ADSORPTION CHARACTERISTICS OF PHOSPHATE FROM AQUEOUS SOLUTIONS BY SPONGE IRON:ISOTHERM, KINETICS, AND THERMODYNAMICS STUDIES

摘要

In order to improve understanding of the mechanisms of phosphate removal by sponge iron, equilibrium and kinetic experiments were carried out in batch-scale. Five isotherm models, namely, Langmuir, Freundlich, Lang-muir-Freundlich, Temkin and Dubinin-Radushkevich, are applied to fit the equilibrium data at 298 K. The results show both Freundlich and Langmuir-Freundlich can well describe the adsorption equilibrium. The kinetic data obtained in various conditions, such as varying particle size, initial pH of solution, initial phosphate concentration and temperature, are analyzed using the pseudo first-order, pseudo second-order, Elovich and Bangham's models. The results represent the pseudo second-order model can give a good simulation for the process of phosphate adsorption on sponge iron. The thermodynamic parameters are evaluated. The results reflect the adsorption process of phosphate on sponge has different characteristics in different temperature ranges. The adsorption process is exothermic at low temperature conditions (288-298 K) whereas it is endothermic at high temperature stage (303-318 K). Besides, the negative AG~0, in the whole temperature range investigated, confirms the adsorption process is spontaneous. Therefore, the adsorption mechanisms of phosphate on sponge iron may be proposed that the physical sorption mainly takes place at low temperature but chemisorption may be dominant at high temperature.