Dr Ian Ross of professional services firm Tetra Tech, and an editor of the Emerging Contaminants Handbook, discusses the emergence of PFAS as contaminants of concern in the UK
Poly- and perfluoroalkyl substances (PFAS) are a large group of emerging contaminants that have been used in a wide array of commercial goods and products since the 1940s. They are thermally stable and repel oils and water with impressive surface tension levelling properties. Given their wide field of application, all kinds of sites can be impacted by PFAS and potentially pose a risk to water bodies or drinking water supplies. Some of these are listed in the image caption, opposite.
As PFAS show no sign of biodegradation at all, they have been described as “forever chemicals”, enduring permanently in the environment1. They are generally water soluble and hence very mobile, meaning they can be transported with groundwater or in surface waters well beyond the original location where they were lost to ground, termed a source area.
As many PFAS are surfactants they can also coat soils and concrete surfaces at source areas, exhibiting multi-layered waterproofing effects as PFAS storage zones, where they slowly release because of rainfall. This means PFAS can potentially continue to leach from source zone soils and concrete surfaces for decades2, 3 potentially impacting immense volumes of groundwater and forming large plumes or continuing to bleed to surface waters.
PFAS may then pose a risk of harm, depending on the site setting (i.e. the topography, geology, hydrology and hydrogeology), coupled with the location of drinking water supply wells, crop spray irrigation, surface waters and proximity of businesses using water4, 5.
Why a problem?
Increasing attention to the environmental and human health effects of PFAS is leading to the development of increasingly conservative (low) regulatory levels for PFAS in drinking water, soil, groundwater, sediments and surface waters4, 6-9. Regulations have generally focused on perfluoroalkyl substances, termed perfluoroalkyl acids (PFAAs) as opposed to the larger class of polyfluoroalkyl substances, which form PFAAs via biotransformation processes in the environment (and hence termed PFAA precursors). The majority of commercial products contain these proprietary polyfluorinated precursor compounds, which evade detection by conventional analytical methods which focus on detecting the PFAAs. To comprehensivley detect PFAS in products such as firefighting foams and in the environment in soils and waters, the total oxidisable precursor (TOP) assay is required.
The “long chain” PFAS, including PFOS (perfluorooctanesulphonic acid), PFOA (perfluorooctanoic acid) and PFHxS (perflurohexanesulfonic acid), are subject to the majority of current environmental regulations. These PFAS accumulate in humans through consumption of impacted food and drinking water. They are not readily excreted so concentrate in human tissues and are often associated with proteins, so can be detected in the blood.
Following the phasing out of PFOS production in the US and Europe from 2003, an increasing number of alternative ‘regrettable’ replacement PFAS have been synthesized for multiple applications. These comprise “short chain” varieties (often termed C6 or C4), alternatives such as perflurooalkyl phosphinic or phosphonic acids and perfluoroalkyl ethers (such as GenX). So whilst an understanding of their toxicology and bioaccumulation potential is evolving, there is some evidence that stable intermediates in the biotransformation of short-chain PFAS bioaccumulate in rats and marine invertebrates whilst other short chain PFAS bioaccumulate in the edible portion of crops. The short chain replacements are significantly more mobile in the environment than the long-chained varieties and more difficult to remove from water, making them a potentially larger threat.
Given growing evidence of human health risks and potential ecological harm, more and more countries are now regulating an increasing number of PFAS including both long and short chain varieties, while the latter are still commonly used as commercial replacements (e.g. C6 in firefighting foams).
Some PFAS have been an ingredient in “film forming” Class B firefighting foams used to extinguish liquid hydrocarbon fires. Since the late 1960s foams have been used for fire training and in sprinkler-based fire suppressant systems.
All fluorinated firefighting foams characterised to date contain polyfluorinated precursors to the regulated PFAS, so conventional analysis such as the use of EPA methods 533 and 537.1 cannot assess the concentrations of principle PFAS in most fluorinated foams4, 10. The ‘C6’ firefighting foams tend to contain PFAS undetectable by conventional commercial analysis11, which are converted by microbial action in soil and groundwater to the regulated PFAS12. There is evidence to show that these bioactive precursors are 10,000-fold more toxic than their perfluoroalkyl daughter products13, meaning it’s likely the precursors in aqueous film-forming foam (AFFF) will drive assessment of how harmful these fluorosurfactants are.
An increasing number of PFAS, including both long chain (C8) and shorter chain (C6, C4 etc.), are regulated in drinking water, surface waters, soils and groundwater. Also, the use of firefighting foams containing PFAS is being curtailed in multiple jurisdictions, with many foam users having transitioned to fluorine-free (F3) foams14, 15, whose extinguishing performance has been proven in International Civil Aviation Organization (ICAO) tests since 200216. As a result of the transition to F3 foams an increasing volume of fluorinated firefighting foams require treatment, with traditional disposal options being challenged, a series of alternative technologies are being developed.
