Diffusion of tritiated water, 137Cs+, and 125I− in compacted Ca-montmorillonite: Experimental and modeling approaches
| Autor: | Takahiro Ohkubo, Yukio Tachi, Yuta Fukatsu, Kenji Yotsuji |
|---|---|
| Rok vydání: | 2021 |
| Předmět: |
Tritiated water
Diffusion Analytical chemistry 020101 civil engineering Geology Sorption 02 engineering and technology Electrolyte 021001 nanoscience & nanotechnology 0201 civil engineering Salinity chemistry.chemical_compound Montmorillonite chemistry Geochemistry and Petrology Bentonite Effective diffusion coefficient 0210 nano-technology |
| Zdroj: | Applied Clay Science. 211:106176 |
| ISSN: | 0169-1317 |
| DOI: | 10.1016/j.clay.2021.106176 |
| Popis: | Mechanistic understanding and predictive modeling of radionuclide diffusion in Na- and Ca-montmorillonite are essential to evaluate the long-term evolution of the bentonite barrier and their impact on radionuclide migration during geological disposal of radioactive wastes. Thus, the diffusion behavior of 137Cs+, 125I−, and tritiated water (HTO) in compacted Ca-montmorillonite was investigated as a function of porewater salinity and dry density via both experiments and models. The effective diffusion coefficient (De) followed in the order of 137Cs+ > HTO > 125I−. The De of 137Cs+ decreased with increasing salinity, whereas the dependence of De of 125I− on salinity was uncertain. The cation excess and anion exclusion effects for Ca-montmorillonite were lower than those for Na-montmorillonite. The integrated sorption and diffusion (ISD) model, assuming the homogeneous pore structure and the electrical double layer (EDL) theory for 2:1 electrolyte (CaCl2), could account for the observed trends for De in Ca-montmorillonite. The lower dependence of De on the porewater salinity in Ca-montmorillonite was caused by the reduction of the EDL thickness for divalent cations (Ca2+) in comparison with that for monovalent cations (Na+). The multipore model could improve the fit for De of 125I− at low salinity due to the reduction of interlayer pore volumes and anion exclusion effect, however, the disparity at higher densities was considerably larger. From these results, cation diffusion for compacted Ca-montmorillonite could be mainly explained by the electrostatic interactions in the homogeneous pore model; in contrast, anion diffusion was sensitive to both electrostatic interactions and heterogeneous pore structures. The proposed ISD model is an effective tool to evaluate the radionuclide diffusion and sorption behavior in both compacted Ca-montmorillonite and Na-montmorillonite. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|
Full Text from ScienceDirect