The microwave-/photo-assisted degradation of bisphenol-A in aqueous TiO2 dispersions revisited: Re-assessment of the microwave non-thermal effect

The microwave-assisted photodegradation of bisphenol-A (BPA) in the presence of TiO2 was examined at ambient temperature to unravel some details on the importance of the microwave non-thermal effect. Normally, bisphenol-A photodegrades under microwave irradiation as a result of a thermal effect emanating from the microwave radiation which sees increase in temperature in the aqueous TiO2 dispersion. However, on cooling the dispersion with circulating silicone oil at −20 °C, the temperature of the dispersion could be maintained constant at 21 °C. Under the latter irradiation conditions, the microwave-assisted photodegradation of 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 leading to enhanced photodegradation under constant ambient temperature. Other origins of the microwave non-thermal effect may include additional formation of charge carriers on the photomediator TiO2 and formation of additional trap sites that may otherwise prolong the lifetimes of the charge carriers through diminished recombination and ultimately increase process kinetics.