Light induced elimination of mono- and polychlorinated phenols from aqueous solutions by PW12O403-. The case of 2,4,6-trichlorophenol

Article abstract of DOI:10.1021/es990802y

PW₁₂O₄₀^3- was used as catalyst in the photodegradation of polychlorinated phenols (2,4-dichlorophenol (2,4-DCP), 2,6-dichlorophenol (2,6-DCP), 3,4-dichlorophenol (3,4-DCP), 3,5-dichlorophenol (3,5-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP)). Photolysis in 270-290 nm led to the photodecomposition of chlorophenols while the addition of PW₁₂O₄₀^3- to oxygenated solutions accelerated the photodecomposition. Chlorination of phenol generally enhanced photodecomposition rates while the effect of chlorine substituents in the ortho position was less pronounced. A detailed study of 2,4,6-TCP photodecomposition showed that the major reactions involved hydroxylation of the aromatic ring, substitution of chlorine by OH, oxidation of chlorinated HQ to the corresponding quinone, and breaking of the aromatic ring to produce carboxylic acids, such as maleic, oxalic, acetic and formic acids. The ultimate products were CO₂, H₂O and Cl^-.

Full text of DOI:10.1021/es990802y

Environ. Sci. Technol. 2000, 34, 2024-2028
monochlorophenols (4), chloracetic acid (5), and orga-
Light Induced Elimination of Mono-
nochlorine pesticides (6). In all cases the final photodegra-
dation leads to complete mineralization of substrates, i.e.,
formation of CO2, H2O, and Cl-.
and Polychlorinated Phenols from
3-
Aqueous Solutions by PW O .
The method is an example of “Advanced Oxidation
Processes” (AOP) that cause mineralization of organic
pollutants through the generation of very active, mainly OH,
radicals.
12 40
The Case of 2,4,6-Trichlorophenol
E V A G E L I A A N D R O U L A K I , † , ‡
Chlorinated phenols are used in the pesticide field as
fungicides and disinfectants and are also important chemicals
in a number of industrial processes. Chlorinated phenol
residues are also liberated via chemical or biological deg-
radation of several other groups of pesticides (for example,
phenoxy, organophosphorus, and carbamate compounds).
Moreover it has been shown that chlorinated phenols are
chemical precursors of the highly toxic polychlorinated
dibenzo-p-dioxins (7-10). Among the chlorinated phenols,
2,4,6-TCP is of special note because of its potential carci-
nogenicity (7).
A N A S T A S I A H I S K I A , †
D I M I T R A D I M O T I K A L I , ‡
C L A U D I O M I N E R O , § P A O L A C A L Z A , §
E Z I O P E L I Z Z E T T I , § A N D
E L I A S P A P A C O N S T A N T I N O U * , †
Institute of Physical Chemistry, NCSR Demokritos,
153-10 Athens, Greece, Chemical Engineering Department,
NTU, 157-80 Athens, Greece, and Department of Analytical
Chemistry, University of Torino, 10125 Torino, Italy
The mineralization of monochlorophenols in the presence
4-
of PW12O403-, SiW12O404-, and W10O40 has been reported
Light induced catalytic decomposition of several mono-,
di-, and trichlorophenols and phenol in the presence
(4a,b). Misono and co-workers have also studied the pho-
todegradation of 4-chlorophenol by PW12O403- (4c,d). It has
of PW12O 3- in aqueous solutions (pH 1) leads to mineralization
40
been shown that PW12O403-, SiW12O404-, and W10O40 have
4-
of substrates. The method is an example of Advanced
Oxidation Processes (AOP) that cause mineralization of
organic pollutants through the generation of very active,
mainly OH, radicals. Generally, chlorination of phenolic ring
enhances the decomposition, whereas the effect of
chlorine substituents in the ortho position is less pronounced.
However, the rates of decomposition of chlorinated
phenols are very much the same. Dioxygen’s main function
seems to be the regeneration of the catalyst, with
limited participation in the initial stages of the photoreactions.
