Phototransformation of metoxuron [3-(3-chloro-4-methoxyphenyl)-1,1-dimethylurea] in aqueous solution

Article abstract of DOI:10.1002/ps.405

UV irradiation of metoxuron in aerated aqueous solution at 254nm or between 300 and 450 nm led initially to an almost specific photohydrolysis of the C-C1 bond, resulting in the formation of 3-(3-hydroxy-4-methoxyphenyl)-1,1-dimethylurea (MX3) and hydroge

Full text of DOI:10.1002/ps.405

Pest Management Science
Pest Manag Sci 57:1119±1126 ꢀonline: 2001)
DOI: 10.1002/ps.405
Phototransformation of metoxuron
[3-(3-chloro-4-methoxyphenyl)-1,1-
dimethylurea] in aqueous solution
Abdelaziz Boulkamh,1 Dominique Harakat,2 Tahar Sehili1 and Pierre Boule3*
1
´
Laboratoire des Sciences et Technologies de l’Environnement, Universite Mentouri, Route de Ain El Bey, 25000 Constantine, Algeria
2
´
´
`
Centre Regional de Mesures Physiques, Service de Spectrometrie de Masse, 63177 Aubiere Cedex, France
3Laboratoire de Photochimie Mole´culaire et Macromole´culaire, Universite´ Blaise Pascal CNRS UMR 6505 (Clermont-Ferrand), 63177
`
Aubiere Cedex, France
Abstract: UV irradiation of metoxuron in aerated aqueous solution at 254nm or between 300 and
450nm led initially to an almost speci®c photohydrolysis of the C±Cl bond, resulting in the formation of
3-%3-hydroxy-4-methoxyphenyl)-1,1-dimethylurea %MX3) and hydrogen chloride. The quantum yield
was determined to be 0.020 %Æ0.005) in solutions irradiated at 254nm. Five minor photoproducts were
also identi®ed, in particular the dihydroxydimethoxybiphenyl derivatives resulting from the photo-
transformation of MX3. Irradiation increased the toxicity of an aqueous solution of metoxuron to the
marine bacterium Vibrio ®scheri.
# 2001 Society of Chemical Industry
Keywords: phenylurea; metoxuron; photolysis; photohydrolysis; toxicity
1
INTRODUCTION
on the direct photolysis of metoxuron, 3-ꢀ3-chloro-4-
methoxyphenyl)-1,1-dimethyl urea.
Halogenophenylureas are a main group of herbicides.
The ®rst to be used was 3-ꢀ4-chlorophenyl)-1,1-
dimethylureaÐcommon name monuron. It was ®rst
marketed in 1952. Thereafter many mono- and
dihalogenophenylureas were proposed. They are
chemically stable, but they absorb light up to
c 300nm and direct photolysis may be involved in
their elimination from natural waters, especially in
summer. The photodegradation of some of them has
been reported in literature. In 1968 Rosen and Strusz1
showed that the main photoproduct formed from
The main aim of the present work was to study the
photochemical behaviour of metoxuron in order to
establish whether the speci®city of the photoreaction
observed with chlorotoluron may be related to the
position of the halogen on the ring. Secondary aims
were to study the in¯uence of irradiation on the
toxicity of solutions and to check whether photo-
chemical transformations observed in laboratory con-
ditions may occur during sunlight irradiation.
metobromuron
[3-ꢀ4-bromophenyl)-1-methoxy-1-
methylurea] was [3-ꢀ4-hydroxyphenyl)-1-methoxy-
1-methylurea], but demethoxylation and demethyla-
tion also occurred. Several photoproducts of monuron
were identi®ed by Crosby and Tang.2 They result from
the oxidation or elimination of methyl groups.
