Phototransformation of Chlorimuron-ethyl in Aqueous Solution

Article abstract of DOI:10.1021/jf950556j

Chlorimuron-ethyl is relatively stable in water buffered to pH 7.0 and 9.0, but hydrolyzes readily (half-life, 14 d) in water buffered to pH 4.0. In addition, chlorimuron-ethyl photodegrades rapidly and extensively in aqueous solution. The predominant photoproducts are 4-methoxy-6-chloro-2-aminopyrimidine, ethyl 2-aminosulfonylbenzoate, N-(4-methoxy-6-chloropyrimidin-2-yl)methyl urea, and o-benzoic sulfimide (saccharin). A minor deesterified product (chlorimuron) was evident. The decrease in chlorimuron-ethyl concentration in aqueous solutions followed first-order kinetics. The rate of degradation in different types of water followed the order irrigation water > tap water > distilled water. Chlorimuron-ethyl photodegraded in pH 4, 7, and 9 buffer solutions under both UV and sunlight. A faster degradation rate in pH 4.0 buffer solution was observed.

Full text of DOI:10.1021/jf950556j

J. Agric. Food Chem. 1996, 44, 3379
−3382
3379
P h ototr a n sfor m a tion of Ch lor im u r on -eth yl in Aqu eou s Solu tion
Partha P. Choudhury and Prem Dureja*
Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi 110012, India
Chlorimuron-ethyl is relatively stable in water buffered to pH 7.0 and 9.0, but hydrolyzes readily
(half-life, 14 d) in water buffered to pH 4.0. In addition, chlorimuron-ethyl photodegrades rapidly
and extensively in aqueous solution. The predominant photoproducts are 4-methoxy-6-chloro-2-
aminopyrimidine, ethyl 2-aminosulfonylbenzoate, N-(4-methoxy-6-chloropyrimidin-2-yl)methyl urea,
and o-benzoic sulfimide (saccharin). A minor deesterified product (chlorimuron) was evident. The
decrease in chlorimuron-ethyl concentration in aqueous solutions followed first-order kinetics. The
rate of degradation in different types of water followed the order irrigation water > tap water >
distilled water. Chlorimuron-ethyl photodegraded in pH 4, 7, and 9 buffer solutions under both
UV and sunlight. A faster degradation rate in pH 4.0 buffer solution was observed.
Keyw or d s: Chlorimuron-ethyl; aqueous solution; photolysis
INTRODUCTION
Ta ble 1. p H a n d EC of Differ en t Typ es of Wa ter
distilled water
tap water
irrigation water
Chlorimuron-ethyl (ethyl 2-[[[[(4-chloro-6-methoxy-2-
pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoate) is
degraded in the agricultural environment primarily via
pH- and temperature-dependent chemical hydrolysis
and is not readily susceptible to other modes of degra-
dation (Beyer et al., 1988). Earlier studies have shown
that selective nonionic surfactants altered the photolysis
rates of chlorimuron-ethyl in aqueous solution and on
glass slides exposed to sunlight (Thomas and Harrison,
pH
EC (dS/m)
7.00
0.02
8.44
1.92
8.70
2.01
P r ep a r a tion of Aqu eou s Solu tion of Ch lor im u r on -
eth yl. An analytical sample of chlorimuron-ethyl (1 g) was
added to 1 L of double-distilled water in a conical flask and
stirred for 24 h at room temperature. The stirred aqueous
solution of chlorimuron-ethyl was filtered through Whatman
No. 42 filter paper. Similarly, solutions of chlorimuron-ethyl
were prepared in tap water and irrigation water. The con-
centration of chlorimuron-ethyl was adjusted to 10 ppm by
further dilution with the respective types of water.
1
990a,b). Following eight days of exposure, there was
a 30% loss of aqueous chlorimuron-ethyl in control
solutions with no surfactants, compared with a 97% loss
of chlorimuron-ethyl in aqueous solution containing
Bu ffer . The buffer solutions used during the photolysis rate
study to maintain pH values of 4.0, 7.0, and 9.0 were prepared
according to the system established by Clark and Lubs (Lange,
1961). The buffered solutions at pH 4.0 and 7.0 were prepared
by addition of appropriate amounts of 0.2 M disodium hydro-
gen phosphate to 0.1 M citric acid, and the buffered solution
at pH 9.0 was prepared by adding 0.05 M sodium tetraborate
to 0.1 M boric acid.
