Introductory Soils Lab 7 Cation Exchange Capacity NRES 201 Page 1 of 8 LABORATORY 7. CATION EXCHANGE CAPACITY OF SOILS 7.1 GENERAL CONCEPTS. Cation exchange is the reversible, low-energy transfer of ions between solid and liquid phases. Cation exchange affects many soil processes: 1. Weathering of soil minerals. 2. Nutrient absorption by plants. 3. Leaching of electrolytes. 4. Buffering of soil pH. Cation exchange is the result of the neutralization of the negative charge on soil colloids by oppositely charged cations. The cations are held to the colloidal surface by columbic attraction with van der Waal forces and induction forces increasing the strength of bonding of some types of cations. Along with the attraction and subsequent concentration of cations which are oppositely charged to the colloidal surface, is the repulsion of anions which are negatively charged like the colloidal surface. 7.2 CONCEPTUAL MODEL OF THE CATION EXCHANGE PROCESS. Cations are not rigidly held on the colloidal surface, but because of their thermal energies have some degree of motion on and away from the surface, such that a hemisphere of motion is defined for each particular combination of ion and colloidal surface. Consider two cases, the first case, a tightly held and hence, nonexchangeable or fixed cation and the second case, a less tightly held exchangeable cation.Introductory Soils Lab 7 Cation Exchange Capacity NRES 201 Page 2 of 8 What is cation exchange? Cation exchange occurs (Fig. 2a) when ions () in the bulk solution move into the hemisphere of motion of a cation on the surface ( סּ) at a point in time when the exchangeable cation is far from the surface. The ion initially in solution becomes trapped on the surface by the negative charge, and the ion initially on the surface moves into the soil solution. If the surface ion is close to the charged colloidal surface when the solution ion randomly moves into the hemisphere of motion, ion exchange does not occur (Fig. 2b), and the ion returns to the solution. The motion of the ions in the bulk solution is due to their thermal energies. Many factors affect the distribution of cations between the soil solution and the colloidal surface. An intuitive feel for the factors affecting cation exchange can be developed using the conceptual model. 1. As the concentration of a particular cation in the bulk solution increases, the probability of that type of cation penetrating the hemisphere of motion of a surface cation at a time when the surface cation is at a distance from the surface also increases. Hence, as the concentration of a cation in solution increases, there is a corresponding increase in the amount of that cation on the colloidal surface (exchange site). 2. The effect of ion valence can also be illustrated. As the valence of an exchangeable cation increases, so does the affinity of the cation for the surface resulting in a smaller hemisphere of motion. Hence, as valence increases the exchangeability of the ion decreases and the cation’s concentration on the colloidal surface increases relative to cations of lower valence. Other factors such as the hydrated size of the cation and the density of charge on the colloidal surface also affect the degree of attraction of the cation to the colloidal surface. The hydrated size of the cation determines how close the cation can approach the negativelyIntroductory Soils Lab 7 Cation Exchange Capacity NRES 201 Page 3 of 8 charged surface, while the density of charge on the surface affects the strength of attraction of the cation. Both factors affect the cation’s hemisphere of motion and therefore, its exchangeability. 7.3 MASS ACTION CONCEPT OF CATION EXCHANGE. Cation exchange can be treated as a mass action chemical reaction, whereby cations in the soil solution compete for exchange sites on the soil colloids based on their concentrations in the soil solution. 2Na-clay + Ca2+aq  Ca-clay + 2Na+aq When the concentration of Ca2+aq in the soil solution is increased the reaction is shifted to the right increasing the concentration of calcium on the clay and releasing Na+ ions to the soil solution. If the concentration of Ca2+aq ions in the soil solution is decreased, then the reaction will shift to the left, releasing Ca2+ ions to the soil solution. The above equation illustrates that the ionic compositions of the soil solution and the colloids are connected and cannot be varied independently. When the concentration of a cation in the soil solution is increased by processes such as fertilization or decreased by processes such as nutrient absorption by plants, the concentration of that cation on the exchange sites must also increase or decrease. Analysis of the soil shows that the amount of cations in the soil solution is small compared to the amount of cations held on the colloidal surfaces as exchangeable cations. The actual ratio of total exchangeable cations to total cations in the soil solution depends primarily on the exchange capacity of the soil. With the exception of salt-affected soils, the ionic composition of the soil solution is reasonably constant from soil to soil, whereas the amount of exchangeable cations is a function of the amount and type of clay and the humus (organic colloid) content of the soil. 7.4 CATION EXCHANGE CAPACITIES (CEC) OF CLAY MINERALS. 7.4.1 1:1 minerals. Minerals such as kaolinite have little or no isomorphous substitution. Because of the multiple hydrogen bonding between the layers they are nonswelling. The only source of charge is the pH-dependent charge sites on crystal edges and at surface irregularities. The low negative charge coupled with the nonswelling nature of these clays results in minerals with low exchange capacities. 7.4.2 2:1 minerals. The cation exchange capacity of these minerals is related to the amount of isomorphous substitution, whether the cations balancing the negative charge are tightly or weakly held, and the tendency of the clay mineral to swell. a. Micas. Most if not all the permanent charge is satisfied by “fixed” cations.Introductory Soils Lab 7 Cation Exchange Capacity NRES 201 Page 4 of 8 There is little or no swelling, hence internal surfaces are not available for exchange reactions. Cation exchange capacity (CEC) is primarily due to pH-dependent charge sites on external surfaces. These minerals have low CEC values. b. Illites and vermiculites. If the dominant interlayer