Study On Photoelectrocatalytic Decolorization Mechanism Of Methylene Blue Under The Visible-light Irradiation By Measuring Chemical Oxygen Demand Index

In this thesis,a new type of composite photoelectrode?CuInSe₂/Ag₃PO₄@Ag IO₄/GC?was prepared by combining the electrochemical deposition and the sequential chemical bath deposition method on the glass carbon substrate,and this composite photoelectrode with heterojunction structures between different materials exhibited the high photoelectrocatalytic stability and excellent photoelectrocatalytic activity to organic dye under the visible-light irradiation.Moreover,we had improved the existing two-step microwave-assisted digestion method for COD determination,so that it could be applied to the COD assay of high-salt organic dye wastewater samples and the experimental research work for photoelectrocatalysis.Using a 60 W incandescent lamp as the radiation light source and adopting the phenothiazine dye methylene blue?MB?as the probe molecule,we had mainly studied the mechanism of MB photoelectrocatalytic decolorization reaction through the ultraviolet-visible absorption spectrum,the chemical oxygen demand index?COD?and the water-phase infrared absorption spectrum.The results shown that the phenothiazine mother nucleus of MB molecular structure was only partially degraded in this photoelectrocatalytic decolorization reaction,and could not been examined into the completely mineralized inorganic substances.Based on the experimental evidence about UV-vis and FT-IR spectroscopy,a new mechanism that can explain the nature of MB Photoelectrocatalytic decolorization reaction was proposed.The work of this thesis consists of three parts:Part one: Preparation of CuInSe₂/Ag₃PO₄@Ag IO₄/GC composite photoelectrode and its visible-light photoelectrocatalytic propertiesIn this section,A new type of composite photoelectrode?CuInSe₂/Ag₃PO₄@Ag IO₄/GC?was prepared by combining the electrochemical deposition and the sequential chemical bath deposition method on the glass carbon substrate.By optimizing the concentration ratio in the chemical bath solution,we could obtained a novel Ag₃PO₄@Ag IO₄ photoelectrode materials that formed the heterojunction structure between different materials,and its surface morphology and photocatalyst materials was characterized by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?techniques,respectively.Using a 60 W incandescent lamp as the visible-light source and adopting the 0.050 mmol/L MB solution as testing sample,we evaluated the photoelectrochemical characteristics and photoelectrocatalytic properties of the photoelectrode.As a result,the photoelectric response current was upto 0.47 ?A,while the total decolorization rate of MB with 60 min irradiation time could reach 88.4%.Specially,because the same photoelectrode could be repeatedly used,but not found performance degradation,the stability of the photoelectrode was considered to be quite better.Part two: Determination of COD vale of a high-salt wastewater sample containing organic dye by two-step wet-chemical microwave-assisted digestion methodIn this section,a rapid determination method of chemical oxygen demand?COD?for high-salt wastewater sample containing organic dye is successfully established.To improve the total digestion rate of organic dye molecules for the COD assay,this method have adopted the microwave-assisted reaction and two-step wet-chemical digestion treatment technique in which both KMn O₄ and Ce?SO₄?2 are used as the main oxidants.With a simulated wastewater sample containing methylene blue,sulphate and phosphate,we have optimized many experimental conditions such as the added amount of Ca?NO3?2,microwave digestion time and the acidity or alkalinity of the digestion solution,herein the services target adding the proper amount of Ca?NO3?2 into the testing sample is to eliminate the influence of SO₄2-and PO₄3-ions.Under the optimum experimental conditions,it is found that the total average digestion rate of MB in the simulated dye wastewater sample can reach to 99.96%,and linear response range of COD values is 1.0-66.0 mg O2 L-1,R2=0.9999.In addition,when the simulated wastewater sample containing both MB dye and H₂O₂ has also validated the linear response range of COD values of 1.2-60.0 mg O2 L-1 R2=0.9999 by using the proposed method.Therefore,these results show that this new method can be suitable for the determination of low-level COD index for a high-salt organic dye wastewater.Part three: Study on photoelectrocatalytic decolorization mechanism of methylene blue under the visible-light irradiation by measuring COD indexIn this section,the photoelectrocatalytic degradation experiment of MB dye is performed with a CuInSe₂/Ag₃PO₄@Ag IO₄/GC composite photoelectrode under the visible-light irradiation,and the main reaction conditions are optimized.The COD value of the time-sharing sampled electrolyte drawing from the photoelectrocatalytic reaction cell was determined by the above-mentioned COD method.Therefore,in view of the fact that there exists the special variation tendency about the ultraviolet-visible absorption spectrum and water-phase infrared spectra in the reaction mixture solution,we have explored the possible mechanism of MB photoelectrocatalytic decolorization reaction.Under the fixed experimental conditions about the visible-light irradiation time and intensity and light-receiving area of photoelectrode,if the other experimental conditions are also set as follows: the applied bias of +0.61 V?vs.SCE?,the 0.42 mmol/L initial concentration of H₂O₂ as a reaction promoter,the 0.15 mol/L Na2SO₄?p H=2.7?as supporting electrolyte and 0.050 mmol/L MB as the initial reaction concentration,we can find that the ratio of decolorization rate to mineralization rate could be reached 8.5 at the irradiation reaction time of 45 min,while a small amount of organic precipitated product can be continuously formed during the photocatalytic decolorization reaction process of MB.So,it is concluded that the phenothiazine mother nucleus of MB molecules structure has only partially degraded in the photoelectric decolorization reaction,but did not been completely mineralized into the inorganic substances.Therefore,referring to the experimental evidence about UV-vis and FT-IR spectroscopy again,a new mechanism that can explain the nature of MB Photoelectrocatalytic decolorization reaction can be deduced out.