Study On New COD Determination Methods Based On Photo-electrocatalysis Of TiO₂

The water pollution will be an important factor to restrain our country's sustainable development of society and economy in the 21^st century, which influences our nation's modernization construction and renewal. The department of environmental detection is attaching greater importance on the water environmental protection. The water pollution is resulted from the waste water effluent, which depends on its chemical oxygen demand value. Chemical Oxygen Demand (COD) is defined as the amount of oxygen equivalents consumed in the oxidation of organic compounds by strong oxidizing agents such as dichromate or permanganate and is expressed by quantity (mg l^-1) of oxygen. The accuracy is determined by the oxidizing extent of the organic compounds or other reductive substances. With its special photoelectric characteristic, titanium dioxide can produce strong oxidizing agent, namely hydroxyl radical HO, through which the pollutant in the aqueous system will experience photocatalytic process and then be reduced into water and carbon dioxide. On the basis of this theory, the fast oxidation of the organic pollutants and the measurement of the quantity of the organic compounds in the aqueous system through amperometry or photometry can be achieved, which will bring about the fast and on-line detection of the organic pollutants and provide a brand new concept and method.In this paper, different titanium dioxide electrodes with photocatalytic or photoeletrocatalytic ability were prepared. The photocatalytic oxidation of organic compounds was discussed. And the two new COD measurement methods using these electrodes were proposed. The measuring principle was based on the signals that were produced in the process of photocatalytic or photoelectrocatalytic oxidation proportional to the quantity of organic compounds. The two new methods were characterized by low detection, wide linear range, easy operation, and low environmental impact. Additionally, the COD values obtained from the proposed and conventional methods agreed well.Chapter 1 The preparation, mechanism and applications in environmental technology of titanium dioxide showing photocatalytic property were reviewed. Also the development of COD determination was described.Chapter 2 A Ti/TiO₂ mesh electrode by laser calcinations was prepared. The resulting TiO₂ film was investigated by x-ray diffraction (XRD), atomic force microscope (AFM) and electrochemical impedance spectroscopy (EIS). The photodegradation of methylene blue using the proposed electrode under different experimental conditions was investigated in terms of both UV absorbance at 664 nm and chemical oxygen demand (COD) removal. Through the comparison between photocatalytic oxidation and electro-assisted photocatalytic oxidation, it was found that PEC oxidation was a convenient and effective way to mineralize the organic matters and that laser-treated photoelectrode exceeded the oven-treated one.Chapter 3 The method of Ti/TiO₂ photoelectrode preparation using laser-assisted sol-gel was introduced. The proposed electrode was employed as a sensor for flow injection analysis (FIA) to determine chemical oxygen demand. The measuring principle was based on the photocurrent responses on the electrode, which were proportional to the COD values. This method was characterized by short analysis time, simplicity, low environmental impact and long lifetime of the sensor. Additionally, the COD values obtained from the proposed and conventional method agreed well as demonstrated by the significant correlation between the two sets of COD values (R = 0.9961, N = 20).Chapter 4 Photoelectro-synergistic catalysis oxidation of organics in the water on Ti/TiO₂/PbO₂ electrode was investigated. The method proposed employed photoelectro-synergistic catalysis, together with flow injection analysis, to determine COD values. It was shown that the method of photoelectro-synergistic catalysis had lower detection limit (15.0 mg l^-1) and wider linear range (30.0-2500.0 mg l^-1) than the methods of electro-assisted photocatalysis and electrocatalysis. The results obtained by the proposed method and conventional one were compared by carrying out the experiment on 20 wastewater samples and also agreed well by high correlation (R=0.9912).