The term PFAS refers to per-and polyfluoroalkyl substances, which comprise thousands of synthetic chemicals that have been used in a wide variety of consumer and industrial products since the 1940s. PFAS is quickly becoming the hottest topics in the environmental field due in part to numerous State-lead initiatives, work groups, and EPA’s February 14, 2019 PFAS Action Plan. Two of the more commonly discussed PFAS, perfluorooctanoic acid (PFOA) and perfluoroctane sulfonate (PFOS), already have a 70 part per trillion (ppt) Health Advisory for drinking water and will be further evaluated by EPA to determine if a maximum contaminant level (MCL) is needed. Several states have established their own MCLs and cleanup levels for various PFAS. With these emerging contaminants headed toward increased regulation, questions regarding treatability are at the forefront. This article is intended to provide a non-exhaustive elementary overview on PFAS treatability in environmental media by commonly employed technologies.
PFAS have oil and water repellency, temperature resistance, friction reduction properties, and one of the strongest bonds in nature (fluoride-carbon). These properties all contribute to PFAS resistance to treatment. Most of the technologies that have demonstrated some level of effectiveness are associated with ex-situ (i.e., above-ground) treatment of water contaminated with PFAS. These technologies include activated carbon, reverse osmosis, and ion exchange. However, the ability to achieve a desired removal rate or effluent concentration is highly site-specific and may not be scalable. Additionally, these technologies all generate wastes that contain concentrated PFAS, which require management (likely by costly offsite thermal destruction). While these resource- and energy-intensive “pump-and-treat” technologies are often associated with the treatment of other constituents in municipal drinking water systems, the remediation industry has generally moved away from pump-and-treat for more cost-effective and sustainable in-situ (i.e., in-ground) technologies. If we are required to rely on these pump-and-treat technologies at commercial and industrial sites based purely on necessity, it would be a step backwards in the evolution of site remedies.
The traditional approaches used for in-situ treatment of groundwater and soil have shown little promise for treatment of PFAS. Like it’s ex-situ counterpart, activated carbon that is used in-situ does not on its own destroy contaminants, it merely transfer’s them from one media to another. For this reason, activated carbon is often impregnated with other material to support contaminant destruction (e.g, zero valent iron). However, the impregnated activated carbon products that are currently in commercial use have not been demonstrated to be effective on PFAS. Therefore, if activated carbon (granular-, micro-, or nano-scale) were used in-situ, the result would be a concentrated mass of PFAS, which may require removal or replenishment of the activated carbon to ensure continued treatment. Studies are ongoing to understand the long-term leachability of PFAS from various types of activated carbon and other forms of adsorbents. It’s possible that with a change in subsurface geochemical conditions, including the introduction of other competing contaminants, PFAS could desorb from the treatment media and become remobilized.
To date, there does not appear to be isolated bacterial strains that are capable of degrading PFAS. Therefore, biotic natural attenuation, biostimulation, or bioaugmentation are not amendable treatment technologies for PFAS in soil and groundwater. Some fungi strains may be capable of PFAS degradation; however, practical application to environmental media is a significant challenge. The low volatility and high solubility of PFAS prohibits the use of air sparging and soil vapor extraction (in-situ) and air-stripping (ex-situ). Various chemical oxidants are routinely used to treat contaminants in soil and groundwater. However, studies suggest that there is either no or incomplete destruction of PFAS via chemical oxidation. In a laboratory setting, when certain engineered conditions are present, specific types of oxidants may result in some degree of PFAS destruction. However, practical scale-up to a natural system has yet to be successfully demonstrated. To the best of my knowledge, excavation with landfill disposal or thermal destruction has been the only full-scale commercial technology to address PFAS in soil. The pump-and-treat technologies previously identified show promise but have yet to be employed commercially for the intended purpose of treating PFAS.
A complicating factor related to treatment of PFAS, is the presence and role of “precursors.” Precursors are included within the thousands of substances that comprise the overall PFAS families. However, unlike many of the PFAS that are currently or may be regulated by EPA and State agencies, precursors are not regulated and can readily degrade/transform biotically and abiotically under aerobic and anerobic conditions. The problem becomes that precursor degradation/transformation creates more recalcitrant PFAS, such as those within the same regulated chemical groups as PFOA and PFOS. Although this process can occur naturally, an unintended consequence of some treatment technologies is creation of additional PFAS from precursors that are present.
Government agencies, universities, work groups, and private companies are all diligently working to better understand PFAS. Like most other contaminants, there will likely not be a “silver bullet” for PFAS treatment. But hopefully, there will be new technologies developed or modifications of existing technologies that can treat environmental media contaminated with PFAS in a cost effective and sustainable manner while achieving the extremely low cleanup standards.
Published in Cox-Colvin’s May 2019 Focus on the Environment newsletter.
Nick M. Petruzzi, PE, CPG is a Principal Engineer at Cox Colvin & Associates, Inc. Mr. Petruzzi holds degrees in both geology and environmental engineering. He has been involved with numerous projects that have required the evaluation, design, construction, and operation of both established and innovative remedial alternatives for the treatment and disposal of contaminated soil and groundwater at industrial facilities. He also provides management and technical services on projects that deal with hydrogeologic investigation as well as hazardous waste, NPDES, and air permits. Mr. Petruzzi was a contributing author and instructor for the Interstate Technology and Regulatory Council (ITRC) Green and Sustainable Remediation (GSR) team, is a registered Professional Engineer in the State of Ohio, Kentucky, and Pennsylvania, and is a Certified Professional Geologist.