Light Induced Oxidative Degradation Studies of Organic Dyes and Their Intermediates

Abstract

ABSTRACT

Problem Formulation

Reactive dyes are the largest single group of dyes used in the textile industry and have been selected for our study because they are difficult to eliminate by biodegradation and sludge adsorption techniques in the wastewater treatment plants. The work on photocatalytic oxidative degradation studies of reactive dyes in literature is limited as compared to other dyes. Reactive dyes are known to form a covalent bond with the fibre in the dyeing process. This leads to favorable properties such as wash-fastness. However, unfixed dye reacts with water to form hydrolyzed or an oxo-dye intermediate that has lost its bonding capacity and thus cannot be reused. Therefore dye recovery is not an option with reactive dyes and the treatment process must lead to final destruction or disposal of these contaminants. One method to remove reactive dyes from the wastewater is sorption into sorbents in fixed bed filters but no successful regeneration has been reported. Moreover, adsorption transfers the toxic dyes from one medium to the other without converting it to harmless non-toxic substances.

The application of novel treatment methods encompasses investigations of advanced oxidation processes (AOP’s), which are characterized by production of the hydroxyl radical (OH) as a primary oxidant. Examples of AOP’s include the use of (H2O2/UV), semiconductor photocatalysis, ozonolysis and ultrasonic irradiation (sonolysis), (ultrasound/O3) as important segmental or parallel processes and are found to enhance OH radical production leading to higher oxidation rates and organic matter mineralization. In this work, we investigated the photocatalytic oxidative degradation and discoloration of various reactive dyes and dye intermediates using light (UV/visible)/semiconductor catalyst by optimizing the operational parameters to ensure the rapid and complete transformation of the toxic organic compounds to benign chemicals. Also the simultaneous sonochemical effect along with photochemical oxidation process (light/semiconductor/ultrasound) is used which leads to faster destruction rate.

Work Accomplished

The work done in this study has been presented in seven chapters as discussed in the following text. After introducing the problem and its content in chapter 1, the study begins with the literature review in chapter 2. Literature of various types of toxic pollutants present in the industrial effluents, dyes, photocatalytic degradation, ultrasound, reactor design, mechanism of photosensitization of TiO2 under visible, UV and solar light have been summarized in this chapter.

In chapter 3 experimental procedures, description of reactors, instruments used and analytical techniques are discussed in detail. Chapters four, five and six deal with degradation and kinetic studies of Reactive Red 198 (RR dye198), J-acid and industrial effluent respectively. Conclusions are drawn in chapter seven.

In chapter 4, photocatalytic degradation of Reactive Red 198 (RR 198) has been investigated in aqueous heterogeneous solution using different catalysts (Degussa P25 TiO2, ZnO, Hombikat UV-100) in immersion well and shallow pond slurry type reactors under UV, visible and solar light. The disappearance of the dye follows approximately pseudo-first order kinetic according to the Langmuir-Hinshelwood model. The adsorption of dye on the semiconductor shows a strong dependence on the pH and follows a Langmuir adsorption model. The studies include dark adsorption experiments at different pH conditions. The degradation was determined by UV-Vis and decrease in COD with time. The acceleration of photocatalytic degradation of RR 198 under visible light using dye sensitized TiO2 activation by the synergistic effect of ultrasound is also reported. The effect of sonolysis, photocatalysis and sonophotocatalysis under visible light has been examined to study the influence on the degradation rates by varying the initial substrate concentration, pH, catalyst loading, H2O2 concentration to ascertain the synergistic effect on the degradation techniques. Further the presence and role of oxidative species, such as singlet oxygen (1O2), superoxide (O2-•) and hydroperoxy (HO2•) radicals was examined with the use of appropriate quenchers of these species. The photocatalytic activity of RR dye 198 dye sensitized TiO2 is demonstrated by the degradation of phenol in the presence of visible light. A comparative study using TiO2, Hombikat UV 100 and ZnO was also carried out. GC-MS of the extract left after photocatalytic degradation was done to study the intermediates formed during the reaction and thus to determine the reaction pathway. Recyclibility of the photocatalyst was also studied to make the process more economical. Besides, modification of the catalyst by sensitizers enhances the applicability of the catalyst in utilizing both UV and visible light. This technology was extended to the degradation of dye intermediates under visible light. In chapter 5, degradation of a dye intermediate J-acid has been carried out in the presence of UV and visible light using Degussa P25 TiO2 and sensitized TiO2 (TiO2 sensitized with Rose Bengal) in an immersion well type reactor. Degradation was carried out by varying the parameters like pH, initial concentration, catalyst loading and optimum conditions were determined. Effect of electron acceptors was also determined. Effect of various photocatalysts (ZnO, Degussa P25, Hombikat UV 100) was also studied on the photodegradation rate of J-acid. The degradation was studied by monitoring the change in substrate concentration employing UV-spectroscopic analysis. Photodegradation of J-acid was also carried out in sunlight in a shallow pond type reactor under optimized conditions. To study the synergistic effect of ultrasound, sonophotocatalytic degradation of J-acid was carried out under the optimized conditions. To extend the application of the process to industrial scale and to make the process cost effective, recyclibility of the photocatalyst was also studied under UV light.

In chapter 6, the degradation of effluent from a textile industry was done on laboratory scale in a shallow pond type reactor using Degussa P25 under sunlight. The degradation was done by varying conditions like pH, catalyst loading, initial concentration, A/V ratio and electron acceptors such as hydrogen peroxide. The degradation was studied by monitoring the decrease in COD values as a function of irradiation time. Recyclibility of the photocatalyst was also studied so as to make the process cost effective. The optimum degradation parameters for maximum degradation were determined. Total solids, total dissolved solids and total suspended solids in the effluent were also determined. In chapter 7, the conclusions of the entire work are drawn.