Smectite clay minerals

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. 

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.

Kostka JE, Wu J, Nealson KH, Stucki JW. 1999. The impact of structural Fe(III) reduction by bacteria on the surface chemistry of smectite clay minerals. Geochimica et Cosmochimica Acta 63 3705-3713. 

Hance (1967) investigated the rate of sorption and desorption of four pesticides (monuron, linuron, atrazine, and chlorpropham) on two soils, a soil organic matter fraction, and bentonite, a 2 1 smectitic clay mineral. An equilibrium in sorption was reached in 24 h for every system except one (Table 6.1). With eight of the 18 systems equilibrium was reached in less than 4 h, and in five cases equilibrium was established in 1 hr. Equilibrium was attained for most of the systems in 4-24 h. Desorption was slower than sorption. In only eight systems was an equilibrium reached in 24 h. Hance... 

Variable Oxidation States of Iron in the Crystal Structure of Smectite Clay Minerals... 

The oxidation state of iron in smectite clay minerals may vary between Fe3+ and Fe2+, either through natural or laboratory processes, with accompanying changes in chemical and physical properties ... 

STUCKI LEAR Oxidation States of Iron in Smectite Clay Minerals 337... 

An actual contribution of humic substances to metal oxide reduction in natural systems has not been demonstrated, and there are processes such as adsorption or decomposition that could limit their effectiveness. Kostka et al. (2002a) observed that AQDS additions elicited a larger increase in Fe(III) reduction by S. oneidensis growing on ferrihydrite than smectite clay minerals. This suggests that the influence of humic substances may depend on soil or sediment mineralogy. Nevertheless, there is ample evidence to suggest that a portion of the anaerobic metabolism that was previously attributed to direct enzymatic Fe(III) and Mn(IV) reduction was actually none-nzymatic reduction by microbially reduced humic substances. 

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

The interaction between some organic contaminants and mineral surfaces has recently attracted attention as a way of cleaning up contaminants in natural waters. The large cation exchange capacity of smectite clay minerals (Section 4.5.2), in particular, has prompted research into their use as a catalyst, i.e. a substance that alters the rate of a chemical reaction without itself changing. Clay catalysts have potential applications as adsorbents to treat contaminated natural waters or soils. 

Recently, several works study the preparation of nano scale acid solids constructed between the interlayers of smectite clay minerals [3]. 

The different classes of clay minerals, namely, 1/1,2/1, and so on, have a differenf arrangemenf of tetrahedral and octahedral layers. Structural units of clays, fherefore, consist of either (1) alternating tetrahedral sheets (OT or 1/1 structure e.g., the kaolinite group) (2) a sandwich of one ocfahedral sheet between two tetrahedral sheets (TOT or 2/1 structure e.g., smectite clay minerals, of which fhe mosf common member is monf moril-lonite) or (3) an arrangement in which three TOT units alternate with a brucite layer (2/1/1 structure e.g., chlorite). 

Figure 10. Illustration of the basic structure of smectite clay minerals. Lattice positions are assigned as silicon ( and °) aluminum, magnesium, or iron (0) oxygen (O) and hydroxyl ( ). (Reproduced with permission from reference 27. Copyright 1968 McGraw Hill.)...

A series of ID and 2D NMR experiments performed on nanocomposites of hectorite, a smectite clay mineral, showed new opportunities for studying large assemblies of layered minerals with polymeric materials.In particular. rotational-echo double-resonance (REDOR) experiments afforded H- Si distances with precision, and a mobility gradient of intercalated polyethyleneoxide (PEG) segments was demonstrated in H-" Si wideline separation (WISE) experiments. 

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