Influence of time and ageing conditions on the properties of ferrihydrite

Natural conversion of ferrihydrite (Fh), a widespread Fe( iii )-oxyhydroxide mineral at the Earth's surface, to thermodynamically more stable iron oxides such as goethite (Gt) and hematite (Hm) is a slow process that spans months to years. Here we examined the effects of synthesis and storage c...

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Bibliographic Details
Published in:Environmental science. Nano 2024-04, Vol.11 (4), p.1682-1692
Main Authors: Sassi, Michel, Qafoku, Odeta, Bowden, Mark E, Pearce, Carolyn I, Latta, Drew, Miller, Quin R. S, Boamah, Mavis D, N'Diaye, Alpha T, Holliman Jr, Jade E, Arenholz, Elke, Rosso, Kevin M
Format: Article
Language:English
Subjects:
Aerobic conditions
Ageing
Aging
Aging (natural)
Anaerobic conditions
Anoxic conditions
Bound water
Dehydration
Earth surface
GEOSCIENCES
Goethite
Haematite
Hematite
Hydration
Inorganic carbon
Iron oxides
Magnetism
Mass spectroscopy
Moisture content
Oxic conditions
Room temperature
Spectra
Spectroscopy
Spectrum analysis
Storage conditions
Thermogravimetric analysis
Water content
X ray absorption
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Summary:Natural conversion of ferrihydrite (Fh), a widespread Fe( iii )-oxyhydroxide mineral at the Earth's surface, to thermodynamically more stable iron oxides such as goethite (Gt) and hematite (Hm) is a slow process that spans months to years. Here we examined the effects of synthesis and storage conditions on the hydration, the ratio of tetrahedral to octahedral iron sites, and the transformation of naturally aged 2-line Fh at room temperature and mildly acidic pH over an ageing period of 5 years. Fh samples synthesized and aged in either aerobic or anaerobic conditions were characterized over time by XRD, SEM, thermogravimetric analysis - mass spectroscopy (TGA-MS), and X-ray absorption spectroscopies (XANES and XMCD). The findings show that the ratio of tetrahedral to octahedral Fe( iii ) sites in Fh is correlated to its extent of hydration, with fresher Fh samples exhibiting a higher ratio and more bound water. Fresh Fh aged in aerobic conditions has similar bound inorganic carbon, is more hydrated, and has less tetrahedral Fe( iii ) than that aged in anaerobic conditions. Hence, for relatively fresh Fh there is a link between Fh properties and storage conditions. However, the long-term ageing characteristics, such as the transformation rate and relative phase fraction of Gt and Hm products, are not noticeably impacted by storage conditions. TGA-MS measurements coupled with O K-edge XANES spectra confirm that Fh tends to lose its hydration as it ages, as expected. Corresponding Fe L 2,3 -edge XMCD spectra reveal that this dehydration is coupled to a steady decrease in the ratio of tetrahedral to octahedral Fe( iii ) sites. In addition to the obvious constraints these findings place on making comparisons across Fh samples of different age and environmental settings, they also highlight that Fh structure, and consequently magnetism, are linked to its bound water content. Storage conditions affect the initial tetrahedral iron and hydroxyl populations of ferrihydrite, both are correlated and decrease over time as function of ageing.
ISSN:2051-8153
2051-8161
DOI:10.1039/d3en00828b