Precipitation pathways for ferrihydrite formation in acidic solutions

Mengqiang Zhu, Cathrine Frandsen, Adam F. Wallace, Benjamin Legg, Syed Khalid, Hengzhong Zhang, Steen Mørup, Jillian F. Banfield, Glenn A. Waychunas

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

Iron oxides and oxyhydroxides form via Fe3+ hydrolysis and polymerization in many aqueous environments, but the pathway from Fe3+ monomers to oligomers and then to solid phase nuclei is unknown. In this work, using combined X-ray, UV-vis, and Mossbauer spectroscopic approaches, we were able to identify and quantify the long-time sought ferric speciation over time during ferric oxyhydroxide formation in partially-neutralized ferric nitrate solutions ([Fe3+] = 0.2 M, 1.8 <pH <3). Results demonstrate that Fe exists mainly as Fe(H2O)63+, mu-oxo aquo dimers and ferrihydrite, and that with time, the mu-oxo dimer decreases while the other two species increase in their concentrations. No larger Fe oligomers were detected. Given that the structure of the mu-oxo dimer is incompatible with those of all Fe oxides and oxyhydroxides, our results suggest that reconfiguration of the mu-oxo dimer structure occurs prior to further condensation leading up to the nucleation of ferrihydrite. The structural reconfiguration is likely the rate-limiting step involved in the nucleation process.
Original languageEnglish
JournalGeochimica Et Cosmochimica Acta
Volume172
Pages (from-to)247-264
Number of pages18
ISSN0016-7037
DOIs
Publication statusPublished - 2016

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