Extractive spectrophotometric method for the determination of carbaryl in environmental samples

Article abstract of DOI:10.4314/bcse.v29i2.2

In the view of the potential hazards associated with the widespread use of carbaryl insecticide, a new simple extractive spectrophotometric method has been developed for its determination in environmental samples viz. soil, water and foodstuffs for its safer and more effective use. The proposed method is based on the microwave assisted alkaline hydrolysis of the insecticide to methylamine. The later is measured as methyl isobutyl ketone (MIBK) extractable yellow nickel(II)-methyldithiocarbamate complex at 380 nm through the reaction with carbon disulfide and nickel(II) acetate. The insecticide can be determined in the linearity range from 2.01 to 60.3 μg mL^-1 with a correlation coefficient of 0.996. The method has been applied to the analysis of carbaryl in its commercial formulation and its recovery from vegetable and water samples for monitoring health hazards. Recoveries of the insecticide from vegetables and spiked water samples were good, ranging from 87.6-92.8%, with RSDs ranging from 0.54-1.02%. The method has also been validated for investigating the sorption of carbaryl on five soils with different characteristics to evaluate its leaching behaviour which is a measure of ground and surface water contamination. The leaching potential of the insecticide in terms of groundwater ubiquity score (GUS) has values in the range 1.8-2.2 classifying it as transition leacher hence it has potential to contaminate groundwater.

Full text of DOI:10.4314/bcse.v29i2.2

Bull. Chem. Soc. Ethiop. 2015, 29(2), 173-186.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v29i2.2
ISSN 1011-3924
 2015 Chemical Society of Ethiopia
EXTRACTIVE SPECTROPHOTOMETRIC METHOD FOR THE DETERMINATION
OF CARBARYL IN ENVIRONMENTAL SAMPLES
Devender Kumar Sharma1, Tilak Dharmani1 and Nisha Sharma2*
1Department of Chemistry, Himachal Pradesh University, Shimla-171005, India
2Govt. P.G. College, Sarkaghat-Mandi-175024, India
(Received July 29, 2014; revised December 29, 2014)
ABSTRACT. In the view of the potential hazards associated with the widespread use of carbaryl insecticide, a
new simple extractive spectrophotometric method has been developed for its determination in environmental
samples viz. soil, water and foodstuffs for its safer and more effective use. The proposed method is based on the
microwave assisted alkaline hydrolysis of the insecticide to methylamine. The later is measured as methyl isobutyl
ketone (MIBK) extractable yellow nickel(II)-methyldithiocarbamate complex at 380 nm through the reaction with
carbon disulfide and nickel(II) acetate. The insecticide can be determined in the linearity range from 2.01 to 60.3
µg mL-1 with a correlation coefficient of 0.996. The method has been applied to the analysis of carbaryl in its
commercial formulation and its recovery from vegetable and water samples for monitoring health hazards.
Recoveries of the insecticide from vegetables and spiked water samples were good, ranging from 87.6-92.8%,
with RSDs ranging from 0.54-1.02%. The method has also been validated for investigating the sorption of
carbaryl on five soils with different characteristics to evaluate its leaching behaviour which is a measure of ground
and surface water contamination. The leaching potential of the insecticide in terms of groundwater ubiquity score
(GUS) has values in the range 1.8-2.2 classifying it as transition leacher hence it has potential to contaminate
groundwater.
KEY WORDS: Carbaryl, Spectrophotometry, Environmental toxicity, Soil-sorption study, Leaching/GUS
INTRODUCTION
Carbaryl (I) - (1-naphthyl-N-methylcarbamate) is a broad spectrum third-most-used insecticide
because of its high insecticidal activity and low mammalian toxicity. It controls over 100
species of insects on citrus, cotton, lawns, ornamentals, paddy, fruits and vegetable crops [1-3].
OCONHCH
3
I
The environmental pollution caused by the extensive use of this indispensable chemical is of
major concern because of its high toxicity. The acute oral LD50 for rats is 850 mg/kg [4]. It has
been reported that carbaryl causes alteration of liver microsomal enzymes, neurotoxicity,
immunotoxic effects, histopathological changes in the testis, haemoatopoietic alterations [5-9].