Hydroxylation of the ring without dechlorination and
formation of 4,4'-dichlorocarbanilide have also been
reported, but no dechlorination was mentioned. Rosen
et al3 observed the photosubstitution of Cl by OH, but
did not quantify the part taken by this reaction. In
methanol the main pathway is the photoreduction of
C±Cl bond.4 With chlorotoluron [3-ꢀ3-chloro-4-
methylphenyl)-1,1-dimethylurea], photohydrolysis of
the C±Cl bond was reported to be initially almost
quantitative.5
2 MATERIALS AND METHODS
2.1 Reactants
Metoxuron ꢀ99.6%) was purchased from Riedel-de
HaeÈn. Water used for solutions was puri®ed using a
Milli-Q system and the purity monitored by measuring
its resistivity ꢀthe resistivity was always >18MOcm).
2.2 Irradiation
Several instruments were used for continuous irradia-
tion. Quantum yield measurements undertaken on
solutions of metoxuron were carried out at 254nm
with a device consisting of a cylindrical mirror with an
elliptic base, a low-pressure mercury lamp ꢀgermicidal
lamp) located along one of the focal axes and a quartz
reactor ꢀ2cm ID) located on the second focal axis. The
average number of photons absorbed by the solution
To our knowledge no results have been published
´
* Correspondence to: Pierre Boule, Laboratoire de Photochimie, Universite Blaise Pascal, CNRS UMR 6505 (Clermont-Ferrand), 63177
`
Aubiere Cedex, France
E-mail: pierre.boule@univ-bpclermont.fr
(Received 12 February 2001; revised version received 11 June 2001; accepted 25 July 2001)
# 2001Society of Chemical Industry. Pest Manag Sci 1526±498X/2001/$30.00
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A Boulkamh et al
was found to be 8Â1015 photons sÀ1cmÀ3 using
uranyl oxalate as actinometer. The advantage of using
light of this wavelength is that it excites metoxuron
more ef®ciently than does light of longer wavelengths.
Solutions of metoxuron were also irradiated with six
¯uorescent lamps ꢀDuke GL 20W) emitting UV light
with spectral wavelengths above 275nm and with a
maximum intensity at c 310nm. With these lamps
mercury lines at 365, 405 and 436nm are also emitted
but not involved in the phototransformation of
metoxuron. Wavelengths below 290nm are cut off
by the Pyrex2 vessel used as a reactor. In these
experiments light reaching the solutions is closer in
wavelength to sunlight. Dilute solutions ꢀc 10À4M)
were used for the kinetic study. For the isolation and
characterisation of photoproducts, more concentrated
solutions ꢀ2.2Â10À3M) were irradiated to obtain up to
60% of metoxuron conversion.
ing the concentration ꢀEC50) of a toxicant that inhibits
50% of the natural luminescence of a marine bac-
terium Vibrio ®scheri ꢀBejerinck) Lehman & Neumann
ꢀPhotobacterium phosphoreum Ford). The luminescence
emission is measured at two or three exposure times,
usually 5, 15 and 30min. Values of EC50 are often not
signi®cantly different for these exposure times. A
decrease of EC50 corresponds to an increase of
toxicity. This test is rapid and gives standardized
values that can be compared with those of a large
database.6 However, the scale of toxicity obtained with
this bacterium is not necessarily the same as scales
obtained with other micro-organisms.
3
RESULTS
3.1 Spectrophotochemical study
The UV spectrum of metoxuron between 210 and
320nm is given in Fig 1ꢀannotated the `initial' time-
point). Absorption maxima are located at 241and
285nm, with measured molar absorption coef®cients
corresponding to 13 900 ꢀÆ200) and 4600 ꢀÆ100)
Some solutions were deoxygenated by argon
bubbling to study the in¯uence of oxygen on photo-
transformation rates.
Solutions in a Pyrex2 vessel were also exposed to
sunlight in Clermont-Ferrand, France ꢀlatitude 46°N,
altitude 420m) in late February and March.
M
À1cmÀ1 respectively.
