Photochemistry of Nitrate Ion: Reduction by Formic Acid under UV Irradiation

Abstract

Mutual transformations of various nitrogen compounds and their reaction mechanisms have been a subject of great concern to the chemical, ecological and environmental communities. In the paper, the reactions of ${\text{NO}}_3^{-}$ ion with small organic acids such as formic acid (HCOOH), acetic acid (CH₃ COOH) and lactic acid (C₃ H₆ O₃ ) under ultraviolet illumination were investigated systematically. It was found that ${\text{NO}}_3^{-}$ ion is easily reduced into ${\text{NO}}_2^{-}$ and NO_x and then further into N₂ and NH₃ (in the form of ${\text{NH}}_4^{+}$ ) in the process. The carboxyl anion radicals and hydrogen formed by photodecomposition of formic acid are responsible for the rapid photoreduction reaction of nitrate. The initial pH and the nitrate concentration considerably affect the product distribution and nitrate conversion. Based on a preliminary simulation study, we speculated that the photoinduced reaction may effectively proceed in oceans, lakes and rivers because of ever-increasing nitrate and organic emissions. This research is helpful to understand nitrogen cycle mechanism and develop water environmental control technologies.