Chemosphere 80 (2010) 845–851
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Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere
Comparative electrochemical degradation of phthalic acid esters using
boron-doped diamond and Pt anodes
Hongna Li, Xiuping Zhu, Yi Jiang, Jinren Ni *
Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China
a r t i c l e
i n f o
Article history:
Received 12 March 2010
Received in revised form 3 June 2010
Accepted 4 June 2010
Available online 29 June 2010
Keywords:
Electrochemical oxidation
Boron-doped diamond electrode
Phthalic acid esters
Wastewater treatment
a b s t r a c t
Phthalic acid esters (PAEs) are a group of endocrine disruptors commonly used as plasticizers. The present study compares the electrochemical oxidation of PAEs at boron-doped diamond (BDD) anode with
that at Pt anode. Both the degradation and the mineralization processes of PAEs became much slower
when using the Pt anode compared with that using the BDD anode. Moreover, the degradation rates of
PAEs decreased at the BDD anode but increased at the Pt anode with increasing alkyl chain length. This
was attributed to the different oxidation mechanisms at the two anodes. The BDD electrode has an inert
surface that holds a large amount of strong oxidants as free hydroxyl radicals (OH), causing electrophilic
attack by OH to be the main reaction. Therefore, degradation of PAEs became slower on the BDD anode
due to there being less available electronic energy as the alkyl chain length increased. However, adsorbed
oxidants (PtOx+1) with low oxidation ability tended to form on the surface of the active Pt anode. Therefore, PAEs with longer alkyl chains promote faster degradation owing to their stronger hydrophobicity.
Detection of intermediates in the GC/MS tests confirmed the above conclusion, in which oxidation of
dimethyl phthalate on BDD occurred on the aromatic ring at first, while the alkyl chain was preferentially
attacked on the Pt anode.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Phthalic acid esters (PAEs), also known as phthalates, are commonly used as plasticizers, and in the manufacture of insecticide
carriers, propellants, and cosmetics (Peakall, 1975; Gian et al.,
1984; Yuan et al., 2002). Over the past few decades, about 80 kinds
of PAEs have been produced by various industries (Fromme et al.,
2002). PAEs exist in a free state in plastic and other products,
and so readily enter the environment, being detectable even in
the Atlantic and Arctic Oceans (Xie et al., 2007). PAEs are relatively
stable in the natural environment (Staples et al., 1997). They have a
high octanol–water partition coefficient and hence may tend to
bioconcentrate in animal fat, promote chromosome injuries to human leucocytes, and interfere with the reproductive system (Jobling et al., 1995). Following the 1999 UNEP Protocol on LongRange Transboundary Air Pollution, a considerable body of knowledge (Jobling et al., 1995; Staples et al., 1997) has been built up
concerning about the characteristics of PAEs in terms of bioaccumulation, toxicity, environmental degradation, and adverse affects
on human health.
Much recent research effort has been directed towards determining effective methods for the degradation of PAEs. Several investiga* Corresponding author. Tel.: +86 010 6275 1185; fax: +86 010 6275 6526.
E-mail address: [email protected] (J. Ni).
0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2010.06.006
tions have considered the biodegradation processes of PAEs and
their by-products in aqueous media, and found that Pseudomonas
(Xu et al., 2005) and white rot fungus (Lee et al., 2007) are microorganisms that are particularly efficient at degradation of PAEs. Immobilized bacteria and other bio-augmented measures have also been
processed in order to strengthen the PAEs removal effect (Wang
et al., 1997; Wang et al., 2004). It has been suggested (Wang et al.,
1997; Xu et al., 2005; Lee et al., 2007) that monoesters and phthalic
acid are the main intermediates in PAEs degradation. Biodegradation experiments involving different PAEs have indicated that those
with shorter alkyl chains such as dimethyl phthalate (DMP) and
diethyl phthalate (DEP) are very easily biodegraded, whereas PAEs
with longer alkyl chains are poorly degraded with some considered
resistant to biological treatment (Ejlertsson et al., 1997; Chang et al.,
2004; Wang et al., 2004; Roslev et al., 2007). Another study of PAEs
biodegradation found that the degradation process could take from
several days to a few months and so be only capable of handling
trace to minor concentrations of these compounds with risk of generating secondary pollution (Staples et al., 1997).
