Heterogeneity and lability of Pb(II) complexation by humic substances: practical interpretation tools
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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 (ı = 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.
KeywordsHumic Substance Titration Curve Natural Organic Matter Metal Loading Binding Data
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