Proton Uptake at the Barite–Aqueous Solution Interface: A Combined Potentiometric, Electrophoretic Mobility, and Surface Complexation Modeling Investigation

A well-characterized barite powder was investigated via base titrations as functions of pH (3 to 10), ionic strength (0.03 and 0.30 m NaCl), and temperature (15 to 50 °C) and with and without added Ca²⁺ (0.001 and 0.002 m), along with ζ-potential measurements (25 °C, pH 2.5 to 11.5 in 0.001 m NaCl). Ba²⁺ concentrations measured in parallel dissolution experiments and Ca²⁺ concentrations measured at the conclusion of titration runs were utilized to constrain solution conditions. X-ray surface diffraction and molecular modeling results for the barite (001) surface from the literature were employed to estimate surface protonation constants via the MUSIC model. This information was integrated into a surface complexation model (SCM) of the barite–aqueous solution interface, with one Helmholtz plane utilized to accommodate Ba²⁺, Ca²⁺, and SO₄^2– adsorption. Proton uptake/release between pH 3 and 11 was < 0.7 μmols/m², which is approximately 10 times less than for typical metal oxides over the same pH range, while ζ-potentials were similar to metal oxide values. Although H⁺ uptake/release exhibited slight differences with respect to ionic strength, temperature, and added Ca²⁺, these differences could not be confidently differentiated from various sources of experimental error or model uncertainties including pretreatment procedures, blank titrations, or MUSIC model assumptions. Therefore, along with binding constants and a Stern layer capacitance value, the SCM included a “δ” parameter, which allowed for slight vertical adjustments of the individual base titration curves during fitting. As a result, the final model parameters for all titration conditions were very similar.