Removal of fluoride ions using calcined paper sludge

Paper sludge (PS) is generated as an industrial waste during the manufacture of recycled paper products, and amounts discharged globally are increasing annually. In this study, PS is converted into an effective sequestration agent for fluoride through calcination. PS contains mainly cellulose fibers and inorganic fillers together with coating materials such as calcite, kaolinite, and talc. The sludge was fired at temperatures between 200 and 1,000 °C for 6 h. Crystalline phases originally present were altered at increasing temperatures (up to 800 °C) in the order of kaolinite < calcite < talc. An amorphous phase was formed at 800 °C, and gehlenite was formed at 1,000 °C. The uptake of fluoride by these fired samples was investigated at room temperature. PS fired at 800 °C shows a higher uptake of fluoride than that calcined at other temperatures. It also shows a high selective removal for fluoride in solutions containing chloride, nitrate, and sulfate. The main process responsible for fluoride sequestration is the formation of fluorite by reaction between labile calcium and fluoride in solution. The fluoride removal behavior fits a Freundlich isotherm model than Langmuir, and indicates the spontaneous nature of this reaction.