Adsorption-Desorption of Lead and Tin in Soils

Abstract

The fate of heavy metals, such as Pb, in the soil-water environment is significant in the assessment of their potential mobility and toxicity in the ecosystem. In this study, adsorption batch and sequential extraction experiments were carried out to assess the retention and reactivity of Sn and Pb in soils. Isotherm results exhibited highly nonlinear sorption for both heavy metals. Lead isotherms indicated a Freundlich exponent parameter b of 0.204 and 0.249 for Windsor and Olivier soils, respectively. The respective values for Sn were greater than 1 (2.46 and 3.72), which implies irreversible sorption. Moreover, Sn was completely sorbed, indicative of strong irreversible retention where more than 99% of the added Sn was retained in both soils. No Sn release after 30 days of desorption was detected. The presence of Sn resulted in reducing sorption of Pb by both soils, as evidenced by the decrease in their maximum sorption capacity; whereas, Pb exhibited kinetic behavior where 3 to 15% and 13 to 28% of Pb were released for Windsor and Olivier soils, respectively, during desorption. The use of a second-order two-site model that accounts for nonlinear equilibrium and kinetic reactions was capable of describing the kinetic behavior of Pb during adsorption and desorption in both Windsor and Olivier soils. Sequential extraction results revealed that the most susceptible Pb fraction for release was highest corresponding to highest Pb input concentrations. This concentration-dependent release is consistent with the observed strong nonlinearity of Pb retention and implies that mobility of Pb tends to increase as the input Pb concentration is elevated.