A novel Electro-Fenton-Like system using PW11O 39Fe(III)(H2O)4- as an electrocatalyst for wastewater treatment

Article abstract of DOI:10.1016/j.electacta.2010.05.094

A novel Electro-Fenton-Like (EFL) system was developed using the Keggin-type iron-substituted heteropolytungstate anion PW₁₁O ₃₉Fe(III)(H₂O)^4- to substitute for Fe ³⁺ in the conventional Electro-Fenton (EF) system for treatment of water polluted with organic compounds. The EFL system overcomes the drawback of low pH in conventional EF approaches and can be directly applied to neutral water treatment without any pH adjustment. Experimental results for dimethylphthalate (DMP) revealed complete degradation in <80 min in pH 6.86 solution containing 0.1 mM DMP at a potential of -0.5 V and O₂ flow rate of 60 mL min^-1. Total organic carbon removal of ～56% was achieved at 120 min. Comparison with conventional EF oxidation revealed better efficiency of the present system for DMP degradation, suggesting its potential in treatment of water and wastewater with a relaxed pH requirement. The cumulative H₂O₂ concentration generated in situ at the electrode was monitored and the observed degradation rate constants k _obs were determined for different initial DMP concentrations. The ligand exchange reaction of PW₁₁O₃₉Fe(III)(H ₂O)^4- with H₂O₂ and the electron transfer resulting in hydroxyl radicals were examined using HPLC and electrochemical impedance spectroscopy. An electrocatalytic model involving inner-sphere electron transfer and a reaction mechanism for PW ₁₁O₃₉Fe(III)(H₂O)^4- electrocatalytic reduction of H₂O₂ are proposed.

Full text of DOI:10.1016/j.electacta.2010.05.094

Electrochimica Acta 55 (2010) 6755–6760
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
4
−
A novel Electro-Fenton-Like system using PW O Fe(III)(H O) as an
1
1
39
2
electrocatalyst for wastewater treatment
Chongtai Wang , Yingjie Hua , Yexiang Tongb
a
a,∗
a
School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, Hainan Province, PR China
Institute of Optoelectronic and Functional Composite Materials, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel Electro-Fenton-Like (EFL) system was developed using the Keggin-type iron-substituted het-
Received 14 April 2010
Received in revised form 27 May 2010
Accepted 29 May 2010
4−
3+
eropolytungstate anion PW11O39Fe(III)(H2O) to substitute for Fe in the conventional Electro-Fenton
EF) system for treatment of water polluted with organic compounds. The EFL system overcomes the
(
drawback of low pH in conventional EF approaches and can be directly applied to neutral water treat-
ment without any pH adjustment. Experimental results for dimethylphthalate (DMP) revealed complete
degradation in <80 min in pH 6.86 solution containing 0.1 mM DMP at a potential of −0.5 V and O2 flow
Available online 8 June 2010
Keywords:
Iron-substituted heteropolytungstate
Electrocatalysis
Dimethylphthalate degradation
Electro-Fenton
Total organic carbon removal
−1
rate of 60 mL min . Total organic carbon removal of ∼56% was achieved at 120 min. Comparison with
conventional EF oxidation revealed better efficiency of the present system for DMP degradation, suggest-
ing its potential in treatment of water and wastewater with a relaxed pH requirement. The cumulative
H2O2 concentration generated in situ at the electrode was monitored and the observed degradation
rate constants kobs were determined for different initial DMP concentrations. The ligand exchange reac-
4
−
tion of PW11O39Fe(III)(H2O) with H2O2 and the electron transfer resulting in hydroxyl radicals were
examined using HPLC and electrochemical impedance spectroscopy. An electrocatalytic model involv-
4−
ing inner-sphere electron transfer and a reaction mechanism for PW11O39Fe(III)(H2O) electrocatalytic
reduction of H2O2 are proposed.
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
[17,18]. Unfortunately, most contaminated waters are nearly neu-
tral in most cases. Therefore, this inherent drawback limits the
application of conventional EF systems. As a strategy to overcome
this limitation, a novel EFL system with a relaxed pH requirement
The Keggin-type iron-substituted heteropolytungstate anion
4−
PW11O39Fe(III)(H O) [PW11Fe(III)(H O)] is an excellent electro-
catalyst for H O2 reduction to hydroxyl radicals at neutral pH [1].
