PROTON BINDING TO HUMIC SUBSTANCES .2. CHEMICAL HETEROGENEITY AND ADSORPTION MODELS

Ion binding to humic substances is influenced by chemical heterogeneity and by the variable charge behavior of the humics. In this paper, we focus on chemical heterogeneity. To study the chemical heterogeneity, the affinity distribution is calculated from the Q(pH(s)) master curves that were obtained from the acid/base titration data measured at a series of salt levels for 11 humic substances. The proton concentration in the diffuse double layer near the binding site, H(s), is calculated with the help of a double-layer model in which the humic particles are considered rigid impermeable spheres or cylinders. For all samples, the calculated affinity distributions are characterized by a large peak with a peak position in the log K range 3-4. In samples that were titrated over a large pH range, a second peak appears with a peak position around log K = 8-9. From the calculated affinity distributions a site-binding model to describe the data can be selected. Because the peaks in the affinity distributions are broad and smooth, a description based on adsorption equations for continuous heterogeneous ligands is to be preferred to equations for discrete heterogeneity. The Q(pH(s)) master curves can be described very well with normalized Freundlich type of binding equations. The Langmuir-Freundlich equation and the Toth equation give slightly better results than the generalized-Freundlich equation. From the combination of the site-binding model with the double-layer model, the Q(pH) curves can be calculated for various values of the ionic strength. This leads to a good description of the experimental data.