Hydrotalcite, or formerly also völknerite, is a layered double hydroxide (LDH) of general formula , whose name is derived from its resemblance with talc and its high water content. Multiple structures containing loosely bound carbonate () ions exist. The easily exchangeable carbonates enable applications of the mineral in wastewater treatment and the immobilisation of radioactive waste after 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. It occurs as an alteration mineral in serpentinite in association with serpentine, dolomite and hematite. 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.
Applications
Anion exchange Layered double hydroxides (LDH) are well known for their anion exchange properties.
Wastewater treatment Treating mining and other wastewater by producing hydrotalcite 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, centrifuging, or other mechanical means.
Anion getter for nuclear waste disposal Hydrotalcite has been studied as potential getter for iodide 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.Krumhansl, James L., Pless, Jason D., Chwirka, J. Benjamin, Holt, Kathleen C. (July 2006). Yucca Mountain Project getter program. Results (Year 1): I-129 and other anions of concern. SANDIA Report SAND2006-3869. 55 pp. https://www.osti.gov/servlets/purl/893147#page=1.00&gsr=0 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 for an iodide getter for radioactive waste is the long-term stability of the sequestrant that must survive over geological time scales.
Medical Hydrotalcite is also used as an antacid, such as Maalox (magnesium-aluminium oxide).
See also Barbertonite Brucite, Fougerite Layered double hydroxide (LDH) Magnesium hydroxide Stichtite
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.
Further reading