Building the bridge between U(VI) and Ca-bentonite – Influence of concentration, ionic strength, pH, clay composition and competing ions

Brix, Kristina; Baur, Sandra; Haben, Aaron; Kautenburger, Ralf

doi:10.1016/j.chemosphere.2021.131445

Cited by 10 publications

(11 citation statements)

References 61 publications

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“…U(VI) forms the twofold-charged uranyl cation determining complexation and speciation behaviour whereas for Eu(III) and Sm(III) the three-fold-charged cations are defining. At pH 10.5-11, U(VI) forms negatively charged species in solution (mainly UO2(OH)3or UO2(CO3)3 4-) which tend not to adsorb on the potential negatively charged tube wall [30]. The main species for Eu(III) are Eu(OH)3 and Eu(CO3)2 - [64] which have a higher affinity for wall adsorption under these conditions.…”

Section: Precipitation and Wall Adsorptionmentioning

confidence: 99%

“…Another hint for the Cs(I) adsorption on the strong sites is the immobilisation's irreversibility. Iwaida et al [30] observed an incorporation in the C-S-H structure at high Cs(I) concentrations (1 mol•L −1 ) which is also irreversible. Incorporation processes take place on a long timescale.…”

Section: Time-dependent Retention Of Cs(i)mentioning

confidence: 99%

“…Nevertheless, high retention of U(VI) on cementitious materials is observed in many studies. It occurs through precipitation [2,9], incorporation [52], direct surface complexation [2,49,51,53,54] or ternary surface complexes involving Ca(II) [30,55,56]. Tits et al [57] even observed three U(VI) species in their study which they attributed to surface complexation, an incorporated species and a uranate-like precipitate.…”

Section: Introductionmentioning

confidence: 99%

“…The used background electrolytes are highly saline (I = 0.1 M and 2.6 M) because host rocks for potential HLW disposals may be salt formations or in contact with salt dome deposits which results in highly saline pore waters [27][28][29]. Retention studies with such ionic strengths are scarce [14,[30][31][32][33][34][35]. We used diluted Gipshut solution (DGS, I = 2.6 M) which is a reference pore water of claystone in northern Germany and 0.1 M NaCl as a simple and lower-saline comparative medium.…”

Section: Introductionmentioning

confidence: 99%

“…Adsorption on the negatively charged surface of C-S-H phases seems unlikely for U(VI) at pH > 10 because it forms negatively charged carbonate or hydroxy complexes [30,[49][50][51]. Nevertheless, high retention of U(VI) on cementitious materials is observed in many studies.…”

Section: Introductionmentioning

confidence: 99%

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Time-Dependent Retention of a Mixture of Cs(I), Sm(III), Eu(III) and U(VI) as Waste Cocktail by Calcium Silicate Hydrate (C-S-H) Phases

Brix,

Haben,

Kautenburger

2023

Minerals

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In the context of the safe storage of high-level radioactive waste, the time-dependent retention of a waste cocktail (WC) consisting of Zr(IV), Mo(VI), Ru(III), Pd(II), Cs(I), Sm(III), Eu(III) and U(VI) was studied on the commercially available C-S-H phase Circosil®. The herein presented results focus on Cs(I), Sm(III), Eu(III) and U(VI). Precipitation and wall adsorption studies in the absence of the solid phase show only a small amount of precipitation for Sm(III) and Eu(III) (34 ± 18%) in the high-saline diluted Gipshut solution (DGS, pH 10.6, I = 2.6 M). For Cs(I) and U(VI), no precipitation was observed. In 0.1 M NaCl (pH 10.9), the measured retention could completely be attributed to wall adsorption for all four elements. The obtained Rd values for the time-dependent retention of Sm(III), Eu(III) and U(VI) on Circosil® of 105 to 106 L·kg−1 are in good agreement with the literature. For Cs(I) in the strongly saline background electrolytes, slightly higher Rd values of up to 8·102 L·kg−1 were determined for the crystalline Circosil® compared to the wet chemical C-S-H phases. Overall, the commercial product Circosil® is suitable as an alternative to synthesised C-S-H phases to observe trends in the retention behaviour of these elements. Comparison between both background electrolytes shows an increase in the amount and velocity of retention for all four elements with decreasing salinity. This confirms adsorption processes as the fastest and initial retention mechanism. Precipitation or incorporation of Eu(III), Sm(III) and U(VI) cannot be ruled out in the long term. Comparing the kinetic of this WC study to single-element studies in the literature, a longer uptake time to reach a steady state of 7 d in 0.1 M NaCl and 28 d in DGS instead of <1 d was observed for Eu(III) and Sm(III). The situation for Cs(I) is similar. This indicates competing effects between the different WC elements for adsorption sites on the C-S-H phases.

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Section: Precipitation and Wall Adsorptionmentioning

confidence: 99%

Section: Time-dependent Retention Of Cs(i)mentioning

confidence: 99%