Large-Scale Molecular Dynamics Simulations of Metal Sorption onto the Basal Surfaces of Clay Minerals (NUREG/CR-6757)

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Publication Information

Manuscript Completed: October 2001
Date Published: February 2002

Prepared by:
D. M. Teter, R. T. Cygan

Sandia National Laboratories
Albuquerque, NM 87185-0750

E. O'Donnell, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code W6811

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Abstract

Large-scale molecular dynamics computer simulations have been completed to evaluate the sorption of Cs⁺, Sr²⁺, and other related ions onto the (001) basal planes of kaolinite and montmorillonite. Simulations were undertaken as a function of metal ion concentration to evaluate sorption mechanisms and to determine distribution coefficients (K_d's). The simulations show that Cs⁺ ions bind to both kaolinite and montmorillonite, while Sr²⁺ ions tend to remain in solution. At high ionic strengths (greater than 0.5 M), the fundamental mechanism for Cs⁺ sorption to the aluminol plane of kaolinite is through the formation of CsCI complexes where the CI ion is loosely bound to the outer hydroxyl groups. At lower concentrations (0.1 M and 0.01 M), the Cs⁺ ions do not sorb to the aluminol plane, but prefer to bind adjacent to the center of the six-membered ring silica tetrahedral sheet as an inner-sphere complex. We have determined a K_d of 10 ± 4 ml/g for kaolinite to sorb Cs⁺ ions from a 0.01M SrCl₂ solution, and a K_d of 0 for kaolinite to sorb Sr²⁺ ions from a 0.01M SrCl₂ solution. For the case of montmorillonite, we have determined a Kd of 293 ± 30 ml/g to sorb Cs⁺ ions from a 0.1M CsCl solution, and a Kd of 11 ± 4 ml/g to sorb Sr²⁺ ions from a 0.1M SrCl₂ solution. The montmorillonite sorbs the cations more effectively than kaolinite because it has negatively charged sorption sites that are created by isomorphic substitutions on both tetrahedral and octahedral sites. Calculations show that the Sr²⁺ ion is solvated more strongly than Cs⁺and is not sorbed by the kaolinite or montmorillonite surfaces. Directions for further work are outlined and prioritized.