Modeling cadmium and nickel toxicity to earthworms with the free ion approach

The use of the free ion approach to quantify the toxic effects of Cd and Ni to the earthworms Lumbricus rubellus and Aporrectodea longa exposed in soils of different types was explored. Median lethal concentration (LC50) of Cd (expressed as the total concentration in soil) varied by approximately 11-fold and 28-fold for L. rubellus and A. longa, respectively. For Ni, these values were 50-fold and 38-fold, respectively. For the 2 earthworm species, no significant influence of cations (H⁺, Ca²⁺, Mg²⁺, K⁺, and Na⁺) on Cd²⁺ toxicity was observed, while Mg²⁺ was found to significantly alleviate Ni²⁺ toxicity. The free ion activity model, which is a special case of the free ion approach with no impact of cations, sufficiently described the variability in Cd²⁺ toxicity across soils but failed in predicting Ni²⁺ toxicity. The free ion approach, in which the protective effects of Mg²⁺ were included, explained 89% and 84% of the variations in LC50{Ni²⁺} (expressed as free ion activity) for L. rubellus (log LC50{Ni²⁺}=1.18log{Mg²⁺}-0.52) and A. longa (log LC50{Ni²⁺}=0.51log{Mg²⁺}-2.16), respectively. Prediction error was within a factor of 2 for both Cd²⁺ and Ni²⁺ toxicity, indicating the applicability of the free ion approach for predicting toxicity of these 2 metals. Although extrapolation of the free ion approach across metals still needs more research efforts, this approach, as an alternative to the biotic ligand model, provides a feasible framework for site-specific risk assessment.