Selective photo-reduction of NO2- to N2 in the presence of Fe2+ and citric acid.

The photo-reduction of NO2- has received increasing attention due to its high photo-activity. However, the intermediate products of NO2- photo-reduction might contain NOx, which are also toxic pollutants. Herein, a novel strategy to selectively photo-reduce NO2- to N2 was proposed using Fe2+ and citric acid (H3Cit) as assistant to eliminate the formation of NOx. In this strategy, NO2- was firstly reduced to NO by the combination of photon, Fe2+ and H3Cit; the generated NO was then immediately captured by Fe2+-H3Cit to form Fe2+-H3Cit-NO complex; finally, H3Cit was activated by Fe3+ and •OH in Fe2+/H3Cit/UV/NO2- system to produce carbon dioxide anion radical (CO2•-), which could reduce the NO in Fe2+-H3Cit-NO complex to N2 with high efficiency and selectivity. The removal efficiencies of NO2- and TN were 98.6% and 87.5%, respectively, and the selectivity of N2 was 81.6% in Fe2+/H3Cit/UV/NO2- system after 60-min reaction at initial pH of 2.2, Fe2+ dosage of 3.0 mmol·L-1 and H3Cit dosage of 3.0 mmol·L-1. Based on the experimental results and spectral analysis, the mechanism of NO2- selective reduction in Fe2+/H3Cit/UV/NO2- system was proposed. Our finding provides a new way for wastewater denitrification and water purification.