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New material could filter water contaminants that others miss

Chemicals from nonstick manufacturing have been difficult to clean up

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4:23pm, June 20, 2017
aviation fire

TOXIC FOAM  The foam used to put out aviation fires often contains toxic perfluorooctanoic acid and related molecules. When these chemicals get into the water supply, they linger for years without breaking down. A new material may offer a better way to filter them out.

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A new material can pull a toxic, hard-to-degrade industrial chemical from drinking water more effectively than current methods.

Perfluorooctanoic acid, or PFOA, hangs around in the environment for years and might cause health problems for people and animals. A new polymer material traps PFOA molecules, making them easy to filter out of water, researchers report in the June 14 Journal of the American Chemical Society

PFOA is one of several fluorine-containing molecules that have come under scrutiny in recent years. Known as perfluorinated compounds, the chemicals have been used to manufacture nonstick coatings on cookware and are in fire-suppressing foams used by the military and aviation industry.

Many manufacturers are phasing out PFOA and its chemical cousins, substituting less harmful molecules, but the problem hasn’t gone away. Once they’re in groundwater or rivers, the substances can linger for years without breaking down, thanks to their strong fluorine-carbon bonds.

“These compounds are very possibly some of the most persistent organic chemicals we've made to date,” says Christopher Higgins, an environmental engineer at the Colorado School of Mines in Golden who wasn’t part of the study. “They don't degrade under any meaningful timescales in the natural environment.”

PFOA exposure increases the risk of certain tumors in rodents, though it’s unclear whether it raises the risk of cancer in humans. Studies in humans have linked the chemical to thyroid disease (SN Online: 1/22/10), immune deficiencies and problems for developing fetuses. Because PFOA can build up in the human body over time, the Environmental Protection Agency’s recommended exposure limit is low — just 0.07 parts per billion in drinking water. But many municipal water systems around the country have levels far higher than that, recent studies have shown.

A common way to remove the substance from water is to use a PFOA-attracting material in a filter. PFOA molecules accumulate on the material’s surface, making for easy removal. Activated carbon is widely used to filter water in this way, but PFOA molecules are only moderately attracted to the material. So some PFOA molecules get left behind, says study coauthor William Dichtel, a chemist at Northwestern University in Evanston, Ill.

“We're finding negative health effects from being exposed to them even when they're in trace concentrations,” Dichtel says. “It's becoming clear that we need to remove them almost completely.”

So Dichtel and his team designed a material that appears to work better. It’s a polymer made of repeated units of two different molecules linked together: a giant ring-shaped molecule called beta-cyclodextrin that’s made from cornstarch, and a fluorine-rich molecule called decafluorobiphenyl, or DFB.  The fluorine in DFB makes the polymer more attractive to fluorine-containing PFOA molecules, helping them to stick better, Dichtel says.

Unlike activated carbon, which is a “grab all,” this new material is specifically attractive to these hard-to-get fluorine-containing molecules, says Karen Wooley, a chemist at Texas A&M University in College Station who wasn’t part of the study.

In tests, the polymer removed 95 percent of PFOA from water; activated carbon removed only a little over 50 percent. The polymer kept working even alongside common chemicals in the water that often dampen activated carbon’s effectiveness. Another benefit: The polymer is reusable — washing with methanol removed the PFOA, leaving the polymer ready for another round of filtration.

Dichtel and colleagues are working to commercialize this polymer and others like it so such materials could someday be used for home or industrial water filtration.

Other perfluorinated molecules are a concern, too, Higgins says. When he and colleagues examined a drinking water supply contaminated with firefighting foam residue, they found PFOA — and nearly 30 other perfluorinated molecules, nearly half of which were only recently discovered. In contrast to the new study, Higgins’ team found that activated carbon did a decent job of removing PFOA. But the material was substantially less effective for these under-the radar molecules that could have similar environmental and health impacts, the team reports in the June 6 Environmental Science and Technology.

When it comes to getting PFOA and its relatives out of water supplies, “we need all the help we can get,” he says. “This is an effort to do something quite new.”

Citations

L. Xiao et al. β-cyclodextrin polymer network sequesters perfluorooctanoic acid at environmentally relevant concentrations. Journal of the American Chemical Society. Vol. 139, June 14, 2017, p. 7689. doi: 10.1021/jacs.7b02381.

X. Xiao et al. Sorption of poly- and perfluoroalkyl substances (PFASs) relevant to aqueous film-forming foam (AFFF)-impacted groundwater by biochars and activated carbon. Environmental Science and Technology. Vol. 51, June 6, 2017, p. 6342. doi: 10.1021/acs.est.7b00970.

X. C. Hu et al. Detection of poly- and perfluoroalkyl substances (PFASs) in U.S. drinking water linked to industrial sites, military fire training areas, and wastewater treatment plants. Environmental Science and Technology Letters. Vol. 3, October 11, 2016, p. 344. doi: 10.1021/acs.estlett.6b00260.

Further Reading

J. Raloff. ‘Nonstick’ pollutants may cut efficacy of vaccines in kids. Science News. Vol. 181, March 10, 2012, p. 15.

R. Ehrenberg. Common stain repellant linked to thyroid disease. Science News Online, January 22, 2010.

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