Advanced oxidation processes (AOPs) of PAEs involve several
treatment procedures that are characterized by the in situ generation of the hydroxyl radical (OH). These include hydrothermal oxidation (Onwudili and Williams, 2007), photochemical degradation
with UV or UV/H2O2 (Lau et al., 2005; Xu et al., 2007), photocatalysis with Fe (III) (Mailhot et al., 2002) or TiO2 (Yuan et al., 2008) as
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H. Li et al. / Chemosphere 80 (2010) 845–851
the catalyst, sonication methods (Psillakis et al., 2004), and electrochemical oxidation (Hou et al., 2009; Kabdasßlı et al., 2009; Trabelsi
et al., 2009). Among these methods, electrochemical oxidation is a
promising environmentally clean process that is versatile, energy
efficient and can be automated. As part of the electrochemical process, organic compounds are broken down by means of the hydroxyl radical formed through water oxidation at the surface of a high
O2-overpotential anode (Marselli et al., 2003; Panizza and Cerisola,
2005). The reaction equation is as follows:
M þ H2 O ! Mð OHÞ þ Hþ þ e
ð1Þ
where M refers to the anode. As a consequence, the most important factors in implementing electrochemical technology for the
effective degradation of organic pollutants are usually the electrode material and its stability. Numerous experiments have been
performed to degrade organic pollutants through anodic oxidation
with different electrode materials. Classical anodes such as Pt,
graphite and oxide films including PbO2, SnO2, and IrO2, have
all been used for anodic oxidation but produce a poor degradation
effect due to weak generation of hydroxyl radicals. Pt tends to
form an adsorbed film on the surface which deactivates the anode
and reduces the useful lifespan of the electrodes. Compared to
conventional anodes, boron-doped diamond (BDD) anodes have
the following advantages: extremely wide potential window, corrosion stability, inert surface, high O2-overpotential, and strong
oxidation capacity (Marselli et al., 2003; Panizza and Cerisola,
2005; Jeong et al., 2009). BDD anodes have been demonstrated
to give a superior performance than conventional anodes applied
to the treatment of many kinds of organic compounds (Brillas
et al., 2007; Hammami et al., 2008; Cui et al., 2009; Hamza
et al., 2009).
Nevertheless, except for several studies published recently concerning electrochemical degradation of PAEs (Hou et al., 2009;
Kabdasßlı et al., 2009; Trabelsi et al., 2009), to the authors’ knowledge little attention has been paid to date to the electro-degradation of PAEs at the BDD anode. The present paper presents a
study of eight PAEs with gradually increasing alkyl chains, including dimethyl phthalate, diethyl phthalate, dipropyl phthalate
(DPrP), dibutyl phthalate (DBP), diamyl phthalate (DAP), dihexyl
phthalate (DHexP), diheptyl phthalate (DHepP), and dioctyl
phthalate (DOP), on both BDD and Pt anodes as a comparison. It
was found that the eight PAEs were completely degraded using
the BDD anode, unlike the Pt anode. Besides, the removal of COD
and organic constituents of the PAEs was more rapid using the
BDD anode than for the Pt anode. Moreover, the degradation rates
were related to the structure of PAEs, whereby the rates decreased
at the BDD anode and increased at the Pt anode as the alkyl chain
length increased.
2. Experimental
2.1. Chemicals
DMP (CAS: 131-11-3), DEP (CAS: 84-66-2), DPrP (CAS: 131-168), DBP (CAS: 84-74-2), DAP (CAS: 131-18-0), DHexP (CAS: 84-753), DHepP (CAS: 3648-21-3), DOP (CAS: 117-84-0) were analytical
standard material supplied by Dr. Ehrenstorfer (German) with a
purity of 99%. Organic solvent was HPLC grade supplied by Sigma
(St. Louis). All other chemicals were analytical grade purchased
from Beijing Chemical, and they were used without further purification. Solutions were prepared using deionized Milli-Q water
(Millipore). The BDD electrode was provided by CONDIAS GmbH,
Germany.
