Dried waste activated sludge as biosorbents for metal removal: adsorptive characterization and prevention of organic leaching

Dried waste activated sludge was used for copper removal from simulated waste water in this study. Unconditioned activated sludge (UAS) bound up to 35 mg Cu g(-1), although there was significant leaching of organic material. Organic and copper leaching from the UAS increased significantly as the pH was reduced. Immobilization of UAS by sodium and calcium alginate was utilized in order to overcome the leaching problems, but reduced the adsorption capacity. Based on metal removal and organic leaching, calcium alginate-conditioned UAS (CACAS) was found to be the most suitable sorbent for copper removal. Kinetic experiments showed that copper removal by both sodium alginate-conditioned UAS (SACAS) and CACAS was faster than that by activated carbons, but slower than most of the other biosorbents described in the literature. FTIR spectroscopy identified a number of atomic groupings and structures in UAS relevant to copper adsorption. It suggested that hydrogen ions are replaced with copper ions. The Freundlich equation fitted the experimental isotherms better than the Langmuir equation. A computational model based on adsorption isotherm, external mass transfer and diffusion processes successfully described the kinetics of copper ion removal and suggested that the biosorption kinetics was controlled by mass transfer. (C) 2002 Society of Chemical Industry.