Reaction kinetics of UV-decolourization for dye materials

Various purified dyes with assorted chromophores, and physical and chemical properties were used to explore the characteristics of photo-decolourization at 253.7 nm. Experimental results indicate that photo-decolourization of all selected dyes follows pseudo-first-order decay and the reaction mechanism is dominated by free radical reactions where acidic photo-products are formed in the reaction. The solubility of the dye molecules is very important for the efficiency of photo-decolourization. Hydrophilic dyes are easier decolourized than hydrophobic dyes, however, photo-sensitizer such as acetone can greatly enhance the photo-decolourization rates for all dyes even for the hydrophobic ones. The rate constants of dye decolourization by UV are also determined at different pH. In general, the higher the initial pH, the higher the reaction rates constants. At high pH range, the decolourization rates of dye materials can be ranked as the following: insoluble anthraquinone < soluble anthraquinone < diazo ≈ triazo < polyazo < Cu monoazo.