Photoaccelerated oxidation of chlorinated phenols

Article abstract of DOI:10.1039/b301705b

Exposure to visible light increases the rate of oxidation of chlorinated phenols by hydrogen peroxide in aqueous solution in either the presence or the absence of iron-based catalysts, which may be explained by the aqueous photoreactions of chloroquinone intermediates.

Full text of DOI:10.1039/b301705b

Photoaccelerated oxidation of chlorinated phenols†
a
b
Gábor Lente‡ and James H. Espenson*
a
Department of Inorganic and Analytical Chemistry, University of Debrecen, Debrecen, Hungary.
E-mail: lenteg@delfin.klte.hu; Fax: 36-52-489-667; Tel: 36-52-512-900 Ext.: 2373
Ames Laboratory, Iowa State University, Ames, IA, USA. E-mail: espenson@ameslab.gov;
Fax: 1-515-294-5233; Tel: 1-515-294-5370
b
Received (in Corvallis, OR, USA) 12th February 2003, Accepted 25th March 2003
First published as an Advance Article on the web 10th April 2003
Exposure to visible light increases the rate of oxidation of
chlorinated phenols by hydrogen peroxide in aqueous
solution in either the presence or the absence of iron-based
catalysts, which may be explained by the aqueous photoreac-
tions of chloroquinone intermediates.
2 2
oxidant instead of H O . All experiments were carried out
without the addition of buffers in order to avoid possible
interference from the oxidation of organic buffers or complexa-
tion of the catalysts with inorganic buffers. The pH decreased
from 4.6 to 2.7 in the experiment shown in Fig. 1. However, the
acid produced in the reaction is not an important factor. In fact,
experiments showed that the rate of the reaction was pH-
Chlorinated phenols are used as wood preservatives, pesticides,
or disinfectants, and they are also present in the waste of paper
mills. Because of their toxicity and high persistency, these
compounds are regarded as priority pollutants for which
efficient chemical treatment processes are needed. Oxidative
degradation is probably the most advantageous reaction type for
independent in this pH range in the most intensely studied
+
Fe(TPPS) /H
2
O
2
system. It should also be noted that the light
effect was caused primarily by visible light, as the lamp was
shown to have very little emission in the UV (the emission
spectrum of the lamp is given in Fig. S2 of ESI†), and all
experiments were done in pyrex glassware.
this purpose and several different methods have been re-
–6
ported.1 These methods usually use H
O or KHSO as an
2 2 5
Efforts were made to identify the source of light sensitivity in
oxidant and iron(II) or iron(III) complex catalysts with ligands
such as TAML, tetrasulfophthalocyanine, or water soluble
2 2
these systems. Stock solutions of TCP, H O and the catalysts
5
2
were stable in room light for months as evidenced by UV-vis
1
4
porphyrins, although TiO
2
-based systems for photodegrada-
spectroscopy. Photo-Fenton effects reported in recent literature
tion and ozonization3 are also employed. 2,4,6-Trichlor-
ophenol (TCP) is one of the most significant chlorinated
phenols and is often used to test the efficiency of oxidative
degradation methods.2 In this paper, we report our findings
about the effect of light on a broad class of oxidation reactions
of TCP and other chlorinated phenols.
4,6
do not explain our observations for several reasons. The
