By now, it's no surprise that many synthetic chemicals are ubiquitous in the environment and are detectable in human blood and urine. But determining how specific chemicals end up in the environment and if they pose any serious environmental or human health risks is still far from a straightforward process.
One example is perfluorooctanoic acid (PFOA) and its homologs. The ammonium salt of PFOA, ammonium perfluorooctanoate, is used in small quantities as a surfactant to aid industrial production of widely used fluoropolymers, primarily polytetrafluoroethylene and polyvinylidine fluoride. The chemical inertness of these compounds makes them useful for a variety of consumer and industrial products, but this property also means the compounds are persistent and can bioaccumulate.
Although PFOA and its salts have low volatility and are not normally present in final manufactured products, PFOA has been detected in the blood of humans and animals worldwide, even in remote areas in the Canadian Arctic. The Environmental Protection Agency has been working to gather information from companies that produce or use PFOA or its salts to determine how these compounds have become so widespread and if they have sufficient toxicity to pose any health risk.
Following several years of intensive study, a research team led by chemistry professor Scott A. Mabury of the University of Toronto and research scientist Timothy J. Wallington of Ford Motor Co. has now pieced together a set of atmospheric degradation pathways to show that the real culprit is not PFOA, but likely polyfluorinated alcohols that convert to PFOA or other carboxylic acids in the environment [Environ. Sci. Technol., 38, 3316 (2004)].
The alcohols are part of a group of compounds known as telomers, short-chain fluorinated compounds built up from tetrafluoroethylene using a telomerization synthesis. Fluorotelomer alcohols (FTOHs) include CF3(CF2)nCF2CH2CH2OH, for example, where n = 2, 4, or 6. These compounds are known from lab studies to degrade to carboxylic acids. The alcohols are produced on a much larger scale than PFOA--some 10 million lb per year worldwide--and are used primarily as protective coatings for carpeting, fabrics, and paper.
"From a chemical and toxicological standpoint, and ultimately from a regulatory standpoint, the intermediates have much interest for their reactivity and potential biological effects," Mabury says. The alcohols may leach from the polymeric materials during manufacture or use, he adds. "Fixing the problem could be as simple as making production processes more efficient or ensuring residual material is removed from the products when they are manufactured."
EPA's concern over long-chain perfluorocarbons dates back a few years. In late 1999, EPA began to take a closer look at perfluorooctyl sulfonate (PFOS) when the agency obtained data that the compound was persistent, unexpectedly toxic, and bioaccumulative. The only U.S. maker of PFOS was 3M, which used the compound in its Scotchgard brand of stain-resistant coatings.
In May 2000, 3M announced that it would voluntarily phase out PFOS production based on data that showed the highest concentrations found in animals in the wild were approaching a level that caused adverse effects in lab animals. EPA then issued regulations to limit future manufacturing or import of PFOS.
As a follow-up, EPA expanded its investigation of perfluorinated chemicals to include PFOA and telomers. In April 2003, following input from manufacturers, EPA released a preliminary risk assessment for PFOA and its salts, citing a concern that these compounds are accumulating in the environment and that there is uncertainty over their toxicity (C&EN, June 16, 2003, page 24).
Mabury first thought about FTOHs as a possible link a few years ago following studies his lab conducted for 3M on PFOS, he says. "We hypothesized that the reason for high concentrations of these compounds in animals, such as polar bears at the top of the food chain in remote regions, is that there had to be an atmospheric component," he says.
To prove this, Mabury and coworkers had to determine the physical properties of PFOA, FTOHs, and related compounds. This required synthesis of many compounds that were not commercially available or could not be obtained from manufacturers, he notes. The researchers measured vapor pressures of FTOHs to show that they are volatile enough to transport into the atmosphere. Mabury's group had to show that the compounds could be found in the atmosphere, and that their lifetimes there--about 20 days for the C10 alcohol--are long enough for them to be transported to remote areas. The researchers followed up with monitoring studies to show that PFOA was in water and in the blood of polar bears, fish, and other animals.
COLLECTIVELY, the studies reported in more than a dozen papers during the past couple of years provided hints as to how PFOA and other perfluorinated carboxylic acids might be formed from the alcohols, Mabury notes. The last step was to determine the degradation pathway from FTOHs to the carboxylic acids, which is the subject of the current ES&T paper.
Mabury, Wallington, and coworkers carried out a series of smog chamber studies in which they oxidized FTOHs using chlorine or hydroxyl radicals under UV light. The radicals set off a chain of reactions that convert the alcohols first into an aldehyde intermediate, followed by various acyl peroxy and peroxyl species, other aldehydes, and acid fluorides. Ultimately, one part of the reaction sequence leads to formation of the carboxylic acids. The various compounds were analyzed by gas chromatography/mass spectrometry, liquid chromatography/tandem mass spectrometry, and 19F nuclear magnetic resonance spectroscopy. Mabury is now convinced that all long-chain perfluoroalkyl compounds with reactive terminal functional groups will ultimately end up as carboxylic acids in the environment.
Further work needs to be done to help pin down point sources for the alcohols and to look at related compounds, such as perfluoroalkyl sulfonamido alcohols, Mabury notes. "We would like to eventually give regulators and industry the information they need to allow the chemistry to be optimized so that these fascinating and highly important materials can continue to be used," Mabury concludes.
For now, EPA continues to work with companies that produce or use PFOA and telomers to identify possible points of release of the compounds to the environment during consumer use or end-of-life processing, such as incineration. EPA's full risk assessment on PFOA is expected to be available for public review next fall. Following evaluation by an EPA advisory committee, a determination will be made to seek voluntary or regulatory control of these compounds, if deemed appropriate.