METAL-ION BINDING TO HUMIC SUBSTANCES - APPLICATION OF THE NONIDEAL COMPETITIVE ADSORPTION MODEL

The application of a new model to describe metal ion binding by humic acids is discussed. Metal ion binding is always of a competitive natur e since the proton is always present. Although of great practical impo rtance, the combination of a chemically heterogeneous system with comp etitive binding poses difficult problems from both experimental and th eoretical points of view. The new Non-ideal Competitive Adsorption mod el (NICA model) used here is able to account for the non-ideal binding to heterogeneous ligands. A good description of the binding of H, Ca, Cd, and Cu to a purified peat humic acid is achieved over a wide rang e of free metal ion concentrations (-2 > log Me(2+) > -14) and pH (2 < pH < 10). The results show that binding of metal ions to humic acid i s strongly influenced by the intrinsic chemical heterogeneity of the h umic material itself as well as by ion-specific non-ideality. The resu lts indicate that copper competes much more efficiently with protons b ound to the phenolic type groups than calcium and cadmium.