Removal of Persistent Organic Contaminants by Electrochemically

Subscriber access provided by The University of Liverpool

Article

Removal of Persistent Organic Contaminants by Electrochemically Activated Sulfate Ali Farhat, Jürg Keller, Stephan Tait, and Jelena Radjenovic Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02705 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

Environmental Science & Technology

1

Removal of Persistent Organic Contaminants by Electrochemically Activated Sulfate

2 Ali Farhata, Jurg Keller a, Stephan Tait a, Jelena Radjenovic a,b,*

3 4 5

a

6

Australia

7

b

8

University of Girona, 17003 Girona, Spain

Advanced Water Management Centre, The University of Queensland, Queensland 4072,

Catalan Institute for Water Research (ICRA), Scientific and Technological Park of the

9 10 11 12 13 14 15 16 17

* Corresponding author:

18

Jelena Radjenovic, Catalan Institute for Water Research (ICRA), Scientific and Technological

19

Park of the University of Girona, 17003 Girona, Spain

20

Phone: + 34 972 18 33 80; Fax: +34 972 18 32 48; E-mail: [email protected]

1

ACS Paragon Plus Environment


21

Abstract

22 23

Solutions of sulfate have often been used as background electrolytes in electrochemical

24

degradation of contaminants, and have been generally considered inert even when employing

25

high oxidation power anodes such as boron-doped diamond (BDD). This study examines the role

26

of sulfate by comparing electrooxidation rates for seven persistent organic contaminants at BDD

27

anodes in sulfate and inert nitrate anolytes. Sulfate yielded 10–15 times higher electrooxidation

28

rates of all target contaminants compared to nitrate anolyte. This electrochemical activation of

29

sulfate was also observed at concentrations as low as 1.6 mM, relevant for many wastewaters.

30

Electrolysis of diatrizoate in the presence of specific radical quenchers (tert-butanol, methanol)

31

had a similar effect on electrooxidation rates, illustrating a possible role of hydroxyl radical

32

(•OH) in the anodic formation of sulfate radical (SO4•-) species. Addition of 0.55 mM persulfate

33

increased the electrooxidation rate of diatrizoate in nitrate from 0.94 h-1 to 9.97 h-1, suggesting a

34

non-radical activation of persulfate. Overall findings indicate the formation of strong sulfate-

35

derived oxidant species at BDD anodes when polarized at high potentials. This may have

36

positive implications in electrooxidation of wastewaters containing sulfate. For example, the

37

energy required for 10-fold removal of diatrizoate was decreased from 45.6 to 2.44 kWh m-3 by

38

switching from nitrate to sulfate anolyte.

39 40

2


Page 2 of 29

Page 3 of 29

41


TOC

42 Boron-doped diamond anode RO, CO2, H2 O BDD ~S2 O82-ACT

Removal of diatrizoate 1,0

0,8

Na2SO4 anolyte

R

CH3

O

0,6

I

I

NH

NH

O

S2 O8 2BDD ~SO4• -

BDD ~SO4 • BDD ~•OH e-

H3C

RO, CO2 , H2O R

O

NaClO4 anolyte

0,4

CH3

I

NaNO3 anolyte

0,2

SO4 2-

0,0 H2 O

0

30

60

90 120 150 180 Electrolysis time (min)

210

240

43

3



44

INTRODUCTION

45 46

There has been increasing interest in recent years in electrooxidation to remove persistent

47

organic contaminants from wastewater.1 In environmental applications of electrochemical

48

processes, boron-doped diamond (BDD) electrodes have attracted a lot of interest because of

49

their high electrocatalytic activity towards organic oxidation.2 As postulated by Comninellis,3

50

water electrolysis at anodes with high oxygen evolution overpotential (such as a BDD anode)

51

yields weakly adsorbed hydroxyl radicals (•OH) capable of mineralizing organic contaminants.

