Photodegradation of the Rhodamine B using the microwave/UV/H2O2 process: effect of the temperature

The oxidative decoloration of Rhodamine B was performed in a photochemical reactor which activates a chemical reaction with microwave (MW) and UV radiation, when these are applied simultaneously. We used an immersed source of microwave, with no need for an oven. Controlling the temperature, MW power, UV emission of the reactor led to a greater overall control of the process. The decoloration of Rhodamine B, due to the action of highly-reactive hydroxyl radicals, was followed on-line using a fiberoptic spectrograph. In the optimised case, complete decoloration occurred in four minutes and 92% of mineralisation was obtained in 70 minutes. The experiments were performed at various temperatures (21 °C, 30 °C, 37 °C and 46 °C), with and without hydrogen peroxide, in order to obtain the rate of reaction at different temperatures. The apparent rate of reaction was used to calculate the apparent activation energy of the decoloration process. The apparent activation energy was 38 kJ/mol and 40 kJ/mol in the presence (400 mg/l) or absence of hydrogen peroxide, respectively. Thus Ea was found to be independent of the addition of hydrogen peroxide. This is the first time that the apparent activation energy Ea has been reported with microwave irradiation, with and without hydrogen peroxide. The lack of deviations from the linear behavior of the Arrhenius plot confirms that the application of MW does not affect the apparent rate of reaction of the decoloration of RhB. The apparent activation energy value found was compared with the few examples of data from the literature obtained in the absence of MW radiation.