Synergetic Transformations of Multiple Pollutants Driven by Cr(VI

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Synergetic transformations of multiple pollutants driven by Cr(VI)-sulfite reactions Bo Jiang, Yukun Liu, Jingtang Zheng, Minghui Tan, Zhaohui Wang, and Mingbo Wu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 17 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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Environmental Science & Technology

1

2

Synergetic transformations of multiple

3

pollutants driven by Cr(VI)-sulfite reactions

4 5

Bo Jiang†,‡, Yukun Liu†, Jingtang Zheng†,*, Minghui Tan†, Zhaohui Wang§,¶,*, Mingbo Wu†

6 7

†

266580, Shandong, P.R. China;

8 9

‡

§

College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China;

12 13

School of Environmental and Municipal Engineering, Qingdao Technological University, Qingdao 266033, China;

10 11

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao

¶

Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia

14 15

16

17

*Corresponding author: [email protected] (J.T. Zheng) [email protected] (Z.H. Wang) Author: [email protected] (B. Jiang)

18 19 20 21 22

1

ACS Paragon Plus Environment


23

Abstract

24

Reduction of Cr(VI) is often deemed necessary to detoxify chromium contaminants,

25

however, few investigations utilized this reaction for the purpose of treating other industrial

26

wastewaters. Here a widely-used Cr(VI)-sulfite reaction system was upgraded to

27

simultaneously transform multiple pollutants, namely the reduction of Cr(VI) and oxidation

28

of sulfite and other organic/inorganic pollutants in an acidic solution. As(III) was selected as

29

a probe pollutant to examine the oxidation capacity of a Cr(VI)-sulfite system. Both •OH and

30

SO4•− were considered as the primary oxidants for As(III) oxidation, based on the results of

31

electron spin resonance, fluorescence spectroscopy and specific radicals quenching. As(III)-

32

scavenging, oxidative radicals greatly accelerated Cr(VI) reduction and simultaneously

33

consumed less sulfite. In comparison with a Cr(VI)-H2O2 system with 50 µM Cr(VI), Cr(VI),

34

the sulfite system had excellent performance for both As(III) oxidation and Cr(VI) reduction

35

at pH 3.5. Moreover, in this escalated process, less sulfite was required to reduce Cr(VI)

36

than the traditional Cr(VI) reduction by sulfite process. This effectively improves the

37

environmental compatibility of this Cr(VI) detoxification process, alleviating the potential for

38

SO2 release and sulfate ion production in water. Generally, this study provides an excellent

39

example of a “waste control by waste” strategy for the detoxification of multiple industrial

40

pollutants.

41 42 43 44 45 46 47 48 2


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TOC/Abstract Art

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51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

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Introduction

72

Hexavalent chromium (Cr(VI)) is extensively used in various industrial processes, such as

73

electroplating, pigments and leather tanning 1. However, Cr(VI), an US Environmental

74

Protection Agency “priority pollutant”, is known to be toxic and carcinogenic and is highly

75

mobile in many soils and waters 2. Hence, disposal of Cr(VI)-bearing waste is a significant

76

and well-recognized environmental issue 2. In contrast to Cr(VI), Cr(III) is less toxic and can

77

be readily precipitated out of solution in the form of Cr(OH)3. Consequently, the reduction of

78

Cr(VI) to non-toxic Cr(III) is of great interest for engineered waste management systems.

79

To date, a variety of methods have been developed for aqueous Cr(VI) detoxification

80

including conventional methods (e.g., chemical reduction and precipitation) and advanced

81

processes (e.g., TiO2-based photocatalysis, glow plasma processes and photochemical

82

methods mediated by organic acids)

83

species, these processes consume a large number of chemical regents or energy input and are

84

therefore not cost-effective. Notably, Bokare et al.8 and Wang et al.

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novel application of Cr(VI) reduction by H2O2 to generate highly reactive •OH towards

86

pollutant oxidation in situ. For example, 100 µM 4-chlorophenol was completely oxidised at

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solution pH 3.0-7.0 within 360 min in a Cr(VI)-H2O2 reaction system ([Cr(VI)]0 = 2.0 mM,

88

[H2O2]0 = 20 mM). Although this reaction system gained some economic benefits, it cannot

89

efficiently relieve the environmental hazard of Cr(VI) and thus still required additional

90

processes to detoxify Cr(VI) in the waste water.

91

−

1-7

. In spite of the success in detoxification of Cr(VI)

-

2-

2CrO4 + 4HSO3 + 6H + → 2Cr 3+ + S2O 6 + 6H 2 O + 2SO 4

9

2-

recently reported the

(1)

2−

92

4CrO 4 + 6NaHSO3 + 3H 2SO 4 + 8H + → 2Cr2 (SO 4 ) 3 + 3Na 2SO 4 + 10H 2O (2)

93

An alternative to H2O2, sulfite, a common industrial contaminant in wastewaters or in

94

exhaust gas, is a typical reducing agent and has been widely utilized for reductive 4


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detoxification of chromate-containing wastewaters or remediation of contaminated sites in

96

many emergency cases

97

Reaction 1 in the presence of an excess of sulfite, whereas the theoretical stoichiometry of

98

Cr(VI)/sulfite is 2:3 for excessive Cr(VI) (Reaction 2)

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primarily focused on kinetic or equilibrium models to study the reaction of sulfite reducing

100

Cr(VI) in an acid solution with excess sulfite 11-14. For example, Beukes et al. 12 investigated

101

Cr(VI) reduction by sulfite as a function of pH in wastewater and reported that for a 26 mg L-

102

1

103

was required for complete reduction within the pH range of 2.0-5.0.

2, 10-12

. The reaction between Cr(VI) and sulfite can be described by

11

. Until now, the literature has

Cr(VI) solution, a sulfite concentration of at least 5 times the initial Cr(VI) concentration

104

Moreover, the significant role of sulfite as an efficient source of active radicals via the

105

reaction of an iron-sulfite-oxygen complex under light irradiation has been reported 13-16. For

106

example, Zhang et al.

