Enhanced Reactivity and Electron Selectivity of Sulfidated Zerovalent

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Enhanced Reactivity and Electron Selectivity of Sulfidated Zerovalent Iron toward Chromate under Aerobic Conditions Jinxiang Li, Xueying Zhang, Meichuan Liu, Bing-Cai Pan, Weiming Zhang, Zhong Shi, and Xiaohong Guan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06502 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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Environmental Science & Technology

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Enhanced Reactivity and Electron Selectivity of

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Sulfidated Zerovalent Iron toward Chromate under

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Aerobic Conditions

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Jinxiang Li,†,‡ Xueying Zhang,†,‡ Meichuan Liu,§ Bingcai Pan,∥ Weiming Zhang,∥

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Zhong Shi,⊥ and Xiaohong Guan†,‡,* †

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State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, P.R. China

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‡

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§

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Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, P.R. China

School of Chemical Science and Engineering, Tongji University, Shanghai 200092, P.R. China

∥State

Key Laboratory of Pollution Control and Resources Reuse, School of Environment, Nanjing University, Nanjing 210023, Jiangsu Province, P.R. China ⊥Department

of Physics, Tongji University, Shanghai 200092, P.R. China

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*Corresponding author: Xiaohong Guan

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Email: [email protected]; Phone: +86-21-65980956; Fax: 86-21-65986313

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ACS Paragon Plus Environment


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Abstract

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When zerovalent iron (ZVI) is used in reductive removal of contaminants from

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industrial wastewater, where dissolved oxygen (DO) competes with target

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contaminant for the electrons donated by ZVI, both the reactivity and electron

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selectivity (ES) of ZVI toward target contaminant are critical. Thus, the reactivity and

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ES of two sulfidated ZVI (S-ZVI) samples, synthesized by ball-milling with elemental

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sulfur (S-ZVIbm) and reacting with Na2S (S-ZVINa2S), toward Cr(VI) under aerobic

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conditions were investigated. Sulfidation appreciably increased the reactivity of ZVI

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and the ratio of the rate constants for Cr(VI) removal by S-ZVIbm or S-ZVINa2S to their

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counterparts without sulfur fell in the range of 1.4-29.9. ES of S-ZVIbm and S-ZVINa2S

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toward Cr(VI) were determined to be 14.6% and 13.3%, which were 10.7- and 7.5-

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fold greater than that without sulfidation, respectively. This was mainly ascribed to

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the greater improving effect of sulfidation on the reduction rate of Cr(VI) than that of

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DO by ZVI. The improving effects of sulfidation on the performance of ZVI were

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mainly due to the following mechanisms: sulfidation increased the specific surface

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area of ZVI, the FeSx layer facilitated the enrichment of Cr(VI) anions on S-ZVI

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surface because of its anions selective property and favored the electron transfer from

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Fe0 core to Cr(VI) at the surface because of its role as efficient electron conductor.

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1. Introduction Zerovalent iron (ZVI) can remove a variety of water contaminants, such as

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halogenated organics,1,

2

nitroaromatics,3,

4

azo dyes,5,

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metalloids,9-14 etc., by a mixture of transformation, adsorption, and coprecipitation

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processes.15 Thus, ZVI has been widely used in the remediation/treatment of

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groundwater and wastewater in the past few decades.16 However, ZVI generally has

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low reactivity due to the inherent oxide layer generated in manufacturing process of

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ZVI17 and that generated during the reaction of ZVI with substrate.18 Therefore,

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multiple countermeasures have been developed to improve the reactivity of ZVI, such

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as pretreatment of pristine ZVI (e.g, premagnetization or acid washing),19-22 synthesis

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of nanoscale ZVI (i.e., nZVI),23 combining ZVI with additives (e.g., Pd or Ni),24

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dosing of chemicals (e.g., Fe2+25 or H2O226), application of weak magnetic field

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(WMF),27 as well as the modification/transformation of ZVI by exposure to sulfur

