EFFECT OF EQUILIBRIUM METAL CONCENTRATIONS ON APPARENT SELECTIVITY COEFFICIENTS OF SOIL COMPLEXES

Abstract

We reevaluated selected data describing complexation (including adsorption) of cadmium and other trace metals by natural solid and soluble soil components, using a consistent format to demonstrate the conditional nature of reported selectivity coefficients. In all cases, the determined selectivity values increased substantially as the concentration of trace metals decreased with respect to the concentration of other competing cations. This dependence upon relative metal concentrations is apparently due to the heterogeneous composition of complexing sites in most soil systems and competition among the various cations present for these sites. Most selectivity coefficients have been obtained after the introduction of metal concentrations well above those normally encountered in natural systems, with little attention paid to the concentration and composition of competing cations. Under such conditions, the metal occupied not only the most specific sites, but a large portion of the more numerous nonspecific sites. The use of selectivity values obtained under such conditions, in predictive models, will greatly underestimate metal complexation in natural systems. This demonstrates the importance of determining selectivity coefficients under conditions comparable to those existing in nature, if results are to be applicable to real systems.