Oxidation System Of Simultaneous Generation Of ·OH/SO₄^-· On A Paired Electrode For Phenol Degradation And Its Mechanism Study

Electrochemical oxidation has attracted extensive attention in treatment of biorefractory organic wastewaters,attributed to its strong oxidation ability,easily controlled and environmental-friendly advantages.However,the current efficiency is still unsatisfactory because there are several shortages,such as the low utilization of oxidant.In this paper,we propsed a paired electrolytic process that paired electrogeneration of H₂O₂/S2O82-and then using UV activate to generate·OH/SO₄^-· for the degradation of organic contaminant.In this process,gas diffusion electrode was used to electrogeneration of H₂O₂ by cathodic oxygen reduction reaction,Pt or Boron-doped diamond?BDD?as anode for the electrogeneration of persulte and persulte was activate by UV?254 nm?to form ·OH/SO₄^-·.The main conclusions are as follows:A simple structure of gas diffusion electrode?GDE?was constructed by rolling carbon black and PTFE as conductive catalyst layer to enhance the producibility of hydrogen peroxide.The electrochemical behavior and electron transfer number of the gas diffusion cathode were analyzed to reveal the catalytic mechanism of cathodic oxygen reduction.Results show that two-electron and four-electron reactions were simultaneous carried out in the oxygen reduction reaction process of carbon black catalyst?XC-72R?.However,the two-electron reaction for H₂O₂ played the dominant role in the oxygen reduction reaction process.The influence of the ratio of carbon black catalyst and PTFE on electrode surface was investigated by microstructure analysis.Single factor experiment was empolyed to investigate the effects of initial p H,current density and gas flow rate on the electrogeneration of H₂O₂.Box-behnken design?BBD?coupled with response surface methodology was employed to assess individual and interactive effects of the three main independent parameters on H₂O₂ concentration.Under optimal operating conditions,the concentration of H₂O₂ was 313.72 mmol/L and 301.85 mmol/L,respectively.The model analysis results show that the model has good regression and strong significance.The relative error of the predicted value between experiment and model is small,as well as the fit is good with the predicted value.Electrochemical degradation of phenol by in situ electro-generated and electro-activated hydrogen peroxide using an improved gas diffusion cathode was proposed.Electro-Fenton method for removaling phenol was first carried out.When the concentration of Fe2+ reached 0.5 mol/L,the removal rate of phenol was reached 100 % after electrolysis of 10 min.The diaphragm electrolytic device was then used to degrade phenol without adding any catalyst.The effects of the operation parameters,such as current density,supporting electrolyte,p H,air flow rate,and phenol concentration,on phenol removal were systematically optimized.Electron transfer mechanism of the electro-activation H₂O₂ to produces ·OH was proposed.In this process,H₂O₂ was first electrogenerated by the two-electron reaction in the surface of gas diffusion cathode,then it was decomposed into ·OH by the electro-activation process.Removal efficiency of phenol in an alkaline electrolyte was higher than that in an acid electrolyte,because H₂O₂ cuold be decompose to HO2-,·OH and O2-·,which have higher oxidizability.Oxidation and generation mechanism of anodic indirect oxidation technique based on SO₄^-· was proposed.The electrochemical properties of Pt and BDD electrodes were text.The effect of oxygen suppressor,current density and sulfate concentration on persulfate production were also investigated.Compared with the other oxidants,such as ozone or active chlorine,the electrogeneration of S2O82-was obtained higher current efficiency.The current efficiency of Pt and BDD anode employed to S2O82-electrogeneration were 58 % and 67%,respectively.Moreover,the process using BDD anode did not need to add the oxygen suppressor.Experimental result demonstrates efficient electro-generation of persulfate and degradation of phenol were obtained simultaneously when the anolyte contains sulfate ion and potassium sulfocyanide.Hydroxyl radicals?·OH?electrogenerated at anode surface can efficiently eliminate organic pollutants.However,these radicals are rapidly consumed and its action occurs only in the region where it is produced,thus only a portion of ·OH that are generated can be utilized and the degradation efficiency will eventually drop upon extended electrolysis.This paper proposed an alternative electro-oxidation process to avoid the mass transfer limitation and ·OH loss.Persulfate electrogenerated was activated by UV?254 nm?,thus the the degradation efficiency will obviously increases.A two-step mediated oxidation method was employed to further reduce consumed energy and sulfate dosage.Oxidation system of ·OH/SO₄^-· generated by paired electrode was established.Paired electrosynthesis of H₂O₂ and S2O82-and its interaction mechanism was performed in a paired electrolysis system separated with a cationic membrane?Nafion-117?.Under the circular flow pattern,the total current efficiency of paired electrosynthesis of H₂O₂ and S2O82-reached 154.7 %.Cathodic reaction conditions have little influence on the electrosynthesis of S2O82-,but reducing p H value of anolyte significant decrease the electrosynthesis of H₂O₂.Then S2O82-and H₂O₂ were activated by UV to form SO₄^-·and ·OH,and the degradation efficiency of phenol and its mechanism was also investigated.Intermediate degradation product of UV/cathode and UV/anode system was comparative analyzed,and the reaction path and mechanism were also revealed.In this paired oxidation system,not only the oxidants including H₂O₂ and S2O82-were utilized more fully,but also the theoretical current efficiency was multiplied.This research will provide the theoretical foundation and implementation method for electrochemical techniques.This oxidation system has great development potential in the application of advanced oxidation process.It not only broadens and improves the application scope of electrochemical degradation technology,but also reduce its energy consumption and cost.