Simulated sunlight exposure as a prerequisite for the biodegradation of persistent microplastics

The environmental fate and biodegradability of microplastics (MPs) are key concerns in regulatory and scientific contexts. Standard biodegradation testing effectively evaluate microbial decomposition, but overlook crucial abiotic processes, particularly photooxidation, which may significantly alter polymer structures and their susceptibility to microbial breakdown. Therefore, we investigated how simulated sunlight exposure influences the subsequent (bio)degradability of radiolabeled ¹⁴C-polyurea (PUA) microcapsules and ¹⁴C-linear low-density polyethylene (LLDPE) particles. Using standardized OECD test guidelines, we conducted coupled photo- and biodegradation experiments in both aqueous and soil environments. Our results demonstrate that simulated sunlight exposure led to fragmentation of PUA microcapsules and the release of low-molecular-weight ¹⁴C-labeled compounds, such as aminocaproic acid. Irradiated PUA microcapsules showed significantly enhanced biodegradation in aqueous (up to 28 ± 4.05 %_AR, OECD TG 301B) and soil-based tests (up to 63.2 ± 13.4 %_AR, OECD TG 307), compared to negligible biodegradation in non-irradiated controls. In contrast, LLDPE MPs demonstrated only minor changes. These findings establish that abiotic weathering processes substantially influence MP biodegradability for certain polymer types, and demonstrate the necessity of incorporating realistic environmental exposure conditions into standardized testing protocols. This study improves the understanding of MP degradation pathways and supports more comprehensive regulatory evaluation strategies.