Reductive Immobilization of Hexavalent Chromium by Polysulfide

X-ray diffractograms from the standard library: (c) PDF no. 76-2301 (lepidocrocite), (d) PDF no. 81-0464 (goethite), (e) PDF no. 65-3408 (iron sulfide...
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Cite This: Ind. Eng. Chem. Res. 2019, 58, 11920−11926

Reductive Immobilization of Hexavalent Chromium by PolysulfideReduced Lepidocrocite Mei Shi,†,‡ Jiao Li,† Xinyang Li,† Dongli Liang,‡ Caiyang Guo,† Jianzhong Zheng,*,† and Baolin Deng*,§ †

College of Resources and Environment, University of Chinese Academy of Sciences, 19 A Yuquan Road, Beijing 100049, China College of Natural Resources and Environment, Northwest A&F University, No. 3 Taicheng Road, Yangling, Shaanxi 712100, China § Department of Civil and Environmental Engineering, University of Missouri, Columbia, Missouri 65211, United States Downloaded via NOTTINGHAM TRENT UNIV on August 8, 2019 at 10:14:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The anoxic reduction of lepidocrocite by polysulfide was studied under alkaline conditions (pH 9.5), and the reducing capacity of the resultant reduced lepidocrocite (RL) for hexavalent chromium (Cr(VI)) reduction was examined in the pH range from 6 to 10. The results indicated that polysulfide had a high reactivity in reducing lepidocrocite at pH 9.5. Elemental sulfur was the main product from polysulfide oxidation, and the produced ferrous iron was mainly distributed in solid and adsorbed forms, with much less observed in the dissolved state. RL maintained a good reducing activity in the pH range from 6 to 9. X-ray photoelectron spectroscopy analysis suggests that both Fe(II) and the reduced sulfur species in RL participated in Cr(VI) reduction. The possible reaction paths of lepidocrocite reduction by polysulfide were proposed and consistent with the observed changes in reactants and products, and the reaction process may involve the adsorption of polysulfide, the electron transfer, and the detachment of produced Fe(II). This work also explained why polysulfide could maintain a prolonged reducing condition in field remediation of Cr(VI)contaminated soil and groundwater. field applications.18 Recently, polysulfide has been observed to maintain a reducing capacity for a prolonged period when it was injected and used to treat Cr(VI)-contaminated subsurface.16 The mechanism for the long-term effectiveness of the polysulfide treatment, however, is still not clearly understood. Since ferric (hydro)oxides exist in a significant amount in some soils, it has been speculated that these Fe(III) minerals could react with the injected polysulfide, and the produced Fe(II) during polysulfide treatment may lead to the long-term reduction of Cr(VI).16,20 There are several reports in the literature examining the reaction between ferric iron and aqueous sulfide compounds21 and others on heterogeneous reactions leading to iron oxides sulfidation.22,23 It is generally accepted that polysulfide, a typical intermediate product formed in the reaction between iron and sulfide, plays a vital role in the phase transition process of Fe−S minerals.24 Our understanding on reduction of ferric (hydro)oxides directly by polysulfide, however, is still

1. INTRODUCTION Chromium contamination in soil and groundwater has been frequently reported at former industrial facilities related to chromium ore processing, metallurgy, leather tanning, electroplating, and corrosion control.1 Carcinogenic Cr(VI) is highly soluble in water and could easily pollute groundwater supplies, whereas Cr(III) is less mobile due to its potential to form hydroxide precipitate in the range of pH 6−11.2,3 Therefore, reducing Cr(VI) to less mobile and less toxic Cr(III) is an effective approach for Cr-contaminated site remediation. Reductants explored for Cr(VI) reduction included zerovalent iron,4−7 divalent iron (Fe(II)),8−10 sulfide,11−13 and polysulfide.14−17 While common reductants such as Fe(II) compounds are effective under acidic conditions or at near neutral pH, their effectiveness is much lower under alkaline conditions such as those related to chromite ore processing residue (COPR) contaminated sites where the pH is around 11−12.18 Polysulfide has been used in treating COPR14 and in situ geochemical fixing of Cr(VI) in soil and groundwater.15,17,19 It is advantageous over other reductants because of its high stability and persistence in subsurface environments, minimal competition from side reactions, and low safety concerns for © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11920

