Removal and recovery of phosphate from water by lanthanum hydroxide materials

The adsorption of phosphate from water by two lanthanum hydroxides (LHs), a commercial LH and a synthesized LH from waste alkaline solution, was investigated. The amorphous synthesized LH had higher specific surface area, and showed greater performance for phosphate adsorption than the crystalline commercial LH. The phosphate adsorption data agreed well with the Langmuir model with the calculated maximum capacity of 107.53 mg/g (dry weight) for synthesized LH and 55.56 mg/g (dry weight) for commercial LH, respectively. The affinity toward phosphate was high over a wide pH value range, from about 2.5 to 9.0 for commercial LH and from about 2.5 to 12.0 for synthesized LH, respectively. Release of La was negligible when pH > 4.0. FTIR measurements showed that the monodentate surface species of La-OPO3 was formed via ligand exchange mechanism. In the coexistence of chloride, nitrate, sulfate and hydrogen carbonate anions, phosphate removal by LHs was only slightly affected. The uptake of phosphate (∼5 mg/L) from real effluent by LHs performs well at a dose of 1 kg/10m3, with the removal efficiency exceeding 99% for synthesized LH and 90% for commercial LH, respectively. The adsorbed phosphate could be successfully recovered by hydrothermal treatment in NaOH solution, and the regenerated LHs could be reused for phosphate removal.