Photodecomposition of carbendazim in aqueous solutions

Article abstract of DOI:10.1016/S0043-1354(00)00058-0

The kinetic of photodecomposition of carbendazim (methyl-2-benzimidazole carbamate) at different pHs (1, 5, 7, 8 and 11) and at two different concentrations of dissolved oxygen has been studied. According to the experimental results, the photodegradation process follows a first-order kinetic and the degradation rate of carbendazim increases with pH and dissolved oxygen concentration. The kinetic of photodecomposition of carbendazim (methyl-2-benzimidazole carbamate) at different pHs (1, 5, 7, 8 and 11) and at two different concentrations of dissolved oxygen has been studied. According to the experimental results, the photodegradation process follows a first-order kinetic and the degradation rate of carbendazim increases with pH and dissolved oxygen concentration. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0043-1354(00)00058-0

Wat. Res. Vol. 34, No. 11, pp. 2951±2954, 2000
7 2000 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
PII: S0043-1354(00)00058-0
0043-1354/00/$ - see front matter
www.elsevier.com/locate/watres
PHOTODECOMPOSITION OF CARBENDAZIM IN
AQUEOUS SOLUTIONS
RICARD PANADES1, ALBERT IBARZ1* and SANTIAGO ESPLUGAS2M
1Escola Tecnica Superior d'Enginyeria Agraria, Universitat de Lleida, Rovira Roure 177, 25198, Lleida,
Spain and Departament d'Enginyeria Quõmica i Metal-lurgia, Universitat de Barcelona, Martõ i
Franques, 1, 08028, Barcelona, Spain
2
(First received 1 November 1998; accepted in revised form 1 October 1999)
AbstractÐThe kinetic of photodecomposition of carbendazim (methyl-2-benzimidazole carbamate) at
di€erent pHs (1, 5, 7, 8 and 11) and at two di€erent concentrations of dissolved oxygen has been
studied. According to the experimental results, the photodegradation process follows a ®rst-order
kinetic and the degradation rate of carbendazim increases with pH and dissolved oxygen concentration.
7 2000 Elsevier Science Ltd. All rights reserved
Key wordsÐcarbendazim, photodecomposition, kinetic
INTRODUCTION
degradation may take place by the e€ect of ultra-
violet radiation or by alkalinity and dissolved oxy-
The carbendazim [MBC (methyl-2-benzimidazole
gen concentration of the medium. In a previously
carbamate)] is a widely used systemic fungicide for
controlling a broad range of fungi a€ecting fruits,
photodecomposition of benomyl in aqueous sol-
nuts, vegetables, turf, and ®eld crops. Additionally,
the carbendazim and n-butylisocyanate (BIC) are
degradation products of benomyl fungicide(methyl-
solutions of carbendazim at di€erent pH values
1-(butylcarbamoyl)-2-benzimidazole carbamate) in
published paper (Ibarz et al., 1996), a study of the
ution was presented showing the degradation of this
fungicide. In this paper, the behavior of aqueous
under ultraviolet radiation has been studied.
water and in organic solvents (Arnold et al., 1957;
Calmon and Sayag, 1974; Chiba and Cherniak,
1978).
As systemic fungicides, benomyl and carbendazim
MATERIALS AND METHODS
can be adsorbed through the roots, leaves, and
The photochemical reactor (Ibarz and Pagan, 1986;
green tissues of plants along with water (Ben-Aziz Ibarz et al., 1996) used in the experiments was a rectangu-
lar box made of metacrylate, measuring 22.9 Â 11.9 Â
and Aharanson, 1974). These products, benomyl,
carbendazim and BIC, are toxic for humans, ani-
mals and plants. This toxicity includes e€ects in the
male mammalian reproductive system, embriotoxi-
city, teratogenesis (Axness and Fleeker, 1979; Hess
et al., 1994; Somerville, 1986) and phytotoxicity
(Shilling et al., 1994a,b).
