Magnéli-phase Ti₄O₇ Conductive Membrane For Effective Electrochemical Degradation Of 4-chlorophenol Wastewater

Electrochemical advanced oxidation has been receiving a growing attention in the field of chlorophenol wastewater treatment because of its advantages of higher efficiency,lower energy consumption and less secondary pollution.Searching for efficient and stable electrode materials and improving mass transfer efficiency in pollutants degradation process are two key factors to promote the industrialization of electrochemical oxidation wastewater treatment technology.Considering the excellent electrochemical properties of Magnéli-phase titanium oxides,we designed a Magnéli-phase Ti4O7 conductive membrane as anode with the principle of membrane filtration used to improve the mass transfer for electrochemical oxidation of 4-chlorophenol pollutant.The Magnéli-phase Ti4O7 conductive membrane was characterized by scanning electron microscope(SEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).It was proved that the microstructure of the porous structure was large pore and mesoporous honeycomb porous structure.The main elements were Ti and O,most of the crystalline phase was Ti4O7 phase.Electrochemical impedance spectroscopy(EIS)and cyclic voltammetry curves(CV)were used to analyze the electrochemical properties of Ti4O7 electrode.The results showed that the charge transfer internal resistance and diffusion internal resistance of the Ti4O7 electrode were far lower than that of the carbon cloth,respectively.And the electrochemical active area was 3 orders of magnitude higher than the surface area of the pure electrode itself.The Magnéli-phase Ti4O7 conductive membrane had a molecular weight cutoff of approximately 200 k Da and good anti fouling performance.The mass transfer rate constant of 3.96×10-5 m·s-1 had been obtained under the condition of a peristaltic pump for enhancing the advection transport of solutes to the anode surface.The rich pore structure,excellent electrochemical performance,unique and excellent membrane properties which are distinguished from other traditional electrodes provides a good condition for the electrochemical reaction to take place quickly and efficiently.The electrochemical oxidation behaviors and kinetics of 4-chlorophenol on the surface of Ti4O7 electrode have been investigated by cyclic voltammetry,linear voltammetry and chronoelectric method.The experimental results showed that the electrochemical oxidation process of 4-chlorophenol was controlled by the diffusion of the electrode/solution interface and the adsorption on the surface of the electrode.The diffusion coefficient from the solution to the electrode surface is 4.3×10-6 cm2·s-1.Based on the optimization of key operating parameters,the optimum electrolyte concentration,current density and membrane flux conditions are determined to be 0.04 mg·L-1,5 m A·cm-2 and 0.023 m L·cm-2·s-1,respectively.The maximum removal rate of 4-chlorophenol can reach to 100%,and the energy consumption was low.Thereafter,the performances of Magnéli-phase Ti4O7 conductive membrane was examined for electrochemical tertiary treatment of actual industrial wastewater under the condition of coexistence of anions.The results showed that the COD and chromaticity of EDR concentrated effluent after 2.5 hours of degradation could reach the standard of direct discharge,the mineralization rate was 63.10% and the energy consumption was 85.12 k Wh·kg COD-1.The free radical capture experimental results proved that the hydroxyl radical is the main strong oxidizing active substance in the electrochemical oxidation of 4-chlorophenol wastewater by the Magnéli-phase Ti4O7 conductive membrane.The qualitative and quantitative detection of the intermediate products of 4-chlorophenol degradation process was carried out to analyze the reaction path under this system: 4-chlorophenols were oxidized and reduced to hydroquinone,quinone and phenol respectively in the anode and cathode areas,and a series of small molecular organic acids were formed under the action of strong oxidizing active substances produced at the interface of the electrode/solution.Finally,they were completely mineralized into CO2 and water.