Smectites surface modifiers

The simplest, self-consistent model of the diffuse-ion swarm near a planar, charged surface like that of a smectite is modified Gouy-Chapman (MGQ theory [23,24]. The basic tenets of this and other electrical double layer models have been reviewed exhaustively by Carnie and Torrie [25] and Attard [26], who also have made detailed comparisons of model results with those of direct Monte Carlo simulations based in statistical mechanics. The postulates of MGC theory will only be summarized in the present chapter [23] ... 

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. 

Another type of reaction that responds to WD cycles is the fixation of K and NH4 ions by smectite (3-7). The fixation of K in smectite has been studied extensively by soil scientists because of its effect on the availability of plant nutrients. The reaction also decreases smectite s ability to swell, decreases its cation exchange capacity (CEC), and modifies its BrjSnsted acidity. Therefore, an understanding of this phenomenon is applicable to many fields of study that are concerned with swelling clays, fields such as soil fertility, soil mechanics, waste disposal, clay catalysis, and the geochemistry of ground and surface waters. 

These examples illustrate that biomolecules may act as catalysts in soils to alter the structure of organic contaminants. The exact nature of the reaction may be modified by interaction of the biocatalyst with soil colloids. It is also possible that the catalytic reaction requires a specific mineral-biomolecule combination. Mortland (1984) demonstrated that py ridoxal-5 -phosphate (PLP) catalyzes glutamic acid deamination at 20 °C in the presence of copper-substituted smectite. The proposed pathway for deamination involved formation ofa Schiff base between PLP and glutamic acid, followed by complexation with Cu2+ on the clay surface. Substituted Cu2+ stabilized the Schiff base by chelation of the carboxylate, imine nitrogen, and the phenolic oxygen. In this case, catalysis required combination of the biomolecule with a specific metal-substituted clay. 

Pyrene luminescence also has been utilized to probe surfactant-modified smectites. Pyrene and anthracene themselves are poorly adsorbed on the hydrophilic surface of smectites, but they are readily adsorbed by organoammonium-intercalated clays. Intocalation of pyrene, anthracene, and azobenzenes into long-chain quatamary alkylammonium-smectites was successfully achieved by... 

In the first method, commercial, partially neutralized AP solutions are modified by addition of La + or Ce +, and the pillaring process of a smectite clay mineral is performed under reflux or under hydrothermal conditions. A material is obtained that is, at least partially, expanded to a t/ooi spacing of 2.6 nm. The structure and composition of the pillaring oligomer is unknown, nor is it known to what extent the La or Ce cations are incorporated into the pillar. In view of the pH conditions of the pillaring process, only traces are expected to be incorporated in the pillar some may be ion exchanged or adsorbed on the pillar. Anyhow, the data of Table 2 clearly show the increase in surface area and pore volume with respect to the regularly pillared materials. 

The increased adsorption of certain pollutants on surfactant modified pillared montmorillonites results from the orientation of fte surfactant cations different from that in the non-piUared smectites. As the polyhydroxoaluminium complexes are positively charged, the cations are assumed to be oriented with their positive head groups pointing away from the pillars (Srinivasan and Fogler, 1990). This makes the surface hydrophilic, whereas the space between the alkyl chains retains its hydrophobic character. It is also assumed that an electrostatic shielding against flocculation further increases the effective interface area. 

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