Degradation of Congo red by UV photolysis of nitrate: Kinetics and degradation mechanism

The efficiency of Congo red (CR) removal by low-pressure ultraviolet photolysis of nitrate (UV/NO 3 − ) process was investigated in this study. Results showed that CR degradation through direct UV (254 nm) photolysis could be ignored, but CR was obviously removed in UV/NO 3 − process. Moreover, the contributions of UV, NO 2 and OH to the degradation of CR were explored to be roughly 0.63%, 17.46% and 81.90%, respectively. In addition, the observed pseudo-first-order rate constants for the degradation of CR ( k obs ) were assessed by the effects of different water matrices and operation conditions, including NO 3 − dosage, CR concentration, solution pH, chloride ion (Cl − ), natural organic matter (NOM) and carbonate/bicarbonate (CO 3 2− /HCO 3 − ). The k obs was increased by nearly 12.9 times with the addition of NO 3 − dosage from 1 mM to 200 mM due to the generation of more active components (e.g., OH and NO 2 ). Higher doses of CR reduced the k obs , which could be attributed to the filter effects of CR on UV irradiation. Although the photolysis of NO 3 − could be expedited under acidic conditions, the k obs was decelerated because of the protonation of CR (pKa = 4.1, 25 °C). While in alkaline conditions, the k obs was also reduced as the decrease in redox potential of OH. The presence of Cl − (0–7 mM) exhibited no scavenging effect on the k obs , which suggested the secondary radicals (i.e., chlorine radicals (e.g., Cl , Cl 2 − )) could oxidize CR effectively. NOM restricted the degradation of CR significantly, resulting from the filter effects of NOM ( ε  = 0.11 L mgC −1 cm −1 ) on UV and the effects of radical scavenging. The k obs was improved obviously with the participation of CO 3 2− /HCO 3 − (0–7 mM), ascribing to the oxidation of carbonate radical (CO 3 − ) and the increased steady state concentration of NO 2 . The degradation of CR in UV/NO 3 − process has the potential for practical applications in real water. Furthermore, eight intermediates were proposed by Liquid chromatography-tandem mass spectrometry (LC-MS/MS). Hydroxylation, nitrification and other aromatic products in the reaction of CR with OH and/or NO 2 were readily produced by H-abstraction, addition reaction and electron transfer.