Modelling ion binding by humic acids

Abstract

Humic ion-binding Model V is a discrete site/electrostatic model of ion binding by humic matter that describes competition amongst interacting species (protons and metal species) and the effects on binding of ionic strength. Six adjustable parameters are required to describe proton binding. These are nA (equivalent acid groups per gram), pKA and pKB (mid-range intrinsic proton dissociation constants), ΔpKA and ΔpKB (ranges of pK values), and P (empirical constant for electrostatics). For each metal species, two further parameters are required; these are intrinsic equilibrium constants for metal-proton exchanges (pKMHA and pKMHB). Both monodentate and bidentate binding sites for metal species are included, and binding by non-specific accumulation of counterions is taken into account. In this study, Model V parameter values are derived from published results for humic acids (acid-base titrations and binding studies with 15 metals). It is found that an approximate relationship between pKMHA and pKMHB can be derived, which eliminates an adjustable parameter and allows useful rationalization of the different data sets. Electrostatic effects with humic acids are greater than those with fulvic acids, and the two types of humic material also differ in the affinities of their discrete binding sites for protons and metal ions.