Humic Substances Considered as a Heterogeneous Donnan Gel Phase

Abstract

A Donnan model in combination with a site-binding model is applied to interpret proton binding to humic substances as a function of pH at different salt levels. The Donnan model accounts for the salt effect on the charging behavior; it can only be successfully applied if it is assumed that the specific volume of humic substances is strongly dependent on the ionic strength. Empirical relationships have been derived that relate the volume of the Donnan phase with the ionic strength. For fulvic acid, the Donnan approach is physically not very realistic. The approach presumably mimics the behavior of the diffuse double layer around the relatively small fulvic acid molecules. For humic acids, however, the approach is physically realistic. The Donnan approach is a relatively simple and numerically efficient technique to account for the effects of ionic strength on the charging behavior. The ionic composition of the gel phase follows directly from the technique. The insight obtained on the swelling behavior of the humic acids may be of use for interpreting binding of organic pollutants in natural systems.