Acetone-induced photodegradation of organic dyes in the presence of hydrogen source

Abstract

Many organic dyes that are usually used by the textile dyeing industry are resistant to UV degradation. The photodegradation of textile dyes with different chromophores in the presence of acetone (ACE), which performs as a solvent and/or a photo-sensitizer, was investigated in this study. All photolytic experiments were carried out in the Rayonet TM RPR-200 merry-go-round photoreactor, at 253.7 nm monochromatic ultraviolet (UV) lamps. Hydrophobic disperse dyes are known to have very low photodegradation rates in the natural environment because of their low solubility. In this study, ACE acts not only as a solvent to increase the dye's solubility but also as a photosensitizer to enhance the photodegradation rate. The results demonstrated that photochemical reaction in the presence of ACE could rapidly and effectively enhance color removal. The photosensitization follows pseudo first-order decay and is dominated by photoreduction process. The rate constants of dye degradation by UV depended on the solution pH and solvent system, (i.e., ACE/H2O ratio). The decay quantum yields of dyes were normally observed with the increase of the ACE/H2O ratio and the photosensitization of disperse dye found to be optimized at alkaline conditions. In general, more than ten times of quantum yield increment is observed in the presence of ACE photo-sensitizer than in water alone. Further increase in ACE/H2O ratio reduces the quantum yields, possibly due to the light attenuation by excess ACE. In addition, the BOD5/COD ratio of the treated solution was increased, indicating that the dye structure was shattered and more biodegradable in the ensuing biological treatment. ACE in the wastewater could be recycled by a simple gas stripping process, which not only recovered the ACE for reuse but also increased the dissolved oxygen in the wastewater stream and facilitated the biological treatment. Photochemical reaction of disperse dye in a cocktail solution containing ACE and triethylamine (TEA) has been also examined. Adding a low concentration of TEA to the aqueous ACE can further enhance the reaction, because TEA can be used as additional hydrogen source. However, an overdose of TEA will quench the reaction. The possible photoreduction mechanisms of disperse dye in aqueous ACE and TEA were proposed, two models based on the Stern-Volmer plot were derived and successfully described the reaction at both low and high concentration of TEA, which made the process performance predictable. Besides the study of photodegradation of disperse dyes, a typical highly soluble and non-biodegradable dye contaminant, azo reactive dye - C. I. Reactive Red 2 (RR2), was used to explore the reaction mechanisms and kinetics of photodegradation in the cocktail solutions. The photodegradation of RR2 in aqueous ACE or TEA solution was found to be kinetically controlled by the pseudo first-order and zero-order kinetics respectively. In the presence of TEA, the rate enhancement is mostly due to the electron transfer from TEA to RR2 and results in the reduction of dye chromophore. Photosensitization is likely the dominant mechanism in the presence of ACE. The photodegradation of RR2 through photoreduction was shown to be the main decay pathway. In addition, some minor pathways were observed including photodechlorination and photodesulphonation. The modeling of cocktail photodecolorization of RR2, in the mixture aqueous of ACE and TEA solution was investigated. It was interested to find that three distinct stages were obviously observed in the cocktail photosensitization profiles. A lag phase was observed at the commencement of the degradation, but its duration was gradually reduced with the increment of the TEA concentration as well as the incident light intensity. Subsequently, a fast decay of RR2 was observed in which over 80% of the dyes were reduced in this stage and it was interesting to find a tailing stage after 90-95% of color was removal. Since the dye photodegradation processes were found to be kinetically controlled, a quantitative estimation of RR2 in the cocktail photosensitization system was also studied. The system of differential equations has been solved by numerical integration to calculate the concentration of RR2 as a function of time. A mathematical Cocktail model was proposed and the predicted data was compared with the experiments, generally very good agreement was achieved. Furthermore, the sensitivity analysis of the Cocktail model confirmed that photosensitization process was the dominant mechanism in the cocktail system and it was mainly contributed to the presence of ACE.