UC Berkeley

Ordered mesoporous materials are porous solids with a regular, patterned structure composed of pores between 2 and 50 nm wide. Such materials have attracted much attention in the past twenty years because the chemistry of their synthesis allows control of their unique physicochemical properties, which can be tuned for a variety of applications. Generally, ordered mesoporous materials have very high specific surface areas and pore volumes, and offer unique structures that are neither crystalline nor amorphous. The large tunable interface provided by ordered mesoporous solids may be advantageous in applications involving sequestration, separation, or detection of actinides and lanthanides in solution. However, the fundamental chemical interactions of actinides and lanthanides must be understood before applications can be implemented. This dissertation focuses primarily on the fundamental interactions of plutonium with organically modified mesoporous silica, as well as several different porous carbon materials, both untreated and chemically oxidized. The interactions were studied by batch contact, Pu LIII edge X-ray absorption spectroscopy, and electron microscopy techniques. Some batch sorption experiments were also performed with europium, and to a lesser extent, cerium and zirconium, to complement the plutonium studies. The extensive efforts that were required in the synthesis and characterization of the materials are also discussed.

A method for functionalizing mesoporous silica by self assembly and molecular grafting of functional organosilane ligands was optimized for the 2D-hexagonal ordered mesoporous silica known as SBA-15 (Santa Barbara amorphous silica). Four different organically-modified silica materials were synthesized and characterized with several techniques. To confirm that covalent bonds were formed between the silane anchor of the ligand and the silica substrate, functionalized silica samples were analyzed with ²⁹Si nuclear magnetic resonance spectroscopy. Infrared spectroscopy was used in combination with ¹³C and ³¹P nuclear magnetic resonance spectroscopy to verify the molecular structures of the ligands after they were synthesized and grafted to the silica. The densities of the functional silane ligands on the silica surface were estimated using thermogravimetric analysis. Batch sorption experiments were conducted with solutions of Pu(IV), Pu(VI), Eu(III), Ce(III), and Zr(IV). The acetamide phosphonate functionalized silica called Ac-Phos-SBA-15 required more extensive synthesis than the other three functionalized silica materials. Development of functionalized mesoporous silica extractants for actinides is contingent on their synthesis and hydrolytic stability, and these two aspects of the Ac-Phos-SBA-15 material are discussed. This material showed the highest binding affinity for all of the target ions, and the sorption and desorption of Pu(VI) to Ac-Phos-SBA-15 was extensively investigated. An experimental Pu uptake capacity of 30 ± 3 mg Pu per g Ac-Phos-SBA-15 was determined in pH 1.8 solutions. A combination of density functional theory modeling and experimental X-ray absorption spectroscopy data indicated that a strong 2:1 bidentate octahedral complex of PuO₂²⁺ is formed with the acetamide phosphonate ligands at the silica surface.

Ordered mesoporous carbons are attractive as sorbents because of their extremely high surface areas and large pore volumes, and could be suitable substrates for the development of actinide sensors based on their electrochemical properties. Three different mesoporous carbon materials were synthesized by collaborators to test their application as radionuclide sorbent materials. The first is called CMK (carbons mesostructured by Korea Advanced Institute of Science and Technology), and was synthesized using a hard silica template with 3D-bicontinuous ordered mesostructure. Highly ordered body-centered cubic mesoporous carbon was synthesized by self-assembly of a phenol resin around a soft polymer template, and this material is known as FDU-16 (Fudan University). Etching of the silica portion of mesoporous carbon-silica composites created the 2D-hexagonal mesoporous carbon called C-CS (carbon from carbon-silica nanocomposites) with a bimodal pore size distribution. The as-synthesized nanocast mesoporous carbon in this work is called UN CMK, and the same material after oxidation treatment with nitric acid is called OX CMK. A portion of both FDU-16-type and C-CS-type ordered mesoporous carbons were oxidized with acidic ammonium persulfate, which created the oxidized carbon materials called FDU-16-COOH and C-CS-COOH, respectively. The mesoporous carbons were characterized by scanning electron microscopy to view their particle sizes and morphologies. Their porosities and structures on the meso-scale were analyzed using transmission electron microscopy, nitrogen adsorption isotherms, and small-angle X-ray scattering. The identity and density of functional groups on the different carbon surfaces were investigated using infrared spectroscopy, elemental analysis, thermogravimetric analysis, and determination of the point-of-zero-charge with the powder addition technique. The porous carbon materials studied present a wide range of particle morphologies, mesostructures, surface areas, pore volumes, and surface chemistries.

