“Waterborne diseases are responsible for 2 million deaths annually, the majority 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 that our novel technology will facilitate revolutionary changes in water disinfection and inspire more innovations in this exciting interdisciplinary field,” he added.

To date, as the researchers point out, existing water-treatment methods include using chemicals that can produce toxic byproducts when exposed to ultraviolet light. This is counterproductive and requires further processing to remove them to make the water safe to drink. They are also relatively slow and can demand access to electricity.

The new powder, however, works by absorbing UV and high-energy visible light from the sun. The powder comprises nano-sized flakes of aluminum oxide, molybdenum sulfide, copper, and iron oxide. When the molybdenum sulfide/copper catalyst absorbs photons from the sun, it acts as a semiconductor/metal junction.

This allows the photons to release electrons, interacting with the nearby water to produce hydrogen peroxide and hydroxyl radicals. These oxygen molecules are extremely harmful to bacteria, damaging their cell membranes and quickly killing them off.

“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, when immersed in water, they all function together,” he explained.

To conduct their study, the team from Stanford and SLAC utilized a 6.8 ounces (200 milliliters) beaker filled with room-temperature water. This water was contaminated with approximately 1 million E. coli bacteria per mL [.03 oz.]. Additionally, as per his statement, fine nanoflakes are highly mobile and capable of swiftly coming into contact with numerous bacteria, causing their rapid demise. Furthermore, the chemical derivatives created by sunlight disperse rapidly.

“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 into water and oxygen and are discarded within seconds. So you can drink the water right away,” he added.

The powder used is eco-friendly and can be recycled. Including iron oxide in the nanoflakes allows them to be extracted from water using a regular magnet. The researchers foresee the powder being useful for other applications beyond providing billions of safe water to impoverished people. Hikers, campers, and largescale water treatment plants too. The team plans to test the powder on other water-borne pathogens like viruses, protozoa, and parasites.

You can review the study for yourself in the journal Nature Water.

Study abstract:

Although heterogeneous water disinfection can avoid secondary pollution and other shortcomings in homogeneous systems, its low efficiency hinders development. Here we successfully address the issues above of heterogeneous disinfection by developing discrete nanoflakes of (Al2O3@v-MoS2)/Cu/Fe3O4. Three exciting features are integrated into such a novel structure: bifacial vertically aligned nano fingerprint MoS2 grown on both sides of the light-transparent Al2O3 nanoflakes that can largely absorb sunlight, where both sides can operate simultaneously; a Cu-MoS2 junction that enhances charge separation for the efficient generation of reactive oxygen species; and magnetic Fe3O4 nanoparticles that have magnetic separation capability and conveniently regenerate after disinfection. The (Al2O3@v-MoS2)/Cu/Fe3O4 nanostructures reported herein exhibit outstanding water disinfection with complete inactivation of over 5.7 log10 colony-forming units ml−1 Escherichia coli within 1 min in real sunlight (the system thermal effect has little impact on disinfection performances) as well as facile separation and stable long cycle reuse, demonstrating broad application prospects.

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