title: Chemical transformations during aging of zerovalent iron nanoparticles in the presence of common groundwater dissolved constituents.
abstract: 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(-), NO(3)(-), SO(4)(2-), HPO(4)(2-), and HCO(3)(-)) or to dissolved oxygen (saturated, approximately 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 Fe(0) 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 Fe(0) and Fe(3)O(4), while the 6 month aged samples contained additional mineral phases such as vivianite (Fe(3)(PO(4))(2).8H(2)O) and iron sulfate species, possibly schwertmannite (Fe(3+)(16)O(16)(OH,SO(4))(12-13).10-12H(2)O). 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 Fe(0) and evolved both magnetite and maghemite (gamma-Fe(2)O(3)) within the oxide layer. These findings have implications toward the eventual fate, transport, and toxicity of NZVI used for groundwater remediation.
authors.affiliation: Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 152313, USA.
authors.forename: Brian C
authors.initials: BC
authors.lastname: Reinsch
authors.forename: Brady
authors.initials: B
authors.lastname: Forsberg
authors.forename: R Lee
authors.initials: RL
authors.lastname: Penn
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Kim
authors.forename: Gregory V
authors.initials: GV
authors.lastname: Lowry
mesh_terms: D000838:Anions; D007501:Iron / Q000737:chemistry*; D053758:Nanoparticles / Q000737:chemistry; D036103:Nanotechnology / Q000379:methods; D010100:Oxygen / Q000737:chemistry; D011208:Powders; D013997:Time Factors; D014241:Trichloroethylene / Q000737:chemistry; D014867:Water / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000032:analysis*; D018508:Water Purification / Q000379:methods; D056928:X-Ray Absorption Spectroscopy / Q000379:methods; D014961:X-Ray Diffraction
chemical_list: D000838:Anions; D011208:Powders; D014874:Water Pollutants, Chemical; D014867:Water; D014241:Trichloroethylene; D007501:Iron; D010100:Oxygen