Co-hydrothermal carbonization of typical plastics waste and garden waste: Effect of plastic additives on hydrochar formation and properties.

Co-hydrothermal carbonization (co-HTC) of plastics and garden waste (GW) is a promising strategy for plastic waste management, while the impact of plastic additives, which are inherent in plastic production, on this process has not been fully elucidated. This study elucidates the role of three plastic additives (bisphenol A, calcium stearate, lead sulfate) in governing the reaction behavior of co-HTC and the hydrochar functionality during the conversion of GW with four representative plastics (PVC, PET, PS, PP). The results revealed that lead sulfate significantly increased PVC derived hydrochar yield (maximum reach 56.3 ± 0.8 %), suppressed particle agglomeration in PS/PP, and improved PVC dechlorination efficiency by 10.9%. In contrast, calcium stearate generally reduced hydrochar yield and promoted undesirable agglomeration. Based on typical downstream application scenarios of hydrochar, the effects of plastic additives on the performance of hydrochar were further evaluated. Bisphenol A enhanced the fuel properties of PET/GW-derived hydrochar by increasing the carbon content (58.7 % to 59.3 %) and higher heating value (19.87 to 20.93 MJ/kg), while also elevating ignition and burnout temperatures, indicating improved combustion stability. For adsorption performance, lead sulfate-modified PVC/GW hydrochar showed a notable increase (up to 42.7%) in tetracycline adsorption capacity over calcium stearate at higher additive levels, despite an overall initial performance decline caused by additives, whereas bisphenol A suppressed it via competitive π-π interactions. These findings resolve a key knowledge gap in additive-mediated hydrochar formation, enabling tailored co-HTC process design for plastic waste valorization.