Catalytic co-hydrothermal carbonization of cigarette butts and sewage sludge for waste valorization and pollutant mitigation

The accumulation of hazardous cigarette butts (CBs) and sewage sludge (SS) poses serious environmental and waste management challenges. This study developed a catalytic co-hydrothermal carbonization (Cat-Co-HTC) strategy to stabilize pollutants and valorize these urban wastes into carbon-rich hydrochar. Using response surface methodology, the optimum non-catalytic conditions (253 °C, 113 min, L:S = 7:1, CBs/SS = 38%) produced a hydrochar with a higher heating value (HHV) of 28.4 MJ/kg and hydrochar yield (HY) of 57.1%. Adding 2.5 wt% Hormuz red soil (HRS) significantly enhanced hydrochar quality, increasing HHV to 33.9 MJ/kg and HY to 60.8%, while promoting heavy metal immobilization, reducing process water COD, and improving carbon stabilization. The HRS-catalyzed process moderately improved textural characteristics, increasing specific surface area from 10.2 to 13.4 m 2 /g and creating a more porous structure with an average pore diameter of 11.5 nm, thus validating enhanced carbonization and aromatic condensation. The catalyst not only improved solid fuel quality but also decreased process water COD and immobilized heavy metals in the hydrochar matrix, reducing their leaching potential. Compared with equivalent pure α-Fe 2 O 3 (HHV: 31.8 MJ/kg; HY: 59.2%), HRS achieved superior performance (HHV: 33.9 MJ/kg; HY: 60.8%), indicating the synergistic contribution of its mineral constituents. This study demonstrates the synergistic catalytic functions of Fe 2 O 3 -rich HRS in enhancing hydrolytic cleavage, facilitating aromatization, and stabilizing pollutants. The proposed Cat-Co-HTC pathway provides an integrated strategy for hazardous waste stabilization, resource recovery, and environmentally safer management of mixed urban residues within a circular economy framework.