Formation and evolution of PVC waste-derived hydrochar

Abstract

Hydrothermal carbonization (HTC) is an effective method for converting solid waste PVC into multifunctional chlorine-free hydrochar. However, the formation and evolution of hydrochar derived from PVC are not clear. In this study, a combination of ultimate analysis, SEM, BET, FTIR, XPS, and 13C NMR analytical techniques was employed for a comprehensive characterization of the surface morphology and molecular structure of rigid PVC(RPVC)-based hydrochars. The influence of additives on the dechlorination of RPVC and structural evolution was investigated, leading to the establishment of the formation and evolution pathway of hydrochars. The results demonstrate that the CaCO3 filler facilitates dechlorination of RPVC by reacting with HCl and providing an additional channel for HCl diffusion. The chlorine removal rate reaches 96.08% at 280 °C and the residual chlorine in hydrochar is detected primarily on its surface. Prior to dechlorination, RPVC underwent a significant isomerization reaction, resulting in the transformation of hydrochar from long straight-chain alkanes into a polymerized aromatic structure. This transformation led to the formation of hydrochar with a relatively stable core structure and a highly active shell structure. These shell layers primarily contain C-O, CO, and O-CO. This study provides vital information for the design of large-scale processes for handling polyvinyl chloride (PVC)-containing waste and upgrading PVC waste into high-value multifunctional materials.