Challenges and Opportunities for Electrochemical Processes as Next

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Challenges and Opportunities for Electrochemical Processes as NextGeneration Technologies for the Treatment of Contaminated Water Jelena Radjenovic, and David L. Sedlak Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02414 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on September 15, 2015

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Environmental Science & Technology

1 2 3 4 5

Challenges and Opportunities for Electrochemical Processes as

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Next-Generation Technologies for the Treatment of Contaminated Water

7

Jelena Radjenovic1,2* and David L. Sedlak3

8 9 10

1

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University of Girona, 17003 Girona, Spain

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2

13

Australia

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3

15

California 94720-1710, United States

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

Advanced Water Management Centre, The University of Queensland, St Lucia, QLD 4072,

Department of Civil and Environmental Engineering, University of California, Berkeley,

16 17 18 19 20 21

* Corresponding author:

22

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

23

Technological Park of the University of Girona, 17003 Girona, Spain

24

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

1 ACS Paragon Plus Environment


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TOC Art

26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 2 ACS Paragon Plus Environment

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Abstract

42 43

Electrochemical processes have been extensively investigated for the removal of a range of

44

organic and inorganic contaminants. The great majority of these studies were conducted

45

using nitrate, perchlorate, sulfate, and chloride-based electrolyte solutions. In actual treatment

46

applications, organic and inorganic constituents may have substantial effects on the

47

performance of electrochemical treatment. In particular, the outcome of electrochemical

48

oxidation will depend on the concentration of chloride and bromide. Formation of chlorate,

49

perchlorate, chlorinated and brominated organics may compromise the quality of the treated

50

effluent. A critical review of recent research identifies future opportunities and research

51

needed to overcome major challenges that currently limit the application of electrochemical

52

water treatment systems for industrial and municipal water and wastewater treatment. Given

53

the increasing interest in decentralized wastewater treatment, applications of electrolytic

54

systems for treatment of domestic wastewater, greywater, and source-separated urine are also

55

included. To support future adoption of electrochemical treatment, new approaches are

56

needed to minimize the formation of toxic byproducts and the loss of efficiency caused by

57

mass transfer limitations and undesired side reactions. Prior to realizing these improvements,

58

recognition of the situations where these limitations pose potential health risks is a necessary

59

step in the design and operation of electrochemical treatment systems.

60 61 62 63 64



65

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Introduction

66 67

Electrochemical processes have gained increasing interest in recent years for the treatment of

68

polluted waters. They are considered to be versatile and capable of degrading a wide range of

69

contaminants, including refractory carboxylic acids 1,2 and even perfluorocarboxylic acids 3,4.

70

Electrochemical systems offer several advantages over other approaches, such as operation at

71

ambient temperature and pressure as well as robust performance and capability to adjust to

72

variations in the influent composition and flow rate. They generally require no auxiliary

73

chemicals and do not produce waste. Electrochemical processes can be adapted to various

74

applications, and can be easily combined with other technologies 5. The modular design and

75

small footprint of electrochemical systems also make them attractive for decentralized

76

wastewater treatment. However, application of electrochemical treatment has been slowed

77

down by the relatively high costs of electrodes and concerns about the presence of toxic

78

byproducts in the treated water.

79 80

Numerous bench-scale studies have demonstrated the ability of electrochemical processes to

81

remove organic contaminants from an electrolyte solution (see references

82

reviews of electrochemical advanced oxidation processes). While these studies provide an

83

understanding of the effects of operational variables on process performance, the importance

84

of these effects, as well as reaction kinetics, mechanisms, and expected byproducts are likely

85

to differ when electrochemistry is used to treat real waste streams

86

and bromine species formed at the anode can produce toxic organic chlorine- and bromine-

87

containing transformation products

88

may yield toxic chlorate and perchlorate

13,14

12

6-11

for recent

. In particular, chlorine

. Furthermore, the presence of chloride in wastewater 15,16

. Most studies that have been conducted in


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89

authentic wastewater have focused on the removal of chemical oxygen demand (COD),

90

ammonia, colour, and dissolved organic carbon (DOC) 11,17.

