Multilayer Heterojunction Anodes for Saline Wastewater Treatment

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Multi-layer hetero-junction anodes for saline wastewater treatment: Design strategies and reactive species generation mechanisms Yang Yang, Jieun Shin, Justin T. Jasper, and Michael R Hoffmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00688 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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Environmental Science & Technology

Multi-layer hetero-junction anodes for saline wastewater treatment: Design strategies and reactive species generation mechanisms

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Yang Yang, Jieun Shin, Justin T. Jasper, Michael R. Hoffmann*

5 6

Linde + Robinson Laboratories

7

California Institute of Technology

8

Pasadena, California 91125, United States.

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A manuscript submitted to Environ. Sci. Technol.

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*Corresponding author: Email: [email protected]

ACS Paragon Plus Environment


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TOC/Abstract art

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ABSTRACT

18

Multilayer hetero-junction SbSn/CoTi/Ir anodes, which consist of Ir0.7Ta0.3O2 bottom layers

19

coated onto a titanium base, Co-TiO2 interlayers, and over-coated discrete Sb-SnO2 islands, were

20

prepared by spray pyrolysis. The Ir0.7Ta0.3O2 bottom layer serves as an Ohmic contact to

21

facilitate electron transfer from semiconductor layers to the Ti base. The Co-TiO2 inter-layer and

22

over-coated Sb-SnO2 islands enhance the evolution of reactive chlorine. The surficial Sb-SnO2

23

islands also serve as the reactive sites for free radical generation. Experiments coupled with

24

computational kinetic simulations show that while ·OH and Cl· are initially produced on the

25

SbSn/CoTi/Ir anode surface, the dominant radical formed in solution is the dichlorine radical

26

anion, Cl2·-. The steady-state concentration of reactive radicals is ten orders of magnitude lower

27

than that of reactive chlorine. The SbSn/CoTi/Ir anode was applied to electrochemically treat

28

human wastewater. These test results show that COD and NH4+ can be removed after 2 h of

29

electrolysis with minimal energy consumption (370 kWh/kg COD and 383 kWh/kg NH4+).

30

Although free radical species contribute to COD removal, anodes designed to enhance reactive

31

chlorine production are more effective than those designed to enhance free radical production.



32

INTRODUCTION

33

Water scarcity has been recognized as an emerging global crisis. In order to facilitate water

34

recycling and reuse, decentralized wastewater treatment has been proposed as a supplement to

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the conventional urban wastewater system.1 Electrochemical oxidation (EO) is usually more

36

efficient than biological treatment and is often less expensive than homogenous advanced

37

oxidation processes.2, 3 In addition, the compact design, ease of automation and small carbon

38

footprint make it an ideal candidate for decentralized wastewater treatment and reuse.2, 4-7

39

The performance of EO is determined by the electrochemical generation of reactive species,

40

which largely depends on the nature of anode materials. Non-active anodes with high over-

41

potentials for oxygen evolution reaction (OER), such as those based on SnO2, PbO2, and boron-

42

doped diamond (BDD), have been extensively investigated in the previous decades.8-13 In spite

43

of their superior current efficiency for hydroxyl radical (·OH) generation, SnO2 and PbO2 anodes

44

have poor conductivity and stability. The application of BDD anodes is hindered by their high

45

cost and complicated fabrication. Conversely, Pt-group metal oxides (e.g., RuO2 and IrO2) are

46

efficient and stable catalysts for OER, exhibiting high chlorine evolution reaction (CER) activity

47

in the presence of chloride,14 although they are typically less efficient for hydroxyl radical

48

generation. Hence the development of durable anodes with high activity for both CER and

49

radical generation is an ongoing challenge.

50

Electrolyte composition is another factor in EO performance. Previously, ·OH was

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considered as the main contributor to organic matter removal during EO.15 Recent studies have

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pointed out that carbonate, sulfate and phosphate radicals are also potent oxidants.4, 16 Compared

53

with these anions, chloride (Cl-) in wastewater can be more readily oxidized to reactive chlorine

54

species. Enhanced electrochemical oxidation of organic compounds observed in the presence of


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Cl- has been attributed to reaction with free chlorine (Cl2, HOCl and OCl-).17, 18 More recent

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studies have suggested that Cl· and Cl2·- might be primarily responsible for organic compound

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degradation.7, 19, 20 However, direct experimental evidence verifying the presence or formation

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mechanism of these radicals during electrochemical is lacking. A quantitative description of

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reactive species formation and reactivity in Cl- solutions during the electrochemical oxidation of

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organic contaminants has not yet been fully elucidated.

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In this study, versatile SbSn/CoTi/Ir hetero-junction anodes with high activity for chlorine

62

and radical generation were prepared and characterized. A combination of experimental and

63

kinetic modeling approaches were undertaken to unravel anodic reactive species generation

64

mechanisms and to model their steady-state concentrations in the electrolyte. This research

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aimed to improve the design of hetero-junction metal oxide anodes and provide new insight into

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the mechanism of wastewater electrolysis.

