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

The photocatalytic activity of surface fluorinated TiO2 (F-TiO2) for both the degradation and mineralization of bisphenol A (BPA) was compared with that of pure TiO2. 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 CO2) 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 TiO2 enhanced the production of the hydroxyl radical ((OH)-O-center dot), which is primarily responsible for the degradation of BPA, by altering the (OH)-O-center dot generation pathway. However, the lower mineralization on F-TiO2, which produced more (OH)-O-center dot, implies that the role of (OH)-O-center dot in the photocatalytic mineralization process is minor. The production of superoxide/hydroperoxyl radical (O-2(center dot-)/HO2 center dot), which is suggested as an essential oxidant for the mineralization of phenolic pollutants, by F-TiO2 was lower than that exhibited by pure TiO2. The reduced photocurrent (I-ph) generation and the enhanced H2O2 production on F-TiO2 indicate that fluorides on the TiO2 surface reduce the interfacial electron transfer rate (i.e., the production of O-2(center dot-)/HO2 center dot) and enhance the reduction of O-2(center dot-)/HO2 center dot to H2O2. 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-TiO2. This result indicates that the negative effect of surface fluorination on the mineralization of phenolic pollutants is pervasive and is not restricted to BPA. (C) 2016 Elsevier B.V. All rights reserved.