| Abstract
| - Model VI and the NICA-Donnan model are two successfulmodels presently available for describing metal ionbinding by humic materials. Both models deal with thecomplexity of the underlying processes (intrinsic heterogeneity, partial correlation between affinity distributions,variable stoichiometry, electrostatics) in a pragmatic way.The parameters of the NICA-Donnan model and theirdetermination are discussed. The current interpretation ofthe ion-specific “non-ideality” parameters, ni, found inthe NICA-Donnan model, emphasizes their role in determiningthe stoichiometry for the competitive binding of ions.The ratio nM/nH is reflected in the corresponding exchangeratios and the pH dependence of the metal ion binding.Experimental complications occur in testing models. AlthoughModel VI and the NICA-Donnan models have much incommon, there are differences that may become moreapparent as the models are more widely tested. Recentlyion binding to complexes of humics and oxides hasbeen described by combining the NICA-Donnan model(ion binding to humics) with the CD-MUSIC model (ionbinding to oxides). The impact of humic to oxide binding onboth cation and anion binding is briefly discussed.
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