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EXPERIMENTAL SECTION

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Electrode preparation

69

A clean Ti base metal electrode (1 cm × 1.5 cm) was polished with sand paper and etched in

70

10% HF solution for 1 min before use. Metal oxide layers were deposited on cleaned Ti surfaces

71

by spray-pyrolysis. Aqueous metal oxide precursors were atomized with 5 psi air and sprayed

72

onto the heated (300 °C) Ti foil. The resulting oxide film was annealed at 500 °C for 10 min.

73

This procedure was repeated to reach the desired mass loading, which was followed by a final

74

annealing at 500 °C for 1 h. The Ir0.7Ta0.3O2 layer precursor contained 3.5 mM IrCl3 and 1.5 mM

75

TaCl5 in isopropanol. The TiO2 precursor contained 25 mM titanium-glycolate complex prepared

76

by the hydroxo-peroxo method.21 The dopant precursor Co(NO3)2 was added to the TiO2

77

precursor at a molar fraction of 0.1. The Sb-SnO2 precursor contained 25 mM SnCl4 and 1.24



78

mM SbCl3 dissolved in isopropanol. Anodes with only an Ir0.7Ta0.3O2 layer are denoted as Ir for

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simplicity. Multilayer anodes with Sb-SnO2 islands, a Co doped TiO2 layer and an Ir0.7Ta0.3O2

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layer are denoted as SbSn/CoTi/Ir. Anodes without Sb-SnO2 doping are simply denoted as

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CoTi/Ir (with Co-doping) or Ti/Ir (without Co-doping). The mass loadings of Ir0.7Ta0.3O2, TiO2

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and SnO2 were 0.3, 0.5 and 1.0 mg/cm2 respectively. For select anodes denoted as

83

SbSn/CoTi/Ir*, the Ir0.7Ta0.3O2 loading was reduced to 0.05 mg/cm2. Commercially available

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IrO2-based CER anode was purchased from Nanopac® Korea.

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Physicochemical characterization

86

X-ray photoelectron spectroscopy (XPS) was performed using a Surface Science M-Probe

87

ESCA/XPS. Morphologies and elemental composition were obtained with a ZEISS 1550VP field

88

emission scanning electron microscope (FESEM) equipped with an Oxford X-Max SDD X-ray

89

energy-dispersive spectrometer (EDS).

90

Electrochemical characterization

91

The electrolysis cell consisted of an anode in parallel with a stainless steel cathode (2×1.5

92

cm2, 5 mm separation). For characterizations obtained in NaCl electrolyte, voltages were

93

controlled versus a Ag/AgCl/Sat. NaCl reference electrode (BASI Inc). For experiments in

94

Na2SO4 electrolyte, voltages were controlled versus a Hg/Hg2SO4 reference electrode (Gamry

95

Instruments). Electrochemical double layer capacitances (Cdl) were measured by cyclic

96

voltammetry (0.1 V window centered on the open-circuit potential) in the non-Faradaic range in

97

static 30 mM Na2SO4 solution at various scan-rate (0.005-0.8 V/s).22 Electrochemical impedance

98

spectroscopy (EIS) measurements were made in a static 30 mM NaCl electrolyte. The amplitude

99

of the sinusoidal wave was 10 mV with frequencies ranging from 0.1 Hz to 100 kHz. EIS spectra


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were fitted by considering the Helmholtz layer of the anode as a Randles circuit that includes

101

solution resistance, charge transfer resistance (Rct) and capacitance.

102

Electrolysis

103

Anodes were preconditioned in 30 mM NaCl at 25 mA/cm2 for 1 h before use. The

104

uncompensated resistance (Ru) of the cell was measured by current interruption with a 200 mA

105

current bias.7 All anodic potentials were adjusted for Ru and were reported versus the normal

106

hydrogen electrode (NHE). All electrolysis experiments were in galvanostatic mode with current

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density of 25 or 50 mA/cm2. CER tests were conducted by galvanostatic electrolysis of 30 mM

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NaCl solution. Samples were taken at 2 min intervals over 15 min. Total chlorine (TC) and free

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chlorine (FC) concentrations were measured using DPD (N,N-diethyl-p-phenylenediamine)

110

reagent (Hach method 10101 and 10102). Chlorine evolution rate and current efficiency (CE)

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were calculated as previously reported.7,

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galvanostatically. BA was chromatographically separated using a Zorbax XDB column with 10%

113

acetonitrile and 90% 0.1% formic acid as eluent. Human wastewater was collected from the

114

public solar toilet prototype located on the California Institute of Technology campus (Pasadena,

115

CA).23 Chemical oxygen demand (COD) was determined by dichromate digestion (Hach method

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8000). Total organic carbon (TOC) was analyzed by an Aurora TOC analyzer. Anions (Cl-, ClO3-,

117

ClO4-, NO3- and PO43- ) and cations (NH4+, Na+, Ca2+ and Mg2+) were simultaneously detected by

118

ion chromatography (ICS 2000, Dionex, USA; Ionpac AS 19 and Ionpac CS 16 columns).