91 92

To gain insight into the use of electrochemical treatment under conditions likely to be

93

encountered in industrial wastewater and water recycling systems, recent research on

94

electrochemical treatment was reviewed. The main focus was placed on anodic oxidation

95

because it is the most mature of the approaches employed for water treatment. Cathodic

96

reduction, electrodialysis and related processes were also considered as alternatives for

97

specific applications. Considering the importance of electrode materials to system

98

performance and cost, a brief overview was made of the most commonly used anodes before

99

assessing treatment applications.

100 101

Electrode materials and electrochemical oxidation mechanisms

102 103

The choice of electrode material will determine the efficiency of electrochemical treatment

104

processes as well as the potential formation of toxic byproducts. Electrochemical oxidation

105

can proceed via direct and indirect electrolysis. Direct electrolysis requires adsorption of

106

pollutants onto the electrode, and can occur at relatively low potentials (i.e., prior to the O2

107

evolution). The rates of direct electrolysis are affected by diffusion limitations, slow reaction

108

kinetics and a decrease in the catalytic activity of the electrode in the presence of dissolved

109

solutes (i.e., poisoning)

110

at the electrode that mediate the transformation of contaminants. The nature of these

111

electrochemically-produced species is affected by the electrode material, such as the O2

112

overpotential and adsorptive properties of the electrode surface.

18

. Indirect electrolysis relies on the production of oxidizing species

113



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114

Mixed metal oxide (MMO) electrodes, known under the trade name Dimensionally Stable

115

Anodes (DSA®), have been commercially available for almost 30 years. They consist of

116

corrosion-resistant base material, such as titanium or tantalum, coated with a layer of metal

117

oxides (e.g., RuO2, IrO2, PbO2, SnO2). Comninellis

118

active, characterized by a strong and weak interaction of electrogenerated hydroxyl radicals

119

(·OH) with the electrode surface, respectively. Active anodes (e.g., IrO2, RuO2) have a low

120

overpotential for O2 evolution, and consequently exhibit low capability for transformation of

121

recalcitrant organic compounds. Non-active anodes (e.g., PbO2, SnO2) exhibit lower

122

electrochemical activity for O2 evolution and therefore have a higher efficiency for oxidation

123

of recalcitrant organic compounds. Due to the limited ability of MMO electrodes to oxidize

124

and mineralize the organics, they have mostly been applied for the treatment of pollutants in

125

the presence of chloride.

19

classified anodes into active and non-

126 127

Boron-doped diamond (BDD) electrodes have been studied extensively in recent years. The

128

distinct features of BDD electrodes such as high O2 overpotential make them better suited for

129

the direct oxidation of contaminants than metal oxide anodes. The high activity of BDD

130

anode towards organics oxidation has been explained by the presence of weakly adsorbed

131

·OH formed by water electrolysis at the anode surface 19:

132

BDD + H2O → BDD(·OH) + H+ + e-

(1)

133

The nature of the BDD(·OH) is still a subject of scientific debate, as there is no clear

134

spectroscopic evidence of free ·OH radicals

135

because of the high reactivity of BDD(·OH), oxidation reactions are confined to an adsorbed

136

film adjacent to the electrode surface

137

molecular oxygen in radical chain reactions, evidenced by the formation of C18O2 and

138

C16O18O when BDD anodes were used for electrooxidation of acetic acid in a solution

20,22

20,21

. Some authors have hypothesized that,

. Kapalka et al.

23,24


suggested the participation of

Page 7 of 36


18

O2. Electrogenerated ·OH may trigger the formation of organic

139

saturated with labelled

140

radicals (R·), which in the presence of O2, form organic peroxy radicals (ROO·)

141

radicals can initiate subsequent chain reactions leading to a non-Faradaic enhancement of the

142

rate of electrooxidation of organic compounds.

23

. Peroxy

143 144

In addition to oxidizing organic contaminants, BDD anodes can generate ozone, H2O2, as

145

well as ferrate

146

(P2O84-) and peroxydisulfate (S2O82-) in the presence of carbonate, phosphate and sulfate ions,

147

respectively

148

formation of sulfate radical (SO4·-) by direct, one-electron oxidation of sulfate ion at the

149

anode, or by the reaction of H2SO4 or HSO4- with the electrogenerated ·OH and, ii)

150

recombination of two SO4·- radicals to yield peroxydisulfate 29,30.

