[Image above] Many places in the world lack access to clean water. Glass-ceramic photocatalysts could offer a safe and cost-effective way to purify water. Credit: World Bank Photo Collection, Flickr (CC BY-NC-ND 2.0)
Fifty years ago this month, the Cuyahoga River in Cleveland, Ohio, caught fire yet again because of sewer and waste debris dumped into the water. What makes this fire notable from previous ones is the fact that this fire sparked the formation of the United States Environmental Protection Agency (EPA) less than a year later.
While waterways in the U.S. and elsewhere have improved since the 1970s, there are many parts of the world where access to clean water is limited. Providing cost-effective methods for water purification will greatly enhance the lives of billions of people.
One way to purify water is by using photocatalysts. Photocatalysts harness the energy of light, particularly the visible and UV wavelengths of sunlight, to break chemical bonds of harmful substances and to kill bacteria.
The ideal photocatalyst deployment method distributes the material evenly throughout the reaction volume, is optically transparent, is stable for long-term effectivity, is easily removed from the water, and is cost-effective.
However, common photocatalyst deployment methods face notable shortcomings. Photocatalyst nanopowders combine in water to form larger, less effective particles and they are difficult to extract from water. Films and coatings containing photocatalysts tend to degrade rather quickly. These shortcomings increase the cost of using photocatalysts.
In prior studies on improving photocatalyst performance, researchers have created glass-ceramic photocatalysts consisting of active nanocrystals in transparent glassy matrices in the laboratory. The glassy matrix locks the catalyst into a well distributed pattern while the glass is more durable than polymer-based coatings. These glass-ceramics then can be formed into pellets or plates that are easily removed, or into tubes and vessels to contain the contaminated water as it is cleaned.
Unfortunately, the materials and methods used in these prior studies were not suitable for safe and cost-effective use of glass-ceramic photocatalysts at larger scales. For example, some glass-ceramics contained bismuth, which is toxic when ingested over long periods and thus unsafe for water treatment purposes. Further, most materials required energy-intensive (and therefore expensive) processing methods.
Recently, a group of scientist in the School of Engineering at Indian Institute of Technology (IIT) Mandi, led by professor of engineering Rahul Vaish, developed a series of glass-ceramic photocatalysts that show promise for cost-effective and safe microbial and organic pollution reduction.
In three recent articles published in the Journal of the American Ceramic Society, Vaish and his colleagues report different potential solutions to improve safety and cost-effectiveness by exploiting the photocatalytic abilities of zinc oxide (ZnO), titania (TiO2), and fluorite (CaF2) with modifiers to improve physical and catalytic properties. These solutions include
- Heat-treating Ca–Ba–B–Al–Ti–Zn oxide glass to form nanocrystalline TiO2, which shows promise for glass-ceramics with excellent mechanical properties,
- Acid-treating CaO–2B2O3 glass to form nanocrystalline CaF2 at the surface, which promises to reduce production costs, and
- Exploring combined piezo- and photocatalysis in tin-doped BCT (Ba–Ca–Ti-Sn oxide), which indicates that ferroelectric materials have untapped potential in the area of photocatalysis.
In all three studies, the researchers reported similar degradation of dyes (representative of typical water pollutants) of up to 78%, and the first two studies showed more than 95% reduction of bacteria cells (the third study did not test biological results).
In a Deccan Herald article on the research, Vaish explains that their glass-ceramics can clean more than just water. “This technology can also be used to clean air. It can remove NOx (oxides of nitrogen) from the air. If we place these glasses in our windows, we can fight air pollution too,” he says.
The three recent articles on water-purifying glass-ceramics, published in Journal of the American Ceramic Society, are
“Antibacterial and photocatalytic active transparent TiO2 crystallized CaO–BaO–B2O3–Al2O3–TiO2–ZnO glass nanocomposites” (DOI:10.1111/jace.16199),
“Transparent CaF2 surface crystallized CaO–2B2O3 glass possessing efficient photocatalytic and antibacterial properties” (DOI: 10.1111/jace.16395), and
“Photocatalytic, piezocatalytic, and piezo‐photocatalytic effects in ferroelectric (Ba0.875Ca0.125)(Ti0.95Sn0.05)O3 ceramics” (DOI: 10.1111/jace.16502).