The extensive use of this insecticide poses a serious risk to both the environment and the human
health due to direct exposure or through residues in the food and drinking water [10]. Carbaryl
is the second most widely detected insecticide in the environmental samples viz foodstuffs and
water [11].
The fate of pesticides is strongly influenced by their interactions with soil as the later is an
ultimate reservoir of these chemicals irrespective of their application target. Following
application, pesticides enter the soil and then into aquatic environment, via soil percolation and
__________
*Corresponding author. E-mail: nishasharma581@gmail.com

174
Devender Kumar Sharma et al.
surface runoff and contaminate them. The pesticides in the soil get fractioned between soil
solution phase (in the free form) and soil solid phase (in the bound form) through sorption. The
sorption of pesticides affects their mobility, persistence, volatilization, toxicity and
bioaccumulation which in turn influence the extent of surface and ground water contamination
[12-13]. The knowledge of sorption characteristics of soil is necessary for predicting the
mobility and fate of pesticides in the soil. This is because detoxification mechanism of the
pesticides viz: degradation, metabolism and microbial uptake are operative on the unadsorbed
fraction (free form) of the pesticide. Thus, from environmental point of view, sorption of
carbaryl on soils is of great importance for the prediction of its movement in the soil and its
subsequent contamination of ground and surface water. The determination of this insecticide in
residues on foodstuffs and water samples and in its commercial formulation for the purpose of
monitoring pollution and health hazards is equally important.
To accomplish above objectives, simple, sensitive and reliable methods of wide applicability
are necessitated for the analysis of carbaryl. Of the various analytical methods viz. fluorescence,
voltammetric, enzymatic, immunosensor, chromatographic, capillary zone electrophoresis [1,
14-19], the spectrophotometric methods [20-25] being simple, sensitive, reliable and find wide
acceptance for the determination of carbaryl. Further, the spectrophotometric procedure can also
tolerate little interfering material and the equipment being cheap in comparison to HPLC
method commonly used for this insecticide and is easily accessible in most laboratories. The
spectrophotometric methods reported in the literature require colored reagents and number of
steps thus making methods tedious and time-consuming [22, 23]. In the present work, a simple
extractive method based on the microwave assisted alkaline hydrolysis of the carbaryl to
methylamine and its subsequent reaction with carbon disulfide and nickel(II) acetate to form
MIBK extractable yellow colored nickel(II) methyldithiocarbamate complex [Ni(DTC)2] has
been developed. The analysis is accomplished by measuring absorbance of the yellow MIBK
extract at 380 nm against a reagent blank. The method has been suitably validated for the
analysis of this insecticide in its commercial formulation and its recovery from grain, vegetable
and water samples. The method has also been validated to study the sorption of this insecticide
on five soils with different characteristics at two temperatures viz 20 oC and 30 oC. The various
sorption and thermodynamic parameters for the sorption of the insecticide have also been
calculated.
EXPERIMENTAL
Apparatus
The spectrophotometric measurements have been made with carry100 Bio UV-Vis
spectrophotometer (Varian Australia Pvt. Ltd. Mulgrave-Victoria, Australia). Domestic
microwave oven (Samsung electronics, New Delhi, India) was used to carry out hydrolysis. The
pH was made on Cyber Scam 2500 pH meter (Eutech instruments, Singapore). A shaking
incubator (Genie (TM), Banglore, India) was used in Soil sorption study. A centrifuge (Labnet Z
