Hydrothermal conversion of lignocellulosic biomass using deep eutectic solvents and iron salts for solid biofuel and chemical recovery

ABSTRACT Hydrothermal carbonization (HTC) of lignocellulosic waste (hemp oil cake) was investigated using a thermally stable choline chloride–glycolic acid deep eutectic solvent (DES) as an alternative reaction medium, in combination with iron salts (FeCl 3 and FeSO 4 ), to assess their synergistic effects on product properties. DES significantly enhanced biomass hydrolysis and solubilisation, while Fe salts modified reaction pathways and nutrient distribution. Compared to water-assisted HTC, DES-assisted HTC resulted in reduced hydrochar yields (50–72 wt%), particularly at higher temperatures and DES dosages, but promoted carbon enrichment. Carbon content increased from 49.6 wt% for water-derived hydrochar to 53.1–59.7 wt% for DES–Fe derived hydrochars, accompanied by a substantial reduction in nitrogen content. The combined DES–Fe HTC resulted in a notable improvement in fuel quality, with heating values reaching up to 26.6 MJ/kg. SEM showed increased porosity and spherical carbon structures. XPS and NMR confirmed enhanced aromatisation and nitrogen stabilisation in heterocyclic and graphitic forms, with Fe salts accelerating graphitic-N formation. DES-assisted HTC enriched process liquids in organic carbon, phenolics and furfural, indicating enhanced extraction but also increased ecotoxicity. No genotoxicity was detected in the Vicia faba assay. This work introduces a novel DES–Fe-assisted HTC approach to tailor hydrochar properties for renewable energy applications.