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

Abstract

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 layermore » 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.« less

Authors:
 [1];  [2];  [1];  [2]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  2. Japan Atomic Energy Agency (JAEA), Chiba (Japan)
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1347867
Report Number(s):
PNNL-SA-113727
Journal ID: ISSN 1552-8367; 600301020
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Clays and Clay Minerals; Journal Volume: 64; Journal Issue: 4
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; cesium; mica; (001) surface; layer charge; adsorption free energy; outer-sphere; inner-sphere; molecular dynamics; CLAYFF; potential of mean force

Citation Formats

Kerisit, Sebastien N., Okumura, Masahiko, Rosso, Kevin M., and Machida, Masahiko. Molecular Simulation of Cesium Adsorption at the Basal Surface of Phyllosilicate Minerals. United States: N. p., 2016. Web.
Kerisit, Sebastien N., Okumura, Masahiko, Rosso, Kevin M., & Machida, Masahiko. Molecular Simulation of Cesium Adsorption at the Basal Surface of Phyllosilicate Minerals. United States.
Kerisit, Sebastien N., Okumura, Masahiko, Rosso, Kevin M., and Machida, Masahiko. 2016. "Molecular Simulation of Cesium Adsorption at the Basal Surface of Phyllosilicate Minerals". United States. doi:.
@article{osti_1347867,
title = {Molecular Simulation of Cesium Adsorption at the Basal Surface of Phyllosilicate Minerals},
author = {Kerisit, Sebastien N. and Okumura, Masahiko and Rosso, Kevin M. and Machida, Masahiko},
abstractNote = {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.},
doi = {},
journal = {Clays and Clay Minerals},
number = 4,
volume = 64,
place = {United States},
year = 2016,
month = 8
}
  • Abstract not provided.
  • Molecular simulations of the adsorption of representative organic molecules onto the basal surfaces of various clay minerals were used to assess the mechanisms of enhanced oil recovery associated with salinity changes and water flooding. Simulations at the density functional theory (DFT) and classical levels provide insights into the molecular structure, binding energy, and interfacial behavior of saturate, aromatic, and resin molecules near clay mineral surfaces. Periodic DFT calculations reveal binding geometries and ion pairing mechanisms at mineral surfaces while also providing a basis for validating the classical force field approach. Through classical molecular dynamics simulations, the influence of aqueous cationsmore » at the interface and the role of water solvation are examined to better evaluate the dynamical nature of cation-organic complexes and their co-adsorption onto the clay surfaces. The extent of adsorption is controlled by the hydrophilic nature and layer charge of the clay mineral. All organic species studied showed preferential adsorption on hydrophobic mineral surfaces. However, the anionic form of the resin (decahydro-2-naphthoic acid)—expected to be prevalent at near-neutral pH conditions in petroleum reservoirs—readily adsorbs to the hydrophilic kaolinite surface through a combination of cation pairing and hydrogen bonding with surface hydroxyl groups. Analysis of cation-organic pairing in both the adsorbed and desorbed states reveals a strong preference for organic anions to coordinate with divalent calcium ions rather than monovalent sodium ions, lending support to current theories regarding low-salinity water flooding.« less
  • Abstract not provided.
  • The effect of fluorination on various types of phyllosilicate minerals has been investigated. Two different fluorination techniques have been used: direct F{sub 2} gas and cold radio frequency plasma involving c-C{sub 4}F{sub 8} or O{sub 2}/CF{sub 4} mixtures. The modifications of the surface composition and properties have been followed mostly by x-ray photoelectron spectroscopy (XPS). Depending of the fluorination reagents, a reactive etching process involving M-F bonding occurs (direct F{sub 2} gas; O{sub 2}-CF{sub 4} rf plasma) or a carbon fluoride deposition takes place (c-C{sub 4}F{sub 8} rf plasma). In the case of F{sub 2}-gas treated minerals, Si 2p XPSmore » signal accounts for the presence of fluorinated Si environments.« less
  • Anthropogenic activities have led to an increased concentration of uranium on the Earth’s surface and potentially in the subsurface with the development of nuclear waste repositories. Uranium is soluble in groundwater, and its mobility is strongly affected by the presence of clay minerals in soils and in subsurface sediments. We use molecular dynamics simulations to probe the adsorption of aqueous uranyl (UO 2 2+) ions onto the basal surface of muscovite, a suitable proxy for typically ultrafine-grained clay phases. Model systems include the competitive adsorption between potassium counterions and aqueous ions (0.1 M and 1.0 M UO 2Cl 2 ,more » 0.1 M NaCl). We find that for systems with potassium and uranyl ions present, potassium ions dominate the adsorption phenomenon. Potassium ions adsorb entirely as inner-sphere complexes associated with the ditrigonal cavity of the basal surface. Uranyl ions adsorb in two configurations when it is the only ion species present, and in a single configuration in the presence of potassium. Finally, the majority of adsorbed uranyl ions are tilted less than 45° relative to the muscovite surface, and are associated with the Si 4Al 2 rings near aluminum substitution sites.« less