Direct electrochemical oxidation of a pesticide, 2,4-dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement

Article abstract of DOI:10.1016/j.crci.2014.05.004

Pesticides' biorecalcitrance can be related to the presence of a complex aromatic chains or of specific bonds, such as halogenated bonds, which are the most widespread. In order to treat this pollution at its source, namely in the case of highly concentrated solutions, selective processes, such as electrochemical processes, can appear especially relevant to avoid the possible generation of toxic degradation products and to improve biodegrad- ability in view of a subsequent biological mineralization. 2,4-D was found to be electroactive in oxidation, but not in reduction, and the absence of hydroxyl radicals formation during the electrochemical step was demonstrated, showing that the pretreatment can be considered as a direct electrochemical process instead of an advanced electrochemical oxidation process. The presence of several degradation products in the oxidized effluent showed that the pretreatment was not as selective as expected. However, the relevance of the proposed combined process was confirmed since the overall mineralization yield was close to 93%.

Full text of DOI:10.1016/j.crci.2014.05.004

G Model
CRAS2C-3947; No. of Pages 7
C. R. Chimie xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Comptes Rendus Chimie
www.sciencedirect.com
´
Full paper/Memoire
Direct electrochemical oxidation of a pesticide,
2,4-dichlorophenoxyacetic acid, at the surface
of a graphite felt electrode: Biodegradability improvement
Jean-Marie Fontmorin a,b, Florence Fourcade a,b, Florence Geneste b,c
,
Isabelle Soutrel a,b, Didier Floner b,c, Abdeltif Amrane a, ,b
*
a
´
CNRS, UMR 6226, Ecole nationale supe´rieure de chimie de Rennes, Universite´ de Rennes-1, 11, alle´e de Beaulieu, CS 50837,
35708 Rennes cedex 7, France
b
´
`
´
`
´
´
CNRS, UMR 6226, Equipe « Matiere condensee et Systemes electroactifs », campus de Beaulieu, Universite de Rennes-1,
35042 Rennes cedex, France
c Universite´ europe´enne de Bretagne, 5, boulevard Lae¨nnec, 35000 Rennes, France
A R T I C L E I N F O
A B S T R A C T
Article history:
Pesticides’ biorecalcitrance can be related to the presence of a complex aromatic chains or
of specific bonds, such as halogenated bonds, which are the most widespread. In order to
treat this pollution at its source, namely in the case of highly concentrated solutions,
selective processes, such as electrochemical processes, can appear especially relevant to
avoid the possible generation of toxic degradation products and to improve biodegrad-
ability in view of a subsequent biological mineralization. 2,4-D was found to be
electroactive in oxidation, but not in reduction, and the absence of hydroxyl radicals
formation during the electrochemical step was demonstrated, showing that the
pretreatment can be considered as a ‘‘direct’’ electrochemical process instead of an
advanced electrochemical oxidation process. The presence of several degradation
products in the oxidized effluent showed that the pretreatment was not as selective as
expected. However, the relevance of the proposed combined process was confirmed since
the overall mineralization yield was close to 93%.
Received 25 February 2014
Accepted after revision 15 May 2014
Available online xxx
Keywords:
2,4-D
Combined process
Electrochemical process
Biological treatment
Carbon felt electrode
´
ß 2014 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
Regional Direction of the Environment (DIREN) observes
river contamination by phytosanitary products, including
The large accumulation of endocrine disruptors, in
continental and marine natural waters are the conse-
quence of the worldwide general application of intensive
agricultural methods, the large-scale development of the
agrochemical and food industry and the high levels found
in some specific effluents (textile, pharmaceutical, hospital
waste. . .). For instance, in the Brittany region (France), the
pesticides, which can interfere with hormone systems of
living beings (endocrine disruptors) [1]. Partly responsible
for this pollution, low volumes containing high concentra-
tions of persistent organic pollutants, in the range of
concentrations found in some specific industrial and
agricultural effluents (unused treatment solution, spray,
machine and container washing. . .) [2], can result in large
polluted volumes that are difficult to treat owing to their
low pollutant concentrations. One solution would be
therefore to treat pollution at its source, as intended in
this study.
*
Corresponding author.
E-mail address: abdeltif.amrane@univ-rennes1.fr (A. Amrane).
http://dx.doi.org/10.1016/j.crci.2014.05.004
1631-0748/ß 2014 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: Fontmorin J-M, et al. Direct electrochemical oxidation of a pesticide, 2,4-
dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement. C. R. Chimie
(2014), http://dx.doi.org/10.1016/j.crci.2014.05.004

G Model
CRAS2C-3947; No. of Pages 7
2
J.-M. Fontmorin et al. / C. R. Chimie xxx (2014) xxx–xxx
Pesticides’ impact on the environment is complex and
varied according to various factors, such as toxicity and
ecotoxicity of the parent molecule or by-product meta-
bolites, synergistic effects with other pollutants, length of
the half-life, exposure time and dose, etc. Various acute or
chronic poisoning effects on human health have been
described [3–6]. There is therefore an urgent need for
efficient processes for their removal and, owing to the
possible toxicity of the by-product metabolites, total
mineralization is mainly targeted.