0
.1% (v/v) octoxynol. The presence of p-coumaric acid
delayed chlorimuron-ethyl photolysis, whereas ribofla-
vin sensitized its photolysis (Thomas and Harrison,
1
990a; Venkatesh et al., 1993). However, no informa-
tion on the identities of the degradation products has
been reported.
The present study was undertaken to determine the
kinetics of sunlight and ultraviolet (UV) light photo-
degradation of chlorimuron-ethyl in water, the products
of photodegradation, and the effect on photodegradation
rate of various solutes including dissolved organic
matter.
Aqu eou s Sta bility. The aqueous stability of chlorimuron-
ethyl was characterized prior to photochemical degradation
experiments. The stability of chlorimuron-ethyl (10 ppm) was
tested in natural water buffered to pH 4.0, 7.0, and 9.0 (by
the addition of appropriate buffer). The test solutions were
maintained in the dark at 28 °C for 20 days. One 2-mL aliquot
was taken from each buffer solution after 6 h and 1, 2, 3, 5, 7,
1
0, 12, 15, and 20 days. All experiments were done in
MATERIALS AND METHODS
duplicate and analyzed by HPLC-UV.
Ir r a d ia tion a n d P h otop r od u cts. An aqueous solution of
chlorimuron-ethyl (50 ppm) was irradiated for 2 h with a
medium-pressure Hg lamp (125 W, Philips) jacketed with a
water cooled quartz filter. After irradiation, the aqueous
solution was extracted with methylene chloride (100 mL × 3).
To produce enough of the photoproduct for structural analysis,
a solution of 500 mg of chlorimuron-ethyl in 5 L of distilled
water was irradiated in five batches (100 mg in 1000 mL) for
Ch em ica ls. A technical sample of chlorimuron-ethyl (95%
purity) was supplied by DuPont Far East Inc., New Delhi,
India, and was purified further by repeated crystallization
from benzene and hexane until a constant mp of 185 °C was
achieved. Laboratory grade reagents and solvents were
procured locally. All the solvents were dried and distilled
before use.
Wa ter . The types of water used in the experiments were
distilled water (double distilled from systronic model distilled
water plant), tap water (collected from New Delhi Municipal
Corporation water supply), and irrigation water (collected from
an irrigation well on Research Farm, Indian Agricultural
Research Institute, New Delhi, India). The properties of the
water are described in Table 1.
2
h through a quartz filter. The irradiated solutions were
combined and extracted with methylene chloride (100 mL ×
). After drying with anhydrous Na SO , the organic phase
9
2
4
was concentrated under reduced pressure. A brown residue
was obtained. A dark control experiment was performed by
covering the flask with aluminum foil to ensure that a given
product was derived by photochemical reactions.
P h otolysis Kin etics. Aqueous solutions of chlorimuron-
ethyl (distilled water, tap water, and irrigation water; 10 ppm)
were irradiated in both quartz and Pyrex tubes under both
*
Author to whom correspondence should be ad-
dressed (fax 011-5740722/5752006).
S0021-8561(95)00556-5 CCC: $12.00
© 1996 American Chemical Society

3380 J. Agric. Food Chem., Vol. 44, No. 10, 1996
Choudhury and Dureja
Ta ble 2. Hyd r olysis of Ch lor im u r on -eth yl
chlorimuron-ethyl, %
pH 7.0
Ta ble 3. Ma ss Sp ectr a l Da ta of Ch lor im u r on -eth yl a n d
P h otolysis P r od u cts
compound
mass (m/z)
dark time (days)
pH 4.0
pH 9.0
benzoic acid (II)
ethyl benzoate (III)
122 (M+)
0
1
2
3
5
7
0
2
5
0
98
94
88
85
77
70
63
54
41
32
98
98
97
95
93
93
92
91
91
90
98
97
97
96
95
93
93
93
92
92
1
05
+
150 (M )
1
1
21
05
+
4
4
-methoxy-2-aminopyrimidine (IV)
125 (M )
9
4
5
3
1
1
1
2
+
-methoxy-6-chloro-2-aminopyrimidine (V)
159 (M )
1
1
9
29
24
4
UV light (24 h) with a medium-pressure Hg lamp and sunlight
30 days, 8 h/day from May to J une, 1994). The sunlight
43
217 (M )
158
129
124
+
(
N-(4-methoxy-6-chloro-pyrimidin-2-yl)-
N-methyl urea (VI)
intensity of 300 to 400 nm wavelength was ∼570, 1420, and
2
2
00 µW/cm at the beginning, middle, and end of the day,
respectively. Nonirradiated samples of the aqueous solution
of chlorimuron-ethyl kept in darkness served as experimental
controls. At various time intervals, samples were withdrawn
in triplicate and analyzed by HPLC. Similarly, the rates of
photolysis of chlorimuron-ethyl in pH 4.0, 7.0, and 9.0 buffered
solutions were studied under UV light and sunlight. The
hydrolysis study served as the dark control.
