Hydrotalcite

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.

Hydrotalcite
Hydrotalcite with serpentine, Snarum, Modum, Buskerud, Norway. Size: 8.4 x 5.2 x 4.1 cm
General
CategoryCarbonate mineral
Formula
(repeating unit)		Mg6Al2CO3(OH)16·4H2O
IMA symbolHtc[1]
Strunz classification5.DA.50
Crystal system3R polytype: Trigonal
2H polytype: Hexagonal
Crystal class3R polytype: Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
2H polytype: Dihexagonal dipyramidal (6/mmm)
Space groupR3m
Unit cella = 3.065 Å,
c = 23.07 Å; Z = 3
Identification
ColorWhite with possible brownish tint
Crystal habitSubhedral platey crystals, lamellar-fibrous, rarely euhedral prismatic; commonly foliated, massive
Cleavage{0001}, perfect
TenacityFlexible, not elastic
Mohs scale hardness2
LusterSatiny to greasy or waxy
StreakWhite
DiaphaneityTransparent
Specific gravity2.03 - 2.09
Optical propertiesUniaxial (-)
Refractive indexnω = 1.511 - 1.531 nε = 1.495 - 1.529
Birefringenceδ = 0.016
Other characteristicsGreasy feel
References[2][3][4] [5]

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 (T1/2 = 15.7 million years) and also other fission products such as 79Se (T1/2 = 327,000 years) and 99Tc, (T1/2 = 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.

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]

See also

References
  1. 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.
  2. Mindat.org
  3. Handbook of Mineralogy
  4. Webmineral data
  5. "IMA Nomenclature Report" (PDF). Archived from the original (PDF) on 2013-05-24. Retrieved 2012-11-25.
  6. 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.
  7. "Hydrotalcite – Drugs.com". July 2021.
  8. 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.

Further reading
  • Jow, H. N.; R. C. Moore; K. B. Helean; S. Mattigod; M. Hochella; A. R. Felmy; J. Liu; K. Rosso; G. Fryxell; J. Krumhansl (2005). Yucca Mountain Project-Science & Technology Radionuclide Absorbers Development Program Overview. Yucca Mountain Project, Las Vegas, Nevada (US).
  • Kaufhold, S.; M. Pohlmann-Lortz; R. Dohrmann; R. Nüesch (2007). "About the possible upgrade of bentonite with respect to iodide retention capacity". Applied Clay Science. 35 (1–2): 39–46. doi:10.1016/j.clay.2006.08.001.
  • Krumhansl, J. L.; J. D. Pless; J. B. Chwirka; K. C. Holt (2006). Yucca Mountain Project getter program results (Year 1) I-I29 and other anions of concern. SAND2006-3869, Yucca Mountain Project, Las Vegas, Nevada.
  • Mattigod, S. V.; G. E. Fryxell; R. J. Serne; K. E. Parker (2003). "Evaluation of novel getters for adsorption of radioiodine from groundwater and waste glass leachates". Radiochimica Acta. 91 (9): 539–546. doi:10.1524/ract.91.9.539.20001. S2CID 97175034.
  • Mattigod, S. V.; R. J. Serne; G. E. Fryxell (2003). Selection and testing of getters for adsorption of iodine-129 and technetium-99: a review. PNNL-14208, Pacific Northwest National Lab., Richland, WA (US).
  • Moore, R. C.; W. W. Lukens (2006). Workshop on development of radionuclide getters for the Yucca Mountain waste repository: proceedings. SAND2006-0947, Sandia National Laboratories.
  • Stucky, G.; H. M. Jennings; S. K. Hodson (1992). Engineered cementitious contaminant barriers and their method of manufacture. Google Patents.
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