Enhanced Reactivity\nand Electron Efficiency of Zerovalent\nIron with Various Methods

Over the past 30 years, various methods have been developed\nfor\nenhancing contaminant removal by zerovalent iron (ZVI), thus accumulating\na large amount of quantitative data including the reactivity (<i>k</i><sub>obs</sub>) and electron efficiency (EE). However,\n comparisons and relationships of the data are still lacking, which\nhinders the selection and development of ZVI enhancement methods for\npractical applications. In this review, a large number of <i>k</i><sub>obs</sub> and EE results are systematically summarized\nand classified into three types based on enhancement mechanisms: regulating\niron (hydr)oxide films of ZVI (RIF), accelerating ZVI corrosion (AZC),\nand coupling of iron reactive species with ZVI (CIRs). Then, the comparisons\nof <i>k</i><sub>obs</sub> and EE by ZVI along with their\nenhancement multiples (referred to as <i>R</i><sub><i>k</i></sub> and <i>R</i><sub>EE</sub>) were conducted\nwithin the context of RIF, AZC, and CIRs. This review identified that\nin cases where ZVI exhibited low reactivity toward pollutants, it\noften possessed a high electron efficiency for pollutant reduction\nand vice versa. Moreover, there existed correlations between lg<i>k</i><sub>obs</sub> (lg<i>R</i><sub><i>k</i></sub>) and lgEE (lg<i>R</i><sub>EE</sub>) by ZVI with\nenhancement methods. These relationships suggest that when both the <i>k</i><sub>obs</sub> (<i>R</i><sub><i>k</i></sub>) and EE (<i>R</i><sub>EE</sub>) parameters are known,\nthe other parameter can be predicted to some extent. Finally, this\nreview discussed the effects of the solution chemistry and iron-related\ncompounds on the <i>k</i><sub>obs</sub> (<i>R</i><sub><i>k</i></sub>) and EE (<i>R</i><sub>EE</sub>) by ZVI with enhancement methods in detail and outlined their potential\nresearch needs in future studies.