Nuclear Waste Remediation Research Articles

Kinetics of the Temperature-Dependent eaq - and ⋅OH Radical Reactions with Cr(III) Ions in Aqueous Solutions.

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (eaq - ) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr3+ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)2+ , did with a rate coefficient of k= (7.2±0.3)×108 M-1 s-1 at 25 °C. The oxidation of Cr(OH)2+ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.

Metal Sulfide Ion Exchangers: High Acid Stability of Na2xMg2y-xSn4-yS8 (NMS) and Topotactic Conversion to 2D Solid Acids with Semiconducting Character.

Metal sulfide ion exchange materials (MSIEs) are of interest for nuclear waste remediation applications. We report the high stability of two structurally related metal sulfide ion exchange materials, Na2xMg2y-xSn4-yS8 (Mg-NMS) and Na2SnS3 (Na-NMS), in strongly acid media, in addition to the preparation of Na2xNi2y-xSn4-yS8 (Ni-NMS). Their formation progress during synthesis is studied with in-situ methods, with the target phases appearing in <15 min, reaction completion in <12 h, and high yields (75-80%). Upon contact with nitric or hydrochloric acid, these materials topotactically exchange Na+ for H+, proceeding in a stepwise protonation pathway for Na5.33Sn2.67S8. Na-NMS is stable in 2 M HNO3 and Mg-NMS is stable in 4 M HNO3 for up to 4 h, while both NMS materials are stable in 6 M HCl for up to 4 days. However, the treatment of Mg-NMS and Na-NMS with 2-6 M H2SO4 reveals a much slower protonation process since after 4 h of contact both NMS and HMS are present in the solution. The resultant protonated materials, H2xMg2y-xSn4-yS8 and H4x[(HyNay-1)1.33xSn4--1.33x]S8, are themselves solid acids and readily react with and intercalate a variety of organic amines, where the band gap of the resultant adduct is influenced by amine choice and can be tuned within the range of 1.88(5)-2.27(5) eV. The work function energy values for all materials were extracted from photoemission yield spectroscopy in air (PYSA) measurements and range from 5.47 (2) to 5.76 (2) eV, and the relative band alignments of the materials are discussed. DFT calculations suggest that the electronic structure of Na2MgSn3S8 and H2MgSn3S8 makes them indirect gap semiconductors with multi-valley band edges, with carriers confined to the [MgSn3S8]2- layers. Light electron effective masses indicate high electron mobilities.

Robust 2D porphyrin metal–organic framework nanosheets for high-efficiency photoreduction-assisted uranium recovery from wastewater

The elimination of highly radioactive and chemically toxic uranium from nuclear waste is of significance for radionuclide remediation and environment protection. Herein, two-dimensional (2D) porphyrin-based nickel (Ni-TCPP) metal–organic frameworks (MOFs) are successfully prepared by a simple surfactant-assisted strategy. Owing to highly accessible active sites and low diffusion barrier, 2D Ni-TCPP nanosheets present fast uranium capture by coordination of uranyl ions with TCPP ligands and Ni paddlewheel metal nodes. Particularly, 2D Ni-TCPP nanosheets display outstanding photocatalytic activity due to the merit of dual ligand-to-metal charge transfer pathways, greatly improving photoreduction-assisted uranium recovery performance. As a result, light-irradiated 2D Ni-TCPP adsorbents display fast removal of uranium from acidic wastewater, excellent selectivity and good reusability. Moreover, a high uranium removal rate can be maintained even in the presence of excess calcium ions. These advantages combined with good structure stability make 2D Ni-TCPP MOFs promising for selective uranium separation in nuclear waste remediation.

Challenges in Planning of Integrated Nuclear Waste Management

Planning for integrated nuclear waste management (INWM) entails consideration of all generated waste from energy generation, nuclear fuel cycle and institutional facilities as well as waste from decommissioning and remediation of nuclear facilities, legacy waste, and eventual accident waste and requires establishment of different planning scenarios as well as control milestones to allow for adequate flexibility to address inevitable changes. An early assessment of waste management needs from development and use of advanced reactors and innovative nuclear fuel cycles is required to aid design and operation of such facilities as well as to understand their impact to overall waste management planning. Major prerequisites for approach to planning and establishment of INWM plans are discussed briefly. It is pointed out that five most important challenges in establishing and implementing the INWM plan needs to be addressed: (i) inventory; (ii) time frame for an integrated plan; (iii) assessment of facility needs; (iv) costs estimation (life-cycle cost analyses) and (v) funding and financing. The INWM has to promote strategic thinking within a broad framework resulting in a sustainable and sensible outcome for nuclear waste management at a strategic and national level.