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decolorized within 60 min in an Fe(II)/sulfite-based photochemical system using a xenon

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lamp (350 W), whereas less efficiency (15%) was achieved without irradiation. Application

109

of industrially available sulfite as a new source of sulfate radicals, instead of using expensive

110

persulfate reagents (ca. $1,300/t), seems very promising in the practice of advanced oxidation

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technologies (AOTs) 14.

15

reported that 84% of Orange II (10 mg L−1) can be efficiently

112

Compared with Fe(III), Cr(VI) may be found more abundant in some industrial

113

wastewaters, is soluble over a larger pH range and has a higher standard oxidation potential

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(E0(HCrO4−/Cr3+) = 1.35 VNHE) 8. Thus, aqueous Cr(VI) species may be an alternative

115

activator for transforming sulfite to active radical species. However, although Cr(VI)

116

reduction by sulfite is a common process for Cr(VI) detoxification throughout the world 11, 12,

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there are few reports describing a sulfite-mediated generation of active radicals in Cr(VI)

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reduction processes .

5



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This study investigated the reactions occurring in a Cr(VI)-sulfite reaction system and

120

examined the feasibility of upgrading this reaction system for the oxidation of other

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inorganic/organic contaminants. The changes in concentrations of various reagents were

122

measured over reaction time, with different dosages of Cr(VI) and sulfite. The effects of pH,

123

additions of other metal ions, and scavenging radicals were examined to probe the underlying

124

reaction mechanisms. This work provides a new route for the oxidation of organic/inorganic

125

pollutants, co-occurring with sulfite oxidation and Cr(VI) reduction.

126

Experimental section

127

Chemicals.

128

Sodium arsenite (NaAsO2, 97%) and sodium arsenate (Na2HAsO4·7H2O, 99%) were

129

supplied by Xiya Reagent and Jiangxi Qianhua Industry Co., Ltd., respectively. Methyl

130

orange (MO), methylene blue (MB), rhodamine B (RB), anhydrous sodium sulfite

131

(Na2SO3, >97%), sodium thiosulfate (Na2S2O3·5H2O, >99%), potassium chromate

132

(K2Cr2O7, >99%), diphenylcarbazide (98%), sodium pyrosulfite (Na2S2O5, >96%), sodium

133

hyposulfite

134

((NH4)6Mo7O24·4H2O, >99%), ethanol (EtOH, 73.0%-75.0%), 5,5’-Dithiobis (2-nitrobenzoic

135

acid) (DTNB, >99%), ethylenediaminetetraacetic acid (EDTA, >99.5%), antimony potassium

136

tartrate (K(SbO)C4H4O6·0.5H2O, >99%), L-Ascorbic acid (C6H8O6, >99.7%), chromium

137

nitrate (Cr(NO3)3·9H2O, >99%), copper sulfate (CuSO4·5H2O, >99%), cobaltous sulfate

138

(CoSO4·5H2O, >99.5%), manganese sulfate (MnSO4, >99%), aluminium sulfate hydrate

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(Al2(SO4)3·H2O, 98%), ferrous sulfate heptahydrate (FeSO4·7H2O, >99%), ferric sulfate

140

(Fe2(SO4)3, >99%), coumarin (C9H6O2), tert-butyl alcohol (TBA, >98%), H2SO4 (73.0%-

141

75.0%), H3PO4 (>98.5%), NaOH (>96%), HCl (36%-38%) and H2O2 were purchased from

142

Sinopharm Chemical Reagent Co. Ltd., China. All chemical reagents were used without

143

further purification. UltraPure water was used for all experiments.

(Na2S2O4,

>86%),

ammoniummolybdate

6


tetrahydrate

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144


Reaction procedures.

145

All experiments were conducted in an open, 150mL, cylindrical glass tube, magnetically

146

stirred and maintained at a temperature of 20±1°C in a circulating water jacket. Stock

147

solutions of 1000 mg L−1 Cr(VI) and As(III) were prepared by dissolving analytical grade

148

K2Cr2O7 and NaAsO2 in pure water. All working solutions were freshly prepared before use

149

by diluting the stock solution with pure water. The pH was adjusted to the desired values with

150

concentrated H2SO4 or NaOH solution. Cr(VI) and As(III) were pre-mixed, sulfite was added

151

to initiate the experiment. For the experiments using a volumetric flask (250 mL) as the

152

reaction vessel in the presence of argon or oxygen (gas velocity, 0.6 L min-1), the solution

153

was purged with the corresponding gas for 10 min prior to initiating the reaction. The

154

samples were withdrawn at various time intervals, and immediately measured.

155

Analytical methods.

156

The pH of the solution was determined by pH meter (PHS-3C). As(V) concentration was

157

determined using a modified molybdenum-blue method 17. Briefly, for each 2.0 mL quenched

158

aliquot (1 mL of sample + 1 mL of methanol), 0.5 mL of the 2% HCl acidifying solution and

159

0.3 mL of the color reagent were mixed sequentially. For total As determination, 0.5 mL of

160

the 2% HCl containing 2 mmol L-1 KIO3 was used instead of 2% HCl solution. The

161

absorbance at 880 nm was determined within 30 min using an UV−vis spectrophotometer

162

(UV-3000, MAPADA). The concentration of Cr(VI) remaining in the solution was measured

163

by a diphenylcarbazide method. The concentrated acids (H3PO4:H2SO4:H2O = 1:1:2, v:v:v)

164

were premixed with diphenylcarbazide reagent to minimize the interference of acid with

165

Cr(VI) determination. The absorbance of sample solutions was detected at 540 nm after full

166

color development (> 15 min) 7, 9.

167

The concentration of S(IV) was determined using a modified colorimetric procedure with

168

DTNB 18. Briefly, 1 mL of sample was added into a cuvette containing a mixture with 1 mL 7



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of EDTA (1 mM), 2 mL of DTNB (1 mM), and 5 mL of Na2HPO4/KH2PO4 buffer (pH = 7).

