Subscriber access provided by UNIV OF NEBRASKA - LINCOLN
Article
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 35
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: Mehmet.Oturan@univ-paris-est.fr (M.A. Oturan)
1
ACS Paragon Plus Environment
Environmental Science & Technology
Page 2 of 35
24 25
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
38
polybrominated diphenyl ethers (PBDEs) in landfill leachate were also investigated. Both
39
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
44
2
ACS Paragon Plus Environment
Page 3 of 35
45
Environmental Science & Technology
1. Introduction
46
A variety of organic pollutants have been detected in municipal solid waste landfill
47
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
52
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
59
relatively cheap and is easy to operate and maintain,15 which has been investigated for landfill
60
leachate treatment.16,17 However, the regeneration of ferrous ion to catalyze Fenton's reaction
61
is regarded as much slower during Fenton chain reactions and a large amount of ferrous ion
62
need to be applied to keep the sufficient hydroxyl radicals production.18 This results in the
63
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
3
ACS Paragon Plus Environment
Environmental Science & Technology
Page 4 of 35
67
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
72
leachate.25-28 When electrochemical oxidation is combined with Fenton process (namely
73
electro-Fenton (EF) process), Fenton’s reagent, i.e., hydrogen peroxide and ferrous ion, could
74
be in-situ electrogenerated (hydrogen peroxide) and electro-regenerated (ferrous iron as
75
catalyst).29-31 Compared to classical Fenton process, EF process needs significantly less
76
(catalytic amount) ferrous ion.19,29 A variant of EF process using a sacrificial iron anode,
77
namely anodic Fenton treatment, has been used for the treatment of landfill leachates.16,30,31 In
78
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
80
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
ACS Paragon Plus Environment
Page 5 of 35
Environmental Science & Technology
89
processes used (AO and EF) on the removal percentage of these organic micropollutants was
90
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
94
of residual refractory organic pollutants.33-35
95 96
2. Materials and methods
97
2.1 Landfill leachates samples
98
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
ACS Paragon Plus Environment
Environmental Science & Technology
Page 6 of 35
112
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
119
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
ACS Paragon Plus Environment
Page 7 of 35
Environmental Science & Technology
134
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
ACS Paragon Plus Environment
Environmental Science & Technology
156
Page 8 of 35
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
ACS Paragon Plus Environment
Page 9 of 35
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
Page 10 of 35
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
ACS Paragon Plus Environment
Page 11 of 35
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
Page 12 of 35
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
ACS Paragon Plus Environment
Page 13 of 35
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
Page 14 of 35
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
ACS Paragon Plus Environment
Page 15 of 35
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
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
ACS Paragon Plus Environment
Page 17 of 35
358
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
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
ACS Paragon Plus Environment
Page 19 of 35
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
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 (