Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model
Résumé
A Donnan-type model for nonspecific binding of electrolyte ions has been combined w ith the non- ideal competitive adsorption (NICA) model for specific binding to produce a model for ion binding to humic substances. The model considers site heterogeneity, non-ideality, multicomponent competition, and electrostatic interactions. The NICA-Donnan model w as fitted to data for H, Ca, Cd, Cu, and Pb binding by a purified peat humic acid. The model fits w ere good and covered a w ide range of pH and free metal concentrations. The parameters from these single metal data sets w ere then used to predict the competitive effect of Ca on Cd and Cu binding at various pHs. These predictions agreed w ell w ith the experimental data although there w ere some small but systematic differences. The new NICA-Donnan model also predicted reasonably w ell the increase in Cd and Cu binding on changing from a 0.1 M KNO3 background electrolyte to 0.01 M KNO3. A shortcom- ing of the model is that in some cases it significantly underestimated the H+/M 2+ exchange ratio, especially at high pH and for Cu binding.