Smectites with exchangeable cations

Ca because their stronger hydration precludes inner-sphere complexation by —NO2 groups. When adsorbed to clays saturated by these cations, the FTIR spectra of TNB are very similar to those in aqueous solution. These data provide molecular-level support for the hypothesis that SNB sorption on smectites is controlled, in part, by interactions with exchangeable cations. 

The original pillared clays were made by (/) mixing smectite with a polymeric cationic hydroxy metal complex such as aluminum chlorhydrol (2) allowing a minimal amount of time for the cationic hydroxy metal complex to exchange with the interlayer cations and (7) calcining the resulting material to decompose the hydroxy metal complex (110). A number of newer methods have been developed to make pillared clays (111—117). 

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

Halloysite-10A represents a structure with few if any interlayer cations, allowing one to investigate the relatively simple case of water interacting with a clay surface. Similarly, ice-like models have been proposed for water adsorbed on smectite and vermie-ulite surfaces (2, 12, 12). These represent cases of charged clay layers with adsorbed exchangeable cations. 

The validity of the assumption that the various thermodynamic properties of the smectite remain invariant, regardless of the state of hydration, has been addressed in detail by Sposito and Prost (1). They point out that one would, for example, expect hydrolysis of the clay to occur at high water contents, and also, it is likely that the exchangeable cations will change their spatial relationship with the clay layers. Thus, the derived thermodynamic properties of the adsorbed water would not represent correct values. 

Figure 8. Percentage illite layers versus equivalents of fixed interlayer cations (Na + K) per illite layer [based on 01o(0H)2]. Solid circles = aluminous smectites with 1 Sr-exchange. Open circles = aluminous smectites with 2 or 3 Sr-exchanges. X = iron-rich smectites with 1 Sr-exchange. Points calculated from data in Tables III, IV, VI, and VIII.

For layer silicates with low structural charge (i. . smectites), this expansion is limited to about four molecular layers of water if the exchangeable cation has a charge of +2. Since the silicate platelet is about 0.96 nm thick, the repeat spacing along the c-axis is then approximately 0.96 + (4x.26) = 2.0 nm. 

The g-values and A values of Table IV reveal that the particular layer silicate has more effect on ESR parameters of adsorbed Cu " - than saturation of exchange sites with different cations such as Na+ and Ca +. Also, the smectites as a group have lower g and higher A values than vermiculite. From the perspective of molecular orbital theory, low g and high A values correspond to more covalent bonds between Cu + and the ligand (19). Thus,... 

Isaacson and Sawhney (60) studied the reactions of a number of phenols and smectite with transition metal (Cu, FeJ+) and nontransition metal exchangeable cations. IR spectra of the clay-phenol complexes showed that all the clays studied transformed the sorbed phenols. The transformation occurred to a much greater extent in clays with transition metal cations than in those with the non-transition metal cations. In a subsequent study, Sawhney et al. (61) studied the polymerization of 2,6-dimethylphenol on air-dried homoionic Na-, Ca-, A1-, and Fe-smectite at 50°C. A portion of the adsorbed 2,6-dimethylphenol was transformed into dimers, trimers, tetramers, and quinone-type compounds. The nature of the exchange cations had an effect on both sorption and transformation and decreased in the order Fe Al > Ca > Na. 

Clay minerals with their own surface properties affect the near surface water in different ways. The adsorbed water in the case of kaolinite consists only of water molecules ( pure water), whereas water adsorbed on a smectite-type mineral is an aqueous solution, due to the presence of exchangeable cations on the 2 1 layer sihcate. Sposito (1989) noted the generally accepted description that the spatial extent of adsorbed water on a phyUosilicate surface is about 1.0 nm (two to three layers of water molecules) from the basal plane of the clay mineral. 

Montmorillonites (smectite clays) have structures resembling that of pyrophyllite but the structure is not electrically neutral. Exchangeable cations are located in interlamellar regions of the clay and, furthermore, the clay can be flocculated such that the plate-like crystals compact with parallel c-axes to give coherent layers. The smectites are then attractive materials with which to modify electrodes. 

Clay minerals are hydrous aluminum phyllosilicates made of sheets or layers composed of tetrahedra and octahedra. This mineral type includes the following groups kaolinite, smectite, illite, and chlorite. In the case of smectite, each layer comprises two sublayers of tetrahedra with an inserted octahedral layer, where, between layers, an interlayer space where the exchangeable cations are located is formed [131-133], In Figure 2.24... 

Figure 3.8. Schematic of the smectite structure showing one gibbsite sheet between two silicate sheets. The basic unit is repeated many times in the horizontal directions to produce layers. The basic unit with the 9.6 A c-axis spacing expands to 14 A when water enters between layers. The exchangeable cations located between the layers produce the counter charge for the isomorphous substitution. It occurs in the layers marked with an asterisk (from Taylor and Ashcroft, 1972, with permission).

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