Quantification of per- and polyfluoroalkyl compounds (PFAS) in flue gasses: experiences from the Belgian frontline

“What are PFAS?”

PFAS comprise a huge family (>10 000) of man-made chemicals consisting of poly- (partially) and per- (fully) fluorinated alkyl compounds (PFAS), produced since 1940 (EEA, 2019, EPA, 2020, Abunada et al., 2020, OECD, 2018). Most well-known chemicals include PFOA, PFOS and GenX. Thanks to their unique water- and grease-repellent properties and thermal, biological and chemical inertness, PFAS are applied in a wide range of industrial applications and consumer products resulting in the widespread prevalence and bioaccumulation of these compounds in consumer products, food chains and our environment (Groffen et al., 2023, Winchell et al., 2021, Abunada et al., 2020, Rauert et al., 2018, Giesy and Kannan, 2001, Buck et al., 2011, Brunn et al., 2023). Past research and legislation focused primarily on food, soil and water prevalence of PFAS, resulting in strategic roadmaps and legislative actions, but the air compartment can today still be considered as a black box in terms of PFAS composition, concentration ranges and exposure routes (D’Ambro et al., 2023, D’Ambro et al., 2021, Lin et al., 2020, Brunn et al., 2023).

“PFAS in air?”

To date, there is no reference methodology available to characterize PFAS in the air or emissions, nor advisory guideline values or limit values for air. In response to findings of elevated PFAS levels in different environmental media, the Flemish Institute for Technological research (VITO) and reference laboratory in Flanders, Belgium, was asked by the government in 2021 to develop methods for the quantification of PFAS in ambient air, depositions and emissions. With regard to PFAS emissions, VITO gained experience with the OTM-45 methodology, published by US EPA (EPA, 2021), in combination with the existing compendium method for the quantification of PFAS via LC-MS/MS in water (WAC/IV/A/025 (VITO, 2022). VITO conducted stack emission monitoring for initially 38 and later 50 individual PFAS compounds on various stacks and industries in Flanders and gained experience on prevailing concentration levels of PFAS, compositional fingerprints and potential impacts from flue gas treatment technologies. In collaboration with commercial laboratories in Flanders, VITO adapted the OTM-45 methodology and validated the proposed compendium method in order to arrive at a scientifically sound and supported methodology for the quantification of PFAS (>C4, boiling point >100°C) in ducted gas streams.
This knowledge resulted in the development of a Belgian compendium method for the sampling and quantification of per- and polyfluoroalkyl substances (PFAS) in a flue gasses (LUC/VI/003), and provided valuable insights in prevailing emission concentration levels and PFAS fingerprints of a variety of stacks and industries in Flanders, Belgium.

“How to measure PFAS?”

The broad chemical diversity of PFAS challenges both the sampling and analytical procedures required to collect and characterize PFAS, their intermediates and breakdown products in different environmental media (Brunn et al., 2023, Smith et al., 2024). The scope of our study was on the quantitative characterization of a set of well-know semi-volatile and particle bound PFAS (C4-18, boiling point>100°C). In order to gain practical experience with the OTM-45 methodology and evaluate the reproducibility of the technique, a dedicated monitoring campaign with repeated sampling was set up at a rotary kiln of Indaver, a waste treatment plant located in Antwerp, Belgium. Indaver processes approximately 750 000 tons of hazardous and industrial waste per year, of which 150 000 tons by means of high-temperature incineration in rotary kilns guaranteeing thermal destruction of hazardous waste under high temperature (>950°C), long residence times (30 min to 1 hour) and proper turbulence (rotating drum) of the material. A dedicated flue gas treatment includes an electrostatic precipitator (dust removal), a four-step wet gas washing with various chemical flows (gaseous compounds), and an active carbon filter (organic compounds like dioxins and furans) before flue gasses are released to the atmosphere via a chimney.
A PFAS emission measurement includes 3 hours (~2Nm³) of isokinetic sampling from the stack by means of a heated sampling probe. The flue gas is guided over a fiberglass filter, cooled in a condenser and guided over a primary adsorbent (XAD-2) module. The adsorbent module is followed by a condensate flask, impinger train and a secondary (breakthrough) adsorbent module to evaluate potential breakthrough of the PFAS sampling train (EPA, 2021). Chimney emissions were sampled on 13 different days between December, 2021, and May, 2023, using a sampling train setup as described by OTM-45 and LUC/VI/003 (Figure 1). The procedure considers several QA/QC steps and controls for potential contamination by means of medium, field and post-rinse blanks.
The collected field samples are aggregated in 6 analytical fractions, extracted and subsequently analyzed via Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS) in the Multiple Reaction Monitoring (MRM) detection mode. The first chimney measurements (December 2021) aimed at evaluating the practical feasibility of the OTM-45 method. After testing the practical feasibility, further incremental improvements and simplifications of the monitoring setup and analytical procedure were tested. Indaver collaborated in this effort by opening up their facilities for development and optimization purposes of the compendium method.
Validation of the final LUC/VI/003 OTM-45 sampling train for 50 individual PFAS compounds was conducted in 2023, by means of 3 hour sampling of ambient air at a background location by means of 3 spiked sampling trains (50 native compounds spiked in filter, XAD-2 and water) evaluating native spike recovery and measurement uncertainty based on the observed bias of the 3 measurements (Figure 2). In February 2024, an interlaboratory comparison (ILC) was performed to evaluate the equivalence of the sampling train variants, by means of preconditioned (temperature and humidity) air sampling with duplo native spiked (50 compounds) sampling train variants considered in the LUC/VI/003 compendium method (Figure 2).   

