Estimation of surface precipitation constants for sorption of divalent metals onto hydrous ferric oxide and calcite

Abstract

Equilibrium constants for modeling surface precipitation of divalent metals, M 2+, onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energy of formation, Δ G f o, of the surface precipitates. The experimental Δ G f o of the surface precipitates was derived from the surface precipitation model of Farley et al. [J. Colloid Interface Sci. 106 (1985) 226], which in turn was based on a surface complexation model coupled with solid solution representation for precipitation on the surface. The Δ G f o values are correlated through the relations ΔG f, M(OH) 2( s) o −77.210 r M 2+ =1.03266 ΔG n, M 2+ o −305.368 and ΔG f, MCO 3( s) o −83.991 r M 2+ =0.915 ΔG n, M 2+ o −343.331 where ‘s’ stands for the end-member component of the solid solution, Δ G f o is in kcal/mol, r represents the Shannon–Prewitt radius of M 2+ in a given coordination state (Å), and Δ G n, M 2+ o denotes the non-solvation contribution to the Gibbs free energy of formation of the aqueous M 2+ ion. The coefficients in the correlations were regressed from the aforementioned experimental Δ G f o. The statistically significant correlations ( R 2=0.99) allow approximate estimation of free energies and, hence, equilibrium constants of the surface precipitation reactions for Be 2+, Mg 2+, Ca 2+, Mn 2+, Co 2+, Ni 2+, Sr 2+, Sn 2+, Ba 2+, Eu 2+, Ra 2+, Pb 2+, Hg 2+, Cu 2+, and UO 2 2+.