For this purpose, biological processes, the most cost-
effective for wastewater treatment, which are destructive
and have been extensively studied [7–11], do not always
appear relevant for the removal of recalcitrant compounds,
owing to their low biodegradability.
Contrarily, physicochemical techniques have proved
their efficiency for such removal. Among them, advanced
oxidation processes constitute the most important and
widely documented group [2,12,13], owing to the high
reactivity of the free ꢀOH radicals produced. However, and
due to the lack of selectivity of the free radicals, possible
toxic by-products can be generated, which in some cases
can appear more toxic than the parent compounds [14,15].
Consequently, the mineralization time is of major impor-
tance; a too low processing time can result in toxic by-
products, while a long processing time to ensure total
mineralization can induce high-energy costs.
It is noteworthy that pesticides biorecalcitrance can be
related to the presence of a complex aromatic chain or to
the presence of specific bonds, such as nitro- or haloge-
nated bonds. For instance, among the most used pesticides,
a three-quarter of them contain halogenated bonds, and
the presence of chlorine atoms on phenyl ring is a factor
that favors the toxicity of aryl compounds [16]. From this,
the development of processes targeting a selective attack
of specific functional groups to improve the biodegrad-
ability of a given effluent can constitute another approach
to treat pesticide-containing effluents.
up to now our work seems to be the only one available
dealing with the combination of a direct electrochemical
process and a biological treatment. It was investigated for
the removal of phosmet, an organophosphorous insecticide
[23], some antibiotics, tetracycline [24], sulfamethazine
[25], and a chlorinated phenoxy herbicide, 2,4-dichloro-
phenoxyacetic acid [26,27].
The promising results obtained regarding 2,4-D should
be underlined [28,29], since halogenated pesticides are the
most widespread pesticides. Indeed, it was shown that
mild oxidation of the target compound can be sufficient to
improve biodegradability [28], allowing subsequent bio-
logical treatment [29]. However in view of the targeted
selectivity, the nature of the electrochemical process
should be clearly elucidated, namely the involvement or
not of free hydroxyl radials in pesticide oxidation; it is
discussed in this study.
However, the production of this highly reactive species
is closely linked to the electrode material used and to
some operating parameters such as oxygen overvoltage. If
some electrodes (boron-doped diamond electrodes for
instance) are well-known as powerful candidates for ꢀOH
generation, it is not at all obvious for some other materials,
such as graphite felt, the material used in the laboratory
[23,25–27]. Indeed, the use of a graphite felt working
electrode with a high specific area in a flow electro-
chemical cell [30] allows the electrochemical reaction of
the electroactive species at macroscale level with low
electrolysis times. Even if mechanisms involving ꢀOH have
been previously suggested for studies based on the use of
graphite felt [31], to our knowledge such production at the
surface of such electrode material has never been
demonstrated up to now. However, this question appears
to be of major importance to understand the electro-
chemical oxidation phenomena occurring when using
such material and to confirm the specificity and selectivity
of the considered processes, namely a direct reaction at
the electrode surface. For this purpose, three points have
to be considered, the nature of the working electrode, the
experimental conditions and the indirect determination
of the hydroxyl radicals; these points are investigated in
this study.
Indeed, if agricultural effluents are for instance
considered, pesticides levels can reach 500 mgꢁLꢂ1 [2], as
it is the case in farm bottom tanks. Hence, a selective attack
of specific functional groups can constitute a relevant
solution to relieve AOPs drawbacks. The generation of
possible toxic by-products could thus be avoided owing to
the expected control of the resulting by-products as well as
the high-energy costs, since total mineralization is not the
objective, due to the expected improvement of biodegrad-
ability.
In addition and to complete the mechanism’s knowl-
edge, the electrochemical reaction of 2,4-D at the electrode
was also examined.
Biological treatment involving activated sludge was
then considered to examine the efficiency of the electro-
chemical pretreatment.
For this purpose and in the case of an electroactive
target compound, its electrochemical oxidation or reduc-
tion can be carried out for its degradation. Total miner-
alization can be subsequently completed during biological
treatment, since the potential advantages of the strategy of
combining physicochemical and biological processes to
treat contaminants in wastewater were previously under-
lined [17–20]. However, the literature dealing with the use
of direct electrochemical oxidation/reduction for effluent
pretreatment remains scarce. Doan et al. [21] and Ghafari
et al. [22] coupled an electrochemical process and a
biological treatment for the removal of heavy metals and
nitrate, respectively, while regarding organic pollutants,
2. Materials and methods
2.1. Chemicals
2,4-Dichlorophenoxyacetic acid (2,4-D) (98%) was
purchased from Alfa Aesar (Schiltigheim, France). Chlor-
ohydroquinone (85%) was purchased from Sigma-Aldrich
(Saint-Quentin Fallavier, France). Acetonitrile (ACN) and
formic acid were LC/MS grade from JT Baker (Deventer,
Netherlands). All standards were prepared with ultra-pure
water (PurelabOptions-Q7/15, Elga, 18.2 M
Vꢁcm).
