Abstract
Simultaneous removal of NOx and SO2 during the wet absorption process has made it possible for nitrogen resource utilization. However, nitrites formation at high ratio in absorption solution would limit its application. In this study, the catalytic oxidation behaviors of aqueous nitrite ions assisted by ozone on HZSM-5 zeolites with different SiO2/Al2O3 ratios have been investigated. The experimental results revealed that the oxidation and disproportionation reactions of nitrite ions took place competitively, both of which were accelerated under acidic condition. Moreover, the introduction of HZSM-5 zeolites and ozone would significantly improve the nitrite oxidation rate, where the zeolites with high SiO2/Al2O3 ratios were found to be more effective owing to the enhanced adsorption of nitrite ions and ozone. Based on the results under different operating conditions (such as O3 concentration, HZSM-5 dosage, pH values and presence of radical scavengers etc.), the reaction mechanism was then proposed. The disproportionation reaction of nitrite ions mainly occurred in the bulk solution. And the catalytic oxidation of nitrite ions over zeolites proceeded via a non-radical surface reaction between the adsorbed nitrite ions and ozone/oxygen molecular.
Highlights
Nitrogen oxides (NOx) caused by fossil fuel combustion are one of the major atmospheric pollutants and can result in acid rain and photochemical smog[1,2,3]
It could be concluded that the surface oxidation reaction was dominated in the total oxidation of nitrite ions, which could be verified by the fact that in the presence of HZSM-5, the oxidation efficiency was greatly improved and the oxidation efficiency increased with the rising of HZSM-5 dosage
It was found that the oxidation and disproportionation reactions of nitrite ions could take place simultaneously
Summary
Nitrogen oxides (NOx) caused by fossil fuel combustion are one of the major atmospheric pollutants and can result in acid rain and photochemical smog[1,2,3]. The NH3-SCR technology seems to be not very cost-effective for low-content NOx abatement from industrial boilers and furnaces. Wet scrubbing of NOx in flue gas desulfurization (FGD) device, due to its low costs and less land occupied, has been considered as a promising supplementary or alternative to the NH3-SCR technology[7]. When combined with additional oxidation processes (transforming NO to NO2, normally), NOx, as well as SO2, could be effectively removed in wet FGD device via a series of free radical reactions between NO2 and sulfite ions[8]. Heterogeneous catalytic wet oxidation has been considered as an effective technology to remove both organic and inorganic pollutants from industrial waste www.nature.com/scientificreports/. Such technology is of the advantages including low cost, mild operational condition, low energy consumption and high efficiency[27,28,29]. ZSM-5 zeolites could be potential superior catalysts for catalytic wet oxidation of nitrite ions
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