Adsorption and Movement of Carbofuran in Two Soils of Rajasthan: A Verification of Co-Solvent Theory and a Comparison of Batch Equilibrium and Soil Thin Layer Chromatography Techniques Results

Abstract

The adsorption and movement of carbofuran in two different types of uncontaminated soils in methanol water mixtures at different volume fractions (f_s =0. 25. 0. 50, 0. 75 and 1. 0) has been studied by batch equilibrium and soil thin -layer chromatography (Soil TLC) techniques, respectively. The measured equilibrium adsorption isotherms for both soils studied were S-shaped for methanol /water mixtures at all f_s values. All isotherms were in close agreement with the Freundlich equation. Higher adsorption and a lower movement of carbofuran was observed on loam soil than sandy loam soil at all f_s values for methanol /water systems, and was anticipated by the Freundlich constants, K_m, and the R_f values. The K_m and R_f values also confirmed that the adsorption of carbofran decreased while it's movement increased with increasing f_s values. The values of the Freundlich constant K_m were used to evaluate the co-solvent theory for describing the adsorption of carbofuran in methanol -water mixtures. For each soil, the values of the adsorption coefficient K_m decreased in a linear logarithmic fashion as f_s increased, the slope of the log K_m versus f_s plots being essentially the same for both the soils studied. Thus, the co-solvent effects on the adsorption could be expressed by a single parameter ( σ_s ) which combines the characteristics of the solvent (methanol ) and the adsorbate (carbofuran ). The aqueous phase partition coefficient K_w (mol^-1 ) normalised on f_oc for carbofuran was evaluated by extrapolating f_s = 0. The affinity of carbofuran towards the organic carbon and clay content of the soils was evaluated by calculating the K_ocand K_cc values. The data obtained indicated that, although the affinity of carbofuran was better correlated with the organic matter content. The contribution of the clay content of soils towards carbofuran adsorption can not be ignored. The negative magnitude of the Gibb's free energy (∆G°) indicated the spontaneity of the adsorption of Carbofuran onto soils. The leaching index (LEACH) and Groundwater Ubiquity Score (GUS) indexes were also calculated for predicting leaching potential for carbofuran instudied soils indicated it's high potential to leach into ground water.