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The Adsorption of o-Phosphate on Alumina: A Solid Solution Model¹

ABSTRACT

Alumina-P interactions were examined over a range of PO₄ concentrations (0.8 µM-0.2 mM) and pH's (7–9) conducive to adsorption rather than precipitation. The equilibria established were interpreted using chemical thermodynamic concepts to distinguish adsorption from precipitation and to evaluate the nature of the surface interaction. The chemical speciation model GEOCHEM, supplied with selected thermodynamic values, was used to predict adsorption-free equilibria for the aqueous PO₄/alumina systems. Relative to these predictions, the adsorption of PO₄ increased soluble Al levels, apparently as a result of increasing the solid alumina surface activity. The formation of a soluble Al-PO₄ complex did not appear to contribute significantly to total soluble Al. The total soluble Al and PO₄ concentration product was pH and total P addition dependent, as expected for equilibria controlled by the presence of a solid solution. An estimate of the ratio of activity coefficients, calculated for differing total PO₄, indicated that the activity coefficient of the Al-OH-PO₄ surface component increased with increasing amount of PO₄ added. It was concluded that adsorption of PO₄ at the inorganic oxide surface could be viewed as the formation of a surface-limited solid solution. An attempt to apply a solid solution quasithermodynamic model to soil PO₄ adsorption appears warranted, with particular emphasis on Fe, Al, and Ca oxide and carbonate minerals as the primary matrix for adsorption/solid solution formation.

¹ Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State Univ., Wooster, OH 44691. Journal Article no. 134–85.

² Graduate Research Associate (Currently Assistant Professor, Agronomy and Soils Dep., Auburn Univ., Auburn, AL 36849, Professor, and Associate Professor, OARDC, OSU, Columbus, OH 43210, respectively.

Received for publication June 3, 1985.