Real-Time\nVisualization of Infiltration and Retention\nof Microplastics with Different Shapes in Porous Media

Infiltration\nand retention of microplastics in porous media are\nimportant for understanding their fate in environments and formulating\ntreatment measures. Given porous media opacity, knowledge is usually\nobtained indirectly by monitoring microplastic concentration in the\neffluent and measuring microplastic distribution after removing grains\nin layers. In this study, real-time visualization of infiltration\nand retention of microplastics in porous media under vertical water\nflow is performed using an improved reflective index matching method,\nconsidering the different shapes and densities of microplastics and\nsize ratios between microplastics and grains. The spherical microplastics\nhave the largest infiltration depths, with trajectories closest to\nvertical and accompanied by long acceleration durations and low deceleration\nfrequencies. The cylindrical microplastics deviate from vertical and\nhave stronger transverse oscillations and more frequent decelerations,\nwhile the flaky microplastics have the most significant transverse\ndisplacements. The infiltration depth can be improved by reducing\nthe size ratio between microplastics and grains and increasing the\nvertical flow rate, while the density of microplastics has a relatively\nlimited effect. Sliding and rotating of microplastics after collision\nwith grains are observed, responsible for deceleration and transverse\ndisplacements. Different retention patterns are found, with the number\nof types being inversely proportional to the number of principal dimensions\nof the shape.