+
ethyl 2-aminosulfonyl benzoate (VII)
229 (M )
2
1
1
1
13
84
21
04
+
o-benzoic sulfimide (VIII) (saccharin)
183(M )
1
1
19
04
Liqu id Ch r om a togr a p h y. Chlorimuron-ethyl and its
degradates in rate kinetic studies were analyzed by HPLC
+
chlorimuron (IX)
386 (M )
(waters model 400, equipped with a model 401 pump, a UV
2
1
28
84
detector (240 nm), and plotter). The stationary phase con-
sisted of a C18 Bondapak column (3.9 × 300 nm), and the
mobile phase was methanol:water (60:40, v/v) maintained at
a flow rate of 1 mL/min.
Ga s Ch r om a togr a p h y-Ma ss Sp ectr oscop y (GC-MS).
The GC-MS system consisted of a Shimadzu model QP-200
mass spectrometer equipped with a 50 m × 0.25 mm i.d. fused
silica capillary column packed with Ulbon HR-1 (0.25 µm film
thickness). The column oven temperature was programmed
to increase from 100 to 250 °C in 6 min at a rate of 10 °C/min.
Helium was used as a carrier gas at a flow rate of 2 mL/min.
158
129
124
104
spectrum showed a molecular ion peak at m/z 217 (M+).
Photoproduct VII was identified as ethyl 2-aminosulfon-
ylbenzoate, and its mass spctrum showed a molecular
+
ion peak at m/z 229 (M ). The mass spectrum of
photoproduct VIII was similar to the mass spectrum of
saccharin published by Anderson and Dulka (1985).
Photoproduct IX was tentatively identified as chlorimu-
ron by its mass spectrum, which showed a molecular
RESULTS AND DISCUSSION
Aqu eou s Sta bility. The stability of chlorimuron-
ethyl in aqueous solution was markedly influenced by
pH (Table 2). At pH 4.0, hydrolysis of chlorimuron-ethyl
was extensive, with an estimated half-life of 14 days.
At pH 7.0 and 9.0, chlorimuron-ethyl was much more
stable throughout the course of 20-day experiments than
at pH 4.0. These results are similar to those reported
by Brown (1990). The hydrolysis rate constant of
chlorimuron-ethyl increased >150-fold as the pH drops
fron 7.0 to 4.0 (J . J . Dulka, personal communication,
+
ion peak at m/z 386 (M ).
The identification of these photoproducts suggests
that photoreaction of chlorimuron-ethyl in water in-
cludes cleavage of the sulfonylurea bridge to give V, VII,
and VIII. Photoproduct V undergoes further degrada-
tion (i.e., dechlorination) to give photoproduct IV. Pho-
toproduct VIII is formed from the intermediate 2-ami-
nosulfonyl benzoic acid by rearrangement. A minor
photoproduct, IX, is formed by deesterifiction of chlo-
rimuron-ethyl (Fuesler and Hanafey, 1990). Formation
of ethyl benzoate (photoproduct III) would be expected
as an intermediate photolysis product by analogy to the
work of Weiss et al. (1980). Photoproducts V and VII
are the major degradation products and photoproducts
II, III, IV, VI, VIII, and IX are the minor ones.
P h otod egr a d a tion Ra te. The photodegradation
rate of chlorimuron-ethyl was determined in distilled
water, tap water, irrigation water, and buffer solutions
(pH 4.0, 7.0, and 9.0) under sunlight and UV light
(quartz and pyrex). The rate constant (K) was calcu-
lated by regression analysis of recovered chlorimuron-
ethyl concentration versus time. The half-life (t1/2) was
calculated with the equation t1/2 ) 0.693/K.
1
985).
