Electrocatalytic Oxidation And Mechanisms Of Emerging Organic Contaminants By Titanium Suboxide

With rapid growth of the global economy,the discharge amount of industrial/agricultural/domestic wastewater is increasing dramatically every year,as well as the type and quantity of organic pollutants in water environment.The health risk caused by this would be one of the major environmental challenges in this century.Emerging organic contaminants（EOCs）including pharmaceutical and personal care products（PPCPs）,perfluorinated compounds（PFCs）,and persistent organic pollutants（POPs）,have been frequently detected in the water cycle systems of developed and developing countries.However,it’s difficult to achieve complete degradation of EOCs by traditional treatment methods due to their wide distribution,refractory nature and bioaccumulation effect.Electrocatalytic oxidation（EO）is one of the leading-edge technologies in the field of water/wastewater treatment,which has been proved to have great potential in the treatment of refractory organic pollutants.The development of low-cost and high-performance electrode materials is the key factor in realizing the practical application of EO technology,while limited mass transfer and high energy consumption remain the major bottlenecks.Titanium suboxide（TiSO）has become an electrode material extensively studied in recent years because of its excellent conductivity,corrosion resistance,wide potential window and low cost.In this study,we fabricate TiSO-based anodes of differenct structures to degrade a variety of typical EOCs,and optimize the structural design of the EO systems to improve the mass transfer efficiency and overall performance,providing theoretical guidance for the large-scale application of EO technology in water treatment.The following is a brief description of the research content:Considering the mass transfer limitation of plate electrodes,TiSO mesh anode（M-TiSO）is prepared by coating TiSO powder（TiSO-Powder）via plasma spraying on the surface of titanium mesh substrate.Scanning electron microscopy（SEM）shows that TiSO-Powder is coated uniformly on the substrate with a diameter of approximately 1.76μm,providing the anode with an ideal electrochemically active surface area（ECSA）.Compared with TiSO plate anode（TiSO-Plate）,the mesh structure of M-TiSO can induce local turbulence near the anode,thus elevating the mass transfer efficiency.Under the same conditions,the reaction rate constant of sulfadiazine（SDZ）degradation by M-TiSO reaches 0.0912 min^-1,which is much higher than that by TiSO-Plate(0.0329 min^-1).Highest SDZ removal is observed in acidic solutions at 10 m A cm^-2 for M-TiSO,and the type of common inert electrolytes has little effect on this process.Compared with other commercial electrodes,M-TiSO also shows more excellent mineralization performance.Subsequently,SDZ degradation is proved to be mainly dependent on hydroxyl radical（·OH）by cyclic voltammetry（CV）and electron paramagnetic resonance（ESR）.The density function theory（DFT）is used to predict the active sites of SDZ in M-TiSO/EO system with Fukui function as descriptor.Combined with the analysis results of liquid chromatography-mass spectrometry/mass spectrometry（LC-MS/MS）,intermediate products and degradation mechanism in the system are proposed.Plate electrodes also have the disadvantage of limited active surface area.Besides,the PFCs are difficult to be degraded by·OH in the EO system.To solve theses issues,we fabricate TiSO porous anode（P-TiSO）,which could also facilitate the production of SO₄^-·.The results of optical microscope,SEM and Hg-injection test show that P-TiSO has a unique hierarchical pore structure,which contributes to an ECSA 2-3 orders of magnitude higher than that of conventional metal oxide bulk electrodes,providing more active sites.Different from M-TiSO,electrolyte type plays a more prominent role in P-TiSO/EO system.Compared with sodium nitrate（Na NO₃）and sodium perchlorate（Na Cl O₄）,sodium sulfate（Na₂SO₄）is more ideal for the EO performance of perfluorooctane sulfonate（PFOS）and perfluorooctanoic acid（PFOA）.The electrochemical activation of SO₄^2-to generate SO₄^-·is thermodynamically feasible at anodic potential of 3.5-4.1 V vs SHE and current density of 5 m A cm^-2.Based on degradation products,F^-concentration,and free radical type,the degradation of PFOS and PFOA by P-TiSO was confirmed to be mainly dependent on SO₄^-·generated in the porous structure.The degradation products are formed by stepwise-CF₂-unzipping of PFOS and PFOA,including F^-and short-chain perfluorocarboxylic acids.In order to increase the ECSA of the anode and enhance the mass transfer of target pollutants from bulk solution to anode surface,TiSO reactive electrochemical membrane（TiSO-REM）is introduced with flow-through operation mode.Micron-sized pores enable larger ECSA and more active sites,while flow-through operation of EO system is beneficial to reducing the thickness of boundary layer and enhancing the mass transfer efficiency.However,gas bubbles produced from oxygen evolution tends to stay in the porous structures,which seriously reduces the membrane flux and hinders the pollutant/anode contact.Therefore,an ultrasound-enhanced TiSO-REM/EO system is constructed in this study.Based on experiment data and COMSOL simulation,ultrasound can effectively enhance the membrane flux and mass transfer in TiSO-REM/EO system,significantly improving the removal efficiency of 4-chlorophenol（4-CP）.ESR confirms the essential role of ultrasound in increasing the·OH production during EO,while DFT theoretically determines the main mechanism of 4-CP removal in the system.4-CP is first oxidized via direct electron transfer to generate[4-CP]^·+,which is then coverted into CO₂ and H₂O through·OH oxidation.The ultrasound-enhanced TiSO-REM/EO system also exhibits excellent stability during the treatment of real wastewater containing EOCs.It indicates that ultrasound not only effectively controls the electrode pollution during EO,but also prolongs the service life of the electrode.