RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

In this work, the surface complexion database by Dzombak and Morel [18] for hydrous ferric oxide was extended to include geothite.Available acis-base and ion sorption data for geothite up to 1995 were fitted with the Generalized Two-Layer Model using a consistent methodology, including a consistent set of values for geothite surface properties. Optimal surface complexion reactions and equilibrium constants for fitting available geothite sorption data were determined.This analysis serves to demonstrate the potential of the Generalilzed Two-Layer Model to uniform interpretation of sorption data covering a wide range of chemical conditions, as well as different kinds of mineral oxide surfaces.

The surface complexation constants obtained for sorption of ions on goethite in this study are coherent with each other and with known chemical properties of the ions.The latter was demonstrated by the succesful development of linear free energy relationships correlating the surface complexation constants with equilibrium constants related solution reactions. The LFERs provide a means to estimate sorption constants for ions for which limited or no sorption data exist.

Plots of cation surface complexation constants against the first hydrolysis constants for the various cations resulted in log-linear relationships. These plots allowed the prediction of surface complexation constants for Ba²⁺, Sr²⁺, Ag⁺, Mn²⁺, UO₂²⁺, PuO₂²⁺, Sn²⁺, and Pd²⁺.

Surface complexation constants for the various anions considered in this study were plotted against second deprotonation constants, and good fits were observed. The resulting LFERs between log K^int and pK_a enabled prediction of surface complexation constants for several anions: SO₃^2-, SiO₃^2-, S₂O₃^2-, WO₄^2-, SbO(OH)₄^-, CNO^-, CNS^-, and CN^-. These LFERs demonstrate the correspondence between the tendency of anions to protonate and their tendency to form surface complexes.

Finally, a comparison of intrinsic surface complexation constants for similar surfacereactions of ions on goethite and hydrous ferric oxide revealed close agreement of the surface complexation constants. This findings suggests that if differences among iron oxides in surface site density and surface area taken into account, the free energy of reaction of an ion at the majority of surface hydroxyl sites is approximately constant. It may be possible that the intrinsic constants for ion sorption on all metal oxides are approximately the same, but further database development with other kinds of oxides (i.e., Mn, Al, and Si oxides) is needed to test this hypothesis.