Influence of electrolysis on the degradation of dye water solutions

The basic subject of this thesis is to investigate the influence of electrolysis on degradation of organic dyes in aqueous solutions. Electrolysis is a physically-chemical process, during which - under the influence of direct current - chemical reactions proceed inside the system. By choosing efficient conditions it is possible to achieve fundamental changes in the structure of organic compound’s molecules and on the basis of that for example their gradual degradation. This effect can be used in the process of wastewater treatment. The main task of the thesis is to analyse the influence of variously chosen chemical and physical conditions on the rate of degradation of azo dyes, which is the largest group of organic dyes used in industry and from this reason in great amounts contained both in mill residues and sewage water. Azo dyes have been selected for this experiment for their good solubility in water. Other advantage is that their degradation is accompanied by visible decoloration of the solution and their concentration in solution can be easily determined by UV-VIS spectrometry. Concretely two direct azo dyes - C.I. Direct Blue 106 and C.I. Direct Red 79 - have been chosen. The experiment was carried out in a simple reactor consisted of a bath with a stirrer into which two electrodes were installed and attached to the source of direct voltage. Every series of measurement proceeded at constant current which values were varied in range from 100 mA to 1 000 mA. The electric voltage ranged from 7 V to 22 V. Several samples were taken away from the solution during the experiment and the decrease of dye concentration was evaluated in every series. This evaluation validated the hypothesis that the dyes contained in the solution really degraded. This process was also connected to the expected bleaching of the solution. Decoloration of aqueous solution of dye is caused by disruptions in the structure of the dye molecule which is loosing its characteristic sections that cause colourfulness (a conjugate system of double bonds and presence of appropriate substitutes). A total amount of measured series was 16 with various input conditions (Fe and Pt electrodes, electrolytes NaCl, Na2SO4, and NaNO3 and their different concentrations, different current values) which more or less influenced the rate of degradation of investigated substances. It was found out that the blue dye is more likely to be electrolytically degraded. Chemical structure of its molecules, which is smaller and less branched than molecules of the used red dye, could be the source of that. The greatest degradation of the blue one proceeded at two chosen maximum values of constant current 800 mA and 1 000 mA. It could be assumed that current enhancement through the system induces higher dye concentration decrease. Degradation of the blue dye proceeded faster by using electrodes made of stainless steel than the platinum electrodes. The most suitable electrolyte was shown to be sodium chloride. When higher concentration (or conductivity) of electrolyte was used higher efficiency of degradation process was observed