150 A, Labnet International Inc Woodbride, NJ, USA) was used.
Reagents and samples
Acetonitrile (Merck, Mumbai, India) was kept over phosphorus pentaoxide (5 g L-1) and
distilled twice. The analytical standard of carbaryl (supplied by the courtesy of Environmental
Protection Agency EPA, NC, USA) was used and its stock solution (10-3 M) was prepared in
acetonitrile. The purity of the sample was also established by reported method [26]. Boric acid-
phosphoric acid-acetic acid and sodium hydroxide buffer of pH ~ 4.5 was prepared by mixing
equal volumes (23.3 mL) of boric acid (Ranbaxy, SAS Nagar, Punjab, India, LR, 0.04 M
solution in water), phosphoric acid (Ranbaxy, SAS Nagar, Punjab, India, LR, 0.04 M solution in
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Spectrophotometric determination of carbaryl in environmental samples
175
water) and acetic acid (Ranbaxy, SAS Nagar, Punjab, India, LR, 0.04 M solution in water)
followed by the addition of 30 mL of sodium hydroxide (Merck, Mumbai, India, LR, 0.2 M
solution in water) to make the final volume to 100 mL [27]. Potassium hydroxide (Merck,
Mumbai, India, LR, 0.1 M solution in water), acetic acid (Ranbaxy, SAS Nagar, Punjab, India,
LR, 0.1 M in water) were prepared in distilled water. Methyl isobutyl ketone (MIBK, Merck,
Mumbai, India, AR) was used as received. Nickel(II) acetate (Central Drug House, Delhi, India,
LR), its 0.01 M solution in distilled water was prepared. Sodium sulfate anhydrous (Merck,
Mumbai, India, AR), sodium chloride (Merck, Mumbai, India, AR) and carbon disulphide
(Merck, Mumbai, India, AR) were used as received. Chloroform, commercial chloroform was
allowed to stand over anhydrous potassium carbonate (AR) overnight and then distilled. The
o
fraction distilling at 55 C was collected. Sevin, 50 WP (carbaryl formulation containing 50%
active ingredient) used in the present investigation was procured from local market.
Sample collection and preparation
The soils samples used in the sorption study were collected with hand trowel at the depth of 0.5
cm from selected regions of Solan District of Himachal Pradesh, India. The soil samples were
air dried and passed through 2 mm sieve to remove stones and large particles and kept in glass
bottles prior to experiment. The soil characteristic viz pH, organic carbon (%), clay (%) and
cation exchange capacity (CEC) were determined by reported methods [28, 29] at the University
of Horticulture and Forestry, Solan, Himachal Pradesh, India. The vegetable samples were
procured from local farm houses and water samples collected from natural drinking water
sources of the region were spiked with carbaryl insecticide for the recovery experiment.
Preparation of calibration graph for pure compound
Aliquots (0.1-3.0 mL) of standard solutions of carbaryl (10-3 M in acetonitrile) were taken in 10
mL measuring flasks and diluted to 3 mL with acetonitrile. Each solution was mixed with
aqueous potassium hydroxide (1 mL, ~0.1 M) and 3 mL of distilled water. Each solution was
mixed with one drop of carbon disulphide (~100 μL) and kept in microwave oven for 40 s (1000
Watts). The hydrolyzed solution was then treated with acetic acid (2 mL, ~0.1 M in water) to
neutralize the excess alkali and also to make the condition slightly acidic. The solution was
poured into a 100 mL separating funnel containing 1 mL, 0.01 M nickel(II) acetate solution and
5 mL of buffer of pH ~ 4.5 (a mixture of boric acid-phosphoric acid-acetic acid and sodium
hydroxide). The contents of the funnel were equilibrated two times with MIBK using 4 mL each
time and total volume of the MIBK extract was made to 10 mL with MIBK. The solution was
dried with anhydrous sodium sulfate and the absorbance of yellow colored solution was
measured at 380 nm against a reagent blank (Figure 1) and calibration curve was prepared
(Figure 2) by plotting absorbance values against concentration. The calibration characteristics
viz. Beer’s law range, molar absorptivity, Sandell’s sensitivity, slope and intercept values and
correlation coefficient calculated from calibration graph are given in Table 1.
Formulation analysis
A wettable powder (WP) formulation ‘‘Sevin’’ containing 50% active ingredient (carbaryl)
procured from an authorized pesticide dealer was used. A single large sample of formulation
equivalent to 10.05 mg active ingredient was dissolved in10 mL acetonitrile and diluted to a
known volume (50 mL) with acetonitrile. Suitable aliquots of the above solution were taken and
processed for analysis in the same manner as described above for pure compound.