Id en tifica tion of P h otop r od u cts. HPLC analysis
of the brown residue obtained after irradiation showed
the formation of eight products in addition to chlorimu-
ron-ethyl. The degradation products were tentatively
identified by GC-MS (Table 3; Figure 1). Photoproduct
+
II was identified as benzoic acid (M 122). The identi-
fication was confirmed by matching the mass spectrum
of photoproduct II with that of a known standard. The
mass spectrum of photoproduct III showed a molecular
+
ion peak at m/z 150 (M ), with fragment ion peaks at
m/z 121 and m/z 105. Photoproduct III was identified
as ethyl benzoate by comparison with a reference
compound. Photoproduct IV was tentatively identified
as 4-methoxy-2-aminopyrimidine, and its mass spec-
Ra te of P h otolysis in Bu ffer Solu tion . The pho-
tolysis rate of chlorimuron-ethyl at different buffers (pH
4.0, 7.0, and 9.0) followed first-order kinetics with
+
trum showed a molecular ion peak at m/z 125 (M ). The
mass spectrum of photoproduct V showed a molecular
+
2
ion peak at m/z 159 (M ). Photoproduct V was identi-
significantly different rate constants and high r values
fied as 4-methoxy-6-chloro-2-aminopyrimidine. Photo-
product VI was tentatively identified as N-(4-methoxy-
under both UV (quartz and Pyrex) and sunlight (Tables
4 and 5; Figures 2, 3, and 4). The photolysis rate was
higher at pH 4.0 than at pH 7.0 and 9.0 (Tables 4 and
6
-chloro-pyrimidin-2yl)-N-methyl urea, and its mass

Chlorimuron-ethyl Phototransformation
J. Agric. Food Chem., Vol. 44, No. 10, 1996 3381
F igu r e 1. Photoproducts of chlorimuron-ethyl in aqueous solution.
Ta ble 4. Ra te Con sta n t (K) a n d Ha lf-Life (t1/2) Va lu es for
Ch lor im u r on -eth yl P h otod egr a d a tion in Differ en t Typ es
of Wa ter a n d p H Bu ffer ed Solu tion s u n d er Ultr a violet
Ligh t
aqueous media
filter
K (h-1)
t1/2 (h)
n
r2
distilled water
tap water
irrigation water
distilled water
tap water
irrigation water
pH buffer 4.0
pH buffer 7.0
pH buffer 9.0
pH buffer 4.0
pH buffer 7.0
pH buffer 9.0
Pyrex
Pyrex
Pyrex
quartz
quartz
quartz
Pyrex
Pyrex
Pyrex
quartz
quartz
quartz
0.0762
0.1665
0.2922
0.0008
0.0019
0.0040
0.2072
0.0736
0.1381
0.0012
0.0045
0.0015
9.08
4.16
2.37
0.24
0.1
0.05
1.25
9.53
5.19
0.13
0.25
0.16
3
3
3
3
3
3
3
3
3
3
3
3
0.96
0.95
0.92
0.96
0.94
0.97
0.98
0.88
0.98
0.86
0.94
0.99
Ta ble 5. Ra te Con sta n t (K) a n d Ha lf-Life (t1/2) Va lu es for
Ch lor im u r on -eth yl P h otod egr a d a tion in Differ en t Typ es
of Wa ter a n d p H Bu ffer ed Solu tion s u n d er Su n ligh t
filter
used
aqueous media
K (days-1)
t1/2 (days)
n
r2
distilled water
tap water
irrigation water
pH buffer 4.0
pH buffer 7.0
pH buffer 9.0
quartz
quartz
quartz
quartz
quartz
quartz
0.0759
0.1151
0.1543
0.1589
0.0736
0.0575
9.20
5.99
4.49
4.35
9.53
11.79
3
3
3
3
3
3
0.96
0.93
0.97
0.94
0.88
0.94
5
). Interestingly, however, the rate of photolysis was
F igu r e 2. Photolysis of chlorimuron-ethyl in aqueous and
buffer solutions with a Pyrex filter under UV light.
higher at pH 9.0 than at 7.0 (Tables 4 and 5). These
results indicate that the rate of photolysis is higher in
both acidic and alkaline conditions. The rate of degra-
dation at different pH values followed the order pH 4.0
Three different types of water (viz., irrigation water,
tap water, and distilled water) were used for this study
to establish the role of dissolved inorganic substances,
such as humic acid, fulvic acid, and other complex
organic constituents in natural water (Draper and
Crosby, 1981, 1983; Zika, 1980). Distilled water does
not contain any dissolved impurities and natural sen-
sitizers, so the rate of photolysis is lowest. In contrast,
irrigation water contains dissolved inorganic sub-
stances, which may be responsible for accelerating the
rate of degradation of chlorimuron-ethyl by acting as
photosensitizers.
>
9.0 > 7.0.
P h ot olysis R a t e in Differ en t Typ es of Wa t er .