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compounds of various oxidation states (i.e., sulfidation of ZVI).28-31 Although all of

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these methods can improve the reactivity of ZVI toward various contaminants to

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different extents, very few studies have reported the improved electron selectivity (ES)

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of ZVI with these methods except Fe2+ dosing25 and sulfidation.32 The reaction of ZVI

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with natural reducible species, such as oxygen, protons, water, and other co-existing

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solutes (e.g., nitrate) would cause low ES of ZVI toward target contaminants, which

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would result in the waste of ZVI materials.16,

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performance of ZVI, improving both the reactivity and the ES of ZVI is of particular

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interest.

31

6

heavy metals,7,

8

and

Therefore, to optimize the

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Among all of these approaches to enhance or expand the applicability of ZVI, the

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sulfidation method recently stands out mainly because it has the potential to greatly

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improve both the reactivity and ES of ZVI.30, 31, 33 The initiated sulfidation should be

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enlightened by the heuristic work on using dithionite (Na2S2O4) for fabricating nZVI

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and rejuvenating aged nZVI.34-36 Then the sulfidation of nano-ZVI (nZVI) was

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deliberately proposed by Kim et al..28 Briefly, an appropriate amount of dithionite

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(Na2S2O4) was dissolved in NaBH4, then the mixed solution was continuously added

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dropwise to the FeCl3 solution.28 In this process, Fe0 was formed in parallel to the

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generation of FeS. In that work, the authors explicitly found that S-nZVI showed a

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much higher reactivity toward trichloroethylene (TCE) than pure nZVI.28 Most of the

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researchers focused on improving the reductive reactivity of nZVI by sulfidation

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before publication of the paper authored by Rajajayavel and Ghoshal.30 These authors

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showed that S-nZVI caused significantly higher TCE degradation rate than bare nZVI

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but evolved H2 at a slower rate, which suggested that sulfidation increased the ES of

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nZVI toward TCE.30 These results directed researchers to pay attention to the

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improving effect of sulfidation on the ES of nZVI under anaerobic conditions and led

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to a boom of research on contaminants transformation by S-nZVI.30-33, 37-44 Several

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other studies have also confirmed that sulfidation could significantly elevate the ES of

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nZVI towards TCE under anaerobic conditions.30, 31, 33 The published literatures on

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S-nZVI illustrated that the S/Fe molar ratio, which is defined as the ratio of the molar

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concentration of total sulfur in sulfidation reagents (e.g., Na2S,39 Na2S2O3,33 and

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Na2S2O4,28 etc.) to that of total iron (including Fe0, Fe(II), and Fe(III)), was one of the

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critical factors affecting the reactivity of S-nZVI toward target contaminants.

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Up to now, all of S-nZVI samples investigated in the literature were synthesized

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with wet methods. The synthesis methods can be divided into two categories, one-step

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synthesis method28 and two-step synthesis method,39 depending on the sequence of

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sulfidation relative to iron reduction. Considering that the micro-sized ZVI (noted as

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ZVI in this study) is more readily available, inexpensive, and environmental friendly

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than nZVI, Xu et al.45 employed sulfidated ZVI (S-ZVI), fabricated by treating ZVI

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particles with Na2S under anaerobic conditions, to remove Orange I from water.

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Besides the wet synthesis method of fabricating S-ZVI with dissolved and reduced

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sulfur species, He’s group46-48 recently developed a novel process for synthesizing

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S-ZVI by ball-milling ZVI particles with elemental sulfur (S). It was found that the

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ball-milled S-ZVI (noted as S-ZVIbm) exhibited much higher reactivity toward TCE

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than that its counterpart without S under either ZVI-limited or TCE-limited

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conditions.46 But the performance of S-ZVI prepared with ball-milling and that

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synthesized by wet methods have never been compared.

