New filtering method inspired by biology
Tufts University scientists have developed a filtering technology that could help curb a drinking water-related disease that affects tens of millions of people worldwide and potentially improve a range of industrial processes.
It is well known that adding fluoride to a water supply can reduce the incidence of tooth decay, including cavities. Less well known is the fact that some groundwater supplies have such high natural levels of fluoride that they can lead to severe health problems, with prolonged exposure to excess fluoride causing fluorosis — a condition that can actually weaken the teeth, calcify tendons and ligaments, and lead to bone deformities.
The ability to remove fluoride with a relatively inexpensive filtering membrane could protect communities from fluorosis without requiring the use of high-pressure filtration or having to completely remove all components and then re-mineralise the drinking water. In designing such membranes, the Tufts team were inspired by biology.
Cell membranes are remarkably selective in allowing the passage of ions (electrically charged atoms) into and out of the cell, and they can even regulate the internal and external concentrations of ions and molecules with great precision. Biological ion channels create a more selective environment for the passage of these small ions by lining the channels with functional chemical groups that have different sizes and charges and different affinity for water. The interaction between the passing ions and these groups is forced by the nanometre dimensions of the channel pores, and the rate of passage is affected by the strength or weakness of the interactions.
The filtration membranes were designed by coating a zwitterionic polymer — a polymer in which molecular groups contain closely linked positive and negative charges on their surface — onto a porous support, creating membranes with channels narrower than a nanometre surrounded by both water-repelling and plus- and minus-charged chemical groups. As with biological channels, the very small size of the pores forces the ions to interact with the charged and water-repelling groups in the pores, allowing some ions to pass much faster than others.
While other filtering membranes have difficulty distinguishing single-atom ions from each other, the Tufts membranes are capable of separating ions that differ by only a fraction of their atomic diameter — even when their electric charges are nearly identical. Indeed, the researchers found that their polymer membranes could separate fluoride from chloride and other ions with twice the selectivity reported by other methods, as reported in Proceedings of the National Academy of Sciences. The composition of the polymer was made to target the selection of fluoride vs chloride, but by altering the composition of the zwitterionic polymer it should be possible to target the selection of different ions, they said.
“The potential for ion-selective membranes to reduce excess fluoride in drinking water supplies is very encouraging,” said team leader Associate Professor Ayse Asatekin. “But the technology’s potential usefulness extends beyond drinking water to other challenges. The method we used to manufacture the membranes is easy to scale up for industrial applications. And because the implementation as a filter can also be relatively simple, low cost and environmentally sustainable, it could have wide applications to improving agricultural water supplies, cleaning up chemical waste and improving chemical production.”
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