Heterovalent Cation Exchange

Based on Reaction 4.38, the Gapon exchange selectivity coefficient (AfG) can be expressed as 

A plot of SAR versus ExNa (see Eq- 4.40) will produce a curvilinear line, asymptotically approaching CEC (see section entitled Relationship Between SAR and ExNa). 

Theoretically, a plot of S AR versus ExNa/ExCa1/2 or exchangeable sodium ratio (ESR) will produce a straight line with slope equal to KQ. The average magnitude of KG for soils of the arid west is approximately 0.015 (mmol L 1) 1/2. However, the experimental Kg appears to be dependent on pH, salt concentration, and clay mineralogy. Furthermore, the experimental KG does not appear to be constant across the various sodium loads. Commonly, as sodium load increases, KG also increases. Furthermore, as pH increases, KG decreases (Fig. 4.26). 

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H. Laudelout, R. van Bladel, G. H. Bolt, and A. L. Page, Thermodynamics of heterovalent cation exchange reactions in a montmorillonite clay, Trans. Faraday Soc. 64 1477 (1968). 

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. 

An equation that is most commonly used to describe heterovalent cation exchange, such as Na+-Ca2+ exchange, is the Gapon exchange equation. For example, for the Na+-Ca2+ system,... 

Equation 4.54 shows that even if Ky is constant across the entire exchange isotherm, is exchangeable Na-load dependent. By taking the limit of Equation 4.57 at the Na of 0.60, it can be shown that Ky - KG, and when the monovalent cation approaches an equivalent fraction of 1, KG = °° (Figs. 4.30 and 4.31). In summary, similar conclusions would be reached on the behavior of a heterovalent cation exchange up to an equivalent monovalent fraction load of 0.20 employing Ky or KG. 

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,... 

Owing to the almost complete absence of heterovalent isomorphic substitution and low amounts of silanol groups, silica minerals have a very low adsorption capacity, either by cation exchange or by specific adsorption. However, cation-exchange capacity increases somewhat at high pH values. In general, these minerals have a small contribution to soil-ion binding. 

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. 

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... 

The CEC (in meq/kg) of a rock is most likely constant, whereas TEC (in mmol/kg) of a heterovalent system varies with the relative amount of cations with different charges that neutralize the constant CEC. In most situations the activities of exchangeable cations are therefore calculated more conveniently as exchangeable fractions with respect to a fixed CEC. 

Because here we have univalent cations, equivalent and molar fractions are the same, but for heterovalent exchange, both fraction types can be found (see below). Of course both plots are mirror images around the diagonal, because = 1. The... 

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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