Ion Exchange Kinetics

When an exchanger is in contact with a solution of exchanging ions the rate of exchange can be rate controlled by one of three steps:

1. film diffusion - controlled by the rate of progress of an ion through a film of water molecules, which by virtue of the surface charge on the exchanger can be regarded as 'stagnant' (the Nernst layer);

2. particle diffusion - controlled by the progress of ions inside the exchanger;

3. chemical reaction - controlled by bond formation. Examples of this process are not simple to define but the most often cited case is when chelating ionogenic groups, present in an ion exchange organic resin, are able to form strong bonds with, say, a transition metal ion to create a very specific extractant.

The three possible steps are illustrated in Figure 2.

Distinction between film and particle control can be made from the following criteria.

• Film diffusion is affected by the speed of stirring in a batch exchange (or the rate of passage of liquid through a column of exchanger). The rate of diffusion will directly depend upon the total concentration in the external solution.

• Particle diffusion has a rate that is dependent on the particle size, and is independent of both stirring speed and external solution concentration.

Kressman has devised a simple interruption test to distinguish between Tlm and particle control. The exchange being studied is interrupted for a short

Figure 2 Possible rate-determining steps in an ion exchange process. Step I, diffusion of ions through a surface film. Step II, diffusion through the solid exchanger. Step III, formation of chelate bond at the ionogenic group.

period of time by separating the liquid and solid phases. The phases are then recombined to recommence the exchange. Provided that the exchange is remote from equilibrium at the time of interruption, diagnostic rate profiles will ensue. The Tlm-driven process will have an undisturbed profile, whereas the particle-driven step will have attained a partial equilibrium even in the absence of an external driving force. The different profiles observed when fractional attainments of equilibrium with time are plotted are illustrated in Figure 3.

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