Kinetics of heterogeneous photocatalytic decomposition of 2,4-dichlorophenoxyacetic acid over titanium dioxide and zinc oxide in aqueous solution

Article abstract of DOI:10.1002/(SICI)1096-9063(1998110)54:3<269::AID-PS811>3.0.CO;2-I

The photocatalytic transformation of 2,4-D in aqueous solution containing a suspension of titanium dioxide or zinc oxide leads to the formation of intermediates which are totally mineralised to carbon dioxide and hydrogen chloride (2,4-dichlorophenol and chlorohydroquinone are the major intermediates). The products at the initial stage of the reaction were 2,4-dichlorophenol (2,4-DCP), chlorohydroquinone, 4-chloropyrocatechol, 2,4-dichloro-pyrocatechol and 1.4-chlorobenzoquinone. The initial rate of photodegradation was studied as a function of the initial concentration of reactants by the linearised form of the Langmuir-Hinshelwood equation, by which rate constants κ and equilibrium adsorption constants K were evaluated. These constants were calculated at different temperatures between 25 and 60°C. The photodegradation rate increased with increase of pH. The photocatalytic transformation of 2,4-D over titanium dioxide or zinc oxide in solution containing hydrogen peroxide was studied. The latter accelerated the reaction rate of 2,4-D significantly. It was found that chloride or bicarbonate ions slowed down the photo-degradation rate of 2,4-D by scavenging hydroxyl radicals. Partial inhibition by ethanol is attributed to scavenging of the OH radicals involved in the first step of the reaction.

Full text of DOI:10.1002/(SICI)1096-9063(1998110)54:3<269::AID-PS811>3.0.CO;2-I

Pestic. Sci. 1998, 54, 269È276
Kinetics of Heterogeneous Photocatalytic
Decomposition of 2,4-Dichlorophenoxyacetic Acid
over Titanium Dioxide and Zinc Oxide in
Aqueous Solution
Kamel Djebbar & Tahar Sehili*
Laboratoire de Photochimie et Environnement, Unite de Recherche de Chimie, Universite de Constantine,
Route de Ain El Bey, Constantine 25000, Algeria
(Received 19 December 1997; revised version received 16 April 1998; accepted 10 July 1998)
Abstract: The photocatalytic transformation of 2,4-D in aqueous solution con-
taining a suspension of titanium dioxide or zinc oxide leads to the formation of
intermediates which are totally mineralised to carbon dioxide and hydrogen
chloride (2,4-dichlorophenol and chlorohydroquinone are the major
intermediates). The products at the initial stage of the reaction were 2,4-dichloro-
phenol (2,4-DCP), chlorohydroquinone, 4-chloropyrocatechol, 2,4-dichloro-
pyrocatechol and 1,4-chlorobenzoquinone. The initial rate of photodegradation
was studied as a function of the initial concentration of reactants by the linear-
ised form of the LangmuirÈHinshelwood equation, by which rate constants k and
equilibrium adsorption constants K were evaluated. These constants were calcu-
lated at di†erent temperatures between 25 and 60¡C. The photodegradation rate
increased with increase of pH. The photocatalytic transformation of 2,4-D over
titanium dioxide or zinc oxide in solution containing hydrogen peroxide was
studied. The latter accelerated the reaction rate of 2,4-D signiÐcantly.
It was found that chloride or bicarbonate ions slowed down the photo-
degradation rate of 2,4-D by scavenging hydroxyl radicals. Partial inhibition by
ethanol is attributed to scavenging of the OH radicals involved in the Ðrst step of
the reaction. ( 1998 Society of Chemical Industry
Pestic. Sci., 54, 269È276 (1998)
Key words: photocatalysis; 2,4-dichlorophenoxyacetic acid; zinc oxide; titanium
dioxide; aqueous solution
1
INTRODUCTION
domestic usage, industrial waste and large-scale total
weed control operations on industrial sites, railways
and road verges. Pollutants include chlorinated organic
compounds which are resistant to chemical, photo-
chemical and biological degradation in the environ-
ment, making disposal, particularly of contaminated
water, difficult. The oxidative photocatalytic degrada-
tion of many chlorinated organic compounds by semi-
conductor particles, such as titanium dioxide or zinc
Increasing concern over surface water pollution by
pesticides has been voiced in recent years. Major
sources contributing to this pollution include surface
run-o† from agricultural land, direct application,
* To whom correspondence should be addressed.