“Results and field experiences”

Repeated stack measurements at Indaver showed that physicochemical properties (chain length, functional group, solubility,…) of the PFAS compounds determined the affinity towards individual sampling train compartments (filter, XAD-2, impinger water, rinse), showed overall low medium and field blank concentrations,  negligible sampling train breakthrough and demonstrated lab optimization efforts to deal with the wetness of the primary XAD-2 module and improve long-chain (C8-C18) recoveries over time. The stability of the PFAS fingerprints (relative composition of individual compounds) illustrated the reproducibility of the method, while flue gas treatment optimizations by Indaver resulted in significant emission reductions over time. Similarly, PFAS that were difficult to quantify in terms of internal standard recovery and blank contamination risk included PFPrA, PFPrS and 6:2FTS, glassware contamination with long-chain PFAS resulted in a more stringent cleaning procedure and sulfonate recovery problems were observed during the validation exercise.
When applying the compendium method on other stacks and industries in Flanders, PFAS emissions are observed at every stack ranging between 11 ng/Nm³ and 43 mg/Nm³ for the total sum of quantified PFAS, with generally stable and stack-specific fingerprints. Measured stack emission concentrations (ng/Nm³) are converted to ambient concentrations by means of a bi-gaussian IFDM model in order to evaluate the emission contribution against the temporary EFSA assessment framework in Flanders (0.4 ng/m³ for the sum of EFSA compounds PFOA, PFNA, PFHxS and PFOS), an inhalation equivalent derived from the EFSA tolerable weekly intake (TWI) values (Figure 3).
The validation exercise of the OTM-45 sampling train resulted in 19 quantitative and 22 indicative PFAS compounds and the LUC/VI/003 compendium method can be consulted via
https://emis.vito.be/nl/erkende-laboratoria/lucht-gop/compendium-luc. Interlaboratory comparison (ILC) results are still underway.
“What’s next?”
As the physicochemical diversity and wide-scale prevalence of PFAS requires a combination of specific sampling and analytical methods, there is an urgent need for harmonized PFAS air monitoring across the EU. VITO’s lab and field experiences have resulted in a validated Flemish compendium method for the quantification of ~50 PFAS (>C4, boiling point >100°C) in flue gasses (LUC/VI/003) and has already shown a variety of prevailing PFAS emission concentrations and fingerprints in stack emissions across Flanders, Belgium. The current target method focusses on a selection of PFAS compounds and is often combined with non-target/suspect screening methods to confirm the representativity of the target PFAS compounds and/or identify other relevant compounds that are present in the emissions. The existing compendium method will be further optimized to include more quantitative compounds, while complementary methods will be developed to extend the scope towards more volatile ultra-short chain (C1-C3) PFAS. Following our recent interlaboratory study (including international participants), we call on all relevant stakeholders to harmonize PFAS monitoring (and related emission standards) as much as possible!

More information:

1.  LUC/VI/003 compendium NL: https://reflabos.vito.be/2024/LUC_VI_003.pdf ENG: https://reflabos.vito.be/2024/LUC_VI_003_ENG.pdf
2.  Recent presentation at EU “Tackling PFAS pollution & Launch         Knowledge Center Innovative Remediation Solutions”: https://assets.vlaanderen.be/image/upload/v1708681087/ VITO_Jelle_Hofman_Jan_Peters_Patrick_Berghmans_Bart_     Baeyens_Griet_Jacobs_Stefan_Voorspoels_Gert_Otten_r3mhrs.pdf  

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