Capacity exchange

The cation exchange capacity (CEC) is important for two reasons. First, hke zeolites, by placing proper cations in the PILC, its adsorption properties can be tailored. Its pore structure can also be altered by ion exchange, albeit only 

The CEC of the precursor clay is equal to the total amount of the charge-compensating cations. Upon PILC synthesis, most of the CEC is lost only about 10-20% remains. The CEC of the clay varies depending on the geological source. For the most used clay, montmorillonite (or bentonite), the CEC is 0.76 meq/g for Wyoming bentonite and is 1.40 meq/g for Arizona (Cheto) bentonite. The reason for the loss of CEC upon PILC synthesis will be discussed first, followed by methods for restoring it in the PILC. 

CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS 

the original CEC is now taken up by the protons. Some or most of these protons migrate, at the calcination temperature, into the octahedral sheet of the clay, and toward the incompletely neutralized hydroxyl groups that are coordinated to magnesinm, aluminum or other octahedral structural metal atoms. The migration of the cations into the octahedral layer is basically responsible for the CEC loss of the calcined product because these protons are no longer accessible for ion exchange. 

It has been reported that approximately 80% of the initial CEC of the clay could be restored by treating the PILC with a base, such as ammonia, potassium carbonate or alkali solutions (Vaughan et al., 1981 Molinard et al., 1994a, b Cheng and Yang, 1995a Li et al., 1996). 

Other suppliers of SPE aids (e.g. Waters, Alltech) offer similar products. 

The exchange capacity of an ion-exchange resin is the number of ions a given quantity of resin is capable of taking up from solution. It obviously 

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Figure C2.12.1. Origin of ion exchange capacity in zeolites. Since every oxygen atom contributes one negative charge to the tetrahedron incoriDorated in the framework, the silicon tetrahedron carries no net charge while the aluminium tetrahedron carries a net charge of-1 which is compensated by cations M.

AMP-1 4.0 Microcrystalline ammonium molybdo-phosphate with cation exchange capacity of 1.2 mequiv/g. Selectively adsorbs larger alkali metal ions from smaller alkali metal ions, particularly cesium. 

Suspension Polymers. Methacrylate suspension polymers are characterized by thek composition and particle-size distribution. Screen analysis is the most common method for determining particle size. Melt-flow characteristics under various conditions of heat and pressure are important for polymers intended for extmsion or injection molding appHcations. Suspension polymers prepared as ion-exchange resins are characterized by thek ion-exchange capacity, density (apparent and wet), solvent sweUing, moisture holding capacity, porosity, and salt-spHtting characteristics (105). 

Polymers. Quinoline and its derivatives may be added to or incorporated in polymers to introduce ion-exchange properties (see Ion exchange). For example, phenol—formaldehyde polymers have been treated with quinoline, quinaldine, or lepidine (81) (see Phenolic resins). Resins with variable basic exchange capacities have been prepared by treating Amherlites with 2-methylquinoline (82). 

Several physicochemical properties of dietary fiber contribute to its physiological role. Water-holding capacity, ion-exchange capacity, solution viscosity, density, and molecular kiteractions are characteristics determined by the chemical stmcture of the component polysaccharides, thek crystallinity, and surface area. 

Material Shape" of particles Bulk wet density (drained), kg/L Moisture content (drained), % by weight Swelhng due to exchange, % Maximum operating temperature, Operating pH range Exchange capacity ... 

Adequate area soil cover and depth to groundwater usually greater than 1.5 m (4 ft). Slope should not exceed 5 to 8 percent. Soil type, including ion-exchange capacity. 

It has been seen that this resin has also some important advantages over the other resins in the literature like high total ion exchange capacity, easy synthesis, lower cost, simple regeneration. Furthermore, very good sepai ations were obtained using a concentration gradient of elution. In these elutions, very low concentrations of sodium trimetaphosphate were used. As a result, the resin synthesized can be used as an adsorbent for the effective removal of Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. 

Ion-exchange resins swell in water to an extent which depends on the amount of crosslinking in the polymer, so that columns should be prepared from the wet material by adding it as a suspension in water to a tube already partially filled with water. (This also avoids trapping air bubbles.) The exchange capacity of a resin is commonly expressed as mg equiv./mL of wet resin. This quantity is pH-dependent for weak-acid or weak-base resins but is constant at about 0.6-2 for most strong-acid or strong-base types. 

TABLE 15. Cation exchange BEAD FORM Capacity (meq/g) ION-EXCHANGE PACK AGINGS Anion exchange Capacity (meq/g)... 

Films of the copolymers are, as with Nafion, saponified and used for permselective membranes. They have a much higher tensile strength than the Du Pont material and are also claimed to have a higher ion exchange capacity. 

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