Occurrence and Removal of Organic Micropollutants in Landfill

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Occurrence and removal of organic micropollutants in landfill leachates treated by electrochemical advanced oxidation processes Nihal Oturan, Eric D. Van Hullebusch, Hui Zhang, Laurent Mazeas, Helene Budzinski, Karyn Le Menach, and Mehmet A. Oturan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02809 • Publication Date (Web): 17 Sep 2015 Downloaded from http://pubs.acs.org on September 17, 2015

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Environmental Science & Technology

1

Occurrence and removal of organic micropollutants in landfill leachates

2

treated by electrochemical advanced oxidation processes

3

4

Nihal Oturan†, Eric D. van Hullebusch†, Hui Zhang‡, Laurent Mazeas§, Hélène Budzinski£,

5

Karyn Le Menach£, Mehmet A. Oturan†,*

6

7

†

Université Paris-Est, Laboratoire Géomatériaux et Environnement, EA 4508, UPEM, 5 Bd Descartes, 77454 Marne-la-Vallée, Cedex 2, France

8 9

‡

Department of Environmental Engineering, Wuhan University, P.O. Box C319 Luoyu Road 129#, Wuhan 430079, China.

10 11

§

Hydrosystems and Bioprocesses Research Unit, IRSTEA, 1 rue Pierre-Gilles de Gennes, CS 10030, F-92761 Antony Cedex, France.

12 13

£

Université de Bordeaux, Environnements et Paléoenvironnements Océaniques et

14

Continentaux, EPOC - UMR 5805 CNRS, Laboratoire de Physico- et Toxico-Chimie de

15

l'environnement (LPTC), Bâtiment A12, 351 crs de la Libération, 33405 Talence, France.

16 17 18 19 20

Corresponding author:

21

Tel.: +33 149 32 90 65; fax: +33 149 32 91 37.

22 23

E-mail address: [email protected] (M.A. Oturan)

1

ACS Paragon Plus Environment


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Abstract: In recent years, electrochemical advanced oxidation processes have been shown to

26

be effective alternative for the removal of refractory organic compounds from water. This study

27

is focused on the effective removal of recalcitrant organic matter (micropollutants, humic

28

substances, etc.) present in municipal solid waste landfill leachates. A mixture of 8 landfill

29

leachates has been studied by electro-Fenton process using a Pt or BDD anode and a carbon felt

30

cathode or by anodic oxidation process with a BDD anode. These processes exhibit great

31

oxidation ability due to the in situ production of hydroxyl radicals (OH), a highly powerful

32

oxidizing species. Both electrochemical processes were shown to be efficient in the removal

33

of dissolved total organic carbon (TOC) from landfill leachates. Regarding the electro Fenton

34

process, the replacement of the classical anode Pt by the anode BDD allows reaching better

35

performance in term of dissolved TOC removal. The occurrence and removal yield of 19

36

polycyclic aromatic hydrocarbons (PAHs), 15 volatile organic compounds (VOCs), 7

37

alkylphenols, 7 polychlorobiphenyls (PCBs), 5 organochlorine pesticides (OCPs), and 2

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polybrominated diphenyl ethers (PBDEs) in landfill leachate were also investigated. Both

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electrochemical processes allow reaching a quasi-complete removal (about 98%) of these

40

organic micropollutants.

41 42

Keywords: Landfill leachate; Electro-Fenton; Anodic oxidation; Organic micropollutants;

43

Mineralization

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2


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1. Introduction

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A variety of organic pollutants have been detected in municipal solid waste landfill

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leachates worldwide. These compounds are aromatic compounds, chlorinated aliphatics,

48

higher alkanes, fatty acids, nonylphenol ethoxycarboxylate acids, pesticides, phenolic

49

compounds, polyaromatic hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxins and

50

dibenzofurans,

51

contaminants such as perfluorinated compounds (PFCs) and pharmaceuticals and personal

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care products (PPCPs).1-9 Most of the reported compounds are known to belong to the current

53

list

54

(http://water.epa.gov/scitech/methods/cwa/pollutants.cfm)

55

biodegradation.10,11 In addition, such compounds may be involved in the contamination of

56

surface and groundwater.7,12

of

polychlorinated

126

Priority

biphenyls

(PCBs),

Pollutants

phthalates,

defined and

are

and

even

emerging

by

US

quite

refractory

EPA to

57

Therefore, advanced oxidation processes (AOPs) were proposed to degrade recalcitrant

