Degradation of Methyl Orange wastewater by dielectric barrier discharge plasma: process optimization, assessment of degradation mechanisms

The degradation performance of Methyl Orange (MO) wastewater by different plasmas (i.e., O2, Ar, Air, O2/Ar, and Air/Ar) was experimentally examined in a parallel plate dielectric barrier discharge (DBD) reactor driven by an AC power. Firstly, the discharge characterizations and degradation performance of different single gases were investigated, and it was found that the removal efficiency sequence of MO and COD in the wastewater was O2> Ar >Air, although Ar discharge was the strongest. Through capture experiments, it was known that hydroxyl radical (·OH) was the most effective active particle for degrading MO in Ar atmosphere, while hyperoxygen group (·O2-) was the most effective particle for degrading MO in O2 and Air atmosphere. In order to improve the degradation performance of the organic dye wastewater, further research was conducted on the degradation of MO wastewater by O2/Ar and Air/Ar mixtures. Compared to pure O2 and Air conditions, the gas discharge was significantly enhanced by adding Ar to O2 and Air, and the degradation rate, degradation amount, energy efficiency, and COD removal rate of O2/Ar=2:1 and Air/Ar=1:2 plasma were higher than those of pure O2 and Air, respectively. This may be because that more particles of·OH and H2O2 resulted in the enhancement of oxidizing properties of O2/Ar and Air/Ar. Furthermore, it was found that the effects of both ·O2- and ·OH were significant in O2/Ar =2:1 and Air/Ar (1:2) conditions, respectively. The FTIR testing showed that compared with Air/Ar =1:2 weaker degradation ability, most N=N in MO molecule was opened, and almost substituent group on the benzene ring at 1195 cm-1 and 1123 cm-1 were broken by O2:Ar=2:1plasma. In addition, it was observed that the plasma degradation system with the mixed gases had good continuous degradation ability. Therefore, it is worth mentioning that DBD plasma discharge can be a useful method for degrading organic dye wastewater by enhancing the oxygen-containing gas discharge.