Contract/grant sponsor: Ministere Algerien de lÏEnseignement
Superieur et de la Recherche ScientiÐque
269
( 1998 Society of Chemical Industry. Pestic. Sci. 0031È613X/98/$17.50. Printed in Great Britain

270
Kamel Djebbar, T ahar Sehili
oxide has been widely recognised as a promising
method for the treatment of water and wastewater.1h7
The herbicide 2,4-D is widely used for spraying
weeds. World Health Organisation (WHO) has given
the recommended maximum concentration of 2,4-D in
drinking water as 0É1 mg litre~1. Extensive studies of
the photocatalytic degradation of 2,4-D on semicon-
ductor materials have been reported.8 It has been
demonstrated that the complete mineralisation of chlo-
rinated hydrocarbons to form carbon dioxide and
hydrogen chloride can be achieved by using titanium
dioxide or zinc oxide in aqueous solution saturated with
oxygen gas.9h11
It is generally accepted that the photocatalytic reac-
tion is initiated by band gap photoexcitation of the tita-
nium dioxide on zinc oxide with UV light having energy
equal to or larger than the band gap of the semicon-
ductor (3É2 eV). Electrons are photoexcited to the con-
duction band from the valence band, producing
electron-hole pairs within the semiconductor solid.
Some of the electrons and holes migrate to the semicon-
ductor and initiate redox reactions with adsorbates
through interfacial electron transfer.2,3 Okamoto et
al.12 suggested that hydroxyl radicals were formed not
only via holes and water but also via electrons and
oxygen.
products of the photocatalysed degradation of 2,4-D
and to propose a mechanism for the transformation.
2
EXPERIMENTAL DETAILS
2.1 Chemicals
2,4-dichlorophenoxyacetic acid (2,4-D) was provided by
Aldrich (purity greater than 99%). Standards used for
HPLC analyses were from Prolabo or Rhone-Poulenc
Industries except for 3,5-dichlorocatechol, prepared by
DakinÏs method, i.e. oxidation of 3,5-dichlorosalicylic
aldehyde by hydrogen peroxide in basic solution.24
Chlorobenzoquinone was obtained by the oxidation
of chlorohydroquinone with lead acetate, using the
same method as used by Bruce and Chaudry25 for the
oxidation of 4,4@-dihydroxybiphenyl into diphenoquin-
one.
Zinc oxide (ZnO) was provided by Vieille Montagne
SA (France), and contained percentages of lead, iron
and cadmium lower than 10~4, 2 ] 10~4 and
3É5 ] 10~3 respectively. Its speciÐc area was
9É4 m2 g~1.
Draper & Fox13 observed that direct electron-
transfer oxidation instead of ~OH radical-mediated oxi-
dation is responsible for the oxidation of a variety of
electron-transfer agents on semiconductors in aqueous
suspension. Sta†ord et al.14 showed that 4-chlorophenol
can be photooxidised to hydroquinone on a titanium
dioxide surface in the absence of water or oxygen.
In this study we have examined the kinetics of photo-
degradation of 2,4-D, which is used as a herbicide.
Although titanium dioxide is recognised as the choice
photocatalyst for water treatment because of its lack of
toxicity, its stability and its low cost, we have also inves-
tigated zinc oxide, since the behaviour of this semicon-
ductor towards hydrogen peroxide15,16 is di†erent from
that of titanium dioxide.17,18 The results obtained from
these two di†erent semiconductors are compared.
