Ultrasound Wave Pulses May Rid Oceans of Microplastics

Sep 16, 2023

Elizabeth Montalbano | Apr 19, 2023

Microplastics floating in the ocean and other bodies of water are a massive source of pollution in the world, posing a threat to wildlife, the environment, and even humans. Now researchers have developed a proof-of-concept device that uses pulsing sound waves to remove plastic particles as a potential solution to this global issue.

A team led by Menake Piyasena, associate professor at New Mexico Institute of Mining and Technology, developed a two-stage device composed of steel tubes and pulsing sound waves that has demonstrated it can remove tiny plastic particles less than 5 millimeters wide from real water samples.

The solution is one of a number of ways scientists are eyeing to tackle the mounting problem of microplastics, which represent tons of plastic waste in the ocean that then are ingested by animals—and later humans when they eat fish from the sea—and also wash up on land.

While predictions for how much microplastics exist today are rather dire—with a 2021 study by Kyushu University finding that there are about 24.4 trillion pieces of microplastics with a combined weight of 82,000 to 578,000 tons in the world's upper oceans—scientists predict their impacts on the environment and even human health can be reduced if current solutions to the problem are widely applied.

Piyasena's team hopes to add its invention to help fight this problem. The idea for using ultrasound waves to collect microplastics came from a discussion that the project's principal investigator had with a colleague about how to solve the problem of removing these plastics from water, he said.

"Because acoustic forces can push particles together, I wondered if we could use them to aggregate microplastics in water, making the plastic easier to remove," he said.

As it turns out, scientists could, using sound waves to transfer energy to particles so they vibrate, move, and then gather together for easier collection, the researchers found. Indeed, scientists already have applied this concept—similar to how speakers playing loud music can shake the ground—to other processes, such as to separate biological particles like red blood cells from liquids such as plasma.

Initially, the New Mexico Tech team tested their theory to separate microplastics tens of microns wide—which is smaller than the width of a human hair—from samples they prepared in the lab with tiny volumes of pure water.

However, knowing that microplastics in the environment are larger than that, Piyasena's team—including Nelum Perera, a graduate student in his lab—set out to build a device that can be used with most of the sizes of microplastic that exist and also could be scaled up for real-world use, she said.

To accommodate higher water-flow rates, Perera created a proof-of-concept device with 8-mm-wide steel tubes connected to one inlet tube and multiple outlet tubes, then attached a transducer to the metal tube's side. This transducer, when turned on, generates ultrasound waves across the metal tube, which applies acoustic forces onto microplastics as they passed through the system. The result is that the sound waves make the particles easier to capture.

Thee prototype device is relatively simple compared to traditional filtration methods, which are the most common way now used to collect microplastics, the researchers said. That's because it doesn't clog as easily as filter-based solutions can, they said.

Researchers tested this device with polystyrene, polyethylene, and polymethyl methacrylate microplastics, observing the difference in behaviors between smaller (6- to 180-µm-wide) and larger (180- to 300-µm-wide) particles, they said.

In those test runs of the system, particles of both sizes arranged themselves along the center of the channel and exited through the middle outlet, while clean water flowed out the surrounding outlets. However, if researchers added laundry detergent or fabric softener to the water, the larger particles changed their behavior. They collected along the sides, exited through the side outlets, and purified water out the middle outlet.

Knowing this about the different behavior of particles, researchers then aimed to develop a system that could take advantage of the varied movements. To do this, they connected two steel tubes in tandem, with the first stage capturing small microplastics less than 180 µm wide, and the water stream that contained remaining larger microplastics moving on to the second stage to be cleaned.

"We removed more than 70% of the small plastics and more than 82% of the large ones this way," Perera said of the result of testing the new system.

The next step for the team was to prove that this two-stage system could work for real-world applications, which Perera and Piyasena set out to do by collecting water from a pond on the New Mexico Tech campus and from the Rio Grande River. Before conducting the test, they filtered out all of the samples to remove large contaminants, leaving behind water that still contained dissolved substances that could have affected the separation. They then spiked the water with microplastics and put it through their system, finding that they were removed as effectively as with the pure water used in the lab, the researchers said.

Perera estimates it would cost around 7 cents to operate the current device for an hour and take around an hour and a half to clean one liter of water, making it cost effective to use on a larger scale—an application that they hope is in the future of their device.

To this end, the researchers' next step for the development of the system is to create one with wider tubes, or bundles of multiple tubes, and to try it on real-world samples that haven't been doctored, including ocean water and wastewater from washing machines, they said.

"We have shown that acoustic forces can be used to concentrate a wide range of microplastic sizes," Piyasena said. "And from here, we want to prove that this can be done on a larger scale with real samples that already have microplastics in them."

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