Microplastic migration in porous media at various scales: a review

Migration of microplastics in porous media is an important, yet overlooked phenomenon because most microplastic research has focused mainly on microplastic behavior in aquatic environments. Here we review experimental advances of microplastic migration in porous media, with emphasis on factors influencing microplastic migration. We observed that microplastic migration is influenced by environmental factors and microplastic properties. The effect of microplastic surface charge and functional groups, and of soil organisms on microplastic migration is unclear. Research at the macro-scale, higher than 1 m, predominantly starts with field sampling, and then carries out measurements or mathematical modeling to explore migration patterns. At the meso-scale, below 1 cm, studies often employ filled sand columns as proxies for porous media to generate breakthrough curves and retention profiles. At the micro-scale, below 1 mm, visualization of microplastic migration in pores is done by lab-on-a-chip devices to build transparent micromodels. Current research predominantly relies on industrially produced regular spherical microplastics, with limited focus on macro- and micro-scale studies.