Removal of per- and polyfluoroalkyl substances from water

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Per- and polyfluoroalkyl substances (PFASs, also perfluorinated alkylated substances) are synthetic organofluorine chemical compounds that have multiple fluorine atoms attached to an alkyl chain. As such, they contain at least one perfluoroalkyl moiety, –CnF2n–. According to OECD, there are at least 4730 different PFASs. A subgroup, the fluorosurfactants or fluorinated surfactants, have a fluorinated “tail” and a hydrophilic “head” . They are more effective at reducing the surface tension of water than comparable hydrocarbon surfactants. They include the perfluorosulfonic acids such as the perfluorooctanesulfonic acid (PFOS) and the perfluorocarboxylic acids such as the perfluorooctanoic acid (PFOA). GenX is a shorter carbon chain replacement for PFOA. PFOS and PFOA are persistent organic pollutants and are detected in humans and wildlife. (Wikipedia)

Space filling model of PFOS

PFASs are accumulated in living organisms and are associated with adverse health effects. This accumulation was brought in relationship with hypercholesterolemia, thyroid diesaeses, cancer, etc.

β-Cyclodextrin (β-CD) has been proposed as a possible remediation strategy for PFAS contamination [18–23]. The non-toxic, water soluble, inexpensive, and sustainable β-CD is an environmentally friendly and economical alternative to current PFAS water treatment methods.

Positively charged β-CD derivatives showed increased encapsulation of short chain PFAS relative to native β-CD. Linear PFAS with a –CF2CF2CF2– group had little or no increase with mono-amino-β-CD and QACD, and moderate increase with hepta-amino-β-CD, compared to β-CD. [1] Favorable interactions between the hydrophobic fluorinated chain and the β-CD cavity dominate these complexes, while ionic bonding interactions between the PFAS carboxylate and the β-CD amino groups are secondary.

Crosslinked β-CD based adsorbents are promising for PFAS remediation. β-CD was crosslinked by hexamethylene diisocyanate in the presence of PFOA and PFOS in order to get molecularly imprinted sorbents for adsorption of these chemicals. [2] A β-cyclodextrin polymer linked with tetrafluoroterephthalonitrile (TFN-CDP) has high affinity for cationic and many neutral micropollutants from contaminated water because of anionic groups incorporated during the polymerization. [3]

Two other β-CD-based polymers with different cross-linkers, epichlorohydrin and 2-isocyanatoethyl methacrylate, showed inferior affinity compared to TFN-CDPs for 8 out of 10 PFASs [4].

Installing amino groups into the crosslinkers of a β-CD polymer network improved the binding of many anionic PFAS, including short-chain and branched derivatives. To this end, βCD was crosslinked with tris(2-aminoethyl)amine (TREN) based tripodal crosslinkers, one containing amino groups for complementary
electrostatic interactions and another functionalized with amido groups. [5] β-CD polymers containing amines showed superior removal for ten anionic PFAS compared to polymers containing amido groups. [6]

Reaction scheme for crosslinking with tripodal coupling agent [5]

The β-CD polymer overperformed activated carbon in sorption of PFASs.

Scheme of PFAS sorption from water by CD polymers and activated carbon [6]


[1] Weiss-Errico, M.J., O’Shea, K.E. Enhanced host–guest complexation of short chain perfluoroalkyl substances with positively charged β-cyclodextrin derivatives. J Incl Phenom Macrocycl Chem 95, 111–117 (2019).

[2] Karoyo, A. H., & Wilson, L. D. (2016). Investigation of the Adsorption Processes of Fluorocarbon and Hydrocarbon Anions at the Solid–Solution Interface of Macromolecular Imprinted Polymer Materials. The Journal of Physical Chemistry C, 120(12), 6553–6568. doi:10.1021/acs.jpcc.5b12246 

[4] Xiao, L., Ching, C., Ling, Y., Nasiri, M., Klemes, M. J., Reineke, T. M., … Dichtel, W. R. (2019). Cross-linker Chemistry Determines the Uptake Potential of Perfluorinated Alkyl Substances by β-Cyclodextrin Polymers. Macromolecules. doi:10.1021/acs.macromol.9b00417 

[5] Yang, A., Ching, C., Easler, M., Helbling, D. E., & Dichtel, W. R. (2020). Cyclodextrin Polymers with Nitrogen-Containing Tripodal Crosslinkers for Efficient PFAS Adsorption. ACS Materials Letters, 1240–1245. doi:10.1021/acsmaterialslett.0c00240 

[6] Wu, C., Klemes, M. J., Trang, B., Dichtel, W. R., & Helbling, D. E. (2020). Exploring the factors that influence the adsorption of anionic PFAS on conventional and emerging adsorbents in aquatic matrices. Water Research, 115950. doi:10.1016/j.watres.2020.115950 

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