2
2
3+
was developed using PW11Fe(III)(H O) to substitute for Fe in
2
2
In view of other properties of PW11Fe(III)(H O), such as stabil-
the conventional EF system. Although systems containing EDTA,
diethylenetriaminepentaacetate and Fe3+ are also suitable for a
wide pH range, these organic chelators seem to be susceptible to
2
ity in aqueous solution at pH 2–8 and high resistance to strong
oxidation [2–5], as well as convenient separation from solution
using adsorption on D301R anion exchange resin [6], a novel
•
oxidation caused by HO radicals [19,20].
Electro-Fenton-Like (EFL) system using PW11Fe(III)(H O) as an
To assess the novel EFL system, dimethylphthalate (DMP) was
chosen as a model contaminant because it is representative of
di-alkyl phthalate esters, endocrine-disrupting chemicals clas-
sified as priority pollutants by various environmental agencies
[21]. The efficiency for DMP degradation was evaluated by deter-
mining degradation kinetic curves and percentage total organic
carbon (TOC) removal. Comparison with the conventional EF sys-
tem demonstrated that DMP degradation efficiency was better in
the novel EFL system than in the conventional EF system.
2
electrocatalyst was developed to overcome the main drawbacks of
conventional Electro-Fenton (EF) system. The conventional EF, one
of the advanced oxidation processes, is very efficient for the degra-
dation of various persistent organic pollutants [7–15]. Recently,
Brillas et al. reviewed comprehensively and systematically the EF
method and related electrochemical techniques [16]. However, the
3+
EF system requires a low solution pH. If the pH is ≥3.5, Fe pre-
cipitates from solution and terminates the reaction. Neutral water,
such as sewage, is thus needed to reduce the pH before treatment
and to restore neutrality after treatment by chemical addition or
electrochemical methods, which is very problematic in practice
2. Experimental
2.1. Chemicals
∗ Corresponding author. Tel.: +86 89865883398.
E-mail address: 521000hua282@sina.com (Y. Hua).
HPLC-grade acetonitrile was purchased from Dima Technology
(Richmond Hill, ON, Canada). Other chemicals were analytical
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.05.094

6
756
C. Wang et al. / Electrochimica Acta 55 (2010) 6755–6760
electrode was polished with 0.05-␮m alumina and then washed
by sonication for 2 min in purified water. The electrolyte solution
was deaerated by purging with pure argon for 10 min before each
experiment.
2.3. Analytical methods
The DMP concentration was analyzed by HPLC (Techcomp, LC
130, Shanghai, China) equipped with a reverse phase column
2
(
Phenomenex® C18, 5 ␮m, 150 mm × 4.6 mm) and a UV detector.
The mobile phase was comprised of 30% acetonitrile and 70% water
containing 0.5% tetrabutylammonium bromide, and the detec-
tion wavelength was 276 nm. The flow rate of mobile phase was
−
1
1
mL min and the sample volume injected was 25 ␮L. The qual-
itative identification of PW11Fe (III) (H O ) was also performed
2
2
using the same HPLC under the same conditions mentioned-above.
Cumulative H O2 concentrations were measured by spectropho-
2
tometric method based on the pink Ti(IV)-H O2 complex [23,24]
2
using a TU1810 UV–vis spectrophotometer (Universal Analysis,
Beijing, China). TOC concentrations were measured using a Shi-
madzu 5000A analyzer.
Fig. 1. The spatial structure of the PW11O39Fe(III) (H2O)4− anion.
reagent grade and were used as received. D301R anion exchange
resin was purchased from Nankai University (China). The Keggin
heteropolytungstate Na7PW11O39 and iron-substituted het-
3. Results and discussion
3.1. DMP degradation
eropolytungstate Na PW11O39Fe(III) (H O) were synthesized as
4
2
described in the literature [2,22]. Fig. 1 shows the spatial structure
PW11Fe(III)(H O) has been proved to be an excellent electrocat-
2
4−
of the PW11O39Fe(III) (H O)
anion. In all experiments, double
alyst for H O reduction to hydroxyl radicals[1]. Herein it was used
2
2 2
distilled water was used.
to constitute a novel EFL system that was directly applied to DMP
degradation under neutral pH conditions. The results are shown in
Fig. 2A.
