Precipitation pathways for ferrihydrite formation in acidic solutions. (1st January 2016)

• Record Type: Journal Article
• Authors: Zhu, Mengqiang; Frandsen, Cathrine; Wallace, Adam F.; Legg, Benjamin; Khalid, Syed; Zhang, Hengzhong; Mørup, Steen; Banfield, Jillian F.; Waychunas, Glenn A.
• Is Part Of: Geochimica et cosmochimica acta. Volume 172(2016:Jan. 01)
• Page Start: 247
• Page End: 264
• Publication Date: 2016-01-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.09.015
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Iron oxides and oxyhydroxides form via Fe 3+ hydrolysis and polymerization in many aqueous environments, but the pathway from Fe 3+ monomers to oligomers and then to solid phase nuclei is unknown. In this work, using combined X-ray, UV–vis, and Mössbauer 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 ([Fe 3+ ] = 0.2 M, 1.8 < pH < 3). Results demonstrate that Fe exists mainly as Fe(H2 O)6 3+, μ -oxo aquo dimers and ferrihydrite, and that with time, the μ -oxo dimer decreases while the other two species increase in their concentrations. No larger Fe oligomers were detected. Given that the structure of the μ -oxo dimer is incompatible with those of all Fe oxides and oxyhydroxides, our results suggest that reconfiguration of the μ -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.