Efficient reclamation of phosphorus from wetland biomass waste via liquid-recirculated hydrothermal carbonization and...

Hydrothermal carbonization (HTC) for the recovery of phosphorus (P) from biomass wastes has attracted considerable attention, while migration of P to the liquid phase greatly weakened P recovery efficiency and elevated the environmental risk. Therefore, a systematic scheme was proposed in this work to accomplish the complete reclamation of P from wetland plant (Ceratophyllum demersum) through coupling liquid-recirculated HTC mediated by H₂O or H₂SO₄ with precipitation, and the migration and speciation of P during this process was determined by P K-edge X-ray absorption near edge structure, ³¹P nuclear magnetic resonance, and the modified sequential extraction. The P concentration in the liquid phase increased with the recirculation of HTC process water, and reached up to 550.6 mg L^-1. >98.1 % of P in the recirculated liquid products was recovered in the forms of hydroxyapatite and struvite with the HTC mediums of H₂O and H₂SO₄, respectively, without the addition of exogenous metals. In addition to the production of P compounds, P-enriched hydrochar was simultaneously obtained during this process. The HTC medium and liquid recirculation had profound impact on the hydrochar characteristics and the transformation of P. Hydroxyapatite and magnesium phosphate were the dominant P species in the hydrochars derived from H₂O-mediated HTC, while FePO₄ and other Ca-P species [Ca₃(PO₄)₂ and Ca(H₂PO₄)₂] dominated the P compounds in the H₂SO₄-mediated hydrochar. These results suggest that coupling liquid-recirculated HTC and precipitation could be a promising strategy for P reclamation, which could provide new insights into the P recovery from biomass waste.