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/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)-N-center dot and (OH)-O-center dot 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 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)-O-center dot and (NO2)-N-center dot). 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 k(obs) was decelerated because of the protonation of CR (pKa = 4.1, 25 degrees C). While in alkaline conditions, the kobs was also reduced as the decrease in redox potential of center dot 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-center dot, Cl-2(center dot-))) could oxidize CR effectively. NOM restricted the degradation of CR significantly, resulting from the filter effects of NOM (epsilon = 0.11 L.mgC(-1).cm(-1)) on UV and the effects of radical scavenging. The kobs was improved obviously with the participation of CO32- /HCO3- (0.7mM), ascribing to the oxidation of carbonate radical (CO3 center dot-) and the increased steady state concentration of (NO2)-N-center dot. 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)-O-center dot and/or (NO2)-N-center dot were readily produced by H-abstraction, addition reaction and electron transfer.

Automated summary

The study found that the efficiency of CR removal in the UV/NO3- process was high, mainly due to the role of (OH)• and (NO2)-N• in the degradation of CR. Increasing NO3- dosage can enhance k(obs), but high CR concentrations can decrease k(obs). Accelerating NO3- photolysis under acidic conditions while decelerating CR degradation, and decreasing kobs under alkaline conditions were also observed.