Abstract:
Systematic analysis of surface titration data from the literature has been performed for ten oxides (anatase, hematite, goethite, rutile, amorphous silica, quartz, magnetite, δ-MnO2, corundum, and γ-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 (Ks,1 and Ks,2) were calculated by combining predicted values of ΔpK(log Ks,2 - log Ks,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 (C2) was set at 0.2 F.m-2. This scheme permitted us to retrieve consistent sets of values for the inner layer capacitance (C1), and for the electrolyte adsorption constants (Ks,L- and Ks,M+_ corresponding, respectively, to the equilibria >SOH+2 + Laq- = >SOH+2 - 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).