Solubilities of neptunium and plutonium were studied inJ-13 groundwater (ionic strength of about 3.7 mmol; totaldissolved carbonate of 2.8 mmol) from the proposed YuccaMountain Nuclear Waste Repository site, Nevada, atthree different temperatures (25, 60, and 90
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C) and pHvalues (6.0, 7.0, and 8.5). Experiments were performed fromboth over- and undersaturation at defined CO
2 partialpressures. The solubility of
237Np from oversaturationranged from a high of (9.40 ± 1.22) × 10
-4 M at pH 6.0and 60
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C to a low of (5.50 ± 1.97) × 10
-6 M at pH 8.5 and90
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C. The analytical results of solubility experimentsfrom undersaturation (temperatures of 25 and 90
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C andpH values 6, 7, and 8.5) converged on these values. The
239/240Pu solubilities ranged from (4.70 ± 1.13) × 10
-8 M at pH6.0 and 25
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C to (3.62 ± 1.14) × 10
-9 M at pH 8.5 and 90
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C. In general, both neptunium and plutonium solubilitiesdecreased with increasing pH and temperature. Greenish-brown crystalline Np
2O
5·
xH
2O was identified as thesolubility-limiting solid using X-ray diffraction. A meanthermodynamic solubility product for Np
2O
5·
xH
2O of log
K
sp= 5.2 ± 0.8 for the reaction Np
2O
5·
xH
2O + 2 H
+ ![](/images/entities/rlhar2.gif)
2NpO
2+ + (
x+1)H
2O at 25
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C was calculated. Sparinglysoluble Pu(IV) solids, PuO
2·
xH
2O and/or amorphousplutonium(IV) hydroxide/colloids, control the solubility ofplutonium in J-13 water.