170

The color was allowed to developed for 15 min and the sample solutions were detected at 412

171

nm with an UV−vis spectrophotometer.

172

Electron spin resonance (ESR) experiments were performed using DMPO as a spin-

173

trapping agent, whose detailed parameters and procedure are shown in Text S1. A UV–

174

visible spectrophotometer (UV-3000, MAPADA) was employed to measure the

175

concentrations of MO, MB and RhB at wavelengths of 505, 663 and 554 nm, respectively.

176

Coumarin (1 mM) was employed as a chemical probe for •OH. 7-Hydroxycoumarin

177

(reaction 3) was measured by monitoring the fluorescence emission at 460 nm under

178

excitation at 332 nm using a spectrofluorometer (F97PRO, Lengguang Tech.) 19. •

179

OH + coumarin → 7 − hydroxycoumarin

180

Results and discussion

181

Oxidation of pollutants by Cr(VI)-sulfite reactions.

(3)

182

In this study, toxic As(III) was selected as a model pollutant. Its oxidation kinetics have

183

been extensively studied and can be easily characterized by measuring the oxidation product-

184

As(V)

185

experimental conditions. In the reaction system of Cr(VI)-sulfite, approximately 400 µM

186

sulfite was depleted for only 20 µM Cr(VI) reduced with a [sulfite]consumption/[Cr(VI)]reduction

187

stoichiometric ratio of 20 in Figure 1(b,c) (no significant sulfite removal due to the oxidation

188

of air or the complexation of Cr(III) in Figure 1(c)). In contrast, when adding 50 µM As(III),

189

as an electron donor, into a Cr(VI)-sulfite solution, much more Cr(VI) (approximately 49 µM)

190

was rapidly reduced, consuming only 330 µM sulfite ([sulfite]consumption/[Cr(VI)]reduction ≈ 6.7).

191

Accordingly, approximately 32 µM of As(III) was gradually transformed to As(V) in this

192

process. As shown in Figure 1(a), neither sulfite nor Cr(VI), nor Cr(III)-sulfite can

7, 20-22

. Figure 1(a) compares the extent of As(III) oxidation under the different

8


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individually lead to measurable As(V) formation. Consequently, the oxidative conversion of

194

As(III) can be attributed to interactions between As (III), Cr(VI) and sulfite. Similarly, rapid

195

decoloration of MO, MB and RB within tens of seconds can be achieved (Figures S1-S3), in

196

contrast to the oxidation performance of Fe(II)/sulfite-based photochemical systems with

197

reaction times in tens of minutes 13-16. This indicates the strong oxidation capacity of Cr(VI)-

198

sulfite reaction system towards oxidation of both inorganic arsenite and organic pollutants.

199

To examine the role of sulfite in simultaneous conversions of Cr(VI) and As(III),

200

hydrosulfite (S2O42-), pyrosulfite (S2O52-) and hyposulfite (S2O32-) were chosen as alternative,

201

reducing sulfur(IV) sources. The results in Figure 1 show that although pyrosulfite and

202

hyposulfite can effectively reduce Cr(VI) to Cr(III), only pyrosulfite can simultaneously

203

trigger As(III) oxidation to As(V). Since a pyrosulfite molecule can be easily transformed to

204

two sulfites through a hydrolysis reaction (Figure S4) 23, sulfite is expected to play a critical

205

role in the synergetic conversion of Cr(VI) and As(III). As for, hydrosulfite, no obvious

206

conversion of As(III) or Cr(VI) was observed at pH 3.5 after 60 min. The importance of free

207

inorganic sulfite was further demonstrated by a formaldehyde addition experiment (see more

208

details in Figure S5). In conclusion, although many reducing sulfur reagents can transform

209

Cr(VI) to Cr(III), only free inorganic sulfite species result in As(III) oxidation.

210

Effect of initial reagent concentration.

211

To investigate the effects of initial reagent concentrations on the oxidation capacity of a

212

Cr(VI)-sulfite system, batch experiments were carried out in aqueous solution by varying

213

their initial concentrations at pH 3.5. Figure S6 shows that for 50 µM Cr(VI) reduction, the

214

addition of As(III) resulted in the reduction of more Cr(VI) to Cr(III) and consumed less

215

sulfite

216

[sulfite]consumption/[Cr(VI)]reduction (5-10) was lower in the Cr(VI)-sulfite-As(III) systems. The

than

a

Cr(VI)-sulfite

system

without

As(III).

9


Therefore,

the

ratio

of


217

reason may be that As(III), acting as an active radicals scavenger, can alleviate the

218

consumption of sulfite, another electron donor in the systems.

219

The optimal As(III) oxidation (approximately 32 µM) observed was achieved at a sulfite

220

concentration of 400 µM (Figure S7). Further increasing sulfite concentration (> 400 µM)

221

retarded As(V) formation. This could be explained by excess sulfite causing rapid Cr(VI)

222

reduction, increasing competition with As(III) for reactive oxidants. Higher concentrations of

223

As(III) may counter this effect, increasing the ionic activity of As(III) in the system, leading

224

to more As(V) formation and a higher ratio of [As(III)]oxidation/[Cr(VI)]reduction (Table 1).

225

Although an increase of Cr(VI) (25-100 µM) greatly enhanced the oxidation of As(III) to

226

As(V), the ratio of [As(III)]oxidation/[Cr(VI)]reduction reduced from 0.82 to 0.55 ([As(III)]0 = 50

227

µM, [sulfite]0 = 400 µM). Moreover, even though [sulfite]0 was optimized for As(V)

228

formation at [Cr(VI)]0 of 400 µM, a downtrend of [As(III)]oxidation/[Cr(VI)]reduction was

229

observed under the conditions of Cr(VI) ranging from 50 to 400 µM with the

230

[Cr(VI)]0/[As(III)]0 ratio of 1:1 (Table 1). These results indicated that the reaction of sulfite

231

reducing Cr(VI) for radicals generation was inhibited at high Cr(VI) concentrations, implying

232

that the reduction mechanism of Cr(VI) may depend on the Cr(VI) levels in Cr(VI)-sulfite

233

systems.

