Mechanochemically Sulfidated Microscale Zero Valent Iron: Pathways

School of Public Health, Oregon Health & Science University 3181 SW Sam Jackson Park Road, Portland, Oregon 97239, United States. Environ. Sci. Techno...
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Mechanochemically Sulfidated Microscale Zero Valent Iron: Pathways, Kinetics, Mechanism, and Efficiency of Trichloroethylene Dechlorination Yawei Gu, Binbin Wang, Feng He, Miranda J. Bradley, and Paul G. Tratnyek Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03604 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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Mechanochemically Sulfidated Microscale Zero Valent Iron:

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Pathways, Kinetics, Mechanism, and Efficiency of

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Trichloroethylene Dechlorination

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Yawei Gu1, Binbin Wang1, Feng He1*,

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Miranda J. Bradley2, and Paul G. Tratnyek2

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College of Environment

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Zhejiang University of Technology

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Hangzhou 310014, China

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School of Public Health

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Oregon Health & Science University

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3181 SW Sam Jackson Park Road, Portland, OR 97239

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*Corresponding author: Feng He

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Email: [email protected], Phone: 86-571-88871509

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Keywords: TCE, Reductive Elimination, Sulfidation, Ball Milled, Corrosion

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Potential, Electron Efficiency, Selectivity, Longevity

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9/27/2017

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Graphical Abstract

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Abstract

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In water treatment processes that involve contaminant reduction by zero-valent iron (ZVI),

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reduction of water to dihydrogen is a competing reaction that must be minimized to maximize

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the efficiency of electron utilization from the ZVI. Sulfidation has recently been shown to

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decrease H2 formation significantly, such that the overall electron efficiency of (or selectivity

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for) contaminant reduction can be greatly increased. To date, this work has focused on

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nano-scale ZVI (nZVI) and solution-phase sulfidation agents (e.g., bisulfide, dithionite or

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thiosulfate), both of which pose challenges for up-scaling the production of sulfidated ZVI

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for field applications. To overcome these challenges, we developed a process for sulfidation

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of microscale ZVI by ball milling ZVI with elemental sulfur. The resulting material

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(S-mZVIbm) exhibits reduced aggregation, relatively homogeneous distribution of Fe and S

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throughout the particle (not core-shell structure), enhanced reactivity with trichloroethylene

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(TCE), less H2 formation, and therefore greatly improved electron efficiency of TCE

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dechlorination (εe). Under ZVI-limited conditions (initial Fe0/TCE = 1.6 mol/mol),

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S-mZVIbm gave surface-area normalized reduction rate constants (k'SA) and εe that were ~2-

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and 10-fold greater than the unsulfidated ball-milled control (mZVIbm). Under TCE-limited

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conditions (initial Fe0/TCE = 2000 mol/mol), sulfidation increased kSA and εe ~5- and 50-fold,

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respectively. The major products from TCE degradation by S-mZVIbm were acetylene, ethene,

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and ethane, which is consistent with dechlorination by β-elimination, as is typical of ZVI,

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iron oxides, and/or sulfides. However, electrochemical characterization shows that the

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sulfidated material has redox properties intermediate between ZVI and Fe3O4, mostly likely

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significant coverage of the surface with FeS.

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Introduction

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Zero valent iron (ZVI) has been used to remediate contaminated groundwater since the early

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1990s.1, 2 In the past few decades, the majority of research on improved, alternative

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formulations of ZVI has focused on increasing rates of reaction with target contaminants.

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This can be achieved by combining ZVI with Pd or Ni, which can give significantly faster

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and deeper reduction of chlorinated solvents3-6 apparently by catalyzing hydrogenation and

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inhibiting passivation.7-9 In practice these benefits are short-lived, however, so interest in

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bimetallic formulations of ZVI has waned. More recently, there has been great interest in the

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nano-sized ZVI (nZVI) because of its high reactivity with contaminants and potential

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mobility in porous media.4, 5, 10-13 However, it is now clear that the surface-area normalized

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reactivity of nZVI is not significantly greater than micron-sized ZVI (mZVI),14 without

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additional modifications (such as palladization), and that the mobility of nZVI in porous

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media is limited to a few meters,5, 15, 16even with modifications (such as surfactants).17-20

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Other alternative formulations of ZVI include combination with carbon,21-24 silica,25

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zeolites,26 bentonite,27 and other treatments.28, 29

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With all of these alternative formulations of ZVI—and especially those involving

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nZVI—there has been growing recognition that the longevity and capacity aspects of

