UV/Nitrate photocatalysis for degradation of Methylene blue in wastewater: Kinetics, transformation products, and toxicity assessment

Abstract

This study explored the degradation kinetics of Methylene blue (MB) and formation of nitrated/nitrosated products via UV/NO3− during wastewater treatment. The contributions of UV irradiation, hydroxyl radical (HO⋅), and reactive nitrogen species were calculated to be approximately 6.21%, 77.86%, and 15.93%, respectively. Increasing NO3− dosage or decreasing MB concentration facilitated the degradation of MB. The removal efficiency of MB was higher under acidic conditions than that under alkaline and neutral conditions, which was ascribed to the higher HO⋅ concentration under acidic conditions. Both chloride ions (Cl−) and carbonate/bicarbonate ions (CO32−/HCO3−) had a slight effect on observed pseudo-first-order rate constants (kobs). However, natural organic matters (NOM), as radical scavengers and UV-irradiation filters, notably inhibited the degradation of MB. Furthermore, the degradation intermediates/products and transformation pathway of MB in UV/NO3− were clarified. Results indicated that nitrated/nitrosated products were more likely to be produced by coupling reactions and electrophilic addition. Fortunately, no significant enhancement in acute toxicity towards Vibrio fischeri was found during the degradation of MB by UV/NO3−. MB could be degraded effectively in different real water, which indicated that UV/NO3− system should be a good alternative for the degradation of pollutants.