Optimization of photocatalytic degradation of red solophenyl direct dye in TiO2 aqueous suspension by application of full factorial experimental design

Optimization of the photocatalytic degradation of a direct dye such as Solophenyl Red 3 BL, under UV light irradiation in presence of TiO₂ (Degussa P25) particles in a batch photo-reactor was studied. The reagent mixture composed of dye and TiO₂ powder dissolved in distilled water. Full factorial experimental design was applied to study the main effects and the interaction effects between three operational parameters viz. the concentration of TiO₂, medium temperature and initial pH, in the degradation process of this dye. The parameters were coded as Xi, X₂ and X₃ consecutively, and were investigated at two levels (-1 and +1). The effects of individual variables and their interaction effects for dependent variables, namely, photocatalytic decolorization efficiency (%) were determined in two domains of pH initial values. When the initial pH values range from 2 to 6.3, the results showed that the concentration of TiO₂ and the initial pH exhibit a significant negative effect on the response (yield of decolorization), whereas, the temperature develops a negligible effect, but its interaction with the initial pH of the solution (X₂X₃) has a positive and significant effect on the efficiency of decolorization. Thus, the effect of temperature (X₂) depends on the initial pH of the solution (X₃) in the chosen experimental conditions. In fact, when the pH values range from 6.3 to 12, the most significant main effects are due to the initial pH of the aqueous dye solution (X₃), temperature (X₂) and to a lesser extent than the concentration of TiO₂ (Xi). The binary interaction effects all seem important; the most remarkable effect but negative, is that corresponding to the interaction between pH and temperature. The interactive effect between the three factors studied seems very important.