Dechlorination of PVC wastes by hydrothermal treatment using alkaline additives

Some chemicals were usually utilized in the hydrothermal dechlorination (HTD) of chlorine-containing wastes without revealing their roles. This work intends to investigate the role of chemical additives in the HTD of PVC (polyvinyl chloride). Several chemicals, including Na₂CO₃, KOH, NaOH, NH₃·H₂O, CaO and NaHCO₃, were added into the PVC HTD process, which was conducted in subcritical Ni²⁺-containing water at 220°C for 30 min. The results show the alkalinity of additives had notable effects on the dechlorination efficiency (DE) of PVC due to the neutralization between HCl and additives. The most effective additive is Na₂CO₃, with the maximum DE of 65.12% at a Na₂CO₃ concentration of 0.025 M in this study. According to SEM, the hydrochar obtained from the HTD with Na₂CO₃ become more porous and looser than the others did, which contributed to the acceleration of PVC dechlorination. The DE vibration with the concentration of additives was different. For Na₂CO₃, it was firstly increased and then decreased with Na₂CO₃ concentration increasing from 0.01 to 0.04 M. For KOH and NaOH, it kept reducing with the concentration increasing from 0.02 to 0.08 M. The drop in DE was ascribed to surface poisoning and a loss in the supported active phase resulting from the formation of metal chloride species. FTIR analysis shows that the elimination of hydrogen chloride was the main route for HTD of PVC. All the results provide some fundamental data to find some cheap but efficient chemicals with aim to recycle the chlorinated organic wastes effectively.