Experimental Study On The Advanced Oxidation Of Aqueous Nitrites

Nitrogen oxides（NO_x）are one group of the main atmospheric pollutants.Wet absorption denitration process in flue gas desulfurization（FGD）device has attracted more and more attention in economy and adaptability.However,the absorption solution of this process contained large amounts of nitrites,which were unstable and would lead to the emission of secondary pollution.If the problem of efficient oxidation of NO₂^-could be overcome,it would provide the possibility for nitrogen-resource untilization and facilitate the application of wet combined desulfurization and denitrification technology.In this thesis,three different advanced oxidation processes（including ozone assisted wet catalytic oxidation process,fenton process and ultraviolet assisted oxidation process）were employed for aqueous nitrite oxidation,which would provide some experimental guildlines for nitrogen-resource reuse.Firstly,HZSM-5 zeolites with different Si/Al ratios were used for the catalytic oxidation of NO₂^-.And the addition of trace ozone was employed to improve the oxidation of NO₂^-efficiently.The oxidation efficiency and disproportionation ratios were investigated under different operating conditions and the relative reaction mechanism was discussed.It was found that the presence of ozone and HZSM-5zeolites would promote the surface oxidation reaction and would greatly inhibit the disproportionation reaction of NO₂^-.In addition,the adsorption of nitrite ions played a vital role in surface catalytic oxidation reaction.What’s more,the catalytic oxidation of NO₂^-on the zeolites proceed through the direct reaction of molecular oxygen and ozone with adsorbed nitrite ions.Secondly,in order to avoid the NO₂^-disproportionation reaction,the nitrite oxidation experiments under near neutral conditions were performed based on the Fenton oxidation system,using Fe-Zr-O as catalysts.It was found that the strong Fe-Zr interaction would improve the surface Fe²⁺content and enlarge the surface area of the catalysts,thereby increasing the oxidation efficiency.The Fe Zr-6 sample（with Fe:Zr molar ratio of 6:4）possessed the best catalytic performance,exhibiting an oxidation efficiency of more than 90%at 60 min.Furthermore,excessive Fe would weaken the Fe-Zr interaction and reduce the surface Fe²⁺content,thus decreasing the catalytic activity and stability.Finally,in order to avoid secondary pollution caused by ion leaching from the catalysts,a comparative study of the VUV/H₂O₂ and VUV/O₃ systems for nitrite oxidation was carried out.It was obtained that VUV could efficiently enhanced the oxidation efficiency of H₂O₂ and O₃,and the promoting effect on H₂O₂ oxidation was more obvious.Furthermore,short-wavelength ultraviolet light and acidic environment were more conducive to the improvement of NO₂^-oxidation efficiency in VUV/H₂O₂system.Moreover,the economic analysis showed that VUV/H₂O₂ system was superior to the VUV/O₃ system in running cost.The dissertation provided three feasible technical routes for the efficient oxidation of aqueous nitrite,and the results could offer solid data-support for the nitrogen resource recycling in wet combined desulfurization and denitration process.