Chemical recycling methods for managing waste plastics: a review

Substantial quantities of discarded plastic have resulted in detrimental effects on the environment and ecosystems, calling for effective recycling of plastic wastes. Chemical methods for managing plastic wastes have been extensively studied, and selective recycling of products has grown in popularity. Pyrolysis, photocatalysis, and supercritical fluids were the main chemical methods reported for processing plastics. This article reviews reaction mechanisms and representative studies of pyrolysis, photocatalysis and supercritical fluid disposal of plastics. Three challenges are identified: first, high temperatures, above 400 °C, pose safety risks and potentially release harmful gases. Second, the photocatalytic decomposition of plastics is generally economical and environmentally friendly, however, reaction rates are uncontrollable. Third, the exceptional solubility and mass transfer properties exhibited by supercritical fluids have the potential to significantly augment the decomposition of plastics. Supercritical fluids can also serve as reactants that directly influence the thermal or kinetic mechanisms of the reaction. Whereas individual treatment methods are limited, coupled methods enable efficient plastic management and facilitate the selective acquisition of products. In particular, this review suggests that coupling photocatalysis and supercritical carbon dioxide should allow to dispose of plastics and convert carbon dioxide into valuable resources.