Analysis of the critical active species for methylene blue decoloration in a dielectric barrier discharge plasma system

In the paper, a hybrid gas–liquid dielectric barrier discharge (DBD) plasma system was set up to treat a methylene blue (MB) solution. The effects of the change of the carrier gas, the gas bubbling rate and different kinds of scavenger addition, including sodium carbonate (Na₂CO₃), para benzoquinone (p-BQ), triethylenediamine and sodium dihydrogen phosphate (NaH₂PO₄), on the MB decoloration were reviewed to clarify the critical active species for the dye decoloration in the DBD plasma system. The obtained results show that higher decoloration of the MB solution could be achieved when O₂ was used as the carrier gas, which could be 100% after 20 min discharge treatment, and the result confirmed the crucial effect of O₃ in the MB decoloration. Based on the experiments of the scavenger addition, it could be concluded that O₂^? and ¹O₂ were two other important reactive oxygen species (ROS) for the MB decoloration. The results of the higher chemical oxygen demand removal and faster disappearance of the characteristic peak of the MB from the UV–vis analysis under O₂ bubbling conditions also proved the critical effect of the ROS formed by O₂ on the MB decoloration.