BNL-108457-2015-JA 1 Precipitation Pathways for Ferrihydrite Formation in Acidic Solutions 2 †, ‡, ∗ ⊥ § ∆ 3 Mengqiang Zhu, Cathrine Frandsen, Adam F. Wallace, Benjamin Legg,∥ Syed Khalid, ⊥ 4 Hengzhong Zhang,∥ Steen Mørup, Jillian F. Banfield ∥,‡ and Glenn A. Waychunas ‡ 5 6 † Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY, 7 82071 8 9 ‡ Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 10 11 ⊥ Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark 12 13 § Department of Geological Sciences, University of Delaware, Newark, DE 19713 14 15 ∥ Department of Earth and Planetary Science, University of California, Berkeley, CA, 94720 16 17 ∆ National Synchrotron Light Sources, Brookhaven National Laboratory, Upton, NY, 11973 18 19 20 Submitted to Geochimica et Cosmochimica Acta 21 * Corresponding author:
[email protected]. Phone: + 1 307 766-5523. Fax: +1 307 766-6403 1 22 Abstract 23 Iron oxides and oxyhydroxides form via Fe3+ hydrolysis and polymerization in many 24 aqueous environments, but the pathway from Fe3+ monomers to oligomers and then to solid phase 25 nuclei is unknown. In this work, using combined X-ray, UV-vis, and Mössbauer spectroscopic 26 approaches, we were able to identify and quantify the long-time sought ferric speciation over time 27 during ferric oxyhydroxide formation in partially-neutralized ferric nitrate solutions ([Fe3+] = 0.2 3+ 28 M, 1.8 < pH < 3). Results demonstrate that Fe exists mainly as Fe(H2O)6 , µ-oxo aquo dimers 29 and ferrihydrite, and that with time, the µ-oxo dimer decreases while the other two species 30 increase in their concentrations.