Hydrothermal liquefaction of Cd-enriched Amaranthus hypochondriacus L. in ethanol-water co-solvent: Focus on low-N bio-oil and heavy metal/metal-like distribution

The high nitrogen content in bio-oil from protein-rich biomass will cause the possible pollution problems by NOX emissions during combustion. In this study, Cd-enriched Amaranthus hypochondriacus L. (AHL) was treated by hydrothermal liquefaction (HTL) in ethanol-water co-solvent aiming to reduce the nitrogen content of the biooil. Besides, aqueous phase recycling (APR) was applied to achieve a higher bio-oil yield. HTL in ethanol-water co-solvent with APR three times resulted in the maximum bio-oil yield (47.26%) and greatly reduced the nitrogenous compounds content of bio-oil. During the APR process, the increase of total nitrogen (TN) content in the aqueous phase indicated that organic-N in the organic phase transformed into NH4+ to the aqueous phase. Acetic acid (13.87-22.37 mg/mL) was dominated in the aqueous phase, leading to a low pH value (6.27-5.29), which could serve as the possible catalyst for the HTL process during APR that can be the reason for the higher bio-oil yield. After the HTL process, Cr, Cu, Cd and Pb remained mostly in bio-char while As was present largely in the aqueous phase. Thus, this study demonstrated that the APR for HTL process in ethanol-water co-solvent can be a hopeful method to dispose the high-protein biomass for improved bio-oil yield and quality.