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International Journal of Environmental Research, Vol. 2, No. 1, 2008, pp. 71-74 Dissipation Kinetics of Benthiocarb in Water at Different pH levels under Laboratory Condition Wasim Aktar, Md.1*, Paramasivam, M.1 and Bhattacharyya, A.2 1 Pesticide Residue Laboratory, Department of Agricultural Chemicals, Bidhan ChandraKrishi Viswavidyalaya, Mohanpur-741252, Nadia, West Bengal, India2 Export
Testing
Laboratory,
APEDA,
Bidhan
Chandra
Krishi
Viswavidyalaya,Mohanpur-741252,
Nadia,
West
Bengal,
India Received 17 May 2007; Revised 20 Oct. 2007; Accepted 5 Nov. 2007 Code Number: er08010 ABSTRACT Laboratory degradation studies were performed in water at pH 4.0, 7.0 and 9.2 using Benthiocarb (Saturn 50 EC) formulation at the rates of 1.0 (T1) and 2.0 (T2) µg/mL. Water samples collected on 0 (2h),3,7,15,30,45,60 and 90 days after treatments were processed for residue analysis of benthiocarb by GC-NPD capillary system. In 60 days, dissipation was 92.82-93.81 % at pH 4.0, 89.12-90.63 % at pH 7.0, and 92.67-93.88 % at pH9.2 in both treatments showing verylittle effectof pH on dissipation. The half-life periods observed were 15.13 and 16.01 days at pH 4.0, 17.92 and 18.81 days at pH 7.0 and 11.32 and 12.54 days at pH 9.2 at T1 and T2 doses respectively. Dissipation followed a triphasic first order kinetics in water at all the three pH levels. Key words: Benthiocarb, pH, Dissipation, Kinetics, Herbicide INTRODUCTIONBenthiocarb (S-4-chlorobenzyl diethyl thiocarbamate) [C12H16NOSCl] is a selective herbicide of thiocarbamate group. It is mainly used as a pre or post emergent herbicide in the paddy field (Sharma and Bhunia, 1999). It is applied 3-4 days after transplantation of rice. The compound is absorbed by coleoptiles, mesocotyl, roots and leaves. The compound translocates to the meristem and inhibits protein synthesis (Ishikawa, et al., 1971 and Ishikawa, 1980). As a result, shoot growth of emerging seedlings are inhibited. It is effective as pre emergence to early post emergence herbicide for the control of Echinochloa, Leptochloa, Cyperus sp. and other monocotyledons and annual broad-leaved weeds in direct seeded and transplanted rice at 3-6 kg a.i./ ha (Attalla and Kholosy, 2002). It is nonphytotoxic to rice. Benthiocarb exhibits excellent properties as a mix partner with other herbicides for controlling dicotyledonous weeds like Galium aparine, Chenopadinum alum, Setaria spp., etc. It is a moderately toxic herbicide having LD50 (acute oral) 1033 mg/kg for male rates. When used on paddy, the residues of benthiocarb in standing water may harm beneficial flora and fauna in general and fish in particular where pisiculture is practiced in paddy crop (Ishikawa, 1980). Benthiocarb was determined by using gas chromatography coupled with mass spectrometry with programmable temperature vaporizer (Hada, et al., 2000) and capillary gas chromatography equipped with atomic emission detection Stan and Linkerhagner, 1993). As there is no information available on this aspect of benthiocarb, it was thought imperative to investigate dissipation behavior of this herbicide in water at different pH levels. MATERIALS & METHODSAnalytical grade of Benthiocarb (99.1%) was obtained through the courtesy of M/S PI Industries Ltd. All the solvents like dichloromethane, acetone and ethyl acetate were glass distilled before use. Sodium sulphate was washed repeatedly with distilled acetone and activated at 110 oC for 2 h before use. Stock solution (100 µg/mL) was prepared in ethyl acetate and working solution was prepared by diluting it. The pH of water was adjusted using buffer. Buffer capsules of pH 4.0, 7.0 and 9.2 from E.Merck were used for the purpose of preparing buffer solution. One capsule was required for 100 mL of distilled water to maintain the above mentioned pH. In a series of Winchester bottles (10 L capacity) 6 L distilled water was kept and sixty capsules were added to each of the bottle. The bottles were then left in room temperature for overnight to homogenize the buffer solutions. For carrying out laboratory experiment, water (6 L) of each pH triplicate was spiked; 1.0 (T1) and 2.0 (T2) µg a.i/mL with Benthiocarb formulation and was stored in Winchester bottles from July to February 2005-06 under room temperature (15-39.5 oC). Untreated control was also carried out simultaneously. Samples were drawn periodically on 0 (2 h), 3, 7, 15, 30, 45, 60, and 90 days after treatments and analyzed for benthiocarb residues. Representative 200 mL water sample was taken in 1 L separating funnel and 5-10g sodium chloride was added to it. It was extracted thrice (100, 50, 50 mL) with dichloromethane by liquidliquid partitioning. Organic phases were combined, passed through anhydrous sodium sulphate and concentrated on a rotary vacuum evaporator under reduced pressure at 40 0C followed by a gas manifold evaporator till near dryness. Final solution was made to 1 mL in ethyl acetate and subjected to GC analysis. No clearing was required