ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 10, 309-3 13 (1985)

Adsorption Thermodynamics of Carbofuran on Sandy Clay Loam and Silt Loam Soils

R. P. SINGH,* K. G. VARSHNEY, AND SIMA RANI

*Section of Plant Pathology and Nematology, Botany Department, and Analytical Laboratories, Chemistry Section, Faculty of Engineering & Technology, Aligarh Muslim University,

Aligarh-202001 India

Received January 31, I985

The adsorption of carbofuran on sandy clay loam and silt loam soils has been studied at 30 and 50°C. Adsorption data for both soils are in close agreement with the Freundlich isotherms and yield “S” class isotherms. The order of adsorption of carbofuran is in accordance with the partial molal free energy changes and conforms to the mobility of carbofuran in different soils. The thermodynamic constant (IQ and standard free energy (AGO), enthalpy (AH”), and entropy changes (AS’) have also been calculated for predicting the nature of adsorption. 0 1985 Academic

Press. Inc.

INTRODUCTION

Soil colloids are known to react with pesticides, affecting their stability and biological activity in soils as well as environmental pollution. The adsorption studies of pesticides (Bailey and White, 1970; Biggar and Cheung, 1973; Nearpass, 1965; Van Bladal and Moreale, 1974) are, therefore, necessary to understand the behavior of pesticides in soils and clays for their more judicious use in field crops. Carbofuran (2,3-dihydro- 2,2-dimethyl-7-benzofuramyl methylcarbamate) has been the least studied (Felsot and Wilson, 1980; Jamet and Piedallu, 1975) material for this aspect, a recently intro- duced systemic nematicide. Therefore, the following summarizes the results of our thermodynamic studies of adsorption of carbofuran on sandy clay loam and silt loam soils.

EXPERIMENTAL

Reagents and chemicals. Carbofuranlfuradon-3G was obtained from Rallis India Ltd., Bombay. All other reagents and chemicals were of BDH (A.R.) grade.

Apparatus. A shaking water-bath incubator having a temperature variation of t- 0.1 “C was used for all the adsorption studies and a Bausch & Lomb Spectronic-20 calorimeter was used for the spectrophotometric studies.

Adsorption studies. Surface soils (at a depth of 0 to 30 cm) from Aligarh Muslim University farm (silt loam) and sandy clay loam near Aligarh Fort were air-dried, crushed, and sieved. Their physicochemical properties were determined by the standard techniques and summarized in Table 1.

One-gram fractions of the soil were shaken in various stoppered conical flasks at the desired temperature (30 or 50°C) for 2 hr with the carbofuran solution (250 pg/ml in CH,OH) in varying amounts (0 to 10 ml). The volume in each flask was made up to 20 ml by adding demineralized water (DMW). After centrifuging for 5

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310 SINGH, VARSHNEY, AND RAN1

TABLE 1

SOME PHYSICCICHEMICAL PROPERTIES OF SANDY CLAY LOAM AND SILT LOAM SOILS

Organic Sand Silt Clay - CEC matter

Soil (%) (%I (%I (meq/lOO d (%I PH

Sandy clay loam 50.00 27.60 22.40 17.3 0.19 8.35 Silt loam 32.68 57.00 10.06 16.0 0.61 7.75

min the supernatant liquid was analyzed spectrophotometrically (Mithyantha and Perur, 1974) for the presence of carbofuran.

The movement of carbofuran in these two soils was studied by the soil thin-layer chromatographic technique (Helling and Turner, 1968; Singh et al., 1977). KOH (5%) and p-nitrobenzene diazonium fluoroborate ( 1%) solutions in methanol were used as detectors.

RESULTS

Adsorption of carbofuran on sandy clay loam and silt loam soils at 30 and 50°C is represented by the adsorption isotherms (Fig. 1) which are concave to the Y axis at both the temperatures up to an equilibrium concentration -0.2 X 10e3 mmol/ml for both soils. Above this concentration a reversal is obtained. Adsorption decreases with the rise in temperature in both soils. Also, it is higher in a sandy clay loam soil than in the silt loam soil. The average partial molal free energy changes, obtained from the thermodynamic relationship

-F = RTlnz (1)

