Abstracts on Extracting Uranium From Seawater

Life cycle cost and energy balance of uranium recovered by a braid adsorbent system
Erich A Schneider, The University of Texas at Austin

Life cycle discounted cash flow and inventory analysis methods are used to estimate the production cost and energy return on investment (EROI) of uranium recovered from seawater via a polyethelene-based braid type adsorbent. The estimates are built on original assessments of the cost and energy intensity of materials, capital equipment, labor and other inputs to the uranium production chain. If fresh adsorbent achieves a capacity of 2 grams of uranium (g U) / kg ads, as in trials off the coast of Japan, and the adsorbent may be reused 6 times with capacity degradation of 5% per recycle, the U production cost is estimated at $1230/kg U with a 95% confidence interval of [$1030/kg U, $1430/kg U]. If this uranium is used in a once-through fuel cycle, the EROI is found to be 22. Improving the capacity of the multi-recycled adsorbent to 6 g U/kg ads would reduce the cost by approximately a factor of three, as would attaining a very high capacity -- 20 g U/kg ads -- in a single-use adsorbent.

Polymer-supported reagents for the selective removal of uranium from seawater
Spiro D. Alexandratos, Hunter College

Crosslinked poly(vinylbenzyl chloride) is explored as a substrate onto which ligands are immobilized that can be used to remove the uranyl ion from seawater. The ligands are phosphorus-based and include phosphates, phosphonates, and bifunctional ligands.

Interaction of uranium with poly(acrylamidoxime) adsorbents
Sinisa Vukovic, Ph.D., Oak Ridge National Laboratory

Acrylamidoxime polymers currently employed for extraction of uranium from seawater contain distinct chelating structures that are capable of forming metal complexes. Density functional theory is used to rank these chelates with respect to their relative affinity for the uranyl cation. The results yield further insight into how these adsorbents function and provide criteria for the design of improved adsorbents through chemical modification of the binding sites.

Improving the chemistry of amidoxime-based uranium adsorbents
Benjamin P. Hay, Oak Ridge National Laboratory

The presence of 4 billion tons of uranium dissolved in Earth's oceans has motivated a sustained effort to develop technology to sequester uranium from seawater over the past 40 years. A demonstrably successful approach has involved the use of poly (acrylamidoxime) adsorbents, which are able to extract and concentrate 3 ppb levels of uranyl carbonate under actual marine conditions. This talk presents research progress toward increasing uranium uptake from seawater through chemical modification of amidoxime-based adsorbent materials. The research entails a combined theoretical and experimental approach to (i) better understand how current these adsorbents function, (ii) identify binding site architectures optimized for uranyl cation interaction using state-of-the-art de novo structure-based design methods, and (iii) synthesize, characterize, and evaluate performance of promising candidates in the laboratory.

Development of rigid cyclic amidoximes for uranyl binding
Prof. David M. Jenkins, University of Tennessee

The ability to extract uranyl ([UO2]2+) from seawater would ensure a ready supply of uranium for nuclear power. Higher capacity adsorbents that are effective in the salinity and pH of seawater may be the quickest method to improve the gravimetric yield of uranyl extraction from seawater. We are developing rigid cyclic amidoximes of the type shown below. The progress on their synthesis, spectroscopic characterization and binding affinity for uranyl will be discussed.

Complexation of amidoxime-related ligands with uranium and transition metals: Potentiometric and spectrophotometric studies
Dr. Xiaoqi Sun1 , Lawrence Berkeley National Laboratory

Extraction of uranium from seawater is very challenging, not only because uranium is in an extremely low concentration, but because it exists in seawater as very stable triscarbonato complexes in the presence of many transition metal ions, some of which are in concentrations higher than or comparable to that of uranium. Sorbents with amidoxime-type ligands were successfully used to extract kilogram quantities of uranium in marine tests in Japan. To improve the extraction efficiency and reduce the extraction cost, fundamental understanding of the coordination chemistry of the extraction process is necessary. Therefore, thermodynamic studies have been conducted to determine the stability constants and enthalpy of complexation with a combination of techniques including potentiometry, spectrophotometry and microcalorimetry. These data provide guidance for the design of ligands and the optimization of sorbent preparation.

