Solid base-assisted photocatalytic degradation of polyethylene via the Norrish mechanism through the generation of alternating polyketones

Abstract

Low-density polyethylene (LDPE) is widely used in plastic products. Due to its nonpolar molecular structure and inert hydrocarbon chains, it is resistant to degradation, leading to serious environmental problems. As a photocatalyst, TiO2 generates electron-hole pairs upon exposure to ultraviolet (UV) light. The holes interact with hydroxide ions $${\left(\right.{\mbox{OH}}}^{-}$$ dissociated from a solid base to produce hydroxyl radicals ( $${}^{\bullet }{{\rm{OH}}}$$ ). These radicals then attack the PE chain, generating carbonyl groups (C=O), which facilitate Norrish reactions and promote PE degradation. In this study, a LDPE-1%TiO2 (LDPE-1T) composite film was first soaked in a NaOH solution to determine its photocatalytic degradation efficiency under alkaline conditions. After 800 h of UV exposure, the film exhibited a significant mass loss of ~87 wt%. In contrast, the LDPE-1T composite film without NaOH pretreatment presented a lower mass loss of ~55 wt%. To increase the applicability of this study, a solid base was incorporated to fabricate LDPE-5%TiO2-3%K2CO3 (LDPE-5T-K) and HDPE-1%TiO2-3%K2CO3 (HDPE-1T-K) composite films through hot pressing. This modification was also shown to increase photocatalytic degradation efficiency by increasing the concentration of $${}^{\bullet }{{\rm{OH}}}$$ radicals in the system, promoting the formation of C=O groups and facilitating the Norrish type I reaction. Caption: In this study, a simple hot-pressing and blending method was utilized to produce polyethylene films, followed by photocatalytic degradation under alkaline conditions with TiO2. During the photocatalytic degradation process, alternating polyketone groups were generated on the polyethylene matrix, promoting Norrish type I reactions and effectively enhancing the photocatalytic degradation efficiency of polyethylene.