摘要:

IntroductionThe high bioavailability and long halflives of radioactive isotopes of Cs make them a significant environmental hazard.1–3To isolate this hazard, nuclear waste containment facilities may be separated from host bedrock by compacted bentonite liners,2shown in laboratory experiments to slow the diffusion of Cs+.1,2,4–7The smectite minerals making up these liners are 2 ∶ 1 layer type clay minerals.8Each layer consists of an alumina (dioctahedral) or magnesia (trioctahedral) sheet sandwiched between two silica tetrahedral sheets. The arrangement of silica tetrahedra in sheets results in the creation of a network of six-oxygen rings, at the center of each a distorted hexagonal cavity, across the surface of the mineral. Isomorphic substitution of Mg for Al, or Li for Mg, results in negative octahedral charge sites, while substitution of Al for Si creates negative tetrahedral charge sites. These charge sites are balanced by cations in the interlayer which attract waters of hydration to this region, causing the clay mineral to swell. Although Cs-smectite9little detailed information about the mechanisms involved in Cs+adsorption, hydration, and diffusion has been adduced from experimental findings. Molecular simulation thus has proven to be a useful adjunct to experimental work concerning the smectite interlayer.10–15A number of XRD studies show that the layer spacing of Cs-smectites remains in the range 11.9–12.5 Å after exposure to water vapor at any relative humidity,9,16–20or even after immersion in an aqueous solution.9Water vapor adsorption isotherms16–20indicate increasing hydration of Cs-smectite with increasing relative humidity, but Calvet17and Prost18argued that most of the water adsorbed resides in micropores, not in the interlayer region. Indeed, measurements of the micropore volume in homoionic montmorillonites21have established the extensive microporosity of Cs-smectites. Calvet17and Prost22suggested that the 12.4 Å layer spacing in Cs-smectite was achieved and stabilized after the adsorption of only about 1.2–1.4 H2O per unit cell of the clay mineral, well below the nominal monolayer water content of 4 H2O per unit cell observed for smectites containing the strongly hydrating Li+cation.17,19Our recent Monte Carlo (MC) simulations of model Cs-smectite hydrates at low water contents15confirmed this speculation, showing that the 12.4 Å hydrate of Cs-smectite likely contains no more than 2.7 H2O per unit cell (about 2/3 monolayer).The Cs-smectites we investigated were (Table 1): Cs-hectorite with 1.3 and 2.7 H2O per unit cell (1/3 and 2/3 water monolayer), Cs-beidellite with 1/3 water monolayer, and Cs-montmorillonite with 1/3 and 2/3 water monolayer. Caesium-smectites with 1.0, 1.31, 0.844, and 0.75 monolayers of water also were modeled for up to 10 million MC steps, but did not meet our MC equilibration criteria of achieving both a minimum average potential energy and a stable layer spacing. That simulations containing more water molecules were inherently unstable is not surprising given experimental data indicating Cs-smectites have low interlayer water contents.17,22Layer spacing and molar potential energy of water (MC simulation)15and self-diffusion coefficients for interlayer water (MD simulation) in Cs-smectite hydratesHydrateMC stepsLayer spacing/ÅPotential energy of water/kJ mol−1Self-diffusion coefficient,Dw/10−9m2s−1Monte Carlo steps required for convergence.(Total potential energy of hydrate − total potential energy of clay mineral) ÷ moles of water per simulation cell.13Value calculated from linear regression of the mean-square displacement (MSD) on elapsed time withP= 0.05 confidence interval.15This hydrate exhibited MC convergence only at the lower water content.15This hydrate did not produce a linear relationship between MSD and elapsed time.15MCY water33yieldsDw= 2 × 10−9m2s−1.1/3 water monolayerCs-hectorite10612.37 ± 0.08−47.43 ± 3.530.229 ± 0.001Cs-beidellite4 × 10612.31 ± 0.10−42.98 ± 4.580.342 ± 0.002Cs-montmorillonite10612.46 ± 0.09−32.33 ± 4.04Nonlineare2/3 water monolayerCs-hectorite3.0 × 10612.41 ± 0.07−46.56 ± 2.000.685 ± 0.006Cs-montmorillonite3.5 × 10612.68 ± 0.10−36.66 ± 2.121.168 ± 0.006Bulk liquid water−35.4 ± 0.2312.332The MC simulations were followed by molecular dynamics (MD) simulations.15These latter results were analyzed for average properties, such as interlayer water self-diffusion coefficients, as well as for individual molecular properties, such as plots ofxyzcoordinates sampled by specific Cs+ions or by water molecules. These traditional simulation outputs were not able to characterize the full complexity of interactions among cations, water molecules, and mineral surface sites. To continue analysis of the MD results, we constructed animations of molecular motions in the five Cs-smectite hydrate systems to examine the effects of charge site and hydration state on the behavior of interlayer species. Stereo-viewing and animated motion have been shown to increase comprehension of three dimensional structures.23Because humans understand and navigate through a dynamic, three dimensional world, depiction of MD animations as a collection of spheres in three dimensional space that change position over time is a familiar metaphor that is intuitive to most viewers.23Other more traditional forms of presentation, such as a display of the entire path of a molecule over time, can result in a “tangled mess” that is not easy to interpret.23Animations thus can be helpful in understanding the complex three dimensional relationships between interacting molecules over time.

ISSN:1467-4866    

DOI:10.1039/b204973m

出版商:RSC    

年代:2002

数据来源: RSC