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Advances in Sulfidation of Zerovalent Iron for Water Decontamination Jinxiang Li, Xueying Zhang, Yuankui Sun, Liping Liang, Bing-Cai Pan, Weiming Zhang, and Xiaohong Guan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02695 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Environmental Science & Technology

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Advances in Sulfidation of Zerovalent

2

Iron for Water Decontamination

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Jinxiang Lia, Xueying Zhanga, Yuankui Suna, Liping Liangb, Bingcai Panc,

4

Weiming Zhangc, Xiaohong Guana*

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a

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

6 b

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c

College of Life Science, Shaoxing University, Shaoxing 312000, P.R. China

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

9 10 11 12 13 14 15 16 17 18

*Corresponding author contact information:

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Xiaohong Guan, Email: [email protected]; Phone: +86-21-65980956; Fax:

20

86-21-65986313

1

ACS Paragon Plus Environment


21

Abstract

22

Sulfidation has gained increasing interest in recent years for improving the

23

sequestration of contaminants by zerovalent iron (ZVI). In view of the bright

24

prospects of the sulfidated ZVI (S-ZVI), this review comprehensively summarized the

25

latest developments in sulfidation of ZVI, particularly that of nanoscale ZVI (S-nZVI).

26

The milestones in development of S-ZVI technology including its background,

27

enlightenment, synthesis, characterization, water remediation and treatment, etc., are

28

summarized. Under most circumstances, sulfidation can enhance the sequestration of

29

various organic compounds and metal(loid)s by ZVI to various extents. In particular,

30

the reactivity of S-ZVI toward contaminants is strongly dependent on S/Fe molar ratio,

31

sulfidation method, and solution chemistry. Additionally, sulfidation can improve the

32

selectivity of ZVI toward targeted contaminant over water under anaerobic conditions.

33

The mechanisms of sulfidation-induced improvement in contaminants sequestration

34

by ZVI are also summarized. Finally, this review identifies the current knowledge

35

gaps and future research needs of S-ZVI for environmental application. The

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performances of S-ZVI for water decontamination should be systematically unraveled

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and assessed from a wide-spectrum perspective.

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1. Introduction

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Since the initiation of employing nanoscale zerovalent iron (nZVI) for

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dechlorination,1 the application of nZVI in contaminant removal or groundwater

43

remediation has arisen great interest in the researcher community.2-4 This interest has

44

led to the rapid development of this technology over the past two decades. Compared

45

to micro-sized ZVI (mZVI), the greater reactivity of nZVI is often thought to be the

46

result of larger overall surface area, greater density of reactive sites on the particle

47

surfaces, and/or higher intrinsic reactivity of the reactive surface sites.2, 5 The rich

48

chemistry has resulted in several behaviors of nZVI that are different from mZVI,

49

such as the highly degradation of contaminants by nZVI (e.g. polychlorinated

50

biphenyls and brominated flame retardants);6,

51

contaminants that mZVI can also treat (e.g. chlorinated ethylenes);8 and more

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favorable degradation products from contaminants, which can be rapidly degraded by

53

mZVI but yields undesirable byproducts (e.g. carbon tetrachloride).9,

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numerous hazardous organic and inorganic contaminants, including halogenated

55

organics,1,

56

perchlorate,24-26 dyes,27, 28 and uranium,29, 30 have been successfully removed by nZVI.

57

Although the nZVI technology from bench-scale tests to field-scale applications

58

has been advanced by a great deal of research, there are still two major technical

59

obstacles needed to be conquered: (i) The unstabilized nZVI tends to aggregate due to

60

van der Waals forces, high surface energy, and intrinsic magnetic interactions, etc.,31,

11, 12

nitroaromatics,13,

14

arsenic,15,

7

16

more rapid degradation of

10

So far,

heavy metals,17-20 nitrate,21-23

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61

32

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not so reactive as expected and the performance of nZVI in the field has not reflected

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the conventional wisdom—based on numerous bench studies—that the reactivity of

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nZVI with target contaminants is much higher than the counterpart with larger particle

65

size;33,

66

capacity consumed by target contaminant reduction to the total available reducing

67

capacity of nZVI for target contaminant reduction,35 should be improved. The reaction

68

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

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solutes (e.g., nitrate) would cause low selectivity of nZVI toward target

70

contaminants.34, 36-42 In light of these limitations, prior and ongoing research efforts

71

have provided several promising strategies that can potentially improve the

72

performance of nZVI. Numerous studies have been carried out to immobilize nZVI

73

onto various solid porous materials,43-52 such as carbon, silica, resin, bentonite,

74

kaolinite, montmorillonite, and zeolite, and coat nZVI with carboxymethyl cellulose

75

(CMC-nZVI),40,

76

agglomeration of nZVI. These measures can result in increased subsurface mobility

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and enhanced reactivity by increasing the specific surface area.8, 9 Doping of nZVI

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with a second (noble) metal such as Pd, Au, or Cu has also been well documented to

79

increase the reactivity of nZVI.55-60 It was reported that the selectivity of nZVI toward

80

trichloroethylene (TCE) was as low as 3.1 ± 1.4%, independent of the tested

81

conditions.39 Owing to the low selectivity, nZVI would be consumed by the non-target

82

compounds, resulting in its short lifespan (i.e., less than a day to several weeks).61

which could limit the mobility of nZVI in subsurface. The aggregated nZVI may be

34

(ii) The selectivity of nZVI, defined as the percentage of the reducing

53

or amphiphilic triblock copolymer,54 so as to prevent the

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83

Therefore, it was recently recognized that the selectivity of nZVI toward target

84

contaminant should be enhanced to increase its cost-effectiveness.34, 62

85

Among the approaches to improve the performance of nZVI, sulfidation of nZVI

86

(i.e., S-nZVI), which is defined as the chemical modification of nZVI particles by

87

reducing sulfur compounds, recently turned out to be technologically simple,

88

inexpensive and environmentally acceptable. Particularly, the published results have

89

showed that sulfidation can lead to a dramatic increase in the reactivity and selectivity

90

of nZVI.35, 61, 63-66 And it seems that sulfidation has the potential to become one of the

91

most promising and cost-effective approaches to significantly enhance the nZVI

92

performance. Sulfidation has been extensively employed for improving the

93

sequestration of various contaminants by mZVI and nZVI (known collectively as ZVI

94

in this review), whereas few papers have systematically summarized and compared

95

the performances of sulfur-modified ZVI (S-ZVI) reported in different studies.

96

Moreover, the critical factors controlling the enhanced reactivity of ZVI toward

97

contaminants caused by sulfidation are not well understood. No general conclusions

98

for the mechanisms of sulfidation-induced improvement in various contaminants

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sequestration by ZVI have been reached so far.

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Accordingly, the objectives of this review are to: (1) review the development of

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ZVI sulfidation and summarize the sulfidation methods reported in the literature; (2)

102

discuss the performances of various contaminants sequestration by S-ZVI, with a

103

focus on the influence of sulfidation on the contaminants removal rate, efficiency,

104

capacity by ZVI and the selectivity of ZVI toward target contaminants; (3) illustrate

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the influence of sulfidation on the properties of nZVI and discuss the mechanisms of

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sulfidation-induced influence on contaminants removal by nZVI. In addition, the

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pertinent engineering challenges and future research needs are also addressed to

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provide a future look to the iron-based technologies.

