Microwave-assisted catalytic wet peroxide oxidation: Energy optimization

Abstract

Over the past few years, microwave technology (MW) has been successfully coupled to different advanced oxidation processes for wastewater treatment. This intensification method provides a rapid and homogeneous heating and, in presence of microwave absorbing materials, hot spots can be generated. These have shown a significant contribution on the overall efficiency of the process. Up to the date, there is no information on the influence of the radiation mode on the system. To gain knowledge on this issue, three different operation modes were studied in the MW-assisted catalytic wet peroxide oxidation of phenol: (i) controlled temperature (120 °C), (ii) controlled continuous MW and (iii) controlled pulsated MW. Other experimental conditions were [Phenol]0: 100 mg·L−1, [H2O2]0: 500 mg·L−1, [graphite]: 500 mg·L−1, pH0:3. The pulsated method reached 90% mineralization degree and complete H2O2 decomposition using 240 kJ, three times less energy than the controlled temperature run. Continuous MW showed a slightly inferior performance (XTOC: 82%), ascribed to a worse energy dissipation from the hot spots on the material. After reaction, only biodegradable short chain carboxylic acids remain in the effluent and there are no significant modifications on the catalyst crystallinity, despite the extreme conditions produced by hot spot formation. Under these conditions TOC abatement follows a pseudo-second kinetic order (EA,TOC ≈ 30–40 kJ·mol−1), whereas H2O2 decomposition fits by a pseudo-first order (EA,H2O2 ≈ 33–52 kJ·mol−1). Additionally, the specific energy consumption (ECTOC) for the controlled power runs was lower than that of other intensified AOP, making MW-CWPO a competitive technology for wastewater treatment.