The trend in spectral change produced over the
successive irradiation time-points, shown in Fig 1, was
observed to be the same for both air-saturated and
deoxygenated solutions. This trend was also observed
whether the incident UV was 254nm ꢀas in Fig 1) or
within the range 290±350nm ꢀspectra not shown). In
all instances irradiation resulted in a reduction in the
spectral absorption between 232 and 257nm accom-
panied by an increase in the spectral absorption
between 260 and 300nm. The presence of two
isosbestic points for all the experiments indicated a
speci®c chemical transformation, the chemical out-
come of which was not signi®cantly in¯uenced by the
presence of dissolved oxygen. ꢀIsosbestic points
correspond to wavelengths where the substrate and
its products have the same molar absorption. The
presence of isosbestic points in a photochemical
transformation implies that the stoichiometry is not
modi®ed during the transformation). Furthermore,
2.3 Analyses
UV spectra were recorded on a Varian Cary 3
spectrophotometer.
HPLC analyses were carried out using a Waters 990
ꢀMillipore) chromatograph equipped with a photo-
diode array detector. Separation was achieved on a
Spherisorb ODS2 5m column ꢀ250mmÂ4.6mm ID)
with a methanol‡water ꢀ40‡60 by volume) eluent.
The same HPLC system was used for both analysis
and isolation of photoproducts. The following proce-
dure was used. The pH of the irradiated solutions
ꢀ2.2Â10À3 M; 200ml) was increased to 11 with
sodium hydroxide ꢀ0.1M) in order to selectively extract
unreacted metoxuron with chloroform. The solution
was then acidi®ed to pH=3 with a few drops of
hydrochloric acid ꢀ1M) and evaporated to about 2ml.
Sub-samples ꢀ50ml per injection) were injected in
HPLC and isolated by fraction collection.
Structural elucidation of isolated photoproducts was
achieved by a combination of MS and 1H NMR.
Direct introduction of samples into a Hewlett-Packard
5989-B mass spectrometer running in chemical
ionisation mode ꢀmethane) was used for the determi-
nation of molecular masses. Ions corresponding to
M‡1and M ‡29 were generally obtained using this
technique.
Samples of isolated photoproducts were prepared
for NMR analysis by dissolving in hexadeutero-
acetone. 1H NMR spectra were obtained from the
prepared samples using a Bruker AC 400 spec-
trometer.
2.4 Test of toxicity
The toxicities of solutions were determined by apply-
ing a Microtox1 test.6 This test consists of determin-
Figure 1. UV spectrum of a 1.1Â10À4 M solution of metoxuron irradiated at
254nm.
1120
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Phototransformation of metoxuron in aqueous solution
the relatively minor modi®cations observed in the
spectra over the time of irradiation would imply that
metoxuron and its principal photoproducts have
broadly similar UV spectral characteristics.
in air-saturated solutions. Thus oxygen had only a
slight inhibiting effect on the overall rate of conversion.
Whilst the qualitative pro®le of photoproducts formed
in deoxygenated solutions was almost the same as in
aerated solutions, the quantitative pro®le was differ-
ent. The reaction was more speci®c towards MX3
formation in the presence of oxygen, where c 90% of
metoxuron conversion was accounted for as MX3.
This contrasted with the c 60% of metoxuron conver-
sion as MX3 in the absence of oxygen. Besides this, the
absence of oxygen favoured the production of MX4
and MX6, but reduced that of MX5. An unidenti®ed
photoproduct, MX7, was only observed in deoxyge-
nated solutions ꢀFig 2).
3.2 Analytical study
It is clear from the HPLC chromatogram of a 1.1Â
10À4 M aqueous solution of metoxuron irradiated at
254nm ꢀFig 2) that 3-ꢀ3-hydroxy-4-methoxyphenyl)-
1,1-dimethylurea ꢀMX3) is the major product formed.
Some minor photoproducts, MX1±MX6, appeared
only after relatively long irradiation periods.
