Hydrotalcite

Hydrated Mg-Al layered double hydroxide (LDH) containing carbonate anions

Mg₆Al₂CO₃(OH)₁₆·4H₂OIMA symbolHtc^[1]Strunz classification5.DA.50Crystal system3R polytype: Trigonal
2H polytype: HexagonalCrystal class3R polytype: Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
2H polytype: Dihexagonal dipyramidal (6/mmm)Space groupR3mUnit cella = 3.065 Å,
c = 23.07 Å; Z = 3IdentificationColorWhite with possible brownish tintCrystal habitSubhedral platey crystals, lamellar-fibrous, rarely euhedral prismatic; commonly foliated, massiveCleavage{0001}, perfectTenacityFlexible, not elasticMohs scale hardness2LusterSatiny to greasy or waxyStreakWhiteDiaphaneityTransparentSpecific gravity2.03–2.09Optical propertiesUniaxial (−)Refractive indexn_ω = 1.511 – 1.531 n_ε = 1.495 – 1.529Birefringenceδ = 0.016Other characteristicsGreasy feelReferences^[2]^[3]^[4]^[5]

Hydrotalcite, or formerly also völknerite,^[6] is a layered double hydroxide (LDH) of general formula Mg
₆Al
₂CO
₃(OH)
₁₆·4H
₂O, 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 deposit in Snarum, Modum, Buskerud, Norway.^[2] It occurs as an alteration mineral in serpentinite in association with serpentine, dolomite and hematite.^[3] The layers of the structure stack in multiple ways, to produce a 3-layer rhombohedral structure (3R polytype), or a 2-layer hexagonal structure (2H polytype) formerly known as manasseite. The two polytypes are often intergrown.^[2]^[3]^[5]

Applications

Nuclear fuel reprocessing

Hydrotalcite has been studied as potential getter 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 to be disposed under oxidising conditions in volcanic tuff at the Yucca Mountain nuclear waste repository. However, 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 is the long-term stability of the sequestrant that must survive over geological time scales.

Anion exchange

Layered double hydroxides (LDH) are well known for their anion exchange properties.^{[citation needed]}

Medical

Hydrotalcite is also used as an antacid, such as Maalox (magnesium-aluminium oxide).^[7]

Wastewater treatment

Treating mining and other wastewater by creating hydrotalcites often produces substantially less sludge than lime. In one test, final sludge reductions reached up to 90 percent. This alters the concentration of magnesium and aluminum and raises the pH of water. As the crystals form, they trap other waste substances including radium, rare earths, anions and transition metals. The resulting mixture can be removed via settling, centrifuge, or other mechanical means.^[8]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ ^a ^b ^c Mindat.org
^ ^a ^b ^c Handbook of Mineralogy
^ Webmineral data
^ ^a ^b "IMA Nomenclature Report" (PDF). Archived from the original (PDF) on 2013-05-24. Retrieved 2012-11-25.
^ Rammelsberg, C. (1856-05-01). "On völknerite or hydrotalkite, and the so-called steatite of Snarum". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 11 (73): 405–407. doi:10.1080/14786445608642090. ISSN 1941-5982.
^ "Hydrotalcite – Drugs.com". July 2021.
^ Hoopes, Heidi (June 12, 2014). "Wastewater that cleans itself results in more water, less sludge". www.gizmag.com. Retrieved 2016-06-11.

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.