Optimal Design of Sulfidated Nanoscale Zerovalent Iron for Enhanced

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Optimal design of sulfidated nanoscale zerovalent iron for enhanced trichloroethene degradation Sourjya Bhattacharjee, and Subhasis Ghoshal Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02399 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Environmental Science & Technology

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Optimal design of

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sulfidated nanoscale zerovalent iron

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for enhanced trichloroethene degradation *

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Sourjya Bhattacharjee, Subhasis Ghoshal

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Department of Civil Engineering, McGill University, Montreal, QC H3A 0C3, Canada

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submitted to

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*Corresponding author phone: 514-398-6867; fax: 514-398-7361; email: [email protected]

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ABSTRACT:

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Sulfidated nanoscale zerovalent iron (S-nZVI) has the potential to be a cost-effective remediation agent for a

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wide range of environmental pollutants including chlorinated solvents. Various synthesis approaches have

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yielded S-nZVI consisting of a Fe0 (or Fe0/S0) core and FeS shell, which are significantly more reactive to

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trichloroethene (TCE) than nZVI. However, their reactivity is not as high as palladium-doped nZVI (Pd-

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nZVI). We synthesized S-nZVI by co-precipitation of FeS and Fe0, using Na2S during borohydride reduction

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of FeSO4 (S-nZVIco). This resulted in FeS structures bridging the nZVI core and the surface, as confirmed by

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electron microscopy and X-ray analyses. TCE degradation capacity of up to 0.46 moles TCE/mole Fe0 was

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obtained for S-nZVIco at high S loading, and was comparable to Pd-nZVI but 60% higher than the currently

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most reactive S-nZVI in which FeS only coats the nZVI (S-nZVIpost). The high TCE degradation was due to

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complete utilization of Fe0 (2 e-/mole Fe0) towards formation of acetylene. Although Pd-nZVI yielded 3 e-

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/mole Fe0, TCE degradation was comparable because it reduced acetylene further to ethene and ethane.

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Under Fe0 limited conditions, S-nZVIco TCE degradation rate was 16 times higher than Pd-nZVI (0.5 wt.%

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Pd ), and 90 times higher than S-nZVIpost.

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2 ACS Paragon Plus Environment

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INTRODUCTION

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Nanoscale zerovalent iron (nZVI) has been investigated extensively over the past several years for the

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removal of a wide range of environmental contaminants such as heavy metals, chlorinated solvents,

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pesticides and dyes.1, 2 Potential benefits arising from the nanoscale size, particularly higher contaminant

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removal capacities and feasibility of injection in finer aquifer material containing subsurface zones

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compared to micro-scale ZVI, has led to significant interest in nZVI-based in situ groundwater remediation.

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However, removal of chlorinated organic contaminants such as trichloroethene (TCE) from water by nZVI is

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limited due to its poor selectivity towards TCE and preferential reactions with water that generate

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hydrogen.3,

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improve reactivity of nZVI to TCE by enhancing TCE concentrations near the nZVI surface by sorption to

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activated carbon, and leads to modest increases in TCE degradation rates.5 Doping the nZVI surface with

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noble metals such as palladium (Pd-nZVI) has been effective in improving TCE degradation rates by several

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orders of magnitude, due to the role of Pd0 as an electron conductor and hydrogenation catalyst.2,

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However, major challenges with the use of Pd-nZVI for in situ groundwater remediation are its high cost,

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and policy directives preventing its implementation, such as the precautionary principle in the EU Water

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Framework Directive.8

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Iron sulfide minerals such as iron monosulfide (FeS) are abundant, cheap and have good electron transfer

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properties.9,

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performance for contaminant remediation. The synthesis and use of sulfidated nZVI (S-nZVI) has been

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explored in recent studies and has emerged as an important advancement towards the applicability of nZVI

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for water treatment and environmental remediation. S-nZVI has demonstrated higher reactivity and removal

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capacity towards a wide range of water contaminants such as metals11,

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Composites of activated carbon and nZVI such as Carbo-Iron®, have been developed to

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Functionalization of nZVI with FeS is thus a cost-effective strategy to enhance nZVI

15-18

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, pharmaceuticals13,

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, and

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halogenated organics

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Kim et al.16 reported a 20-fold increase in the TCE degradation rate by S-nZVI compared to that achieved by

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bare nZVI. The S-nZVI was synthesized by adding sodium dithionite as the sulfidation agent during

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borohydride reduction of FeCl3, in a one-pot approach. This approach generates nanoparticles with a mixed

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core of Fe0 and S0 surrounded by a shell of iron oxides and FeS.10, 12, 14, 16 However, the inclusion of dithionite

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suppresses the crystallization of Fe0 and/or oxidizes Fe0, resulting in particles with low Fe0 content and thus

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low TCE reduction capacities.19, 20 The Fe0 depletion provides a constraint on the extent of sulfidation that

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can be achieved using dithionite towards a reactive S-nZVI. Kim et al.16 observed linear increases in the

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pseudo first order TCE degradation rate constants (kobs) by S-nZVI, for dithionite concentrations up to 2.0

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g/L, exceeding which the kobs decreased.

, than pure nZVI.



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Rajajayavel et al.15 observed up to 40-fold increase in the TCE degradation rate by S-nZVI prepared by

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contacting nZVI (synthesized by borohydride reduction of FeSO4) with aqueous solutions of sodium sulfide.

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This two-step synthesis approach results in the formation of nanoparticles with a surface FeS layer and a Fe0

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core.10, 11, 15 Na2S dissociates to form bisulfide ions in water which react with Fe0 to form FeS. However, use

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of high doses of Na2S with the aim to incorporate more sulfur in the nanoparticle retarded the TCE reduction

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rates. Rajajayavel et al.15 reported maximum TCE degradation rates between S/Fe molar ratios of 0.04 to

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0.083 and proposed that the sulfide layer thickness on the Fe surface played a critical role in determining

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TCE degradation rates. The authors also reported that S-nZVI had lower selectivity towards water resulting

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in a 2.5 times lower hydrogen evolution rate compared to nZVI. In an earlier study, Fan et al.11 reported the

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existence of an optimal S/Fe ratio of 0.056 wherein highest reduction and surface precipitation rates for

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pertechnetate (anionic form of 99technetium) were observed. Han et al.21 also suggested the existence of an

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optimum S/Fe ratio and reported that TCE degradation rate constants increased rapidly up to S/Fe ratio of

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0.025 beyond which they plateaued off.