A detailed study of 2,4,6-trichlorophenol (2,4,6-TCP)
photodecomposition showed that key reactions involved
were hydroxylation, substitution of chlorine by OH radicals
mainly in the ortho and para positions, and breaking of
the aromatic ring. Ring-opened products detected were
maleic, oxalic, acetic, and formic acids. Acetic acid has been
so far a common intermediate in the photodecomposition
of aromatic compounds with this method. The ultimate
products were CO , H O, and Cl-.
similar performance in the light induced mineralization of
several organic pollutants (3, 4a,b, 5). In this study, PW12O40
has been used as catalyst in the photodegradation of
polychlorinated phenols, namely, 2,4-dichlorophenol (2,4-
DCP), 2,6-dichlorophenol (2,6-DCP), 3,4-dichlorophenol (3,4-
DCP), 3,5-dichlorophenol (3,5-DCP), 2,4,6-trichlorophenol
3-
(2,4,6-TCP), and included monochlorophenols and phenol
4-
for comparison. Limited work was done with SiW12O40
.
Specifically this paper reports on the following: (a) the
photodecomposition of chlorophenols with cut off filters 320
and 345 nm by direct photolysis and through the catalyst
PW12O403-, (b) the effect of the number and position of
chlorine substituents on the rates of photodecomposition,
(c) the role of dioxygen, and (d) the final degradation products
and the intermediates involved in the case of 2,4,6-TCP.
Experimental Section
Materials. All chemicals were reagent grade or pure and used
as received. 2,6-Dichlorohydroquinone (2,6-DCHQ) was
prepared by the reduction of 2,6-dichlorobenzoquinone (2,6-
DCQ) with sodium dithionate and was characterized by 1H
NMR (11). PW12O403- and SiW12O404- were prepared according
to methods reported in the literature (12).
2
2
Introduction
Polyoxometalates (POM) offer a wide range of characteristics
such as molecular composition, size, shape, charge density,
redox potentials, acidity, and solubility that render them
potentially promising catalysts. In addition, POM can accept
and release a certain number of electrons without decom-
position (1).
It is well-known that illumination of POM at the OfM CT
band (i.e. below 400 nm) enhances their oxidizing ability,
making them useful in the oxidation of a great variety of
organic compounds including various organic pollutants (2).
We and others have demonstrated the ability of POM to cause
photocatalytic decom position of phenol, p-cresol (3),
Instrum entation. Photolysis experiments were performed
with an Oriel 1000 W Xe arc lamp, equipped with a cool
water circulating filter to absorb the near-IR radiation. This
lamp gives a flat responce from ca. 320 to 750 nm corre-
sponding to irradiance at 0.5 m, ca. 200 mW‚m-2 nm-1
according to the supplier. The incident radiation was reduced
to about 40% with a slit diaphragm in order to obtain
reasonable photolysis times. Control experiments have shown
comparable photodegradation rates of organic pollutants
3-
by TiO2 and PW12O40
.
HPLC analysis was carried out with a Waters apparatus
equipped with a UV detector. GC analysis for the determi-
nation of CO2 was carried out using a Varian Model 3300 gas
chromatograph equipped with TCD and a 2 m Porapack Q
column. Identification of intermediates was performed using
a Micromass Platform II quadrupole mass spectrometer
equipped with a DB-5 fused silica capillary column. The
* Corresponding author phone: 011-3-01-6503642-3; fax: 011-
3-01-651176-6; e-mail: epapac@mail.demokritos.gr.
† NCSR Demokritos.
‡ NTU.
§
University of Torino.
