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Sulfidation of Nano Zerovalent Iron (nZVI) for Improved Selectivity during In-Situ Chemical Reduction (ISCR) Dimin Fan, Graham S. O'Brien Johnson, Paul G. Tratnyek, and Richard L. Johnson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02170 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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Environmental Science & Technology Fan et al. (Sulfidation)

Sulfidation of Nano Zerovalent Iron (nZVI) for Improved Selectivity during In-Situ Chemical Reduction (ISCR)

Dimin Fan,1,†, Graham O’Brien Johnson1, Paul G. Tratnyek1, and Richard L. Johnson1,2,* 1 2

Institute of Environmental Health

OHSU/PSU School of Public Health

Oregon Health & Science University 3181 SW Sam Jackson Park Road, Portland, OR 97239

*Corresponding author: Email: [email protected], Phone: 503-346-3432 †

Current address: Office of Superfund Remediation and Technology Innovation, U.S. Environmental Protection Agency, Arlington, VA, 22202 Keywords: Sulfidation, Carboxymethylcellulose, CMC-nZVI, Fe(0) Content, Longevity, Reductant demand, Capacity, Kinetics Environmental Science & Technology Revised Version 17 June 2016

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Environmental Science & Technology

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Abstract The high reactivity of nano zerovalent iron (nZVI) leads to inefficient treatment due to competition with various natural reductant demand (NRD) processes, especially the reduction of water to hydrogen. Here we show that this limitation can be alleviated by sulfidation (i.e., modification by reducing sulfur compounds). nZVI synthesized on carboxylmethylcelluose (CMC-nZVI) was sulfidated with either sulfide or dithionite. The reactivity of the resulting materials was examined with three complementary assays: (i) direct measurement of hydrogen production, (ii) reduction of a colorimetric redox probe (indigo disulfonate, I2S), and (iii) dechlorination of trichloroethylene (TCE). The results indicate that sulfidation at S/Fe molar ratios of ≥0.3, effectively eliminates reaction with water, but retains significant reactivity with TCE. However, sulfidation with sulfide leaves most of the nZVI as Fe(0), whereas dithionite converts majority of the nZVI to FeS (thus consuming much of the reducing capacity originally provided by the Fe(0)). Simplified numerical models show that the reduction kinetics of I2S and TCE are mainly dependent on the initial reducing equivalents and that the TCE reduction rate is affected by the aging of FeS. Overall, the results suggest that pretreatment of nZVI with reducing sulfur compounds could result in substantial improvement in nZVI selectivity.

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Introduction The wide range of possible applications of nano zerovalent iron (nZVI) for treatment of contaminated water (and soil, sludge, etc.) has led to a great deal of research on nZVI formulation, properties, fate, and effects.1-10 While there have been many apparently-successful field applications of nZVI for groundwater remediation,11-15 the full potential of this technology has not yet been realized. The main technical obstacles have been (i) limited mobility of nZVI in subsurface (due to aggregation of primary nZVI particles and attachment to soil grains, etc.),16-18 (ii) challenges to deliver nZVI to target contaminant zones,19-21 and (ii) rapid loss of nZVI reactivity due to side reactions such as reduction of dissolved oxygen, nitrate, or water (which, together, constitute the major part of natural reductant demand, NRD).22-27 While the former two have been greatly improved by recent developments, such as surface modification by organic polymers,22,23 synthesis in the presence of organic polymers (e.g., carboxymethylcellulose (CMC))30-32 or colloidal activated carbon37-39, there has been relatively little effort to reduce the role of side reactions, which would increase the selectivity of nZVI towards target contaminants. This has been the case, in part because of the lack of understanding regarding the very short lifetime of nZVI in the subsurface, which can be further attributed to the lack of Fe(0)-specific analytical methods.7, 40 To provide a method specific for the Fe(0) content of CMC-nZVI, under typical subsurface conditions, we recently developed an assay based on chemical reactivity probes (CRPs).41 Using this approach, the lifetime of Fe(0) in CMC-nZVI was shown to be only a few days, even under anaerobic conditions.41 The rapid loss of Fe(0) was mainly due to corrosion of the nZVI by water. This is consistent with previous work with CMC-nZVI, such as a field study showing that abiotic reduction of contaminants ceased after the first week of injection due to depletion of Fe(0).32 Similar results have also been reported for NANOFER, a commercially available nZVI product, where the majority of the Fe(0) was shown to be consumed by reaction with water instead of trichloroethylene (TCE).42 While the Fe(II) and H2 produced from Fe(0) oxidation may contribute to long-term biogeochemical conditions that favor contaminant reduction (e.g., stimulated in-situ biodegradation due to H2 produced by Fe(0) corrosion), this benefit may be limited by other effects, such as a shift from the preferred dechlorination pathway (reductive elimination) to the pathway the leads to “stall” intermediates (hydrogenolysis).12, 27