Please cite this article in press as: Fontmorin J-M, et al. Direct electrochemical oxidation of a pesticide, 2,4-
dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement. C. R. Chimie
(2014), http://dx.doi.org/10.1016/j.crci.2014.05.004

G Model
CRAS2C-3947; No. of Pages 7
J.-M. Fontmorin et al. / C. R. Chimie xxx (2014) xxx–xxx
3
2.2. Electrochemical pretreatment
2.4.1. Dissolved organic carbon (DOC) measurements
Solutions were filtered on Sartorius Stedim Minisart
0.45- m GF prefilters (Go¨ttingen, Germany). DOC was
measured by means of a Schimadzu TOC-VCPH/CPN Total
Organic Analyzer. Reproducible DOC values were always
obtained using the standard NPOC (Non-Purgeable Organic
Carbon) method.
The electrochemical pretreatment was based on a
home-made flow cell [28]. Graphite felt used as the
working electrode was supplied by Mersen (RVG 4000 –
m
´
Mersen, Paris La Defense, France) [27]. The dimensions of
the graphite felt were 48 mm in diameter and 12 mm in
width.
Two interconnected stainless steel plates were used
as counter-electrodes and compartments were separated
by cationic exchange membranes (Ionac 3470 – Lanxess
SAS, Courbevoie, France). The reference electrode (Satu-
rated Calomel Electrode–SCE) was positioned in the
middle of the graphite felt. To ensure a good homo-
geneity of the potential distribution in the three-
dimensional working electrode, the felt was located
between the two counter-electrodes [30]. The electrolyte
solution percolated the porous electrode and the flow
rate was monitored by a Gilson minipuls 2 peristaltic
pump (Middleton, WI, USA). A VersaSTAT 3 potentiostat
2.4.2. Chemical oxygen demand (COD) and biological oxygen
demand (BOD5)
Chemical oxygen demand (COD) was measured by
means of a Test Nanocolor1 CSB 160 from Macherey-Nagel
(Du¨ren, Germany).
BOD5 measurements were carried out in Oxitop IS6
`
(WTW, Ales, France). Activated sludge provided by a local
wastewater treatment plant (Rennes Beaurade, Bretagne,
France) was used to inoculate duplicate flasks and the
initial microbial concentration was set to 0.5 g Lꢂ1. More
details regarding the experimental procedure can be found
in previous papers [26,27].
´
(Ametek/Princeton Applied, Elancourt, France) was used
to control the potential.
3. Results and discussion
2.3. Biological process
3.1. Electrochemical behavior of 2,4-D
After only one pass through the electrochemical flow
cell, the effluent was collected for the subsequent
biological treatment, which was carried out in aerobic
conditions, using activated sludge purchased from the
local wastewater treatment plant (station de Beaurade,
Rennes, France). Before use and to avoid any residual
carbon or mineral nutrient, it was treated as previously
detailed [26].
The electrochemical behavior of 2,4-D in acidic medium
(pH ꢅ pKa = 2.8, naturally obtained when 2,4-D was
dissolved in water) showed an electroactivity in reduction
and in oxidation [27]. 2,4-D Electrolysis in reduction
showed a disappearance of the first reduction signal, which
was however not confirmed by HPLC analysis, which
indicated no 2,4-D degradation. The disappearance of the
reductive signal was found to be closely related to its
anionic or molecular form, and thus to the pH of the
medium, since the reductive signal disappeared at alkaline
final electrolysis pH (11), while it was recovered after
acidification of the medium to the initial pH value (2.8)
(Fig. 1).
Experiments were carried out in 250-mL erlenmeyer
flasks containing 100 mL of medium, stirred at 250 rpm,
kept at 30 8C and triplicated to ensure the reproducibility
of the results. The electrolyzed solution (initially 500
mgꢁLꢂ1 2,4-D) was diluted (five times dilution) to allow a
direct comparison with non-treated 2,4-D (100 mgꢁLꢂ1
–
This electrochemical behavior closely related to the pH
and coupled to the analytical results obtained by liquid
chromatography led to exclude the initial assumption of a
C–Cl bond cleavage and to consider instead a reduction of
[26]). Minerals were spiked in the medium as highly
concentrated solutions to reach the following initial
composition (mgꢁLꢂ1): Na2HPO4, 334; K2HPO4, 208;
KH2PO4, 85; CaCl2, 27.4; MgSO4.7H2O, 22.6; NH4Cl, 75;
FeCl3.6H2O, 0.26; and the initial pH was adjusted to
7.0 ꢃ 0.2. Activated sludge was added in order to have an
initial concentration of 0.5 g Lꢂ1 of dry matter. Samples
(5 mL) were taken regularly and filtered through 0.45 mm-
syringe filters for measurements.
