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Reductive Dechlorination of Trichloroethene by Zero-valent Iron Nanoparticles: Reactivity Enhancement through Sulfidation Treatment Yanlai Han, and Weile Yan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03997 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 8, 2016
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Reductive Dechlorination of Trichloroethene by Zero-valent Iron
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Nanoparticles: Reactivity Enhancement through Sulfidation Treatment
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Yanlai Han1, Weile Yan1,*
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Department of Civil, Environmental, and Construction Engineering, Texas Tech University,
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Lubbock, TX, 79409, USA
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Corresponding author. Tel: +1 806 834 3478; Fax: +1 806 742 3449
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Email address:
[email protected] 23
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Abstract
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Zero-valent iron nanoparticles (nZVI) synthesized in the presence of reduced sulfur compounds
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have been shown to degrade trichloroethene (TCE) at significantly higher rates. However, the
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applicability of sulfidation as a general means to enhance nZVI reactivity under different particle
28
preparation conditions and the underlying cause for this enhancement effect are not well
29
understood. In this study, the effects of sulfidation reagent, time point of sulfidation, and sulfur
30
loading on the resultant particles were assessed through TCE degradation experiments. Up to 60-
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fold increase in TCE reaction rates was observed upon sulfidation treatment, with products being
32
fully dechlorinated hydrocarbons. While the reactivity of these sulfur-treated nZVI (S-nZVI) was
33
relatively unaffected by the sulfidation reagent (viz., sodium sulfide, dithionite, or thiosulfate) or
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the sequence of sulfidation relative to iron reduction, TCE reaction rates were found to depend
35
strongly on sulfur to iron ratio. At a low sulfur loading, TCE degradation was accelerated with
36
increasing sulfur dose. The rate constant reached a limiting value, however, as the sulfur to iron
37
mole ratio was greater than 0.025. Different from previous propositions that iron sulfidation
38
leads to more efficient TCE or tetrachloroethene (PCE) degradation by enabling depassivation of
39
iron surface, affording catalytic pathways, or facilitating electron transfer, we show that the role
40
of sulfur in nZVI lies essentially in its ability to poison hydrogen recombination, which drives
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surface reactions to favor reduction by atomic hydrogen. This implies that the reactivity of S-
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nZVI is contaminant-specific and is selective against the background reaction of water reduction.
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As the effect of sulfur manifests through surface processes, sulfidation represents a broadly
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applicable surface modification approach to modulate or increase the reactivity of nZVI for
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treating TCE and other related contaminants.
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1. Introduction
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Due to widespread historical applications in a broad range of industrial and commercial
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processes and their persistence again natural attenuation [1, 2], trichloroethene (TCE) and
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tetrachloroethene (PCE) in the form of dissolved chemicals or non-aqueous phase liquid (NAPL)
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are the most frequently encountered contaminants at the U.S. superfund sites [3,4, 5]. Compared
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to microbial-mediated reduction of PCE and TCE, which tends to produce toxic intermediates
53
such as dichloroethenes (DCEs) and vinyl chloride (VC), abiotic dechlorination undergoes
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predominantly a reductive elimination pathway to yield completely dechlorinated products of
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benign nature (e.g., acetylene, ethene, or ethane) [6,7,8]. With recently reported TCE and PCE
56
transformation by reduced iron minerals under field conditions [9,10], there is a surge of interest
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in using indigenous or engineered abiotic materials to improve the remediation performance of
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sites impacted by chlorinated ethenes. To this end, two broad categories of iron materials have
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been evaluated. Ferrous-containing minerals such as iron sulfides (e.g., mackinawite or pyrite),
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magnetite, and green rust are able to reduce chlorinated ethenes to acetylene [7,11,12,13,14].
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These Fe(II)-containing minerals are naturally present in subsurface soils, or their formation can
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be stimulated under conditions favorable for biologically mediated sulfate or Fe(III) reduction in
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processes known as in situ biogeochemical treatment [3,9,10]. However, reductive
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dechlorination on ferrous minerals is relatively slow and the minerals appear to possess limited
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reduction capacities [12,15], thus it requires high mass loadings of the solids to outcompete the
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less desirable biological reduction pathways [16]. Another form of iron materials extensively
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studied for the degradation of chlorinated ethenes is zero-valent iron (ZVI) [17-21]. Various
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forms of ZVI, including iron granules and powder, colloidal iron nanoparticles (nZVI), and
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bimetallic iron carrying a small amount of catalyst metal (e.g., Pd-Fe and Ni-Fe) have been 3 ACS Paragon Plus Environment
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studied in the past. In spite of the intrinsic reactivity of ZVI materials, corrosion of iron in
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aqueous solutions causes spontaneous surface passivation and the catalyst additives on ZVI are
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prone to deactivation by common groundwater solutes [22, 23,24,25].
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Recent studies have reported enhanced reactivity of ZVI towards chlorinated contaminants in the
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presence of sulfur compounds [26,27,28,29,30,31]. Hassan [26] observed that iron filings
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containing sulfur impurities were more efficient at TCE degradation than high purity iron. Butler
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and Hayes noted enhanced reduction of chlorinated ethenes when the reaction mixture was
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amended with sulfide ion [11]. By adding sodium dithionite into the synthesis broth of nZVI,
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Kim et al. created a Fe/FeS nanocomposite material with up to 20-fold increases in TCE removal
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rates [27,28]. Similarly, iron nanoparticles that had been conditioned in dilute sulfide or
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dithionite solutions were found to degrade TCE [29,31] and 1,2-dichloroethane [30] more rapidly.
