New consideration on the application of nano-zero-valent iron (nZVI) in groundwater remediation: refractions to existing technologies

Nano-zero-valent iron (nZVI, < 100 nm) has been used extensively in groundwater remediation over the past two decades. In this review, recent research (past 5–6 years) were discussed and evaluated. (1) nZVI could already be prepared by top-down and bottom-up methods. Liquid-phase reduction and carbothermal reduction still possessed a high proportion. Ball milling and green synthesis were more promising pathways. Carbothermal reduction was cost-effective compared to other paths. In-situ green synthesis was expected to in-situ synthesis nZVI, providing new ideas for groundwater remediation. (2) Composites and modification could improve the applicability and lower storage requirements of nZVI to a certain extent. Recyclability was effectively enhanced and costs indirectly reduced (5 cycles test). The suitability of habitats should be considered. Natural minerals and/or its component carriers finally acted as mineral cover layer. Carbon materials and biopolymer coatings eventually served as microbial nutrients. They were all potential directions. (3) In-situ groundwater remediation could maximize the value of nZVI. Mobility and stability of nZVI should be further optimized. So as to completely purified argillaceous soil and aquifer matrix. Geological properties, biological population, and hydrochemical aging all influenced groundwater remediation. Batch injection could avert rebound issue (usually two injections, spaced several months apart). DC electric field–assisted obviously reduced the cost (5 times). (4) nZVI removed pollutants in aqueous matrix (purified water, surface water, groundwater) through various ways and mechanisms. Hydrodynamic and hydrochemical conditions could not change the mainstream process. Long-term fixation of heavy metals, reasonable removal of inorganic salt ions, and effective mineralization of organic substances were the most expected results. The macro-to-micro study of mechanisms was more in-need. This review will offer fresh perspectives on the use of nZVI in groundwater remediation.