9
2 0 2 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 10, 2000
10.1021/es990802y CCC: $19.00
2000 Am erican Chem ical Society
Published on Web 04/08/2000

FIGURE 1. A comparative diagram of the photodegradation of
oxygenated solutions of 2,4,6-TCP, in the presence and absence of
FIGURE 3. First-order plots for the photodecomposition of phenol
and various chlorophenols, in the presence of catalyst: Substrates
10-3 M, PW12O 3- 7 × 10-4 M, oxygenated solutions, pH 1 (HClO ),
catalyst, with cut off filters 320 and 345 nm: 2,4,6-TCP 10-3 M,
40
4
PW12O
7 × 10-4 M, pH 1 (HClO ), T ∼ 20 °C.
3-
40
4
λ > 320 nm, T ∼ 20 °C.
TABLE 1. Initial Rate of Photodegradation of Phenol and
Various Chlorophenols in Oxygenated Solutions (First Column),
Calculated from the First-Order Plots (Figure 3), and the
Initial Rate of Reduction of Catalyst in Deaerated Solutions
(Second Column), Calculated from the “Zero-Order” Plots of
the Initial Development of the Blue Color with Photolysis
Timea
initial rate of
degradation of substrates,
M‚min-1 × 105
reduction of catalyst,
M‚min-1 × 105
substrate
(values within 20%)
(values within 15%)
phenol
2-CP
3-CP
2.4
8.6
8.2
9.5
8.6
2.4
4.9
8.3
6.8
4.4
3.7
8.3
13.6
1.8
FIGURE 2. The gradual formation of the reduced form of catalyst,
PW12O 4-, upon photolysis of deaerated solutions of 2,4,6-TCP.
40
4-CP
Substrate 10-3 M, PW12O 3- 7 × 10-4 M, pH 1 (HClO ), λ > 320 nm,
40
4
2,4-DCP
2,6-DCP
3,4-DCP
3,5-DCP
2,4,6-TCP
T ∼ 20 °C. Photolysis time is indicated on spectra.
6.6
12.6
12.8
7.5
maleic, oxalic, acetic, and formic acids were analyzed by
suppressed ion chromatography, performed with a Dionex
apparatus.
a
3-
Substrates: 10-3 M, PW12O40 7 × 10-4 M, pH 1 (HClO4), λ > 320
Photolysis Experim ent. Aqueous solutions of chlorophe-
nols, 10-3 M, unless it is stated otherwise, in the presence of
the catalyst (PW12O403-, 7 × 10-4 M), were prepared by
dissolving certain quantities of substrate in HClO4 (0.1 M).
Of the above solutions, 4.0 mL was added to an 8 mL
spectrophotometer cell (1 cm path). After 20 min of deaera-
tion or oxygenation, the cell was covered with an airtight
serum cap. Photolysis was performed at 20 °C with stirring.
The concentration of dioxygen saturated solution is ca. 1.2
mM at room temperature (13). Thus, a typical oxygenated
photolysis experiment contained 4 mL of dioxygen saturated
aqueous solution with 4 mL of dioxygen gas over it. No specific
measurements of dioxygen concentration were performed,
since we did not involve dioxygen in any specific kinetic
studies.
To have adequate quantities for the identification of
intermediates by GC-MS, samples of 30 mL of the 2,4,6-TCP
(10-3 M) were photolyzed under the same conditions.
Analysis of the Photolyzed Solutions. Photolyzed solu-
tions (4.0 mL) of chlorinated phenols were analyzed by HPLC
with the UV detector wavelength at 280 nm. When GC-MS
was used for analysis, the photolyzed solution (30 mL) was
extracted with dichloromethane (3 × 20 mL), and the organic
layers were combined, dried with sodium sulfate, and
evaporated to 100 µL. Chloride ions were analyzed spec-
trophotometrically (14).
nm , T ∼ 20 °C.
Gases from the headspace were analyzed for CO2 via GC-
TCD. Carbon dioxide content was calculated using a calibra-
tion curve, made of known quantities of CO2, and processed
under the same experimental conditions.
3-
The degree of reduction of PW12O40 in photolyzed
deaerated solutions was calculated from the known extinction
coefficients of the blue products (ꢀ751 nm ) 2 × 103 M-1 cm-1
and ꢀ653 nm ) 4.4 × 103 M-1 cm-1 for the one and two-electron
reduction products, respectively) (15).
