Photoelectrocatalytic Determination Of COD In Water With Visible-light Responsive BiVO₄/rGO Film Electrode

China is faced with serious water environmental pollution in recent years,which is in urgent need of proper supervision and treatment in effective ways.Chemical oxygen demand（COD）,as an important index in water pollution detection,is often used to evaluate the degree of water pollution.The traditional method of potassium dichromate oxidation has some disadvantages in the actual determination of COD,such as complicated determination process and long time,high cost of reagent raw material and toxicity of reagent used.Moreover,the traditional water quality detection is mostly to collect water samples for laboratory analysis,but in recent years,the research focus is more concerned with replacing the traditional method and meeting the timeliness and environmental friendliness of new testing methods.Photocatalytic method with environmentally friendly,strong oxidation and operability is a new technology based on photocatalytic and electrocatalytic technology,which can continue to degrade and purify organic pollutants in water environment,such as organic waste,industrial waste water,medical waste water and other sewage.It also has a broad application prospect for the detection of organic matter.In addition,the electrochemical workstation as a detection device can be a portable instrument for in-situ determination of COD in water,which is conducive to timely monitoring and management of water pollution.The characteristics of different electrodes can determine the degradation rate and salinity of organic matter to some extent.Previous researches mainly focus on titanium dioxide（Ti O₂）,but its band gap of 3.2 e V makes it only respond to ultraviolet light and produce strong photocorrosion phenomenon,which has many limitations in practical application.Recent studies have confirmed that BiVO₄,a visible light responsive semiconductor material,exhibits excellent performance in photocatalytic degradation and transformation of organic pollutants.However,the weak adsorption capacity of BiVO₄ for pollutants and fast recombination of e^-and h⁺seriously limit the application of BiVO₄ photocatalyst.Graphene oxide（rGO）,as a semiconductor material with a unique structure,has excellent electrical conductivity and electron mobility.The synergistic effect of the two materials is expected to improve the adsorption performance of pollutants and inhibit the recombination time of photogenerated carriers.In this study,BiVO₄/rGO composite material with narrow band gap and excellent electron mobility was selected as the photoanode.The preparation of BiVO₄/rGO coating electrode and the influencing factors of COD in water samples were investigated by PEC method,which is also applied to actual water sample determination.The research is mainly divided into the following three parts:1.BiVO₄/rGO precursor was prepared using bismuth nitrate as bismuth source,ammonium metadvanadate as vanadium source,citric acid as metal ion complex,urea as combustion agent and adding rGO.A visible light responsive BiVO₄/rGO coating electrode was prepared by loading the mixed precursor solution onto FTO conductive glass by spin-coating method and subsequent solution combustion method.The visible light-responsive BiVO₄/rGO coated electrode is characterized by X-ray diffraction（XRD）,scanning electron microscope（SEM）,transmission electron microscope（TEM）and specific surface area analyzer（BET）.The classical three-electrode photocatalytic system together with electrochemical workstation was constructed to study the photoelectric and stability properties of BiVO₄/rGO film electrode by electrochemical testing methods such as cyclic voltammetry curve（CV）and photocurrent-time curve（i-t）.2.Under the optimal preparation conditions,the relevant influencing factors（working bias,light intensity,type and concentration of electrolyte,p H value of solution and other interfering impurity ions）related to the determination of COD in the photocatalytic system of 1mmol/L glucose as the target organic matter were studied.The results show that the BiVO₄/rGO electrode can effectively determine COD in organic solution in the range of 12.18-719.8mg/L.3.Aiming at four typical organic compounds of glycine,glucose,sodium acetate and potassium hydrogen phthalate,the calibration curves of transfer net charge(Q_net)and its theoretical COD（Th COD）of organic compounds were constructed.The linear correlation coefficient is greater than0.99.When the BiVO₄/rGO film electrode is used for the determination of COD in the actual wastewater,the steady-state photocurrent of organic matter can be quickly reached within 60s.Determination results here were in conformity with that by the national standard potassium dichromated method,and the relative deviation is within 5%.