Enhanced indigo carmine dye removal via chitosan-modified NiO-g-C₃N₄ catalyst: Adsorption and photocatalysis studies

In this work, an ultrasonic approach was utilized to synthesize a chitosan-modified NiO-g-C₃N₄ (CSGN10), which was then effectively employed for the photocatalytic and adsorptive removal of indigo carmine (IC). Several techniques, including HRTEM, XPS, PL, DRS, and FTIR spectroscopy, were employed to thoroughly characterize the as-fabricated nanocomposite. The appropriate operational variables for dye elimination were carefully examined and optimized. Different models were used to assess the kinetic models and adsorption isotherms, with the Sips model best fitted the data indicating a mixed physi- and chemi-sorption processes, rather than a straightforward monolayer. CSGN10 exhibited superior adsorption features to IC dye with q_max of 227 mg/g, which is 3 and 10 times higher than those of pure CS and g-C₃N₄, respectively. Moreover, it showed promising synergistic impacts of photodegradation and adsorption with synergistic elimination ratio up to 91.1 %. It was revealed that ionic attraction, π-π interaction, Van der Waals forces, and hydrogen bonding played vital roles, with the IC dye forming multilayers on the CSGN10 NPs' surfaces. Moreover, Z-scheme photocatalytic mechanism showed that visible light irradiation improved the separation of generated electron-hole pairs and caused a noticeable lowering in the band gap energy. Chitosan has a high ability to move electrons on its surfaces; therefore, it greatly inhibited the recombination of electron-hole pairs photogenerated on the composite catalyst surfaces. Thus, more reactive oxygen species are created when photons interact with the composite photocatalyst surfaces. Experiments with scavenging species ascertained the noticeable involvement of superoxide radicals in the photocatalytic degradation of IC dye.