New powder uses sunlight for disinfecting water

When exposed to sunlight, a low-cost, recyclable powder can kill thousands of waterborne bacteria per second. Stanford and SLAC scientists say the ultrafast disinfectant could be a revolutionary advance for 2 billion people worldwide without access to safe drinking water.

When exposed to sunlight, a low-cost, recyclable powder can kill thousands of waterborne bacteria per second. Stanford and SLAC scientists say the ultrafast disinfectant could be a revolutionary advance for 2 billion people worldwide without access to safe drinking water.

At least 2 billion people worldwide routinely drink water contaminated with disease-causing microbes.

Now, scientists at Stanford University and SLAC National Accelerator Laboratory have invented a low-cost, recyclable powder. When exposed to ordinary sunlight, that powder kills thousands of waterborne bacteria per second. The discovery of this ultrafast disinfectant could be a significant advance for nearly 30 per cent of the world’s population with no access to safe drinking water, according to the Stanford and SLAC team. Their results are published in a May 18 study in Nature Water.

“Waterborne diseases are responsible for 2 million deaths annually. The majority of them are found in children under the age of 5,” said study co-lead author Tong Wu, a former postdoctoral scholar of materials science and engineering (MSE) at the Stanford School of Engineering. “We believe our novel technology will facilitate revolutionary changes in water disinfection and inspire more innovations in this exciting interdisciplinary field.”

A positive step when it comes to disinfecting water

Conventional water-treatment technologies include chemicals, which can produce toxic byproducts, and ultraviolet light, which takes a relatively long time to disinfect and requires a source of electricity.

The new disinfectant developed at Stanford is a harmless metallic powder that absorbs UV and high-energy visible light from the sun. The powder comprises nano-size flakes of aluminium oxide, molybdenum sulphide, copper, and iron oxide.

“We only used a tiny amount of these materials,” said senior author Yi Cui, the Fortinet Founders Professor of MSE and Energy Science & Engineering in the Stanford Doerr School of Sustainability. “The materials are low cost and fairly abundant. The key innovation is that they all function together when immersed in water.”

Fast, nontoxic, and recyclable

After absorbing photons from the sun, the molybdenum sulphide/copper catalyst performs like a semiconductor/metal junction. It enables the photons to dislodge electrons. The freed electrons then react with the surrounding water, generating hydrogen peroxide and hydroxyl radicals – one of the most biologically destructive forms of oxygen. The newly formed chemicals quickly kill the bacteria by seriously damaging their cell membranes.

For the study, the Stanford and SLAC team used a 200 millilitre [6.8 ounces] beaker of room-temperature water contaminated with about 1 million E. coli bacteria per mL [.03 oz.].

“We stirred the powder into the contaminated water,” said co-lead author Bofei Liu, a former MSE postdoc. “Then we conducted the disinfection test on the Stanford campus in real sunlight. Within 60 seconds, no live bacteria were detected.”

He added that the powdery nanoflakes could move around quickly, make physical contact with many bacteria and kill them fast.

The chemical byproducts generated by sunlight also dissipate quickly.

“The lifetime of hydrogen peroxide and hydroxy radicals is very short,” Cui said. “If they don’t immediately find bacteria to oxidize, the chemicals break down into water and oxygen and are discarded within seconds. So you can drink the water right away.”

The nontoxic powder is also recyclable. Iron oxide removes the nanoflakes from water with an ordinary magnet. The researchers used magnetism to collect the same powder 30 times to treat 30 different samples of contaminated water.

Mobile solution for disinfecting water in sunlight

“For hikers and backpackers, I could envision carrying a tiny amount of powder and a small magnet,” Cui said. “During the day, you put the powder in water, shake it up a little bit under sunlight, and within a minute, you have drinkable water. You use the magnet to take out the particles for later use.”

He added that the powder might also be helpful in wastewater treatment plants. It would replace the UV lamps currently used to disinfect treated water.

“During the day, the plant can use visible sunlight. It would work much faster than UV and probably save energy,” Cui said. “The nanoflakes are fairly easy to make and can be rapidly scaled up by the ton.”

The study focused on E. coli, which can cause severe gastrointestinal illness and even be life-threatening. The U.S. Environmental Protection Agency has set the maximum contaminant level goal for E. coli in drinking water at zero. The Stanford and SLAC team plans to test the new powder on other waterborne pathogens. They want to look at viruses, protozoa and parasites that cause serious diseases and death.

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