A Mass-Produced Microplastic Antioxidant Degrades Rapidly under UV and Produces More Toxic Products via Nonphotolytic Processes

Plastic additives are emerging contaminants in aquatic environments, making it critical to understand their persistence in plastics and surrounding waters. In this work, we conducted dual-phase monitoring of antioxidant additive octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox 1076, I-1076) in a system where low-density polyethylene (LDPE) microplastic films were suspended in pure water and exposed to ultraviolet (UV) radiation (295–365 nm). Over 240 days of exposure, I-1076 concentrations in the plastic phase decreased exponentially, with a half-life of 20.2 days, about 5 times shorter than in the dark control (99.2 days). In contrast, I-1076 concentrations in the aqueous phase remained low and nearly constant in both light and darkness, suggesting slow I-1076 leaching and/or transformation of I-1076 in the water outpacing leaching. Two degradation products were identified: 7,9-di-tert-butyl-1-oxaspiro[4,5]deca-6,9-diene-2,8-dione and 2,6-di-tert-butyl-1,4-benzoquinone. They both predominantly formed under dark conditions, suggesting nonphotochemical oxidative degradation as the primary pathway. Extrapolation to environmental UV conditions suggests that most I-1076 in waterborne LDPE microplastics is degraded before reaching the open ocean, limiting its ecological risks primarily to inland and coastal waters. However, the formation of secondary products may pose indirect toxicological risks. These results underscore the importance of quantifying plastic additives in both the plastic and aqueous phases to understand their fate and ecotoxicological impacts.