In recent years, the zero-valent iron(ZVI)-based chemical reduction technology has been applied for the in-situ groundwater remediation because of its high efficiency and low cost. However, the technology still faces up some bottleneck restrictions in engineering applications considering the complex groundwater constituents. ZVI, as a highly reactive electron donor, not only reacts with the target contaminants, but also reacts with other co-existing oxidants(O₂, H⁺, NO₃^-, etc.) in the groundwater, which would negatively impact the remediation efficiency and increase the cost of remediation. In addition, electron competition usually occurs among the contaminants of the same or different categories, consequently resulting in a lower removal efficiency in a multi-solute system than in a single-solute system. Therefore, in this paper, the electron transfer process and the electron competition mechanism among different oxides in ZVI-based groundwater remediation systems are reviewed, including ZVI corrosion and electron transfer, the concept of ZVI reduction selectivity and its quantification methods, the electron competition among various coexisting oxides in groundwater, and the influence factors and enhancement methods of electron efficiency. Finally, the future development direction of the technology is forecast, so as to provide reference for future engineering application of in situ chemical remediation of groundwater.
Contents
1 Introduction
2 Zero-valent iron corrosion and electron-transfer process
2.1 Zero-valent iron corrosion
2.2 Effects of corrosion on the electron-transfer process
3 “Electron selectivity” and “electron efficiency” of ZVI
3.1 Conceptulization and quantification of electron selectivity
3.2 Calculation of “electron efficiency”
4 Electron competition of coexisting oxides
4.1 DO
4.2 H₂O or H⁺
4.3 NO₃^-
4.4 Heavy metals
4.5 Organic pollutants
5 Influence factors for “electron efficiency”
5.1 The relative proportions of the reducing agent and the oxidant
5.2 pH
5.3 Other factors
6 Technical methods to improve the “electron efficiency”
6.1 “Sulfidation”
6.2 “Magnetization” or “pre-magnetization”
6.3 Zero-valent iron/Fe(Ⅱ)
7 Conclusion and outlook