119

Kinetic modeling

21

Electrolysis of benzoic acid (BA) was performed

120

Kinetic modeling of CER and radical production during NaCl electrolyte electrolysis was

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performed using Kintecus 5.75 chemical kinetic modeling software equipped with Bader-

122

Deuflhard integrator.24 The model established in this study contained 37 reactions (Table S2).19,



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25-36

124

electrolysis (pH rapidly increased to 8.5 within 1 min of electrolysis) and wastewater electrolysis.

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Unknown rate constants were obtained by fitting the experimental data with the kinetic model.

126

RESULTS AND DISCUSSION

127

Anode Characterization

The pH was held constant at 8.5, which was typical of experimental conditions during NaCl

128

Morphologies of anodes prepared by spray pyrolysis were denser and smoother than the

129

“cracked-mud” texture typical of brush-coated anodes (Figure S1). Element mapping also

130

indicated better dispersion of sub-layer Ir, top-layer Ti and Co dopant for anodes prepared by

131

spray pyrolysis. (a)

(b)

1000

CPS

800

Sn Lα 1 Ti Kα 1 Ir Mα 1

600 400

Line scan

200 0 0.0

1.5

2.0

2.5

3.0

(d)

(e)

(f) CoTi/Ir Ti/Ir

Intensity (a.u.)

Intensity (a.u.)

Ir Ti/Ir CoTi/Ir

35 30

CoTi

25

CoTi/Ir 2.0

20 1.5

15 1.0

10 0.5

5

0.0 0

0

70 68 66 64 62 60 58 56

132

Binding energy (eV)

3.5

5 μm

CoTi/Ir CoTi

Intensity (a.u.)

1.0

-Im(Z) (kOhm)

(c)

0.5

Distance (µm)

25 μm

468 466 464 462 460 458 456 454 538 536 534 532 530 528 526 524

Binding energy (eV)

Binding energy (eV)

0

5

10

1

2

3

15

4

20

5

25

Re(Z) (kOhm)

133

Figure 1. (a) FESEM image and elemental mapping of SbSn/CoTi/Ir anode. (Green: Sn;

134

Yellow: Ti; Red: Ir). (b) Cross section image of SbSn/CoTi/Ir multilayer deposited on a

135

glass slide (inset: line scan EDS spectrum). XPS spectrum of (c) Ir 4f, (d) Ti 2p and (e) O 1s


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orbitals. (f) EIS of CoTi/Ir and CoTi without Ir0.7Ta0.3Ox (inset: expanded plot from Re(Z)

137

= 0-5 kOhm).

138

Deposition of Sb-SnO2 produced isolated islands on top of the Co-TiO2 layer instead of a

139

thin film (Figure 1a). This is in agreement with previously reported morphologies of Sb-SnO2

140

anodes prepared by spray pyrolysis.37 Sb-SnO2 islands were determined to be in the range of 2-4

141

µm high and were located on top of a 0.5 µm Co-TiO2 layer overlying a 0.5 µm Ir0.7Ta0.3O2 layer

142

(Figure 1b). Overlap of the EDS signal of Ir and Ti was likely due to thermal diffusion of IrO2

143

into the TiO2 layer.

144

The CoTi/Ir anode surface was primarily composed of TiO2, as evidenced by loss of 38, 39

145

distinctive iridium oxide peaks in XPS spectra (62 and 65 eV)

146

coating layers (Figure 1c). The Ti 2p peaks of CoTi/Ir (Figure 1d) shifted to slightly lower

147

binding energies compared with that of CoTi without an IrO2 under-layer. This shift was

148

ascribed to charge transfer from IrO2 to TiO2, since IrO2 has a higher work function than TiO2.40,

149

41

150

barrier between the Co-TiO2 layer and the Ti base. Electron transfer is thus facilitated, based on

151

the observed reduction of the charge transfer resistance (Rct) of anodes containing Ir (i.e., Rct was

152

reduced from 122 kΩ for CoTi to 4 kΩ for CoTi/Ir; Figure 1f).

within TiO2 or Co-TiO2

This interaction suggests the IrO2 layer acts as an electron shuttle to overcome the Schottky

153

The properties of the TiO2 layer can be modified by metal ion doping. Co doping

154

significantly increased the fraction of oxygen vacancies (531-533 eV) versus lattice oxygen

155

(529-531 eV; Figure 1e).42 This shift reflected the weakening of the oxygen binding energies of

156

CoTi/Ir versus Ti/Ir.

