Behaviour of heavy metals in soils

Abstract

Fractions of Zn, Cd, Cu, Pb, Fe and Mn extractable with water, a salt solution and dilute acid, and residual fractions were determined in soils with raised contents of heavy metals, near zinc smelters, along a river formerly discharging heavy metals, and in a sewage farm. Special attention was paid to the role of oxides of Fe and Mn in the binding of other heavy metals. The theory of regular solid solutions was applied to exchange adsorption of ions in soil and some approximative equations of practical interest were derived. The possible role of hydrolysis and the adsorption of hydrolysis products was studied. Ion exchange in soil was described in terms of sites of high and low selectivity (two-site model) giving rise to a decreasing free enthalpy of adsorption with increasing surface coverage (Freundlich behaviour). Cation-exchange experiments with Ca2+, Zn2+, Cd2+, Cu2+ and Pb2+ on Winsum clay soil showed that heavy metals were subject to regular and selective (specific) exchange adsorption, and to superequivalent adsorption. A pronounced lowering of pH upon adsorption of heavy metals was assumed to be by desorption of weakly acidic protons or selective adsorption of hydrolysis products, in combination with a diminished acid-neutralizing capacity.