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To date, most of the studies on contaminants removal by S-ZVI or S-nZVI were

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performed under anaerobic conditions and only four studies had investigated the

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influence of sulfidation on the performance of contaminants removal by ZVI or nZVI

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open to the air.41, 42, 45, 49 These four studies showed that sulfidation could appreciably

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accelerate the removal of Orange I45 and antimonite49 by ZVI and enhance the

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removal of Cd(II)42 and diclofenac41 by nZVI under aerobic conditions. When ZVI is

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employed in industrial wastewater treatment, oxygen is one of the major electron

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acceptors. This is very different from the situation for groundwater remediation,

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where there is little oxygen or no oxygen at all. Although sulfidation can increase the

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ES of nZVI and ZVI towards the target contaminants over the reduction of water

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under anaerobic conditions, the influence of sulfidation on the ES of ZVI under oxic

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conditions keeps unknown, where oxygen is a much stronger electron accepter than

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H2O.

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Consequently, taking chromate (Cr(VI)), a suspected carcinogen to organisms

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and widely present in wastewater, as the targeted contaminant, the objectives of this

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work are to: (i) compare the performance of S-ZVI fabricated with ball-milling and

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that with wet-synthesis method at different S/Fe molar ratios; (ii) explore the

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transformation of sulfur species in Cr(VI) sequestration of contaminants by S-ZVI;

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and (iii) clarify the contribution of sulfidation in the electron selectivity of ZVI

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toward Cr(VI) in consecutive runs under aerobic conditions. It should be noted that

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Na2S-treated ZVI was employed as a typical S-ZVI sample fabricated with wet

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methods since previous study showed that the sulfidation reagent (viz., sodium sulfide,

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dithionite, or thiosulfate) or the sequence of sulfidation only slightly affected the

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reactivity of S-nZVI samples.33

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2. Experimental Section

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2.1. Chemicals and Particle Preparation

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All chemicals employed in this study were of analytical grade and used as

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received. The granular ZVI used in this study, if it was not specified, was purchased 7



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from the Shanghai Jinshan Reduced Iron Powder Factory, China. The BET surface

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area of Jinshan ZVI was determined to be 0.30 m2/g. All the other chemicals including

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potassium bichromate (K2Cr2O7), Sodium sulfide (Na2S·9H2O), sodium sulfate

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(Na2SO4), sodium chloride (NaCl), 2-(N-morpholino)ethanesulfonic acid (MES),

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sodium hydroxide (NaOH), hydrochloric acid (HCl), and elemental sulfur (S), were

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purchased from the Sinopharm Chemical Reagent Company, China. The details of

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preparing S-ZVIbm and S-ZVINa2S are present in Text S1. The synthesized S-ZVI

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samples were stored in a N2-filled glovebox (Mikrouna, china) for subsequent

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characterization and use.

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2.2. Procedures of Batch, Consecutive Tests, and the

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Competitive Experiments between Cr(VI) and Dissolved

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Oxygen (DO)

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To investigate the influence of sulfidation method and S/Fe molar ratio on Cr(VI)

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removal by ZVI, batch tests were initiated by adding 0.25 g of S-ZVI of different S/Fe

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molars into a 500 mL solution with 5.0 mg L−1 Cr(VI) and 0.5 mM Na2SO4 as the

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background electrolyte, which was freshly prepared for each batch test, and the

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mixture was mixed at 400 rpm with a mechanical stirrer. The initial pH (pH0) was

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adjusted to 6.0 with NaOH and H2SO4, and no attempt was made to maintain a

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constant pH during the experiments. The influence of sulfidation on Cr(VI) removal

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by ZVI was also determined at pH0 ranging from 4.0 to 10.0 and at initial Cr(VI)

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concentration of 2.0-12.0 mg/L. Moreover, the effect of sulfidation on Cr(VI) removal

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by ZVI samples from 6 different origins was also examined so as to check the benefit

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of sulfidation on improving the reactivity of ZVI from a wide spectrum view.