2.2. DMP degradation and electrochemical impedance
spectroscopy (EIS)
As can be seen from Fig. 2A, without application of a cathodic
potential, the DMP concentration remained nearly unchanged in
DMP degradation experiments were performed using an elec-
trochemical workstation (CHI, Shanghai, China) under a controlled
potential using a conventional three-compartment cell (volume
solution containing 0.1 mM DMP and 1.0 mM PW11Fe(III)(H O)
2
under ambient conditions (curve a). Once a cathodic potential of
−0.5 V was applied, a slow decrease in DMP concentration was
observed (curve b) and reached ∼78% degradation after a reaction
time of 120 min. This result indicates that application of a cathodic
potential is necessary for DMP degradation because it causes reduc-
4
0 mL). The working electrode was a graphite rod with a diameter
2
of 6 mm and a surface area of ca. 2.5 cm , and the counter electrode
was a platinum flake placed in the anodic compartment. The anodic
compartment was separated from the cathodic compartment by a
porous glass frit. The reference electrode was Ag/AgCl (3 M KCl). The
cathodic compartment had a volume of 20 mL and was equipped
with a magnetic stirrer.
tion of O2 in solution to H O2 in situ at the graphite electrode and
2
reduction of Fe(III) to Fe(II), which further catalyzes H O reduc-
2
2
•
tion to HO that destroys DMP. The slow degradation rate is due
to insufficient O2 under ambient conditions, which resulted in a
−
1
Before electrolysis, the solution containing 1.0 mM
slow H O2 generation. When an O2 flow of 60 mL min was fed to
2
PW11Fe(III)(H O) and different initial DMP concentrations was
added to the electrochemical cell in a single batch. Pure O2 (99.9%)
was blown through the solution in the cathodic compartment. The
the solution at the same potential, the decrease in DMP concentra-
tion immediately accelerated and complete DMP degradation was
obtained after 80 min (curve c and inset). We believe that this is the
2
O flow rate was controlled by a valve with scale. After the solution
2
result of a faster H O2 generation.
2
was saturated with O , electrolysis was started. During reactions,
To confirm the generation of H O2 via O2 reduction at the
2
2
1
00-␮L samples were withdrawn at different time intervals and
graphite cathode, the cumulative H O2 concentration in the pH
2
diluted to 500 ␮L for HPLC analysis. For TOC analysis, a 5-mL
6.86 phosphate buffer solution was monitored. Fig. 2B shows the
sample was withdrawn at the end of the electrolysis.
experimental results. It can be seen that the H O2 concentration
2
In order to prove the existence of H O generated in situ at elec-
trode during reaction, a series of experiments were performed in
increased with the reaction time. Under both ambient conditions
and a more positive potential, H O was produced at a low concen-
2
2
2
2
the pH 6.86 phosphate buffer solution free of PW11Fe(III)(H O)
tration. With an increase in O2 and application of a more negative
2
in the electrochemical cell by purging oxygen (99.9%) onto the
graphite cathode by a glass frit diffuser under cathodic potentials.
A valve with scale was used to ensure the oxygen flow rate. The
solution sample of 200 ␮L was taken and analyzed immediately to
measure the H O concentration.
potential, H O2 was generated at higher concentrations. However,
an O2 flow too fast is detrimental to oxygen adsorption onto the
2
cathode and is thus unfavorable for H O2 generation [17].
2
Because the cathodic potential and O2 flow rate directly influ-
ence the rate of H O generating at the graphite electrode, and thus
2
2
2
2
Electrochemical impedance spectroscopy (EIS) measurements
were conducted using the same electrochemical workstation and
a one-compartment cell with glassy carbon (GC) as the working
the DMP degradation rate, the optimal cathodic potential and O2
flow rate values were investigated. The results are shown in Fig. 3.
We can see from Fig. 3A that a more negative cathodic potential
2
electrode (0.07 cm ), a platinum wire as the counter electrode, and
accelerated the rate of DMP degradation by facilitating H O gener-
2 2
Ag/AgCl (3 M KCl) as the reference electrode. Prior to use, the GC
ation. However, when the potential was more negative than −0.5 V,

C. Wang et al. / Electrochimica Acta 55 (2010) 6755–6760
6757
Fig. 2. (A) Concentration changes for dimethylphthalate (c0 = 0.1 mM) in pH 6.86 mixed phosphate solution containing 1.0 mM PW11Fe(III)(H2O) during electrolysis: (a)
−
1
ambient conditions; (b) ambient conditions and E = −0.5 V; (c) O2 at 60 mL min and E = –0.5 V (and inset). (B) Cumulative hydrogen peroxide concentration electrogenerated
−
1
−1
in pH 6.86 mixed phosphate solution: (a) ambient conditions and E = −0.3 V; (b) O2 at 60 mL min and E = −0.3 V; (c) ambient conditions and E = −0.5 V; (d) O2 at 60 mL min
and E = −0.5 V.