58

micropollutants from landfill leachate.10,12-14 Among various AOPs, Fenton process is

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relatively cheap and is easy to operate and maintain,15 which has been investigated for landfill

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leachate treatment.16,17 However, the regeneration of ferrous ion to catalyze Fenton's reaction

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is regarded as much slower during Fenton chain reactions and a large amount of ferrous ion

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need to be applied to keep the sufficient hydroxyl radicals production.18 This results in the

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generation of ferric iron hydroxide sludge when the hydrogen peroxide to ferrous ion mole

64

ratio is low. Moreover the mineralization efficiency is weak because of involvement of several

65

wasting reactions consuming hydroxyl radicals, especially those with reagents (H2O2 and

66

ferrous iron).19

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In parallel, electrochemical oxidation has been employed for the treatment of landfill

68

leachate, in which a variety of anode materials were employed.20 Among various anodes used,

69

boron-doped diamond (BDD) electrode was regarded as the more efficient anode for the

70

mineralization of wastewater due to its high oxidation power and high oxygen

71

overvoltage.21-24 Several authors have applied BDD electrodes to the treatment of landfill

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leachate.25-28 When electrochemical oxidation is combined with Fenton process (namely

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electro-Fenton (EF) process), Fenton’s reagent, i.e., hydrogen peroxide and ferrous ion, could

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be in-situ electrogenerated (hydrogen peroxide) and electro-regenerated (ferrous iron as

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catalyst).29-31 Compared to classical Fenton process, EF process needs significantly less

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(catalytic amount) ferrous ion.19,29 A variant of EF process using a sacrificial iron anode,

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namely anodic Fenton treatment, has been used for the treatment of landfill leachates.16,30,31 In

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these applications, a sacrificial cast iron anode was used to provide ferrous ion from anodic

79

oxidation of iron anode. Other applications were carried out using DSA (Dimensionally Stable

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Anode) in which ferrous ions are obtained from cathodic reduction of externally added ferric

81

ions.18,32 However, there is no report on treatment of landfill leachates by classical EF process

82

using BDD anode with the continuous electro-generation of hydrogen peroxide and

83

electro-regeneration of ferrous iron (catalyst).

84

In this study, anodic oxidation (AO) with BDD electrode and EF process either with a Pt

85

or a BDD anode were used for the mineralization of greatly complex landfill leachates. The

86

effect of applied current and anode materials on the mineralization efficiency of landfill

87

leachates was investigated. In parallel, the occurrence of organic micropollutants in landfill

88

leachate was determined before and after treatment, and the comparative efficiency of the

4


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processes used (AO and EF) on the removal percentage of these organic micropollutants was

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monitored. The issue of nitrogen removal was out of scope of the present work because it was

91

previously showed that nitrogen might be quite easily removed by implementing a biological

92

process as a pre-treatment for removing biodegradable organic matter as well as ammonia and

93

the sequent implementation of an advanced oxidation process was responsible for the removal

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of residual refractory organic pollutants.33-35

95 96

2. Materials and methods

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2.1 Landfill leachates samples

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Leachate samples were taken in glass bottles from eight municipal solid waste landfills

99

located in France. Samples taken were preserved in refrigerator at 4 °C in accordance with the

100

Standard Methods.32 Single and mixture of leachates were studied by two electrochemical

101

advanced oxidation processes (AO and EF) to evaluate the effect of complex composition of

102

the mixture on the treatment efficiency.

103

The leachate from one landfill (denoted as leachate #1) (see average landfill leachates

104

composition in Table S1) and the mixture of leachate from eight landfills (denoted as leachate

105

#2) were used for the electro-Fenton as well as anodic oxidation experiments. The mixture of

106

leachates was prepared just before experiments by mixing equal volumes of different samples

107

preserved in refrigerator.

108 109

2.2. Electrochemical advanced oxidation processes set-ups

110

For all the EAOPs, the electrolyses were performed in an open, undivided and cylindrical

111

electrochemical cell of 6 cm diameter and 250 mL capacity in which the landfill leachates

5



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were placed. Two electrodes, all with 24 cm2 (4 cm × 6 cm) area were used as anode:

113

commercial pure Pt and boron-doped diamond (BDD thin-film deposited on a niobium

114

substrate (CONDIAS, Germany). A tri-dimensional, large surface area carbon-felt (14 cm × 5

115

cm × 0.5 cm in width, Carbone-Lorraine, France) was used as cathode.

