Synergistic co-catalysis by seawater salts and CO2 for degradation of marine microplastics in hydrothermal gasification: Mechanism and modeling based on ReaxFF molecular dynamics

To advance the resource utilization of microplastics in marine environments via hydrothermal gasification, this work investigates the catalytic effects of individual and combined additions of chloride salts (KCl and NaCl) and CO2. The role of these salts, naturally present in seawater, in promoting the gasification of polyethylene terephthalate (PET) and polyethylene (PE) was evaluated using reactive force field molecular dynamics (ReaxFF-MD) simulations. Results demonstrate that KCl significantly enhances the degradation efficiency and H2 yield for both individual and mixed PE/PET plastics. Furthermore, the introduction of CO2 strengthens the catalytic performance by reacting with KCl to form K2CO3 during the process. The hydrothermal gasification of PE/PET mixtures also shows considerable potential for the recovery of valuable products, such as terephthalic acid (TPA) and ethylene, with TPA recovery rates reaching up to 95 % at a simulated temperature of 2000 K (corresponding to an actual temperature of 640 K). The apparent activation energy for gasification of the plastic mixture was approximately 219 kJ/mol; the addition of KCl reduced it to about 62 kJ/mol, while the combined introduction of KCl and CO2 further decreased it to around 60 kJ/mol. This work provides essential theoretical insights and fundamental data for deploying seawater-based hydrothermal gasification as a sustainable technology for recovering mixed plastic waste.