Prediction of the speciation of alkaline earths adsorbed on mineral surfaces in salt solutions

Abstract

Despite the fact that the bulk compositions of most low temperature natural surface waters, groundwaters, and porewaters are heavily influenced by alkaline earths, an understanding of the development of proton surface charge in the presence of alkaline earth adsorption on the surfaces of minerals is lacking. In particular, models of speciation at the mineral–water interface in systems involving alkaline earths need to be established for a range of different minerals. In the present study, X-ray standing wave results for Sr 2+ adsorption on rutile as a tetranuclear complex [Fenter, P., Cheng, L., Rihs, S., Machesky, M., Bedyzk, M.D., Sturchio, N.C., 2000. Electrical double-layer structure at the rutile–water interface as observed in situ with small-period X-ray standing waves. J. Colloid Interface Sci. 225, 154–165] are used as constraints for all the alkaline earths in surface complexation simulations of proton surface charge, metal adsorption, and electrokinetic experiments referring to wide ranges of pH, ionic strength, surface coverage, and type of oxide. The tetranuclear reaction 4 > SOH + M 2 + + H 2 O = ( > SOH ) 2 ( > SO - ) 2 _ M ( OH ) + + 3 H + predominates for the large cations Sr 2+ and Ba 2+ (and presumably Ra 2+), consistent with X-ray results. In contrast, the mononuclear reaction > SOH + M 2 + + H 2 O = > SO - _ M ( OH ) + + 2 H + predominates for the much smaller Mg 2+ (and presumably Be 2+), with minor amounts of the tetranuclear reaction. Both reaction types appear to be important for the intermediate size Ca 2+. For all the alkaline earths on all oxides, the proportions of the different reaction types vary systematically as a function of pH, ionic strength, and surface coverage. The application of Born solvation and crystal–chemical theory enables estimation of the equilibrium constants of adsorption of all the alkaline earths on all oxides. On high dielectric constant solids (rutile, magnetite, manganese dioxide), where the solvation contribution is negligable, ion adsorption correlates with crystal radius: the equilibrium constants increase in the sequence Be 2+ < Mg 2+ < Ca 2+ < Sr 2+ < Ba 2+ < Ra 2+. On low dielectric constant solids (hematite, gibbsite,and silicas), the solvation contribution opposing adsorption is largest for ions with the smallest hydrated radii: the equilibrium constants increase in the sequence Ra 2+ < Ba 2+ < Sr 2+ < Ca 2+ < Mg 2+ < Be 2+. These predicted sequences are consistent with adsorption affinities in the literature. In combination with previously published results, the present study enables the predictive use of the triple-layer model for 1:1 and 2:1 electrolytes, and mixtures of these, permitting calculation of proton surface charge and adsorption under conditions more relevent to natural water compositions than previously possible.