Remediation of per- and polyfluoroalkyl substances
Remediation of per- and polyfluoroalkyl substances refers to the destruction or removal of perfluoroalkyl substances (PFASs) from the environment. PFAS's are a group of synthetic organofluorine compounds that are used to produce diverse products such as non-stick cookware and firs-fighting foams. Known as "everywhere chemicals", they have attracted great concern as chronic poisons. Because they are pervasive and have adverse effects, much interest has focused on their removal.
PFAS's are by design highly stable. They often occur as extremely dilute (ppm to ppb) solutions.[1] These factors - resilience and diluteness - make remediation extremely challenging. Nonetheless, diverse methods are being tested including sonolysis, electrochemical oxidation, and advanced oxidation processes, all of which promote the formation of hydroxyl radicals which can oxidize PFAS and break its C-C bonds.[2][3]
Destruction
[edit]Both oxidative and reductive approaches can be taken to destroy PFAS's. The oxidation of perfluorooctane sulfonic acid (PFOS), as one prominent example, is described as follows:
- C8F17SO3H + 8 H2O + 4 O2 → 17 HF + 8 CO2 + SO3
The challenge implicit in this approach is that PFAS's have been used in aqueous film forming foam (AFFF) because they both make foams and they resist oxidation.[4]
For the perfluorocarboxylic acids, such as perfluorooctanoic acid (PFOA), decarboxylation has been identified as a possible route to their eventual degradation.[5]
- C8F17CO−2 → C6F13CF=CF2 + F− + CO2
It is unclear if this technology is applicable to real-world concentrations and media.
Adsorption
[edit]Through the process of adsorption, PFAS's can in principle be concentrated to facilitate their physical removal from the environment.
Adsorption is generally more efficient in an acidic environment and with large mesopores. Carbons such as activated carbon and biochar have a very high specific surface area and are nonpolar, allowing them to interact with the hydrophobic tail of PFAS molecules. They can then be regenerated through incineration.[6][3] Anion exchange resins, metal–organic frameworks, and layered double hydroxides may also be used for the adsorption of PFAS (PFAS can become an anion through losing a hydrogen from its head). In situ, adsorption is less effective due to the presence of other pollutants in the water to be treated.[6]
A 2024 study has shown that thermophilic anaerobic digestion, combined by adsorption by activated carbon can remove up to 61% of PFAS from sewage sludge.[7]
Regeneration
[edit]Activated carbon granules or particles can be incinerated to regenerate and reuse the surface while breaking down PFAS at the same time. However, various harmful products can be produced as a result, such as tetrafluoromethane, a strong greenhouse gas, and the heating process is expensive. Meanwhile, regeneration with a solvent does not break down PFAS, so further waste treatment is required.[2][6]
Reverse osmosis
[edit]Reverse osmosis and nanofiltration effectively separate PFAS but are typically too expensive to be viable solutions.[2][6]
References
[edit]- ^ Fromme H, Tittlemier SA, Völkel W, Wilhelm M, Twardella D (May 2009). "Perfluorinated compounds—exposure assessment for the general population in Western countries". Int. J. Hyg. Environ. Health. 212 (3): 239–70. Bibcode:2009IJHEH.212..239F. doi:10.1016/j.ijheh.2008.04.007. PMID 18565792.
- ^ a b c Wanninayake, Dushanthi M. (1 April 2021). "Comparison of currently available PFAS remediation technologies in water: A review". Journal of Environmental Management. 283. doi:10.1016/j.jenvman.2021.111977. ISSN 0301-4797. PMID 33517051. S2CID 231766709.
- ^ a b Kucharzyk, Katarzyna H.; Darlington, Ramona; Benotti, Mark; Deeb, Rula; Hawley, Elisabeth (15 December 2017). "Novel treatment technologies for PFAS compounds: A critical review". Journal of Environmental Management. 204 (Pt 2): 757–764. doi:10.1016/j.jenvman.2017.08.016. ISSN 0301-4797. PMID 28818342.
- ^ Darlington, R.; Barth, E.; McKernan, J. (2018). "The Challenges of PFAS Remediation". The Military Engineer. 110 (712): 58–60. PMC 5954436. PMID 29780177.
- ^ Trang, Brittany; Li, Yuli; Xue, Xiao-Song; Ateia, Mohamed; Houk, K. N.; Dichtel, William R. (2022). "Low-temperature mineralization of perfluorocarboxylic acids". Science. 377 (6608): 839–845. Bibcode:2022Sci...377..839T. doi:10.1126/science.abm8868. PMID 35981038.
- ^ a b c d Lei, Xiaobo; Lian, Qiyu; Zhang, Xu; Karsili, Tolga K.; Holmes, William; Chen, Yushun; Zappi, Mark E.; Gang, Daniel Dianchen (15 March 2023). "A review of PFAS adsorption from aqueous solutions: Current approaches, engineering applications, challenges, and opportunities". Environmental Pollution. 321. doi:10.1016/j.envpol.2023.121138. ISSN 0269-7491. PMID 36702432.
- ^ Deligiannis, Michalis; Gkalipidou, Evdokia; Gatidou, Georgia; Kostakis, Marios G.; Triantafyllos Gerokonstantis, Dimitrios; Arvaniti, Olga S.; Thomaidis, Nikolaos S.; Vyrides, Ioannis; Hale, Sarah E. (August 2024). "Study on the fate of per- and polyfluoroalkyl substances during thermophilic anaerobic digestion of sewage sludge and the role of granular activated carbon addition". Bioresource Technology. 406: 131013. doi:10.1016/j.biortech.2024.131013. ISSN 0960-8524.