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Despite the benefits demonstrated by sulfidation on reactivity of nZVI to TCE, studies to date have not been

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able to achieve S-nZVI performance close to Pd-nZVI in terms of rates and extents of TCE degradation. For

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instance, Ma et al.19 showed that S-nZVI removed 38% of the initial TCE over a 7-day period whereas Pd-

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nZVI showed 100% removal in 2.5 hours. The authors prepared S-nZVI in a modified one-pot synthesis

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approach by solely employing dithionite for simultaneous reduction and sulfidation of Fe2+ salt. The

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resulting particles primarily consisted of Fe3O4 with low quantities of Fe0, and a shell of iron oxides and iron

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sulfides. While both Pd0 and FeS are electron conductors to TCE, the intrinsic roles of Pd0 (hydrogenation

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catalyst) and FeS (higher selectivity of TCE over water10, 15, 22, 23) are different with respect to improving

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nZVI reactivity. Thus, approaches to further improve S-nZVI reactivity and use in groundwater remediation

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are not obvious, and is a current area of interest.

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Several studies, as discussed above, indicate that at high S/Fe ratios, the TCE degradation rate constants for

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S-nZVI are diminished compared to the optimum. For S-nZVI where the Fe0 has not been depleted, this is

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counter-intuitive because higher amounts of FeS should result in higher reactivity due to increased

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conduction of electrons. Electron microscopy images of S-nZVI reveal the presence of laminar or flake-like

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FeS structures primarily on the outside of the spherical Fe0 particles10-12 suggesting that at high S/Fe ratios,

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there may only be limited contact of FeS to Fe0 . Therefore, we rationalized that by distributing FeS in the

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nanoparticle such that it bridges the core to the exterior can maximize the association between Fe0 and FeS

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and may result in increased TCE degradation rates. In this study we demonstrate that S-nZVI prepared in a

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one-step approach using Na2S as the sulfidation agent yields such an architecture, while maintaining

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sufficient amounts of Fe0. This results in an S-nZVI as reactive as Pd-nZVI, the most reactive form of nZVI. 4 ACS Paragon Plus Environment

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We evaluated the TCE degradation rates, extents, and reaction end products in batch reactors, as well as

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characterized changes in nanoparticle morphology and surface chemistry for S-nZVI synthesized through the

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one-step approach. We compared its reactivity performance against the previously reported S-nZVI

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synthesized through the two-step approach11,

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experiments were carried out under Fe0 limited conditions and Fe0 excess conditions to obtain a more

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complete understanding of the Fe0 reductive capacity.

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MATERIALS AND METHODS

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Chemicals and gases. FeSO4·7H2O (99%), NaBH4 (≥98.5%), Na2S (99%), palladium acetate (99%),

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chloroethenes (vinyl chloride and cis 1, 2- & trans 1, 2- dichloroethene) and hexenes (cis 3- & trans 3-

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≥95%) were obtained from Sigma-Aldrich. Gas standards of ethane, ethene, methane (99%, grade CP) and

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1-, cis-, trans- butene (1000 ppm in N2) were obtained from Scotty Specialty Gases. Acetylene (99%) was

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obtained from Praxair Inc. Methanol (99%) was purchased from Fisher Scientific. Water used in experiments

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was double deionized.

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Synthesis of nZVI. Bare nZVI particles were synthesized using a procedure described previously15 with

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some modifications. Briefly, 1.3 M NaBH4 was added drop-wise at 5 mL/min using a syringe pump to a

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continuously mixed aqueous solution of 0.3 M FeSO4·7H2O under anaerobic conditions followed by mixing

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for 30 minutes. The resulting NZVI suspension was washed with methanol, dried under nitrogen, and stored

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in an anaerobic chamber.

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Synthesis of S-nZVIco. Sulfidated nZVI prepared by co-precipitation of iron sulfides and Fe0 in a one-step

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approach is referred to herein as S-nZVIco, and is distinct from the two-step approach referred to herein as

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post sulfidation (S-nZVIpost) in which sulfidation occurs after nZVI formation.

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In the co-precipitation approach, aqueous solutions of NaBH4 (1.7 M) and Na2S (between 0.026 M to 0.64

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M) were mixed together and then added drop-wise over a time period of 45 min, at 5 mL/min to a

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continuously mixed aqueous solution of 0.3 M FeSO4·7H2O under anaerobic conditions, followed by 30

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minutes of mixing. The resulting particles termed as S-nZVIco, were washed with methanol and dried in the

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anaerobic chamber for 12 hours and then stored in the chamber in sealed vials until use. The sulfur

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incorporated into the particles was measured and were 0.035, 0.05, 0.07, 0.1, 0.2, 0.3 and 0.4 as mole

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fractions of S/Fe.

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Synthesis of S-nZVIpost. The S/Fe mole ratios of S-nZVIco served as the molar dosing ratios ([S/Fe]dosed) in

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the post-sulfidation approach. S-nZVIpost was prepared according to Rajajayavel et al.15 wherein, nZVI

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particles were first prepared separately using the nZVI synthesis method described above, and then

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as well as with Pd-nZVI and unamended nZVI. The



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sulfidated directly in the vials used for TCE reaction experiments. This was done by adding different doses

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of aqueous Na2S solutions to the nZVI suspensions equivalent to [S/Fe]dosed, followed by sonication of the

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vials in a bath sonicator for 15 min. The uptake of sulfur by the particles was also measured. As discussed

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later, complete uptake of sulfur did not take place, and S-nZVIpost suspensions contained undeposited

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bisulfide ions.

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Palladium doped nZVI (Pd-nZVI). An ethanolic solution of palladium acetate, at 1 wt. % of nZVI, was

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added directly to nZVI suspensions in TCE reaction vials and sonicated for 15 min. All the Pd added was

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deposited on the particles.

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Reactivity studies. TCE degradation experiments were carried out in 60 mL vials crimp-sealed with butyl

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rubber septa and samples were prepared in the anaerobic chamber and all solutions were purged with

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nitrogen prior to handling. 100 µL TCE was added to reaction vials containing a total aqueous volume of

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24.9 mL and 40 mg of nanoparticles.

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Upon addition of TCE, a small globule of NAPL was formed. This provided a constant saturated aqueous

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solution of TCE throughout the duration of the reactivity experiments. No measurable changes in the

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headspace TCE concentrations occurred in the reactors, due to the saturated TCE solution and

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stoichiometrically limited amounts of Fe0 compared to TCE. Thus TCE disappearance was tracked by

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quantifying TCE degradation products with time and using a carbon mass balance approach as described in

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our previous study.24 Calibration standards were prepared by adding known quantities of the gas standard in

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the reactors set-up exactly like the reactivity systems.

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Analytical methods. The mass of sulfur deposited on nZVI particles was measured using an ICP-OES

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(PerkinElmer Optima 8300). The limit of detection for sulfur and iron was 25 ppb and 10 ppb measured at

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wavelengths of 181 nm and 259 nm respectively. The nanoparticles were separated from solution using

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centrifugation followed by magnetic separation and then the nanoparticles and supernatant were separately

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acid digested in aqua regia (3:1 HCl: HNO3).

