Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

at the Earth's surface, and a likely constituent in

extraterrestrial materials Extraterrestrial material refers to natural objects now on Earth that originated in outer space. Such materials include cosmic dust and meteorites, as well as samples brought to Earth by sample return missions from the Moon, asteroids and comet ...

. It forms in several types of environments, from freshwater to marine systems,

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...

s to hydrothermal

hot spring A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by c ...

s and scales,

soil Soil, also commonly referred to as earth or dirt Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty. Common types of dirt include: * Debri ...

s, and areas affected by mining. It can be precipitated directly from oxygenated

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

-rich aqueous solutions, or by

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

either as a result of a metabolic activity or passive

sorption Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are treated in the following articles: ; Absorption: "the incorporation of a substance in one state into another of a d ...

of dissolved iron followed by nucleation reactions. Ferrihydrite also occurs in the core of the

ferritin Ferritin is a universal intracellular protein that stores iron and releases it in a controlled fashion. The protein is produced by almost all living organisms, including archaea, bacteria, algae, higher plants, and animals. It is the primary ' ...

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

from many

living organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi ...

s, for the purpose of intra-cellular iron storage.

Structure

Ferrihydrite only exists as a fine grained and highly defective

nanomaterial * Nanomaterials describe, in principle, materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale). Nanomaterials research takes a materials science-based approach to nan ...

. The powder X-ray diffraction pattern of Fh contains two scattering bands in its most disordered state, and a maximum of six strong lines in its most crystalline state. The principal difference between these two diffraction end-members, commonly named two-line and six-line ferrihydrites, is the size of the constitutive crystallites.V. A. Drits, B. A. Sakharov, A. L. Salyn, et al. ''Clay Minerals'', 28, 185-208 (1993) The six-line form has been classified as a mineral by the IMA in 1973 with the nominal chemical formula 5•9. Other proposed formulas are •4Kenneth M Towe and William F Bradley (1967): "Mineralogical constitution of colloidal 'hydrous ferric oxides'". ''Journal of Colloid and Interface Science'', volume 24, issue 3, pages 384-392. and •2•2.6.J. D. Russell (1979): "Infrared spectroscopy of ferrihydrite: evidence for the presence of structural hydroxyl groups". ''Clay Minerals'', volume 14, issue 2, pages 109-114. However, its formula is fundamentally indeterminate as its water content is variable. The two-line form is also called

hydrous ferric oxides {{Short description, Hydrous oxide inorganic compounds Hydrous ferric oxides, also called hydrous iron oxides and iron(III) oxide-hydroxides are hydrous oxide inorganic compounds. They may be found as a class of minerals that form from the weatherin ...

(HFO). Due to the nanoparticulate nature of ferrihydrite, the structure has remained elusive for many years and is still a matter of controversy. Drits ''et al.'', using X-ray diffraction data, proposed in 1993 a multiphase structure model for six-line ferrihydrite with three components: (1) defect-free

crystallite A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...

s (f-phase) with double-hexagonal stacking of oxygen and hydroxyl layers (ABAC sequence) and disordered

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

Fe occupancies, (2) defective crystallites (d-phase) with a short-range feroxyhite-like (δ-FeOOH) structure, and (3) subordinate ultradisperse hematite (α-Fe₂O₃). The diffraction model has been confirmed in 2002 by

Neutron diffraction Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of thermal or cold neutrons to o ...

, and the three components were observed by

High-resolution transmission electron microscopy High-resolution transmission electron microscopy is an imaging mode of specialized transmission electron microscopes that allows for direct imaging of the atomic structure of samples. It is a powerful tool to study properties of materials on the a ...

. A single phase model for both ferrihydrite and hydromaghemite has been proposed by Michel ''et al.'',F. M. Michel, L. Ehm, S. M. Antao, et al. ''Science'', 316, 1726-1729 (2007) in 2007-2010, based on

pair distribution function The pair distribution function describes the distribution of distances between pairs of particles contained within a given volume. Mathematically, if ''a'' and ''b'' are two particles in a fluid, the pair distribution function of ''b'' with respect ...

(PDF) analysis of x-ray total scattering data. The structural model, isostructural with the mineral akdalaite (Al₁₀O₁₄(OH)₂), contains 20% tetrahedrally and 80% octahedrally coordinated iron. Manceau ''et al.'' showed in 2014 that the Drits ''et al.'' model reproduces the PDF data as well as the Michel ''et al.'' model.

