Microbial Mediated Synthesis of Visible Light Active Silver Based Titania Nanocomposites and their Photocatalytic Applications in Degradation of Azo Dyes

Abstract

A large quantity of dyes released with textile industry effluents has raised a lot of concern due to their harmful and toxic effects on the ecosystem. Heterogeneous photocatalysis with TiO2 has been reported to decontaminate these effluents by mineralization of these dyes. However, owing to its limitations such as the wide band gap and high charge recombination rate, TiO2 is less effective and solar light cannot be utilized as the irradiation source. The modification of TiO2 with oxides of silver and copper can overcome these limitations. Microbial synthesis of nanoparticles is a greener approach. The present study reports a new method for the synthesis of silver-based TiO2 and silver-based TiO2 modified with Cu nanocomposites extracellularly using cell free culture supernatant (CFS) of Alcaligenes aquatilis under ambient conditions and their application in the degradation of Reactive Blue 220 (RB 220) and Acid Yellow 17 (AY 17) dyes under visible and solar light. Two step method of synthesis of Ag based TiO2 nanocomposites was developed to achieve maximum photocatalytic activity under visible light. The optimum synthesis conditions were found to be molar ratio of Ag to Ti of 1:1.6, CFS pH of 7, synthesis time of 4 h for silver ion reduction and 4 h after the addition of Ti precursor. AgO/Ag2O@TiO2 nanocomposites thus formed were spherical with AgO/Ag2O embedded in TiO2 and average particle size of 39.6 nm. Around 99% of 100 ppm RB 220 could be degraded in 90 min with AgO/Ag2O@TiO2. It showed better activity than Bio-TiO2 and AgO/Ag2O, found to be active under visible as well as solar light. It did not exhibit appreciable activity in the degradation of AY-17. AgO/Ag2O@TiO2 was further modified with Cu by microbial synthesis and the nanocomposites synthesized with molar ratio of Ag:Ti:Cu of 1:1.6:0.7 and calcined at 400°C was found to be the optimum to achieve maximum dye degradation. AgO/Ag2O@TiO2 modified with Cu (ATC-75) was with average particle size of 14.3 nm and showed the presence of Cu as CuO. ATC-75 nanocomposites could degrade 100 ppm RB 220 completely in 45 minutes under visible light and >95 % degradation in 90 min under solar light, whereas it could degrade 90 % of 100 ppm of AY17 dye in 90 min under visible and solar light. The degradation of AY-17 was greatly affected by the presence of RB-220 in mixed dye solution. ATC-75 showed a good reusability potential. These photocatalysts can be used in the treatment of wastewater containing dyes by harnessing solar energy. The synthesis is green, environmental friendly and energy efficient method.