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performance are among the most challenging.15, 18, 30, 31 If the ZVI becomes passivated or

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consumed before treatment goals are reached, then overall performance is limited by

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longevity or capacity, respectively.31-33 These factors are significantly influenced by the

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selectivity of reaction between ZVI and the various oxidants that contribute to the reductant

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demand of the system (e.g., target contaminants, co-contaminants, dissolved oxygen,

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water).34 The selectivity of ZVI for these oxidants can be quantified as “efficiencies” and

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recent studies have begun to quantify this performance metric (along with rate constants for

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target contaminant degradation).30, 33, 35 The control and optimization of these efficiencies is

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the main fundamental and practical goal of many recent studies, including this one.

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One modification to ZVI that has great promise for improving the efficiency of 4

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contaminant treatment process is sulfidation, which is defined, for ZVI based systems, as the

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result of any treatment that forms iron sulfide secondary phases in place of, on the surface of,

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or in close association with the primary ZVI and iron oxide phases. Sulfidation of nZVI may

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enhance rates of contaminant reduction, but also improves the selectivity of nZVI for

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reduction of contaminants over water, a benefit that has not been observed with bimetallic or

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nano formulations of ZVI.33, 36-41 The benefits of sulfidation of ZVI have been observed with

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materials produced by a variety of methods. An early method involved reduction of ferrous

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chloride with sodium borohydride (the most common recipe for synthesizing nZVI42, 43) in

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the presence of sodium dithionite,36 while most subsequent studies have added the sulfidation

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agent after nZVI synthesis33, 37 and some have used other reduced sulfur species, such as

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sodium sulfide and thiosulfate.33, 37, 40 In general, all of these methods increase the observed

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dechlorination rate (e.g., kobs by 60-fold for TCE40) and decreased the H2 production rate.33, 37,

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40

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Most of the variations on sulfidated ZVI that have been described to date have been

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based on nanoscale ZVI,33, 36, 37, 40 but nZVI is expensive and therefore only modest quantities

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are used in targeted applications. In addition, all of the sulfidation agents used to date have

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carried out in aqueous phase using dissolved, lower-valent sulfur species that are easily

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oxidized by oxygen.44, 45 Sulfide solutions are usually prepared from Na2S, which is strongly

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hygroscopic (absorbing moisture and CO2 from air) and are prone to release H2S, which is

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noxious, toxicity, and regulated.40 Dithionite (S2O42−) solutions are more conveniently

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prepared and managed, but sulfidation of nZVI by dithionite oxidizes a significant portion of

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the Fe(0), which depletes the resulting material’s capacity to reduce contaminants.33 These

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limitations might be overcome by sulfidation of micron-sized ZVI (mZVI) using elemental

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sulfur with mechanochemical mixing by ball milling. Elemental S is a safe, readily available,

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inexpensive, and relative stable solid that has been used as electron donor for

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bio-denitrification in groundwater remediation46 and bio-bleaching of heavy metals from

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contaminated soil.47 Ball milling has been used to remediate heavy metal and PCB

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contaminated soils48 and synthesize nZVI,49 bimetallic ZVI50 and Fe-C composites.23 It was 5

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reported that FeS can be synthesized by ball milling Fe and S powders at S/Fe molar ratio of

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1,51 but we hypothesized that ball milling mixtures with S/Fe ratio significantly less than 1

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will produce a composite material composed of iron metal, oxides, and sulfides. Since

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mechanochemical mixing can produce solid-solid reactions that are not obtained under

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ambient conditions,23, 52 the material described in this study may have novel characteristics.

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An additional benefit of ball milling is that it can efficiently produce the sub-micron-sized

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particles that are optimal for injection into subsurface for in-situ remediation applications.23

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Herein, we describe the synthesis of a relatively air stable sulfidated microscale ZVI

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(S-mZVIbm) by ball milling ZVI and S powders under dry conditions. The physical properties

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of S-mZVIbm, and mZVI ball milled without S (mZVIbm), were characterized using

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crystallographic, microscopic, and surface chemistry methods. The reactivity of the material

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was characterized in terms of the kinetics and products of TCE dechlorination, the production

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of H2 in the presence and absence of TCE, and current-potential measurements made using

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electrodes packed with the ball milled powders. The overall objective was to understand how

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sulfidation and ball milling influences the structure and properties of ZVI, especially its

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reactivity with respect to pathway, kinetics, electron selectivity, and mechanism of TCE

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dechlorination. The specific objectives are to: (i) quantify and compare reaction rates and

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products formed from both S-mZVIbm and mZVIbm, (ii) determine efficiencies and capacity

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of reduction (i.e., reaction with TCE vs. water), and (iii) gain insight into the mechanism of

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the enhanced reactivity and selectivity of S-mZVIbm.