In order to identify the photoproducts, a more
concentrated solution ꢀ2.2Â10À3M) was irradiated
and treated as described in Section 2.3. Several
products were obtained in suf®cient amounts to allow
1H NMR and MS analyses. Results are summarised in
Table 1.
3.2.2 Influence of wavelength
The transformation of metoxuron in aqueous solu-
tions irradiated in the range 290±350nm was sig-
ni®cantly slower than at 254nm. This is mainly due to
the weaker absorption at the longer wavelength range,
as metoxuron does not appreciably absorb beyond
313nm and absorptivity is much lower between 290±
313 than at 254nm ꢀFig 1). The same photoproducts
were formed in both high- and low-wavelength
experiments, but MX3 accounted for a slightly higher
proportion of the photoproducts in the longer wave-
length experiment.
3.2.1 Influence of oxygen
At 254nm metoxuron phototransformation was initi-
ally about 20% faster in deoxygenated solutions than
3.3 Kinetic study
The quantum yield of phototransformation was
f=0.020 ꢀÆ0.005) for an air-saturated solution
ꢀ10À4 M) irradiated at 254nm. In our experimental
conditions it was assumed that all photons reaching
the solution were absorbed. The determination of
quantum yield in polychromatic light ꢀl>290nm) was
thought to be less accurate.
The disappearance of metoxuron in an irradiated
solution ꢀ1.1Â10À4M) at 254nm or in the range 290±
350nm obeyed apparent ®rst-order kinetics. In our
experimental conditions the rate constants were 0.148
and 0.05minÀ1 respectively.
The main photoproduct ꢀMX3) was obtained in
suf®cient quantities to enable preparation of solutions
of de®ned concentrations and to plot an HPLC peak
versus MX3 concentration calibration curve. It was
then possible to evaluate the concentration of MX3
formed for each irradiation time and to compare it
with that of transformed metoxuron. Results obtained
in air-saturated solutions irradiated in the range 290±
350nm are plotted in Fig 3. It appeared that, in the
early stages of the reaction ꢀup to 35% of metoxuron
transformation), MX3 accounted for c 95% of metox-
uron converted. The speci®city was a little lower at
254nm.
The formation of photoproducts in air-saturated
solutions irradiated at 254nm is summarised in Fig 4
in relative units of absorbance ꢀdetection at 260nm).
Only MX3 showed the kinetics of a primary photo-
product. The other products did not appear in the
early stages of the reaction. It could then be deduced
Figure 2. HPLC chromatograms of a solution of metoxuron 1.1Â10À4
M
irradiated at 254nm: a: Initial; b: air-saturated solution irradiated for 4min
(35% conversion); c: air-saturated solution irradiated for 17min (89%
conversion); d: deoxygenated solution irradiated for 15min (92%
conversion).
Pest Manag Sci 57:1119±1126 ꢀonline: 2001)
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A Boulkamh et al
that they did not result directly from the phototrans-
formation of metoxuron, ie they are secondary
photoproducts.