157

The Ir anode exhibited an onset potential of 1.32 V at 1 mA/cm2 during linear sweep

158

voltammetry in 30 mM NaCl (Figure S2), corresponding to a 0.5 V over-potential for oxygen



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evolution (0.82 V at pH = 7). This is comparable to over-potentials previously reported for nano-

160

crystalline IrO2 catalysts.43,

161

potential, deposition of Sb-SnO2 raised the onset potential to 1.38 V, which closely matches the

162

CER potential (1.36 V). The observed shift in onset potential was likely due to inhibition of OER,

163

as evidenced by a decrease in the electrochemically active surface area for OER of SbSn/CoTi/Ir

164

versus Ir anodes (Figure S2). Although the Co-TiO2 interlayer only slightly affected the OER

165

onset potential, it was crucial for inhibition of OER activity. Without a Co-TiO2 coating the

166

Ir0.7Ta0.3O2 layer had access to electrolyte through cracks among the Sb-SnO2 islands, increasing

167

the electrochemically active surface area for OER and lowering the onset potential.

44

While the TiO2 or Co-TiO2 coatings barely affected the onset

168

An 83% reduction in the mass loading of Ir0.7Ta0.3O2 (SbSn/CoTi/Ir*) resulted in a relatively

169

inactive anode, based on its high onset potential (1.56 V) and low electrochemically active

170

surface area for OER (Figure S2). In addition, the loading of Ir0.7Ta0.3O2 is crucial to overall

171

anode stability. Accelerated lifetime tests show that the lifetime of SbSn/CoTi/Ir* anode at 25

172

mA/cm2 was 720 h while that of SbSn/CoTi/Ir could be up to 4 years (Figure S3).

173

Chlorine evolution

174

Coating the Ir anode with TiO2 significantly increased CER activity and current efficiency

175

during electrolysis of 30 mM NaCl solutions (Figure 2a). The increase in CER activity resulted

176

from interaction between the top TiO2 layer and the Ir0.7Ta0.3O2 sub-layer, as only TiO2 sites

177

were exposed, and TiO2 anodes without an Ir0.7Ta0.3O2 sub-layer had no CER activity (data not

178

shown).

179 180 181

It is generally accepted that CER follows the Volmer-Heyrovsky (V-H) mechanism.45, 46 The Volmer step includes the adsorption of Cl- and the discharge of an electron:

MO x + Cl- → MO x (Cl⋅) + e -


(1)

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182

In the Heyrovsky step, the adsorbed Cl· combines with Cl- from the bulk electrolyte and releases

183

Cl2:

MO x (Cl⋅) + Cl − → MO x + Cl 2 + e −

184 185

(2)

The recombination of two Cl·via the Volmer-Tafel reaction can also produce Cl2:47

186

2 MOx (Cl⋅) → 2 MO x + Cl 2

187

Catalysts with optimal oxygen binding energies for OER normally have high activity for CER,14

188

resulting in competition between OER and CER. However, recent density functional theory

189

(DFT) calculations48 reported that selectivity towards CER could be enhanced by a monolayer

190

TiO2 coating above RuO2, slightly increasing the energy barrier for CER, but drastically raising

191

the energy barrier for OER. In support of these calculations, the TiO2 coating applied onto IrO2,

192

which has a similar oxygen binding energy to RuO2,47 significantly increased the current

193

efficiency for chlorine production. Decreased active surface area for OER and increases in OER

194

onset potential with TiO2 over-coating (Figure S2) also supported DFT calculations.

(3)

195

At the molecular level, the desorption of Cl· (eqn 2 and 3) is considered to be the rate-

196

limiting step of CER.49 Considering the positive linear relationship between oxygen binding

197

energy and chlorine binding energy,47 lowering the oxygen binding energy by Co doping (Figure

198

1e) is likely to facilitate Cl· desorption, enhancing the CER activity of CoTi/Ir compared with

199

that of Ti/Ir.



60

(b)

(1.61)

0.6 (1.59)

(1.51)

40

(2.0)

0.4 (1.57)

(2.5)

20

0.2

C oT i/I r C oT i/I r Sb Sb Sn Sn /Ir /C oT i/I r*

(c) 30

ClFC ClO3-

20

10

0

(d)

ClFC ClO3-

20

10

1

2

3

4

Time (h)

20

10

0

0 0

ClFC ClO3ClO4-

30

0

1

2

3

Time (h)

4

0

1

2

3

Time (h)

Sb

Sn /

Ti /Ir

er m C om

200

Ir

0

ci al

0.0

CE (%)

CER (mmol/m2/s)

(1.9)

30

Concentration (mM)

CE

Concentration (mM)

CER

0.8

Concentration (mM)

(a)

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201

Figure 2. (a) Chlorine evolution rate and current efficiency measured in 30 mM NaCl at 25

202

mA/cm2. Anodic potentials are shown as numbers above each bar. Error bars represent

203

standard deviation. Electrolysis of 30 mM NaCl with (b) CoTi/Ir, (c) SbSn/CoTi/Ir and (d)

204

SbSn/CoTi/Ir* anodes. Dotted lines represent model results.

