Chemical transformations during aging of zerovalent iron nanoparticles in the presence of common groundwater dissolved constituents

Nanoscale zerovalent iron (NZVI) that was aged in simulated groundwater was evaluated for alterations in composition and speciation over 6 months to understand the possible transformations NZVI could undergo in natural waters. NZVI was exposed to 10 mN of various common groundwater anions (Cl^-, NO₃^-, SO₄^2-, HPO₄ ^2-, and HCO₃^-) or to dissolved oxygen (saturated, ∼9 mg/L). Fresh and exposed NZVI samples, along with Fe-oxide model compounds, were then analyzed using synchrotron radiation X-ray absorption spectroscopy (XAS) to yield both relative oxidation state, using the X-ray absorption near edge structure (XANES), and quantitative speciation information regarding the types and proportions of mineral species present, from analysis of the extended X-ray absorption fine structure (EXAFS). Over 1 month of aging the dissolved anions inhibited the oxidation of the NZVI to varying degrees. Aging for 6 months, however, resulted in average oxidation states that were similar to each other regardless of the anion used, except for nitrate. Nitrate passivated the NZVI surface such that even after 6 months of aging the particles retained nearly the same mineral and Fe⁰ content as fresh NZVI. Linear least-squares combination fitting (LCF) of the EXAFS spectra for 1 month-aged samples indicated that the oxidized particles remain predominantly a binary phase system containing Fe⁰ and Fe₃O₄, while the 6 month aged samples contained additional mineral phases such as vivianite (Fe₃(PO₄)₂8H₂O) and iron sulfate species, possibly schwertmannite (Fe³⁺₁₆O ₁₆(OH,SO₄)_12-1310-12H₂O). The presence of these additional mineral species was confirmed using synchrotron-based X-ray diffraction (XRD). NZVI exposed to water saturated with dissolved oxygen showed a rapid (<24 h) loss of Fe⁰ and evolved both magnetite and maghemite (γ-Fe₂O₃) within the oxide layer. These findings have implications toward the eventual fate, transport, and toxicity of NZVI used for groundwater remediation.