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Clays
Ray L. Frost


Can humans do it better than nature? Can we synthesise clays better than what we find in nature?

Clay scientists can synthesise clays to a high degree of purity and design them for specific purposes. An example is the synthesis, characterisation and application of one group of clay minerals known a hydrotalcites or layered double hydroxides. Hydrotalcites are anionic clays, which have been relatively unknown although these minerals are quite prevalent in nature. Early reports of natural hydrotalcites date back to 1944. Hydrotalcites are fundamentally known as anionic clays and are less well-known and more diffuse in nature than cationic clays such as smectites. Hydrotalcites can be synthesized from a broad range of compositions.

Although hydrotalcites both natural and synthetic have been known for an extended period of time, only now are more people realizing their usefulness. Fuelling this realization is an increasing importance for their use as catalysts.

Applications

Interest in the study of hydrotalcites results from their potential use as catalysts (Alejandre et al., Applied Catalysis, v. 30, p. 195-207, 2001; Das and Parida, Rect. Kinet. Catal. Lett., v. 69, p. 223-229, 2000; Patel et al., J. Vinyl Addit. Technol., v. 1, p. 201-206, 1995; Rives et al., 1998). The reason for the potential application rests with the ability to make mixed metal oxides at the atomic level, rather than at a particle level. Such mixed metal oxides are formed through the thermal decomposition of the hydrotalcite (Rey et al.,; J. Chem. Soc., Faraday Trans., 88, 2233-2238, 1992; Valcheva-Traykova et al., Journal of Materials Science, 28, 2157-2162, 1993). Hydrotalcites may also be used as a components in nano-materials such as nano-composites (Oriakhi et al., Clays Clay Miner., 45, 194-202, 1997). Hydrotalcites are also important in the removal of environmental hazards in acid mine drainage (Lichti and Mulcahy, Chemistry in Australia, v. 65, p. 10-131998; Seida and Nakano, Journal of Chemical Engineering of Japan, v. 34, p. 906-911, 2001).

Hydrotalcite formation also offers a mechanism for the disposal of radioactive wastes (Roh et al., Clays and Clay Minerals, ,v. 48, p. 266-271, 2000) or as a means of heavy metal removal from contaminated waters (Seida et al., 2001).

Of particular importance is the use of hydrotalcites in wastewater treatment, specifically for the removal of anions such as arsenate or cyanide (Theo Kloprogge and Frost, Phys. Chem. Chem. Phys., v. 1, p. 1641-1647, 1999a).

Synthesis of hydrotalcite samples

The hydrotalcites were synthesised by the co-precipitation method (Hickey et al., J. Mater. Sci., v. 35, p. 4347-4355, 2000), synthesizing hydrotalcites with materials varying in amounts of magnesium, zinc and aluminium (or, in place of magnesium, nickel or cobalt; and in place of aluminium, iron or chromium). The composition of the hydrotalcites may be checked by ICP and ICP-AES analysis. The phase composition is checked by X-ray diffraction.

Hydrotalcites are based on the brucite structure, in which some magnesium is replaced by a trivalent cation such as aluminium (Al3+), iron (Fe3+) or chromium (Cr3+). This replacement introduces a positive charge on the layered double hydroxides, and this charge is counterbalanced by an anion that may be carbonate or sulphate.

It should be understood that the interlayer spaces in these anionic clays are filled with water and anions. In certain natural hydrotalcites the divalent cations are replaced by other divalent cations such as zinc (Zn3+) or nickel (Ni3+). In many natural hydrotalcites the interlayer is composed of a mixture of anions.

Characterisation of hydrotalcites

Hydrotalcites are characterised in a number of ways, firstly by X-ray diffraction to show the anionic clays are a layered structure; secondly by transmission electron microscopy (TEM) to show the crystallinity of the anionic clay and by vibrational spectroscopy to show the structure of the anions in the interlayer (Kloprogge et al., American Mineralogist, v. 87, p. 623-629, 2002).

Another excellent tool for studying the anions in the interlayer is Raman spectroscopy. Many of the anions in the interlayer are oxyanions, which lend themselves to Raman spectroscopic analysis. The sulphate ion SO42- is one example.

Hydrotalcites and the layered double hydroxides form a fascinating set of clay minerals to study. The natural minerals are probably best described as hydrotalcites and the synthetic anionic clays as layered double hydroxides.

The question arises as to whether or not man can synthesise clay minerals better than nature and the answer is yes. Why? Because the hydrotalcites that are synthesized are of high purity.

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Frost teaches at the Queensland University of Technology in Australia and is Secretary General for AIPEA (Association Internationale Pour L'etude Des Argiles, or the International Association for the Study of Clays, affiliated with the International Union of Geological Sciences). E-mail: r.frost@qut.edu.au

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