Novel phenanthroline-derived pyrrolidone ligands for efficient uranium separation: Liquid-liquid extraction, spectroscopy, and molecular simulations

A challenge in nuclear waste remediation is the construction of extraction systems with exceptional stability and remarkable selectivity under harsh conditions, to achieve efficient uranium separation from nuclear wastewater. In this work, two novel N, O-hybrid extractants, L1 and L2 attaching two or one pyrrolidone groups to the phenanthroline skeleton were synthesized and applied in uranium extraction. Solvent extraction tests showed that these ligands in 3-nitrobenzotrifluoride can extract uranium with fast kinetics, high selectivity over twelve competing ions, and facile stripping. Slope analysis indicated that each uranyl ion binds to one ligand during extraction, and the extraction conforms to the cation exchange mechanism. The effect of temperature on uranium extraction was also carried out to determine the thermodynamic parameters. Notably, outstanding radiation stability was demonstrated for these extractants even after being exposed to 500 kGy γ ray irradiation. In addition, NMR titration and DFT calculations were performed to understand the complexation behaviors between ligands and uranyl ions. This study reveals the potential of these two phenanthroline-based extractants for the use in uranium separation from nuclear wastewater.

Combined Effects of Fe(III)-Bearing Nontronite and Organic Ligands on Biogenic U(IV) Oxidation.

Bioreduction of soluble U(VI) to sparingly soluble U(IV) solids was proposed as a remediation method for uranium contamination. Therefore, the stability and longevity of biogenic U(IV) are critical to the success of uranium remediation. However, co-occurrence of clay minerals and organic ligands could potentially reoxidize U(IV) to U(VI). Herein, we report a combined effect of Fe(III)-rich nontronite (NAu-2) and environmentally prevalent organic ligands on reoxidation of biogenic U(IV) at circumneutral pH. After 30 days of incubation, structural Fe(III) in NAu-2 oxidized 45.50% U(IV) with an initial rate of 2.7 × 10-3 mol m-2 d-1. Addition of citrate and ethylenediaminetetraacetic acid (EDTA) greatly promoted the oxidative dissolution of U(IV) by structural Fe(III) in NAu-2, primarily through the formation of aqueous ligand-U(IV) complexes. In contrast, a model siderophore, desferrioxamine B (DFOB), partially inhibited U(IV) oxidation due to the formation of stable DFOB-Fe3+ complexes. The resulting U(VI) species intercalated into an NAu-2 interlayer or adsorbed onto an NAu-2 surface. Our results highlight the importance of organic ligands in oxidative dissolution of U(IV) minerals by Fe(III)-bearing clay minerals and have important implications for the design of nuclear waste storage and remediation strategies, especially in clay- and organic-rich environments.

Quantitative UO bond activation in uranyl complexes via silyl radical transfer.

Reductive silylation of the uranyl dication with 1,4-bis(trimethylsilyl)dihydropyrazine, or "Mashima's reagent", is detailed. The reagent simultaneously delivers silylium ions and electrons to multiple uranyl complexes, i.e. a pyridine dipyrrolide uranyl complex and a common uranyl-containing starting material, UO2Cl2(OPPh3)2. This reaction results in quantitative activation of thermodynamically robust UO bonds. The reductive functionalization of environmentally persistent actinyl species with Mashima's reagent is seen as a promising solution for nuclear waste remediation.

Synthesis and Structural Characterisation of Yttrium-Doped α-Zirconium Phosphate

There has been a considerable amount of interest in the ion-exchange properties of layered zirconium phosphates. Potential applications in the remediation of nuclear waste have renewed interest in these inorganic materials, due to their high stability under the acidic conditions typically found in legacy waste pools. It has been well documented that the substitution of metals with different ionic radii into the frameworks of inorganic materials can alter the chemical properties including ion-exchange selectivity. The work presented here focusses on the synthesis and characterisation of yttrium-doped α-zirconium phosphates which are reported for the first time. Two different synthetic methods were used, reflux and hydrothermal syntheses, and the products were characterised by various methods such as powdered X-ray diffraction, MAS-NMR and scanning electron microscopy. It was found that up to 15% of zirconium could be replaced by yttrium before any noticeable impurity phases could be observed. Rietveld refinement from the doping showed that the products did not obey the Vegard’s law. However, the ion-exchange results clearly showed enhanced capacities and selectivity towards Co2+ ions for the substituted materials.