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To simulate the performance of S-ZVI for Cr(VI) sequestration in continuous

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operation and determine the influence of sulfidation on ES of ZVI toward Cr(VI)

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under aerobic conditions, up to 7-8 re-spikes of Cr(VI) were performed on one batch

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of S-ZVI suspension. To initiate the first run, 5.0 g Fe0 particles were dosed into a 500

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mL wide-mouth bottle containing 2.0 mg L−1 Cr(VI). When Cr(VI) was exhausted, 5

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mL Cr(VI) stock solution (200 mg/L) was added to the solution to achieve the similar

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Cr(VI) concentration (∼2 mg/L) at the beginning of each cycle. The concentration of

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Cr(VI) after Cr(VI) spiking at the beginning of each cycle was recorded. It should be

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specified that 1.0 mM NaCl was employed as the background electrolyte and HCl was

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used instead of H2SO4 for pH adjustment in this part to avoid the influence of SO42-

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on determining the ES of S-ZVI toward Cr(VI). The details of the experiments for

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identifying the competition of Cr(VI) and dissolved oxygen (DO) for the electrons

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donated by ZVI or S-ZVI are present in Text S2.

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For all the Cr(VI) removal experiments, at given time intervals, approximately 2

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mL of suspension was sampled, filtered through a 0.22 µm membrane filter, and

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acidified for analysis. Each experiment was performed in duplicate. If it was not

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otherwise specified, all experiments were performed open to the air and at 25 °C,

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which was controlled with a water bath.

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2.3. Analysis and Material Characterization

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The prepared S-ZVI samples and the control samples before and/or after reacting

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with Cr(VI) were characterized with scanning electron microscopy (SEM), BET

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specific surface area (SSA), S K-edge X-ray Absorption Near Edge Structure

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(XANES), X-ray photoelectron spectroscopy (XPS), and electrochemical tests. The

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details are summarized in Text S3. After the experiments, the concentration of Cr(VI) was determined with the

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diphenylcarbazide

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spectrophotometer.50

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1,10-phenanthroline colorimetric method using an UV/visible spectrophotometer

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(TU-1901, Purkinje General Instrument) at 510 nm. The concentrations of total

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soluble Cr and Fe were determined with ICP-AES (Agilent, 720ES). Various sulfur

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species including SO42-, SO32-, and S2O32- in aqueous solution were determined using

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an ion chromatography equipped with an Ion Pac AS19 analytical column (4 × 250

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mm) and an Ion Pac AG19 guard column (4 × 50 mm) (Thermo Scientific Dionex

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ICS-5000).

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3. Results and Discussion

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3.1. Influence of Sulfidation Treatment on the Properties of

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ZVI

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colorimetric The

method

Fe(II)

at

540

concentration

nm was

using

an

determined

UV-visible with

the

The properties of the prepared S-ZVI samples and the control samples were

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characterized with SEM, BET SSA, S K-edge XANES, XPS, and electrochemical

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tests. The ZVIbm particles are present mainly as flakes with smooth surface but there

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are more spherioids in the prepared S-ZVIbm samples and the surface becomes coarse,

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as illustrated in Figure S1. Similarly, there was an increase in surface roughness of

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ZVINa2S due to sulfidation treatment (Figure S1). It is interesting to find that the

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surface roughness of S-ZVIbm and S-ZVINa2S increased with increasing S/Fe molar

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ratio at small S/Fe molar ratio and dropped at large S/Fe molar ratio. The increase in

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surface roughness of S-ZVI could be due to the formed FexSy precipitates, whereas

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the transition should be likely due to the pore plugging and masking by the excessive

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precipitates.28 Generally, the larger surface roughness of the S-ZVI sample, the larger

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BET SSA. Figure S2 shows the variation of BET SSA of the fabricated S-ZVI with

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S/Fe molar ratio. Specifically, the BET SSA was enhanced progressively from 0.97 m2

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g-1 to 33.28 m2 g-1 for S-ZVIbm and 0.57 m2 g-1 to 13.24 m2 g-1 for S-ZVINa2S, with

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increasing the S/Fe molar ratio from 0 to 0.010 and 0.050, respectively. However, the

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BET SSA of both S-ZVIbm and S-ZVINa2S evidently decreased with further increasing

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the S/Fe molar ratio.

