Is surface fluorination of TiO2 effective for water purification? The degradation vs. mineralization of phenolic pollutants

Abstract The photocatalytic activity of surface fluorinated TiO 2 (F-TiO 2 ) for both the degradation and mineralization of bisphenol A (BPA) was compared with that of pure TiO 2 . The degradation rate of BPA (i.e., the conversion rate of BPA to intermediates) was enhanced, but the mineralization rate (i.e., the conversion rate of BPA to CO 2 ) was reduced by surface fluorination. These behaviors are different from the general trend in photocatalysis, in which the photocatalyst with a higher activity for the degradation also shows a higher activity for the mineralization. The surface fluorination of TiO 2 enhanced the production of the hydroxyl radical ( OH), which is primarily responsible for the degradation of BPA, by altering the OH generation pathway. However, the lower mineralization on F-TiO 2 , which produced more OH, implies that the role of OH in the photocatalytic mineralization process is minor. The production of superoxide/hydroperoxyl radical (O 2 − /HO 2 ), which is suggested as an essential oxidant for the mineralization of phenolic pollutants, by F-TiO 2 was lower than that exhibited by pure TiO 2 . The reduced photocurrent ( I ph ) generation and the enhanced H 2 O 2 production on F-TiO 2 indicate that fluorides on the TiO 2 surface reduce the interfacial electron transfer rate (i.e., the production of O 2 − /HO 2 ) and enhance the reduction of O 2 − /HO 2 to H 2 O 2 . The degradation rate increased, but the mineralization efficiency decreased with increasing the surface coverage of fluorides, which depends on the pH and fluoride concentration in the solution. The reduced mineralization efficiency of other phenolic pollutants (4-chlorophenol, phenol, methylene blue, rhodamine B, and acid orange 7) was also observed on F-TiO 2 . This result indicates that the negative effect of surface fluorination on the mineralization of phenolic pollutants is pervasive and is not restricted to BPA.