Limitations of existing methods used to remove heavy metals from water – such as high cost, pollutant specificity or insufficient filtration capacity -- may be overcome by a hybrid filter membrane developed at ETH Zurich. The membrane is composed of relatively inexpensive activated charcoal and denatured whey proteins 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. The filtration process is extremely simple: contaminated water is drawn through the membrane by vacuum, and metal ions can be almost completely eliminated from water in a single pass. Furthermore, the system is almost infinitely scalable, allowing even large volumes of water to be filtered cost effectively.

Contaminated water is drawn through the membrane by vacuum and metal ions can be almost completely eliminated from water in a single pass.Various heavy metals are absorbed in a non-specific manner, including industrially relevant elements, such as lead, mercury, gold and palladium. Radioactive substances, such as uranium or phosphorus-32, which are relevant in nuclear waste or certain cancer therapies, respectively, are also removed from water.

The researchers note that the membrane eliminates highly toxic metal cyanides from water. This class of materials includes gold cyanide, which is used in the electronics industry to produce conductor tracks on circuit boards. The device provides a simple way of filtering out and recovering the gold, and could one day play an important role in gold recycling as well.

In tests with mercury chloride, the mercury concentration present in the filtrate declined 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 greater than 99.97 percent. And with radioactive uranium, 99.4 percent of the original concentration was bound during filtration.