Hydrotalcite or formerly also Völknerite is 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 l ...

(LDH) of

general formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

·4, whose name is derived from its resemblance with talc and its high water content. Multiple structures containing loosely bound carbonate ions exist. The easily exchanged carbonates allow for applications of the mineral in wastewater treatment and nuclear fuel reprocessing.

Structure and discovery

It was first described in 1842 for an occurrence in a serpentine -

magnesite Magnesite is a mineral with the chemical formula (magnesium carbonate). Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts. Occurrence Magnesite occurs as veins in and an alteration product of ultramafic ro ...

deposit in Snarum,

Modum Modum is a municipality in Buskerud in Viken county, Norway. The administrative centre of the municipality is the town of Vikersund. The municipality of Modum was established on 1 January 1838 (see formannskapsdistrikt). The area has a long tr ...

, Buskerud,

Norway Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...

. It occurs as an alteration mineral in

serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''ser ...

in association with

serpentine Serpentine may refer to: Shapes * Serpentine shape, a shape resembling a serpent * Serpentine curve, a mathematical curve * Serpentine, a type of riding figure Science and nature * Serpentine subgroup, a group of minerals * Serpentinite, a ...

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

and

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 . ...

. The layers of the structure stack in multiple ways, to produce a 3-layer rhombohedral structure (3''R''

Polytype In materials science, polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism. Any crystalline material can exhibit the phenomenon. Allotropy refers to polymorphism ...

), or a 2-layer hexagonal structure (2''H'' polytype) formerly known as manasseite. The two polytypes are often intergrown.

Applications

Nuclear fuel reprocessing

Hydrotalcite has been studied as potential

getter A getter is a deposit of reactive material that is placed inside a vacuum system to complete and maintain the vacuum. When gas molecules strike the getter material, they combine with it chemically or by . Thus the getter removes small amounts of ...

for iodide in order to scavenge the long-lived ¹²⁹I (T_1/2 = 15.7 million years) and also other

fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

s such as ⁷⁹Se (T_1/2 = 327,000 years) and ⁹⁹Tc, (T_1/2 = 211,000 years) present in spent nuclear fuel to be disposed under oxidising conditions in

volcanic tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

at the Yucca Mountain nuclear waste repository. However,

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...

anions easily replace iodide anions in its interlayer and therefore the selectivity coefficient for the anion exchange is not favorable. Another difficulty arising in the quest of an iodide getter for

radioactive waste Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons r ...

is the long-term stability of the

sequestrant A sequestrant is a food additive which improves the quality and stability of foods. A sequestrant forms chelate complexes with polyvalent metal ions, especially copper, iron and nickel, which can prevent the oxidation of the fats in the food. Se ...

that must survive over

geological time The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronol ...

scales.

Anion exchange

Layered double hydroxides (LDH) are well known for their anion exchange properties.

Medical

Hydrotalcite is also used as an antacid, such as Maalox (magnesium-aluminium oxide).

Wastewater treatment

Treating mining and other wastewater by creating hydrotalcites often produces substantially less sludge than

lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...

. In one test, final sludge reductions reached up to 90 percent. This alters the concentration of

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

and

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

and raises the pH of water. As the crystals form, they trap other waste substances including

radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rather t ...

, rare earths,

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s and

transition metals In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

. The resulting mixture can be removed via settling, centrifuge, or other mechanical means.

References

* Douglas, G., Shackleton, M. and Woods, P. (2014). Hydrotalcite formation facilitates effective contaminant and radionuclide removal from acidic uranium mine barren lixiviant. Applied Geochemistry, 42, 27-37. * Douglas, G.B. (2014). Contaminant removal from Baal Gammon acidic mine pit water via in situ hydrotalcite formation. Applied Geochemistry, 51, 15-22.