52

In addition, BDD anodes can also produce ozone, hydrogen peroxide and other peroxy species

53

(e.g., C2O62-, P2O84- and S2O82-).4-7

54 55

Persulfate formation at BDD anode is considered to occur via oxidation of sulfate ions to sulfate

56

radicals (SO4•-) and recombination of two SO4•- to yield persulfate.7-9 Yet, as in the case of •OH

57

generated at a BDD anode,10,11 there is no spectroscopic evidence of electrochemically formed

58

SO4•-. The principal pathway for the formation of SO4•- is by advanced oxidation processes, via

59

heat, UV, alkaline, or metal-catalyst activation of persulfate (S2O82-) or peroxymonosulfate

60

(HSO5-).12 Since persulfate has slow oxidation kinetics with organic compounds in the absence

61

of an activator, electrogenerated persulfate may contribute only to a minor extent to the bulk

62

oxidation, and BDD electrooxidation mechanisms in the presence of sulfate have typically been

63

interpreted by the action of •OH.5,13,14 A limited number of studies have suggested that inorganic

64

radicals generated at the anode (e.g., PO4•2-, SO4•-, Cl2•-) may be contributing to a minor

65

additional electrooxidation of organic contaminants.15-17 Sulfate radicals are strong oxidants with

66

a redox potential Eo (SO4•-/SO42-) = 2.5–3.1 V,18,19 similar to the redox potential of hydroxyl

4


Page 4 of 29

Page 5 of 29


67

radicals at acidic pH (Eo(•OH/H2O) = 2.7 V).20,21 Both radicals have been reported to react with

68

many pharmaceuticals at comparable oxidation rates.21-24 Yet, SO4•- tends to react primarily via

69

electron transfer mechanisms, and •OH is more likely to react via addition to unsaturated bonds

70

and H-abstraction.25 Thus, electrolysis of sulfate ions to sulfate radical species (HSO4•, SO4•-)

71

may have a significant effect on the oxidation kinetics and degradation pathways of

72

contaminants in electrochemical treatment of wastewater. Given that sulfate can be present in

73

municipal and industrial wastewater at significant concentrations of several hundred mg L-1 up to

74

g L-1 level, it is important to elucidate the role of sulfate ions in electrooxidation at a BDD anode.

75 76 77

The objective of this study is to investigate the role of sulfate and persulfate ions in the

78

electrochemical oxidation of contaminants at BDD anodes and elucidate the participation of

79

electrogenerated sulfate radical species and non-radically activated persulfate in electrooxidation

80

of persistent organic contaminants. We have quantified the rates of electrooxidation of several

81

organic contaminants at BDD anode, including diatrizoate, carbamazepine, N,N-diethyl-meta-

82

toluamide (DEET), iopromide, tribromophenol, triclosan and triclopyr. These contaminants were

83

selected due to their high persistence to chemical oxidation, e.g., by ozone or •OH. Experiments

84

are performed in sulfate anolyte, and compared with inert nitrate and perchlorate anolytes. The

85

study evaluates the effect of anolyte concentration/conductivity and applied current density on

86

the

87

electrooxidation experiments were performed with iodinated contrast media (ICM) diatrizoate as

88

a model contaminant in the presence of the radical scavengers, tert-butanol and methanol. In an

89

attempt to further segregate the effects of •OH and SO4•-, the study also examines

electrooxidation

performance.

To

determine

the

major

5


participating

oxidants,


90

electrooxidation of nitrobenzene at a BDD anode, as a typical •OH probe compound. Finally,

91

electrolysis of diatrizoate was investigated in the presence of persulfate to investigate its

92

activation via non-radical mechanisms.

93 94

MATERIALS AND METHODS

95 96

Chemicals

97

All solutions were prepared using analytical grade reagents and Milli-Q water. Analytical

98

standards for diatrizoate, carbamazepine, N,N-diethyl-meta-toluamide (DEET), iopromide,

99

tribromophenol, triclosan, and triclopyr were purchased from Sigma-Aldrich (Steinheim,

100

Germany). Sodium sulfate, hydrogen peroxide, tert-butanol, and nitrobenzene were also

101

purchased from Sigma-Aldrich (Steinheim, Germany). Sodium nitrate and sodium perchlorate

102

were purchased from Chem-Supply (Gillman, Australia). Potassium persulfate, nitric acid

103

(HNO3, 69%) and formic acid were purchased from Ajax Finechem (Auckland, New Zealand).

