophobicity of SNZVI. The interactions of SNZVI materials with coexisting arsenite inhibited their reactivity with water (∼5.8–230.7-fold), trichloroethylene (∼3.6–67.5-fold), and florfenicol (∼1.1–5.9-fold). However, the overall selectivity toward trichloroethylene and florfenicol relative to water was improved (up to 9.0-fold) because the surface-associated As increased the SNZVI hydrophobicity. These results indicate that reactions of SNZVI with arsenite can remove As from groundwater and improve the properties of SNZVI for dehalogenation selectivity.


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The sulfur content of SNZVI did not affect the rate or extent of the As removal efficiency (close to 100%) at the low As/Fe ratio (i.e., 0.1 mg or 1 mg As per g SNZVI) (Figure S1a, c).The removal kinetics of arsenite by SNZVI with different S content followed a second-order-kinetic model (r 2 >0.999) (Figure S1b, d, f).The similar removal efficiency and kinetics indicate the comparable performance of these materials for trace arsenite removal.
The adsorption isotherms of SNZVI materials for arsenite were performed to assess the impact of sulfur on the maximum capacity of arsenite.The equilibrium adsorption data could be well fitted with the Langmuir model for the materials without S or with low S (i.e.NZVI and 0.010 SNZVI) (Figure S1g).Detailed fitted parameters are shown in Table S1.In contrast, the adsorption of arsenite by the materials with high S content (i.e.0.049 and 0.099 SNZVI) was better fitted with the Freundlich model (r 2 =0.951 and 0.994, respectively) rather than the Langmuir model (r 2 =0.539 and 0.842, respectively).While previous studies have proved that different metals (e.g., U, Au, and Ag) would have different distributions (e.g., cluster and encapsulation) over NZVI particles, 1,2 the results here show that sulfidation of NZVI can alter the distribution and surface coverage of As(III), which would change the surface property and reactivity as discussed later.
In addition, the maximum adsorption capacity of arsenite by NZVI and SNZVI (0.010 [S/Fe]particle) according to the Langmuir model could be up to 115 mg g −1 (i.e., 6.9 mg m −2 ) and 135 mg g −1 (i.e., 12.1 mg m −2 ), respectively.The higher capacity of 0.010 SNZVI than that of NZVI was probably because the 0.010 SNZVI material was the more reactive than NZVI, 3 generating more iron (hydr)oxides (i.e., FeOOH and ferrihydrite) on the surface with high affinity toward arsenite (Figure S8), and reducing more arsenite to reduced forms (e.g., As 0 ).The reaction of S with arsenite to form As sulfides (e.g., realgar) would also improve the adsorption capacity of SNZVI as discussed later.The adsorption capacity of SNZVI was higher than most of functionalized iron/carbon materials either via direct adsorption 4 or pre-oxidation then adsorption process, 5,6 indicating the good potential of SNZVI for the arsenite remediation.
Arsenite capacity follows the order of NZVI > 0.010 SNZVI > 0.099 SNZVI > 0.049 SNZVI at a relatively high As concentration, and this trend was well correlated with the hydrophobicity of these materials (Figure 3i), i.e., a hydrophobic SNZVI possessed a low removal capacity and rate for the hydrophilic arsenite ions.][9] However, this trend was not consistent with a previous study where the removal efficiencies of As(III) by SNZVI materials (using an Fe 2+ precursor and a NaBH4/Fe molar ratio of 1.0) were all larger than that by NZVI. 10 This is because that different Fe precursors and NaBH4/Fe would result in different physicochemical properties (e.g., Fe 0 content, S content and speciation, surface area, lattice constant, and electron transfer ability) of SNZVI materials, 11 which could possibly affect the performance of SNZVI materials.For example, our recent study found that the Brunauer-Emmett-Teller surface area of SNZVI synthesized by a low NaBH4/Fe ratio (1.5) was ~5 times higher than that by a high NaBH4/Fe ratio (5.0), while the surface area of NZVI was close at different NaBH4/Fe ratios. 12This is possibly one of the reasons for the discrepancy between this work and the previous study. 10 (mg As)

