pH = 11 - SKB

PDF rendering: DokumentID 1415884, Version 1.0, Status Godkänt, Sekretessklass Öppen5.2.5 Lanthanides (Sm and Ho)Carbonate bearing phases, SmOHCO 3 (s) and Ho 2 (CO 3 ) 3 (s), are the expected solubilitylimiting phases of Sm and Ho under the geochemical conditions studied here.The solubility of both solids is shown in Figure 5-4 as a function of temperature andcarbonate aqueous concentration at two different pHs. As can be observed, the trendsof both solid phases are nearly equal. pH and aqueous carbonate variations slightlyaffect the solubility of those solid phases in the studied temperature range. On theother hand, the increase of temperature produces an increase in the solubility of bothsolid phases.In the case of Sm, at neutral pH and temperatures below 25ºC, SmCO +3 andSm(CO 3 ) - 2 control the aqueous chemistry of this radionuclide with a small contributionby SmSO + 4 . When increasing the temperature above 25ºC SmSO + 4 is negligible beingSmCO + 3 the most relevant Sm aqueous species. It should be considered here thatthere are no enthalpy data available for SmCO + -3 , Sm(CO 3 ) 2 and the solid phaseSmOHCO 3 (s) (data available only for SmSO + 4 ). It is thus clear that the change in Smspeciation justifies the increases observed with increasing temperature. At lowtemperatures, the presence of SmSO + 4 in the results justifies the slight increase ofsolubility from 0 to 25ºC. On the other hand, the inexistency of enthalpy data for Smaqueous carbonates explains the increases observed in the solubility. At pH=11, thereasoning behind Sm behaviour is nearly the same, the sole difference being thatinstead of SmSO + 4 , the hydrolysed species Sm(OH) 3 (aq) appears to be relatively morerelevant at low temperatures.66