Fig. 3. Effects of (A) cathodic potential and (B) O2 flow rate on DMP degradation in mixed phosphate solution (pH 6.86) containing 0.1 mM DMP and 1.0 mM PW11Fe(III)(H2O),
−1
6
0 mL min O2 flow rate in (A) and E = −0.5 V in (B).
there was no distinct difference in the DMP degradation rate. This is
because a more negative potential may result in H O2 reduction to
2
water [17]. Therefore, −0.5 V was chosen as the optimal potential
under our experimental conditions.
It is evident from Fig. 3B that the rate of DMP degrada-
tion increased with the O flow rate, but the rates for 50 and
2
−
0 mL min were nearly identical, indicating O saturation of the
2
solution. Thus, 60 mL min was chosen as a suitable O2 flow rate
in our experiments.
1
6
−1
A comparison of the novel EFL system with the conventional EF
system in which DMP degradation proceeded under the optimal
conditions was shown in Fig. 4. Obviously, DMP degradation was
faster for the EFL system than for the EF system. Specifically, DMP
degradation was complete in <80 min for the former, whereas more
than 120 min was needed for the latter. On the other hand, we found
in our previous works that PW11Fe(III)(H O) was unchanged and
2
Fig. 4. Comparison of the novel system and a conventional Electro-Fenton sys-
readily separated from solution by D301R anion exchange resin
tem. (a) 0.1 mM DMP + 1.0 mM PW11Fe(III)(H2O), supporting electrolyte was pH 6.86
3
+
mixed phosphate; (b) 0.1 mM DMP + 1.0 mM Fe , supporting electrolyte was pH 3.0
after the treatment [6]. PW11Fe(III)(H O) adsorbed onto the resin
2
−
1
Na2SO4–NaHSO4. The O2 flow rate was 60 mL min and E = –0.5 V.
can be eluted by ion exchange and reused. All of these demonstrated
the advantage of the EFL system.
To further evaluate the efficiency of this EFL system for DMP
degradation, a series of degradation experiments using different
initial DMP concentrations and TOC measurements were carried
out. The results are presented in Fig. 5.
DMP concentrations (inset). The observed rate constants calculated
from the slope of these lines are listed in Table 1. It is evident that
the observed rate constants kobs are inversely proportional to the
initial DMP concentration. It is known that the half-life (or c/c0) of a
We can see from Fig. 5A that the DMP degradation trend for
different initial concentrations is consistent with first-order expo-
nential decay in all cases. However, the reaction time required to
Table 1
The observed rate constants at different initial DMP concentrations.
degrade to the same concentration (c/c ) was longer for higher than
for lower initial DMP concentrations. All of the log c/c0 vs. t plots
is straight line through zero for the degradation of different initial
0
Initial DMP concentration/mM
0.10
0.23
0.43
−
1
2.8 × 10−2
1.3 × 10−2
6.9 × 10−3
kcbs/min

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C. Wang et al. / Electrochimica Acta 55 (2010) 6755–6760
Fig. 5. (A) c/c0 vs. t curves during DMP degradation at different initial concentrations (inset: log c/c0 vs. t): (a) 0.10 mM; (b) 0.23 mM; and (c) 0.43 mM DMP. (B) Changes
−
1
in solution TOC during DMP degradation. Initial solution: 0.1 mM DMP + 1.0 mM PW11Fe(III)(H2O). Other conditions: the O2 flow rate was 60 mL min ; E = −0.5 V, pH 6.86
mixed phosphate buffer.
first-order reaction is independent of the initial reactant concentra-
tion, while the c/c in DMP degradation is related to the initial DMP
0
concentration. This is because the degradation of DMP is actually
the second-order reaction, which is considered to be a quasi-first-
•
order reaction due to constant H O2 or HO concentration under
2
the constant O2 flow rate and cathodic potential.
