Degradation of Orange IV by UV photolysis in nitrite-containing wastewater: Influencing factors, mechanism, and response surface methodology

Abstract

In this study, we examined the ultraviolet (UV) light-induced degradation of Orange IV (O-IV) in an aqueous solution containing nitrite (NO2-). The observed pseudo-first-order rate constants of O-IV (kobs) degradation were assessed based on the effects of different influencing factors, including the initial concentration of O-IV, light intensity, temperature, pH, NO2- concentration, and inorganic anions. The results showed that O-IV was significantly removed with a kobs of 0.05268 min-1 in the UV/NO2- system. Quenching experiments and electron paramagnetic resonance (EPR) spectroscopy determined the possible active species in the UV/NO2- system, including hydroxyl radical (̇OH) and singlet oxygen (1O2). kobs decreased with an increase in the initial concentration of O-IV, possibly because the number of ˙OH produced by NO2- was not sufficient to remove all the dye molecules. kobs increased with an increase in light intensity and NO2- concentration, and the degradation of O-IV under acidic or alkaline conditions was inhibited. The most influential inorganic anions were carbonate ions (CO32-), which reacted with ˙OH and rendered photochemical reactions less efficient. Additionally, a possible photodegradation mechanism of O-IV was proposed using liquid chromatography-mass spectrometry (LC-MS) analysis. Finally, the response surface methodology (RSM) was performed, the reaction parameters were optimized, the RSM model (R2 =0.9746) was constructed, and the optimal experimental conditions were obtained using the model with an NO2− concentration of 12.91 mmol/L, pH of 7, and light intensity of 32 W.