OLIGOELECTROLYTE MODEL FOR CATION BINDING BY HUMIC SUBSTANCES

An oligoelectrolyte model of humic substances is developed to explain ionic strength effects on copper and hydrogen ion titrations. After discussing the relevance of various polyelectrolyte models to humic substances, we choose a model in which the molecules are represented as impenetrable spheres. The electrostatic effect is calculated using numerical solutions of the appropriate nonlinear Poisson-Boltzmann equation. Incorporation of available data on molecular weight distribution, size, and functional group content of humic substances reduces the number of arbitrary fitting parameters. A simple model, containing two copper binding sites and an additional acidic site, and two size classes of impenetrable spheres, can successfully explain pH and copper titration data. In particular, the model results demonstrate the importance of size heterogeneity as a means of explaining the relatively small effect of ionic strength on pH titrations as compared to copper titrations. This difference is not attributable to the difference in charge between H+ and Cu2+ and suggests that an electrostatic model calibrated only on a pH titration cannot describe a copper titration or vice versa.