2.2. Bulk electrolysis
Bulk electrolysis was carried out under galvanostatic conditions
at room temperature (20 °C) in a 400 mL beaker equipped with two
electrodes, BDD or Pt with an exposed geometric area of 1 cm2 as
the anode and stainless steel as the cathode. Eight PAEs were separately electrolysed as the substrate with a concentration of
0.03 mM. Higher DMP concentration than those found in sewage
treatment plant (STP) and natural effluents was chosen to evaluate
better the oxidation ability of the BDD anode and the concentration
was identified according to the water solubility experiment of the
eight PAEs.
The electrode gap was set to 10 mm and the current density
was kept constant at 20 mA cm2. 0.2 M Na2SO4 was introduced
into the initial electrochemical system as the supporting electrolyte. The selection of the operative parameters such as current density and the electrolyte concentration was originated from the
optimized parameters in other previous research (Chen et al.,
2003; Tian et al., 2006; Zhu et al., 2007). The 250 mL electrolytic
system was continuously stirred by a magnetic bar throughout
the process. Samples were collected from the cell at prescribed
intervals for chemical analysis. Before the experiments started,
the BDD electrode was subjected to ultrasound for 5 min to remove
contaminants, and then washed with deionized water; the Pt electrode was polished and then washed with deionized water. A blank
control with only 0.2 M Na2SO4 in 250 mL solution was prepared
for each electrolysis test.
2.3. Analytical procedures
Concentrations of the PAEs were monitored by an Agilent
HP1100 HPLC instrument with a DAD detector and a ZORBAX SBC18 column at 30 °C. The mobile phase comprised a mixture of
methanol and water, flowing at a rate of 1 mL min1 at isocratic
mode. The methanol:water (v:v) was 70:30 for DMP and DEP,
80:20 for DPrP and DBP, 85:15 for DAP and DHexP, and 88:12 for
DHepP and DOP, respectively. The UV detector was set at
230 nm. The measured concentration time history, and hence degradation rate, of the eight PAEs was derived from the HPLC data.
COD was measured by a titrimetric method, using dichromate as
the oxidant in acidic solution at 150 °C for 2 h (Hachi, USA).
In order to detect the intermediates of DMP degradation, a liquid–liquid solvent extraction was applied to concentrate the samples. The extraction was carried out three times with ethyl acetate
in a 250 mL separating funnel. After extraction, the solvent was
evaporated on a rotary evaporator to a known volume and was
ready for analysis by gas chromatography (Agilent 6890) and mass
spectrometry (Agilent 5973), using an HP-5MS (crosslinked 5% PH
ME Siloxane) column with a length of 30 m and a film thickness of
0.25 mm.
The conditions for GC were as follows: helium as the carrier gas,
flow rate of 1 mL min1, injector temperature 280 °C, detector temperature 285 °C, oven temperature initially 50 °C for 10 min and
increased to 140 °C by 10 °C min1 for 5 min, increased to 160 °C
by 4 °C min1, increased again to 290 °C by 10 °C min1 and held
for 2 min. The effluent from the GC column was connected to
MS, and the spectra were obtained by EI mode, 70 eV ionization energy, and 50–550 amu scan for 2 s. The degradation products of
DMP were identified through comparison with the mass spectra library stored in the MS system.
2.4. Multifactor analysis
In the multifactor analysis, fourteen kinds of molecular structure descriptors of the eight PAEs were identified, including molecular connectivity indices, obtained from the online database of
H. Li et al. / Chemosphere 80 (2010) 845–851
847
Milano Chemometrics and QSAR Research Group (http://
michem.disat.unimib.it/mole_db/); parameters of physical chemistry, extracted from Staples et al. (1997) and parameters of quantum chemistry (calculated by software Chemoffice 2004). The
stepwise regression analysis was carried out using Statistical Product and Service Solutions (SPSS) Statistics 17.0 software.
