An Experimental Investigation into the Scavenging of Microplastics by a Rising Bubble

Abstract

Marine microplastic pollution is a crucial subject of study due to its global pervasiveness and persistence, accelerating presence, and known and theorized adverse effects on ecological and human systems [2, 7, 12, 18, 34, 41, 50, 52, 62, 69]. Urgent and informed action is needed to better monitor, mitigate, and ideally eliminate plastics flows within the marine environment [34]. This action begins with understanding the mechanisms of particle transport. Various microplastic transport mechanisms cause these particles to transfer from their anthropogenic sources to components of Earth’s systems, one of which is the transfer of microplastics from the ocean to the atmosphere via the bursting of bubbles [2, 54, 61].

This mechanism plays a key role in transporting microplastics to remote regions where they can possibly contribute to adverse climate impacts [1, 2, 14, 25]. An important process that contributes to the efficiency of transporting these plastics from water to the air is the scavenging of particles by the rising bubble [61]. However, much uncertainty remains in the description of this efficiency [21].

The present study seeks to make progress towards such an end by filling the knowledge gap in describing particle scavenging by a rising bubble. It asks, “what is the scavenging efficiency of microplastics by a bubble?” and “how can the saturation of the bubble with microplastics be characterized?”.

These questions are answered through a laboratory experiment wherein individual bubbles of a controlled millimetric diameter are observed using a high speed camera rising through a water column. The water is filled with spherical microplastic particles of constant size in varying concentrations. A water tank and stand are designed and built specifically to accommodate the imaging needs of the experiment, and photographic optics are analyzed to ensure visibility of microplastic particles on the bubble. Image analysis is completed to quantify the number of microplastics attached to the bubble as a function of concentration and bubble rise distance.

The experimental conditions maintain a high Reynolds number resulting in a helical and zig-zag trajectory of the bubble and a valid assumption of potential flow. A small Stokes number is found, indicating microplastics act as tracer particles and inertial effects can be ignored. The number of particles found on the bubble at the maximum height of rise range from 42 to 523 particles across the different concentrations. Notably, the experiment finds that the number of particles attached to the bubble scales linearly with concentration and with a power law of approximately 1/2 with bubble rise distance. The present work concludes by situating the subject of study within the broader conversation of sustainability and environmental pollution.