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Solid phase reactions of Cr(Ⅵ) with Fe(0) were investigated with spherical-aberration-corrected scanning transmission electron microscopy (Cs-STEM) integrated with X-ray energy-dispersive spectroscopy (XEDS).Near-atomic resolution elemental mappings of Cr(Ⅵ)-Fe(0) reactions were acquired.Experimental results show that rate and extent of Cr(Ⅵ) encapsulation are strongly dependent on the initial concentration of Cr(Ⅵ) in solution.Low Cr loading in nZVI (＜1.0 wt％) promotes the electrochemical oxidation and continuous corrosion of nZVI while high Cr loading (＞1.0 wt％) can quickly shut down the Cr uptake.With the progress of iron oxidation and dissolution,elements of Cr and 0 counter-diffuse into the nanoparticles and accumulate in the core region at low levels of Cr(Ⅵ) (e.g.,＜ 10 mg/L).Whereas the reacted nZVI is quickly coated with a newly-formed layer of 2-4 nm in the presence of concentrated Cr(Ⅵ) (e.g.,＞ 100 mg/L).The passivation structure is stable over a wide range of pH unless pH is low enough to dissolve the passivation layer.X-ray photoelectron spectroscopy (XPS) depth profiling reconfirms that the composition of the newly-formed surface layer consists of Fe (Ⅲ)-Cr(Ⅲ) (oxy)hydroxides with Cr(Ⅵ) adsorbed on the outside surface.The insoluble and insulating Fe(Ⅲ)-Cr(Ⅲ) (oxy)hydroxide layer can completely cover the nZVI surface above the critical Cr loading and shield the electron transfer.Thus,the fast passivation of nZVI in high Cr(Ⅵ) solution is detrimental to the performance of nZVI for Cr(Ⅵ) treatment and remediation.