Degradation mechanism of Alizarin Red in hybrid gas–liquid phase dielectric barrier discharge plasmas: Experimental and theoretical examination

The degradation of the dye Alizarin Red (AR) has been studied in the hybrid gas–liquid dielectric barrier discharge (DBD) plasma. The relationship between the degradation efficiency of AR and the ozone concentration in solutions is analyzed. Based on the bond dissociation energies (BDEs) theory and experimental results, the degradation pathway of AR in DBD plasma is theoretically proposed. It is found that the degradation course consists of three steps in the order of gas-discharge phase, ring-open phase, and mineralization phase. The AR molecule is cleaved into two parts from the weakest bonds 4C–5C and 12C–13C after losing −NaSO3 first in the gas-discharge phase, when the aqueous ozone concentration is low, leading to low degradation efficiency. Both cleaved transitional groups undergo further oxidation reactions subsequently, forming some unstable intermediates with low molecular weights in the ring-open phase. In this phase the degradation efficiency rises quickly with a sharp increase of the aqueous ozone concentration. Most intermediates are carrying with free radicals, which promote the degradation during the mineralization phase. The main longer-lived species are glyoxylic acid and vinyl formate besides CO2.