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The amount of sulfur (wt%) incorporated in the synthesized S-ZVI samples was

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also quantified with carbon-sulfur analyzer and the results are summarized in Figure

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S3. As expected, although the bulk sulfur content progressively elevated with

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increasing S/Fe molar ratio from 0 to 0.200, the amount of sulfur contained in

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S-ZVIbm was always larger than that contained in S-ZVINa2S samples and the

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difference became larger at higher S/Fe molar ratio. The reason was that all of the

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added sulfur was incorporated in the prepared S-ZVIbm sample while the portion of

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sulfur incorporated in the S-ZVINa2S samples decreased with increasing S/Fe molar

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ratio.

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The S K-edge XANES spectra of the obtained S-ZVI samples were collected to

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identify the S species and their content in these samples, as demonstrated in Figure

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1(a). Evidently, the S K-edge XANES spectra of S-ZVI samples were all very close to

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those of the low-valent sulfur references, implying that these pristine S-ZVI were

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likely to be mainly composed of reduced sulfur (e.g., S2- and S22-) and/or intermediate

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oxidized sulfur (i.e., S0). LCF analysis of these S K-edge XANES spectra was carried

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out to semi-quantitatively identify the S species in the synthesized samples and the

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results are summarized in Figure S4(a). LCF analysis revealed that S in the

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synthesized S-ZVI samples was predominantly present as S2- and these samples also

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contained some S22-, S0, and SO42- (Figure S4(a)). As the S/Fe molar ratio increased

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from 0.010 to 0.200, the S2- content in the bulk of S-ZVIbm enhanced progressively

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from 0.22% to 7.30% and that in the bulk of S-ZVINa2S increased to ~1.7% at S/Fe

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molar ratio of 0.050-0.100 and then dropped to 1.1% at S/Fe molar ratio of 0.200.

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XPS measurements were also performed to determine the chemical state of S on the

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surface of prepared S-ZVI particles with S/Fe molar ratio of 0.05 to better understand

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the surface properties of the materials (Figure S5). Note that XPS only detects

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materials in the first several nanometers of the surface.37 Peak assignment was based

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on literature reported binding energies of sulfide minerals and the spectra of reference

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materials acquired under the same conditions as the samples.33 The elemental

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composition of sulfur was calculated using peak areas with appropriate sensitivity

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factors.28 Similar to the results offered by the XANES spectra, four kinds of sulfur

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(i.e., S2-, S22-, S0, and SO42-) were all detected on the surface of both “as synthesized”

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S-ZVI samples. The possible reactions leading to the generation of sulfur species in

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synthesized S-ZVI samples are summarized in Table S1. Gu et al. found that the

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surface of S-ZVIbm fabricated in their study consisted predominantly S2- but the

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presence of other S species is negligible,46 which is very different from the S-ZVIbm

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prepared in our study. It may be ascribed to the different atmosphere and milling

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durations employed in these two studies. Gu et al. milled the S-ZVIbm sample in Ar

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atmosphere for 30 hours46 but we milled the sample in air atmosphere for 4 hours. The

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literatures on fabricating S-nZVI sample by treating nZVI with Na2S also reported the

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formation of S2-, S22-, and SO42- on the surface of S-ZVI33, 37 and elemental sulfur (S0,

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and possibly polysulfides) was observed on the surface of thiosulfate-treated nZVI.33

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The atomic concentrations of S on the surface of S-ZVIbm and S-ZVINa2S (with

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S/Fe molar ratio) determined with XPS analysis were 3.14% and 6.30%, respectively.