104

Sulfuric acid (H2SO4, 98%) and solvents for liquid chromatography (acetonitrile, methanol) were

105

purchased from Merck (Darmstadt, Germany).

106 107

Experimental Setup

108

The experiments were performed in a laboratory-scale plate-and-frame electrolytic cell, with an

109

inter-electrode distance of 3.5 cm (Supporting Information (SI), Figure S1) and divided by a

110

cation exchange membrane (Ultrex CMI-7000, Membranes International, Ringwood, NJ,

111

U.S.A.). The net volume of the anodic and cathodic compartment was 200 mL each. Working

112

electrode was DIACHEM® BDD (polycrystalline, 5µm thick, 1000–4000 ppm boron doping on

6


Page 6 of 29

Page 7 of 29


113

monocrystalline niobium plate) purchased from Condias (Itzehoe, Germany). Prior to the

114

experiments, the BDD electrode was polarized anodically for 2 h in 0.1 M H2SO4 at constant

115

anodic potential of 3.0 V vs Standard Hydrogen Electrode (SHE). Stainless steel was used as the

116

counter electrode. The dimensions of both electrodes were 48×85×2mm. Chronopotentiometric

117

electrolysis experiments were conducted using a VSP potentiostat/galvanostat, using an external

118

booster channel (BioLogic, Claix, France). The reference electrode was a 3M Ag/AgCl (+0.210

119

V vs SHE), supplied by BASi (West Lafayette, IN, U.S.A.), which was placed in the proximity

120

of the working electrode.

121 122

The applied current density was 200 A m-2, unless otherwise stated. All experiments were

123

performed in batch mode at anodic and cathodic flow rates of 200 mL min−1. The total volume of

124

both anolyte and catholyte was 500 mL each. To investigate the effect of sulfate-based anolyte in

125

electrooxidation at BDD anode, experiments were performed using sodium sulfate anolyte

126

(Na2SO4, pH 2, 9 mS cm-1, 40 mM, unless otherwise stated) and compared with sodium nitrate

127

anolyte (NaNO3, pH 2, 9 mS cm-1, 60 mM, unless otherwise stated). The experiments were

128

performed with several persistent organic contaminants: diatrizoate, carbamazepine, DEET,

129

iopromide, tribromophenol, triclosan, and triclopyr, each added at an initial concentration of 2

130

µM. To confirm the inertness of nitrate ions in electrooxidation at a BDD anode, preliminary

131

experiments compared electrooxidation rates of diatrizoate as a model compound in nitrate (60

132

mM, pH 2, 9 mS cm-1) and perchlorate anolytes (74 mM, pH 2, 9 mS cm-1).

133 134

Persulfate can decompose to hydrogen peroxide in strongly acidic aqueous solutions.27 To

135

determine the maximum amount of H2O2 and S2O82- generated at the BDD anode in Na2SO4 (40

7



136

mM, pH 2, 9 mS cm-1), chronopotentiometric experiments were performed at the highest applied

137

current density (i.e., 200 A m-2) without added organic, and the concentration of H2O2 or S2O82-

138

was measured. To investigate the contribution of chemical oxidation of S2O82- to

139

electrooxidation, 2 µM solutions of each persistent organic was prepared in Na2SO4 anolyte (40

140

mM, pH 2, 9 mS cm-1) using amber glass bottles. Then, K2S2O8 was added to a final

141

concentration of 0.55mM, and the mixture was left to react while periodically collecting samples

142

for the analysis of target organic contaminant (see Chemical Analysis). To investigate the

143

activation of persulfate via non-radical mechanisms, electrooxidation of diatrizoate was

144

performed in inert NaNO3 anolyte (pH 2, 60 mM, 9 mS cm-1) and with the addition of persulfate

145

at the final concentration of 0.55 mM.