109

2.

110

sulfur-modified ZVI technology

111

2.1. Investigating the effect of sulfur compounds on the

112

reactivity of ZVI

Milestones

in

development

of

113

The milestones of S-ZVI technology development are summarized in Figure 1.

114

Since various organic and inorganic sulphur compounds are present in the

115

environment, an understanding of their possible impact on contaminants sequestration

116

by ZVI is critical.67 Therefore, the researchers initiated the research on investigating

117

the influence of several sulphur compounds including Na2SO4, Na2S, FeS, FeS2

118

(pyrite), and an organosulphonic acid (C8H18N2O4S, HEPES) on the kinetics of

119

carbon tetrachloride (CT) degradation by ZVI under aerobic conditions in 1994.67 It

120

was observed that all of the examined sulphur compounds accelerated CT reduction

121

by ZVI. Hassan also showed that the introduction of sulfur anions, including sulfide,

122

sulfite, sulfate, thiosulfate and elemental sulfur, accelerated TCE reduction by the

123

extra-pure ZVI.68 It was further demonstrated that addition of 1 mM NaHS to both

124

Fisher and Peerless iron granules increased the rate constants of TCE transformation

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125

by these materials.69 Generally, the dosing of sulfur compounds such as Na2SO4, Na2S,

126

FeS, FeS2, and organosulphonic acid, could increase the contaminants (e.g., TCE, CT,

127

pentachloroethylene (PCE), trichloroethane (TCA), cis-dichloroethylene (cDCE), etc.)

128

removal rates by 1-125 fold with the S content ranging from 0.2% to 6.4%.70 Given

129

that, sulfidation treatment of ZVI was proposed to improve the performance of ZVI.

130 131

Figure 1. The milestones in the development of S-ZVI technology.

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2.2. Fabricating of nZVI and rejuvenating aged nZVI with

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Na2S2O4

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In light of the improving effect of sulfur compounds on contaminants removal by

135

ZVI, pretreatment of nZVI with dissolved sulfur compounds should be a promising

136

approach for environmental cleanup. Nevertheless, there was limited information

137

about the modification of nZVI with sulfur compounds until the genesis of nZVI with

138

dithionite compound (Na2S2O4) via Eq. (1) in Table 1.71 Employing dithionite as the

139

reductant for nZVI production is favorite and promising since the reducing agent is

140

less expensive and widely available.72 Subsequently, Ma et al. compared the reactivity

141

of nZVI prepared by using NaBH4 as the reducing agent (nZVIBH4) with that using

142

Na2S2O4 as the reducing agent (nZVIS2O4) toward TCE.73 However, their results

143

revealed that nZVIS2O4 was less reactive than nZVIBH4 for TCE degradation. The

144

reason is that the nZVIBH4 particles are predominantly composed of an Fe0 inner core

145

covered by a thin iron oxide shell, while the nZVIS2O4 particles contain a significant

146

amount of magnetite and other minor Fe species (Fe0 and/or FeS species).73

147

Table 1. Main reactions occurring in synthesizing sulfur-modified nZVI. Materials Dithionite-reduced nZVI Dithionite-modified nZVI (One-step synthesis) Sulfide-modified nZVI (Two-step synthesis)

148

Reactions Fe2+ + S2O42- + 4OH- → Fe0 + 2SO32- + 2H2O

Eq. (1)

Ref. 71

2S2O42- + H2O → 2HSO3- + S2O32S2O42- + S2O32- + 2H2O + H+ → H2S + 3HSO3H2S → 2H+ + S2Fe2+ + S2- → FeS Fe0 + 2H2O → Fe(OH)2 + H2 Fe(OH)2 → Fe2+ + 2OHNa2S + H2O → 2Na+ + HS- + OHFe2+ + 2HS- → FeS + H2S 2FeS + 2H+ → FeS2 + Fe2+ + H2

(2) (3) (4) (5) (6) (7) (8) (9) (10)

76

63

Taking advantage of the reducing ability of dithionite, dithionite was also 9



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employed to reduce the passive layer of aged nZVIBH4 so as to restore the reactivity of

150

nZVIBH4.74 It was found that reduction of passive layer by low dithionite

151

concentrations (1 g/g of nZVIBH4) restored the suspension reactivity to levels equal to,

152

and occasionally greater than, that of unaged nZVIBH4.74 Multiple dithionite additions

153

could further improve Cr(VI) removal, achieving at a 15- fold increase in Cr(VI)

154

removal capacity relative to that of as-received nZVI (i.e., ∼300 mg vs ∼20 mg of

155

Cr(VI)/g of nZVI).74

156

However, it should be addressed that the above-mentioned work did not

157

highlight the role of sulfur contained in the synthesized nZVIS2O4 or the

158

dithionite-treated aged nZVIBH4 in the process of contaminants removal.

159

2.3. Proposing the synthesis methods of sulfur-modified

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ZVI

161

Considering that multicomponent nanoparticles may provide novel functions not

162

available in single-component nanoparticles and the presence of sulfur compounds

163

greatly improves chlorinated contaminants reduction by ZVI, S-nZVI has been

164

synthesized. The synthesis methods of S-nZVI can be divided into two categories,

165

one-step synthesis method and two-step synthesis method. The sulfidated nZVI

166

samples produced with the one-step method and two-step method are noted as

167

S-nZVIOne-step and S-nZVITwo-step, respectively.

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2.3.1. One-step synthesis of dithionite-modified nZVI

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Since nZVI produced by reducing Fe2+ with dithionite alone based on the

170

methods proposed in a patent contains very little Fe0,71 it is a milestone in the history

171

of developing S-ZVI technology that Kim et al. proposed a novel and facile one-step

172

route for fabricating S-nZVI.75 Briefly, an appropriate amount of dithionite was

173

dissolved in NaBH4, then the mixed solution was continuously added dropwise to the

174

FeCl3 solution.75 In this process, Fe0 is formed in parallel to the generation of FeS.

175

The remaining solution was decanted, and the precipitates were rinsed with degassed

176

water several times. The particles were dried in a vacuum oven for 1 d and stored in

177

an anaerobic chamber prior to characterization. The FeS precipitates on the Fe surface

178

are formed by the interaction between dissolved iron species and hydrogen sulfide,

179

one of the decomposition products of dithionite in solution (Eqs. 2-5 in Table 1).76

180

2.3.2. Two-step synthesis of sulfide-modified nZVI

181

Despite the prevalence of sulfidic conditions in subsurface environment, which

182

can be created by either direct injection of sulfides or stimulated in situ by microbial

183

sulfate reduction, its impact on contaminants sequestration by nZVI is not well

184

understood. To provide the fundamental geochemical understanding of Tc

185

sequestration during the development of sulfidogenic conditions in the presence of

186

nZVI, Fan et al. proposed to synthesize S-nZVI with a two-step method (i.e.,

187

S-nZVITwo-step).77 Firstly, nZVIBH4 is fabricated by reducing Fe(III) with NaBH4, then

188

the solid nZVI particles were recovered by flash drying, as previously described.9 11



189

Secondly, the synthesized nZVIBH4 and the sulfidation reagent (Na2S) are

190

pre-equilibrated under anaerobic conditions for a period of time. During this period,

191

nZVIBH4 reacts with water to generate Fe2+ ions, which form precipitate with S2- via

192

Eqs. 6-10 (Table 1). Hereafter, the nZVI/sulfide suspension is employed for

193

contaminants sequestration. The procedures of two-step synthesis method of

194

preparing S-nZVI are schematically illustrated in Figure 2.