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Comparing the S contents determined by XANES and XPS, it could be concluded that

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S was distributed almost evenly in the S-ZVIbm sample while the concentration of S

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on the surface of S-ZVINa2S sample was much larger than that in the bulk. Moreover,

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there was much more sulfate on the surface of S-ZVIbm and S-ZVINa2S samples than in

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the bulk, which may be due to the oxidation of sulfur species on the surface S-ZVI

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samples during preparation. Based on the atomic concentration of S on the surface of

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S-ZVI and the fraction of S2- determined by simulating the XPS spectra, the atomic

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concentrations of S2- on the surface of S-ZVIbm and S-ZVINa2S samples are 1.14% and

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1.41%, respectively.

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To unravel the influence of sulfidation on the corrosion reactivity of ZVI, the

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linear sweep voltammograms (LSVs) of Pt electrode chemically modified with S-ZVI

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were collected as a function of S/Fe molar ratio (Figure S6). The resulting current

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density (i) vs potential data (E) were used to determine the polarization resistance as

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defined by Eq. 1.51

264

=

. ( )

(1)

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where is the polarization resistance (Ω cm2), is the corrosion current

266

density (mA cm-2), and are the anodic and cathodic Tafel constant (V dec-1),

267

respectively.52 As summarized in Table S2 and depicted in Figure 2(a), it was found

268

that is more negative for S-ZVI than that without sulfidation, indicating that

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S-ZVI had a greater overall rate of electron transfer than non-sulfidated ZVI. With

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increasing S/Fe molar ratio, the dropped sharply and then increased slightly. It

271

was also noticed that sulfidation had little effect on the corrosion potential (Ecorr) of

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S-ZVINa2S but the Ecorr of S-ZVIbm decreased about 20-60 mV with sulfidation at S/Fe

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molar ratio of 0.010-0.100. The different effects of sulfidation on the corrosion

274

potential of S-ZVINa2S and S-ZVIbm may be due to the different distributions of sulfur

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on their surface and in their bulk.

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3.2. Influence of Sulfidation Variables and Water Matrices on

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Cr(VI) Removal by S-ZVI

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The kinetics of Cr(VI) removal by S-ZVI samples synthesized with different

279

methods and different S/Fe molar ratios and their counterparts without S are present

280

in Figure S7. Obviously, sulfidation could remarkably enhance the kinetics of Cr(VI)

281

sequestration by ZVI at pH0 6.0, regardless of the sulfidation method and S/Fe molar

282

ratio. Only 23.4% and 34.6% of Cr(VI) were removed by ZVIbm and ZVIH2O in 3.0 h,

283

respectively. However, it took only 45 min to remove 47.3% of Cr(VI) by S-ZVIbm

284

with S/Fe molar ratio as low as 0.010. As the S/Fe molar ratio of S-ZVIbm increased

285

from 0.010 to 0.050, the rate of Cr(VI) by S-ZVIbm increased progressively but a

286

further increase in S/Fe molar ratio would cause a drop in the removal rate of Cr(VI)

287

by S-ZVIbm. Likewise, the most rapid Cr(VI) removal by S-ZVINa2S sample was also

288

observed at S/Fe molar ratio of 0.050. However, the S/Fe molar ratio has smaller

289

effect on the kinetics of Cr(VI) removal by S-ZVINa2S than that by S-ZVIbm.

290

To quantify the influence of sulfidation on the reactivity of ZVI, the kinetics of

291

Cr(VI) removal by S-ZVI or ZVI were simulated with pseudo-first-order rate law.

292

Nevertheless, the kinetics of Cr(VI) removal by S-ZVI did not follow the

293

pseudo-first-order rate law throughout the reaction duration because of tailing.

294

Therefore, the Cr(VI) removal rates were quantified using only the initial reaction

295

stage, as shown by the solid lines in Figure S7. The obtained rate constants of Cr(VI)

296

removal by S-ZVIbm and S-ZVINa2S at pH0 6.0 are summarized in Figure 3(a).