25,26

26-30

. BDD can also produce peroxydicarbonate (C2O62-), peroxydiphosphate

. The formation of peroxydisulfate is thought to proceed in two stages: i)

151

SO42- → SO4·- + e-

(2)

152

HSO4- + ·OH → SO4·- + H2O

(3)

153

H2SO4 + ·OH → SO4·- + H3O+

(4)

154

SO4·- + SO4·- → S2O82-

(5)

155

The rate of persulfate generation at BDD anode is affected by the condition of the electrode

156

surface; up to 50 times lower rates of persulfate formation were observed on an aged BDD

157

compared to a new electrode 31. Although SO4·- and other inorganic radicals are considered to

158

be the intermediates responsible for the formation of peroxy-species, some studies have

159

suggested their participation in the oxidation of organics

160

faster oxidation rates of the X-ray contrast agent diatrizoate and several other organic

161

compounds were observed when a BDD anode was used in a sulfate electrolyte, relative to

162

perchlorate or nitrate electrolyte (Figure 1)

35

32-34

. For example, significantly

. The presence of SO4·- and other inorganic



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163

radicals on conductive diamond anodes has not been confirmed through spectroscopic

164

methods.

165 166

The main impediment to large-scale application of BDD electrodes is the high cost of the

167

substrate onto which the BDD film is deposited (Nb, W, Ta), and its poor mechanical

168

strength in the case of Si substrate. In addition, chemical vapour deposition (CVD)

169

manufacturing of synthetic diamond is still limited to small-scale production. The price of

170

conductive diamond electrodes is currently about ten times higher than that of MMO

171

electrodes, in the range 12,000-18,000 € m-2 36. As a result of advances in manufacturing, the

172

price is expected to drop by approximately half in the next decade, but it will remain

173

expensive for the foreseeable future. While titanium possesses all the necessary features of a

174

good substrate material (i.e., good electrical conductivity, mechanical strength and

175

electrochemical inertness), the deposition of stable BDD films on Ti has not been achieved 37.

176 177

Ebonex®, a low-cost ceramic material comprised of Magneli phase titanium oxides (i.e.,

178

Ti4O7 and Ti5O9) has been investigated as an anode for organics oxidation due to its high

179

overpotential for O2 evolution

180

graphite and can be produced in a number of forms. When polarized anodically, it behaves as

181

a non-active anode that is capable of oxidizing organic compounds

182

uncertain how the electrocatalytic activity of Ebonex compares to other anodes with high O2

183

overpotential (e.g., BDD, SnO2, PbO2). Considering that Ebonex is produced from

184

inexpensive starting materials (titania and hydrogen), has excellent corrosion resistance and

185

good electrical conductivity, this material may be well suited for treatment of highly acidic

186

and basic solutions, where the life-time of conventional electrode materials is significantly

38,39

. Ebonex exhibits conductivity comparable to that of


21,40

. Yet, it is still

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187

shortened 39. Further research is needed to assess the possibilities of using this material as an

188

anode for wastewater treatment.

189 41

190

A recent study

191

TiO2 nanotube array (NTA). The electrocatalytic activity of the blue TiO2 NTA electrode for

192

chlorine and ·OH production was observed to be comparable to the commercial DSA® and

193

BDD anodes, respectively. Given the simple fabrication of the electrode by cathodic

194

polarization of an anatase TiO2 NTA, this material has the potential to become a cost-

195

effective anode material for industrial and environmental applications. Nevertheless, further

196

research is required to determine the capability of the blue TiO2 NTA anode to oxidize the

197

organic contaminants. Also, the long-term performance of the blue TiO2 NTA has not been

198

assessed under conditions relevant to water treatment.

reported the synthesis of a new electrode material, a dark blue coloured

199 200

Formation of halogenated byproducts and other toxic byproducts

201

As mentioned previously, the formation of halogenated byproducts during electrooxidation is

202

a serious concern due to the toxicity of many of the compounds

203

risks of forming toxic halogenated byproducts during electrooxidation, available data from

204

studies conducted in authentic wastewater and chloride-containing solutions (Table 1, Table

205

S1) were reviewed and categorized related to conditions typically observed in different

206

matrices.

