Non-toxic nano system powder uses sunlight to quickly disinfect contaminated drinking water

(Nanowerk News) At least 2 billion people worldwide regularly drink water contaminated with disease-causing microbes. Now, scientists at Stanford University and the SLAC National Accelerator Laboratory have discovered an inexpensive, recyclable powder that kills thousands of waterborne bacteria per second when exposed to regular sunlight. The discovery of this ultrafast disinfectant could be a significant advance for the nearly 30 percent of the world’s population who don’t have access to safe drinking water, according to the Stanford and SLAC teams.

The results are published in Natural Water (“A solar-driven heterogeneous subminute water disinfection nanosystem assembled with MoS fingerprint2“). Disinfectant powder is stirred in water contaminated with bacteria (top left). The mixture is exposed to sunlight, which quickly kills all bacteria (above right). Magnets collect metal powder after disinfection (bottom right). The powder is then put back into another beaker containing the contaminated water, and the disinfection process is repeated (bottom left). (Image: Tong Wu/Stanford University)

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

Conventional water treatment technologies include chemicals, which can produce toxic by-products, 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 metal powder that works by absorbing high-energy UV and visible rays from the sun. The powder consists of nano-sized flakes of aluminum oxide, molybdenum sulfide, copper and iron oxide.

“We only use small amounts of these materials,” said senior author Yi Cui, Fortinet Founding Professor in MSE and Energy Science & Engineering at Stanford Doerr School of Sustainability. “The materials are cheap and quite abundant. The main innovation is that, when immersed in water, everything works together.”

Fast, non-toxic and recyclable

After absorbing photons from the sun, the molybdenum sulfide/copper catalyst acts like a semiconductor/metal junction, allowing the photons to eject electrons. The freed electrons then react with the surrounding water, producing hydrogen peroxide and hydroxyl radicals – one of the most biologically damaging forms of oxygen. The newly formed chemicals quickly kill bacteria by seriously damaging their cell membranes. Microscopic images of E. coli before (left) and after disinfection. Bacteria die quickly after sunlight produces chemicals that cause serious damage to the bacterial cell membrane, as shown in the red circle. (Image: Tong Wu/Stanford University)

For the study, the Stanford and SLAC teams used a 200-milliliter (6.8-ounce) beaker filled with room-temperature water contaminated with about 1 million E. coli bacteria per mL (0.03 oz.).

“We stirred the powder into the contaminated water,” said co-lead author Bofei Liu, a former MSE postdoc. “Then we did a disinfection test on the Stanford campus in real sunlight, and within 60 seconds there was no detectable live bacteria.”

The nanopowder flakes can move quickly, make physical contact with lots of bacteria and kill them quickly, he added.

The chemical by-products produced by sunlight also dissipate quickly.

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

Non-toxic powder can also be recycled. The iron oxide allows the nanoflakes to be removed from the water with an ordinary magnet. In the study, researchers used magnets to collect the same powder 30 times to treat 30 different samples of contaminated water.

“For hikers and backpackers, I can imagine bringing a little bit of powder and a little magnet,” said Cui. “During the day you put the powder in water, shake it a little in the sun and within a minute you have potable water. You use a magnet to eject particles for later use.”

The powder might also be useful in wastewater treatment plants that currently use UV lamps to disinfect treated water, he added.

“During the day plants can use visible sunlight, which will work faster than UV light and will probably save energy,” said Cui. “Nanoflakes are fairly easy to make and can be scaled up quickly by tons.”

Studies focused on E. coli, which can cause severe and even life-threatening gastrointestinal illness. The US Environmental Protection Agency has set maximum contaminant level goals for E. coli in zero drinking water. The Stanford and SLAC teams plan to test the new powder on other water-borne pathogens, including viruses, protozoa and parasites that also cause serious illness and death.