116

In EF experiments, the anode was centered in the electrochemical cell and was

117

surrounded by carbon felt cathode, which covered with the inner wall of the cell. H2O2 was

118

produced in situ from the reduction of dissolved O2 in the solution. The concentration of O2 in

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the solution was maintained by continuously bubbling compressed air through a frit at about 1

120

L.min-1. A period of 10 min before electrolysis was sufficient to reach a stationary O2 level. A

121

0.2 mM FeSO4.7H2O salt was added to the solution to have 0.2 mM Fe2+ as a catalyst in the

122

EF experiments. The use of 0.2 mM Fe2+ is recommended as the catalyst optimum amount in

123

the EF process with a minimum contribution of parasitic reactions.29 On the other hand,

124

leachates contain soluble iron at about t 0.1 mM.36 This means the total iron concentration in

125

leachates treated in this study is more than 0.2 mM (about 0.3 mM). The initial iron

126

concentration of leachates being not known exactly, external addition of 0.2 mM Fe2+ was

127

preferred to the solutions to enhance the conductivity and maintain enough catalyst

128

concentration. A pH of 3.0 was considered as the optimum pH for the EF process.19,29 Finally,

129

a 0.05 M Na2SO4 as a supporting electrolyte was added. A constant current of 500 or 1000

130

mA was applied for mineralization experiments. The AO experiments were conducted at

131

natural pH 8.05 of landfill leachates solution at same operating conditions but without iron

132

salt addition and air bubbling. In addition, AO experiments were also performed without

133

addition of Na2SO4 to verify if the studied leachates are enough conductive to avoid

6


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supporting electrolyte addition. All experiments were carried out at room temperature (23 ±

135

2 °C) and solutions were vigorously mixed by a magnetic PTFE stirrer during the treatment to

136

ensure the mass transport toward electrodes.

137

Electrolyses were performed with a Hameg HM8040 triple power supply at constant

138

currents. This instrument displayed the cell voltage along the treatments as well. The solution

139

pH was measured by using a CyberScan pH 1500 pH-meter (Eutech Instruments). The

140

mineralization of treated solutions was assessed from the abatement of their dissolved organic

141

carbon, which can be considered as the dissolved total organic carbon (TOC) in the case of

142

highly water-soluble organic compounds. A Shimadzu VCSH TOC analyzer was used to

143

determine TOC values. Samples withdrawn from the treated solution at different electrolysis

144

times were microfiltered onto a hydrophilic membrane (Millex-GV Millipore, pore size 0.22

145

µm) before subjected to analysis. Reproducible TOC values, within ±1% accuracy, were found

146

using the non-purgeable organic carbon method.

147 148

2.3. Analysis of the organic micropollutants

149

Polycyclic Aromatic Hydrocarbons (PAHs)

150

Polycyclic Aromatic Hydrocarbons were analyzed by solid phase micro-extraction

151

(SPME) - gas chromatography –mass spectrometry (Agilent Technologies MSD 5975. The

152

SPME Fiber was a PDMS (polydimethylsloxane) 100 µm from Supelco. The fiber was

153

immerged directly in the sample during 60 min at 40 °C. Organic compounds were then

154

desorbed in the splitless injector of the gas chromatograph maintained at 280°C for 3 min and

155

separated on a HP5MS-UI (30 m × 0.25 µm × 0.25 mm) column. The detection was

7



156

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performed on the single ion monitoring mode targeting PAHs and internal standards.

157 158

Volatile Organic Compounds (VOCs)

159

Volatile Organic Compounds were analyzed by solid phase micro-extraction (SPME) -

160

gas chromatography –mass spectrometry (GC-MS) (Agilent Technologies MSD 5973). The

161

SPME Fiber was a PDMS/DVB 65 µm from Supelco. The fiber was introduced in the

162

headspace of the sample vial at 50°C for 30 min. Organic compounds were then desorbed in

163

the splitless injector of the gas chromatograph maintained at 220 °C for 3 min and separated

164

on a DB-624 (30 m × 1.8 µm × 0.32 mm) column. The detection was performed on the single

165

ion monitoring mode targeting VOC and internal standards.

