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Chemical Modeling of Anion Competition on Oxides Using the Constant Capacitance Model-Mixed-ligand Approach¹

ABSTRACT

The constant capacitance model was used to describe selenite-o-phosphate and silicate-o-phosphate competition on synthetic goethite (-FeOOH) and synthetic gibbsite (Al(OH)₃) by direct optimization of mixed-ligand adsorption data. The model assumes ligand exchange mechanisms for selenite, o-phosphate, and silicate adsorption on oxide surfaces. The computer program, FITEQL, was used to fit intrinsic surface complexation constants to the competitive adsorption data. Values for the intrinsic protonation-dissociation constants, surface capacitance density, surface site density, and specific surface area had been determined previously. The constant capacitance model was able to represent competitive anion adsorption for selenite-o-phosphate over the pH range of 4 to 10 and for silicate-o-phosphate from pH 5.5 to 9.5. The fit to the experimental data using the mixed-ligand approach was much better than that obtained by prediction from single anion systems. The intrinsic surface complexation constants for o-phosphate adsorption were found to be dependent upon the surface composition of the adsorbent oxide. We suggest that this composition dependence is due to heterogeneity of surface adsorption sites. Our findings suggest the utility of mixed-ligand modeling for describing anion competition reactions using direct optimization of competitive adsorption data.

¹ Contribution from the U.S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501, and the Dep. of Agronomy, Ohio State Univ., Columbus, OH 43210.

² Soil Scientist (Riverside) and Assistant Professor Soil Physical Chemistry (Columbus), respectively. OSU-OARDC Journal Article no. 43-87.

Received for publication December 11, 1986.

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