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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Fulvic acid, isolated from the Suwannee River, Georgia, was assessed for its ability to bind Ca 2+ , Cd 2+ , Cu 2+ , Ni 2+ , and Zn 2+ ions at pH 6 before and after extensive fractionation that was designed to reveal the nature of metal binding functional groups. The binding constant for Ca 2+ ion had the greatest increase of all the ions in a metal binding fraction that was selected for intensive characterization for the purpose of building quantitative average model structures. The "metal binding" fraction was characterized by quantitative 13 C NMR, 1 H NMR, and FT-IR spectrometry and elemental, titrimetric, and molecular weight determinations. The characterization data revealed that carboxyl groups were clustered in short-chain aliphatic dibasic acid structures. The Ca 2+ binding data suggested that ether-substituted oxysuccinic acid structures are good models for the metal binding sites at pH 6. Structural models were derived based upon oxidation and photolytic rearrangements of cutin, lignin, and tannin precursors. These structural models rich in substituted dibasic acid structures revealed polydentate binding sites with the potential for both inner-sphere and outersphere type binding. The majority of the fulvic acid molecule was involved with metal binding rather than a small substructural unit.