Zero-valent iron nanoparticles (nano-Fe⁰) in aqueous solution rapidly inactivated Escherichia coli. A strong bactericidal effect of nano-Fe⁰ was found under deaerated conditions, with a linear correlation between log inactivation and nano-Fe⁰ dose (0.82 log inactivation/mg/L nano-Fe⁰·h). The inactivation of E. coli under air saturation required much higher nano-Fe⁰ doses due to the corrosion and surface oxidation of nano-Fe⁰ by dissolved oxygen. Significant physical disruption of the cell membranes was observed in E. coli exposed to nano-Fe⁰, which may have caused the inactivation or enhanced the biocidal effects of dissolved iron. The reaction of Fe(II) with intracellular oxygen or hydrogen peroxide also may have induced oxidative stress by producing reactive oxygen species. The bactericidal effect of nano-Fe⁰ was a unique property of nano-Fe⁰, which was not observed in other types of iron-based compounds.