Heterovalent exchange, clay

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. 

Eriksson applied Donnan equilibrium calculations to heterovalent exchange, reasoning that clays in a salt solution could be thought of as an ion species restricted from free diffusion. His equation was... 

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. 

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