Evaluation of internally consistent parameters for the triple-layer model by the systematic analysis of oxide surface titration data

Systematic analysis of surface titration data from the literature has been performed for ten oxides (anatase, hematite, goethite, rutile, amorphous silica, quartz, magnetite, delta-MnO2, corundum, and gamma-alumina) in ten electrolytes (LiNO3, NaNO3, KNO3, CsNO3, LiCl, NaCl, KCl, CsCl, NaI, and NaClO4) over a wide range of ionic strengths (0.001 M-2.9 M) to establish adsorption equilibrium constants and capacitances consistent with the triple-layer model of surface complexation. Experimental data for the same mineral in different electrolytes and data for a given mineral/electrolyte system from various investigators have been compared. In this analysis, the surface protonation constants (K-s,K-1 and K-s,K-2) were calculated by combining predicted values of Delta pK(log K-s,K-2 - log K-s,K-1) (Sverjensky and Sahai, 1996) with experimental points of zero charge; site-densities were obtained from tritium-exchange experiments reported in the literature, and the outer-layer capacitance (C-2) was set at 0.2 F.m(-2). This scheme permitted us to retrieve consistent sets of values for the inner layer capacitance (C-1), and for the electrolyte adsorption constants (K-s,K-L- and K-s,K-M+) corresponding, respectively, to the equilibria >SOH2+ + L-aq(-) = >SOH2+ - L- and >SO- + M-aq(+) = >SO- - M+ Aqueous activity coefficients were calculated using the extended Debye-Huckel equation (Helgeson et al., 1981), which is valid to high ionic strengths (>0.5 M). Systematic analysis of the data reveals important trends and differences between triple-layer model predictions and experimental data and between data for the same mineral/electrolyte from different investigators. Furthermore, the analysis yields an internally consistent set of triple-layer parameters which will be used in developing a predictive model for electrolyte adsorption based on Born solvation and electrostatic theory (Sahai and Sverjensky, 1997a). Copyright (C) 1997 Elsevier Science Ltd.