DIFFERENTIAL COMPLEXATION OF Zn~(2+) AND Cd~(2+) TO FULVIC ACID IN WATER

摘要

As complexants of metallic ions in soil and water, aquatic and terrestrial fulvic (FA) and humic acids (HA), deriving from the solubilization of organic matter are environmentally significant pollution transporters and bioavailability agents. Cadmium ions are important natural water pollutants, occurring in zinc metallurgy and in electroplating liquid wastes, as cadmium is always associated to zinc in nature, by isomorphic substitution in its compounds. In this paper, a study of complexes formed by Zn~(2+) and Cd~(2+) with fulvic acids, using computational chemistry is presented. The relative free energy calculations on a thermodynamic cycle were carried out using Free Energy Perturbation (FEP) and Density Functional Theory (DFT-B3LYP/6-31G~(**)) methods. The computed interaction energies between orbitals from Natural Bond Order (NBO) calculations showed that Cd-FA complexes, octahedral with four water molecules in the coordination sphere, are more stable than Zn-FA complexes, tetrahedral with two water molecules. Consequently, FA should be a more efficient ligand for Cd~(2+) bioavailability and toxicity in natural aquatic environments.