Neptunium and plutonium solubilities in a Yucca Mountain groundwater

Solubilities of neptunium and plutonium were studied in J-13 groundwater (ionic strength of about 3.7 mmol; total dissolved carbonate of 2.8 mmol) from the proposed Yucca Mountain Nuclear Waste Repository site, Nevada, at three different temperatures (25, 60, and 90 degrees C) and pH values (6.0 7.0,and 8.5). Experiments were performed from both over- and undersaturation at defined CO2 partial pressures. The solubility of Np-237 from oversaturation ranged from a high of (9.40 +/- 1.22) x 10(-4) M at pH 6.0 and 60 degrees C to a low of (5.50 +/- 1.97) x 10(-6) M at pH 8.5 and 90 degrees C. The analytical results of solubility experiments from undersaturation (temperatures of 25 and 90 degrees C and pH values 6, 7, and 8.5) converged on these values. The Pu-239/240 solubilities ranged from (4.70 +/- 1.13) x 10(-8) M at pH 6.0 and 25 degrees C to (3.62 +/- 1.14) x 10(-9) M at pH 8.5 and 90 degrees C. In general, both neptunium and plutonium solubilities decreased with increasing pH and temperature. Greenish-brown crystalline Np2O5. xH(2)O was identified as the solubility-limiting solid using X-ray diffraction. A mean thermodynamic solubility product for Np2O5. xH(2)O of log K(sp)degrees = 5.2 +/- 0.8 for the reaction Np2O5. xH(2)O + 2 H+ reversible arrow 2NpO(2)(+) + (x+1)H2O at 25 degrees C was calculated. Sparingly soluble Pu(IV) solids, PuO2. xH(2)O and/or amorphous plutonium(IV) hydroxide/colloids, control the solubility of plutonium in J-13 water.