Visualization of coupled mass transfer and reaction in a gas–liquid dielectric barrier discharge reactor

• Visualized gas–liquid mass transfer and reaction in a plasma reactor by reactive-PLIF.
• Evaluated the effects of discharge voltage and atmosphere on water decoloration.
• Revealed the convective transportation of reactive species in the liquid induced by plasma.
• Determined the role of reactive species, e.g., O3, H2O2, OH, etc. in gas–liquid plasma.

A novel approach, i.e., the reactive planar laser induced fluorescence (reactive-PLIF) technique, is employed to visualize the coupled mass transfer and reaction in a gas–liquid dielectric barrier discharge (DBD) reactor, by quantitatively recording the dynamic change of concentration field of fluorescence dye (e.g., Rhodamine B) in the liquid layer during its decoloration process. Firstly, the decoloration dynamics are revealed under the conditions of different discharge voltages in oxygen atmosphere. Increasing the discharge voltage can give rise to the increase of the density of DBD plasma filaments impacting on the surface of the liquid layer. Thus, more reactive species (O3, H2O2, OH and other radicals detected by optical emission spectroscopy) are generated, together with the intensification of the convective transport in the liquid layer confirmed by Particle Image Velocimetry (PIV) measurement. Consequently, it can be demonstrated that the simultaneous intensification of the coupled mass transfer and reaction is the reason for the significant enhancement of the decoloration efficiency when increasing the discharge voltage. Secondly, the decoloration dynamics in the discharge atmosphere of inert gas (i.e., argon) are revealed in comparison with the oxygen discharge at the same discharge voltage. It is found that O3 plays a more important role in the reactive species for Rhodamine B degradation. This study provides straightforward analysis on the mechanism of the interaction between discharge plasma and polluted water, which will be beneficial to develop highly efficient cold plasma techniques for advanced oxidation processes (AOPs).