42,43

. To assess the potential

207 208

Electrochemical oxidation of chloride-containing water produces chlorine (Cl2) and

209

hypochlorous acid/hypochlorite (HOCl/OCl-) and other reactive halogens (Figure 2).

210

Hypochlorous acid reacts with unsaturated bonds and electron-rich moieties (e.g., reduced

211

sulphur moieties, aliphatic amines, phenols and other aromatics)


44

to form halogenated


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212

products that are often more persistent and toxic than the parent compound. Up to 33 mg L-1

213

and 1.2 mg L-1 of organohalogen byproducts measured as adsorbable organic halogen (AOX)

214

were formed during electrooxidation of brine

215

respectively, with a BDD electrode. Trihalomethanes (THMs) and haloacetic acids (HAAs)

216

were formed at concentration ranging from 0.05 to 2 mg L-1

217

respectively, in electrooxidation of brine and landfill leachate. These values exceed the World

218

Health Organization (WHO) drinking water guidelines for THMs (i.e., 0.06 - 0.3 mg L-1) 51.

219

In the presence of ammonium, especially in the case of high-strength wastewater (e.g.,

220

landfill leachate, effluents from food, pharmaceutical and

221

organic matter by affected by the rapid conversion of free chlorine into less reactive

222

chloramines.

45,46

and municipal wastewater effluent

45,48-50

and 0.2 to 3 mg L-1

47

,

45,48

,

fertilizer industry), oxidation of

223 224

In most studies employing MMO anodes, free available chlorine is considered to be the main

225

oxidant in the presence of chlorides

226

potent chlorine radicals (i.e., Cl2▪−, Cl▪) when bismuth-doped TiO2 anodes (BiOx/TiO2) are

227

used to treat wastewater. In solution, chloride radical (Cl▪) has similar properties to ·OH; it

228

can undergo rapid addition, hydrogen abstraction and direct electron transfer reactions with

229

aromatics at second-order rate constants ranging from 108 to 109 M-1 s-1 56. Chloride radical is

230

converted to dichloride radical anion (Cl2▪−) in the presence of higher background chloride

231

concentration. Cl2▪− reacts with organic compounds similar to Cl▪ but typically at rates that are

232

two to four orders of magnitude slower

233

oxidation rates of organic compounds on BiOx/TiO2 anode and their relative bimolecular rate

234

constants of reaction with Cl2▪− in 50 mM NaCl solution. Further research is needed to verify

235

the presence of reactive chlorine radicals during electrooxidation reactions under conditions

236

of interest for wastewater treatment.

48,52

. Recent studies

53,56

. Park et al

53-55

53

suggest the formation of more

reported a correlation between the


Page 11 of 36


237 238

Similar to chloride, bromide can be oxidized at the anode to bromine (Br2) and hypobromous

239

acid (HOBr), which react rapidly with the bulk organics

240

bromide can also be oxidized by HOCl/OCl- to hypobromous acid in the bulk liquid 44. HOBr

241

may be more reactive than hypochlorous acid with some organic compounds, for example

242

with phenolic compounds 57. Bromide is also an important scavenger of ·OH (k=1.1*1010 M-1

243

s-1)

58,59

13,45

. In the presence of chloride,

.

244 245

The formation of chlorate, perchlorate and bromate are also of great concern as these

246

chemicals are suspected carcinogens and mutagens

247

chlorate can be formed both chemically and electrochemically, and it was observed at both

248

conductive diamond and metal oxide-coated electrodes

249

electrochemical (i.e., limited to the oxidation of chlorate at the anode surface) and seems to

250

occur only at highly oxidizing BDD anodes

251

and perchlorate were formed, respectively, in electrooxidation of sewage effluent at a BDD

252

anode 47. Higher formation of chlorate and perchlorate was observed at BDD compared with

253

MMO anodes

254

anodes 66, although its expected concentrations will be in the low µg L-1 range due to the low

255

bromide concentrations in most wastewaters.

