Particles, ion exchange

In both cation and anion exchangers, the acid or base groups are chemically bonded to the resin matrix, and the resins have a high concentration of fixed negative or positive ions. These are balanced by mobile counterions such as Na+, H+, or Ca2+ for cation resins and CL, OH", or N03 for anion resins, so that electrical neutrality is always maintained in the resin particles. Ion exchange takes place when the activity of ions in the external solution differs from that of the mobile ions in the solid phase. For example, exposing a hydrogen-form resin HR to a solution with Na+ and H+ will result in diffusion of some Na+ ions into the resin and diffusion of some H+ ions into the solution. 

Figure 3.1 Charged surface of latex particle ion-exchange from Na" " to H" " form...

Experimental studies by Fiiredi and Valentine at various frequencies involved the bunching and orientation of microscopic particles made of polystyrene spheres, aluminum or carbon particles, ion exchange resin beads, ferric oxide or potato starch particles in water, saline solutions, or in castor oils. Orientation effects appeared to be absent in the salt solutions, but to be accentuated in castor oil, relative to that in water especially at high frequencies. GrifBn and Stowell confirmed and extended the studies of Teixera-Pinta et on the bunching and orientational effects of... 

Mass transfer in a fluid flowing around solid particles, e.g., catalyst particles, ion exchange resins, particles of a dissolving solid. 

Other methods to confer specificity include ion exchange and the deposition of metallic islands and semiconductor particles. Ion exchange occurs spontaneously between counteranions or countercations (in the case of macromolecular anions) and ions that bathe the CEP membrane. Repeated redox cycling encourages the exchange of mobile ions within the film with ions drawn from the solution in which the film is bathed. This becomes a convenient means to introduce catalytic ions into CEPs to confer a measure of specificity [29,126,183]. Another approach used has been to deposit islands of catalytic metal onto the surface or within the electroconductive polymer membrane [187-190], Wrighton et al. [11] deposited platinum particles. Semiconductor particles have also been used [174]. 

Liquid porous ion exchange resin particles ion exchange chromatography (LSC) ion exchange chromatography... 

After preparation, colloidal suspensions usually need to undergo purification procedures before detailed studies can be carried out. A common technique for charged particles (typically in aqueous suspension) is dialysis, to deal witli ionic impurities and small solutes. More extensive deionization can be achieved using ion exchange resins. 

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

However, the quantity of Pa produced in this manner is much less than the amount (more than 100 g) that has been isolated from the natural source. The methods for the recovery of protactinium include coprecipitation, solvent extraction, ion exchange, and volatility procedures. AH of these, however, are rendered difficult by the extreme tendency of protactinium(V) to form polymeric coUoidal particles composed of ionic species. These caimot be removed from aqueous media by solvent extraction losses may occur by adsorption to containers and protactinium may be adsorbed by any precipitate present. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

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). 

The 2eohtes are prepared as essentially bindedess preformed particles. The kaolin is shaped in the desired form of the finished product and is converted in situ in the pellet by treatment with suitable alkaU hydroxide solutions. Preformed pellets of 2eohte A are prepared by this method. These pellets may be converted by ion exchange to other forms such as molecular sieve Type 5A (1). ZeoHtes of higher Si02/Al202 ratios, eg, 2eohte Y, can be obtained by the same method, when sodium metasiUcate is incorporated in the preshaped pellets, or when acid-leached metakaolin is used. 

Solutions of monosilicic acid may also be obtained by carehil hydrolysis of tetrahalo-, tetraalkoxy-, or tetraacyloxysilanes by electrolysis or acidification of alkah sihcate solutions or by ion exchange (qv). By operating under carefully controlled conditions at low temperature and pH, solutions may be obtained that remain supersaturated with respect to amorphous sihca for hours at temperatures near 0°C. Eventually, however, polymerization reactions involving the formation of siloxane linkages occur, leading ultimately to the formation of coUoidal particles and further aggregation or gel... 

The dissolution of soluble sihcates is of considerable commercial importance. Its rate depends on the glass ratio, sohds concentration, temperature, pressure, and glass particle size. Commercially, glasses are dissolved in either batch atmospheric or pressure dissolvers or continuous atmospheric processes. Dissolution of sodium sihcate glass proceeds through a two-step mechanism that involves ion exchange (qv) and network breakdown (18). 

The gels precipitated as described above are not useful in ion-exchange systems because their fine size impedes fluid flow and allows particulate entrainment. Controlled larger-sized particles of zirconium phosphate are obtained by first producing the desired particle size zirconium hydrous oxide by sol—gel techniques or by controlled precipitation of zirconium basic sulfate. These active, very slightly soluble compounds are then slurried in phosphoric acid to produce zirconium bis (monohydrogen phosphate) and subsequently sodium zirconium hydrogen phosphate pentahydrate with the desired hydrauhc characteristics (213,214). 

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