Environmental fate of microplastics, with a focus on polymer microcapsule formulations used in pesticide applications

This thesis investigates the biodegradability and environmental fate of microplastics (MPs), with a particular focus on intentionally added polyurea (PUA) microcapsules in plant protection products (PPPs) to the terrestrial environment. Despite growing attention to plastic pollution in soils, this compartment has received comparatively little attention in both scientific and regulatory discussions. The work was initiated based on the proposed restriction under the REACH regulation on intentionally added MPs and was aimed to contribute to the development of reliable and regulatory-relevant test methods. MPs can be derogated if their biodegradability is demonstrated using proposed standardized testing methods (e.g., OECD or ISO), with a primary focus on complete mineralization to CO2 or the corresponding oxygen consumption. To assess the biodegradation of microplastics, the following main objectives were addressed, each investigated in a separate section of this thesis: (i) the development of analytical methods to assess alternative endpoints of biodegradation besides mineralization as the main endpoint of the regulation, e.g., physical or chemical changes of the MP; (ii) the investigation of the influence of abiotic factors, such as simulated sunlight exposure on MP biodegradation; (iii) the development of an appropriate extraction method applying the analytical methods developed in (i) and ideally identify transformation products within a biodegradation test in soil. In total two polymer types were investigated: PUA microcapsules, representing industrially applied encapsulants in PPP formulations and linear low-density polyethylene (LLDPE) microparticles, representing rather persistent commodity plastics. Radiolabeling with 14C enabled mass balance investigations and allowed for exact tracking of polymer degradation and development of transformation products under controlled laboratory conditions. The major outcomes of this thesis are threefold: (1) This thesis highlights the importance of purification and thorough characterization, or where feasible, the separation of complex materials into their constituent fractions to better understand which components are mineralized and which persist in the environment, thereby avoiding misleading results caused by carbonaceous co-constituents. This was exemplified in the first part of the thesis, where radiolabeled low-molecular-weight synthesis residues caused increased mineralization results that did not reflect actual polymer biodegradation. In this context, the applied purification approach proved effective in sufficiently isolating microcapsules from these formulation residues through filtration and resuspension, which could also be applied to regular testing without radiolabeling. (2) Simulated sunlight irradiation influenced the degradability of MPs, particularly for PUA in the second part of this thesis. Irradiated PUA microcapsules released aminocaproic acid, a compound readily mineralized under test conditions, resulting in elevated 14CO2 evolution. The LLDPE particles were disintegrated only to a minor degree and did not show elevated mineralization after irradiation on the soil surface. While 14C-fragments from aqueous irradiation of LLDPE were detected, biodegradation of this suspension remains untested. These results highlight the importance of considering photo-oxidative effects when evaluating MP degradation, especially if the respective products may exhibit exposure scenarios involving sunlight. (3) In the last part of this thesis, a modified oil extraction protocol was developed which proved to recover 14C-labeled PUA and LLDPE MPs from two different types of agricultural reference soils. The method was effective for both pristine and simulated sunlight-exposed particles and showed potential for extracting the most challenging small MPs (<500 µm). Radiolabeling also proved beneficial here since it enabled quantification of recovery and identification of losses. Taken together, this work leads to three main conclusions and recommendations. First, thorough characterization or, where feasible, fractionation of test materials by properties such as particle size or composition is essential for accurately interpreting biodegradation data and avoiding misclassification of microplastic degradability. Such efforts are recommended for future studies to improve the reliability and clarity of biodegradation assessments. Second, simulated sunlight was shown to influence microplastic degradation and should be considered in environmental fate evaluations. Lastly, it may be beneficial to integrate microplastic extraction methods, such as the modified oil extraction method described in this thesis, into soil biodegradation testing to investigate physicochemical changes and transformation products. This could support more robust biodegradation testing under simulated environmental conditions. Since sunlight exposure in realistic application scenarios remains uncertain, the environmental relevance of simulated sunlight effects on PUA microcapsules must be validated. Controlled lysimeter or field studies are essential to determine whether the photodegradation observed under laboratory conditions also occurs in agricultural environments. This is crucial for assessing the transferability of experimental results to real-world settings.