3. Results and discussion
3.1. Bulk electrolysis
Electrolysis was performed on 0.03 mM different PAEs in a
250 mL solution at an initial pH of 7. Fig. 1 presents time histories
of the COD ratios measured during the electrochemical degradation of PAEs at BDD and Pt anodes. Fig. 2 shows the concentration
decay of the eight PAEs at the two anodes.
For each phthalate, the electrochemical oxidation processed faster on the BDD anode than the Pt anode for both COD removal and
substrate concentration decay, indicating that BDD had a much
stronger oxidizing ability than Pt, in accordance with the findings
of other studies (Brillas et al., 2004; Hammami et al., 2008; Hamza
et al., 2009). For example, DMP was completely oxidized in 20 min
with a COD removal of 50% at the BDD anode, whereas under the
Fig. 2. Decay of PAEs with electrolysis time. Initial solution contains 0.03 mM
substrate with 0.2 M Na2SO4 at pH 7. Anode: (a) BDD, (b) Pt; cathode, stainless
steel; j = 20 mA cm2. Symbols: DMP (j), DEP (d), DPrP (N), DBP (), DAP (h),
DHexP (s), DHepP (4), DOP (}).
Fig. 1. COD removal of eight PAEs as a function of time. Initial solution contains
0.03 mM substrate with 0.2 M Na2SO4 at pH 7. Anode: (a) BDD, (b) Pt; cathode,
stainless steel; j = 20 mA cm2. Symbols: DMP (j), DEP (d), DPrP (N), DBP (), DAP
(h), DHexP (s), DHepP (4), DOP (}).
same conditions DMP concentration decay was a mere 3% with a
COD removal of only 1% at the Pt anode. This trend was due to
the different actions of oxidants at the two anodes. The obviously
faster mineralization rate observed at the BDD anode could be
attributed to ample OH produced at the BDD surface according
to reaction (1). Compared with the Pt anode, the BDD anode presented a much larger electrochemical window, ranging from
2.8 V for oxygen evolution to about 1.3 V for hydrogen generation
(Tröster et al., 2002). This promoted production of hydroxyl radicals with a small amount of oxygen generated by side reactions.
In general, the degradation at the BDD anode implies that organic
compounds are oxidized by free OH. This was because the surface
of the BDD anode was inert, and so the BDD(OH) was less strongly
adsorbed on the electrode surface than the oxide PtOx+1 formed on
the Pt anode (Brillas et al., 2007; Zhu et al., 2008). In contrast, electrolysis with the Pt anode did not permit total degradation. On the
Pt anode, adsorbed hydroxyl radicals were formed through the
transfer from water molecular discharge on the electrode surface,
through which adsorbed radical species such as O, OOH, OH were
produced and PtOx created. Then in the reaction of PtOx with adsorbed hydroxyl radicals, higher oxide sites PtOx+1 formed, which
then oxidized the organic compounds (de Lima Leite et al., 2003).
The generation of PtOx+1 was accompanied by oxygen evolution.
Given the smaller amount of adsorbed PtOx+1 and its much lower
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H. Li et al. / Chemosphere 80 (2010) 845–851
oxidizing capability compared with that of OH on the BDD anode,
the Pt anode was less able to destroy the phthalates.
The concentration decays in Fig. 2 fit well with a degradation kinetic equation related to a pseudo-first-order reaction of the PAEs.
Table 1 lists the degradation rate constants for DMP, DEP, DPrP,
DBP, DAP, DHexP, DHepP and DOP at the BDD and Pt anodes.
Fig. 3 shows the relationships between the degradation rate constants (k) and the number of carbon atoms on the alkyl chain of
PAEs (nC) obtained for the BDD and Pt anodes. Very strong linear
correlations are obtained between k and nC for both the BDD and
Pt anodes; the square of the correlation coefficient r2 was 0.993
(negative correlation) for the BDD anode and 0.987 (positive correlation) for the Pt anode. It may be discerned that an increase of nC
led to a reduced k for the BDD anode but an increased k for the Pt
anode. The relative reactivity of the compounds present in the
aqueous solution can be explained in terms of their physicochemical properties in conjunction with the electrochemical reaction
mechanisms at the two anodes. As the alkyl chain lengths increase,
the PAEs become more and more hydrophobic as their molecular
weight also increases, making them increasingly easier to adsorb
on the anode. BDD exhibits chemical inertness and, thus its surface
does not favor adsorption of organic pollutants or interaction with
the hydroxyl radicals (Gandini et al., 1999). As a result, PAEs with
longer alkyl chains exhibited a slower degradation rate at the BDD
anode. As for the Pt anode, the higher oxide PtOx+1 formed on the
active electrode surface promoted the oxidation of adsorbed organics (Brillas et al., 2004; Brillas et al., 2007). Hence, PAEs with longer
alkyl chains degraded relatively faster on the Pt anode.