formed. Metoxuron converted almost exclusively into
MX3 during the ®rst few days, but thereafter MX3 did
not accumulate as much as in shorter wavelength
laboratory experiments. This may be due merely to
slow thermal oxidation of MX3 during long ex-
periments in sunlight or to degradation of MX3
Solutions were also exposed to sunlight from mid-
February to the end of March. After 40 days of
exposure, metoxuron was approximately 90% trans-
Table 1. Identification of metoxuron photoproducts
Product
MS ꢀm/z) chemical ionisation ꢀmethane) 1H NMR ꢀdppm) in acetone-d6
‡
‡
MX2
181 ꢀM ‡1); 209 ꢀM ‡29)
136 protonated isocyanate*
72 CONꢀCH3)2
7.96 ꢀs) OH; 7.45 ꢀs) NH; 7.29 ꢀm) H2
and H6; 6.69 ꢀm) H3 and H5; 2.95 ꢀs)
ꢀCH3)2
‡
‡
MX3
MX4
211 ꢀM ‡1); 239 ꢀM ‡29); 195 ꢀloss of 7.44 ꢀs) OH or NH; 7.40 ꢀs) NH or OH;
CH3)
166 protonated isocyanate*
72 CONꢀCH3)2
7.14 ꢀd, J =2.50Hz) H2; 6.90 ꢀdd,
J =2.50 and 8.76Hz) H6; 6.77 ꢀd,
J =8.76Hz) H5; 3.75 ꢀs) OCH3; 2.95
ꢀs) NꢀCH3)2
‡
‡
419 ꢀM ‡1); 447 ꢀM ‡29)
374 protonated isocyanate*
7.68 ꢀs) 2ÂOH; 7.44 ꢀs) H6, H6' or H3,
H3'; 6.71 ꢀs) H3, H3' or H6 H6'; 6.64 ꢀs)
2ÂNH; 3.80 ꢀs) 2ÂOCH3; 2.74 ꢀs)
2ÂNꢀCH3)2
‡
402 isocyanate, M ‡29
329 protonated double isocyanate
MX4'
303 protonated isocyanate*
331 isocyanate ꢀM ‡29)
7.49 ꢀd, J=8.70Hz) H5'; 7.35 ꢀs) H3 or
H6; 7.25 ꢀs) H6 or H3; 6.81 ꢀs) NH;
6.79 ꢀdd, J =2.90 and 8.70Hz) H6';
6.71 ꢀd, J =2.90Hz) H2'; 3.25 ꢀs)
2ÂOCH3
‡
Product not resolved from MX4
Some signals are merged
MX5 ꢀunidenti®ed)
372, 344, 327, 299
8.40 ꢀs broad); 7.58 ꢀs broad); 7.54 ꢀs
broad); 7.43 ꢀs sharp); 7.42 ꢀdd,
J =2.70 and 8.60Hz); 7.28 ꢀs broad);
7.25 ꢀd, J =2.70Hz); 6.90 ꢀd,
J =8.60Hz); 6.74 ꢀs sharp); 3.56 ꢀs
sharp); 3.00 ꢀs sharp)
‡
MX5'
215, 217 ꢀM ‡1) chlorinated product
7.64 ꢀd, J =2.50Hz) H2; 7.24 ꢀdd,
J =2.50 and 8.80Hz) H6; 6.86 ꢀd,
J =8.80Hz)H5
‡
243 ꢀM ‡29)
Product not resolved from MX5
Some signals are merged
‡
‡
MX6
419 ꢀM ‡1); 447 ꢀM ‡29)
374 protonated isocyanate*
7.71 ꢀs) OH; 7.45 ꢀs) OH; 7.39 ꢀs) H6' or
H3'; 7.33 ꢀd, J =8.80Hz) H5; 6.92 ꢀd,
J =8.80Hz) H4; 6.80 ꢀs) NH; 6.69 ꢀs)
NH; 6.68 ꢀs) H3' or H6'; 3.85 ꢀs) OCH3;
3.78 ꢀs) OCH3; 2.72 ꢀs) NꢀCH3)2; 2.70
ꢀs) NꢀCH3)2
‡
402 isocyanate ꢀM ‡29)
329 double isocyanate ꢀprotonated)
‡
357 double isocyanate ꢀM ‡29)
R=ÐNHÐCOÐNꢀCH3)2.
s=singlet, d=doublet, dd=double doublet, m=multiplet.
* Isocyanate resulting from the pyrolysis of urea derivative in the mass spectrometer.
1122
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Phototransformation of metoxuron in aqueous solution
are based on the total concentration of all products,
assuming that this concentration is not signi®cantly
modi®ed by irradiation. The concentration of toxicant
that inhibited 50% of the luminescence of the
bacterium ꢀEC50) was determined after a 5-min
exposure. The relationship between EC50 and percen-
tage of metoxuron transformation is represented
graphically in Fig 5. As metoxuron is replaced by its
photoproducts, the EC50 of the solution decreases,
which implies that at least one of the photoproducts is
more toxic to V ®scheri than is metoxuron. The rate of
increase in toxicity during the early irradiation period
correlates well with the rate of disappearance of
metoxuron and the corresponding rate of formation
of the principal photoproduct, MX3. This would
imply that MX3 is the main cause of toxicity. However
the possibility of a contribution from other photo-
products to the overall toxicity cannot be excluded.
