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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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Environmental Science & Technology

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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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1

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

ACS Paragon Plus Environment


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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

36

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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3



51

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

245

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.,

247

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

249

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

254

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

258

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

259

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

261

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

269

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

270

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

271

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

272

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.

276

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

277

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

279

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

280

separate the data into categories that do not overlap.

281

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

282

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

283

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

284

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

285

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

286

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

287

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

288

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

289

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

290

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

291

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

292

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

293

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

294

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

295

of this study.

296

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

297

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

298

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

299

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

300

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

301

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



302

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

303

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

304

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

305

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

306

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

307

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

308

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

309

product distributions suggest that TCE dechlorination occurs mainly by reductive

310

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

311

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

312

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

313

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

314

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

315

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

316

studies.40

14


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317 318 319 320 321 322 323 324 325

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.

326

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

328

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

331

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 (

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