146 147

Diatrizoate was also selected to further study the effect of sulfate concentration (i.e., sulfate

148

added at 1.6, 5, 15 and 40 mM), anodic current density (100 and 150 A m-2) and the addition of

149

radical scavengers (tert-butanol and methanol). In all experiments, of the Na2SO4 and NaNO3

150

anolytes was adjusted to pH 2 with concentrated H2SO4 and HNO3, respectively. This pH was

151

chosen as both radicals exhibit similar redox potentials at acidic pH, and because the production

152

of protons at the anode did not lead to further lowering of the pH that remained constant in all

153

experiments. The selected pH was above the second pKa of sulfuric acid (pKa (HSO4-/SO42-)=

154

1.92),28 and thus SO42- ions were the dominant species in the solution. To prevent further

155

addition of sulfate in the case of 1.6 mM Na2SO4 anolyte, pH was adjusted with concentrated

156

HNO3. In the experiments with radical scavengers, methanol or tert-butanol was added to

157

Na2SO4 anolyte (36 mM) to a final concentration of 0.1 mM.

158

8


Page 8 of 29

Page 9 of 29


159

Given the lack of a suitable SO4•- probe compound, electrooxidation experiments were

160

performed with nitrobenzene, a common •OH probe compound,29 at 200 A m-2, in Na2SO4 (35

161

mM, pH 2, 9 mS cm-1) or NaNO3 anolytes (54 mM, pH 2, 9 mS cm-1). Due to the poor

162

sensitivity of the employed analytical method, nitrobenzene was added at a higher initial

163

concentration (i.e., 400 µM).

164 165

All electrochemical and chemical oxidation experiments were conducted at room temperature

166

(25ºC). Reactors and glassware used in chemical and electrochemical oxidation experiments

167

were protected from light. During periodic sampling in all cases, 750 µL samples were collected

168

and were immediately quenched with 250 µL of methanol. The concentration of the respective

169

organic of interest was then measured (see Chemical Analysis). For data processing, measured

170

concentrations (C) were normalized against the initial concentration (C0) of target organic

171

contaminant, and all the concentration ratios values (C/C0) from duplicate experiments were then

172

fitted with a pseudo-first order kinetic relationship as the concentration of the formed radicals

173

were expected to be in high excess compared to that of the persistent organic studied. The best-

174

fit values of a pseudo-first order kinetic decay rate were determined by a nonlinear parameter

175

estimation routine in Aquasim 2.1d,30 and these values were expressed with estimates of error at

176

the 95% confidence level. The confidence limits were calculated using a standard error estimated

177

by Aquasim and an appropriate t-value for the respective number of degrees of freedom (DOF >

178

10) of the duplicate experiment.

179 180

Chemical Analysis

9



Page 10 of 29

181

Target organic contaminants were analyzed by liquid chromatography-tandem mass

182

spectrometry (LC-MS/MS) using a Shimadzu Prominence ultra-fast liquid chromatography

183

(UFLC) system (Shimadzu, Kyoto, Japan) coupled with a 4000 QTRAP MS equipped with a

184

Turbo Ion Spray Source (Applied Biosystems-Sciex, Foster City, CA, U.S.A.). Details of the

185

analytical methods are summarized in SI Text S1 and Table S1.

186 187

Nitrobenzene was analyzed by high performance liquid chromatography (HPLC) equipped with

188

a diode array UV-detector (SPD-M10AVP) purchased from Shimadzu, Japan. The detection of

189

nitrobenzene was performed at 254 nm. Nitrobenzene was eluted using an Alltima C18 column

190

(5 µm; 4.6 mm × 250 mm) and a mobile phase of methanol/water (1/1, v/v) at 1 mL min-1.

191 192

Hydrogen peroxide concentration was quantified with ammonium metavanadate method.31 This

193

method was found to be insensitive to S2O82- in the concentration ranges investigated (SI Text

194

S2, Figure S2). Thus, the metavanadate method was used to determine the concentration H2O2

195

without the interference of S2O82-. Persulfate was detected with a thiocyanate method.32 The

196

thiocyanate method was found to detect both H2O2 and S2O82- (SI Text S2, Figure S3). Thus, it

197

was used as to measure the combined concentration of H2O2 and S2O82- in the solution.