234

Effect of initial pH.

235

The pH was a potentially critical variable for As(III) oxidation in this Cr(VI)-sulfite

236

reaction system, because of its effect on the speciation and redox potentials of Cr(VI) and

237

sulfite. Figure S8 demonstrates that SO2·H2O and HSO3- were prominent species at lower pH

238

values (pH < 7.0). Figure S9 depicts that the major Cr(VI) species at pH < 6.0 is HCrO4-,

239

while CrO42- is the major species at pH > 6.0. Since CrO42- is a much weaker oxidant than

240

HCrO4- (E0(CrO42-/Cr2O3) = 0.56 VNHE at pH 7.0 vs E0(HCrO4-/Cr3+) = 0.94 VNHE at pH 3.0),

241

CrO42- is less active than HCrO4- for the Cr(VI)-induced oxidation of sulfite 8. Therefore, a 10


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faster reduction of Cr(VI) is expected, with enhanced oxidation of As(III) at more acidic pH,

243

as shown in Figure 2 and S10. Again, the pH dependence of As(III) oxidation demonstrates

244

the driving role of Cr(VI) in the transformations of sulfite and As(III).

245

Identification of the reactive oxidants.

246

The rapid oxidation of As(III) at acidic pH suggests that it might be a radical-involved

247

process initiated by the oxidative activation of sulfite with Cr(VI). To gain direct evidence for

248

the involvement of radicals, fluorescence measurement and electron paramagnetic resonance

249

(ESR) were employed. In oxygen or an oxygen-free atmosphere, the characteristic signal of

250

the sulfite anion radical (SO3•−) (aN = 14.7 G, aH =15.9 G) from Cr(VI)-initiated oxidation of

251

sulfite was observed by spin-trapping ESR in Figure 3(a) 24. Signal intensity of DMPO/SO3•−

252

adduct became stronger in the deoxygenated solution than the oxygenated solution. Other

253

radicals were not observed perhaps because excess DMPO (100 mM) trapped all of the SO3•−

254

and terminated any subsequent radical propagation reactions.

255

In order to trap secondary radicals formed from the sulfite chain reactions, the

256

concentration of DMPO was decreased to 25 mM, enhancing the reaction of the primary

257

SO3•− with O2. As presented in Figure 3(b), typical DMPO−•OH adducts (aN = 1.49 mT, aH =

258

1.49 mT) and DMPO−SO4•− adducts (aN = 1.38 mT, aH = 1.02 mT, aH = 0.14 mT, aH = 0.08

259

mT) signals can be confirmed based on the previous literature 25, indicating the generation of

260

SO4•− and •OH in the Cr(VI)-sulfite reaction system. Coumarin was used as a chemical probe

261

for •OH (Reaction 3) 26, to compare the formation of •OH in the Cr(VI)-sulfite system under

262

various gas atmospheres. As shown in Figure 3(c), the production of 7-hydroxycoumarin

263

(•OH + coumarin) was determined in an air atmosphere, however it was greatly inhibited in

264

an argon atmosphere. These results confirm the participation of oxygen for •OH and SO4•−

265

generation in the Cr(VI)-sulfite system. Thus, explaining the small quantity of As(III)

266

oxidized in an argon atmosphere – the observed oxidation may be attributed to remnants of 11



267

oxygen dissolved in the reaction mixture. Significant amounts of As(V) were formed in air

268

and oxygen atmospheres, respectively in Figure 3(c). Besides, oxidation of sulfite by oxygen

269

and its derived reactive radicals accounted for the enhanced depletion of sulfite, which led to

270

the inferior reduction of Cr(VI) in air and oxygen atmospheres to that in an argon atmosphere

271

(Figure S11).

272

To quantitatively differentiate the roles of various active species in As(III) oxidation,

273

radicals scavenging experiments for As(V) formation were performed by adding EtOH and

274

TBA with 400:1 molar ratio of the alcohols versus sulfite. Previous works 14, 27-32 have shown

275

that EtOH can effectively scavenge HO• and SO4•- at a rate of (1.2−2.8) × 109 M−1 s−1 and

276

(1.6−7.7) × 107 M−1 s−1, respectively. As for the alcohols without α-hydrogen, the rate

277

constant of TBA reacting with HO• was (3.8−7.6) × 108 M−1 s−1, approximately 3 orders of

278

magnitude greater than that with sulfate radicals ((4.0−9.1) × 105 M−1 s−1). Other radicals, e.g.,

279

SO5•− and SO3•−, generated in this system are fairly inert toward alcohols (k ≤ 103 M−1 s−1).

280

Table S1 indicates that As(V) generation in the Cr(VI)-sulfite system was greatly affected by

281

the presence of EtOH, implying that HO• and SO4•− were primary responsible for As(III)

282

oxidation, approximately 89% at 10 min and 91% at 60 min, respectively. Although SO3•−

283

and SO5•− were produced in this system, they cannot efficiently oxidize As(III) to As(V)

284

(E(SO3•−/SO32-) = 0.72 VNHE, E(SO5•−/SO52-) = 1.1 VNHE)

285

remaining 11.0% and 9.2% of As(V) formation within 10 and 60 min, respectively. As for

286

adding TBA in solution, the transformation of the As(III) decreased by 31.2% and 31.6%

287

within 10 and 60 min, respectively. In the Cr(VI) sulfite system, SO4•− contributed much

288

more to As(III) oxidation, e.g., 57.8% and 59.2% within 10 and 60 min, respectively than

289

HO•.

