Modeling the acid-base properties and metal complexation of humic substances with the Stockholm Humic Model

To describe the binding of protons and metals to humic substances, the Stockholm Humic Model (SHM) has been developed. The model employs a discrete-site approach similar to that of Model V/VI, although it has another electrostatic submodel, based on the Basic Stern concept. An empirical set of equations has been introduced to account for the extra screening of charge inside the gel-like structures of the humic substances. Six adjustable parameters are needed to describe proton binding to humic or fulvic acids. To simulate metal binding, equilibrium constants are defined for mono- and bidentate coordination and an extra parameter, Delta LK2, accounts for binding-site heterogeneity. It is shown that the SHM appears to be able to describe proton binding well; the quality of the fits is similar to those obtained by Model V/VI and the NICA-Donnan model. The SHM was capable of correctly describing metal binding and competitive interactions over a wide range of conditions, although the model performance was not very convincing concerning the ionic strength dependence of metal binding and stepwise proton-metal exchange stoichiometries. (C) 2001 Academic Press.