Estimation of equilibrium surface precipitation constants for trivalent metal sorption onto hydrous ferric oxide and calcite

Equilibrium constants for modeling surface precipitation of trivalent metal cations ( M 3 + ) onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energies of formation, ( Δ G f , MvX ( ss ) 0 ) of the surface precipitates. The surface precipitation reactions were derived from Farley et. al. [K.J. Farley, D.A. Dzombak, F.M.M. Morel, J. Colloid Interface Sci. 106 (1985) 226] surface precipitation model, which are based on surface complexation model coupled with solid solution representation for surface precipitation on the solid surface. The Δ G f , MvX ( ss ) 0 values were correlated through the following linear free energy relations Δ G f , M ( OH ) 3 ( ss ) 0 - 791.70 r M 3 + = 0.1587 Δ G n , M 3 + 0 - 1273.07 and Δ G f , M 2 ( CO 3 ) 3 ( ss ) 0 - 197.241 r M 3 + = 0.278 Δ G n , M 3 + 0 - 1431.27 where ‘ss’ stands for the end-member solid component of surface precipitate, Δ G f , MvX ( ss ) 0 is in kJ/mol, r M 3 + is the Shannon-Prewitt radius of M 3 + in a given coordination state (nm), and Δ G n , M 3 + 0 is the non-solvation contribution to the Gibbs free energy of formation of the aqueous M 3 + ion. Results indicate that the above surface precipitation correlations are useful tools where experimental data are not available.