PrePrint: xsun_acs2012f_abs.docx

Preparation of uranyl carbonate ions absorbent containing amidoxime groups via pre-irradiation induced grafting method
Prof. Jingye Li

Acrylonitrile is grafted onto polyethylene non-woven fabric and the amidoxime groups are introduced onto the non-woven fabric by the reaction of nitrile groups with hydroxylamine hydrochloride. The degree of grafting (DG) and the conversion of amidoximation are measured by gravimetric method and the kinetics is studied. The existence of the amidoxime groups is proven by Fourier transform infrared (FT-IR) spectroscopy analysis. Adsorption results show that the absorbent can extract uranyl carbonate ions when the concentration is lower than 3 ppb.

Amidoxime functionalized materials for the selective extraction of the uranium
Patrick S. Barber, The University of Alabama

The selective extraction of uranium from the environment can lead to a variety of applications from waste remediation to the mining of UO22+ from seawater. Several studies over the past sixty years has concluded that the amidoxime moiety, RC(NH2)=NOH is highly selective for the uranyl cation, even under such complex conditions as seawater. Despite these studies, only recently has solid-state structural data been acquired that supports the η2 coordination of amidoxime to the uranyl ion. These recent results suggest a higher stability complex and are potentially linked to the selectivity of the amidoxime for the uranyl cation. We have modified both solid (small organic molecules and biopolymers) and liquid (ionic liquids) materials with this functional group and have applied them towards the separation of the uranyl cation. We will discuss the synthesis and modification of these materials and their applied separations of uranium.

Electrospun chitin nanofibers for uranyl absorbant materials
Mr. Chris S Griggs, The University of Alabama

It has been recently shown that ionic liquids (ILs) allow the dissolution of biopolymers without the loss of the important high molecular weight of the natural polymer which leads to improved strength. Electrospinning polymers produces high surface area fibers which can be functionalized with selective ligands for preferential complexation of the uranyl ion. Here we will present our efforts to prepare electrospun nano and micron sized chitin fibers directly from the dissolution of shrimp shells in the IL 1-ethyl-3-methylimidazolium acetate. The results of this single step process suggest that chitin can be extracted with higher molecular weight and purity over current processes that result in chitin with a lower degree of polymerization. We will discuss the correlation of physical properties and electrospinning conditions with the surface morphology and size range of the fibers.

Extraction of uranium with regenerated chitin from the dissolution of shrimp shells in ionic liquid
Robin D Rogers, The University of Alabama

Ionic liquids (ILs) have the ability to dissolve large polysaccharides such as cellulose and chitin under mild conditions giving unprecedented access to high molecular weight biopolymers directly from biomass. We are interested in using this ability to dissolve chitin directly from shrimp shell waste and generate chitin-based resins for the extraction of uranium from seawater. In addition to being a strong, environmentally friendly, and renewable material, the easily accessible deacetylated derivative chitosan provides a useful tether for attaching any number of functional groups. While chitin and chitosan have been used to extract heavy metals from waste water streams, chitin processed by ILs has not been studied. We will present our efforts towards the "green" processing of shrimp shell waste to manufacture a uniquely high purity/high surface area absorbent chitin material for the extraction of uranium from seawater.

Amidoxime-grafted mesoporous carbon and porous organic gel sorbents for extraction of uranium from seawater
Suree Brown, University of Tennessee

Mesoporous carbon (MC) sorbents have gained much attention due to their inertness, high surface areas, and controllable pore sizes. In this study, pristine, activated, and oxidized MCs were synthesized by a soft template method. Free radical grafting of monomers under an electron beam irradiation or a chemical initiator, followed by amidoximation, yielded high surface area amidoxime (AO)-grafted MCs. In comparison to grafted MCs, porous AO-containing phenolic-formaldehyde monolithic gels were also prepared. The availability of various nitrile-containing phenolic precursors enabled the preparation of amidoxime and phthalimide dioxime (i.e., cyclic structure) sorbents. Results from uranyl sorption study, compared side-by-side with known uranyl sorbents, will be presented.

