One-pot production of 5-methylfurfural (5-MF) and enhanced dewaterability of waste activated sludge by hydrothermal treatment with natural deep eutectic solvents (NADES): Experimental and theoretical studies

Natural deep eutectic solvents (NADES) coupled with hydrothermal treatment was innovatively proposed to process waste activated sludge for sustainable production of 5-methylfurfural (5-MF) and enhanced dewaterability. The yield of 5-MF up to 11.54 kg/t volatile solids (VS) with a NADES dosage of 2.0 mL, hydrothermal temperature of 180 °C and holding time of 30 min. The decreased capillary suction time (CST) and bound water content indicated that NADES with hydrothermal treatment substantially improved the dewaterability of sludge. The variations of sludge physicochemical properties and extracellular polymeric substances (EPS) compositions revealed that NADES disrupted the structure of sludge biopolymers. Density functional theory (DFT) calculations indicated that NADES weakened the hydrogen bond strength between EPS components, and thus promoted the solubilization of proteins and polysaccharides. Besides, transition state analysis showed that NADES also lowered the energy barrier of polypeptide hydrolysis and carbohydrates dehydration reactions for the production of 5-MF.