complexes used here, except hexaaquairon(III), do not catalyze
decomposition of H
selective oxidations.
2 2 2 2
O and they usually activate H O toward
,5
1,7,9,10
+
The catalysts, except Fe(TPPS) , do
not have absorption in the visible region at the low concentra-
tion levels used in this study. Indeed, the samples were initially
colorless, but they turned pink as the oxidation proceeded. The
light effect was also observed in the uncatalyzed oxidation of
The uncatalyzed, very slow oxidation of TCP by H
2 2
O was
monitored by UV-vis spectrophotometry (lmax = 293 nm, e =
2
1
21
+
2
2
200 M cm for TCP) using the oxidant in large excess. In
TCP by H
2
O
2
and in the Fe(TPPS) /HSO
5
system, where
a sample kept in the dark, a 4% decrease in the concentration of
TCP was observed over the course of 18 days. In contrast, an
identical sample exposed to fluorescent room light showed a
Fenton chemistry is not expected. These facts strongly suggest
that the origin of the light sensitivity is a colored intermediate in
all of the studied systems.
23% decrease in TCP concentration under the same condi-
The major product of the chemical oxidation of TCP in non-
tions.
2 3
aqueous solvents with oxidizing agents such as PbO , or CrO
The catalytic oxidation of TCP was monitored using a
chloride ion selective electrode. Earlier work showed that Cl is
is 2,6-dichloro-1,4-benzoquinone (DCQ).11 DCQ is usually
2
produced from TCP during destructive oxidation almost
quantitatively.2 The iron catalysts used in this study were
hexaaquairon(III), iron(III) meso-tetra(4-sulfonatophenyl)por-
,5
+
2
phine (Fe(TPPS) ), iron tetrasulfophthalocyanine, the iron(II
)
7
complex of tris(2-pyridylmethyl)amine, and the cis-cyclam
complex of iron(III). The use of these catalysts results in
8
destructive oxidation of the chlorophenols, which is evidenced
by the fact that total organic carbon measurements (depending
on the particular system studied) showed that 35–60% of the
2
original organic carbon content was converted to CO after the
completion of the reaction. In all of these systems, the rate of
2
Cl formation during catalytic oxidation is faster in room light
than in the dark, and it is much faster if the sample is illuminated
with a lamp (500 W, halogen). Fig. 1 shows one experiment as
+
an example where Fe(TPPS) was used as a catalyst. The
experiment was started in the dark. Exposure to room light after
2
about 14 h resulted in a sudden 25% increase in the rate of Cl
buildup (inset in Fig. 1). Direct illumination gave a 10-fold rate
2
increase, which levelled off because a quantitative yield of Cl
was obtained within 5 h after the illumination began. Similar
light effects were observed when KHSO
5
was used as an
2
Fig. 1 Effect of light on Cl formation during the oxidation of TCP by
+
+
H O
2 2
/Fe(TPPS) . [TCP] = 50 mM, [Fe(TPPS) ]
0
= 0.83 mM, [H
O ]
2 2 0
0
=
†
Electronic supplementary information (ESI) available: experimental
information. See http://www.rsc.org/suppdata/cc/b3/b301705b/
Fulbright Scholar at lowa State University in 2002–2003.
5.0 µM, T = 25.0 °C, µ = 0.1 M (NaNO ); F = fluorescent room light
turned on; L = halogen lamp turned on. Inset: part of the curve between 700
and 1000 min showing the accelerating effect of room light.
3
‡
1
162
CHEM. COMMUN., 2003, 1162–1163
This journal is © The Royal Society of Chemistry 2003