Hydrogen peroxide corresponds to two electron trans-
formations of oxygen and water. It is formed under
photocatalytic conditions.17,19 The irradiating wave-
lengths employed cannot directly photolyse this com-
pound. Favourable e†ects were also reported when
hydrogen peroxide was added to the water treated by
heterogenous photocatalysis, again under conditions
that precluded direct photolysis.20h23
Most of the experiments were carried out with tita-
nium dioxide, (TiO ; P25 Degussa AG Frankfurt,
Germany). It was predominantly anatase (80% anatase,
2
20% rutile), with a surface area of 50(^5) m2 g~1 and
small amounts of SiO and Al O .26,27
2
2 3
Substrates and standards for analyses were com-
mercial compounds used as received.
2.2 Reactors and light sources
The device used for irradiation was a low pressure
mercury lamp (Philips TLAD 15W/05) located along
one of the focal axes of a cylindrical mirror with an
elliptic base. The lamp had a maximum emission at
365 nm with a 50 nm half-width. The reactor, in Pyrex,
was located along the other focal axis of the mirror. Its
temperature was maintained between 15 and 20¡C by
cooling water-jacket. The air-saturated solution Ðlled
about 90% of the reactor. The volume over the solution
was Ðlled with pure oxygen and the reactor was closed
with a stopcock to maintain an excess of oxygen in the
solution and to limit the evaporation of 2,4-D. A mag-
netic stirrer maintained the zinc oxide or titanium
dioxide in suspension and favoured the renewal of con-
sumed oxygen in the solution. The reaction mixture,
composed of 20 ml of 2,4-D and 2 g litre~1 of the oxide,
was stirred for 30 min before being irradiated. The
initial pH of the suspension was 7É6 with ZnO and 4É2
An important parameter which a†ects the yield of
2,4-D photodegradation is the pH of the suspension.
The inhibition of the photocatalysed degradation
process of 2,4-D by Cl~ ions and HCO~ ions appears
3
possible as the degradation rate decreases when enough
sodium chloride or hydrogen carbonate is added. The
main aim of the present work was to analyse the photo-
with TiO . The light intensity received by the reactor
2

Kinetics of photocatalytic 2,4,D decomposition over aqueous T iO or ZnO
271
2
was evaluated with potassium ferrioxalate to be about
4 ] 1015 photons cm~3 s~1.
2.3 Analyses
Irradiated solutions were analysed by HPLC and
GC-MS. HPLC used
a
Waters chromatograph
equipped with an lBONDAPAK 250 mm ] 4É6 mm
C18-type column, after being Ðltered on a 0É45-lm
Millipore Ðlter. Acetonitrile ] water (40 ] 60 by
volume) was used as the eluent. Products were detected
at 280 nm with an L.C. Lambda-Max 481 spectrometer.
UV spectra were recorded on a Secomam 1000 PC. For
GC-MS analyses, the solutions were extracted with
diethyl ether and injected into a FS SE 30 capillary
column, length 25 m, internal diameter 0É25 mm. The
spectrometer was a Nermag service 32 equipped with
FID detection.
3
RESULTS AND DISCUSSION
3.1 Kinetics of 2,4-D disappearance and 2,4-D
adsorption
The kinetics of disappearance of 2,4-D (initial concen-
Fig. 1. Kinetics of the transformation of 2,4-D (5 ] 10~4 M)
tration C \ 5 ] 10~4 M) were studied under three dif-
with ZnO (2 g litre~1) and TiO (2 g litre~1) (a) In the dark;
2
0
(b) illuminated.
ferent experimental conditions: (i) under UV
illumination in the absence of semiconductors, (ii) in the
dark with semiconductors, (iii) under UV illumination
in the presence of semiconductors (Fig. 1). These experi-
ments show that the photocatalytic degradation of 2,4-
All experimental data reported in Fig. 2 as plots
according to eqn (2). In the graphs, mean values of 1/R
are shown together with the range of their standard
0
D in the presence of the semiconductors (TiO , ZnO) is
2
more efficient than direct photolysis, the transformation
deviation. Values of k and K were calculated by linear
regression analysis of the experimental data.
