A risk assessment framework for fragmenting (micro-)plastics. A case study for polymer coated fertilizers in soil

Polymer coated fertilizers (PCFs) consist of a soluble mineral fertilizer core surrounded by a polymer coating that controls nutrient release to match with crop production cycles. While soil is recognized as a major sink for microplastics (MP), current risk assessments (RA) primarily focus on secondary MPs from sources like atmospheric deposition, mulch film debris, and biosolids. However, primary MPs, such as those from PCF coatings, enter soil directly as MPs (2 mm ≤ Ø ≤ 5 mm) and are irremovable. The extensive use of various PCFs raises concerns about potential risks to soil health. These risks can be assessed using species sensitivity distributions (SSD), which compile toxicity data from species exposed to MPs of different characteristics (e.g., polymer type, size, shape). However, not all fragmented PCF-MPs are bioavailable to soil organisms, while the often monodisperse MPs used in effect tests do not represent the diversity of actual environmental exposure. Therefore, it is crucial to consider the bioavailable fraction of MPs to organisms, and rescale toxicity and exposure data to the same toxicologically relevant metrics (TRMs) and units, for a consistent risk characterization. Here, we introduce a model for the fragmentation of PCF coatings over time, parameterized using literature data. The exposure to the increasing number of ever smaller particles is re-scaled using realignment method by Koelmans et al. and compared with Hazardous Concentrations for 5 Also see: https://micro2024.sciencesconf.org/559358/document