Plutonium redox chemistry under anoxic conditions in the presence of iron(II) bearing minerals

The environmental fate of plutonium, the major transuranium actinide in nuclear waste, is largely impacted by its sorption onto and redox reactions with iron oxide, carbonate or sulfide minerals that form as corrosion products of steel in the "near field" and occur widely in sediments. To obtain information on oxidation state and local structure of Pu in the presence of Fe(II) bearing minerals, electrolytically prepared Pu(V) or Pu(III) (242Pu, 1-3·10-5 M) were, under anoxic conditions, reacted with magnetite (FeIIFeIII2O4) (pH 6-8.5), chukanovite (Fe2(CO3)(OH)2) (pH 8.5) and mackinawite (FeS) (pH 6-8.5). Pu-LIII-edge X-ray absorption spectra (XAS) were collected after 40 d and 8 months of reaction.
In all 14 samples, more than 98 % of Pu was associated with the solid phase and its redox speciation thus accessible by XAS. With magnetite, only the sample prepared at the highest pH and highest Pu loading contained Pu(IV)O2 while in all others Pu was solely present as a tridentate Pu(III) surface complex [1]. The three chukanovite samples all contained both Pu(III) (15 to 40 %) and PuO2. With mackinawite at pH 6 only Pu(III) was present, while all samples prepared at pH7 and higher contained mostly PuO2 and up to approx. 10 % Pu(III).
Through comparison of the different types of minerals (oxide, carbonate, sulfide), reaction pH and Pu/mineral ratios, it becomes apparent that the type of surface complexation (e.g. inner-sphere on magnetite vs. outer-sphere on mackinawite) and total mineral surface area are key parameters in controlling concentrations of dissolved Pu and in determining whether a PuO2 solid phase precipitates. While PuO2 provides an upper limit for concentrations of dissolved Pu, the available mineral surface area and sorption complex stability control what percentage of Pu is present in surface complexes. Under reducing conditions as established through the Fe(II) bearing minerals used here, this mineral surface associated Pu was found to be trivalent. Surface complexed Pu(III) and PuO2 can be thought of being in equilibrium with each other via two processes: a sorption reaction between dissolved and surface complexed Pu(III) and a heterogeneous redox reaction between dissolved Pu(III) and solid phase Pu(IV)O2. It remains to be investigated if and through what mechanisms the Pu solid phase speciation (sorbed Pu(III) vs. solid phase PuO2) might impact the migration behavior of Pu and how, for risk assessment purposes, Pu(III) surface complexes with iron minerals can be implemented into geochemical models.