Efficient reclamation of phosphorus from wetland plant via CaCl2/NaOH-mediated hydrothermal carbonization: insights from the evolution of phosphorus

Abstract Hydrothermal carbonization (HTC) of wetland plant could achieve the recovery of phosphorus (P) via the production of P-enriched hydrochar to alleviate the crisis of phosphate resources, while the migration and transformation of P should be determined. In this study, Canna indica was derived into hydrochar through HTC at different temperatures (200°C-260°C) and liquid mediums (H 2 O, CaCl 2 , and NaOH). The P forms were systematically characterized using P K-edge X-ray absorption near edge structure (XANES), 31 P liquid nuclear magnetic resonance (NMR), and sequential extraction. The total P content in hydrochar was up to 23 mg g −1 with mainly inorganic P (> 97.8%), and the recovery rate was almost 100% during NaOH-mediated HTC. The P species, monoester-P and soluble orthophosphate (ortho-P), in biomass were transformed to more stable ortho-P in hydrochars, which was highly dependent on temperature and liquid medium. With increasing temperature, Al/Mg-P was gradually replaced by Ca/Fe-P. The CaCl 2 solution facilitated the transformation of Ca(H 2 PO 4 ) 2 into CaHPO 4 by elevating the Ca/P ratio. While for the NaOH-mediated HTC, the CaHPO 4 and Ca(H 2 PO 4 ) 2 were transformed to hydroxyapatite (74.3%-81.5%), and the proportion of MgHPO 4 elevated with increasing temperature. The diffusive gradients in thin films (DGT) results implied that the addition of hydrochar greatly elevated the soil available P content, which was further promoted by high temperature and NaOH medium. These results indicate that the species and availability of P in hydrochar could be adjusted through varying liquid medium and reaction temperature, which provide guidance for the target design of P-enriched hydrochar and P reclamation. Graphical Abstract