Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays

Luis Ruiz Pestana, Kedarnath Kolluri, Teresa Head-Gordon, Laura Nielsen Lammers

Research output: Contribution to journalArticlepeer-review

24 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)393-400
Number of pages8
JournalEnvironmental Science and Technology
Volume51
Issue number1
DOIs
StatePublished - Jan 3 2017
Externally publishedYes

ASJC Scopus subject areas

  • Chemistry(all)
  • Environmental Chemistry

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