Figure S1 Figure S2 Figure S3 Figure S4 Figure S5 Figure S6 Figure S7 FigureFigureFigure S12 Figure S15 Figure S16 Figure S17 Figure S18 Figure S19 Figure S20 Figure S21 Figure S22 Figure S24 Figure S25
Figure S1 As removal kinetics and capacity by SNZVI.Page S5 Figure S2 HAADF and elemental maps of NZVI at low As/Fe.Page S6 Figure S3 HAADF and elemental maps of 0.010 SNZVI at low As/Fe.Page S7 Figure S4 HAADF and elemental maps of 0.049 SNZVI at low As/Fe.Page S8 Figure S5 HAADF and elemental maps of NZVI at high As/Fe.Page S9 Figure S6 HAADF and elemental maps of 0.010 SNZVI at high As/Fe.Page S10 Figure S7 HAADF and elemental maps of 0.049 SNZVI at high As/Fe.Page S11 Figure S8 qe and kd of As by different iron and sulfur compounds.Page S12 Figure S9 spICP-TOF-MS of Fe mass distribution at low As/Fe ratio.Page S13 Figure S10 spICP-TOF-MS of Fe mass distribution at high As/Fe ratio.Page S14 Figure S11 spICP-TOF-MS of As mass distribution at high As/Fe ratio.Page S15 Figure S12 Water contact angle measurements for As-reacted SNZVI pellets.Page S16 Figure S13 Impact of arsenite on the water reactivity of NZVI and SNZVI.Page S17 Figure S14 BET surface area of fresh and As-reacted SNZVI materials.Page S18 Figure S15 Impact of arsenite on the TCE removal by NZVI.Page S19 Figure S16 Impact of arsenite on the TCE removal by 0.010 SNZVI.Page S20 Figure S17 Impact of arsenite on the TCE removal by 0.049 SNZVI.Page S21 Figure S18 Impact of arsenite on the TCE removal by 0.099 SNZVI.Page S22 Figure S19 Aqueous Fe concentration after TCE reaction by SNZVI.Page S23 Figure S20 FF removal by NZVI or SNZVI without As.Page S24 Figure S21 FF removal by NZVI or SNZVI in As-unsaturatedlow scenario.Page S25 Figure S22 FF removal by NZVI or SNZVI in As-unsaturatedhigh scenario.Page S26 Figure S23 FF removal by NZVI or SNZVI in As-saturated scenario.Page S27 Figure S24 Elemental distribution of As-SNZVI in real groundwater.Page S28 Figure S25 As(III)-impacted hydrophobicity of SNZVI in real groundwater.Page S29 Figure S26 As(III)-impacted TCE reactivity of SNZVI in real groundwater.Page S30 Figure S27 As(III)-impacted surface area of SNZVI in real groundwater.Page S31

Figure
Figure S1 (a, c, and e) Arsenite removal performance and (b, d, and f) kinetics (during

Figure
Figure S10 spICP-TOF-MS analysis (triplicates) of Fe mass distribution and the

Figure
Figure S11 spICP-TOF-MS analysis of As mass distribution and the percentage of "As

Figure S14 Figure S15
Figure S14Brunauer-Emmett-Teller surface area of fresh and As-reacted SNZVI 0

Figure S16
Figure S16Impact of co-existed arsenite on the TCE removal by 0.010 SNZVI in the

Figure S17 Figure S18 Figure S19 Figure S20 Figure S21 Figure S22 Figure S23 Figure S24
Figure S17Impact of co-existed arsenite on the TCE removal by 0.049 SNZVI in the

Table S1
Compositions of the used groundwater.

Table S3
Linear combination fitting results of XANES spectra at As K-edge.

Table S3
Linear combination fitting results of XANES spectra in R space at As K-edge.