The interactions between nanoscale zero-valent iron and microbes in the subsurface environment: A review

Nanoscale zero-valent iron (NZVI) particles, applied for in-situ subsurface remediation, are inevitable to interact with various microbes in the remediation sites directly or indirectly. This review summarizes their interactions, including the effects of NZVI on microbial activity and growth, the synergistic effect of NZVI and microbes on the contaminant removal, and the effects of microbes on the aging of NZVI. NZVI could exert either inhibitive or stimulative effects on the growth of microbes. The mechanisms of NZVI cytotoxicity (i.e., the inhibitive effect) include physical damage and biochemical destruction. The stimulative effects of NZVI on certain bacteria are associated with the creation of appropriate living environment, either through providing electron donor (e.g., H₂) or carbon sources (e.g., the engineered organic surface modifiers), or through eliminating the noxious substances that can cause bactericidal consequence. As a result of the positive interaction, the combination of NZVI and some microbes shows synergistic effect on contaminant removal. Additionally, the aged NZVI can be utilized by some iron-reducing bacteria, resulting in the transformation of Fe(III) to Fe(II), which can further contribute to the contaminant reduction. However, the Fe(III)-reduction process can probably induce environmental risks, such as environmental methylation and remobilization of the previously entrapped heavy metals.