Adsorption of Cadmium, Copper and Chromium by an Agricultural Soil Impacted by Mining Activities

Batch adsorption studies for cadmium (Cd), copper (Cu) and chromium (Cr) onto an agricultural soil impacted by mining activities were conducted in single-and multi-component systems. The effect of initial concentration, pH and competing ions (Fe3+, Ca2+, Co2+, Mg2+, K+, Ni2+ and Zn2+) on adsorption was studied. The soil exhibited high adsorption capacities for the elements at all initial concentrations with the adsorption process better described by the Freundlich isotherm. Adsorption was found to proceed via an ion exchange mechanism. The pseudo second-order kinetic model described the adsorption of the elements (R-2 > 0.999), indicating a chemisorption process. The adsorption of Cd increased with pH in both systems while that for Cu decreased. The adsorption of Cr decreased with pH in the single-component system, but increased in the multicomponent system. The adsorption of Cd was affected more by competing ions while Cu and Cr were not significantly affected (p > 0.05). Elemental speciation under varying conditions was studied using the PHREEQC geochemical modelling code. The observed high capacity of the soil for the elements pointed to the soil's potential as a repository, a feature that would change depending on the speciation of the elements and soil conditions.