Decoloration and degradation of some textile dyes by gamma-irradiation

In this study, the possibility of using gamma rays to degrade or decolorize reactive dyes in water was investigated. Two different reactive dyes (Reactive Blue 15 and Reactive Black 5) in aqueous solutions were irradiated with doses 0.1 kGy to 15 kGy at 2.87 kGy/h and 0.14 kGy/h dose rates. The change of absorption spectra, pH, chemical oxygen demand (COD), and the degree of decoloration (per cent reduction in optical density) were examined in the presence of air and H2O2. The absorption bands at 664, 640, 340, 260 nm and 596, 392, 312 nm for RB15 and RB5 were decreased rapidly with an increasing irradiation dose. The degree of decoloration of each dye solution with an irradiation dose was estimated at 100 per cent for the lower concentration (50ppm) dye solutions. The complete decoloration was observed after 1 and 15 kGy doses for RB5 and RB15, respectively. The pH of RB5 and RB15 solutions was decreased from 6.15 and 6.98 to 3.40 and 3.68 with the irradiation dose. The COD reduction for all the dye solutions was approximately 76-80% at 1 kGy and 15 kGy for RB5 and RB15. The COD reduction and the change of pH for all the dye solutions was examined similarly to each other. The effect of pH, N2, and N2O on the degradation and decoloration process was studied. The degradation and decoloration in the presence of nitrous oxide occurred faster than in air and nitrogen-saturated solutions. The degree of decoloration for RB5 increased with the increasing irradiation dose and dose rate. Above 2 kGy/h dose rate, the degree of decoloration is independent of the dose rate and irradiation dose. The degree of decoloration increased to 100% for the nitrous oxide-saturated RB5 solution from 80% for the nitrogen-saturated RB5 solution at 0.8 kGy irradiation dose and at 2 kGy/h dose rate. Although the absorptions at maximum wavelength (596 nm) decreased with increasing irradiation dose for nitrogen-saturated RB5 aqueous solutions, for nitrous oxide-saturated RB5 aqueous solutions they decreased and shifted to lower wavelength. This suggests that the skeleton of the RB5 molecule is mainly destroyed by the attack of the OH radicals. AR and RB5 dyes are destroyed in the pH range from 7 to 4 with a reduced sensitivity at a lower and higher pH range. In the second dye group, Apollofix-type dyes were chosen. Two different Apollofix dyes, Apollofix Red (AR) and Apollofix Yellow (AY), in aqueous solutions, were irradiated in air with doses of 1.0 kGy to 8.0 kGy at 0.14 kGy/h dose rate. The change in absorption spectra, pH, chemical oxygen demand (COD), and the degree of decoloration (per cent reduction in optical density) were examined in the presence and absence of H2O2. The absorption bands at 534 nm and 420 nm for AR and AY were observed to decrease rapidly with an increasing irradiation dose. The degree of decoloration of each dye solution with an irradiation dose was estimated of 100 per cent for the lower concentration (50ppm) dye solutions. The complete decoloration was observed after 3.0 kGy and 1.0 kGy doses for AR and AY, respectively. The COD and BOD reduction and the change of pH for all dye solutions on irradiation showed similar behavior. (author)