This research was supported by the Fuel Resources Campaign in the Fuel Cycle Research and Development Program, Office of Nuclear Energy, U.S. Department of Energy (DOE).

Radiation grafting of acrylonitrile to UHMWPE fiber for the preparation of uranium collection material
Guozhong Wu and Mouhua Wang, Shanghai Institute of Applied Physics

In this work, we developed a novel amidoxime adsorbent using ultra-high molecular weight polyethylene (UHMWPE) fiber as grafting substrate. The fiber adsorbent was prepared by pre-irradiation under γ-ray, graft copolymerization in binary monomer solution and followed amidoximation reaction. The UHMWPE fibers were firstly irradiated in air at room temperature, and then reacted with the monomers in DMF solution purged by nitrogen at 60℃. The grafting degrees of acrylonitrile (AN) and acrylic acid (AA) reached 68.2% and 26.5%, respectively, on the UHMWPE fiber in a grafting period of 3 hr. The infrared analysis of UHMWPE-g-(PAN-co-PAA) fiber show C≡N stretching band at 2243 cm-1 and C=O stretching band at 1732 cm-1. The SEM micrograph of the non-grafted UHMWPE fiber exhibited a smooth surface and microgrooves along axial orientation. That of the UHMWPE-g-(PAN-co-PAA) fiber showed that the microgrooves were covered by the AN and AA co-grafted layer and the surface became rough.

Uranium adsorbents based on crosslinked polymers
Xiao-Guang, Oak Ridge National Laboratory

The separation and purification of uranium has gained considerable importance with increasing demand of this element for clean energy applications. The storage of uranium in sea water is humongous due to its vast volume covering the earth's surface (estimated around 4.5 billion tons), therefore, developing effective adsorbents for uranium extraction from seawater is very promising. So far, the most efficient uranium adsorbents were developed by Japanese based on grafting acrylonitrile and methacrylic acid on electron beam irradiated polyethylene fibers, followed by amidoximation with hydroxylamine and potassium hydroxide conditioning. To eliminate the high energy consumption step, electron beam irradiation, we focus on developing high surface area adsorbents based on crosslinked polymers. Divinylbenzene (DVB) as crosslinker and different monomers containing either nitrile group (-CN) or acid groups (-COOH, -SO3H, -PO(OH)2) were used to synthesize the crosslinked polymers. The effect of monomer types, monomer ratios, as well as crosslinking density on the surface area and uranium adsorption capacity of the polymeric adsorbents were systematically investigated and will be presented in this talk.

Synergistic effect of sonochemical grafting and hierarchical porosity of carbons on uranium adsorption from seawater
Joanna Gorka, Oak Ridge National Laboratory

Here we report for the first time the synergistic effect of the sonochemical grafting and hierarchical porosity of carbons on uranium adsorption from seawater. Series of different carbon materials has been synthesized by combined soft- and hard-templated method. Typical soft-templating approach is based on the polymer-polymer interaction between a phenolic resin-type carbon precursor and a triblock copolymer used as soft template. The thermal decomposition of the latter creates uniform cylindrical mesopores ~8-10nm in diameter. The dissolution of silica colloids added the synthesis gel introduces porosity with spherical geometry, which can be easily tailored by the choice of silica colloids used. Our former studies suggest that the presence of tiny pores (up to 2nm) and large surface area is a key factor to improve degree of grafting. The goal of this work is to determine the role of mesopores and small macropores, and to discriminate the optimum porosity range for maximum uranium adsorption from seawater.

Isolating trace seawater uranium with polymer functionalized porous carbon
Yanfeng Yue, Oak Ridge National Laboratory