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TCE degradation products were quantified periodically by injection of 300 µL reactor headspace into a gas

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chromatograph (GC-FID and GC-MS). Details on GC, X-ray photoelectron spectroscopy (XPS) and

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transmission electron microscopy coupled with energy dispersive spectroscopy (TEM-EDS) instrumentation

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have been provided previously.24 Prior to measurement with XPS, S-nZVI samples were dried in an

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anaerobic chamber. Before being analyzed with TEM, a drop of S-nZVI suspension was directly placed on

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copper TEM grids and excess water was removed using a tissue. Fe0 content was measured using acid

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digestion protocol mentioned elsewhere.25 The nanoparticles were acid digested in HCl and the liberated H2

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gas was measured using a GC-TCD. Size and surface charge of nanoparticle suspensions were measured in a 6 ACS Paragon Plus Environment

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Zetasizer Nano (Malvern). Nanoparticle suspensions were prepared at concentration of 150 ppm in 10 mM

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NaHCO3 (pH 8.2), sonicated for 15 seconds and immediately analyzed to obtain a hydrodynamic diameter

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(cumulant Z-average size) and zeta potential, with an acquisition time of 60 seconds. The particles were in

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suspension throughout the duration of analysis. The refractive index of nanoparticles was set at 2.87.26

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RESULTS AND DISCUSSION

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Morphology of S-nZVIco is different from S-nZVIpost. A detailed comparison of the characteristics in

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morphologies and surface chemistry of S-nZVIco and S-nZVIpost was carried out using TEM-EDS and XPS.

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The representative TEM images of S-nZVIco and S-nZVIpost are shown in Figures 1a and 1b, respectively.

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Overall, four distinct morphologies were observed in the images; needle and plate-like structures which were

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often observed together (Type I), spherical particles in the size range of 100-150 nm with a rough surface

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(Type II), small irregular particles in the size range of 20-30 nm (Type III), and particles in the size range of

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50-200 nm with well-defined spherical boundaries (Type IV). S-nZVIco was abundant in Type I, Type II and

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Type III morphologies whereas Type IV was rarely observed. In contrast, S-nZVIpost was primarily

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composed of Type I and Type IV morphologies.

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Figures 1a1-1a4 are blown up images of sections in Figure 1 that highlight the different morphologies

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observed for S-nZVIco. As seen in Figures 1a1 and 1a2, Type I morphology consisted of laminar structures

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which resembled a mix of needles and thin plates. Type I structures also appeared to be embedded within the

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Type II particles. EDS analyses of Type I structures generated S signals along with Fe suggesting that they

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were likely iron sulfides (Figure S1). Relatively small O signals were also present. Previous studies have

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attributed the laminar shapes to FeS.11, 20, 27 Moreover, pure nano-FeS also shows a similar structure.28 XPS

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analysis of S-nZVIco (Figure S2 and Table S1) showed predominance of FeS and presence of polysulfides

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and sulfates to a lesser extent. This suggests that the Type I structures are FeS. The speciation of S-nZVIco

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did not change at different [S/Fe]dosed(Table S1). Figure 1a2 shows the Type II morphology observed for S-

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nZVIco and these were spherical particles with a granular appearance and surface roughness. EDS analysis of

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the particle surface provided signals for Fe and S. The surface roughness of Type II particles may have been

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caused due to the entrapment of Type I FeS structures. Figure 1a3 shows the Type III morphology which

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were 20-30 nm irregularly shaped particles, the EDS for which provided Fe and S signals. The Type III

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particles were observed to be enmeshed with Type I structures, usually at higher [S/Fe]dosed (Figure S1). The

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presence of the smaller Type III particles likely resulted in the small increases in the BET surface area for S-

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nZVIco with increasing [S/Fe]dosed (Table S2). Overall, the location of FeS structures were observed both

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within and on the surface of the particles. In addition, the FeS structures formed extensive mesh like

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networks with increasing [S/Fe]dosed (Figure S1), suggesting that multiple Fe0 rich sites may have been 7 ACS Paragon Plus Environment


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bridged together in aqueous suspension, maximizing Fe0 and FeS contact. The structure of S-nZVIco is

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distinct from the morphology of S-nZVI prepared in a one-pot approach using dithionite, wherein a defined

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spherical core comprised of Fe0 and S0 was observed, with FeS flakes only on the outer shell for the latter10,

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.

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In contrast, S-nZVIpost particles were abundant in Type IV morphology. The particles generally had a smooth

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spherical particle edge and were also observed to be associated with Type I FeS structures. However, the

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FeS structures were located primarily on the outer surface of the spherical particles (Figure S3) unlike that

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observed for S-nZVIco. XPS analysis of S-nZVIpost primarily showed the presence of FeS (Figure S4). Unlike

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the XPS spectra for S-nZVIco (Figure S2), peaks for polysulfide or sulfate species in S-nZVIpost were not

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observed. nZVI and Pd-nZVI employed in this study have been thoroughly characterized elsewhere.24 Both

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nZVI and Pd-nZVI consisted of spherical particles with primary particle sizes in the range of 20-100 nm.

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Analysis of aggregate size and surface charge of S-nZVIco and S-nZVIpost was also carried out at two

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[S/Fe]dosed as shown in Table S3. S-nZVIco was present as smaller aggregate sizes than S-nZVIpost. For

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example, at [S/Fe]dosed of 0.4, the mean aggregate diameter of S-nZVIco was 1.3±0.1 µm (PDI = 0.38),

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whereas for S-nZVIpost, it was 4.4±0.4 µm (PDI = 0.5). The aggregate sizes of nZVI and Pd-nZVI were

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similar at 7.2±0.8 µm (PDI = 0.7) and 7±0.5 µm (PDI = 0.6) respectively. We also observed that the zeta

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potential of S-nZVIpost was higher (-37.1 mV at [S/Fe]dosed of 0.4) than S-nZVIco (-11.3 mV at [S/Fe]dosed of

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0.4), nZVI (-6 mV) and Pd-nZVI (-6.5 mV). The differences could likely be related to the differences in the

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surface characteristics, particularly the amounts and locations of sulfide and oxides deposited.

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TCE degradation capacity of S-nZVIco is similar to Pd-nZVI and higher than S-nZVIpost. The extent of

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TCE degradation achieved by S-nZVI is an important benchmark for assessing its remediation performance.

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We evaluated the specific degradation capacity (defined in Equation 1) of nZVI, Pd-nZVI, S-nZVIco and S-

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nZVIpost for TCE degradation and they are presented in Figure 2a. It should be noted that Pd-nZVI particles

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did not contain any sulfur and their results are shown along with S-nZVI for comparison only.

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, =

!"!#! $ %

(1)

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The SDC was estimated by Equation 1, based on data from systems employing stoichiometrically excess

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TCE mass compared to Fe0, for time points where no further TCE degradation was observed in each of the

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reaction systems (Figure S5).