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To preserve the capacity of Fe(0) for contaminant reduction—and, thereby, extend the longevity of nZVI-based remedies—methods are needed to “tune” the reactivity of nZVI to favor contaminant degradation over reaction with water. However, most laboratory studies to date have focused on enhancing the performance of nZVI by increasing reactivity (i.e., kinetics of contaminant reduction)43-45, rather than reducing the effects of reaction with water. Among the various approaches that have been investigated for improving nZVI reactivity, modification by reducing sulfur compounds (i.e., “sulfidation”) is of particular interest because it can also significantly decrease the rate of corrosion in water.46, 47 We have shown previously that sulfidation by low doses of aqueous sulfide completely inhibited further corrosion of nZVI,46 and recent work by Rajajavel et al. showed sulfide amendments inhibits H2 evolution from anaerobic corrosion of nZVI.47 These results are consistent with prior literature showing corrosion under sulfidic conditions may form a protective layer of FeS that inhibits further corrosion of Fe(0).48, 49

At the same time, freshly precipitated FeS has been shown to be reactive with a number of

contaminants, including chlorinated solvents.50-56 Therefore, sulfidation of nZVI could produce a material with improved selectivity towards contaminants, and therefore efficiency, for in situ remediation applications. Sulfidation of nZVI has been achieved by reacting nZVI with either sodium sulfide or sodium dithionite. In the former case, the aqueous sulfide (HS–) forms FeS directly by reaction with the Fe(II) that arises from corrosion of Fe(0).46, 47 The sulfidation of nZVI by aqueous dithionite has not been well characterized, and is a more complex process involving both direct and indirect routes to FeS formation.57, 58 In addition, dithionite has been studied as an additive or alternative to borohydride in the synthesis of nZVI.44, 45, 59-62 In the present study, CMC-nZVI was sulfidated with sulfide and dithionite and the resulting materials was compared using three assays: (i) formation of hydrogen (H2), a common method used to quantify total Fe(0) in nZVI22 and for studying the corrosion rate of Fe(0) in water;63 (ii) reduction of chemical reactivity probes (CRP), an approach adapted from our earlier study for detection of Fe(0) in water;41 and (iii) dechlorination of TCE, the most common use of nZVI for groundwater remediation.

