A QICAR approach for quantifying binding constants for metal-ligand complexes

Relative metal-ligand complex stability is predicted by evaluating the relationships between physicochemical properties of metal ions and their experimental biotic and abiotic binding constants, K. Linear regression analysis showed that the softness index (σ_p) and the covalent index (χ²_mr) were especially useful in model construction for rainbow trout (Oncorhynchus mykiss), fathead minnows (Pimephales promelas) and crustaceansaquatic (Daphnia magna) based on RMSE and F-ratio criterion (F_observed/F_critical of ≥4). The absolute value of the log of the first hydrolysis constant |log K_OH| correlated best with log K values for barley (R²=0.74, p=0.02) and earthworm (R²=0.82, p=0.01). In contrast, the ionic index Z²/r explained most of the variability of log K values for the two clays kaolinite and montmorillonite, while |log K_OH| was a better predictor of the generic NICA-Donnan parameters for HA and FA (0.67<R²<0.80, 0.002<p<0.01). This implies dissimilarity of the nature of the binding sites on biotic and chemical ligands and the different binding mechanisms between metal and ligands.