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Figure 2a demonstrates that pure nZVI ([S/Fe]dosed = 0) achieved a low SDC of 0.01. This was likely due to

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the preferential reactions of nZVI with water over TCE. Although nZVI only dechlorinated 5 µmoles of TCE

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over 8 days during the degradation studies (Figure 2a, Figure S5c), we measured a total of 57 µmoles of H2

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in the reactor in the same time period, suggesting reaction of nZVI with water was dominant compared to

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reaction with TCE.

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However as the [S/Fe]dosed increased from 0.035 to 0.1, the SDC of S-nZVIco increased substantially from

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0.08±0.01 to 0.36±0.02. The increase in SDC was smaller between [S/Fe]dosed of 0.1 to 0.4, with a maximum

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SDC of 0.46±0.02 being achieved at [S/Fe]dosed of 0.4. Conversely, S-nZVIpost achieved a maximum SDC of

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only 0.28±0.02 within a similar range of [S/Fe]dosed. Pd-nZVI exhibited an SDC of 0.45±0.02 and varying the

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Pd loading between 0.5 to 5 wt.% Pd did not significantly alter the SDC value (Figure S5c).

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Among the three major TCE degradation products reported for Pd-nZVI and S-nZVI (acetylene, ethene and

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ethane), a theoretical maximum SDC (SDCtheor,max) can be achieved when TCE is converted to the least

240

saturated product, i.e. acetylene. If 2 electrons per mole of Fe0 are available (Fe0 → Fe2+ + 2e-) and

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acetylene is the major end product, then 2 moles of Fe0 are required to convert 1 mole TCE to acetylene

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(& '() + 4 , + ' - → & '& + 3( , ), providing a SDCtheor,max of 0.5. However, if 3 electrons per mole of

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Fe0 are available (Fe0 → Fe2+ + 2e- ; Fe2+ → Fe3+ + e-), then 1.33 moles of Fe0 are required to convert 1 mole

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TCE to acetylene, providing a SDCtheor,max of 0.75.

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In our previous study we observed that Pd-nZVI could provide up to 3 electrons per mole of Fe0 to

246

dechlorinate TCE.24 Therefore, as a first step towards understanding the differences in SDC observed in this

247

study, we analyzed the electrons available per mole of Fe0 for TCE degradation in the case of Pd-nZVI, S-

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nZVIco and S-nZVIpost. This involved first the identification and quantification of the moles of end products

249

formed in the TCE degradation experiments (Table S4-S6). Subsequently, the electrons needed for formation

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of those end products were calculated from stoichiometry. Thereafter, summation of electrons from the end

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products yielded the total electrons used in the conversion of TCE by the nanoparticles. The total electrons

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were then employed in electron balance calculation (Table S7), which confirmed that Pd-nZVI provided 3

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electrons per mole of Fe0 for TCE dechlorination, whereas S-nZVIco and S-nZVIpost had 2 electrons per mole

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of Fe0 available. The electron balance on average was ≥90%.

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Despite the availability of lower number of electrons, S-nZVIco achieved a maximum SDC similar to Pd-

256

nZVI, between [S/Fe]dosed of 0.3 and 0.4, as seen in Figure 2a. We thus compared the end products generated

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by Pd-nZVI and S-nZVIco to better understand how the available electrons were being utilized in the

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degradation process. As shown in Figure 2b, Pd-nZVI primarily transformed TCE to ethene and ethane,

259

which accounted for 80% of the total dechlorination products. Whereas S-nZVIco degraded TCE 9 ACS Paragon Plus Environment


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predominantly to acetylene (97% of total dechlorination products). Formation of ethene and ethane require 6

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and 8 electrons per mole of Fe0 respectively, whereas acetylene requires only 4 electrons. Thus, the

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generation of acetylene by S-nZVIco enabled it to achieve an SDC equivalent to that of Pd-nZVI despite

263

having fewer electrons available for TCE dechlorination. Thus, S-nZVIco achieves a maximum experimental

264

SDC (0.46) close to SDCtheor,max (0.5). However the experimental maximum SDC for Pd-nZVI (0.45) is 40%

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lower than the SDCtheor,max (0.75). This was due to the inefficient electron distribution among the degradation

266

products by Pd-nZVI. Other than ethene and ethane, Pd-nZVI generates several byproducts including

267

dichlorethenes, which maybe undesirable in TCE degradation applications.

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In contrast, S-nZVIpost did not achieve an SDC equivalent to S-nZVIco despite a similar availability of 2

269

electrons per mole of Fe0 and formation of similar degradation products. Additionally, the SDC for both S-

270

nZVIco and S-nZVIpost was dependent on the [S/Fe]dosed. For S-nZVIco and S-nZVIpost, FeS plays a critical

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role in the shuttling of electrons from Fe0 to halogenated contaminants at the surface.14,

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hypothesized that the amount of FeS associated with S-nZVIco and S-nZVIpost was different at different

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[S/Fe]dosed which influenced the ability of the nanoparticles to release the available electrons per mole of Fe0

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for TCE degradation. Therefore we measured the particle S/Fe ratio ([S/Fe]particle) to estimate the amounts of

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FeS associated with S-nZVIco and S-nZVIpost at different [S/Fe]dosed. As shown in Figure 2c, S-nZVIpost

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showed a limited uptake capacity for sulfur over the large range of [S/Fe]dosed employed. Relative to S-

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nZVIco, the [S/Fe]particle values of S-nZVIpost were much lower. For instance, at the [S/Fe]dosed of 0.4, the

278

[S/Fe]particle for S-nZVIpost was only 0.05 whereas for S-nZVIco it was 0.4. This also implies that the S-

279

nZVIpost suspensions contained the remainder of the undeposited bisulfide ions, whereas the S-nZVIco

280

suspensions did not. The [S/Fe]particle for S-nZVIpost did not change between 15 min to 12 hrs of contact time

281

between nZVI and the Na2S solutions. Thus, the significantly lower amounts of sulfur associated with S-

282

nZVIpost is likely to have caused lower SDC in comparison to S-nZVIco. Furthermore by plotting the SDC’s

283

as a function of [S/Fe]particle (Figure 2d), S-nZVIpost and S-nZVIco displayed similar SDC values upto

284

[S/Fe]particle of 0.05, validating the hypothesis that amount of FeS associated with Fe0 influenced the SDC.

285

Increasing the [S/Fe]particle allowed for higher TCE transformation extents due to higher amounts of electrons

286

being extracted from the nanoparticle via FeS.

