Predicting Cd partitioning in spiked soils and bioaccumulation in the earthworm Eisenia fetida

Multiple linear regression analysis yielded a statistically based, semimechanistic model describing Cd partitioning coefficients (Kp) in nine Cd-spiked soils. The Ca, Mg and Fe content in pore water and soil pH were the main variables that account for the variation in Cd partitioning between the soil solid phase and pore water. The affinity constants of Ca2+ (log KCa-s = 1.47), Mg2+ (log KMg-s = 2.04), Fe3+ (log KFe-s = 5.78) and H+ (log KH-s = 5.59) binding to active sites on the soil surfaces were evaluated based on the soil ligand modeling approach (SLM), which was derived from the concept of the biotic ligand model (BLM). Cadmium uptake by the earthworm Eisenia fetida was only weakly related to total pore water Cd concentrations (R2 = 0.26, P < 0.01) or free Cd activities (R2 = 0.21, P < 0.05) in pore water. Introduction of pore water pH as an explanatory variable increased R2 to 0.45, indicating that Cd uptake by E. fetida may be moderated by the presence of other cations in pore water, especially H+ ions. A competitive equilibrium model based on the free ion activity model (FIAM) was developed to quantify the effects of H+ ions on the uptake of Cd by earthworms. One of the model parameters is the conditional binding constant of H+ (log KH-w = 6.06) to the biotic uptake sites on the surface of the earthworm E. fetida. The results indicated that pH-adjusted free Cd2+ was a better predictor of tissue concentration in E. fetida than unadjusted free Cd2+.