The rate constants k were 0É7 ] 10~4 mol litre~1
min~1 for 2,4-D over ZnO and 2É2 ] 10~4 mol litre~1
is slower with ZnO than with TiO . The level of 2,4-D
2
degradation depends on the mass of semiconductors in
suspension.
The LangmuirÈHinshelwood kinetic model can be
used to Ðt the experimental data. This model states that
the initial rate of a unimolecular surface reaction (R ) is
0
proportional to the surface coverage (h) and that the
adsorption equilibrium of the solute is described by a
Langmuir isotherm:29
[dC
dT
KkC
1 ] KC
0
R \
\ K \
(1)
0
h
0
where k and K are the rate and adsorption constants
respectively. A linearised form of eqn (1) has been
employed:
1
1
\ ]
k
1
Fig. 2. Linearised plots of eqn (1) for the transformation of
(2)
R
KkC
2,4-D.
0
0

272
Kamel Djebbar, T ahar Sehili
TABLE 1
Rate Constant (k) and Adsorption Equilibrium Constant (K) for 2,4-D Photooxidation at
Di†erent Temperatures (ZnO, 2 g litre~1, pH \ 7É6 and TiO , 2 g litre~1), pH \ 4É2)
2
ZnO
T iO
2
T
k
K
k
K
(¡C)
(mol litre~1 min~1)
(litre~1 mol~1)
(mol litre~1 min~1)
(litre~1 mol~1)
25
30
40
50
60
0É7 ] 10~4
6É2 ] 10~4
8É0 ] 10~4
2É0 ] 10~3
4É6 ] 10~3
2É0 ] 103
7É8 ] 102
4É5 ] 102
2É0 ] 102
8É0 ] 101
2É2 ] 10~4
1É8 ] 10~3
3É4 ] 10~3
6É4 ] 10~3
8É2 ] 10~3
0É5 ] 103
1É0 ] 103
8É2 ] 102
6É0 ] 102
3É6 ] 102
min~1 over TiO at 25¡C. The inÑuence of the tem-
No formation of biphenyl derivatives was observed
by GC-MS. These results are in good agreement with
those obtained previously on 2,4-D29,30 but are in con-
trast with the case of 2,4-dichlorophenol.31
2
perature on the process was studied by following the
kinetics of 2,4-D disappearance at di†erent tem-
peratures between 25 and 60¡C (Table 1).
Titanium dioxide is the photocatalyst most exten-
sively studied and its use has been suggested for the
elimination of chlorinated solvents32,33 and chloroaro-
matic derivatives.34h35
3.2 Intermediate products of photocatalytic 2,4-D
degradation
The concentration of identiÐed photoproducts was
measured at various irradiation times with ZnO and
The products of the photodegradation of 2,4-D over
TiO (P25) (Fig. 3).
TiO and ZnO were identiÐed by HPLC and by com-
2
2
paring the retention time of each peak with those of
authentic standards. Analysis showed that 2,4-dichloro-
3.3 E†ect of pH
phenol
(2,4-DCP),
chlorohydroquinone,
4-
chloropyrocatechol,
2,4-dichloropyrocatechol
and
chlorobenzoquinone were present in the medium at
various irradiation times. Chlorobenzoquinone results
from the redox tautomerism of chlorohydroquinone.28
The major product was 2,4-DCP for both photo-
catalysts. The use of GC/MS corroborated the forma-
tion of these intermediates. The formation of 2,4-DCP
can be explained by considering the attack of an ~OH
radical on the alkyl chain of the molecule, while chlo-
rohydroquinone might be formed by the attack of an
~OH radical on DCP.29 The nature and results of inter-
mediates are summarised in Table 2.
