Study On The Mechanism Of High-Salty 3,4-DMA Wastewater Degradation By Hybrid Photoelectrocatalytic Oxidation

3,4-DMA wastewater is a kind of high salt content of refractory organic wastewater and have carcinogenesis,teratogenicity and mutation trace toxic effect,which can cause serious harms to human body and local environment.It is difficult to be removed effectively by traditional biochemical treatment and becomes a key and difficult research topic in industrial and environmental fields.Based on the technologies of electrocatalytic oxidation(ECO)and photocatalytic oxidation(PCO),a novel wastewater treatment process called hybrid photoelectrocatalytic oxidation(HPECO)was proposed forthe treatment of 3,4-DMA wastewater containing high chlorine content.The photocatalytic subsystem was composed of suspended nano-titanium dioxide photocatalysts and UV light,while the titanium-based Iridium oxide DSA and graphite electrodes could set up the electrocatalytic subsystem.And the synergistic catalytic oxidation of two sub-systems could occur in a chaotic system.On the basis of this process,the influencing factors,the reaction kinetics,oxidation mechanisms,degradation mechanisms and synergistic effect in the HPECO catalytic oxidation were deeply explored in this paper,which has certain theoretical significance and engineering application value for the further study of HEECO,ECO and PCO.(1)The influencing factors and function rules in the HPECO system were investigated,such as current density,light intensity,aeration intensity,Ti O₂ dosage and p H,Na Cl concentration,3,4-DMA concentration etc.The research results of the mathematical model based on response surface methodology analysis and the dynamics model on account of L-H equation showed that the degradation rate of 3,4-DMA complied with pseudo-first order reaction kinetic.The order of importance on the degradation rate was current density>p H>Na Cl concentration>light intensity.It was found that there were interactions between current density and Na Cl concentration,p H and light intensity.The optimal reaction conditions in this experiment were that initial p H of the solution was 3.3,Na Cl concentration was 5733 mg/L,current intensity was3.46 m A/cm²,and UV light intensity was 10.6 W.(2)According to the molecular probes testing experiments and kinetics of different inhibitors,active oxidants like hydroxyl radicals(·OH),photogenerated valance band holes,metal peroxides,and free chlorines were studied and the contribution degree based on kinetics was proposed for evaluating the oxidation effect of each active substance.It was concluded that the contributions of chlorine active oxides and·OH were 88.24%and 6.19%respectively,while those of hole oxidation and direct oxidation of metal peroxides could be neglected.Among them,the hydroxyl radicals in HPECO system were mainly derived from photocatalytic subsystem,and the yield rate followed with a zero-order kinetic.For chlorine active substances,free chlorine and ClO₂ played important roles in 3,4-DMA degradation.(3)By GC-MS,UV-vis,the rules of 3,4-DMA degradation rateand TOC removal rate,the mechanism of hydroxyl radicals and chlorine active substances on 3,4-DMA degradation in HPECO and its degradation pathway were speculated.GC-MS analysis showed that the intermediates of 3,4-DMA degradation were mainly o-xylene,benzaldehyde,toluene,alkanes and biphenyls.It showed that,firstly,the active chlorines attacked the side chains rapidly,so that 3,4-DMA could be converted into benzaldehyde and other benzene derivatives.And then the benzene derivatives were transformed into small molecules which could be mineralized finally through the attack towards the?bonds by·OH.Moreover,few chlorine substitutes were detected by GC-MS which suggested that·OH and ClO₂ probably inhibited the formation of chlorine substitutes.(4)Obvious synergistic effect between photocatalysis subsystem and electrocatalysis subsystem was detected in HPECO.The mechanisms of synergistic effect of HPECO were quantitative analyzed by series experiments and theoretical analysis of photochemistry and quantum mechanics.It was found that the main mechanisms of synergism were:(1)High-concentration oxygen produced by the side oxygen evolution reaction in the electrocatalytic process provided electron acceptors for the photocatalytic system,and which promoted the generation of hydroxyl radicals synergistically,thereby improving the overall degradation efficiency of the system.(2)In the photocatalytic process,not only were the photocatalysts excited to generate photogenerated holes and electron pairs by the UV light photons,but also the 3,4-DMA molecules and their degradation intermediates were irradiated by ultraviolet light to the excited state,which had higher redox ability and enhanced the utilization of active oxides such as hydroxyl radicals,free chlorides,ClO₂ etc.(3)The voltage applied on the electrode during the electrocatalysis process not only provided energy for the oxidation reaction of Ir O₂ anode,but also made the suspended photocatalyst dispersing uniformly in an electrostatic field.Under the electrostatic field,the recombination of photogenerated electron-hole pairs was curbed,and the photocatalytic efficiency was improved accordingly.(5)The actual treatment effect of the HPECO of 3,4-DMA wastewater was studied by using the effluent from the secondary sedimentation tank of a pharmaceutical factory in Hubei province as raw water.The results showed that after 10 min HPECO degradation,the residual concentration of 3,4-DMA is about 2 mg/L,which met the secondary emission requirement of the Integrated Wastewater Discharge Standard GB8978-1996,and had excellent treatment effect.The operation cost of the HPECO process was 3.75 yuan/ton of water.Considering comprehensively the treatment effect and operation cost,this method has good treatment performance and remarkable economic benefits.HPECO can be used broadly as a feasible and valuable industrial wastewater treatment technology.