Denis M. Strachan

Separation of rare gases and chiral molecules by selective binding in porous organic cages

The separation of molecules with similar size and shape is an important technological challenge. For example, rare gases can pose either an economic opportunity or an environmental hazard and there is a need to separate these spherical molecules selectively at low concentrations in air. Likewise, chiral molecules are important building blocks for pharmaceuticals, but chiral enantiomers, by definition, have identical size and shape, and their separation can be challenging. Here we show that a porous organic cage molecule has unprecedented performance in the solid state for the separation of rare gases, such as krypton and xenon. The selectivity arises from a precise size match between the rare gas and the organic cage cavity, as predicted by molecular simulations. Breakthrough experiments demonstrate real practical potential for the separation of krypton, xenon and radon from air at concentrations of only a few parts per million. We also demonstrate selective binding of chiral organic molecules such as 1-phenylethanol, suggesting applications in enantioselective separation.

Potential of Metal–Organic Frameworks for Separation of Xenon and Krypton

CONSPECTUS: The total world energy demand is predicted to rise significantly over the next few decades, primarily driven by the continuous growth of the developing world. With rapid depletion of nonrenewable traditional fossil fuels, which currently account for almost 86% of the worldwide energy output, the search for viable alternative energy resources is becoming more important from a national security and economic development standpoint. Nuclear energy, an emission-free, high-energy-density source produced by means of controlled nuclear fission, is often considered as a clean, affordable alternative to fossil fuel. However, the successful installation of an efficient and economically viable industrial-scale process to properly sequester and mitigate the nuclear-fission-related, highly radioactive waste (e.g., used nuclear fuel (UNF)) is a prerequisite for any further development of nuclear energy in the near future. Reprocessing of UNF is often considered to be a logical way to minimize the volume of high-level radioactive waste, though the generation of volatile radionuclides during reprocessing raises a significant engineering challenge for its successful implementation. The volatile radionuclides include but are not limited to noble gases (predominately isotopes of Xe and Kr) and must be captured during the process to avoid being released into the environment. Currently, energy-intensive cryogenic distillation is the primary means to capture and separate radioactive noble gas isotopes during UNF reprocessing. A similar cryogenic process is implemented during commercial production of noble gases though removal from air. In light of their high commercial values, particularly in lighting and medical industries, and associated high production costs, alternate approaches for Xe/Kr capture and storage are of contemporary research interest. The proposed pathways for Xe/Kr removal and capture can essentially be divided in two categories: selective absorption by dissolution in solvents and physisorption on porous materials. Physisorption-based separation and adsorption on highly functional porous materials are promising alternatives to the energy-intensive cryogenic distillation process, where the adsorbents are characterized by high surface areas and thus high removal capacities and often can be chemically fine-tuned to enhance the adsorbate-adsorbent interactions for optimum selectivity. Several traditional porous adsorbents such as zeolites and activated carbon have been tested for noble gas capture but have shown low capacity, selectivity, and lack of modularity. Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are an emerging class of solid-state adsorbents that can be tailor-made for applications ranging from gas adsorption and separation to catalysis and sensing. Herein we give a concise summary of the background and development of Xe/Kr separation technologies with a focus on UNF reprocessing and the prospects of MOF-based adsorbents for that particular application.

Metal–Organic Frameworks for Removal of Xe and Kr from Nuclear Fuel Reprocessing Plants

Removal of xenon (Xe) and krypton (Kr) from process off-gases containing 400 ppm Xe, 40 ppm Kr, 78% N(2), 21% O(2), 0.9% Ar, 0.03% CO(2), and so forth using adsorption was demonstrated for the first time. Two well-known metal-organic frameworks (MOFs), HKUST-1 and Ni/DOBDC, which both have unsaturated metal centers but different pore morphologies, were selected as novel adsorbents. Results of an activated carbon were also included for comparison. The Ni/DOBDC has higher Xe/Kr selectivities than those of the activated carbon and the HKUST-1. In addition, results show that the Ni/DOBDC and HKUST-1 can adsorb substantial amounts of Xe and Kr even when they are mixed in air. Moreover, the Ni/DOBDC can successfully separate 400 ppm Xe from 40 ppm Kr and air containing O(2), N(2), and CO(2) with a Xe/Ke selectivity of 7.3 as indicated by our breakthrough results. This shows a promising future for MOFs in radioactive nuclide separations from spent fuels.

