Photocatalytic Oxidation of Phenol Red on Nanocrystalline TiO2 Particles

In the current research study, the photocatalytic (photodecolorization) degradation of the phenolic model ecotoxicant, phenol red dye has been investigated using laboratory synthesized anatase TiO2 nanoparticles in aqueous solution. The prepared nano TiO2 powder has been characterized using several advanced instruments including, Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDXS), X-Ray Diffraction (XRD), Diffused Reflectance UV-VIS spectrometry (DUR-UV-VIS), Transmission Electron Microscopy (TEM), Brunauer–Emmett–Teller (BET) theory and Raman Spectrometry. Average particle size of 9, 9.95 and 11 nm was found using TEM, XRD and SEM, respectively. The surface area and pore size were also measured employing BET nitrogen adsorption apparatus which resulted in surface area of 48 m2/g and a pore size of 3.7 nm, which indicates the mesoporosity of the prepared nano anatase TiO2 particles. The impacts of several operational parameters for the photodegradation process were explored encompassing, TiO2 loading, solution pH, initial phenol red concentration, UV light source intensity, photocatalyst particle size and added oxidants. Under optimum experimental conditions, 4.5 solution pH, 500 mg/l TiO2 loading and 2.9 x 10-5 mol /l (10.3 mg/l) phenol red, the value of the apparent rate constant, kapp , obtained has been 0.01052 min-1 (17.53x 10-5 sec-1) and the half life of the process, accordingly is equal to 1.098 hours.
The kinetic of phenol red photobleaching has also been studied and it was found that it follows the pseudo first order pattern regardless of reaction conditions.
Furthermore, the apparent quantum yield for the photodecolorization process was also determined and found to be approximately 0.1. Results reveal that the photooxidation process of phenol red follows hydroxyl type chemistry in which the addition of hydrogen peroxide has contributed massively in the promotion of the process rate and yield, and further, suppressed to a great extent by addition of isopropanol as .OH radical scavenger.
The main process activation thermodynamic parameters namely, Gibbs energy, enthalpy and entropy were also deduced following the computation of photolysis activation energy employing the well known Arrhenius relation.