February 26, 2019 May 9, 2019 May 13, 2019 May 13, 2019 DOI: 10.1021/acs.iecr.9b01055 Ind. Eng. Chem. Res. 2019, 58, 11920−11926

Article

Industrial & Engineering Chemistry Research very limited. It is not clear whether the products resulted from polysulfide reduction could maintain a high reductive capacity and what the factors are that could affect the reduction process. As part of our overall effort to understand the fundamental mechanisms involved in the reductive immobilization of Cr(VI) by polysulfide in the subsurface, this study aimed to examine the production of ferrous products during lepidocrocite reduction by polysulfide and the kinetics of Cr(VI) reduction by polysulfide-reduced lepidocrocite (RL) as a function of solution pH. Lepidocrocite (γ-FeOOH) was selected as a representative iron (hydro)oxide because it has high oxidation−reduction (redox) reactivity25 and exists widely in subsurface environments (such as groundwaters, soils, and lacustrine sediments) as a product of weathering processes.26

were collected at various time intervals for ferrous iron and elemental sulfur analyses. 2.3.2. Chromate Reduction by Reduced Lepidocrocite. To evaluate the capacity of RL in further reducing chromate in solution, RL was obtained following 24 h of reaction between lepidocrocite and polysulfide, by centrifuging of 2 mL of the slurry, decanting the supernatant, and rinsing of the solid fraction with the borate buffer (pH 9.5). The RL sample thus prepared was analyzed for its total concentration of Fe(II) that included both the solid and adsorbed forms of Fe(II). Chromate reduction by RL was initiated by rapidly mixing a desired volume of Cr(VI) stock solution with a known amount of RL in a 120 mL septum-sealed bottle, with a final reaction volume of 100 mL. A typical reaction system had 90 μM Fe(II) (introduced in the form of RL according to its total Fe(II) concentration), 30 μM Cr(VI), and 5 mM buffer with a target pH ranging from 6 to 10. MES, MOPS, TAPS, or borate was used to buffer solution pHs at 6.0, 7.0, 8.0 or in the range from 9 to 10, respectively. All reactions occurred at an ionic strength of 0.2 M controlled by NaCl. The reaction was monitored by analyzing chromate concentrations in the solution. 2.4. Chemical Analyses. Ferrous iron concentration was determined spectrophotometrically at 510 nm using 1,10phenanthroline as a color development agent.30 To analyze Fe(II) in the aqueous phase ([Fe(II)aq]), 1 mL of the reaction mixture was collected and filtered through a 0.22 μm nylon filter disk to obtain 0.5 mL of filtrate for Fe(II) analysis after acidifying the sample with HCl (0.5 N final concentration).30 To quantify the adsorbed Fe(II) ([Fe(II)CaCl2]), 1 mL of the reaction mixture was collected and centrifuged to obtain the Fe(II)-containing solid fraction.31 The solid was then thoroughly rinsed in the anaerobic chamber with the same buffer solution used in the corresponding redox experiment, followed by 2 h extraction with 1 M CaCl2 solution buffered at pH 7. [Fe(II)CaCl2] was determined by analyzing Fe(II) extracted to the aqueous phase from the solid fraction. Total HCl-extracted solid Fe(II) ([Fe(II)HCl]) was analyzed similarly except that the Fe(II)-containing solid fraction was extracted with 6 N HCl. Solid Fe(II) was calculated by mass balance: [Fe(II)solid] = [Fe(II)HCl] − [Fe(II)CaCl2]. The polysulfide and sulfide stock solutions were quantified by the iodometric titration method.29 Total elemental sulfur (S0) was determined by high performance liquid chromatography (HPLC, Shimadzu, LC AT20) with UV detection (SPD20A, Shimadzu).22 Briefly, 300 μL of reaction mixture was samples and immediately transferred to 1.2 mL of methanol to equilibrate for 1 h. The suspension was then filtered (0.22 μm) and stored at −20 °C before HPLC analysis. Chromate concentration in the filtrate was measured at 540 nm by UV−vis spectroscopy (Shimadzu, UV mini-1240), following EPA method 7196A. 2.5. Characterization. Lepidocrocite (L), RL, and the solid products of reaction between RL and Cr(VI) (RL_Cr) were characterized by high resolution transmission electron microscopy (HRTEM, JEM-2010/JEOL, Japan), X-ray diffraction spectroscopy (XRD, Bruker, D8 focus z20090463), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA). Lepidocrocite in its powder form was directly used for HRTEM, XRD, and XPS characterization, whereas RL and RL_Cr samples were collected by centrifugation. After decanting the supernatant in the anaerobic chamber, the solid sample was divided into three portions. One portion was

