Aerosols complicate PFOA picture

Riding the waves—could the oceans be a source of PFOA to the air?

A largely discounted wrinkle in the chemistry of perfluorooctanoic acid (PFOA) indicates that this ubiquitous chemical moves into the air more readily than previously thought, according to new research in ES&T (DOI: 10.1021/es7032026). Experts say that the findings could have implications for PFOA's fate in the environment, but they caution against extrapolating these laboratory results to the real world without field-based verification.

PFOA is the best known of the perfluoroalkyl carboxylates. A U.S. EPA Science Advisory Board advised the agency in 2006 to classify it as a likely human carcinogen (PDF Size 300 KB). Animal studies indicate that perfluorinated chemicals affect the liver, neonatal development, the immune system, and hormone levels. Until recently, PFOA had been widely used in stain repellents, polishes, and paper coatings. Manufacturers have begun voluntary efforts to eliminate its use.

Throughout this decade, scientists interested in the widespread distribution of PFOA and other perfluorochemicals have assumed that when PFOA enters bodies of water it stays there. This is because the predominant form of PFOA in water is PFO^–, an ionized species and potent surfactant with negligible vapor pressure.

Now, chemist David Ellis at Trent University (Canada) and colleagues have turned this thinking on its head. From lab experiments with high concentrations of PFOA, they report that aerosols can concentrate PFO^– on the surfaces of bubbles, which loft the aerosols into the air. Once airborne, some of the PFO^– converts to PFOA, which then moves into the atmosphere. The scientists estimate that this PFOA should stay airborne long enough to travel great distances.

The collaborators used an ultrasonic aerosol generator to create a fine aerosol mist in a sealed vessel. In a series of experiments funded by DuPont, they spiked deionized water and samples from a river, a lake, and the ocean with PFOA in concentrations ranging from 200 to 800 parts per billion. Once they analyzed the condensed aerosols, they found enrichment in PFO^– and PFOA as a gas phase in the vessel. The aerosols formed in ocean water had the highest levels of enrichment, as much as 55-fold.

Ellis and colleagues' work helps explain PFOA movement, even if the experimental concentrations are about 1 million times greater than what is found in the environment, notes oceanographic chemist Nobuyoshi Yamashita of Japan's National Institute of Advanced Industrial Science and Technology. "The high concentration is a limitation, but [it's] still useful as a preliminary trial," he says.

Ellis notes that the experiment has direct relevance to the marine environment, where waves and wind kick up sea spray, a natural marine aerosol. As a surfactant, PFO^– should concentrate on the surface of the water. The first measurements confirming this surface concentration were published by Xiaodong Ju at Dalian University of Technology in China (Environ. Sci. Technol. 2008, DOI 10.1021/es703006d). Ellis concludes that the PFO^– should be further concentrated in the sea spray aerosols that, once airborne, should release some PFOA into the atmosphere.

Aerosolization is a potentially important route of loss of PFOA from aquatic systems, says Environment Canada chemist Derek Muir. "This process may help explain the global distribution of PFOA and other perfluorocarbons, because the results of this lab study suggest that PFOA will be transported in the gas phase and also continuously exchanged between water and the atmosphere via aerosol release," he says.

Ellis and colleagues also speculate that this process could muddy one of the environmental signatures that can be used to tell the difference between PFOA produced with an electrochemical process (used by 3M) and that manufactured with a different telomer process (used by DuPont).

PFOA produced with the telomer process consists almost entirely of straight-chain isomers, and research to date suggests that these isomers are dominant in the Arctic. The electrochemical process results in a mixture of branched- and straight-chain isomers. Ellis and colleagues speculate that because of the differences in physical properties, the branched isomers will tend to concentrate in water bodies such as the ocean and even in rain, whereas linear isotopes will fractionate into the air.

The fractionation hypothesis is relatively easy to test, says environmental chemist Scott Mabury at the University of Toronto. If isomeric fractionation takes place, then "bulk rainwater, from temperate regions, should be enriched in branched isomers," he says. "This is a straightforward measurement to make and would directly shed light on the overall potential significance of gas-phase transport of PFOA," Mabury adds.

Muir agrees that Ellis and colleagues' paper presents fertile ground for field studies. "I'm sure environmental chemists will immediately begin thinking of ways to confirm whether this is important in the ambient environment. The lab study offers strong evidence, but the next step is to confirm predictions with environmental measurements," he says. —REBECCA RENNER