Modeling of the Metal Binding to Humic Substances: Comparison between the Discrete and Continuous Affinity Distribution Models

Abstract

In fifty years metal binding to humic substances (HS) has been investigated and several binding models have been developed. Among them the Model VI developed by E. Tipping and the NICA-Donnan model developed by D.G. Kinniburgh and his coworkers are prominent in that they can successfully describe the binding of various metal ions to HS over a wide range of conditions: the equilibrium concentration of metal ion interest, pH, salt concentration and the concentration of competitive metal ions. These models are contrastive in their approach to the distribution of the affinity constant of the binding sites of HS (i.e. the "heterogeneity" of the binding sites). The Model VI is based on the discrete affinity distribution and the NICA-Donnan model on the continuous affinity distribution. In this review these two models will briefly be introduced, and subsequently the comparison between the NICA-Donnan model and the charge neutralization model, which is developed by J.I. Kim and frequently used in the binding of the actinide ions to HS, will be made using a data set of Cu²⁺ binding to the purified Aldrich humic acid.