Selective and Efficient Reduction of Nitrate to Gaseous Nitrogen from Drinking Water Source by UV/Oxalic Acid/Ferric Iron Systems: Effectiveness and Mechanisms

Nitrate (NO₃⁻) reduction in water has been receiving increasing attention in water treatment due to its carcinogenic and endocrine-disrupting properties. This study employs a novel advanced reduction process, the UV/oxalic acid/ferric iron systems (UV/C₂O₄²⁻/Fe³⁺ systems), in reducing NO₃⁻ due to its high reduction efficiency, excellent selectivity, and low treatment cost. The UV/C₂O₄²⁻/Fe³⁺ process reduced NO₃⁻ with pseudo-first-order reaction rate constants of 0.0150 ± 0.0013 min⁻¹, minimizing 91.4% of 60 mg/L NO₃⁻ and reaching 84.2% of selectivity for gaseous nitrogen after 180 min at pH_ini. 7.0 and 0.5 mg/L dissolved oxygen (DO). Carbon dioxide radical anion (CO₂^•−) played a predominant role in reducing NO₃⁻. Gaseous nitrogen and NH₄⁺, as well as CO₂, were the main nitrogen- and carbon-containing products, respectively, and reduction pathways were proposed accordingly. A suitable level of oxalic acids (3 mM) and NO₃⁻ (60 mg/L) was recommended; increasing initial iron concentrations and UV intensity increased NO₃⁻ reduction. Instead, increasing the solution pH decreased the reduction, and 0.5–8.0 mg/L DO negligibly affected the process. Moreover, UV/C₂O₄²⁻/Fe³⁺ systems were not retarded by 0.1–10 mM SO₄²⁻ or Cl⁻ or 0.1–1.0 mM HCO₃⁻ but were prohibited by 10 mM HCO₃⁻ and 30 mg-C/L humic acids. There was a lower reduction of NO₃⁻ in simulated groundwater (72.8%) than deionized water after 180 min at pH_ini. 7.0 and 0.5 mg/L DO, which meets the drinking water standard (<10 mg/L N-NO₃⁻). Therefore, UV/C₂O₄²⁻/Fe³⁺ systems are promising approaches to selectively and efficiently reduce NO₃⁻ in drinking water.