TY - JOUR
T1 - Structure of Fe(III) precipitates generated by the electrolytic dissolution of Fe(0) in the presence of groundwater ions
AU - van Genuchten, Case M.
AU - Peña, Jasquelin
AU - Amrose, Susan E.
AU - Gadgil, Ashok J.
N1 - Funding Information:
We gratefully acknowledge the following researchers for their technical assistance and/or advice along the various stages of this work: Sharon Bone, Joe Rogers, Matthew Lattimer, Jeff Maske, Erik Nelson, Kevin Beyer, Karena Chapman, Peter Chupas, Garrison Sposito, Alejandro Fernandez-Martinez, F. Marc Michel, Siva Rama Satyam Bandaru, and Caroline Delaire. This work was supported by a National Science Foundation Graduate Research Fellowship to C.M. van Genuchten. We acknowledge The Richard C. Blum Center for Developing Economies and the Sandoz Family Foundation for support of this research. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. Use of the Advanced Photon Source, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the US DOE under Contract No. DE-AC02-06CH11357.
PY - 2014/2/15
Y1 - 2014/2/15
N2 - 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 1mM 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 (>6Å) 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.
AB - 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 1mM 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 (>6Å) 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.
UR - http://www.scopus.com/inward/record.url?scp=84891752688&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2013.11.044
DO - 10.1016/j.gca.2013.11.044
M3 - Article
AN - SCOPUS:84891752688
SN - 0016-7037
VL - 127
SP - 285
EP - 304
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -