RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The adsorption of carbonate on goethite has been evaluated, focussing on the relation between the structure of the surface complex and corresponding adsorption characteristics, like pH dependency and proton co-adsorption. The surface structure of adsorbed CO⁻²₃ has been assessed with (1) a reinterpretation of IR spectroscopy data, (2) determination of the charge distribution within the carbonate complex using surface complexation modeling, and (3) evaluation of the proton co-adsorption of various oxyanions, including carbonate, in relation with structural differences. Carbonate adsorption leads to a degeneration of the ν₃ IR vibration. Currently, the magnitude of the Δν₃ band splitting is used as a criterion for metal coordination. However, the interpretation is not unambiguous, since the magnitude of Δν₃ is influenced by polarization and additional field effects, due to, e.g., H bonding. Our evaluation shows that for goethite the magnitude of band splitting Δν₃ falls within the range of values that is representative for bidentate complex formation, despite contrarily assignments made in literature. Determination of the charge distribution (CD), derived by modeling available carbonate adsorption data, shows that a very large part (2/3) of the carbonate charge resides in the surface. Interpretation of this result with a bond valence and a ligand charge analysis strongly favors the bidentate surface complexation option for adsorbed carbonate. This option is also supported by the proton co-adsorption of carbonate. The H co-adsorption is very high, which corresponds closely to an oxyanion surface complex in which 2/3 of the ligands are common with the surface. The high H co-adsorption is in conflict with the monodentate option for adsorbed CO⁻²₃. The study shows that the H co-adsorption of CO⁻²₃ is almost equal to the experimental H co-adsorption obtained for SeO⁻²₃ adsorption, which can be rationalized supposing for both XO⁻²₃ complexes the same ligand distribution in the interface, i.e., bidentate complex formation.