Optimization and mechanism elucidation of the catalytic photo-degradation of the dyes Eosin Yellow (EY) and Naphthol blue black (NBB) by a polyaniline-coated titanium dioxide nanocomposite

A polyaniline-coated titanium oxide nanocomposite (PTO) was synthesized, characterized and used in the catalytic photo-degradation of the anionic dyes Eosin Yellow (EY) and Naphthol blue black (NBB). An emphasis was placed on the key parameters governing the degradation process. It was noted that the extent of degradation was higher at lower pH for both dyes. The rate of degradation of both dyes also increased with increasing catalyst dosage, Langmuir-Hinselwood models and other second-order kinetic models verified that heterogeneous photocatalysis occurred. The rate of degradation increased with increasing initial dye concentrations and only increased gradually with an increase in UV light intensity in the absence of the catalyst. The optimum operating conditions for the degradation was also predicted using response surface methodology (RSM) analysis as a statistical tool. This analysis revealed that the initial dye concentration, pH and dosage of the catalyst are all significant parameters in the degradation process. The combined effect of pH and initial concentration was antagonistic whilst the combined effect of initial concentration and catalyst dosage and the combined effect of catalyst dosage and pH were both synergistic. The optimum degradation percentages were found to be 99.85 and 99.74 for EY and NBB respectively. These optimum percentage degradations were observed at pH 3.0, initial dye concentration 15mgL^-1 and a catalyst dosage of 1.0gL^-1. LC-MS data was also used to identify the photodegradation products and to propose a mechanism of photodegradation.