Transformation of Phosphorus in Wetland Biomass during Pyrolysis and Hydrothermal Treatment

The reclamation of phosphorus (P) in wetland biomass is a potential strategy to alleviate the P resource shortage, and (hydro)thermal treatment is considered a promising technique for biomass waste treatment and valuable product manufacturing. In order to understand the evolution of P during the (hydro)thermal process, we systematically determined P speciation during pyrolysis (300-600 degrees C) and hydrothermal carbonization (HTC, 200-260 degrees C) of Napier grass via combining P-31 NMR, XRD, and sequential chemical extraction, and the derived chars were characterized. The biochars showed higher pH, higher ash content as well as greater stability, and the hydrochars had more oxygen-containing groups and better energy density. Temperature-dependent (hydro)thermal treatments can effectively reduce the risk of P leaching loss by transforming P into stable pools. Pyrolysis could increase the portion of potentially plant-available P (NaHCO3-Pi) from 3.6% to 29.0% with increasing pyrolysis temperature, while HTC treatment favored the formation of more stable P pools (HCl-P). Ortho-P (93.4%) and monoester-P (6.6%) were the main P species in Napier grass, while a considerate amount of amorphous pyrophosphate (20.0-40.3%) formed after pyrolysis. In contrast, orthophosphate (e.g., Ca-3(PO4)(2)) was the dominant P species in hydrochars. Therefore, P availability and speciation are greatly affected by (hydro)thermal treatment conditions, and these findings provide guidance for P reclamation and biomass waste utilization.