Microwave-ultraviolet co-catalysis of chlorite-oxygen mixture for highly cost-efficient simultaneous removal of NO and Hg⁰

Developing novel advanced oxidation process (AOP) to simultaneously remove various air pollutants has received considerable attention in past years. This paper develops a novel cost-efficient AOP method of microwave (MW) induced ultraviolet (UV) co-catalyzing trace NaClO2-O2 to simultaneously remove NO and Hg0, the obtained removal efficiencies are 91.4% and 99%, respectively, under a molar ratio of NaClO2 to NO/Hg0 of 1.3:1. The effects of some key parameters including NaClO2 dosage, reaction temperature, input MW power, UV light intensity, concentrations of O2, NO and SO2, and gas flow on the removal of NO and Hg0 were studied. ESR and radical quenching tests demonstrated that superoxide radical (center dot O2-), hydroxyl radical (center dot OH), oxychloride radical (ClO center dot), chlorine dioxide (ClO2) and ozone (O3) were the major reactive species. With the assistance of Na2SO3 absorber, the produced NO2 was efficiently absorbed and controlled to below 5 mg/m3. IC and AFS analyses revealed the distribution of removal products: (i) 51% of NO3- and 85% of Hg2+ were preserved in the MW-UVEL reactor; (ii) the other NO3- and Hg2+ as well as the NO2- generated via the redox reaction between NO2 and SO32- were totally absorbed by the Na2SO3 solution. Mechanism analyses elucidated that the NO removal process was attributed to the radical-induced oxidation reaction, and the contribution order was center dot O2- > center dot OH > ClO center dot/ClO2 > O3; however, the Hg0 removal process was mainly determined by the produced NO2/NO3-, and UV light coupled with other active species contributed less 40%.