Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

at the Earth's surface, and a likely constituent in extraterrestrial materials. It forms in several types of environments, from

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

to marine systems,

aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...

s to hydrothermal

hot spring A hot spring, hydrothermal spring, or geothermal spring is a Spring (hydrology), spring produced by the emergence of Geothermal activity, geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow ...

s and scales,

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

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

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

-rich aqueous solutions, or by

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

either as a result of a

metabolic activity Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

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 In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...

reactions. Ferrihydrite also occurs in the core of the

ferritin Ferritin is a universal intracellular and extracellular 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. ...

protein from many

living organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

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, chemical substances or 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 ...

. The powder

X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

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 (HFO). Due to the nanoparticulate nature of ferrihydrite, the structure has remained elusive for many years and is still a matter of controversy.A. Manceau ''ACS Earth and Space Chemistry'', 4, 379–390 (2020). 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 Wikt:longulite ...

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

octahedral In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...

Fe occupancies, (2) defective crystallites (d-phase) with a short-range feroxyhite-like (δ-FeOOH) structure, and (3) subordinate ultradisperse

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

(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 Neutron temperature, thermal or ...

, and the three components were observed by high-resolution transmission electron microscopy.D.E. Janney, J.M. Cowley, P.R. Buseck. ''American Mineralogist'', 86, 327–335 (2001). 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, the pair distribution function of ''b'' with respect to ''a'', ...

(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, and he suggested in 2019 that the tetrahedral coordination arises from

maghemite Maghemite (Fe2O3, γ-Fe2O3) is a member of the family of iron oxides. It has the same formula as hematite, but the same spinel ferrite structure as magnetite () and is also ferrimagnetic. It is sometimes spelled as "maghaemite". ''Maghemite'' ...

and

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

impurities observed by electron microscopy.

Porosity and environmental absorbent potential

Because of the small size of individual nanocrystals, 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 i ...

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 para ...

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; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

phosphate Phosphates are the naturally occurring form of the element phosphorus. 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 orthop ...

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

humic Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown lead ...

and fulvic acids). 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 is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui has b ...

, and at small scale to clean

wastewater Wastewater (or waste water) 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 do ...

s and

groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...

s, for example to remove arsenic from industrial effluents and

drinking water Drinking water or potable water is water that is safe for ingestion, either when drunk directly in liquid form or consumed indirectly through food preparation. It is often (but not always) supplied through taps, in which case it is also calle ...

.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; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

can be used to elaborate Fh-supported nanosized Au particles for the

catalytic oxidation Catalytic oxidation are processes that rely on catalysts to introduce oxygen into organic and inorganic compounds. Many applications, including the focus of this article, involve oxidation by oxygen. Such processes are conducted on a large scale ...

of CO at temperatures below 0 °C. Dispersed six-line ferrihydrite nanoparticles can be entrapped in a vesicular state to increase their stability. Presence of ferrihydrite can enhance growth of certain microbes, eg. '' Dissulfuribacter thermophilus'', by scavenging sulfide, a waste product of their metabolism.

Metastability

Ferrihydrite is a

metastable In chemistry and physics, metastability is 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 is onl ...

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

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 Crystal growth is a major stage of a crystallization, crystallization process, and consists of the addition of new atoms, ions, or polymer strings into the characteristic arrangement of the crystalline lattice. The growth typically follows an ini ...

. 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 , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

as most of natural ferrihydrites are

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

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

, a layered double hydroxide (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 resource extraction, extracting coal from the ground or from a mine. Coal is valued for its Energy value of coal, energy content and since the 1880s has been widely used to Electricity generation, generate electr ...

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, ...

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 Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

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