Results and Discussion
Photodecom position of Chlorophenols with Cut Off Filters
320 and 345 nm by Direct Photolysis and through the
Catalyst PW12O403-. Chlorinated phenols absorb strongly
from about 270 to 290 nm. Photolysis in that region, results
in photodecomposition of chlorophenols (16). Despite the
lack of absorption of 2,4,6-TCP beyond 320 nm, 25% of the
substrate was decomposed after 1 h of irradiation with a 320
nm cut off filter, whereas the decomposition with a 345 nm
cut off filter was negligible (Figure 1).
Addition of PW12O403- to oxygenated solutions accelerates
the photodecomposition significantly (Figure 1). Effective
photodecomposition also takes place in the absence of
9
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FIGURE 4. The percent reduction of catalyst and the concomitant
FIGURE 6. Variation of maximum percent reduction of catalyst upon
photolysis of deaerated solutions of various chlorophenols with
increasing number of chlorine substituents, i.e., phenol, 4-CP, 2,4-
photodecomposition of 2,4,6-TCP in deaerated solutions: 2,4,6-TCP
3-
10-3 M, PW12O
7 × 10-4 M, pH 1 (HClO ), λ > 320 nm, T ∼ 20
40
4
DCP, 2,4,6-TCP. Substrates 2 × 10-3 M, PW12O 3- 7 × 10-4 M, pH
°C (100% reduction indicates complete one-electron reduction of
40
catalyst).
1 (HClO ), λ > 320 nm, T ∼ 20 °C (100% reduction indicates complete
4
one-electron reduction of catalyst).
of the one-electron reduced catalyst, PW12O404-, in deoxy-
genated solutions (except for 2,4,6-TCP which is four times
faster). The last item suggests that dioxygen’s main function
is the regeneration of the catalyst with limited effect in the
initial decomposition of the substrates.
The basic photocatalytic reactions involving POM have
been reported elsewhere (4b). It is sufficient here to mention
that excited POM has been shown to react with substrate S
via two modes, direct reaction and indirect reaction through
OH radicals (17).
POM* + S f POM(e-) + S(+) redox reaction (1)
POM* + H2O h POM(e-) + OH + H+
formation of OH (2)
FIGURE 5. The gradual reduction of PW12O 3- and Cl- production,
40
upon photolysis of deaerated solution of 2,4-DCP: Catalyst 7 × 10-4
OH + S f hydroxylation, H abstraction,
M, 2,4-DCP 4 × 10-4 M, pH 1 (HClO ), λ > 320 nm, T ∼ 20 °C (100%
4
Cl substitution, oxidation products (3)
reduction indicates complete one-electron reduction of catalyst).
POM(e-) + oxidant f POM + oxidant products
dioxygen. A deep blue color of the one-electron reduced
tungstate, PW12O404-, develops upon photolysis of a deaerated
aqueous solution of 2,4,6-TCP (Figure 2). Under the experi-
mental conditions used, the decomposition of chlorophenols
followed first-order kinetics (Figure 3).
regeneration of catalyst (4)
Several attempts have been made to correlate the rates of
photodecomposition of chlorinated phenols with the position
and the number of chlorine atoms in the ring.
The Effect of Chlorination on the Rates of Photode-
com position of the Title Substrates. The Role of Dioxygen.
