Plasmacatalytic bubbles using CeO₂ for organic pollutant degradation

Interactions between effects generated by non-thermal plasma (NTP) and catalysts are increasingly investigated, ranging from energy conversion and material synthesis to pollutant degradation (e.g. volatile organic compounds, VOCs) and water purification. To make the water ecosystem more environmentally and economically sustainable, green and efficient wastewater treatment technologies are urgently required. In this study, a plasmacatalytic bubble column reactor was designed to generate underwater plasma bubbles (PBs) for efficient delivery of plasma-produced gaseous reactive oxidative species to the liquid phase for subsequent applications. This system shows great potential for degrading a stubborn water contaminant azophloxine (AP), which was further enhanced with an addition of CeO₂ catalysts, resulting in the catalytic oxidation of the azo dyes. Moreover, the performances and durability of CeO₂ were further improved via plasma pre-activation. The plasma-treated CeO₂ (CeO₂-P) displayed a combination of favourable properties, such as a higher concentration of oxygen vacancies and Ce³⁺ (observed by XPS), a narrowed energy bandgap, and an increased specific surface area (by 41.39%). Compared with the sole PBs and PBs/CeO₂ systems, the highest removal efficiency and kinetic constant up to 99.5% and 0.907 min⁻¹, respectively, were achieved in the PBs/CeO₂-P system. Reactive species trapping corroborated that ·OH and ·O₂^– played an integral part in AP degradation. Finally, intermediates in the process of AP degradation were identified and a degradation pathway was proposed. This research would offer a promising remediation methodology for water purification via the integration of underwater plasma bubbles and CeO₂ catalysts.