Researchers from the RMIT University and University of New South Wales (UNSW) present a new filter technology that harnesses naturally occurring nanostructures that grow on liquid metals. The team also shows that their new creation can filter both oils and heavy metals from water. The filter works over 100 times faster than current ones.

Low- (a,b) and high-resolution (c,d) transmisson electron microscope photos of the filter’s layers.
Image credits Ali Zavabeti et al., 2018, AFM.

Not only is this filter way faster than its currently-available counterparts, but it’s also simple to produce and scale up, meaning that it can be deployed rapidly and en masse in areas or situation that require it.

Quick’n’clean

Water contamination and pollution are a significant threat to public health in many areas of the world today. Roughly 1 in 9 people have no clean water close to their homes, the team writes.

“Heavy metal contamination causes serious health problems and children are particularly vulnerable,” says Dr. Ali Zavabeti, a researcher at RMIT and the paper’s lead author. “Our new nano-filter is sustainable, environmentally-friendly, scalable and low cost.”

The filters rely on a unique internal architecture to perform their job. The team drew on liquid metal chemistry to grow differently shaped nano-structures (either as the atomically thin sheets used for the nano-filter or as nano-fibrous structures) in the process of creating the new filters.

Zavabeti’s team created the alloy by combining gallium-based liquid metals with aluminum. When exposed to water, aluminum oxide moves to the surface of the nano-sheets and start ‘growing’, forming aluminum oxide layers. These layers, which start out 100,000 times thinner than a strand of human hair, end up all wrinkled, making them very porous.

The shapes physically stop pollutants from passing through. Lead and other heavy metals have a very high affinity to aluminum oxide so, as the water passes through billions of layers, each one of these lead ions get attracted to one of these aluminium oxide sheets. Lab testing revealed that the nano-filter could remove lead from water even when concentration passed 13 times safe drinking levels, and was also very effective in separating oil from water.

“We’ve shown it works to remove lead and oil from water but we also know it has potential to target other common contaminants. Previous research has already shown the materials we used are effective in absorbing contaminants like mercury, sulfates and phosphates,” Zavabeti explains.

“With further development and commercial support, this new nano-filter could be a cheap and ultra-fast solution to the problem of dirty water.”

The method Zavabeti’s team developed allows them to grow the material either in nano-sheets or as nano-fibers, each with their own characteristics. The ultra-thin sheets used in the nano-filter experiments have high mechanical stiffness, while the nano-fibers are highly translucent. These two flavors of the material can be mixed to create filters with certain properties for applications in fields such as electronics, membranes, optics, and catalysis.

One particularly exciting trait of the manufacturing process is that it “be readily upscaled, the liquid metal can be reused, and the process requires only short reaction times and low temperatures,” according to Zavabeti, which should keep costs down. The manufacturing process generates no waste, requiring only aluminum and water — the liquid metals are reused for each new batch of nano-sheets.

UNSW Professor Kalantar-zadeh, paper co-author, believes the technology could be put to good use in Africa and Asia in places 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 aluminium 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.”

There are portable filtration products available that do remove heavy metals from water, but they are comparatively expensive, often costing more than $100. In contrast, the team’s aluminum oxide filters could be produced for as little as 10 cents a piece.

The paper “Green Synthesis of Low-Dimensional Aluminum Oxide Hydroxide and Oxide Using Liquid Metal Reaction Media: Ultrahigh Flux Membranes” has been published in the journal Advanced Functional Materials.