Identifying influencing physical and environmental parameters on fate and characterization factors for microplastic emissions in the marine environment

Purpose: Within the international working group Marine Impacts in Life Cycle Assessment MarILCA, a mechanistic fate framework was proposed to refine the fate factors (FFs) and subsequently characterization factors (CFs) of microplastics (MPs) emitted to the marine environment, for the impact category “physical effects on marine biota.” To operationalize this framework with parsimony, this paper quantifies different identified fate mechanisms and determines the most influencing parameters on the fate. This will help determine a minimum set of variables based on which FFs could be categorized and clarify the need for regionalization in the operationalization of the framework. Methods: Based on different studies and models, fate mechanisms are quantified. A simulation plan is adopted to test the influence of microplastic and environmental properties on the settling of the particles using TrackMPD. Fate and CF matrices are developed for defined microplastic categories based on the simulation plan. A local sensitivity analysis is then applied in order to test the influence of various fate mechanisms on the fate and CF matrices. Results and discussion: The physiology of the particles (size, density, shape) and oceanic properties significantly affect the fate of the particles. The interaction between various influencing parameters highlights the complexity of quantifying the fate of MPs in the marine environment. Large particles of low density presented the highest residence time in water sub-compartments compared to smaller particles and negatively buoyant ones due to their slow settling. The final fate for all microplastics analyzed is benthic sediments. This highlights the need to develop effects factors (EFs) for sediment species to better understand the sensitivity of species exposed through sediments compared to species exposed through water. The sensitivity of fate mechanisms on the FFs and the variability of influencing parameters indicate the need for categorizing the fate, and subsequent CFs, based on the physiology of the particles. It also implies that regionalization is needed in future steps to account for the variability of water currents, biofouling celerity, and turbulence. Conclusion: This article supports one of MariLCA’s objectives of integrating marine litter in life cycle impact assessment (LCIA). Testing the variability of fate parameters and identifying the importance of their influence assists in the operationalization of the framework previously proposed. This will help refine the fate factors and CFs already existing in the literature, increasing the accuracy linked to the variability and influence of combined physical and environmental parameters (biofouling, size, density, shape, etc.).