166 167

Organo Halogenated Compounds (OCHs): Polychlorobiphenyls, Organochlorine pesticides,

168

and Polybrominated diphenyl ethers

169

OCHs were analyzed by solid phase micro-extraction (SPME) - gas chromatography –

170

micro-electron capture detector (micro-ECD; Agilent Technologies). The SPME Fiber was a

171

PDMS 100µm from Supelco. The fiber was introduced in the headspace of the sample vial at

172

80°C for 60 min. Organic compounds were then desorbed in the splitless injector of the gas

173

chromatograph maintained at 280°C for 3 min and separated on a HP5MS-UI (30 m × 0.25

174

µm × 0.25 mm) Column.

175 176 177

Alkylphenols (AKPs) 4-Nonylphenol (4NP 4-ter-Octylphenol (4OP) and 4-ter-Butylphenol (4BP) were

8


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178

analyzed by solid phase micro-extraction (SPME) - gas chromatography –mass spectrometry

179

(Agilent Technologies MSD 5975). The SPME Fiber was a PA 85µm from Supelco. The fiber

180

was introduced in the headspace of the sample vial at 80 °C for 80 min. Organic compounds

181

were then desorbed in the splitless injector of the gas chromatograph maintained at 270 °C for

182

3 min and separated on a HP5MS-UI (30 m × 0.25 µm × 0.25 mm) Column. The detection

183

was performed on the single ion monitoring mode targeting AKPs and internal standards

184

(pn-nonylphenol-C13, 4-Nonylphenol-d8, 4-ter-Octylphenol-d2, 4-ter-Butylphenol-d13).

185

For the remaining AKPs, 100 mL acidified samples spiked with surrogate standards

186

(nonylphenoxyacetic acid-d2, 4-nonylphenol monoethoxylate-d2, bisphenol A-d16) were first

187

extracted using BondElut® C18 (200 mg, 3 mL) cartridges previously conditioned with

188

methanol and acidified natural mineral water was used as reference water at the laboratory.

189

The cartridges were rinsed with a mixture of methanol–water and dried under vacuum. The

190

alkylphenols were then eluted using a mixture of dichloromethane–methanol. The extracts

191

were evaporated near to dryness under a gentle stream of nitrogen and dissolved in 2 mL

192

80:20

193

nonylphenoxyacetic acid (NP1EC). The remaining extracts were further purified on

194

BondElut® HF-PSA (500 mg, 3 mL) cartridges. The columns were conditioned with 3 mL

195

methanol and 3 mL 80:20 methanol– dichloromethane. After the sample loading, the

196

cartridges were rinsed with 80:20 methanol–dichloromethane (2 × 0.5 mL) and the

197

compounds were then eluted using successively 5 mL 80:20 methanol-dichloromethane and 5

198

mL 79:19:2 methanol-dichloromethane-trifluoroacetic acid (v/v/v). The purified extracts were

199

evaporated near to dryness under nitrogen stream. They were then reconstituted with 300 µL

methanol–dichloromethane

(v/v).

1

aliquot

is

kept

for

the

analysis

of

9



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200

methanol. 3 aliquots were prepared: a 100 µL aliquot was kept at −20 °C as stock sample,

201

another 100 µL aliquot was used for the determination of BPA (Bisphenol A),

202

nonylphenoxyacetic acid (NP1EC) by UPLC–MS/MS in negative electrospray ionization

203

(ESI−) mode, and the last 100 µL aliquot was used for the determination of

204

4-nonylphenolmonoethoxylate (NP1EO) and 4-nonylphenol diethoxylate (NP2EO) by

205

UPLC–MS/MS in positive electrospray ionization (ESI+) mode.

206

207

3. Results and discussion

208

3.1. TOC removal by electro-Fenton process

209

To investigate dissolved TOC abatement by electro-Fenton process, the pH of the

210

leachate #1 was first adjusted to around 3.0. As can be seen in Figure 1, the initial dissolved

211

TOC value of leachate #1 was 4650~4750 mg L-1 with an initial pH of 8.05. When the pH was

212

adjusted to around 3.0, organic matter precipitation occurred and the dissolved TOC values

213

decreased to 3425~3455 mg L-1. Generally, landfill leachates contain a variety of organics

214

including humic substances.38-40 Humic substances display different solubility depending on

215

pH and the dissolved TOC decrease was attributed to the formation of organic compounds

216

precipitates at low pH. Rivas et al. (2005) reported that 33% of COD was removed by simple