Table 1 compares (a) the rates of decomposition of oxygen-
ated aqueous solutions of chlorinated phenols and phenol
as they were calculated from the first-order plots of Figure
3 and (b) the initial rates of reduction of catalyst from the
corresponding deoxygenated solutions, calculated from the
zero-order plots of the initial development of the blue color
with photolysis time. From Figure 3 and Table 1, one can
observe the following general trends for the photodecom-
position of aqueous solutions (pH 1) of phenol and chlori-
nated phenols in the presence of PW12O403-: (a) the rates of
photodecomposition of chlorinated phenols are within the
same order of magnitude. In that respect, as a referee has
kindly pointed out, the lack of selectivity is an advantage for
waste management. (b) The photodecomposition of sub-
strates follows first-order kinetics as stated earlier, (c)
chlorination of the phenolic ring enhances slightly the rate
of decomposition, whereas the effect of chlorine substituents
in the ortho position is less pronounced, and (d) the rates
of decomposition of chlorophenols in oxygenated solutions
are not dramatically different from the rates of production
If we assume, as is generally true, that the main oxidant
in Fentons reagent, UV-H2O2, UV/ near visible-TiO2, and UV/
near visible-POM are the OH radicals, then we could expect
similar behavior in the photodecomposition process reported
here.
As is known for electrophilic reagents, the OH group in
the phenolic ring is o, p directing with activation, whereas
the Cl substituent is o, p directing with deactivation. OH
radicals are electrophilic, and so one might assume that
phenol would degrade quicker than chlorophenols. However,
the opposite was observed. There are, of course, several other
factors affecting the initial rates of photodecomposition, the
most important being: (a) the nature of the associated
complex or associated equilibrium involved between the
catalyst and substrate for the systems using TiO2 or POM
and (b) the pH of the solution.
Our experimental results for deoxygenated samples were
based on the rates of reduction of catalyst in aqueous
solutions, monitored by the initial development of the blue
color (751 nm) (Figure 2). These experiments were repeated
three times, and the average rates of reactions were recorded
(Table 1). These results are easily obtained and have low
9
2 0 2 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 10, 2000

SCHEME 1
experimental error. The order of decreasing initial rates of
photodecomposition in deaerated solutions is as follows:
3,5-DCP > 3-CP > 3,4-DCP > 4-CP > 2-CP > 2,4-DCP >
2,6-DCP > phenol > 2,4,6-TCP.
chlorine is not the oxidant of the reduced catalyst. The low
percent reduction of catalyst with 2,4,6-TCP (Figure 6) could
be attributed to the formation of the highly oxidizing 2,6-
DCQ.
The acceleration of the decomposition of 2,4,6-TCP by
dioxygen suggests its participation not only in the reoxidation
of the catalyst (eq 4) but also in the initial stages of the reaction
(Table 1). Reductive activation of dioxygen proceeds in this
manner (3-5).
Because of the low solubilities of trichlorophenols we used
an order of magnitude lower concentrations, i.e., 7 × 10-5
M, for both 2,4,5-TCP and 2,4,6-TCP to compare their
photodegradation rates, keeping all other parameters the
same. The rates were 5.19 × 10-5 M min-1 and 1.25 × 10-5
M min-1 for 2,4,5-TCP and 2,4,6-TCP, respectively. Thus 2,4,6-
TCP reacts four times slower than 2,4,5-TCP, indicating that
ortho chlorosubstituents retard the initial rates of photo-
decomposition through stereochemical inhibition (18) and/
or intramolecular hydrogen bonding (20). Similar results have
been reported with TiO2 (19).
-
PW12O404- + O2 f PW12O403- + O2
O2- + S f oxidation products
(5)
(6)
Interm ediates in the Photodecom position of 2,4,6-TCP.
A detailed study of the photodegradation of 2,4,6-TCP reveals
the formation of several intermediates shown in Scheme 1.
2,6-Dichlorohydroquinone (2,6-DCHQ), 2,6-dichlorobenzo-
quinone (2,6-DCQ), 3,5-dichlorocatechol (3,5-DCC), a di-
hydroxytrichlorobenzene, a trihydroxydichlorobenzene, ma-
leic, oxalic, acetic, and formic acids were identified by GC-
MS and ion exchange chromatography. No chlorocyclopenta-
diene carboxylic acids, which are metabolites of direct
photolysis of 2,4,6-TCP (16), were detected among the
intermediates in these studies. The incident photons were
overwhelmingly absorbed by the catalyst, so that direct
photolysis was negligible.
