Assessing Microplastic (Bio)Degradation: Scientific and Regulatory Hurdles

"Microplastics are a global pollutant with complex environmental behavior. For regulatory testing of their biodegradability, conventional methods are limited in their applicability. At Fraunhofer IME, we investigated how abiotic aging, such as photooxidation, influences subsequent microbial degradation. By combining standardized test methods with innovative analytical approaches, we deepened our understanding of microplastic biodegradation and enhanced its evaluation",

        says first author Eva-Maria Teggers, research associate at Fraunhofer IME in Schmallenberg.

Rethinking biodegradability: When microplastics meet regulation

Microplastics (MPs) can be found even in the most remote parts of the world. Regulations like the new EU’s REACH restriction on synthetic polymer microparticles aim to limit the intentional release of these tiny plastic particles. Some MPs are allowed to be used in products if their biodegradability can be proven using specific test methods. Most of these tests were originally developed for small, soluble molecules—not for solid, rather long-lasting plastic particles— so the methods require adaptation to accurately assess microplastic degradability. However, developing methods to determine whether microplastics break down or release transformation products under environmentally relevant conditions remains a scientific and technical challenge.

At Fraunhofer IME, we focus on improving the scientific foundation for microplastic biodegradation testing. In two studies, we evaluated polyurea (PUA) microcapsules considered as microplastics in context of the regulation and often used for agricultural purposes or fragrances. Their biodegradation was examined using standardized screening and simulation tests according to OECD guidelines. To enable precise tracking of degradation, the polymers were radiolabeled with ¹⁴C-isotopes. To mimic environmental abiotic aging before microbial degradation, we performed simulated sunlight irradiation followed by the biodegradation tests. This combination of different tests reflects real-world processes, which are currently not addressed in regulatory assessments.

Photooxidation as a prerequisite

Microplastics that have entered natural environments are often exposed to UV radiation, heat, mechanical forces, and chemical oxidation. These abiotic stressors often precede biodegradation and can modify polymer properties in ways that influence environmental behavior and bioavailability. Like wood that weathers under sunlight and rain before it rots and fungi and microbes begin to decompose it. Photooxidation, in particular, is known to fragment microplastics, alter their surface chemistry, and potentially generate transformation products.

To mimic environmental exposure, we irradiated the test polymers with simulated sunlight (Figure 1) in both aqueous and soil-based systems following OECD TG 316 and its draft equivalents. The aging step was then combined with biodegradation studies using OECD TG 301B (for a screening test setting; Figure 2) and OECD TG 307 (for a simulation test setting in soil; Figure 3). This allowed us to test whether prior exposure to light-induced aging processes would affect subsequent biodegradation, compared to untreated control materials.

Tracking microplastic degradation: Radiolabeling and sequential filtration for more clarity

One major obstacle in biodegradation testing of microplastics lies in differentiating the degradation of the polymer and other sample constituents—especially for complex products like pesticide microcapsules, where the polymer shell may account 1-5% of the formulation. To achieve the needed specificity, we used ¹⁴C-labeled polymers and tracked the formation of ¹⁴CO₂ to unambiguously quantify the mineralization of the polymer content of the formulation. This provided both high sensitivity and a clear analytical endpoint.

To complement the mineralization data, we also applied a sequential filtration methodology to monitor changes in particle size distributions - an aspect often neglected in standard test methods. This gave insights into fragmentation processes and the release of transformation products.

Our findings show that PUA microcapsules that had been exposed to simulated sunlight showed higher fragmentation and a shift toward smaller particle sizes, indicating greater accessibility for microbial attack. Furthermore, the irradiation led to the release of compounds that microbes could mineralize and use as an energy source generating more ¹⁴CO₂ than non-irradiated materials.

These combined analytical approaches support a more nuanced interpretation of biodegradation beyond ¹⁴CO₂ release alone. They also reinforce the importance of integrating intermediate degradation steps and abiotic pre-treatment, demonstrating that these factors can be relevant for simulating real-world degradation scenarios – which are currently absent from most regulatory considerations.