287

The difference in the amounts of sulfur in S-nZVIco and S-nZVIpost can be attributed to the distribution

288

pattern of FeS. As is seen in Figure 1b, FeS structures are associated with the S-nZVIpost particles, primarily

289

on the outer surface. The availability of FeS deposition sites (Fe0 and Fe-oxide sites) being limited to the

290

outer surface of S-nZVIpost likely caused the limited sulfur uptake.11 In contrast, for S-nZVIco FeS

291

precipitation occurs along with formation of Fe0 during the nanoparticle nucleation and growth phase. This

292

allows association of higher amounts of FeS with S-nZVIco through incorporation within the nanoparticles as

15, 18

We thus


Page 11 of 24


293

well formation of FeS structures around the particles. We observed that with increases in the [S/Fe]dosed from

294

0 to 0.4, the initial Fe0 content (moles of initial Fe0 per mole of Fe) for S-nZVIco decreased from 0.82 to 0.4

295

respectively (Figure S5d). This is expected due to the stoichiometric consumption of Fe0 by HS- during FeS

296

formation. Interestingly, the initial Fe0 content of S-nZVIpost particles remained relatively unchanged for the

297

different [S/Fe] doses. This could be a result of the small [S/Fe]particle achieved by S-nZVIpost (Figure 2c) as

298

well as the replacement of only the surface Fe-oxides by FeS. Despite a lower Fe0 content, S-nZVIco is much

299

more reactive than S-nZVIpost. For instance, at [S/Fe]dosed of 0.4, S-nZVIco has an initial Fe0 content of 0.4

300

compared to S-nZVIpost which has 0.82. However S-nZVIco achieved an SDC of 0.46 compared to 0.28 of S-

301

nZVIpost. This demonstrates that Fe0 content or the sulfide dose alone does not determine TCE degradation

302

capacity and highlights the benefit of a rational particle design approach, wherein creating particle

303

architectures that maximize association between Fe0 and FeS can facilitate complete utilization of the nZVI

304

reductive power.

305

In a recent study, Gu et al.29 investigated the reactivity of sulfidated microscale zerovalent iron (S-mZVI) to

306

TCE. The authors reported that at [S/Fe]dosed of 0.1, the Fe0 in S-mZVI was completely consumed during

307

degradation reactions with TCE providing a maximum SDC of 0.39. Additionally, cis-DCE accounted for

308

16% of the total dechlorination products formed, and S-mZVI generated hydrogen during reactions with

309

TCE which accounted for 10% of the Fe0 consumption. Mackenzie et al.5 investigated the TCE degradation

310

extent of an nZVI and activated carbon composite, and an SDC of 0.2 was estimated from their study based

311

on the degradation products (mainly ethene) identified. In an earlier study, Liu et al.25 reported SDC’s of two

312

different nZVI, one containing a Fe-boron alloy which was catalytically active like bimetallic nZVI, and the

313

other being the commercially available RNIP which contained reduced sulfur species.30 Both particles

314

exhibited similar SDC’s of 0.25.

315

S-nZVIco has higher rates of TCE degradation than Pd-nZVI and S-nZVIpost. Along with quantifying

316

the extents of TCE degradation, we also evaluated the rates of degradation for each type of nanoparticle. The

317

TCE degradation profile over time was best fitted with a pseudo first order rate law (Figure S5 a and b) as

318

shown in the following integrated rate equation.

319

0# = 0 + 102 − 0 4 ,5678 #

320

In Equation 2, Mt is the moles of TCE in the reactor at any time t, M0 is the initial moles of TCE in the

321

reactor, Me is the moles of TCE in the reactor at the end of the degradation reaction, and kobs is the observed

322

pseudo-first-order TCE degradation rate constant (h-1). The observed rate constant was then normalized by

323

the particle BET surface areas (m2/g) and Fe0 mass concentrations (g/L) to obtain ksa presented in Figure 3.

(2)



Page 12 of 24

324

As seen in Figure 3, the ksa for S-nZVIco showed a rapid increase from 0.61×10-3 to 23.06×10-3 L m-2 h-1

325

between [S/Fe]dosed of 0.035 and 0.4, respectively. The increase in rate constants can be attributed to the

326

increasing association of FeS and Fe0 with increasing [S/Fe]dosed. Electron conduction by iron oxides and iron

327

polysulfides deposited on nZVI is less efficient compared to electron conduction by FeS

328

the rapid increase in TCE degradation rate constants observed for S-nZVIco, the contribution of iron oxides

329

and iron polysulfides to the overall ksa is likely negligible. FeS has also been reported to be more water-

330

wetting (less hydrophobic) compared to iron oxides 28 and thus, the observed rate constants are unlikely to be

331

an effect of increased sorption of TCE to FeS. The rapid rise in the ksa from 0.2 to 0.4 is likely related to the

332

increase in the FeS structures embedded in and present around the particles (Figure 1a) that substantially

333

improve electron conduction to TCE molecules at the surface. The ksa for Pd-nZVI with 0.5 wt.% Pd in

334

Figure 3, was 1.43×10-3 L m-2 h-1 which was comparable to degradation rates of S-nZVIco between [S/Fe]dosed

335

of 0.05 and 0.1. By increasing the Pd content to 2.5% and 5%, we observed a linear increase in the ksa with it

336

being 16×10-3 L m-2 h-1 at 5 wt.% Pd (Figure S6). This suggests that under Fe0 limited conditions, the ksa was

337

limited by the number of Pd sites.33 Thus, in order to achieve rate constants comparable to S-nZVIco at

338

[S/Fe]dosed greater than 0.1, higher loadings of Pd are needed on nZVI which can significantly add to the

339

costs of remediation.

340

The ksa for S-nZVIpost and S-nZVIco were similar between [S/Fe]dosed of 0.035 and 0.1. However between

341

[S/Fe]dosed of 0.1 and 0.4, the ksa of S-nZVIpost decreased from 1.03×10-3 to 0.26×10-3 L m-2 h-1. This decrease

342

in degradation rates was likely due to increased oxide growth near the particle surface (discussed in the

343

section below). Because the location of FeS is at the particle surface in S-nZVIpost, the growth of more

344

oxides at the surface likely slows down the efficiency of electron conduction, thereby retarding the

345

degradation kinetics. Additionally between [S/Fe]dosed of 0.1 and 0.4, a significant amount of HS- remained

346

dissolved in the aqueous suspension (Figure 2b), which may have also contributed to the slowing down of

347

TCE degradation rate.11

348

We also evaluated the performance of the nanoparticles under Fe0 excess conditions. As seen in Figure S7,

349

the ksa values for S-nZVIco increased from 0.34×10-3 L m-2 h-1 at [S/Fe]dosed of 0.035, to 3.46×10-3 L m-2 h-1 at

350

[S/Fe]dosed of 0.2 and did not change thereafter likely due to the attainment of optimal number of FeS binding

351

sites for the limited mass of TCE employed. S-nZVIpost displayed an optimum ksa of 1.98×10-3 L m-2 h-1 at

352

[S/Fe]dosed of 0.1. Pd-nZVI (0.5 wt.% Pd) had a ksa of 79.61 ×10-3 L m-2 h-1 which was 56 times higher than

353

that under Fe0 limited conditions. Under Fe0 excess conditions, the rapid reaction kinetics of Pd-nZVI with

354

water to produce hydrogen (shown in Figure 4) likely resulted in rapid hydrogenation of TCE and high

355

dechlorination rate.

