Bio-crude oils production from wheat stem under subcritical water conditions and batch adsorption of post-hydrothermal liquefaction aqueous phase onto activated hydrochars

Hydrothermal liquefaction (HTL) is known to be a promising technology to produce crude bio-oils as intermediate to drop-in transport fuels. However, the co-production of liquefaction wastewater (HTL-AP) and hydrochar residues (HCs) limits the economic viability and technical scalability. Hence, the objective of this work is to study the effect of catalysts NaOH,KOH,Na ₂CO ₃,K ₂CO ₃,H ₃PO ₄,FeCl ₃andFe ₂O ₃ in the HTL reaction medium and on the characteristics of derived crude bio-oils from wheat stem under subcritical conditions at 350 °C for 15 min. Likewise, the mentioned chemical agents were used to enhance the structural, morphological, and chemical surface properties of the HCs for the uptake of the organic adsorbates and nutrients from the HTL-AP. A yield of 30.85 wt% crude bio-oil, having the highest HHV of 34.36 MJ/kg, and lowest 22.03 wt% hydrochar are achieved under Na ₂CO ₃-catalyzed HTL. In contrast, the acidic and Fe-based catalysts revealed a lesser bio-oil yield because of the low pH, which promotes dehydration and polymerization reactions. Reduced Na, K, Fe, and S contents were found in H ₃PO ₄, FeCl ₃, and Fe ₂O ₃-catalyzed biocrudes. This result supports the hypothesis of the in situ demetallation during HTL reaction due to their adsorption onto the mesoporous hydrochars with D _p = 13.77–33.58 nm. The removal efficiency levels for COD, TOC, phenols, total N, P, and dissolved K are 66.67–92.77 %, 62.58–91.84 %, 65.59–99.91 %, 37.63–80.80 %, 96.67–99.90 %, and 45.57–92.36 %, respectively after HTAL-AP treatement. The results demonstrate new insights and directions for the use of activated hydrochar as a low-cost adsorbent for HTL-AP remediation purposes.