Hydrothermal conversion of Cd/Zn hyperaccumulator (Sedum alfredii) for heavy metal separation and hydrochar production.

The harmless treatment of heavy metal-enriched hyperaccumulator biomass is the main barrier to the industrialization of phytoremediation. Hydrothermal conversion of Sedum alfredii using different solvents (i.e ·HO and HCl) at 210-300 ℃ was performed to investigate the behaviors of Cd and Zn, and the characteristics and potential application of the derived hydrochars were determined. Low temperature and HCl addition favored the removal of Cd/Zn from the solid phase. The highest removal efficiencies of Cd (95.0%) and Zn (89.3%) were achieved at 210 ℃ with the presence of HCl. The yield, pH, ash content, element concentration, functional groups, and crystalline minerals of the derived hydrochar were influenced by the reaction temperature and addition of HCl. The leaching risk of Cd and Zn was significantly reduced by hydrothermal conversion. The addition of HCl facilitated the immobilization of Zn, while it enhanced the mobility of Cd. Moreover, the hydrochar derived at 210 ℃ showed increased sorption capacity towards Cu, and the addition of HCl greatly improved the energy density of hydrochar. These results suggest that HCl-mediated hydrothermal conversion could be a promising technique to achieve the separation of Cd and Zn from hyperaccumulator biomass as well as the production of value-added hydrochar.