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| Home > Publications database > Retention of 226Ra by montmorillonite |
| Poster (Other) | FZJ-2022-03480 |
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2022
Please use a persistent id in citations: http://hdl.handle.net/2128/31896
Abstract: Although 226Ra is a critical radionuclide in the safety analysis for deep geological disposal of spent nuclear fuel, adsorption data for 226Ra onto clay minerals are sparse in the open literature. So far, the adsorption and diffusion of 226Ra in clay minerals is assumed similar to that for Sr and Ba, based on chemical analogy. In this study, we present new experimental data that improves the quantitative understanding of the retention of 226Ra in 2:1 clay minerals relevant for radioactive waste disposal. Montmorillonite as a major constituent of bentonite (up to 90 wt.-%) is an important sink for potentially released radionuclides. To study the 226Ra adsorption behaviour on bentonites, adsorption experiments were carried out onto the homo-ionic Na form of Wyoming montmorillonite (SWy-2) in diluted systems and compared to the adsorption of Ba under the same conditions. 226Ra and Ba adsorption edges in the pH range 5 to 10 at different ionic strengths (0.01 - 0.3 M NaCl) and an isotherm (pH 7, 10-9 M < [Ra,Ba] > 10-2 M) were carried out on Na-SWy. Our results c The 2-site Protolysis Non Electrostatic Surface Complexation and Cation Exchange adsorption model reproduces the experimental data for both elements quite well. The Kc values for Ba2+-Na+ and 226Ra2+-Na+ exchange equilibria are in good agreement at low ionic strength (log Kc = 0.70 - 0.84), whereas at high ionic strength, the Kc (Ba-Na) is slightly higher (log Kc of 0.90 (0.3 M) vs 0.70 (≤ 0.02 M)). In contrast, the Kc (Ra-Na) exhibits a clear dependency on ionic strength, with log Kc values of 0.7, 1.14 and 1.34 at NaCl background concentrations of 0.01/0.02 M, 0.14 M and 0.3 M, respectively. The results presented here indicate that Ba can be used as a good analogue for 226Ra regarding the adsorption on montmorillonite at ionic strengths < 0.1 M and pH < 8, but deviates in its behaviour at higher ionic strength. This evident higher selectivity of Ra on Na-SWy may be due to the larger ionic radius of Ra compared to Ba. The contribution of surface complexation to the overall adsorption is the same for Ba and Ra and is independent of ionic strength.
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