Catalytic non-thermal plasma process for the degradation of organic pollutants in aqueous solution

The aim of the present work was to optimize the performance of the Non-Thermal Plasma (NTP) technology coupled with a structured catalyst for the degradation of organic and recalcitrant pollutants in aqueous solution. Specifically, the catalyst consists of CeO2 supported onto gamma-Al2O3 spheres, synthesized by the wet-impregnation method, and characterized through Raman spectroscopy, specific surface area (BET) by N-2 adsorption at 196 C and Scanning Electron Microscopy (SEM). All experiments were carried out in a Dielectric Barrier Discharge (DBD) reactor. The first sets of tests were accomplished to optimize the degradation of Acid Orange 7 (AO7) azo dye in aqueous solution. The effect of catalyst formulation and oxygen flow rate in the reactor, as well as the reusability of the catalyst, were studied in detail. Moreover, the effect of radical scavengers was examined to suggest a plausible degradation mechanism, and an HPLC analysis was performed to understand to which extent degradation proceeds with the formation of intermediates. The best performance of the system in terms of total degradation and mineralization efficiencies is found for 15 min run time using 2.9 wt% CeO2/gamma-Al2O3, with a voltage equal to 12 kV and 0.045 NL/min oxygen flow rate. The catalyst showed reusability properties after 5 runs and no intermediates were found by HPLC analysis, suggesting that AO7 degradation takes place with complete mineralization. Scavenger analysis showed that ozone is the most important reactive species responsible for degradation since its decomposition leads to the generation of effective secondary oxygen reactive species. Furthermore, given the semiconductor features of CeO2, it may also be excited by the plasma-generated UV radiation, suggesting that electron-hole pairs were also involved in the degradation mechanism. The remarkable efficiency of the proposed catalytic NTP treatment shows the potential of the system for the decontamination of aqueous solutions containing organic and recalcitrant pollutants. To confirm the potentiality of the system, the degradation of glyphosate (GLY) was also assessed under the optimal conditions set up for AO7 degradation, obtaining 84% degradation efficiency after 30 min run time.