pH = 11 - SKB

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PDF rendering: DokumentID 1415884, Version 1.0, Status Godkänt, Sekretessklass Öppenlog Solubility (mol dm−³)log Solubility (mol dm−³)-2-3-4-5-6-7-8-9-10-11-2-3-4-5-6-7-8-9-10-11pH = 7pH = 11U0⁰C 15⁰C 25⁰C40⁰C 70⁰C 90⁰C1.0E-021.0E-04[CO₃²−] (mol dm−³)U0⁰C 15⁰C 25⁰C40⁰C 70⁰C 90⁰C1.0E-021.0E-04[CO₃²−] (mol dm−³)Figure 5-5. UO 2 (am,hyd) solubility evolution with temperature at two different pHs andtwo different concentrations of carbonate. Ca concentration controlled through calciteequilibrium. Goethite/Magnetite equilibrium controls both Fe and Eh of the system.5.2.6.2 NeptuniumAs for uranium, the highly reducing conditions of the studied systems forces Np to bein +4 oxidation state. Under such conditions, the predicted solubility limiting phase isNpO 2·xH 2 O(am). The solubility of this solid phase as a function of temperature isshown in Figure 5-6.69
PDF rendering: DokumentID 1415884, Version 1.0, Status Godkänt, Sekretessklass ÖppenAt the lowest pH studied here, the lowest NpO 2·H 2 O(am) solubility is obtained at 25ºC.Below and above 25ºC, the solubility of this solid phase increases due to the differentchemical speciation of Np. At 25ºC and above, Np(OH) 4 (aq) (for which enthalpy dataare available) is the main Np aqueous species in solution. When decreasing thetemperature, NpCO 3 (OH) - 3 species (for which there are not enthalpy data) governs theaqueous chemistry of Np. It must be pointed out that NpO 2·H 2 O(am) enthalpy isavailable and has been considered in the calculations. Therefore, at 25ºC and abovethe main reaction occurring has an endothermic character that justifies the increase ofsolubility with temperature. On the other hand, below 25ºC, where the main reactionchanges involves an aqueous species for which enthalpy data are not available, thetrend of the results changed: the solubility of NpO 2·H 2 O(am) increases as temperatureincreases.At pH 11, the same arguments used for explaining NpO 2·H 2 O(am) solubility at pH 7are valid for calculations with the highest carbonate content in solution. The onlydifference is that, instead of NpCO 3 (OH) - 3 species, Np(OH) 4 (CO 3 ) 2- species is the onecontrolling the aqueous chemistry of Np below 25ºC. When dealing with low carbonateconcentrations at pH 11, Np-carbonate aqueous species are not relevant anymore,and only Np(OH) 4 (aq) appears to control the aqueous chemistry of Np. That changejustifies the increase of solubility when increasing the temperature at pH 11 and lowamount of carbonate in the system (Figure 5-6).70
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