Determination of H+ and metal ion affinity distributions for humic substances

Abstract

Ion binding to humic materials is influenced by the variable charge and chemical heterogeneity of the humics. In order to characterize the charge effects and the chemical heterogeneity of humic material separately the so called mastercurve method can be used. This method is based on the analysis of acid/base titration data measured at a series of salt levels. In the first step of the method a double layer model, which describes the charge effects, is assessed. In the second step the intrinsic H+ affinity distribution is determined. The mastercurve method is applied to three datasets for aquatic fulvic acids. For the description of the charge effects a spherical double layer model was used. Using the particle density as a constraint the estimated particle radius for the different samples varied from 0.5 to 1 nm and the obtained molecular weights seem reasonable. The intrinsic H+ affinity distribution is dominated by carboxylic acid type of groups with a mean intrinsic H+ affinity in the log KHint range 3 to 4. Metal binding to humic and fulvic acids is far more complicated than proton binding. A few approaches are suggested that can be used to analyze metal binding to humic and fulvic materials.