Batch Pu(VI) sorption and desorption tests in 0.1 M NaClO₄ solutions ranging from pH 1 to 10 were performed with all six mesoporous carbons, and a commercial amorphous activated carbon powder for comparison. More extensive batch sorption studies were completed with the different carbon samples, mostly in pH 4 NaClO₄ solutions. Plutonium sorption kinetics, uptake capacities, competition with ethylenediaminetetraacetic acid in solution, Pu desorption, and effects of ionic strength were examined. Batch Eu(III) sorption studies with C-CS-type carbon and its oxidized counterpart C-CS-COOH were performed to complement the Pu studies, by demonstrating how pristine and highly oxidized ordered mesoporous carbons interact with lanthanide cations. The oxidation state and coordination environment of plutonium sorbed to pristine and oxidized ordered mesoporous carbon surfaces were probed using X-ray absorption spectroscopy and transmission electron microscopy. Both untreated and oxidized mesoporous carbons demonstrated high affinity for Pu over a wide range of solution pH conditions, and proved to be superior to activated carbon in terms of Pu sorption kinetics and capacities. The 23-hour Pu uptake capacity of mesoporous carbon materials was measured to be at least 60 ± 5 mg ²³⁹Pu per g carbon, compared to 12 ± 5 mg super>239Pu per g activated carbon. The batch and spectroscopic studies with four different mesoporous carbons showed two main Pu sorption mechanisms, depending on the concentration of oxygen-containing functional groups on the surface. Analysis of the X-ray absorption near-edge spectra determined that both untreated and oxidized carbons reduce Pu(VI) and Pu(V) to Pu(IV) in solutions with pH 2-7. The preferred binding mechanism is complexation by surface groups, which controls the sorption kinetics on a time scale of 0 to 40 minutes. After surface sites are saturated, the uptake is controlled by the formation of PuO₂ colloids, which exhibit a high affinity for mesoporous carbon surfaces. The colloids form when Pu is reduced to Pu(IV) by carbon materials with low concentrations of surface functional groups. Although Pu(V) and (VI) are also reduced at oxidized carbon surfaces, colloid formation is inhibited by surface complexation of Pu. Based on Eu sorption studies, highly oxidized ordered mesoporous carbon is a very effective general scavenger for actinide and lanthanide cations. Pu and Eu uptake by highly oxidized ordered mesoporous carbon appears to be dictated by chemisorption, and the Eu uptake capacity (138 mg/g from pH 4 solution) is higher than those previously reported for several other carbon materials. Pristine ordered mesoporous carbon exhibits a low affinity for Eu(III), but is an excellent sorbent of PuO₂ nanocrystals due to its high surface area, as well as its large pore size relative to activated carbon. The high affinity of mesoporous carbon for Pu, and the spontaneous reduction of Pu(VI) or Pu(V) to Pu(IV) at these carbon surfaces could be valuable for a variety of applications.

The plutonium sorbed to mesoporous carbon surfaces could be removed with 1 M HClO₄ or 1 M HCl, yielding solutions of Pu(III). The redox interaction between porous carbons and aqueous Pu was further investigated in batch experiments with acidic solutions. These experiments showed that porous carbons reduce Pu(VI) to Pu(III) in 1-1.5 M HCl or 1-1.5 M HClO₄, and reduce Pu(VI) to Pu(IV) in 1.3 M HNO₃. The contact time required for complete reduction of Pu generally decreases with higher surface areas, and increases with carbon surface oxidation. The pore size and ordered structure of the carbon had little or no effect on the redox reaction, provided the carbon had a high surface area. As expected, no redox activity was observed with mesoporous silica. It was demonstrated with activated carbon powder that acidic solutions of oxidized Pu can easily be reduced to Pu(III) by being passed through a column packed with porous carbon particles. This is a fast (~16 min per mL solution with the column size used in this work) and convenient way to reduce oxidized forms of Pu and dissolve Pu(IV) colloids without changing the solution matrix, adding chemical reagents, or using a potentiostat. This may be useful in a variety of settings, as manipulation and control of the oxidation state of plutonium is applicable to almost all processes involving remediation, separation and chemical purification of plutonium.