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Nanofilters Make Toxic, Polluted Water Potable

aluminum oxide forming on gallium
Researchers in Australia have developed a low-cost nanofilter that can potentially bring clean drinking water to areas of the world that still desperately need it.

With about 10 percent of the world’s population—or about 8.4 million people—without access to clean water, finding fast and easy ways to purify water for drinking has been a top priority for scientists. Now, researchers in Australia have made an innovation in liquid chemistry that could help bring clean water to more of the world’s population at a very low cost.

A team of scientists from the University of New South Wales (UNSW) Sydney and RMIT University Melbourne has developed cheap and energy-efficient filters that can transform water contaminated with heavy metals into safe drinking water in a matter of minutes, they said in a UNSW press release.

“Many of these people who find themselves in this position [without access to water] live in disadvantaged societies, so an expensive, complex solution to the problem is not suitable,” explained UNSW professor Kourosh Kalantar-zadeh in the release. “Therefore, keeping the solution simple and cheap is the goal if we want to ensure everyone on the planet has at least the most basic access to clean, healthy drinking water.” 

Shown is a liquid metal droplet of gallium with 0.03 mm flakes of aluminum oxide compounds grown on its surface. Each 0.03mm flake is made up of about 20,000 nano-sheets stacked together. (Image source: UNSW)

Porous Aluminum Oxide

To do this, researchers designed a filter from aluminum oxide, which is well suited to the application because it is extremely porous, he said. They also found an inexpensive, simple way to develop the filters that won’t take a lot of money or resources to reproduce, Kalantar-zadeh said.

“Because it is so porous, water flows through it very rapidly—you can use it to filter a cup of water in a matter of minutes,” he explained. “Lead and other heavy metals have an affinity to aluminum oxide, so these heavy-metal ions in the water are attracted to the aluminum oxide sheets.”

The filters are durable—able to withstand repeated use without the heavy metals entering the purified water, he added—although they will need replacing at some point. The good news about this is, “production of the filters is cheap and uses very low amounts of energy,” he said.

To produce them, researchers “put a chunk of aluminum in liquid metal of gallium at room temperature and waited for aluminum oxide layers to start forming on the surface of the liquid metal,” Kalantar-zadeh said. “When we removed the aluminum oxide layers, the gallium did not change or react, and can be used again and again, ensuring this remains a cheap process.” 

Kalantar-zadeh worked with Ali Zavabeti from RMIT Melbourne to develop the filters—the design of which they realized after seeing how the porous structure of many layers of aluminum oxide formed when experimenting with liquid gallium.

Filtering Lead from Water

“When we tested it, we pushed water contaminated with 10 times the safe level of lead in it through the filters and found the filtered water was completely safe and drinkable,” Kalantar-zadeh said. “ In our testing, we filtered contaminated water 10 times and each time, the filtered water was safe to drink.”

The team envisions the technology being put to good use in places like Africa, India, or Asia, where heavy-metal ions in the water are at levels well beyond safe human consumption. “If you've got bad quality water, you just take a gadget with one of these filters with you,” he said. “You pour the contaminated water in the top of a flask with the aluminum oxide filter. Wait two minutes, and the water that passes through the filter is now very clean water, completely drinkable. And the good thing is, this filter is cheap.”

In addition to purifying water, the invention could have other uses, Kalantar-zadeh said. Applications for electronics, materials science, and the beverage industry are just a few. “Our work with liquid metals using gallium has also enabled us to make large-scale sheets of piezoelectric materials—used in things like accelerometers in mobile phones—that can then be placed directly onto silicon chips,” he said. “Another application of the aluminum-oxide sheets is to use them in mixtures with polymers for fortifying them.”

The incorporation of aluminum-oxide sheets also can significantly reduce the gas permeation through bottles that are used for storing carbonated beverages, Kalantar-zadeh said, improving their longevity. “Currently, we can keep fizzy drinks for just a few weeks in plastic bottles before they lose their gas,” he said. “With polymers-aluminum-oxide mixtures, this lifetime can increase to years.”

Researchers published a paper on their work in the journal Advanced Functional Materials.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.

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