LAUSR

Abstract The generation process for the highly reductive carbon dioxide radical ( CO2−) in the Fe(III)-oxalate system is enhanced under UV irradiation and used in nitrate (NO3−) reduction with improved product selectivity to nitrogen (N2). The combination of Fe(III) and residual oxalate forms Fe(III)-oxalate complexes again for an Fe(II)/Fe(III) species cycle and achieves generation of CO2− radical continually. The electron paramagnetic resonance (EPR) spectrum demonstrates the existence of CO2− radicals in this process. Some of the important control factors, such as initial concentrations of ferric ions (Fe(III)) (0.5, 1, 2, and 3 mM), NO3− (14, 21, and 42 mg N/L), oxalate (3, 7, and 10 mM), and pH values (2, 3, 4, 5, and 6), are systematically assessed for NO3− reduction efficiency and N-species selectivity. Among all the control factors mentioned, pH is a key parameter with an important role in the NO3− reduction process. Excellent NO3− reduction efficiency (∼90 %) was achieved at pH 3 in the presence of 1 mM Fe(III) and 10 mM oxalate under UV irradiation for 120 min. High Fe(III) and oxalate concentrations promote NO3− removal. In addition, there was an increase in the NO3− removal amount with a greater initial NO3− concentration. About 60 % of NO3− can be selectively reduced to N2 with low selectivity to NO2− (∼3.4 % of total N) and NH4+ (∼15.7 % of total N). In general, the UV/Fe(III)-oxalate system provided an alternative way to improve NO3− removal performance and selectivity to form N2 in water and wastewater remediation processes.