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

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 (Delta H-ads(o)) and standard total entropy of adsorption (Delta S-ads(o)). 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)-O-center dot radicals formed on surface.