Photocatalytic degradation kinetics of cationic and anionic dyes using Au–ZnO nanorods: Role of pH for selective and simultaneous degradation of binary dye mixtures

Photocatalysis with ecofriendly and low cost materials is attractive for degradation of organic pollutants without aid of strong reagents. In this regard, Au nanoparticle decorated ZnO nanorods (Au–ZnO) with good crystalline quality was synthesized by cost-effective and scalable hydrothermal method followed by photo-reduction of Au salt. This study addresses variation of photocatalytic degradation kinetics of different nature of dyes with various scavengers and pH conditions. Further, the process versatility is demonstrated by selective or simultaneous degradation of binary dye mixtures with optimized parameters. From Langmuir-Hinshelwood kinetic model, dye concentration range for first order limiting case was determined, and further validated from measured surface area of photocatalyst and change in dye absorbance before irradiation. Better adsorption with faster degradation exhibited by Methylene Blue (MB) dye showed efficient mineralization, revealed from chemical oxygen demand measurements. In Au–ZnO photocatalysis, generation of hydroxyl radical and its significant role in dye degradation was demonstrated using different scavengers. Variation of dye adsorption with pH dependent surface charge characteristics of photocatalyst resulted about one order higher degradation rate constant of MB at high pH. Strong pH dependent MB degradation is shown to be useful for its selective or simultaneous degradation with other dyes. Results of this study are useful for designing photo-reactors and these nanoparticles are efficient for degradation of different dyes, their combinations and industrial effluents under low power UV lamp for treatment different organic pollutants.