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Mechanism of Enhanced Electrochemical Oxidation of 2,4-dichlorophenoxyacetic Acid with in situ Microwave Activated Boron-doped Diamond and Platinum Anodes

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10466

J. Phys. Chem. A 2009, 113, 10466–10473

Mechanism of Enhanced Electrochemical Oxidation of 2,4-dichlorophenoxyacetic Acid with
in situ Microwave Activated Boron-doped Diamond and Platinum Anodes
Junxia Gao, Guohua Zhao,* Meichuan Liu, and Dongming Li
Department of Chemistry, Tongji UniVersity, 1239 Siping Road, 200092 Shanghai, China
ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: August 7, 2009

Remarkable enhancement in degradation effect is achieved at in situ activated boron-doped diamond (BDD)
and Pt anodes with different extent through electrochemical oxidation (EC) of 2,4-dichlorophenoxyacetic
acid (2,4-D) with microwave (MW) radiation in a flow system. Results show that when EC is activated with
MW radiation, the complete mineralization time of 2,4-D at the BDD is reduced quickly from 10 to 4 h while
Chemical oxygen demand (COD) removal at Pt is increased from 37.7 to 58.3% at 10 h; the initial current
efficiency is both improved about 1.5 times while the pseudo-first-order rate constant is increased by 153 and
119% at the BDD and Pt, respectively. To gain insight into the higher efficiency in microwave activated EC,
the mechanism has therefore been systematically evaluated from the essence of electrochemical reaction and
the accumulated hydroxyl radical concentration. 2,4-Dichlorophenol, catechol, benquinone, and maleic and
oxalic acids are the main intermediates on the Pt anode measured by high performance liquid chromatography
(HPLC), while the intermediates on the BDD electrode include 2,4-dichlorophenol, hydroquinone, and maleic
and oxalic acids. The reaction pathway with microwave radiation is the same as that in a conventional
electrochemical oxidation on both electrodes. While less and lower aromatic intermediates produce at the
BDD with MW, which suggests the higher ring-open ratio and the faster oxidation of carboxylic acids. With
microwave radiation, the ring-open ratio at the BDD is increased to 98.8% from 85.6%; the value at Pt is
increased to 67.3% from 35.9%. So microwave radiation can activate the electrochemical oxidation, which
leads to the higher efficiency. This promotion is mainly due to the higher accumulated hydroxyl radical
concentration and the effects by microwave radiation. All the results prove that the BDD electrode presents
much better mineralization performance with MW. To the best of our knowledge, it is the first time the
systematic analysis of the mechanism of microwave activated EC has been reported....
Mechanism of Enhanced Electrochemical Oxidation of 2,4-dichlorophenoxyacetic Acid with
in situ Microwave Activated Boron-doped Diamond and Platinum Anodes
Junxia Gao, Guohua Zhao,* Meichuan Liu, and Dongming Li
Department of Chemistry, Tongji UniVersity, 1239 Siping Road, 200092 Shanghai, China
ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: August 7, 2009
Remarkable enhancement in degradation effect is achieved at in situ activated boron-doped diamond (BDD)
and Pt anodes with different extent through electrochemical oxidation (EC) of 2,4-dichlorophenoxyacetic
acid (2,4-D) with microwave (MW) radiation in a flow system. Results show that when EC is activated with
MW radiation, the complete mineralization time of 2,4-D at the BDD is reduced quickly from 10 to 4 h while
Chemical oxygen demand (COD) removal at Pt is increased from 37.7 to 58.3% at 10 h; the initial current
efficiency is both improved about 1.5 times while the pseudo-first-order rate constant is increased by 153 and
119% at the BDD and Pt, respectively. To gain insight into the higher efficiency in microwave activated EC,
the mechanism has therefore been systematically evaluated from the essence of electrochemical reaction and
the accumulated hydroxyl radical concentration. 2,4-Dichlorophenol, catechol, benquinone, and maleic and
oxalic acids are the main intermediates on the Pt anode measured by high performance liquid chromatography
(HPLC), while the intermediates on the BDD electrode include 2,4-dichlorophenol, hydroquinone, and maleic
and oxalic acids. The reaction pathway with microwave radiation is the same as that in a conventional
