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

Abstract

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 pHini. 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 pHini. 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.