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Experimental Chemical Reagents and Protocols. All chemical reagents except indigo-5,5ʹ-disulfonate (indigo carmine, I2S) were ACS reagent grade and were used as received. I2S was purchased from TCI America (Portland, OR), with 95% purity. Deoxygenated deionized (DO/DI) water was prepared by sparging DI water with N2 for at least 2 hr and left in the anaerobic chamber (100% N2, O2 < 0.8 ppm) overnight. All further procedures were carried out inside the anaerobic chamber unless indicated otherwise. Fresh CMC-nZVI suspensions were prepared following a protocol we adapted from He et al.,31 which was described in detail in a previous publication.41 Sulfidation of CMC-nZVI. Stock solutions of Na2S and Na2S2O4 (0.1 M) were freshly prepared using DO/DI water. The freshly prepared CMC-nZVI (1 g/L as total iron) was divided into five 8 mL batches. In addition to untreated control, four other batches were amended with aliquots of Na2S and Na2S2O4 to create low (S/Fe = 0.33) and high (S/Fe = 1) Na2S and Na2S2O4 treatments. The quantities of iron and sulfur in each treatment are documented in Table S1 of the Supporting Information (SI). The vials were then sealed and allowed to equilibrate for 24 h while rolling on a hematology mixer (15 rpm). Aging experiments were then initiated by further diluting each batch five-fold to ~200 mg/L (which was selected to be a more representative concentration in subsurface), and the pH of each vial was adjusted to 7.2 by addition of 0.2 mL of 500 mM HEPES buffer (final HEPES concentration 10 mM). Over the next three weeks, aliquots of each treatment were periodically withdrawn for assays of H2 evolution, I2S reduction, and TCE dechlorination. Hydrogen Evolution Assay. H2 evolution was measured under two regimes: (i) with acidification, to dissolve and therefore quantify total Fe(0); and (ii) without acidification, to determine the rate of nZVI corrosion by reduction of water.7, 61 For the acidification assay, 5 mL of each sample (~200 mg/L) was added to an 11 mL serum vial. The vial was crimp sealed with a butyl rubber septa, and then 1 mL of 1 M HCl was injected into the vial (to completely digest the particles and produce H2). After removing the vails from the anaerobic chamber, H2 concentration in the vial headspace was determined by gas chromatography (GC, SRI 310) with a Carboxin 1010 column (Supelco). To accomplish this, 2 mL of nitrogen was injected into the vial and then, after ~2 seconds, 2 mL of headspace was withdrawn and injected into a gas sampling valve on the GC (loop size ~500 µL). The measured

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H2 concentration was converted to Fe(0) content assuming the stoichiometry between the two reactants is 1:1 (eq 1): Fe + 2H ⟶ Fe + H (g) ↑

(1)

The rate of nZVI corrosion in DO/DI water at neutral pH was determined by measuring H2 evolution over time on 200 mg/L of unaged (“0 days”), un-acidified samples. In these experiments, 5 mL of nZVI suspension was placed in an 11 mL vial. The vial was crimp-sealed, and then removed from the anaerobic chamber for analysis. Periodically over ~2 days, the headspace in the vial was analyzed for hydrogen content using the same procedure and sampling valve described above. After each analysis, the vials were sparged with N2 to remove H2, so that the measured H2 would be the quantity evolved during each sampling interval. These data were then converted to Fe(0) consumed during each sampling interval using the stoichiometry represented in eq 1. At the conclusion of the corrosion experiments, the samples were acidified to recover any remaining Fe(0) and these results were used to determine the overall mass balance for the experiments. I2S Reactivity Probe Assay. The I2S assay used here was described in our earlier study where it was first used to quantify the Fe(0) content of CMC-nZVI.41 Briefly, for each analysis the CMC-nZVI suspension (~200 mg/L) was diluted 50-fold with DO/DI water to give ~4 mg/L total iron and a final volume of 3.8 mL in a 1 cm path length optical glass cuvette. The pH was then adjusted to 7.2 by adding 78 µL of 500 mM HEPES buffer (final HEPES concentration 10 mM). The cuvette was sealed with a Viton-lined screw cap and transferred out of the anaerobic chamber. An aliquot of deoxygenated I2S stock solution was then injected into the cuvette through the septum to make an initial I2S concentration of ~140 µM. The decrease in absorbance at 610 nm (λmax for the oxidized form of I2S) was continuously recorded as a function of time using a UV-vis spectrophotometer (Lambda 20, Perkin-Elmer) for the next 15 min. The solution was then reoxidized by exposing it to air for ~5 s, and the absorbance was re-measured to determine the exact concentration of initial I2S solution (due to the reversibility of oxidation/reduction of I2S). In our previous work with I2S and CMC-nZVI,41 it was shown that the products of nZVI oxidation do not interfere the absorbance of I2S, and the effective stoichiometry of reaction between I2S and Fe(0) is given by eq 2:

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3 I2S + 2 Fe + 6 H → 3 I2S + 2 Fe

(2)

where I2Sox and I2Sred are the oxidized and reduced form of I2S, respectively (Complete chemical structures are presented in the SI as Figure S1). Unlike hydrogen production which is only dependent on Fe(0), I2S might also be reduced by FeS if present. As a result, sulfidated nZVI has the potential to reduce I2S by either Fe(0)—if still present after sulfidation—or FeS. The stoichiometry of I2S reduction with FeS is less certain (as discussed in detail below) but is assumed to follow eq 3: I2S + 2 Fe + 2 H → I2S + 2 Fe