10 cm. The distance between the lamp and the solution
surface was 22.3 cm and the volume, V, of the irradiated
solutions was 4 l. The lamp used in the experiments was a
mercury PHILIPS HPM-12 with 400
W of nominal
power. The emission wavelength range for the lamp is
250±750 nm.
The carbendazim used in the experimentation was sup-
plied by Du Pont Iberica (>99%).The bu€er solutions
(pHs 1, 5, 7, 8 and 11) were prepared with bi-distilled
Therefore, the quantitative determination is very
important of carbendazim and benomyl in water, water with additional degasi®cation during 30 min in an
ultrasonic bath. The reagents used in the preparation of
soil, wastewater, crops and foods. Recently, instru-
the bu€er solutions were of a quality for analysis and
mental analytical methods as well as immunoassays
for the quantitation of carbendazim and benomyl
were supplied by Merck. The methanol used as co-solvent
(Fluka) was of a quality for UV spectroscopy. Previously,
have been developed (Itak et al., 1993). Both car-
bendazim and benomyl are in widespread use and
persist in the environment. In an aqueous medium,
it was degassed for 30 min in an ultrasonic bath.
The solutions were prepared by dissolving the carbenda-
zim in methanol and diluting with the corresponding buf-
fer solutions to 4 l. Initial concentrations of carbendazim
were 5 mg/l.
The pH of the bu€er solutions was tested with a Crison
*Author to whom all correspondence should be addressed. pH-meter at 208C. In all cases, pH of bu€er solution pre-
Tel.: +34-973-70-25-21; fax: +34-973-23-82-64; e-mail: pared were in 20.1 interval. The dissolved oxygen concen-
aibarz@tecal.udl.es
tration was determined with an oxygen-meter Oxy-91.
2951

2952
Ricard Panades et al.
Table 1. El values for carbendazim at di€erent wavelength and pH
5
l (nm)
El 10 (m2/kmol)
pH=1
pH=5
pH=7
pH=8
pH=11
255
265
275
285
295
305
3.47
7.73
12.16
6.40
±
4.34
5.08
10.02
10.39
3.13
±
4.39
4.94
10.02
11.56
5.10
±
4.47
4.79
10.03
11.65
5.69
±
4.98
4.14
8.88
11.72
5.89
0.40
±
Fig. 2. Photodegradation of carbendazim for 6 mg/l of dis-
solved oxygen.
The decay process was followed with HPLC, using a
Waters 490 chromatograph. The column used was
a
wavelength l (m2/kmol), ql is the modulus of the
radiation energy ¯ux density vector to the wave-
length l (einstein/m2 s), a function of the position
(x, y, z ) in the reaction chamber, Cc is the carben-
dazim concentration (kmol/m3), f is the quantum
yield (mol decomposed/einstein absorbed), and rc is
the intensive degradation rate of carbendazim
(kmol/(m3 s)).
Taking into account the mass balance in the
photochemical reactor and assuming perfect mixing
in the reaction medium, it is possible to relate the
carbendazim concentration with irradiation time
(Esplugas and Vicente, 1991):
Spherisorb C-18. The ¯ow (isocratic) was 1.5 ml/min, with
composition acetonitrile:bu€er solution 60:40. The wave-
length of detector was 286 nm. In all cases, the amounts
of sample were 100 ml.
For determination of the molar extinction coecient El
at di€erent pH values a spectrophotometer PHILIPS UV/
VIS 8700 with a 1 cm quartz cell was used (Table 1).
Figure 1 shows the absorbance spectra for MBC at di€er-
ent pH of aqueous medium. In the wavelength range of
carbendazim absorption (250±310 nm), the photon ¯ux
entering the reactor was determined by uranyl±oxalate
actinometry (Calvert and Pitts, 1967) to be We ˆ 5:3
meins/s.
RESULTS AND DISCUSSION
ꢀ
X
Figures 2 and 3 show the variation of the carben-
dazim concentration with the irradiation time at
di€erent pH levels and at two values of the concen-
tration of dissolved oxygen (6 mg/l and 3 mg/l).