3.2. Multifactor analysis
A multifactor analysis was undertaken involving the molecular
descriptors and the degradation rate constants. Although many
molecular structural descriptors can be devised, not all of them reveal information about the properties of interest. Appropriate
choice of descriptors is therefore vital to multifactor analysis. Herein, the factors were selected following Boethling and Sabljić (1989),
and included molecular connectivity indices (v), octanol–water
partition coefficient (Koc), and molecular weight (MW). The molecular connectivity indices represent the molecular structure by
different mathematical models, and compile the resulting information into single numbers indicating the number and type of bonds
as well as the pattern of bonding. This proven, well-established approach has been used in many studies (Raymond et al., 2001). In
accordance with the rules governing Quantitative Structure–Properties Relationships, each group of variables was examined and the
most relevant variable(s) was (were) rejected. We implemented
the method by undertaking correlation analysis of 24 molecular
connectivity indices (obtained from the online database of Milano
Chemometrics and QSAR Research Group) and then identifying the
five descriptors that had the least correlation values. Other descriptors were obtained from quantum mechanical procedures; namely,
Table 1
Pseudo-first-order rate constants for electrolysis of PAEs at different anodes at
20 mA cm2.
Substrate
DMP
DEP
DPrP
DBP
DAP
DHexP
DHepP
DOP
BDD
Pt
k (h1)
r2
k (h1)
r2
6.39
5.31
4.65
3.69
2.94
2.13
1.48
0.93
0.940
0.989
0.983
0.979
0.920
0.967
0.962
0.973
0.01
0.04
0.10
0.15
0.17
0.25
0.27
0.34
0.986
0.993
0.997
0.987
0.954
0.975
0.985
0.901
Fig. 3. Relationships between k and nC (the number of carbon atoms on the alkyl
chain). Symbols: BDD (s), Pt (h).
heat of formation (DHf, kJ mol1), electronic energy (EE, eV), Core–
core Repulsion (CCR, eV), dipole moment (l, Debye), and energy of
the highest/lowest occupied molecular orbital (EHOMO/ELUMO, eV).
These descriptors were calculated from energy-minimized threedimensional conformations, optimized using the semi-empirical
PM3 method implemented by Chemoffice 2004 software. Moreover, diffusion coefficient (D, cm2 s1) measured according to Cynthia (2007) was also included, as for diffusion is the sole form of
mass transport in electrochemical experiments often designed. Table 2 lists all the descriptors used in the test.
Stepwise regression analysis by SPSS was used to select the
variables. After stepwise regression, EE and MW were selected to
model the BDD and Pt anodes, respectively. The resulting empirical
models are:
BDD : k ¼ 0:002EE þ 10:7
ð2Þ
Pt : k ¼ 0:002MW 0:3
ð3Þ
The square of the correlation coefficient (r2) was 0.998 for the
BDD anode and 0.983 for the Pt anode, mean square errors were
0.024 and 0.008 and the corresponding Fisher values (F, level of
statistical significance) were 2432 and 344, respectively. Absence
of diffusion coefficient in both models indicated that mass transfer
limitations did not play a key role in the electrochemical degradation processes of PAEs. This confirmed that the pseudo-first order
rate constant k, which describe the degradation rate of PAEs, do
correspond to electrochemical degradation, not mass transport kinetic process.
