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

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.