Denser microplastics migrate deeper? Effect of particle density on microplastics transport in artificial and natural porous media

Predicting the fate of microplastics (MPs) in porous media has been challenging and previous research mainly focused on the transport of MP considering plastic size and shape effects, media size effects, and solution chemistry effects. However, few studies examined the plastic density impact on the transport behavior of MPs in porous media. This is significantly important to gain insight into how the MP density influences its fate in the environment. Therefore, column experiments under saturated conditions were conducted to explore the MP transport in columns packed with glass beads and gravel and using polyethylene microspheres with different densities within the same size range together with a conservative tracer. Experimental results were fitted well (R2 > 82.3-98.7 %, and low RMSE value) with a two-site transport model with a depth-dependent blocking function in HYDRUS-1D. The results showed that particle density influences the transport of MPs, and the deposition rate varied with particle density in the following order: 1.12 g/cm3 > 0.995 g/cm3 > 1 g/cm3. This suggests that compared to neutrally buoyant and buoyant MP, denser MPs tend to deposit in the selected material under the tested flow rate. The coupled experimental and simulated results indicate that denser MPs may be retained but neutrally buoyant MPs can be potentially migrated with infiltrated water into subsurface systems, thus posing groundwater contamination risk. Hence, further studies are needed with viable densities and diverse conditions to advance the understanding the impact of plastic density on its transport fate. Keywords: Microplastics, density, transport, sediments, glass beads