195 196

Figure 2. Illustration of the major procedures of the two-step synthesis of S-nZVI. (a)

197

Preparation of S-nZVITwo-step by treating nZVI with Na2S; (b) Synthesis of FeS@nFe0

198

by precipitating FeS in the presence of nZVI; (c) Fabrication of S-CMC-nZVITwo-step

199

by treating CMC-nZVI with Na2S.

200

Employing this method, Fan et al. treated the freshly prepared CMC-nZVI with

201

Na2S or Na2S2O4 and showed that sulfidation with Na2S left most of the nZVI as

202

Fe0,35 whereas Na2S2O4 converted a majority of the nZVI to FeS, thus consuming

203

much of the reducing capacity originally provided by the Fe0. Very recently, inspired

204

by the idea of Fan et al.,35 a modified two-step synthesis strategy was proposed for

205

S-nZVI.78 To prepare S-nZVI with FeS as shell and nZVIBH4 as core (FeS@nFe0),

206

Fe2+ and Na2S stoichiometrically precipitated in the presence of nZVIBH4.78 Moreover,

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thioacetamide79 and thiosulfate80 have been proposed to treat the synthesized nZVIBH4

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to prepare S-nZVI following the two-step synthesis protocol.

209

2.3.3. Initiating the synthesis of sulfur-modified mZVI

210

Enlightened by the merits of sulfidated nZVI compared to its counterpart without

211

sulfidation, Xu et al. recently proposed the sulfidation of mZVI with sulfide to

212

improve Orange I removal from water.64 S-mZVI was synthesized in 250 mL glass

213

serum bottles. Each bottle was filled with 250 mL of HAc-NaAc buffer solution (0.2

214

M, pH 6.0) and then deoxygenated by bubbling with N2 for 30 min.64 One gram mZVI

215

was added to the deoxygenated medium and the bottle was immediately crimp-sealed

216

with polyethylene septa. The mixture was mixed in a constant temperature rotary

217

mixer at 120 rpm and 25 ± 0.2 °C. Afterwards, 1.5 mL Na2S (1 M) was added to each

218

bottle, and the bottles were then returned to the mixer for another 12 h. Hereafter, the

219

solids were collected with membrane filters under an N2 atmosphere. The filtered

220

particles were freeze-dried for 2 h, and then stored for later use. Very recently, He et

221

al.81-83 proposed that S-mZVI could be prepared by simultaneously milling mZVI and

222

elemental sulfur, i.e., S-mZVIbm.

223

Although the one-step synthesis method for S-nZVI is superior to the two-step

224

one due to the complex procedure of the latter, the synthesis of S-nZVI in aqueous

225

phase is difficult to be scaled up. On the contrary, preparing S-mZVI via ball milling

226

is more likely to be scaled up. Thus, the ball-milled S-mZVI81-83 is very promising

227

and may promote the application of S-mZVI in real practice.

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228

3. Effect of sulfidation on the characteristics

229

of ZVI

230

Sulfidation can greatly influence the properties of ZVI, such as surface area,

231

electrical conductivity, aggregation behavior (of nZVI), shape, and core-shell

232

composition, etc. Thus, this section is divided into two subsections to summarize the

233

effects of sulfidation on the (sub)surface chemistry ( Section 3.1) and aggregation of

234

nZVI (Section 3.2).

235

3.1 Effect of sulfidation on the (sub)surface chemistry of

236

ZVI

237

Because of the formation of FeSx on Fe0 surface, sulfidation may greatly

238

influence the specific surface area of ZVI. We have summarized the influence of S/Fe

239

ratio on the specific surface area of S-ZVI prepared with one-step, two-step, or ball

240

milling method, as shown in Figure 3. It was revealed that the BET specific surface

241

areas of the nZVI sulfidated at different sulfide doses with two-step synthesis method

242

were similar to that of the unamended nZVI and were in the range of 21-26 m2/g, with

243

a mean value of 25 m2/g.63 However, Du et al. observed that sulfidation resulted in a

244

considerable drop in the BET surface area of nZVI when S-nZVI was prepared with a

245

two-step method.78 When S-ZVI was prepared with one-step method, its specific

246

surface area was generally larger than its unsulfidated counterpart, as demonstrated in

247

Figure 3. However, Gong et al. observed that the specific surface area of S-nZVI

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fabricated with one-step method decreased progressively with increasing the S/Fe

249

molar ratio from 0 to 0.138 but the further increase of S/Fe molar ratio to 0.207

250

resulted in a great increase in the specific surface area of S-nZVI.84 On the other hand,

251

the BET surface area of S-mZVIbm was reported to elevate from 0.21 m2/g to 1.43

252

m2/g 83 and 2.1 m2/g 82, respectively, with increasing S/Fe molar ratio from 0 to 0.1

253

and 0.2. Due to the presence of S, ball milling significantly improves the surface

254

roughness of particles, and thus increases the BET surface area of S-mZVIbm.82, 83

-1

2x102

2

BET surface area (m g )

4x102

102

S-nZVIOne-step Na2S2O4

4x101

Reference 75 Reference 65 Reference 6 Reference 82

2x101

101 4x100

S-nZVITwo-step Na2S

2x100

Reference 63 Reference 78

100

bm

4x10-1

S-mZVI

References 82, 83

2x10-1

10-1 0.0

0.1

0.2

0.3

0.4

0.5

S/Fe molar ratio

255 256

Figure 3. Influences of S/Fe molar ratio, sulfidation method and reagent on the BET

257

surface area of ZVI.

258

Kim et al. showed that there was a significant increase in surface roughness of

259

S-nZVI prepared with one-step method with increasing dithionite concentration due to

260

the formation of FeS precipitates,75 which was consistent with the results reported by

261

Han and Yan.80 The increase in the roughness of S-nZVI corresponded to the

262

elevation in its specific surface area.

