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.