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

摘要

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 for_mation, (ΔG_(f,MvX(ss))~O) 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 ΔG_(f,MvX(ss))~O values were correlated through the following linear free energy relations ΔG_(f,MvX(ss))~O-791.70r_(M~(3+)) = 0.1587ΔG_(nM~(3+))~O -1273.07 and ΔG_(fM_2(CO_3)_3(ss))~O=-197.241τ_(M~(3+)) = 0.278AG° M3.-1431.27 where 'ss' stands for the end-member solid com- ponent of surface precipitate, ΔG_(f,MvX(ss))~O is in kJ/mol, r_(M~(3+))is the Shannon-Prewitt radius of M~(3+) in a given coordination state (nm), and ΔG_(nM~(3+))~O is the non-solvation contribution to the Gibbs free energy of forma_tion of the aqueous M~(3+) ion. Results indicate that the above surface precipitation correlations are useful tools where experimental data are not available.