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.

Efficient extraction and separation of actinides, lanthanides, and fission products from acidic or high ionic strength media is desirable for applications in nuclear waste reprocessing, environmental remediation, and laboratory scale processing of accelerator targets or forensics samples. The adaptation of traditional solvent extraction agents for use in solid phases would potentially reduce or eliminate the generation of mixed hazardous, radioactive waste streams. This dissertation presents the synthesis and evaluation of organically functionalized silica for the extraction of U(VI) from aqueous solutions. Diethylphosphonate and acetamide diethylphosphonate were grafted to an ordered mesoporous silica substrate, Santa Barbara Amorphous (SBA)-15. The phosphonate functional groups were chosen based on their demonstrated utility in solid and liquid phase extraction studies. SBA-15 has shown great potential as a framework for organic modification due to its high surface area, tunable pore size and particle morphology, resilience in acidic media, and hydrothermal robustness.

The work presented here is motivated by the need to obtain a better fundamental understanding of 1) the functionalization process, 2) the structure of the organometallic complexes formed at the functionalized silica surface, and 3) the limitations of metal extraction. Most previous studies on heavy metal extraction using organically modified silica have relied on measurement of macroscopic properties. In contrast, this work uses a combination of batch contact and nuclear magnetic resonance spectroscopic techniques to more thoroughly characterize the functional layer, and to probe the interaction between uranyl and the surface. In doing so, a greater understanding of the limitations of these types of materials has been achieved.

The fundamental interaction of U(VI) with diethylphosphatoethyl triethoxysilane (DPTS) functionalized SBA-15 mesoporous silica is studied by macroscopic batch experiments and solid-state nuclear magnetic resonance (NMR) spectroscopy. DPTS-functionalized silica has been shown to extract U(VI) from nitric acid solutions at or above pH 3. Extraction is dependent on pH and ionic strength. Single-pulse 31P NMR on U(VI) contacted samples revealed that U(VI) only interacts with a fraction of the ligands present on the surface. At pH 4 the U(VI) extraction capacity of the material is limited to 27-37% of the theoretical capacity, based on ligand loading. Single pulse 31P NMR spectroscopy on U(VI)-contacted samples was combined with batch studies to measure a ligand-to-metal ratio of approximately 2:1 at pH 3 and 4. Batch studies and cross-polarization NMR measurements reveal that U(VI) binds to deprotonated phosphonate and/or silanol sites. 31P-31P DQ-DRENAR (Double-Quantum-based Dipolar Re-coupling effects Nuclear Alignment Reduction) NMR studies were used to compare the average dipolar coupling between phosphorus spins for both U(VI)-complexed and non-complexed ligand environments. These measurements reveal that U(VI) extraction is not limited by inadequate surface distribution of ligands, but rather by low stability of the surface phosphonate complex.

Acetamide diethylphosphonate (AcPhos)-functionalized silica has been shown to have a high affinity for U(VI) in pH 2-3 nitric acid solutions. Previous work has focused on actinide and lanthanide extraction under various conditions, but has shown poor reproducibility in the functionalization process. For this work, four AcPhos-functionalized SBA-15 materials were synthesized and evaluated based on their U(VI) sorption capacity and their resilience to contact with pH 3 nitric acid solutions. Materials synthesized with a pyridine base catalyst were shown to form a greater fraction of polymeric structures at the silica surface, which correlated with higher structural integrity upon contact with acidic solutions. The single-pulse 31P and 1H NMR spectra of these materials show evidence of the formation of phosphonic acid groups, as well as hydrogen-bonding interactions between ligands or with the silica surface. Additionally, these materials were found to have a significantly higher U(VI) sorption capacity and Keq than the materials synthesized without pyridine, most likely due to the ion-exchange properties of the phosphonic acid groups. The 31P-31P DQ-DRENAR NMR technique was again employed to compare the average strength of dipolar coupling interactions between phosphorus atoms for the four different materials. Because the strength of dipolar coupling interactions depends on the number and proximity of neighboring spins, this technique provides information about the average density of ligands on the surface. The conventional functionalization procedure yielded materials with the lowest average surface ligand density, while those using extended reaction times and the pyridine base catalyst yielded materials with higher surface ligand densities.

Physical separation of short-lived isotopes produced in heavy-ion-induced fusion reactions is a powerful and well know method and often applied in investigations of the heaviest elements, called the transactinides (Z>=104). By extracting these isotopes from a recoil separator, they can be made available for transport to setups located outside the heavily shielded irradiation position such as chemistry setups. This physical preseparation technique overcomes many limitations currently faced in the chemical investigation of transactinides. Here we describe the basic principle using relatively short-lived isotopes of the lighter group 4 elements zirconium (Zr) and hafnium (Hf) that are used as analogs of the lightest transactinide element, rutherfordium (Rf, element 104). The Zr and Hf isotopes were produced at the LBNL 88-Inch Cyclotron using a cocktail of 18O and 50Ti beams and the appropriate targets. Subsequently, the isotopes were physically separated in the Berkeley Gas-filled Separator (BGS) and guided to a Recoil Transfer Chamber (RTC) to transfer them to chemistry setups. The magnetic rigidities of the reaction products in low-pressure helium gas were measured and their identities determined with gamma-pectroscopy. Using preseparated isotopes has the advantages of low background and beam plasma free environment for chemistry experiments. The new possibilities that open up for chemical investigations of transactinide elements are descr ibed. The method can readily be applied to homologous elements within other groups in the periodic table.