Hydrotalcite or formerly also Völknerite is a layered double hydroxide (LDH) of general formula ·4, whose name is derived from its resemblance with

talc Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent a ...

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 deposit in Snarum, Modum,

Buskerud Buskerud () is a former county and a current electoral district in Norway, bordering Akershus, Oslo, Oppland, Sogn og Fjordane, Hordaland, Telemark and Vestfold. The region extends from the Oslofjord and Drammensfjorden in the southeast to Ha ...

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 the ...

. 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 ''se ...

in association with serpentine, dolomite and hematite. The layers of the structure stack in multiple ways, to produce a 3-layer

rhombohedral In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a three-dimensional figure with six faces which are rhombi. It is a special case of a parallelepiped where all edges are the same length. It can be us ...

structure (3''R'' Polytype), 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 products such as ⁷⁹Se (T_1/2 = 327,000 years) and ⁹⁹Tc, (T_1/2 = 211,000 years) present in

spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...

to be disposed under oxidising conditions in volcanic tuff at the

Yucca Mountain nuclear waste repository The Yucca Mountain Nuclear Waste Repository, as designated by the Nuclear Waste Policy Act amendments of 1987, is a proposed deep geological repository storage facility within Yucca Mountain for spent nuclear fuel and other high-level radio ...

. 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 ...

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 geochron ...

scales.

Anion exchange

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

Medical

Hydrotalcite is also used as an

antacid An antacid is a substance which neutralizes stomach acidity and is used to relieve heartburn, indigestion or an upset stomach. Some antacids have been used in the treatment of constipation and diarrhea. Marketed antacids contain salts of alu ...

, such as

Maalox Maalox is a brand of antacid owned by Sanofi. Their main product is a flavored liquid containing a suspension of aluminum hydroxide and magnesium hydroxide, which act to neutralize or reduce stomach acid, for the purpose of relieving the symptom ...

(magnesium-aluminium oxide).

Wastewater treatment

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

sludge Sludge is a semi-solid slurry that can be produced from a range of industrial processes, from water treatment, wastewater treatment or on-site sanitation systems. For example, it can be produced as a settled suspension obtained from conventional ...

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 ha ...

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 (rathe ...

rare earths The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

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 conve ...

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 ca ...

. 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.