Cationic clays characteristics

The principal class of swelling clays is termed smectites. These clays characteristically have alumina octahedra sandwiched between silica tetrahedra as shown in Figure 1. The smectites are distinguished by the type and location of cations in the layered framework. In a unit cell formed from twenty oxygens and four hydroxyl groups, there are eight tetrahedral sites and six octahedral sites. Idealized formulas are given in Table I. 

Another characteristic of 2 1 clays is isomorphous substitution, where iso means same and morphous means shape. During the formation of clay, cations other than aluminum and silicon become incorporated into the structure. In order for this to work and still ensure a stable clay, the cation must be about the same size as either aluminum or silicon, hence the term isomorphous. There are a limited number of cations that satisfy this requirement. For silicon, aluminum as Al3+ and iron as Fe3+ will tit without causing too much distortion of the clay structure. For aluminum, iron as Fe3+, magnesium as Mg2+, zinc as Zn2+, and iron as Fe2+ will fit without causing too much structural distortion (see Figure 3.4). 

To assess the influence of some soil physico-chemical characteristics, viz., pH, organic matter, clay content, etc., and exchangeable cations, on the spatial variation and vertical distribution of the solid-phase Al species in the profile of Delhi soils at selected sites. 

Clays have a loose layer structure (Figure 4.6). Characteristic minerals are montmoril-lonite and beidellite. Aluminosilicates such as montmorillonite, kaolinite, and feldspar can act as cation and anion exchangers. 

Zeolite surface chemistry resembles that of smectite clays. In contrast to clays, however, natural zeolites can occur as millimeter- or greater-sized particles and are free of shrink-swell behavior. As a result, zeolites exhibit superior hydraulic characteristics and are suitable for use in filtration systems (Breck 1974) and as permeable barriers to dissolved chemical migration. Internal and external surface areas up to 800 m2 g have been measured. Total cation exchange capacities in natural zeolites vary from 250 to 3000 meq kg 1 (Ming and Mumpton 1989). External cation exchange capacities have been determined for a few natural zeolites and typically range from 10 to 50 percent of the total cation exchange capacity (Bowman et al. 1995). 

Sorption on oxides and hydrous oxides has been extensively studied both experimentally and theoretically. In order to define an ideal oxide or hydrous oxide exchanger, we will rely on experiments with well-defined sorbents such as chromatographic alumina (1-4). Briefly, the adsorption characteristics of oxides and hydrous oxides are (1) At high pH, they act as cation exchangers but with less sensitivity to the total salt concentration than clay minerals. This behavior is illustrated in Figure 2, where sorption of strontium on alumina is shown. 

Soils also display a wide range of physico-chemical characteristics that influence the bioavailability of toxic metals to microorganisms, fauna, and plant roots. These characteristics include soil texture, organic matter, cation and anion type, pH, or alkalinity. The presence of colloidal clay minerals in soils greatly increases the... 

When the dye is ion-exchanged on clays, it is concentrated in a relatively small volume around the clay particles. The size of the volume depends on the swelling characteristics of the clay and thus on such factors as (1) the size and the charge density of the clay particles (2) the type of exchangeable cation and (3) the ability of the dye to penetrate in the interlamellar space of the aggregates. 

This paper is concerned with a quantitative study of the fluorescence intensity of proflavine on several clays. The intensity of the fluorescence changes with loading in a characteristic way, which depends on the type of clay and the type of exchangeable cation. An attempt is presented to calculate effective surface areas in aqueous suspension. 

The characteristic evolution of the quantum yields with loading, shown in figures 5-8, are clear fingerprints of the type of exchangeable cation and the type of clay in aqueous suspension. The following conditions must be... 

Spectroscopy. The important parameters, which determine the characteristic evolution of the fluorescence intensities with loading are (1) the particle size of the clays (hectorite vs. laponite) (2) the exchangeable cation (Na+, Ca2+, K+ vs. Cs+) and (3) for BS the pH. The latter is due to the extensive protonation of proflavine on the surface even in neutral suspension (Figure 3 and reference 5). 

A wide variety of soils and sludge have now been treated. Soil characteristics that can impact the SET chemistry include the general soil type, which is treated (loam, sand, silt, and clay), the presence of humic material, the pH value, the soil s cation exchange capacity, its particle size, the amount of water present, and the iron content. Processes have been engineered to accommodate this wide range of variables [7,8,34]. Some soils can be treated... 

---