Degradation Process Optimization And Mechanism Of Azo Dye Wastewater By Electrochemical And Heterogeneous Electric Fenton Method

Dye wastewater,especially azo dyes,has the characteristics of high concentration,high toxicity and poor biodegradability.With the development of China’s textile industry,the pollution caused by azo dyes will be more and more serious.Harmless treatment of azo dyes wastewater must be carried out.This paper summarizes the treatment methods of dye wastewater.Taking acid orange 74（AO74）azo dye as the model organic compound,the degradation process and mechanism of low and high concentration AO74 by electrochemical oxidation（EO）and heterogeneous electro Fenton（H-E-Fenton）were studied in detail.The key is to improve the degradation/separation efficiency for high concentration azo dyes wastewater by H-E-Fenton and the deep oxidation for low concentration azo dyes wastewater by EO.The optimization process was used to treat the dyeing wastewater of Ningbo Qiaotaixing Textile Co.,Ltd.,which showed excellent results.Electrochemical oxidation（EO）is widely used because of its friendly environment,simple equipment,convenient operation and easy control.The process conditions on the electrochemical oxidation of low concentration AO74 were studied.The removal effect of organic pollutants depends on the·OH of the system.Compared with the active electrode,which only relies on the electrode surface for direct oxidation,the inert anode BDD performs best because it produces a large amount of·OH.The kinetic analysis showed that the degradation process accorded with the first-order kinetic model.It was found that the degradation process belongs to diffusion control.The optimum process was as follows:using BDD as an anode,current density 40 m A·cm^-2,C（Na₂SO₄）=2.5 g·L^-1,stirring speed 600 rpm and C（AO74）=50 mg·L^-1.The removal efficiency of azo double bond in AO74 could reach 100%.The degradation of high concentration dyes has always been the focus of attention.EO cannot meet the requirements.H-E-Fenton has the advantages of generation of H₂O₂ in-situ,avoiding the transportation and storage of hazardous chemicals,reducing energy consumption,reducing sludge production and cathode Fe²⁺continuous production.It is widely used in the degradation of high concentration dye organic wastewater.The catalyst is the key.Active carbon loaded iron has a broad application prospect in H-E-Fenton process because of its simple preparation method and easy recovery.The iron complex was loaded on the activated carbon in situ,Fe-N-AC catalyst was obtained.By characterization analysis of Boehm,IR,XRD,XPS,SEM and EDS,the results showed that activated carbon acidified by nitric acid was conducive to Fe³⁺loading.The surface of Fe-N-AC contained a large number of hydroxyl and carboxyl groups.The iron complex was successfully loaded on the activated carbon and the iron ions exist in the form of 3+of Fe₂O₃.The catalyst has excellent reusability.The regulation of H-E-Fenton process conditions is an important measure to improve the removal effect of dye organic wastewater.It is found that the catalyst concentration and current density are the most important.When the catalyst concentration is too low,the amount of active substances is insufficient.When the concentration is too high,the system will produce excessive Fe³⁺,and occur free radical scavenging side reactions.With the increase of current density,the electroregeneration ability of Fe³⁺to Fe²⁺increases,and the dye removal efficiency increases.However,if the current density is too high,side reactions will occur to generate weak oxidant·OOH.The oxidation of benzene ring was more difficult in AO74,and the oxidation degradation rate of-N=N-bond was 1.5 times that of benzene ring.In H-E-Fenton,the heterogeneous Fenton can improve the removal efficiency of organic functional groups,which are difficult to decompose.EO and Fenton have a synergistic effect.The optimum process was as follows:using BDD as anode,C（Fe-N-AC）=10 g·L^-1,current density 60 m A·cm^-2,p H=3,C（AO74）=1g·L^-1,C（Na₂SO₄）=2 g·L^-1,air flow=200 ml·min^-1.Azo double bond removal efficiency was 96%,benzene ring removal efficiency was 89%,COD removal efficiency was 89%,COD value was 90.2 mg·L^-1,COD value reach waste water draining standard.The optimized process has excellent results in the treatment of two kinds of organic industrial wastewater from Ningbo Qiaotaixing Textile Co.,Ltd.The COD value of the red wastewater can meet the emission standard.Although blue wastewater was difficult to treat,the B/C value was greatly improved,which can be continuously degraded by the economical biochemical method.Research results have good guiding significance for the treatment of industrial printing and dyeing organic wastewater.The degradation mechanism of dyes organic wastewater has very important theoretical significance.In this paper,the species and proportion of active substances in EO and H-E-Fenton were studied by three kinds scavenger such as isopropanol,p-benzoquinone and EDTA-2Na.For low concentration AO74,the active substance was mainly·OH and BDD has the strongest ability to generate·OH.For the oxidation of high concentration AO74 by H-E-Fenton,the addition of solid-phase catalyst can induce the production of oxygen hole active substances and promote the generation of·OH,so as to greatly improve the absolute quantity and relative proportion of active substances.Through the analysis of degradation products,the degradation mechanism of AO74 is put forward.When low concentration AO74 is oxidized by EO,·OH which is generated by BDD anode not only breaks the-N=N-bond,but also opens other ring structures.Intermediate degradation products include formic acid and acetic acid.Formic acid and acetic acid continue to mineralize into CO₂ and H₂O.When high concentration AO74 is oxidized by H-E-Fenton,there is a similar mechanism,·OH which is generated by BDD anode and H-E-Fenton not only breaks the-N=N-bond,but also opens other ring structures.Intermediate degradation products include formic acid and acetic acid.Formic acid and acetic acid continue to mineralize into CO₂ and H₂O.