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Devender Kumar Sharma et al.
Figure 1. Absorption maximum for carbaryl.
Figure 2. Calibration graph of carbaryl (as nickel(II) methyl dithiocarbamate) complex) at 380
nm.
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Spectrophotometric determination of carbaryl in environmental samples
177
Table 1. Spectrophotometric determination of carbaryl as colored Ni(DTC)2 complex: calibration
characteristics.
S. No.
Optical characteristics
λmax
Corresponding value
380 nm
1
2
3
4
5
6
7
8
Beer’s law range
Molar absorptivity (ε)
Sandell’s sensitivity
Stability of color
Slope
2.01-60.3 μg mL-1
2.66×103 L mol-1cm-1
0.076 µg cm-2
120 min
0.013
0.011
0.996
Intercept
Correlation coefficient (R)
Determination of carbaryl in vegetable and water samples
A carbaryl standard solution was prepared by dissolving 10.05 mg insecticide in 50 mL
acetonitrile; this solution was further diluted 10 times with acetonitrile to a final concentration
of 20 μg mL-1 carbaryl. Aliquots (0.25-1.00 mL) of this standard solution were added to 20 g
portion of cauliflower and peas placed in glass containers. After thorough mixing with a Teflon-
blended mechanical stirrer, each sample was blended with 50 mL of chloroform in the same
containers according to the general procedure of Schenk et al. [30]. The samples were filtered
through coarse filter paper (Whatman Grade No. 4) and each filtrate was transferred into 250
mL separating funnels. Sodium chloride (5 g) was added to each sample, the contents were
shaken for one minute and the phases were allowed to separate for 15 min. The lower aqueous
phase and any emulsion were discarded. Anhydrous sodium sulfate (4 g) was added and the
funnel was shaken for 30 s. The dried extract was filtered through coarse filter paper and filtrate
evaporated to dryness on a water bath at 60 oC. The residue was dissolved in 3 mL of
acetonitrile and was processed for analysis as described above for the pure compound.
The proposed method has also been applied to the determination of the insecticide in spiked
water samples. Aliquots (0.25-1.00 mL) of standard solution having concentration 20 μg mL-1
were added to 25 mL of water samples and were mixed thoroughly. The samples were extracted
with chloroform and the extract processed for analysis in the same manner as described above.
Interference study due to diverse metal ions
Various aliquots of each diverse ion solution (10-1 M) were added to the standard solution
containing 20 μg of carbaryl prior to hydrolysis. Each solution was processed for analysis of
carbaryl in the same manner as described for the compound.
Soil sorption study
Adsorption isotherms of carbaryl on five Indian soils with different soil characteristics were
obtained by the batch equilibration technique using 50 mL of conical flask at two different
o
o
temperatures, i.e. 20 C and 30 C. Triplicate soil samples (2 g) were equilibrated with carbaryl
formulation in the concentration range equivalent to 10.05-60.3 μg mL-1 of carbaryl on shaker
(Genie (TM), Banglore, India) at 150 rpm at respective temperature for 6 h equilibrium time
(estimated time required for equilibrium to be reached between carbaryl adsorbed and in
solution). After equilibration, the suspensions were centrifuged and the equilibrium
concentrations (γe) were determined in supernatants by the spectrophotometric procedure
described above. Adsorption isotherms were evaluated by using Freundlich’s sorption equation
which is written as
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Devender Kumar Sharma et al.
X = Kf γenf
(1)
where X is the amount of pesticide adsorbed mg/kg on the adsorbent; γe is the equilibrium
solution concentration (mgL-1); Kf and nf are sorption coefficients that characterize the sorption
capacity of adsorbent. The sorption coefficients Kf and nf are calculated from the least square
methods applied to the linear form of the Freundlich’s sorption equation
log X = log Kf + nf log γe
(2)
Another parameters for the sorption process viz. distribution coefficient or soil-sorption
coefficient (Kd), soil organic carbon partition coefficient (Koc) and Groundwater Ubiquity Score
(GUS) have been calculated by using equations 3-5, respectively [31-33].
