Quasi-amorphous La2O3 nanoparticles loaded steam-exploded straw biochar for simultaneous adsorption and selective separation of phosphate and arsenate.

Co-contamination of phosphate and arsenate is common in wastewater and aquatic environments due to their chemical similarity. However, it remains challenging to simultaneously remove and selectively separate them via adsorption. Here, quasi-amorphous La2O3 nanoparticles loaded steam-exploded straw biochar (La-BC) were synthesized via impregnation-precipitation-low temperature pyrolysis. La-BC exhibited exceptional adsorption capacities of 79.7 mg P g-1 and 205.4 mg As g-1. When phosphate and arsenate coexisted, La-BC reduced their concentrations from 1 mg L-1 to 0.002 and 0.009 mg L-1, respectively. Phosphate adsorption proceeds mainly via surface precipitation, whereas arsenate removal relies on surface precipitation combined with ligand exchange. La-BC exhibits extremely rapid adsorption rates for phosphate, particularly within the first 3 min. In contrast, the equilibrium time of arsenate is 10 h. This pronounced kinetic disparity enables La-BC to selectively separate phosphate from phosphate-arsenate binary solutions in a time-dependent manner, with a maximum separation factor of 397. Leaching and biochar aging tests demonstrate the high stability of La-BC-P and La-BC-As. This study proposes a sustainable and feasible strategy to fabricate adsorbents from biomass for simultaneous removal of phosphate and arsenate and selective recovery of phosphate.