Valorization of low heavy metal-accumulating plants through catalytic hydrothermal liquefaction with attapulgite: Product characterization and migration behavior of heavy metals

Abstract

Harmless treatment of heavy metal-accumulating plants is significant for further development and application of phytoremediation for contaminated soil remediation. In this study, hydrothermal liquefaction (HTL) of low heavy metal-accumulating plants (Jute, Solanum nigrum, and Sorghum) was conducted at different temperatures (280–360 °C) for bio-oil production. Attapulgite and Co/Attapulgite were prepared and tested in HTL process. The maximum bio-oil yield and higher heating value (HHV) was found to be 32.04% and 29.61 MJ/kg from HTL of Solanum nigrum with Co/Attapulgite at 300 °C. The improvement in quality of bio-oil was marked with decreased O/C content (0.26) and newly formed hydrocarbon fraction with the content of 6.24% with Co/Attapulgite catalyst. It was beneficial for immobilization of Mn, Zn and Cr in hydrochar with addition of catalyst. Notably, Mn and Cd were not detected in any bio-oil, and the proportion of Cr in the bio-oil obtained at 360 °C decreased to zero with Co/Attapulgite. XPS and XRD analysis indicated that Mn and Zn existed in the form of Mn3O4, Mn, ZnO and Zn(OH)2 in hydrochar. Finally, potential pathway of heavy metals migration-transformation behaviors during HTL was proposed.