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(2020) Characterizing Uranium Surface Speciation on Na-Montmorillonite

Tinnacher R, Massey M, Bhattacharyya A, Grangeon S & Tournassat C


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08b: Room 3, Monday 22nd June 22:18 - 22:21

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Submitted by Laura Kennell-Morrison on Monday 22nd June 02:09
What are the salinities of the solutions used? Is there any consideration to look at higher pH (9-10) - the behaviour looks very consistent in the range 5-8?
Thank you very much for your interest, Laura. We used a background electrolyte of 0.1 M NaCl for all of these samples in order to minimize the relevance of uranium ion exchange reactions. We have not looked at systems in the pH range from 9-10 yet, because the U(VI) sorption onto Na-montmorillonite is very low in this range in the presence of atmospheric CO2/carbonate ligands in solution. But I agree that these high pH values are probably relevant from a repository perspective.

Submitted by Jeffery A Greathouse on Monday 22nd June 16:31
How do your EXAFS results differ from previous EXAFS or other spectroscopic studies of uranyl adsorption on montmorillonite?

Submitted by Ruth Tinnacher on Monday 22nd June 21:35
This is a great question, Jeff. Catalano & Brown, 2005 state that they observed the formation of ternary U(VI)-carbonate surface complexes on montmorillonite. However, Marques Fernandes et al., 2012 state that their EXAFS measurements "did not allow to verify the formation of ternary uranyl-carbonate complexes on the montmorillonite surface". I/we believe that it is possible that the specific chemical conditions and the details of the sample preparation might play a role with regard to these apparent differences. For instance, if the concentration ratio between uranium and montmorillonite surface sites is different, uranium might be (predominantly) bound to different site types. Additionally, different researchers use different approaches to separate clay fractions from supernatant solutions, e.g. in terms of centrifugation, filtration, washing and drying steps. It seems possible that depending on the sample preparation, different amounts of pore water (with a high concentration of U(VI)-carbonate solution complexes) might still have been present in the wet paste of the analyzed clay samples.

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