205

As expected based on the inhibition of OER by TiO2 and Co-TiO2 coatings (Figure S2), Ti/Ir

206

and CoTi/Ir anodes exhibited increased CERs compared to Ir anodes (Figure 2a). However, the

207

lower conductivity of Sb-SnO2 islands resulted in higher operating anodic potentials. The

208

relatively inferior performances of SbSn/Ir and SbSn/CoTi/Ir* indicates that the absence of a

209

CoTi layer or a reduction in Ir mass loading was detrimental to CER activity. In general, the

210

CER activities of CoTi/Ir and SbSn/CoTi/Ir were higher that of commercial anodes (Figure 2a).

211

Electrolysis of NaCl solutions with CoTi/Ir anodes resulted in gradual loss of Cl- with

212

corresponding production of HOCl/OCl- and ClO3- (Figure 2b). Similar results were observed

213

with SbSn/CoTi/Ir anodes (Figure 2c) except with a higher production of ClO3-. Testing of

214

SbSn/CoTi/Ir* anodes showed formation of ClO4- (Figure 2d), in agreement with previous

215

studies demonstrating that non-active electrodes produce ClO4- more readily than active

216

electrode.50, 51


4

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217 218

219 220


Kinetic modeling was performed to estimate rate constants of key reactions involved in the evolution of Cl2 (eqn 4) and the pH-dependent equilibria of Cl2, HOCl and OCl- (Table S2). k'

1 2Cl-  → Cl2

(4)

The direct oxidation of HOCl/OCl- into ClO3-,52 and ClO3- into ClO4-50 were considered as well: k'

221

2a 2 MO x+1 + OCl -  → 2 MO x + ClO3-

222

2b 2MO x+1 + HOCl  → 2MO x + ClO-3 + H +

223

3 MO x+1 + ClO3−  → MO x + ClO-4

(5)

k'

(6)

k'

224

(7)

The overall kinetics could be treated as first-order reaction in series:

225

d[Cl- ] = −k1[Cl- ] dt

(8)

226

d[FC] = k1[Cl- ] − k2' [MO x+1 ][FC] = k1[Cl- ] − k2 [FC] dt

(9)

227

d[ClO3− ] ' = k2 [MO x+1 ][FC]− k3'[MO x+1 ][ClO3− ] = k2 [FC] − k3[ClO3− ] dt

(10)

228

d[ClO-4 ] ' = k3[MO x+1 ][ClO-3 ] = k3[ClO-3 ] dt

(11)

229

FC formation rates (k1) for the CoTi/Ir and SbSn/CoTi/Ir anodes were found to be more than

230

two orders of magnitude higher than ClO3- formation rates (k2) (Table 1). ClO4- formation rates

231

(k3) were only calculated for SbSn/CoTi/Ir* anodes and were lower than FC and ClO3- formation

232

rates (k1 and k2), in line with previous research showing that the oxidation of ClO3- to ClO4- is

233

sluggish.53 Increased current density (50 vs 30 mA/cm2) with SbSn/CoTi/Ir anodes did not

234

markedly increase the FC concentration, but instead resulted in greater ClO3- production (Figure



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235

S4). Model fitting showed that an increase in current density led to an increase in apparent rate

236

constants (Table 1), which can be explained by the Butler-Volmer formulation.54 That is, the

237

simultaneous increase of FC production rate (k1) and FC oxidation rate (k2) results in a less

238

pronounced increase of d[FC]/dt in eqn 9, which explains the inefficient chlorine accumulation

239

at 50 mA/cm2 (Figure S4).

240

It appears that an increase in the Cl- concentration increases the FC concentration more

241

efficiently than an increased current density. As expected, doubling Cl- concentrations (i.e., 60 vs.

242

30 mM) during electrolysis with CoTi/Ir, SbSn/CoTi/Ir and SbSn/CoTi/Ir* anodes at 25 mA/cm2

243

resulted in approximately double the peak FC concentration (Figure S4).

244

Table 1. Rate constants estimated by kinetic modeling. 25 mA/cm2

50 mA/cm2

CoTi/Ir SbSn/CoTi/Ir SbSn/CoTi/Ir* SbSn/CoTi/Ir k1 (10-3 M-1s-1)

2.69

7.65

3.15

14.5

k2 (10-5 M-1s-1)

1.88

6.06

32.8

19.2

k3 (10-4 M-1s-1)

-

-

2.18

0.0877

rHO· (10-8 M s-1)

-

-

1.38

0.792

kCl· (10-6 s-1)

-

-

6.48

14.9

245 246

Modeling of the electrolytic process in 60 mM NaCl gave results that were consistent with

247

the experimental data (Figure S4). However, the actual increase of ClO4- production with

248

SbSn/CoTi/Ir* anode was less than predicted. This may have been because high Cl-

249

concentrations inhibited ClO4- formation by blocking active sites for the oxidation of ClO3- to


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250

ClO4-.51 Nevertheless, the kinetic model as presented provides a simple but powerful tool for the

251

optimization of CER.

