Kinetics and thermodynamics of adsorption of cadusafos on soils

Laboratory batch experiments were designed to study the adsorption of cadusafos on two types of soils: clay loamy and sandy, collected from two regions around the Nile Delta in Egypt. The adsorption process is fast reaching equilibrium in 60 min, decreases with increasing soil mass and independent on pH in the range 3-11. The experimental results were well fitted to linear partition model, and to a lesser extent to Freundlich non-linear model, Temkin and Dubinin-Radushkevich models. However, the 1/n parameter of Freundlich relation being of value around one supported the linear partition model. The K(d) values obtained from the linear model were 4.20 and 2.74 L/g for sandy and clay soils, respectively. The energy of adsorption calculated from D-R equation was 4.36 and 5.04 kJ/mol for clay and sandy soils, respectively, in the range assigned to physical forces. This kind of weak interaction, together with pH-independence implies that for the studied soils the organic content (that is higher in the clay soil) is not a major parameter in the adsorption of cadusafos. Water movement taking place more readily in sandy soil caused increased uptake relative to the case of clay soil. The higher uptake in sandy soil denotes that cadusafos may be mobile with a potential to leach and eventually pollutes ground water and surface water. The kinetics of adsorption was well fitted by the pseudo-second order equation. The intraparticles diffusion is of lower significance in case of sandy soil than in clay soil. Thermodynamic parameters indicated that the adsorption is spontaneous, endothermic accompanied by increase in entropy.