2. MATERIALS AND METHODS 2.1. Chemicals. Buffer reagents TAPS (N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid) and MES (2-(N-morpholino)ethanesulfonic acid), potassium dichromate, sodium thiosulfate, and sodium chloride were purchased from Sigma-Aldrich. Sodium sulfide and elemental sulfur (S8) were purchased from Acros, and MOPS (3-(Nmorpholino)propanesulfonic acid) was from Amresco. All other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. Solutions were prepared using 18.2 MΩ·cm deionized water (Millipore Corp.) after purging with highpurity N2 (99.99%) for at least 30 min to remove dissolved oxygen. 2.2. Synthesis of Lepidocrocite and Polysulfide. Lepidocrocite (γ-FeOOH) was synthesized following the procedure reported elsewhere.26 The prepared lepidocrocite had a specific surface area of 92 m2·g−1 by N2-BET analysis (Quadrasorb SI-MP). The polysulfide stock solution (100 mM) was prepared based on previous reports.27,28 Briefly, 50 mL of 0.10 M sodium sulfide solution was mixed with 0.16 g of elemental sulfur (1:1 mol ratio of S2−:S0) in a sealed glass flask, followed by mixing at 65 °C on a shaker (100 rpm) for about 16 h until the elemental sulfur dissolved. The prepared polysulfide stock solution was kept in a glovebox filled with nitrogen. A stock solution of high sulfide concentration was prepared for each set of experiments by dissolving precleaned Na2S·9H2O crystals in deoxygenated water, and its concentration was quantified by the iodometric titration method.29 Sulfide working solution (0.10 M) was prepared by diluting the stock solution with deoxygenated water. 2.3. Batch Experiments. All experiments in this study were performed in an anaerobic chamber (N2 balanced by 10% H2), including experimental setup and sample collection. 2.3.1. Lepidocrocite Reduction by Polysulfide. A predetermined amount of lepidocrocite was introduced to approximately 250 mL of solution in a 500 mL bottle. The bottle was then septum-sealed and sonicated for 2 h before being transferred to the glovebox. A predetermined amount of polysulfide stock solution was then added to the mixture (final volume of mixture 250 mL) to initiate the reaction. A typical reaction mixture contained 3 mM Fe(III) (added in the form of lepidocrocite) and 12 mM polysulfide, with its ionic strength fixed at 0.2 M by NaCl and pH buffered at 9.5 by a 0.05 M borate−NaOH buffer. The reduction of lepidocrocite by polysulfide occurred in batch systems at 25 °C. Samples 11921

DOI: 10.1021/acs.iecr.9b01055 Ind. Eng. Chem. Res. 2019, 58, 11920−11926

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Industrial & Engineering Chemistry Research

