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This article in SSSAJ

  1. Vol. 60 No. 2, p. 442-452
    Received: June 27, 1994

    * Corresponding author(s): zelazny@vt.edu
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Hydroxyl-Sulfate Exchange Stoichiometry on γ-Al2O3 and Kaolinite

  1. L. M. He,
  2. L. W. Zelazny ,
  3. D. C. Martens,
  4. V. C. Baligar and
  5. K. D. Ritchey
  1. Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061
    Appalachian Soil and Water Conservation Research Lab., USDA-ARS, Beckley, WV 25802



There is a release of OH- from soil colloids with the addition of SO2-4. The ratio of moles of OH- released per mole of SO2-4 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 SO2-4 amendments such as gypsum. The OH/SO4 stoichiometry on γ-Al2O3 and kaolinite was determined by a back titration method with a pH-stat autotitrator as well as calculated from SO2-4 adsorption edges at various SO2-4 concentrations by the thermodynamic approach. The stoichiometry increased with increasing solution pH and SO2-4 adsorption density. At a low pH, the stoichiometry was low and was similar for γ-Al2O3 and kaolinite. At higher pH levels, it was higher for γ-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 γ-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.

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