A filtration system for heavy metals

Researchers from ETH Zurich have developed a filtration system that is very efficient at removing various toxic heavy metal ions and radioactive substances from water. Their research, published in the journal Nature Nanotechnology, could be used to combat one the most serious problems in the world today.

Until now, 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 have several disadvantages: they are too targeted at a specific element, their filter capacity is too small or they are too expensive.

Now, Professor Raffaele Mezzenga and researcher Sreenath Bolisetty have created a simple filtration system with a hybrid membrane at its heart. Heavy metal ions can be almost completely removed from water in just a single pass through the filter membrane.

The hybrid membrane is made up of activated charcoal and tough, rigid whey protein fibres — both of which are cheap to obtain and simple to produce. The whey proteins are denatured, which causes them to stretch and ultimately come together in the form of amyloid fibrils. Together with activated carbon, these fibres are applied to a suitable substrate material, such as a cellulose filter paper.

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

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.

The filtration process is very simple: contaminated water is drawn through the membrane by a vacuum, which could be something as simple as a hand pump. As they are drawn through the filter, the toxic substances ‘stick’ to the protein fibres, which have numerous binding sites. The large surface area of the activated charcoal can also absorb large quantities of toxins. In addition, the protein fibres lend mechanical strength to the membrane and at high temperatures allow the trapped ions to be chemically converted into metallic nanoparticles.

In tests with mercury chloride, the mercury concentration present in the filtrate fell by more than 99.5%. The efficiency was even higher with a toxic potassium gold cyanide compound, where 99.98% of the compound was bound to the membrane, and with lead salts, where the efficiency was larger than 99.97%. With radioactive uranium, 99.4% of the original concentration was bound during filtration.

The efficiency of the filtration system can be increased to the desirable requirements by simply increasing the protein content in the membrane, allowing even large volumes of water to be filtered cost-effectively. For example, to filter half a litre of contaminated water, the researchers used a membrane weighing just a tenth of a gram, of which 7% 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 lifetime,” said Professor Mezzenga.

Image caption: Sreenath Bolisetty and Raffaele Mezzenga examine a sample of their novel filter membrane in the laboratory. Image credit: ETH Zurich.

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