802
Chemistry Letters Vol.34, No.6 (2005)
Decomposition of Trichlorobenzene with Different Radicals Generated
by Alternating Current Electrolysis in Aqueous Solution
Akinobu Nakamura,ꢀ Keiji Hirano, and Masatoshi Iji
Fundamental and Environmental Research Laboratories, NEC Corporation, Miyukigaoka 34, Tsukuba 305-8501
(Received January 18, 2005; CL-050085)
Trichlorobenzenes can be easily decomposed by alternating
1,2,3-trichlorobenzene of reagent grade purchased from
Kanto Chemicals was used in this study. Water was purified
by Milli-Q (Millipore) filters immediately before use. NaOH
and NaCl were obtained from Wako Chemicals. Electrolysis
was carried out at room temperature with our assembled system,
in which a 30-kHz 15-V AC potential of between the electrodes
can be controlled by a unit connected with a function generator
and amplifiers. Three electrodes were used: the working elec-
trode, the counter electrode, and a ground electrode. Each was
a titanium plate plated with platinum (35 ꢃ 175 ꢃ 1 mm3, dis-
tance between neighboring electrodes = 25 mm, contact area
with electrolytic solution = 56 cm2), and arranged vertically in
a cell for electrolysis. The cell was a cylindrical glass reactor
(about 190 mm long and 95 mm in diameter) with a top.
TCB solutions (0.66 mM) containing NaOH or NaCl
(20 mM) were subjected to batch electrolysis in the cell. A 30-
kHz anti-phase rectangular impulse at 15-V was applied between
the two electrodes and the other was grounded. The evolution of
gases accompanying the electrolysis of water and the small in-
crease of temperature with the Joule’s heat were observed. The
TCB decomposition products were characterized by gas chroma-
tography (GC) with an FID detector (Shimadzu GC-14B) and
GC-MS (Agilent Technologies GC/MS system 5972-6890)
analysis of the headspace. Free radicals generated by electrolysis
in the solution were analyzed by using an X-band electron spin
resonance (ESR) spectrometer, the JES-FR30EX (JOEL).
Observation of the TCB decomposition by-products made it
clear that two different reactions resulted in selective redox reac-
tions. The formation of 2-ethyl-1-hexanol in the NaOH solution
shows the occurrence of both a reductive ring-opening reaction
and oxidation of the end methyl group (Figure 2a). That is, it
shows that oxidation and reduction of TCB can take place com-
petitively in the degradation, and the barely detected by-product
makes it possible to characterize such an unusual process in AC
electrolysis. Products other than 2-ethyl-1-hexanol were not
observed, presumably because successive radical reactions
occurred without forming stable intermediates (e.g., Eq 3). The
decomposition rate of TCB after electrolysis for 0.5 h was
29.6%, and about 2% of that was estimated to be converted to
2-ethyl-1-hexanol. The complete decomposition of TCB with
only a little by-product is probably due to the complex reaction
of hydrogenation and oxidation with hydrogen atoms and
hydroxyl radicals formed on the electrodes. As shown in Figure
2b, on the other hand, the formation of CHCl3 in the NaCl solu-
tion showed a successive chlorination of TCB with chloride rad-
current electrolysis in aqueous solution. The mechanism of the
decomposition was found to be based on selective redox reac-
tions with different radicals—hydrogen atoms and hydroxyl rad-
icals—generated by water electrolysis.
Some of the advantages of electrochemical techniques over
other techniques for environmental remediation are environmen-
tal compatibility, selectivity, versatility, and amenability. The
toxicity of a wide range of pollutants can be substantially
decreased by oxidation or reduction processes, especially in
water.1 Inoue et al. reported a method for hydrogenating organic
compounds by using water electrolysis with direct current,2 and
Hayakawa developed an electrolysis system that uses high-
frequency alternating current to purify water.3 He showed that
impurities could be oxidized in electrolyzed reduced water,
something that had never been observed in the conventional
direct current electrolysis where the anode and cathode are
separated. This unusual phenomenon suggests the possibility
of developing a new redox process for environmental remedia-
tion by determining its mechanism, but alternating current
(AC) electrolysis is such a new technique that the mechanism
has not yet been investigated.
We propose a mechanism based on selective redox reactions
with different radicals generated by AC electrolysis that allows
both oxidation and reduction at the same reaction site between
adjacent electrodes. As shown in Figure 1, highly reactive free
radicals formed by water electrolysis can be related with the de-
composition of organic compounds. In this letter, we present the
mechanism of the AC electrolytic decomposition of trichloro-
benzene (TCB), an example of a hard-to-degrade organic com-
pound. Analysis of the intermediates of the degradation revealed
that the mechanism involves selective redox reactions with
ꢁ
short-lived and highly reactive radicals: hydrogen atoms ( H)
ꢁ
and hydroxyl radicals ( OH), that are electrochemically formed
on the electrodes (Eqs 1 and 2).4,5 Our experiments showed that
these radicals control selectivity of the decomposition and its
products.
ꢂ
ꢁ
H2O þ e ! H þ OH
ð1Þ
ð2Þ
OHꢂ ! OH þ e
ꢁ
• H
• OH
organic
compounds
decomposition
products
+
H2O
ꢁ
icals ( Cl). About 43% of the decomposed TCB was estimated to
be converted to CHCl3. Toxic intermediates such as COCl2 were
not produced in this reaction because TCB in the reductive con-
dition due to the surrounding hydrogen atoms was able to react
to chloride radicals without being oxidized (e.g., Eqs 4 and 5).
Figure 1. Proposed mechanism of the AC electrolytic degrada-
tion of organic compounds.
Copyright ꢀ 2005 The Chemical Society of Japan