Hydrothermal Dechlorination of Polyvinyl Chloride in Plastic Mixtures: Generation of Environmentally Benign Precursors

Understanding dechlorination behavior is important in the generation of environmentally benign precursors from chlorinated plastics by thermal treatment. In this work, the real-time chlorine distribution during hydrothermal treatment (HTT) of chlorinated plastics was first studied by installing an online sampler on the exhaust pipe of the reactor. Linear low-density polyethene (LLDPE), polypropylene (PP), and polyvinyl chloride (PVC) were selected as parent materials to investigate the dechlorination behavior within a broad temperature range of 175°C–280°C for 30 min, and 205°C–235°C for 15–60 min. The results showed that the existence of LLDPE and PP did not change the chlorine release form but affected the chlorine distribution and dechlorination efficiency (DE). Below 220°C, the existence of LLDPE, PP, and LLDPE/PP showed neglectable effect on the PVC dechlorination behavior and efficiency. A positive synergistic effect on dechlorination was observed at the HTT temperature of 220°C for mixed plastics. DE was improved by 11.90%, 9.50%, and 10.30% for LLDPE/PVC, PP/PVC, and LLDPE/PP/PVC, respectively under 220°C for 30 min. Further increasing HTT temperature, the molten LLDPE and PP could cover PVC, thus hindering the escape of HCl. Kinetic analysis shows that the dechlorination process followed the first-order reaction, controlled by chemical reactions, and higher temperatures could accelerate chlorine removal. This work provides a theoretical basis for the harmless disposal and resource utilization of chlorinated organic waste.