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

In order to expand the application of CO anion radical (CO), as a novel green reductant in the control of environmental pollution, CO radical was induced into the reduction of nitrate. The reduction efficiency, products and mechanism of nitrate or nitrite by CO 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 CO 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 CO 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 (N). On this basis, a novel strategy of rapid reduction of nitrate into N using CO radical was proposed. Specifically, nitrate was firstly reduced into nitrite with the assistance of Zn/Ag bimetal, and then nitrite was further reduced into N by CO radical. In this way, the removal efficiency of nitrate was all achieved nearly 100% in the initial nitrate concentration ranging from 25 to 100 mg (N)/L, while the highest N selectivity could reach 97.5%. This work provided a promising approach for the reduction of nitrate into nitrogen with high efficiency and high N selectivity by CO radical.