Converting red mud wastes into mesoporous ZSM-5 decorated with TiO₂ as an eco-friendly and efficient adsorbent-photocatalyst for dyes removal

Red mud wastes have been converted into mesoporous zeolite socony mobile-5 (ZSM-5) followed by deposited titanium dioxide (TiO₂) nanoparticles to generate synergy adsorption-photodegradation for removal of dye removal in waste water. The amount of TiO₂ loading was varied to achieve optimum photocatalytic activity while maintaining the mesoporosity and high surface area of ZSM-5. Sol-gel method facilitated the formation of anatase TiO₂ on the ZSM-5. The fourier transform infrared spectra clarified the formation of Si–O–Ti at 957 cm⁻¹ by the exchanging the hydrogen ion with titanium ion, which proved by decreasing the absorption band of Si–OH and Si–O interaction at 964 and 944 cm⁻¹, respectively. Sol-gel method also preserved the mesopore diameter of ZSM-5 at 3.5 nm which allow the diffusion of methylene blue (MB) molecules into the pores. However, the surface area and the pore volume were slightly reduced with increasing the TiO₂ loading. The adsorption performance of samples showed that the increasing in the TiO₂ loading led to the decreasing in the adsorption capacity. All samples showed the suitability towards the pseudo second order kinetic. The Langmuir isotherm was suitable to describe the adsorption mechanism by monolayer adsorption. Mesoporosity of ZSM-5 accelerated the adsorption of dye via the increase of mass transfer in the pore channel which confirmed by the low intercept of intraparticle diffusion model at the first stage. The photocatalytic test showed that 10% TiO₂ loading on the ZSM-5 exhibited the highest methylene blue removal followed by 5% and 20% TiO₂ loading. Optimization on the amount of photocatalyst and the pH of solution indicated the reaction favoured 1 g L⁻¹ of catalysts and at alkaline pH. 10% TiO₂/ZSM-5 also exhibited high stability and reusability up to four reaction cycles. Photocatalytic performance of 10% TiO₂/ZSM-5 was further investigated on photodegradation of malachite green and rhodamine B organic dyes, which showed the photocatalytic efficiency of 73 and 88%, respectively. Superoxide radical, hydroxyl radical, and photogenerated electron were identified as the main active species for MB photodegradation based on the reduction of degradation rate following the addition scavenger molecules.