Spinosad Adsorption on Humic and Clay Constituents of Lacustrine Egyptian Soils and Its Leaching Potential

Adsorption of pesticides on soils is a key process for the assessment of their fate and transport in the environment. The kinetics and adsorption/desorption isotherms of spinosad on Abis soil and its clay and humic acid (HA) soil fractions were studied using batch experiments. Equilibrium time was investigated at 10 mg l^- 1 spinosad initial concentration in 0.01M CaCl2 background solution. Adsorption isotherms were carried out at different initial spinosad concentrations (0.1, 0.5, 1, 5, 10, 50 and 100 mg l^-1). Desorption experiment was started immediately after adsorption equilibrium was reached through two successive dilution steps with 0.01M CaCl2 solution. Equilibrium time of spinosad adsorption was at 24 h. Soil clay fraction had higher affinity to adsorb spinosad than HA and Abis soil had the least affinity. Adsorption data were successfully fitted to the Freundlich equation. Slopes of the linearized Freundlich equation, 1/nads were 1.11, 1.54 and 1.49 and the intercepts, Kads were 2.09, 5.02 and 9.21 for Abis soil, HA and clay fractions, respectively. Spinosad was not completely desorbed from the sorbents tested and the coefficient of hysteresis was 0.68 for Abis soil. Spinosad had KOC of 1050 l kg^-1. The calculated groundwater ubiquity score (GUS) index of spinosad in Abis soil ranged from 0.93 to 1.20 and from 2.16 to 2.35 indicating spinosad very low to low and moderate leaching potential under aerobic and anaerobic conditions, respectively.