64

15,16,62,63

15,60,61

62

. In electrooxidation systems,

. Perchlorate generation is strictly

. Up to 120 and 100 mg L-1 of chlorate

. Similar to chlorate, bromate can be formed at both MMO

65

and BDD

256 257

In most studies where formation of chlorine, chlorate or perchlorate was identified as an

258

issue, the applied current densities were in the range from 50-300 A m-2 13,45-48,67-69. For more

259

contaminated wastewater, such as landfill and olive pomace leachate, higher current densities

260

have been applied (i.e., 1.2-2.6 kA m-2 50,70). The formation of perchlorate at BDD anodes can

261

be controlled by working at lower current densities, which can range from 100 to 300 A m -2,



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depending on the type of wastewater 47,71. Further lowering of the current density (≤50 A m-2)

263

may minimize the detrimental effect of chlorides, but it also limits the oxidation performance

264

of the treatment due to pronounced charge transfer limitations and decreased formation of

265

·OH radicals 72. The formation of chlorine is difficult to avoid, as oxygen evolution reaction

266

is associated with a higher overpotential than chlorine evolution reaction

267

formation in electrooxidation at BDD anodes increases linearly with charge density

268

Thus, the amount of chlorinated organic byproducts may be minimized by applying short

269

electrolysis time while targeting a suitable degree of removal of contaminants. Also,

270

decreasing the inter-electrode distance may reduce the mass transfer limitations of the

271

contaminants towards the electrode surface and enhance their electrochemical transformation

272

75,76

273

capital costs of the treatment.

73

. Moreover, AOX 45,74

.

. Nevertheless, high electrode surface/reactor volume ratio also implies an increase in the

274 275

The reaction kinetics of chloride and bromide will be affected by the presence of organic

276

matter, ammonia and other inorganic anions. Thus, formation of inorganic and organic

277

halogenated byproducts should be evaluated on a case-by-case basis, and used to assess the

278

suitability of electrochemical treatment for environmental applications. If formed oxidation

279

byproducts are of concern, they could be removed by sorption to activated carbon, similar to

280

full-scale ozonation systems. Compared to activated carbon alone, electrochemical process

281

provides disinfection of the treated effluent, and oxidative degradation of the organic matter.

282

However, although halogenate and perhalogenate species adsorb on the activated carbon, the

283

treatment efficiency is highly dependent on the composition of source water and modification

284

of carbon surface may be required to achieve satisfactory performance 61,77. A range of other

285

treatment techniques has been explored to date for the removal of chlorate, perchlorate and

286

bromate such as ion-exchange, membrane filtration, and microbial, chemical and


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electrochemical reduction 61,77. Yet, further development of these techniques is required prior

288

to their full-scale use.

289 290

Figures-of-merit for electrochemical treatment processes

291 292

The technical feasibility of electrochemical oxidation of wastewater is often based on the

293

removal of COD, ammonia and/or specific target organic contaminants. The removed COD

294

can be converted to the portion of electrical current that results in decreases in solute

295

concentrations by use of current efficiency (i.e., columbic efficiency, Faradaic yield). Current

296

efficiency is defined as the ratio of the electricity consumed by the electrode reaction of

297

interest divided by the total electricity passed through the circuit. In anodic oxidation, current

298

efficiency for COD removal can be expressed using general current efficiency (GCECOD) 8,78:

299

GCE COD  n O 2FV

COD 0  COD t MO 2It

(6)

300

where nO2 is the number of electrons required for water oxidation (n=4, 2H2O → O2 + 4H+ +

301

4e-), F is the Faraday constant (96487 C mol-1), V the electrolyte volume (L), COD0 and

302

CODt are COD values measured at time t=0 and time t (in g O2 L-1), MO2 is the molecular

303

weight of oxygen (32 g mol-1), I is the applied current (A), and t is the time over which

304

treatment occurrs (s). An annalogous approach can be used for ammonia or organic

305

contaminants, with values of n adjusted accordingly.

