There is no doubt that microplastics pose a rising ecological and health risk, but this wasn’t always the case. It’s only recently that scientists began to appreciate the scope of these microscopic particles and their influence on animals ranging from marine life to humans. A 2019 study found that we consume around five grams of microplastic weekly! That’s the weight of a credit card, for scale.
The problem now is to find methods to properly remove microplastics (MPs) from water and the atmosphere, which is no easy undertaking given that these tiny bits of plastic range in size from one micrometer to five millimeters.
A team from Shinshu University used sound to remove MPs, experimenting with acoustic filtering to push these tiny shards of plastic into a central channel, with branched parts filled with MP-free water that can later be released.
How does acoustic filtering work?
“Our proposed microfluidic device, which is designed based on a hydraulic-electric analogy, has three 1.5 mm-wide microchannels connected via four serial 0.7-mm-wide trifurcated junctions,” explained lead researcher Professor Yoshitake Akiyama of the Department of Mechanical Engineering and Robotics at the Faculty of Textile Science and Technology at Shinshu University. “The MPs are aligned at the center of the middle microchannel using a bulk acoustic wave of 500-kHz resonance frequency. As a result, a 3.2-fold enrichment of MPs must occur at each junction, resulting in a 105-fold overall enrichment in the device.”
In simpler terms, ultrasonic waves flow through the water and drive MPs to the center of a fluid stream, where they can be collected or filtered out when MP-free water filters into the device’s branches.
What gives acoustic filtering an edge over existing methods?
Traditionally, MPs are collected using mesh filters, which can readily become clogged and are restricted in what they can capture because of the mesh size.
Instead, this device employs microfluidic technology, a new science that manipulates the behavior of water through microchannels. When independent studies on grouped MPs were conducted, the collection rate for those sized 10 m, 15 m, 25 m, 50 m, and 200 m was greater than 90 percent. Further testing with different particle sizes (25-200 m and 10-25 m) revealed a collection rate of approximately 80 percent.
It is not the scientists’ first acoustic filtration model; they previously created and tested a system for laundry wastewater. The team believes the device’s progress demonstrates its far-reaching applications, such as purifying wastewater from industrial-scale operations before it is discharged.
“This proposed microfluidic device based on acoustic focusing can efficiently, rapidly, and continuously collect 10–200 μm MPs without recirculation after pre-filtration of larger MPs through a mesh,” stated Akiyama. “It can be installed in washing machines, factories, and other sources of MPs for efficiently enriching and removing various-sized MPs from laundry and industrial wastewater. This will make it possible to prevent the discharge of MPs to the environment.”
Source study: Separation and Purification Technology— A collection device for various-sized microparticles that uses four serial acoustic separations: Working toward microplastic emission prevention