A novel strategy for sequential reduction of nitrate into nitrogen by CO2 anion radical: Experimental study and DFT calculation.

Abstract In order to expand the application of CO2 anion radical (CO2 −), as a novel green reductant in the control of environmental pollution, CO2 − radical was induced into the reduction of nitrate. The reduction efficiency, products and mechanism of nitrate or nitrite by CO2 − radical were investigated based on the results of batch experiments and theoretical calculation using density functional theory (DFT) methods, respectively. It was found that: (1) the efficiency of nitrate reduction by CO2 − radical from the HCOOH/UV system was far lower than that of nitrite under the same reaction conditions, (2) the rate-control step of nitrate reduction by CO2 − radical was the transformation process of nitrate into nitrite with an activation energy of 23.9 kcal/mol, (3) the final products of nitrate reduction were mainly composed of nitrogen (N2). On this basis, a novel strategy of rapid reduction of nitrate into N2 using CO2 − radical was proposed. Specifically, nitrate was firstly reduced into nitrite with the assistance of Zn/Ag bimetal, and then nitrite was further reduced into N2 by CO2 − radical. In this way, the removal efficiency of nitrate was all achieved nearly 100% in the initial nitrate concentration ranging from 25 - 100 mg (N)/L, while the highest N2 selectivity could reach 97.5%. This work provided a promising approach for the reduction of nitrate into nitrogen with high efficiency and high N2 selectivity by CO2 − radical.