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Materials and Methods

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Chemicals. Details regarding the chemicals used are provided in SI.

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Particle Preparation. For the preparation of S-mZVIbm, ZVI and sulfur powder were mixed

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and sealed in a jar with an argon atmosphere. The S/Fe molar ratio was typically 0.1 except

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that for XRD samples was 0.2. The milling was carried out in a planetary ball mill

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(Boyuntong Instrument Technology, Nanjing, China) with stainless steel jars (100 mL) and 6

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zirconia balls (6 mm in diameter). The milling was performed at 400 rpm, with Ar headspace,

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and at room temperature (no temperature rise was detected). After up to 30 h of milling, the

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S-mZVIbm samples were collected in a N2-filled glovebag and stored in an argon-filled

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glovebox before use. For comparison, mZVIbm was obtained using same procedures without

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adding S. The resulting materials were subject to comprehensive physical/chemical

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characterizations, with some details provided in SI.

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Reaction Systems. The reactivity of the material was tested under two conditions that

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represent the range of regimes that might apply during full-scale implementation: (i) excess

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iron and (ii) iron-limited. For excess iron conditions (TCE limited)—to represent the high

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iron to TCE ratios such as after injection of ZVI into a plume of contaminated

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groundwater—the TCE and ZVI concentrations were set at 76 µM and 10 g/L, respectively.

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For excess TCE conditions (ZVI limited)—to represent low iron to TCE ratios such as might

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arise during treatment of a source zone—1.9 mM TCE and 0.2 g/L ZVI were used. Details

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regarding the batch experiments and chemical analyses for TCE degradation and H2

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generation are provided in SI.

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Reaction Kinetics. For S-mZVIbm, the disappearance of TCE and the formation of daughter

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products were fit concurrently to the proposed reaction pathways using the software package,

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Scientist V3.0 (Micromath, Missouri, USA). The transformation of TCE by mZVIbm was not

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modeled because either (i) there was not sufficient reaction under iron-limited conditions or

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(ii) the particles were gradually deactivated (excess iron conditions). In this case, initial TCE

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dechlorination rates were determined by fitting the product generation data to

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pseudo-first-order kinetics.

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Quantification of Efficiencies. Terms such as “electron efficiency”, “electron selectivity”,

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and “particle efficiency”, and “utilization ratio” have been used in recent studies, but not

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always precisely or consistently.30, 32, 33, 35, 53 We define here two types of efficiencies (or

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selectivities) for the anaerobic ZVI-TCE-H2O system (only significant oxidants in the system

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are H2O and the contaminant). One is the efficiency of Fe(0) utilization (εFe(0)), which is the 7

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molar fraction of total Fe0 (provided by the ZVI) that reacts with TCE or H2O (Equation 1) CFe0 ,i − C Fe0 ,f

ε Fe (0) =

(1)

C Fe0 ,i

where C Fe ,i and CFe ,f is the initial and final molar quantities of Fe0. 0

0

The other type of efficiency is termed electron utilization (εe), which is the fraction of

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electron equivalents from Fe0 that are used by reduction of TCE (to all products). To quantify

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εe requires that stoichiometries be assumed for the characteristic half reactions:

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Fe 0 → Fe 2+ +2e -

(2)

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TCE + n e- + m H+ → products + g Cl-

(3)

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2H+ + 2e− → H2 ↑

(4)

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The values of n, m and g depend on the products formed, as shown in Table S1. For the

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major products observed in this study, n is 4, 6, and 8 for acetylene, ethene, and ethane,

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respectively. The value of n for the overall dechlorination TCE is calculated using:

∑n p ∑p i

n=

i

i

i

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(5)

i

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where ni is the stoichiometry for product i, pi is the molar quantity of that product.

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In general, the efficiency of electron utilization (εe) is calculated using Equation 6:

εe =

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∑n p i

i

i

∑n p i

i

+ 2 M H2

(6)

i

where MH2 is the molar quantity of H2 produced during the TCE dechlorination. Both of the efficiencies reported in this study could vary with time over the course of

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an experiment, so the elapsed time must be specified. In this study, the elapsed time was 24 hr

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and 8 d for excess and limited iron conditions, respectively.

