Engineering hydrochar properties through waste-derived reactive media during co-hydrothermal carbonization of oil palm residues.

Integrating multiple waste streams into high-value energy carriers represents an important strategy for advancing circular economy principles. This study investigates the synergistic co-hydrothermal carbonization (co-HTC) of oil palm empty fruit bunches (EFB) using two chemically distinct waste-derived reactive media: OH-active bilimbi juice and N-functionalizing tofu wastewater. HTC was conducted at 180 and 210 °C to elucidate how the chemistry of these liquid wastes governs hydrochar evolution and energy recovery. Physicochemical characterization (SEM-EDX, BET, FTIR), thermogravimetric combustion analysis, and molecular modeling (HyperChem) were employed to reveal the underlying transformation mechanisms. The results demonstrated that the OH-active medium, rich in organic acids, promoted catalytic dehydration, decarboxylation, and carbon densification, resulting in hydrochars with enhanced thermal stability and combustion characteristics approaching those of sub-bituminous coal. In contrast, the amino acid-rich tofu wastewater facilitated in situ nitrogen incorporation and secondary char formation, yielding hydrochars with superior combustion reactivity and the highest higher heating value of 24.18 MJ kg-1. The highest energy densification ratio (1.43) and energy yield (103.01%) obtained for TR180 further indicate a synergistic transfer of energy-rich compounds from the liquid phase to the hydrochar matrix during co-HTC. Molecular analysis revealed that nitrogen functionalization reduced the HOMO-LUMO energy gap, thereby enhancing electronic reactivity and combustion performance, whereas acidic media favored the formation of more condensed aromatic structures with improved thermal resistance. These findings demonstrate that waste-derived reactive media can serve not only as process water substitutes but also as active chemical agents for selectively engineering hydrochar properties toward either stability or reactivity-oriented solid fuels. The proposed approach offers a sustainable pathway for the simultaneous valorization of agricultural residues and food-processing effluents.