The hydroxyl radical has large electronic affinity (569 kJ), and so
is able easily to attack regions of the organic compound structure
that have higher electron density (Uri, 1952). This implies that
the hydroxyl radical is electrophilic (Rodgers et al., 1999). Zhu
et al. (2007) investigated the effect of the electronic nature of substituents on the electrochemical reactivity of compounds using a
BDD electrode. They found that p-substituted phenols with electron-withdrawing groups degraded faster than p-substituted phenols with electron-donating groups, and that the degradation
was mainly carried out by indirect electrochemical oxidation with
hydroxyl radicals at BDD. In other words, electron-withdrawing
groups, which are straightforward to be released, make the organic
compounds preferentially degraded. Hence, the electrochemical
oxidation implemented by OH in the BDD system was an electrophilic reaction. The higher the electronic energy, the faster the organic compounds degraded. As the alkyl chain length increased,
the electronic energy of the PAE molecules reduced, as did their
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H. Li et al. / Chemosphere 80 (2010) 845–851
Table 2
Data matrix used for the multiple factors analysis.
Substrate
k-BDD (h1)
DMP
DEP
DPrP
DBP
DAP
DHexP
DHepP
DOP
6.39
5.31
4.65
3.69
2.94
2.13
1.48
0.93
Substrate
5
DMP
DEP
DPrP
DBP
DAP
DHexP
DHepP
DOP
0.023
0.023
0.024
0.026
0.028
0.034
0.038
0.042
vvA
k-Pt (h1)
0.01
0.04
0.10
0.15
0.17
0.25
0.26
0.34
MW
194.2
222.2
250.3
278.4
312.4
334.5
362.5
390.6
3
Koc
1.61
2.38
3.27
4.45
3.23
6.30
7.60
8.54
vA
5
vA
0.201
0.193
0.199
0.202
0.205
0.208
0.211
0.213
0.087
0.084
0.089
0.087
0.090
0.091
0.092
0.094
5
vv
1
0.637
0.740
0.880
0.970
1.132
1.425
1.625
1.925
vvA
0.28
0.32
0.34
0.36
0.37
0.38
0.39
0.40
DHf (kJ mol1)
EE (eV)
CCR (eV)
l (eV)
EHOMO (eV)
ELUMO (eV)
D 105 (cm2 s1)
155.40
165.62
175.33
181.59
196.85
207.89
218.89
229.93
2029.40
2400.72
2820.46
3179.27
3574.13
3908.25
4189.34
4503.19
484.77
412.51
291.79
231.87
136.29
101.25
119.25
104.49
2.30
5.02
3.44
2.57
1.85
1.87
1.85
1.87
10.28
10.28
10.26
10.24
10.30
10.30
10.31
10.31
0.64
0.68
0.80
0.88
0.65
0.65
0.65
0.65
2.04
0.78
0.19
0.94
0.63
0.34
0.50
0.34
Table 3
Main intermediates in the electrolysis of DMP on BDD and Pt anode.
Retention time (min)
Detected ions m/z (% abundance)
Molecular weight
10.705a
115(100), 55(36), 87(18), 15(22)
146
Molecular structure
O
O
CH3
O
CH3
O
13.583b
120(100), 92(72), 152(57), 65(19)
O
152
O
CH3
OH
15.371b
120(100), 92(77), 139(40),64(37)
O
138
OH
16.133a,b
104(100), 76(74), 50(27), 148(15)
OH
O
148
O
O
21.594c
22.978b
163(100), 77(20), 92(10), 194(7)
114(100), 76(100), 50(72), 149(42)
O
194
O
CH3
O
CH3
O
CH3
O
O
180
OH
O
28.624a
179(100), 210(20), 149(8), 108(6)
O
210
OH
a
b
c
Intermediates on the BDD anode.
Intermediates on the Pt anode.
DMP.
O
O
CH 3
O
CH 3
850
H. Li et al. / Chemosphere 80 (2010) 845–851
Fig. 4. Degradation mechanisms of DMP on BDD (a) and Pt (b) anodes.
oxidation. The positive correlation between k and EE in the BDD
model illustrates this point.
