Heterogeneity and liability of Pb(II) complexation by humic substances: practical interpretation tools

The complexation of Pb(II) by natural organic matter (NOM) is better described by taking into account the dependence of the strength of binding on metal loading conditions. The utility of a linear differential equilibrium function for interpretation of metal ion binding data is demonstrated. This approach considers the binding intensity (log K*) as a function of metal ion loading (theta = bound metal/binding site concentration). Three methods for calculating this function are presented: -direct calculation from metal titration curves, -direct calculation from polarograms, and -compilation of data derived from interpretation of complexation in terms of one- or two- binding sites (e.g. Scatchard analysis), i.e. Cc (complexation capacity = effective site concentration)-K pairs. Heterogeneity also impacts on the apparent lability of complexes; complexes formed at the lowest metal loadings are the least labile.