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 chemical heterogeneity is explicitly included in the binding model. For proton binding, a model based on a bimodal Langmuir-Freundlich isotherm plus a cylindrical electrical double-layer model provides a good description. Data for Cd and Ca binding to a purified peat humic acid could not be satisfactorily described by merely shifting the affinity distribution along the affinity axis (the fully coupled case). Assuming that the low pH ("carboxylic") and high pH ("phenolic") parts of the distribution are shifted independently (the highly correlated case) gave 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 effectively with protons for binding at these sites. The uncoupled case assumes that proton and metal binding occur on different sites and only 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 data was obtained. It is likely that the incorporation of ion specific nonidealities into the fully coupled model would improve it.