The formation and decay of two key intermediates
involved in the photodisintegration process, 2,6-DCQ and
2,6-DCHQ, are shown in Figure 7. Measured amounts were
very low due to their instability under photocatalytic condi-
tions, as has been observed by others (19). The presence of
2,6-DCQ at zero photolysis time indicates contamination of
2,4,6-TCP (purity 98%) in the preparation process (7).
Misono and co-workers, on the other hand, have indicated
that the rate of decomposition of several chlorophenols by
PW12O403- increases with the extent of chlorine substitution
(4d). Matthews observed that the rate of production of CO2
for several chlorophenols and chlorobenzenes increases when
the number of chlorine substituents in the aromatic ring
increases from one to two, whereas addition of a third chlorine
group results in slight decrease (21). Thus, it appears from
this work and the work of others that chlorination of phenol
generally enhances the rates of photodecomposition, whereas
the effect of chlorine substituents in the ortho position is
less pronounced (18, 19, 21). It should be mentioned that
our experiments were made at pH 1 where all chlorophenols
and phenol are in un-ionized form, according to their pKa
range (6.1 < pKa < 9.9) (22).
Details of the Photocatalytic Decom position of 2,4,6-
TCP by PW12O403-. The Role of Dioxygen. Effective decom-
position of the target com pound is accom plished by
PW12O403- in the presence and absence of dioxygen. Dioxygen
accelerates the decomposition of 2,4,6-TCP by about four
times.
Figure 4 plots the percent reduction of the catalyst and
the percent decomposition of deoxygenated aqueous solu-
tions of 2,4,6-TCP vs the photolysis time, whereas in Figure
5 the percent reduction of catalyst and the Cl- production
are monitored for 2,4-DCP.
Figures 4 and 5 show that within ca. 30 min of irradiation,
4-
the percent reduction of the catalyst PW12O40 reaches a
plateau, whereas the initial decomposition of 2,4,6-TCP
(Figure 4) and the formation of Cl- (Figure 5) continued.
This suggests, as has been observed in other cases (4), that
a dynamic equilibrium is established in which the rate of the
primary photoreactions that reduce the catalyst and oxidize
the substrate (eqs 1-3) are matched by a thermal reoxidation
of the catalyst by some intermediate produced during
photolysis (eq 4).
The case of reoxidation of catalyst by H+ is ruled out,
since H+ oxidizes only the two-electron reduced tungstate,
PW12O405-, as per the redox potentials of the species (23).
Further, Figure 6 shows the extent of reduction of catalyst
for several chlorophenols. No correlation is found with the
number of chlorine substituents, suggesting that liberated
FIGURE 7. Formation and decay of 2,6-DCQ and 2,6-DCHQ upon
photolysis of aqueous oxygenated solution 2,4,6-TCP 10-3 M, in the
presence of PW12O 3- 7 × 10-4 M, pH 1 (HClO ), λ > 320 nm, T ∼
40
4
20 °C.
9
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FIGURE 8. Formation of CO , Cl- and decay of 2,4,6-TCP upon
2
photolysis of aqueous oxygenated solution of substrate, in the
presence of catalyst: 2,4,6-TCP 4 × 10-4 M, PW12O 3- 7 × 10-4 M,
40
pH 1 (HClO ), λ > 320 nm, T ∼ 20 °C.
4
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Acknowledgments
We thank Ministry of Development, General Secretariat of
Research and Technology of Greece, and the Italian-Greek
exchange program for supporting part of this work. We thank
S. Boyatzis for helping out with the GC-MS.
Received for review July 16, 1999. Revised manuscript re-
ceived December 7, 1999. Accepted January 14, 2000.
Literature Cited
(1) (a) Baker, L. C. W.; Glick, D. C. Chem. Rev. 1998, 98, 3. (b) Pope,
M. T. Heteropoly and Isopoly Oxometalates, Inorganic Chemistry
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