The presence of actinides in the environment is a major concern due to their radiotoxicity and long lived radioisotopes. Two notable examples of such are plutonium 239 (Pu 239) with half life of ~24,000 years and neptunium 237 (Np 237) with half life of ~2,000,000 years. Actinide migration is strongly affected by their interactions at the solid/water interface, such as sorption onto soils and sediments, and leads to the importance of understanding actinide sorption processes with environmental matrices, which contributes to accurate prediction of actinide mobility rates and development of transport models. Additionally, migration of actinides must be considered for safety assessment of long term geological nuclear waste storage and help demonstrate good stewardship at radioactively contaminated sites. Therefore, an understanding of the interactions of actinides in the environment, with a particular focus on the sorption processes that may immobilize actinides, is of utmost importance to increasing knowledge of contaminant transport in environmental systems.
Investigations of the interactions of plutonium and neptunium with the iron oxide minerals goethite and hematite are presented in this work. A primary focus was to examine Pu(VI) sorption to goethite and 1% Al substituted goethite at a fundamental level. The chemistry of plutonium is extremely complex, especially regarding solubility and oxidation state, with four different oxidation states accessible under various environmental conditions. The more oxidized forms [Pu(V) and Pu(VI)] are generally more soluble in aqueous solutions than the reduced forms [Pu(III) and Pu(IV)], and thus more mobile in oxic conditions. This underscores the importance of investigating plutonium sorption behavior of the more oxidized and soluble Pu(VI). Goethite is one of the most prevalent iron oxides in the environment. Because aluminum is often found as a minor component associated with minerals, a 1% Al substituted goethite serves as an example of goethite as it might occur in nature.
Batch sorption experiments of plutonium on goethite and 1% Al substituted goethite were conducted as a function of pH and contact time. Plutonium exhibits a complex sorption behavior with both minerals, and the data suggest sorption equilibration is reached after 7 days. As plutonium sorbed to a mineral surface will be less mobile, the presence of goethite in the environment can contribute to plutonium retardation. The complex sorption behavior indicates the sorption process might involve a redox transformation. X ray absorption spectroscopy (XAS) experiments were performed to directly determine, in situ, the oxidation state of plutonium sorbed to goethite. X ray absorption near edge structure (XANES) measurements show the coexistence of multiple plutonium oxidation states, indicating reduction of Pu(VI) to the more insoluble Pu(IV). The extent of plutonium reduction was correlated with solution pH increasing across the point of zero charge (PZC) of goethite. This suggests the reduction of Pu(VI) to Pu(IV) is via a surface mediated mechanism.
Another primary focus of this work was to examine the effect of temperature and ionic strength on Np(V) sorption to hematite. Numerous studies have been undertaken concerning the sorption behavior of neptunium with a multitude of mineral surfaces. However, most experiments have been conducted at ambient conditions, not under nuclear waste repository conditions. There is relatively little information regarding neptunium sorption at the varying temperatures encountered in nature or at the elevated temperatures (~80 °C) or ionic strengths expected in sealed geological repositories for nuclear waste. Understanding actinide sorption over a range of temperatures and ionic strengths is critical for predicting the chemical behavior of actinides for remediation efforts and for the storage of nuclear waste. Np 237 was chosen as the radionuclide of interest because of its long half life and high levels in spent nuclear fuel (SNF). Np(V) also serves as a stable chemical analog for Pu(V), which is difficult to study experimentally due to plutonium’s extreme redox sensitivity. Hematite was chosen as the solid phase because of its prevalence in nature, and also is representative of corrosion by products expected to occur in future nuclear waste repositories when SNF will be stored in steel canisters.
Batch sorption experiments were conducted as a function of pH for temperatures of 25 °C, 35 °C, 50 °C, and 75 °C for ionic strengths of 0.01 M NaClO4, 0.1 M NaClO4, and 1 M NaClO4. Np(V) sorption to hematite is temperature dependent, as greater sorption was observed with increasing temperature. Displacement of the hydrating waters may be the driving force behind the increased sorption, and the cause is postulated based on removal of water molecules from the primary hydration sphere to bulk water being entropically driven. The positive entropy change that occurs during the sorption reaction is due to the increased disorder in the system as water molecules move from a fully coordinated state in the neptunium hydration sphere to the more disordered state of bulk water. Ionic strength does not have a significant effect on sorption, indicating that the sorption mechanism is based on the formation of inner sphere Np(V) hematite complexes. In support, XAS experiments also indicate inner sphere complexation. Extended X ray absorption fine structure (EXAFS) spectra and fits show no variation of the Np(V) hematite complex as temperature and ionic strength are increased.
This work is by no means a complete study of actinide behavior in the environment, but represents a step toward a thorough understanding of the interactions of plutonium and neptunium with the iron oxide minerals goethite and hematite.