Xenon Trapping in Metal‐Supported Silica Nanocages (Small 39/2021)

Xenon Trapping Xenon (Xe) atoms are trapped in silicate nanocages in contact with a metal surface. The atoms can be trapped at moderate temperatures (> 300 K) and released by heating to elevated temperatures (> 750 K). This can benefit several applications, including Xe production, nuclear power, nuclear waste remediation, and nonpro liferation of nuclear weapons. More details can be found in article 2103661 by Matheus Dorneles de Mello, J. Anibal Boscoboinik, and co-workers.

Xenon Trapping in Metal-Supported Silica Nanocages.

Xenon (Xe) is a valuable and scarce noble gas used in various applications, including lighting, electronics, and anesthetics, among many others. It is also a volatile byproduct of the nuclear fission of uranium. A novel material architecture consisting of silicate nanocages in contact with a metal surface and an approach for trapping single Xe atoms in these cages is presented. The trapping is done at low Xe pressures and temperatures between 400 and 600 K, and the process is monitored in situ using synchrotron-based ambient pressure X-ray photoelectron spectroscopy. Release of the Xe from the cages occurs only when heating to temperatures above 750 K. A model that explains the experimental trapping kinetics is proposed and tested using Monte Carlo methods. Density functional theory calculations show activation energies for Xe exiting the cages consistent with experiments. This work can have significant implications in various fields, including Xe production, nuclear power, nuclear waste remediation, and nonproliferation of nuclear weapons. The results are also expected to apply to argon, krypton, and radon, opening an even more comprehensive range of applications.

Ion Pairing Mediates Molecular Organization Across Liquid/Liquid Interfaces.

Liquid/liquid interfaces play a central role in scientific fields ranging from nanomaterial synthesis and soft matter electronics to nuclear waste remediation and chemical separations. This diversity of functions arises from an interface's ability to respond to changing conditions in its neighboring bulk phases. Understanding what drives this interfacial flexibility can provide novel avenues for designing new functional interfaces. However, limiting this progress is an inadequate understanding of the subtle intermolecular and interphase interactions taking place at the molecular level. Here, we use surface-specific vibrational sum frequency generation spectroscopy combined with atomistic molecular dynamics simulations to investigate the self-assembly and structure of model ionic oligomers consisting of an oligodimethylsiloxane (ODMS) tail covalently attached to a positively charged methyl imidazolium (MIM+) head group at buried oil/aqueous interfaces. We show how the presence of seemingly innocuous salts can impart dramatic changes to the ODMS tail conformations in the oil phase via specific ion effects and ion-pairing interactions taking place in the aqueous phase. These specific ion interactions are shown to drive enhanced amphiphile adsorption, induce morphological changes, and disrupt emergent hydrogen-bonding structures at the interface. Tuning these interactions allows for independent control over the oligomer structure in the oil phase versus interfacial population changes and represents key mechanistic insight that is needed to control chemical reactions at liquid/liquid interfaces.

99TcO4- Separation through Selective Crystallization Assisted by Polydentate Benzene-Aminoguanidinium Ligands.

99Tc is one of the most abundant radiotoxic isotopes in used nuclear fuel with a high fission yield and a long half-life. Effective removal of pertechnetate (TcO4-) from an aqueous solution is important for nuclear waste separation and remediation. Herein, we report a series of facilely obtained benzene-linked guanidiniums that could precipitate TcO4- and its nonradioactive surrogate ReO4- from a high-concentration acidic solution through self-assembly crystallization. The resulting perrhenate and pertechnetate solids exhibit exceptionally low aqueous solubility. The benzene-linked guanidiniums hold one of the highest TcO4- removal capacities (1279 mg g-1) among previously reported materials and possess a removal percentage of 59% for ReO4- in the presence of Cl- over 50 times. The crystallization mechanism was clearly illustrated by the single-crystal structures and density functional theory calculations, indicating that TcO4- is captured through a charge-assisted hydrogen bonding interaction and stabilized by π-π stacking layers. In addition, the removal process is easily recycled and no toxic organic reagents are introduced. This work provides a green approach to preliminarily separate TcO4- from high-level nuclear wastes.