also implicated as a major intermediate of the destructive
To extend our experimental observations for chlorinated
phenols other than TCP, it was shown that the oxidations of
2
oxidation of TCP. In citrate buffer and CH
3
CN solvent, DCQ
or H
was a final product of TCP oxidation with KHSO
5
2
O
2
2,6-dichlorophenol, 2,4-dichlorophenol, 2,4,5-trichlorophenol,
using Fe(TPPS)+ catalyst. Our data showed that the same
catalytic system in unbuffered aqueous solution gives destruc-
tive oxidation and an almost quantitative yield ( > 95%) of Cl2
from TCP. Quinones are usually colored (the absorption
spectrum of DCQ is given in Fig. S14 of ESI†) and known to
undergo a variety of photoinduced reactions,12 and we hypothe-
sized that the formation of DCQ as an intermediate may explain
the photoacceleration.
1
and 2,3,4,6-tetrachlorophenol by H
O
2
/Fe(TPPS) are also
+
2
subject to light effects similar to that observed for TCP. The
source of the light sensitivity may be very similar in these cases.
To test the internal consistency of our interpretation, we
+
oxidized DCQ and DCHQ with H
2
O
2
/Fe(TPPS) . These
2
reactions were also accelerated by illumination. The rate of Cl
formation was very similar for DCQ and DCHQ oxidation. This
is not surprising as DCHQ is likely to be oxidized to DCQ
rapidly under catalytic conditions. Because the photoreaction of
Indeed, DCQ was shown to undergo light-induced reactions
in water, the major final product of which is 2,6-dichlorohy-
droquinone (DCHQ), but a small amount of 3,5-dichloro-
2
DCQ does not produce Cl either in the presence or in the
absence of TCP, the photochemical effect during the oxidation
2 2
of TCP may be that DCBT is oxidized by H O
/Fe(TPPS)+
1,2,4-benzenetriol (DCBT) is also formed. Thus, the early phase
of the oxidation can be summarized as:
much faster than DCQ. Thus light helps the oxidation to
proceed from DCQ to form further oxidized products. Oxida-
tion of DCQ seems to be a slow step in the catalytic system, and
it is quite possibly the overall rate controlling step.
In conclusion, we have shown that the rate of oxidation of
chlorinated phenols in water is enhanced by visible light most
probably because of the light sensitive quinone intermediates.
Any mechanistic study of these reactions should take this fact
into account and experimental procedures should be designed
accordingly. Without adequate precaution, a photochemical
reaction could be easily mistaken for a thermal reaction. In
addition, it can also be postulated that exposure to light (even
sunlight) in possible industrial waste treatment methods may
considerably help the oxidation of chlorinated phenols.
This research was supported by the Center for Catalysis,
Institute for Physical Research and Technology, Iowa State
University. We acknowledge helpful discussions with Prof. W.
S. Jenks and Mr. Youn-chul Oh. GL also wishes to thank the
Fulbright Program for financial support.
To study the photoreaction, an aqueous solution of DCQ (2
mM) was illuminated with the lamp, and changes in this sample
were monitored by different on-line methods. Considerable
spectral changes were seen within 5 minutes as the yellow color
of the solution darkened and new resonances appeared in the
2
NMR spectrum. No Cl was produced in any detectable
amounts ( > 10 mM) during the photoreaction. Small amounts of
an intermediate, identified as 3,5-dichloro-2-hydroxy-1,4-ben-
1
zoquinone (DCHB), were detected by both H NMR and UV-
vis spectroscopy. This intermediate is not surprising as a
reaction between DCQ and DCBT is expected to form DCHB
and DCHQ based on redox potentials.13 DCHB is also expected
to give DCBT in a photoreaction similar to that of DCQ. A
sample exposed to room light showed quite similar but slower
changes. A sample kept in the dark did not change for several
hours. After spectral changes were no longer seen in the
Notes and references
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2
3
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illuminated sample (10–12 h), water was evaporated and H
4 R. Bauer, G. Waldner, H. Fallmann, S. Hager, M. Klare, T. Krutzler, S.
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NMR, 13C NMR and MS were used to identify the remaining
solid as DCHQ along with a small amount ( ~ 5–10%) of DCBT
and even smaller amounts of unidentified byproducts. A
standard sample of DCHQ was also prepared independently14
and used to confirm the identity of the photoreaction product.
6
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solution. However, a 60% increase in the concentration of
dissolved O was detected in the sample after 10 minutes of
2 2
O was found in the final aqueous
2
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the literature. The photoreaction of 1,4-benzoquinone in water
1
13.
15
11 W. H. Hunter and M. J. Morse, J. Am. Chem. Soc., 1926, 48, 1615.
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recent study on the photochemical oxidation of water by
-methyl-1,4-benzoquinone provided evidence against the
A
1
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2
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16
involvement of free hydroxyl radical. It is reasonable to
assume that other quinones will show similar reactions and light
sensitivity can be expected whenever quinones are inter-
mediates in aqueous processes.
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