Identification of Water-Soluble Chemicals in Leachate of Plastics and Their Kinetics Enhanced under Photooxidative Conditions

Abstract

Plastics can be extensively discharged in various environments such as rivers, coastal areas and offshore. When plastics in aquatic environment are subject to photooxidative conditions, various substances (e.g., plasticizers) can be leaching-out. The leaching kinetics would be affected by aging process enhanced by photo-oxidation. However, the release and fate of leachable substances and their leaching kinetics are not well understood. The identification of these chemicals can be achieved through suspect screening and non-target screening methods using high resolution mass spectrometry (HRMS). This study aims to investigate the substances released when plastics are exposed to a xenon lamp and natural sunlight, as well as the leaching kinetics over time under photooxidation condition. five types of plastics (PP, PS, PE, PVC, PLA) were selected based on their global use amount. The leaching experiment was performed using each 350 g of plastics in Boro bottles with 700 mL of DI water. The samples were subjected to a xenon lamp with a light intensity of 600W/m² for 21 days. Samples were collected at 7-day intervals. To enrich the sample, a solid phase extraction (SPE) was used. The sample was adjusted to 100 mL, and citrate buffer with internal standards was added. Elution used alkaline and acidic ethyl acetate/methanol solutions. Extracts were concentrated to 1 mL, reconstituted with water/methanol (9:1), and filtered through a 0.45μm filter. The samples were then analyzed using high performance liquid chromatography-high resolution mass spectrometry (HPLC-HRMS). A suspect list was established using the NORMAN Suspect List Exchange and literature survey. Subsequently, suspect screening was performed based on the peak selection criteria (i.e., peak intensity, isotopology pattern, and MS2 fragments ratio). Acquired MS2 fragments were evaluated with MS2 library (e.g., mzCloud) for qualitatively chemical structure elucidation. A total of 9 substances were tentatively identified. Out of identified suspects, 2 plasticizers: Caprolactam, Melamine and 1 solvent: Isophorone were confirmed using reference standard (confidence level 1). Occurrences of Butyl-methacrylate and 3 compounds (i.e., Heptapropylene-glycol, Hydrogenated-terphenyl, Styrene) were evident with confidence level 3. 1,2,4-Trimethylbenzene and N-Dodecanoyl-N-methylglycine were identified with confidence level 4. The peak area of 4 compounds (i.e., Caprolactam, Hydrogenated-terphenyl, Melamine, N-Dodecanoyl-N-methylglycine) showed the time-independent trends, whereas the other substances continuously increased over time. These results are expected to expend our understanding of the fate of substances released from plastics through photodegradation.