New filtering method promises safer drinking water, improved industrial production

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The Tufts University School of Engineering developed a new filtering technique. It is based on biology and could be used to reduce a water-related disease that has affected tens of thousands of people around the world.

The researchers reported in the Proceedings of the National Academy of Sciences that their polymer membranes could separate fluoride from chlorineide and other ionselectically-charged atoms with twice the selectivity as other methods. The technology could be used to prevent fluoride poisoning in water supplies, where fluoride is naturally present at too high levels for human consumption.

Fluoride can be added to water supplies to reduce tooth decay and cavities. It is less well-known that groundwater supplies can contain high levels of fluoride, which can cause severe health problems. Fluorosis can be caused by prolonged exposure to fluoride. This condition can weaken teeth and calcify ligaments and tendons, as well as cause bone deformities. According to the World Health Organization, fluoride levels in drinking water have led to tens of millions cases of skeletal and dental fluorosis.

Fluoride can be removed with a fairly inexpensive filtering membrane. This could help protect communities against fluorosis.

Ayse Ayse Asatekin is associate professor of chemical-biological engineering at the School of Engineering. "The potential of ion selective membranes for reducing excess fluoride within drinking water supplies are very encouraging," she said. The technology's potential utility extends to other areas as well. It is simple to scale up the manufacturing process for industrial applications. It can be used as a filter for agricultural water supply, chemical waste cleanup, and chemical production.

Asatekin stated that theoretically, the process could increase yields from small geological lithium reserves for sustainable lithium battery manufacturing or uranium for nuclear power generation.

Asatekin's team was inspired to design the synthetic membranes by biology. Cell membranes allow ions to flow into and out of cells with remarkable precision. They can also regulate the internal and exterior concentrations of molecules and ions.

By lining biological ion channels with functional chemical group that are different in size, charge and affinity for water, biological ions channels provide a more selective environment for passing small ions. These channels have nanometer-sized pores that force the interaction between passing ions with these groups. The rate of passage is affected based on the strength and weakness of the interactions.

Asatekin's team created filtration membranes by coating a zwitterionic polmera polymer, in which molecular group have closely linked negative and positive charges on its surfaceonto porous supports. This creates channels that are narrower than a nanometer and surround by both water repelling and chemical groups with plus and minus-charged. The pores are very small, which allows the ions in them to interact with the water repelling and charged groups within the pores. This makes some ions pass more quickly than others. The current study focused on the composition of the polymer to determine whether it was fluoride or chloride. Researchers believe it is possible to select different ions by altering the composition.

Current filtering membranes can separate molecules by significant differences between their molecular or particle size, but they have trouble distinguishing single atoms from one another because of their small sizes and the fact that their electric charges are almost identical.

The membranes developed by Tufts researchers can, however, separate ions with atomic diameters that are only a fraction of each other even though their electric charges are almost identical.

Zwitterco, a Cambridge company that helped to fund this research, will explore the possibility of scaling up the manufacturing of the ion-separating membranes in order to test their use in industrial settings.

Learn more about Sieving ions using a polymer membrane