Fig. 5B reveals changes in TOC in the solution containing 0.1 mM
DMP during degradation. TOC gradually decreased with increasing
reaction time and ca. 56% TOC removal was obtained at 120 min,
indicating that DMP degradation was accompanied by partial min-
eralization. The results above also prove that the efficiency of DMP
degradation through fresh H O2 electrochemically generated in
2
Fig. 6. HPLC of mixed phosphate solution (pH 6.86) containing (a) 5 mM H2O2; (b)
situ at the electrode was better than that through H O2 addition
2
1.0 mM PW11Fe(III)(H2O); and (c) 5 mM H2O2 + 1.0 mM PW11Fe(III)(H2O).
reported in our previous study [1]. Therein, complete DMP degra-
dation took more than 120 min and the TOC removal was only about
3
0%.
to the PW11Fe(III)(H O) solution, a clear peak at the retention time
2
of ∼3 min appeared. According to Toth and Anson [4,5], the water
3
.2. Mechanism of PW11Fe(III)(H O) electrocatalytic reduction of
molecule located at the sixth coordinate site in PW Fe(III)(H O)
2
11
2
H O
is labile and readily substituted by other ligands existing in
solution. Therefore, we deduced that a ligand exchange reac-
tion between H O2 and H O in PW Fe(III)(H O) occurred when
2
2
For
a
better understanding of the electrocatalysis of
2
2
11
2
PW11Fe(III)(H O) towards H O reduction to hydroxyl radicals
H O exists in the PW11Fe(III)(H O) solution, generating a new
2
2
2
2
2
2
that lead to DMP degradation, establishment of the mechanism of
species, PW11Fe(III)(H O ), the so-called seven-coordinate Fe–O
2 2
electron transfer from the electrode to H O2 via the active Fe(III)
peroxo species [25,26]. In the UV–vis spectrum PW11Fe(III)(H O)
2
2
center is required, and a series of HPLC experiments were carried
out firstly to examine the ligand exchange reaction between
PW11Fe(III)(H O) and H O before electron transfer. Fig. 6 shows
exhibits four strong absorption bands centered at near 200, 258,
400 and 460 nm, corresponding to O → W, O → W, O → Fe and
d
b,c
d
O
→ Fe charge transfer, respectively(Fig. 7). After H O in the
2
2
2
b,c
2
the HPLC experimental results.
PW11Fe(III)(H O) was replaced by H O , the positions of the two UV
2 2 2
The HPLC chromatograms in Fig. 6 for solution containing only
H O show no peaks other than the background (curve a), and for
absorption peak at 200 and 258 nm and of the Vis absorption peak
at 460 nm were unchanged, but the absorption peak at near 400 nm
2
2
solution containing only PW11Fe(III)(H O) a tiny peak at the reten-
shifted toward the blue wavelength because of involving in O → Fe
2
d
tion time of about 4 min (curve b). However, as H O2 was added
charge transfer, leading to a strong absorption at 276 nm. Therefore,
2
Fig. 7. The UV (A) and vis (B) spectra of PW11Fe(III)(H2O).

C. Wang et al. / Electrochimica Acta 55 (2010) 6755–6760
6759
K
2
−
PW11Fe(III)(H O ) + e ꢀPW Fe(II)(H O )
(2)
2
2
11
2
2
k
PW11Fe(II)(H O ) + H O −→ PW Fe(III)(H O ) + OH + HO• (3)
2
−
2
2
2
2
11
2
2
k
−1
PW11Fe(III)(H O ) + H O−→ PW Fe(III)(H O) + H O
(4)
2
2
2
11
2
2
2
The net reaction resulted from Eqs. (1)–(4) is
H O + e → OH− + HO•
−
(5)
2
2
Eq. (5) just right represents the electrocatalytic character of
PW11Fe(III)(H O) towards H O reduction.
2
2
2
Further reactions of (6)–(8) [19] are included in the process of
DMP degradation.
k
HO +DMP−→ P, k = 10 to 1010
•
3
9
M
−1 −1
s
(6)
(7)
(8)
3
Fig. 8. EIS of mixed phosphate solution (pH 6.86) containing (a) 1.0 mM
PW11Fe(III)(H2O); and (b) 1.0 mM PW11Fe(III)(H2O + 5 mM H2O2. E = −0.09 V,
k
•
•
4
7
−1 −1
s
ꢀE = 5 mV, frequency = 100,000–0.1 Hz.
