Scientists Unveil An Eco-Friendly Way to Disinfect Water Using Light
A Chinese research team has developed a way to use light to purify water in half an hour without leaving behind metal pollutants.
With over two billion people worldwide living without reliable access to clean, safe drinking water, big strides need to be made in water purifying technology. Some of the most promising innovations could metal-free photocatalysts, which use energy from light to split microbe-killing chemicals out of water. Image Credit: Nestlé, flickr
Nearly a third of the world’s population lives without the guarantee of safe, drinkable water. Solving this problem will require overhauls both technological and sociopolitical—including water purification methods that are accessible, simple, and cheap.
That’s not a trivial trifecta. But in a study published today in the journal Chem, a team of Chinese scientists unveils new technology that might just meet all three criteria, without creating environmental pollutants. While the method—which uses energy from visible light to generate reactive microbe-killing chemicals like hydrogen peroxide—is far from commercialization, it avoids the pitfalls of other purification strategies that can leach heavy metals into water.
“So far, this seems like an exciting new technology,” says Alison Wood, a civil and environmental engineer at Olin College who was not involved in the study. “More safety tests need to be done before this is deployed widely, but [this type of purification] could offer better outcomes with fewer negative consequences.”
Many traditional water purification methods rely on corrosive chemicals like chlorine and ozone that, while effective against disease-causing bacteria, can also pollute the environment. To get around this, researchers have begun developing materials that can split unstable molecules of oxygen out of water, which go on to act as short-lived disinfectants that pose no threat to human health. One particularly appealing category of these materials is composed of photocatalysts, which rely entirely on light as a source of energy to power the purification process.
Nanomaterials based in graphitic carbon nitride, a metal-free photocatalyst, can kill over 99 percent of bacteria in a contaminated water sample in just half an hour. Image Credit: U.S. Agency for International Development, flickr
Traditional photocatalysts are often built with metals, however, which means there’s a chance that these ingredients could contaminate the water supply, says study author Zhenyuan Teng, a chemical engineer at Yangzhou University in China. An ideal water purification system, he says, needs to be something that’s not only highly effective, but also environmentally friendly and non-toxic to humans.
And so, in developing their method, the researchers turned to a photocatalytic nanomaterial called graphitic carbon nitride. Like its metal-based counterparts, graphitic carbon nitride absorbs light to unleash a hefty dose of microbe-fighting hydrogen peroxide, but accomplishes this with only carbon and nitrogen atoms, keeping the chemical reaction metal free.
In a test run, a thin sheet of graphitic carbon nitride killed 99.9999 percent of the bacteria present in a contaminated water sample in about half an hour. With these numbers, the team’s metal-free nanomaterial performs about as well as your garden-variety metal-based photocatalyst.
The researchers have already shown that their nanomaterial can be coated onto everyday items like plastic bags. Such prototypes could end up being the precursor for personal products such as water bottles or bladders.
“Given the percentage of the population that still doesn’t have access to safe drinking water, this could be very powerful even in individual applications, even with very small quantities,” Wood says. That may be especially true in communities that can’t afford large water plants.
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But the researchers’ prototype won’t be hitting the shelves of your local supermarket anytime soon. The team is still refining its model, and additional safety testing must be conducted. “The material itself appears to be pretty economical and safe for consumption, but any time we’re producing [reactive oxygen molecules], there’s potential to create disinfection byproducts,” says Brooke Mayer, an environmental engineer at Marquette University who was not involved in the study.
Additionally, “there’s a huge gap between a material that performs well in a lab, and water purification on a commercial scale,” says Mamadou Diallo, a civil and environmental engineer at Caltech who was not involved in the study. The material has promise, he says, but even a moderate price tag could be prohibitive when a technology is mass-produced. What’s more, Diallo says, there’s more to the water purification equation than killing bacteria; chemical contaminants need to be eliminated as well. And while graphitic carbon nitride could have the potential to perform both functions, this has yet to be confirmed.
And nailing the technology aspect of a water purification system is only one part of a much larger equation, Wood says. “Once we’ve considered the direct safety of the water a person is drinking, we still need to think about social acceptance, cost, and ease of use of the technology,” she says. “The water purifier needs to be safe, but also inexpensive enough to purchase and easy to use correctly. And people need to be willing to use it.”
