Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays

Journal Article · Wed Dec 14 04:00:00 EST 2016 · Environmental Science and Technology

DOI:https://doi.org/10.1021/acs.est.6b04747· OSTI ID:1480084

Ruiz Pestana, Luis ^[1]; Kolluri, Kedarnath ^[1]; Head-Gordon, Teresa ^[1]; Lammers, Laura Nielsen ^[1]

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1. 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.

• Research Organization: Lawrence Berkeley National Laboratory-National Energy Research Scientific Computing Center
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 1480084
• Journal Information: Environmental Science and Technology, Journal Name: Environmental Science and Technology Journal Issue: 1 Vol. 51; ISSN 0013-936X
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English