Figure 4 shows the e†ect of pH on the rate of photo-
degradation of 2,4-D (5 ] 10~4 M) in the pH range 6É0È
12É0. The semiconductor zinc oxide dissolves in the
presence of highly acidic media and therefore the photo-
catalytic decomposition of 2,4-D could not be investi-
gated in the lower pH range. It is observed that the rate
of decomposition of 2,4-D increases with increasing pH
of the medium. This observation is similar to that of
Terzian et al.36 who have reported that the degradation
of cresol by TiO -UV is optimum at alkaline pH. Sabin
2
et al.37 have studied the oxidation of chlorophenols
TABLE 2
Photoproducts Observed in the Photocatalytic Transformation of 2,4-D, after a 15-min Irradiation
Conversion extent
2,4-D
(mole fraction)
Amount (% of 2,4-D converted)
ZnO
0É14
0É34
40
47
18
1É2
6
1É2
Traces
Traces
Traces
Traces
TiO
2

Kinetics of photocatalytic 2,4,D decomposition over aqueous T iO or ZnO
273
2
Fig. 3. Concentration of identiÐed photoproducts from 2,4-D
versus irradiation time (a) ZnO, (b) TiO .
2
Fig. 4. InÑuence of pH on the photocatalytic transformation
of 2,4-D (a) with ZnO and (b) with TiO .
2
under controlled pH, and reported that the oxidation
rate is maximum under alkaline conditions.
The increase of photoreactivity with increasing pH is
explained by process 3:
and dissociation characteristics forming hydroxyl rad-
icals.
In 1994 Amalric et al.38 observed that the e†ect of
H O added to ZnO was negative and in the case of
2 2
h` ] OH~ ] ~OH
(3)
TiO was either favourable or unfavourable depending
2
on the initial ratio of
[H O ]/[1,2-DMB](1,2-
2 2
dimethoxybenzene). This can be explained on the basis
of a competition between these two compounds for the
adsorption sites and/or the photoproduced holes, the
formation of additional ~OH radicals and the detrimen-
3.4 E†ects of additives
The e†ects of additives are very important in estimating
the photodegradation rate of pollutants because many
di†erent kinds of organic and inorganic compounds
may be present in water and wastewater.
tal modiÐcation of the TiO surface.
As mentioned before, the beneÐcial e†ect of photo-
catalysis was demonstrated by comparing sensitised
degradation with direct photolysis. Furthermore we
2
In this study, the e†ects of some additives showing
di†erent reaction rates with aqueous electrons and
hydroxyl radicals were studied.
observed that addition of H O to the photocatalysts
led to further increase in the photooxidation rate of 2,4-
D. As shown in Fig. 5 the complete disappearance of
2,4-D was achieved within 140 to 155 min in the pres-
ence of ZnO and within 80 to 100 min in the presence
2 2
3.4.1 E†ect of added hydrogen peroxide on the
photocatalytic degradation of 2,4-D
It is known that hydrogen peroxide can accelerate the
photodegradation rate of organic compounds in photo-
assisted reaction because of its strong oxidising power
of TiO .
2
Moreover, at the higher concentration of H O the
intermediate compounds disappeared at approximately
the same time as the parent product, thus leading to a
faster total mineralisation. We can conclude that these
2 2

274
Kamel Djebbar, T ahar Sehili
Fig. 6. InÑuence of [Cl~] on the photocatalytic transform-
Fig. 5. InÑuence of [H O ] on the photocatalytic transform-
ation of 2,4-D (a) with ZnO and (b) with TiO .
2
2
ation of 2,4-D (a) with ZnO and (b) with TiO .
2
2
Experiments on the degradation of 2,4-D over ZnO
and TiO have been conducted in the presence of
2
intermediate compounds are less photostable under
these conditions.
sodium chloride at 10~1 M and 10~2 M respectively.
