Oligoelectrolyte model for cation binding by humic substances

Abstract

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. IntroductionHumic substances abound in natural waters and may have significant effects on metal speciation and mobility, especially in waters with high levels of dissolved organic carbon (DOC). At present, our ability to predict these effeds using equilibrium models is quite limited. A simple

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Humic substance extraction from leonardite, lignite Mae Mho Mine by base-acid treatment process

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Elimination of Phenols on a Porous Material

“…The natural organic matter is also known for its role in the transport and the trapping of organic and/or inorganic pollutants (Bartschat and Al, 1992;Tippinge, 1993). It represents also a potential substrate for the biological growth in the distribution network of drinking water.…”