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While multiple explanations have been postulated on the origin of the enhanced reactivity caused
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by iron sulfidation, including a catalytic effect ascribed to the iron sulfides formed on the particle
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surface [26], more efficient charge transfer mediated by the sulfides [27, 29,32], and increased
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depassivation of iron surface [33,34], these views remain largely hypothetical awaiting
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experimental verification. Moreover, variations in experiment conditions, type of iron substrates
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used, and sulfidation procedures in these studies preclude the identification of critical factors
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controlling the reactivity of sulfur-modified iron. As a result, the broader implications of
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sulfidation as a means to increase the performance of ZVI materials for the treatment of
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chlorinated contaminants are unclear.
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The objective of this study was to examine the effects of sulfidation on the physicochemical
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characteristics of nZVI and their reactivity in TCE dechlorination experiments. Our choice of
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nanosized ZVI stems from their consistent quality and an ability to manipulate particle synthesis 4 ACS Paragon Plus Environment
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conditions to accommodate different sulfidation procedures. While the focus of the present study
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was on nZVI, our ongoing investigations suggest that sulfidation is applicable to other forms of
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iron materials such as commercial ZVI products, thus the findings presented here will lend
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relevant insights into the general role of sulfur in modulating the reactivity of ZVI materials.
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2. Materials and methods
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2.1 Chemicals and Materials
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All chemicals used are listed in the Supporting Information. Deoxygenated deionized-distilled
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water (DDI), prepared by purging DDI with N2 for 30 min, was used in all procedures including
101
material synthesis and TCE dechlorination experiments. nZVI was synthesized using the
102
borohydride reduction method [23]. Sulfur-treated nZVI particles (denoted as S-nZVI) were
103
prepared using two approaches. The first approach follows that of Kim et al 27, which involves
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amending an appropriate amount of sulfidation reagent to an Fe(III) solution prior to the addition
105
of borohydride and is referred to as pre-synthesis S-nZVI. Three common sulfur compounds
106
were evaluated as sulfidation reagents in this study, namely, sodium sulfide (Na2S), sodium
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dithionite (Na2S2O4), and sodium thiosulfate (Na2S2O3). The dose of the sulfur compound was
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varied such that the mole ratio of the sulfur reagent to the initial concentration of ferric salt in the
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synthesis mixture (denoted as S/Fe mole ratio) was the range of 1.25 x 10-3 to 0.75. In the
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second approach, a sulfidation reagent was dosed into the synthesis mixture at 20 min after the
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onset of Fe(III) reduction via the addition of borohydride. The resultant particles are denoted as
112
post-synthesis S-nZVI. Amorphous iron sulfide (FeS) was synthesized in the lab following the
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method by Butler and Hayes [35]. All iron sulfide or sulfided iron particles were used
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immediately in subsequent experiments upon preparation.
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To investigate the mechanism of sulfur-induced reactivity improvement, a small amount of
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arsenic-modified nZVI was prepared following the same post-synthesis method as that of S-
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nZVI, except that the sulfur compound (sodium thiosulfate) was replaced with equivalent moles
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of sodium arsenite (NaAsO2).
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2.2. TCE dechlorination experiments
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Batch TCE dechlorination experiments were performed to compare the reactivity of S-nZVI
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prepared under different conditions. All experiments were conducted in 45-mL EPA vials
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containing 30 mL of aqueous solution and the balance as headspace. The initial pH of all
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solutions was adjusted to between 7.8-8.2 using dilute NaOH or HCl to simulate the typical pH
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in groundwater. The solutions were amended with 5 g/L of particles (dry weight). The vials were
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capped with PTFE-lined mininert valves. Experiments were started by injecting a small volume
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of TCE stock solution in methanol to reach an initial TCE concentration of 25 mg/L. The
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reactors were placed on a wrist-action shaker at 250 rpm at 22 +/- 1 oC. Control experiments
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without iron materials or with unmodified nZVI were performed in parallel. Periodically, an
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aliquot (25 – 50 μL) of headspace gas was withdrawn using a gastight syringe. The samples were
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directly injected into a GC-FID system (Agilent 6890) equipped with an Agilent PoraPlot Q
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column (25 m x 0.32 mm) to analyze for the concentrations of TCE, chlorinated intermediates
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(not detected in this study), acetylene, ethene, ethane, and longer chain hydrocarbons (up to C6).
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The analysis conditions are described in SI. This method provides adequate separation between
134
TCE and the daughter products. TCE calibration line was constructed by headspace analysis of
135
TCE aqueous standard solutions prepared in the same type of vials as the experimental reactors.
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Calibrations for C2-C6 hydrocarbons were performed using commercial gas standards as
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mentioned in the SI. The results were used to compute their total concentrations in the reaction 6 ACS Paragon Plus Environment
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vials after accounting for partition between headspace and aqueous phases using the respective
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Henry’s Law constants 36, 37 (Table S1). Details on H2 evolution measurements and isotope
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fractionation analysis of TCE during reactions with S-nZVI are available in the SI.
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The solids were subject to microscopic, crystallographic, and surface chemistry characterizations
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and the details are described in the SI.
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3. Results and discussion
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3.1. Effect of sulfidation conditions
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The effect of sulfidation reagent on the reactivity of nZVI for TCE dechlorination was evaluated.