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Results and Discussion

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Particle Characterization. The solid phases that resulted from mechanochemical sulfidation

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of ZVI were characterized by X-ray diffraction (XRD), scanning electron microscopy with

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energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy

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(XPS). The XRD spectra (Figure 1A) show diffraction lines for α-Fe0 and S0 after 2 hr of

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milling at S/Fe = 0.2 mol/mol, but between 5 and 10 hr milling, the diffraction lines for S0

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disappear and only α-Fe0 and FeS were observed. This suggests a mechanochemical,

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solid-state reaction corresponding to Fe0 + S0 ⇌ FeS, which forms the Fe/FeS composite

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material herein referred to as S-mZVIbm. After further milling (up to 30 h), the peaks

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corresponding to α-Fe0 become broader, which is consistent with the decrease in crystallite

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size expected from extended milling. Rietveld analysis of the XRD spectra for S-mZVIbm

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indicates that the crystallite size of α-Fe0 and FeS phases after 20 h milling was 28.8 nm and

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15.2 nm, respectively.

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Figure 1. (A) XRD spectra for pristine ZVI and S-mZVIbm (S/Fe = 0.2 mol/mol) milled for 2 to 30 hr ; (B) SEM image of S-mZVIbm after 20 h milling at S/Fe = 0.1 mol/mol; (C) Molar fraction of Fe(0), Fe(II) and Fe(III) in pristine ZVI, mZVIbm, and S-mZVIbm derived from fitting of XPS Fe 2p spectra; (D) S 2p XPS spectra of S-mZVIbm. 9

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The morphology of the ZVI before and after 20 hr milling at S/Fe = 0.1 mol/mol was

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determined by SEM (Figure S1 and 1B). Before milling, ZVI and S particles can be

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distinguished easily in the SEMs because of the sharp difference in their electron

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conductivity (S0 is not conductive and therefore shows as bright spots in Figure S1) and the

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EDS, which shows that Point 1 and 2 on Figure S1 correspond to predominantly Fe and S

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particles, respectively (Table S2). After milling for 20 h, the particles appear uniform by

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SEM and the average atomic ratio of S/Fe determined by EDS at randomly selected two spots

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(Points 3 and 4 on Figure 1B) was 0.104 (Table S2), which is close to the theoretical value of

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0.1 that would be expected if S were homogeneous distributed throughout the milled particles.

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The uniform distribution of Fe and S is further supported by mapping of these elements by

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SEM-EDS (Figure S2).

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Additional data on the speciation of Fe and S in these materials was obtained by XPS.

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The results in Figure S3A show that the molar fraction of O in the pristine ZVI was

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significantly (~10%) higher than that of S-mZVIbm and mZVIbm. The high resolution Fe2p

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XPS spectra for pristine ZVI, mZVIbm, and S-mZVIbm are shown in Figure S3B-D. Fitting

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these peaks gave the distribution of iron oxidation states summarized in Figure 1C (details of

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spectra fitting are provided in SI). Figure 1C shows that Fe0 was absent only on the surface

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of pristine ZVI, and increased to ~12% on the surface of mZVIbm. This suggests that pristine

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ZVI is protected by a relatively thick oxide layer and this layer was destroyed or displaced

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during ball milling. This would allow contact between Fe0 and S0 resulting in formation of

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FeS. This reaction is not likely to be significant without ball milling, but is supported in this

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case by the decrease of Fe(0) and increase of Fe(II) content in S-mZVIbm compared to

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mZVIbm.

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The S 2p spectrum of S-mZVIbm in Figure 1D was fitted with doublets representing

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2p3/2 and 2p1/2 and the details are in SI.54, 55 The surface of S-mZVIbm consists predominantly

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S2- with negligible S22- and Sn2-. Significant amount of disulfide has been seen in previous

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studies of sulfidated nZVI that was synthesized using the borohydride method.36, 37, 40

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Disulfide presumably is formed by oxidation of S2- by incidental O2, so the absence of S22-

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from S-mZVIbm suggests the material is relatively stable in air. The S/Fe molar ratio on the

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surface of S-mZVIbm was ~0.06, which is less than the theoretical value of 0.1 likely due to

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the presence of the iron oxide layer. This S/Fe ratio also confirms that S is likely

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homogeneously distributed in S-mZVIbm and not significantly concentrated on the surface.

