Abstract

The presence of plastic fragments in the environment is a growing global concern. The coast is particularly vulnerable to microplastic (MP) pollution. However, field experiments are less capable of disentangling the complex interplay of various factors. Therefore, this study aims to investigate the transport of MPs in porous media combined with typical coastal processes. Firstly, a weathering experiment was conducted to comprehensively reveal the changes in MPs in the environment. The results indicated that seawater aging mainly affected the physical properties of MPs, increasing its surface pores and hydrophilicity. Ultraviolent (UV) aging significantly increased its hydrophilicity and crystallinity and introduced oxygen-containing functional groups onto MPs.
Then, the detachment of MPs from porous media under various water content conditions combined with flow patterns was studied. For both the wet and dry conditions, the increase in flow rates decreased the detachment of hydrophobic polyethylene (PE) of two sizes and hydrophilic polymethylmethacrylate (PMMA). Transient flows with varied flow rates and ionic strength led to flow peaks and more MP detachment compared to steady flow. Furthermore, substrate drying significantly impeded the detachment of MPs compared to wet conditions irrespective of the flow regimes. The release of MPs decreased pronouncedly with prolonged air drying duration of the column since drying heightens the energy barrier for MPs to detach.
Tide is a typical coastal process that has profound influence on many biological and abiotic processes. In the following study, the effects of tidal cycles on transport of MPs (4−6 μm PE1; 125 μm PE2; and 5−6 μm polytetrafluoroethylene, PFTE) in porous media were systemically investigated. Smaller substrate sizes exhibited higher retention percentages compared to those of larger substrate sizes under different tidal cycles. In terms of the size of MPs, a larger size (same density) was found to result in enhanced retention of MPs in the column. As the number of tidal cycles increased, although the transport of MPs from the substrate to the water phase was enhanced, less hydrophobic MPs was washed out more with the change in water level. The results implied that MPs with size far smaller than the substrate tend to end up in the open ocean.
To expand our understanding of MP mobilization by tidal movement, the influence of dynamic fluctuations of capillary fringe on the transport of MPs was explored. An increase in the cycles of water table fluctuations enhanced the MPs transport from substrate to the water below. More MPs with larger size were retained in substrate compared to the smaller one. The retention percentages of both PE1 and PTFE in column increased with the elevated ionic strength and the decrease of fluctuation velocity. The results highlight that capillary fringe fluctuation can serve as a pathway to relocate MPs to the tidal aquifer.
Finally, a mesoscale tank experiment was conducted to simulate the infiltration and resuspension of MPs in a slope substrate under the influence of repeated tidal forces. The results imply that large, high-density, and less flat particles tend to be distributed in the lower tidal zone and deeper substrate layers. The obtained observation contributes valuable insights into the behavior, transport, and redistribution of MPs in complex environmental systems. The findings enhance our understanding of MP fate and distribution, assisting in the development of strategies for mitigating MP pollution and managing its impact in coastal areas.