KINETICS OF PHOSPHATE SORPTION BY GOETHITE AND BENTONITE-GOETHITE (B-G) SYSTEM

The study of phosphate (PO4) fixation due to reactions which occur to the surface of soil minerals has been limited by a lack of evidence on which a structural model may be based. A structural model should atte mpt at least to describe the coordination on the absorbed PO43- ion, t hat is, to indenfity the immediate neighbour atoms or ions in contact with it. Many kinetic equations have been used either to deduce the me chanisms or simply to summarise results of this continued removal of P O4 from solution. In this study five kinetics models of first-order, E lovich, parabolic diffusion, modified Freundlich and a new kinetic equ ation, were evaluated to describe the rate of PO4 sorption from synthe tic goethite and bentonite-goethite (b-g) system into 0.01M calcium ch loride (CaCl2) solutions of 0.0403-0.4435 mmol P L-1. The rate of PO4 sorption is a dynamic factor that regulates the continuous supply of P O4 and was found initially to be rabid but gradually declined with tim e. The adsorption of PO4 per gram of goethite was faster and greater t han the adsorbed amount of PO4 per gram of bentonite-goethite system. Comparison of the coefficients of determination (r) indicates that Elo vich, Freundlich, and the new kinetic models provided better fits for phosphate sorption by goethite while First order, Parabolic, and Freun dlich were found to be the best for bentonite goethite (b-g) system.