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    Microplastics are all around. Here’s how

    Microplastics are everywhere, but how they move through, and accumulate in, the environment is not completely clear.

    Microplastic particles, seen here in green, travel long distances in soil, but take a few breaks. Credit: Princeton University / Datta Lab

    It’s been assumed that when minute particles travelling through porous materials such as soil and sediment get stuck, they tend to stay there. New research by Princeton University, US, suggests otherwise, however.

    In a paper in the journal Science Advances, Sujit Datta and colleagues report that when the rate of fluid flowing through the media remains high enough, the particles can break free again, often moving substantially further.

    They found that the process of deposition and erosion is cyclical: clogs form then are broken up by fluid pressure over time and distance, moving particles further through the pore space until clogs reform.

    Knowing this will, they hope, help researchers understand how to deploy engineered nanoparticles to remediate contaminated groundwater aquifers.

    In their study they tested two types of particles – accurately named “sticky” and “non-sticky” – and were surprised to find no difference in the clogging and unclogging process itself. What was different was where the clusters formed.

    The non-sticky particles tended to get stuck only at narrow passageways, whereas the sticky ones seemed able to get trapped at any surface of the solid medium they encountered.

    As a result of these dynamics, the researchers say, it is now clear that even “sticky” particles can spread out over large areas and throughout hundreds of pores.

    Porous media are usually opaque, so researchers have only been able to measure what goes in and what comes out, then infer what happened inside. To overcome this, Datta’s team developed a transparent media that closely mimics the structure of soils and sediments, then pumped fluorescent polystyrene microparticles through it.

    And the findings might just be the tip of the iceberg, he says. The goal is to use these observations to improve parameters for larger-scale models to predict the amount and location of contamination.

    Beyond that, the principle could yield insight into how clays, minerals, grains, quartz, viruses, microbes and other particles move in media with complex surface chemistries.

    “Now that we found something so surprising in a system so simple, we’re excited to see what the implications are for more complex systems,” he says.

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