pH = 11 - SKB

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PDF rendering: DokumentID 1415884, Version 1.0, Status Godkänt, Sekretessklass Öppenlog Solubility (mol dm−³)0-2-4-6-8-100⁰C 15⁰C25⁰C 40⁰C70⁰C 90⁰CSm pH = 71.0E-021.0E-04[CO₃²−] (mol dm−³)log Solubility (mol dm−³)0-2-4-6-8-100⁰C25⁰C70⁰C15⁰C40⁰C Ho90⁰C1.0E-021.0E-04[CO₃²−] (mol dm−³)pH = 7log Solubility (mol dm−³)0-2-4-6-8-100⁰C25⁰C70⁰C15⁰C40⁰C90⁰CSm pH = 111.0E-021.0E-04[CO₃²−] (mol dm−³)log Solubility (mol dm−³)20-2-4-6-8-100⁰C25⁰C70⁰C15⁰C40⁰C90⁰CHo pH = 111.0E-021.0E-04[CO₃²−] (mol dm−³)Figure 5-4. SmOHCO 3 (s) (left plots) and Ho 2 (CO 3 ) 3 (s) (right plots) solubility evolutionwith temperature at two different pHs and two different concentrations of carbonate.Ca concentration controlled through calcite equilibrium.It must be pointed out that, as for Sm, Ho aqueous carbonate species enthalpy dataare not available. In addition, solid phase enthalpy is also not available. Only data forHoSO + 4 have been considered in the calculations. Therefore, Ho aqueous chemistrybehaves equal to Sm and so does their trends in solubility, as expected.As a final remark, it is worth mentioning that the increase of SmOHCO 3 (s) andHo 2 (CO 3 ) 3 (s) solubility with temperature should be considered with caution given thescarcity of data for those solid phases. In addition, it is expected a transformation ofthe solid when increasing the temperature, which would favour a more crystallinephase, so that the calculated solubilities at higher T would presumably decrease.Giffaut and co-workers (Giffaut, 1994; Vercouter et al., 2005) studied Am(III)-carbonate systems with temperature and identified solid phase transformations whenincreasing the temperature, with a consequent decrease of the solubility.At higher temperatures crystalline phases would prevail over amorphous phases andthus the expected radionuclide solubilities will be presumably lower than thosecalculated in Figure 5-4.67
PDF rendering: DokumentID 1415884, Version 1.0, Status Godkänt, Sekretessklass Öppen5.2.6 Actinides (U, Np, Pu, Am and Cm)5.2.6.1 UraniumUnder the redox reducing conditions studied in the present work (i.e.goethite/magnetite equilibrium) uranium is mainly present as U(IV), and UO 2 (am,hyd)is the solid phase expected to exert a solubility control over this radionuclide. Theevolution of UO 2 (am,hyd) solubility as a function of temperature is shown in Figure 5-5.As can be seen, under the geochemical conditions considered in the present study,neither the pH nor the carbonate concentration affect the solubility of UO 2 (am,hyd) to asignificant extent. However, temperature changes in the range 0-90ºC may producesolubility variations of more than 5 log units. It is worth to mention that under the wholeset of studied conditions, U(OH) 4 (aq) is the main uranium aqueous species formed.Regarding the availability of enthalpy data, there are enthalpy data available for theformation of U(OH) 4 (aq), but not for the selected solid phase.Thus, the important increase of UO 2 (am,hyd) solubility as temperature increasesshould be considered with caution given the scarcity of data for the studied solidphase. In fact, as reported by Goodwin (1982), UO 2 (am,hyd) solubility is only slightlydependent on temperature at reducing and near neutral conditions (~0.5 log units).Experimental data at higher temperatures than 25ºC (Parks and Pohl, 1988; Rai et al.,2003) have shown almost no increase in solubility with temperature. Thus, the resultsobtained for uranium are a clear example of erroneous solubility predictions due to theincompleteness of the used dataset.68
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