290

Effect of metal ions

12


33

, only accounting for the

Page 12 of 28

Page 13 of 28


291

Divalent/trivalent metals are often abundant and coexist with Cr(VI) species in many

292

wastewaters, e.g., acid mine drainage, electroplating effluent. Sulfite preferentially forms

293

complexes with Fe(III) over Cr(VI) 11. It is well understood that iron ions are very active for

294

the catalytic oxidation of sulfite via the formation of a ‘generalized’ sulfito-complex of Fe(III)

295

34

296

sulfite radical (SO3•−) can lead to the production of active radicals such as SO4•−, HO• and

297

SO5•−, responsible for the enhanced oxidation of As(III)/sulfite and significantly retarding the

298

reduction of Cr(VI) in As(III)-Fe(II)/Fe(III)-sulfite-Cr(VI) systems (Figure 4 and Figure S12)

299

13

300

addition of Mn(II) depletes sulfite, at an even greater rate (Figure S12), greatly suppressing

301

conversions of As(III) and Cr(VI) in Figure 4

302

explained by the fact that the rate constant of Mn(III) oxidizing sulfite is ca. 5×103 times

303

larger than that for the reaction between Fe(III) and sulfite. Rapid reaction between SO5•− and

304

Mn(II) (≈108 M-1 s-1) regenerates Mn(III), blocking the reaction pathways for the generation

305

of highly active radicals (Reactions 4 and 5)

306

system, the active radicals produced, i.e., HO• and SO4•−, can oxidize Mn(II) to Mn(III) at a

307

rate of 3×107 M-1 s-1 and 2×107 M-1 s-1, respectively, further driving sulfite consumption 31, 32.

308

In contrast, Cu(II), Co(II) and Al(III) did not cause any significant change in the co-

309

conversion of As(III) and Cr(VI).

310

311

312

313 314

. In this case, the fast intramolecular redox decomposition of Fe(III)-sulfite to Fe(II) and the

. Similarly, the formation of SO3•− also involved in Mn(II)-sulfite reaction system, but the

35

. The exceptional activity of Mn(II) can be

35

. Moreover, in the Cr(VI)-As(III)-sulfite

•−

SO3 + O 2 → O3SOO•− k = 2.5×109 M-1 s-1

2−

•−

HSO 3 + O3SOO •− → SO 4 + HSO 4

2−

k = 1.3×107 M-1 s-1

(4)

(5)

Comparison with the Cr(VI)-H2O2 system. Cr(VI) species are known to activate H2O2 to generate HO• for oxidation of organic compounds in water

8, 36

. Thus, a comparison of As(III) oxidation in two Cr(VI)-related 13



Page 14 of 28

315

oxidation systems, i.e., Cr(VI)-sulfite and Cr(VI)-H2O2. Figure 5 shows that although Cr(VI)

316

readily reduces with high concentrations of H2O2, the transformation of As(III) to As(V) is

317

low within 60 min. In contrast, relatively low sulfite concentrations were required for

318

complete Cr(VI) reduction, and resulted in more As(III) oxidation within 60 min, in the

319

Cr(VI)-H2O2 system. In addition, the Cr(VI)-sulfite system required no additional chemical

320

reagents because sulfite and Cr(VI) themselves are common industrial contaminants, while

321

expensive, large volumes of H2O2 ($1520/t) are required to drive the oxidation of As(III) in a

322

Cr(VI)-H2O2 system. It is well-known that the peroxochromate(V) complex in Cr(VI)-H2O2

323

systems is potentially the most significant intermediate for •OH formation 36, 37. However, the

324

mechanisms of active radicals formation in Cr(VI)-sulfite reaction systems is not well

325

understood.

326

Mechanism discussion.

327

The studies on Cr(VI) reduction by organic and inorganic acids are well documented. The

328

reaction begins with the formation of chromium(VI) esters followed by either a redox

329

reaction or a unimolecular redox reaction

330

proposed that the mechanism of sulfite oxidation by Cr(VI) involves the sequent

331

condensation of HSO3- with HCrO4- to give activated complexes, i.e., CrSO62- and

332

CrO2(SO3)22- (Reactions (6-8))

333

CrO2(SO3)22- to give Cr(III)-bound SO42- and SO3•− in Reaction (9) was confirmed by a spin

334

trapping technique (Figure 3(a)), which leads to Cr(VI) reduction via an intramolecular

335

electron transfer reaction.

336

337

338

38-40

. Thus, as seen in Scheme 1, it is reasonably

41

. Specifically, one-step, spontaneous decomposition of

−

HCrO 4 + HSO 3− ↔ CrSO 6 2− + H 2 O

H + + HSO 3− ↔ SO 2 ⋅ H 2 O CrSO 6 2− +SO 2 ⋅ H 2 O ↔ CrO 2 (SO 3 )2 2− + H 2O 14


(6)

(7)

(8)

Page 15 of 28


339

CrO 2 (SO 3 )2 2− +4H 2O + 2H + ↔ SO 4 Cr(H 2 O)5 + SO 3

340

CrSO6 2− + H + + HCrO 4 ↔ O3CrOCrO3SO 3 2− + H 2 O

341

O3CrOCrO3SO3 2 − → 2Cr(V) + SO 4

342

Cr(V) + S(IV) → Cr(III) + S(VI)

+

•−

(9)

−

(10)

2−

(11)

(12)

343

Increasing the concentration of Cr(VI) in aqueous solution, Cr(VI) reduction was

344

accelerated, with less sulfite consumption (shown in Table 1). Cr(VI) species probably

345

participates with CrSO62- in the formation of the complex (O3CrOCrO3SO32-), which then

346

decomposes into Cr(V) and sulfate ions (see Reactions 10 and 11). The formed Cr(V) is

347

probably directly bound to sulfite, leading to the direct reduction to Cr(III) in Reaction 12,

348

by-passing the highly energetic intermediates Cr(IV) 41. Reactions 10 and 11 should compete

349

with Reaction 8 and therefore block the one-step, three-electron reaction of Cr(VI) and SO3•−

350

production in Reaction 9. Thus, this step can account for relatively less generation of active

351

radicals and a higher ratio of [Cr(VI)]reduction/[sulfite]consumption at high [Cr(VI)]0 (Table 1). The

352

two fates of CrSO62- in Reactions 8 and 10 were demonstrated by Haight et al.