believed to be ascribed to the oxidation of polysulfide by Fe(III) in lepidocrocite. 3.2. HRTEM and XRD Analyses. Figure 2 shows highresolution TEM images and EDS data of solid samples collected before and after lepidocrocite reaction with polysulfide. Under HRTEM, unreacted lepidocrocite particles were blade-structured single crystals. The lengths of crystals were 0.1−0.3 μm, and the widths were about 20 nm (Figure 2a). The edge of the initially blade-structured lepidocrocite crystal turned blurred after reacting with polysulfide. As shown in the high resolution TEM image of lepidocrocite after 6 h of reaction with polysulfide, the amorphous regions formed around the grain surface were found (red arrows of Figure 2b). After 24 h, the crystal structure of lepidocrocite was further destroyed (Figure 2c). EDS spectroscopy (Figure 2d) collected over several locations on the particle surface, including the edge regions without substrate lepidocrocite, indicated that most of the surface reaction products had a Fe:S ratio of 1:1. Figure 3 shows XRD spectra of the synthesized lepidocrocite (a), its reaction product with polysulfides at 24 h of reaction time (b), and related minerals. After reaction with polysulfide, the peak intensity of characteristic spectral lines of lepidocrocite decreased (RL, 24 h, Figure 3b), but the relative intensity of the peak at 21.24° increased, possibly due to the existence of a small amount of goethite impurity in the synthesized lepidocrocite which was less reactive with polysulfide than lepidocrocite. A close comparison of the XRD pattern of the polysulfide-reduced product (RL, 24 h) with standard PDF cards in the library indicated that the reaction product contained elemental sulfur and iron sulfide (FeS). 3.3. Chromate Reduction by RL. RL was able to reduce chromate in the pH range from 6 to 10 (Figure 4), very notably in pH 6−9. As clearly shown in the first 30 min (see inset), the reaction rate increased with decreasing pH in the examined pH range, with the highest reaction rate appearing at pH 6 and the lowest at pH 10. After 30 min, however, there was a relatively higher reaction rate in the pH 9 treatment than that at pH 8. After 9 h, the removal rates of Cr(VI) in different pH treatments (6−10) were 99%, 70%, 54%, 72%, and 27%, respectively. The observed effect of pH on chromate reduction by RL was generally similar to other systems using different reductants to reduce Cr(VI).33,34 3.4. XPS Analysis. The XPS survey scan of the synthetic lepidocrocite indicated the presence of O and Fe at the sample surface (Figure S1). After reaction with polysulfide, sulfur spectra appeared on the surface of lepidocrocite. Spectra also revealed the existence of chromium in the solid products after RL reacted with chromate. High resolution Cr 2p spectra showed that the chromium present in RL_Cr was Cr(III) (Figure S2). Figure 5 shows the high-resolution Fe 2p spectra. Each species in the Fe 2p spectra was fitted with a doublet of Fe(2p1/2) and Fe(2p3/2), with the gap of binding energy (BE) between the Fe(2p1/2) and Fe(2p3/2) lines fixed at 13.4 eV and their peak area ratio set at 1/2. Table S1 lists values of binding energy used in this study for the fitted Fe(2p3/2) peaks. Fitting of Fe 2p spectra for all samples tested suggested the existence of satellite peaks. For the synthetic lepidocrocite (Figure 5a, L), spectrum fitting resulted in two Fe(2p3/2) peaks occurring near 710.6 and 712.5 eV due to Fe(III)−O compounds.35 After reduction by polysulfide, a new iron signal having a Fe(2p3/2) binding energy near 707.7 eV appeared (Figure 5b,

mounted on grids for HRTEM imaging and energy dispersive spectroscopy (EDS) analysis, whereas the other two were used for XRD and XPS analysis. For XRD analysis, the X-ray source was a long-time-focus Cu X-ray tube, operated at 40 kV and 40 mA. The detector was an energy selective solid-state Si(Li) detector, and diffraction data were collected in the range from 5° to 70° with a resolution of 0.05° and 0.5 s collection time per step. For XPS analysis, the instrument used an Al Kα X-ray source. Survey spectra were collected from 0 to 1361 eV at a pass energy of 100 eV with 1 eV scan step, while the highresolution spectra for Fe 2p, S 2p, Cr 2p, and C 1s were collected with a scan step of 0.05 eV. Both survey and highresolution spectra were calibrated to the binding energy (BE) of C 1s at 284.6 eV to compensate for the surface charging effect. XPS spectra were fitted by XPSpeak 4.1.

3. RESULTS AND DISCUSSION 3.1. Ferrous Iron Produced from Lepidocrocite Reduction by Polysulfide. In the reduction of lepidocrocite by polysulfide at pH 9.5, the addition of polysulfide to lepidocrocite changed the slurry color immediately from orange to dark green and gradually to black after 3 h of reaction (Figure 1). The total ferrous iron increased over time.

