High-gravity Intensified Fe-C Micro-electrolysis Catalytic Ozone Treatment Of DNT Actual Wastewater

Due to the electron-withdrawing effect of nitro groups,the wastewater containing nitro compounds produced during the production of dinitrotoluene（DNT）is particularly stable.Fe-C micro-electrolysis can change refractory nitro groups to easily degradable amino groups,and the electrolysis tail water is deeply oxidized by the catalysis system of ozone and Fe²⁺dissolved in the electrolysis tail water.However,there are problems in the long-term operation of iron-carbon packing in wastewater treatment,such as iron-carbon fillers are easily passivated and difficult to be continuously available.The cleaning and regeneration of the fillers increases the cost of wastewater treatment,making it difficult for the iron-carbon micro-electrolysis process to operate continuously and efficiently for a long time.At the same time,the catalytic ozone method has the problem of insufficient gas-liquid contact,resulting in low ozone mass transfer efficiency and low utilization rate.Based on the above existing problems,combined with the theory of enhanced mass transfer and surface renewal by high-gravity technology,the idea of high-gravity intensified Fe-C micro-electrolysis catalyzed ozone treatment of dinitrotoluene wastewater is proposed.Commercially available iron-carbon spheres are used as the packing in the supergravity rotating packed bed（RPB）.The high-speed rotating filler causes liquid disturbance,which enables the active sites on the surface of the iron-carbon packing to be renewed,resulting in large and rapid renewal.liquid-solid interface to solve the problem of passivation of iron-carbon packing.At the same time,the RPB filled with wire mesh is used as the reactor to solve the problems of low ozone solubility and low mass transfer efficiency in the process of catalytic ozone degradation of organic matter.The specific experiments are as follows:（1）In the High-gravity intensified Fe-C micro-electrolysis experiment,the single factor analysis method was used to explore the influencing factors of DNT wastewater.It is found that under optimal conditions（high-gravity factorβ=46.22,liquid flow rate Q=80 L/h,initial solution p H=1.1）,the removal rate of nitro compounds reaches 68.4%at 100 min.On this basis,the effects of two different reactors,RPB and stirred-tank reactor（STR）,on the treatment of the wastewater by Fe-C micro-electrolysis were compared under the same conditions.The research results show that the high-gravity technology or RPB can significantly strengthen the effect of Fe-C micro-electrolysis,improve the removal rate of nitro compounds in DNT wastewater,effectively solve the passivation problem of iron-carbon fillers,and prolong the service life of iron-carbon packing.（2）In order to further treat DNT wastewater and make it meet the discharge standard,it is proposed to use Fe²⁺dissolved in the wastewater after Fe-C micro-electrolysis to catalyze ozone advanced treatment of tail water.The degradation effect of RPB-O₃/Fe²⁺system on Fe-C micro-electrolysis tail water was investigated.In this paper,the effects of ozone concentration,high gravity factor and liquid flow rate on the degradation efficiency of Fe-C micro-electrolysis tail water were studied.The research shows that under the condition that the wastewater quality（p H,total iron concentration）remains unchanged after iron-carbon micro-electrolysis,the high-gravity factorβ=30,the liquid flow rate Q=80L/h and the ozone concentration C_O3=80mg/L,the nitro compounds in the tail water after Fe-C micro-electrolysis can be completely removed,which meets the national first-class standard（GB 8978-1996）,and the degradation efficiency of total organic carbon（TOC）reaches 83%.The comparison results of different processes show that the High-gravity intensified the mass transfer of ozone and improves the degradation effect of wastewater.The response surface method study shows that there is an obvious interaction between the high gravity factor and the gas phase ozone concentration.After optimizing the process conditions by response surface methodology,under the conditions of high-gravity factorβ=29.36,liquid flow rate Q=78 L/h,and ozone concentration C_O3=96.6 mg/L,the degradation efficiency of TOC is 90%,and the predicted value（88.22%）is similar to the actual value differs by 1.78%.Therefore,it is reasonable to use the response surface methodology to optimize the process conditions of Fe²⁺catalyzed ozone degradation of Fe-C micro-electrolysis tail water in hyper gravity field.（3）After the coupling operations of（1）and（2）above,the concentration of nitro compounds in DNT wastewater has been completely removed,and the TOC degradation rate is90%,which greatly reduces the biological toxicity of DNT wastewater and satisfies the subsequent biochemical treatment requirements.In order to clarify the mechanism and degradation pathway of the above two processes,in this study,nitrobenzene was used as a typical representative of nitro compounds.The intermediate products of benzene wastewater were analyzed,and their degradation pathways were speculated:Fe-C micro-electrolysis provides 6 electrons to NB,and NB is reduced to nitrosobenzene,hydroxyaniline and end product aniline in turn under the action of electrons.Aniline becomes p-iminoquinone in Fe（II）/O₃ system,and then oxidatively deaminated to generate active and unstable p-benzoquinone;the unreduced NB in tail water is oxidized to nitrophenol,which is further oxidized to p-benzoquinone However,p-benzoquinone,which is more active in physical and chemical properties,is easily decomposed to generate small molecular organic acids,and finally the small molecular organic acids are oxidized to CO₂ and H₂O.