217

acidification of the landfill leachate.39 Figure 1 indicates that TOC values of humic acids in

218

the precipitate contributed from 26.3% to 27.2% of total TOC in leachate #1. To investigate

219

the performance of electro-Fenton process with or without filtration, the electrolysis was

220

carried out with 0.2 mM catalyst at 500 mA. As illustrated in Figure 1, the residual TOC

221

(1000 mg L-1) of the leachate after precipitate removal by filtration was lower than that (1390 10


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222

mg L-1) of the leachate without filtration. However, the total TOC removal was only 2425 mg

223

L-1 in the first case since the precipitation was removed a part of TOC from the reaction

224

system. The total TOC removal was 3360 mg L-1 without filtration and the treated leachate

225

was clear at the end of the treatment along with the disappearance of precipitate. Since needs

226

further treatment, the filtration was not implemented to separate the precipitate in the below

227

described experiments. However, in the point of view of real application of electrochemical

228

oxidation, a pre-filtration would be required for removing particles in order to avoid clogging

229

of the electrochemical cell in which two parallel plates (anode and cathode) are usually

230

positioned at a narrow channel.

231 232

3.2 Effect of applied current intensity on TOC removal by electro-Fenton process using

233

different electrode materials as anode

234

Figures 2 illustrate mineralization of landfill leachate by EF process using Pt or BDD

235

anode when the applied current was operated at 500 or 1000 mA. As can be seen, the initial

236

TOC value of leachate #1 was 4708~4750 mg L-1 with pH of 8.00~8.05, but it decreased to

237

3455~3500 mg L-1 when the solution was acidified to pH around 3.0. For leachate #2 (initial

238

TOC = 3375 mg L-1), the corresponding TOC value dropped to 2005~2025 mg L-1. After 18 h

239

EF treatment using Pt anode, the final TOC values of the treated leachate #1 dropped to 1390

240

and 520 mg L-1, respectively, at the applied current of 500 and 1000 mA. The corresponding

241

final TOC values of the treated leachate #2 were 447 and 244 mg L-1, respectively, at same

242

conditions. Similar trends (Figure 2) were observed when BDD anode was employed. This

243

indicated that the increase in applied current intensity would lead to the enhancement in of

244

TOC removal. 11



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245

The TOC removal is due to the mineralization of organic pollutants of leachates by OH

246

radicals generated in EF process which is based on the continuous production of H2O2 in by

247

two-electron reduction of oxygen at a suitable cathode:19,29,40

248

O2 (g) + 2H+ + 2e− → H2O2

(1)

249

Carbon felt is used as a cathode material for H2O2 generation because it presents a high

250

electrochemical activity for O2 reduction and low catalytic activity for H2O2 decomposition

251

(Brillas et al., 2009). Fe2+ present in the landfill leachate or externally added to the solution),

252

reacts with H2O2 to generate OH according to the Fenton reaction (2):

253

Fe2+ + H2O2 + H+ → Fe3+ + OH + H2O

(2)

254

The catalyst Fe2+ is then electrogenerated by reduction of Fe3+ (electrocatalysis) formed by

255

reaction (2) at the cathode surface:

256

Fe3+ + e− → Fe2+

(3)

257

In the case of operating in undivided cell, organics are destroyed by OH produced

258

homogeneously from Fenton's reaction but also by the action of heterogeneously formed

259

hydroxyl radical (M(OH)) from water oxidation by reaction (5), when using a high

260

O2-overvoltage anode (M):

261

M + H2O → M(OH) + H+ + e-

(4)

262

Thus hydroxyl radicals formed by reactions (2) and (4) react with organics until

263

mineralization (i.e., transformation to CO2, water and inorganic ions).24,29 Therefore the

264

applied current intensity is the main parameter influencing process efficiency in EF setup,

265

since the formation of hydroxyl radicals is governed by this parameter through reactions (1-4).

