Proton binding to humic acids: Electrostatic and intrinsic interactions

The proton adsorption behavior of eight humic acids (HA) and a fulvic acid (FA) was studied as a function of pH and KNO3 concentration, The emphasis i s on the comparison of the different humics with respect to their ion bindi ng properties and on the comparison of two different models to describe the electrostatic interactions: the Donnan model and the impermeable sphere (I S) model, Viscosimetric data were used to estimate the hydrodynamic volumes and radii of the HA molecules, These data were incorporated in the electro static models and calculations could be carried out without any adjustable parameter. The Donnan model in combination with hydrodynamic volumes obtain ed by viscometry cannot adequately describe the electrostatic effects relat ed to changes of the electrolyte concentration, This model leads to good pr ediction of the HA behavior if unrealistically large volumes are used for f ulvics and unrealistically large volume-salt concentration dependencies are used for humics, The IS model can successfully reproduce experimental prot on adsorption data with physically realistic radii. The good performance of the IS model and the poor performance of the Donnan model is directly rela ted to the fact that the hydrodynamic volumes of the molecules are too smal l to allow for charge compensation within the molecular limits. The combina tion of viscometry with the IS model leads to a consistent description of t he electrostatic in humics and to a consistent way of positioning the maste r curves, Therefore, the electrostatic potentials and the intrinsic affinit y distributions of the different samples can be compared on an equal basis. The similarities in the intrinsic affinity distributions give faith for th e possibility to develop a generic model to describe the ion binding to hum ics, (C) 1999 Academic Press.