Switching Kr/Xe selectivity with temperature in a metal-organic framework.

Krypton (Kr) and xenon (Xe) adsorption on two partially fluorinated metal-organic frameworks (FMOFCu and FMOFZn) with different cavity size and topologies are reported. FMOFCu shows an inversion in sorption selectivity toward Kr at temperatures below 0 °C while FMOFZn does not. The 1D microtubes packed along the (101) direction connected through small bottleneck windows in FMOFCu appear to be the reason for this peculiar behavior. The FMOFCu shows an estimated Kr/Xe selectivity of 36 at 0.1 bar and 203 K.

Raman spectroscopic study of gadolinium(III) in sodium-aluminoborosilicate glasses

Abstract A Raman spectroscopy study was performed on a series of sodium-aluminoborosilicate glasses with Gd2O3 from 0 mass% up to its solubility (47 mass% or 13.58 mol%). Experimentally measured spectra were fitted with a Gaussian function for each individual band without any restriction on the band position, width, and intensity. The evolution of the spectroscopic bands appears to result from partitioning of the rare earth cation, as its dissolution mechanism, in these borosilicate glasses. Specifically, the evolution of the Raman bands was correlated well with Gd cations partitioning in the borate-rich environment at low Gd2O3 concentration, Gd2O3/[1/3B2O3] 1. Raman bands near 1420 and 710 cm −1 suggest the presence of a local Gd-metaborate environment, which appeared to occur at all Gd2O3 concentrations. The bands near 300 and 910 cm −1 further suggest the formation of Gd–O–Gd clusters in the silicate-rich environment at high Gd2O3 concentrations.

Dissolution Kinetics of Pyrochlore Ceramics for the Disposition of Plutonium.

Abstract Single-pass β ow-through (SPFT) experiments were conducted on a set of non-radioactive Ti-based ceramics at 90 °C and pH = 2 to 12. The specimens contained 27.9 to 35.8 wt%CeO2 as a surrogate for UO2 and PuO2. Compositions were formulated as TiO2-saturated pyrochlore (CeP1) and pyrochlorerich baseline (CePB1) ceramic waste forms. Pyrochlore + Hf-rutile and pyrochlore + perovskite + Hf-rutile constituted the major phases in the CeP1 and CePB1 ceramics, respectively. Results from dissolution experiments between pH = 2 to 12 indicate a shallow pH-dependence with an ill-defined minimum. Element release rates determined from experiments over a range of sample surface areas (S) and β ow rates (q) indicate that dissolution rates become independent of q/S values at 10.8 to 10.7 m/s. Dissolution rates dropped sharply at lower values of q/S, indicating rates that are subject to solution saturation effects as dissolved constituents become concentrated. Forward dissolution rates were 1.3(0.30) x 10-3 and 5.5(1.3) x 10-3 g/m2·d for CeP1 and CePB1 ceramics, respectively. Dissolution rates obtained in other laboratories compare well to the findings of this study, once the rate data are placed in the context of solution saturation state. These results make progress toward an evaluation of CeO2 as a surrogate for UO2 and PuO2 as well as establishing a baseline for comparison with radiation- damaged specimens.