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The particles shown in Figure 1 were prepared using different sulfur compounds at a constant
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S/Fe mole ratio of 0.05 following the post-synthesis sulfidation procedure. As a comparison, the
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inset of Figure 1 shows TCE degradation by fresh nZVI prepared under equivalent conditions but
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without exposure to any sulfur reagent. It is evident that all sulfur-amended nZVI displayed
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remarkable improvements in TCE dechlorination rates. The observed mass-normalized pseudo-
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first-order reaction rate constants (km) of various s-nZVI were approximately 60 folds higher
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than that of the untreated nZVI (Table 1). Distribution of products was qualitatively similar
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among different S-nZVI, with ethene being the dominant product, accompanied by lesser
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amounts of ethane, acetylene, and heavier hydrocarbons (mixture of C3-C6 alkanes and alkenes)
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(Table 1). Dichloroethene isomers (DCEs) and vinyl chloride (VC), common intermediates
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generated by hydrogenolysis reactions, were not detected in nZVI or S-nZVI systems during the
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course of experiments, which agrees with prior studies that reduction of TCE on abiotic surfaces
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occurs predominantly via a dichloro-elemination pathway bypassing the formation of chlorinated
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intermediates [21,38]. 7 ACS Paragon Plus Environment
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In prior studies, dithionite and sulfide ions have been employed to restore the reactivity of
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passivated ZVI [39] or to synthesize Fe(0)/FeS nanocomposite materials [27,29,31]. Aqueous
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sulfide (as H2S or HS- at near neutral pH) is a corrosive chemical and its attaking on iron results
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in deposition of a layer of iron sulfide (FeS) on the surface. Hydrolysis of dithionite in acidic
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solutions gives rise to thiosulfate and sulfite [27, 40] (R1). At an alkaline pH, dithionite may
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hydrolyze via another pathway producing sulfite and sulfide [39, 41] (R2). Disproportionation of
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thiosulfate leads to the formation of elemental sulfur and sulfite (R3) [42]. Elemental sulfur may
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react with iron directly (R4) or convert to sulfide that subsequently binds with iron to form FeS
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[43,44]. As FeS is cathodic to Fe(0), its formation propels further corrosion of Fe(0).
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2S2O42- + H2O S2O32- + 2HSO3-
(R1)
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S2O42- + 6OH- 5SO32- + S2- + 3H2O
(R2)
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S2O32- S0 + SO32-
(R3)
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Fe(0) + S0 FeS
(R4)
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In principle, sulfidation of the ZVI material can be achieved with the use of either thiosulfate,
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dithionite, or free sulfide. Dithionite is a fairly strong reductant, especially under alkaline pH
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[39,45], for which it has been proposed as a reductant to prepare nZVI [46]. Thiosulfate does not
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have an as strong reducing capability, but it readily decomposes to release elemental sulfur or
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sulfide (R3), and the former is reduced in the presence of Fe(0) to sulfide [42]. Thus thiosulfate
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effectively serves as a source of sulfide in aqueous nZVI suspension. The sulfide salt used, Na2S,
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is highly hygroscopic and tends to absorb moisture and CO2 in the air, posing material storage
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and handling difficulties. Furthermore, the rapid release of toxic fume upon addition of a sulfide
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chemical raises process safety concerns. In field applications, the above considerations are 8 ACS Paragon Plus Environment
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significant enough to justify the choice of dithionite or thiosulfate. Considering the availability of
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thiosulfate (both as a synthetic chemical and a naturally occuring sulfur compound) and the
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concern that excess dose of dithionite may consume Fe(0) [31], thiosulfate was chosen as the
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sulfidation reagent in all subsequent experiments.
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In the method proposed by Kim et al. [27], dithionite was introduced into the synthesis solutions
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prior to Fe(III) reduction by borohydride. More recently, post-synthesis sulfidation involving
188
reacting pre-formed nZVI in sulfide solutions has been employed [29,47]. To assess whether the
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time point of suflidation exerts an effect on the nature of the particles formed, we prepared pre-
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and post-synthesis S-nZVI using thiosulfate at the same S/Fe mole ratio (0.05). The morphology
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of the solids emerging from the two preparations was considerably different. Under TEM, the
192
pre-synthesis S-nZVI consists of a heterogeneous mixture of spherical particles that are typical
193
of solution-derived iron nanoparticles (indicated by a white arrow) together with some cubic (red
194
arrow) and platy (blue arrow) structures that resemble iron sulfides or oxides (Figure 2b). In
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contrast, the post-synthesis S-nZVI sample in Figure 2c shows more uniform structure
196
characterized by spherical particles aggregating in string-like clusters. The appearance was akin
197
to that of the unmodified nZVI prepared in our earlier studies [48,49]. The surface chemistry of
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the pre- and post-synthesis S-nZVI was analyzed with X-ray photoelectron spectroscopy (XPS).
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The S 2p spectra of pre- and post-synthesis S-nZVI are shown in Figures 2d and 2e. The spectra
200
were fitted with S 2p3/2 and S 2p1/2 spin-orbit doulets that are sperated by 1.2 eV with an intensity
201
ratio of 2:1 [50]. Peak assignement was based on literature reported binding energies of sulfide
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minerals [50, 51,52] and the spectra of reference materials acquired under the same conditions as
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the samples. Pre-synthesis S-nZVI carried prodominantly monosulfide (S2-) and disulfide (S22-),
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accounting for 63 and 37 atomic percents (at.%) of total sulfur species, respectively. The surface 9 ACS Paragon Plus Environment
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of post-synthesis S-nZVI consists mainly of S2- (34 at.%) and S22- (46 at.%), with S22-
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contributing a higher portion than that in the pre-synthesis S-nZVI. The post-synthesis sample
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also features a group of low-rising peaks in the binding energy range of 163.3 – 164.3 eV,
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corresponding to polysulfides (Sn2-) and possibly elemental sulfur, and a sulfate (SO42-)
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component at the highest binding energy (167.6 eV). Comparison of the two S 2p spectra
210
suggests that oxidation of the sulfur precursor has occcurred to a greater extent during the post-
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synthesis sulfidation process.
