Role of reactive oxygen species in the dechlorination of trichloroethene and 1.1.1-trichloroethane in aqueous phase in UV/TiO2 systems

The performance of trichloroethene (TCE) and 1.1.1-trichloroethane (TCA) degradation and dechlorination in aqueous solution using UV and UV/TiO2 was investigated. The effects of pH and the roles of the reactive oxygen species generated during the degradation of the target contaminant were evaluated. The TiO2 was synthesized using a simple hydrothermal solution containing tetrabutyl-titanate and hydrofluoric acid and was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results showed that the TiO2, which was in the form of anatase, consisted of well-defined sheet-shaped structures with a rectangular outline equivalent to Ti-F bonds that were formed on the surfaces of photocatalysts. TCE and TCA were completely degraded (100%) in 60 and 90 min of illumination, and the degradation of TCE followed the pseudo-first order kinetic model, while the degradation of TCA showed a slow degradation stage initially followed by a fast stage later. The inhibitive effects of pH on TCE degradation were negligible because complete degradation occurred in all the tested pH solutions. Instead, the pH affected the degradation of TCA. Probe compound tests using nitrobenzene (NB) and tetrachloromethane (CT) identified the generation of hydroxyl radicals (center dot OH) and the superoxide radical anion (O-2(center dot-)) in the UV/TiO2 system. Free radical quenching studies demonstrated that all of the studied reactive oxidative species contributed to the TCE and TCA degradation. However, O-2(center dot-) and center dot OH were main detected radicals and contributed more to the degradation process than the O-3(2) radical. It is believed that the results from this study can be used to support further UV/TiO2 studies for remediating groundwater contaminated with chlorinated solvents. (C) 2014 Elsevier Ltd. All rights reserved.