On the influence of the microwaves' thermal and non-thermal effects in titania photoassisted reactions

In this paper we contend that the microwaves' non-thermal effect(s) is an important factor in the enhancement of TiO2 photoassisted reactions involving the decomposition of organic pollutants in model wastewaters by an integrated microwave-/UV-illumination method (UV/MW). This method proved far superior in degrading organic pollutants than the more traditional TiO2-assisted photodegradations under UV irradiation alone (UV method). For instance, all the functions in the rhodamine-B (RhB) dye structure decompose competitively by the UV method for which the transformation of the nitrogen atoms was not an insignificant part of the process; however, their transformation to NH4+ ions (and some NO3- ions) was significantly enhanced by the UV/MW method. The latter also proved far superior to the UV method in the degradation of 1,4-dioxane under identical temperature conditions as for the UV/MW method but with conventional heating (UV/CH). In addition, the microwave-assisted photodegradation of bisphenol-A (BPA) was shown to be not only due to a microwave thermal effect, but also to a significant non-thermal effect that might implicate hot spots on the TiO2 particle surface when MW irradiated leading to enhanced photodegradation at near-ambient temperatures. Such enhancements in process dynamics may be due to enhanced formation of center dot OH radicals under UV/MW irradiation, at least for the Evonik P25 titania system. The response of this metal oxide specimen to the microwave non-thermal effect(s) has been correlated to changes in lattice distortions in the TiO2 crystalline structure. (C) 2013 Elsevier B.V. All rights reserved.