A monitoring tool that can prescreen aqueous film-forming foam (AFFF) for the contaminants perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) would be incredibly useful to industry. Ideally, the tool could be used onsite or in nonspecialised laboratories, and have high sensitivity, specificity and selectivity. CRC CARE has recently developed a smartphone app to selectively detect PFOA/PFOS down to 0.5 parts per billion (ppb), which could be this prescreening tool.
Poly- and perfluoroalkyl substances (PFAS) are a human-made family of compounds, to which PFOA and PFOS belong. PFAS generally contain multiple F–C bonds, which lead to their unique properties, such as simultaneous hydrophobicity and oleophobicity, that is not seen in other compounds.1 Consequently, PFAS have become widely used in items such as clothing, upholstery, carpeting, painted surfaces, food containers, cookware and firefighting foams.2 Unfortunately, the F–C bond is among the most stable covalent bonds known in chemistry, and is highly resistant to degradation. For example, PFOA has been reported to have a half-life of about 40 years and PFOS of about 91 years in water (USEPA 505-F-14-001). Because of their widespread use and high persistence, they are detected all over the world – from in humans to the deep sea. PFOA/PFOS have been reported to have adverse effects on humans, and have been listed as emerging contaminants and persistent organic pollutants. Thus, their monitoring is urgently needed.
Problems with current monitoring methods
Currently, high-performance liquid chromatography-mass spectrometry (HPLC-MS) is used to analyse substances for the presence of PFOA/PFOS.3 However, HPLC-MS is time-consuming, expensive (>$100/sample) and must be carried out in professional laboratories. Therefore, a mechanism that allows them to be detected by a nonspecialised laboratory or even onsite using prescreening tools before quantitative measurement is highly desirable. Recently, CRC CARE has explored several testing kits or sensors, such as astkCARE™, an improved version of methylene blue active substrates, molecular-polymer-based ion-selective electrode (ISE) and surface-enhanced Raman scattering. However, the selectivity and sensitivity have been challenged in these solutions, or cumbersome set-up or instrumentation is needed.
A colour-based assay may be the simplest way to test for PFOA/PFOS, such as that used in astkCARE, an assay previously developed by CRC CARE. Here, anionic surfactants, including PFOA/PFOS, react with the cationic dye in the test to form an ion pair. This ion pair is hydrophobic, because the hydrophilic terminate has been blocked by the dye. Consequently, this ion pair is immiscible in the aqueous phase, so that it can be extracted into a non-aqueous phase for colour justification. However, this visual detection is highly dependent on the colour justification with the colour chart provided as reference, which means it is a semi-quantitative test with a large variation. Furthermore, the visual fatigue and the interference from illumination in the background may also be problematic.4
Taking advantage of mobile technology
Mobile phones are now incredibly widespread. As of 2016, more than 1 billion iPhones alone had been sold. Modern high-tech smartphones are equipped with features such as a high-resolution camera, high-speed processor, touchscreen display, high memory capacity and long-life battery. Therefore, smartphones offer a platform to increase sensor availability and accessibility, particularly for portable sensors and kits. In addition, with the development of user-friendly smartphone apps, combined with other gadgets – such as GPS to mark the test position, network connectivity to share the information, online help and demos – smartphones could drive a new direction in sensor development for onsite testing.
App-based sensors
An app-based sensor might lose some sensitivity when compared with a more sophisticated instrument. For example, the concentration of PFOA/PFOS is usually at the ppb level, which is much lower than that of inorganic ions, such as chloride at the parts per million (ppm) level or higher. Fortunately, the possible interference from inorganic ions can be removed by solid phase extraction (SPE). Furthermore, by using SPE, PFOA/PFOS can be concentrated (by 100–1000 times in volume) to a higher concentration level (such as ppm) before the assay, to improve the testing sensitivity.
Another issue is the selectivity of the sensor. In principle, any anionic surfactant – such as sodium dodecyl benzene sulfonate (SDBS), a commonly used detergent – will demonstrate interference when it forms an immiscible ion pair and is extracted using SPE to enter the non-aqueous layer. Fortunately, F-SPE is now well established and offers a good opportunity to extract F–C skeletons because of the unique F–F interaction. In this case, only PFOA/PFOS that contain F–C chains will be extracted, whereas other anionic surfactants, including SDBS, which do not share an F–C skeleton, will not be extracted. Consequently, PFOA/PFOS will be tested without interference originating from the nonfluorinated compounds, such as SDBS.
CRC CARE has improved astkCARE by developing a smartphone app to read the PFOA/PFOS concentration with a digital output. By using a reading kit to stabilise the background illumination, the assay establishes a link between the colour and the concentration. The output concentration variation is successfully restricted to <10% in a range of 10–1000 ppb. Furthermore, CRC CARE recommends using SPE to preconcentrate the PFOA/PFOS to improve the assay’s sensitivity, followed by F-SPE to remove the interference from nonfluorinated anionic surfactants. Thus, CRC CARE’s app can detect PFOA/PPFOS with a limit of detection of 0.5 ppb, suggesting the app could be used as a prescreening tool in the common laboratory and for onsite testing.
PFAS analysis and remediation technologies
In addition to astkCARE, CRC CARE has developed several technologies for analysing or cleaning up PFAS contamination.
matCARE™
matCARE is a modified clay sorbent that irreversibly immobilises PFAS, including PFOS and PFOA, in soil and water. This technology remediates PFOS and PFOA in AFFF-contaminated wastewater to <0.002 μg/L, which is well below the US EPA’s 2016 drinking water health advisory levels of 0.07 parts per billion.
pfasCARE™
pfasCARE is a new technology developed by the CRC CARE. It uses electricity to generate extremely strong oxidising agents (including free radicals) that strip the PFAS molecules of electrons, thus breaking them down into smaller – and safer – components. Previously, this approach has been prohibitively costly, requiring expensive materials, such as diamonds, to be effective. pfasCARE uses lead peroxide (a common, inexpensive industrial material) to dramatically cut the cost of production.
References
- Buck RC, Franklin J, Berger U, Conder JM, Cousins IT, de Voogt P, et al. Perfluoroalkyl and polyfluoroalkyl substances in the environment: terminology, classification, and origins. Integrated Environmental Assessment and Management 2011;7:513–541.
- McKenzie ER, Siegrist RL, McCray JE, Higgins CP. Effects of chemical oxidants on perfluoroalkyl acid transport in one-dimensional porous media columns. Environmental Science & Technology 2015;49:1681–1689.
- Fang C, Megharaj M, Naidu R. Chemical oxidization of some AFFFs leads to the formation of 6:2FTS and 8:2FTS. Environmental Toxicology and Chemistry 2015;34:2625–2628.
- Motomizu S, Fujiwara S, Tôei K. Liquid–liquid distribution behavior of ion-pairs of triphenylmethane dye cations and their analytical applications. Analytica Chimica Acta 1981;128:185–194.