263

Iron sulfides are generally known as either semiconductors or metallic

264

conductors due to the presence of delocalized electrons in the layers. Consequently,

15



265

the deposition of iron sulfides on the iron surface can facilitate conduction of

266

electrons from iron core to adsorbed contaminant, resulting in the remarkable rate

267

enhancement of contaminant reduction.75 Kim et al. employed electrostatic force

268

microscopy to characterize the influence of sulfidation on surface potential and

269

electrical conductivity change and revealed that the electron flow on the Fe/FeS

270

surface exhibited good mobility.75 Li et al.65 performed electrochemical tests and

271

revealed that S-nZVI had a higher electron transfer capacity and a greater electron

272

transfer rate than unamended nZVI. It had also been shown that S-nZVI underwent

273

more rapid corrosion and was more strongly influenced by solution chemical

274

conditions than its counterpart without sulfidation.85

275

Sulfur in S-nZVI or S-mZVI is generally present in two major forms,

276

monosulfide (S2-) and disulfide (S22-). It was reported that the relative atomic

277

abundance of S as disulfide increased with sulfide dose, indicating higher amounts of

278

FeS2 formation compared to FeS.63 Sulfur in S-nZVI synthesized with two-step

279

method consists mainly of monosulfide (34 at. %) and disulfide (46 at. %), with

280

disulfide contributing a higher portion than that in the S-nZVI prepared with one-step

281

method.80 Sulfidation of mZVI with Na2S also generated both FeS and FeS2 on Fe0

282

surface.64 However, no FeS2 formation was observed in S-nZVI synthesized by Fan et

283

al..77 They reported that the surface atomic ratio of sulfide leveling off at S/Fe ≥ 0.112

284

and higher S/Fe ratio (>0.112) did not result in the formation of significantly more

285

FeS.77

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3.2 Effect of sulfidation on the aggregation of ZVI

287

The influence of sulfidation on the aggregation and sedimentation behavior of

288

nZVI in aqueous solution had been investigated. The distance between nZVI particles

289

was increased due to the presence of FeSx flakes, which may impart some steric

290

stability on S-nZVI nanoparticles.86 Thus, sulfidation could inhibit the aggregation

291

and sedimentation of nZVI in solution with common sodium salts (e.g., Na2CO3 and

292

NaNO3).86 However, the aggregation of nZVI and S-nZVI showed negligible different

293

in the presence of CaCl2, which agreed with the earlier findings by Kim et al..87

294

Aggregation of both nZVI and S-nZVI in divalent cations including Ca2+ and Mg2+

295

should be mainly due to their suppression of electrostatic repulsion between the

296

particles.88 Recently, Gong et al.84 and Zhang et al.79 also showed that sulfidation

297

could effectively prevent the aggregation of nZVI particles. However, Rajajayavel and

298

Ghoshal found that there was no trend in the particle (aggregate) size with the extent

299

of sulfidation of nZVI, indicating that sulfidation had negligible influence on the

300

agglomeration of nZVI.63 Therefore, the results in the literature on the influence of

301

sulfidation on the agglomeration of nZVI were very inconsistent, which may be

302

ascribed to the different reaction conditions and different sulfidation reagents used in

303

different studies.

304

Taken together, it seems that the property of S-ZVI was highly affected by the

305

choice of the sulfidation synthesis methods (i.e., one-step synthesis method or

306

two-step synthesis method) and sulfidation reagents (viz., sodium sulfide, dithionite,

307

or thiosulfate). Additionally, the S/Fe molar ratio also largely impacted the 17



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308

characteristics of S-nZVI. The influence of sulfidation on the properties of ZVI is

309

reflected by its impact on the reactivity of ZVI toward different contaminants, which

310

will be generalized and assessed in the Section 4 in detail.

311

4.

312

performance of contaminants removal by ZVI

Influence

of

sulfidation

on

the

313

The reported data on the removal rate (k), removal efficiency (W), and removal

314

capacity (Q) of various contaminants by S-ZVI, ZVI in the presence of sulfur

315

compounds, ZVI with sulfur impurities, and nZVI fabricated by reducing Fe2+ with

316

Na2S2O4 are summarized in Table S1. To provide a basis for showing the influence of

317

sulfur compounds on reductive removal of contaminants by ZVI, the ratio (Rsulfur) of k,

318

W, or Q of various contaminants by ZVI with the presence of sulfur to that without

319

was determined, as shown in Eq. 11. =

// //

(11)

320

It should be noted that the obtained ratios are unitless. Obviously, sulfidation affects

321

the reactivity of ZVI toward various contaminants but to diverse extents, with most of

322

values falling in the range of 1.0-76.0, indicating that the presence of sulfur

323

compounds improved the reactivity of ZVI under most conditions, as reported in most

324

literatures. Only several values in Table S1 are smaller than 1.0, which are

325

related to the systems of Cr(VI)/S-nZVIOne-step84 and TCE/Indigo-5,5′-disulfonate (I2S)

326

/S-CMC-nZVITwo-step.35 Gong et al. reported that sulfidation of nZVI at low S/Fe

327

molar ratio (0.07 or 0.138) depressed Cr(VI) removal by nZVI, and they ascribed this 18


Page 19 of 52


328

to the drop in the specific surface area at low S/Fe molar ratio.84

329

To further illustrate the influence of sulfidation on the performance of

330

contaminants removal by ZVI, the influence of sulfidation methods (one-step,

331

two-step), S/Fe molar ratio, and water chemistry on contaminants sequestration by

332

ZVI was separately discussed. At the end of this section, the influence of sulfidation

333

on the selectivity of nZVI toward target contaminant was summarized.

334

4.1. Influence of sulfidation method on reductive

335

sequestration of contaminants by ZVI

336

nZVI samples synthesized in the presence of reduced sulfur compounds have

337

been shown to degrade TCE at significantly higher rates than their counterparts

338

without reduced sulfur compounds. The methods of fabricating S-ZVI may affect the

339

performance of synthesized S-ZVI. To test this hypothesis, Han and Yan investigated

340

the effects of sulfidation methods, including sulfidation reagent and time point of

341

sulfidation (one-step synthesis method vs. two-step synthesis method), on TCE

342

degradation experiments.80 However, unexpectedly, their results demonstrated that the

343

reactivity of these S-nZVI samples toward TCE reduction was only slightly affected

344

by the sulfidation reagent (viz., sodium sulfide, dithionite, or thiosulfate) or the

345

sequence of sulfidation (Figure 4(a)). In spite of significant structural differences, the

346

two forms of S-nZVI exhibit similar reactive behavior in TCE dechlorination

347

experiments. Fan et al.35 also compared the reactivity of sulfidated CMC-nZVI,

348

prepared by treating CMC-nZVI with sulfide or dithionite (i.e., S-CMC-nZVITwo-step),

19



Page 20 of 52

toward TCE and I2S. Their results revealed that sulfide-treated CMC-nZVI and

350

dithionite-treated CMC-nZVI showed similar reactivity toward TCE or I2S at the

351

beginning of the reaction, which may be ascribed to the fact that all sulfidated nZVI

352

have similar surface properties that dictate the rate of electron transfer.35 Nonetheless,

353

it should be specified that S-nZVI sulfidated by dithionite showed more significant

354

decreases in I2S reduction with aging than that by sulfide.35 The discrepancy involved

355

in the decreased reactivity with aging for these two sulfidated CMC-nZVI are likely

356

to be due to transformation of reactive amorphous FeS to more crystalline iron sulfide

357

phases. A greater fraction of Fe0 in CMC-nZVI was oxidized, presumably to FeS, due

358

to reaction with dithionite compared to the case using sulfide as sulfidation reagent.35

su lfa t

e

-1

-1

k (L g min )

(a)

-1

-1

10-4

10-5

-1

no ne

10-6

p

I nZV

Z S-n

p

p

-ste -ste -ste -ste VI One nZVI One nZVI One nZVI Two SSS-

p

102

(b)

101

→

100

TCE TBBPA Tc(VII) Cd(II) → Cr(VI) → 12S

10-1 10-2

103 102 101 100

→

10-3

10-1

10-4 10-5

Removal capacity (mg/g)

thi dit su os hio lfid ulf nit e ate e

10-3

thi o

-1

10-2

k (h or L g min or µM min )

