The contaminated water (colored water in vials) is drawn through the hybrid membrane by negative pressure; the heavy metal ions (red spheres) bind to the protein fibers in the process. The filtered water is of drinking quality. Credit: Graphics copyright: Bolisetty & Mezzenga, Nature Nanotechnology, 2016
Water pollution is one of today’s most serious global problems. A new water filtration system has been developed that is superior to existing systems at removing various toxic heavy metal ions and radioactive substances from water and can even be used in gold recovery. Prof. Raffaele Mezzenga’s hybrid filter membrane not only has an extremely simple structure, but uses 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.
They whey proteins are denatured, which causes them to stretch, and ultimately come together in the form of amyloid fibrils. Together with activated carbon (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%, and a mere 2% of protein.
This hybrid membrane absorbs various heavy metals in a non-specific manner, including industrially relevant elements, eg 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. It also eliminates highly toxic metal cyanides from water. This class includes gold cyanide, 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.
The filtration process is simple: contaminated water is drawn through the membrane by vacuum. “A sufficiently strong vacuum could be produced with a simple hand pump,” says Mezzenga, “which would allow the system to be operated without electricity.” It 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, protein fibres lend mechanical strength to the membrane and at high temps allow trapped ions to be chemically converted into valuable metallic nanoparticles.
In tests with mercury chloride, for example, 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, or with lead salts, where the efficiency was larger than 99.97%. And with radioactive uranium, 99.4% of the original concentration was bound during filtration. “We achieved these high values in just a single pass,” emphasises Bolisetty.
“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 the ETH professor. This also implies that the efficiency can be further increased to the desirable requirements, by simply increasing the protein content in the membrane, he adds, emphasizing the flexibility of this new approach.
Mezzenga is confident that his technology will find its way onto the market. “There are numerous applications for it, and water is one of the most pressing problems we face today,” he says in light of the torrent of mud experienced in Brazil. The ETH professor has patented his technology and was nominated in March this year for ETH Zurich’s Spark Award. However, because the scientific publication had to undergo a nine-month review process, only now can Bolisetty and Mezzenga make public their discovery. https://www.ethz.ch/en/news-and-events/eth-news/news/2016/01/highly-efficient-heavy-metal-ions-filter.html
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