Removal behavior of phosphate from aqueous solution by calcined paper sludge

Paper sludge, which is an industrial waste generated during the manufacture of recycled paper products, was converted into an effective phosphate (PO43−) sequestration agent through calcination and its removal behavior toward phosphate ion in aqueous solution was examined. Paper sludge contains mainly cellulose fibers and inorganic fillers together with coating materials such as calcite and kaolinite. The sludge was calcined at temperatures between 200 and 1000 °C for 6 h. Crystalline phases originally present were altered at increasing temperatures (up to 800 °C) in the order kaolinite < calcite. An amorphous phase formed at 800 °C, and gehlenite formed at 1000 °C. Paper sludge calcined at 800 °C (PS-800), which has amorphous phases with high reactivity, removed the greatest amount of phosphate ion. The removal of phosphate by PS-800 was nearly constant in the equilibrium pH range of 5–12, and possessed high selectively for PO43− in solution with coexisting Cl−, NO3−, and SO42−. With increasing temperature of the aqueous solution, the kinetics of removal increased and the amount of phosphate removed was greater, which indicated that the removal reaction was an endothermic process. The main process responsible for sequestration of PO43− was the formation of brushite [CaHPO4·2H2O] by the reaction between labile Ca and PO43− in the solution.