| Coking wastewater is a kind of organic wastewater with complex composition,high concentration of pollutants and difficult to degrade.It is usually treated by biochemical denitrification process.In recent years,with the continuous improvement of water environment management requirements,the drainage of traditional biochemical processes has been difficult to achieve stable discharge standards.Advanced treatment of wastewater is required.Among various advanced treatment technologies,electrocatalytic oxidation technology has attracted the attention of scholars in the research of degrading water organic matter due to its strong oxidizing ability,simple operation,mild reaction conditions and environmental friendliness.With the addition of particle electrodes,the three-dimensional(3-D)electrode electrooxidation technology significantly improves mass transfer,increases the effective reaction area,improves current efficiency and reaction rate,and enhances the electrocatalytic oxidation ability of pollutants.It has excellent application prospects in the field of treating organic wastewater.Activated carbon fiber(ACF)is a new type of carbon material with high adsorption and desorption rates,large microporous structure and special surface reaction activity.As a 3-D particle electrode,it should have strong electrocatalytic activity.However,there are few studies on the research of ACF particle electrodes and the degradation of coking wastewater.Therefore,this paper plans to modify the ACF material and carry out the metal oxide loading,in the hope of preparing a new and efficient 3-D particle electrode.Then,the biochemical effluent of coking wastewater was degraded by electrocatalytic degradation.The main research results are as follows.The ACF was pretreated by concentrated nitric acid oxidation modification,liquid phase electrochemical modification(anodic oxidation and cathodic reduction),and 5 modified ACF particle electrodes were prepared under different modification conditions.Characterization analysis revealed that the surface oxidation degree of ACF was the highest after chemical modification of concentrated nitric acid,followed by anodic electrooxidation modification,and the lowest was cathodic electroreduction modification.M-cresol was selected as the model wastewater,and the 3-D electrode electrocatalytic oxidation reaction was performed at a current density of 20 mA/cm2.It was found that the TOC removal rate of the sample ACF-pH4 after the cathodic reduction was improved by 10%,and Samples modified by concentrated nitric acid will reduce the reaction effect.The cyclic voltammetry(CV)curve shows that the modified ACF-pH4 has a larger capacitance than the unmodified ACF.By programmed temperature desorption mass spectrometry(TPD-MS)and X-ray photoelectron spectroscopy(XPS)analysis,a variety of surface oxygen groups(SOGs)were found on the surface of the modified ACF,but the proportion of graphitic carbon decreased.Electron paramagnetic resonance(EPR)measurements of hydroxyl radicals(·OH)during the reaction showed that the reaction was not a free radical mechanism.Combining the above analysis,this study proposes the SOGs reaction degradation mechanism of ACF 3-D particle electrodes,and calculates and discusses the effects of different SOGs on its electrocatalytic oxidation performance by using front-line molecular orbital energy calculations.Among them,carboxylic anhydride has the highest oxidation capacity and ether has the oxidation capacity lowest.Various metallic oxides nanoparticles supported on commercial activated carbon fibers(M/ACF)were fabricated and employed as particle electrodes to degrade aqueous m-cresol in 3-D electrochemical systems.It has been experimentally demonstrated the ACF loaded with MnOx exhibited relatively higher 3-D electrooxidation performance than other metallic oxides(FeOx,CuO and SnO2).The electrocatalytic oxidation and electrochemical performance tests show that Mn/ACF can effectively reduce the side reactions of oxygen.The optimal preparation conditions for Mn/ACF are 6%loading,the precursor is Mn(NO3)2,the firing temperature is 450?,and the corresponding TOC removal rate can reach 68%,while the TOC removal rate of the unloaded ACF particle electrode is only maintained at about 40%.According to XPS and transmission electron microscope(TEM)analysis,we can see that the MnOx supported on the Mn-450/ACF surface is mainly composed of MnO2 and Mn2O3 nanoparticles,and the higher valence MnOx is more conducive to the rapid degradation of pollutants.The EPR measurement results show that ACF can generate ·OH through electro-oxidation with the aid of surface-loaded MnOx,thereby enhancing the removal effect of organic pollutants.The reaction mechanism should have both the role of SOGs on the surface of ACF and the degradation process of ·OH.Mn-450/ACF material with the best electro-oxidation performance was used as the particle electrode to investigate the adsorption and electrooxidation degradation behavior of different phenolic pollutants on the 3-D electrode.As the number of chlorine atoms in the phenolic branch increases,the electrode adsorption performance increases accordingly.The presence of methyl substituents on the benzene ring was conducive to the degradation of pollutants,while the presence of chlorine substituents was not conducive to the degradation of pollutants.The degradation process was theoretically explained by the frontier molecular orbital theory.When using Mn-450/ACF as the particle electrode and the actual coking wastewater biochemical effluent as the research object,it was found that as the initial pH value of the solution decreased and the current density increased,the effect of the 3-D electrode electrocatalytic oxidation reaction showed an upward trend.Under the optimal reaction conditions,the particle electrode can run stably and continuously.The average COD removal rate in the wastewater can reach about 60%.The daily average COD of the effluent is below 100 mg/L,which meets emission standard of pollutants for coking chemical industry(GB 16171-2012)limits for direct discharge of wastewater. |
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