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Hydroxyl-Sulfate Exchange Stoichiometry on -Al₂O₃ and Kaolinite

Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061

V. C. Baligar and K. D. Ritchey

Appalachian Soil and Water Conservation Research Lab., USDA-ARS, Beckley, WV 25802

*Corresponding author (zelazny{at}vt.edu).

ABSTRACT

There is a release of OH^- from soil colloids with the addition of SO^2-₄. The ratio of moles of OH^- released per mole of SO^2-₄ 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 SO^2-₄ amendments such as gypsum. The OH/SO₄ stoichiometry on -Al₂O₃ and kaolinite was determined by a back titration method with a pH-stat autotitrator as well as calculated from SO^2-₄ adsorption edges at various SO^2-₄ concentrations by the thermodynamic approach. The stoichiometry increased with increasing solution pH and SO^2-₄ adsorption density. At a low pH, the stoichiometry was low and was similar for -Al₂O₃ and kaolinite. At higher pH levels, it was higher for -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 -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.

Received for publication June 27, 1994.

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