In Section 3.4, the cation exchange capacity, or CEC, of particulate soil humus is defined as the maximum number of moles of proton charge per kilogram that can be desorbed by a metal cation under prescribed conditions. Thus, CEC for particulate humus is equal to the maximum absolute value of the negative net proton charge. Operationally, this maximum value is measured typically as the surface excess of Ba2+ adsorbed by humus at pH 8.2 (Eq. 3.5). Extending this concept to soils, one can define the CEC as the maximum number of moles of readily exchangeablemetal cation charge per unit mass of dry soil that can be extracted under prescribed conditions. In this more general context, CEC refers to metal cations that adsorb on soil particles in either outer sphere surface complexes or the diffuse ion swarm (Fig. 7.2). In alkaline soils, the common readily exchangeable cations are Ca2+, Mg2+, Na+, and K+, whereas in acidic soils, this group expands to include Al3+, and its complexes AlOH2+, Al(OH)2+, and AlSO+4. Following the operational paradigm for soil humus, one concludes that the measurement of soil CEC involves not only the desorption of protons, but also the replacement of the population of readily exchangeable adsorbed metal cations at a selected pH value (usually pH 7–8) by a chosen cation. Laboratory procedures for measuring CEC are described in Methods of Soil Analysis, listed in For Further Reading at the end of this chapter. In alkaline soils, the replacing cation chosen is often Na+ or Ca2+, whereas in acidic soils and for soil humus, the replacing cation of choice is Ba2+. These cations, in turn, are typically displaced from soil particle surfaces by Mg2+ to measure the surface excess. A conceptual definition of CEC can be developed in terms of the surface charge balance concepts introduced in Chapter 7. Consider first a soil in which a net positive surface excess of anions does not occur, such as the Mollisol example discussed in Section 8.1. In this case, the only adsorbed ions are Ca2+ and Cl-. The CEC of this soil may be defined by a special case of the charge-balance condition in Eq. 7.3a: ∆qex (max) ≡ CEC
Keywords: Barium exchange method, Vanselow model, cation exchange capacity, diffuse double layer, exchange selectivity, ideal solution, mole fraction, net proton charge, phenolic hydroxyl, selectivity sequence
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