as no interference peaks were observed during analysis. The residues of Benthiocarb were analyzed on GC (Agilent Technologies 6890N Network GC system) with nitrogen phosphorous detector (NPD) coupled with Chemito 5000 data processor. The HP-5 capillary column (30m x 0.32mm i.d.) of 0.25 µmfilm thickness was used. The temperatures were: Oven 210 0C, Injector 230 0C, Detector 300 0C. Flow rate of carrier gas (nitrogen), hydrogen and air were 2, 4 and 60 mL/min, respectively. The retention time, limit of detection (LOD) and limit of quantification (LOQ) were 4.30 min, 0.01 µg/g and 0.05 µg/g, respectively. RESULTS & DISCUSSIONAverage recoveries of benthiocarb from water fortified at 0.25 and 1 µg/mL varied from 90-96% at pH 4.0, 88-98% at pH 7.0 and 88-97% at pH 9.2. Residues of Benthiocarb in water at different pH levels are presented in table 1. As evident from the data, at pH 4.0, initial residues of 0.97 µg/mL in treatment T1 dissipated to 0.89 µg/mLin 3 days, 0.32 and 0.06 µg/mL in 30 and 60 days, respectively. Corresponding dissipation were 8.25, 67.01 and 93.81%, respectively. In treatment T2, initial residues of 1.95 µg/mL dissipated to 1.67, 0.49 and 0.14 µg/mL in 3, 30 and 60 days after application with corresponding dissipationof 14.36, 72.31 and 92.82 %, respectively. At pH 7.0, initial residues of 0.96 µg/mL in T1 treatment, dissipated to 0.87, 0.35 and 0.09 µg/ mL in 3, 30 and 60 days after treatment with corresponding dissipation of 9.38, 63.54 and 90.63 % respectively, whereas in T2 initial residues of 1.93 µg/mL dissipated to 1.68, 0.69 and 0.21 µg/ mL in 3, 30 and 60 days after treatment showing dissipation of 12.95, 64.25 and 89.12 %, respectively. At pH 9.2, from treatment T1 initial residues of 0.98 µg/mL dissipated to 0.78, 0.47 and 0.06 µg/mL in 3, 15 and 45 days after treatment with corresponding dissipation of 20.41, 52.04 and 93.88 %, respectively. In T2 residues of 1.91 µg/mL on 0 (2 h) day dissipated to 1.53, 1.01 and 0.14 in 3, 15 and 45 days after treatment and corresponding dissipation was 19.89, 47.12 and 92.67 %, respectively. It is clear that Benthiocarb residues in water dissipated more than 90% in 60 days at pH levels of 4.0, 7.0 and 9.2 in both treatments under laboratory conditions under temperature ranging from 15 to 39.5 °C. The degradation was slow during first 3 days followed by relatively faster degradation from 3rd day onward to 45 days after which it became slow. Almost
identical
degradation
ranging
from
89
to
94
%
at
all
the
three
pH
levels
during
60
days
study
indicate
that
there
was
no
significant
effect
of
pH
on
degradation,
although
dissipation
was
a
little
faster
at
pH
9.2
throughout
the
studies.
Halflife
values
at
pH
levels
of
4.0,
7.0
and
9.2
varied
from
15.13
to
16.01,
17.92
to
18.81
and
11.32
to
12.54
days,
respectively
(table 2).
At
all
pH
levels
degradation
was
observed
to
be
faster
in
T1 than
T2.
The
slow
dissipation
at
higher
rate
could
attribute
to
inhibition
of
microbial
activity.
The
dissipation
of
Benthiocarb
followed
a
tri-phase
first
order
kinetics. Similar observations have been reported in degradation of metolachlor in water where dissipation was independent of pH and was subject to photodegradation following pseudo first order kinetics (Liu, et al., 2000). Dissipation ofbutachlor residues has also been reported (Chang, 1973) following pseudo first order kinetics in river waters. Whereas in another studies (Kumari and Prasad, 2002 and Prasad, 2005), decomposition of trifluralin and metribuzin in water as pH and dose dependent, resulted in faster dissipation under alkaline conditions and at low dose of application. Slightly faster dissipation of benthiocarb at alkaline pH has been observed in our studies also. In one study, it has been shown that Benthiocarb has no residues in paddy field soil as well as water during harvest (Tanigawa and Nishimura, 1999). CONCLUSIONConsidering rapid dissipation of Benthiocarb at the tested doses in water system, its much faster degradation can be expected under field condition. Consumption of paddy grain from Benthiocarb treated field could not be harmful for health as no residues of the herbicide would be found in the harvested plant samples as the dissipation of Benthiocarb would be higher in plant system due to higher enzymatic activity. Hence, the tested doses can be considered safe from the point of view of health hazards, environmental pollution and ground water contamination due to its residual effects. ACKNOWLEDGEMENTThe authors are grateful to M/s P. I. Industries Ltd., Gurgaon, India for financial assistance and providing analytical standard of Benthiocarb. We are very much thankful to Director of Research, Bidhan Chandra Krishi Viswavidyalaya, Kalyani, and Nadia for conductance of the experiment. REFERENCES
Copyright 2008 - Graduate Faculty of Environment University of Tehran The following images related to this document are available:Photo images[er08010t1.jpg] [er08010t2.jpg] |
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