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, M2+, onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energy of formation, ΔGf°, of the surface precipitates. The experimental ΔGf° 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 ΔGf° values are correlated through the relationsΔGf,M(OH)2(s)°-77.210rM2+=1.03266ΔGn,M2+°-30 5.368andΔGf,MCO3(s)°-83.991rM2+=0.915ΔGn,M2+°-343.331wh ere 's' stands for the end-member component of the solid solution, ΔGf° is in kcal/mol, r represents the Shannon-Prewitt radius of M2+ in a given coordination state (Α), and ΔGn, M2+° 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 ΔGf°. 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+.