Effect of Major Cations and pH on the Acute Toxicity of Cadmium to the Earthworm Eisenia fetida: Implications for the Biotic Ligand Model Approach

The purpose of this study was to characterize the individual effect of different cations (Ca2+, Mg2+, Na+, K+, and H+) on the acute toxicity of Cd to the earthworm Eiseniafetida. Higher activities of the considered cations increased the 48-h LC50 (expressed as Cd2+ activities) linearly to different extents in simulated soil solution. The conditional constants for the binding of H+ (log K = 5.41), Cd2+ (log K = 4.0), Ca2+ (log K = 3.35), Mg2+ (log K = 2.82), Na+ (log K = 1.57), and K+ (log K = 2.31) to the biotic ligand (BL) of E. fetida were derived from the toxicity data based on the biotic ligand model (BLM). Furthermore, it was calculated that on average 72% of the BL sites needed to be occupied by Cd2+ to induce 50% lethal effect. Autovalidation of the model with the results of the test sets showed that 48-h LC50 could be predicted within a factor of two.