Characteristics and Sinking Behavior of Typical Microplastics Including the Potential Effect of Biofouling: Implications for Remediation

Microplastics
are ubiquitous pollutants within the marine environment,
predominantly (>90%) accumulating in sediments worldwide. Despite
the increasing global concern regarding these anthropogenic pollutants,
research into the remediation of microplastics is lacking. Here, we
examine those characteristics of microplastics that are essential
to adequately evaluate potential remediation techniques such as sedimentation
and (air) flotation techniques. We analyzed the sinking behavior of
typical microplastics originating from real plastic waste samples
and identified the best-available drag model to quantitatively describe
their sinking behavior. Particle shape is confirmed to be an important
parameter strongly affecting the sinking behavior of microplastics.
Various common shape descriptors were experimentally evaluated on
their ability to appropriately characterize frequently occurring particle
shapes of typical microplastics such as spheres, films, and fibers.
This study is the first in this field to include film particles in
its experimental design, which were found to make up a considerable
fraction of marine pollution and are shown to significantly affect
the evaluation of shape-dependent drag models. Circularity χ
and sphericity Φ are found to be appropriate shape descriptors
in this context. We also investigated the effect of biofouling on
the polarity of marine plastics and estimated its potential contribution
to the settling motion of initially floating microplastics based on
density-modification. It is found that biofouling alters the polarity
of plastics significantly; this is from (near) hydrophobic (i.e.,
water contact angles from 70 to 100°) to strong hydrophilic (i.e.,
water contact angles from 30 to 40°) surfaces, rendering them
more difficult to separate from sediment based on polarity as a primary
separation factor. Thus, besides providing a better understanding
of the fate and behavior of typical marine microplastics, these findings
serve as a fundamental stepping-stone to the development of the first
large-scale sediment remediation technique for microplastics to address
the global microplastic accumulation issue.