Figure 3. Specificity of the phototransformation of metoxuron into the
hydroxylated product MX3 in air-saturated solution 1.1Â10À4 M irradiated
in the range 290–350nm.
photoinduced by some minor photoproducts. Photo-
transformation is expected to be faster in summer,
since the sunlight UV spectrum starts at shorter
wavelengths in that season ꢀc 290nm) than in winter
ꢀc 300nm) and the total photon ¯ow is higher.
Metoxuron and its main photoproduct should not
accumulate much, at least in summer.
4 DISCUSSION AND MECHANISMS
4.1 Formation of the main photoproduct MX3
The photoexcitation of metoxuron leads almost
exclusively to the formation of MX3 ꢀsubstitution of
Cl by OH) in the presence of dissolved oxygen ꢀabout
90% yield) and somewhat less exclusively in its
absence. The oxygen in OH is necessarily provided
by water and the reaction cannot be explained by a
homolytic scission of the CÐCl bond, since this
requires the homolytic scission of water, which is
highly unlikely under these conditions. Similar reac-
tions have previously been observed with 3-halogeno-
phenols. Comparable values for transformation
ef®ciency were obtained in the case of 3-¯uorophenol
and 3-chlorophenol.7 The latter reaction occurs even
in very acidic solutions.8 It is explained by a mechan-
ism of photohydrolysis involving the concerted hetero-
lytic scission of the CÐCl bond and a HÐOH bond in
a molecule of water. It can be deduced from the partial
inhibition by oxygen that the reaction may occur from
the excited triplet state of metoxuron. This assump-
tion is consistent with the possible sensitization of the
photohydrolysis of halogenophenols7 ꢀFig 6ꢀa)).
3.4 Test of toxicity
Solutions ꢀ2.2Â10À3M) irradiated at 254nm were
used to study the in¯uence of irradiation on the
toxicity to V ®scheri ꢀthe Microtox test). It is more
important to study, as a ®rst step, the in¯uence of
irradiation on the overall toxicity of the solution by
subjecting the bacterium directly to irradiated solu-
tions containing the crude mixture of products, rather
than to test the individual toxicities of the main
photoproducts, since minor photoproducts, not de-
tected or not identi®ed, may have a signi®cant
in¯uence on toxicity. In these conditions calculations
Figure 4. Kinetics of formation of the main photoproducts in a
deoxygenated solution 1.1Â10À4 M of metoxuron irradiated at 254nm.
Values of absorbance at 260nm are given in arbitrary units.
Figure 5. Toxicity of the photoproducts of metoxuron: evolution of EC50
with the course of transformation.
Pest Manag Sci 57:1119±1126 ꢀonline: 2001)
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A Boulkamh et al
Figure 6. Mechanism of formation of
photoproducts of metoxuron (a) MX3,
(b) MX4 and MX6, (c) MX4', (d) MX5',
(e) MX2.
1124
Pest Manag Sci 57:1119±1126 ꢀonline: 2001)

Phototransformation of metoxuron in aqueous solution
Figure 7. Main pathways for the photochemical transformation of metoxuron (MX).
4.2 Photoproducts MX4, MX6 and MX4
'
to one product, 3-ꢀ3-hydroxy-4-methoxyphenyl)-1,1-
dimethylurea, ꢀMX3) via a mechanism of photo-
hydrolysis. By comparing the phototransformation of
metoxuron with that of chlorotoluron [3-ꢀ3-chloro-
4-methylphenyl)-1,1-dimethylurea] and monuron
[3-ꢀ4-chlorophenyl)-1,1-dimethylurea], it appears
that photohydrolysis is more likely to occur exclu-
sively when the chlorine atom is in the meta
position. This conclusion is consistent with pre-
viously reported observations from the phototrans-
formation of chlorophenols and chloroanilines.
