Molecular Simulation of Cesium Adsorption at the Basal Surface of Phyllosilicate Minerals

Journal Article · Tue Aug 16 04:00:00 EDT 2016 · Clays and Clay Minerals

OSTI ID:1347867

Kerisit, Sebastien N. ^[1]; Okumura, Masahiko ^[2]; Rosso, Kevin M. ^[1]; Machida, Masahiko ^[2]

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1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2. Japan Atomic Energy Agency (JAEA), Chiba (Japan)

A better understanding of the thermodynamics of radioactive cesium uptake at the surfaces of phyllosilicate minerals is needed to understand mechanisms of its selective adsorption and help guide the development of practical and inexpensive decontamination techniques. In this work, molecular dynamics simulations were carried out to determine the thermodynamics of adsorption of Cs⁺ at the basal surface of six 2:1 phyllosilicate minerals, namely pyrophyllite, illite, muscovite, phlogopite, celadonite, and margarite. These minerals were selected to isolate the effects of the magnitude of the permanent layer charge (≤ 2), its location (tetrahedral versus octahedral sheet), and the structure of the octahedral sheet (dioctahedral versus trioctahedral). Good agreement was obtained with experiment in terms of the hydration free energy of Cs⁺ and the structure and thermodynamics of Cs⁺ adsorption at the muscovite basal surface, for which published data were available for comparison. With the exception of pyrophyllite, which did not exhibit an inner-sphere free energy minimum, all phyllosilicate minerals showed similar behavior with respect to Cs⁺ adsorption; notably, Cs⁺ adsorption was predominantly inner-sphere whereas outer-sphere adsorption was very weak with the simulations predicting the formation of an extended outer-sphere complex. For a given location of the layer charge, the free energy of adsorption as an inner-sphere complex was found to vary linearly with the magnitude of the layer charge. For a given location and magnitude of the layer charge, adsorption at phlogopite (trioctahedral sheet structure) was much less favorable than at muscovite (dioctahedral sheet structure) due to the electrostatic repulsion between the adsorbed Cs⁺ and the hydrogen atom of the hydroxyl group directly below the six-membered siloxane ring cavity. For a given magnitude of the layer charge and structure of the octahedral sheet, adsorption at celadonite (layer charge located in the octahedral sheet) was favored over muscovite (layer charge located in the tetrahedral sheet) due to the increased distance with surface potassium ions.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1347867
• Report Number(s): PNNL-SA--113727; 600301020
• Journal Information: Clays and Clay Minerals, Journal Name: Clays and Clay Minerals Journal Issue: 4 Vol. 64; ISSN 1552-8367
• Country of Publication: United States
• Language: English

Related Subjects

(001) surface
58 GEOSCIENCES
CLAYFF
adsorption free energy
cesium
inner-sphere
layer charge
mica
molecular dynamics
outer-sphere
potential of mean force