| Summary: |
The European Commission (EC) identified poly-and perfluoroalkyl substances (PFAS) as a priority contaminant within the general strategy for a toxic-free environment, with the recast drinking water (DW) Directive entering in force earlier in 2021 and preconizing a parametric value for Total PFAS and Sum of PFAS. On February 22nd, 2021, EPA reissued final regulatory determinations for contaminants on the Contaminant Candidate List 4 and is making final determinations to regulate two contaminants, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). The approach to achieving a sustainable use of PFAS requires, in combination with advances in monitoring, management, and regulation, the development of alternative solutions for mitigation of contaminations found in drinking water sources and other natural water reservoirs. The large variety of contamination sources of PFAS, their environmental mobility, and persistence severely impacts the effectiveness of strategies targeting treatments at the points of origin. Current water treatment plants' efficiency in tackling PFAS contamination is at best disputed, with evidence showing that complete removal is only achieved in plants with ultrafiltration/reverse osmosis systems. There is no available efficient solution for the removal of PFAS without the generation of secondary concentrated waste streams (i.e. concentrate from membrane filtration) or saturated adsorbents requiring regeneration.
The F-CAT team proposes a systematic study of a catalytic defluorination process as a path to develop a sustainable solution for the mitigation of PFAS contamination. The proposal seeks to bypass the shortfalls in the defluorination of PFAS by clarifying the reaction pathways involved, employing custom-designed catalysts to optimize their efficiency towards achieving the complete conversion of PFAs in water. Total conversion into carboxylic acids (or perfluorinated if partial conversion is achieved) opens up avenues to  |
Summary
The European Commission (EC) identified poly-and perfluoroalkyl substances (PFAS) as a priority contaminant within the general strategy for a toxic-free environment, with the recast drinking water (DW) Directive entering in force earlier in 2021 and preconizing a parametric value for Total PFAS and Sum of PFAS. On February 22nd, 2021, EPA reissued final regulatory determinations for contaminants on the Contaminant Candidate List 4 and is making final determinations to regulate two contaminants, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). The approach to achieving a sustainable use of PFAS requires, in combination with advances in monitoring, management, and regulation, the development of alternative solutions for mitigation of contaminations found in drinking water sources and other natural water reservoirs. The large variety of contamination sources of PFAS, their environmental mobility, and persistence severely impacts the effectiveness of strategies targeting treatments at the points of origin. Current water treatment plants' efficiency in tackling PFAS contamination is at best disputed, with evidence showing that complete removal is only achieved in plants with ultrafiltration/reverse osmosis systems. There is no available efficient solution for the removal of PFAS without the generation of secondary concentrated waste streams (i.e. concentrate from membrane filtration) or saturated adsorbents requiring regeneration.
The F-CAT team proposes a systematic study of a catalytic defluorination process as a path to develop a sustainable solution for the mitigation of PFAS contamination. The proposal seeks to bypass the shortfalls in the defluorination of PFAS by clarifying the reaction pathways involved, employing custom-designed catalysts to optimize their efficiency towards achieving the complete conversion of PFAs in water. Total conversion into carboxylic acids (or perfluorinated if partial conversion is achieved) opens up avenues to achieve their complete mineralization in tandem with other (advanced) oxidation techniques, e.g. catalytic ozonation. A heterogeneous catalytic system presents several advantages which have been demonstrated in the removal of inorganic contaminants from drinking water. Such systems are able to efficiently transform contaminants such as bromate, nitrate, and perchlorate without the generation of concentrated waste streams or requiring further treatment to further degrade captured contaminants. Moreover, a heterogeneous catalytic system using an external reducing agent has a fairly straight-forward path to scaling-up which places it as a very attractive alternative for the mitigation of challenging drinking water contaminations, bypassing several limitations in reactor design found for example in electrochemical or photocatalytic applications.
The reported kinetics for room temperature and pressure catalytic dehalogenation of PFAS are remarkably slow, with rate constants often calculated on a per-day scale. The rate-limiting step is suggested to be the cleavage of the bond formed by complexation with the reduced catalyst, which is mediated by a proton transfer and results in the release of F-. The overall strategy proposed by F-CAT takes advantage of the team's know-how in the engineering of catalysts for reductive hydrogenation of water contaminants. The combination of hydrodefluorination (HDF) catalytic systems with a hydrogen splitting catalyst has the potential to improve kinetics by increasing the availability of protons and electrons for reaction, in particular when using an external (i.e. fed into the system) hydrogen source. Moreover, the choice and tuning of an adequate catalyst support can further improve kinetics by providing a platform for electron and proton migration between sites. |