349

10-2 0.0

0.2

Iron samples

0.4

0.6

0.8

S/Fe molar ratio 101

-

(Cl /NO3 /HCO3 )

N on e

+

2+

2+

at io ns

(Na /Ca /Mg )

s

10-2

90 80

←

70 10-1

←

60

←

TCE HBCD TCE

(Humic acid/Ethanol)

Cd(II)

rg an ic O

359

100

100

50

Cd(II) removal (%)

-

(d)

→

Fresh -1 kTCE or HBCD (h )

ns

-

Aged

An io

Co-existing solutes C

M et al s

(c) (Co/Ni/Cu/Pd/Mn)

40

10-2 10-1

100

101

102

3

4

5

6

7

8

9

10

11

pHini

kTCE or HBCD (1/hr)

360

Figure 4. Comparison of the rate constants for TCE dechlorination by S-ZVI

361

synthesized with different sulfidation approaches (a), effect of S/Fe molar ratio (b),

362

water chemistry including co-existing solutes (c), and pHini (d) on the reactivity of 20


Page 21 of 52


363

S-ZVI synthesized with one-step, two-step, or ball milling method toward various

364

contaminants.

365

In sum, the reaction rate of contaminants by S-ZVI at the beginning of the

366

reaction was slightly affected by the sulfidation reagent and the sequence of

367

sulfidation once the other reaction conditions of fabricating S-ZVI were fixed.

368

However, the influence of sulfidation reagent and sulfidation sequence on the

369

reactivity of S-ZVI in a long term keeps unclear and warrants further investigation.

370

4.2. Influence of S/Fe molar ratio on reductive removal

371

of contaminants by S-ZVI

372

S/Fe molar ratio, which is defined as the ratio of the molar concentration of

373

applied sulfidation reagent (as S) to that of total iron (including Fe0, Fe(II), and

374

Fe(III)), is one of the most critical factors affecting the reactivity and removal

375

capacity of S-ZVI for various contaminants. As summarized in Table S1 and Figure

376

4(b), Kim et al.75 reported that the observed pseudo-first-order rate constants (kobs) of

377

TCE reduction by S-nZVI increased linearly with increasing dithionite concentrations

378

up to 2.0 g/L (S/Fe molar ratio of 0.33), and then decreased when the dithionite

379

loading exceeded 2.0 g/L. Rajajayavel and Ghoshal also showed that TCE reduction

380

by S-nZVI was strongly dependent on the S/Fe molar ratio and S/Fe ratios in the

381

range of 0.04-0.083 provided the highest TCE dechlorination rates.63 Han and Yan80

382

found that more rapid TCE dechlorination occurred with increasing S/Fe ratio when

383

thiosulfate was applied at a small dose (S/Fe < 0.025). However, the rate constant

21



384

levels out approaching a plateau when the S/Fe molar ratio exceeds 0.025.

385

Fan et al. reported that aqueous TcO4− was rapidly removed by S-nZVI from

386

solution at the S/Fe value ranging from 0.011 to 0.56.77 As S/Fe increased from 0 to

387

0.045, the Tc disappearance rate increased. The reaction rate was too fast to be

388

accurately determined from the data obtained at S/Fe = 0.056 and 0.224, but higher

389

values of S/Fe resulted in decreased Tc removal rates.77 The reaction rate constants for

390

Cr(VI) removal by FeS@Fe0 first increased with increasing S/Fe ratio from 0 to 0.1,

391

and then decreased for the S/Fe ratio increased further to 0.167 or 0.25.78 With respect

392

to the performances of S-mZVIbm, Gu et al.83 found that the surface-area normalized

393

rate constants for TCE reduction by S-mZVIbm (S/Fe molar ratio 0.1) were ~2 and ~5

394

fold greater than that of the unsulfidated ball-milled control (i.e., mZVIbm) under ZVI-

395

and TCE- limited conditions, respectively.

396

Since the reaction conditions, including the contaminants, are very different for

397

different researchers to get the optimum S/Fe molar ratio and thus the values of the

398

optimum S/Fe molar ratio in different studies are non-comparable. Despite the

399

variability in the optimum S/Fe molar ratios, the kinetic data of contaminants

400

sequestration by S-ZVI under various conditions do form clusters and trends that offer

401

potentially useful insights into the existence of an optimum S/Fe molar ratio. On one

402

hand, the S/Fe molar ratio affects the properties of prepared S-ZVI. Higher S/Fe ratio

403

may lead to the generation of more FeSx and larger surface area of synthesized S-ZVI,

404

and thus favors the reductive sequestration of contaminants. However, the active sites

405

on the surface can also be blocked by excessive FeSx and thus slow the reduction of

22


Page 22 of 52

Page 23 of 52


406

contaminants by iron core. Moreover, the transformation of Fe0 to ferrous/ferric

407

(hydr)oxides may be aggravated at high S/Fe ratio, resulting in a waste of reducing

408

capacity of ZVI. On the other hand, the optimum S/Fe molar ratio is strongly

409

dependent on the properties of target contaminants even other reaction conditions are

410

fixed. Different mechanisms, including adsorption, co-precipitation, and reduction,

411

are responsible for different contaminants removal by S-ZVI. Therefore, it is vital to

412

find the relationship among the S/Fe molar ratio, the properties of synthesized S-ZVI,

413

and the performance of S-ZVI for contaminants removal with different mechanisms.

414

4.3. Influence of water chemistry on contaminants

415

sequestration by S-ZVI

416

The reactivity of reducing materials can be affected by solution conditions. The

417

electrochemical tests had showed that S-nZVI underwent more rapid corrosion and

418

was more strongly influenced by solution chemical conditions than pure nZVI.89 Thus,

419

this section reviews the influence of water chemistry including background solutes

420

and solution pH on contaminants sequestration by S-ZVI. As demonstrated in Figure

421

4(c), the rates of TCE reduction by S-nZVI were unaffected by ionic strength over the

422

range of 0.1-10 mM NaCl, increased with increasing Ca2+ or Mg2+ concentrations, but

423

inhibited by the presence of humic acid.74 However, Li et al. recently reported that the

424

inorganic ions (NO3-, HCO3-, Cl-, Na+, Mg2+) and organic solvent (ethanol) had an

425

inhibitory effect on the transformation of hexabromocyclododecane (HBCD) by

426

S-nZVI.6 The humic acid-induced impairment in the reactivity of S-nZVIOne-step may

23



427

be mediated by the co-presence of humic acid and Ca2+/Mg2+, presumably due to the

428

formation of humic acid−Ca2+/Mg2+complexes and consequent decreased adsorption

429

of humic acid onto S-nZVIOne-step surface.74 Nonetheless, Cao et al.90 recently found

430

that the reactivity of S-nZVI toward antibiotic florfenicol (FF) was relatively

431

unaffected by the presence of both dissolved ions and organic matter in in four real

432

water samples including groundwater, river water, seawater, and wastewater,

433

indicating that the S-nZVI reactivity was robust.

