Photocatalytic degradation of perfluorooctanoic acid on Pb‐doped TiO<sub>2</sub> coated with reduced graphene oxide
Nusrat Chowdhury, Hyeok Choi
Abstract
Abstract Poor reactivity of extraordinarily strong per‐ and polyfluoroalkyl substances (PFAS) with TiO 2 makes it necessary to advance photocatalytic materials. In this present work, lead (Pb)‐doped TiO 2 coated with reduced graphene oxide (TiO 2 ‐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 TiO 2 ‐Pb/rGO was compared with neat TiO 2 , Pb‐doped TiO 2 (TiO 2 ‐Pb), and rGO‐coated TiO 2 (TiO 2 /rGO). TiO 2 ‐Pb/rGO (0.33 g/L) under ultraviolet (UV) showed superior removal of PFOA (10 mg/L) at 98% after 24 h, following TiO 2 ‐Pb/UV at 80%, TiO 2 /rGO/UV at 70%, TiO 2 /UV at <10%, and UV at <10%. Doping of TiO 2 with Pb and introduction of rGO to TiO 2 greatly changed the physicochemical properties of TiO 2 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 TiO 2 ‐Pb/rGO under UV led to formation of short‐chain perfluorocarboxylic acids (PFCAs) as reaction intermediates through step‐by‐step removal of CF 2 units. Polyfluoroalkyl substance (6:2 fluorotelomer sulfonate [ 6:2FTS ]) and long‐chain PFCAs such as PFOA were significantly removed and defluorinated by TiO 2 ‐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 TiO 2 /rGO showed better performance than Fe doping. Overall, this study implied that proper designing of TiO 2 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 TiO 2 ‐Pb/rGO was studied. TiO 2 ‐Pb/rGO shows better photoactivity towards PFAS than TiO 2 ‐Pb and TiO2/rGO system. Scavenger test indicated that h+, • O2−, and iO 2 are responsible for PFOA removal. Using TiO 2 ‐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.