(3)

TCE Reduction Assay. For all five treatments, aliquots of saturated TCE solution (10 µL) were added to serum vials containing ~200 mg/L nZVI suspensions to obtain a final TCE concentration of ~10 mg/L. The vials were crimp sealed using Viton septa and placed upside down (to minimize TCE losses at the septum) on an orbital shaker (125 rpm) outside the anaerobic chamber. At each sampling time, 0.25 mL of solution was withdrawn from the reaction vial and injected into a sealed headspace vial containing 2 mL DI water. After 10 s of vortex mixing, 80 µL of headspace was withdrawn by a gas tight syringe and injected into a gas chromatograph (Hewlett Packard 5890) with electron capture detection to measure the TCE concentration. Although the reaction mechanisms and products of TCE with Fe(0) and FeS are complicated, for the purposes of calculating reducing equivalents, we have assumed that reduction of TCE to ethene is the only important reaction and that Fe(0) and FeS are oxidized to Fe(II) and Fe(III), respectively, by TCE, resulting in eqs 4 and 5: C Cl H + 3Fe + 3H ⟶ C H + 3Fe + 3Cl

(4)

C Cl H + 6 Fe (solid) + 3 H ⟶ 4C H + 3 Cl + 6 Fe (soild)

(5)

Reducing Equivalent Calculation. Quantitative comparisions between the three assays used in this study were based on reducing equivalents (e–), which is operationally defined for this study as the molar quantity of electrons available for each assay reaction (eqs 1–5) from two likely reductants: Fe(0) and FeS. For modeling of the kinetic data below, the assumed number of reducing equivalents per molecule of reductant are summarized in Table 1.

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Table 1. The reducing equivalents per molecule of the reductants for three assays.

a

Assay

e– in Fe(0)

e– in FeS

H2

2

0

I2S

3

1a

TCE

2

1a

Assuming that only Fe(II) is available for the reactions based on eqs 3 and 5.

Results and Discussion Corrosion Rates and Fe(0) Content from Hydrogen Evolution. The effect of sulfidation on the kinetics of H2 formation during oxidation of CMC-nZVI in circum-neutral pH DO/DI water is show in Figure 1. All of the sulfidated treatments produced negligible H2 over two days. In contrast, H2 production from untreated nZVI occurred rapidly in the first day and gradually leveled out, due to depletion of Fe(0). The result is generally consistent with our previous work, using the I2S assay, showing a half-life of ~0.5 day for 200 mg/L CMC-nZVI in DO/DI water at neutral pH.41 The corrosion reaction between nZVI and water at neutral pH (eq 1) was modeled by first-order appearance kinetics and a least-squares fit to the data produced a kobs = 1.6 d–1 (t1/2 = 0.42 d) and is shown as the smooth curve in Figure 1A. At the conclusion of the corrosion experiments, the samples were acidified to quantify any remaining Fe(0). The H2 produced by acidification was added to the quantity of H2 accumulated during the experiment to calculate the total Fe(0) (solid symbols in Figure 1). For the untreated nZVI (Figure 1A), the total H2 produced was ~80% of the theoretical maximum yield of H2, assuming complete reaction of 200 mg/L Fe(0) and the 1:1 stoichiometry shown in eq 1. The undetected ~20% was likely due to consumption of Fe(0) during the 24 hr preequilibration in DO/DI water. For sulfide-treated nZVI (Figure 1B), acidification at the end of the experiment produced a large quantity of H2 despite negligible H2 evolution during the experiment. This demonstrates that treatment with aqueous sulfide greatly inhibited nZVI corrosion by water, thereby preserving most of the original Fe(0). In contrast, acidification of the low and high dithionite-treatments recovered ~30% and 0% of the theoretical H2, respectively (Figure 1C), indicating significant transformation of Fe(0) due to reaction with dithionite, presumably to FeS, as shown in prior studies.57

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Figure 1. Effects of sulfidation on hydrogen evolution kinetics with (A) ~200 mg/L untreated nZVI, (B) sulfide-treated nZVI, and (C) dithionite-treated nZVI. The smooth curve in A corresponds to the least-squares fit to a first-order model with a rate constant of 1.6 d−1; The dashed horizontal black lines represent theoretical H2 production based on the initial Fe(0) content. Experimental conditions are [I2S]0 = ~140 µM, [Fe]total = ~4 mg/L from unaged CMCnZVI, pH = 7.2.

