Fe-impregnated hydrochar catalysts prepared using liquor from hydrothermal carbonization of biomass as the reaction medium

This work presents a novel single-step approach for the synthesis of Fe-impregnated hydrochars using spent liquor from the hydrothermal carbonization (HTC) of residual biomass as a reaction radium. The resulting Fe-modified hydrochars were evaluated as catalysts for 4-nitrophenol (4-NP) removal via the Fenton reaction. Herein, it was demonstrated that incorporating HTC spent liquor significantly enhanced the hydrochar's catalytic activity, particularly at an HTC temperature of 250 °C with a 0.5 M metal-salt concentration. With the catalyst synthesized under these latter conditions, ∼88 % removal efficiency of 4-NP was achieved, including a reaction rate constant 17 times higher than at an HTC temperature of 220 °C. Characterization confirmed the in-situ formation of magnetite (Fe₃O₄) on the hydrochar surface, providing improved thermal stability and magnetic properties that enable catalyst recovery and reuse. The Fe reduction from the metal salt and subsequent impregnation may probably be attributed to the biomolecules in the spent liquor, especially in the liquor obtained from the HTC of Jatropha fruit husk. This single-step impregnation method forms the magnetite phase during HTC without requiring a post-pyrolysis treatment, as reported in previous studies. This energy-efficient and environmentally sustainable method provides valuable insights into the potential of HTC spent liquor as a renewable medium for functional material synthesis.