Discrete Fragment Model for Complex Formation of Europium(III) with Humic Acid

The discrete fragment model was applied to the complex formation constant of a trivalent f-element, europium, with humic acid. The model decomposes the entire molecule of humic acid into several sites of functional groups such as carboxyl and phenolic groups, and chelating bridges connecting the functional groups. The acid dissociation and metal complexation constants of all the sites and pairs were taken from the literature and experimental values in order to calculate the overall values for the humic acid system. The heterogeneity of site distribution was also statistically considered. The simulated curves of the overall formation constant depending on the concentration of Eu were consistent with the experimental data. The present model would be useful for metal complexation reactions of polyvalent metal ions with strong but less abundant binding sites.