The photolytic degradation rate of chlorimuron-ethyl in
different types of water followed first-order kinetics with
significantly different rate constants and high r values
2
(
Tables 4 and 5). Degradation was more extensive in
irrigation water and tap water compared with distilled
water under both UV light and sunlight (Figures 2, 3,
and 4). The rate of photodegradation in different types
of water followed the order distilled water < tap water
<
irrigation water.
The rate of photodegradation under UV light was

3382 J. Agric. Food Chem., Vol. 44, No. 10, 1996
Choudhury and Dureja
of the aromatic ring. These π-π* and n-π* transitions
can lead to the production of either singlet or triplet
excited states responsible for photodegradation.
To determine the effect of natural conditions (tem-
perature, air, wind), the rate of photodegradation of
chlorimuron-ethyl was also studied under sunlight
(
Table 5). Compared with UV light, the rate of degra-
dation under sunlight was slower, had the same pattern
Figures 2, 3, and 4), and was higher in irrigation water
(
and in pH 4.0 buffer solutions. We conclude that
natural forces did not play any role in the photolytic
degradation of chlorimuron-ethyl.
Parallel control experiments in the dark with chlori-
muron-ethyl in distilled water, tap water, and irrigation
water showed no degradation (Figures 2, 3, and 4).
CONCLUSIONS
This study confirms and extends earlier findings that
chlorimuron-ethyl is less persistent with regard to
hydrolysis and photolysis in acidic conditions. Chlori-
muron-ethyl photodegraded rapidly in water primarily
by cleavage of the sulfonylurea bridge to give ethyl
2
-aminosulfony benzoate and 4-methoxy-6-chloro-2-ami-
nopyrimidine. Deesterification was a minor degradation
pathway. The significance of this study is that pho-
tolysis of chlorimuron-ethyl was shown to obey first-
order kinetics, and that the degradation half-life is
concentration independent.
F igu r e 3. Photolysis of chlorimuron-ethyl in aqueous and
buffer solutions with a quartz filter under UV light.
LITERATURE CITED
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5
96.
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Brown, M. H. Mode of action, crop selectivity, and soil relations
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2
81.
Draper, W. M.; Crosby, D. G. Hydrogen peroxide and hydroxyl
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Draper, W. M.; Crosby, D. G. The photochemical generation
of hydrogen peroxide in natural water. Arch. Environ.
Contam. Toxicool. 1983, 12, 121.
Fuesler, T. P.; Hanafey, M. K. Effect of moisture on chlorimu-
ron degradation in soil. Weed Sci. 1990, 38, 256-261.
Lange, N. A. Composition and pH of Clark and Lubs buffer
mixture at 20 °C. In Handbook of Chemistry, 10th ed.;
McGraw-Hill: New York, 1961; p 951.
Thomas, S. M.; Harrison, S. K. Surfactant altered rates of
chlorimuron and metsulfuron photolysis in sunlight. Weed
Sci. 1990a , 38, 602-606.
Thomas, S. M.; Harrison, S. K. Interaction of surfactants and
reaction media on photolysis of chlorimuron and metsulfu-
ron. Weed Sci. 1990b, 38, 620-624.
Venkatesh, R.; Harrison, S. K.; Loux, M. M. Photolysis of
aqueous chlorimuron and imazaquin in the presence of
phenolic acids and riboflavin. Weed Sci. 1993, 41, 454-459.
Weiss, B.; Durr, H.; Hass, H. J . Angew. Chem., Int. Ed. Engl.
F igu r e 4. Photolysis of chlorimuron-ethyl in aqueous and
buffer solutions with a quartz filter under sunlight.
1
980, 19, 64.
Zika, R. G. In Marine Organic Chemistry; Duursma, E. K.,
Dawson, R., Eds; Elsevier: Amsterdam, 1980; pp 299-326.
higher when a quartz filter was used than when a Pyrex
filter was used (Table 4). This difference indicates that
chlorimuron-ethyl has absorption maxima that are
significantly >290 nm; therefore, it undergoes photolysis
more easily with a quartz filter than with a Pyrex filter.
This result is supported by the absorption spectrum of
chlorimuron ethyl in water, which exhibits a 213 nm
band (ꢀ 14 755) for the allowed π-π* transition of the
phenyl ring and a 239.5 nm band, which is essentially
n-π* in character, resulting from the lower energy band
Received for review August 14, 1995. Revised manuscript
X
received J uly 17, 1996. Accepted J uly 19, 1996.
J F950556J
X
Abstract published in Advance ACS Abstracts, Sep-
tember 1, 1996.