Kd = X/ γe
(3)
(4)
(5)
Koc = Kd×(100/%OC)
GUS = log(t1/2) [4-log(Koc)]
where t1/2 is pesticide persistence (half life, days), OC is organic carbon content of the soil. GUS
score is used to study the leaching behaviour of pesticides and these can be classified as leacher
in which GUS values are higher than 2.8, transition with GUS values between 1.8 and 2.8 and
non-leacher pesticides with GUS values lower than 1.8 [33].
The thermodynamic parameters viz. Gibb’s free energy (∆Go), enthalpy change (∆Ho) and
entropy change (∆So) have also been calculated by using equations 6-8, respectively [34].
∆Go = -RT ln Kd
(6)
(7)
(8)
ln{(Kd)2/(Kd)1} = ∆Ho/R {(T2-T1)/ T1T2}
∆So = ((∆Ho- ∆Go)/ T
where R = gas constant, T = absolute temperature. All these parameters for the sorption of
carbaryl on five Indian soils with different characteristics were calculated.
RESULTS AND DISCUSSION
Standardization of the reaction conditions
Effect of various experimental parameters on the development, stability and sensitivity of the
color, vis-à-vis development of the proposed method has been studied before applying it to the
analysis of carbaryl in commercial formulation, vegetables, water and soil sorption study. The
experimental conditions optimized for developing the proposed method are:
Effect of time of hydrolysis
The optimum time required for complete hydrolysis (in microwave) of carbaryl to methylamine
(dithiocarbamate) was obtained by varying the time of hydrolysis from 10-80 s and measuring
corresponding absorbance of yellow nickel(II) complex at 380 nm. It has been observed that
maximum color intensity and its stability was obtained corresponding to 40 s hydrolysis time
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Spectrophotometric determination of carbaryl in environmental samples
179
(Figure 3). Hydrolysis time less than 40 s gives less intensity of the color indicating non
completion of hydrolytic reaction to form methylamine.
Figure 3. Effect of the time of hydrolysis in microwave on the absorbance value of carbaryl as
(as nickel(II) methyl dithiocarbamate) complex) at 380 nm (concentration of carbaryl
= 10 g).
Effect of pH
It has been observed that maximum yellow color with high stability was obtained in acidic
medium at pH ~ 4.5 using universal buffer mixture (boric acid–phosphoric acid-acetic acid–
sodium hydroxide) [27]. Further, the extraction of colored product into Methyl isobutyl ketone
is also best at this pH. Methyl isobutyl ketone is an extracting solvent of choice because it is not
only safe in comparison of commonly used toxic extracting solvents like benzene, chloroform,
dichloromethane, ethyl acetate etc. and it relatively extract the colored product quantitatively
from aqueous solution. Nickel(II) acetate (coloring reagent) remains in aqueous phase having no
affinity for organic phase.
Quantification
Under the optimized experimental conditions, the proposed spectrophotometric method obeys
Beer’s law in the range of 2.01-60.3 µg mL-1 for carbaryl. The method is quite sensitive and the
molar absorptivity (ε) and Sandell’s sensitivity were found to be 2.66×103 L mol-1cm-1 and 0.076
µg cm-2 at 380 nm, respectively. The method has subsequently been applied to the determination
of carbaryl in commercial formulation, vegetables and spiked water samples; the recoveries of
the insecticide from commercial formulation were in the range 93.2-96.6% of the nominal
content with relative standard deviations (RSDs) in the range 0.4-1.5% (Table 2). The results
have however been compared by an independent method [26]. The formulation analysis is
essential not only to ensure the quality of the marketed samples of the insecticide but also to get
reliable residue/sorption data. The recoveries of the insecticide from vegetables and spiked
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Devender Kumar Sharma et al.
water samples were good, ranging from 87.6-92.8%, with RSDs ranging from 0.54-1.02%. The
recoveries of carbaryl from vegetable and water samples are given in Table 3.
Table 2. Assay of a commercial formulation of carbaryl containing 50% active ingredient (Sevin).
