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, ^[rad]NO₂ and ^[rad]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 NO₃⁻ dosage, CR concentration, solution pH, chloride ion (Cl⁻), natural organic matter (NOM) and carbonate/bicarbonate (CO₃²⁻/HCO₃⁻). The k_obs 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., ^[rad]OH and ^[rad]NO₂). 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 NO₃⁻ 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 ^[rad]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^[rad], Cl₂^[rad]−)) could oxidize CR effectively. NOM restricted the degradation of CR significantly, resulting from the filter effects of NOM (ε = 0.11 L^[rad]mgC^−1[rad]cm⁻¹) on UV and the effects of radical scavenging. The k_obs was improved obviously with the participation of CO₃²⁻/HCO₃⁻ (0–7 mM), ascribing to the oxidation of carbonate radical (CO₃^[rad]−) and the increased steady state concentration of ^[rad]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 ^[rad]OH and/or ^[rad]NO₂ were readily produced by H-abstraction, addition reaction and electron transfer.