Isotherms of phosphate sorption by hematite and bentonite-hematite (b-h) system

Iron (hydr)oxides are active phosphate (P) sorbents in soils and sediments. Unfortunately measuring their P-sorption capacity is difficult, because of the relatively low proportion ofthese minerals in many natural mineral mixtures and the difficulties involved in concentrating and separating them from accompanying minerals. Under these circumstances, synthetic samples are commonly used to study P-sorption. In this study, the objectives were to elucidate the Langmuir, Freundlich, Temkin, Gouy-Chapman, and Sposito isotherms of P adsorption by hematite and bentonite-hematite (b-h) system at pH 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0 and initial P concentrations of 1250, 2500, 3750, 5000, 7500, and 13750 mu g/L of sample solution. The used isotherms were fitted by regression to the adsorption data and compared on the basis of goodness of fit (using the correlation coefficient of Pearson, r). Each was found to describe P sorption by hematite and bentonite-hematite (b-h) system with comparable success. The Sposito model was found superior. The total amount of P sorbed by hematite and bentonite-hematite (b-h) system after 75 days in equilibrium concentrations 1.2-14 mg/L and pH 4.0-9.0 ranged between 34-115 mu mol/g, and 0.7-20 mu mol/g, respectively. Bentonite-hematite (b-h) system in comparison with hematite shows slower sorption and lower affinity for P. The mechanisms for P sorption by hematite and bentonite-hematite (b-h) system were studied by FT-IR spectroscopy.