Carbon dioxide radical reducing nitrate to nitrogen gas in a UV/Fe(III)-oxalate system

The generation process for the highly reductive carbon dioxide radical ( CO 2 − ) in the Fe(III)-oxalate system is enhanced under UV irradiation and used in nitrate (NO 3 − ) reduction with improved product selectivity to nitrogen (N 2 ). 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 CO 2 − radical continually. The electron paramagnetic resonance (EPR) spectrum demonstrates the existence of CO 2 − 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), NO 3 − (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 NO 3 − reduction efficiency and N-species selectivity. Among all the control factors mentioned, pH is a key parameter with an important role in the NO 3 − reduction process. Excellent NO 3 − 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 NO 3 − removal. In addition, there was an increase in the NO 3 − removal amount with a greater initial NO 3 − concentration. About 60 % of NO 3 − can be selectively reduced to N 2 with low selectivity to NO 2 − (∼3.4 % of total N) and NH 4 + (∼15.7 % of total N). In general, the UV/Fe(III)-oxalate system provided an alternative way to improve NO 3 − removal performance and selectivity to form N 2 in water and wastewater remediation processes.