Reactivity is often more complex with para halo-
genated derivatives.
MX4 and MX6 have kinetics of secondary products.
Their structures imply their formation from MX3.
The formation of similar biphenyl derivatives has
previously been reported in the case of the photolysis
of phenol.9 The intermediate formation of a phenoxyl-
like radical is favoured by the presence of an oxidant
which traps the ejected electron or the hydrogen atom
released ꢀFig 6ꢀb)). Formation of the third expected
product ꢀ2-2' coupling) is probably unfavourable
because of steric hindrance.
The formation of MX4' is explained by the reaction
of the radical with the NHÐCO bond of metoxuron
ꢀMX) ꢀFig 6ꢀc)).
In the course of the metoxuron study several other
minor photoproducts were identi®ed:
4.3 Formation of MX5' and MX2
3-ꢀ4-hydroxyphenyl)-1, 1-dimethylurea ꢀMX2),
3-ꢀ3-chloro-4-methoxy)-1-methylurea ꢀMX5'),
two biphenyl derivatives ꢀMX4 and MX6) derived
from the main photoproduct MX3,
MX5' results from the elimination of a methyl group
from nitrogen. A similar reaction has previously been
observed with several halogenophenylureas.1,2 This
was tentatively explained in the case of diuron by the
intermediate formation of a methyl enolate.10
A
a substituted diphenylamine ꢀMX4') also formed
from MX3.
similar mechanism might therefore be proposed for
metoxuron, as shown in Fig 6ꢀd).
The formation of MX2 is not affected by oxygen. It
may be explained by hydrolysis of the methoxy bond
concerted with CÐCl scission ꢀFig 6ꢀe)).
The formation of the last three compounds ꢀMX4,
MX6 and MX4') is expected to be negligible in diluted
environmental conditions since their formation in-
volves bimolecular reactions.
The main routes for the photochemical transforma-
tion of metoxuron are summarised in Fig 7.
The irradiation of metoxuron signi®cantly increases
the toxicity of solutions to V ®scheri. This is an
important feature concerning the environmental im-
pact of this herbicide. However the scale of toxicity
may be different according to the test micro-organisms
and it is probable that the photoproducts identi®ed
would photo- or biodegrade in further stages.
5
CONCLUSIONS
The phototransformation of metoxuron, 3-ꢀ3-chloro-
4-methoxyphenyl)-1,1-dimethylurea, in aqueous
aerated solution proceeds initially almost exclusively
Pest Manag Sci 57:1119±1126 ꢀonline: 2001)
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A Boulkamh et al
1,1-dimethylurea ꢀmonuron) and 3-phenyl-1,1-dimethylurea
ꢀfenuron). J Agric Food Chem 20:957±959 ꢀ1972).
5 Tixier C, Meunier L, Bonnemoy F and Boule P, Phototrans-
formation of three herbicides: chlorbufam, isoproturon and
chlorotoluron. In¯uence of irradiation on toxicity. Internat J
Photoenergy 2:1±8 ꢀ2000).
ACKNOWLEDGEMENTS
The authors particularly acknowledge F Bonnemoy
Â
ꢀLaboratoire de Biologie Comparee des Protistes,
Universite Blaise Pascal) for her ef®cient assistance
Â
for tests of toxicity.
6 Kaiser KLE and Palabrica VS, Photobacterium phosphoreum,
toxicity data index. Water Poll Res J Canada 26:361±431
ꢀ1991).
7 David-Oudjehani K and Boule P, Photolysis of halogenophenols
in aqueous solution sensitized by hydroquinone or phenol. New
J Chem 19:199±206 ꢀ1995).
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