Transformation of Phosphorus in Wetland Biomass during Pyrolysis and Hydrothermal Treatment

Abstract

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 °C) and hydrothermal carbonization (HTC, 200–260 °C) of Napier grass via combining 31P 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., Ca3(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.