Homovalent cation exchange

Homovalent cation exchange refers to exchanging cations with similar valence. For example, with Ca2+-Mg2+ exchange, the reaction can be expressed as follows  

Based on Reaction 4.33, the cation-exchange selectivity coefficient (ATCa Mg or KG) can be expressed as 

A plot of ExCa versus CRCa will produce a curvilinear line, asymptotically approaching CEC. The pathway of such line from ExCa = 0 to ExCa = CEC depends on ATCa Mg and CEC (see the section entitled Relationship Between (CR, and ExCa). 

An important component of homovalent exchange is the magnitude of the exchange selectivity coefficient. Commonly, homovalent cation-exchange reactions in soils or soil minerals exhibit a selectivity coefficient somewhere around 1 (Table 4.2). This value signifies that the soil mineral surface does not show any particular adsorption preference for either of the two cations. However, for a mineral where the A Ca Mg is 

TABLE 4.2. Thermodynamic Equilibrium Constants of Exchange (ATCJ and Standard Enthalpy of Exchange (AH°e ) Values for Binary Exchange Processes on 

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For homovalent cation exchange, a = b and Eq. 5.14 reduces to the simple expression... 

Follow the approach in Section 4.2 to derive a rate law for a homovalent cation exchange reaction described by Eq. 5.6 under the assumption that the kinetics are surface controlled. Compare your result with the Bunzl rate law in Eq. 5.54. (Hint Show that Eq. 4.26 generalizes to the rate law ... 

The limits of Equations B and C at zero and infinity imply that a plot of Equation B in terms of CRCa versus ExCa (with ExCa being the dependent variable) at constant CEC and ACa Mg will give a curvilinear line approaching the CEC asymptotically (Fig. 4A). A plot of CRCa versus ExCa/CEC would also produce a curvilinear line asymptotically approaching 1. The pathway from ExCa/CEC = 0 to ExCa/CEC = 1 depends only on Ca Mgor (Fig 4B). Similar conclusions apply to all other homovalent cation-exchange reactions (e.g., K+ - NH4 and K+ - Na+). 

The generalization of Eqs. 4.96 and 5.24 to include the possibility of imbibed water in an exchanger (thus making it a three-component mixture) is described in Chap. 5 of G. Sposito, The Thermodynamics of Soil Solutions, Clarendon Press, Oxford, 1981. The presence of charge fractions in Eq. 5.25 instead of mole fractions, as in Eq. 4.11, derives from the possible inequality of the stoichiometric coefficients, a and b, in the cation exchange reaction (cf. Eqs. 4.6 and 5.9). For homovalent exchange reactions, only mole fractions appear in the expressions for the adsorbate species activity coefficients. 

Cation exchange in soils or clay minerals involves replacement of a given cation on a given mineral surface by another cation. Exchange equations are commonly used to evaluate ion availability to plant roots and/or release of metals to soil water (e.g., heavy metals to groundwater or surface water). There are two major types of cation-exchange reactions in soil systems—homovalent and heterovalent cation exchange. 

TABLE 4.3. Cation Exchange Selectivity Coefficients for Homovalent (K-Na) and Heterovalent(K-Ca) Exchange... 

Carbonic acid, 31 Carboxylic acids, 137 Cation exchange, 102,103,140,149, 513 Homovalent exchange, 191 Heterovalent exchange, 196 Thermodynamics of exchange,... 

The transformation of the equilibrium (or selectivity) constants and the ion-exchange isotherms can easily be made only for homovalent ion exchange because the ion-exchange isotherms usually do not take into consideration the heterovalent character of the ion exchange. This causes additional serious problems in the evaluation of isotherm parameters. It is shown for the exchange of monovalent and divalent cations that... 

Rather small selectivity differences are observed for homovalent-and heterovalent exchanges involving alkali, alkaline earth, bivalent transition metal ions, aluminium and rare earth cations, as is amply evidenced from the extensive compilation by Bruggenwert and Kamphorst (16). This compilation includes various clay minerals illite, montmorillonite, vermiculite and kaolinlte. 

It has been found that for binary exchange, a mass action expression in which the ratio of sorbed cation mole fractions is raised to the nth power (n 1), can fit a wider range of empirical sorption data than when n = 1. Thus, for the homovalent exchange reaction. 

Some mineral structures allow many chemical substitutions, or cation/anion exchanges, such as Mg replaced by Fe, Mn, Fe, or Af and replaced by OH", F , or Cl". Where the valency is the same (homovalent or isovalent exchange, as opposed to heterovalent), this is known as isomorphic substitution isomorphism for the phenomenon, but this term is also used in a different way concerning crystal faces isotypism or isostructuralism are sometimes used and the last is, by far, the most appropriate). 

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