Hard, soft, hot, cold

F Block Chemistry Efficient sequestration of radioactive americium (Am) ions remains a challenge in nuclear waste remediation. Part of the difficulty is that the ions have a similar size and charge to stable lanthanide ions. Goodwin et al. systematically explored whether differences in covalency might manifest when Am and neodymium (Nd) are coordinated by more or less polarizable ligands (often termed soft or hard). They compared the crystal structures and spectral and computational electronic properties of Am(III) and Nd(III) with soft selenium (Se) or hard oxygen ligands. The upshot was that Am showed signs of covalency with Se, whereas Nd did not. Angew. Chem. Int. Ed. 10.1002/anie.202017186 (2021).

Hydrothermal Synthesis and Structural Investigation of a Crystalline Uranyl Borosilicate

The relevance of multidimensional and porous crystalline materials to nuclear waste remediation and storage applications has motivated exploratory research focused on materials discovery of compounds, such as actinide mixed-oxoanion phases, which exhibit rich structural chemistry. The novel phase K1.8Na1.2[(UO2)BSi4O12] has been synthesized using hydrothermal methods, representing the first example of a uranyl borosilicate. The three-dimensional structure crystallizes in the orthorhombic space group Cmce with lattice parameters a = 15.5471(19) Å, b = 14.3403(17) Å, c = 11.7315(15) Å, and V = 2615.5(6) Å3, and is composed of UO6 octahedra linked by [BSi4O12]5− chains to form a [(UO2)BSi4O12]3− framework. The synthesis method, structure, results of Raman, IR, and X-ray absorption spectroscopy, and thermal stability are discussed.

Magnetic CoFe2O4/Graphene oxide nanocomposite for highly efficient separation of f-block elements

A magnetic nanocomposite, CoFe2O4/Graphene oxide (CoFe2O4/GO) was fabricated for highly efficient separation of f-block elements. The growth of CoFe2O4 nanoparticles on the GO nanosheets was confirmed by the X-ray diffraction pattern. The sorbent was found to be superparamagnetic with a large magnetization saturation (38 emu g‒1). This magnetic nanocomposite was found to have high sorption capacity for f-block elements following their ionic potential. The sorption kinetics followed the pseudo-second-order model, whereas the sorption data fitted well in the Freundlich isotherm due to the surface heterogeneity. The thermodynamic analysis revealed the process to be entropically driven and spontaneous. The complex with trivalent f-cation was found to be asymmetric with no inner-sphere water molecules. The spectroscopic analysis confirmed the strong complexation of Sm(III) with oxygen-containing functionalities of CoFe2O4/GO. The excellent radiolytic stability and regeneration ability of CoFe2O4/GO showed that the sorbent could be successfully applied for nuclear waste remediation.

Clean methodology for nuclear laboratory waste remediation: Part-II: Recovery of Americium

As a part of various analytical activities of nuclear laboratories, large volumes of radioactive waste are generated that are extremely hazardous to the environment. The alpha emitting actinides from these wastes must be removed completely (<0.37 Bq/mL) before their discharge for further waste management. In this context, we developed and deployed a novel, simple and clean two step methodology for the recovery of U, Pu and Am from the laboratory waste generated during various chemical quality control activities in nuclear industry. In the first step (reported in Part-I), U and Pu are recovered quantitatively in highly pure form using hollow fibre supported liquid membrane leaving behind all Am and other metallic impurities in the raffinate. In this communication, we report the quantitative recovery of Am from the left-over raffinate using tetraoctyl diglycolamide extraction chromatography. The raffinate after the recovery of actinides (U, Pu and Am) was free from alpha activity that could be managed easily. Additionally, the recovered actinide products were in desired purity for recycling.