HO + H2O2−→ H2O + HO , k4 = 3.3 × 10 M
2
k5
•
•
9
−1 −1
s
HO + HO −→ H O , k5 = 5.3 × 10 M
2
2
under the detection wavelength of 276 nm the solution containing
So the reaction rate of DMP degradation can be expressed as:
PW11Fe(III)(H O) exhibited only a very weak absorption peak at the
2
retention time of about 4 min, whereas the solution after forma-
tion of PW11Fe(III)(H O ) exhibited a very strong absorption peak
at the retention time of about 3 min on the HPLC chromatograms,
as shown in Fig. 6.
dP
•
=
dt
ꢁ = k [HO ][DMP]
(9)
3
2
2
At steady state the concentration of hydroxyl radical is considered
to be constant, thus the DMP degradation follows the first-order
kinetics regime, i.e.
Formation of PW11Fe(III)(H O ) is advantageous for electron
2
2
transfer in an inner-sphere electron transfer manner, as demon-
strated by EIS experiments(Fig. 8). EIS in Fig. 8 shows that
•
[DMP]
[DMP]0
k3[HO ]
2.303
log
= −
t
(10)
the solution containing only PW11Fe(III)(H O) exhibited a high-
2
frequency semicircle corresponding to electron transfer and a
low-frequency line corresponding to mass transfer (curve a). After
H O addition to the solution, the EIS spectrum contained only
According to the hypothesis of steady state, [HO·] is easily given
from reaction Eqs. (1)–(8) by the expression:
2
2
a high-frequency semicircle and the low-frequency line disap-
peared (curve b), indicating that the electrode reaction rate is
completely controlled by the electron transfer process. Further-
more, the high-frequency semicircle was much smaller in diameter
k2K2[Fe(III)][H2O2]
•
[
HO ] =
(11)
k [DMP] + k [H O ]
3
4
2
2
At the beginning of reaction the concentration of DMP is very larger
than that of the hydrogen peroxide and the constant k3 is larger
than k , that is, k [DMP] ꢀ k [H O ] , so Eq. (11) becomes
than that without H O , indicating that the electron transfer resis-
2
2
4
3
0
4
2
2 0
tance reduced greatly. These phenomena can be explained by rapid
recovery of oxidized Fe(III) at the electrode surface due to electron
scavenging from reduced Fe(II) by coordinated H O . The seven-
k2K2[Fe(III)]0[H2O2]0
k3[DMP]0
•
[
HO ] =
(12)
2
2
coordinate Fe–O peroxo species bound at the Fe center facilitates
Fe(III) reduction to Fe(II) and rapid recovery of Fe(III) from Fe(II)
via direct scavenging of an electron by H O , leading to lower
Using Eq. (12) to substitute for [HO·] in formula (10) leads finally
to the result
2
2
[
DMP]
k
impedance than for PW11Fe(III)(H O) solution.
2
log
= −
t = −kobs
t
(13)
[
DMP]0
[DMP]0
In summary, the process for PW11Fe(III)(H O) electrocatalytic
2
reduction of H O2 may be expressed by Scheme 1.
2
where
The mechanism for PW11Fe(III)(H O) electrocatalytic reduction
2
k K [Fe(III)] [H O ]
2
2
0
2
2
0
of H O to hydroxyl radicals is thus considered to be
2
2
k =
(14)
(15)
2
.303
k
1
and
PW11Fe(III)(H O) + H O −→ PW Fe(III)(H O ) + H O
(1)
2
2
2
11
2
2
2
k
kobs = [
DMP]0
Eq. (15) indicates that the observed rate constant kobs of DMP
degradation is inversely proportion to the initial DMP concen-
tration, being consistent with the experimental results shown in
Table 1.
4. Conclusions
The novel EFL system comprising PW11Fe(III)(H O) can be used
2
for complete DMP degradation in aqueous solution at near-neutral
pH, which avoids the drawback of low working pH and is more
effective than the conventional EF system. Therefore, it should be
possible to develop this technology for treatment of wastewater
with a relaxed pH requirement.
Scheme 1. Model of PW11Fe(III)(H2O) electrocatalytic reduction of H2O2.

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C. Wang et al. / Electrochimica Acta 55 (2010) 6755–6760
Acknowledgements
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This work was supported by the Natural Science Foundation
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[
[
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