Selective removal of phosphate using La-porous carbon composites from aqueous solutions: Batch and column studies

Porous carbon composites containing varying La contents were synthesized and studied for the sorption of phosphate. The phosphate sorption capacities increased with increasing La content and showed an almost complete La consumption efficiency (P/La molar ratio of 0.936) for the composite containing a La(OH)₃/porous carbon mass ratio of 0.1 (La_0.1-PC). Phosphate sorption by these composite materials occurred through precipitation of LaPO₄ supported by PXRD, XPS and EDS analyses. Kinetic studies revealed that phosphate sorption by the La_0.1-PC composite was rapid and reached equilibrium within 1 h compared to the composites containing higher mass ratio of La(OH)₃. The sorption capacity of phosphate was not reduced in the presence 20 mM chloride and 20 mM sulfate, but was disturbed by 20 mM carbonate. The phosphate sorption in the presence of 25 mg/L humic acid was maintained along with the adsorption of dissolved humic acid by electrostatic adsorption onto the carbon support. Interestingly, the phosphate sorption capacity in seawater was twice that in fresh water, likely because the Ca²⁺ and Mg²⁺ ions in seawater enhanced the precipitation of phosphate on the surface of La in the La-PC composite. Column experiments verified that the La_0.1-PC composite was effective for the continuous treatment of phosphate. The current outcome suggests that the La-PC composite can be used as a multifunctional sorbent for the remediation of phosphate along with alkaline metal ions (Ca²⁺ and Mg²⁺) as well as the removal of humic substances in wastewaters. Moreover, the spent sorbent could be repurposed as a phosphate plant fertilizer, where it would reduce the cost of disposal and increase the commercial value.