252

Radical generation.

253

In addition to free chlorine, electrolysis of NaCl aqueous solution generates radicals such

254

as ·OH, Cl· and Cl2·-.7, 19 BA was selected as a radical probe compound since it reacts with ·OH,

255

Cl· and Cl2·- (rate constants given in Table S2) but does not react with free chlorine.55

256

BA degradation in a 30 mM Na2SO4 electrolyte solution at 25 mA/cm2 was observed only

257

with SbSn/CoTi/Ir* anodes (Figure 3a). When current density was increased to 50 mA/cm2, BA

258

degradation was observed with SbSn/CoTi/Ir anodes (Figure 3b), but was not observed with

259

CoTi/Ir and Ti/Ir anodes without added Sb-SnO2 islands (data not shown). BA could be degraded

260

via direct oxidation on BDD electrodes at high oxidation potentials (i.e., 2.4 VNHE).56 However,

261

this pathway was excluded in the current study as the same current responses in linear sweeping

262

voltammetry were observed in Na2SO4 in the absence or presence of 1 mM BA (data not shown).

263

The contribution of sulfate radical is excluded as the same BA decay kinetic was observed in 30

264

mM NaNO3 electrolyte (Figure S5). Thus, degradation of BA is attributed to reaction with ·OH.

265

Assuming that the generation of ·OH is a zero-order reaction, the generation rate (rHO·; M/s)

266

could be estimated by fitting BA degradation data with a kinetic model (Table 1). SbSn/CoTi/Ir*

267

was found to be more efficient than SbSn/CoTi/Ir in terms of ·OH generation (Figure 3a vs. b).

268

The steady state ·OH radical concentrations were calculated to be 2.6 × 10-15 and 1.4 × 10-15

269

mol/L for SbSn/CoTi/Ir* and SbSn/CoTi/Ir anodes, respectively.



Page 16 of 31

270 271

Figure 3. BA degradation with (a) SbSn/CoTi/Ir* and (b) SbSn/CoTi/Ir anodes under

272

variable current densities (L: 25 mA/cm2, H: 50 mA/cm2) and initial Cl- concentration (30

273

and 60 mM). Error bars represent the standard deviation.

274

generated by (c) SbSn/CoTi/Ir* and (d) SbSn/CoTi/Ir to BA degradation. Schematic

275

illustrations of (e) reactive species generation mechanism and (f) active site distribution.

Contributions of radical

276

BA degradation was accelerated in the presence of 30 mM NaCl (Figure 3a and b). This

277

implies that more radicals were generated in the presence of Cl-. It is well known that Cl· reacts

278

at similar rates as ·OH with organic molecules. However, Cl·, which is involved in the V-H step

279

(green line in Figure 3e), is assumed to be surface-bound and to rapidly combine with local

280

Cl· or Cl-, and hence is unlikely to contribute to BA degradation. This assertion was supported

281

by the lack of BA degradation that was observed with CoTi/Ir and Ti/Ir anodes, despite FC

282

production (data not shown). The ·OH radicals can be quenched by Cl- to generate less reactive

283

chlorine radicals (black lines in Figure 3e). A kinetic model in which ·OH is the only radical

284

species failed to simulate the observed enhanced BA degradation rate (Figure S5). Therefore,


Page 17 of 31


285

there must be additional reactive radical inputs. One possibility involves the contribution by Sb-

286

SnO2 promoting the one-electron oxidation of Cl- to produce free Cl· (Figure 3e and f): Sb−SnO

287

2 Cl-  → Cl ⋅ + e-

288

Generation of free Cl· on SnO2 during electrolysis has been reported previously,57 although

289

without substantiating experimental evidence. The kinetic model result, however, provides a self-

290

consistent kinetic argument for the contributions of the various chlorine radical species to the

291

overall rates. The kinetic model was also used to simulate degradation of BA in both Na2SO4 and

292

NaCl electrolytes provided that eqn (12) was included.