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X-ray diffraction analysis (Figure S2) detected the presence of Fe(0) and magnetite in both types
213
of S-nZVI, with the post-synthesis S-nZVI exhibiting a lower degree of crystallinity due to
214
broadening of diffraction peaks. No mono-, di-, or polysulfides of iron can be discerned in the
215
diffraction spectra, which in conjunction with the XPS analysis confirms that sulfide formation
216
on the surface is amorphous. This observation is consistent with the notion that rapid corrosion
217
of iron in sulfidic water tends to produce poorly ordered iron sulfides [53].
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We noticed that post-synthesis S-nZVI can be efficiently separated from the aqueous phase
219
through a filtration step, while the pre-synthesis S-nZVI had a significant portion of the solids
220
passing through a 0.2 μM filter. This observation was likely caused by a tendency of post-
221
synthesis nZVI to form aggregates and the presence of fine, loose iron sulfide or oxide particles
222
in the pre-synthesis S-nZVI as suggested by TEM images.
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In spite of significant structrual differences, the two forms of S-nZVI exhibit similar reactive
224
behavior in TCE dechlorination experiments. km for pre-synthesis and post-synthesis S-nZVI
225
was 0.9 ± 0.1 x 10-3 and 0.8 ± 0.05 x 10-3 L/g-min, respectively, and the product composition
226
matches closely with each other (Table 1). In view of the uniform texture of particles prepared
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via the post-synthesis method and their amenability to fast solid/liquid separation, this method
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was adopted to produce S-nZVI in all subsequent experiments.
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3.2. Effect of sulfur to iron ratio
230
A series of S-nZVI were prepared using different doses of thiosulfate such that the S/Fe mole
231
ratio in the synthesis solutions varied in the range of 1.25 x 10-3 to 0.75. The concentration of
232
thiosulfate after S-nZVI synthesis was measured, and the amount of sulfur deposited on nZVI,
233
estimated based on thiosulfate consumption, correlates well with the initial S/Fe mole ratio
234
(Table S2 in SI). It was found that the loading of sulfur on iron has a strong impact on TCE
235
degradation rates. With particles prepared using the highest sulfur dose (S/Fe = 0.75, Figure 3a),
236
ethene and ethane were the dominant products, accounting for 70% and 17%, respectively, of
237
total products identified (product yields were determined at approximately 90% TCE conversion).
238
Close inspection of Figure 3a indicates there was an accumulation of acetylene during the initial
239
phase of the reaction, nonetheless, its concentration declined over time accompanied by
240
concurrent increases in ethene and ethane concentrations. Partially dechlorinated intermediates
241
(i.e., DCEs or VC) were not detected in the headspace mixture. C3-C6 hydrocarbons contributed
242
to a minor fraction (11%) of the products formed. In comparison, particles treated with the
243
lowest sulfur dose, corresponding to a S/Fe mole ratio of 1.25 x 10-3, produced ethene, ethane,
244
and C3-C6 products (Figure 3b), and acetyelne was below detection limit at any sampling point.
245
Overall, a carbon recovery (as C2 equivalent) of 50% to 90% was achieved for all S-nZVI used
246
in this study. Incomplete carbon recovery has been noticed in prior studies of TCE
247
dechlroination using nZVI [38] or iron sulfide materials [7,11]. As noted in later discussion, the
248
missing carbon is likely products of acetylene polymerization reactions, which are affinitive to
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metal surfaces, thus their quantities cannot be reliably measured using the headspace sampling
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method.
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The results of control experiments, namely, the reactions of TCE with unmodified nZVI and pure
252
FeS prepared from aquoues precipitation, were shown in Figure 3c and 3d. Greater than 90%
253
TCE degradtion was achieved within a time frame of 0.3 to 2 days when S-nZVI was the
254
reductant, whereas it required 21 days to attain a similar extent of TCE removal by nZVI.
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Degradation of TCE in FeS suspsension was even slower, with only 25% TCE being degraded in
256
19 days. The apparent mass-normalized reaction rates of various solids vary by approximately
257
three orders of magnitude (Table 1). The composition of product mixture is similar for the
258
original nZVI and those receiving a low dose of sulfur (Figures 3b and 3c). At a high sulfur dose,
259
considerable accumulation of acetylene during the intermeidate stage of the reaction was
260
observed and ethene was the dominant final product (Figure 3a). Contrary to nZVI or S-nZVI,
261
TCE degradation by FeS yielded exclusively acetylene. This slow transformation of TCE to
262
acetylene by FeS without further hydrogenation of aceytlene agrees with earlier findings by other
263
investigators [7,11].
264
As all TCE degradation data conform to a first-order rate model, the effect of sulfur loading on
265
TCE reduction kinetics was assessed by plotting the mass-normalized rate constant, km, against
266
the S/Fe mole ratio. The results, shown in Figure 4a, reveal a biphasic trend. When thiosulfate
267
was applied at a small dose (S/Fe < 0.025), more rapid TCE dechlorination occurred with
268
increasing S/Fe ratio. However, when the S/Fe mole ratio exceeds 0.025, the rate constant levels
269
out approaching a limiting value with increasing sulfur loading. The highest rate constant was
270
1.3 x 10-3 L/g-min, in comparison to 2.2 x 10-5 and 1.5 x 10-6 L/g-min achieved by unmodified
271
nZVI and FeS, respectively. The effect of S/Fe ratio on product distribution is depicted in Figure 12 ACS Paragon Plus Environment
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4b. The final yields of ethene or ethane, defined as the amount of product formed over TCE
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consumed determined at the point of 90% TCE conversion, do not bear a strong correlation with
274
the S/Fe ratio. However, the maximum accumulation of acetylene during the course of TCE
275
transformation is strongly affected by the sulfur dose. For particles prepared under a low sulfur
276
loading condition (S/Fe < 0.025), no acetylene was detected in the product mixture, whereas
277
particles containing a higher sulfur dose caused a substantial buildup of acetylene before its
278
gradual conversion to downstream products.
