Reduction of nitrate by zero valent iron (ZVI)-based materials: A review

Zero valent iron (ZVI) and ZVI-based materials have been widely used for the reduction of nitrate, a major contaminant commonly detected in groundwater and surface water. The reduction of nitrate by ZVI is influenced by various factors, such as the physical and chemical characteristics of ZVI and the operational parameters. There are some problems for the nitrate reduction by ZVI alone, for example, the formation of iron oxides on the surface of ZVI at high pH condition, which will inhibit the further reduction of nitrate; in addition, the end reduction product is mainly ammonium, which itself needs to be concerned. Several strategies, such as the optimization of the structure of ZVI composites and the addition of reducing assistants, have been proposed to increase the reduction efficiency and the selectivity of end product of nitrate reduction in a wide range of pH, especially under neutral pH condition. This review will mainly focus on the high efficient reduction of nitrate by ZVI-based materials. Firstly, the reduction of nitrate by ZVI alone was briefly introduced and discussed, including the influence of physical and chemical characteristics of ZVI and some operational parameters on the reduction efficiency of nitrate. Then, the strategies for enhancing the reduction efficiency and the N₂ selectivity of the reductive products of nitrate were systematically analyzed and evaluated, especially the optimization of the structure of ZVI composites (e.g., doped ZVI composite, supported ZVI composite and premagnetized ZVI), and the addition of reducing assistants (e.g., metal cations, ligand, hydrogen gas and light) were highlighted. Thirdly, the mechanisms and pathways of nitrate reduction were discussed. Finally, concluding remarks and some suggestions for the future research were proposed.