Nanostructured Metal Oxide Sorbents for the Collection and Recovery

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Nanostructured Metal Oxide Sorbents for the Collection and Recovery of Uranium from Seawater Wilaiwan Chouyyok, Cynthia L. Warner, Katherine E. Mackie, Marvin G. Warner, Gary Gill, and R Shane Addleman Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03650 • Publication Date (Web): 07 Feb 2016 Downloaded from http://pubs.acs.org on February 13, 2016

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Industrial & Engineering Chemistry Research

Nanostructured Metal Oxide Sorbents for the Collection and Recovery of Uranium from Seawater

Wilaiwan Chouyyok, Cynthia L. Warner, Katherine E. Mackie, Marvin G. Warner, Gary A. Gill, R. Shane Addleman*

Pacific Northwest National Laboratory, Richland, Washington 99352 *) Raymond Shane Addleman, Chemical and Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington, 99352, 509-375-6824, [email protected]

Abstract The ability to collect uranium from seawater offers the potential for a long-term green fuel supply for nuclear energy. However, extraction of uranium, and other trace minerals, is challenging because of the high ionic strength and low mineral concentrations in seawater. Herein we evaluate the use of nanostructured metal oxide sorbents for the collection and recovery of uranium from seawater. Chemical affinity, chemical adsorption capacity and uptake kinetics of sorbent materials were evaluated. Materials with higher surface area clearly produced better sorbent performance. Uptake kinetics showed that the materials could rapidly equilibrate in a few hours with effective solution contact. Manganese, iron oxide, and especially Mn-Fe nanostructured composites provided the best performance for uranium collection from seawater. The preferred materials were demonstrated to uranium from natural seawater with up to 3 mg U/g-sorbent in 4 hours of contact time. Inexpensive nontoxic carbonate solutions were demonstrated to be an effective and environmentally benign method of stripping the uranium from the metal oxide sorbents. Various formats for the utilization of the nanostructured metals oxide sorbent materials are discussed, including traditional methods and nontraditional methods such as magnetic separation. Keywords: Uranium, nano, manganese, iron, sorbent, seawater, magnetic, separations, nuclear energy

1. Introduction Nuclear power provides a clean energy source that emits little or no greenhouse gases or other environmental contaminants when properly operated and managed, but its successful use requires that 1

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a stable, cost effective source of uranium is readily available.1-4 The uranium content of seawater has a fairly uniform concentration of ~3.3 ppb. Given the high volume of the earth’s oceans, the total dissolved uranium present is estimated to be far greater than the amount accessible from terrestrial ores.5 The large amount of available uranium in seawater makes it a promising alternative resource for nuclear power. A number of cost effective and efficient extraction methods for the recovery of uranium from seawater have been explored.5-12 In natural seawater uranium mainly occurs as uranyl tricarbonate species, UO2(CO3)34-.5, 6, 10, 12-19 This species is a very stable and soluble complex5, 6, 16-18, 20, 21, making its recovery from seawater challenging. Solid phase adsorption is an attractive, proven technique that is industrially scalable and amenable to easy integration with industrial-scale liquid handling systems. Different solid phase sorbents based on inorganic, organic, and composite materials have been investigated and explored for uranium recovery from seawater.5, 8, 12 Solid phase extraction offers a number of potential advantages for the processing of very large volumes of seawater containing low concentrations of uranium at a practicable cost. These advantages include flexible configuration, easy application and operation, scalability, and a flexible form factor for solid phase materials. Metal oxide sorbents, including hydrated titanium oxides (HTO), iron oxides, manganese oxides, and their composites, have been investigated for uptake of uranium from various aqueous solutions, including seawater, through packed columns and magnetic separations.5, 6, 8, 12, 22-28 The advantages of these sorbents include large surface areas22, 25, 29, 30, high capacities23, 24, 29, 30, adjustable surface chemistries and particle sizes29, 30, uncomplicated preparation28, 31, and reusability5, 22, 23, 29, 30. These qualities make them very attractive for application to the challenge of collection of uranium from seawater.5, 8 Hydrated titanium oxides in particular have been successfully tested at the pilot plant scale via packed bed techniques in several countries.5 The metal oxide sorbents adsorb the marine uranium via surface hydroxyl groups.5, 15, 32, 33 However, the slow uptake rate and low collection uptake efficiency5, 6, 8