306 307

In situations where target contaminants are present at low concentrations, a more appropriate

308

parameter for estimating the energy efficiency of electrochemical treatment may be electric

309

energy per order (EEO). EEO expresses the electric energy (in kWh m-3) required to reduce the



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310

concentration of a contaminant by one order of magnitude in a unit volume of contaminated

311

water 79. EEO can be calculated as:

312

E EO 

313

E EO 

Pt C V log( 0 ) C P C q log( 0 ) C

for batch processes

(7)

for flow-through processes

(8)

314

where P is the rated power of the system (kW), V is volume of water (L) treated in time t (h),

315

q is the water flow rate (m3 h-1), C0 and C are initial and final (or influent and effluent)

316

contaminant concentrations. Equations 8 and 9 are valid only for perfectly mixed batch and

317

plug-flow flow-through processes. For an electrochemical treatment process, EEO will depend

318

on the treatment efficiency of the process and the concentrations of other solutes (e.g., COD)

319

that undergo reactions.

320 321

The energy efficiency of electrochemical treatment is often determined using the specific

322

energy consumption (ESP), expressed per kg of COD removed 80:

323

ESP 

FVCELL 1 3600 8ACE COD

(9)

324

where VCELL is the total cell potential (V), 8 is the equivalent mass of oxygen (32 g O2 per 4

325

mol e-), and ACECOD is the average current efficiency of COD removal.

326 327

Given the reactor-specific nature of the figures-of-merit used in electrochemical systems and

328

the effect of water composition on treatment efficiency, it is difficult to compare the

329

operational costs among existing publications. Most studies employ a batch mode of

330

operation, using an external reservoir. Under these conditions, the ratio of the reactor volume


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331

(active volume, VACT) and total volume of the electrolyte (VTOT), as well as electrode

332

surface/reactor volume ratio (AEL/VACT or AEL/VTOT) will play a critical role in treatment

333

efficiency. Furthermore, electrolysis experiments are typically performed in the galvanostatic

334

mode. In many cases, current density is reported, but anodic potentials are measured very

335

rarely, and in some cases information about the reference electrode is omitted. This

336

information is crucial for gaining insight into the possible generation of oxidant species at the

337

anode surface, particularly when comparing electrodes of different resulting potentials for a

338

given applied current (e.g., active vs non-active anodes). Therefore, we recommend that it

339

should always be reported." Thus, it is crucial to provide complete information on the reactor

340

design (e.g., interelectrode distance, AEL/VACT ratio, VACT/VTOT ratio for the batch mode),

341

operational parameters (e.g., electrode potential and current, electrolyte flow rate), electrolyte

342

characteristics (conductivity, presence of chloride and bromide) and full composition of the

343

test to enable comparisons with the reported data.

344 345

Application of electrochemical water treatment processes

346 347

There are very few examples in which electrochemical treatment has been applied

348

successfully in full-scale treatment applications. Use of electrooxidation for disinfection of

349

swimming pool water is widely applied 7. Electrochemical treatment is also one of the ballast

350

water treatment technologies approved by International Maritime Organization

351

anodes have also been employed in some situations for the on-site disinfection of rainwater,

352

sewage and industrial process water

353

of effluents from tanneries, petrochemical plants, dairies, and pulp and papermills 83-86. These

354

industrial wastewaters, which often contain high concentrations of refractory organics, also

355

contain elevated concentrations of chloride. As a result, indirect electrochemical oxidation

82

81

. BDD

. Anodic oxidation has been applied for the treatment



Page 16 of 36

356

with active halogen species is typically responsible for much of the removal of COD and

357

ammonia

358

chlorinated byproducts that increase the toxicity of the water 88. Activated carbon was used in

359

some studies to lower the concentration of halogenated byproducts in the effluent 89,90.

83,87

. As previously discussed, this mode of anodic treatment is also likely to yield

360 361

Some of the major impediments to the commercialization of electrochemical processes

362

include low current efficiencies and limited space-time yields (i.e., quantities of removed

363

contaminant per unit volume and unit time), which consequently lead to high energy

364

consumption. To obtain adequate removal kinetics under conditions encountered in

365

wastewater, high current densities are often applied, leading to a drastic increase in energy

366

consumption. For example, the energy consumption of pilot-scale system that used BDD

367

electrode to oxidize organic matter in landfill leachate ranged from approximately 22 kWh

368

kgCOD-1 at high current efficiency (charge transfer controlled regime) to 95 kWh kgCOD-1 at

369

high current densities where mass transfer controlled regime occurred 91. To lower the overall

370

energy consumption current modulation can be used, based on the stepwise lowering of

371

current to values close to the limiting current for organics oxidation 92.

