The Photocatalytic, Thermal Degradation Kinetics and Lifetime Prediction of Some Commercial Reactive Dyes

Abstract

<em>The photocatalytic degradation of the commercial reactive Black5 (RB5), Blue19 (RB19), Green19 (RG19), Red120 (RR120), and Yellow81 (RY81) in aqueous solution and sunlight were examined in the presence of Fe₂O₃ and the mixture Fe₂O₃/H₂O₂ as catalysts. The results showed that the photocatalytic degradation followed first-order kinetics with faster degradation in case of the mixture Fe₂O₃/H₂O₂than that in Fe₂O₃ catalyst. The photocatalytic degradation percentages with the exposure time intervals obtained for each dye were extrapolated to determine the time at the complete degradation (100% degradation). The thermal decomposition behavior of these dyes was investigated by using non-isothermal thermogravimetric analysis (TG, DTG, and DTA), at 10 °C/min heating rate and under nitrogen. The Coats-Redfern integral method was used for the five reactive dyes in order to evaluate the kinetic parameters for each step in the sequential decomposition TGA curves. The initial molecular structure destruction of these five reactive dyes occurred at their second steps. The kinetic parameters of this structural destruction step at multiple heating rates (10, 15, 20 and 30 ^°C/min) were only worked out for three selected dyes (RB5, RB19, and RR120) using the Flynn-Wall-Ozawa equation and compared with that of Kissinger's equation. Based on the data obtained by the Coats-Redfern method, the thermodynamic parameters (∆G^*, ∆H^* and ∆S^*) for these second steps were determined. Finally, the kinetic data extracted from the TGA curves were also used as appropriate to predicate the lifetime of the five dyes at 25, 50, 100 and 200 °C.</em>