Simultaneous removal of U(VI) and Re(VII) by highly efficient functionalized ZIF-8 nanosheets adsorbent

The simultaneously efficient removal of cationic and anionic radionuclides is an important and challenging topic for nuclear waste remediation as well as environmental protection. Herein, monoclinic ZIF-8 nanosheets modified with ethyleneimine polymer (denoted as ZIF-8/PEI) was achieved and used to determine the capture behaviors of both U(VI) oxycations and Re(VII) oxyanions from aqueous solution. ZIF-8/PEI assemblies showed a maximum U(VI) and Re(VII) uptake capacity of 665.3 (pH 5.0) and 358.2 mg/g (pH 3.5), respectively. Experimental, spectroscopic and theoretical calculation results directly unraveled that U(VI) adsorption onto ZIF-8/PEI assemblies was mainly ascribed to the coordination with abundant amino groups and weakly due to the Zn terminal hydroxyl groups, while anion exchange mechanism contributed predominantly to the Re(VII) sequestration. This work not only sheds light on the interaction mechanisms of simultaneous capture of U(VI) and Re(VII) but also highlights the versatile material design of cationic and anionic radionuclide immobilization in radioactive wastewater remediation.

Large cemented gibbsite agglomerates in alkaline nuclear waste at the Hanford site and the impacts to remediation

Recent work with the remediation of legacy alkaline nuclear waste has focused on nanometer and micrometer particle sizes, emphasizing how these small particles can impact efforts to treat the waste. Building upon this work, we present here findings that show very large particles (several centimeters in size) also exist in these waste which likewise play an important role in the remediation process. While large cemented gibbsite nodules have been periodically reported in acid soils in the literature, this study found similar large gibbsite agglomerates (7 cm in diameter) in alkaline nuclear waste, the first time that such large agglomerates have been identified in an alkaline environment. The morphology of the gibbsite in the agglomerates that were grown over more than 40 years of storage in the waste tank were similar to the much smaller agglomerates that have been reported in previous shorter term studies. Fluid dynamics calculations indicate that these cemented particles would be difficult to mobilize with standard jet slurry technologies, which is consistent with their persistence in the waste heel after jet sluicing of the tank.

Evaluation of three novel benzene-centered tripodal diglycolamide ligands for the pertraction of americium(III) through flat sheet membranes for nuclear waste remediation applications

Three benzene-centered tripodal diglycolamide (Bz-T-DGA) ligands were evaluated for the pertraction of Am3+ across PTFE flat sheet membranes. The feed phase contained a tracer spiked nitric acid solution (3 M), while the receiver phase contained 1 M α-hydroxyisobutyric acid (AHIBA). The transport of Am3+ was quantitative with ligand LI, with ethyl groups appended to the benzene ring at the 1,3,5-positions and two spacer atoms between the ring and the DGA group. Ligand LIII, with a butoxy tether between the ring and the DGA unit, displayed a much better transport efficiency than ligand LII having an ethylamide linkage, although in both cases the transport rates are lower than that of LI. The transport efficiency of the actinide ions was Am3+ > Pu4+ ≫ UO22+ suggesting that Am and Pu can be easily decontaminated from U in a mixture. The membrane stability was reasonably good with LI, while it was poor for the other two ligands.

Ionization‐Facilitated Formation of 2D (Alumino)Silicate–Noble Gas Clathrate Compounds

AbstractThe nanoscale confinement of noble gases at noncryogenic temperatures is crucial for many applications including noble gas separations, nuclear waste remediation, and the removal of radon. However, this process is extremely difficult primarily due to the weak trapping forces of the host matrices upon noble gas physisorption. Herein, the formation of 2D clathrate compounds, which result from trapping noble gas atoms (Ar, Kr, and Xe) inside nanocages of ultrathin silica and aluminosilicate crystalline nanoporous frameworks at 300 K, is reported. The formation of the 2D clathrate compounds is attributed to a novel activated physisorption mechanism, facilitated by ionization of noble gas atoms. Combined X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT) studies provide evidence of an initial ionization process that significantly reduces the apparent trapping barrier. Noble gas ions become neutralized upon entering the cages, and their desorption requires unprecedentedly high temperatures, even in ultrahigh vacuum conditions. From 2D aluminosilicate films these temperatures are 348 K (Ar), 498 K (Kr), and 673 K (Xe). DFT calculations also predict that Rn can be trapped in 2D aluminosilicates with an even higher desorption temperature of 775 K. This work highlights a new ionization‐facilitated trapping mechanism resulting in the thinnest family of clathrates ever reported.