Theoretical prediction of single-site enthalpies of surface protonation for oxides and silicates in water

Details Theoretical prediction of single-site enthalpies of surface protonation for oxides and silicates in water Theoretical prediction of single-site enthalpies of surface protonation for oxides and silicates in water

Dec 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62.3703s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 62, Issue 23-24, pp.3703-3716

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6

Scientific paper

Surface protonation is the most fundamental adsorption process of geochemical interest. Yet remarkably little is known about protonation of mineral surfaces at temperatures greater than 25°C. Experimentally derived standard enthalpies of surface protonation, H r,1 ° , H r,2 ° , and H r,ZPC ° , correspond to the reactions > SOH + H + => SOH 2 + > SO - + H + => SOH > SO - +2 H + => SOH 2 + respectively, and provide a starting point for evaluating the role of surface protonation in geochemical processes at elevated temperatures. However, the experimental data for oxides do not have a theoretical explanation, and data are completely lacking for silicates other than SiO 2 . In the present study, the combination of crystal chemical and Born solvation theory provides a theoretical basis for explaining the variation of the enthalpies of protonation of oxides. Experimental values of H r,1 ° , H r,2 ° , and H r,ZPC ° consistent with the triple layer model can be expressed in terms of the inverse of the dielectric constant (1/ ) and the Pauling bond strength per angstrom (s/r M-OH ) of each mineral by equations such as H r , ZPC ° = r , Z [(1/ )-( T / ) 2 ( / T )]- B ' Z ( s / r M - OH )+ H ' Z . The Born solvation coefficient r,Z was taken from a prior analysis of surface equilibrium constants. The coefficients B Z ' and H Z ' were derived by regression of experimental enthalpies for rutile, -alumina, magnetite, hematite, and silica. This approach permits widespread prediction of the enthalpies of surface protonation. Predicted standard enthalpies of surface protonation for oxides and silicates extend over the ranges (in kcal.mole -1 ): H r,1 ° -3 to -15; H r,2 ° -0.5 to -18; H r,ZPC ° -4 to -33. Minerals with the largest values of s/r M-OH (e.g., quartz and kaolinite) are predicted to have weakly negative enthalpies and a weak temperature dependence for their protonation equilibrium constants. Conversely, minerals with the smallest values of s/r M-OH (e.g., garnets and olivines) should have strong negative enthalpies and a strong temperature dependence for their protonation equilibrium constants.

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