Hydrothermal Treatment of E-Waste Plastics for Tertiary Recycling: Product Slate and Decomposition Mechanisms

Amounts of e-waste plastics have been one of the fast growing global waste streams and threaten to grow into an unmanageable problem. This phenomenon needs to be solved urgently by efficient and cost-effective ways. In this study, a novel hydrothermal treatment technology was implemented to convert the e-waste plastics into organic products which can be used as monomers of plastic production or chemical feedstock. We systematically investigated the recovery efficiencies of organic products, the product slate, the possible hydrothermal degradation mechanisms, and the microstructure of solid residues. The results showed that the yields of organic products derived from four kinds of e-waste plastics ranged from 81.4 to 97.6 wt % at 350 °C. The recovered products contained styrene monomers, styrene derivatives, bisphenol A (BPA), caprolactam (CPL), and other valuable commodity chemicals. On the basis of the systematic analysis of hydrothermal organic products, the possible degradation mechanisms were proposed which involved a variety of reactions such as depolymerization, hydrothermal cracking, hydrolysis, nucleophilic substitution, and free radical reaction in it. The degradation mechanisms provided the theoretical basis for the hydrothermal treatment of e-waste plastics. All of the results demonstrated that hydrothermal treatment is a viable and prospective method for sustainable recycling of e-waste plastics.