Hydroxyl-sulfate exchange stoichiometry on gamma-Al2O3 and kaolinite

There is a release of OH- from soil colloids with the addition of SO42-. The ratio of moles of OH- released per mole of SO42- adsorbed is referred to as the OH/SO4 exchange stoichiometry. The OH/SO4 stoichiometry not only provides an important constraint on surface complexation models but also is a critical area of research to explain the effects on soil pH from the application of SO42- amendments such as gypsum. The OH/SO4 stoichiometry on gamma-Al2O3 and kaolinite was determined by a back titration method with a pa-stat autotitrator as well as calculated from SO42- adsorption edges at various SO42- concentrations by the thermodynamic approach. The stoichiometry increased with increasing solution pH and SO42- adsorption density. At glow pH, the stoichiometry was low and was similar for gamma-Al2O3 and kaolinite. At higher pH levels, it was higher for gamma-Al2O3 than for kaolinite. The average values of the stoichiometry calculated by the thermodynamic approach were in agreement with those measured by back titration for both gamma-Al2O3 and kaolinite. The thermodynamic approach provides an easier experimental method to evaluate the OH/anion stoichiometry as a function of pH and adsorption density. It also indicated that at low pH levels (<5), the stoichiometry might be overestimated due to dissolution of adsorbents, whereas at higher pH levels (5-8), it might be underestimated because of precipitation of Al hydroxy sulfates and gibbsite.