Hydration energy determines isovalent cation exchange selectivity by clay minerals
2006
Teppen, B.J. | Miller, D.M.
Cation exchange is one of the most venerable concepts in soil science, yet it needs rethinking. This paper presents an extremely simple conceptual framework for interpreting many observed trends in cation exchange. Taking the example of Cs-K exchange, the methods of computational molecular mechanics found that Cs-montmorillonite is considerably higher in energy than K-montmorillonite at constant water content, in agreement with inferences from a new thermodynamic cycle representation of cation exchange. Since montmorillonite selects Cs(+) over K(+) in real experiments, these results mean that alkali cation-exchange selectivity is controlled by selectivity of the solution phase for the more strongly hydrated cation. Thus the clay does not "select" for Cs(+) over K(+) in any positive sense and it may be more useful to consider cation exchange as a partitioning reaction: Given two cations of equal valence, the more weakly hydrated will tend to partition into the "subaqueous" smectite interlayer phase. This concept seems not only parsimonious, but also more accurate than other hypotheses for cation exchange selectivity that impute more favorable interactions between smectite surfaces and the selected cations; such theories err by ignoring energy changes in the solution phase. This simple partitioning concept rationally explains the alkali and alkaline earth selectivity sequences as well as the selectivities of smectites for organic cations over inorganic, for larger organic cations over smaller, and for organometallic complexes over the uncomplexed metal.
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