On the Pt anode, it appears that the high-molecular-weight
DHepP, DOP were all readily susceptible to electrochemical degradation. Conversely, the low-molecular-weight DMP and DEP were
more recalcitrant and so complete degradation would need a much
more prolonged electrolysis. Similar degradation behavior of the
PAEs has also been detected in other studies (Ooka et al., 2003;
Psillakis et al., 2004), all of whom find the ones with high-molecular-weight degraded faster. This phenomenon may be explained in
terms of hydrophobicity of PAEs, which gradually increases as the
PAEs molecular weight increases, causing the adsorption on the Pt
surface to be raised, thus enhancing the degradation process. This
is confirmed by the satisfactory correlation between Koc and the
degradation rate constants k obtained for both anodes, data not
shown here. The correlation is positive for the Pt anode but negative for the BDD anode, providing a further explanation for the different mechanisms. Although Koc was not included in the Pt model,
the partial correlation coefficient between k-Pt and Koc (r2 = 0.92)
was only second to that between k-Pt and MW (r2 = 0.98). An increase in Koc leads to an improvement of the hydrophobic quality
of the organic compounds, and hence increases adsorption at the
electrode surface. On the Pt anode, an increase in adsorption of organic compounds enhances the interaction between PtOx+1 and the
organic compounds, resulting in an increase in the degradation
rate constants. On the BDD anode, OH exists in a free state in
the bulk electrolyte, thus causing the degradation rate constants
to decrease monotonically with increase in Koc.
3.3. DMP degradation mechanisms
The present analytical results of the intermediate products of
DMP electrolysis listed in Table 3 generally agree with those reported previously.
Many researchers report the mechanisms of OH on aromatic
derivatives, in which the aromatic ring is readily attacked by OH,
leading to the formation of hydroxylated derivatives (Oturan and
Pinson, 1995; Gozmen et al., 2003; Cui et al., 2009). With BDD anode, DMP underwent a possible attack of OH on its C(4)-position
yielding 4-hydroxy-1,2-benzoic dicarboxylic acid, dimethyl ester.
Then the aromatic ring was broken down and succinic acid, dimethyl ester formed. After that succinic acid generated referring
to the process put forward by Hou et al. (2009). Finally, succinic
acid was mineralized to carbon dioxide and water. Meantime,
demethylation of DMP also occurs in the beginning to give phthalic
acid anhydride, which might then converted to carbon dioxide and
water by OH gradually in the oxidation process.
It was deduced that the alkyl chain rather than the aromatic
ring of DMP was preferred attacked with Pt as the anode. This phenomenon was also detected in the photodegradation of PAEs. With
Pt anode, monomethyl phthalate (MMP) was firstly formed in the
electrolysis of DMP. Then the carboxyl chain was substituted by
OH, forming methyl salicylate and subsequently salicylic acid.
Similarly with BDD, phthalic acid anhydride also formed at the
start. The final oxidation was the mineralization of salicylic acid
to carbon dioxide and water.
It is the different degradation mechanism that leads to the different degradation order on BDD and Pt anode for the eight PAEs
with increasing alkyl chain length. The main degradation pathway
is illustrated in Fig. 4.
4. Conclusions
Electrochemical oxidation of PAEs under optimized electrolysis
condition at both BDD and Pt anodes was investigated in this work.
Complete degradation and mineralization of PAEs were realized on
BDD due to a large amount of strong oxidants as free hydroxyl radicals in the system. However, the degradation processes became
H. Li et al. / Chemosphere 80 (2010) 845–851
much slower when using the Pt anode due to the weak oxidation
ability of adsorbed oxidants (PtOx+1) which was produced from
the instant reaction of OH on its surface with the created PtOx in
electrolysis. Besides, degradation rates of PAEs decreased on BDD
with the increase of the alkyl chain lengths of the chemicals, while
it increased on Pt. This was attributed to the different oxidation
mechanisms at the two anodes, which was confirmed by the detection in the GC/MS tests. On BDD anode, the aromatic ring was attacked firstly; and on Pt anode, the alkyl chain was preferentially
oxidized.
Acknowledgements
This research was funded by the National Natural Science Foundation of China through Grant No. 20877001.
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