Naturally occurring iron oxide nanoparticles: morphology, surface chemistry and environmental stability

Abstract

The widespread nanostructures of iron oxides and oxyhydroxides are important reagents in many biogeochemical processes in many parts of our planet and ecosystem. Their functions in various aspects are closely related to their shapes, sizes, and thermodynamic surroundings, and there is much that we can learn from these natural relationships. This review covers these subjects of several phases (ferrihydrite, goethite, hematite, magnetite, maghemite, lepidocrocite, akaganéite and schwertmannite) commonly found in water, soils and sediments. Due to surface passivation by ubiquitous water in aquatic and most terrestrial environments, the difference in formation energies of bulk phases can decrease substantially or change signs at the nanoscale because of the disproportionate surface effects.Phase transformations and the relative abundance are sensitive to changes in environmental conditions.Each of these phases (except maghemite) displays characteristic morphologies, while maghemite appears frequently to inherit the precursor's morphology. We will see how an understanding of naturally occurring iron oxide nanostructures can provide useful insight for the production of synthetic iron oxide nanoparticles in technological settings.

The influence of silicate on transformation pathways of synthetic 2-line ferrihydrite

“…Usually, it transforms with time into stable minerals: goethite (a-FeOOH) and/or hematite (a-Fe 2 O 3 ), through dissolution-reprecipitation and dehydration-rearrangement mechanisms, respectively [1][2][3]. At elevated temperatures, ferrihydrite converts into hematite but, despite many studies, the details of the process are still controversial and seem to depend on many factors [5,33].…”

“…Irrespective of the model, the number of broad peaks in the XRD pattern gives the descriptive names for the ferrihydrite varieties, with 2-line and 6-line ferrihydrite being the most common. Because of its low crystallinity, high surface area and surface reactivity, ferrihydrite plays significant role in, for example, inorganic weathering processes, biogeochemical cycling of iron and as a scavenger of trace metals and metalloids in various near-surface environments [1][2][3]. For this reason, substantial amounts of admixtures are present in ferrihydrite chemical composition.…”

“…This particularly applies to e-Fe 2 O 3 , which exhibits a giant room-temperature coercive field and the ability to absorb electromagnetic waves in gigahertz region. Due to a number of intermediate features, 3 with NaOH in the presence of Na 2 SiO 3 at pH 8.2 [18,23]. After 12 h of stirring, the samples were incubated for 96 h in room temperature and then dialyzed to remove excess of salts (SO 4 2-and Na ?…”

“…For this reason, substantial amounts of admixtures are present in ferrihydrite chemical composition. In nature, the most common and well-documented impurities include silicate, phosphate, arsenate, sulfate, calcium, aluminum and organic compounds [e.g., [1][2][3][14][15][16]. These ions affect ferrihydrite composition, surface chemistry, reactivity and sorption properties [14,[17][18][19][20][21][22].…”

Laboratory synthesis of goethite and ferrihydrite of controlled particle sizes

“…Because of their abundance and reactivity, these minerals play an important role and are extensively studied in numerous disciplines, including environmental science, geochemistry, geology, engineering and health sciences (Schwertmann and Cornell, 2007), in an attempt to understand their different physical, chemical and mineralogical properties. According to Guo and Barnard (2013) there are 14 species of iron oxides, ten of which occur in nature, the most abundant being goethite (α-FeOOH), hematite (α-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ), followed by ferrihydrite [Fe 10 O 14 (OH) 2 ] (Michel et al, 2007), maghemite (γ-Fe 2 O 3 ) and lepidocrocite (γ-FeOOH). These iron oxides are responsible for the mobility and fate of numerous chemical species in soils and aquatic environments through adsorption processes, particularly onto goethite and ferrihydrite (Maji et al, 2008;Swedlund et al, 2009;Villalobos and Antelo, 2011) or through adsorption followed by reduction mechanisms as is the case of susceptible species on magnetite (Villacís-García et al, 2015).…”