Nitrate Removal via a Formate Radical-Induced Photochemical Process

Removal of excess nitrate is critical to balance the nitrogen cycle in aquatic systems. This study investigated a novel denitrification process by tailoring photochemistry of nitrate with formate. Under UV light irradiation, short-lived radicals (i.e., HO^•, NO₂^•, and CO₃^•-) generated from nitrate photolysis partially oxidized formate to highly reductive formate radical (CO₂^•-). CO₂^•- further reduced nitrogen intermediates generated during photochemical denitrification (mainly NO^•, HNO, and N₂O) to gas-phase nitrogen (i.e., N₂O and N₂). The degradation kinetics of total dissolved nitrogen was mainly controlled by the photolysis rates of nitrate and nitrite. The distribution of final products was controlled by the reaction between CO₂^•- and N₂O. To achieve a simultaneous and complete removal of dissolved nitrogen (i.e., nitrate, nitrite, and ammonia) and organic carbon, the formate-to-nitrate stoichiometry was determined as 3.1 ± 0.2 at neutral pH in deionized water. Solution pH impacted the removal rates of nitrate and nitrite but not that of total dissolved nitrogen or formate. The presence of dissolved organic matter at levels similar to those in groundwater had a negligible impact on the photochemical denitrification process. A high denitrification efficiency was also achieved in a synthetic groundwater matrix. Outcome from this study provides a potential denitrification technology for decentralized water treatment and reuse facilities to abate nitrate in local water resources.