Microplastics naturally scatter in flowing water (left), but after turning on sound waves, the particles concentrate along the tube’s sides (right), making them easier to remove. Credit: Menake Piyasena
Key points:
- A proof-of-concept device can filter microplastics that are up to 300-µm-wide out of water.
- Tests removed more than 70% of small plastics and more than 82% of large ones.
- The system costs 7 cents to operate for 90 minutes in order to clean 1 L of water.
Plastic pollution in the world’s waterways is rapidly increasing, causing problems for both the marine ecosystem as well as human health. Now, researchers have created a two-stage device made with steel tubes and pulsing sound waves that can remove most of the plastic particles from real water samples, according to a presentation at the American Chemical Society (ACS) Spring meeting.
Previous research has used sound waves to separate plastic particles from water, but the technique was tested in pure lab water with particles tens of microns wide—about the width of a single human hair.
“I read that most of the microplastics in the environment are larger than that,” said Nelum Perera, a graduate student at the New Mexico Institute of Mining and Technology. “So, I wanted to develop a device that could be useful for most of the sizes and could be scaled up to meet real-world goals.”
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, she attached a transducer to the metal tube’s side. When the transducer was turned on, it generated ultrasound waves across the metal tube, applying acoustic forces onto microplastics as they passed through the system, making them easier to capture.
In initial experiments with polystyrene, polyethylene and polymethyl methacrylate microplastics, the researchers discovered that smaller (6- to 180-µm-wide) particles behaved differently than the larger (180- to 300-µm-wide) ones in the presence of acoustic forces.
Spiked into pure water, particles of both sizes arranged along the center of the channel, exiting through the middle outlet, while clean water flowed out the surrounding outlets. But if laundry detergent or fabric softener were added to the water, the larger particles focused toward the sides, exiting through the side outlets, and purified water out the middle outlet.
Based on these results, the researchers set out to develop a system that could take advantage of these differing movements. They connected two steel tubes in tandem: The first stage captured small microplastics less than 180 µm wide, and the water stream with the remaining larger microplastics went 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,” said Perera.
To show that the two-stage system could work for real-world applications, Perera and team collected water from a pond on the New Mexico Tech campus and from the Rio Grande River. They filtered all of the samples to remove large contaminants, leaving behind water that still contained dissolved substances that could have affected the separation. Next, they spiked the water with microplastics. When the environmental water samples went through the acoustics device, plastic particles were removed as effectively as from pure water.
With this prototype, 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. The team’s next step is to develop a system with wider tubes, or bundles of multiple tubes, and to try it on unspiked real-world samples, including ocean water and wastewater from washing machines. ‘
Information courtesy of ACS.