Multicolored particles of plastic float beneath the surface of most waterways, from headwaters to the Arctic Ocean. These nearly invisible microplastics less than 5 mm wide are potentially harmful to humans as well as aquatic plants and animals. As such, researchers are devising ways to remove them and stop them at their source. Today, the team reports his two-stage device, made of steel tubes and pulsed sound waves, that removes most plastic particles from real water samples.
The researchers plan to present their results at the Spring Meeting of the American Chemical Society (ACS). ACS Spring 2023 will feature over 10,000 of his presentations on a wide range of scientific topics in a hybrid virtual and in-person meeting taking place March 26-30.
“The idea arose from a discussion with colleagues that we needed a new way to collect microplastics from water,” said Dr. Menake Piyasena, principal investigator of the project. “Since acoustic forces can push particles together, we wondered if we could use them to clump together microplastics in water, making them easier to remove.”
Filtration is the most commonly used technique to remove these substances from water. For example, a washing machine outlet filter can prevent fibers that are shed from clothes during washing from entering the wastewater. However, this method is costly at scale and requires regular cleaning of filters, which can clog.
Another option is to use acoustic forces or sound waves to concentrate plastic particles in flowing water and transfer energy to nearby particles, causing some to vibrate or move. Think of speakers that play loud music. It shakes the ground at a loud volume, causing small pieces of dust and dirt to bounce off each other. Scientists are already using this phenomenon to separate biological particles from liquids, such as red blood cells from blood plasma.
Recently, some teams applied this approach to isolate microplastics from samples prepared using pure water in the lab. However, this work was done with a small amount of water. A graduate student at New Mexico Tech’s Piacena lab, he explains that Nelum Perera also used microplastics that were just a few tens of microns wide (smaller than the width of a human hair).
“I read that most microplastics in the environment are larger than that,” says Perera, who publishes the work. “So we wanted to develop a device that could accommodate most sizes and scale up to meet practical goals.”
To accommodate higher water flow rates, Perera created a proof-of-concept device with an 8 mm wide steel tube connected to one inlet and multiple outlet tubes. Then I attached the transducer to the side of the metal tube. When the transducer was turned on, ultrasonic waves were generated across the metal tube, and acoustic forces were applied to the microplastics as they passed through the system, facilitating their capture. Piyasena explains that the prototype device is relatively simple compared to traditional filtration methods.
In initial experiments with microplastics of polystyrene, polyethylene, and polymethyl methacrylate, the researchers found that small particles (6–180 µm wide) were separated from large particles (180–300 µm wide) in the presence of acoustic forces. I have found that it behaves differently. When spiked in pure water, particles of both sizes were positioned along the center of the channel and exited through the central outlet, leaving clean water out of the surrounding outlets. However, when laundry detergent or fabric softener was added to the water, the large particles were concentrated towards the sides and exited the side outlets while purified water exited the center outlet.
Based on these results, researchers set out to develop a system that could take advantage of these different movements. I connected two steel pipes in tandem. Small microplastics less than 180 μm wide were captured in the first stage, and the water stream containing the remaining large microplastics was sent to the second stage and washed. “With this method, he removed more than 70% of the small plastic and more than 82% of the large plastic,” Perera says.
To show that the two-stage system works in real-world applications, Perera and Piyasena collected water from a pond on the New Mexico Institute of Technology campus and the Rio Grande River. They filtered all samples to remove large contaminants, leaving water that still contained dissolved material that could have affected the separation. When environmental water samples passed through an acoustic device, plastic particles were removed as effectively as pure water. Perera estimates that the current device would cost him about 7 cents to operate for an hour and take about an hour and a half to clean a liter of water using the prototype.
The team’s next step is to develop systems with wider tubes, or bundles of multiple tubes, and try them on real, unspiked samples, such as seawater or wastewater from a washing machine. We have shown that we can use it to concentrate microplastics of different sizes,” says Piacena. “And from here we want to prove that this can be done on a larger scale using real samples that already contain microplastics.”
Original: Pulsating ultrasound may one day remove microplastics from waterways
Than: New Mexico Institute of Mining Technology | American Chemical Society
