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

Abstract

The efficiency of Congo red (CR) removal by low-pressure ultraviolet photolysis of nitrate (UV/NO3−) 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/NO3− process. Moreover, the contributions of UV, NO2 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 (kobs) were assessed by the effects of different water matrices and operation conditions, including NO3− dosage, CR concentration, solution pH, chloride ion (Cl−), natural organic matter (NOM) and carbonate/bicarbonate (CO32−/HCO3−). The kobs was increased by nearly 12.9 times with the addition of NO3− dosage from 1 mM to 200 mM due to the generation of more active components (e.g., OH and NO2). Higher doses of CR reduced the kobs, which could be attributed to the filter effects of CR on UV irradiation. Although the photolysis of NO3− could be expedited under acidic conditions, the kobs was decelerated because of the protonation of CR (pKa = 4.1, 25 °C). While in alkaline conditions, the kobs was also reduced as the decrease in redox potential of OH. The presence of Cl− (0–7 mM) exhibited no scavenging effect on the kobs, which suggested the secondary radicals (i.e., chlorine radicals (e.g., Cl, Cl2−)) could oxidize CR effectively. NOM restricted the degradation of CR significantly, resulting from the filter effects of NOM (ε = 0.11 LmgC−1cm−1) on UV and the effects of radical scavenging. The kobs was improved obviously with the participation of CO32−/HCO3− (0–7 mM), ascribing to the oxidation of carbonate radical (CO3−) and the increased steady state concentration of NO2. The degradation of CR in UV/NO3− 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 NO2 were readily produced by H-abstraction, addition reaction and electron transfer.