198 199

RESULTS AND DISCUSSION

200 201

Effect of anolyte on electrooxidation rates of persistent organic contaminants

202

Nitrate and perchlorate ions are known to not react with •OH.33 Moreover, both ions are usually

203

considered as inert in electrooxidation at a BDD anode.13 In the present work, this was confirmed

10


Page 11 of 29


204

in preliminary experiments comparing electrooxidation of diatrizoate in NaNO3 and NaClO4

205

anolytes (Figure 1). The apparent rate constants for oxidation of diatrizoate were observed to be

206

0.94±0.07 h-1 and 1.9±0.07 h-1 in the nitrate and perchlorate anolytes, respectively. Thus, NaNO3

207

was selected as an inert background anolyte for subsequent comparison with Na2SO4 anolyte.

208 209

In all the experiments (including with NaNO3 and NaClO4 anolytes as noted above), the

210

disappearance of each organic contaminants could be described by pseudo-first order rate

211

kinetics. Table 1 summarizes the apparent rate constants for the electrooxidation of target

212

organic contaminants in NaNO3 and Na2SO4 anolyte of the same initial conductivity and pH (9

213

mS cm-1, pH 2). The lowest rate constants were observed for the ICM, iopromide and diatrizoate,

214

in NaNO3 anolyte (kNaNO3, h-1) with 0.83±0.15 and 0.94±0.07 h-1, respectively. ICM have been

215

reported to be recalcitrant in various oxidation processes.34-36 For example, transformation of

216

diatrizoate and iopromide have been observed to be slow in ozonation,34 with some improvement

217

in the presence of UV light 37 or H2O2 38,39 that induces the formation of •OH radicals. Similar to

218

ICM, halogen groups in triclopyr, triclosan and tribromophenol exhibit a negative inductive

219

effect that decreases the electron density at the benzene ring, thus increasing persistence to

220

oxidation. Oxidation of other model contaminants yielded kNaNO3 constants of the same order of

221

magnitude (Table 1).

222 223

Electrooxidation of all the organic contaminants was substantially faster in Na2SO4 anolyte than

224

in NaNO3 anolyte (Table 1). That is, apparent rate constants were 10–15 times higher in sulfate

225

anolyte than in nitrate anolyte. This resulted in a drastic decrease in the electrolysis time and a

226

lower electric energy per order (EEO) required for the anodic oxidation process (Figure 1; SI Text

11



Page 12 of 29

227

S3). For example, removal of diatrizoate required only 8.5 minutes and 2.44 kWh m-3 in sulfate

228

anolyte, compared to 180 minutes and an energy consumption of 45.6 kWh m-3 for the

229

electrooxidation in nitrate anolyte (SI Table S2). Note that the experimental conditions (i.e.,

230

applied current density, anolyte conductivity and pH, and recirculation flow rate) were identical

231

with both NaNO3 and Na2SO4 anolytes. Significantly higher electrooxidation rates of target

232

contaminants in the sulfate anolyte may be explained by the: i) formation of SO4•- at the anode,

233

ii) persulfate formation and its activation via non-radical mechanisms, and/or iii) persulfate

234

formation and its acid-catalyzed hydrolysis to hydrogen peroxide, which activates persulfate to

235

SO4•-.27,40

236 237

Previously, electrooxidation of coumaric acid at Pt anode was attributed to the formation of

238

persulfate from sulfate ions and its acid-catalyzed hydrolysis to H2O2, which decomposes to •OH

239

radicals in the presence of dissolved iron.27,41 In addition, H2O2 was reported to activate

240

persulfate to form SO4•-.40 In the absence of organic contaminants persulfate concentration

241

reached 0.2 mM at the highest applied charge density, i.e., 1.6 Ah L-1, during electrooxidation in

242

Na2SO4 anolyte (SI Figure S4). Yet, H2O2 was not detected in NaNO3 and Na2SO4 anolytes.

243

Given that persulfate has slow oxidation kinetics with organic compounds,12 it is unlikely to

244

cause the higher electrooxidation rates in sulfate anolyte without activation by UV, heat, alkaline

245

conditions or metal catalysts. This was confirmed by chemical oxidation experiments with

246

persulfate that yielded significantly lower rate constants (i.e.,

Removal of Persistent Organic Contaminants by Electrochemically

Recommend Documents