Influence of pH on the UV photolysis of N-nitrosamines in water: Kinetics and products

Amine-based post-combustion CO₂ capture is recognized as a promising technique for abating CO₂ emissions from anthropogenic sources. A severe drawback of this technique, however, is the formation of carcinogenic N-nitrosamines as a by-product. In this study, the effect of pH was investigated on the UV photodegradation of N-nitrosamines (N-nitrosodiethanolamine (NDELA), N-nitrosodiethylamine (NDEA), and N-nitrosomorpholine (NMOR)) which are closely related to the amine-based CO₂ capture technology. A decrease in pseudo-first-order degradation rate constants was observed for NDELA (2.49 × 10⁻²–6.48 × 10⁻³ L/W-min), NDEA (1.56 × 10⁻²–5.25 × 10⁻⁴ L/W-min), and NMOR (1.68 × 10⁻²–7.00 × 10⁻⁴ L/W-min) with an increase in pH2−10. The formation of NO₂⁻ was more prevalent than that of NO₃⁻ at pH4−10, while under strong acidic condition (pH2) NO₃⁻ formation was more prevalent. The concentration of secondary amines (diethanolamine, diethylamine, and morpholine) increased from basic (pH10) to weakly acidic conditions (pH4) and then decreased at pH2. Whereas, the concentration of primary amines (monoethanolamine and ethylamine) followed the opposite trend. Furthermore, mechanistic pathways were revealed for N-nitrosamine photodegradation based on the degradation products. A good total nitrogen (TN) balance over the irradiation period confirmed that all the main degradation products were quantified.