Recent Study Reveals Organic Soil Variability in Sorbing PFAS Contaminants

Researchers from the Swedish University of Agricultural Sciences (SLU) and the Technical University of Munich have conducted a comprehensive study shedding light on the sorption behaviour of per- and polyfluoroalkyl substances (PFAS) in various organic soil horizons. The study, published in the journal Chemosphere, investigates how different PFAS substances interact with organic soils of varying quality and provides valuable insights into the environmental risk assessment of these persistent contaminants (1).

Handful of dark brown soil isolated on white background | Image Credit: © toomler - stock.adobe.com

The study’s primary objective was to examine the sorption behaviour of several legacy and precursor PFAS within three distinct organic soil horizons: a Spodosol Oe (Mor Oe) and two Sphagnum peats with different degrees of decomposition. In particular, the researchers aimed to understand the charge- and concentration-dependent sorption behaviour of different PFAS subclasses and correlate this behaviour with the structural properties of organic matter found in the soils, as determined through chemical extractions and ¹³C nuclear magnetic resonance (NMR) spectroscopy (1).

There were several key findings that were revealed in this study. One of them was that among the tested soils, there was a significant amount of variability in sorption strength (1). This was especially true when the two Sphagnum peats were compared with the Mor Oe soil. The Sphagnum peats exhibited four times stronger sorption of PFAS compared with the Mor Oe soil (1). Furthermore, it was observed that longer-chained PFAS displayed greater binding affinity to the peat materials compared with the Mor Oe sample, underscoring the influence of the PFAS chain length on sorption behaviour (1).

Another key finding was that the carbohydrate content in the organic matter, specifically the O-alkyl carbon, had a direct correlation with sorption. According to the researchers, this relationship highlighted the importance of organic matter quality in determining the binding affinity of PFAS in organic soil horizons (1). However, the researchers do acknowledge that to fully elucidate the connection between organic matter quality and PFAS binding, more research will need to be done (1).

The study also revealed the role that the pH values played in PFAS sorption of soils. The inverse relationship between the pH levels and sorption allowed researchers to understand the impact the pH had on perfluoroalkyl carboxylates (PFCAs) (1). The most significant pH effects observed for PFCAs were for those that contained C₁₀ and C₁₁ perfluorocarbon chain lengths. The study’s geochemical modelling attributed this pH-dependent binding to changes in surface charge, primarily because of the protonation of acidic functional groups (1). This effect became more pronounced as the PFAS chain length increased up to C₁₀.

The study revealed that the hydrophobic effect resulting from the perfluorocarbon chain might override the pH/charge dependency for longer-chained PFCAs (1). This finding suggests that multiple factors contribute to PFAS sorption behaviour, making it a complex interplay of chemical properties (1).

The investigation also extended to two PFAS precursors, 6:2 fluorotelomer sulfonate (FTSA) and 8:2 FTSA, which exhibited pH-dependent binding similar to their perfluorinated analogues, perfluorohexanesulfonic acid (PFHxS) and perfluorooctanesulfonic (PFOS) acid (1). This observation supported the notion that perfluorocarbon chain length played a critical role in the pH dependency of PFAS binding. The inclusion of a hydrogenated spacer group (–CH₂CH₂–) in 8:2 FTSA did not significantly affect its sorption behaviour compared with PFOS (1). In contrast, the shorter-chained 6:2 FTSA exhibited slightly weaker sorption, indicating that the spacer group influenced its behaviour (1).

This study provides valuable insights into the complex sorption behaviour of PFAS contaminants in different organic soil horizons. The results highlight the significance of organic matter quality, chain length, and pH in determining PFAS sorption, shedding light on the environmental risks associated with these persistent substances (1). Further research in this area is crucial for a comprehensive understanding of PFAS behaviour in soil and for developing effective strategies to mitigate their impact on the environment.

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Reference

(1) Campos-Pereira, H.; Makselon, J.; Kleja, D. B.; et al. Binding of Per- and Polyfluoroalkyl Substances (PFASs) by Organic Soil Materials With Different Structural Composition – Charge- and Concentration-Dependent Sorption Behavior. Chemosphere 2023, 297, 134167. DOI: 10.1016/j.chemosphere.2022.134167