279
Acetylene is a reactive chemical and it readily undergoes polymerization reactions on metal
280
surfaces to form longer chain hydrocarbons [54], or in the presence of a hydrogen source,
281
hydrogenates to more saturated products. Our separate experiments reacting acetylene with nZVI
282
and S-nZVI confirm that both solids are able to rapidly transform acetylene into ethene, ethane,
283
and higher order hydrocarbons (Figure S3). Notably, significant gaps in carbon recovery were
284
also observed during these reactions, which was attributed to deposition of non-volatile
285
polymerized products (>C6) on the particle surface. FeS, as expected from previous studies, did
286
not show any appreciable reaction with acetyelene. The composition of TCE daughter products
287
and the reactivity of acetylene towards different iron materials suggest that TCE reduction on
288
nZVI, S-nZVI, or FeS shares an identical pathway of β-elimination leading to acetylene
289
formation [6, 7]. In the presence of nZVI or S-nZVI, acetylene is further converted to ethene,
290
ethane, and higher order hydrocarbons, while it remains intact on FeS. The reaction pathway and
291
its pertinent kinetic parameters are shown schematically in Figure 5. Although multiple steps are
292
invovled in TCE transformation to acetylene, including surface adsorpton of TCE and
293
conversion of chloroacetylene (i.e., the immediate product of TCE β-elimination) to acetylene,
294
the rate of these sequential steps can be captured by a single kinetic parameter (k1) that 13 ACS Paragon Plus Environment
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represents the rate-limiting step. Note that in Figure 5 we consider ethene and ethane as being
296
formed via two parallel pathways instead of going through sequential hydrogenation because
297
ethene hydrogenation by nZVI or S-nZVI was exceedingly slow (Figure S4) and the ratio of
298
ethene to ethane remained constant during TCE degradation. The values of rate constants in
299
Figure 5 were estimated from TCE reduction and acetylene hydrogenation data and are
300
summarized in Table 2. It is intersting to note that sulfiation of nZVI effectively increases the
301
value of k1, but the treatment has no enhancement effect on k2 or k3 value. Thus, the effect of
302
sulfidation is specific for TCE conversion to acetylene and it does not accelerate the subsequent
303
hydrogenation steps.
304
3.3. Role of sulfidation in iron reactivity improvement
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A pertinent question then arises on why the incorporation of sulfur into nZVI substrate would
306
cast such a prominent effect on TCE dechlorination. Earlier studies suggest that reduced sulfur
307
compounds such as free sulfide and dithionite are able to depassivate iron surface by reducing
308
Fe(III) to Fe(II) leading to disintegration of the native oxide layer and/or the formation of Fe(II)-
309
containing oxides (e.g., magnetite) that have greater charge transfer abilities [39,53,45].
310
Depassivation effect alone is, however, unable to account for our findings here, since treating
311
nZVI with dilute acid or amending the nZVI suspension with ascobate (a reductant of Fe(III))
312
[55] did not bring about substantial improvements in TCE reduction rates compared to the
313
freshly synthesized particles (Figure S5). This suggets that the presence of sulfided iron is
314
necessary to enable the large increases in TCE degradation rates. It has been proposed that iron
315
sulfide may catalyze PCE or TCE reduction by ZVI [26]. Nevertheless, our analysis of carbon
316
isotope fractionation during TCE experiments did not record consistent shifts in TCE bulk
317
enrichment factors of the unmodified nZVI and its sulfur-treated counterparts (Table S3), nor did 14 ACS Paragon Plus Environment
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318
saturating the reaction mixture with H2 bring about more rapid TCE removal, refuting a possible
319
catalytic role played by the sulfide formation.
320
An alternative explanation points to enhanced electron transfer from the Fe(0) core to the
321
solution phase by FeS surface deposits owing to its good electron conducting ability [27,32]. The
322
proposition is supported by the obsevatin of increased anodic currents in recent electrochemical
323
investigations [53,56], however, more rapid iron oxidation may not beget higher rates of TCE
324
transformation since the latter reaction is not limited by electron transfer but the availability of
325
atomic hydrogen [57,58]. Furthermore, the rate enhancement effect caused by accelerated iron
326
corrison is expected to apply to other reducible contaminants, such as carbon tetrachloride (CT,
327
CCl4), whose reduction is governed by a direct electron transfer process [57,59]. To this end, we
328
evaluated reactions of CT with nZVI and sulfided nZVI. The results reveal nearly identical
329
performance by nZVI and those receiving varying levels of sulfur dose (Figure S6). The effect of
330
sulfidation is therefore specific for TCE dechlorination and cannot be ascribed to a general cause
331
related to increased iron corrosion.
