Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays
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- 作者：Luis Ruiz PestanaKedarnath Kolluri ; Teresa Head-Gordon ; Laura Nielsen Lammers
- 刊名：Environmental Science & Technology
- 出版年：2017
- 出版时间：January 3, 2017
- 年：2017
- 卷：51
- 期：1
- 页码：393-400
- 全文大小：495K
- ISSN：1520-5851
文摘
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.