Decontaminating fire suppression systems
Finding effective decontamination methods for fire suppression systems is challenging as fluorosurfactant PFAS adhere to surfaces and can form multiple layers. This means the interior of fire suppression systems can be coated with a significant mass of PFAS that cannot be removed with repeated water rinses. This can result in significant rebound of PFAS into F3 foams, raising regulatory concerns and potentially negating the benefit of transitioning to PFAS-free foams. Fortunately, decontamination of fire suppression equipment is achievable with some recently developed approaches so replacement of costly systems is not required.
Globally, the regulatory net is tightening, bringing down the acceptable level of contamination and going beyond the traditional focus on PFAAs. Environmental regulators’ heightened focus on PFAS has been apparent in the last five years. In 2016, the detection of PFAS in drinking water in the USA took a step forward. The US Environmental Protection Agency (US EPA) issued a long-term health advisory level of 70 ng/L (for combination of PFOS and/or PFOA)17, which led to 6.5 million people’s drinking water being considered unsafe18.
Up to this point, drinking water supplies in the US had been appraised by a program called the third unregulated contaminant monitoring rule (UCMR3). This assessed all major and some minor water supplies to determine the concentrations of 6 PFAS and 15 other emerging contaminants.
The recent release of new UK drinking water quality guidance for PFOS and PFOA may have an impact on contaminated site management and will mean that a significantly larger volume of UK drinking water requires treatment to remove PFAS.
The new drinking water standards follow a tiered system where the concentration of PFOS or PFOA requiring monitoring is set at 10 ng/L (previously 300 ng/L). The concentration requiring treatment, given the potential danger to human health, is set at 100 ng/L for PFOS or PFOA (down from 1,000 ng/L PFOS and 5,000 ng/L PFOA)19, 20. Given the widespread potential sources of PFAS to aquifers and surface waters which are used for drinking water, a UK national program of monitoring for PFAS and other unregulated contaminants, similar to the US UCMR program appears warranted. The forthcoming fifth UCMR round of analysis in the US (UCMR5), assesses drinking water supplies for some 29 PFAS.
There is an immense amount of research focussed on developing treatment technologies to address PFAS. This includes remediation using chemical, physical and biological approaches. Treatment and clean-up solutions are likely to comprise multiple technologies working in tandem. Techniques which remove PFAS from water effectively can be applied first, for example, where there are high volumes of to-treat liquid and low PFAS concentrations. Then a destructive technology could be applied to more cost effectively destroy the PFAS which has been concentrated by the first treatment technology.
Biological approaches to mineralise PFAS (i.e. convert it to innocuous carbon dioxide and fluoride) appear highly unlikely to be developed for use in situ in soil and groundwater, as a result of the perfluoroalkyl group being a true xenobiotic, whose arrival on the planet is relatively recent (100 years ago). PFAS is also extremely stable, having persisted in the environment since the 1960s21-24.
Thermal technologies to address impacted soil and waters will meet with regulatory questions regarding emissions of organofluorine compounds, and there may be residual PFAS in any ashes formed, presenting similar issues to those being encountered with incineration25, 26. Proving that PFAS are destroyed without creating emissions may be critical to securing regulatory approval.
Foam fractionation is an innovative approach to the selective removal of PFAS from aqueous wastes as a concentrated foam. Fluorosurfactants adhere to gas bubbles, which rise to the water surface in a column, and form a foam which can be separated off27. The separated foam comprises a liquid concentrate, which comprises a PFAS concentrate, with treated water passing through multiple columns to effectively remove PFAS28. The concentrated waste can then be treated with technologies such as sonolysis, electrochemical oxidation, plasma or supercritical water oxidation, which are all evolving as potentially effective destruction approaches.
An alternative approach requires an ozone generator to add the chemical oxidant, ozone, in a process termed ozofractionation29, 30. Major drawbacks arise with this approach as a result of using ozone, which is a chemical oxidant. Ozone converts PFAA precursors to short and ultrashort chain PFAS which are much more difficult to remove from water. So, using ozone will create additional PFAS that are not amenable to removal by fractionation. If air was used then these PFAS could be effectively treated. This issue is especially pertinent to treating waste from AFFF use31 and landfill leachate32, which are commonly dominated by precursors.
As remediation technologies are researched and scaled for commercial application there will be many opportunities to establish the efficacy of different approaches in laboratory and field environments. Critical thinking combined with a detailed understanding of PFAS behaviour will be essential to delivering successful remedies.
Who can help?
Tetra Tech has a long history of managing PFAS, with staff working to characterise and develop treatment approaches since 2005. Our strength is centred on our knowledge of complex PFAS chemistry combined with significant expertise in environmental risk assessment and our long-standing involvement with research and development of remedial technologies. A large part of what we do is focused on cost-effective advocacy strategies, leveraging environmental risk assessment and modelling to establish the impact on human health and ecology, and whether it is actionable. Tetra Tech has developed decontamination solutions for PFAS-impacted infrastructure and has relationships that support the deployment of treatment technologies.