Φωτοηλεκτροκαταλυτική αδρανοποίηση ρύπων σε υδατικά διαλύματα

Abstract

The last decades the concern for the use of the “Advanced Oxidation Technologies-AOT” has increased because of the need of new, alternative to the conventional ones and environmentally friendly technologies. This term involves the technologies whose effectiveness is based on the production of the OH• radicals. The powerful oxidizing ability of the OH• radicals in combination with their non-selectiveness allows not only the oxidation of the various organic pollutants present in water or wastewater, but also the simultaneously destruction and lysis of the cell membrane of various pathogenic microorganisms. Recently it has demonstrated that semiconducting materials mediating photocatalytic oxidation of organic compounds can be an alternative to conventional methods for the removal of organic pollutants from water and air. The illumination of these particles with energy greater than the bandgap energy of the semiconductor (hv>Eg) produces excited high energy states of electron and hole pairs (e-/h+) that can migrate to the surface of the particle and initiate a wide range of chemical redox reactions, which can lead to complete mineralization of the organic pollutants. An advantage of the photocatalytic process is its mild operating conditions and the fact that it can be powered by sunlight, thus reducing significantly the electric power required and therefore the operating cost. A variety of semiconductor powders (oxides, sulphides, etc) acting as photocatalysts has been used. Most attention has been given to TiO2 because of its high photocatalytic activity, its resistance to photocorrosion, its biological immunity and low cost. Photocatalysts such as TiO2 can either be used as powders in a slurry form or as supported films. This study aims at the improvement of the heterogeneous photocatalytic method in cleaning and disinfect potable water and effluent wastewater through the use of different types of TiO2 electrodes and the parallel use of electric field. The use of TiO2 as supported film is advantageous for catalyst separation, but poses surface area and mass transfer limitations. To counterbalance these, the use of a conducting substrate offers the possibility of enhancing the photocatalytic activity of TiO2 overlayer by applying a positive bias on the catalyst in an appropriate electrochemical cell, thus drawing the photogenerated electrons away from the catalyst interface and minimizing recombination with photogenerated holes. The objectives of this study were: a) the synthesis of thermal and particulate TiO2 electrodes under various experimental conditions, b) the morphological and electrochemical/photoelectrochemical characterization of the electrodes and c) the use of the electrodes with the best photoelectrocatalytic properties in electrocatalytic/photocatalytic and photoelectrocatalytic experiments in order to study the effectiveness of the system in the destruction of pollutants which are characterized by great stability in the environment and the disinfection of potable water. Final objective was the construction of a novel photoelectrocatalytic reactor that can work in a recirculation batch mode, under artificial or solar light, for the oxidation of representable pollutants (chlorophenols, pesticides, dyes) in potable water and wastewater. During the photoelectrochemical oxidation a number of parameters were tested (initial concentration of the pollutants and the microorganisms, pH of the solution, applied potential, intensity of the illumination, the influence of the solution’s volume and electrodes’ type) and proved to be crucial to the efficiency of the above systems. In the long term, the realization of this study’s systems can lead to construction of autonomous systems for water cleaning and disinfection through the exploitation of solar energy; these can contribute to the protection of public health in distant areas, settlements, etc. The use of low cost and no toxic catalysts with activated the aid of solar radiation can be beneficial for areas such as Mediterranean countries which are subject to long sunshine periods and lack of water.