332
In the catalysis literature, sulfur is a potent poison of hydrogen recombination reactions on metal
333
surfaces [60,61]. In the case of iron, corrosion in anaerobic water consists of two fundamental
334
processes, namely, the transfer of electrons to protons resulting in surface-adsorbed hydrogen
335
atoms, and the recombination of hydrogen adatoms to form molecular hydrogen that bubbles off
336
the surface. The addition of sulfur on metal surface inhibits hydrogen recombindation and as a
337
result, slows down H2 evolution. This forces more atomic hydrogen to remain on the surface or
338
penetrate into the bulk substrate. Such effect has been investigated extensively and is known to
339
cause hydrogen embrittlement and stress-induced cracking of steel [62,63,64]. Nonetheless,
340
when iron is used as a chemical reductant, inhibition of hydrogen recombination would favor 15 ACS Paragon Plus Environment
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341
reactions involving atomic hydrogen, and we believe this effect is the primary mechanism
342
responsible for the remarkable improvements in TCE degradtion rates, since the reduction of
343
chlorinated ethenes on iron are considered to be predominantly mediated by surface adsorbed
344
atomic hydrogen [58,59]. This action of sulfur is consistent with the highly specific effect of
345
sulfidation on TCE reduction relative to that of chlorinated compounds undertaking different
346
reduction mechanisms (e.g., carbon tetrachloride). It also corroborates with recent findings that
347
H2 production was surpressed in the presence of S-nZVI, in contrary to what would be expected
348
if the increase in reactivity is contributed by enhanced iron corrosion [29,31]. In this study,
349
H2 production by S-nZVI was also evaluated under conditions relevant to TCE dechlorination
350
experiments. The results (Figure 6a) clearly demonstrate that sulfur amendment exerts a strong
351
impact on the rate of H2 evolution. The trend of H2 production shown in Figure 6a, that there is a
352
notable decrease in H2 generation rate when S/Fe ratio increases from 0.01 to 0.05 while further
353
increase in sulfur loading does not give rise to significant reduction in H 2 generate rate, agrees
354
with the effect of S dosage on TCE dechlorination kinetics (Figure 4a). An additional argument
355
in support of the poisoning effect of sulfur comes from TCE reduction by nZVI loaded with
356
arsenic (As-nZVI), another potent deactivator of H recombination reactions [63]. The As-nZVI
357
was prepared using the same protocol as that of S-nZVI except that sodium arsenite was
358
employed in place of sodium thiosulfate. The results indicate that arsenic-modified nZVI
359
exhibited similar reactive behavior as the sulfur-modified particles towards TCE (Figure 6b),
360
releasing ethene as the dominant product. Thus, sulfur does not act as a direct facilitator of TCE
361
dechlorination reaction, but rather modifies iron surface chemistry to favor the production of a
362
key reactive species invovled in TCE reduction. When sulfur is deposited on the iron surface, it
363
induces dissolution of the native oxide and causes the surface to be more favorable for atomic
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364
hydrogen adsorption. Meantime, inhibtion of hydrogen recombination promotes the build-up of
365
atomic hydrogen on the iron surface and its reaction with TCE. The process is illustrated
366
schematically in Figure 7.
367
Finally, the dependence of the rate constant on S/Fe ratio in Figure 4a suggests the existence of
368
an optimal surface sulfur coverage. Exceeding this optimal loading, there is no further increase in
369
reactivity, possibly due to the formation of polysulfides, discrete FeS precipitates, or oxidized
370
sulfur species that do not contribute as effectively towards reactivity enhancement. The relatively
371
low concentration of sulfur required to achieve this optimal loading implies effective sulfidation
372
can be attained in dilute solutions of sulfur reagents.
373
3.4. Environmental implications
374
ZVI is one of the most frequently applied and environmentally benign reductants for treating a
375
broad vareity of water contaminants. The ZVI chemistry has been studied extensively for
376
chlorinated ethenes. Degradation of TCE by iron alone is relativley slow, and iron reactivity
377
tends to be short-lived due to rapid passivation in air or water. Attemps to improve the
378
performance of ZVI in the past has largely concentrated on a group of bimetallic ZVI particles.
379
The catalyst metals amended on iron surface, such as Pd or Ni, are able to improve the rates of
380
contaminant reduction by catalyzing the activation of H2 [65,66]. The sulfidation method
381
examined here represents a different approach to modify ZVI reactivity. Instead of serving as a
382
catalyst, sulfur poisons a parallel reaction that competes with TCE for the electron source (i.e.,
383
water reduction and H2 evolution), thereby increasing the accessibility of atomic hydrogen for
384
TCE reduction. Although under optimal conditions, the reactivity of these sulfur-modified ZVI
385
may fall short of that of the highly reactive bimetallic materials (for example, km of fresh Pd-
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386
nZVI prepared in our lab is approximately 1.4 x 10 -2 L/g-min [25]), iron sulfidation promises
387
some crucial advantages from an enviornmental chemistry perspective. It circumvents the use of
388
catalyst metals that are expensive or toxic to aquatic environment. Inhibition of hydrogen
389
evolution effectively favors the reduction of the target contaminant (e.g., TCE) against the
390
prevalent background reaction between iron and water. The last point will be of interest in large-
391
scale field implementations as material efficiency and longevity in environmental matrices are
392
important considerations in those circumstances. As the effects of sulfur manifest primarily
393
through surface processes, sulfdation can be applied as a surface treatment procedure to pre-
394
synthesized nZVI, and the method should in principle be extendable to other ZVI materials,
395
including bulk iron granules or filing that are frequently used in remediation applications.
396
Further investigations on sulfidation of other forms of ZVI material are underway.
397
Reduced sulfur compounds are ubiquitous in anoxic environment. Partially reduced sulfur anions
398
such as thiosulfate, polysulfides, and sulfite may arise as intermediates during sulfide oxidation
399
and their interconversion is strongly coupled with biogeochemical processes [67,68]. Interstingly,
400
these species are known to be strong inhibitor of catalytic systems including the bimetallic Pd-Fe
401
[25]. From this viewpoint, iron sulfidation not only offers a material that can sustain its reactivity
402
in underground matrix where reduced sulfur ligands are abundant, but also suggests possibilities
403
to optimize the outcome of iron-based remediation technologies via biogeochemical
404
manipulations.
