Hydrothermal Treatment of the Pristine and Contaminated Cd/Zn Hyperaccumulators for Bio-Oil Production and Heavy Metal Separation

Abstract

Hydrothermal conversion is a promising technology to dispose of biomass wastes. To achieve the harmless treatment of heavy metal (HM) hyperaccumulators and production of bio-oil, hydrothermal conversion of two types of Sedum alfredii [i.e., uncontaminated S. alfredii (SAL) and the harvested S. alfredii after phytoremediation (SAH)] at 210–300 °C was conducted in this study. Only 0.53–1.59% of Cd and 0.34–5.94% of Zn in SAH migrated to bio-oils, which were much lower than those in hydrochar and the aqueous products. High hydrothermal conversion temperature promoted the bio-oil yield, and the S. alfredii-derived bio-oils showed considerable higher heating values (32.07–33.83 MJ/kg). The main components of bio-oils were aliphatic hydrocarbons, followed by oxygen-containing compounds (e.g., alcohols, phenols, and ketones/aldehydes) and nitrogen-containing compounds. The bio-oils derived from SAL contained a higher content of aliphatic hydrocarbons (56.39–60.11%) and N-containing compounds (9.03–15.17%), while the SAH-derived bio-oils had more ketones/aldehydes (15.17–19.79%), which could be ascribed to the component difference of feedstocks and the catalytic effect of Zn. Moreover, the distillable fraction (boiling point <538 °C) in the bio-oils was up to 79.89–86.90%. These results suggest that hydrothermal conversion of S. alfredii could be a promising technique for the HM separation and value-added bio-oil production.