297

As S/Fe molar ratio increased from 0 to 0.050, the rate constants for Cr(VI) 15



298

sequestration by S-ZVIbm enhanced gradually from 0.0104 to 0.1383 min-1 but higher

299

values of S/Fe resulted in decreased Cr(VI) removal rates. The rate constants of Cr(VI)

300

removal by S-ZVINa2S was either larger than or close to those by S-ZVIbm over the

301

S/Fe molar ratio of 0.010-0.200. The largest rate constant of Cr(VI) removal by

302

S-ZVINa2S was 0.1344 min-1, very close to that by S-ZVIbm. The high Cr(VI) removal

303

rate observed at low S/Fe ratios can be explained by either enhanced ZVI corrosion

304

due to sulfide facilitated corrosion and/or increasing formation of reactive FeS and/or

305

FeS2.39 The enhanced ZVI corrosion induced by sulfidation was evidenced by the

306

more intense change of pH, DO, and ORP during the process of Cr(VI) removal by

307

S-ZVI, as demonstrated in Figure S8.

308

Since the most rapid Cr(VI) removal was achieved at S/Fe molar ratio of 0.05 for

309

both S-ZVIbm and S-ZVINa2S samples, the influence of sulfidation on Cr(VI) removal

310

as functions of initial pH, initial Cr(VI) concentration, and ZVI origins was further

311

evaluated at S/Fe molar ratio of 0.05 and the data are demonstrated in Figures S9-S11.

312

The removal of Cr(VI) by either ZVIbm or ZVIH2O was always very slow over the pH0

313

range of 4.0-10.0 and only 13.3-34.7% Cr(VI) was removed in 3 h, as depicted in

314

Figure S9. Sulfidation greatly accelerated Cr(VI) removal by either ZVIbm or ZVIH2O

315

over the pH0 range of 4.0-10.0. About 59.9% and 88.1% Cr(VI) could be removed by

316

S-ZVIbm and S-ZVINa2S at pHini 10.0 in 3 h, respectively, though it was much slower

317

than that at lower pHini levels. As the initial Cr(VI) concentration increased from 2.0

318

mg/L to 12.0 mg/L at pH0 6.0, sulfidation could always enhance the removal of Cr(VI)

319

by ZVIbm or ZVIH2O (Figure S10). However, the removal of Cr(VI) by S-ZVIbm or

16


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320

S-ZVINa2S was retarded at higher initial Cr(VI) concentration, especially when the

321

initial Cr(VI) concentration was ≥ 8.0 mg/L. The slowed removal of Cr(VI) by ZVIbm

322

or S-ZVINa2S at elevated Cr(VI) concentration or pH should be ascribed to the

323

passivating effect of Cr(VI), which had also been reported in our previous study.53

324

Furthermore, sulfidation could enhance the kinetics of Cr(VI) removal by ZVI from

325

different origins to diverse extents, as shown in Figure S11. The initial portion of all

326

these data was simulated with pseduo-first-order rate law, as shown by the solid lines

327

in Figures S9-S11 and the obtained rate constants (kobs) for Cr(VI) removal are

328

summarized in Table S3, together with the experimental details. The influence of

329

sulfidation on the rate constants of Cr(VI) removal by ZVIbm or ZVIH2O under various

330

reaction conditions are further summarized in Figure 3(b), using the format of logk

331

with sulfidation treatment vs logk without sulfidation treatments. In this Figure,

332

Rsulfidation is defined as the ratio of the rate constant of Cr(VI) removal by S-ZVIbm or

333

S-ZVINa2S (kobs +S) to that obtained by ZVIbm or ZVIH2O (kobs –S). Rsulfidation always falls

334

in the range of 1.4-29.9 (Figure 3(b)). It was found that the sulfidation methods,

335

ball-milling and Na2S-treatment, have no obvious influence on the reactivity of

336

prepared S-ZVI samples for Cr(VI) removal under various reaction conditions.

