Secondary degradation of anaerobic-digested and UV-pretreated plastics under cycles of freeze-thaw

Cycle of freeze-thaw (CFT) is a typical environmental characteristic in cold regions during the transition from winter to spring, which may drive physical stress acting on the plastics. This study investigated the impacts of CFT on the changes to polyvinyl-chloride (PVC) and polylactic acid (PLA) plastic-film surface, sourced from anaerobic digestion (AD) and AD + UV exposure. The findings showed that under the action of AD+UV, the surface of PVC and PLA plastic-film became yellow, fragile and rough. The consequent CFT process reduced surface yellowing and abundance of oxygen-containing groups on plastic-surface increased during AD and UV exposure, as well as impelled the PLA plastic-film fragmentation. The hydrophobicity of the plastic-film surface was compromised by AD and UV treatment, with partial restoration observed after CFTs. The phenomenon could be attributed to CFT stress stripping away the fragile oxidized surface layer of the plastic-film, as evidenced by the presence of micro/nanoplastics as well as the detection of total dissolved solids (TDS), plastic-additive bisphenol A and increased turbidity in the CFT extracts. The study highlights that CFT stress may be a significant yet often overlooked environmental process that influences the environmental degradation of "organic fertilizer source" plastics.