(12)

293

The first-order rate constant for Cl· formation (kCl·; s-1) obtained by model fitting was found

294

to be more than two orders of magnitude higher than that for ·OH evolution (Table 1). The

295

higher kCl· of SbSn/CoTi/Ir compared with that of SbSn/CoTi/Ir* may be explained by the Sb-

296

SnO2 islands accepting more Cl· from the Co-TiO2 sites (Figure 3f). A similar argument was

297

used to explain the activity of RuO2-coated BDD electrodes. In this example, Cl· generated on

298

active RuO2 sites was proposed to spill over to non-active BDD sites..58

299

Even though electron transfer reactions leading to the generation of Cl· and ·OH were the

300

initial radical formation steps on anode, additional modeling of the entire set of free radical

301

concentrations (·OH, O·-, Cl·, Cl2·- and ClOH·-) showed that the dominant radical species by

302

concentration was Cl2·- (Figure S6). Model simulation further indicates that the combination of

303

Cl· and Cl- is the main pathway for Cl2·- formation. An increase in Cl- concentrations from 30 to

304

60 mM resulted in a lowering of the observed BA degradation rate (Figure 3a and b). This

305

apparent inhibition could be attributed to the role played by FC scavenging ·OH and Cl·.29, 33 The

306

decreased BA degradation rate and reduced radical concentrations observed at 60 mM NaCl

307

were successfully predicted by the kinetic modeling results (dotted line in Figure 3a and b).



Page 18 of 31

308

The relative contributions of various free radicals to the BA degradation rate were estimated

309

by simulating the oxidation of BA by a specific target radical while excluding other radical

310

reactions in the model (Figure 3c and d). It appears that Cl· was the major contributor to BA

311

oxidation, followed by Cl2·- and ·OH, even though Cl2·- has the highest concentration. This was

312

due to the much higher reaction rate of BA with Cl· as compared to Cl2·- (1.9 × 1010 vs. 2.0 × 106

313

M-1s-1).

314

Based on the calibrated rate constants, the kinetic model is able to predict reactive species

315

formation and steady-state concentrations. In general, the FC concentrations were found to be ten

316

orders of magnitude higher than radical concentrations.

317

Wastewater electrolysis

318 319

The active SbSn/CoTi/Ir anode and non-active SbSn/CoTi/Ir* anode were tested in terms of their potential for domestic (i.e., exclusively human waste) wastewater treatment.

320 321 322 323



SbSn/CoTi/Ir* L30

(c)

L30 50

L60 H30

8 6 L30

4

L60

2

0

1

2

Time (h)

3

4

40

20

H30

0

0

60

0

0

1

2

3

4

Time (h)

Sb H 30 Sn /C L3 o 0 Ti/I r*

100

TOC removal (%)

SbSn/CoTi/Ir* L30

150

NH4+ (mM)

10

200

COD (mg/L)

Commercial L30

(b) 12

Commercial L30

L6 0

(a) 250

L3 0

Page 19 of 31

324 325

Figure 4. Concentration vs. time profiles of (a) COD and (b) NH4+ in human wastewater

326

electrolyzed by SbSn/CoTi/Ir and SbSn/CoTi/Ir* anodes under various current densities (L:

327

25, H: 50 mA/cm2) and initial Cl- concentration (30, 60 mM). (c) Removal of TOC after 4 h

328

electrolysis. Error bars represent standard deviation. All data are collected from

329

SbSn/CoTi/Ir anode except the one labeled with SbSn/CoTi/Ir*.

330

In terms of COD removal, the SbSn/CoTi/Ir anode outperforms the commercial anode but

331

was less efficient than the SbSn/CoTi/Ir* anode at 25 mA/cm2 in 30 mM Cl- (Figure 4a).

332

Assuming that direct oxidation of COD is insignificant, then the COD removal obtained with the

333

SbSn/CoTi/Ir anode at 25 mA/cm2 should take place exclusively via FC mediated oxidation.

334

Conversely, our calculations showed that the radical-mediated oxidation pathways contributed

335

up to 80% of COD removal on SbSn/CoTi/Ir* anode (Figure S7 and Text S1). This result

336

suggests that the radicals produced by the ‘non-active’ SbSn/CoTi/Ir* anodes were more

337

efficient for COD removal than FC alone. However, TOC analysis showed that complete

338

mineralization of the organic carbon in human wastewater was not observed with the

339

SbSn/CoTi/Ir* anode (Figure 4c), in spite of complete COD removal. This may be due, in part,

340

to the contribution of Cl2·- as the dominated radical species. Cl2·- reacts with organics via



341

hydrogen abstraction, electrophilic addition and electron transfer.19 The residual TOC may be

342

due to the formation of chlorinated byproducts or to an accumulation of formate and oxalate.

343

For example, our recent study using commercially available Ir-based anodes found that 4 h

344

human wastewater treatment via chlorine-mediated EO will form trihalomethanes and haloacetic

345

acids.59 Considering that the concentrations are generally within the range of those reported for

346

secondary effluent after disinfection process and swimming pool waters, the treated water should

347

be safe for non-potable reuse.

