The CS/K Exchange in Muscovite Interlayers: An Ab Initio Treatment

Abstract

Abstract--Plane-wave pseudopotential total energy calculations have been applied to investigate the structure and energetics of the Cs/K exchange into interlayer sites in muscovite mica. Novel muscovite structures were designed to isolate the effects of 2:1 layer charge, cation size/interlayer site shape, and tetrahedral A1/Si substitutions on the exchange. All atom and cell-parameter optimizations were performed with the intention to mimic the constant pressure, non-isovolumetric exchange conditions thought to be found at frayed-edge sites. Under conditions where the cell parameters are allowed to relax, the overall Cs/K exchange reaction is surprisingly close to isoenergetic. The forward reaction is more strongly favored with increasing layer charge. For the condition of zero layer charge and no interlayer site distortion, the difference in the optimal interlayer spacing for Cs relative to K is very small, indicating a baseline indifference of the muscovite structure to cation size. The presence of 2:1 layer charge or tetrahedral rotations arising from A1/Si substitutions clearly change this outcome. Analysis of the dependence of the interlayer spacing on layer charge shows that while the spacing collapses with increasing layer charge for K as the interlayer cation, the reverse is true for Cs. We attribute the contrasting behavior to inherent differences in the ability of these cations to screen 2:1 layer-layer repulsions. Such effects might be involved during exchange at frayed-edge sites where interlayer spacings are increased. This is known, from experiment, to be very selective for Cs. Overall, the exchange energetics are so low that the Cs/K exchange rate and degree of irreversibility are likely to be dominated by diffusion kinetics.

Mechanism of Strong Affinity of Clay Minerals to Radioactive Cesium: First-Principles Calculation Study for Adsorption of Cesium at Frayed Edge Sites in Muscovite

“…This is consistent with the results of virtual equivalent replacement in the bulk periodic system reported in ref. 25) These findings clearly suggest the importance of frayed structures emerging at surfaces and edges for Cs replacement.…”

“…In particular, density functional theory (DFT) 19,20) and molecular dynamics have been frequently employed for clay minerals. [21][22][23][24][25][26][27] However, no direct work toward clarifying the mechanism of the strong affinity of Cs in FESs has been published yet. In this paper, we employ a first-principles calculation scheme based on DFT and microscopically examine the strong selective affinity of FES to Cs.…”

“…The employed calculation program is the Vienna Ab initio Simulation Package (VASP), 28,29) which supports the projected augmented wave method 30,31) and generalized gradient approximation, 32) which are also employed in this work similar to several previous works on clays. 25,26) The cutoff energy is 1600 eV, and 3×1×1 sampling points are used on kpoint grids. Structural relaxation is repeated until the gradient forces on atoms are less than 2.0 × 10 −2 eV/Å.…”

“…25) This hydration energy is given in "bulk" water. This simplification is useful for clarifying the mechanism of Cs exchange in FES.…”

Quantum-Mechanical Methods for Quantifying Incorporation of Contaminants in Proximal Minerals

“…In the case of clays, incorporation typically occurs via cation exchange, where interlayer cations are replaced when in contact with aqueous solutions. This type of reaction has been modeled in several studies including one by Rosso et al [11]. They simulated the replacement of interlayer K + with Cs + in muscovite,…”