The destructive degradation of some organic textile dye compounds using gamma ray irradiation

The destructive degradation of 8 coloured reactive and direct dye compounds currently used in the textile industry has been investigated. These dyes are: Levafix Blue ERA (LB), Levafix Brilliant Red E4BA (LBR), Levafix Brilliant Yellow EGA (LBY), Drimarene Scarlet F3G (DS), Drimarene Brilliant Green X3G (DBG), Fast Yellow RL (FY), Fast Violet 2RL (FV) and Fast Orange 3R (FO). The process of degradation of the respective dye has been followed spectrophotometrically at the characteristic lmax. The variation of the colour intensity of aerated aqueous solution of the investigated dyes has been measured as a function of gamma irradiation dose. In all cases, the amplitude of the absorption bands of the dye compound was found to decrease with the increase of the gamma dose. Irradiation was carried out for actual waste and distilled water. By comparing the heights of the absorption maxima in both the visible and ultraviolet ranges, it was found that complete decolouration is attained at lower doses than that needed for the process of degradation of the dye. The kinetics of the degradation process has been traced and the kinetic constant, k1, was calculated and found to be concentration dependent indicating a first order reaction in all cases. The radiation-chemical yield (G-value) as a measure of the efficiency of gamma ray to degrade the respective dye was calculated for all dye compounds and it was found that the G-value in all cases increases exponentially for low radiation doses and changes linearly for high radiation doses. Also the K* value (the efficiency coefficient of dye radiolysis) was calculated and compared for the different dye compounds e.g. for FO, FY and FV dyes, the K* values were found to range from 5.5x109 to 1.92x10-7 mol·L-1'·cm-1. In addition to the study of a single dye compound in solution, mixtures of different dyes (3 dyes) were also subjected to g-ray irradiation simulating more closely actual waste effluents. Also the effect of some other chemicals used in the textile industry such as H2O2 and NaOCI on the degradation process was investigated. The specific bimolecular rate constants of the reaction of dyes with the hydroxyl radical (OH) were determined by studying the effect of ethanol concentration on G-value using competition kinetics. The effect of pH, NaOCI and H2O2 on the degradation process was studied. In addition to the main objective of the use of gamma rays to achieve the destruction of the dye compounds, the eco-toxicity of the dyes and their irradiation products was assessed using some invertebrate animals (snails) for the three FO, FY and FV dyes. The FV dye and its degradation products were found to be toxic (results of hemocyte count, hemagglutination titter, total protein content and accumulation levels) and should be completely degraded and removed from the wastewater streams using high irradiation doses. Also, and for the purpose of comparison, the sorption of the FV, FY and FO dyes on conventional sorbents such as charcoal was tested, gamma ray irradiation, as compared with conventional sorption techniques, was found to be very rapid and the removal of organic pollutants from wastewaters can be achieved nearly instantaneously. It appears that, for the complete destruction and full removal of organic pollutants from waste effluents, the sequential adsorption using chemical techniques followed by gamma irradiation treatment may prove to be more effective and advantageous. (author)