Switzerland-based university ETH Zurich announced last month that its researchers had developed a new water filtration system that efficiently removes a number of toxic heavy metal ions and radioactive substances from water; it can also be used in gold recovery.
The new hybrid filter membrane, developed in the laboratory of ETH Zurich food and soft materials professor Raffaele Mezzenga, comprises low-cost raw materials such as whey protein fibres and activated charcoal. Heavy metal ions are almost completely removed from water on passing through the filter membrane.
Mezzenga and fellow ETH Zurich researcher Sreenath Bolisetty were the researchers working on the membrane filter.
Activated charcoal and tough, rigid whey protein fibres make up the heart of the hybrid membrane filtration system. The two components are cheap to obtain and simple to produce.
The whey proteins are denatured, causing them to stretch and, subsequently, come together in the form of amyloid fibrils. Together with activated carbon, commonly found in medical charcoal tablets, these fibres are applied to a suitable substrate material such as a cellulose filter paper. The filter paper comprises 98% carbon and 2% protein.
The hybrid membrane absorbs various heavy metals in a nonspecific 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 isolates highly toxic metal cyanides in the 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 and recovering the gold. “The profit generated by the recovered gold is more than 200 times the cost of the hybrid membrane,” Mezzenga asserts.
The filtration process involves contaminated water being drawn through the membrane using a vacuum. “A sufficiently strong vacuum could be produced using a simple handpump, which would allow for the system to be operated without electricity,” he explains.
Further, the system is scalable, allowing for the cost-effective filtering of larger volumes of water.
As the contaminated water is drawn through the filter, the toxic substances cling 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 for the saturation limits of the membranes to be delayed.
In addition, the protein fibres provide mechanical strength for the membrane and, at high temperatures, allow for the trapped ions to be chemically converted into valuable metallic nanoparticles.
In tests using mercury chloride, the mercury concentration in the membrane filtrate dropped by more than 99.5%.
The efficiency was even higher when a toxic potassium gold cyanide compound was used, where 99.98% of the compound was bound to the membrane. The efficiency was greater than 99.97% when lead salts were used. 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.
During multiple passes, the hybrid membrane removes toxic substances with a high degree of reliability. Although the mercury concentration in the filtrate increased by a factor of 10, from 0.4 parts per million (ppm) to 4.2 ppm after ten passes, the quantity of protein used was extremely low.
To filter 0.5 ℓ of contaminated water, the researchers used a membrane weighing just 0.1 g, with 7% of the weight comprising protein fibres. “One [kilogram] of whey protein would be enough to purify 90 000 ℓ of water, more than the amount of water needed in a human life time,” says Mezzenga.
He adds that this implies that the filtration system’s efficiency can be increased through a higher protein content in the membrane, highlighting the flexibility of this new solution.