Figure 1. Total concentration of ferrous ions (column) and elemental sulfur (line) produced from lepidocrocite reduction by polysulfide as a function of reaction time. Photos in the inset depict changes in color of the reaction mixture. Experimental conditions: [Sx2−]0 = 12 mM, [Fe(III)]0 = 3 mM, pH = 9.5, [borate buffer] = 0.05 M, ionic strength I = 0.2 M.

Distribution of the produced ferrous iron in the system indicated that more than 80% of the produced ferrous iron existed in the solid and adsorbed forms, with less than 20% being in the aqueous phase in the first 24 h. To our knowledge, this is the first report on the distribution of Fe(II) formed from the reduction of lepidocrocite by polysulfides under an alkaline condition of pH 9.5. The results were consistent with a previous report showing that iron (hydro)oxides coated on subsurface media strongly adsorbed Fe(II), especially under an alkaline condition.32 Concurrently, the concentration of elemental sulfur increased as the content of the system’s ferrous iron increased (Figure 1). At the end of the 24 h reaction, the concentration of elemental sulfur reached 3.9 mM. The appearance of elemental sulfur in the system was 11922

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Figure 2. High-resolution TEM images of lepidocrocite (a) before and (b−d) after reaction with polysulfide at pH 9.5. The TEM image of the reaction product at 6 h (b) shows the appearance of amorphous materials formed between neighboring grains. After 24 h (c), the crystal structure of lepidocrocite was further destroyed. Part d shows the EDS spectra of the 24 h reaction product for the selected area. Experimental conditions: [Sx2−]0 = 12 mM, [Fe(III)]0 = 3 mM, pH = 9.5, [borate buffer] = 0.05 M, ionic strength I = 0.2 M).

Figure 4. Effect of pH on Cr(VI) reduction by RL in buffered solutions. Experimental conditions: initial concentration of Fe(II) from RL = 90 μM; [Cr(VI)]0 = 30 μM; ionic strength I = 0.2 M; solutions buffered by 5 mM MES, MOPS, TAPS, and borate for pH 6.0, 7.0, 8.0, and 9−10, respectively.

Figure 6 shows the high-resolution S 2p spectra of the reduced-lepidocrocite before and after reacting with Cr(VI), with corresponding fitting parameters presented in Table S1. In fitting of the S 2p spectra, the gap of binding energy between S(2p1/2) and S(2p3/2) was fixed at 1.18 eV, while the ratio of the corresponding peak areas was set at 1/2. Fitting of the S 2p spectra of reduced lepidocrocite resulted in four species (Figure 6a, RL), with their S 2p3/2 binding energies and concentrations found to be 160.5 eV (a1, 60.6%), 161.2 eV (a2, 22.0%), 163.3 eV (a3, 15.1%), and 168.1 (a4, 2.3%), which could be attributed to S2−, Sn2− (n ≥ 2), elemental sulfur (S0), and SO42− (cf. Table S1 and references therein). The occurrence of S2− species (Figure 6a,a1) could be attributed to the newly formed Fe(II)−S bond on lepidocrocite after reaction with polysulfide. The signature at BE = 161.2 eV (Figure 6a,a2) could be due to the polysulfide bound to the solid surface. Elemental sulfur (Figure 6a,a3) could be formed from the oxidation of polysulfide, while the small amounts of

Figure 3. X-ray diffractograms of lepidocrocite before (a, L) and after (b, RL) 24 h of reaction with polysulfide. X-ray diffractograms from the standard library: (c) PDF no. 76-2301 (lepidocrocite), (d) PDF no. 81-0464 (goethite), (e) PDF no. 65-3408 (iron sulfide), (f) PDF no. 78-0793 (sulfur).

