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 to neat TiO2 , Pb-doped TiO2 (TiO2 -Pb) and rGO-coated TiO2 (TiO2 /rGO). TiO2 -Pb/rGO (0.33 g/L) under UV showed superior removal of PFOA (10 mg/L) at 98% after 24 hr, 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 while it was ineffective towards perfluorosulfonic acids and short-chain PFCAs. Removal kinetics decreased in 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 decomposition of persistent organic pollutants in water, in particular highly challenging fluorinated chemicals.