2.4. Analysis
Measurements of the residual 2,4-D and chlorohydro-
quinone concentration were performed by an HPLC (High
Pressure Liquid Chromatography) (Milford, USA) system
involving a pump Waters 600, fitted with a Phenomenex
Kinetex1 C18 2.6
m
m column (4.6 mm ꢄ 100 mm), along
with a Waters 996 Photodiode array detector, a Waters
717 ph Autosampler and controlled through an Empower1
2 program. The mobile phase consisted of acetonitrile and
trifluoroacetic acid (TFA) 0.1% in ultra-pure water with a
Fig. 1. Cyclic voltammetry of a 2,4-D solution (500 mgꢁLꢂ1) at pH 2.8
(dots), at pH 11 (dashes) and at pH 2.8 after re-acidification (full line).
Voltammograms were recorded at 100 mVꢁsꢂ1 in Na2SO4 0.1 mol Lꢂ1 on a
glassy carbon electrode (7 mm2).
ratio of 30/70 at 1 mL minꢂ1
.
Please cite this article in press as: Fontmorin J-M, et al. Direct electrochemical oxidation of a pesticide, 2,4-
dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement. C. R. Chimie
(2014), http://dx.doi.org/10.1016/j.crci.2014.05.004

G Model
CRAS2C-3947; No. of Pages 7
4
J.-M. Fontmorin et al. / C. R. Chimie xxx (2014) xxx–xxx
Fig. 2. (Color online). Possible reactions for the reduction of the
Fig. 4. Cyclic voltammetry of Na2SO4 0.1 mol Lꢂ1 as supporting
electrolyte (dots), in the presence of 100 mgꢁLꢂ1 2,4-D at pH 3.6 (full
line), at pH 11 (empty dashes) and at pH 2.8 after re-acidification (full
dashes). Voltammograms were recorded at 100 mVꢁsꢂ1 on a glassy carbon
electrode (7 mm2).
carboxylic function of 2,4-D.
the carboxylic acid function of 2,4-D. Indeed, the
carboxylic acid can undergo a one-electron reaction to
yield the corresponding carboxylate anion, which is
accompanied by hydrogen release (Reaction 1 – Fig. 2),
and the reduction of the formed carboxylate appears
difficult in the considered solvent, water. The carboxylic
acid can also undergo 2-, 4- or 6-electron reductions to give
the corresponding aldehyde, alcohol or alkane, respec-
tively (reactions (2) to (4) in Fig. 2); these reactions can
however occur in acidic conditions avoiding the carbox-
ylate reduction [32].
The lack of a reduction wave was expected at alkaline
pH, since its anionic form largely predominated owing to
the 2,4-D pKa (2.8); this characteristic wave can only be
observed in the presence of the protonated form of the
compound, namely for pH values below 4.
(–1.75 V/ECS) than the initial compound (–1.5 V/ECS).
Consequently, the wave observed during 2,4-D reduction
was linked to the reduction of the acid to acetate instead of
the aromatic ring dechlorination.
The electrochemical behavior of 2,4-D was then
examined in oxidation, and hence a similar approach
was considered, namely cyclic voltammetry analysis prior
to electrolysis to confirm the pretreatment’s feasibility. An
oxidation wave was observed at 1.6 V/ECS (Fig. 4).
To confirm 2,4-D electroactivity in oxidation, the
absence of pH impact on its electrochemical behavior
was checked and confirmed, since the oxidation wave was
observed at both acidic and alkaline pHs (Fig. 4).
The transformation of the carboxylic acid into its ester
form should allow one to avoid its carboxylate reduction,
which would be helpful to validate the above assumption.
Thus, the diethyl ester of 2.4-D was prepared by the
reaction of 2.4-D with oxalyl chloride to form the acyl
chloride derivative and then with ethanol. However, it was
not possible to analyze the synthesized ester in water
owing to its low solubility, leading to perform cyclic
voltammetry analyses in acetonitrile. As shown in Fig. 3,
the synthesized ester was reduced at a lower potential
3.2. Nature of the electrochemical process
Since no 2,4-D degradation was detected during
cathodic reduction experiments in aqueous medium,
electrolyses were therefore performed in oxidation. How-
ever, one major question remained, which was the nature
of the electrochemical process involved in 2,4-D oxidation:
are free radicals involved in 2,4-D degradation?
