Mopping up tiny particle pollution

Air pollution can be a hidden killer. Ultrafine particles smaller than 100 nanometres wide pose some of the greatest risks to health. Researchers in Hong Kong have developed a filtration material made from a substance called a metal-organic framework that can remove particles as small as 15 nanometres across, with close to 99 percent efficiency.

They report their work in the journal Applied Materials Today. “This could be a breakthrough in air pollution control with great promise for reducing health risks,” says Li Zhang, whose research group at the Chinese University of Hong Kong collaborated in the work with Chun Chen’s group at the same institution.

Zhang points out that the particles that the filter can remove are a leading cause of respiratory, heart and circulatory disease and death worldwide. They are estimated to be a key factor in more than three million premature deaths each year, in addition to a substantially greater number of chronic lung and circulatory system problems. The particles are produced outdoors by vehicle exhausts, industrial smokestacks and fuel combustion; and indoors by such activities as tobacco smoking and burning candles or incense.

Metal-organic frameworks (MOFs) have metal ions or clusters connected by carbon-based (organic) linker groups. Varying the metallic and organic groups can yield a huge variety of MOFs with internal structures that can be tuned to interact with selected chemicals and fine particles.

Other research groups have devised MOF-based filters, but those developed by the collaboration between Zhang and Chen’s group are significantly more effective at removing ultrafine particles and are also much more straightforward to manufacture. “Our MOF composite filters are prepared in a fast one-step method at room temperature,” says Zhang. This simplified fabrication process significantly reduces manufacturing costs and could also be readily scaled up for commercial mass production.

The procedure uses existing electrospinning technology to create the MOFs in the form of nanofibres arranged in a meshwork. In addition to its remarkable efficiency in removing particles in the 15–100 nanometre size range, the material also delivered a drop in pressure that was one sixth as large as that suffered by existing alternatives. The air flow is therefore much less impeded as it moves through the filter. Additional advantages are a mechanical and chemical resilience that should allow a long service life and easy cleaning during service.

The researchers believe that their material could be further developed to build in antibacterial and antiviral properties. In the context of the current Covid-19 pandemic, Zhang comments: “If we are able to embed functional antiviral materials into the filter, we could help combat future disease outbreaks and pandemics.”

The research undertaken so far has also revealed details of the mechanism by which the various pollutant particles interact with the fibres of the filter, including electrostatic interactions. This information could prove useful in designing new versions of the material best-suited for different applications. The possibilities include specialist filters for hi-tech industries, face-mask filtration and large-scale ventilation and air-conditioning systems.

Article Details: Bian, Y. et al.: “Effective removal of particles down to 15 nm using scalable metal-organic framework-based nanofiber filters,” Applied Materials Today (2020).