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The particle size (d50) of the S-mZVIbm decreased from 38.0 to 6.3 µm after 20-h

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milling, while that of mZVIbm only decreased to 12.3 µm under the same milling conditions

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(Table S3), suggesting that the presence of S during milling results in greater erosion of the

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particles. SEM images of mZVIbm (Figure S4) also show a flake shape of the particles in

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contrast to the spherioid shape of S-mZVIbm. The BET surface area of the S-mZVIbm and

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mZVIbm particles was determined to be 1.43 and 0.21 m2/g, respectively. The total Fe and Fe0

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contents of mZVIbm obtained from chemical digestion were 98.3% and 92.7%, respectively.

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Due to the incorporation of S and the reaction between Fe0 and S0, the total Fe and Fe0

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contents of S-mZVIbm decreased to 95.5% and 85.5%, respectively.

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Electrochemical Characterization. Chronopotentiometry (CP) for 13 hr after initial

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immersion of the electrode gave stable and consistent open circuit potentials (EOCP), with

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final values of −479±4 and −516±1 mV (vs. SHE) for mZVIbm and S-mZVIbm respectively

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(Figure 2A). At 13 and 15 hr, linear polarization resistance (LPR) measurements gave similar

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values for the respective corrosion potentials (Ecorr, Table S4), so both methods indicate that

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ball-milling with S0 lowered the electrode potential of the mZVI by about 40 mV. The LPR

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data (raw data not shown) were fitted to obtain the polarization resistance (Rp) of each

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electrode, and these results are summarized in Table S4. By testing independent preparations

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of the PDE using fresh electrolyte (e.g., labeled 1.1 vs. 1.2) and reused electrolyte (e.g.,

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labeled 2.1 vs. 2.2), the significance of variations in Rp could be determined. LPR at 13 and

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15 hr produced very similar values of Rp, sequential reuse of the electrolyte produced modest

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increases or decreases, and a follow-up experiment with new agarose (Run 4) gave

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consistently higher Rp’s than earlier experiments. With these sources of background

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variability, the data are not sufficient to resolve an effect of sulfidation on Rp which is 11

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consistent with the complex interpretation given to the Rp data in our previous work.56 After LPR, linear sweep voltammetry (LSV) was performed and the data were fitted by

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Tafel analysis to obtain Ecorr and β (cathodic and anodic slopes). The results (Figure 2B and

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Table S5) also show that Ecorr decreased about 40 mV with sulfidation, but the individual

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values of Ecorr obtained from LSV are consistently about 30-60 mV more negative than the

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values from LPR or CP (e.g., cf. Figure 2A and B). This effect is the expected result of

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initiating LSV with polarization at −200 mV (with respect to the last measured EOCP), and has

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been discussed previously.56, 57 The Rp values obtained from Tafel analysis of LSV data

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correlate well with those obtained by LPR (Table S4) and any effect of sulfidation is still less

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than other factors.

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Figure 2. Electrochemical characterization of stationary PDEs made from mZVIbm and S-mZVIbm. (A) Open circuit chronopotentiograms (B) Linear sweep voltammogram at 0.1 mV s−1. (C) Redox Ladder of published thermodynamic potentials for representative couples. Measured results for Fe/FeO and Fe/FeS are from Turcio-Ortega et al..56 Literature data in C from Amonette58 and Ning et al..59

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For comparison with the CP and LSV results obtained in the study, potential data from other

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sources are shown in Figure 2C. The data in the measured category are from our prior work

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on the electrochemistry of ZVI that was sulfidated by a different method than used in this

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study. In the older study,56 the PDEs were packed dry with a sulfidated ZVI that was a

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multiphase material with some core-shell character, and the medium composition was varied,

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including deionized water and various concentrations of chloride salts, Tris buffer, and 12

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natural organic matter. The resulting range in measured potentials was significant (~150 mV),

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but the average value is about 100 mV more negative than the values obtained in this study.

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Among the possible explanations for this difference, the most likely is that the current work

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was done with micron-sized ZVI, whereas the older work was done with nano-sized ZVI,

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which has been shown to give more reducing measured potentials over a range of

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conditions.14, 60 In both studies, sulfidation favors more negative potentials, but not enough to

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separate the data into categories that do not overlap.

