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

Equilibrium constants for modeling surface precipitation of divalent metals, M2+, onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energy of formation, ΔGfo, of the surface precipitates. The experimental ΔGfo 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 ΔGfo values are correlated through the relationsΔGf, M(OH)2(s)o−77.210 rM2+=1.03266ΔGn, M2+o−305.368andΔGf, MCO3(s)o−83.991 rM2+=0.915ΔGn, M2+o−343.331where ‘s’ stands for the end-member component of the solid solution, ΔGfo is in kcal/mol, r represents the Shannon–Prewitt radius of M2+ in a given coordination state (Å), and ΔGn, M2+o denotes the non-solvation contribution to the Gibbs free energy of formation of the aqueous M2+ ion. The coefficients in the correlations were regressed from the aforementioned experimental ΔGfo. The statistically significant correlations (R2=0.99) allow approximate estimation of free energies and, hence, equilibrium constants of the surface precipitation reactions for Be2+, Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Sr2+, Sn2+, Ba2+, Eu2+, Ra2+, Pb2+, Hg2+, Cu2+, and UO22+.