Siliceous termite hill supported ZnO-TiO2 as a solar light responsive photocatalyst: Synthesis, characterization and performance in degradation of methylene blue dye

Abstract

Colored contaminants in wastewater need to be removed prior to discharge due to their negative impacts on public health and the environment. Herein, immobilized TiO2 on termite hill (TiO2/TH) and ZnO-promoted TiO2/TH (ZnO-TiO2/TH) were synthesized, evaluated by different characterization techniques (XRF, XRD, FTIR, UV-Vis DRS, N2 adsorption-desorption analysis and SEM), and the possibility of using them as photocatalysts for methylene blue (MB) degradation under solar irradiation investigated. The influence of photocatalytic reaction parameters (initial MB concentration, photocatalyst dosage and solution pH) was studied at fixed irradiation time. XRD and SEM results, among other analyses revealed successful incorporation of ZnO and TiO2 on structured support (TH), while UV-Vis analysis affirmed reduction in band gap energy due to immobilizing effect. The optimized photocatalytic degradation variables were 50 mg/L dye concentration, 1.2 g/L photocatalyst dosage and 6.0 pH. The ZnO-TiO2/TH showed better photoactivity for MB degradation than TiO2/TH under the same conditions which could be attributed to a reduction in band gap energy from 2.94 eV to 2.85 eV upon addition of ZnO. More than 80% degradation efficiency was achieved after the 7th cycle, depicting better stability. A pseudo-first-order Langmuir-Hinshelwood model with a rate constant of 0.0774 min−1 and R2 of 0.9971 at optimum dye concentration (50 mg/L) appeared to best describe the photocatalytic degradation kinetic data. The UV-Vis spectra changes and decrease in total organic carbon indicated that ZnO-TiO2/TH photocatalyst has the prospect to degrade organic pollutants in industrial effluent completely.