29, 31, 32

, and given


Page 13 of 24


356

S-nZVIco does not produce H2 during TCE degradation but its reactivity to pure water is higher than

357

S-nZVIpost. An important observation from the reactivity studies under Fe0 limited conditions was the

358

absence of hydrogen generation during TCE degradation reactions, for S-nZVIco, S-nZVIpost and Pd-nZVI.

359

Hydrogen is produced by the corrosion of Fe0 by water and amendment of nZVI with Pd or S can influence

360

its reactivity to water. Previous studies investigating the hydrogen generation rate of S-nZVI particles

361

(synthesized using the two-step approach) reported the suppression of reaction rates with water.15,

362

Therefore we conducted separate kinetic experiments with nZVI, Pd-nZVI, S-nZVIco and S-nZVIpost to

363

evaluate their hydrogen generation ability in pure water in the absence of TCE (Figure S8).

364

As shown in Figure S8, H2 evolution profiles of S-nZVIco and S-nZVIpost followed zero order kinetics.

365

However we observed that Pd-nZVI and nZVI showed deviations from the zero order kinetics after 24 h and

366

60 h respectively. Pd-nZVI particles generated large amounts of H2 within 24 hrs and likely deviated from

367

zero order kinetics due to rapid depletion of Fe0 as it reached the stoichiometric maximum. In the case of

368

nZVI particles, the deviation from zero order kinetics was likely due to the effect of progressively thickening

369

surface oxides which slowed down electron conduction. Therefore, in order to draw a comparison with the

370

zero-order kinetics displayed by S-nZVIpost and S-nZVIco, we used the initial zero-order rate constants for

371

Pd-nZVI (estimated from time points up to 24 h) and nZVI (estimated from time points up to 60 h). Their

372

surface area normalized rate constants (9:; ) were computed by normalizing the observed rate constants with

373

BET surface areas and Fe0 mass concentrations and are presented in Figure 4.

374

As seen in Figure 4, the 9:; for nZVI was 0.054 µmoles L m-2 h-1 while at [S/Fe]dosed of 0.035, the 9:; for S-

375

nZVIco was 0.035 µmoles L m-2 h-1. A possible explanation for the higher 9:; of nZVI compared to S-

376

nZVIco is the higher affinity of Fe-oxides than FeS, to water.15 However by increasing the [S/Fe]dosed, the

377

number of electron conduction sites in S-nZVIco increase due to higher amounts of FeS, which increases the

378

9:; to 0.071 µmoles L m-2 h-1 at [S/Fe]dosed of 0.4. Similar to reactivity with TCE, S-nZVIpost displayed a

379

lower 9:; (0.019 to 0.023 µmoles L m-2 h-1) compared to S-nZVIco. This was likely due to lower amounts of

380

FeS (Figure 2c) and lower accessibility of electrons due to the FeS distribution pattern (Figure 1b).

381

Overall, H2 generation rates of S-nZVIpost in contact with pure water was lower than nZVI, in the range of

382

[S/Fe]dosed investigated. This is in good agreement with prior studies.15,

383

comparable or slightly higher H2 generation rates than nZVI, at higher [S/Fe]dosed.

384

Morphology of S-nZVIco after reaction with TCE: The oxidized nanoparticles were retrieved after their

385

reaction with TCE, in order to characterize changes in their surface chemistry and morphology. The

386

morphological changes are shown in TEM images in Figures 5a and 5b. As seen in the figures, the particles

387

looked significantly different at the end of their reaction lifetime. For the oxidized S-nZVIco (Figure 5a) the

21

21, 22

In contrast, S-nZVIco showed



Page 14 of 24

388

plate like precipitates, provided high O signals alongwith Fe and S in the EDS analysis, suggesting that they

389

were a mixture of iron oxides and sulfides (Figure S9 a). XPS spectra (Figure S9 c and d) revealed the

390

presence of higher amounts of polysulfide species after reaction with TCE compared to unreacted S-nZVIco.

391

The scattered distribution of the precipitates for S-nZVIco suggests the outgrowth of oxides away from the

392

particle surface. In the case of S-nZVIpost (Figure 5b), spherical shapes were discernible along with some

393

hollowed out particles which are attributable to the Kirkendall effect.11 The nanoparticle surfaces appeared

394

to be covered with a thick mesh of needle like structures which were a mix of iron oxides and sulfides

395

(Figure S9 b). It is likely that the presence of FeS structures at the surface of S-nZVIpost behaved as a surface

396

coating which caused restricted outgrowths of the oxides. XPS analysis revealed peaks corresponding to FeS

397

and polysulfides (Figure S9 e and f). The Pd-nZVI particles also had outgrowth of oxides in the absence of

398

any restricting surface coatings as shown in our previous study.24

399

Implications

400

To our knowledge, this is the first study that demonstrates S-nZVI reactivity with TCE comparable to Pd-

401

nZVI under Fe0 limited conditions. Fe0 limited (TCE excess) conditions can be expected at source zones

402

where TCE NAPL is present. Although the mechanism of TCE degradation facilitated by Pd0 is different

403

from FeS, the results from this study highlight that in composite nanoparticles, structural attributes can play

404

as significant a role in enhancing reactivity, as chemical composition. In scenarios where contaminant mass

405

is low (Fe0 excess), Pd-nZVI exhibits high TCE dechlorination rates. However Pd-nZVI also undergoes

406

rapid reactions with water to produce hydrogen, which consumes Fe0. This may add considerably to

407

treatment costs in field operations wherein the poorly mixed subsurface conditions can result in rapid loss of

408

Pd-nZVI reductive capacity through reactions with water without efficiently treating the contaminant. This

409

may lead to requirement of frequent re-injections of Pd-nZVI suspensions resulting in undesirable

410

consequences such as decreased aquifer porosity. Conversely, S-nZVIco exhibits lower reactivity to water

411

and higher selectivity to TCE which may make it more suitable for field applications. They are also more

412

reactive than S-nZVIpost.

413

Prior studies have demonstrated that sulfidation of nZVI enables degradation of other recalcitrant chlorinated

414

contaminants such as 1,2 DCA over extended periods of time17. Thus, further research investigating the

415

effectiveness of S-nZVIco in treating other challenging chlorinated ethenes and ethanes are required to better

416

understand its reactivity potential.

