Kinetics of Simazine Sorption on Soils and Soil Particle Size Fractions

Abstract

Sorption behavior of simazine in whole soils and four selected soil particle size fractions of two Hiroshima soils, a loam and a decomposed granitic (DG) soil, was measured during two-step sorption experiments. In the first sorption step, adsorption kinetics were followed for 36 hours in initially simazine-free soil samples. A rapid adsorption was observed for all soil particle size fractions after 10 minutes. Between 10 minutes and 36 hours adsorption slowly progressed in all loam size fractions while adsorption stopped in all DG size fractions. In the second sorption step, 70% of the solution in the soil samples was substituted with simazine-free solution and sorption behavior was followed for an additional 36 hours. A rapid desorption took place in all soil samples during the first 10 minutes, followed by a slow adsorption in the loam size fractions and no apparent adsorption or desorption in the DG size fractions. The amount of simazine rapidly adsorbed (step I) and desorbed (step II) was best correlated with the soil specific surface area for the low-organic DG size fractions and with soil organic matter content for the loam size fractions. For both sorption steps I and II, the adsorption kinetics in the loam size fractions were well described by a modified Freundlich type equation. Adsorption kinetics during step II were dependent on both the soil organic matter content and the total amount of simazine adsorbed during step I, likely due to diffusion-controlled sorption of simazine on the soil organic matter.