Solubility of amorphous Th(IV) hydroxide – application of LIBD to determine the solubility product and EXAFS for aqueous speciation

Neck, V.; Müller, Ralf; Bouby, M.; Altmaier, M.; Rothe, Jörg; Denecke, Melissa A.; Kim, Jae-Il

doi:10.1524/ract.2002.90.9-11_2002.485

Cited by 104 publications

(151 citation statements)

References 23 publications

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“…hydrolysis, and subsequent oligomerization. These reactions and their thermodynamics have been studied in great detail (Baes et al, 1958;Brown et al, 1983;Ryan and Rai, 1987;Felmy et al, 1991;Ekberg et al, 2000;Neck et al, 2002;Wilson et al, 2007;Rand et al, 2008;Walther et al, 2008;Knope et al, 2011;Knope and Soderholm, 2012), and it has become increasingly clear that condensation reactions can significantly impact Th(IV) solution chemistry, even at relatively low pH and low Th concentration.…”

Section: Introductionmentioning

confidence: 99%

Effects of the background electrolyte on Th(IV) sorption to muscovite mica

Schmidt

Hellebrandt

Knope

et al. 2015

Geochimica et Cosmochimica Acta

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The adsorption of tetravalent thorium on the muscovite mica (0 0 1) basal plane was studied by X-ray crystal truncation rod (CTR) and resonant anomalous X-ray reflectivity (RAXR) measurements and alpha spectrometry in the presence of perchlorate background electrolytes LiClO 4 , NaClO 4 , and KClO 4 ([Th(IV)] = 0.1 mM, I = 0.1 M or 0.01 M, pH = 3.3 ± 0.3). RAXR data directly reveal a strong influence of the background electrolyte on the actinide sorption. No significant Th adsorption was observed in 0.1 M NaClO 4 , i.e., the Th coverage h(Th), the number of Th per unit cell area of the muscovite surface (A UC = 46.72 Å 2 ), was 60.01 Th/A UC , whereas limited uptake (h(Th) $ 0.04 Th/A UC ) was detected at a lower ionic strength (I = 0.01 M). These results are in stark contrast to the behavior of Th in 0.1 M NaCl which showed a coverage of 0.4 Th/A UC (Schmidt et al., 2012a). Th uptake was also influenced by the electrolyte cation. Weak adsorption was observed in 0.1 M KClO 4 (h(Th) $ 0.07 Th/A UC ) similar to the results in NaClO 4 at lower ionic strength. In contrast, strong adsorption was found in 0.1 M LiClO 4 , with h(Th) = 4.9 Th/A UC , a $10-fold increase compared with that previously reported in NaCl. These differences are confirmed independently by ex situ alpha spectrometry, which shows no measurable Th coverage in 0.1 M NaClO 4 background in contrast to a large coverage of 1.6 Th/A UC in 0.1 M LiClO 4 . Alpha spectrometry cannot be obtained in situ, but sample preparation requires several washing steps that may affect Th(IV) sorption, RAXR, however, is considered to reflect the in situ sorption structure. The CTR/RAXR analyses of Th-LiClO 4 show the sorption structure consisting of Th species that are broadly distributed, centered at heights of 4.1 Å and 29 Å distance from the interface. Neither the very large distribution height of the second species nor the high coverage can be explained with (hydrated) ionic adsorption, suggesting that the enhanced uptake is presumably due to the formation and sorption of Th nanoparticles.

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Section: Introductionmentioning

confidence: 99%

Effects of the background electrolyte on Th(IV) sorption to muscovite mica

Schmidt

Hellebrandt

Knope

et al. 2015

Geochimica et Cosmochimica Acta

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“…These reactions and their products have been extensively studied in solution, with particular emphasis on the determination of thermodynamic stability constants for the hydrolysis products (Baes et al, 1958;Brown et al, 1983;Ryan and Rai, 1987;Felmy et al, 1991;Ekberg et al, 2000;Neck et al, 2002;Wilson et al, 2007b;Rand et al, 2008;Walther et al, 2008;Knope et al, 2011). Overall, it is clear that condensation of hydrolysed Th plays an important role in its solution chemistry, particularly with the observed presence of dissolved polynuclear species even at relatively low concentrations and acidic pH.…”

Section: Introductionmentioning

confidence: 99%

Sorption of tetravalent thorium on muscovite

Schmidt

Lee

Wilson

et al. 2012

Geochimica et Cosmochimica Acta

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Adsorption of tetravalent thorium to the (0 0 1) basal surface of the phyllosilicate muscovite from an aqueous solution (1 Â 10 À4 mol/L Th(IV) in 1 Â 10 À1 mol/L NaCl, pH = 3.2) was studied by crystal truncation rod (CTR) and resonant anomalous X-ray reflectivity (RAXR) measurements. Th uptake to the muscovite surface from solutions with total Th concentrations [Th] tot = 1 Â 10 À6 -4.88 Â 10 À3 mol/L and 1 Â 10 À1 mol/L NaCl, pH = 3.2 was quantified by alpha-spectrometry. The uptake measurements showed that Th adsorption to the muscovite surface follows a Langmuir isotherm with an apparent adsorption constant K app = 2 Â 10 4 L/mol up to [Th] tot = 1.02 Â 10 À3 mol/L. The CTR and RAXR results identified one dominant Th species with a very broad distribution centered $10 Å above the surface, in agreement with strongly hydrated extended outer sphere sorption. The findings indicate that the large energy of hydration (DG hyd = À5815 kJ/mol (Marcus, 1991)) for the small and highly-charged Th 4+ cation is a controlling parameter in its surface speciation. The surface occupancy (0.4 Th per unit cell area, A UC ) measured by RAXR exceeds the expected level for surface charge compensation by tetravalent Th (0.25 Th/A UC ). However, the radiometric uptake measurements show smaller occupancies (0.21 Th/A UC ) after rinsing by deionized water, indicating a partial removability of sorbed thorium. Thorium oligomerization was observed at total Th concentrations [Th] tot P 2.0 Â 10 À3 mol/L in presence of the surface, although solubility studies suggest that Th is soluble under these solution conditions.

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“…However, the experimental solubilities of An(IV) in neutral and alkaline solutions are by 6-7 orders of magnitude higher than the solubility of <10 À15 M calculated from the solubility product of crystalline AnO 2 and the corresponding hydrolysis constants (Neck and Kim, 2001) for pH values of around 7. At pH values above the onset of An(IV) hydrolysis, the solid is no longer in equilibrium with An 4+ but with products of An(IV) hydrolysis (the most important of which is An(OH) 4 (aq)), and a surface layer of amorphous actinide oxyhydroxide on the solid particle surface becomes solubility-limiting (Neck and Kim, 2001;Neck et al, 2002;Reiller et al, 2008). Consequently, it has been emphasized in the literature that performance assessment calculations on the long-time behavior of actinides should not take into account the extremely low solubilities resulting from the thermodynamic data of the crystalline actinide dioxides, An(IV)O 2 (cr), but should rather be based on the higher solubility values of amorphous An(IV) oxyhydroxide (Altmaier et al, 2004).…”

Section: Introductionmentioning

confidence: 99%