The degradation was shown not to take place at
very acidic conditions (pH=1). In both cases, the
degradation rate increases with the pH of the med-
ium. At alkaline conditions, the degradation looks
like a ®rst-order reaction with respect to the carben-
dazim concentration. The maximum extensive rate
achieved was 2 mmol/min at the initial time of the
UV irradiationÐat pH=11 and 6 mg/l of oxygen.
Additionally, no degradation was detected operat-
ing in the absence of ultraviolet radiation. Similar
to other organic compounds at very low concen-
tration (Ibarz et al., 1996), a simple rate equation
may be used to describe the ultraviolet photodegra-
dation of carbendazim:
dCc
dt
1
ˆ
fElqlCc dV
ꢀ2†
V
V
l
where V is the reaction volume and the summation
is extended to the wavelength range of carbendazim
absorption (250±310 nm).
The amount of radiation energy absorbed by the
compound is obtained from the expression:
ꢀ
X
Wabs
ˆ
ElqlCc dV
ꢀ3†
V
l
and because the carbendazim concentration does
not remain constant during the experimentation
and some of the photoproduct formed may absorb
radiation, the value of Wabs varies with irradiation
time. But, as ql varies with the composition of the
system, the ratio Wabs/Cc may be assumed constant
with time and equal to its value a zero time Wa0bs
C0c (Arantegui et al., 1995). That means that the
/
rc ˆ SfElqlCc
ꢀ1†
where El is the molar extinction coecient to the
Fig. 3. Photodegradation of carbendazim for 3 mg/l of dis-
solved oxygen.
Fig. 1. UV Spectra for MBC at di€erent pH.

Photodecomposition of carbendazim
Table 2. Quantum yield at di€erent pH
2953
pH
CO2=3 mg/l
CO2=6 mg/l
f (mmol/einstein)
Coecient regression
f (mmol/einstein)
Regression coecient
5
7
8
11
0.5620.05
0.7620.06
0.9020.09
4.020.7
0.9898
0.9946
0.9917
0.9943
1.120.2
1.320.3
1.620.3
6.321.2
0.9888
0.9959
0.9881
0.9951
photons absorbed by the carbendazim are pro- Carbendazim is
a product dicult to degrade
portional to its concentration. With this assump- (quantum yield lower than 0.007 mol/einstein and
tion, the di€erential equation (2) may be solved small wavelength range spectra of absorption) and
giving a logarithmic relation between the carbenda- the use of more oxidative methods, as UV/ozone or
zim concentration and the irradiation time:
UV/ hydrogen peroxide should be studied.
Cc
C c0
fWabs
C 0cV
ln
ˆ
t
ꢀ4†
REFERENCES
Arantegui J., Prado J., Chamarro E. and Esplugas S.
(1995) Kinetics of the UV degradation of atrazine in
aqueous solution in the presence of hydrogen peroxide.
J. Photochemistry and Photobiology, A: Chemistry 88,
65±74.
Arnold R. G., Nelson J. A. and Verbanc J. J. (1957)
Recent advances in isocyanate chemistry. Chem. Rev.
57, 47±76.
According to this simple equation, the experimental
results were ®tted to equation (4). Table 2 summar-
izes the regression coecient obtained in the ®tting,
and the value of the quantum yield with a con®-
dence interval of 95%.
Bearing in mind the experimental results, the
degradative process follows a ®rst-order kinetics. As
for the in¯uence of pH in the decay process,
Axness M. E. and Fleeker J. R. (1979) Metabolism of the
butylcarbamoil moiety of benomyl in rat. Pestic. Bio-
chem. Physiol. 11, 1±12.
increasing pH promotes the degradative process, Ben-Aziz A. and Aharanson N. (1974) Dynamics of
uptake, translocation, and disappearance of thiabenda-
whereas at acidic pH, the decay process is negli-
zole and methyl-2-benzimidazole carbamate in pepper
gible. It can be explained by the fact that at high
acidity the protonation takes place in the benzimi-
and tomato plants. Pestic. Biochem. Physiol. 4, 120±126.
