Adsorption and Movement of Endosulfan in Soils: A Verification of the Co-solvent Theory and a Comparison of Batch Equilibrium and Soil Thin Layer Chromatography Results

摘要

Batch equilibrium and soil thin-layer chromatography (soil TLC) techniques were used to investigate the influence of different volume fractions (f(s) = 0.25, 0.50, 0.75 and 1.0) of organic co-solvents (acetone and methanol) oil the adsorption and movement of endosulfan in three different types of Indian soils (silt loam, loam and sandy loam). The measured equilibrium adsorption isotherms for all three soils studied were L-shaped for both co-solvent/water mixtures at all f(s) values. All isotherms were in close agreement with the Freundlich equation. Higher adsorption of endosulfan was observed on silt loam soil followed by loam and sandy loam soils at all f(s) values for both co-solvent systems, as was anticipated from the values of the Freundlich constants, K, and the partition coefficients, K-D The order of the K and K-D values also confirmed that endosulfan adsorption was higher in methanol/water mixtures than in acetone/water Mixtures and decreased with increasing f(s) values. The R-f values obtained from soil TLC studies were inversely proportional to the K and K-D values for both co-solvent systems. The higher K and K-D values and lower R-f values in methanol/water mixtures relative to acetone/water mixtures for all the three soils studied indicated that acetone had a greater potential for ground water contamination through leaching from potential sites than methanol. The affinity of endosulfan towards the organic carbon and clay content of the soils was evaluated by calculating the K-OC and K-C values. The data obtained indicated that, although the affinity of endosulfan was better correlated with the organic carbon content, the contribution of the clay content of soils towards endosulfan adsorption cannot be ignored. The adsorption data were used to evaluate the co-solvent theory for describing the adsorption of endosulfan in acetone/water and methanol/water mixtures. The aqueous-phase partition coefficients, K-DW(mol/a), normalized with respect to f(oc) for endosulfan were evaluated via the extrapolation f(s) -> 0.