PFAS - the new POP on the block
Review of the article “Influences of Chemical Properties, Soil Properties, and Solution pH on Soil−Water Partitioning Coefficients of Per- and Polyfluoroalkyl Substances (PFASs)”
Nguyen, T. M. H., Bräunig, J., Thompson, K., Thompson, J., Kabiri, S., Navarro, D. A., … Mueller, J. F. (2020). Influences of Chemical Properties, Soil Properties, and Solution pH on Soil–Water Partitioning Coefficients of Per- and Polyfluoroalkyl Substances (PFASs). Environmental Science & Technology. https://doi.org/10.1021/acs.est.0c05705
Key points
- Short-chain PFASs are much more mobile in soils and groundwater
- Soil-sorption increased with increasing carbon-fluorine chain lengths (C6 - C11) and PFAS hydrophobicity
- Soil properties that are important for PFAS sorption are discussed
What is the article about?
This article is a thorough assessment of soil properties that affect PFAS solubilities. It improves our ‘know-how’ needed for remediation strategies of PFASs.
PFASs are the new POPs on the block. True to their POP nature, they are everywhere and they clearly want to stick around! And without knowing how mobile and available these chemicals are in soils or groundwater we are unable to develop effective remediation strategies.
Polyfluoroalkyl substances (PFASs), which are a group of fluorinated chemicals including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), are found literally everywhere in soils and groundwater (but also in animals and humans) - even in the “farthest reaches of terrestrial Earth”1. They are found everywhere because they were used in everything, from electric and electronic parts, fire fighting foam, photo imaging, hydraulic fluids and waterproof textiles/garments (think of the biggest brands), non–stick kitchen ware (ok let’s name it: Teflon), food processing equipment, paper and paints, text markers, stain resistant carpets, carpet cleaning liquids and even microwave popcorn bags. PFASs are most prevalent in the US - learn more here.
And this study looks at the soil-sorption factors of a whole range of them - even those PFASs that are not commonly analysed; a total of 34 PFASs were included!
What were the main conclusions?
Smaller PFASs with shorter carbon-fluorine chains (C3-C5) sorbed much less to soils (i.e. have lower Kd values) and were therefore less affected by soil properties. Hence, short-chain PFASs would be much more mobile in the environment.
Soil-sorption increased with increasing carbon-fluorine chain lengths (C6 - C11). For those long-chained PFASs, hydrophobicity (which increases with carbon-fluorine chain length) may have determined sorption to soils.
Charge of PFAS molecules and multiple soil properties (not just one soil property) also played important roles in sorption. Apart from the commonly associated soil properties (such as organic carbon, cation exchange capacity, soil pH, and soil texture) soil pore volume was highlighted to be an important factor for PFAS sorption to soils.
What were the main limitations?
Kd values (i.e. the strengths of sorption to a soil derived from batch sorption experiments) are “the most important parameters used in estimating the migration potential of contaminants” 2. So they are great to do some general predictions of fate and mobility in soils and groundwater. However, there are two things that limit interpretation:
Kd values in this type of experiments can only ever be specific to the specific soils and conditions used in the experiment. However, any remediation strategy in the field must take into account site specific conditions. So in an ideal world one has to do this experiment again and again for any specific location to model the migration potential of PFAS into groundwater.
Kd values do not inform about bioavailability of contaminants for microbiology, plants and animals. Kd values are based on the total extraction of small amounts of soil in a tube (e.g through sonication) which contains freshly added contaminant and water. However, this total extraction of extractable contaminants does not reflect what is actually available to life. Contaminants in the field undergo time-dependant “bioavailability processes”3 which seem to be extremely complex and involve physical, chemical and biological processes. This may be one reason why it is so difficult to assess risks and agree on toxicologically valid exposure limits 4. Hence, a lot more information is needed for an appropriate risk assessment to wildlife and human exposure and to identify suitable bioremediation strategies.
What do I think about it?
I think this study is an example of awesome science. These results will inform future research and policy. Thanks to the authors for a thorough assessment of Kd values of PFASs!
What do you think about it?
Comment in the comment box below and let me what you think about this study or if you have any feedback on this review. Alternatively get in touch with me anytime.
References
Rankin, K., Mabury, S. A., Jenkins, T. M., & Washington, J. W. (2016). A North American and global survey of perfluoroalkyl substances in surface soils: Distribution patterns and mode of occurrence. Chemosphere. https://doi.org/10.1016/j.chemosphere.2016.06.109 ↩︎
USEPA, U. (1999). Understanding variation in partition coefficient, Kd, values. United States Environ. Prot. Agency I. ↩︎
NationalResearchCouncil. (2003). Bioavailability of contaminants in soils and sediments: Processes, tools, and applications. In The National Academies Press, Washington. ↩︎
Mikkonen, A. T., Martin, J., Dourson, M. L., Hinwood, A., & Johnson, M. S. (2020). Suggestions for Improving the Characterisation of Risk from Exposures to Per and Polyfluorinated Alkyl Substances (PFAS). Environmental toxicology and chemistry. ↩︎
