Effect of cation competition on cadmium uptake from solution by the earthworm Eisenia fetida

Metal speciation alone is insufficient to predict metal accumulation in aquatic and terrestrial organisms, because competition between cations can play an important role. In the present study, the effects of competing cations (Ca2+, Mg2+, and H+) on Cd uptake by the earthworm Eisenia fetida were quantified through short-term experiments over a large Cd concentration range in aqueous exposure solutions. Free Cd2+ activity was a better predictor for Cd accumulation in the earthworms than was total Cd concentration in solution, thus supporting the free ion activity model. The presence of the other cations at environmentally pertinent concentrations inhibited Cd uptake to different degrees. A competitive equilibrium model based on the biotic ligand model was developed to quantify the effect of the cations. The model parameters are the conditional constants for binding of H+ (log K = 4.97), Ca2+ (log K = 3.00), and Mg2+ (log K = 2.64) to the biological uptake sites of E. fetida. Autovalidation of the model with experimental results showed that 48-h Cd accumulation in the earthworm could be successfully predicted with activities of free Cd2+ and the major cations (Ca2+, Mg2+, and H+) as input.