Synergistic\nDegradation of Nitrite and Dissolved Formaldehyde\n(aka Methanediol) upon Ultraviolet Irradiation

Synergistic degradation of multiple pollutants in a single\nprocess\nis the ultimate goal for pollution treatment and environmental sustainability.\nHerein, ultraviolet spectroscopy (UV) and gas chromatography–mass\nspectrometry (GC–MS) were employed to track the reaction evolution\nof an aqueous mixture of NaNO<sub>2</sub> and dissolved H<sub>2</sub>CO, aka methanediol (CH<sub>2</sub>(OH)<sub>2</sub>), upon continuous-wave\nUVA irradiation. The process began with the reactions of CH<sub>2</sub>(OH)<sub>2</sub> with hydroxyl (OH) and nitric oxide (NO) radicals,\ngenerated upon photolysis of NO<sub>2</sub><sup>–</sup>. At\nan early exposure period within 200 s, a band at λ < 270\nnm could be attributed to the monoprotonated trioxodinitrate anion\n(HN<sub>2</sub>O<sub>3</sub><sup>–</sup>) and the trioxodinitrate\nanion (N<sub>2</sub>O<sub>3</sub><sup>2–</sup>) via the reactions\ninvolving NO, OH, CH<sub>2</sub>(OH)<sub>2</sub>, and hydrated formyl\nradical, CH(OH)<sub>2</sub>. Kinetic isotope effects on NO<sub>2</sub><sup>–</sup> depletion and HN<sub>2</sub>O<sub>3</sub><sup>–</sup> generation owing to methanediol isotopologues, denoting\nthe substitutions with deuterium atoms on C–H and O–H\nmoieties, revealed that the two −OH groups at geminal diols\naffected the energetics along the hydrogen abstraction coordinate\nat the C–H moiety. Concomitantly, the GC–MS results\nrevealed the generation of N<sub>2</sub>O, CO<sub>2</sub>, and N<sub>2</sub>. The latter two molecules were ideal end products for wastewater\ntreatment. Herein, we have demonstrated the feasibility of synergistic\ndegradation of due pollutants upon UVA treatment without adding extra\nphotocatalysts and advanced oxidation process reagents.