Shape Heterogeneity Facilitates the Transport of Certain Sized Nanoplastics and Eliminates Their Inhibition Effect on the Transport of Coexisting Other Sized Nanoplastics in Porous Media

This study explored the cotransport and coadsorption of various sized (50, 200, and 500 nm) poly(methyl methacrylate) nanoplastics (PMMA NPs) in porous media with different sizes and the role that shape heterogeneity played in this process. It was found that this coadsorption process was dominated by competition for adsorption sites and the blocking effect. Heterogeneity and irregularity in shape brought greater roughness, specific surface area, and inhomogeneity in surface charge, promoting NP adsorption and inhibiting more other sized NP adsorption on porous media. However, there were significant differences between cotransport and coadsorption. PMMA NPs with 500 nm inhibited the transport of 50 and 200 nm PMMA NPs in fine quartz sand due to narrowing and even plugging of the pore throat, whereas they increased their transport in medium and coarse quartz sand owing to competition for adsorption sites. Shape heterogeneity increased the transport routes of certain sized NPs and weakened the narrowing and plugging of the pore throat by them, thus facilitating their transport in porous media, while eliminating their inhibitory effect on the transport of coexisting other sized NPs. This suggested that shape-heterogeneous NPs in real environments may have more severe environmental impacts than many results from studies using homogeneous NPs.