The results obtained and reported in Fig. 6 show that
the inhibition e†ect is not signiÐcant.
3.4.3 E†ect of hydrogencarbonate ion concentration on
the photocatalytic degradation of 2,4-D
3.4.2 E†ect of chloride ion concentration on the
photocatalytic degradation of 2,4-D
Since ~OH radicals are known to oxidise even carbonate
ions and thus become scavenged, the protective e†ect of
hydrogencarbonate ions was studied at two di†erent
The inhibiting e†ect of chloride ions on photocatalysed
oxidation by ZnO and TiO has been reported in the
2
literature39 and has been explained on the basis of the
concentrations of HCO~ (Fig. 7). As can be seen, by
quenching of the degradation process by Cl~ ions via
their reaction with ~OH radicals. The following rate
constant is reported for this:40
3
increasing the concentration of HCO~, the rates signiÐ-
3
cantly decreased. This inhibition is undoubtedly due to
the scavenging of hydroxyl radicals. These results are
interesting because bicarbonate ions are ubiquitous in
natural waters.
Cl~ ] OH~ ] Cl~ ] OH~
(4)
K
\ 4É3 ] 109 M~1 s~1
~
Cl
HCO~ ] ~OH ] CO~ ] H O
(5)
3
3
2
This can also be explained by protonation of the TiO
particle surface at high acidic concentrations (when the
K
\ 8.5 ] 106 M~1 s~1 4,3
~
2
HCO3
pH of the solution is less than the pKa of the TiO
3.4.4 E†ect of ethanol on the photocatalytic degradation
of 2,4-D
Ethanol was chosen since it does not absorb wave-
2
surface), or by competition of Cl~ with hydrocarbons
for the active adsorption sites of TiO surfaces at low
2
acidic concentrations.41
lengths longer than 260 nm and its reaction with ~OH

Kinetics of photocatalytic 2,4,D decomposition over aqueous T iO or ZnO
275
2
Fig. 8. Mechanism of 2,4-D degradationÈmajor route.
nation into hydrogen peroxide and oxygen.47 From the
inhibiting e†ect of ethanol of the transformation of 2,4-
D, it can be concluded that about 70% of the conver-
sion involves hydroxyl radicals and about 30% can be
attributed to the capture of a positive holes by adsorbed
2,4-D on semiconductors.16,44 (Fig. 9).
Fig. 7. InÑuence of [HCO~] on the photocatalytic transform-
3
ation of 2,4-D (a) with ZnO and (b) with TiO .
2
4
CONCLUSION
radicals is almost di†usion-controlled (k \ 1É9 ] 109).40
The photocatalysed transformation of 2,4-D is about
70% inhibited by low concentrations of ethanol (0É08%
v/v; (i.e. 1É4 ] 10~2 mol litre~1). The inhibition ratio is
unchanged for ethanol concentrations as high as 2%
v/v. It may be concluded, therefore, that two di†erent
pathways are involved in the photocatalysed transform-
ation of 2,4-D. One of them involves hydroxyl radicals
and is inhibited by ethanol. The other is attributed to a
direct oxidation by positive holes formed on the irradi-
ated photocatalyst as has previously been sug-
gested.42,43,44
The photocatalytic degradation of a pollutant; 2,4-D, in
aqueous solutions was studied using two semicon-
ductors, TiO and ZnO, with di†erent efficiencies; it
appears that titanium dioxide (Degussa P25) is the
better photocatalyst taking into account its good activ-
ity and lack of toxicity.
The same products are formed with the two photo-
catalysts. The majors intermediates are 2,4-dichloro-
phenol (2,4-DCP) and chlorohydroquinone.
