Transport of microplastics in the marine environment

Microplastic pollution has become increasingly pronounced in the last decade, although their transport behaviors in different environments are still not fully understood. Therefore, the present PhD thesis aims to close this research gap further by examining the basic transport mechanisms of microplastics in marine environments, specifically focusing on their incipient motion, settling, rising as well as cross-shore transport and accumulation patterns.

Experiments involving the incipient motion of 65 different microplastic groups, together with an existing dataset from the literature, have shown that the incipient motion of microplastics lying on a sediment bed depends on the relative static friction, hydraulic roughness (through grain Reynolds number), and hiding-exposure effects. A new predictive incipient motion formulation has been proposed accounting for these effects. Unlike prior methodologies, the present formulation can be reconciled with the classical Shields diagram, generally used for natural sediments.

Experiments involving the settling velocity of 66 different microplastic groups have revealed that microplastics settle with their largest projected area perpendicular to the line of motion to reach the maximum drag possible. This finding is not surprising, as this has long been known for sediments since the 1930s, though this preferential settling orientation has not generally been accounted for in previous predictive formulations. Therefore, after combining the present results with the available data sets in the literature, a new methodology uniformly accounting for the preferential settling orientation of differently shaped microplastics has been developed to estimate their settling velocity. Comparisons with the literature also showed that this new methodology is superior to previous approaches, generally based on the prevailing methodology of using nominal diameter as a universal length scale.

A fully-coupled resolved large eddy simulation-discrete element method (LES-DEM) model has been tested with 12 different cases (involving eight settling and four rising microplastic particles). The drag coefficients, trajectories, and wake patterns of these cases are examined and compared with the experimental findings from the literature, and these examinations show that the present LES-DEM model is effective in simulating microplastic settling and rising.

Experimental results involving cross-shore transport and accumulation of 18 different nonbuoyant microplastic groups under irregular waves have shown that microplastics tend to accumulate in four different hotspots spanning the cross-shore profile, with the accumulation patterns primarily determined by particle Dean number and secondarily by particle shape. It should be pointed out that the importance of particle Dean number has long been established for sediments, but this is the first study to demonstrate its importance for microplastics.

Finally, experimental results involving cross-shore transport of three different buoyant microplastic groups in irregular waves have shown that buoyant microplastics are transported onshore, eventually becoming beached. It is found that the transport velocities of buoyant microplastics are proportional to the velocity of the wave-induced Lagrangian fluid particle velocity before wave breaking, while in the surf zone (after wave breaking), their velocities become proportional to the wave celerity, with dependence on particle Dean number. A new formulation has also been developed to estimate the Lagrangian particle transport velocities of buoyant microplastics.