Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model

International audienceA Donnan-type model for nonspecific binding ofelectrolyte ions has been combined w ith the non-ideal competitive adsorption (NICA) model for specificbinding to produce a model for ion binding to humicsubstances. The model considers siteheterogeneity, non-ideality, multicomponent competition,and electrostatic interactions. The NICA-Donnanmodel w as fitted to data for H, Ca, Cd, Cu, and Pbbinding by a purified peat humic acid. The modelfits w ere good and covered a w ide range of pH andfree metal concentrations. The parameters fromthese single metal data sets w ere then used to predictthe competitive effect of Ca on Cd and Cu bindingat various pHs. These predictions agreed w ell w ith theexperimental data although there w ere some smallbut systematic differences. The new NICA-Donnanmodel also predicted reasonably w ell the increasein Cd and Cu binding on changing from a 0.1 M KNO3background electrolyte to 0.01 M KNO3. A shortcom-ing of the model is that in some cases it significantlyunderestimated the H+/M 2+ exchange ratio, especiallyat high pH and for Cu binding