ANALYSIS OF METAL-ION BINDING BY A PEAT HUMIC-ACID USING A SIMPLE ELECTROSTATIC MODEL

Ion binding to humics can only be satisfactorily described if the chem ical heterogeneity is explicitly included in the binding model, For pr oton binding, a model based on a bimodal Langmuir-Freundlich isotherm plus a cylindrical electrical double-layer model provides a good descr iption, Data for Cd and Ca binding to a purified peat humic acid could not be satisfactorily described by merely shifting the affinity distr ibution along the affinity axis (the fully coupled case), Assuming tha t the low pH (''carboxylic'') and high pH (''phenolic'') parts of the distribution are shifted independently (the highly correlated case) ga ve a better description but the shift for the phenolic-type sites was so large that it appears that Cd2+ and Ca2+ are not able to compete ef fectively with protons for binding at these sites, The uncoupled case assumes that proton and metal binding occur on different sites and onl y interact through the common surface electrical field, This model did not fit the data well. However, when the metal concentration, M, was substituted by the ratio, M/(H)(x) (H is the concentrations of protons and x is an adjustable parameter), a good fit to the experimental dat a was obtained. It is likely that the incorporation of ion specific no nidealities into the fully coupled model would improve it.