Revisiting the mechanisms of nitrite ions and ammonia removal from aqueous solutions: photolysis versus photocatalysis

Nitrite ions and ammonia are widespread forms of inorganic water pollutants. Nevertheless, the mechanisms of their photolytic and photocatalytic reactions under UV-A irradiation are still fully undisclosed, particularly, at different pH values under aerobic and inert atmospheres. Herein, we have studied the photolytic decomposition of nitrite ions under different conditions using 365 nm UV-A LED as a light source instead of mercury lamps that emit photons in the UV-B region and generate a lot of heat. The results indicated that the rate of nitrite disproportionation in the dark at pH ≤ 3.0 is remarkably high relative to the rate of the photolytic decomposition. At pH ˃ 3, the photolytic reaction is negligible and nitrite ions showed considerable stability. In contrast, the photocatalytic oxidation of nitrite ions over TiO2 photocatalysts, namely, TiO2P25, TiO2UV100, and TiO2 anatase/brookite mixture proceeds at pH ˃ 3.0. TiO2 P25 exhibited the highest photocatalytic activity at pH 5. Interestingly, the photolytic simultaneous removal of nitrite ions and ammonia was possible at pH 9.0 in the absence of oxygen (Ar atmosphere). A 42.69 ± 0.66%, 27.75 ± 1.7%, and 32.74 ± 0.59% of nitrogen calculated based on nitrite, ammonia, and both of them, respectively, can be removed after 6 h of UV-A irradiation. The selectivity of N2 evolution was 77.6%. The nitrogen removal rate was significantly reduced in the presence of TiO2 photocatalyst evincing that TiO2 photocatalysis is applicable for nitrite ions oxidation, whereas the photolytic process is better suited for the simultaneous removal of nitrite ions and ammonia.