PULLMAN, Wash. -- The first-known direct observations of the movement of microfiber plastics through a thin layer of soil-like particles show that they tend to tumble, roll and sometimes get stuck in spaces.
The findings, reported in the journal, Water Resources Research, mean that the fibers could get easily trapped in sediment. The work helps to improve understanding of the exposure risks and possible health impacts of the pervasive pieces of plastic, which are the largest pollutant in the world by mass.
"The fibers tend not to want to move. If anything, they want to stay put and want to attach to something," said Nick Engdahl, corresponding author on the paper and an associate professor in Washington State University's Department of Civil and Environmental Engineering.
In the new study, the researchers carefully inserted 1,200 pieces of fishing line – one at a time – through a thin, vertical slice of material with four pores that represent spherical grains of gravel. The fluorescent fishing line pieces that ranged between three and eight millimeters in length were chosen as a material for the study because they were easy to see. Videotaping the fibers' motion showed that the pieces tended to alternate between short periods of tumbling or rolling and longer periods of smooth motion.
Microplastic fibers in the environment have been a growing concern in recent years. Less than five millimeters in size, the thread-like fibers come from synthetic clothing materials like fleece, cosmetics, packaging materials and carpeting. Other research has estimated that about 90% of water worldwide contains microplastic, and 91% of that plastic is made up of microfibers. The fibers have been found to negatively affect small marine organisms, but it's unclear what their impact is on human health and ecosystems, partly because researchers don't know how mobile they are.
"We need to know how they're moving and where they're going to end up to really see their impact in the environment," said Tyler Fouty, first author on the paper, who recently received his Ph.D. at WSU and is now a water resource engineer at Jacobs.
In recent years, Engdahl developed a computer model to simulate how the fibers might move. Researchers have also run plastic fibers through columns of dirt that provided some information, but they couldn't actually watch the fibers move. In the environment, researchers have been able to find where the contaminants start and end up but not how they get there.
"Those experiments don't provide any sense of the mechanisms that are actually causing the fibers to move," said Engdahl.
The researchers extracted travel paths of the fibers from their video and analyzed them to determine their travel times in comparison to microbeads. The researchers found that the fibers travel more slowly than beads, with longer fibers exhibiting even slower movement. With their analysis, they also found that the most common numerical models that might be used to describe the fibers' movement are not accurate.
Capturing the movement of the microfibers directly wasn't easy, said Fouty.
"The most important lesson we learned in this work was that direct observations of the transport behaviors are very difficult to obtain," he said. "We expected that the challenges of trajectory capture could be greatly simplified if relatively large fibers and a relatively large domain were used, but even then, unexpected behaviors were encountered."
The researchers would like to continue the work to include other aspects of transport, including more precise control of flows and using different types of fibers. The work was partially funded by the National Science Foundation.
