Reactivity of nickel in soils: evidence of retention kinetics

The bioavailability and mobility of nickel (Ni) is highly dependent on the mechanisms associated with Ni adsorption-desorption and its kinetics in soils. To examine the characteristics of Ni retention and release, kinetic sorption batch experiments were performed on three soils having different properties, followed by Ni desorption using successive dilutions. Sorption of Ni by all soils was highly nonlinear and strongly kinetic, where the rate of Ni retention was rapid initially and was followed by gradual or somewhat slow retention behavior with increasing reaction time. Desorption of Ni was strongly irreversible and hysteretic in nature, indicating lack of equilibrium retention and/or irreversible or slowly reversible processes. A sequential extraction procedure provided evidence that a significant amount of Ni was irreversibly adsorbed on all soils. A nonlinear multireaction model with equilibrium-kinetic-irreversible reaction sites successfully described the retention (adsorption) and subsequent release of Ni on the different soils. The model was also capable of predicting Ni desorption kinetics based on adsorption data sets only.