Amount taken (µg)
Recovery (%)a
Present method
93.8±0.8
Comparison methodb
90.4±0.6
4.02
10.05
20.10
30.15
60.30
96.6±0.6
93.2±0.4
94.2±1.2
95.5±1.5
94.2±1.2
91.4±0.7
92.6±0.8
96.0±2.0
aValues are mean of five determinations with standard deviation (±). bReference method [26].
Table 3. Recovery of carbaryl from vegetable and water samples.
Recovery (%)a
Carbaryl
(μg)
5.0
10.0
15.0
20.0
Present method
Pea
90.4 ± 0.6
91.6 ± 0.8
90.8 ± 0.7
89.2 ± 1.0
Comparison methodb
Cauliflower
90.4 ± 0.5
89.3 ± 0.7
88.2 ± 0.8
87.6 ± 0.9
Water
Cauliflower
Pea
Water
89.4±0.8
87.8±0.8
85.6±1.0
84.8±0.9
91.6 ± 0.7
90.2 ± 0.8
91.4 ± 0.9
92.8 ± 1.0
86.2±0.8
87.0±0.8
85.2±0.9
85.8±1.0
84.8±0.8
85.2±0.84
85.8±0.9
84.8±1.0
aValues are mean of five determinations with standard deviation (±). bReference method [26].
Proposed mechanism for the spectrophotometric method
The present spectrophotometric method is based on the color reaction of methylamine
(hydrolytic product of carbaryl) with carbon disulphide to form dithiocarbamate which reacts
with nickel(II) acetate in water to form yellow colored [Ni(DTC)2] complex which is extractable
into MIBK that absorbs at 380 nm (Figure 1). That methylamine is formed from the alkaline
hydrolysis of carbaryl [35] and reacts smoothly and quantitatively with carbon disulphide and
nickel(II) acetate (in 2:1 molar ratio) forming metallic complex nickel(II) methyl
dithiocarbamate, [Ni(DTC)2] is quite well known [36-37]. The extraction process of the metallic
complex in MIBK has been found to occur in acidic medium at pH ~ 4.5 using universal mixture
buffer (boric acid–phosphoric acid-acetic acid–sodium hydroxide) [27] and measured
spectrophotometrically under optimized conditions at 380 nm.
OH
OCONHCH
3
+ 2 KOH
2CH NH + K CO + 2CS
2
+ K CO + CH NH
2
2
3
3
3
2
2
3
2CH NH.CS.SK + CO + H O
3
2
2
2CH NH.CS.SK + Ni(OAc)
[(CH NH.CS.S) Ni] + 2KOAc
3
2
3 2
Scheme I. Proposed mechanism for the spectrophotometric method.
Interference study due to diverse metal ions
To assess the validity of the proposed method in soil sorption study, effect of various common
ions on the determination of the insecticide was studied. Known amounts of the diverse ions
(10-1 M) as their aqueous solutions were added to the standard solution containing 20 μg of
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Spectrophotometric determination of carbaryl in environmental samples
181
carbaryl prior to hydrolysis and the solution was analyzed for carbaryl by the proposed method.
The method was found to be free from interferences due to common ions, e.g. Na+, K+, Mg2+,
Ca2+, Al3+, Pb2+,OAc-, PO43-, NO3 , NO2 , CO32-, SO42-, Cl- ions present in soil. The tolerance
limit of these ions is given in Table 4.
-
-
Table 4. Effect of various diverse ions on the determination of carbaryl.
Diverse ions
Na+, K+,
Mg2+, Ca2+
Al3+
Tolerance limit* (mg/L)
300
150
100
50
300
150
200
200
200
150
300
Pb2+
OAc-
3-
PO4
-
NO3
NO2
CO3
-
2-
2-
SO4
Cl-
*The amount causing an error 2±1.5%.
With a view to evaluate the leaching potential of carbaryl a measure of ground and surface
water contamination, its sorption have been studied on five soils with different characteristics
(Table 5) by using batch-equilibrium method at two temperatures viz. 20 oC and 30 oC,
corresponding to the maximum temperature experienced in the field.
