Microcleaners to Catch Microplastics

Microplastics are everywhere. Defined as pieces of plastic debris less than five millimeters long, microplastics seem to have made their way into practically every body of water on the planet. 

Research published in 2023 estimated that over 170 trillion plastic particles are floating around our oceans. Made up primarily of microplastics, all those trillions of tiny particles add up to an estimated total exceeding 2 million metric tons. 

In North Carolina, the first-ever study to sample microplastics in our state’s freshwater rivers and streams — funded by the National Atmospheric and Oceanic Administration and North Carolina Sea Grant — found that 670 million microplastics flow into the Pamlico Sound from the Neuse River Basin each year.

As pervasive as microplastics have become, trying to rid them from our waterways might sound like a losing battle. But at NC State University, researchers in the Department of Chemical and Biomolecular Engineering are developing a tool that could help level the playing field. If the technology turns out to be viable, it would offer a way to capture and remove microplastics from water by simply spreading small pellets made of environmentally safe materials. 

Can we make the cleaning materials … that self-disperse in water, capture microplastics as they sink, and then return to the surface with the … contaminants?

In a paper published in the journal Advanced Functional Materials, the researchers demonstrated a proof of concept for a single-cycle system to actively remove microplastics from water.

“We demonstrated how multiple principles can be integrated into a system that works in a single cycle,” says Orlin Velev, corresponding author of the paper and the S. Frank and Doris Culberson Distinguished Professor of Chemical and Biomolecular Engineering at NC State. “The idea behind this work is: Can we make the cleaning materials in the form of soft particles that self-disperse in water, capture microplastics as they sink, and then return to the surface with the captured microplastic contaminants?”

The researchers’ innovative approach relies on something called soft dendritic colloids, which can stick to almost any surface. Prior research from Velev’s lab helped pioneer these unique materials.

According to Velev and the paper’s first author, Ph.D. student Haeleen Hong, the cleansing particles’ sticky nature allows them to attract and cling to microplastics — even in the wet, salty conditions of seawater. What’s more, the particles are made from chitosan, which comes from processed shellfish waste.

When dried in droplets suspended over a water-repellent surface, soft dendritic colloids take the shape of small pellets. When those pellets are then dropped into water, particles inside of them self-disperse — spreading out to hunt down microplastics. A plant-based oil is infused into the pellets to aid dispersion.

“This oil makes the pellets move in the water by the so-called ‘camphor boat effect,’ decreasing the surface tension on one side of the pellet and driving it forward. This allows our microcleaners to spread out across a larger area, capturing microplastics as they move and descend,” Hong says.

The cleansing particles descend naturally, picking up more and more microplastics as they go down. But how do they return to the surface? The microcleaners also contain a bit of magnesium, which reacts with water to make them bubble up. However, to ensure the particles don’t make their return trip too fast, the researchers coat the magnesium with a layer of gelatin that delays the chemical reaction. The delay caused by the gelatin coating gives the microcleaners more time to pick up more microplastics throughout their swirling descent.

“As the gelatin dissolves, the magnesium generates bubbles and the microcleaners rise, bringing the captured plastics particles to the surface in a dense, scummy mixture,” Hong explains. 

Their research paper showed that the cleansing particles can “swim” and collect microplastics for up to half an hour. Once the microplastic-laden cleaners float to the surface, the particles can then be collected with a skimmer.

It’s still unclear how exactly microplastics affect humans and the rest of the environment. In 2020, the World Health Organization thoroughly reviewed the scientific evidence available and concluded there was not enough research at that time to determine whether microplastics pose a risk to human health. But NOAA certainly considers them a potential threat to aquatic life.

The paper, “Designing of Self-Dispersing Soft Dendritic Microcleaners for Microplastics Capture and Recovery,” was co-authored by former NC State Ph.D. student Rachel Bang and current NC State Ph.D. student Lucille Verster.

This article is based on a news release from NC State University.