For the chosen pollutant, 2,4-D, two methods of oxi-
dation were observed: oxidation with ~OH radicals
originating on the surface of the photocatalyst; direct
2
3.5 Mechanism of 2,4-D photocatalytic degradation
Many reports on the photocatalytic degradation of
organic compounds in aqueous solutions have sug-
gested the action of ~OH radicals (Fig. 8).45,46 These
radicals can be produced by the surface reaction of the
photoproduced holes with adsorbed OH~ ions or
adsorbed water. The electrons reduce oxygen into
superoxide ions which disproportionate after proto-
Fig. 9. Mechanism of 2,4-D degradationÈminor route.

276
Kamel Djebbar, T ahar Sehili
17. Harbour, J. R., Tromp, J. & Hair, M. L., Can. J. Chem., 63
(1985) 204.
oxidation by holes created by the action of light on the
semiconductor.
18. Jenny, B. & Pichat, P., L angmuir, 7 (1991) 947.
19. Sehili, T., Boule, P. & Lemaire, J., Photochem., Photobiol.
A: Chem., 50 (1989) 103.
Nevertheless, after
a
long irradiation time
(t [ 250 min), all compounds are mineralised. This
result is important in the Ðeld of water decontami-
nation.
20. Gratzel, C. K., Jirousek, M. & Gratzel, M., J. Mol. Catal.,
60 (1990) 375.
21. Pelizzetti, E., Carlin, Y., Minero, C. & Gratzel, M., Nouv.
J. Chim., 15 (1991) 351.
This photocatalytic method has good potential for
application on a large scale. For this, it will be neces-
sary to decrease the electron-hole recombination to
increase the quantum yield of the photocatalytic reac-
tion. Moreover, the problem of photocatalyst recovery
remains. The solution to this problem will probably be
achieved by the use of supported or immobilised semi-
conductors.48,49
22. Wei, T. Y., Wang, M. Y. Y. & Wan, C. C., J. Photochem.
Photobiol, A: Chem., 55 (1990) 115.
23. Sclafani, A., Palmisano, L. & Davi, E., Nouv. J. Chim., 14
(1990) 265.
24. Dakin, H. D., Amer. Chem. J., 42 (1909) 477.
25. Bruce, J. M. & Chaudry, A., J. Chem. Soc. Perkin, I, (1974)
295.
26. Augugliaro, V., Palmisano, L., Schiavello, M., Sclafani, A.,
Marchese, L., Matra, G., & Miano, F., Appl. Catal., 69
(1991) 323.
27 Brezova, V., Stasko, A. & Lapcik, L. Jr., J. Photochem.
ACKNOWLEDGEMENTS
Photobiol. A: Chem., 59 (1991) 115.
28. Clapes, P., Soley, J., Vicente, M., Rivera, J., Caixach, J. &
Venture, F., Chemosphere, 15 (1986) 395.
29. Trillas, M., Peral, J., & Domenech, X., Appl. Catal. B:
Environmental, 5 (1995) 377.
The authors acknowledge the Ministere Algerien de
lÏEnseignement Superieur et de la Recherche Scienti-
Ðque for its Ðnancial support. The authors express their
thanks to Mr Hacene Bellili for his skillful technical
assistance.
30. Trillas, M., Peral, J. & Domenech, X., J. Chem. T ech. Bio-
technol., 67 (1996) 237.
31. Sehili, T. & Boule, P., Chemosphere, 22, 11 (1991) 1053.
32. Jason, C. S., Wong, Amy, L., Guangquan, L., Jingfu, F. &
Yates, J. T., Jr., J. Phys. Chem., 99 (1995) 335.
33. Bolan, N. S. & Baskaran, S., Aust. Soil. Res., 34 (1996)
1041.
REFERENCES
34. Uchida, H., Katoh, S. & Watanabe, M., Chemistry L etters,
1. Jason, C. S., Wang, A. L., Guangquan Lu, Jingfu Fan &
Yates, J. T. Jr. J. Phys. Chem., 99 (1995) 335.
2. Ollis, D. F. & Al-Ekabi, H., (eds), Photocatalytic PuriÐ-
cation and T reatment of W ater and Air. Elsevier, Amsterd-
am, 1993.