Table 5. Characteristics of the different Indian soils used in the sorption study of carbaryl.
Soil sample
pH
7.2
7.6
6.5
6.8
7.6
Clay (%)
32.6
18.2
20.0
23.4
Organic carbon (%)
Cation exchange capacity (meq/100 g)
I
II
III
IV
V
0.8
0.9
1.5
1.6
0.7
13.1
12.9
11.0
12.8
11.6
14.1
To study the sorption phenomena Freundlich sorption equation is most reliable because of
possible rigorous derivation and more realistic assumptions compared with other equations [38].
The Freundlich adsorption isotherms are shown in Figures 4 and 5. These isotherms have been
classified as S-type of Gile’s classification (Table 6) according to the initial slope of the curve.
S-type of isotherms represents a system where solid surface has higher affinity for the solvent
than for solutes [11] (e.g. water competes strongly with solute for sorption sites). Freundlich’s
sorption coefficients Kf and nf were calculated from the plot of log X versus log γe (Figure 6)
from Freundlich’s sorption equation and the results are presented in Table 6. The sorption
coefficient Kf represents the amount of pesticide adsorbed at an equilibrium concentration of 1
mg L-1 and nf represents the variation in sorption with varying concentration of pesticide [13].
The observed values of nf were more than 1, (>1.0) in all five soils represent S-type isotherms,
which infer that there is competition between pesticides and water molecules for the sorption
sites on surface of the adsorbents [39].
Other parameters such as soil sorption coefficients (Kd), soil organic carbon partition
coefficient (Koc), Gibb’s free energy for sorption process (ΔGo), enthalpy change (ΔHo) and
entropy change (ΔSo) for the sorption of carbaryl were also calculated and are presented in
Tables 6 and 7. Kd for a pesticide is soil-specific and vary with texture and organic matter
content of the soil, while the Koc is less soil specific [40] and is calculated by normalizing
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Devender Kumar Sharma et al.
sorption coefficient (Kd) with the organic carbon (OC) content of the soil. The values of Kd
represents the extent of sorption and in general higher the Kd value, greater is pesticide sorption.
Of the five soils of different characteristics studied the decreasing sorption order is: Soil IV >
Soil III > Soil I > Soil II > Soil V.
Figure 4. Adsorption isotherm of carbaryl sorption on soils I–V at 20 oC (error bars represent the
standard deviation of three replicates).
Figure 5. Adsorption isotherm of carbaryl sorption on soils I–V at 30 oC (error bars represent the
standard deviation of three replicates).
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Spectrophotometric determination of carbaryl in environmental samples
183
Figure 6. Plot of log X versus log γe for the evaluation of Freundlich’s sorption coefficients Kf
and nf for five soils (a) at 20 ºC (b) at 30 ºC.
The higher sorption in the soils IV and III can be explained on the basis of the higher
organic content in these soils [41]. The disparity among the Koc values obtained in these soils
o
o
suggest that the sorption of carbaryl on five studied soils at 20 C and 30 C is not only
influenced by OC content of the soil alone but also governed by some other factors. The value
of cation exchange capacity (CEC) is directly proportional to hydrophobic nature of adsorbent.
Greater is the value of CEC of the soil, its surface will be more hydrophobic and carbaryl being
more hydrophobic (low water solubility) thus have higher sorption affinity for soils with higher
CEC [42].
The sorption of carbaryl in five soils has been found maximum at 20 oC than at 30 oC. This
may be due to the fact that an increase in temperature might result in weakening of binding
forces between carbaryl and soil particles. Similar results have also been reported in literature in
the sorption studies of other pesticides [43].
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Devender Kumar Sharma et al.
Table 6. Sorption parameters for the sorption of carbaryl insecticide on five Indian soils at two different
temperatures.