434

In addition, the effect of metal amendments (i.e., Pd2+, Cu2+, Ni2+, Co2+, and Mn2+)

435

on the reactivity of fresh and aged S-nZVIOne-step toward TCE had also been

436

investigated.85 It was found that Pd2+, Co2+, and Ni2+ increased the rates of TCE

437

removal by both fresh and aged S-nZVIOne-step, while Mn2+ and Cu2+ decreased TCE

438

reduction rates by about 1 order of magnitude, relative to the that without metal

439

amendments.85

440

Solution pH is one of the most important parameters of natural environment that

441

can significantly affect the rates of contaminants removal by ZVI,70 and thus may

442

impact the reactivity of S-nZVI toward target contaminant. The rate of TCE reduction

443

by S-nZVIOne-step was found to increase with increasing pH (Figure 4(d)), which is

444

consistent with the pH effect reported previously for iron sulfide systems.91

445

Rajajayavel and Ghoshal also demonstrated that increasing solution pH significantly

446

increased TCE degradation rate constant from 0.104 h-1 at pH 7.0 to 0.137 h-1 at pH

447

9.0 and to 0.240 h-1 at pH 11.0, which should be ascribed to the fact that increased

448

deprotonation of FeS at higher pH increased the electron availability at reactive

24


Page 24 of 52

Page 25 of 52


449

surfaces.63 As pH increased from 4.0 to 5.0, Cd2+ removal by S-nZVI increased

450

sharply.92 Only ~40% Cd2+ was removed at pH 4.0, probably due to the relative

451

instability of S-nZVI at this pH level. The removal efficiency of Cd2+ by S-nZVI

452

elevated progressively from 95% to 100% as pH increased from 5.0 to 9.0.92 Recently,

453

It was found that the HBCD sequestration rate constant was enhanced appreciably

454

from 0.078 to 0.176 h-1 with increasing pHini from 3.0 to 5.0.6 However, the reactivity

455

of S-nZVI toward HBCD dropped slightly as pHini increased from 5.0 to 9.0.6 The

456

decrease in transformation rate at high pH might be ascribed to the iron oxides

457

precipitates on the surface of S-nZVI inhibiting the mass transfer of HBCD to the iron

458

surface and blocking the reactive sites on the surface of S-nZVI.6 A drop in the

459

reductive sequestration rate of Cr(VI) by hierarchical S-nZVI was also observed with

460

increasing pH.78

461

4.4. Influence of sulfidation on the selectivity of nZVI

462

toward target contaminant

463

Generally, the reaction of nZVI with natural reducible species, including water

464

and naturally present oxidants (e.g., oxygen, nitrate), results in low selectivity of

465

nZVI toward target contaminants, which has become one of the major obstacles to the

466

widespread utilization of nZVI in the field.61 For example, it had been reported that

467

more than 95% of the reducing equivalents from untreated nZVI in CMC-nZVI were

468

consumed due to corrosion by water, rather than TCE reduction.35 Therefore, it is

25



469

critical to increase the selectivity of nZVI toward the target contaminants over the

470

reduction of water and naturally present oxidants.

471

Most of the researchers focused on improving the reductive reactivity of nZVI by

472

sulfidation before publication of the paper authored by Rajajayavel and Ghoshal.63

473

They reported that the surface-area normalized rate constants for TCE degradation by

474

S-nZVI (S/Fe molar ratio ranging from 0.015 to 1.61) were up to 40- fold greater than

475

that by non-sulfidated nZVI in 12 hours.63 Furthermore, S-nZVI exposed to water in

476

the absence of TCE showed significantly lower hydrogen evolution rate (2.75 µmol

477

L-1 h-1) compared to the unamended nZVI (6.92 µmol L-1 h-1) in 10 hours, indicating

478

that sulfidation suppressed the corrosion reaction of nZVI with water.63 The fact that

479

S-nZVI caused significantly higher TCE degradation rate than bare nZVI but evolved

480

H2 at a slower rate suggested that sulfidation increased the selectivity of nZVI toward

481

TCE. These results direct researchers to pay attention to the improving effect of

482

sulfidation on the selectivity of nZVI under anaerobic conditions. Han and Yan80 also

483

found that up to 60-fold increase in the TCE removal rates with nZVI was observed

484

upon sulfidation treatment. In addition, there was a notable decrease in the rate of H2

485

generation from water when S/Fe ratio increased from 0.01 to 0.05.80 However, these

486

studies did not calculate the influence of sulfidation on the selectivity of nZVI and the

487

influence of sulfidation on TCE reduction and H2 generation was determined in a very

488

short time, ranging from 6 to 60 hours.63, 80

489

Fan et al. did a very comprehensive study to show that the selectivity of

490

CMC-nZVI under anaerobic conditions could be greatly improved by sulfidation.35

26


Page 26 of 52

Page 27 of 52


491

The reduction of water by CMC-nZVI to hydrogen was greatly depressed due to the

492

modification of CMC-nZVI with either sulfide or dithionite.35 All of the sulfidated

493

CMC-nZVI produced negligible H2 over 2 days while H2 production from untreated

494

CMC-nZVI occurred rapidly in the first day and gradually leveled out, due to

495

depletion of Fe0. The authors further investigated the influence of aging on the

496

reducing capacity of sulfidated and untreated CMC-nZVI samples. Specifically, it was

497

found that the reducing capacity of untreated CMC-nZVI rapidly diminished after

498

seven days of aging, whereas it only slightly decreased for sulfide-treated CMC-nZVI

499

samples even after 3 weeks of aging. This should be due to the formation of an iron

500

sulfide film, which inhibits further corrosion of Fe0. It should be additionally

501

addressed that sulfidation with sulfide left most of the CMC-nZVI as Fe0, whereas

502

dithionite converted a large fraction of CMC-nZVI to FeS, thus consuming much of

503

the reducing capacity originally provided by the Fe0. Although sulfidation greatly

504

inhibited CMC-nZVI corrosion by water, all sulfidated CMC-nZVI showed

505

substantially higher rates of TCE degradation over longer time periods compared to

506

the untreated CMC-nZVI. Much more TCE was removed by sulfide-treated

507

CMC-nZVI at the end of reaction than the dithionite-treated CMC-nZVI. Therefore,

508

compared to dithionite, sulfide may be a better reagent for sulfidation of CMC-nZVI

509

since sulfide preserved much more Fe0 and provided greater contaminant degradation

510

even after aging.

511

The non-selectivity of nZVI often results in its short lifespan (i.e., less than a day

512

to several weeks). Since combining dithionite with nZVI can sustain the reactive

27



513

lifespan of the treatment system for much longer periods than nZVI alone, nZVI

514

coupling with dithionite was proposed to treat 1,2-Dichloroethane (1,2-DCA).7

515

Coupled nZVI-dithionite was able to degrade >90% 1,2-DCA over the course of one

516

year. More importantly, characterization analysis of the nZVI-dithionite nanoparticles

517

shows that most of the iron was still preserved in the zerovalent state even after more

518

than one year of reaction with some FeS formation. Therefore, the application of

519

dithionite significantly increases the selectivity of nZVI toward 1,2-DCA.

520

Furthermore, Gu et al.83 recently revealed that sulfidation could improve the electron

521

selectivity of ball-milled mZVI toward TCE by 10- and 50-fold under mZVI-limited

522

conditions and TCE-limited conditions, respectively, compared to its unsulfidated

523

counterpart under anaerobic conditions.

