| 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 Sr2+ 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 + M2+ + H2O → (>SOH)2(>SO−2–M(OH)+ + 3 H+
predominates for the large cations Sr2+ and Ba2+ (and presumably Ra2+), consistent with X-ray results. In contrast, the mononuclear reaction
>SOH + M2+ + H2O → >SO−–M(OH)+ + 2 H+
predominates for the much smaller Mg2+ (and presumably Be2+), with minor amounts of the tetranuclear reaction. Both reaction types appear to be important for the intermediate size Ca2+. 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.
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