| Summary: | 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 (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>2</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NH</mi></mrow><mn>4</mn><mrow><mo>+</mo></mrow></msubsup></mrow></semantics></math></inline-formula>) 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. <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> was fully converted into <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>2</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> 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 (<i>k</i> = 0.12 min<sup>−1</sup>, R<sup>2</sup> = 0.9995). <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>2</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> was partially converted into <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> only at pH 3 and in the presence of oxygen and was stable at an alkaline pH. This interconversion of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>2</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> did not yield nitrogen loss in the solution. The addition of formic acid as an electron donor led to the reduction of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NH</mi></mrow><mn>4</mn><mrow><mo>+</mo></mrow></msubsup></mrow></semantics></math></inline-formula>. Conversely, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NH</mi></mrow><mn>4</mn><mrow><mo>+</mo></mrow></msubsup></mrow></semantics></math></inline-formula> was converted into <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>2</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>3</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> and to an unidentified subproduct in the presence of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi mathvariant="normal">O</mi><mn>2</mn><mo> </mo></msubsup></mrow></semantics></math></inline-formula> at pH 10. Finally, it was demonstrated that <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NO</mi></mrow><mn>2</mn><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>NH</mi></mrow><mn>4</mn><mrow><mo>+</mo></mrow></msubsup></mrow></semantics></math></inline-formula> 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.
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