Clay smectite

Transmission electron micrographs show hectorite and nontronite as elongated, lath-shaped units, whereas the other smectite clays appear more nearly equidimensional. A broken surface of smectite clays typically shows a "com flakes" or "oak leaf surface texture (54). High temperature minerals formed upon heating smectites vary considerably with the compositions of the clays. Spinels commonly appear at 800—1000°C, and dissolve at higher temperatures. Quartz, especially cristobalite, appears and mullite forms if the content of aluminum is adequate (38). 

FIGURE 2.2 The smectite clay structure. (From http //www.pslc.ws/macrog/mpm/composit/nano/stmct3 l. htm, access date 4.11.06.)... 

Many varieties of clay are aluminosilicates with a layered structure which consists of silica (SiOa" ) tetrahedral sheets bonded to alumina (AlOg ) octahedral ones. These sheets can be arranged in a variety of ways in smectite clays, a 2 1 ratio of the tetrahedral to the octahedral is observed. MMT and hectorite are the most common of smectite clays. 

Crocker FH, WA Guerin, SA Boyd (1995) Bioavailability of naphthalene sorbed to cationic surfactant-modified smectite clay. Environ Sci Technol 29 2953-2958. 

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

Chiral recognition of A-[Co(phen)3]3+ has been observed in a modified /3-cyclodextrin.772 Chiral discrimination has also been seen in photoinduced energy transfer from luminescent chiral lanthanoid complexes773 to [Co(phen)3]3+ and between photoexcited [Ru(bpy)3]2+ and [Co(phen)3]3+ co-adsorbed on smectite clays.774 The [Co(bpy)3]3+ ion has been incorporated into clays to generate ordered assemblies and also functional catalysts. When adsorbed onto hectorite, [Co(bpy)3]3+ catalyzes the reduction of nitrobenzene to aniline.775 The ability of [Co(phen)3]3+ to bind to DNA has been intensively studied, and discussion of this feature is deferred until Section 6.1.3.1.4. 

The effect of pH on both clay swelling and fines production has been widely discussed(89-95). Little consensus is found in this literature. Suggested treatments range from application of fluoboric acid(96) to 15% KOH(92) solutions — both treatments are believed to create a protective silicate film that inhibits release of fines. Polyacrylate polymers can provide protection against swelling of smectite clays and shales(97-100). 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

Smectite clays that disperse more easily than non-swelling clays are a common constituent of Alfisols. 

Gonzales and Laird145 have shown that smectites abiotically catalyze dehydration of glucose to form furfural under conditions similar to those found in soils. Four smectite clay minerals were used (saturated with Na, Ca, Fe, or Al), and the formation of HMF and furfural was detected by high-pressure liquid chromatography. The polymerization of furfural may thus be a pathway to the formation of new humic materials in soils. 

Boyd SA, Sheng G, Teppen BJ, Johnston CT (2001) Mechanisms for the adsorption of substituted nitrobenzenes by smectite clays. Environ Sci Technol 35 4227-4234... 

Fig. 23.6. Calculated saturation indices (log Q/K) of aluminum-bearing minerals plotted versus temperature for a hot spring water from Gjogur, Hveravik, Iceland. Lines for most of the minerals are not labeled, due to space limitations. Sampling temperature is 72 °C and predicted equilibrium temperature (arrow) is about 80 °C. Clinoptilolite (zeolite) minerals are the most supersaturated minerals below this temperature and saponite (smectite clay) minerals are the most supersaturated above it.

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

Alvarez-Cohen et al. [91] explicitly showed that microbial transformation rates of trichloroethylene (TCE) were proportional to the aqueous TCE concentrations and independent from zeolite-sorbed TCE concentrations. Apparently in contrast to these findings, Crocker et al. [92] reported on the direct bioavailability of naphthalene sorbed to hexadecyltrimethylammonium (HDTMA)-modified smectite clay to Pseudomonas putida 17848, but not to Alcaligenes sp. strain NP-Alk. It should be noted that sorption to the hexadecyl chains of HDTMA resembles more the solubilisation by a surfactant than adsorption to a solid surface. Possibly, hydrophobic surface structures of strain 17848 allowed the close contact with HDTMA, thereby facilitating the uptake of naphthalene by a lipophilic pathway. 

Magnesium silicate smectite clay, 22 475 Magnesium silicide, 75 364 Magnesium statistics, U.S. historical, 75 324 Magnesium stearate... 

The Smectite Clays. The smectite-type clays are distinctive in that they expand and cause significant destruction to synthetic (human-made) structures. In this type of 2 1 clay, isomorphous substitution occurs in the aluminum sheet. If there is substitution of lower-oxidation-state metal such as magnesium, there will be an unsatisfied pair of bonding electrons in the interior of the crystal and there will be no noticeable change in the surface. Because the charge is in the interior of the crystal, its attraction for cations is diminished by distance. Thus, smectite crystals are not held together strongly by cations and are able to incorporate more water and ions between sheets when the environment is wet and less when it is dry. 

LDH materials are not readily exfoliated in contrast to other layered materials such as smectite clays or MS2 type-chalcogenides (M0S2, NbSe2, etc...) [15-17] the delamination of LDH sheets requires elaborate syntheses as discussed above in Sect. 2.1. 

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