The volume of the world's oceans is so huge that seawater can almost provide limitless amounts of uranium - some around 4.5 billion tons, even though one ton seawater contains only about 3.3 mg Uranium. Therefore, developing effective adsorbents for uranium extraction from seawater is very promising. So far, the most efficient adsorbents are solid polymers containing amidoxime groups (-(NH2)=N-OH), which show high affinity and selectivity for uranyl ion, i.e. approximately ten fold greater than the conventional titanium oxide adsorbent. However, these polymeric adsorbents usually have lower Brunauer-Emmett-Teller (BET) surface area and low mechanical strength. To increase the surface area of the polymeric adsorbent, we copolymerized the amidoxime generating monomers on the surface of high surface area mesoporous carbon. With increasing the ratio of monomers/carbon, the uranyl ion adsorption capacity increases whereas the BET surface area decreases correspondingly. The capacities of uranyl ion adsorption from the best sample are 19.1 g U/kg and 63.3 g U/kg, respectively, for 50 mL and 250 mL uranium stock solutions. The results are higher than the Japanese sample under the same conditions (18.2 g U/kg and 35.4 g U/kg for 50 mL and 250 mL uranium stock solutions, respectively), indicating our sample has higher selectivity for Uranium. The real sea water test on the best polymer coated carbon composite adsorbent is under investigation.

Uranium elution from amidoxime-based polymer sorbents using supercritical CO2
Chien M. Wai, University of Idaho

Amidoxime-based polymer sorbents have been extensively studied for sequestering uranium from seawater. Recovery of uranium from the sorbents is usually done by leaching with an acid such as 0.5 M hydrochloric acid. The aid leaching method is effective for recovering uranium but causes damage to the sorbent due to hydrolysis of the functional groups attached to the polymer. Supercritical fluid carbon dioxide (sc-CO2) is a non-aqueous solvent which has been used to extract uranium from solid substances with CO2-soluble ligands. Our initial results using this ligand-assisted sc-CO2 elusion method to recover loaded uranium from an amidoxime-based sorbent are presented. Ligands tested in the initial experiments include beta-diketones, Cyanex reagents and TBP-HNO3 complexes. Spectroscopic techniques, (fluorescence, Raman and FTIR) are used to measure uranium distributions and uranyl coordination environment in the sorbent.

Enhancement of the extraction of uranium from seawater
Mohamad Al-Sheikhly, University of Maryland

The amount of dissolved uranium in seawater far exceeds the amount available in terrestrial ores. In addition to contributing additional uranium to the nuclear fuel cycle, retrieving uranium from seawater greatly reduces the technological complexity and environmental impact associated with conventional uranium mining and milling. Although several methods of extraction have been developed and tested over the past several decades, significant interest has been generated in developing adsorbent fabrics using radiation-induced polymerization after over a kilogram of uranium was obtained by Japanese groups using similar methods. Despite their success, a need remains to develop more efficient adsorbents to make this technology commercially viable.

Advanced adsorbent materials are being developed using polymeric substrates with high chemical stability, excellent degradation resistance and improved mechanical properties. Fabrics include polypropylene, nylon and advanced Winged Fibers™ from Allasso industries featuring extremely high surface areas for improved grafting density. Using the University of Maryland's 100 kCi Co-60 gamma source and 1-10 MeV electron beam linear accelerator, the various fabrics have been irradiated over a wide range of dose rates, total doses and temperatures and subsequently analyzed with EPR for determination of free radical concentration.

Also being utilized are innovative vinyl phosphonate monomers with high distribution coefficients and selectivity for uranium with excellent potential for free radical polymerization. Optimization of the grafting procedure involves precise control over reaction temperature, duration and methodology. Attachment of the chelating adsorbent to the substrate polymer is maximized by use of high monomer concentrations and quantified by determination of the grafting density of the sample. Grafted samples are subsequently analyzed for uranium adsorption with ICP-AES. Preliminary results with the new adsorbent fabrics have yielded distribution coefficients (kd) of around 1000. These results were obtained with real ocean water doped with approximately 10 mg/L uranium introduced in the form of uranyl acetate.

Current work includes optimization of irradiation conditions in addition to material characterization on the molecular level and analysis of the sample microstructure. Further testing in real seawater will be conducted to compare the selectivity of the adsorbent fabric towards uranium compared to that of other species, in addition to determining the loading and adsorption rates under various conditions such as pH, temperature and salt concentration. Experiments in seawater will also be performed to characterize the effects of organics on the adsorbent materials, test for durability and reusability and determine kinetics and efficiency of the uranium extraction as a function of the time of exposure to seawater in order to study the degradation of the sorbent in realistic environments.