Calmon J. P. and Sayag D. R. (1974) Kinetics and mech-
dazole ring, and the protonated product is more
stable under UV radiation than the previous one
(Fleeker et al., 1977). Moreover, in alkaline media
there is the loss of proton of the carbendazim and,
consequently, the formation of 2-aminobenzimida-
zole takes place, this product being more sensitive
toward UV radiation. In the following scheme it is
possible to appreciate the pH in¯uence.
anisms of conversion of methyl-1-(butylcarbamoil)-2-
benzimidazole carbamate (benomyl) to 2-benzimidazole-
carbamate (MBC). J. Agric. Food Chem. 24, 311±314.
Calvert J. and Pitts Jr J. N. (1967) Photochemistry. John
Wiley and Sons, New York.
Chiba M. and Cherniak E. A. (1978) Kinetic study of re-
versible conversion of methyl-1-(butylcarbamoil)-2-ben-
zimidazole-carbamate (benomyl) and n-butylisocyanate
(BIC) in organic solvents. J. Agric. Food Chem. 26, 573±
576.
Esplugas S. and Vicente M. (1991) An easy equation to
evaluate the direct radiation between a linear source and
a plane photoreactor. A®nidad 436, 371±372.
Fleeker J. R., Laci H. M., Schults I. R. and Houkom E.
C. (1977) Photolysis of methyl-2-benzimidazol carba-
mate. J. Agric. Food Chem. 25(1), 51±55.
Hess R. A., Moore B. J., Forrer J., Linder R. E. and
Abuel-Atta A. A. (1994) The fungicide benomyl causes
testicular dysfunction by inducing the sloughing of germ
cells and oclussion of e€erent ductules. Fundam. Appl.
Toxicol. 47, 1197±1206.
Ibarz A. and Pagan J. (1986) Calibrado de una lampara
para el tratamiento de contaminantes en alimentos lõqui-
dos. Alimentaria 174, 53±58.
In alkaline conditions, there will be more deproto-
nated species, which will be degraded by the ultra-
violet radiation.
The increase of the oxygen concentration in the
aqueous solution gives an increase in the degra-
dation rate. As already found in the bibliography
(Watkins, 1974), carbometoxyguanidine, diguani-
dine, and dimethyloxalate are photoproducts of the
reaction of carbendazim with oxygen.
Instead of the experimental results showing
degradation of the carbendazim in aqueous solution
at alkaline conditions and with the presence of dis-
solved oxygen, the rate achieved is not very high. Ibarz A., Panades R. and Tejero J. M. (1996) Fotodes-

2954
Ricard Panades et al.
composicion de benomilo en solucion acuosa. A®nidad
53, 51±53.
benlate formulations and on plants. J. Agric. Food
Chem. 42, 1204±1208.
Itak J. A., Selisker M. Y., Jourdan S. W., Fleeker J. R.
and Herzog D. P. (1993) Determination of benomyl (as
carbendazim) and carbendazim in water, soil and fruit
juice by a magnetic particle-based immunoassay. J.
Agric. Food Chem. 41, 2329±2332.
Shilling D. G., Aldrich H. C., Moye H. A., Ga€ney J. F.,
Tolson J. K., Querns R. and Mossler M. A. (1994b)
N,N-dibutylurea from n-butylisocyanate, a degradation
product of benomyl. E€ects on plant growth and physi-
ology. J. Agric. Food Chem. 42, 1209±1212.
Shilling D. G., Moye H. A., Aldrich H. C., Gauder J. E.,
Buszko M., Toth J. P., Brey W. S., Bechtel B. and Tol-
son J. K. (1994a) N,N Dibutylurea from n-butylisocya-
nate, a degradation product of benomyl. Formation in
Somerville L. (1986) The metabolism of fungicides. Xeno-
biotica 16, 1017±1030.
Watkins D. (1974) Photolysis of methylbenzimidazol-2-
ylcarbamate. Chemosphere 38(5), 239±240.