Electrodes can be classified as ‘‘active’’ and ‘‘non-active’’
`
according to their reactivity vis-a-vis the reaction of
oxygen formation [33]. In a first step, a plane graphite
electrode was used to allow a fine observation of oxygen
formation on the material (graphite). The obtained results
were then compared to well-known electrode materials,
such as platinum or vitreous carbon. At pH 2.7, namely
close to the experimental conditions in the presence of
2,4-D, it can be observed an oxygen release from about
1.5–1.6 V/SCE on graphite and platinum and from
1.6–1.7 V/SCE for vitreous carbon (it should be remem-
bered that it is usually observed near 2.5 V/SCE on a BDD)
[34]. The shape of the curve recorded on graphite
suggested a resistive character of the material before
oxygen release, which may be related to the nature of the
electrode. This phenomenon was systematically observed,
irrespective of meticulous electrode polishing, long
degassing time or electrochemical surface cleaning,
namely a chronoamperometry at 0 V/SCE before cyclic
voltammetry analyses. This relatively early oxygen release
Fig. 3. Cyclic voltammetry of 2,4-D (20 mmol Lꢂ1–dots) and its ester form
(20 mmol Lꢂ1 – full line) in acetonitrile (KPF6 0.1 mol Lꢂ1 – blank: dashes)
on a glassy carbon electrode (7 mm2). Voltammograms were recorded at
100 mVꢁsꢂ1
.
Please cite this article in press as: Fontmorin J-M, et al. Direct electrochemical oxidation of a pesticide, 2,4-
dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement. C. R. Chimie
(2014), http://dx.doi.org/10.1016/j.crci.2014.05.004

G Model
CRAS2C-3947; No. of Pages 7
J.-M. Fontmorin et al. / C. R. Chimie xxx (2014) xxx–xxx
5
Fig. 6. Oxygen evolution observed in phosphate buffer (pH 2.7) on a plane
graphite electrode (full line, 257 mm2) and on a graphite felt for various
current densities: considering the geometric surface (full dashes), the BET
surface (empty dashes) and the electrochemical surface (dots). The scan
direction was indicated by the arrow. Voltammogram recorded at 100
Fig. 5. Oxygen evolution observed in phosphate buffer (pH 2.7) on
various electrode materials: graphite (full line, 12.6 mm2), vitreous
carbon (dashes, 7 mm2) and platinum (dots, 4.5 mm2). The scan direction
was indicated by the arrow. Voltammograms were recorded at 100
mVꢁsꢂ1
.
mVꢁsꢂ1
.
on graphite suggested an ‘‘active’’ electrode nature, which
was not in favor of ꢀOH production in the considered
experimental conditions.
area of the electrode owing to the high current density
obtained (Fig. 6). In addition to the external volume, the
internal felt volume seems therefore involved in the
electrochemical reaction. Contrarily, the BET surface seems
to overestimate the active surface area owing to the
negligible current density obtained. The electrochemical
surface appears consistent if the beginning of oxygen
release, occurring at 1.5–1.6 V/SCE, namely close to that
observed on a plane graphite felt, is considered.
From this, the oxidation potential of water on graphite
felt remains low (1.5–1.6 V/SCE), showing that it can be
considered as an ‘‘active’’ electrode and hence the nature of
the working electrodeꢀwas also in favor of a direct 2,4-D
oxidation rather than OH generation.
3.2.1. The nature of the working electrode
Current densities can vary according to the considered
surface as highlighted in Fig. 5. A cylindrical graphite felt of
10 mm in diameter and 12 mm in height was considered.
The total geometric surface of the cylinder was 5.34 cm2.
From BET analysis and the felt’s density (given by the
manufacturer), 0.7 m2ꢁgꢂ1 and 0.088 gꢁcmꢂ3, respectively,
the BET surface was 580 cm2 (the cylinder volume was
0.942 cm3), namely a ratio close to 109 between both
surfaces. However, it can be observed that the surface
given by BET measurements is most likely overestimated,
since it is deduced from the adsorbed nitrogen amount,
while, owing to its low size, nitrogen can penetrate pores
that are not accessible to water. Another probably more
realistic approach is to use cyclic voltammetry and
especially the oxidation (or reduction) current of potas-
sium ferricyanide (K3[Fe(CN)6], 4.9 mM) to estimate
the electrochemical electrode surface. Indeed, the ferri-
cyanide peak current is linked to the electrode surface
through the following relation [35]:
3.2.2. The operating conditions
The electrochemical oxidation of 2,4-D was performed
at 1.6 V/SCE. Only few studies involving chronoampero-
metry are available in the literature. However, analytical
studies show that the potentials corresponding to the
currents applied to BDD electrodes in view of herbicides
degradation can easily exceed 3.2 V/SCE [36], namely well
above the oxidation potential of water on a graphite
electrode.