Marine testing of uranium adsorption from seawater
Costas Tsouris, Oak Ridge National Laboratory

A polymeric adsorbent with amidoxime functional groups, developed at Oak Ridge National Laboratory (ORNL), is employed for marine testing. Laboratory experiments with seawater are conducted in batch 5-gallon tanks to provide kinetic and equilibrium data at temperatures ranging from 10 to 35 °C. Flow-through laboratory experiments are conducted using adsorbent beds and 110-gallon seawater tanks. In these experiments, seawater is continuously recycled through the adsorbent. Marine testing is currently conducted at two sites: (1) the Marine Sciences Laboratory of the Pacific Northwest National Laboratory (PNNL) at Sequim, WA, and (2) the Rosenstiel School of Marine & Atmospheric Science of the University of Miami, FL. The batch experiments indicated that six weeks were required for equilibrium, so a period of six weeks is allowed for equilibrium to be reached in all types of the experiments. The parameters investigated are temperature, amount of adsorbent, and flow rate. Results are compared with experimental data obtained in parallel with Japanese adsorbents. Initial results from the PNNL experiment showed that the capacity of the ORNL adsorbent is two to three times the capacity of Japanese adsorbent tested under identical conditions. Kinetic models coupled with mass transfer models and adsorption isotherm equations are used to analyze the experimental results.

Influence of temperature on uranium adsorption from seawater
Jungseung Kim, Oak Ridge National Laboratory

Uranium recovery from seawater has been investigated over several decades. Although it has been reported that temperature has significant effects on uranium adsorption kinetics and equilibrium, further studies are needed to investigate these effects through thermodynamic and kinetic analyses. In this study, a polymeric adsorbent with amidoxime functional groups, developed at Oak Ridge National Laboratory, has been employed for batch experiments of uranium adsorption from seawater in 5-gallon tanks. A quantity of approximately 5 mg of adsorbent is typically used in each experiment, and a period of six weeks is allowed for the equilibrium to be reached. Adsorption kinetics and equilibrium are studied at different temperatures, ranging between 10 and 35 ºC. From the experimental data, the parameters of adsorption isotherms are obtained. Kinetic models coupled with mass transfer models and adsorption isotherm equations are used to analyze the experimental results. Changes in enthalpy, entropy, and free energy are also estimated through the van't Hoff equation to provide a better understanding of the adsorption thermodynamics. This study improves our understanding of the influence of temperature on uranium adsorption from seawater by polymeric amidoxime-based adsorbents.

R&D towards recovery of uranium from seawater in Japan
Noriaki Seko, Japan Atomic Energy Agency

The recovery of uranium has been carried out by adsorption method. In this method, the adsorbent needs the high selectivity and capacity for uranium adsorption in the seawater. It was found that hydrous titanium oxide was noble material for the recovery of uranium from seawater. Then, screening researches were carried out to evaluate the many kinds of uranium adsorbents and concluded that the amidoxime was a promising functional group for recovery of uranium from seawater. We have developed the high performance adsorbent having amidoxime with radiation-induced graft polymerization and the collection of 1 kg uranium from seawater was demonstrated. In the recovery system, the stacks of fabric adsorbent were changed to a braid type adsorbent to improve the contact between the adsorbent and the seawater. The sea area being considered for the recovery of uranium was preliminary investigated on the conditions of seawater temperature and depth of the sea.

Application of unusual metal speciation in ILs to f-element separations
Steven P. Kelley, The University of Alabama

Ionic liquids (ILs) are unique in that they offer a high concentration of free ions at a temperature where many metal complexes and ligands are thermally stable. This has resulted in a number of reports on unusual metal chemistry, including the formation of metal-containing ILs. This also has important implications for liquid-liquid extraction of metals, a leading subfield of IL research. Since the behavior of metals in ILs can be unexpected, research on metal speciation in ILs is needed to design reliable separations and take advantage of the new synthetic opportunities of these materials. We have studied the speciation of uranium, thorium, and lanthanides in ILs due to the major relevance of separating these metals. Here we present our results in this area including the unusual coordination of f-elements to soft donors and attempts to incorporate f-elements into ILs.

