The Adsorption of o‐Phosphate on Alumina: A Solid Solution Model

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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.
Funding Information
  • Ohio Agricultural Research and Development Center, Ohio State University