372 373

Mass transfer limitations in electrochemical systems are exacerbated by the use of plate-and-

374

frame filter press reactors. When plate electrode reactors are operated in a flow-by mode, the

375

current flow direction is perpendicular to the electrolyte flow direction, and the reactions

376

rates are subject to strong mass transfer limitations due to the presence of a thin stagnant

377

boundary layer at the electrode surface. Flow-through electrochemical reactors partially

378

overcome this limitation through enhanced convective transport of the contaminants to the

379

electrode surface due to cross-flow filtration

93,94

. However, many industrial electrochemical


Page 17 of 36


380

processes use a channel flow between two plane, parallel electrodes

381

common configuration for environmental applications.

95

, and this is also a

382 383

To enhance the process performance and increase the current efficiency, porous three-

384

dimensional electrodes can be employed in the flow-through mode

385

efficient phenol removal by electrochemical adsorption and oxidation was achieved in a

386

cross-flow electrochemical filtration unit equipped with porous Ebonex anode

387

Electrochemical carbon nanotube (CNT) filters were applied for the removal of dyes by their

388

direct oxidation at CNTs 94,97. Also, CNT membrane stack was used for flow-through electro-

389

Fenton degradation of persistent contaminants 98. The sequential in situ generation of H2O2 at

390

the first CNT network cathode, H2O2 reduction to ·OH at the subsequent Fe-coated CNT

391

network cathode, and oxidation at the CNT network anode yielded 4-fold greater reaction rate

392

for oxalate oxidation compared to the sum of individual anodic oxidation at CNT and Fenton

393

oxidation processes. Nevertheless, when porous electrodes are employed in actual waste

394

treatment, bed blocking, increases in back pressure due to bubble generation and corrosion of

395

carbonaceous material may lead to diminished performance.

90,91

. For example,

96

.

396 397

As an alternative, three-dimensional particle and granular electrodes can be employed for

398

combined adsorption-electrochemical oxidation. By loading a conventional two-dimensional

399

plate and frame reactor with granular or particulate electrode material, such as granular

400

activated carbon (GAC) or graphite, granules act as bipolar electrodes and promote oxidative

401

degradation of the adsorbed contaminants. The mass transfer of contaminants is greatly

402

increased due to a large specific surface area, thus reducing the energy consumption. For

403

example, use of GAC and quartz sand filling between a MMO anode and a stainless steel

404

cathode lowered the energy consumption for industrial wastewater treatment from 300 kWh



405

kgCOD-1 to 180 kWh kgCOD-1

406

oxidation performance of the treatment and increases the adsorption capacity and effective

407

life-time of carbon due to continuous oxidation of the contaminants adsorbed on the GAC

408

filling

409

pollutants can be oxidized by the in situ generated oxidants (e.g., H2O2, ·OH, Cl2/HOCl) 101.

410

Hydrogen peroxide formed by oxygen reduction at the cathode side of GAC particles can be

411

decomposed to ·OH at the activated carbon, and thus promote indirect oxidation of

412

contaminants

413

fillings merit further research, as they could be well suited for the treatment of wastewater

414

with a high potential of formation of halogenated organics byproducts 104.

100

99

Page 18 of 36

. Electrocatalysis at activated carbon granules enhances the

. In addition to direct electrooxidation on the polarized GAC, adsorbed organic

100,102,103

. Three-dimensional electrochemical reactors with carbon-based

415 416

Complete regeneration of GAC is difficult to achieve due to the slow desorption of the

417

contaminants from a large internal surface area of the adsorbent

418

particle diffusion and increase the efficiency of electrochemical regeneration of the

419

adsorbent, researchers at the University of Manchester, UK, used a non-porous, highly

420

conducting graphite adsorbent Nyex® . The process was based on the adsorption of

421

contaminants, and desorption and oxidation by applying current (Figure 3) 104,106,107. With this

422

approach, a pilot-scale system was capable of removing low µg L-1 levels of pesticides from

423

groundwater, with an estimated energy consumption of 55 kWh kgCOD-1

424

emphasized that the process may not be economic for effluents with high organic load due to

425

the high recycle rate of the adsorbent.

