title: Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays.
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.
authors.affiliation: 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.
authors.forename: Luis
authors.identifier: 0000-0001-8028-5242
authors.initials: L
authors.lastname: Ruiz Pestana
authors.affiliation: 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.
authors.forename: Kedarnath
authors.initials: K
authors.lastname: Kolluri
authors.affiliation: 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.
authors.forename: Teresa
authors.initials: T
authors.lastname: Head-Gordon
authors.affiliation: 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.
authors.forename: Laura Nielsen
authors.initials: LN
authors.lastname: Lammers
mesh_terms: D000327:Adsorption; D000538:Aluminum Silicates / Q000737:chemistry*; D002412:Cations; D000077215:Clay
chemical_list: D000538:Aluminum Silicates; D002412:Cations; D000077215:Clay