A new filter material may be better at straining contaminants from water than the activated carbon in your faucet filter—and may be cheaper and easier to clean, to boot. If it can be developed into a successful technology, the new material might help remove from the water supply small organic molecules such as Bisphenol A (BPA), a byproduct of some plastic manufacturing that has been linked to environmental damage and health risks.
“This was pretty exciting,” says Susan Richardson, an environmental chemist at the University of South Carolina in Columbia, who was not involved in the study. “It looks very promising. I can’t see a downside yet.”
The new material was cooked up by William Dichtel, an organic chemist at Cornell University, and colleagues. It is a β-cyclodextrin polymer, meaning it is composed of long strands of repeating molecules. The basic molecular unit consists of two main pieces: cyclodextrin (shown in blue in the figure below) and a six-carbon ring called tetrafluoroterephthalonitrile (red). Cyclodextrins are sugar molecules arranged in a ring such that the sugar’s hydroxyl groups are perpendicular to the plane of the ring. Their binding power has been exploited before, for example, to trap smelly molecules in the odor-neutralizing spray known as Febreeze. To create the filter material, scientist heated the two components in a solution of potassium carbonate and tetrahydrofuran to link the two types of molecules together in stiff chains and creating a porous 3D lattice.
The researchers then tested the new material and found that it blew the doors off of current filter materials. The scientists ran a sample of BPA-contaminated water through both the new porous cyclodextrins as well as three existing activated carbons and two nonporous cyclodextrin polymers. The porous cyclodextrin polymers reached 95% of their adsorption capacity in just 10 seconds. The other materials took at least 30 minutes to reach their maximum adsorption, and the second best performer only adsorbed 53% of its capacity in the first 10 seconds, the team reports online today in Nature. The results make the β-cyclodextrin the fastest BPA remover ever discovered, the researchers claim. The polymer sucked up a variety of other small organic molecules faster than current materials, as well.
"We knew that [water filtering] would be a likely application if we were successful,” Dichtel says. “We were definitely pleasantly surprised with just how good the performance is.”
Like carbon-based water filters, the new cyclodextrin polymers owe much of their stickiness to a large surface area. The new cyclodextrin polymers have an enormous surface area per mass: 250 m2 per gram, meaning that 2 grams of the stuff contains more than enough surface area to completely cover an NBA basketball court. However some of the existing activated carbon filters actually have even more surface area than the cyclodextrins. The real improvement comes from the properties of the material itself, Dichtel says. “The difference between our material and activated carbons is that the cyclodextrins are actually much better for binding pollutants,” he says.
The new material may even be easier to clean than activated charcoal. Whereas charcoal filters must be heated to liberate contaminant, the polymer can be regenerated simply by washing the filter in methanol at room temperature.
To top it off, β-cyclodextrin polymers are also cheap to make—a detail that surprised Richardson. The researchers estimate that the manufacturing costs may drop as low as $5-$25 per kilogram of β-cyclodextrin as chemists continue to refine the polymerization process, about half the cost of charcoal filters. “I’m thinking this is going to be something like 100 times the cost of activated carbon,” Richardson says. “But it’s cheaper, apparently, so go figure.”
Dichtel is hopeful that the new material could eventually be useful not just in commercial water filters, but industrial ones as well, especially in the developing world. The technology could also be a boon to wildlife: BPA mimics the hormone estrogen and has been wreaking havoc on fish populations for years now, causing male fish to become female in some cases. “We’ve got a big problem with estrogen of fish in our rivers,” Richardson says. “If this works and it’s cheaper than activated carbon, I could see it having promise for reducing the impact of the estrogenicity and helping our fish recover, too.”