Porosity and environmental absorbent potential

Because of the small size of individual

nanocrystal A ''nanocrystal'' is a material particle having at least one dimension smaller than 100 nanometres, based on quantum dots (a nanoparticle) and composed of atoms in either a single- or poly-crystalline arrangement. The size of nanocrystals dist ...

s, Fh is

nanoporous Nanoporous materials consist of a regular organic or inorganic bulk phase in which a porous structure is present. Nanoporous materials exhibit pore diameters that are most appropriately quantified using units of nanometers. The diameter of pores in ...

yielding large surface areas of several hundred square meters per gram.T. Hiemstra, W. H. Van Riemsdijk, ''Geochimica et Cosmochimica Acta'', 73, 4423-4436 (2009) In addition to having a high surface area to volume ratio, Fh also has a high density of local or

point defect A crystallographic defect is an interruption of the regular patterns of arrangement of atoms or molecules in crystalline solids. The positions and orientations of particles, which are repeating at fixed distances determined by the unit cell param ...

s, such as dangling bonds and vacancies. These properties confer a high ability to adsorb many environmentally important chemical species, including

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

, and organic molecules (''e.g.'',

humic Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th an ...

and

fulvic acid Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th an ...

s). Its strong and extensive interaction with trace metals and metalloids is used in industry, at large-scale in

water purification Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for hu ...

plants, as in North Germany and to produce the city water at Hiroshima, and at small scale to clean

wastewater Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industrial ...

s and

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated ...

s, for example to remove arsenic from industrial effluents and

drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...

.A. Manceau, M. Lanson, N. Geoffroy, ''Geochimica et Cosmochimic Acta'', 71, 95-128 (2007) Its nanoporosity and high affinity for

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

can be used to elaborate Fh-supported nanosized Au particles for the catalytic oxidation of CO at temperatures below 0 °C. Dispersed six-line ferrihydrite nanoparticles can be entrapped in a vesicular state to increase their stability.

Metastability

Ferrihydrite is a

metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...

mineral. It is known to be a precursor of more crystalline minerals like hematite and

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

by aggregation-based

crystal growth A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists of the ...

. However, its transformation in natural systems generally is blocked by chemical impurities adsorbed at its surface, for example

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

as most of natural ferrihydrites are

siliceous Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

.L. Carlson, U. Schwertmann, ''Geochimica et Cosmochimica Acta'', 45, 421-429 (1981) Under reducing conditions as those found in

gley soil A gleysol is a wetland soil ( hydric soil) that, unless drained, is saturated with groundwater for long enough to develop a characteristic colour pattern. The pattern is essentially made up of reddish, brownish, or yellowish colours at surfaces ...

s, or in deep environments depleted in oxygen, and often with the assistance of microbial activity, ferrihydrite can be transformed in

green rust Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the hydroxide () anion, and another anion such as carbonate (), chloride (), or sulfate (), in a layered double hydroxide str ...

, a

layered double hydroxide Layered double hydroxides (LDH) are a class of ionic solids characterized by a layered structure with the generic layer sequence cB Z AcBsub>''n'', where c represents layers of metal cations, A and B are layers of hydroxide () anions, and Z are ...

(LDH), also known as the mineral fougerite. However, a short exposure of green rust to atmospheric oxygen is sufficient to oxidize it back to ferrihydrite, making it a very elusive compound. File:Ferrihydrite precipitate.jpg, Ferrihydrite precipitate from

coal mine Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from ...

water. File:Spring in the Zillertaler Alps.jpg, Spring in the Zillertaler Alps with Fh precipitate. File:Seepage of iron-rich water.jpg, Seepage of iron-rich water. File:Ferrihydrite to goethite.jpg, Transformation of ferrihydrite (top) to

(bottom). File:Sand filter.jpg,

Water treatment Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, inc ...

plant using the

slow sand filter Slow sand filters are used in water purification for treating raw water to produce a potable product. They are typically deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water. The length and bre ...

technology to treat raw water. File:Qtz coating_rAsgFebMn.jpg, Tricolor (RGB)

X-ray fluorescence X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis ...

image of the distribution of As (red), Fe (green), and Mn (blue) in coated

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

grains from a water treatment sand bed.

References

{{Reflist Iron(III) minerals Iron oxide pigments Hydroxide minerals Hexagonal minerals Minerals in space group 186

Structure

Porosity and environmental absorbent potential

Metastability

See also

References