353

for the variation in stoichiometric values of [sulfite]consumption/[Cr(VI)]reduction, i.e., 3/2 and 2/1

354

(see Rreaction 1 and 2). Reactions 7, 9 and 10 consume protons, which can explain the

355

acidity dependence of Cr(VI)-sulfite reactions.

41

to account

356

In an Ar-saturated solution, the transformation of SO3•− proceeds via Reactions 13 and 14,

357

which would terminate the radical propagation processes. As a consequence, a negligibly

358

measurable •OH signal was observed in the fluorescence spectrum and oxidation of As(III)

359

was significantly inhibited in this anaerobic environment (see Figure 3(c)).

360

•−

•−

SO3 + SO3 → S2O6

2−

k = 1.5×109 M-1 s-1

15


(13)


•−

2−

−

SO3 + HCrO 4 → SO 4 + Cr(V)

361

Page 16 of 28

(14)

362

However, when Cr(VI)-sulfite reaction is exposed to an oxygen atmosphere, the unstable

363

SO3•− is prone to being oxidized by oxygen and then gives rise to the formation of the

364

oxygen-centered peroxymonosulfate (O3SOO•−) and sulfate (SO4•−) anion radicals through

365

chain propagation steps shown in Reactions 4 and 5 23, 42: 2−

SO 4 + H 2O → SO 4 + • OH + H + k = (103-104) M-1 s-1 •−

366

(15)

367

In this scenario, SO4•− can oxidize almost any biomolecular H2O for •OH production

368

(Reaction 15) 31. Although, in a Cr(VI)-sulfite reaction system, the radicals of •OH and SO4•−

369

are very strong oxidants, they may not lead to the re-oxidation of Cr(III) to Cr(VI)

370

(E(HCrO4−/Cr3+) = 1.35 VNHE) due to the excess of sulfite (E(SO42−/SO2) = 0.158 VNHE).

371

Thus, scavenging the radicals of •OH and SO4•− by free sulfite can be responsible for deviant

372

stoichiometric consumption of sulfite (k•OH/sulfite = 4.5×109 M−1 s−1 and kSO4•−/sulfite > 2.0×109

373

M−1 s−1).This is verified by a greater production of sulfate ions than in the argon atmosphere,

374

Cr(VI)-sulfite reaction system, as shown in Figure S13(a,b). These findings also explain the

375

requirement of excess sulfite for complete Cr(VI) reduction in acidic solutions in previous

376

studies 2, 10-12.

377

However, when As(III) is present in a Cr(VI)-sulfite system, it can scavenge highly active

378

radicals such as •OH (k•OH/As(III) = 8.5×109 M-1 s-1) and SO4•− (kSO4•−/As(III) > 8.0×108 M−1 s−1) 31.

379

As a consequence, sulfite oxidation, mediated by active radicals can be interrupted with

380

oxidative transformation of As(III) to As(V), which in turn accelerates the reduction of

381

Cr(VI). Thus, compared with a Cr(VI)-sulfite reaction in an air atmosphere, much less sulfate

382

was produced in a Cr(VI)-sulfite-As(III) reaction system (Figure S13(c)). Generally, in a

383

Cr(VI)-sulfite system, the presence of other organic/inorganic pollutants decreases the

16


Page 17 of 28


384

amount of sulfite required for Cr(VI) reduction and therefore the sulfate concentration in the

385

resulting waste water.

386

Environmental implications.

387

In the present study, we shed light on the involvement of SO4•− (2.5−3.1 VNHE) and •OH

388

(1.8−2.7 VNHE, pH dependent) in Cr(VI)-sulfite systems, which can be potentially utilized for

389

the oxidation of other pollutants with rates of 104-1010 M-1 s-1 and 107-1010 M-1 s-1,

390

respectively

391

concentrated Cr(VI) electroplating effluent from the electroplating industrial park of Qingdao

392

Development Zone ([Cr(VI)] = 162.7 mg L-1, [Fe] = 79 µM, pH = 2.23) was utilized to

393

examine organic pollutant removal in an air atmosphere. For example, 40 µM rhodamine B

394

(100 mL, pH = 3.0), a common contaminant, can be quickly bleached with the addition of a

395

portion of initiator (1.6 mL real Cr(VI) electroplating effluent and 1.2 mM sulfite).

396

Approximately 24% TOC was diminished after adding a second portion of initiator. This

397

result validates that the traditional Cr(VI) reducing process can be potentially, and

398

economically upgraded for the treatment of Cr(VI) and other inorganic/organic pollutants

399

mixed wastewaters (e.g., chromate-containing textile wastewater and tannery wastewater)

400

with externally added sulfite as the reductant or the synergetic transformations of multiple

401

pollutants, such as Cr(VI) reduction, sulfite oxidation and the oxidation of other

402

inorganic/organic pollutants if mixing their relevant wastewaters is cost effective.

31, 32

. To evaluate the industrial flexibility of this proposed process, real

403

There is an environmental risk that a sulfite process, can cause the toxic release of SO2 in

404

an acidic environment (pH ≤ 3.5 in Figure S8) and high concentrations of sulfate will remain

405

in the waste water. In spite of these, compared with the traditional sulfite reducing Cr(VI)

406

process, the upgraded process, utilizing SO4•− and •OH for the oxidation of other

407

inorganic/organic pollutants in acidic solutions (pH ≤ 4.5) with a [sulfite]0/[Cr(VI)]0 ratio of

408

3.0-8.0, requires less sulfite addition for more Cr(VI) reduction. Thus, it effectively alleviates 17



409

the environmental risks, making the process of sulfite reducing Cr(VI) more environmentally

410

favourablele. As such, the Cr(VI)-sulfite system is a new cost-effective technology for the

411

treatment of various industrial wastewaters.

