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.
Petrovič et al. (Mon,) studied this question.