ABSTRACT The sorption of phosphate ions by goethite (a-FeOH) bentonite-goethite (b-g) and kaolinite-goethite (k-g) systems depends on several factors including pH, specific surface, cation exchange capacity, time of sorption, surface charge, temperature and concentration. We investigated the effects of pH, time of sorption and concentration on the adsorption of phosphate on goethite the bentonite-goethite (b-g) and kaolinite-goethite (k-g) systems. Batch equilibrium phosphate adsorption experiments were conducted with pH, and initial concentration as changeable parameter in order to produce adsorption envelops. The Langmuir, Freundlich, Temkin, Gouy Chapman and Sposito isotherms were fitted to P sorption data. The order of better fitting of those isotherms for the experimental data of P adsorption by bentonite-goethite (b-g) system was Sposito>Freundlich>Temkin and by goethite Langmuir> Freundlich>Sposito>Temkin. In addition five kinetics models as first-order, Elovich, parabolic diffusion, modified Freundlich and a new kinetic equation, were evaluated to describe the rate of phosphate sorption from synthetic goethite, bentonite- goethite (b-g), and kaolinite-goethite (k-g) systems into 0.01 M CaCl2 solutions of 0.0403-0.4435 mmol P L-1. Since the rate of phosphate sorption is a dynamic factor that regulates the continuous supply of phosphate, in the present study it was found initially rabid but gradually declined with time. Comparison of the coefficients of determination (r) indicates that Elovich, modified Freundlich and the new kinetic models provided better fits for phosphate sorption by goethite while First order, Parabolic, and Freundlich were found to be the best for bentonite-goethite system while modified Freundlich, new kinetic, and Parabolic diffusion for kaolinite-goethite system. The adsorption of phosphate per gram of goethite was faster and greater than the adsorbed amount of phosphate per gram of bentonite-goethite, and kaolinite-goethite systems. The total amount of P sorbed by kaolinite- goethite system and goethite, after 103 days, in equilibrium concentrations 0.0403-0.4435 mmol P L-1 and pH 4.0-9.0 was found to range between 2.74-37.00 mmol Kg-1 and 22.97-106.93 mmol Kg-1 respectively.
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