Effects of dissolved oxygen, pH, and anions on the 2,3-dichlorophenol degradation by photocatalytic reaction with anodic TiO2 nanotube films

In this study, the highly-ordered TiO2 nanotube (TNT) arrays on titanium sheets were prepared by an anodic oxidation method. Under UV illumination, the TNT films demonstrated the higher photocatalytic activity in terms of 2,3-dichlorophenol (2,3-DCP) degradation in aqueous solution than the conventional TiO2 thin films prepared by a sol-gel method. The effects of dissolved oxygen (DO) and pH on the photocatalytic degradation of 2,3-DCP were investigated. The results showed that the role of DO in the 2,3-DCP degradation with the TNT film was significant. It was found that 2,3-DCP in alkaline solution was degraded and dechlorinated faster than that in acidic solution whereas dissolved organic carbon removal presented an opposite order in dependence of pH. In the meantime, some main intermediate products from 2,3-DCP degradation were identified by a 1H NMR technique to explore a possible degradation pathway. A major intermediate, 2-chlororesorcinol, was identified from the 2,3-DCP decomposition as a new species compared to the findings in previous reports. Photocatalytic deactivation was also evaluated in the presence of individual anions (NO3-, Cl−, SO42-, and H2PO4-). The inhibition degree of photocatalytic degradation of 2,3-DCP caused by these anions can be ranked from high to low as SO42- > Cl− > H2PO4- > NO3-. The observed inhibition effect can be attributed to the competitive adsorption and the formation of less reactive radicals during the photocatalytic reaction.