Advances in the catalytic reduction of nitrate by metallic catalysts for high efficiency and N2 selectivity: A review

Contamination of surface and groundwater by nitrate ( NO3-) is one of the most serious environmental issues worldwide because of the increase in discharge of N-containing wastewater due to agricultural and industrial activities. Among various NO3- removal techniques, catalytic denitrification by metallic catalyst has been considered as one of innovative and promising techniques to convert NO3- to an ideal gas (N-2) in natural and engineered environments. Extensive studies have developed diverse metallic catalysts for the enhanced catalytic removal of aqueous NO3- Generally, the removal kinetics of NO3- and selectivity towards N-2 are varied with experimental conditions such as type of the catalyst (mono- or bimetallic), support material, promoter, and noble metals and their contents, concentrations of catalyst and NO3-, suspension pH, and additional reductant (H-2) flow. Therefore, selection of a proper combination of metals and support material and optimization of experimental factors are main challenges to achieve an efficient catalytic NO3- reduction, leading to the successful application of the technology to real water and wastewater treatment problems. This review presents an analysis of recent research works on NO3- reduction by the metallic catalysts, and discusses: (i) an assessment of various combinations of promoter and noble metals for mono- and bimetallic catalysts, based on their reactivity and selectivity to N-2; (ii) investigation of the reaction mechanism; (iii) evaluation of effect of diverse environmental factors on the performance of the catalyst; and (iv) stability and durability of the catalyst. In addition, environmental implication and future prospect of the denitrification by metallic catalyst are provided.