Soil sample
Kf
nf
Kd
Koc
20 ºC
GUS
Adsorption isotherm
I
II
III
IV
V
1.1
2.2
2.1
3.4
2.1
1.5
1.2
1.4
1.2
1.7
4.5
4.2
5.8
5.9
4.1
559.0
471.1
386.7
366.8
581.5
1.8
1.9
2.1
2.1
1.8
S-type
S-type
S-type
S-type
S-type
30 ºC
I
II
III
IV
V
2.0
1.7
1.9
1.2
3.7
1.7
1.6
1.8
1.6
1.8
4.1
3.9
5.2
5.3
3.6
516.7
436.3
347.8
330.1
511.9
1.9
2.0
2.1
2.2
1.9
S-type
S-type
S-type
S-type
S-type
The thermodynamic parameters for the sorption of carbaryl insecticide on five soils were
also evaluated and are presented in Table 7. The ΔGo values for sorption of carbaryl were
negative at both temperatures. The magnitude of ΔGo also showed that the interactions of
pesticides with the soil were thermodynamically spontaneous process and sorption occurred
through a bonding mechanism. At constant temperature, the higher the value of ΔGo the slower
is the rate of sorption. The results obtained in the present study indicate an increase in the values
of ΔGo with temperature. Variation of ΔGo with temperature may be due to the increase in the
degree of freedom of adsorbed molecules, which enhances desorption rather than sorption at
higher temperatures [34, 44, 45]. The enthalpy change ∆Ho calculated for carbaryl were in the
range (-5.60 to -9.47 kJ mol-1). The negative values of ∆Ho indicated that the interaction of
pesticides with the soil is an energetically favorable exothermic process. The ∆Ho values explain
the binding strength of pesticides to the soil; the lower negative value of ∆Ho indicates stronger
binding. This indicates that the interactions between the pesticide and the studied soil samples
were stronger at lower temperature [34, 44, 45]. The entropy change ∆So calculated for the
adsorption of carbaryl on all five soils were negative indicated that the sorption involves
stabilization, association, fixation or immobilization of the pesticide molecules as a result of
which sorption decreased the degree of freedom, causing negative entropy effect and increasing
the order of the system [34, 44, 45].
Table 7. Thermodynamic parameters for the sorption of carbaryl on five Indian soils.
Soil samples
ΔGo (kJ mol-1)
ΔHo (kJ mol-1)
25 ºC
ΔSo (kJ K-1 mol-1)
25 ºC
20 ºC
-3.65
-3.52
-4.28
-4.31
-3.42
30 ºC
I
II
III
IV
V
-3.58
-3.45
-4.16
-4.19
-3.22
-5.84
-5.60
-7.78
-7.82
-0.0075
-0.0071
-0.0119
-0.0120
-9.47
-0.0206
A number of models are available to evaluate the leaching potential of pesticides and
associated surface and ground water contamination. The GUS is the most commonly used model
which relates pesticide persistence (half life) and Koc. The leaching potential of the insecticide in
terms of GUS index was evaluated by using the experimentally observed Koc value for each soil
of different characteristics and the reported half life of carbaryl [33]. The GUS score for
carbaryl has been observed in the range 1.8-2.2 which classifies it as a transition insecticide and
has a potential to contaminate ground and surface water [46]. This observation calls for the
Bull. Chem. Soc. Ethiop. 2015, 29(2)

Spectrophotometric determination of carbaryl in environmental samples
185
judicious use of this insecticide by adjusting the application dose according to the soil type
especially near the water bodies. Also the soils should be amended with farmyard manure and
compost which are rich in organic matter content in order to increase the insecticide retention
and thus reducing its mobility. The later application will also improve the soil fertility and
health by serving as a source of soil nutrients.
CONCLUSION
The proposed method is simple, rapid, reliable and cost effective and is free from interferences
of most of the common ions present in the soil. The microwave assisted hydrolysis of the
carbaryl insecticide helps in reducing time from minutes to seconds, instantaneous development
of the color coupled with well established stoichiometry of the reaction are some other salient
features of the method. The recoveries of carbaryl from water and foodstuffs are good. The
leaching potential of carbaryl in terms of GUS score (1.8-2.2) classify it as a transition leacher,
suggesting its weak sorption on the soils leading to its greater mobility, thus having potential to
leach to the ground water.
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