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

An advanced photoreduction (APR) system of UV/H2O2/HCOOH has been explored to remove nitrate (NO3-) in water by reduction of (CO2-)-C-center dot 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 (N-2), ammonium (NH4+), nitrite (NO2-), and hydrazine (N2H4). Among various factors examined, the molar ratio was critical to enhance the selectivity to form N-2 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.