Ion-exchange rates

Fig. 3. Plot of logio normalized ion-exchange rate at amorphous silica saturation vs. the amount of excess alkalis (Na, K), denoted by the molar ratio XAlk/(Al + IVB + FeT). All boron is treated as four-fold coordinated (IVB) and total iron (FeT) is regarded as ferric. The ion-exchange rate subtracts out the contribution of alkalis to solution from matrix dissolution. As the amount of excess alkali increases, the ion-exchange rate increases. This increase in rate reflects the increasing amount of alkalis in non-bridging oxygen (NBO) configurations. Error bars represent 2-

In the case of amorphous intermediate 4, the weakly acidic cation exchange resin DIAION WKIOO was selected as the CEC stationary phase. The resin comprises a methacrylate resin matrix and carboxylic acid, which functions as an exchange group with a high ion exchange rate. Figure 9.4 illustrates the concept behind the CEC procedure for 4 ... 

Jeffers, P.M. and N.L. Wolfe. 1996a. Hydrolysis and chloride ion exchange rate of methyl bromide in sea water. Geophys. Res. Lett. 23 1773-1776. 

Kelley (1948) published his beautiful book Cation Exchange in Soils and astutely and accurately hypothesized that while ion exchange rates... 

The first detailed study on ion exchange rates, and particularly mechanisms, appeared in the very definitive and elegant studies of Boyd et al. (1947) with zeolites. Working in conjunction with the Manhattan Project, these researchers clearly showed that ion exchange is diffusion-controlled, and that the reaction rate is limited by mass-transfer phenomena that are either film (FD) or particle (PD) diffusion-controlled. Boyd et al. (1947) were also the first to derive rate laws for FD, PD, and CR. Additionally, they demonstrated that particle size had no effect on reaction control, that in FD the rate was inversely proportional to particle size, and that the PD rate was inversely proportional to the square of the particle size. 

There are two ways in which a chemical reaction can affect ion exchange rates (Helfferich, 1983). One possibility is that the reaction is slow compared with diffusion. Thus, in the limit, diffusion is fast enough to cause a leveling out of any concentration gradients within the ion exchanger particle. Thus, the reaction is the sole rate-controlling factor, and rate is independent of particle size. 

Figure 1. Dependence of durability of Cu ZSM5 during HC-SCR on Cu ion exchange rate. Reaction condition NO 150 ppm, CO 500 ppm, H2 250 ppm, 02...

Schafer, G.W., Kim, H.J., and Aldinger, R, Protonated P"-aluminas, correlation of ion-exchange rates, chemical composition and resulting lattice constants, Solid State Ionics, 97, 285-289 (1997). 

The resin beads used in most columnar operations range in size from 0.3 to 0.9 mm in diameter, which is a compromise based on the effect of ion-exchange rates, capacities, and hydraulic characteristics. The especially made resins used in resin-in-pulp operations range in size from 0.8 to 1.6 mm in diameter. The apparent density of a resin is defined as that weight of backwashed and settled wet resin per cubic foot, which for resins used in the uranium industry is about 38-45 Ib/ft . In column operations, the attrition losses due to swelling and contraction of resin, abrasion of resin-resin surfaces, and abrasion of resin-equipment surfaces are negligible. In resin-in-pulp operations, an appreciable amount of attrition loss is encountered. 

Ion-Exchange Rate and Transient Concentration Profiles The numerically implicit finite difference method was used to solve the set of nonlinear differential equations (8) for a wide range of model parameters such as diffusivities, Dg, D,, and Dy, dissociation constants. Kg, exchanger capacity, ag, and bulk concentration, Cg, of the solution. 

Secondly, unimolecular first order kinetics given by equation 6.9 is found to fit ion exchange rate data generated under film diffusion control. 

Figure 6.2 Interpretation of ion exchange rate measurements according to first order chemical kinetics (equation 6.9)...

Figure 6.3 Interpretation of ion exchange rate data according to film diffusion theory (equation 6.21) for sodium-hydrogen exchange on a styrenesulfonate resin (12% DVB)...

Modern opinion views the Nernst-Plank theory as a special case of applying the thermodynamics of irreversible processes to ion exchange. It may also be argued theoretically and experimentally that the observed characteristics of ion exchange rate behaviour can only be fully explained by including chemical reaction as a flux-coupling mechanism as well as the diffusion potential. From a research standpoint it is most probable that future theoretical advances in ion exchange kinetics will result from the further application of non-equilibrium thermodynamics. 

Usually, the ion exchange rate between a solvated metal ion and the ion fixed in the complex compoimd is very fast. This is the reason why most of the cation complexes with cyclic polyethers have high exchange rates which are in the order of 10 s >20) Although the exchange rate for metal ions with a bicyclic cryptand is significantly lower than with monocyclic crown ethers the kinetic isotopic... 

Table 4-2. Dependence of ion-exchange rates on various experimental parameters.

Table 4-2 shows, as an example, a summary of the effects of experimental variables on the ion-exchange rate controlled by intraparticle diffusion, and by liquid-phase mass transfer. Further details and special situations will become apparent in the discussion of rate laws to follow. 

Fig. 6. Logarithmic correlation of ion exchange rates in fixed-bed columns. Courtesy of A. I. Ch. E. Journal (H4).

Number of Reaction Units, Column-Capacity Parameter, or Bed-Thickness Modulus, Nb, or s. The general treatment of ion-exchange rates is based upon a surface reaction-kinetic driving force which approximates either an external or an internal material-transfer driving force. By... 

Table 5. Dependence of Ion Exchange Rates on Experimental Conditions ...

Ion exchange phenomena are stoichiometric, i.e. 37 mg of potassium are exchanged by 23 mg of sodium and 40 mg of calcium by 46 mg of sodium. The ion exchange rate depends on the type of exchanger grain size, porosity and distensibility. 

The reactivity of a liquid-solid-liquid triphase reaction (i.e., polymer-supported catalytic reaction) is influenced by the structure of the active sites, particle size, degree of cross-linkage, degree of ring substitution, swollen volume, and spacer chain of a catalyst pellet. In the past, the characteristics of a triphase reaction, subjected to the mass transfer limitation of the reactants and ion-exchange rate in the aqueous phase, have been discussed [146,158,162,178,179]. The ion-exchange rate in the aqueous phase affects the reactivity of the triphase reaction. 

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