Revising the paradigm for cation exchange selectivity of smectites.
Brian J. Teppen1, Vaneet Aggarwal1, and David M. Miller2. (1) Michigan State University, Department of Crop and Soil Sciences, East Lansing, MI 48824-1325, (2) University of Arkansas, Dept. of Crop, Soil, and Environmental Sciences, 115 Plant Science Building, Fayetteville, AR 72701-1201
We attempt to reframe concepts for the fundamental mechanism controlling cation exchange selectivity. Historically, the conceptual framework for interpreting data has been that "the clay prefers one cation over another." We briefly review the development of mechanistic concepts of cation exchange selectivity, which were the subject of many investigations but have been neglected in the recent literature. We argue that, at least for isovalent cation exchange, changes in system free energy are dominated by changes in the solution rather than changes inside the clay. That is, we recast the conceptual framework for understanding cation exchange selectivity to "the aqueous phase prefers one cation over another." This paradigm shift is supported by molecular dynamic calculations for exchange reactions involving a relatively high charge montmorillonite with one layer of interlayer water undergoing K-Rb-Cs cation exchange. We show agreement between these molecular simulation results and new thermodynamic cycle representations for cation exchange, both showing that the clay phase thermodynamics strongly prefer K+, the unselected cation. Thus the clay does not “select” for Cs+ over K+ in any positive sense and it is 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.