Experimental Study On Pretreatment Of Nitrobenzene Wastewater With Three-Dimensional Electro-Fenton As The Core

The rapid development of the fine chemical industry has produced a large number of refractory organic pollutants.Most of these refractory organic pollutants have the’three-induced’effect,among which nitrobenzene organics are typical representatives.Due to its special cyclic molecular structure,biodegradability is limited,and the treatment efficiency in wastewater is constrained.The residues in the effluent pose a serious threat to the safety of the regional water environment.Because nitrobenzene wastewater generally has the characteristics of high concentration,difficult degradation and complex composition,it is necessary to carry out efficient pretreatment before biochemical treatment.In this study,a new three-dimensional electro-Fenton（3D-EF）pretreatment technology was developed,and it was used as the core process combined with electrocoagulation-flotation technique（EC）to pretreat high-concentration nitrobenzene wastewater in order to effectively degrade nitrobenzene substances in wastewater for subsequent further treatment.The main results of this paper include:（1）The use of double aluminum electrode EC process can effectively reduce the concentration of nitrobenzene in wastewater.Increasing pH,current density and reaction time can improve the removal rate of nitrobenzene（mononitro+dinitro）and COD_Crby EC process,but the plate spacing has little effect on the removal rate.The optimum operating parameters of EC process were determined as follows:influent pH was 9,current density was 10 m A/cm²,plate spacing was 3 cm,and reaction time was 100 min.Under these conditions,the average removal rates of nitrobenzene（mononitro+dinitro）and COD_Crreached 60.34%±1.84%and 12.10%±1.12%,respectively.In addition,increasing the pH and current density of the raw water will increase the removal rate of nitrobenzene and COD_Crin the solution,but the concentration of aniline in the effluent shows an upward trend.Under the optimal reaction conditions,the average concentration of nitrobenzene in the raw water was239.27 mg/L,the average concentration of aniline in the effluent was 120.01 mg/L,and the average concentration of nitrobenzene was 94.87 mg/L.After the treatment of nitrobenzene wastewater by EC process,83.11%was converted into aniline,and only16.89%was degraded.The EC process cannot achieve a significant reduction in COD_Cr,but it can realize the effective conversion of nitrobenzene organics to aniline organics with good biodegradability,providing favorable conditions for subsequent further treatment.At the same time,the substantial reduction of nitrobenzene substances is conducive to the production process treatment to improve the three-effect evaporation concentration multiple,reduce the production of concentrated mother liquor,and greatly reduce the cost of nitrobenzene wastewater treatment..（2）The efficient degradation of nitrobenzene pollutants on the basis of EC process was realized by 3D-EF process.The experimental results showed that the optimum operating parameters of 3D-EF process were as follows:the dosage of three-dimensional particles was 100 g/L,the mass ratio of sponge iron to activated carbon was 3:1,the current density was 30 m A/cm²,the dosage of hydrogen peroxide was 50 mmol/L,the cathode aeration rate was 0.8 L/min and the power-on time was120 min.Under the optimum parameters,the average removal rates of nitrobenzene（mononitro+dinitro）and COD_Crreached 67.38%±1.05%and 70.60%±1.15%,respectively.The 3D-EF process increased the TDS content to 891.8 mg/L,which was much lower than the increase of the traditional Fenton process,reduced the pressure of the subsequent desalination process,and reduced the cost of artificial reagents and costs.The continuous and effective precipitation of Fe²⁺can promote the Fenton reaction in the system while the combination of sponge iron and activated carbon plays the role of’iron-carbon microelectrolysis’.（3）The surface morphology of granular electrode sponge iron and activated carbon before and after 3D-EF reaction changed significantly.After the reaction,the surface morphology of sponge iron becomes more rough and dense,and a large number of white crystal particles are attached to the surface,and the corrosion is serious.In addition,due to the continuous dissolution of Fe²⁺from sponge iron to participate in the Fenton reaction and the decomposition of some organic matter on the surface of the particle electrode,the content of Fe element in sponge iron decreased significantly and the content of O element increased after the reaction.The surface morphology of activated carbon changed from a flat,smooth and evenly distributed void structure to a rough surface with a large area of ravines and a lot of small irregular flake and flocculent particles.In addition,since the combination of sponge iron and activated carbon can form a miniature primary battery,a large amount of iron ions are adsorbed on the surface of the activated carbon,resulting in an increase in the content of Fe on the surface of the activated carbon after the reaction.At the same time,the metal elements on the surface of the particle electrode are likely to be involved in catalysis,oxidation or replacement.Under the optimal conditions,the accumulation of·OH can reach 1.721×10^-2mmol/L,which provides conditions for further oxidative degradation of nitrobenzene and aniline organic pollutants.（4）The double aluminum electrode EC process has a significant removal effect on nitrobenzene and phenol in raw water.Nitrobenzenes were efficiently converted in EC process,and the main products were anilines and azobenzenes.This conversion process can not only improve the biodegradability of wastewater,but also provide a good prerequisite for further treatment.The degradation pathway of nitrobenzene by EC process mainly includes two parts:a part of nitrobenzene is removed in the form of precipitation and scum by air flotation and flocculation precipitation;another part of nitrobenzene is reduced to aniline by·H near the cathode,and aniline will oxidize with·OH near the anode plate to form azoxybenzene,but this process cannot achieve ring opening of nitrobenzene organics.（5）The 3D-EF system has a significant degradation of nitrobenzene and aniline in wastewater,and achieves efficient degradation of nitrobenzene and aniline.The degradation products of nitrobenzene substances are mainly nitromethane and methyl butyrate.The mechanism of degradation process is speculated as follows:some nitrobenzene and aniline molecules will be oxidized and degraded by·OH generated by the reaction of Fe²⁺dissolved in the solution with H₂O₂:a part of the pollutant molecules adsorbed on the particle electrode will also be attacked,and some of the degraded pollutants will be desorbed,and then the adsorption and oxidation process will be carried out again.A small part of the pollutant molecules will also be oxidized and degraded by the·OH generated by the anode,that is,the titanium-based ruthenium dioxide plate.Under the continuous attack of·OH,the organic pollutants undergo hydroxyl addition,bridge fracture and ring opening reaction to form nitromethane and methyl butyrate,which are finally oxidized to CO₂and H₂O.The degradation process of nitrobenzene organics can effectively improve the biodegradability of wastewater,which is conducive to further biochemical treatment.In this paper,the treatment effect of nitrobenzene wastewater with three-dimensional electro-Fenton system as the core was studied from the removal rate of nitrobenzene（mononitro+dinitro）and COD_Cr,and the degradation effect of refractory organic matter.The degradation pathway and mechanism of nitrobenzene organic matter in complex wastewater were mainly explored,in order to provide scientific basis and technical support for the treatment of nitrobenzene chemical wastewater.