[Science Column] Capturing Even Dryer Microplastics... Korea University Develops Microbubble-Based Water Purification Technology

Chemical-Free Microplastic Separation
A High-Efficiency Water Purification Technology in the Spotlight

A domestic research team has developed a new water purification technology that could provide a breakthrough solution to the problem of microplastics in everyday environments. The key innovation lies in dramatically improving the efficiency of microplastic separation, which was previously challenging with conventional hydrocyclone methods, by utilizing microbubbles-tiny air bubbles.

The research team led by Professor Hyejung Kim from the Department of Mechanical Engineering at Korea University announced on December 12 that they have implemented a "mini hydrocyclone" water purification system combined with microbubbles, successfully achieving a significant leap in microplastic removal efficiency.

Image of the microplastic separation principle of the mini hydrocyclone-microbubble fusion system. The flow within the mini hydrocyclone and the forces exerted on microplastics according to density and bubble attachment are diagrammed. Provided by the research team

Microplastic pollution has emerged as a global environmental issue, causing ecological disruption and raising concerns about human health risks. However, existing water purification methods face structural limitations, such as the use of chemical agents, membrane contamination, and high maintenance costs. As a result, there has been a continuous demand for the introduction of new water purification technologies.

The "hydrocyclone" is a device that separates substances of different densities using only the rotational motion of water, offering the advantages of not requiring chemical additives and relatively easy equipment management. However, it has limitations in efficiently separating microplastics, which have densities very similar to water.

To address this issue, the research team incorporated the principle of dissolved air flotation. In particular, they utilized a downsized device referred to as the "mini hydrocyclone." The mini hydrocyclone features a compact body and high internal flow velocity, resulting in a shorter separation process and higher removal efficiency for even the smallest microplastics.

Additionally, the team injected microbubbles-tiny air bubbles-into the system. When microbubbles attach to the surface of microplastics, the plastics become relatively lighter than water and are more easily separated within the swirling flow. During this process, multiple bubbles gather at the center of the hydrocyclone to form an "air column" structure. The researchers explained that this air column is the key factor determining the efficiency of microplastic separation.

Photo of the research team. From the left, Hyejung Kim, Professor of Mechanical Engineering at Korea University (corresponding author), Jeongmin Seo, Master's student at Korea University (first author). Courtesy of Korea University

The experimental results were clear. Under conditions where microbubbles were applied, the efficiency of microplastic separation improved by up to 34% compared to conventional methods. Through high-speed camera analysis, the team confirmed that microplastics stably attach to microbubbles even in high-speed rotational flow environments, and they identified a direct link between the air column structure and enhanced separation performance.

This technology also demonstrated applicability to various types of microplastics with different shapes and materials. In particular, experiments targeting fibrous microplastics generated during the clothes drying process showed a 23.3% increase in separation efficiency, proving the potential for real-world and industrial applications.

Professor Hyejung Kim stated, "We have confirmed that the mechanism of microbubble-microplastic attachment, as well as the air column structure formed inside the hydrocyclone, are the core mechanisms determining separation performance. This technology can greatly improve microplastic removal efficiency without chemicals, offering significant potential to evolve into a high-efficiency, low-cost water treatment technology for the future."

This research was conducted with joint support from the National Research Foundation of Korea (NRF) Young Researcher Program and the Korea Basic Science Institute (KBSI). The results were published online on November 12 in the international journal 'Water Research' (Impact Factor 12.4, JCR Top 1.1%) in the field of water treatment and aquatic environment.

The title of the paper is "Enhanced microplastic removal using a mini-hydrocyclone with microbubbles."