Visible-light photocatalytic degradation of acid violet 7 dye in a continuous annular reactor using ZnO/PPy photocatalyst: Synthesis, characterization, mass transfer effect evaluation and kinetic analysis

Abstract

In this work, a composite of ZnO and polypyrrole (ZnO/PPy) was synthesized by polymerization method and characterized by X-ray diffraction (XRD), Ultraviolet–Visible with Near-infrared (UV–Vis NIR) spectroscopy and Fourier Transform Infrared (FT-IR) spectroscopy, both by diffuse reflectance mode. The ZnO/PPy composite was tested as photocatalyst for the degradation of acid violet 7 dye under visible light radiation in two systems: a) by using a batch reactor and b) in a continuous annular reactor. The hole formation in the valence band was evidenced using a hole scavenger test for ZnO/PPy composite and also the hydroxyl radicals formation capacity was determined. A three-resistance model of mass transfer (MT) was evaluated, it considers the traditional external mass transfer coefficient (She) to obtain an overall mass transfer coefficient (Shapp), the model is considered a full 3-D convection–diffusion–reaction applied to the photocatalyst of ZnO/PPy composite attached onto the interior wall of a quartz ring (annular space). The MT analysis showed in the continuous reactor that the degradation reaction was kinetically controlled. A Langmuir-Hinshelwood (L-H) model was analyzed according the fitting of the experimental data in the annular continuous photocatalytic reactor. The adsorption parameter obtained with L-H model was validated based on thermodynamic criteria for the changes in the standard enthalpy of adsorption (ΔHadso) and standard total entropy of adsorption (ΔSadso). The analysis of thermodynamic properties revealed that the adsorbed AV7 molecule on ZnO/PPy photocatalyst still presents mobility in order to react to the very short time OH radicals formed on surface.