Plastic pollution is everywhere. The study reveals how it travels

Plastic pollution is omnipresent today, with microplastic particles from disposable products found in natural environments around the world, including Antarctica. But how these particles move and accumulate in the environment is poorly understood. Now a Princeton University study has revealed the mechanism by which microplastics, such as polystyrene, and pollutant particles are transported long distances through soil and other porous media, with implications for preventing the spread and accumulation of contaminants in food and water sources.

The study, published in Advances in science on November 13, it reveals that microplastic particles freeze as they travel through porous materials such as soil and sediment, but later break free and often continue to move substantially further. Identifying this shutdown and restart process and the conditions that control it is new, said Sujit Datta, assistant professor of chemical and biological engineering and associate faculty at the Andlinger Center for Energy and the Environment, High Meadows Environmental Institute, and Princeton. Institute for Materials Science and Technology. Previously, the researchers thought that when microparticles got stuck, they generally stayed there, which limited understanding of particle diffusion.

Datta led the research team, who found that microparticles are released when the velocity of the fluid flowing through the medium remains high enough. Princeton researchers have shown that the process of deposition, or hoof formation, and erosion, their rupture, is cyclical; clogs are formed and then broken by fluid pressure over time and distance, further moving particles through the pore space until they reform.

“Not only have we found these fantastic dynamics of particles that freeze, clog, build up deposits and then get pushed, but this process allows the particles to expand over much greater distances than we would have thought otherwise,” Datta said.

The team included Navid Bizmark, a postdoctoral researcher associated with the Princeton Institute for the Science and Technology of Materials, graduate student Joanna Schneider, and Rodney Priestley, professor of chemical and biological engineering and vice dean for innovation.

They tested two types of particles, “sticky” and “non-sticky”, which correspond to the actual types of microplastics in the environment. Surprisingly, they found that there was no difference in the process itself; that is, both still clogged and unobstructed at sufficiently high fluid pressures. The only difference was where the clusters formed. The “non-sticky” particles tended to get stuck only in narrow passages, while the sticky ones seemed to be able to get trapped on any solid medium surface they encountered. As a result of these dynamics, it is now clear that even “sticky” particles can spread over large areas and through hundreds of pores.

In the paper, the researchers describe pumping fluorescent, fluid polystyrene microparticles through a transparent porous medium developed in Datta’s laboratory, and then watching the microparticles move under the microscope. Polystyrene is the plastic microparticle that makes up Styrofoam, which is often littered in soil and waterways through shipping materials and fast food containers. The porous media they created closely mimic the structure of media found in nature, including soils, sediments and groundwater.

Typically porous media are opaque, so you can’t see what the microparticles are doing or how they flow. Researchers usually measure what goes in and out of the media and try to infer the processes that take place inside. By making transparent porous media, the researchers overcame this limitation.

“Datta and colleagues have opened the black box,” said Philippe Coussot, a professor at Ecole des Ponts Paris Tech and a rheology expert who is not affiliated with the study.

“We have come up with tricks to make the media transparent. Then, by using fluorescent microparticles, we can observe their dynamics in real time using a microscope,” Datta said. “The great thing is that we can actually see what the individual particles are doing under different experimental conditions.”

The study, which Coussot described as a “remarkable experimental approach,” showed that although the styrofoam microparticles got stuck in some places, they were eventually released and moved the full length of the support during the experiment.

The ultimate goal is to use these particle observations to improve parameters for larger scale models to predict the amount and location of contamination. The models would rely on different types of porous media and different particle and chemical sizes, and would help to more accurately predict contamination under various conditions of irrigation, rainfall, or environmental flow. Research can help provide information to mathematical models to better understand the likelihood of a particle moving over a certain distance and reaching a vulnerable destination, such as a nearby farmland, river or aquifer. The researchers also investigated how the deposition of microplastic particles affects the permeability of the medium, including the ease with which irrigation water can flow through the soil when microparticles are present.

Datta said this experiment is the tip of the iceberg in terms of particles and applications that researchers can now study. “Now that we’ve found something so amazing about such a simple system, we’re excited to see what the implications are for more complex systems,” Datta said.

He said, for example, that this principle could provide information on how clays, minerals, grains, quartz, viruses, microbes and other particles move in media with complex surface chemicals.

The knowledge will also help researchers understand how to deploy engineered nanoparticles to remediate contaminated aquifers, possibly spilled from a manufacturing plant, farm, or urban wastewater stream.

Beyond environmental remediation, the findings are applicable to processes in a wide range of industries, from drug delivery to filtration mechanisms, to effectively any medium where particles flow and accumulate, Datta said.