Adsorption and Movement of Carbofuran in Four Different Soils Varying in Physical and Chemical Properties

Abstract

Laboratory studies were conducted to determine the adsorption and movement of carbofuran on four divergent textured Indian soils at a fixed volume fraction (f_s = 0.1) of methanol/water mixtures using batch equilibrium and soil thin layer chromatography (soil TLC) techniques. The measured equilibrium adsorption isotherms for silt loam (FSL and ASL) and loam (KL) soils were L-shaped and for sandy loam (BSL) soil S-shaped, all being well fitted by the Freundlich isotherm. A higher adsorption of carbofuran was observed on FSL followed by ASL, KL and BSL soils as anticipated by the values obtained for the Freundlich constant, K_F and partition coefficient, K_D. The Frontal Retardation factor (FR_f) values obtained from soil TLC studies were inversely proportional to the K_F and K_D values.

The affinity of carbofuran towards organic carbon, organic matter and clay content of the soils was evaluated by calculating the K_OC, K_OM and K_C values. The negative magnitude of the Gibbs'free energy (ΔG⁰) indicated the spontaneity of the adsorption of carbofuran onto the soils studied. The leaching index (LI) of carbofuran calculated for the soils studied indicated its high potential to leach into shallow aquifers and ground water. Pearson's correlations calculated for K_F and K_D against the soil properties showed a highly significant correlation with the clay + silt content of the soils. Multiple regression of K_F and K_D against the soil properties indicated that the mineral content of the soils (clay and clay + silt) is the best predictor of carbofuran adsorption, followed by soil pH. Predicted log K_OM values were also evaluated by using the aqueous solubility, the 1-octanol/ water partition coefficient (K_OW), the adsorbability index (AI) and the first-order molecular connectivity index (¹χ) of carbofuran. Use of the aqueous solubility, and the 1-octanol/water partition coefficient (K_OW) to predict the extent of adsorption gave values which displayed a considerable error in comparison to those measured experimentally, whereas the use of the adsorbability index (AI) and the first-order molecular connectivity index (¹χ) of carbofuran improved the predictions considerably.

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