Structure of Fe(III) precipitates generated by the electrolytic dissolution of Fe(0) in the presence of groundwater ions

Structure of Fe(III) precipitates generated by the electrolytic dissolution of Fe(0) in the presence of groundwater ions

TitleStructure of Fe(III) precipitates generated by the electrolytic dissolution of Fe(0) in the presence of groundwater ions
Publication TypeJournal Article
Year of Publication2014
AuthorsCase M van Genuchten, Jasquelin Peña, Susan Amrose, Ashok J Gadgil
JournalGeochimica et Cosmochimica Acta
Volume127
Pagination285 - 304
Date Published02/2014
ISSN00167037
Abstract

We apply Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and pair distribution function (PDF) analysis of high-energy X-ray scattering to investigate the effects of bivalent cation–oxyanion pairs on the structure of Fe(III) precipitates formed from the oxidation of Fe(II) generated by the electrolytic dissolution of Fe(0) electrodes. We found that Fe(II) oxidation in the presence of weakly adsorbing electrolytes (NaCl, CaCl2, MgCl2) leads to pseudo-lepidocrocite (Lp; γ-FeOOH), a poorly crystalline version of Lp with low sheet-stacking coherence. In the absence of bivalent cations, P and As(V) have similar uptake behavior, but different effects on the average Fe(III) precipitate structure: pseudo-Lp dominates in the presence of P, whereas a disordered ferrihydrite-like precipitate akin to hydrous ferric oxide (HFO) is the dominant phase that forms in the presence of As(V). Despite its lower affinity for Fe(III) precipitates, Si leads to Si-HFO in all conditions tested. The presence of 1 mM Ca2+ or Mg2+ enhances oxyanion uptake, destabilizes the colloidally stable oxyanion-bearing particle suspensions and, in some P and As(V) electrolytes, results in more crystalline precipitates. The trends in oxyanion uptake and Fe(III) precipitate structure in the presence of Ca2+/Mg2+ suggest a systematic decrease in the strength of bivalent cation:oxyanion interaction in the order of Ca2+ > Mg2+ and P > As(V) ≫ Si. Using the PDF technique, we identify the polyhedral linkages that contribute to the intermediate structures (>6 Å) of disordered, nanoscale oxyanion-bearing Fe(III) precipitate samples. Our results suggest that oxyanions present during Fe(III) polymerization bind to corner-sharing Fe surface sites leading to a precipitate surface deficient in corner-sharing Fe, whereas the edge- and corner-sharing Fe sites in the precipitate core likely remain intact.

DOI10.1016/j.gca.2013.11.044
Short TitleGeochimica et Cosmochimica Acta
Refereed DesignationRefereed