Rates of Ion Exchange on Soils and Soil Constituents

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. 

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 rates of ion exchange on soils range from a few seconds to days, depending on a number of factors. These include type and quantity of inorganic and organic constituents, ion charge and radius, and kinetic methodology (Chapter 3). 

Rates of ion exchange on kaolinite, smectite, and illite are usually quite rapid. Sawhney (1966) found that sorption of cesium on illite and smectite was rapid, while on vermiculite, sorption had not reached an equilibrium even after 500 h (Fig. 5.5). Sparks and Jardine (1984) found that potassium adsorption rates on kaolinite and montmorillonite were rapid, with an apparent equilibrium being reached in 40 and 120 min, respectively. However, the rate of potassium adsorption on vermiculite was very slow. Malcom and Kennedy (1969) studied Ba-K exchange rates on kaolinite, illite, and montmorillonite using a potassium ion-specific electrode to monitor the kinetics. They found 75% of the exchange occurred in 3 s, which represented the response time of the electrode. The rate of Ba-K exchange on vermiculite was characterized by a rapid and slow rate of exchange. 

The above results are related to the structural properties of the clay minerals. In the case of kaolinite, the tetrahedral layers of adjacent clay sheets are held tightly by hydrogen bonds. Therefore, only readily available planar external surface sites exist for exchange. With smectite, the inner peripheral space is not held together by hydrogen bonds, but instead it is able to swell with adequate hydration and thus allow for rapid passage of ions into the interlayer. 

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Quantification and Elucidation of Rate-Limiting Steps 109 Chemical Reaction and Diffusion 112 Rates of Ion Exchange on Soils and Soil Constituents 113 Mineralogical Composition 114 Ion Charge and Radius 116 Binary Cation and Anion Exchange Kinetics 117... 

Rates of Ion Exchange on Soils and Soil Constituents TABLE 5.2 Examples of Ion Exchange with Reaction"... 

The above studies have clearly shown that p-jump relaxation measures chemical kinetics and thus one derives the actual rate constants. The implications these types of measurements have for ascertaining mechanisms of ion exchange and of catalytic reactions on soil constituents is tremendous. The application of p-jump relaxation to studying ion exchange kinetics of NH on zeolite (Ikeda et al., 1984b) and the adsorption/ desorption of Pb2+ on y-Al203 (Hachiya et al., 1979) is presented below,... 

Differentiating between CR and mass-transfer processes (PD and FD) is relatively easy since mass transfer is rate-limiting for most exchange processes on soils and soil constituents. Unfortunately, differentiating between FD and PD rate processes is difficult. In many cases, ion exchange is limited by both types of diffusion, particularly when the rate of ion transfer by both types is the same. 

The rates of most ion exchange reactions on soils and soil constituents are FD- and/or PD-controlled. Any factor that lowers the rate of PD without causing a decrease in the FD rate will favor PD control. Any factor that increases the FD rate will also favor control by PD. Helfferich (1966) proposed the following criteria for determining whether PD or FD is rate-limiting. 

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