266

Consequently, the rate of TOC was found to increase with increasing applied current due to 12


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267

the production higher amount of homogeneous OH in the bulk of solution from Fenton

268

reaction (2) and heterogeneous Pt(OH) or BDD(OH) at anode surface from reaction (4)

269

since high current promotes generation rate of H2O2 (reaction 1) and Fe2+ (reaction 3) leading

270

to the formation of more OH from reaction (2) as well as formation rate of Pt(OH) and/or

271

BDD(OH) from reaction (4).19,24,29,41

272

It is worthy to notice that the comparative performance of Pt and BDD anodes (Figure 2)

273

can be explained by the nature of M(OH) generated on their surface. Heterogeneous hydroxyl

274

radicals BDD(OH) are physisorbed on the surface and thus are more available compared to

275

Pt(OH) that are chemisorbed. In addition BDD anode has a great overpotential (1.27 V)

276

allowing generation of high quantities of BDD(OH) and making this anode more powerful

277

than Pt.19,29

278

It should be noted that doubling the current intensity does not mean that the TOC removal

279

rate will evolve proportionally. This can be explained by enhancement of the following

280

parasitic reactions such as oxygen evolution from water discharge (reaction 5), hydrogen

281

evolution from water reduction (reaction 6), reduction of H2O2 on cathode (reaction 7) or

282

oxidation on anode (reaction 8) of H2O2:24, 29

283

2H2O → O2 + 4H+ + 4e–

(5)

284

2H2O + 2e- → H2 + 2OH–

(6)

285

H2O2 + 2H+ + 2e- → 2H2O

(7)

286

H2O2 → HO2 + H+ + e–

(8)

287 288 289 290

3.3 Effect of anode material on mineralization of landfill leachate by electro-Fenton process To clarify the effect of anode material, 250 mL of leachate #1 and leachate #2 were 13



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291

treated by EF with Pt and BDD anode with 0.2 mM Fe2+ (catalyst) at 500 and 1000 mA

292

constant current. Mineralization experiments with landfill leachates depicted on Figures S-1

293

and S2 show that more TOC was removed by using BDD anode compared with Pt anode. As

294

can be seen in Figure S1, TOC removal efficiency was as high as 86.8% using BDD anode

295

while only 71.0% TOC removal was achieved using Pt anode when leachate #1 with initial

296

TOC of about 3500 mg L-1 (after leachate acidification) was treated by EF at solution pH

297

being adjusted to 3. When leachate #2 was treated under same operating conditions, a

298

mineralization degree of 93.2% was achieved by BDD anode at 18 h while the mineralization

299

efficiency was only 78.2% with Pt anode.

300

The significant enhancement of electro-Fenton process achieved by the replacement of

301

the classical anode Pt with the anode BDD is due to the concomitant formation of large

302

amounts of BDD(•OH) at the BDD anode surface (reaction (4)) and in the bulk through the

303

homogeneous Fenton reaction (2).24,29 The extra advantages of application of BDD in the

304

treatment are: (i) Formation of large amount of BDD(•OH) than others M(•OH) including

305

Pt(•OH) due to the greater O2 overvoltage; (ii) high oxidation window (about 2.5 V)

306

conferring large potential window to form BDD(•OH); (iii) higher oxidizing power of

307

BDD(•OH) than Pt(•OH) since the former are physisorbed contrarily the later that are

308

chemisorbed, and consequently less available.

309

However, the advantages of BDD anode become less significant at higher applied current.

310

As can be seen in Figures S1 and S2, when the applied current was 1000 mA, the difference

311

of TOC removal efficiency between BDD and Pt anode was only 9.1% for leachate #1 (Figure

312

S1) compared to 15.4% at 500 mA, and the corresponding difference value was 8.4% for

14


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313

leachate #2 at 1000 mA (Figure S2) compared to 15% at 500 mA. This phenomenon is due to

314

the enhancement of the wasting reaction (9) at high applied current and relative decrease of

315

AO process compared to the formation of homogeneous OH from reaction (2).

316

2 BDD(•OH) → 2 BDD + O2 + 2 H+ + 2 e–

(9)

317 318

3.4. Comparison of electro-Fenton and anodic oxidation performance for TOC removal

319

Figure 3 displays the effect of applied current intensity on TOC removal by AO process.

320

Note that the initial TOC content is higher in the case of AO treatment than for the EF

321

treatment due to the fact that the pH of the bulk solution was not modified in the former case,

322

while it is required for EF processes. As in the case of EF set-ups, TOC removal efficiency is

323

improved during AO treatment of landfill leachates when the current intensity is increased.

