Interconversion and Removal of Inorganic Nitrogen Compounds via UV Irradiation

Dissolved inorganic nitrogen (DIN) species are key components of the nitrogen cycle and are the main nitrogen pollutants in groundwater. This study investigated the interconversion and removal of the principal DIN compounds (NO3−, NO2− and NH4+) via UV light irradiation using a medium-pressure mercury lamp. The experiments were carried out systematically at relatively low nitrogen concentrations (1.5 mM) at varying pHs in the presence and absence of oxygen to compare the reaction rates and suggest the reaction mechanisms. NO3− was fully converted into NO2− at a pH > 3 in both oxic and anoxic conditions, and the reaction was faster when the pH was increased following a first-order kinetic at pH 11 (k = 0.12 min−1, R2 = 0.9995). NO2− was partially converted into NO3− only at pH 3 and in the presence of oxygen and was stable at an alkaline pH. This interconversion of NO3− and NO2− did not yield nitrogen loss in the solution. The addition of formic acid as an electron donor led to the reduction of NO3− to NH4+. Conversely, NH4+ was converted into NO2−, NO3− and to an unidentified subproduct in the presence of O2  at pH 10. Finally, it was demonstrated that NO2− and NH4+ react via UV irradiation with stoichiometry 1:1 at pH 10 with the total loss of nitrogen in the solution. With these results, a strategy to remove DIN compounds via UV irradiation was proposed with the eventual use of solar light.