The applied current densities varied widely according
to the electrode materials used, the operating conditions
(electrolyte and pollutant concentrations, temperature. . .),
but usually remain in the range of some tens to a hundred
mAꢁcmꢂ2 [37–39]. The difficulty to determine the current
density applied on the graphite felt has been previously
discussed. Indeed, for the graphite felt implemented in the
percolation flow cell (48 mm ꢄ 12 mm), the geometric
surface was 54.3 cm2 and those given by the BET and
electrochemically were 13,300 and 1270 cm2. From this
and for a usual current of 200 mA, the current density
values were 3.7, 0.015, and 0.157 mA cmꢂ2 for the
geometric, the BET, and the electrochemical surfaces,
respectively. Since the active surface should be considered
with caution, the current densities appeared low if
compared to those generally reported in the literature
regarding AOPs, namely involving hydroxyl radicals
formation (in the magnitude of some tens of mAꢁcmꢂ2).
ꢀ
ꢁ
1
1
n
1
2
/
2
2; 69 ꢄ 105
2AD0 C0ꢆ
v
(1)
/
/
ip
¼
with ip the peak current intensity, n the number of
exchanged electrons (1), A the electrode surface (cm2), D0
the ferricyanide diffusion coefficient (0.76 10ꢂ5 cm2ꢁsꢂ1 in
KCl 0.1 M), C0 the ferricyanide concentration (molꢁcmꢂ3
and n0 the sweep rate (0.1 Vꢁsꢂ1).
)
A 0.5 M phosphate buffer at pH 2.7 was used and an
approximate diffusion coefficient value of 0.76 ꢄ 10ꢂ5
cm2/s was considered for ferricyanide (in KCl 0.1 M at
25 8C). From this, the electrochemical surface was 55.05
cm2, namely between the geometric surface and that
deduced from the BET measure.
If compared to the plane graphite electrode case, the
more linear oxygen release observed on the working
electrode highlighted a lower conductivity (Fig. 6). The
geometric surface seems to minimize the active surface
Please cite this article in press as: Fontmorin J-M, et al. Direct electrochemical oxidation of a pesticide, 2,4-
dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement. C. R. Chimie
(2014), http://dx.doi.org/10.1016/j.crci.2014.05.004

G Model
CRAS2C-3947; No. of Pages 7
6
J.-M. Fontmorin et al. / C. R. Chimie xxx (2014) xxx–xxx
3.2.3. Dosage of hydroxyl radicals
whichanacclimationperiod of sevendayswasneeded before
noticeable 2,4-D degradation can be observed [26], no
significant lag phase can be noticed in the case of the
electrolyzed effluent. Indeed, a 66% decrease of the initial
DOC amount in the pretreated effluent was measured after
only two days of culture (Fig. 7), without significant
involvement of biosorption, illustrating a readily assimila-
tion of some degradation products. Mineralization continued
until day 5 of culture, since a 77% mineralization yield was
measured. Beyond this time, no really significant decrease of
the initial dissolved organic carbon was measured, since only
less than 10% further mineralization was obtained after 21
days of culture, leading to a final mineralization yield during
the biological treatment of 85%, namely 93% overall
mineralization yield by means of the combined process. It
should be observed that a biological treatment carried out in
similar conditions but lacking any carbon source, especially
2,4-D, was used as a ‘‘blank’’ test; its DOC value, namely 17
mgꢁLꢂ1, was considered as the reference (100% mineraliza-
tion) [26]. Residual 2,4-D and the main degradation product,
chlorohydroquinone, were also monitored, showing their
total removal was completed within the first two days of
biological treatment for the former and after 14 days for the
latter, in agreement with the total 2,4-D removal also
observed after 14 days of culture for the non-pretreated
effluent [26]. Owing to the presence of a non-negligible
residual DOC amount, the presence of refractory degradation
products in the oxidized effluent can be assumed, which was
notassimilatedeven after21 daysof activated sludgeculture.
Activated sludge acclimation to the by-products should be
therefore subsequently considered to improve the biological
mineralization of the electrolyzed solution.
The above arguments are based on analytical observa-
tions and assumptions; while the best proof of the
formation of radicals remains their dosage. ꢀOH are known
for their strong reactivity with benzene (kinetic rate
constant k = 8 ꢄ 109 Lꢁmolꢂ1ꢁsꢂ1), showing the relevance of
this compound as a radical scavenger [40,41]. Since phenol
is the sole product of benzene hydroxylation, the amount
of ꢀOH produced can be deduced from the HPLC monitoring
of benzene degradation (
l = 200 nm) linked to phenol
production ( = 270 nm). The absence of benzene oxida-
l
tion at the electrode at 1.6 V/SCE was first confirmed.