105

. To minimize the intra-

106

. The authors

426 427

More widespread application of electrochemical systems require advances in operational

428

issues such as electrode fouling. Several researchers have reported the formation of deposits

429

of Ca2+ and Mg2+ ions in the treatment of real waste streams in electrolytic cells 46,108,109. The 18 ACS Paragon Plus Environment

Page 19 of 36


430

most simple and effective method to prevent the cathode scaling in practical applications of

431

electrochemical reactors is polarity reversal. For example, in an on-site electrochemical

432

treatment of wastewater at MMO anodes, cathode scaling was overcome by applying polarity

433

reversal, with an estimated lifetime of 6 years for Ta/Ir and Pt/Ir electrodes . Also, periodical

434

dosing of acid generated at the anode when the reactor is not treating wastewater (e.g., during

435

the night) to the cathode compartment could be used as an alternative to frequent polarity

436

switching. In addition, when high electric charge density is applied, an imbalance between

437

protons and hydroxyl ions formed by water electrolysis can lead to a change in pH,

438

particularly in the case of membrane-divided electrolytic cells. Thus, prior to discharge of

439

electrochemically treated water the properties of the influent (i.e., pH, redox potential) need

440

to be restored.

441 442

Decentralized treatment

443 444

Electrochemical processes may be particularly well suited for decentralized water treatment

445

because the mechanisms through which electrochemical processes are controlled - electrode

446

potential and cell current - are easier to control remotely than conventional chemical and

447

biological processes 110. Furthermore, electrochemical treatment systems are compact and are

448

easier to adjust to variations in influent water composition than most existing technologies.

449

Conventional chemical treatment, such as chlorination and ozonation, have intrinsic

450

limitations to small-scale operation related to the possible build-up of toxic levels of ozone or

451

chlorine in small enclosures typical of decentralized treatment systems. Given the growing

452

interest in decentralized wastewater treatment and source separation, interest has been

453

growing in the use of electrochemical systems for the treatment of potable water, greywater,



454

source-separated urine, settled sewage, hospital wastewater and point-of-use drinking water

455

systems 110.

Page 20 of 36

456 457

The formation of toxic disinfection byproducts (DBPs) is a major challenge in drinking water

458

treatment. The existing strategies aiming at minimization of DBPs are only partially efficient

459

and expensive

460

chlorine and disinfection byproducts (DBPs) as a point-of-use treatment of water intended for

461

direct consumption. Reductive dehalogenation of THMs and HAAs was achieved at carbon-

462

based and metal electrodes polarized at around -1 V vs Standard Hydrogen Electrode (SHE)

463

112-115

464

116,117

465

low conductivity of tap water

466

dimensional reactors. For example, Sonoyama et al

467

activated carbon electrodes for reductive electrochemical removal of THMs from low

468

conductivity tap water (Figure 4). Greater contact between the fluid and the electrode

469

alleviates the detrimental effects of low conductivity and low contaminant concentration on

470

the process performance 118-120.

111

. Electrochemical treatment may be applied for the removal of residual

. Moreover, electrochemical reduction may also be applied for the removal of bromate

. Yet, the reaction rates are significantly limited by the low concentration of DBPs and 113

. The process performance can be improved by using three 112,113

used column-type carbon fibre and

471 472

Electrochemical systems may be an attractive option for the decentralized treatment of

473

greywater. Prior to land application or indirect reuse, it may be necessary to treat greywater

474

to remove chemical contaminants. Electrochemical oxidation of greywater at Pt, Pt/Ir and

475

Ru/Ir anodes removed personal care and household products (e.g., bisphenol A, triclosan,

476

parabens), without the formation of high concentrations of halogened organic byproducts

477

(AOX

Challenges and Opportunities for Electrochemical Processes as Next

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