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The right side of Figure 2C shows potentials for selected redox couples calculated

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using standard potentials from thermochemical data. The conditions used in these

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calculations were chosen to approximate those under which the potential measurements were

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made in this study, but there are significant uncertainties in these assumptions, such as what

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Fe(II) concentration to use (cf., the Fe(II) vs. time data in Figure S5). In simpler systems, the

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alignment between measured and calculated potentials may be attributed to specific reactions

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at the iron surface.57, 61 However, with multiphase sulfidated ZVI, the measured values will

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be mixed potentials,60 probably dominated by redox couples involving Fe(0) and FeS, but

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also including other iron oxides and sulfides. This undoubtedly applies to our earlier work,56

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and to this work, to a lesser degree. However, given the comparatively uniform composition

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of the milled sulfidated ZVI described in this study, the measured potentials (Figure 2A,B)

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may not be mixed potentials of common redox couples (Figure 2C) so much as a “pure”

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potential of a relatively homogeneous Fe/S mixed phase. This hypothesis may merit further

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study, but this will require electrochemical characterization that falls well beyond the scope

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of this study.

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Dechlorination Pathways and Products. For mZVIbm and S-mZVIbm, time series plots for

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TCE and measured degradation products are shown in Figure 3, and the distributions of these

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species at the end of each experiment are summarized in Table S6. In all cases, the data

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suggest parallel and sequential reductive elimination and hydrogenolysis, but the relative

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significance of these pathways varied. Under excess iron conditions (Figure 3A,C),

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S-mZVIbm transformed TCE to acetylene, ethene, ethane, cis-DCE and even-numbered 13

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heavier hydrocarbons (C4, C6). Acetylene was the major intermediate (35% at peak level)

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and cis-DCE was the minor intermediate; whereas ethene and ethane were the major products.

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For mZVIbm, similar product distributions were observed except that 1,1-DCE was the major

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intermediate (1.6% vs. 0% acetylene) and that the final products included more C3

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hydrocarbons (Table S6). These distributions of reaction products are similar to those

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reported in previous studies using micron-sized reagent-grade ZVI62 and nZVI synthesized

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from reduction of Fe3O4 nanoparticles by hydrogen (RNIP, Toda, Japan).32 In general, these

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product distributions suggest that TCE dechlorination occurs mainly by reductive

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β-elimination, for both S-mZVIbm and mZVIbm. The lack of acetylene in the experiments with

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mZVIbm is likely due to its high reactivity with ZVI, so that the intermediate did not

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accumulate to detectable concentrations.62 In contrast, the acetylene observed in the

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experiments with S-mZVIbm, is consistent with reactivity controlled by sulfidated iron phases,

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which reduce acetylene more slowly than ZVI (Figure S6A vs. 6B). This effect of sulfidation

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has not been noted previously, although it is consistent with the data reported in several prior

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studies.40

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Figure 3. Kinetics of TCE dechlorination and formation of major products by (A) S-mZVIbm in excess, (B) S-mZVIbm as limiting reactant, (C) mZVIbm in excess, and (D) mZVIbm as limiting reactant; H2 evolution in the presence and absence of TCE: (E) excess iron and (F) iron limited conditions. Smooth curves on (A) and (B) are calculated from the kinetic model with rate constants in Table 1; Connecting lines on (C)-(F) are only interpolated. H2 concentration in (E) and (F) is expressed as percent of the theoretical yield if all added Fe0 reacts according to Equations 2-4. All experiments at pH 7 buffered by 50 mM HEPES and under anoxic conditions.

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Under iron-limited conditions (Figure 3B,D), S-mZVIbm was depleted after 8 days

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(based on lack of hydrogen generated after adding concentrated HCl to the particles

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recovered after reaction). S-mZVIbm reduced TCE predominantly to acetylene (accounting for

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32.4% of the 57.4% TCE removed, Table S6), but acetylene did not appear to be further 15

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reduced, presumably due to strong competition from excess TCE for the reaction sites. Under

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same conditions, mZVIbm was also depleted, but there was little reduction of TCE

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dechlorination (only ~5%), with accumulation of only trace quantities of products (mainly

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ethene, ethane, and C4 hydrocarbons). These results suggest that reductive β-elimination is

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still the dominant pathway of TCE dechlorination under iron-limited conditions, even though

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the rates and quantities of contaminant transformation varied significantly.

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The final steps of the TCE dechlorination pathway were further characterized with

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batch experiments initiated by adding acetylene and ethene to S-mZVIbm and mZVIbm. Under

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excess iron conditions, mZVIbm rapidly and completely transformed acetylene to ethene,

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ethane, and longer chain hydrocarbons (mainly C3-C6) in 1.5 hr (Figure S6A). Under the

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same reaction conditions, S-mZVIbm also reduced acetylene to ethene, ethane, and C3-C6

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products, but the reaction rate was significantly slower (