Radionuclide Interaction with Clays in Dilute and Heavily Compacted Systems: A Critical Review

Abstract

Given the unique properties of clays (i.e., low permeability and high ion sorption/exchange capacity), clays or clay formations have been proposed either as an engineered material or as a geologic medium for nuclear waste isolation and disposal. A credible evaluation of such disposal systems relies on the ability to predict the behavior of these materials under a wide range of thermal-hydrological-mechanical-chemical (THMc) conditions. Current model couplings between THM and chemical processes are simplistic and limited in scope. This review focuses on the uptake of radionuclides onto clay materials as controlled by mineral composition, structure, and texture (e.g., pore size distribution), and emphasizes the connections between sorption chemistry and mechanical compaction. Variable uptake behavior of an array of elements has been observed on various clays as a function of increasing compaction due to changes in pore size and structure, hydration energy, and overlapping electric double layers. The causes for this variability are divided between "internal" (based on the fundamental structure and composition of the clay minerals) and "external" (caused by a force external to the clay). New techniques need to be developed to exploit known variations in clay mineralogy to separate internal from external effects.

Na-Montmorillonite Edge Structure and Surface Complexes: An Atomistic Perspective

“…The structural characterization of synthetic MMTs with divalent cation substitution ratios greater than 0.20 revealed octahedral sheet occupancies greater than 2.00, which is indicative of an MMT structure with some trioctahedral character, and isomorphic substitutions in the tetrahedral sheet. The octahedral substitution ratio (i.e., the number of substitutions per total octahedral sites) is one half the layer charge, x, in number of moles of excess electron charge per the M-MMT chemical formula: 2 , where M is the interlayer counter ion and the term in brackets represents the octahedral sheet cations. Any assignments that violated either of the two constraints were ignored, and a new random assignment was made.…”

“…The exposed surface hydroxyl coordinated to Al sol is stabilized via an intrasurface H-bond with the nearest silanol group and the H-bonds of a water molecule that forms a bridge to the second silanol group of the surface TOT structure. At a lower layer charge, however, the Mg sol substitution is preferable ( 2 and ≡(Al-OH) 2 edge structures are the most energetically favorable. In this lowest energy structure, the Mg sol is in six-fold coordination with oxygen, including two water molecules (Figure 3a).…”

“…The average number of H-bonds and standard deviation (+/−) was calculated from the final 1.0 ns of the 5.0 ns trajectory. Figure 6; the subscript corresponds to the upper (1) or lower (2) hydroxyl coordinated to the edge Al. Figure 6.…”

“…The permanent negative charge imparts significant cation exchange capacity to the MMT layers. These physicochemical properties of the MMT interlayer have been used to great advantage in engineered barriers [2][3][4], nanocomposite materials [5][6][7], and pharmaceutical and biomedical applications [8].…”

Generic Argillite/Shale Disposal Reference Case

“…A subset of their compilation is given in Table 15. Miller and Wang (2012) also reviewed sorption coefficients for some argillites. …”