Electrolysis was then performed in the flow cell with
recycling of the electrolyzed solution. Neither noticeable
benzene degradation nor phenol formation was observed,
confirming the absence of ꢀOH formation in the considered
experimental conditions, despite the recycling of the
electrolysis solution through the cell to favor phenol
accumulation.
In the light of the above results, a possible involvement
of ꢀOH can be rejected in the mechanism of 2,4-D
oxidation. Consequently, the proposed pretreatment can
be considered as a ‘‘direct’’ electrochemical process instead
of an advanced electrochemical oxidation process.
3.3. Feasibility of the combined process
The BOD5/COD ratio increased from 0.04 initially to 0.25
for the oxidized solution. Owing to this biodegradability
improvement, and even if the limit of biodegradability (0.4
[19,42]) was not reached, a biological treatment of the
pretreated solution was performed.
Prior to the biological treatment step, possible biosorp-
tion on an activated sludge of 2,4-D, on the one hand, and
the major by-product, chlorohydroquinone [27], for the
electrolyzed effluent, on the other hand, was assessed and
found to be negligible [26].
The evolution of the mineralization, as well as of 2,4-D
and of chlorohydroquinone during activated sludgeculture is
displayed in Fig. 7; the low 2,4-D amount (Fig. 7) should
be related to its almost total removal at the end of the
electrochemical pretreatment (96% removal), while the
mineralization level remained limited, 34% [26]. Contrary
to the behavior found for non-pretreated 2,4-D solutions, for
4. Conclusion
The feasibility of an electrochemical process forpesticide
pretreatment was shown, sinceit improved significantlythe
mineralization rate and thus significantly shortened the
length of the biological treatment if compared to non-
pretreated 2,4-D solutions. The absence of hydroxyl radical
formation was demonstrated, showing that the pretreat-
ment can be considered as a ‘‘direct’’ electrochemical
process instead of an advanced electrochemical oxidation
process. However, its selectivity was not as high as expected
owing to the various degradation products identified [27]
and to the biorefractory degradation products remaining at
the end of activated sludge culture. Improving selectivity
constitutes the main objective in the continuation of this
work. For this purpose and since most of the persistent
organic pollutants are halogenated compounds, the specific
removal of halogen groups from the substrate could be
considered. Works are in progress in the laboratory
regarding the electrocatalytic reduction of carbon–halogen
bondsof some halogenatedtarget compounds, aswellas the
influence of the dehalogenation on the biodegradability of
these compounds.
Acknowledgments
Fig. 7. Time-courses of 2,4-D (circles), chlorohydroquinone (squares),
and dissolved organic carbon (triangles, right axis) concentrations during
activated sludge culture on oxidized 2,4-D solutions (five-fold dilution of
a 500 mgꢁLꢂ1 2,4-D solution electrolyzed at 1.6 V/SCE in 0.1 M Na2SO4).
We thank Dr Elisabet Dunach-Clinet for helpful
discussions.
Please cite this article in press as: Fontmorin J-M, et al. Direct electrochemical oxidation of a pesticide, 2,4-
dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement. C. R. Chimie
(2014), http://dx.doi.org/10.1016/j.crci.2014.05.004

G Model
CRAS2C-3947; No. of Pages 7
J.-M. Fontmorin et al. / C. R. Chimie xxx (2014) xxx–xxx
7
References
[22] S. Ghafari, M. Hasan, M.K. Aroua, Bioresour. Technol. 99 (2008) 3965.
[23] N. Alonso-Salles, F. Fourcade, F. Geneste, D. Floner, A. Amrane, J. Hazard.
Mater. 181 (2010) 617.
[24] D. Belkheiri, F. Fourcade, F. Geneste, D. Floner, H. Aı¨t-Amar, A. Amrane,
Sep. Purif. Technol. 83 (2011) 151.
[1] C. Ferragu, I. Tron, Pesticides et sante´: e´tat des connaissances sur les
effets chroniques en 2009, Observatoire re´gional de sante´, Bretagne,
2010.
[2] S. Chiron, A.R. Fernandez-Alba, A. Rodriguez, E. Garcia-Calvo, Water Res.
34 (2000) 366.
[25] I. Saidi, I. Soutrel, F. Fourcade, A. Amrane, D. Floner, N. Bellakhal, F.
Geneste, J. Electroanal. Chem. 707 (2013) 122.
[3] T.E. Arbuckle, Z. Lin, L.S. Mery, Environ. Health Perspect. 109 (2001).
[4] T.E. Arbuckle, L.E. Sever, Crit. Rev. Toxicol. 28 (1998) 229.
[5] L. Rolda´n-Tapia, T. Parro´n, F. Sa´nchez-Santed, Neurotoxicol. Teratol. 27
(2005) 259.
[26] J.-M. Fontmorin, S. Huguet, F. Fourcade, F. Geneste, D. Floner, A.
Amrane, Biochem. Eng. J. 70 (2013) 17.
