January 25, 2016

Highly efficient heavy metal ions filter

The contaminated water (coloured water in vials) is drawn through
the hybrid membrane by negative pressure; the heavy metal ions (red spheres)
bind to the protein fibres in the process. The filtered water is of drinking quality.
(Graphics: Bolisetty & Mezzenga, Nature Nanotechnology, 2016)

(January 25, 2016)  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, Professor 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.

Gold removed and recovered from polluted water.
(Photograph: ETH Zurich/R. Mezzenga/S. Bolisetty)

“The project is one of the most important things I might have ever done,” says Mezzenga, enthusing about the new development. He and his researcher Sreenath Bolisetty were the only people to work on it, and their publication has just 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%, with a mere 2% 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.

journal reference >>