REE recovery and hydrochar production from a hyperaccumulator by microwave-assisted hydrothermal carbonization

Phytomining is a promising approach that uses hyperaccumulators to extract rare earth elements (REEs) from various substrates. Nonetheless, the processing and reutilization of REE hyperaccumulators after hyperaccumulation remains a challenge. In this study, the effects of acid type (HCl, H2SO4, and HNO3) and acid concentration (0.01 mol/L-0.2 mol/L) on REE extraction from grass leaves (GL) during microwave-assisted hydrothermal carbonization (MHTC) were investigated. The structural properties and thermal decomposition behavior of both the GL and the hydrochar produced from the MHTC process were characterized. It was found that at the same concentration, H2SO4 yielded the highest REE extraction efficiency due to the greater availability of hydrogen ions. Notably, the utilization of 0.2 mol/L H2SO4 led to the extraction of nearly 100 % of REEs from the GL into the resulting biocrudes. Concurrently, this also caused the degradation of amorphous hemicellulose and crystalline cellulose present in the GL, thereby enhancing the thermal stability of the resulting hydrochar. The physiochemical properties of the hydrochar were also influenced by acid type and acid concentration. The increase in acid concentration significantly contributed to the reduction in the volatile matter and ash content of the hydrochar. Using 0.2 mol/L H2SO4 as the reaction medium, MHTC resulted in a hydrochar with enhanced high heating value and energy densification. These results suggest that MHTC in the presence of an appropriate concentration of H2SO4 is an effective way to extract REEs and produce hydrochar from the GL.