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Molecular dynamics simulations of anion exclusion in clay interlayer nanopores

Christophe Tournassat, Ian C. Bourg, Michael Holmboe, Garrison Sposito and Carl I. Steefel
Molecular dynamics simulations of anion exclusion in clay interlayer nanopores
Clays and Clay Minerals (August 2016) 64 (4): 374-388

Abstract

The aqueous chemistry of water films confined between clay mineral surfaces remains an important unknown in predictions of radioelement migration from radioactive waste repositories. This issue is particularly important in the case of long-lived anionic radioisotopes ( (super 129) I (super -) , (super 99) TcO (sub 4) (super -) , (super 36) Cl (super -) ) which interact with clay minerals primarily by anion exclusion. For example, models of ion migration in clayey media do not agree as to whether anions are completely or partially excluded from clay interlayer nanopores. In the present study, this key issue was addressed for Cl (super -) using MD simulations for a range of nanopore widths (6 to 15 A) overlapping the range of average pore widths that exists in engineered clay barriers. The MD simulation results were compared with the predictions of a thermodynamic model (Donnan Equilibrium model) and two pore-scale models based on the Poisson-Boltzmann equation under the assumption that interlayer water behaves as bulk liquid water. The simulations confirmed that anion exclusion from clay interlayers is greater than predicted by the pore-scale models, particularly at the smallest pore size examined. This greater anion exclusion stems from Cl (super -) being more weakly solvated in nano-confined water than it is in bulk liquid water. Anion exclusion predictions based on the Poisson-Boltzmann equation were consistent with the MD simulation results, however, if the predictions included an ion closest approach distance to the clay mineral surface on the order of 2.0 + or - 0.8 A. These findings suggest that clay interlayers approach a state of complete anion exclusion (hence, ideal semi-permeable membrane properties) at a pore width of 4.2 + or - 1.5 A.


ISSN: 0009-8604
EISSN: 1552-8367
Coden: CLCMAB
Serial Title: Clays and Clay Minerals
Serial Volume: 64
Serial Issue: 4
Title: Molecular dynamics simulations of anion exclusion in clay interlayer nanopores
Affiliation: Universite d'Orleans, Institut des Sciences de la Terre d'Orleans, Orleans, France
Pages: 374-388
Published: 201608
Text Language: English
Publisher: Clay Minerals Society, Chantilly, VA, United States
References: 76
Accession Number: 2017-004879
Categories: Sedimentary petrologyEnvironmental geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Secondary Affiliation: Lawrence Berkeley National Laboratory, USA, United StatesPrinceton University, USA, United StatesUmea University, SWE, SwedenUniversity of California at Berkeley, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Clay Minerals Society. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201704
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