RL), which could be attributed to sulfide-bound ferrous iron species (Fe(II)−S) formed on lepidocrocite.36 Further examination indicated that BE of the fitted Fe(2p3/2) peaks ascribed to the Fe(III)−O species of RL shifted to 710.1 and 712.8 eV. Simulated Fe 2p spectra of RL after reaction with Cr(VI) chromate (Figure 5c, RL_Cr) had BE of 710.7 and 712.7 eV, almost identical to that of lepidocrocite, suggesting that ferrous iron formed in RL was completely reoxidized to ferric iron due to its reaction with chromate. 11923

DOI: 10.1021/acs.iecr.9b01055 Ind. Eng. Chem. Res. 2019, 58, 11920−11926

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SO42− (Figure 6a,a4) might be caused by the short-time air exposure during the sample testing process. After reaction with chromate, S 2p spectra of RL changed substantially (Figure 6b, RL_Cr). Spectrum fitting resulted in three sets of S 2p doublets, with their S(2p3/2) components located at 162.1, 163.8, and 167.6 eV. The sulfur species at 163.8 eV (Figure 6b,b1), which accounted for 63.6% of the overall sulfur species, could be assigned to elemental sulfur. The signature at 162.1 eV (Figure 6b,b2) could be assigned to thiosulfate (S2O32−, first sulfur) and/or S22−. The remaining component of the S 2p spectra at 167.6 eV (Figure 6b,b3), with a total contribution of 11.2%, could be ascribed to thiosulfate (S2O32−, second sulfur) and/or sulfate, consistent with the reported overlap of spectra of these two sulfur species.37 On the basis of analysis of Fe and S XPS spectra of RL before and after its reaction with chromate, it is reasonable to believe that both Fe(II) and S(−II) on the RL participated in Cr(VI) reduction. 3.5. Redox Processes. The XPS spectra of RL revealed the appearance of Sn2−, S2−, and S0 on this reduced solid product from leipidocrocite by polysulfide. Since RL was rinsed with buffer solution to remove the physically retained polysuifide on particle surfaces, Sn2− and S2− identified on RL surface were speculated to be ligands strongly adsorbed or complexed to the RL surface. The elemental sulfur S0 should be the oxidation product formed from the redox reaction between lepidocrocite and polysulfide. On the basis of the experimental observation and XPS characterization, a possible pathway of the lepidocrocite reduction by polysulfide was proposed. In the first step, polysulfide was adsorbed to the lepidocrocite surface forming a lepidocrocite−polysulfide precursor FeIII−(Sn)− via surface ligand exchange reaction between FeIII−OH and Sn2−. Then inside the precursor, electrons were transferred from polysulfide to Fe(III) forming Fe(II). Changes in oxidation state of the Lewis acid center from Fe(III) to Fe(II) caused a charge deficiency in the solid phase, which was subsequently balanced by H+ uptake from solution.38 Concurrently, the reduced polysulfide was released to the solution and then subjected to further oxidation and eventually formed other sulfur species, including elemental sulfur. Finally, depending on solution pH, Fe(II) was detached from the crystal lattice either as mobile ion or as adsorbed Fe(II) to the newly formed lepidocrocite surface due to a decreased bond energy (Madelung energy) between Fe(II) and O2− in the crystalline lattice. For chromate reduction by RL, both sulfide and iron species played important roles, and the system’s proton concentration had a great impact on the redox process. The general trend of the reaction rate of chromate reduction by RL, as a function of solution pH, could be ascribed to changes in physicochemical properties of the redox system. For the pH range tested in this study, lower solution pH favored not only dissolution of Fe(II) from RL, but also the redox capacity of the surface-complexed S(−II) and adsorbed Cr(VI) on RL surface. In addition, at lower solution pH, RL surface was positively charged, favoring chromate adsorption on its surface and consequently also contributing to the increased reaction rate. Reasons behind the apparent increase in reaction rate from pH 8 to 9 were not fully understood. It is speculated that this abnormality could be attributed to changes in the reductive power of Fe−S on RL, and/or the catalytic effect of elemental sulfur existing on the RL surface. Future studies are needed to clarify the reductive

Figure 5. High-resolution XPS spectra of Fe 2p and the fitted iron species for L (freshly synthesized lepidocrocite), RL (polysulfidereduced lepidocrocite), and RL_Cr (solid products retrieved after the reaction of RL and Cr(VI)). Solid and dotted lines represent the Fe 2p3/2 and Fe 2p1/2, respectively. Redox conditions: pH 7.0, 30, and 90 μM initial concentration for Cr(VI) and Fe(II) (from RL), respectively.