3. Fox, M. A. & Dualy, M. T., Chem. Rev., 93 (1993) 341.
4. Kamat, P. V., Chem. Rev., 93 (1993) 267.
5. Bahnemann, D., Cunningham, J., Fox, M. A., Pelizzetti,
E., Pichat, P. & Serpone, N. In Aquatic and Surface Photo-
chemistry, ed. G. R. Hels, R. G. Zepp & D. G. Crosby.
Lewis Publications, 1994, p. 261.
(1995) 261.
35. Kinkennon, A. E., Green, D. B. & Hutchinson, B., Chemo-
sphere, 31, 7 (1995) 3663.
36. Terzian, R., Serpone, N., Minero, C. & Pelizzetti, E., J.
Catal., 128 (1991) 352.
37. Sabin, F., Turk, T. & Vogler, A., J. Photochem. Photobiol.
A: Chem., 63 (1992) 99.
38. Amalric, L., Guillard, C. & Pichat, P. Res. Chem. Inter-
med., 20, 6 (1994) 579.
39. Melouki, M. A., Sehili, T. & Boule, P., J. T oxicol. Environ.
Chem., 55 (1996) 235.
6. Laszlo, V., Terence, J. & Kemp, J., Photochem. Photobiol.
40. Buxton, G. V., Greenstock, C. L., Helman, W. P. & Ross,
A. B., J. Phys. Chem. Ref. Data, 17 (1988) 513.
41. Ollis, D. F., Hsiao, C. Y., Budiman, L. & Lee, C. L., J.
Catalysis, 88 (1984) 89.
A: Chem., 87 (1995) 257.
7. Glaze, W. H., Kennebe, J. F. & Ferry, J. L., Environ. Sci.
T echnol., 27 (1993) 177.
8. Trillas, M., Sanchez, L., Peral, J. & Domenech, X., Proc.
Electrochem. Soc., 94, 19 (1994) 290.
42. Richard, C. & Boule, P., Nouv. J. Chim., 18 (1994) 547.
43. Draper, R. B. & Fox, M. A., L angmiur, 6 (1990) 1396.
44. Mihaylov, B. V. & Hendrix, J., J. Photochem. Photobiol.
A: Chem., 72 (1993) 173.
45. Pelizzetti, E. & Minero, C., Electrochim. Acta., 38 (1993)
47.
9. Turchi, C. S. & Ollis, D. F., J. Catal., 119 (1989) 483.
10. Turchi, C. S. & Ollis, D. F., J. Catal., 122 (1990) 178.
11. Mills, A. & Morris, S. J., Photochem. Photobiol. A: Chem.,
71 (1993) 75.
12. Okamoto, K., Yamamoto, ., Tanaka, H. & Tanaka, M.,
Bull. Chem. Soc. Jpn, 58 (1985) 2015.
46. Mansour, M., Schmitt, P. & Mamouni, A., Sci. T otal.
Environ., 123 (1992).
13. Draper, R. B. & Fox, M. A., L angmuir, 6 (1990) 1396.
14. Sta†ord, U., Gray, K. A., Kamat, P. V. & Varma, A.,
Chem. Phys. L ett., 205 (1993) 55.
47. Peral, J., Munoz, J. & Domenech, X., J. Photochem. Pho-
tobiol. A: Chem., 55 (1990) 251.
48. Strinivasan, S., Datye, A. K. & Hampden-Smith, M., J.
Catal., 131 (1991) 260.
15. Harbour, J. R. & Hair, M. L., J. Phys. Chem., 83 (1979)
652.
49. Percherancier, J. P., Chapelon, R. & Pouyet, B., J. Photo-
chem. Photobiol. A: Chem, 87 (1995) 261.
16. Kormann, C., Bahnemann, D. W. & Hofmann, M. R.,
Environ. Sci. T echnol., 22 (1988) 798.