PHOSPHATE SORPTION BY SOILS .1. A MODEL FOR PHOSPHATE REACTION WITH METAL-OXIDES IN SOIL

A model for phosphorus (P) sorption is described which is based on previous studies concerning the interaction of phosphate with gibbsite and with a sandy soil. Formation of coatings of metal phosphate on metal oxide particles constitutes the presumed mechanism of the sorption process. Diffusion of phosphate ions through the surface coating is taken as the rate-limiting step. The relating diffusion equation is first solved with the condition of constant concentration for reactive particles of well defined geometry. Then it is shown that for any assembly of reactive particles the sorption as a function of the concentration-time product should yield one curve, irrespective of the reaction pathway. The form of this curve can only be predicted for very specific cases.