Figure 6. High-resolution XPS spectra of S 2p and the fitting results for polysulfide-reduced lepidocrocite before (RL) and after (RL_Cr) reaction with chromate. Solid and dotted lines represent S 2p3/2 and S 2p1/2, respectively. Colors of species in the pie chart are the same as in the spectra. Figure 5 describes the preparation of the RL and RL_Cr samples.

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Fe−S species on RL surfaces under varied pH conditions and the aqueous iron and sulfur species, to elucidate the mechanism of chromate reduction by polysulfide-reduced lepidocrocite. 3.6. Implications. The current work helps explain why polysulfide could maintain a prolonged reductive condition in the subsurface as demonstrated in the field remediation sites.17,39 Considering the remediation of chromate-contaminated soil and groundwater, the injected polysulfide compounds can react not only with chromate in pore water but also with ubiquitous ferric (hydro)oxides coated on porous media in the subsurface. The current work indicates that RL can reduce Cr(VI) in the pH range from 6 to 10, especially in the pH domain from 6 to 9, with generally higher reaction rates observed at lower pHs. Both Fe(II) and the reduced sulfur species in RL participated in Cr(VI) reduction. As a result, remediation of residual chromate could be sustained even when the pollutant is brought in from upstream. According to this study, the established reducing condition stems from the existence of large amounts of solid reductive species (i.e., iron and sulfur) on the polysulfide-reduced ferric (hydro)oxide minerals, such as lepidocrocite. The redox reactions between lepidocrocite and polysulfide, and Cr(VI) with the reduced lepidocrocite are complex. Future studies are needed to verify the proposed mechanism for lepidocrocite reduction by polysulfides and to understand the role of elemental sulfur in the process of chromate reduction by RL.

AUTHOR INFORMATION

Corresponding Authors

*J.Z.: phone, +86-10-69672965; e-mail, [email protected]. *B.D.: phone, +01-573-882-0075; e-mail, dengb@missouri. edu. ORCID

Jianzhong Zheng: 0000-0003-3415-8482 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the National Natural Science Foundation of China (Grant 41473098), the United States National Science Foundation (Grant CBET-0828411), and the Fundamental Research Funds for the Central Universities (Grant 2452017207).

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REFERENCES

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4. CONCLUSIONS In the current study, the anoxic reduction of lepidocrocite by polysulfide was examined under ambient conditions at pH 9.5, and the reducing capacity of the resulted polysulfide-reduced lepidocrocite for chromate reduction was examined in the pH range from 6 to 10. The results show that at pH 9.5, a typical pH for polysulfide application, polysulfide had a high reactivity in reducing lepidocrocite, and XPS, XRD, and TEM analysis supported that the reaction products contained elemental sulfur and iron sulfide. More than 80% of produced ferrous iron from lepidocrocite by polysulfide existed in the solid and adsorbed forms, with only less than 20% found in the aqueous phase in the first 24 h. RL demonstrated a promising reducing activity for chromate reduction in the pH range from 6 to 9. Analysis of iron and sulfur XPS spectra of RL before and after its reaction with chromate suggests that both Fe(II) and S(−II) in RL participated in Cr(VI) reduction. Elemental sulfur was the major oxidation product of S(−II) on RL, with smaller fraction oxidized to S(VI). The current work helps to explain why polysulfide was able to maintain a prolonged reducing condition in the subsurface observed in filed remediation of Cr(VI)-contaminated soil and groundwater.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01055. Survey XPS spectra of samples for L, RL, and RL_Cr; high resolution Cr 2p spectra of solid products from Cr(VI) reduction by RL; XPS fitting parameters of Fe, S, and Cr species (PDF) 11925

DOI: 10.1021/acs.iecr.9b01055 Ind. Eng. Chem. Res. 2019, 58, 11920−11926

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DOI: 10.1021/acs.iecr.9b01055 Ind. Eng. Chem. Res. 2019, 58, 11920−11926