Preparation Of Modified PbO₂ Electrodes And Their Performance And Mechanism For The Degradation Of Phenolic Pollutants In Water

Phenolic compounds are highly toxic and biorefractory,and the anthropogenic discharge method has led to their widespread distribution in relevant wastewater and sediments,and some phenolic compounds have been detected in surface waters.It affects the water quality and ecological balance seriously.The traditional methods of physical,chemical and biological water treatment have deficiencies,such as high treatment cost,secondary pollution with a high probability,enormous difficulty with single treatment method and low efficiency of removing pollutants,etc.The treatment methods of biorefractory organic wastewater still need to be improved.Electrocatalytic oxidation is famous for its strong oxidation capacity,no secondary pollution,high degradation efficiency,straight forward operation,environmental friendly,and other advantages.The physical and chemical properties of electrode materials determine the electrocatalytic degradation efficiency directly.In the research of electrocatalytic oxidation method,to prepare structurally stable anodes with chemical corrosion resistance and perfect catalytic activity is the main research directions at present.Meanwhile,the toxicological analysis,mechanism discussion of the degradation process of phenolic organic pollutants are also new research hotspots.Therefore,the titanium/lead dioxide electrode(Ti/PbO₂)was used as the basic electrode,and the ruthenium oxide-tin oxide(RuO₂-SnO₂)composite gel was used as the interlayer.In this thesis,the preparation conditions and utilization of the electrode were optimized by three modifications to obtain three composite modified PbO₂electrode materials,i.e.praseodymium(Pr)and polyethylene glycol(PEG)co-doped lead dioxide electrode(Ti/Pr-PEG/PbO₂).Lead dioxide//titanium//TiO₂ nanotube bifacial electrode(Pr-PEG-PbO₂//Ti//TiO₂-NTs)and titanium/polyacrylamide-methacrylamide acid/lead dioxide functional electrode(Ti/P(AM-Co-MAA)-Ru-Sn/PbO₂).The physical morphology and chemical structure of the electrodes were characterized by X-ray diffraction(XRD),field emission electron microscopy(SEM),X-ray energy spectroscopy(EDS)and X-ray electron spectrum(XPS)techniques.The electrochemical properties of the prepared materials were investigated by electrochemical impedance(EIS)and accelerated lifetime experiments.The light absorption properties of the modified electrodes were studied by photoelectrochemical experiments and ultraviolet-visible diffuse reflectance characterization(UV-VIS/DRS).The catalytic activity,degradation conditions(p H,current density,electrolyte concentration,etc.)and degradation mechanism of the modified electrodes were investigated using phenolic compounds(4-chlorophenol and acetaminophen)as simulated pollutants.Theoretical predictions of the toxicity and degradation pathways of phenolic compounds were analyzed by Ecological Structure Activity Relationships Model(ECOSAR,a software for theory calculations)and Density functional theory(DFT).It provides important data support for the promotion of electrochemical oxidation technology and the practical application of new composite modified electrode materials.Details of the study are as follows.(1)The electrode interlayer(RuO₂-SnO₂)was prepared by a sol-gel method,and the modified surface layer(Pr-PEG-PbO₂)was prepared by electrochemical deposition.The electrocatalytic oxidative degradation experiments were carried out using 4-chlorophenol(4-CP)as a model pollutant to evalute the electrocatalytic activity.The results indicated that the addition of PEG in the electrodeposition solution signidicantly promoted the deposition of Pr,resulting in a compact and robust film with vanished boundaries.The impedance of the electrode decreased greatly after the modification of Pr and PEG,which contributed to a good electrocatalytic performance.As compared with those of the undoped electrode,the average current efficiency and the energy consumption of the co-modified electrode increased by 96.8%and decreased by 60.0%,respectively.Additionally,the electrode stability experiments revealed that the lifetime of the co-modified electrode(320 h)was 3.3 times that of the undoped one.The improved performance could be attributed to both Pr doping and the addition of PEG in the deposition solution.This study provides basic data for the modification of electrode materials by co-doping with rare earth elements and surfactants,which improves the disadvantages of low adhesion,easy flaking and short lifetime of Ti/PbO₂electrodes during electrolysis,and reduces energy consumption.(2)A bifacial electrode Pr-PEG-PbO₂//Ti//TiO₂-NTs was prepared by co-deposition and anodic oxidation to carry out a photoelectrocatalytic oxidation technique using acetaminophen(ACP)as a simulated pollutant.The results showed that the bifacial electrode,approximately 97.0%of acetaminopen and 73.0%of chemical oxygen demand could be removed in 180 minute.Compared with photocatalysis and electrocatalysis alone,the photoelectrocatalytic process exhibited a higher average current eddiciency and lower energy consumption.This improved performance was mainly attributed to the generation of reactive oxygen species(e.g.·OH and H₂O₂).In addition,the intermediates generated in photoelectrocatalytic processes were identified by liquid chromatography-mass spectrometry(LC-GM),and two possible degradation pathways were proposed(i.e.direct degradation by·OH attarack and acteminophen dimerization).The ECOSAR predicition based on the molecullar structure of intermediates revealed that some products more toxic than parent compounds were formed during photoelectrocatalysis.The actue toxicity test results confirmed that the global toxicity of the treated solution increased in the first 60 minutes of treatment.This study provides data support for the development of electrode materials for photoelectrocatalytic oxidation,and provides new research ideas for the preparation of energy-saving and efficient functional electrodes.Identifying the intermediates and characterizing the evolution of toxicity is important in the acetaminophen-related wastewater treatment for minimizing the potential ecological risks of effluents.(3)A functional electrode material Ti/P(AM-Co-MAA)-Ru-Sn/PbO₂ was prepared by reversed-phase suspension polymerization and cathodic electrode-position.The study on the degradation of organic wastewater containing amino groups(e.g.ACP)was carried out by using the technology of electrocatalytic oxidation.The results showed that the degradation rate of ACP by Ti/P(AM-Co-MAA)-Ru-Sn/PbO₂ electrode was 100.0%within 40 minutes,which was 2.79 times that of Ti/Ru-Sn O₂.Additionally,the totale nitrogen removal rate was 71.7%with 300 minutes,which was 10 times that of Ti/PbO₂ electrode and 2.3 times that of Ti/Ru-Sn O₂.Using(NH₄)₂SO₄ as ammonia nitrogen source,the results showed that the removal rate of NH₄⁺–N and nitrogen removal efficiency were 62.7%and 53.3%,respectively,in 90 minutes.Compared with Ti/PbO₂,Ti/P(AM-Co-MAA)-Ru-Sn/PbO₂ exhibited better nitrogen removal performance.It also possessed higher removal rate of ACP than Ti/Ru-Sn O₂.This improved catalytic performance should be mainly attributed to the rapid production and enormous accumulation of active species(·ClO).The formation mechanism of·ClO on the Ti/P(AM-Co-MAA)-Ru-Sn/PbO₂ electrode was verified by free radical trapping experiments.In the solution,·OH and·Cl reacted rapidly to generate·ClO which could attack the central N atom of NH₄⁺to produce the free radicals of·NH and·NHCl with weak oxidation activity,and then N₂ would be formed and removed from water at room temperature and pressure.Combined with DFT calculation results,the generation path of·ClO is clarified.This study might help to guide the development of novel electrocatalytic oxidation techniques for the efficient degradation of organic pollutants.Meanwhile,ammonia nitrogen(or amino group)could be removed through nitrogen removal mediated by·ClO.Furthermore,the research could also provide the theoretical support for the practical application of Ti/P(AM-Co-MAA)-Ru-Sn/PbO₂.