412

Acknowledgment

413

This work is ﬁnancially supported by the National Natural Science Foundation of China (Nos.

414

21376268, 21176260, 51372277, 21302224), the National Basic Research Development

415

Program of China (973 Program) (No. 2011CB605703), the Taishan Scholar Foundation (No.

416

ts20130929), and Shandong Provincial Natural Science Foundation (ZR2013BQ028). The

417

authors thank Michelle Bush from Southern Cross University, Australia for her kind help in

418

proof-reading.

419

Supporting Information Available

420

Additional experimental evidences. This material is available free of charge via the Internet at

421

http://pubs.acs.org.

422

References

423

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6. Testa, J. J.; Grela, M. A.; Litter, M. I. Heterogeneous photocatalytic reduction of chromium 18


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7. Jiang, B.; Guo, J.; Wang, Z.; Zheng, X.; Zheng, J.; Wu, W.; Wu, M.; Xue, Q. A green

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8. Bokare, A. D.; Choi, W. Chromate-Induced Activation of Hydrogen Peroxide for Oxidative

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9. Wang, Z.; Bush, R. T.; Sullivan, L. A.; Liu, J. Simultaneous Redox Conversion of

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16. Chen, L.; Peng, X.; Liu, J.; Li, J.; Wu, F. Decolorization of Orange II in Aqueous Solution

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measuring arsenic concentrations in groundwater. Anal. Chim. Acta 2004, 526 (2), 203-209.

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18. Humphrey, R. E.; Ward, M. H.; Hinze, W. Spectrophotometric determination of sulfite

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with 4, 4'-dithio-dipyridine and 5, 5'-dithiobis (2-nitrobenzoic acid). Anal. Chem. 1970, 42

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19. Kim, D. H.; Lee, J.; Ryu, J.; Kim, K.; Choi, W. Arsenite Oxidation Initiated by the UV

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Photolysis of Nitrite and Nitrate. Environ. Sci. Technol. 2014, 48 (7), 4030-4037. 19



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20. Kim, K.; Choi, W. Enhanced Redox Conversion of Chromate and Arsenite in Ice.

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Mater. 2014, 267 (0), 62-70.

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22. Wu, Y.; Ma, X.; Feng, M.; Liu, M. Behavior of chromium and arsenic on activated

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carbon. J. Hazard. Mater. 2008, 159 (2–3), 380-384.

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23. Hayon, E.; Treinin, A.; Wilf, J. Electronic spectra, photochemistry, and autoxidation

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mechanism of the sulfite-bisulfite-pyrosulfite systems. SO2-, SO3-, SO4-, and SO5- radicals. J.

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Am. Chem. Soc. 1972, 94 (1), 47-57.

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24. Mottley, C.; Mason, R. P. Sulfate anion free radical formation by the peroxidation of (Bi)

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sulfite and its reaction with hydroxyl radical scavengers. Arch. Biochem. Biophys. 1988, 267

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25. Zou, J.; Ma, J.; Chen, L.; Li, X.; Guan, Y.; Xie, P.; Pan, C. Rapid acceleration of ferrous

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iron/peroxymonosulfate oxidation of organic pollutants by promoting Fe(III)/Fe(II) cycle

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with hydroxylamine. Environ. Sci. Technol. 2013, 47 (20), 11685-11691.

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26. Ishibashi, K. I.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Detection of active oxidative

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species in TiO2 photocatalysis using the fluorescence technique. Electrochem. Commun.

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2000, 2 (3), 207-210.

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27. Yang, Y.; Jiang, J.; Lu, X.; Ma, J.; Liu, Y. Production of Sulfate Radical and Hydroxyl

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Radical by Reaction of Ozone with Peroxymonosulfate: A Novel Advanced Oxidation

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28. Muller, J. G.; Hickerson, R. P.; Perez, R. J.; Burrows, C. J. DNA Damage from Sulfite

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Autoxidation Catalyzed by a Nickel(II) Peptide. J. Am. Chem. Soc. 1997, 119 (7), 1501-1506.

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29. Stemmler, A. J.; Burrows, C. J. Guanine versus deoxyribose damage in DNA oxidation

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mediated by vanadium(IV) and vanadium(V) complexes. J. Biol. Inorg. Chem. 2001, 6 (1),

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100-106.

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30. Anipsitakis, G. P.; Dionysiou, D. D. Radical Generation by the Interaction of Transition

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Metals with Common Oxidants. Environ. Sci. Technol. 2004, 38 (13), 3705-3712.

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31. Neta, P.; Huie, R. E.; Ross, A. B. Rate Constants for Reactions of Inorganic Radicals in

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Aqueous Solution. J. Phys. Chem. Ref. Data 1988, 17 (3), 1027-1284.

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32. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of rate

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constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (•OH/O•−

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in Aqueous Solution. J. Phys. Chem. Ref. Data 1988, 17 (2), 513-886.

505

33. Das, T. N.; Huie, R. E.; Neta, P. Reduction Potentials of SO3•-, SO5•-, and S4O6•3- Radicals 20


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in Aqueous Solution. J. Phys. Chem. A 1999, 103 (18), 3581-3588.

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34. Lente, G.; Fábián, I. Kinetics and mechanism of the oxidation of sulfur(IV) by iron(III) at

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metal ion excess. J. Chem. Soc., Dalton T. 2002 (5), 778-784.

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35. Fronaeus, S.; Berglund, J.; Elding, L. I. Iron-manganese redox processes and synergism in

510

the mechanism for manganese-catalyzed autoxidation of hydrogen sulfite. Inorg. Chem.

511

1998, 37 (19), 4939-4944.

