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Environ Sci Technol. 2017 Jan 3;51(1):393-400. doi: 10.1021/acs.est.6b04747. Epub 2016 Dec 20.

Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays.

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Chemical Sciences Division and ‡Earth and Environmental Science Area, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
Departments of Chemistry, Bioengineering, Chemical and Biomolecular Engineering, and ∥Department of Environmental Science, Policy, and Management, University of California , Berkeley, California 94720, United States.


The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K+ and has high affinity for Cs+. Although exchange of interlayer K+ for Cs+ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs+-K+ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs+ for the smaller K+ significantly lowers the migration barrier of neighboring K+, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs+ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

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