Layered double hydroxides

File:Hydrotalcite-200667.jpg

Hydrotalcite, Snarum, Modum, Buskerud, Norway. Size: 8.4 x 5.2 x 4.1 cm.

Layered double hydroxides (LDH) comprise an unusual class of layered materials with positively charged layers and charge balancing anions located in the interlayer region. This is unusual in solid state chemistry: many more famililies of materials have negatively charged layers and cations in the interlayer spaces (e.g. kaolinite, Al₂Si₂O₅(OH)₄).

LDHs are commonly represented by the formula [M^z+_1-xM³⁺_x (OH)₂]^q+(X^n-)_q/n·yH₂O. Most commonly, z = 2, and M^{2+ = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+; hence q = x.}

Pure phases have been shown to exist over the range 0.2 ≤ x ≤ 0.33. However, values of x have been reported in the range 0.1 ≤ x ≤ 0.5. Also possible is z = 1, where M⁺ = Li⁺ and M³⁺ = Al³⁺. In this case q = 2x - 1.

The latter family of materials can be described by the formula [LiAl₂(OH)₆]X∙yH₂O (LiAl₂-X)). X represents a generic anion and the value of y is normally found to be between 0.5 – 4.

LDHs may be formed with a wide variety of anions X (e.g. Cl, Br, and NO₃).

The anions located in the interlayer regions can generally be easily replaced. A wide variety of anions may be incorporated, ranging from simple inorganic anions (e.g. CO₃^2-) through organic anions (e.g. benzoate, succinate) to complex biomolecules, including DNA. This has led to an intense interest in the use of LDH intercalates for advanced applications. Drug molecules such as ibuprofen may be intercalated; the resulting nanocomposites have potential for use in controlled release systems, which could reduce the frequency of doses of medication needed to treat a disorder. Further effort has been expended on the intercalation of agrochemicals, such as the chlorophenoxyacetates, and important organic synthons, such as terephthalate and nitrophenols. Agrochemical intercalates are of interest because of the potential to use LDHs to remove agrochemicals from polluted water, reducing the likelihood of eutrophication.

LDHs have been shown to have remarkable shape selective intercalation properties. For instance, reacting LiAl₂-Cl with a 50:50 mixture of terephthalate (1,4-benzenedicarboxylate) and phthalate (1,2-benzenedicarboxylate) results in intercalation of the 1,4-isomer with almost 100 % preference. This means that there is significant potential to use LDHs to separate isomeric mixtures. The selective intercalation of ions such as benzenedicarboxylates and nitrophenols has importance because these are produced in isomeric mixtures from crude oil residues, and it is often desirable to isolate a single form, for instance in the production of polymers.

Hydrotalcite is a layered double hydroxide of general formula (Mg₆Al₂(CO₃)(OH)₁₆·4(H₂O) whose name is derived from its resemblance with talc and its high water content. 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 = 295 000 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. Unfortunately carbonate easily replaces iodide in its interlayer. Another difficulty arising in the quest of an iodine getter for radioactive waste is the long-term stability of the sequestrant that must survive over geological time scales. LDHs are well known for their anion exchange properties. The formation of LDHs can be identified easily by X-Ray Diffraction studies. Sharp peaks indicates that the material obtained is ordered. The broad asymmetric peaks indicated that the material is disordered.

Hydrotalcite is also used as an antacid.

See also

• Fougerite, an iron-bearing LDH mineral similar to green rust.

Useful links

• Hydrotalcite on Webmineral
• Hydrotalcite on Mindat

References

• LDH, DNA and Hydrothermal Vents - Science Daily

• 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). 

• 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.; 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. 

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