Factorial design of experiments for optimization of photocatalytic degradation of tartrazine by zinc oxide (ZnO) nanorods with different aspect ratios

The photocatalytic degradation of the azo dye tartrazine using zinc oxide (ZnO) as photocatalyst under ultraviolet light was investigated using a 24 factorial design. The variables studied were the aspect ratio of ZnO nanorods, the ZnO dosage (400 or 700 mg/L), the initial pH (7 or 10) of the tartrazine solution, and the H2O2 volume (0 or 30 µL). These variables were considered aiming to maximize the tartrazine removal efficiency and the pseudo-1st-order rate constant of the removal process. The ZnO aspect ratio was tuned by varying the Lewis base during the synthesis, hexamethylenetetramine (HMTA) was used to prepare ZnO with a low aspect ratio (ZnO_LowAR), and NaOH was used to prepare ZnO with a high aspect ratio (ZnO_HighAR). The microstructural characterizations indicated that ZnO_LowAR and ZnO_HighAR nanorods have similar structural, textural, and optical properties. The only exception was the dimensions of the nanorods obtained, which could result in differences in the facets exposed on each type of nanorod surface. The factorial design revealed that the ZnO aspect ratio, the initial pH of the tartrazine solution, and the H2O2 volume all have significant primary effects. In contrast, the ZnO dosage is not significant neither in the tartrazine removal efficiency nor in the pseudo-1st-order rate constant. Statistical models considering the coefficients of the significant interactions were obtained, leading to consistent predicted results in comparison to the results experimentally obtained. The conditions leading to highest removal efficiency (~92 %) and pseudo-1st-order rate constant (3.81 × 10-2 min-1) were carried out with ZnO_HighAR, initial pH 7, and without H2O2, which outperformed the TiO2 P-25 under the same conditions.