Researchers from Australia have discovered a revolutionary and cheap way to make filters that can turn water polluted with dangerous heavy metals into safe drinking water in a matter of minutes.

Postdoctoral researcher Ali Zavabeti and a host of others at the Royal Melbourne Institute of Technology have invented a way to produce nano-filters made of an aluminum oxide compound using a process needing virtually no energy, with a catalyst of gallium.

In a paper published last week in the journal Advanced Functional Materials, the team explained that when a small piece of aluminum metal is added to a liquid alloy of gallium held at 35 degrees Celsius, layers of an oxide–hydroxide material are quickly produced in nano-sheet form which are highly porous and ideal for use in water filtration equipment. When these nano-sheets were stacked in multi-layer form, they removed both heavy metal ions and oil contamination at unprecedented ultra-fast rates.

Readers may remember an earlier column: Gallium: The Metal that Melts in Your Hand (Elko Free Press, November 2011), that described the strange properties of this rare metal. Just for the record, the other metals in the periodic table that are liquid at ordinary temperatures are mercury and cesium. Within the last decade chemists have discovered many ways of using the low melting point of gallium (85 degrees Fahrenheit), to fashion exciting new alloys, almost in the same way mercury is used to create amalgams.

There would be no blue or white LEDs if it weren’t for gallium nitride. And, you may not know it but gallium is an essential part of your cellphone. Circuit components containing the semiconducting compound gallium arsenide offer the highest communication speeds possible, often leaving old-fashioned silicon devices in the dust.

Gallium can be mixed with many transition metals to produce alloys or liquid-solid metal colloids with a wide range of interesting characteristics. Galinstan is one such example, a nontoxic eutectic alloy composed of 68.5 percent gallium, 21.5 percent indium, and 10.0 percent tin, all by weight. It is a liquid at room temperature and doesn’t freeze until the temperature is dropped to below the ice point.

When aluminum is added to the liquid galinstan, low dimensional materials in sheet form can be synthesized. In the work being discussed here, the forming of atomically thin sheets and nanofibers of boehmite (aluminum oxide-hydroxide) was easily constructed at the liquid interface. By careful processing the sheets of boehmite could be formed as thin as one atomic layer, or unit cell, and featured large surface area as a function of weight.

The scientists tout the fact that since the liquid metal alloy solvent can be used again and again, (like a catalyst it doesn’t enter the reaction), the method is low cost and can be considered green since it uses low amounts of energy to run. The recyclability of the used galinstan is of prime importance, due to the comparatively high retail price of indium and gallium. When the full boehmite has been formed the galinstan metal solvent is found to be well separated from the product, allowing the alloy to be simply reused for further reactions.

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The aluminum oxide–hydroxide sheets that are formed offer some interesting chemical, optical and mechanical characteristics. Because of the large surface area per mass and basic rigid mechanical stability of aluminum oxide, they are also capable of forming highly translucent aerogels, one of the lowest density substances ever produced.

When the two-dimensional sheets were examined for water purification applications they demonstrated high-performance removal of heavy metal ions (lead) of up to 99 percent, and oil removal from water of over 98.5 percent. Because the sheets have a high Young’s Modulus and therefore are intrinsically rigid, they are capable of supporting a tenfold improvement in water flow-through rates per unit area when compared to other filter materials such as 1 micron titanium carbide MXene.

In addition, since heavy metal ions have a high affinity to bind to alumina, the two dimensional sheets quickly grab the charged particles as fast as the water flows through.

As recorded in ScienceDaily.com, one member of the group, Professor Kalantar-zadeh, 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 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.”

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Gary Hanington is Professor Emeritus of physical science at Great Basin College and chief scientist at AHV. He can be reached at garyh@ahv.com or gary.hanington@gbcnv.edu.


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