Funcitonalized carbon materials as uranium adsorbents
Richard Mayes, Oak Ridge National Laboratory

The extraction of valuable materials from seawater provides a pathway for alternatives to terrestrial sources supporting future energy needs. Uranium extracted from seawater will be vital for future nuclear power as the terrestrial ore supply diminishes over time. While polymer-based sorbents have been actively developed for over 20 years, carbon-based sorbents have been neglected. Recently, the introduction of controlled porosity in porous carbons, specifically in the mesopore regime, has led to an increased interest in carbon materials in energy storage, catalysis, and separations. A focus on the functionalization of carbon materials is currently underway, directed toward surface polymerization inside the mesopores, where amidoxime ligands are grafted in high density to the pore walls. Comparisons between mesoporous carbons and other carbon-based materials will be discussed along with some of the challenges associated with achieving a deployable format.

Nanoporous sorbents for selective extraction of actinides
Wenbin Lin, University of North Carolina

Advanced nanoporous sorbents are being developed for uranium extraction from seawater and other actinide elements from nuclear wastes. We have investigated mesoporous silica nanoparticles, mesoporous carbon nanoparticles, and other porous materials for selective actinide extraction. These materials provide highly tunable platforms for incorporating selective chelators, either by grafting or by direct incorporation into the structures. The porous structures of these materials are able to encapsulate chelators at high densities, allowing high actinide sorption capacities. Material properties such as sorption capacity, selectivity, uptake and elution kinetics, durability, and reusability are being optimized by controlling the morphology, pore size, and surface functionalization of nanomaterials.

PrePrint: u_acs_abstract.docx

Investigation of polymer structure and properties of amidoxime-functionalized hydrophilic random copolymers on performance of uranium recovery from seawater
Tomonori Saito, Oak Ridge National Laboratory

Development of technology for recovery of uranium from seawater is crucial for satisfying future energy demand. Amidoxime-functionalized polymer adsorbents prepared via radiation-induced graft polymerization (RIGP) has been the most promising adsorbents for recovering uranium from seawater. RIGP adsorbent properties depend on chemical properties of graft chains on the graft copolymers. Especially the incorporation of hydrophilic groups in addition to amidoxime ligands was the key to obtaining high recovery and uptake rates of uranium. Due to insolubility of immobilized graft chains on a trunk polymer, the correlation between polymer properties of the graft chains and uranium uptake performance has been little investigated. Thus, this work aims to investigate polymer structure and properties of non-grafted amidoxime-functionalized hydrophilic random copolymers. Elucidating the effect of polymer structure and properties to uranium uptake performance will help in designing more efficient polymeric adsorbents for uranium recovery from seawater. Several amidoxime-functionalized hydrophilic random copolymers were synthesized and the effect of the polymer structure and properties including chemical composition, molecular weight, glass transition temperature, and solubility to uranium uptake from model seawater are investigated.

Sponsored by: Department of EnergyThe Office of Nuclear Energy with Oak Ridge National Laboratory

Synthesis, development, and testing of uranium adsorbent materials
Yatsandra Oyola, Oak Ridge National Laboratory

An interest to improve capabilities for producing uranium adsorbents increased after an extensive survey of Japan's 10+ years of research on this technology. Significant efforts by Japanese researchers facilitated the development of protocols to design an absorbent to extract Uranium from seawater. Uranium is present in the seawater in very low concentrations, 3.3 parts per billion. The extraction of Uranium from such low concentrations presents an economical challenge, where as the adsorbent method is not competitive with the actual method of In Situ recovery. For us, this is an opportunity to provide new investigations on extraction absorbents that conquer the economical challenges.

In order to enhance the performance of the material our efforts were first directed to better understand the technology implemented in Japan. We have been exploring the adsorbent synthesis and processing variables, as well as the correlation between chemical modification of the material and the Uranium absorption capacity. The synthesis variation research targets the understanding of each manufacturing step; radiation induced grafting of acrylonitrile, acrylonitrile to amidoxime functional group interconversion, and alkaline conditioning.

Sponsored by: The U.S. Department of Energy - The Office of Nuclear Energy with Oak Ridge National Laboratory