This study investigated the long-term compositional and structural evolution of bare nZVI (nanoscale zero-valent iron) and carboxymethyl cellulose (CMC) coated nZVI (CMC-nZVI) in simulated groundwater under anaerobic conditions. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman and Fourier transform infrared spectroscopy (FTIR) were used to characterize the corrosion products of nZVI and CMC-nZVI. Results show that both structure and chemical composition of the aging products of nZVI/CMC-nZVI transformed over time. The SEM images show that the original spherical particles transformed into the sheet-like and needle-like materials for the aged nZVI, but formed bulk aggregates with some flaky-shaped structures for the aged CMC-nZVI. Compare with the aging products of nZVI and CMC-nZVI (i.e., magnetiteflepidocrocite) in the deionized water under anaerobic conditions, the formed secondary products in the simulated groundwater were complicated. The composition analysis confirm that magnetite, lepidocrocite, feroxyhyte and iron carbonate hydroxide hydrate/iron oxide hydroxide chloride were the main corrosion products for both bare nZVI and CMC-nZVI, except that the amount of each secondary mineral varied for different particles. Besides, small amount of calcium carbonate was also identified in the aging products of nZVI. This indicates that the groundwater ions could be involved in the formation of secondary minerals during nZVI corrosion. (C) 2016 Elsevier B.V. All rights reserved.