Use of immobilized zinc oxide photocatalysts for wastewater treatment: Application to methylene blue degradation

Abstract

AbstractZinc‐oxide (ZnO) nanoparticles were immobilized on borosilicate glass spheres with treatment of zinc acetate dihydrate (ZAC) solution, and calcination at 400–450°C. The isotherm and kinetic parameters of methylene blue (MB) sorption on ZnO‐coated glass beads were estimated from batch sorption tests in darkness. An ultraviolet (UV)‐lamp with nominal power of 6 W and emission peak at 375 nm was used as a light source. Batch experiments of MB photodegradation were performed in a double‐wall glass reactor. Continuous flow tests were conducted in a poly(methyl methacrylate) (PMMA) annular photoreactor by recirculating the MB solution between it and a feed tank. All results were interpreted with dynamic models based on a two‐step process consisting of MB adsorption/desorption, and adsorbed MB surface reaction. The intense stirring caused by air bubbling or aqueous phase flow triggers the detachment of catalyst mass from substrate, the partial renewal of immobilized catalyst surface exposed to UV‐radiation, and the enhancement of the apparent maximum MB sorption capacity by total ZnO mass. The fast MB adsorption leads to an almost constant concentration of adsorbed MB at equilibrium, while the surface photocatalytic reaction is the MB degradation rate‐controlling step. Comparable values of the kinetic constant of surface reaction are estimated from batch and continuous flow tests, for repeated cycles of photocatalysis.