Plutonium solubility

Several attempts to calculate plutonium solubility and speciation in environmental systems are given in the literature (e.g. 

Mathew and Pillai observed a threefold increase in plutonium concentration at low, normal, and high carbonate concentrations when 20mg/liter of organic matter were added to sea water samples (29). Again this indicates the effect of organic complexation upon plutonium solubility in natural waters. 

The effect of fluoride was further demonstrated by the increase in plutonium solubility in deionized water from about 11 percent to essentially 100 percent by addition of sufficient NaF to raise the fluoride concentration to that of basalt ground water. It is likely that the enhanced solubility of plutonium in waters containing high fluoride concentrations is the result of stabilization of Pu(IV) in solution by formation of fluoride complexes. Normally Pu(IV) is the least soluble of the four... 

Regarding plutonium solubilities, judgment needs to be made upon how much time and money to spend determining exact Pu solubilities when all that may be required is to demonstrate that Pu solubilities are acceptably low for repository licensability purposes. 

The plutonium solubility increased in the presence of increased NaN03, NaOH, and NaA102 concentrations. According to the literature, Pu(V) should be the stable oxidation state in alkaline NOjT-NO solutions (8). It has been observed that the solubility of Pu(V) increases as tne NaOH concentration increases (8.11) probably this occured due to formation of the more soluble anionic hydroxide complexes of Pu(V) such as PuO,(OH)2 (11). Sodium nitrate and NaAlO, may have increased Pu(V) solubility through complexation. Sodium nitrate also may have increased plutonium solubility by oxidizing the less soluble Pu(IV), initially present in the tracer solids, to Pu(V). 

Judging from the R2 values, plutonium solubility was described adequately by its prediction equation while the strontium and americium solubility equations were less satisfactory. Analysis of the individual test data was found to be more useful than the prediction equations in assessing strontium and americium behavior. 

Interpretation of the plutonium solubility prediction equations (and the data used to generate those equations) showed that increase in NaN03 and NaA102 concentration from 10 4M to 1M increased plutonium concentrations about threefold. However, increase in NaOH concentration from 1 to 4M increased plutonium solubility by a factor of five. Plutonium concentration in these test solutions ranged from about 2 to lOpM, equivalent to about 40 to 200pCi 239,240Pu/L in Hanford HLW, as NaOH concentration increased from 1 to 4M. Plutonium concentrations in actual HLW (see Table HI), are as high as 7 iCi/L and may be limited by the solubility of plutonium. 

The presence of either HEDTA or EDTA resulted in significantly lower neptunium and plutonium sorption. Complexation of the neptunium and plutonium by HEDTA and EDTA may have caused the reduced sorption. However, this evidence for complex formation was not consistent with the observations made in the solubility studies (HEDTA increased and EDTA decreased neptunium solubility neither affected plutonium solubility). Thus, HEDTA and EDTA may have decreased neptunium and plutonium sorption through some undetermined effect on the sediment minerals. 

Figure 7. Solubility of Plutonium as a Function of Eh at 25°C. (The upper patterned area represents plutonium solubility in Grande Ronae groundwaters using the published value for the stability constant of PuCO,2+. The lower patterned area represents plutonium solubility in the absence of PuC032+ species.)...

M, or 4x10 Bq/m. This is below the lowest ALI value presently used. Considering the selected conditions — highest possible plutonium solubility and lowest observed Kd-value for plutonium sorption on rock — this value contains a considerable safety margin ... 

In the presence of high concentrations of both sulfate and fluoride, the speciation results are somewhat more difficult to interpret. Generally, the effect of fluoride predominates over that of sulfate, but not always. In deionized water containing added fluoride and sulfate to equal the concentrations of these two anions in basalt and shale ground waters, respectively, essentially all the plutonium was soluble—similar to its behavior in deionized water containing only added fluoride. Also, addition of sodium sulfate to basalt ground water to increase its concentration to that of shale ground water had no appreciable effect on plutonium solubility. 

Experimentally measured solubilities over this range of solution compositions are typically two orders of magnitude higher than calculated values (Runde et al, 2002a), presumably due to the presence of Pu(lV) colloids (Capdevila and Vitorge, 1998 Efurd et al, 1998 Knopp et al., 1999). In experimental studies in J-13 water, at ambient temperature, plutonium solubility decreased from 5xl0 M at pH = 6, to 9 X 10 M at pH = 9 (Efurd et al., 1998). 

PuiGDOMENECH, I., and J. Bruno. 1991. Plutonium solubilities, Swedich Nuclear Fuel and Waste Mgmt. Co., SKB Tech. Rept. 91-04. 

Mediterranean Sea, the plutonium activity in unfiltered water samples was twenty five times greater than that of filtered samples (2 ). In experiments in which plutonium was added to seawater, the solubility of ionic species after 30 days was found to be 5x10 (.3. Addition of relatively large quantities of humic material in these experiments increased the plutonium solubility sixfold. 

Particularly in the case of plutonium hydroxide, the amount of plutonium solublized from an internal deposit by biological ligands depends upon the oxidation state of the deposited plutonium. The charge to ionic-radius ratio and the tendency towards hydrolysis decreases in the order32 33) ... 

Plutonium solubility in marine and natural waters is limited by the formation of Pu(OH)4(am) (for amorphous) or Pu02(c) (for crystalline). The K q of these species is difficult to measure, in part due to the problems of the polymer formation. A measured value for Pu(OH)4(am) is log = -56. This value puts a limit on the amoimt of plutonium present, even if Pu(V) or Pu(IV) are the more stable states in the solution phase. Moreover, hydrolyzed Pu(IV) sorbs on colloidal and suspended material, both inorganic and biological. 

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