337

Furthermore, the values of Rsulfidation have no clear relationship with the origins of ZVI

338

samples and solution chemistry. However, sulfidation could enhance the reactivity of

339

ZVI under all the reaction conditions investigated in this study. Therefore, sulfidation

340

is a promising method for enhancing the reactivity of ZVI towards Cr(VI) under oxic

341

conditions.

17



342

3.3. Influence of Sulfidation on the Removal Capacity and

343

Electron Selectivity of Cr(VI) by ZVI in Consecutive Runs

344

Both the reactivity and capacity of ZVI towards target contaminant determine the

345

successful application of ZVI in industrial wastewater treatment. After clarifying the

346

promoting effect of sulfidation on the reactivity of ZVI with batch tests, the influence

347

of sulfidation on the removal capacity of Cr(VI) by ZVI was evaluated in consecutive

348

runs and the results are shown in Figure 4(a-b). It was interesting to find that Cr(VI)

349

was removed by ZVIbm or ZVIH2O much more slowly when Na2SO4 was replaced with

350

NaCl as the background electrolyte, which was consistent with the observations in our

351

previous study.54 However, even with NaCl as background electrolyte, S-ZVIbm or

352

S-ZVINa2S could remove Cr(VI) rapidly in several consecutive runs. Although the rate

353

constants for Cr(VI) removal by S-ZVIbm or S-ZVINa2S decreased gradually from the

354

1st run to the 7th run (Figure S12), the S-ZVIbm and S-ZVINa2S held a higher reactivity

355

toward Cr(VI) even in the 7th run than their counterparts without sulfidation in the 1st

356

run. Moreover, the accumulated amounts of Cr(VI) removal by S-ZVIbm and

357

S-ZVINa2S in these 7 runs were determined to be 30.8 mg g-1 and 27.2 mg g-1, much

358

higher than that of 0.26 mg g-1 and 0.56 mg g-1 by the ZVIbm and ZVIH2O, respectively

359

(Figure 5(c)). Therefore, S-ZVIbm and S-ZVINa2S with S/Fe molar of 0.050 have both

360

much higher reactivity and much larger removal capacity of Cr(VI) than their

361

counterparts without sulfidation. About 66% and 57% of Fe0 in ZVIbm and ZVIH2O

362

were unreacted (Table S4) but no more Cr(VI) could be removed by prolonging the

363

reaction time, which should be due to the passivating effect of Cr(VI) on ZVIbm and 18


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364

ZVIH2O.

365

The sulfidation-induced improvement in the removal capacity of Cr(VI) by ZVI

366

may arise from the increase in the ES of ZVI toward targeted contaminant over

367

oxygen and/or the enhancement in the utilization rate (UR) of ZVI under aerobic

368

conditions.25 To calculate the ES and UR of ZVI, the soluble Fe, Cr, and S species and

369

those in the precipitates were quantified before and after the reaction. The results are

370

summarized in Table S4. It should be noted that the Fe and S species in the

371

precipitates were quantified with HCl digestion method and LCF of S K-edge XANES

372

spectra (Figure S13), respectively. Since XPS analysis revealed that only Cr(III) was

373

detected on the surface of Cr(VI)-treated ZVI and S-ZVI samples (Figure S14), it was

374

assumed that all of the removed Cr(VI) was reduced to Cr(III) when the ES of ZVI

375

and S-ZVI was calculated.

376 377 378

The ES of S-ZVIbm and S-ZVINa2S (

! / #

%$, %) can be calculated

via the Eqs. 2-5. + ++ &' = 3(&)*() + &)*() )

(2)

379

+ ++ + + ++ + + + & = 8.&() / + &()/ − &() 1 + 2.&()/ − &() 1 + (&()/ + &()/ − &() )

380

(3)

381 382

+ + + &34 = 25.&34() − &6 ) − (&34() − &6$ 17 + 3(&34() − &34() ) $

! /#

%$=8

89:

% 8;

Enhanced Reactivity and Electron Selectivity of Sulfidated Zerovalent

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