where C, and Co are the equilibrium and initial concentrations of carbofuran in sus- pension, respectively, are as follows for the two soils at 30 and 50°C:

~~~~, _i:,.l ( 0.0 0.10 0.20 0.30 040 0.0 0.10 0.20 0.30 040

&X103 Equilibrium concentration of Carboturan in m. moles/ml

FIG. I. Adsorption isotherms of carbofuran on sandy clay loam and silt loam soils at 30 and 50°C.

THERMODYNAMICS OF CARBOFURAN ON LOAM 311

30°C 50°C

Sandy clay loam Silt loam

124. I 691.3 639.3 636.1

The frontal R/values of carbofuran in DMW are 0.65 and 0.76 for sandy clay loam and slit loam soils, respectively.

The adsorption behavior of carbofuran on these two soils is in close agreement with the freundlich equation

x/m = KC”” (2)

where x/m = millimoles of carbofuran adsorbed per gram of soil, C is the concentration of carbofuran in equilibrium suspension per milliliter, K and l/n are the constants evaluated from the intercept and slope of the curve (Fig. 2) respectively. The values are summarized in Table 2.

The thermodynamic equilibrium constant (Ko) for the adsorption reaction was ob- tained by plotting ln(C,/C,) vs C, and extrapolating C, to zero (Biggar and Cheung, 1973), where C, = millimoles of carbofuran adsorbed per gram and C, = millimoles of carbofuran per milliliter in equilibrium suspension.

The standard free energy changes (AGO) for the interaction were calculated (Glas- stone, 1960) from the relationship

AG” = -RT In K. (3)

where R is the universal gas constant and T is the temperature in degrees Kelvin. The standard enthalpy changes (AH”) were then calculated from the Van? Hoff isochore

ln[g]=-?[$--!.-I

SILT LOAM SOIL SANDY CLAY LOAM -2.0

VI -3.2 - 0

F d -3.6 -

(4)

II I II I I, I I I, -5.0 -4.6 -4.2 -3.6 -3.4 -5 0 -4.6 -4.2 -3.6 -3.4

Log G

FIG. 2. Freundlich isotherms of carbofuran adsorption on sandy clay loam and silt loam soils at 30 and 50°C.

312 SINGH, VARSHNEY, AND RAN1

TABLE 2

FREUNDLICHISOTHERMCONSTANTSFORCARBOFURANADSORPTIONONSANDYCLAY LOAMANDSILTLOAMSOILSAT 30 AND 50°C

Freundlich isotherm constants

K l/n

Soil 30°C 50°C 30°C 50°C

Sandy clay loam 2.63 X lo-’ 1.20 x 10-5 1.58 1.64 Silt loam 2.60 X 1O-5 1.05 x 10-5 1.38 1.54

and standard entropy changes (AS”) by the equation

AG” = AH” - TAP. (5)

Table 3 summarizes the values of the various parameters obtained as above.

DISCUSSION

The S shape of the isotherms for both the soils suggests that the adsorption of carbofuran is easier at a higher concentration. It may be due to a marked localization of the forces of attraction over the carbonyl group of carbofuran leading to an inter- action with soil cations. Also, the adsorption decreases with a rise in temperature from 30 to 50°C in both soils (Fig. 1). It may be due to the fact that the attractive forces are weaker at higher temperature. The interaction is thus characterized by a negative heat of adsorption (AH”). The adsorption is higher in sandy clay loam than in silt loam soil at both the temperatures. It is also supported by the frontal Rf values of carbofuran in the two soils. Further, the values of l/n (Freundlich isotherm constant) are greater than unity and increase with temperature and Kdecreases with temperature. These results also confirm a higher adsorption behavior of sandy clay loam than the silt loam soil. A similar trend was reported earlier for the adsorption of fenuron and monuron on montmorillonites (Van Blade1 and Moreale, 1974). A negative free energy change (AGO) indicates that the reaction is spontaneous with a high affinity for car- bofuran. It also suggests a high persistance and resistance to degradation of carbofuran

TABLE 3

VALUESOFTHEVARIOUSTHERMODYNAMICPARAMETERSFORTHEADSORPTIONOF CARB~FURANONSANDYCLAYLOAMANDSILTLOAMSOILS

Soil

Thermodynamic parameters

Sandy clay loam

30°C 50°C

Silt loam

30°C 50°C

KO 1.0103 1.0075 1.0101 1.0080 AGo (Cal/mob -6.1533 -4.8 182 -6.0689 -5.1486 A Ho (Cal/mol) -26.3802 -20.0602 AS’ (Cal/mol/deg) -0.1074 -0.0462

THERMODYNAMICS OF CARBOFURAN ON LOAM 313

in soils. Negative values of the standard enthalpy change (AH”) show that carbofuran interaction with both soils is exothermic and the products are energetically stable with a high binding of the pesticide to the soil sites. A negative entropy change (As’) indicates a greater order of reaction during the adsorption of carbofuran in both soil systems and at both temperatures. It may be due to the fixation of carbofuran to the soil sites resulting to a decrease in the degrees of freedom of the pesticide.

CONCLUSION

These studies illustrate the dependence of temperature and nature of the soil on adsorption behavior of carbofuran. The adsorption is less at a higher temperature. Also, it is higher in a sandy clay loam soil than in a silt loam soil. The behavior is similar to that of fenuron and monuron on montmorillonites studied earlier.

ACKNOWLEDGMENTS

The authors thank Dr. S. K. Saxena and Dr. K. T. Nasim for providing research facilities, and to the CSIR and ICAR, India, for the financial assistance.

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