Photocatalytic degradation of azo dye (Reactive Red 120) in TiO2/UV system: Optimization and modeling using a response surface methodology (RSM) based on the central composite design

The aim of our research was to apply experimental design methodology in the optimization of photocatalytic degradation of azo dye (Reactive Red 120). The reactions were mathematically described as the function of parameters such as amount of TiO2 (X1), dye concentration (X2) and UV intensity (X3), and were modeled by the use of response surface methodology (RSM). These experiments were carried out as a central composite design (CCD) consisting of 20 experiments determined by the 23 full factorial designs with six axial points and six center points. The degradation of azo dye (RR120) followed an apparent first-order rate law in every pH condition. The results show that the responses of color removal (%) (Y1) in photocatalysis of dyes were significantly affected by the synergistic effect of linear term of UV intensity (X3) and the antagonistic effect of quadratic term of UV intensity (X32). Significant factors and synergistic effects for the TOC removal (%) (Y2) were the linear terms of TiO2 (X1), and UV intensity (X3). However, the quadratic terms of TiO2 (X12) and UV intensity (X32) had an antagonistic effect on Y2 responses. Canonical analysis indicates that the stationary point was a saddle point for Y1 response whereas a maximum point for Y2 response. The estimated ridge of maximum responses and optimal conditions for Y1 and Y2 using canonical analysis were 100% and 67.27%, respectively. The experimental values agreed with the predicted ones, indicating suitability of the model employed and the success of RSM in optimizing the conditions of photocatalysis.