Development of Photocatalyst for Effective Utilization of Solar Energy for Wastewater Treatment

Abstract

to rapid industrialisation, large amount of wastewater is generated. The textile industries which uses dyes and other dyes manufacturing industries, in particular, discharge highly coloured dyes that can adversely affect plants, marine life and humans. Treatment of this wastewater prior to its discharge into natural water bodies is essential since it could be carcinogenic and toxic. To combat the issue, governments have laid down discharge norms that need to be followed while treating the wastewater. While conventional treatment methods are energy and cost intensive, and generally do not completely degrade the pollutant within optimal operating parameters, advanced oxidation processes (AOP) can mineralise the organic chemicals to harmless water and carbon dioxide. Photocatalysis is one such AOP which thus treats the effluent with the help of solar energy. In the vicinity of city of Ahmedabad and in the state of Gujarat many dyes and textile industries are present. Current work is an attempt to propose better option to conventional techniques for degradation of such dyes from wastewater. Most of the development in this area has been focussed on TiO2 as a photocatalyst. However, owing to its characteristic bandgap, it is able to utilise only the ultra-violet region, which is only ~4% of the solar spectrum. Ag2CO3, on the other hand, has a relatively narrow bandgap and can work in visible region (~47% of the solar spectrum). However, narrow bandgap often results in rapid recombination of the photogenerated electrons and holes. In order to arrest recombination, Ag2CO3 can be coupled with other photocatalysts having appropriate band positions and together they can exhibit better results. The main objective of this work is to evaluate the treatability of a model dye sample in an efficient and effective manner. In the present study, a series of hybrid Ag2CO3/SiC and Ag2CO3/ZnO nanostructures have been successfully synthesised through a simple precipitation route. The photocatalytic performance was evaluated by the degradation of MB since it is non-toxic and hence suitable for educational labwork. All experiments were performed under natural solar irradiation. It was observed that formation of a heterojunction improves the photoactivity of the hybrid photocatalysts by inducing a charge transfer between Ag2CO3 and SiC/ZnO. The photocatalysts were characterized by XRD, SEM, TEM, TGA-DTG, BET and UVVis/ NIR. Among all the hybrid combinations prepared, the best photocatalytic results and formal quantum efficiency (FQE) under natural sunlight were obtained with AGSC-12 and AGZN-22 composites. It is observed that under experimental conditions, AGSC-12 was able to completely decolourise the dye in 90 min with an FQE of 0.154%. Similarly, AGZN-22, under the experimental conditions could decolourise MB completely in just 20 min. The FQE of AGZN-22 was found to be 0.884%. The performance of both these photocatalysts was observed to be much better than that of the conventionally popular TiO2. Factors viz., photocatalyst dosage, solution pH, solar intensity, substrate and its initial concentration and speed of agitation were found to influence phototreatment. Corresponding optimum parametric values have been found and reported in terms of FQE. The performance of the photocatalysts have also been investigated on real industrial effluent and rate expression developed in terms of TOC. Also, a CPC solar photoreactor (CPC-P) has been designed and fabricated and investigated for its performance. Its performance was found to be better than that of the laboratory reactor. While the lab reactor could decolourise MB by 40% with AGSC-12, the discoloration was 60% in the CPC-P. With AGZN-22, the discolorations were 70% and 80% in the lab reactor and CPC-P, respectively. All the work has been carried out in direct natural solar light and would be useful in real-time solar photocatalytic design of reactors and other solar applications for environmental remediation.