417

For in situ remediation there are other important considerations beyond reactivity. For example, effective

418

injection and transport of nanoparticles in the sub-surface requires that they be colloidally stable. S-nZVIco

419

has higher surface charge and lower aggregate sizes compared to nZVI suggesting that it has good transport


Page 15 of 24


420

potential. Our recent studies show that dissolved organic matter at environmentally relevant concentrations is

421

unlikely to influence the reactivity of S-nZVI.34 Furthermore, previous studies demonstrated that

422

groundwater composition only has a limited effect on the reactivity of S-nZVI.15, 35 A cost analysis based on

423

prices of reagents (high purity chemicals) shows that at comparable dechlorination rates and extents, the cost

424

of degradation of a unit mass of TCE using Pd-nZVI is 70% higher than that using S-nZVIco (Table S8).

425

Thus, S-nZVIco appear to be cost–effective agents for TCE degradation, particularly for source zones.

426

Supporting Information Available

427

TEM-EDS and XPS of S-nZVIco and S-nZVIpost before and after reaction with TCE; BET surface areas; TCE

428

degradation profiles; TCE degradation end product quantification; electron balance calculations; hydrogen

429

evolution profiles. This information is available free of charge via the Internet at http://pubs.acs.org.

430

Acknowledgement

431

The research was funded by the Natural Sciences and Engineering Research Council of Canada (Grant

432

Numbers RGPIN-2016-05022, RGPAS-492998-16, I2IPJ-501965-16). We thank Josianne Lefebvre

433

(Polytechnique Montreal) and David Liu (McGill) for assistance with XPS and TEM-EDS analyses,

434

respectively.

435 436 437 438 439

References

440 441 442 443 444 445 446 447 448 449 450 451 452

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6. Lien, H.-L.; Zhang, W.-x., Nanoscale iron particles for complete reduction of chlorinated ethenes. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2001, 191, (1-2), 97-105. 7. Sakulchaicharoen, N.; O'Carroll, D. M.; Herrera, J. E., Enhanced stability and dechlorination activity of pre-synthesis stabilized nanoscale FePd particles. Journal of contaminant hydrology 2010, 118, (3-4), 117-127. 8. EU, Water Framework Directive 2000/60/EC. 9. Pearce, C. I.; Pattrick, R. A.; Vaughan, D. J., Electrical and magnetic properties of sulfides. Reviews in Mineralogy and Geochemistry 2006, 61, (1), 127-180. 10. Fan, D.; Lan, Y.; Tratnyek, P. G.; Johnson, R. L.; Filip, J.; O’Carroll, D. M.; Nunez Garcia, A.; Agrawal, A., Sulfidation of iron-based materials: A review of processes and implications for water treatment and remediation. Environmental Science & Technology 2017, 51, (22), 13070-13085. 11. Fan, D.; Anitori, R. P.; Tebo, B. M.; Tratnyek, P. G.; Lezama Pacheco, J. S.; Kukkadapu, R. K.; Engelhard, M. H.; Bowden, M. E.; Kovarik, L.; Arey, B. W., Reductive sequestration of pertechnetate (99TcO4–) by nano zerovalent iron (nZVI) transformed by abiotic sulfide. Environmental science & technology 2013, 47, (10), 5302-5310. 12. Su, Y.; Adeleye, A. S.; Keller, A. A.; Huang, Y.; Dai, C.; Zhou, X.; Zhang, Y., Magnetic sulfide-modified nanoscale zerovalent iron (S-nZVI) for dissolved metal ion removal. Water research 2015, 74, 47-57. 13. Song, S.; Su, Y.; Adeleye, A. S.; Zhang, Y.; Zhou, X., Optimal design and characterization of sulfidemodified nanoscale zerovalent iron for diclofenac removal. Applied Catalysis B: Environmental 2017, 201, 211-220. 14. Cao, Z.; Liu, X.; Xu, J.; Zhang, J.; Yang, Y.; Zhou, J.; Xu, X.; Lowry, G. V., Removal of antibiotic florfenicol by sulfide-modified nanoscale zero-valent iron. Environmental science & technology 2017, 51, (19), 11269-11277. 15. Rajajayavel, S. R. C.; Ghoshal, S., Enhanced reductive dechlorination of trichloroethylene by sulfidated nanoscale zerovalent iron. Water research 2015, 78, 144-153. 16. Kim, E.-J.; Kim, J.-H.; Azad, A.-M.; Chang, Y.-S., Facile synthesis and characterization of Fe/FeS nanoparticles for environmental applications. ACS applied materials & interfaces 2011, 3, (5), 1457-1462. 17. Nunez Garcia, A.; Boparai, H. K.; O’Carroll, D. M., Enhanced dechlorination of 1, 2-dichloroethane by coupled nano iron-dithionite treatment. Environmental science & technology 2016, 50, (10), 5243-5251. 18. Li, D.; Mao, Z.; Zhong, Y.; Huang, W.; Wu, Y.; Peng, P. a., Reductive transformation of tetrabromobisphenol A by sulfidated nano zerovalent iron. Water research 2016, 103, 1-9. 19. Ma, X.; He, D.; Jones, A. M.; Collins, R. N.; Waite, T. D., Reductive reactivity of borohydride-and dithionite-synthesized iron-based nanoparticles: A comparative study. Journal of hazardous materials 2016, 303, 101-110. 20. Su, Y.; Adeleye, A. S.; Huang, Y.; Zhou, X.; Keller, A. A.; Zhang, Y., Direct synthesis of novel and reactive sulfide-modified nano iron through nanoparticle seeding for improved cadmium-contaminated water treatment. Scientific reports 2016, 6, 24358. 21. Han, Y.; Yan, W., Reductive Dechlorination of Trichloroethene by Zero-valent Iron Nanoparticles: Reactivity Enhancement through Sulfidation Treatment. Environmental science & technology 2016, 50, (23), 12992-13001. 22. Fan, D.; O’Brien Johnson, G.; Tratnyek, P. G.; Johnson, R. L., Sulfidation of nano zerovalent iron (nZVI) for improved selectivity during in-situ chemical reduction (ISCR). Environmental science & technology 2016, 50, (17), 9558-9565. 23. Li, J.; Zhang, X.; Sun, Y.; Liang, L.; Pan, B.; Zhang, W.; Guan, X., Advances in Sulfidation of Zerovalent Iron for Water Decontamination. Environmental science & technology 2017, 51, (23), 13533-13544. 24. Bhattacharjee, S.; Ghoshal, S., Phase transfer of palladized nanoscale zerovalent iron for environmental remediation of trichloroethene. Environmental science & technology 2016, 50, (16), 86318639. 16 ACS Paragon Plus Environment

