Thermal Air Oxidation-Mediated Synchronous Coordination and Carbonation of Lanthanum on Biochar toward Phosphorus Adsorption from Wastewater

Biochar has attracted increasing attention as the sustainableandstructure-tunable carrier for lanthanum (La) species for diverse applications.Carbonated La species possesses a higher biocompatibility and a lowerleaching potential than other commonly used La species, while lessattention is paid on the application of carbonated La in phosphorus(P) adsorption. Herein, thermal air oxidation (TAO) was applied asa novel strategy for synchronously tuning the coordination environmentand chemical species of La on biochar surface. The results demonstratedthat TAO induced the coordination of La with oxidation-generated oxygenatedfunctional groups (OFGs) and carbonation of La species by the oxidation-generatedCO(2) on the biochar surface. The batch adsorption resultsshowed that the Q (m) of resultant biocharremarkably increased from 68.92 to 132.49 mg/g at 1 g/L dosage. Italso showed a robust adsorption stability in pH 2-6, a strongresistance to the co-existing Cl-, SO4 (2-), NO3 (-), CO3 (2-), or HCO3 (-), a stableadsorption recyclability, and an ultralow La leaching potential. TheP adsorption was dominated by ligand exchange-induced inner-spherecomplexation. In practical swine wastewater, the resultant biocharcomposite (1 g/L) removed 99.87% of P from 92.3 to 0.12 mg/L at apractical pH of 7.12. The density functional theory calculation furtherrevealed the significant role of the binding of carbonated La by thebiochar surface OFGs in reducing the P adsorption energies, indicatingthe synergism between the oxygenated biochar carrier and the carbonatedLa in P adsorption. Finally, this study provided a novel route tosynchronously tune the coordination environment and chemical speciesof La on biochar via a facile TAO process for high-efficient P adsorptionfrom wastewater. Thermalair oxidation was applied for synchronously tuningthe coordination environment and La species on biochar. The resultantbiochar was superior in P adsorption, involving surface precipitation,ligand exchange, H-bonding, and electrostatic attraction. The DFTcalculation revealed the significant role of the binding of carbonatedLa by the biochar surface OFGs in reducing the P adsorption energies,indicating the synergism between the oxygenated biochar and the carbonatedLa in P adsorption.