Highly efficient heavy metal ions filter

Highly efficient heavy metal ions filter

10:32 AM, 29th January 2016
Highly efficient heavy metal ions filter
Raffaele Mezzenga (right) and Sreenath Bolisetty examine a sample of their novel filter membrane in the laboratory.

ZURICH, SWITZERLAND: ETH researchers have developed a new water filtration system that is superior to existing systems in many respects: it is extremely efficient at removing various toxic heavy metal ions and radioactive substances from water and can even be used in gold recovery.

In November, Brazil experienced an unparalleled environmental disaster. When two dams broke at an iron ore mine, a poisonous cocktail of heavy metals was sent pouring into the Rio Doce, reaching the Atlantic some days later.

The consequences were devastating for nature and humans alike: countless fish, birds and animals died and a quarter of a million people were left without drinking water.

This case demonstrates that water pollution is one of today’s most serious global problems. No satisfactory technical solution has been found for the treatment of water contaminated with heavy metals or radioactive substances. Existing methods used to remove water from heavy metals, for example, have several disadvantages: either they are too targeted at a specific element or their filter capacity is too small; additionally, they are often too expensive.

Effective filtration of heavy metals

Now, a solution may have been found in a new type of hybrid filter membrane developed in the laboratory of Raffaele Mezzenga, prof of food and soft materials at ETH Zurich. This technology not only has an extremely simple structure, but also comprises low-cost raw materials, such as whey protein fibres and activated charcoal. Heavy metal ions can be almost completely removed from water in just a single pass through the filter membrane.

“The project is one of the most important things I might have ever done,” said Mezzenga. He and his researcher Sreenath Bolisetty were the only people to work on it and their publication has appeared in the journal Nature Nanotechnology.

Whey and activated charcoal required

At the heart of the filtration system is a new type of hybrid membrane made up of activated charcoal and tough, rigid whey protein fibres. The two components are cheap to obtain and simple to produce.

First of all, the whey proteins are denatured, which causes them to stretch, and ultimately come together in the form of amyloid fibrils. Together with activated carbon (which is also contained in medical charcoal tablets), these fibres are applied to a suitable substrate material, such as a cellulose filter paper. The carbon content is 98 percent, with a mere 2 percent made up by the protein.

Gold recovery thanks to the filter membrane

This hybrid membrane absorbs various heavy metals in a non-specific manner, including industrially relevant elements, such as lead, mercury, gold and palladium. However, it also absorbs radioactive substances, such as uranium or phosphorus-32, which are relevant in nuclear waste or certain cancer therapies, respectively.

The filtration process is extremely simple: contaminated water is drawn through the membrane by vacuum. “A sufficiently strong vacuum could be produced with a simple hand pump,” said Mezzenga, “which would allow the system to be operated without electricity.” Furthermore, the system is almost infinitely scaleable, allowing even large volumes of water to be filtered cost effectively.

As they are drawn through the filter, the toxic substances ‘stick’ primarily to the protein fibres, which have numerous binding sites where individual metal ions can dock. However, the large surface area of the activated charcoal can also absorb large quantities of toxins, which allows delaying the saturation limits of the membranes. In addition, the protein fibres lend mechanical strength to the membrane and at high temperatures allow the trapped ions to be chemically converted into valuable metallic nanoparticles.

© ETH Zurich News

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