324

After 18 h electrolysis, at 500 mA, a TOC removal degree of 85.5% and 83.8% is calculated

325

for leachate #1 and leachate #2, respectively. At 1000 mA, a slightly higher TOC removal

326

degree of 89.5% and 91.2.8% is calculated for leachate #1 and leachate #2, respectively. This

327

result demonstrates a higher current efficiency for the lowest current density.23,29,41 As noticed

328

for EF process, the TOC removal percentage is not proportional to the increase in the applied

329

current. This behavior can be explained by enhancement of parasitic reactions, mainly O2

330

evolution at the anode and recombination of BDD(•OH). These reactions compete with the

331

organic pollutants for hydroxyl radicals leading to the decrease in current efficiency. The

332

addition of Na2SO4 does not significantly improve the anodic oxidation process performance

333

in terms of TOC removal degree (Figure S3). This result indicates that leachates investigated

334

in this study are enough conductive and does not necessitate the addition of a supporting

335

electrolyte for treatment by electrochemical processes AO and EF. 15



Page 16 of 35

336

Also, the AO process displays better performance in term of TOC removal rate compared

337

to EF with BDD set-up (216 mg TOC L-1 h-1 versus 168 mg TOC L-1 h-1 for leachate #1 and

338

151 mg TOC L-1 h-1 versus 106 mg TOC L-1 h-1 for leachate #2 at 500 mA). Therefore it is

339

worth to note that this is a relative performance due to the initial TOC that is significantly

340

higher in the case of AO. However, regarding the remaining TOC after treatment, EF with

341

BDD perform better performance with about 200 mg L-1 residual TOC against 260 mg L-1

342

residual TOC for AO process.

343

From the above-discussed results, we could see that BDD anode has an added value in

344

terms of TOC removal in the case of EF process when comparing Pt anode with BDD anode

345

or when comparing EF process using Pt anode in AO process. By applying a current

346

corresponding to anode potential closed to the region of oxygen evolution, the organic

347

pollutants (R) present in landfill leachate can be oxidized via different mechanisms:22,43

348

(i)

to the reaction (4):

349

350

351 352 353

by reaction (10) with physisorbed hydroxyl radicals BDD(OH) formed according

M(OH)ads + R → M + CO2 + H2O + inorganic ions

(10)

The physisorbed BDD(OH) causes unselective oxidation of organics resulting in the complete combustion, (ii)

by the indirect oxidation with active chlorine (gaseous chlorine, hypochlorous acid or

354

hypochlorite) electrogenerated from the oxidation of chloride ions present in the

355

landfill leachates:

356

2Cl- → Cl2 + 2e-

(11)

357

Cl2 + H2O → HOCl + H+ + Cl-

(12) 16


Page 17 of 35

358


HOCl + R → CO2 + H2O + H+ + Cl-

(13)

359

Considering that the studied landfill leachates contain significant amount of chloride ion

360

(5.0-6.2 g L-1) (Table S1), it can contribute to the oxidation of organics in the solution

361

(mediated oxidation). Indeed chloride ion can be oxidized at the anode surface to form active

362

chlorine species (Cl2, HClO, and ClO−) that contribute to TOC removal (reaction 13). Such

363

reaction is probably significantly contributing to the removal of TOC as mentioned by

364

Francisca et al.34

365

However, as reported by Pérez et al.,44 ammonia can be converted by active chlorine

366

species into N2 or nitrate and probably alleviate the contribution of such chlorine in TOC

367

removal.

368

2/3 NH4+ + HOCl → 1/3 N2 + H2O + 5/3 H+ + Cl−

(14)

369

NH4+ + 4 HOCl → NO3− + H2O + 6 H+ + 4 Cl−

(15)

370

In the case of EF process, the following additional mineralization reaction (16) takes

371

place with homogeneous OH enhancing the oxidation/mineralization performance of the

372

process:

373

R + OH → CO2 + H2O + inorganic ions

(16)

374 375

3.5. Removal of the main organic micropollutants from landfill leachates by EF process

376

Organic micropollutants, with high toxicity and environmental concern, are present in the

377

landfill leachates investigated in this study at much lower levels (see Table 1 and Table S1)

378

than other organic constituents (humic substances usually quantified as chemical oxygen

379

demand (COD), biochemical oxygen demand (BOD), or total organic carbon (TOC)), and

380

little is known regarding their behaviors in EAOPs treatment. 17



Page 18 of 35

381

Table 1 displays the initial concentration and the removal yield after EAOPs treatment of

382

PAHs, VOCs, alkylphenols, PBCs organochlorine pesticides (OCPs) and PBDEs, respectively.