[27] J.-M. Fontmorin, S. Huguet, F. Fourcade, D. Floner, F. Geneste, A.
Amrane, Chem. Eng. J. 195–196 (2012) 208.
`
[6] L. Lauria, L. Settimi, A. Spinelli, I. Figa-Talamanca, Reprod. Toxicol 22
[28] J.-M. Fontmorin, S. Huguet, F. Fourcade, F. Geneste, D. Floner, A.
Amrane, Chem. Eng. J. 195–196 (2012).
(2006) 425.
[7] B.V. Chang, C.M. Yang, C.H. Cheng, S.Y. Yuan, Chemosphere 55 (2004) 533.
[8] N. Dafale, N.N. Rao, S.U. Meshram, S.R. Wate, Bioresour. Technol. 99
(2008) 2552.
[29] J.-M. Fontmorin, F. Fourcade, F. Geneste, D. Floner, S. Huguet, A.
Amrane, Biochem. Eng. J. 70 (2013).
[30] C. Moinet, J. Phys. IV 4 (1994) 175.
[9] K.M. Crowe, A.A. Bushway, R.J. Bushway, K. Davis-Dentici, J. Food Sci. 72
(2007) 293.
[10] T. Eizuka, A. Ito, T. Chida, J. Pestic. Sci. 28 (2003).
[11] H.N. Gavala, F. Alatriste-Mondragon, R. Iranpour, B.K. Ahring, Chemo-
sphere 52 (2003) 673.
[12] T. Oppenla¨nder, Photochemical Purification of Water and Air. Advanced
Oxidation Processes (AOPs): Principles, Reaction Mechanisms, Reactor
Concepts, Wiley-VCH, 2003.
[31] A.M. Polcaro, S. Palmas, Ind. Eng. Chem. Res. 36 (1997) 1791.
[32] O. Hammerich, H. Lund, Organic electrochemistry, CRC Press, New
York, 2000.
[33] A.J. Bard, M. Stratmann, Encyclopedia of Electrochemistry, Wiley-VCH
Verlag GmbH & Co, KGaA, Weinheim, Germany, 2007.
[34] R.L. McCreery, Chem. Rev 108 (2008) 2646.
[35] A.J. Bard, L.R. Faulkner, Electrochemical methods: fundamentals and
applications, Wiley, New York, 2001.
[13] M.I. Badawy, M.Y. Ghaly, T.A. Gad-Allah, Desalination 194 (2006) 166.
[14] N.P. Tantak, S. Chaudhari, J. Hazard. Mater. 3 (2006) 698.
[15] D. Chebli, F. Fourcade, S. Brosillon, S. Nacef, A. Amrane, J. Chem. Technol.
Biotechnol. 85 (2010) 555.
[36] B. Boye, E. Brillas, B. Marselli, P.-A. Michaud, C. Comninellis, G. Farnia,
Electrochem. Acta 51 (2006) 2872.
[37] C. Flox, P.L. Cabot, F. Centellas, J.A. Garrido, R.M. Rodriguez, C. Arias,
Chemosphere 64 (2006) 892.
[16] R.W. Okey, H.D. Stensel, Water Res. 30 (1996) 2206.
[17] J.P. Scott, D.F. Ollis, Environ. Prog. 14 (1995) 88.
[18] J.P. Scott, D.F. Ollis, J. Adv. Oxid. Technol. 2 (1997) 374.
[19] C. Pulgarin, M. Invernizzi, S. Parra, V. Sarria, R. Polania, P. Pe´ringer, Catal.
Today 54 (1999) 341.
[38] E. Brillas, B. Boye, I. Sire´s, J.A. Garrido, R.M. Rodriguez, C. Arias, Electro-
chem. Acta 49 (2004) 4487.
[39] M. Panizza, in: C. Comninellis, G. Chen (Eds.), Electrochemistry for the
environment, Springer, New York, 2010, pp. 25–54.
[40] J. Kochany, J.R. Bolton, Environ. Sci. Technol. 26 (1992) 262.
[41] X.-M. Pan, M.N. Schuchmann, C. von Sonntag, J. Chem. Soc. Perkin Trans.
2 (1993) 289.
[20] M.J. Farre´, S. Brosillon, X. Dome`nech, J. Peral, J. Photochem. Photobiol. A
Chem. 189 (2007) 364.
´
[21] H.D. Doan, J. Wu, R. Mitzakov, J. Chem. Technol. Biotechnol. 81 (2006)
1398.
[42] V. Sarria, S. Parra, N. Adler, P. Peringer, N. Benitez, C. Pulgarin, Catal.
Today 76 (2002) 301.
Please cite this article in press as: Fontmorin J-M, et al. Direct electrochemical oxidation of a pesticide, 2,4-
dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement. C. R. Chimie
(2014), http://dx.doi.org/10.1016/j.crci.2014.05.004