METAL-ION BINDING BY HUMIC-ACID - APPLICATION OF THE NICA-DONNAN MODEL

A Donnan-type model for nonspecific binding of electrolyte ions has be en combined with the nonideal competitive adsorption (NICA) model for specific binding to produce a model for ion binding to humic substance s. The model considers site heterogeneity, non-ideality, multicomponen t 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 were good and covered a wide range of pH a nd free metal concentrations. The parameters from these single metal d ata sets were then used to predict the competitive effect of Ca on Cd and Cu binding at various pHs. These predictions agreed well with the experimental data although there were some small but systematic differ ences. The new NICA-Donnan model also predicted reasonably well the in crease in Cd and Cu binding on changing from a 0.1 M KNO3 background e lectrolyte to 0.01 M KNO3. A shortcoming of the model is that in some cases it significantly underestimated the H+/M(2+) exchange ratio, esp ecially at high pH and for Cu binding.