The reactivity towards nitrate and aging characteristics of two types of Fe nanoparticles (Fe-BH and Fe-H2 nanoparticles) were investigated. During shell modification for producing air-stable Fe nanoparticles, the shell layers of Fe-BH nanoparticles were passivated more extensively than those of Fe-H2 nanoparticles were. Column experiments showed that the nitrate-reduction capacity of Fe-BH nanoparticles was 35 times higher than that of Fe-H2 nanoparticles. The Fe(0) core of Fe-BH nanoparticles was consumed completely before breakthrough during the nitrate-reduction column experiments, which indicates that the thickness of Fe-BH nanoparticles' shell layers remained constant during aging. The Fe-H2 nanoparticle surfaces were readily passivated, and subsequently depassivated and reactivated by acid flushing, which indicates that the thickness of Fe-H2 nanoparticles' shell layers increased during aging. Exhausted nanoparticles were examined to obtain information on equilibrium mineral phases. Magnetite was prevalent among the mineral phases in the exhausted Fe-BH nanoparticles, unlike in the case of the Fe-H2 nanoparticles. The formation of goethite as an aging product of Fe-BH nanoparticles is reported for the first time herein. Conceptual aging models for Fe-BH and Fe-H2 nanoparticles are proposed on the basis of current and previous experimental results. (C) 2012 Elsevier B.V. All rights reserved.