Photocatalytic degradation of perfluorooctanoic acid on Pb-doped TiO2 coated with reduced graphene oxide

Abstract

Poor reactivity of extraordinarily strong per- and polyfluoroalkyl substances (PFAS) with TiO₂ makes it necessary to advance photocatalytic materials. In this present work, lead (Pb)-doped TiO₂ coated with reduced graphene oxide (TiO₂ -Pb/rGO) was prepared using hydrothermal method, and then its photocatalytic activity with various PFAS in water, especially perfluorooctanoic acid (PFOA), was investigated. PFAS decomposition kinetics by TiO₂ -Pb/rGO was compared with neat TiO₂ , Pb-doped TiO₂ (TiO₂ -Pb), and rGO-coated TiO₂ (TiO₂ /rGO). TiO₂ -Pb/rGO (0.33 g/L) under ultraviolet (UV) showed superior removal of PFOA (10 mg/L) at 98% after 24 h, following TiO₂ -Pb/UV at 80%, TiO₂ /rGO/UV at 70%, TiO₂ /UV at <10%, and UV at <10%. Doping of TiO₂ with Pb and introduction of rGO to TiO₂ greatly changed the physicochemical properties of TiO₂ and the subsequent charge transfer mechanism. Radical scavenger experiments indicated that holes, superoxide radical anion, and singlet oxygen were responsible for the observed PFOA decomposition. Decomposition of PFOA by TiO₂ -Pb/rGO under UV led to formation of short-chain perfluorocarboxylic acids (PFCAs) as reaction intermediates through step-by-step removal of CF₂ units. Polyfluoroalkyl substance (6:2 fluorotelomer sulfonate [6:2FTS]) and long-chain PFCAs such as PFOA were significantly removed and defluorinated by TiO₂ -Pb/rGO, whereas it was ineffective toward perfluorosulfonic acids and short-chain PFCAs. Removal kinetics decreased in the order of 6:2FTS > PFOA >> PFOS > PFHpA ≈ PFHxS ≈ PFBA ≈ PFBS. Pb doping to TiO₂ /rGO showed better performance than Fe doping. Overall, this study implied that proper designing of TiO₂ photocatalytic materials enables to expedite the decomposition of persistent organic pollutants in water, in particular highly challenging fluorinated chemicals. PRACTITIONER POINTS: Photocatalytic decomposition of various PFAS using TiO₂ -Pb/rGO was studied. TiO₂ -Pb/rGO shows better photoactivity towards PFAS than TiO₂ -Pb and TiO2/rGO system. Scavenger test indicated that h+, ^• O2-, and iO₂ are responsible for PFOA removal. Using TiO₂ -Pb/rGO, PFOA removal was comparable under UVA, UVB, and UVC, and it can be explained by spanning UV absorption to 415 nm. Formation of intermediate PFCAs and F- ions confirmed PFOA removal via chemical decomposition.

Keywords: Lead doping; Per- and polyfluoroalkyl substances (PFAS); Perfluorooctanoic acid (PFOA); Reduced graphene oxide; TiO2 photocatalysis.