In a parallel set of experiments, the Fe(0) content of the five treatments, at three different aging times, were determined by quantification of H2 after acidification (Figure 2). For untreated nZVI, the Fe(0) content was nearly depleted after 7 days and no Fe(0) remained after aging for 21 days. In contrast, for all of the samples treated with sulfide, the Fe(0) contents were relatively stable over the course of three weeks. This is consistent with the corrosion data shown in Figure 1, and is because a film of iron sulfide forms that inhibits further corrosion (and sulfidation) of

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Fe(0).49 For nZVI treated with low dithionite, the initial sulfidation resulted in ~70% loss of Fe(0), but there was not further loss of Fe(0) during the aging period (Figure 2). Presumably, the initial sulfidation by dithionite also formed a sufficient quantity of FeS to protected the Fe(0) from further corrosion. The high dose of dithionite apparently resulted in complete consumption of the Fe(0), because no H2 was produced after acidification. Assuming that FeS is the only product of sulfidation, the respective contents of Fe(0) and FeS were calculated based on the H2 data (Table S2).

Figure 2. Hydrogen produced after acidification of ~200 mg/L CMC-nZVI (untreated and sulfidated) that had been aged for 0, 7, and 21 days at pH 7.2. The dashed black line represents the theoretical concentration of H2 (3.6 mM) expected from complete acidification of 200 mg/L Fe(0).

The hydrogen production data for corrosion (Figure 1) and acidification (Figure 2) show substantial differences between sulfidation of CMC-nZVI by sulfide vs. dithionite. With sulfide, the initial sulfidation appears to prevent further transformation of Fe(0) to FeS, creating an FeS shell and resulting in similar hydrogen production for the low- and high-sulfide treated samples. Similar results were found in our previous study on sulfidation of non-stabilized nZVI by aqueous sulfide, where Mӧssbauer and X-ray photoelectron spectra showed that nZVI treated by low- and high-sulfide concentrations produced similar FeS content.46 In contrast, dithionite

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appears to be a more aggressive sulfidation reagent, resulting in complete Fe(0) conversion at the high dithionite concentration. This is consistent with several prior studies showing that high concentrations of dithionite (> 1 g/g nZVI)—amended either during44 or after nZVI synthesis57— result in extensive transformation of nZVI to FeS. The fundamental mechanisms for the difference between sulfidation of nZVI by sulfide and dithionite is yet to be investigated, but is likely related to complex chemistry of dithionite in the aqueous phase (discussed in more details in SI). I2S Chemical Reactivity Probe Assay. Adapting our earlier protocol,41 I2S was added in stoichiometric excess relative to nZVI, so that the amount of I2S reduction could be used to calculate the number of reducing equivalents consumed by the reaction. I2Sox concentrations were meaured over time by monitoring aqueous absorbance, as shown in Figure 3. For untreated nZVI, the I2Sox concentration quickly decreased and approached a plateau by about 10 min, indicating that the available Fe(0) was mostly consumed by that time (Figure 3A). For I2S reduction by sulfide- and dithionite-treated nZVI (Figure 3B and Figure 3C, respectively), the apparent kinetics of I2S reduction were slower and the plateaus in I2S concentration were not fully reached after 15 minutes, presumably due to slower reaction of I2S with FeS than Fe(0). Note that the high dithionite-treated nZVI still gave appreciable reduction of I2S (Figure 3C)— even though this material was fully depleted of Fe(0) (Figure 1C)—indicating that I2S is reduced directly by FeS. The possibility that I2S was reduced by dissolved reducing sulfur species was excluded by control experiments that showed no reduction of I2S by aqueous sulfide or dithionite under identical experimental conditions. Therefore, for sulfidated nZVI, the I2S assay reflects contribution from both Fe(0) and FeS, and not just Fe(0) as measured by the H2 assay.

