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Filter medium is effective at removing heavy metal ions

27 January 2016

An inexpensive hybrid filter medium shows great promise for the removal of heavy metal ions and radioactive substances, and even the recovery of gold.

Raffaele Mezzenga (right) and Sreenath Bolisetty examine a sample of their novel filter membrane in the laboratory (photo: ETH Zurich)

Existing methods used to remove heavy metals from water 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.

Now, there could be a solution to this problem in a new type of hybrid filter membrane developed by Professor Raffaele Mezzenga and co-researcher, Sreenath Bolisetty 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.

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.

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.

Moreover, the membrane eliminates highly toxic metal cyanides from water. This class of materials includes gold cyanide, which is used commonly in the electronics industry to produce conductor tracks on circuit boards. The membrane provides a simple way of filtering out and recovering the gold, thus the filter system could one day play an important role in gold recycling as well. “The profit generated by the recovered gold is more than 200 times the cost of the hybrid membrane,” says Mezzenga.

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

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, which would allow the system to be operated without electricity,” says Mezzenga. Furthermore, the system is almost infinitely scalable, 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.

In tests with mercury chloride, the mercury concentration present in the filtrate fell by more than 99.5 percent. The efficiency was even higher with a toxic potassium gold cyanide compound, where 99.98 percent of the compound was bound to the membrane, or with lead salts, where the efficiency was larger than 99.97 percent. And with radioactive uranium, 99.4 percent of the original concentration was bound during filtration. These high values were achieved in just a single pass.

Even over multiple passes, the hybrid membrane filters out toxic substances with a high degree of reliability. Although the mercury concentration in the filtrate increased by a factor of 10 from 0.4 ppm to 4.2 ppm after ten passes, the quantity of protein used was extremely low. To filter half a litre of contaminated water, the researchers used a membrane weighing just a 10th of a gram, of which seven percent by weight was made up of protein fibres.

“One kilo of whey protein would be enough to purify 90,000 litres of water, more than the amount of water needed in a human life time,” says Mezzenga. This also implies that the efficiency can be further increased to the desirable requirements, by simply increasing the protein content in the membrane, demonstrating the flexibility of this new approach.

An article describing this work is published in the journal, Nature Nanotechnology.

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