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Ionic Strength Effects on Sulfate and Phosphate Adsorption on -Alumina and Kaolinite: Triple-Layer Model

Marine Biology Research Division and Center for Marine Biotechnology and Biomedicine, Scripps Inst. of Oceanography, Univ. of California at San Diego, La Jolla, CA 92093-0202

L. W. Zelazny^* and D. C. Martens

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

V. C. Baligar and K. D. Ritchey

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

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

ABSTRACT

It is known that PO₄ is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO₄ adsorption at the mineral-water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO₄ and PO₄ adsorption on -Al₂O₃ and kaolinite at varying solution pH (3–11), and to infer SO₄ and PO₄ adsorption mechanisms at the mineral-water interface. The adsorption of SO₄ on -Al₂O₃ and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO₄ adsorption on -Al₂O₃ and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO₄ adsorption varied with pH. At low pH, PO₄ adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO₄ adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple-layer model (TLM) was applied to model the adsorption of SO₄ and PO₄ with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO₄ adsorption was better modeled by assuming inner sphere complex formation.

Received for publication August 25, 1995.

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