Enhanced Reactivity and Electron Efficiency of Zerovalent Iron with Various Methods

Over the past 30 years, various methods have been developed for enhancing contaminant removal by zerovalent iron (ZVI), thus accumulating a large amount of quantitative data including the reactivity (k_obs) and electron efficiency (EE). However, comparisons and relationships of the data are still lacking, which hinders the selection and development of ZVI enhancement methods for practical applications. In this review, a large number of k_obs and EE results are systematically summarized and classified into three types based on enhancement mechanisms: regulating iron (hydr)oxide films of ZVI (RIF), accelerating ZVI corrosion (AZC), and coupling of iron reactive species with ZVI (CIRs). Then, the comparisons of k_obs and EE by ZVI along with their enhancement multiples (referred to as R_k and R_EE) were conducted within the context of RIF, AZC, and CIRs. This review identified that in cases where ZVI exhibited low reactivity toward pollutants, it often possessed a high electron efficiency for pollutant reduction and vice versa. Moreover, there existed correlations between lgk_obs (lgR_k) and lgEE (lgR_EE) by ZVI with enhancement methods. These relationships suggest that when both the k_obs (R_k) and EE (R_EE) parameters are known, the other parameter can be predicted to some extent. Finally, this review discussed the effects of the solution chemistry and iron-related compounds on the k_obs (R_k) and EE (R_EE) by ZVI with enhancement methods in detail and outlined their potential research needs in future studies.