Hydrochar derived from willow using in phytoremediation: Hydrochar properties, metals behavior and potential applications

The safe disposal of biomass waste containing heavy metals is a major challenge for the large-scale phytoremediation. In this study, different hydrothermal media (H2O, HCl, H2SO4, and H3PO4) were used to conduct hydrothermal carbonization (HTC) of willow biomass containing Cd and Zn. The cadmium and zinc distribution and behavior, the characteristics and potential applications of hydrochars were investigated. The results showed that Cd and Zn are beneficial to migrate to the liquid phase with the addition of acid. Reaction medium and acid concentration have great influence on the yield, ash content, element content, functional groups, crystalline minerals and thermal stability of hydrochars. In the safe application, the hydrochar produced at 0.05 mol/L H3PO4 has the maximum adsorption capacity for Cd2+(4.497 mg/g) and Cu2+ (49.708 mg/g), respectively. Besides, adding acid improves the energy density of hydrochars with higher calorific values (23.83 - 28.27 MJ/ kg), making it a promising biofuel choice.