A study from Princeton University has brought to light how microplastic particles move and accumulate in natural environments. The researchers looked at how these pollutants are carried long distances through soil and other porous earth materials. The results have implications for preventing the spread and accumulation of microplastic contaminants in food and water sources.

Microplastics from plastic waste are detrimental to water supplies and the environment.

Researchers found that microplastic particles get stuck when traveling through porous materials, later break free and travel further. It was previously thought that when microplastics got stuck, they stayed there. Identifying the start, stop and restart process, and the conditions surrounding it is brand new information.

Microparticles are pushed free when the rate of fluid flowing stays high for a longer period of time. The process of deposition is cyclical. The particles move further through spore space until clogs reform. This process enables particles to spread out over a larger distance than once believed.

The team tested two particle types that correspond with actual types of microplastics that are often found in the environment. One type was sticky and one was not sticky. The team found that there was no difference in the clogging-unclogging process among both types. Both types still clogged and unclogged themselves. The only difference they found was where the cluster forms. Non-sticky particles tended to get stuck on narrow passageways, while stick particles could get trapped on any surface. In the end, it was clear that the sticky particles can spread out over large areas and through hundreds of pores.

During the study, the team pumped fluorescent polystyrene microparticles and fluid through transparent porous media in the lab. Polystyrene is the plastic that makes up Styrofoam. It is a common plastic found in waste or litter. The transparent media they used allowed the team to watch the microparticles move under a microscope. The media closely mimics the structure of naturally occurring media. Typically, porous media are opaque, so it is almost impossible to watch the microplastic movement. In the past, researchers usually measure what does in and out of a media and have to infer what is going on inside. Results showed that Styrofoam microparticles did get stuck at a point and ultimately push free and move through the entire length of media.

The goal is to use particle observations to improve parameters for larger-scale models to predict the amount and location of contamination. The models are based on varying types of porous material and varying particle sizes and chemistries would be tested. This could help accurately predict contamination under various irrigation, rainfall and ambient flow conditions.

The research could help inform math models to understand the likelihood of a particle moving over a distance and reaching a vulnerable destination. This knowledge will help researchers understand how to deploy engineered nanoparticles to remediate contaminated groundwater aquifers. The findings are applicable to a variety of industries where any media particles flow and accumulate.

This study was published in Science Advances.