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Figure 3. Reduction of I2S (left axis) and e– consumption for (A) untreated, (B) sulfide-treated, and (C) dithionite-treated nZVI. Colored symbols and curves represent data and model simulations for I2S (left axis); Black curves represent model simulations for consumptions of reducing equivalents (right axis). Common conditions are [I2S]0 = ~140 µM, [Fe(0)]0 = ~4 mg/L from unaged CMC-nZVI, and pH = 7.2. Fitted parameters that define curves are given in Table S5.

For each treatment shown in Figure 3, the quantity of reducing equivalents (e−) consumed during reduction of I2S was calculated from the difference between the final I2Sox concentration and the initial I2Sox concentration (the latter was determined by re-oxidation of the suspension at the end of the experiment because rapid initial reduction precluded accurate

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measurements of initial absorbance.41) The measured I2S reduction was converted to consumed e−, given in Table S3. Using the e- concentrations determined by I2S experiments as initial conditions, we modeled the I2S reduction kinetics for untreated and sulfidated nZVI with the second-order model (colored lines in Figure 3, details described in the SI). The rate constant for reduction of I2S (k″I2S) was globally fit using the data from all four sulfidation treatments by minimizing the sum of the least squares values and the value is given in Table S5. The rate constant for I2S reduction by untreated nZVI (0.0015 µM–1 min–1) is roughly twice the value for the sulfidated nZVIs (0.00087 µM–1 min–1). Although the apparent rates of I2S disappearance differ significantly for the two doses of dithionite (Figure 3C), they were both well fit by the same value of k″I2S because the initial concentrations of e− for the two treatments were different. This kinetic model was also used to simulate the consumption of reducing equivalents during I2S reduction for the five treatments (black curves in Figure 3). As expected based on the conditions of the model, the availability of reducing equivalents proceeds asymtotically towards complete consumption in all cases. I2S reduction as a function of aging time for the untreated and sulfidated nZVI is presented in Figure 4. The sulfide treatments showed small decreases in the amount of I2S reduced, while the dithionite treatments showed more significant decreases in I2S reduction with aging. Since the acidification data in Figure 2 did not show decreases in H2 recovery during aging for sulfidated nZVI, the decreases in reducing capacity for I2S with aging are likely due to transformation of reactive amorphous FeS to more crystalline iron sulfide phases, which are known to be less reactive and have less reducing capacity.54, 64, 65 As discussed above, the high dithionite-treated nZVI was entirely converted to FeS. Consequently, the amount of I2S reduction by high dithionite-treated nZVI over time was used to estimate the loss of reducing capacity of the FeS due to transformation to other sulfide phases. We chose to fit the data with a first-order transformation model, which gave a rate of 0.067 d-1. Using this value and the Fe(0)/FeS ratios estimated from the H2 data (Table S3), the loss of capacity for the other three sulfidation cases were simulated and are shown as red and green lines in Figure 4. The simple first-order model obviously showed some deviations from the actual experimental data, but it does provide a quantitative estimate of the FeS transformation process,

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which, as will be seen below, has a significant impact on contaminant (TCE) reduction kinetics over time.

Figure 4. Reduction of I2S for untreated and sulfidated samples aged for 0, 7, and 21 days at pH 7.2. (The nZVI suspensions were diluted 50 times for the assay). The dashed horizontal black line represents the theoretical concentration of I2S reduced (107 µM) by 4 mg/L Fe(0) (71 µM).

Reduction of TCE. The results presented in the previous two sections—obtained with the H2 and I2S assays, respectively—clearly show that sulfidation inhibits the corrosion reaction with water. To determine if sulfidated nZVI still retains sufficient reactivity with contaminants to be useful for remediation, we performed a series of experiments similar to those in Figures 1 and 3, but with TCE as a model contaminant. Figure 5A shows that TCE reduction by untreated CMC-nZVI was significant (~40%) over the first two days, but then a plateau developed with no further degradation of TCE. This is due to depletion of Fe(0) by corrosion with water (eq 1), and is consistent with the data in Figure 1A. In contrast, all sulfidated nZVI treatments (Figures 5B and 5C), showed substantial rates of degradation of TCE over longer time periods.

