Diffusion ion exchange

The second case is where the reaction is faster than diffusion but is binding ions on whose diffusion ion exchange depends. This binding inhibits the diffusion of the ions and lowers the rate of exchange (Schwarz et al., 1964). The rate is thus controlled by slow diffusion, which is affected by the equilibrium of the fast reaction. Since the process is diffusion-controlled, the exchange rate is dependent on particle size. This type of ion exchange can be referred to as reaction-retarded diffusion and is much more likely to happen than is reaction control. In fact, Helfferich (1983) notes that no case of genuine reaction control has been definitively shown. 

The main factors governing the turnover of nitrogen at the sediment-water interface are the total amount of proteinaceous matter, diffusion, ion exchange with clay minerals, and bioturbation. 

With the emergence of a mineral, processes of its dissolution and formation run on its smface. The mechanisms of these processes include similar elemental reactions, which nm in opposite directions. Both include diffusion, ion exchange, adsorption and desorption and chemical reactions in the Helmholtz layer. Both are accompanied by absorption or release of heat. As a result, the solution s temperature changes. That is why, despite a guarantee of their mechanisms total identity, in modeling at the level of elemental reactions is acceptable and the principle of microsccopic reversibility of reactions introduced in 1924 by Richard Chace Tolman (188-1948) is used. It is assumed under this principle that the processes of dissolution and minerogenesis run through a series of the same elemental reactions (in trail) but in the opposite directions and maybe described by one common equation ... 

Diffusion of atoms, molecules, and ions control many processes in glasses, including ionic diffusion, ion exchange, electrical conduction, chemical durability, gas permeation, and permeation-controlled reactions. Since the mechanisms underlying all of these processes are based on similar principles, a fundamental understanding of diffusion phenomena serves as the basis for understanding all diffusion-controlled properties of glasses. 

The radionuclides incorporated in the oxide layer are in part released again to the coolant in a manner very similar to the release of corrosion products from corroding stainless steel. The exact mechanism of release is not certain, but it may be a combination of dissolution, diffusion, ion exchange and desorption or spalling of smaller oxide particles. The overall time constants for the Co activity release from the inner and the outer layer oxides were empirically determined from a number of reactors to be about 2 10 d and 8.6 10 d, respectively (Lin, 1990). 

Glueckauf, E. Derived first comprehensive equation for the relationship between KEPT and particle size, particle diffusion, and film diffusion ion exchange... 

The rates of ion exchange are generally determined by diffusion processes the ratedetermining step may either be that of diffusion across a boundary film of solution or... 

The wastes from uranium and plutonium processing of the reactor fuel usually contain the neptunium. Precipitation, solvent extraction, ion exchange, and volatihty procedures (see Diffusion separation methods) can be used to isolate and purify the neptunium. 

Activated diffusion of the adsorbate is of interest in many cases. As the size of the diffusing molecule approaches that of the zeohte channels, the interaction energy becomes increasingly important. If the aperture is small relative to the molecular size, then the repulsive interaction is dominant and the diffusing species needs a specific activation energy to pass through the aperture. Similar shape-selective effects are shown in both catalysis and ion exchange, two important appHcations of these materials (21). 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

There are other methods of preparation that iavolve estabhshing an active phase on a support phase, such as ion exchange, chemical reactions, vapor deposition, and diffusion coating (26). For example, of the two primary types of propylene polymerization catalysts containing titanium supported on a magnesium haUde, one is manufactured usiag wet-chemical methods (27) and the other is manufactured by ball milling the components (28). 

Graham-Uranoff They studied multicomponent diffusion of electrolytes in ion exchangers. They found that the Stefan-Maxwell interaction coefficients reduce to limiting ion tracer diffusivities of each ion. 

Diffusion in porous solids is usually the most important factor con-troUing mass transfer in adsorption, ion exchange, drying, heterogeneous catalysis, leaching, and many other applications. Some of the... 

In ion-exchange resins, diffusion is further complicated by electrical coupling effec ts. In a system with M counterions, diffusion rates are described by the Nernst-Planck equations (Helfferich, gen. refs.). Assuming complete Donnan exclusion, these equations canbe written... 

Combined Pore and Solid Diffusion In porous adsorbents and ion-exchange resins, intraparticle transport can occur with pore and solid diffusion in parallel. The dominant transport process is the faster one, and this depends on the relative diffusivities and concentrations in the pore fluid and in the adsorbed phase. Often, equilibrium between the pore fluid and the solid phase can be assumed to exist locally at each point within a particle. In this case, the mass-transfer flux is expressed by ... 

TABLE 16-8 Self Diffusion Coefficients in Polystyrene-divinylbenzene Ion Exchangers... 

In binary ion-exchange, intraparticle mass transfer is described by Eq. (16-75) and is dependent on the ionic self diffusivities of the exchanging counterions. A numerical solution of the corresponding conseiwation equation for spherical particles with an infinite fluid volume is given by Helfferich and Plesset [J. Chem. Phy.s., 66, 28, 418... 

Dijfusion Dialy The propensity of and OH" to penetrate membranes is useful in diffusion dialysis. An anion-exchange membrane will block the passage of metal cations while passing hydrogen ions. This process uses special ion-exchange membranes, but does not employ an applied electric current. 

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