383

In the present study, occurrence and removal efficiency of 19 PAHs, 15 VOCs, 7 alkylphenols,

384

7 PBCs, 5 (OCPs), and 2 PBDEs in landfill leachate were investigated.

385

Table 1A shows that PAHs concentration in the studied landfill leachates is rather low

386

compared to the values reported in the literature (for referencing details see Table S2). Also

387

the results of EAOPs treatment shows that these compounds could be almost completely

388

removed with removal efficiency ranging from 87.4 to 100% confirming previous results

389

regarding the removal of PAHs from soil washing solutions.31,46 EF process is showing

390

slightly better performance than AO process.

391

Table 1B shows that VOCs concentration in the studied landfill leachates is rather high

392

compared to the values reported in the literature. Also the EAOPs treatment shows these

393

compounds could be almost efficiently removed with removal degree ranging from 78.1 to

394

100%. Anodic process is showing slightly better performance than electro Fenton process.

395

Concerning alkylphenols, their concentrations are rather important compared to the

396

values reported in the literature (Table 1C). However for the concentrations of bisphenol A, a

397

value of 0.135 µg L-1 has been measured which is in the lower range of the concentrations

398

reported in the literature ranging from 0.01 to 17,200 µg L-1.11,47,48 The source of bisphenol A

399

in landfill leachates may be the waste plastics in waste landfill and it is completely removed

400

by both electrochemical processes as reported previously.49

401

Tables 1D and 1E display the PCBs and OCPs concentration, respectively, in the studied

402

landfill leachate. Regarding the PCBs, the measured concentrations are significantly higher

18


Page 19 of 35


403

than the one measured by Wojciechowska.50 Also for lindane, the measured concentration

404

belongs to the lower range of the concentrations reported in the literature.47 When comparing

405

the performance of AO versus EF, EF displays in general better performance than AO but in

406

some cases contrasting results are obtained where AO displays slightly better results than EF,

407

with PCBs and OCPs removal efficiency ranging from 86.9 to 100%.

408

The last few decades have seen dramatic growth in the scale of production and the use of

409

PBDEs as flame retardants. Consequently, PBDEs such as PBDE 28, 47, 66, 71, 75, 77, 85,

410

99, 100, 119, 138, 153, 154, and 183 have been detected in various environmental matrices

411

with ∑PBDEs concentration ranging from 0.03–1020 ng L-1.51 However, Table 1F shows that

412

PBDEs concentration of BDE 47 and 99 in the studied landfill leachates is rather low with a

413

concentration of 0.6 ng L-1 compared to the values reported in the literature.52,53 Also these

414

compounds were completely removed by the both EAOPs treatment.

415

Results obtained with EAOPs used in this study were shown to be efficient reaching a

416

quasi-complete removal of organic micropollutants with an average of 98% removal and

417

therefore to yield a final treated leachate with enough quality regarding the organic

418

micropollutants concentrations to be discharged into natural water bodies. However the

419

amount of the electrical energy (E) consumed in kWh per g dissolved carbon (TOC) removed

420

was calculated for leacheat # 1 using equation (17):29

421

E = V x I x ∆t / (∆(TOC) x Vsol)

(17)

422

where V (volt) is the potential between anode and cathode (3.5 V), I is the applied current (0.5

423

A), ∆t is the treatment time ( 18 h), ∆(TOC) is the amount of TOC removed (3,1 g carbon L-1)

424

and Vsol (0.250 L) is the volume of the treated solution. Calculated E value for BDD anode was

19



Page 20 of 35

425

40 kWh (g TOC)-1. The values calculated for EF with Pt anode was slightly higher while the

426

AO process resulted in almost same value. This value is significantly higher than that reported

427

in Oturan et al.,54 for the mineralization of a single compound. The high energy consumption

428

is probably related to the complex composition of leachates that contain highly recalcitrant

429

pollutants. Therefore, for a cost effective treatment, such technologies have to be combined

430

with other processes (biological and physical/ chemical treatments) in order to reduce the

431

treatment time and the operational costs. Experiments performed in our laboratory for single

432

compound indicate that a short treatment time such as 1 h is able to increase the

433

biodegradability (in terms of BOD5/COD ratio) to the level of 0.33. This decreases significantly

434

the treatment time and energy cost of electrochemical treatment.

435 436

3.6. Practical implications for the treatment of landfill leachates

437

Usually, young landfill leachates (

Occurrence and Removal of Organic Micropollutants in Landfill

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