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25. Liu, Y.; Majetich, S. A.; Tilton, R. D.; Sholl, D. S.; Lowry, G. V., TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environmental science & technology 2005, 39, (5), 1338-1345. 26. Fatisson, J.; Ghoshal, S.; Tufenkji, N., Deposition of carboxymethylcellulose-coated zero-valent iron nanoparticles onto silica: roles of solution chemistry and organic molecules. Langmuir 2010, 26, (15), 12832-12840. 27. Shi, X.; Sun, K.; Balogh, L. P.; Baker Jr, J. R., Synthesis, characterization, and manipulation of dendrimer-stabilized iron sulfide nanoparticles. Nanotechnology 2006, 17, (18), 4554. 28. Zhang, Y.; Zhi, Y.; Liu, J.; Ghoshal, S., Sorption of Perfluoroalkyl Acids to Fresh and Aged Nanoscale Zerovalent Iron Particles. Environmental science & technology 2018, 52, (11), 6300-6308. 29. Gu, Y.; Wang, B.; He, F.; Bradley, M. J.; Tratnyek, P. G., Mechanochemically sulfidated microscale zero valent iron: Pathways, kinetics, mechanism, and efficiency of trichloroethylene dechlorination. Environmental Science & Technology 2017, 51, (21), 12653-12662. 30. Nurmi, J. T.; Tratnyek, P. G.; Sarathy, V.; Baer, D. R.; Amonette, J. E.; Pecher, K.; Wang, C.; Linehan, J. C.; Matson, D. W.; Penn, R. L., Characterization and properties of metallic iron nanoparticles: spectroscopy, electrochemistry, and kinetics. Environmental Science & Technology 2005, 39, (5), 12211230. 31. Lee, W.; Batchelor, B., Abiotic reductive dechlorination of chlorinated ethylenes by iron-bearing soil minerals. 1. Pyrite and magnetite. Environmental Science & Technology 2002, 36, (23), 5147-5154. 32. Butler, E. C.; Hayes, K. F., Kinetics of the transformation of trichloroethylene and tetrachloroethylene by iron sulfide. Environmental Science & Technology 1999, 33, (12), 2021-2027. 33. Xie, Y.; Cwiertny, D. M., Chlorinated solvent transformation by palladized zerovalent iron: Mechanistic insights from reductant loading studies and solvent kinetic isotope effects. Environmental science & technology 2013, 47, (14), 7940-7948. 34. Bhattacharjee, S.; Ghoshal, S., Sulfidation of nanoscale zerovalent iron in the presence of two organic macromolecules and its effects on trichloroethene degradation. Environmental Science: Nano 2018, 5, (3), 782-791. 35. Kim, E.-J.; Murugesan, K.; Kim, J.-H.; Tratnyek, P. G.; Chang, Y.-S., Remediation of trichloroethylene by FeS-coated iron nanoparticles in simulated and real groundwater: effects of water chemistry. Industrial & Engineering Chemistry Research 2013, 52, (27), 9343-9350.

531 532



Page 18 of 24

533

List of Figures

534

Figure 1: (a) representative TEM image of S-nZVIco - (a1)-(a4) are blown-up images of the sections

535

highlighted by the dotted boxes in Figure 1a; (b) representative TEM image of S-nZVIpost – (b1) is the

536

blown-up image of the dotted boxed area in Figure 1b.

537

Figure 2: (a) Specific degradation capacities (SDC) of S-nZVIco, S-nZVIpost and Pd-nZVI particles; inset

538

images represent conceptualized schematics for the different nanoparticles (b) TCE degradation end product

539

distribution for Pd-nZVI, S-nZVIco and S-nZVIpost ([S/Fe]dosed = 0.4) (c) [S/Fe]particle for S-nZVIco (inset

540

figure) and S-nZVIpost; undeposited sulfur was measured in the aqueous phase and accounted for >97% mass

541

balance (d) Specific degradation capacities plotted as a function of [S/Fe]particle. Lines are drawn to guide the

542

eye and error bars represent standard deviation from triplicate reactors.

543

Figure 3: Surface area normalized pseudo first order TCE degradation rate constants (ksa) at different

544

[S/Fe]dosed under Fe0 limited conditions. Error bars represent standard deviation from triplicate reactors.

545

Figure 4: Surface area normalized zero order hydrogen evolution rate constants for nZVI, S-nZVIco, S-

546

nZVIpost and Pd-nZVI. Error bars represent standard deviation from triplicate reactors.

547

Figure 5: TEM image of (a) S-nZVIco and (d) S-nZVIpost , after reaction with TCE ([S/Fe]dosed = 0.4).

548 549


Page 19 of 24

550


Figure 1

(a)

Type I

(a1)

Type I

(a2)

Type II

(a3)

Type III

(a4)

a3 a1

a2

a4

551

(b1)

(b)

Type IV b1

0.5 µm

552 553

Type I

554 555 556 557



0.6

(a)

% product formed

0.5 0.4 0.3 0.2 S-nZVI co S-nZVI post

0.1

Pd-nZVI 0.0

(moles of each product /moles of total products)

Figure 2

Specific Degradation Capacity 0 (moles of TCE degraded/mole of Fe )

558

Page 20 of 24

(b)

100

Ethene Acetylene Ethane 1-butene trans-2-butene cis- 2-butene 1,1-DCE cis 1,2 DCE Other

80 60 40 20 0

-0.1 Pd-nZVI

0.0

0.1

0.2

0.3

0.4

Pd-nZVI

0.5

S-nZVI co

S-nZVI post

0.06 (c)

S-nZVIpost

0.04

0.4

[S/Fe]particle

[S/Fe]particle

0.05

0.03 0.02

0.3 0.2 0.1 0.0 0.0

0.01

S-nZVIco 0.1

0.2

0.3

0.4

[S/Fe]dosed

0.00 0.0

0.1

0.2

0.3

0.4

0.5

Specific Degradation Capacity 0 (moles of TCE degraded/mole of Fe )

[S/Fe]dosed

0.6 (d) 0.5 0.4 0.3 0.2 S-nZVI co S-nZVI post

0.1

Pd-nZVI 0.0 -0.1 0.0 Pd-nZVI

[S/Fe]dosed

0.1

0.2

0.3

0.4

0.5

[S/Fe]particle

559 560 561


Page 21 of 24

562


Figure 3

25 S-nZVIco S-nZVIpost Pd-nZVI

3

k sa (L m -2 h -1) x 10 -3

k sa

20

2 1

15

0 Pd-nZVI -0.1 0.0

0.1 0.2 0.3 0.4

[S/Fe]dosed

10

5

0 -0.1 Pd-nZVI 563

0.0

0.1

0.2

0.3

0.4

0.5

[S/Fe]dosed

564



565

Page 22 of 24

Figure 4

20.00 nZVI S-nZVIco S-nZVIpost Pd-nZVI

-2

-1

k H 2 (µmoles L m h )

18.00

0.08 0.06 0.04 0.02 0.00 -0.1 Pd-nZVI

566

0.0

0.1

0.2

0.3

0.4

0.5

[S/Fe]dosed

567


Page 23 of 24

568


Figure 5 (a)

(b)

569 570



571

Page 24 of 24

TOC

572


Optimal Design of Sulfidated Nanoscale Zerovalent Iron for Enhanced

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