405
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406
Acknowlegement
407
The authors acknowledge the start-up fund by TTU and funding support from the National
408
Science Foundation (CHE-1611465). The authors appreciate assistance from Drs. Juske Horita
409
and Kaz Suroweic at TTU for carbon isotope and H2 analyses.
410
Supporting Information
411
Data of XRD characterization, carbon isotope analysis, TCE degradation products by S-nZVI
412
prepared under different synthesis conditions, acetylene and ethene hydrogenation, and carbon
413
tetrachloride reduction by S-nZVI are available in the supporting information. The Supporting
414
Information is available free of charge on the ACS Publications website.
415
Reference
416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439
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29. Rajajayavel, S. R. C.; Ghoshal, S., Enhanced reductive dechlorination of trichloroethylene by sulfidated nanoscale zerovalent iron. Water Research 2015, 78, 144-153. 30. Garcia, A. N.; 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. 31. Fan, D.; Johnson, G. O.; 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), 9558-9565. 32. Turcio-Ortega, D.; Fan, D.; Tratnyek, P. G.; Kim, E.-J.; Chang, Y.-S., Reactivity of Fe/FeS Nanoparticles: Electrolyte Composition Effects on Corrosion Electrochemistry. Environmental Science & Technology 2012, 46, (22), 12484-12492. 33. Lipczynskakochany, E.; Harms, S.; Milburn, R.; Sprah, G.; Nadarajah, N., Degradation of carbon tetrachloride in the presence of iron and sulfur-containing compounds Chemosphere 1994, 29, (7), 1477-1489. 34. Hansson, E. B.; Odziemkowski, M. S.; Gillham, R. W., Influence of Na2S on the degradation kinetics of CCl4 in the presence of very pure iron. Journal of Contaminant Hydrology 2008, 98, (34), 128-134. 35. Butler, E. C.; Hayes, K. F., Effects of Solution Composition and pH on the Reductive Dechlorination of Hexachloroethane by Iron Sulfide. Environmental Science & Technology 1998, 32, (9), 12761284. 36. Yaws, C. L., Yaws' Handbook of Thermodynamic and Physical Properties of Chemical Compounds. In Knovel. 37. Williams, M. L., CRC Handbook of Chemistry and Physics, 76th edition. Occupational and Environmental Medicine 1996, 53, (7), 504-504. 38. Arnold, W. A.; Roberts, A. L., Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(O) particles. Environmental Science & Technology 2000, 34, (9), 17941805. 39. Xie, Y.; Cwiertny, D. M., Use of Dithionite to Extend the Reactive Lifetime of Nanoscale ZeroValent Iron Treatment Systems. Environmental Science & Technology 2010, 44, (22), 8649-8655. 40. Lister, M. W.; Garvie, R. C., Sodium dithionite, decomposition in aqueous solution and in the solid state. Canadian Journal of Chemistry-Revue Canadienne De Chimie 1959, 37, (9), 1567-1574. 41. Greenwood, N. N.; Earnshaw, A., Chemistry of the Elements. 2nd ed.; Elsevier Butterworth Heinemann 1997. 42. Kappes, M.; Frankel, G. S.; Sridhar, N.; Carranza, R. M., Reaction Paths of Thiosulfate during Corrosion of Carbon Steel in Acidified Brines. Journal of the Electrochemical Society 2012, 159, (4), C195-C204. 43. Macdonald, D. D.; Roberts, B.; Hyne, J. B., Corrosion of carbon-steel by wet elemental sulfur Corrosion Science 1978, 18, (5), 411-425. 44. Schmitt, G., Effect of elemental sulfur on corrosion in sour gas systems Corrosion 1991, 47, (4), 285-308. 45. Szecsody, J. E.; Fruchter, J. S.; Williams, M. D.; Vermeul, V. R.; Sklarew, D., In situ chemical reduction of aquifer sediments: Enhancement of reactive iron phases and TCE dechlorination. Environmental Science & Technology 2004, 38, (17), 4656-4663. 46. Sun, Q.; Feitz, A. J.; Guan, J.; Waite, T. D., Comparison of the reactivity of nanosized zero-valent iron (nZVI) particles produced by borohydride and dithionite reduction of iron salts Nano 2008, 3, (5), 341-349. 47. Fan, D.; Anitori, R. P.; Tebo, B. M.; Tratnyek, P. G.; Pacheco, J. S. L.; Kukkadapu, R. K.; Engelhard, M. H.; Bowden, M. E.; Kovarik, L.; Arey, B. W., Reductive Sequestration of Pertechnetate ((TcO4)-Tc-99) by Nano Zerovalent Iron (nZVI) Transformed by Abiotic Sulfide. Environmental Science & Technology 2013, 47, (10), 5302-5310.
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68. Cantrell, K. J.; Yabusaki, S. B.; Engelhard, M. H.; Mitroshkov, A. V.; Thornton, E. C., Oxidation of H2S by iron oxides in unsaturated conditions. Environmental Science & Technology 2003, 37, (10), 2192-2199.
593
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Figure 1. TCE dechlorination by S-nZVI prepared using different sulfidation reagents. The initial mole ratio of sulfidation reagent to iron was fixed at 0.05. Initial TCE concentration was 25 mg/L. Inset shows TCE degradation by fresh nZVI without sulfidation treatment. The particle dose was 5 g/L in all experiments.