Page 20 of 31

348

Increasing the Cl- concentration to 60 mM significantly enhanced organic matter removal for

349

the SbSn/CoTi/Ir anode system. This is likely due to enhanced FC evolution, which effectively

350

compensated for the anode’s inability to generate radicals at 25 mA/cm2. At 50 mA/cm2,

351

SbSn/CoTi/Ir anodes produced more FC accompanied with sufficient radicals to achieve

352

complete COD removal and greater than 50% TOC removal. In this case, the contributions of

353

chlorine and radical mediated oxidation to COD removal were calculated to be 94% and 6%

354

(Figure S7 and Text S1).

355

As expected, SbSn/CoTi/Ir anodes outperformed SbSn/CoTi/Ir* and commercial anodes for

356

NH4+ removal (Figure 4b), since NH4+ removal during electrochemical treatment is achieved via

357

breakpoint chlorination,6, 7 which, in turn, is an indirect measure of the CER activity. Most NH4+

358

was converted into N2 with a smaller fraction oxidized to NO3- (Figure S8). The SbSn/CoTi/Ir

359

anode that was operated at L60 and H30 was capable of removing about 74% of total nitrogen

360

after 4 h electrolysis (Figure S8).

361

TC and FC concentrations were low (< 2 mM) during electrochemical wastewater treatment

362

(30 mM Cl-, 25 mA/cm2) with SbSn/CoTi/Ir* and SbSn/CoTi/Ir anodes. Chorine was consumed

363

during breakpoint chlorination or by wastewater organic matter degradation within the electrical


Page 21 of 31


364

double layer, and thus was unable to diffuse into the bulk solution phase. Generation of both TC

365

and FC was only observed with SbSn/CoTi/Ir anodes after complete NH4+ and COD removal

366

(Figure S9). Up to 5 mM ClO3- was produced after 4 h electrolysis (30 mM Cl-; 25 mA/cm2)

367

with both types of anodes (Figure S9). Increasing either current or the Cl- concentration

368

increased ClO3- production (i.e., 10-11 mM maximum). These trends were in agreement with

369

experiments in NaCl solutions, although concentrations were about 50% lower.

370

Significant concentrations of ClO4- (6 mM) were produced by SbSn/CoTi/Ir* anode after 4 h

371

electrolysis of wastewater under low Cl- and low current conditions (i.e., 30 mM Cl-; 25 mA/cm2;

372

Figure S9). However, SbSn/CoTi/Ir anodes only formed relatively low concentrations of ClO4-

373

(0.85 mM) under high current conditions (i.e., 30 mM Cl-; 50 mA/cm2).

374

The SbSn/CoTi/Ir anode, which was operated at 60 mM Cl- and 25 mA/cm2, consumed less

375

energy than the SbSn/CoTi/Ir* anode for COD and NH4+ removal (370 kWh/kg COD; 383

376

kWh/kg NH4+). These values are still higher than those reported in the EO of leachate and

377

reverse osmosis concentrate,11, 60 probably due to the lower conductivity (3.2 mS/cm) of human

378

wastewater. Reducing the electrode spacing or increasing the wastewater conductivity may be

379

able to further lower the energy consumption.

380

In conclusion, the SbSn/CoTi/Ir anode was observed to be the most effective anode in terms

381

of durability, reactive species generation, pollutant removal, by-product formation, and energy

382

consumption. More efficient wastewater treatment provided by an active anode (SbSn/CoTi/Ir)

383

as compared to a non-active anode (SbSn/CoTi/Ir*) highlights the limitation of non-active anodes

384

for wastewater treatment due to ·OH quenching by Cl- and FC. In addition, non-active anodes

385

produce a significant amount of ClO4-. Under appropriate conditions, wastewater electrolysis



Page 22 of 31

386

mediated by electrochemically produced FC may be able to outperform radical-assisted

387

electrolysis.

388

From an engineering point of view, wastewater treated in an appropriately designed reactor

389

equipped with SbSn/CoTi/Ir anodes should be suitable for non-potable water reuse (e.g., as

390

recycled toilet flushing water based on color (Figure S10) and COD removal), as well as for

391

disinfection, which is provided by the residual FC. The semiconductor electrolytic reactors can

392

be easily automated (e.g., reaching the breakpoint for NH4+ chlorination can be used as a signal

393

to end batch treatment). They should be an excellent fit for use in decentralized wastewater

394

treatment.

395

ASSOCIATED CONTENT

396

Figures provided in supporting information include SEM, LSV, and ECSA measurement of

397

anodes, model simulation results, time profiles of ions, the calculation of FC contribution to

398

pollutant removal and energy consumption in wastewater electrolysis. Tables include human

399

wastewater composition and details of the kinetic model.

400

ACKNOWLEDGEMENT

401 402

The authors gratefully acknowledge the financial support of Bill and Melinda Gates Foundation (BMGF-RTTC Grant, OPP1111246).


Page 23 of 31


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Multilayer Heterojunction Anodes for Saline Wastewater Treatment

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