Abstract
This study reports the removal of per- and polyfluoroalkyl substances (PFAS) in water using various photocatalytic and photochemical processes. PFAS were chosen, based on chain lengths, functional groups, and structural properties: four perfluorocarboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), three perfluorosulfonic acids (PFSAs), including perfluorooctanesulfonic acid (PFOS), hexafluoropropylene oxide dimer (GenX), and 6:2 fluorotelomer sulfonate (6:2 FTS), and dependency of the photocatalytic decomposition of PFAS on their properties was investigated. Oxidants and reductants were introduced to study the photochemical decomposition of PFAS, and reactive species and reaction byproducts were identified to elucidate the decomposition mechanism of PFAS. Some notable findings include: long chain PFCAs (95% in 48 h) and 6:2 FTS (100%) were removed via chemical decomposition in TiO2/UVC while GenX (37%), long chain PFSAs (60%), short chain PFSAs (0-10%) and short chain PFCAs (5-18%) were removed via physical adsorption. Sulfate radicals generated with persulfate (PS) played an important role in decomposing PFCAs (60-90%). Sulfite activated by UVC worked for defluorination of PFOA (75%) and PFOS (80%). PFOA was removed faster by UVC/sulfite > UVC/TiO2/sulfite ≈ UVC/TiO2/PS ≥ UVC/PS > UVC/TiO2 while PFOS was removed faster by UVC/sulfite ≫ UVC/TiO2/sulfite ≈ UVC/TiO2/PS ≈ UVC/TiO2 ≫ UVC/PS. Susceptibility of PFAS to the chemical reactions could be explained by their properties and the reactive species produced in each system.
Highlights
Per- and polyfluoroalkyl substances (PFAS) with chemically inert C-F bonds have been widely used in various industries thanks to their many useful properties such as both water and oil repelling property (Fujii et al 2007)
In the context that the photocatalytic decomposition of emerging short chain PFAS should be revealed and compared with that of conventional long chain PFAS, this present study reports the reactivity of 9 PFAS, including 4 perfluorocarboxylic acids (PFCAs), 3 perfluorosulfonic acids (PFSAs), GenX, and 6:2 FTS and the dependency of the photocatalytic decomposition of PFAS on their chain lengths, functional groups, and structural properties
In the TiO2/UVC system, overall PFAS removal rate constant decreased in order of 6:2 FTS (2.25 hÀ1).perfluorononanoic acid (PFNA) (0.144).perfluorooctanoic acid (PFOA) (0.063).perfluorooctanesulfonic acid (PFOS) (0.0252).GenX (0.0105)..perfluorohexane sulfonic acid (PFHxS) (0.0045)≈perfluorobutanoic acid (PFBA) (0.0027)≈Perfluoroheptanoic acid (PFHpA) (0.0021).perfluorobutane sulfonic acid (PFBS) (0.0002)
Summary
Per- and polyfluoroalkyl substances (PFAS) with chemically inert C-F bonds have been widely used in various industries thanks to their many useful properties such as both water and oil repelling property (Fujii et al 2007). Long chain PFSAs containing !6 carbons such as perfluorooctanesulfonic acid (PFOS) and PFCAs containing !8 carbons such as perfluorooctanoic acid (PFOA) have been popularly used (US EPA 2017a; Trojanowicz et al 2018). The occurrence, fate, and transformation of long chain PFAS, mainly PFOA and PFOS, have been relatively well documented (Rahman et al 2014). The challenge becomes serious especially for PFOA and PFOS as two presenting perfluoroalkyl substances due to their highly oxidized states and complete substitution of C-H bonds with C-F bonds. United States Environmental Protection Agency (US EPA) released a drinking water health advisory for PFOA and PFOS and limits their concentration to 70 parts per trillion in drinking water separately or in combination (US EPA 2017b)
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