PHOSPHATE SORPTION BY GOETHITE AND KAOLINITE-GOETHITE (K-G) SYSTEM ASDESCRIBED BY ISOTHERMS

Goethite and kaolinite are active sorbents in soils and sediments. Unf ortunately measuring their phosphate sorption capacity is difficult, b ecause of the relatively low proportion of these clay minerals in many natural mixtures and the difficulties involved in concentrating and s eparating them from accompanying minerals. Under these circumstances, synthetic samples are commonly used to study phosphate sorption. In th is study, the objectives were to elucidate the Langmuir, Freundlich, T emkin, Gouy-Chapman, and Sposito isotherms. The sorption of phosphate ions by goethite (a-FeOH) and kaolinite-goethite (k-g) system depends on several factors including pH, specific surface, cation exchange cap acity, time of sorption, surface charge, temperature and concentration . We investigated the effects of pH, time of sorption and concentratio n on the adsorption of phosphate on goethite and the k-g system. 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 an d Sposito isotherms were fitted to P sorption data. Each was found to describe phosphorus (P) sorption by goethite and k-g system with compa rable success, with the Sposito, Freundlich and Langmuir being slightl y superior for kaolinite-goethite (k-g) system and Langmuir for goethi te. The total amount of P sorbed by k-g 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 22.97-106.9 3 mmol kg(-1), respectively.