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A breakthrough for degrading “forever chemicals”. What’s it in for the water sector?

Per- and polyfluorinated alkyl substances (PFAS) are a very large and diverse group of chemicals used in many industrial processes and daily life products. These chemicals are hugely popular due to their unique properties. However, these properties also make PFAS very persistent to environmental degradation, highly mobile across different environmental compartments, difficult to remove from water, and toxic to organisms and human health. While very harsh conditions are typically required to break down these compounds, a recent study has shown that a large part PFAS can be degraded using low temperatures and mild conditions. This is promising news, however, how does this discovery affect the water sector and human exposure to these chemicals? These are some of the questions we will address in this short article, but first: what are PFAS and why are they so dangerous?

PFAS chemical properties and uses

PFAS are anthropogenic substances containing multiple carbon-fluoride bonds, which are one of the strongest chemical bonds in nature. Thousands of chemicals fall under this definition, and several dozens of sub-classes are used to further characterise these substances. Some of the most commonly reported PFAS contain a polar functional group, such as a carboxylic or a sulfonic acid group, and are referred to as per- and polyfluoroalkyl carboxylic acids (PFCA), and per- and polyfluoroalkyl sulfonic acids (PFSA), respectively. Due to their unique physicochemical properties, these substances are widely used in everyday products, such as water-resistant fabrics, non-stick cookware, paper and cardboard coatings, food containers, stain repellent sprays and coatings, aqueous film–forming foams for fire suppression, paints, inks, adhesives, biocides, and more.

Environmental issues

As a result of their widespread use and their physicochemical properties, these substances are ubiquitous in the environment and have been found in virtually every environmental sample, even in remote areas such as Antarctica. The carbon-fluoride bond provides these chemicals with a strong resistance to environmental degradation, and this is why PFAS are often referred to as “forever chemicals”. In addition, their surfactant properties (i.e., the tendency to be both hydrophilic and lipophilic) grant PFAS high solubility in water and bioaccumulative properties (i.e., the ability to accumulate in biological tissues, where toxic effects can be exerted). Furthermore, these substances are difficult to remove from water or soil and are challenging to break down into harmless products. As a result, PFAS pose a serious threat to the environment and human health and are listed as substances of high concern in many countries (including the EU and the US).

Low-temperature mineralization of PFCA

A recent study published in Science by Trang et al. (2022) describes an innovative low-temperature approach for the effective mineralization of PFCAs, a very large sub-class of PFAS. The mechanism relies on the PFCA decarboxylation in polar aprotic solvents followed by degradation of perfluoroalkyl ion intermediates to fluoride ions. This method is very promising due to the mild conditions and inexpensive reagents used, the minimal formation of fluorocarbon by-products, and the possibility to extend this approach to other PFAS sub-classes. Successful degradation was achieved for PFCA with varying chain lengths, i.e., from 4 to 9 carbon atoms. However, for very short chain PFCA (C < 4) degradation was less efficient.

Potential advantages for the drinking water sector

At the moment there are no full-scale water treatment techniques available that fully mineralize PFAS. PFAS are typically removed from water using adsorption (e.g., activated carbon) or filtration (reverse osmosis) techniques which produce highly concentrated waste, that has to be handled separately. These techniques are not selective to PFAS only (e.g., also important minerals are removed) and an additional step involving the reactivation of adsorbents or mixing of membrane concentrate streams with non-demineralized water is typically applied. During both processes (part of the) PFAS may (re-)enter the environment. The application of the degradation process described by Trang et al. would allow to fully degrade PFCAs present in waste products and avoid their reintroduction into the environment, and ultimately, prevent these substances from reaching drinking water sources. In addition, complete degradation would also prevent the formation of undesired transformation products (e.g., short-chain PFAS) resulting from the incomplete mineralization of PFCAs, and their emission into the environment. This is a very good perspective for the water sector, however, at the moment not all drinking- or wastewater treatment plants have the technology necessary to efficiently remove PFAS from water, and allow their complete removal from the environment. Hence, for the foreseeable future, a drastic decrease in the production and use of these chemicals remains the most important strategy to reduce the pressure of PFAS on the environment and drinking water sources. This could be achieved by restricting the use of these substances only to limited activities (i.e., essential use) and, at the same time, avoiding the production and/or release of similar chemicals as a replacement for PFAS (i.e., regrettable substitutions).

Potential implementation in drinking water treatment plants

The newly developed treatment method may be applied to membrane concentrate or extracts of PFCA-loaded adsorbents. However, more research will still be required before this new process can be implemented on a scale suitable for e.g. drinking water or wastewater treatment. For instance, the degradation of PFCAs was obtained using laboratory conditions that largely differ from those typically found in water treatment plants – i.e., a pure solution and very high concentrations as opposed to a complex mixture of organic and inorganic chemicals and lower concentrations.  Hence, more research will be necessary to investigate the effects of the lower PFCA concentrations and the presence of other contaminants on the mineralization efficiency.

Limitations

The low-temperature mineralization approach described by Trang et al. has proved to be effective for PFCA with varying chain lengths. However, the very short chain PFCAs perfluoropropanoic acid (PFPrA) and trifluoroacetic acid (TFA) showed lower degradation. These chemicals are very polar and mobile in the aquatic environment, and extremely difficult to remove during water treatment processes. Yet, evidence suggests that highly polar organic compounds may be less toxic than hydrophobic ones. Furthermore, PFCAs account for a large part of PFAS typically found in the environment, however, applications of this methodology to other important sub-classes such as PFSA were not reported in the study. As PFSAs are present at similar concentrations to PFCAs and are considered to be more toxic than PFCAs, for water treatment the mineralization process should also be extended to this class of PFAS.

Conclusions

PFAS are typically difficult to remove from water (especially the most polar ones). Therefore, first and foremost, the reduction of emissions of these compounds to the environment should be a priority. Nonetheless, progress in the degradation of dangerous chemicals such as PFAS, as presented by Trang et al., is very promising in the reduction of the chemical burden on society and the environment. Progress in mineralization processes should go hand in hand with improvements in removal technologies and their implementation to prevent excessive chemical stress to the environment and risk to human health. KWR is very active in research areas involving priority substances such as PFAS, their monitoring, fate, impact on the environment, and removal technologies. Check out our latest research here!

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