Particulate flow in porous media: experimental study and numerical modelling of microplastic transport in geomaterials

Plastic waste (PW) has emerged as a significant environmental pollutant, with a portion of it fragmenting into microplastics (MPs). MPs, defined as plastic particles with diameter smaller than 5 mm, can migrate through hydrogeological environments similarly to micron-sized soil particles. This migration increases their bioavailability and the potential to threaten human health and ecosystems. Despite growing awareness, substantial research is still needed to fully comprehend the sources, fates, and transport processes of MPs in porous media, as well as their complex interactions with hydrogeological environments. This study first unveils the MPs transport behaviours under extreme weather events coupled with pore network changes of soil filters by conducting experiments using a newly developed permeameter in the laboratory. The transport of MPs in soil can be significantly affected by the potential clogging, indicated by the variation of soil permeability and MPs mass flux. Then, modified continuum models based on advection-dispersion equations are used to fit breakthrough curves and retention profiles of MPs in soils. Akaike weights and post hoc analysis show that the dual-site model considering both time- and depth-dependent deposition effects has the best modelling performance. Finally, the computational fluid dynamic coupled with discrete element modelling is applied to simulate the transport of MPs in a simplified sand matrix. The microscopic perspective especially characterizes the evolution of MPs behaviour near the pore throat and through the sand layer with the development of hydraulic loadings. Efforts are also made to explore new tracing techniques for the particulate flow of micron-sized particles in the laboratory and potential structural changes in soils and MSW caused by particle loss. In the context of climate change, the findings of this study will offer researchers and policymakers new information, knowledge, and methodology to rationally evaluate and effectively mitigate MPs hazards.