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To describe TCE reduction kinetics for the untreated case (Figure 5A), the numerical model was extended to include two parallel reactions: a first-order reaction of Fe(0) with water and a second-order reaction with TCE (details given the SI). For modeling TCE reduction, the total reducing equivalents (e−) determined from the H2 assay were used as initial condition (Table S4). For the untreated nZVI (Figure 5A), the rate constant for nZVI reaction with water (kʹwater, eq 1) was again taken from the H2 assay results. The two-reaction model (shown as the solid line in Figure 5A) described the TCE disappearance data well. This model was also used to simulate the consumption of reducing equivalents (details in SI), and the result is shown as the dashed line in Figure 5A. The model indicates that more than 95% of the reducing equivalents from untreated CMC-nZVI were consumed by water corrosion, rather than TCE reduction. The data for TCE reduction by sulfidated CMC-nZVI in Figures 5B and 5C show that the reaction kinetics slowed over time, which is likely due to the aging of FeS, as indicated by Figure 4. To model the effects of aging on TCE reduction, we again used two parallel reactions: a second-order model for the reaction between sulfidated nZVI and TCE and the first-order model for the FeS aging used in Figure 4 (details in SI, rate constant = 0.067 d-1). For the second-order model, the initial e–, given in Table S4, was again based on the H2 data. TCE data of all sulfidated cases were globally fit by varying the initial second-order rate constant (kʹʹ0-TCE in eq S18) and the sum of the least-squares fits for all four data sets was minimized. As the colored lines in Figures 5B and 5C show, the parallel-reaction model provided a reasonably good global fit to all of the experimental data with an initial TCE rate constant of 0.051 mM-1 d-1. This suggests that all sulfidated nZVI have similar surface properties that dictate the rate of electron transfer, and the apparent kinetics of TCE reduction, like I2S reduction, is largely affected by the electron equivalents.

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Figure 5. Reduction of TCE (left axis) and e– consumption (right axis) for (A) untreated nZVI, (B) sulfide-treated nZVI, and (C) dithionite-treated nZVI. Reductant concentrations and compositions (Fe(0) vs FeS) varied depending on the treatment and are given in Table S3 in Supporting Information. Experimental conditions are [TCE]0 = 10 mg/L and pH = 7.2. Fitted parameters that define curves are given in Table S5.

Implications. The three assays used in this study (H2, I2S and TCE) represent a complementary suite of tools for characterizing the reducing properties of untreated and sulfidated nZVI. Taken together, they demonstrate that the sulfidation of nZVI provides an effective way to minimize the major side reaction of Fe(0) oxidation by water, thereby preserving more reducing capacity for contaminant degradation. Although CMC-nZVI was the

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only formulation of nZVI used in this study, the observed improvement in nZVI selectivity by sulfidation is expected to be applicable to nZVI or ZVI prepared in other ways. The data also suggest that aqueous sulfide might be a better reagent for sulfidation of nZVI than dithionite because the sulfide preserved significantly more Fe(0), and as a consequence provides greater contaminant degradation after aging. Although the modeling of the suite of reactions involved in the assays represents a simplified view of the underlying complex chemistry, it improves the conceptual understanding of the processes controlling contaminant reduction, such as the effects of FeS aging on the reducing capacity (I2S) and reactivity (TCE) of the sulfidated nZVI. We expect that the same modeling structure could be used both to examine the behaviors of other nZVI formulations and to interpret more complicated experimental conditions (e.g., field performance).

Acknowledgements This material is based on work supported by the Strategic Environmental Research and Development Program of the U.S. Department of Defense, Award Number ER-2308. This report has not been subject to review by SERDP and therefore does not necessarily reflect agency views and no official endorsements should be inferred. The authors thank J.Z. Bandstra for his input on formulation of the kinetic model for aging.

Associated Content (Supporting Information): Additional information on experimental conditions, I2S properties, dithionite chemistry, detailed procedures for modeling I2S and TCE reduction, and summary of reduction rate constants can be found in the Supporting information. This material is available free of charge at http://pubs.acs.org.

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