Reductive removal of nitrate by carbon dioxide radical with high product selectivity to form N2 in a UV/H2O2/HCOOH system

Abstract An advanced photoreduction (APR) system of UV/H2O2/HCOOH has been explored to remove nitrate (NO3−) in water by reduction of CO2− radical, which was detected by electron paramagnetic resonance (EPR) analysis. The removal of NO3− and product selectivity were systematically investigated under varied conditions including initial NO3− concentrations, molar ratios of H2O2 to HCOOH, and initial pH values. The removal rate of NO3− increased with higher initial concentrations of NO3−, while decreased with greater molar ratios of H2O2/HCOOH and higher initial pH. Almost 100 % of NO3− was reduced in the presence of 53.0 mM H2O2 and 106.1 mM HCOOH in 120 min. The main products of reduction were nitrogen (N2), ammonium (NH4+), nitrite (NO2−), and hydrazine (N2H4). Among various factors examined, the molar ratio was critical to enhance the selectivity to form N2 as the major final product. At the molar ratio of H2O2 to HCOOH as 1.1, 80 % of gaseous-N product selectivity was achieved with 50 % removal rate of NO3−. In addition, toxic byproduct of NO2− was low (less than 2.7 % of total N) during reactions. When the initial pH increased from 2 to 7, the removal rate of NO3− declined from 100 % to 70 %, while gaseous-N product selectivity increased to 40 % after 120 min. Overall, the APR process has provided an attractive approach to remove NO3− in natural waters.