524

However, the influence of sulfidation on the selectivity of ZVI in the presence of

525

natural reducible species other than water, such as oxygen and nitrate, keeps unknown.

526

Oxygen and nitrate are more oxidative than water, and thus sulfidation may not be an

527

efficient method for inhibiting the reaction between ZVI and oxygen or nitrate, which

528

warrants further investigation. In addition, more studies should be carried out to

529

examine the capability of sulfidation for preserving the reductive capacity of ZVI in a

530

long term.

28


Page 28 of 52

Page 29 of 52


531

4.5. Influence of sulfidation on degradation of organic

532

contaminants by nZVI activated oxygen, persulfate, or

533

H2O2

534

Although nZVI can react with molecular oxygen (O2) to produce reactive oxygen

535

species (ROS), the low yield of ROS in nZVI/O2 system restricts its application to the

536

oxidative removal of organic contaminants.93,

537

sulfidation to enhance diclofenac (DCF, an emerging groundwater pollutant) removal

538

by nZVI under aerobic conditions in the presence of common anions.86 It was found

539

that the removal of DCF under aerobic conditions at pH ∼6.5 was increased from 21.2%

540

to 73.5% as the S/Fe molar ratio increased from 0 to 0.3 when all the other reaction

541

conditions were fixed. In addition, sulfidation could weaken the negative impact of

542

common anions and humic acid on DCF removal by nZVI, and thus S-nZVI was

543

capable of removing DCF from simulated groundwater more efficiently under aerobic

544

conditions than nZVI. Moreover, the reactivity of S-nZVI (with Fe/S molar ratio of

545

6.9) toward p-nitrophenol (PNP) over the pH range of 6.8 to 9.1 was always greater

546

than that of nZVI under aerobic conditions.66 It had also been reported that Fe/FeS

547

nanoparticles showed a much higher reactivity on the activation of molecular oxygen

548

for Rhodamine B (RhB) removal than the pure Fe nanoparticles.79 All of the above

549

three studies confirmed that hydroxyl radical was the predominant active oxidant in

550

the S-ZVI-O2 system.

94

Therefore, Song et al. employed

551

Similar to nZVI, S-nZVI could also activate persulfate or H2O2 to generate

552

sulfate or hydroxyl radicals so as to achieve the effective oxidation of organic

29



553

contaminants (BA).82,

95

554

persulfate to achieve effective BA abatement under alkaline conditions and in the

555

presence of various organic and inorganic solutes.95 Recently, Huang et al. showed

556

that the initial surface area normalized phenol degradation rate by S-ZVI/H2O2 was 5

557

times of that of ZVI/H2O2, suggesting the much higher efficiency of S-ZVI in

558

catalyzing the decomposition of H2O2 for oxidative degradation of organic

559

contaminants.82 The authors concluded that FeS contained in S-ZVI as a better

560

electron conductor than iron (hydr)oxides facilitated the electron transfer from Fe0 to

561

H2O2, resulting in faster Fe2+ releasing and H2O2 activation, which enhanced

562

contaminant degradation.82

S-nZVI was found to be superior to nZVI for activating

563

Compared to the direct reductive removal of contaminants by S-nZVI, the

564

research on oxidative removal by activating molecular oxygen, persulfate, or H2O2 is

565

very rare and the mechanisms of the improving effect of sulfidation on activating

566

molecular oxygen, persulfate, or H2O2 needs to be further investigated. Moreover, the

567

influence of sulfidation on the generated ROS in nZVI/O2 system has not been

568

quantified up to now.

30


Page 30 of 52

Page 31 of 52


569

5.

Mechanisms

of

sulfidation-induced

570

improvement in contaminants sequestration

571

by ZVI

572

5.1. Enhancing the reactivity and selectivity of ZVI for

573

organic contaminants by sulfidation

574

It has been well documented that sulfidation can enhance the reactivity and

575

selectivity of ZVI for organic contaminants and the possible mechanisms can be

576

divided into five aspects, as demonstrated in Figure 5.

577 578

Figure 5. The major mechanisms involved in the reactions of S-ZVI with various 31



579

Page 32 of 52

contaminants.

580

Firstly, covering the nZVI surface with FeSx can mediate the aggregation of

581

nZVI, increase the surface roughness and surface area, which results in a much higher

582

reactivity of the S-nZVI toward contaminants than pure nZVI particles.75, 84, 87, 90 For

583

example, Kim et al.75 reported that the reactivity of S-nZVI with different S/Fe ratios

584

toward TCE corresponded well with the surface area results.

585

Secondly, the FeSx surface layer acts as a more efficient electron conductor for

586

transferring electrons from the electron rich Fe0 core to chlorinated organics at the

587

particle surface than the iron (hydr)oxide surface layer in unamended nZVI.35, 61, 75, 78,

588

84, 90

589

capacity and a greater electron transfer rate than unamended nZVI by electrochemical

590

analysis.

591 592

For example, Li et al.65 confirmed that S-nZVI had a higher electron transfer

Thirdly, as a reducing agent, FeSx alone can reduce various organics, such as TCE,35, 63 1,2-DCA,7 I2S,35 TBBPA,65 HBCD,6 FF,90 etc..

593

Fourthly, the FeSx layer on the S-nZVI is more hydrophobic than the

594

(hydro)oxide layer on the surface of unamended nZVI and thus has relatively higher

595

binding capacity for hydrophobic organic contaminants over water.63,

596

example, Rajajayavel and Ghoshal63 inferred that the FeSx layer on the S-nZVI could

597

enhance local binding of TCE rather than water molecules compared to the pure nZVI,

598

resulting in the improved selectivity of nZVI toward TCE. The enrichment of organic

599

contaminants on the surface of S-nZVI facilitates the following electron transfer from

600

the iron core to the organic contaminants.63, 75, 84, 96

32


75, 90, 96

For

Page 33 of 52


601

Fifthly, sulfur in S-nZVI can poison hydrogen recombination and thus drives

602

surface reactions to favor reduction by atomic hydrogen.80 This implies that the

603

reactivity of S-nZVI is contaminant-specific and is selective against the background

604

reaction of water reduction.80 Thus, TCE transformation, which is not limited by the

605

electron transfer rate but the availability of atomic hydrogen, by nZVI was greatly

606

enhanced by sulfidation.80 On the other hand, the removal of carbon tetrachloride (CT,

607

CCl4), whose reduction is governed by a direct electron transfer process,97 by nZVI

608

was not affected by sulfidation.80 The mechanisms proposed by Han and Yan are

609

contradictory to those by Rajajayavel and Ghoshal,63 which need to be verified in

610

future.

