New insight into the behavior and cost-effectiveness of different radicals in the removal of NO and Hg⁰

Radical-induced removal of NO and Hg-0 is a hot topic, but few people comparatively study the behavior and costeffectiveness of three categories radicals (Cl-radicals, SO4 center dot(-) and HO center dot) in the removal of NO and Hg-0. For this, we used ultraviolet (UV) to irradiate NaClO2, NaClO, Na(2)S2O(8), KHSO5 and H2O2, respectively, to produce the desirable radicals, and used electron spin resonance (ESR) to reveal the generation and evolution of the radicals in the five UV-AOP systems under different pH. The results demonstrated that Cl-radicals (ClO2, ClO center dot and Cl-2 center dot(-)) produced in UV/NaClO2 and UV/NaClO were more efficient than SO4 center dot(-) and HO center dot produced in UV/Na2S2O8, UV/KHSO5 and UV/H2O2 in term of removal of NO and Hg-0. UV/NaClO2 had the best cost-effectiveness (hundreds to thousand times higher than others), possibly due to NaClO2 possessed the highest absorbance at 254 nm (124.3 abs/mol), implying a possible highest quantum yield. Cl-radicals also exhibited a good selectiveness toward the removal of NO and Hg-0 under high concentration of SO2. Hence, Cl-radical-based UV-AOP system, especially UV/NaClO2, is hopefully to be an emerging and promising UV-AOP method which can be used in both air pollution control field and water treatment field.