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 (M3+M3+) onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energies of formation, (ΔGf,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 ΔGf,MvX(ss)0 values were correlated through the following linear free energy relations ΔGf,M(OH)3(ss)0-791.70rM3+=0.1587ΔGn,M3+0-1273.07 and ΔGf,M2(CO3)3(ss)0-197.241rM3+=0.278ΔGn,M3+0-1431.27 where ‘ss’ stands for the end-member solid component of surface precipitate, ΔGf,MvX(ss)0 is in kJ/mol, rM3+rM3+ is the Shannon-Prewitt radius of M3+M3+ in a given coordination state (nm), and ΔGn,M3+0 is the non-solvation contribution to the Gibbs free energy of formation of the aqueous M3+M3+ ion. Results indicate that the above surface precipitation correlations are useful tools where experimental data are not available.