Investigation of the Physical Mechanisms of Dye Degradation in TiO2-Based Photocatalytic Reactors Under Nigerian Solar Radiation

Heterogeneous photocatalysis using titanium dioxide under solar ultraviolet illumination offers a low-cost water treatment technology applicable in Nigerian communities lacking access to centralized water treatment infrastructure. This study investigates the physical mechanisms and kinetics of dye degradation in TiO2-based photocatalytic reactors operating under natural outdoor solar radiation in Nsukka, Enugu State, South East Nigeria. Methylene blue and Congo red dyes were selected as model organic pollutants representative of textile wastewater common in Nigerian manufacturing zones. Commercial Degussa P25 titanium dioxide was used as catalyst at loadings of 0.5, 1.0, and 2.0 g/L. Solar UV irradiance was simultaneously measured using a UV radiometer to normalize degradation rates to incident photon flux. The Langmuir-Hinshelwood kinetic model was fitted to measured dye concentration versus irradiation time data at initial dye concentrations of 5, 10, 20, and 50 mg/L. Reactive oxygen species including hydroxyl radical generation were indirectly quantified using coumarin fluorescence probe methodology. Results indicate apparent first-order rate constants for methylene blue degradation of 0.028 min-1 at optimal catalyst loading, with a quantum yield of 0.0034 molecules per photon at 365 nm. The study identifies pH, ionic strength, and bicarbonate concentration as critical operational variables affecting photocatalytic efficiency in Nigerian groundwater matrices. Scale-up design equations for a continuous-flow solar photocatalytic reactor treating 500 liters per day are developed from laboratory kinetic data. Keywords: photocatalysis, titanium dioxide, dye degradation, solar radiation, water treatment