599 600 601 602 603 604 605 606 607 608 609
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(a)
0.8
(b) Pre-synthesis S-nZVI
Pre-synthesis S-nZVI Post-synthesis S-nZVI
0.6
C/C0
(d) CPS (a.u.)
1.0
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S22- S2-
Pre-synthesis S-nZVI
100 nm
(e)
0.4
(c) Post-synthesis S-nZVI
0.2
S22-
Post-synthesis S-nZVI SO42-
Sn2-
S2-
0.0
0
200 Time (min)
400 50 nm
169 167 165 163 161 159 Binding Energy (eV)
610 611 612 613 614
Figure 2. (a) TCE dechlorination by S-nZVI that receives sulfidation treatment at different stages of particle synthesis. (b) and (c) TEM micrographs of the particles used in (a). (d) and (e) XPS S 2p3/2 spectra of the corresponding particles in (a). All materials were prepared with a S/Fe mole ratio of 0.05.
615 616 617 618 619 620
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621 622 623 624
Figure 3. TCE dechlorination and product formation in reactor of (a) S-nZVI with low sulfur dose, (b) S-nZVI with high sulfur dose, and (c) unmodified nZVI, and (d) FeS. The particle dose was 5 g/L.
625 626
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km (10-3 L/g-min)
(a) 1
0.1 nZVI 0.01 0.0001
0.01
1
S/Fe mole ratio 0.7
Product Yield
0.6 0.5
Acetylene (max yield) Ethene (final yield) ethane (final yield)
(b)
0.4 0.3 0.2 0.1
0.0 0.0001 627 628 629
0.01
1
S/Fe mole ratio Figure 4. Effect of S/Fe mole ratio on (a) TCE degradation rate and (b) product yields. The particle dose was 5 g/L.
630
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Cl
H C
C
Cl
Cl
k1 H
k2 H H 631 632 633 634
C
Polymerization products
H
k3 H
C
C
C H
H H H C C H H H
Figure 5. Proposed reaction pathways of TCE decomposition on S-nZVI. Only experimentally observed intermediates or products are shown. Dashed line indicates possible involvement of multiple reaction steps. Values of reaction rate constants (k1 – k3) are tabulated in Table 2.
635
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H2 production, mmole
0.6
nZVI
0.5
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(a)
S-nZVI (S/Fe=0.01) S-nZVI (S/Fe=0.05)
0.4
S-nZVI (S/Fe=0.25)
0.3 0.2 0.1 0.0
0
24
48
72
Time, h
636
1.0
(b)
S-nZVI (S/Fe = 0.05)
As-nZVI (As/Fe = 0.05)
C/Co
0.8 0.6 0.4 0.2 0.0
0
200 Time (min)
400
637 638 639 640
Figure 6. (a) H2 production by nZVI and S-nZVI of varying S/Fe mole ratio. (b) TCE degradation by S-nZVI and arsenic-modified nZVI (As-nZVI) at an As or S to Fe mole ratio of 0.05. Particle dose was 5 g/L in all experiments.
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Environmental Science & Technology
Unmodified nZVI H+
H• H•
H•
e-
Fe(0)
H2
H+
C2HCl3 C2H2
H+
H• H•
e-
Sulfur-treated nZVI H+
C2HCl3 C2H2
H• H• H• H• H• Fe Sulfide e-
Fe(0)
H+
H2
H+
slow
H• H•
e-
642 643 644
Figure 7. Schematics of reactions on nZVI and S-nZVI.
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646 647
Table 1. Pseudo-first-order rate constants and product distribution of TCE dechlorination by nZVI and S-nZVI prepared under different conditions Particle type FeS
nZVI
S-nZVI (thiosulfate)
S-nZVI (dithionite)
S-nZVI (sulfide)
S-nZVI (thiosulfate)
648 649 650 651 652
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Sulfidation condition
Products
Yielda
Carbon recovery
km (10-3 L g-1 min-1 )c
(%)b Ethene Ethane Acetylene
N.D. N.D. 126
C3-C6
N.D.
Ethene Ethane
17 6.5 N.D. 5.7 44 10 4.5 7.2 33 8.2 10 9.6 39 9.4 7.2 7.5 45 10 1.3 8.1
Acetylene C3-C6 Ethene Post-synthesis sulfidation, S/Fe Ethane Acetylene = 0.05 C3-C6 Post-synthesis Ethene sulfidation, S/Fe Ethane = 0.05 Acetylene C3-C6 Post-synthesis Ethene sulfidation, S/Fe Ethane = 0.05 Acetylene C3-C6 Ethene Pre-synthesis sulfidation, S/Fe Ethane Acetylene = 0.05 C3-C6
107
0.0024 ± 0.0007
36
0.023± 0.008
68
0.80 ± 0.05
68
0.78 ± 0.21
67
1.04 ± 0.13
70
0.90 ± 0.08
a
calculated from product formation over TCE consumption. b sum of products (as C2 equivalents) and TCE remain. Both a and b were determined at the point of ca. 90% TCE conversion or, for slow reactions, the last sampling point. c mass-normalized pseudo-first-order rate constants, uncertainties represent 95% confidence intervals.
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Table 2. Mass-normalized rate constants of steps involved in TCE degradation by S-nZVI of varying S/Fe mole ratio S/Fe mole ratio
656
k1 (10-3 L g-1 min-1)a
k2 (10-3 L g-1 min-1)b
k3(10-3 L g-1 min-1)b
0.00125 0.18 1.03 0.22 0.05 1.16 0.68 0.15 0.25 1.11 0.60 0.11 a obtained from TCE degradation experiments. b estimated from acetylene hydrogenation experiments.
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