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 re ferred to as the OH/SO4 exchange stoichiometry. The OH/SO4 stoichiomet ry not only provides an important constraint on surface complexation m odels but also is a critical area of research to explain the effects o n 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 calculat ed from SO42- adsorption edges at various SO42- concentrations by the thermodynamic approach. The stoichiometry increased with increasing so lution pH and SO42- adsorption density. At glow pH, the stoichiometry was low and was similar for gamma-Al2O3 and kaolinite. At higher pH le vels, it was higher for gamma-Al2O3 than for kaolinite. The average va lues of the stoichiometry calculated by the thermodynamic approach wer e in agreement with those measured by back titration for both gamma-Al 2O3 and kaolinite. The thermodynamic approach provides an easier exper imental 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 ads orbents, whereas at higher pH levels (5-8), it might be underestimated because of precipitation of Al hydroxy sulfates and gibbsite.