electrochemical oxidation on both electrodes. While less and lower aromatic intermediates produce at the
BDD with MW, which suggests the higher ring-open ratio and the faster oxidation of carboxylic acids. With
microwave radiation, the ring-open ratio at the BDD is increased to 98.8% from 85.6%; the value at Pt is
increased to 67.3% from 35.9%. So microwave radiation can activate the electrochemical oxidation, which
leads to the higher efficiency. This promotion is mainly due to the higher accumulated hydroxyl radical
concentration and the effects by microwave radiation. All the results prove that the BDD electrode presents
much better mineralization performance with MW. To the best of our knowledge, it is the first time the
systematic analysis of the mechanism of microwave activated EC has been reported.
1. Introduction
In recent years the electrochemical method (EC) has been
applied in the wastewater treatment, which is environmentally
compatible with the cleanest oxidant, electron, and without
secondary pollution. The technology presents some significant
advantages, such as versatility and higher energy efficiency,
without production of sludge, is easy to automate, and attracts
more attention for wastewater treatment.
1,2
However, in the
treatment of pollutants with electrochemical oxidation, the cur-
rent efficiency is usually higher at the initial reaction stage and
then becomes lower later, even to a complete halt of the
process.
3
The reason is that pollutants and their intermediates
are often adsorbed on the electrode surface during the oxidation
and reduce the active sites on the electrode surface, resulting
in partial or complete poisoning of electrode and a decrease in
current efficiency. For an economical electrochemical oxidation,
persistently higher oxidation efficiency is required. Recently,
traditional heating,
4,5
microwave,
6,7
ultrasound,
5,8
laser,
5,9
and
some other physical methods
10
are used for activation of the
electrode surface reaching a higher oxidation efficiency.
Microwave (MW) radiation has been applied in chemical
reactions and the treatment of environmental pollutants because
of its thermal
11
and catalytic effects.
12
The application of
microwave radiation combined with some other methods has
also been explored. For example, the combination of microwave
radiation and photocatalysis was effective in increasing the
degradation efficiency.
13
The localized superheating at the
microelectrode surface by microwave radiation
5,6,14
enhanced
the electroanalytical performance of the electrode. In our
previous study, it was also shown that microwave radiation can
activate electrochemical oxidation; the flow system was adopted
to avoid the continuous increase in temperature and pressure
induced by microwave irradiation, and this method was
preliminarily used to removal of pollutants.
15
In addition,
microwave radiation can improve the photoassisted degradation
of pollutants, which is because the microwave radiation provides
not only the heat source (as the thermal effect) but also a specific
effect (the nonthermal effect).
16
The nonthermal effect may be
that microwave can enhance the ion mobility and the diffusion
of charge carriers to the surface, increasing the formation of
OH
radicals and the concentration of electrons at the surface.
17
So the nonthermal effect may be beneficial to the electrochemi-
cal oxidation.
Horikoshi
17
et al. has been pointed out microwave radiation
has thermal and nonthermal effects in the photoassisted deg-
radation of pollutants. This is the same for the electrochemical
oxidation process. The thermal effect causes the solution
temperature to increase, leading to improved mass transport.
And microwave radiation also can accelerate OH radicals
generated on the electrode surface, which can alleviate electrode
passivation, as the extra electrochemical effect. All these are
favorable to the electrochemical oxidation. So the effects by
microwave radiation, and the mechanism of enhancement in
* Corresponding author. Phone: (86)-21-65981180. Fax: (86)-21-
65982287. E-mail: g.zhao@ tongji.edu.cn.
J. Phys. Chem. A 2009, 113, 10466–1047310466
10.1021/jp9057675 CCC: $40.75 2009 American Chemical Society
Published on Web 09/08/2009
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