Phosphate removal from aqueous solutions by adsorption using ferric sludge

The binding efficiency of phosphate onto the ferric sludge was investigated by batch and column experiments to control the deterioration of water quality caused by eutrophication. Several adsorption isotherm models and kinetics models including pseudo-first order, pseudo-second order, Elovich mass transfer model and intra-particle diffusion model for the batch system and Thomas equation for the fixed column bed system were used to evaluate the equilibrium and kinetics data, respectively. Results showed that M-Langmuir model and pseudo-second order model were recommended to describe the adsorption equilibrium and kinetics characteristics. Moreover, the column experimental data fitted well with Thomas model with obtaining the maximum adsorption capacity of about 30 mg/g at pH 5.5, which was in conformity with a result presented in batch experiment. The insignificant effect of SO42- and Cl- on phosphate adsorption indicated that binding phosphate was through a kind of inner-sphere complexation. During the adsorption process, the pH changed significantly, indicating an anion/OH- exchange reaction. Further analysis of the adsorption kinetics indicated that the intra-particle diffusion was the rate limiting process. The above study implies that the ferric sludge has excellent potential in binding phosphate during wastewater treatment process. (C) 2011 Elsevier B.V. All rights reserved.