The "net-capturing" effect of bacteria on the transport of fibrous and fragmental microplastics in porous media: Revealed with the real-time pore-scale visualized and quantitative experiments.

The transport behaviors of microplastics (MPs) in porous media can be significantly influenced by bacteria, yet the underlying pore-scale mechanisms remain unclear. To address this knowledge gap, this study employed a real-time pore-scale visualized and quantitative system to directly investigate the transport and retention patterns of fragmental and fibrous MPs in porous media attached with bacteria (Escherichia coli, Bacillus subtilis) and extracellular polymeric substances (EPS). The videos revealed that biofilms inhibited the surface movement of MPs and intercepted suspended MPs at pore throats. Notably, the "net-capturing" effect of bacteria on MPs retention was observed, wherein biofilms dynamically captured moving MPs through in-situ swinging induced by near-wall shear flow like a "butterfly net", or by filling narrow flow paths like a "fishing net". Some MPs were retained in "cages" constructed with the bacteria and pore throats. Compared to fragmental MPs (TMR ranged from 9.15% to 33.24%), fibrous MPs (TMR ranged from 6.45% to 16.73%) exhibited lower mobility due to morphology-dominated mechanical straining. The presence of bacteria and EPS significantly reduced the mobility of fragmental MPs. This study advanced the novel insights into a critical biogeochemical retention mechanism in subsurface environments.