UNDERSTANDING FATE AND TRANSPORT OF PFAS TO DEVELOP EFFECTIVE MANAGEMENT PLANS
Introduction Poly- and Perfl uoroalkyl substances (PFAS) are used in a wide range of industrial applications and commercial products due to their unique oil and water repelling properties. As PFAS are extremely persistent and mobile in the environment they are being discovered in drinking water supplies above safe levels in many countries, with a drinking water supply well in Cambridgeshire recently described to have been impacted at four times the UK legal limit of 100 ng/L [1].
It’s clear that the use of all PFAS will be subject to regulatory scrutiny to determine if alternatives are available or if specifi c uses are essential.
PFAS, termed fl uorosurfactants are also major components of fi refi ghting foams used to extinguish fl ammable liquid fi res such as aqueous fi lm forming foam (AFFF) and fl uoroprotein foams. For this highly dispersive application, advancing regulations are curtailing their use, with alternative fl uorine free fi refi ghting (F3) foams demonstrating comparable extinguishment performance using large scale tests [2-4].
This article aims to describe the fate and transport of PFAS, discuss analytical tools and treatment options / uncertainties such that PFAS sources and plumes can be effectively managed.
Regulatory Focus
Drinking water standards for PFAS continue to be set at exceptionally low levels in what may be perceived as a “race to the bottom”. The concern being that as compliance levels are set so low, they are at comparable levels to those identifi ed in multiple environmental matrices as “background” detections such as rainwater. The regulatory level for perfl uorooctanoic acid (PFOA) in drinking water was recently set at 2 ng/L in Illinois [5] whilst in Denmark a 2 ng/L level has been set for the sum of PFOA, perfl uorooctane sulphonic acid (PFOS), perfl uorohexanesulphonic acid (PFHxS) and perfl uorononanoic acid (PFNA) in drinking water [6].
In the US the EPA recently released a PFAS strategic roadmap [7] which, amongst many other actions requires the EPA to set enforceable drinking water limits for certain PFAS under the Safe Drinking Water Act in the winter of 2022 and by the spring of 2022, draft a proposed rule designating certain PFAS as hazardous substances under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Whilst in October 2021 the UK the drinking inspectorate instructed water companies to assess every raw drinking water abstraction for some 47 different PFAS [8] following announcement of new lower drinking water standards for PFOS and PFOA (at 100 ng/L) in January 2021 [9]. Meanwhile European regulations on PFAS in drinking water required that the sum of 20 individual PFAS are below a 100 ng/L limit value [10].
Figure 1. Environmental Transformation of Polyfl uoroalkyl PFAS to Create Persistent PFAAs PFAS Uncloaking
The PFAS present in all fl uorinated fi refi ghting foams comprise a mixture of perfl uoroalkyl substances such as PFOS and polyfl uoroalkyl substances, with foams still currently in use being dominated by polyfl uoroalkyl substances. The polyfl uoroalkyl substances tend to be proprietary molecules which cannot be detected using conventional chemical analyses, but they biotransform in the environment, to create the detectable and regulated perfl uoroalkyl substances, such as PFOS, PFOA and shorter chain, more mobile PFAS such as perfl uorohexanoic acid (PFHxA). These perfl uoroalkyl substances are collectively termed perfl uoroalkyl acids (PFAAs) and include PFOS PFOA, PFNA, PFHxS and PFHxA. The polyfl uoroalkyl substances have been termed PFAA-precursors as they create PFAAs and the fact that these precursors remain hidden from conventional chemical analyses has lead to them being termed as “Dark Matter”. [11]. A biotransformation funnel showing the generation of PFAAs via common daughter products from a parent PFAA- precursor found in some fl uorinated fi refi ghting foams called 6:2 fl uoromercaptoalkylamido sulphonate (6:2FTSAS) is shown in Figure 1.
PFAS Source Bleeding
The PFAAs are extremely persistent in the environment, tend to be negatively charged (anionic) thus highly mobile and their mobility increases as the length of the perfl uoroalkyl chain decreases, so shorter chains can travel further in groundwater. The polyfl uoroalkyl substances can be positively charged (cationic) or have a combination of charges (zwitterionic) meaning they are retained by most soils and aquifers, thus they can remain in locations where fi refi ghting foams have been applied, such as fi re training areas. They can then represent an ongoing source of PFAAs, as charge switching can occur as these PFAA-precursors biotransform, meaning they are converted from less mobile cationic and zwitterionic forms to much more mobile anions, such as PFOS, PFOA, PFHxS, PFNA and PFHxA. The further complexity is that amphiphilic PFAS, self-assembled in bilayer structures which can stack into multiple layers, such that multi-layered coatings can be present on surfaces. Amphiphilic PFAS have been shown to concentrate on concrete surfaces which then act as source of PFAAs for decades to come.
PFAS have been shown to be associated with the air water interface, and can also be stored on the surface of groundwater [12]. So, whilst many PFAS are highly mobile and subject to long range transport, there can also be a signifi cant reservoir
IET MARCH / APRIL 2022
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