611

5.2. Increasing the stability of reduced metal(loid)s by

612

S-ZVI

613

Su et al. revealed that the S/Fe molar ratio strongly influenced Cd2+ removal

614

performance of S-nZVI.92 With the increase of S/Fe ratio, Cd2+ removal efficiency

615

decreased initially and then increased at the same initial Cd2+ concentration, which is

616

very different from the trends observed for the removal of other contaminants by

617

S-nZVI (Figures 4(b) and 5). S-nZVI has a minimum Cd2+ removal capacity of 15

618

mg/g at S/Fe ratio of 0.07 and a maximum capacity of 85 mg/g at S/Fe ratio of 0.28.92

619

The removal capacity of Cd2+ by S-nZVI increased with increasing the S/Fe ratio

620

from 0.14 to 0.28, which ascribed to the enhancement in surface sulfydryl group.92

621

Sulfidation can facilitate the enrichment of metal cations (e.g., Cd2+, Pb2+, Cu2+, and

33



Page 34 of 52

622

Zn2+) on nZVI surface via formation of M2+-substituted FeS by surface ion exchange,

623

complexation of M2+ with reactive sites (≡FeSH+), and co-precipitation (formation of

624

Fe(1-x)MxS complexes by FeSH+ and M2+ in solution).92

625

The FeS@Fe0 hybrid particles showed a much higher efficiency toward Cr(VI) sequestration

627

predominantly at the solid-liquid interface, and that Fe(II) generated from

628

FeS@Fe0 corrosion accounted for 52% of the Cr(VI) reduction, while electrons from

629

Fe0 and FeS accounted for the rest.78 Du et al. revealed that Cr(VI) was completely

630

transformed and immobilized as solid Fe-Cr hydroxide precipitates, thus avoiding

631

secondary contamination.78 However, another study on Cr(VI) removal by S-nZVI

632

showed that Cr(VI) was reduced to Cr(III), which was subsequently immobilized in

633

the solid phase of (CrxFe1-x)(OH)3 and FeCr2S4.84 Thus, S-nZVI holds the promise to

634

be employed as an effective sorbent for immobilization of Cr(VI) in contaminated

635

water and soil.

636

99

compared

to

un-treated

nFe0.78

626

Cr(VI)

reduction

occurred

Tc is one of the most problematic radioisotopes in used nuclear fuel owing to its

637

combined features of high fission yield, long half-life, and high environmental

638

mobility.98 Soluble pertechnetate (99TcO4–) can be reduced to less soluble TcO2·nH2O

639

to reduce its hazard, while the reduced form is highly susceptible to reoxidation by

640

oxygen (or nitrate).76 Therefore, as shown in Figure 5, Fan et al. proposed to

641

remediate Tc-contaminated groundwater by S-nZVI since Tc sulfide is favored by

642

sulfidic conditions stimulated by nZVI and it is more recalcitrant to oxidation by

643

oxygen than TcO2·nH2O.77 Their results did demonstrate that sulfidation of nZVI can

34


Page 35 of 52


644

change TcO4– sequestration products from TcIV oxide to TcIV sulfide phases under

645

groundwater conditions.77

646

Therefore, compared to ZVI, S-ZVI has the potential to increase the stability of

647

removed heavy metals and oxyanions. This needs further verification by examining

648

the stability of the precipitates collected from the systems of heavy metals and

649

oxyanions removal by S-ZVI.

650

6. Environmental implications and future

651

challenges

652

This review comprehensively summarized the latest developments and

653

knowledge on the sulfidation of ZVI including the milestones in development of

654

sulfur-modified ZVI technology, the synthesis approaches of S-nZVI and S-mZVI,

655

influence of sulfidation on the performance of contaminants sequestration by ZVI, as

656

well as the mechanisms of sulfidation-induced improvement in contaminants

657

sequestration by ZVI. Sulfidation appears to be a resurgence on the environmental

658

application of ZVI in different water matrices.99-101 However, there are still several

659

research gaps and challenges, which are important for further advancing the S-ZVI

660

based cleanup technology, should be addressed:

661

(i) Abundant data suggested the sulfidation can significantly enhance the removal

662

rate, removal efficiency, and removal capacity of several contaminants by ZVI.

663

However, the sulfidation-induced improvement was strongly dependent on the

664

properties of contaminants and origins of ZVI, as well as the methods, extents, and 35



665

reagents of sulfidation. Moreover, there are several studies revealed that sulfidation

666

did not always promote the removal of contaminants by nZVI. Thus, a wide-spectrum

667

investigation should be carried out to better show the benefit of sulfidation.

668

(ii) Although the past studies on S-ZVI worked with different contaminants, the

669

relationship between the influence of contaminants’ property (e.g., structure, Kow) and

670

sulfidation-induced acceleration or deceleration on contaminants removal by nZVI or

671

mZVI should be unveiled in future. (iii) Although most studies acknowledge the contribution of FeSx to the

672 673

performance of S-ZVI,61,

84

674

distribution and transformation of sulfur species associated with the sequestration of

675

contaminants by S-ZVI.

there is also a need for in-depth investigation of the

676

(iv) S-ZVI is attracting a lot of attention due to its ease of production and high

677

reactivity toward contaminants.80, 92 To assess the environmental application of S-ZVI,

678

a systematic life-cycle analysis is necessary to benchmark the S-ZVI-based

679

technology against other conventional technologies. Furthermore, pilot- and/or

680

field-scale deployments of S-ZVI should be performed to assess the cost and

681

application potential, enabling engineers and regulators to make more informed

682

decisions on technology selection at individual sites.

683

(v) To date, most studies on the performance of S-ZVI have mainly been carried

684

out in the laboratory in a short term ranging from several hours to several weeks.7, 35,

685

92

686

technology is recommended for groundwater remediation or wastewater treatment.

It is imperative to consider the long-term performance of S-ZVI before this

36


Page 36 of 52

Page 37 of 52


687

(vi) Given that sulfide is an extremely powerful reductant for ferric (hydr)oxides

688

and present at different concentration levels in many reducing environments, its

689

presence in groundwater may affect the contaminants removal by S-ZVI and/or ZVI

690

by affecting the generation of ferric (hydr)oxides. Thus, the influence of sulfide on

691

contaminants sequestration by S-ZVI and/or ZVI in groundwater should be clarified.

692

(vii) The duration of treating ZVI with reducing sulfur compounds may affect the

693

properties of generated FeSx and thus influence the performance of prepared S-ZVI,

694

which needs verification.

695

(viii) Up to now, the influences of sulfidation on the selectivity of ZVI were all

696

investigated under anoxic conditions. In future, the influence of sulfidation on the

697

selectivity of mZVI under aerobic conditions, where dissolved oxygen is one of the

698

major electron acceptor, should be evaluated because S-mZVI can be employed in

699

industrial wastewater treatment where there is oxygen.

700

Associated Content

701

Supporting Information

702

The Supporting Information is available free of charge on the ACS Publications

703

website.

704

Summary of data on the removal rate (k), removal efficiency (W), and removal

705

capacity (Q) of various contaminants by S-nZVI and S-mZVI.

37



706

Author Information

707

Corresponding Author

708

*Email: [email protected]; phone: +86-21-65980956; Fax: +86-21-65986313.

709

Notes

710

The authors declare no competing financial interest.

711

Acknowledgement

712

This work was supported by the National Natural Science Foundation of China

713

(Grants No. 51708416, 21777117, 21522704, U1532120, and 51478329), the State

714

Key Laboratory of Pollution Control and Resource Reuse Foundation (Grant No.

715

PCRRK16001), the Fundamental Research Funds for the Central Universities, the

716

Zhejiang Provincial Natural Science Foundation of China (LQ15E080003), China

717

Postdoctoral Science Foundation, and the Tongji University Open Funding for

718

Materials Characterization.

719

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