Fluence-based photo-reductive decomposition of PFAS using vacuum UV (VUV) irradiation: Effects of key parameters and decomposition mechanism

Abstract

PFAS are a group of anthropogenic chemicals capturing global attention due to their numerous applications, ubiquitous detection, and toxicity. PFOS, a well-known compound of this class, is the most frequently detected PFAS in drinking water and human blood samples of several countries. In this study, a fluence-based approach using well-controlled quasi-collimated beam of UV (254 nm) and UV/VUV (254/185 nm) irradiations was followed to investigate the photodegradation of PFOS. Very low ε185 nm of PFOS, 57.2 ± 6.4 M-1 cm-1, implied the negligible contribution of direct photolysis in its decomposition, whereas the use of different scavengers indicated the prominent role of eaq−. Compared with other photo-reductive mediators, the addition of sulfite led to greater decomposition of PFOS which favored both UV and UV/VUV processes. Greater molar absorption coefficient and quantum yield of sulfite photolysis in 185 nm rather than corresponding values in 254 nm resulted in greater decomposition of PFOS in UV/VUV/sulfite process. Fluence-based decomposition rates of PFOS isomers (branched: 2.044 ± 0.336 J-1 cm2 and linear: 0.053 ± 0.003 J-1 cm2) were independent of fluence rate, while time-based rates were directly dependent on the fluence rate. Besides, the effects of key parameters including sulfite dosage, solution pH, temperature, initial concentration of PFOS, and dissolved oxygen were investigated. Defluorination and mass spectrometry results showed faster evolution of intermediates, i.e., their formation and consequent degradation, in the UV/VUV/sulfite process compared with the UV/sulfite process. H/F exchange with consequent C-C bond scission, along with desulfonation with consequent PFCAs formation are proposed decomposition pathways of PFOS upon reaction with eaq−.