Radioactive Iodine and Krypton Control for Nuclear Fuel Reprocessing Facilities

The removal of volatile radionuclides generated during used nuclear fuel reprocessing in the US is almost certain to be necessary for the licensing of a reprocessing facility in the US. Various control technologies have been developed, tested, or used over the past 50 years for control of volatile radionuclide emissions from used fuel reprocessing plants. The US DOE has sponsored, since 2009, an Off-gas Sigma Team to perform research and development focused on the most pressing volatile radionuclide control and immobilization problems. In this paper, we focus on the control requirements and methodologies for 85Kr and 129I. Numerous candidate technologies have been studied and developed at laboratory and pilot-plant scales in an effort to meet the need for high iodine control efficiency and to advance alternatives to cryogenic separations for krypton control. Several of these show promising results. Iodine decontamination factors as high as 105, iodine loading capacities, and other adsorption parameters including adsorption rates have been demonstrated under some conditions for both silver zeolite (AgZ) and Ag-functionalized aerogel. Sorbents, including an engineered form of AgZ and selected metal organic framework materials (MOFs), have been successfully demonstrated to capture Kr and Xe without the need for separations at cryogenic temperatures.

Chalcogen -based aerogels as a multifunctional platform for remediation of radioactive iodine

Aerogels employing chalcogen-based (i.e., S, Se, and/or Te) structural units and interlinking metals are termed chalcogels and have many emerging applications. Here, chalcogels are discussed in the context of nuclear fuel reprocessing and radioactive waste remediation. Motivated by previous work on removal of heavy metals in aqueous solution, we explored the application of germanium sulfide chalcogels as a sorbent for gas-phase I2 based on Pearsons Hard/Soft Acid–Base (HSAB) principle. This work was driven by a significant need for high-efficiency sorbents for 129I, a long-lived isotope evolved during irradiated UO2 nuclear fuel reprocessing. These chalcogel compositions are shown to possess an affinity for iodine gas, I2(g), at various concentrations in air. This affinity is attributed to a strong chemical attraction between the chalcogen and I2(g), according to the HSAB principle. The high sorption efficiency is facilitated by the high porosity as well as the exceptionally large surface area of the chalcogels. This paper briefly discusses the current and alternative waste forms for 129I, elaborates on preliminary work to evaluate a Pt-Ge-S chalcogel as a I2(g) sorbent, and discusses the unknown chalcogel properties related to these materials in waste form.

Glass dissolution: testing and modeling for long-term behavior

The basic concepts of thermodynamics and kinetics are discussed in relationship to glass dissolution testing. While it seems like these subjects are too basic to be discussed in a journal article, it is often the case that we forget these concepts when planning and designing dissolution tests and interpreting the results that come from these tests. The possible connection between the composition of the dissolving glass and its long-term behavior is discussed. Results from a preliminary study suggest that the aluminum content of a glass is important to its long-term behavior. The formation of a zeolite can cause the glass dissolution rate to increase under certain conditions that can be modeled. Results indicate that complex glasses can be modeled with a glass containing as little as six components.

Crystallization of gadolinium- and lanthanum-containing phases from sodium alumino-borosilicate glasses

Abstract Lanthanide-containing glasses, commonly used for optical and laser applications, are also important in the vitrification of actinide-bearing radioactive wastes. In previous studies, we measured the glass forming regions of La2O3 and Gd2O3 in some sodium alumino-borosilicate glasses. Above their highest concentrations in these glasses, lanthanide silicate crystals with an apatite structure were found. In this paper, we characterize these crystals using powder X-ray diffraction (XRD), electron microscopy, energy dispersive spectroscopy (EDS), and selective area diffraction (SAD) to evaluate baseline glass composition effect and mixed La/Gd effect on the structure and chemistry of these crystals. When different lanthanide elements (lanthanum and gadolinium) co-exist in the glasses, complete lanthanide silicate solid solution is observed. Small amounts of boron can enter the gadolinium silicate structure if aluminum is present in the melt. The boron is probably substituting for the silicon in the crystal lattice. This substitution will cause a decrease in the unit cell parameters a0 and c0. A small amount of Na can also enter the crystal lattice, causing a decrease in the cell parameter a0, but an increase in c0. These results may help us to develop better understanding on the solution mechanism of lanthanide oxides in these glasses.