Microwave/Persulphate assisted ZnO mediated photocatalysis (MW/PS/UV/ZnO) as an efficient advanced oxidation process for the removal of RhB dye pollutant from water

Microwave (MW), ultraviolet (UV) light and MW-UV combination are tested as sources of activation for the ZnO-mediated, persulphate (PS)-assisted (MW/PS/UV/ZnO) degradation/mineralization of the pollutant dye Rhodamine B (RhB) in water. The simultaneous irradiation by MW and UV is more efficient and synergistic for the mineralization of the dye compared to the respective individual energy sources. The degradation is influenced by catalyst dosage, concentration of the dye, pH of the medium, presence of contaminant salts, dissolved oxygen etc. The synergy of the MW/PS/UV/ZnO process in comparison to the respective individual processes (MW/PS and UV/ZnO) is attributed to the 'nontherme' and 'specific' effect of MW which induce increased formation of reactive oxygen species (ROS) such as O-2(-), and (OH)-O-center dot radicals. Formation of surface defects which inhibits the recombination of photoinduced electrons and holes in ZnO is another major reason for the synergy. MW enhances the degradation/mineralisation of RhB even in systems with less dissolved oxygen because of the efficient transference of oxygen from the bulk of the catalyst to the surface. Another interesting observation is that simultaneous MW + UV irradiation of ZnO suspensions deaerated with N-2 results in N-doped ZnO, with extended absorption in the visible range of sunlight. The potential of this simple convenient method without any cumbersome procedure for preparing N-doped ZnO needs further in-depth investigation. The study demonstrated the potential use of the combination process 'microwave-photocatalysis' for the efficient decontamination of water from dye pollutants.