The solubility of 242PuO2 in the presence of aqueous Fe(II): the impact of precipitate preparation

Abstract

Abstract The solubility of different forms of precipitated 242PuO2(am) were examined in solutions containing aqueous Fe(II) over a range of pH values. The first series of 242PuO2(am) suspensions were prepared from a 242Pu(IV) stock that had been treated with thenoyltrifluoroacetone (TTA) to remove the 241Am originating from the decay of 241Pu. These 242PuO2(am) suspensions showed much higher solubilities at the same pH value and Fe(II) concentration than previous studies using 239PuO2(am). X-ray absorption fine structure (XAFS) spectroscopy of the precipitates showed a substantially reduced Pu–Pu backscatter over that previously observed in 239PuO2(am) precipitates, indicating that the 242PuO2(am) precipitates purified using TTA lacked the long range order previously found in239PuO2(am) precipitates. The Pu(IV) stock solution was subsequently repurified using an ion exchange resin and an additional series of 242PuO2(am) precipitates prepared. These suspensions showed higher redox potentials and total aqueous Pu concentrations than the TTA purified stock solution. The higher redox potential and aqueous Pu concentrations were in general agreement with previous studies on 242PuO2(am) precipitates, presumably due to the removal of possible organic compounds originally present in the TTA purified stock. 242PuO2(am) suspensions prepared with both stock solutions showed almost identical solubilities in Fe(II) containing solutions even though the initial aqueous Pu concentrations before the addition of Fe(II) were orders of magnitude different. By examining the solubility of 242PuO2(am) prepared from both stocks in this way we have essentially approached equilibrium from both the undersaturated and oversaturated conditions. The final aqueous Pu concentrations are predictable using a chemical equilibrium model which includes the formation of a nanometer sized Fe(III) reaction product, identified in the 242PuO2(am) suspension both by use of 57Fe Mössbauer spectroscopy and transmission electron microscopy (TEM) analysis.