512

36. Bokare, A. D.; Choi, W. Advanced Oxidation Process Based on the Cr(III)/Cr(VI) Redox

513

Cycle. Environ. Sci. Technol. 2011, 45 (21), 9332-9338.

514

37. Zhang, L.; Lay, P. A. EPR spectroscopic studies on the formation of chromium (V)

515

peroxo complexes in the reaction of chromium (VI) with hydrogen peroxide. Inorganic

516

Chemistry 1998, 37 (8), 1729-1733.

517

38. Mason, J. G.; Kowalak, A. D.; Tuggle, R. M. Kinetics of the chromium(VI)-arsenic(III)

518

reaction. II. Dihydrogen phosphate-hydrogen phosphate buffer solutions. Inorg. Chem. 1970,

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9 (4), 847-850.

520

39. Westheimer, F. H. The Mechanisms of Chromic Acid Oxidations. Chem. Rev. 1949, 45

521

(3), 419-451.

522

40. Connett, P.; Wetterhahn, K. Reaction of chromium(VI) with thiols: pH dependence of

523

chromium (VI) thio ester formation. J. Am. Chem. Soc. 1986, 108 (8), 1842-1847.

524

41. Haight Jr, G.; Perchonock, E.; Emmenegger, F.; Gordon, G. The Mechanism of the

525

Oxidation of Sulfur(IV) by Chromium(VI) in Acid Solution1. J. Am. Chem. Soc. 1965, 87

526

(17), 3835-3840.

527

42. Buxton, G.; McGowan, S.; Salmon, G.; Williams, J.; Wood, N. A study of the spectra and

528

reactivity of oxysulphur-radical anions involved in the chain oxidation of S(IV): A pulse and

529

γ-radiolysis study. Atmos. Environ.1996, 30 (14), 2483-2493.

530 531

532

533

21



534 535

Figure 1. (a) Oxidation of As(III), (b) reduction of Cr(VI) and (c) consumption of sulfite

536

under varying experimental conditions ([As(III)]0 = 50 µM, [Cr(III)]0 or [Cr(VI)]0

537

=50 µM, [sulfite]0 = 400 µM, [pyrosulfite]0 = 400 µM, [hydrosulfite]0 = 400 µM,

538

[hyposulfite]0 = 400 µM, pHini = 3.5). 22


Page 22 of 28

Page 23 of 28


539

540 541

Figure 2. The effect of solution pH on the oxidation of As(III) ([As(III)]0 = 50 µM, [Cr(VI)]0= 50 µM, [sulfite]0 = 400 µM).

542

543

544

545

546

547

548

549

550

551

552

23



553 554

Figure 3. ESR spectrum of sulfite radical (a) (100 mM DMPO, [Cr(VI)]0= 0.3 mM, [sulfite]0

555

= 1.8 mM, pHini = 3.5), ESR spectrum of sulfate and hydroxyl radicals (b) (25 mM

556

DMPO, [Cr(VI)]0= 0.3 mM, [sulfite]0 = 1.8 mM, pHini = 3.5, initiating reaction in

557

argon atmosphere and then in an oxygenating condition, ★ represents DMPO-•OH

558

adduct and ▼ represents DMPO-SO4•− adduct), determination of hydroxyl radical

559

using fluorescence spectrometry (c) and the formation of As(V) (c) in Cr(VI)-sulfite

560

system under various gas atmospheres ([coumarin]0 = 1 mM, [Cr(VI)]0= 50 µM,

561

[As(III)]0 = 50 µM, [sulfite]0 = 400 µM, pHini = 3.5).

562

24


Page 24 of 28

Page 25 of 28


563 564

Figure 4. Effects of coexisting metal ions on simultaneous transformations of As(III), Cr(VI)

565

and sulfite in Cr(VI)-sulfite systems ([As(III)]0 = 50 µM, [Cr(VI)]0= 50 µM,

566

[sulfite]0 = 400 µM, [metal ions] = 50 µM, pHinit. = 3.5).

567

25



568 569

Figure 5. Comparisons of oxidation capacity for As(III) (a) and Cr(VI) reduction capacity (b)

570

between the Cr(VI)-sulfite system and Cr(VI)-H2O2 system ([As(III)]0 = 50 µM,

571

[Cr(VI)]0= 50 µM, pHinit = 3.5).

572

573

26


Page 26 of 28

Page 27 of 28


Table 1. The effect of reagents concentrations on the transformations of As(III) and Cr(VI) and sulfite within 60 min (pHini = 3.5). 574 Cr(VI) (µM)

25

50

75

100

50

50

50

100

200

400

400

400

400

As(III) (µM)

50

50

50

50

25

75

100

100

200

400

400

400

400

sulfite (µM)

400

400

400

400

400

400

400

800

1600

1000

1200

1600

2400

Cr(VI) reduction

25

48.9

63.6

69.6

44.6

49.3

49.6

100

200

297.9

380.8

399.8

399.9

400 575 400 576 3200 577 400

As(III) oxidation

20.5

31.6

35.0

38.4

23.1

38.4

47.3

59.3

73.6

124.5

182.5

130.7

84.6

578 75.4

sulfite consumption

220.5

336.1

388.0

396.0

381.8

316.6

301.2

662.2

997.6

991.0

1151.0

1262.5

1181.8

1055.5 579

[As(III)]oxidation/[Cr(VI)]reduction

0.82

0.65

0.55

0.55

0.52

0.78

0.95

0.59

0.37

0.42

0.48

0.33

0.21

0.19 580

[Cr(VI)]reduction/ [sulfite]consumption

0.11

0.15

0.16

0.18

0.12

0.16

0.16

0.15

0.20

0.30

0.33

0.32

0.34

581 0.38 582

583 584 585 586 587

27



588 589

Scheme 1. Proposed pathways for the reduction of Cr(VI) and the formation of active species, i.e., HO• and SO4•−, in the Cr(VI)/sulfite system.

590 591

28


Page 28 of 28

Synergetic Transformations of Multiple Pollutants Driven by Cr(VI

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