Direct Photolysis of an Azo-Dye, Eriochrome Black T

The direct photolysis of a water soluble organic pollutant, the azo-dye Eriochrome Black T (EBT), was studied. The obtained results indicate that EBT elimination from an industrial effluent may be efficiently achieved by irradiation with an artificial UV light, but mineralization takes much longer times. In a pure aqueous solution, the reaction induces a pH decrease with the probable formation of acidic photoproducts. The reaction rate is affected by many parameters: EBT disappears faster as the solution pH is raised stepwise from 2-12.5, due to the acid-base structural changes accompanying the pH changes. In solutions buffered at pH = 9, the reaction kinetics are apparent first order. The half-life time increases steadily with the initial concentration which points to the growing difficulty to remove the pollutant from an effluent as its concentration rises. Quantum yield, absorptivity and incident light intensity are to be considered for selecting the light source and optimizing the operating conditions: in our experimental devices, a faster decolourisation was obtained at 254 nm than at 365 nm and by increasing the light intensity.