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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Anand Prakash

2nd Supervisor

Ajay Ray

Co-Supervisor

Abstract

The overall goal of the research is to develop a novel photocatalytic reactor system, namely, a bubble column photoreactor for the degradation of resistant pollutants, with a particular emphasis on azo dyes. To achieve the objective, different sub-objectives were performed employing lab and pilot-scale reactors with the application of simulated wastewater. Direct Blue (DB15) was used as a model compound for the azo dyes class for this study. Firstly, the influence of various variables such as pH, dye concentration, and catalyst loading to determine optimal combinations of DB15 removal. The experiments were conducted in a swirl flow photoreactor under the UV light effect. Then, the removal kinetics of DB15 were studied over a suspended and immobilized TiO2 catalyst. Next, DB15 adsorption behavior and DB15 mineralization in terms of total organic carbon (TOC) were examined. After that, the decolorization by-products were evaluated using the GC-MS approach. The identified compounds were then classified based on their toxicity using the National Library of Medicine/National Center for Biotechnology Information (NCBI) - the USA. DB15 removal mechanism was then proposed accordingly. For the pilot-scale reactor, a bubble column photoreactor was designed and evaluated for DB15 degradation. The findings showed that the optimum pH value for DB15 decolorization is 4. In terms of the response surface methodology (RSM) approach, the catalyst loading, dye concentration, and light intensity significantly affect DB15 degradation. It was found that DB15 decolorization follows the pseudo first-order kinetics model. In terms of three variables, apparent rate () had the following formula while the empirical model was expressed as follows . For immobilized catalysts, the DB15 degradation kinetics was described by a pseudo first-order model for various dye concentrations while a power-law model described the influence of light intensity on dye kinetics. It was obtained that the DB15 adsorption behavior follows Langmuir isotherm. The DB15 removal recorded 77% after 2 hr irradiation, while its percentage was 98% for the reaction time of 7 hr. TOC removal reached values of 83.64% and 90.9% as the irradiation time was extended to 16 hr and 19 hr, respectively. DB15 removal started with the simultaneous cleavage of C-N and N=N bonds. The generated fragments were then oxidized into small molecular weight products through different steps such as ring-opening and demethylation before their mineralization. It was revealed that the by-products majority have the lowest toxicity level. In contrast, five substances had the same toxicity level as DB15, while ten by-products were more toxic than DB15. For the pilot-scale, it was demonstrated that the factors: of dye concentration, catalyst loading, and airflow have a substantial impact on the dye decolorization. For the RSM model, catalyst loading and airflow had a favorable impact on the removal process, while the dye concentration recorded an inhibitory effect.

Summary for Lay Audience

Industrial wastewater has attracted considerable interest worldwide due to its highly significant effect on the environment. Industrial wastewater typically contains a high level of contaminants such as heavy metals and phenols. These pollutants are characterized as highly toxic compounds. Industrial wastewater is also highly complicated compared to other types of wastewater regarding composition. Moreover, industrial wastewater generally has higher values of water quality parameters, and it is highly variable depending on the type of industry. For example, the steel industry usually generates a considerable amount of ammonia and phenols, while colored dispersed dyes are the main constituents of the wastewater from the textile industry. Thus, industrial wastewater treatment is considered a difficult task due to high concentrations of contaminants and each industry’s highly diverse water qualities. The main goal of the research is to develop a novel photocatalytic reactor system, namely, a pilot-scale photoreactor for the degradation of azo dyes. Direct Blue (DB15) was used as a model compound for the azo dyes class. The effect of different variables such as pH, dye concentration, and catalyst loading to determine optimal combinations of DB15 removal. The experiments were conducted in a swirl flow photoreactor under the UV light effect. The DB15 removal registered 77% after 2 hr irradiation, while its removal was 98% for the reaction time of 7 hr. In terms of total organic carbon (TOC), the outcomes showed that the TOC value reaches 75.6% removal for 14 hr reaction time. TOC removal reached values of 83.64% and 90.9% as the reaction time was increased to 16 hr and 19 hr, respectively. The dye and catalyst concentrations had a big impact on the mineralization time. For the pilot-scale reactor, it was observed that dye concentration, catalyst loading, and airflow factors substantially impact dye decolorization.

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