Solvothermal carbonization of waste activated sludge using deep eutectic solvents: A solvent-directed route to tunable biochar properties

The disposal of waste activated sludge (WAS) poses a persistent environmental challenge. While hydrothermal carbonization (HTC) enables sludge valorization without pre-drying, the resulting hydrochar often exhibits low carbon content, high nitrogen levels and inferior fuel properties. This study aimed to determine whether the acidity/basicity and composition of preformed DES-based media could redirect the carbonization pathways of WAS compared with conventional aqueous HTC. Unlike previous DES-assisted HTC systems, in which low-dose DESs are added to water-dominated media, the preformed DES-based media replaced the conventional bulk aqueous medium. Acidic DES showed characteristics consistent with enhanced dehydration, decarboxylation, and condensation characteristics, producing a biochar with substantially higher carbon content (21.97% vs . 9.60% for HTC), low H/C (1.57) and O/C (0.41) atomic ratios, relatively enhanced aromatic character, reduced hydrophilicity, and higher thermal stability (dominant combustion peak at 445.1 °C). In contrast, alkaline DES was associated with enhanced hydrolysis and solubilization, yielding a carbon-depleted, highly hydrophilic residue. Neutral DES exhibits intermediate behavior, producing an oxygen-enriched surface. Overall, DES chemistry produced distinct transformation pathways and tunable biochar properties relative to conventional HTC, providing insights into solvent-mediated regulation of sludge-derived carbon materials.