Saturday, 15 July 2006

Modelling Phosphate Adsorption by Agricultural and Natural Soils.

You Jiao, William H. Hendershot, and Joann K. Whalen. McGill Univ, 21111 Lakeshore Road, Ste. Anne de Bellevue, QC H9X 3V9, Canada

The Non-Ideal Competitive Adsorption model (NICA) developed from the Langmuir adsorption theory plus the Freundlich empirical equation has some advantages in describing adsorption of ions on solid surface in solutions. The reason is that the NICA model separates the non-ideality of adsorption into the intrinsic chemical heterogeneity of the surface and the ion-specific non-ideality. We believe that the NICA model is more suitable to describe ion adsorption in complex systems, like soils, which have multiple ions, highly heterogeneous surfaces and a variety of sites on the particle surfaces to adsorb ions. Although it has been successfully applied in the binding of protons and metal ions to humic substances, its application for the adsorption of anions on the positively charged surfaces has not been documented. The purpose of this study was to find a suitable way to adapt the NICA model for describing hydroxyl and phosphate adsorption, which involves major assumptions and redefination of surface reaction. Results show that by considering 2 types of surface, the NICA model can excellently fit (R2>0.99) the hydroxyl data obtained from -11 to -4 of log [OH], which corresponds to the pH from 3 to 10. By using the parameters generated from hydroxyl adsorption, including adsorption maxima (Qmax), binding strength (KOH) and non-ideality (m), the NICA model gave a remarkable goodness of fit (R2>0.98) for the P adsorption data obtained at different pH levels. The successful application of P adsorption combined with hydroxyl adsorption provides a promising tool for analyzing competitive adsorption among anions in soils.

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