Anionic clay

Lazaridis, N.K., Hourzemanoglou, A. and Matis, K.A. (2002) Flotation of metal-loaded clay anion exchangers. Part II the case of arsenates. Chemosphere, 47(3), 319-24. 

We have followed the alternative route through equations (2) and (3) to emphasize that at equilibrium, there will also be colons (with cation exchangers such as clays, anions) "invading" the solid phase. 

As proposed in Figure 3.19, ion-exchange reactions might result in the formation of clay anions and metallocene cations (soft ions), and sodium and chloride ions (hard ions). The ion exchange of metallocenium cations with clay surfaces is also discussed by Mariott et al. [83, 84]. 

Similar to the natural cationic clays, anionic clays, namely hydrotalcites (Fig. 7.3), are also present in nature. They are based on positively charged bmcite-like layers containing Mg and A1 hydroxides, described by the general formula [Mg6Al2(OH)6] the electroneutrality of the material as a whole is guaranteed by included anions, typically CO3 in the natural rocks, localized in the interlayer region. 

During the mid-1990s, a research group at Mitsubishi discovered that certain clays could be calcined and used to activate metallocenes [104]. McDaniel et al. [104] proposed that the metallocenes may be activated by the reaction between the clay cations and the metallocene, resulting in the formation clay anions and metallocene cations and sodium chloride. In this way a neutral metallocene can be activated, forming ion-pair active species such as metallocene cation and clay anion. A schematic representation of activation of metallocene by clay is shown in Fig. 13. The ion exchange of metallocenium cations with clay surfaces is also... 

Thermosetting nanocomposites exhibit a reduced rate of heat release compared to neat polymer. However, the approach to nanocomposites itself is not sufficient to comply with the actual fire test standards. For this reason, traditional flame retardants are currently used in combination with nanofillers, and researchers are focusing on the individuation of synergistic systems. As an alternative to the most common cationic clays, anionic clays show improved performance in terms of flame retardancy. Epoxy nanocomposites based on anionic clay exhibit unique self-extinguishing behavior in a UL-94 horizontal burning test never observed before in a pure nanocomposite. The formation of a continnous intu-mescent ceramic layer on the surface of a polymer during combustion reduces the heat release rate to a higher extent than do montmorillonite nanocomposites. 

Anionic and nonionic polyacrylamides effectively remove suspended soHds such as silt and clay from potable water. SuppHers provide special grades which meet EPA/FDA regulations for residual acrylamides. A recent pubHcation (102) states that hydrolyzed polyacrylamides with narrow interchain charge distributions provide better performance in flocculation of clay. These polymers were prepared by alkaline hydrolysis. (See Flocculating agents.)... 

The inorganic reductions of NaBH are numerous and varied (Table 7). Comparatively few anions are reduced, yet the reduction of bisulfite to dithionite (hydrosulfite) (25), which is used in the pulp (qv) and paper (qv), clay (see Clays), and vat dyeing industries, is an important inorganic appHcation ofNaBH,. 

The most widely used pitch control method is the addition of pitch dispersants, which can be either organic, ie, typically anionic polymers such as naphthalene sulfonates, ligninsulfonates, and polyacrylates (33,34), or inorganic, ie, typically clay or talc. The polymers maintain the pitch as a fine dispersion in the pulp, preventing agglomeration and potential deposition on the paper machine or the sheet. When talc, clay, or other adsorbent fillers are added to the furnish, moderate amounts of pitch can adsorb on these materials, producing a nontacky soHd that can be retained in the sheet. 

The method of action of the polymers is thought to be encapsulation of drill cuttings and exposed shales on the borehole wall by the nonionic materials, and selective adsorption of anionic polymers on positively charged sites of exposed clays which limits the extent of possible swelling. The latter method appears to be tme particularly for certain anionic polymers because of the low concentrations that can be used to achieve shale protection (8). 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

With the help of equiUbrium constants, the extent of adsorption can be predicted as a function of pH and solution variables (7,25,43). Based on this model, the partitioning of metal ions and of ligands (organic and inorganic anions between water and pelagic clays and suspended particles) can be explained. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Cationic polymerization of cyclosiloxanes is well known but used much less frequently than anionic reactions. The most widely used catalysts include sulfuric acid and its derivatives, alkyl and aryl sulfonic acids and trifluoroacetic acid1 2,1221. Due to their ease of removal, in industrial applications acid catalysts are generally employed on supports such as bentonite clay or Fuller s earth. 

Two classes of clays are known [3] (i) cationic clays (or clay minerals) that have negatively charged alumino-silicate layers balanced by small cations in the interlayer space (e.g. K-10 montmorillonite) and (ii) anionic clays which have positively charged brucite-type metal hydroxide layers balanced by anions and water molecules located interstitially (e.g. hydrotalcite, Mg6Al2(0H)igC034H20. 

Vaccari A. Clays and Catalysis a Promising Future Appl. Clay Sci. 1999 74161-198 Keywords cationic and anionic clays, catalysis... 

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