Efficiency Evaluation And Mechanism Exploration Of Photo-assisted Electrooxidation Degradation Of Typical Bisphenols

Bisphenol A and its substitute bisphenol AF for industrial production are two typical environmental hormones.The effective treatment of wastewater containing these substances is of great significance to the ecological environment and human health.Although the conventional electrochemical oxidation treatment possesses good degradation efficiency,its energy consumption is too high and utilization is too low.A new method simulating sunlight-assisted electrocatalytic degradation is proposed.In order to prove their effectiveness and operating condition,the degradation efficiency of bisphenol A and bisphenol AF was explored through smallscale degradation experiments.A single-factor experiment was carried out to optimize the factors influencing the degradation of bisphenol A in the photo-assisted electrocatalytic system.The oxidation mechanisms of photo-assisted electrochemical degradation system and pure electrochemical system were explored by using free radical quenching experiment and free radical identification experiment.The degradation intermediates of bisphenol A and bisphenol AF were analyzed and identified by High Performance Liquid Chromatography Quadrupole-Time-of-Flight Mass Spectrometry(HPLC-QTOF).The energy of degradation pathways of bisphenol A and bisphenol AF was calculated and analyzed based on density functional theory from molecular level.Combined with the results from LG-MS and theory simulation analysis,the possible degradation pathways of bisphenol A and bisphenol AF were proposed.The degradation experiments showed that photo-assisted electrochemical degradation had a synergistic effect compared to electrochemical degradation and photodegradation alone.Its degradation rate within 90 minutes was higher than that of electrochemical degradation and photodegradation by 8.2% and 13.2%.The effect of mineralization was increased by 17.86% and 13.24% compared with pure electrical degradation.With cation exchange membrane,the degradation rate of bisphenol A and bisphenol AF increased by 23% and 30% respectively.The types of electrodes were ranked as BDD>Pt>DSA according to the electrochemical performance and photoelectric synergistic efficiency.BDD electrodes performed the best in both two cases.Current density,electrolyte type and concentration,light intensity and light wavelength were all important factors that affect the degradation efficiency of the photo-assisted electrochemical system.Among them,current density,electrolyte concentration and light intensity were positively correlated with the degradation efficiency in this study.The degradation efficiency of Cl-,NO3-,SO42-and CO32-as electrolyte under the same ionic strength was investigated and they were ranked as Cl-> NO3-> SO42-> CO32-.The closer the illumination was to the ultraviolet region,the stronger the promotion of electrochemical degradation.Illumination in nonultraviolet region(from 420 to 780 nm)also promoted the electrochemical degradation.Oblique or back irradiation on the electrode had equal effect on promoting electrochemical degradation.It proved that the enhancement of oxidation efficiency by light irradiation was mainly due to the solution rather than the electrode.By analyzing the degradation mechanisms of photo-assisted electrochemical system and pure electrochemical system,it was found that degradation mechanisms for both systems were the oxidation of three free radicals including ·OH,1O2 and ·SO4-as well as the direct oxidation of electrodes.Among them,·OH and 1O2 played major roles.The free radical identification experiment proved that the addition of simulating sunlight increased the content of ·OH,·SO4-,especially the 1O2.The free radical quenching experiment proved that in photo-assisted electrochemical system,oxidation of the electrode and the contribution of ·SO4-increased gradually with time.The contribution of 1O2 maintained at about 40%,while for ·OH it gradually decreased with time.In pure electrochemical system,the contribution of ·OH reached more than 50% at the first 20 minutes.After 30 minutes the contribution of ·OH and 1O2 maintained at 40%.In conclusion,the addition of simulating sunlight increased the relative content and contribution of 1O2 and ·SO4-in the system.By analyzing degradation pathways of bisphenol A and bisphenol AF,it was found that degradation of bisphenol A mainly included three reactions: hydroxyl and aldehyde reaction,demethylation reaction and carbon bond breaking reaction.Typical intermediates were aromatics including phenol,benzoic acid,p-isopropenylphenol,et al.The degradation of bisphenol AF mainly involved hydroxylation,decarburization,defluorination and other reactions.Typical intermediates were various ring-opening carboxylic acids.Based on the density functional theory(DFT),the active sites of bisphenol A and bisphenol AF were identified.The energy of some reactions were calculated and the results were well consistent with that from LG-MS.A new method of simulating sunlight-assisted electrochemical degradation for bisphenol A and bisphenol AF was proposed in this research.The addition of sunlight increased the degradation and reduced the treatment energy,which has considerable economic significance.It will provide theoretical and data support for using this method in industrial application.