, coupled with the degradation of the materials during elution processes5, make development of

viable process using solid phase sorbents (polymer or inorganic) materials challenging. Moreover, recent estimates for sorbent based uranium production from seawater are as high as $400–1000/kg with the main cost drivers depending upon the sorbent materials; these drivers include production cost, uptake capacity, and reusability/lifetime of the sorbents.6, 8, 34 The development of inexpensive sorbents with

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high capacity, long lifetime, and excellent chemical affinity and selectivity for uranium is required to provide a recovery process that is economically viable and environmentally friendly. In this work, we have explored the use of inorganic metal oxide sorbents for the collection of uranium from seawater. We competitively evaluated the performance of selected metal oxide materials and assessed their relative advantages, and disadvantages, when they are formulated with nanostructured features. We quantified the chemical affinity, uptake kinetics, and collection capacities of selected nanostructured inorganic sorbents and compared them to conventional materials. Methods for stripping collected uranium from the sorbent materials were also explored. Various configurations for the utilization of preferred nanostructured materials for the trace level extraction of uranium from large seawater volumes are discussed. 2. Materials and Experimental Methods 2.1 Sorbents and material characterization The structures of selected inorganic metal-oxide sorbents used for uranium collection are shown in Figure 1. The syntheses of these sorbents are detailed subsequently. The composition of these materials was determined after digestion in concentrated nitric acid with continued shaking at 200 rpm for 48 hours. The dissolved materials were then analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Results are shown in Table 1.

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Figure 1. Images of selected inorganic metal oxides. (A) Scanning Electron Microscopy of Fe-MnO2 nanoporous (NP) composite, (B) Scanning Electron Microscopy of MnO2 NP composite, (C) Mn SEM of NP Mn-Fe3O4 clusters, (D) and Transmission Electron Microscopy of 8 nm Mn-Fe3O4 magnetic nanoparticle (MNP). Table 1. Composition of selected inorganic metal oxide sorbents Weight ratio (%) of element in sorbent Mn Fe Fe-MnO2 NP composite 23.4 4.0 MnO2 NP composite 33.8 N.A. NP Mn-Fe3O4 clusters 30.0 25.2 8 nm Mn-Fe3O4 MNP 35.2 39.4 Material

N.A.-not applicable Weight ratio is defined as the percentage weight of an element per the total weight of the sorbent.

Iron (II, III) oxide nanopowder, nanoporous silica gel (Davisil® Grade 635), TiO2 sorbents, MnO2 sorbents, FeCl3-6H2O, FeCl2-4H2O, FeCl3, FeSO4-7H2O, polyethylene glycol (PEG, 10,000 MW), MnSO4H2O, KMnO4, and NaOH were all purchased from Sigma-Aldrich (St. Louis, Missouri) and used as received. Different forms and sizes of commercial MnO2 sorbents were tested and purchased from 4


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different companies (see detail in Table 2). The strong anionic exchange (SAX, AG® MP-1), and Chelex® 100 resins were purchased from Bio Rad. The strong cationic exchange (SCX, Supelclean™ LC-SCX), activated carbon (Darco®KB-B), and titanium oxides were obtained from Sigma-Aldrich. Glass beads (105–150 µm diameters) were purchased from Polysciences, Inc. All water used in the preparation of the sorbent materials was deionized water (DI, 18.2 MΩ-cm). Materials were characterized using Transmission Electron Microscopy (TEM) for size analysis, Vibrating Sample Magnetometry (VSM) for magnetic measurements, Brunauer-Emmett-Teller (BET) for surface area measurements, and/or Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDS) for imaging and elemental mapping. Synthesis of the Fe3O4 nanoparticles (8 nm core). The Fe3O4 nanoparticles were prepared using a method described previously.35, 36 Briefly, the Fe2+ and Fe3+ chloride salts were combined in 1:2 molar ratio in water, and NaOH was added with vigorous stirring to achieve a pH of 10. The resultant particles were washed with DI water using magnetic decantation and air dried. The prepared nanoparticles were characterized by TEM, BET, and VSM. Synthesis of Fe3O4 clusters. The Fe3O4 clusters were prepared using a modification of a previously described method.37 Briefly, 3.2 g of FeCl3 and 2.4 g of FeCl2-4H2O were dissolved in 100 mL DI H2O. Then 100 mL of a 5% PEG solution in water was added and the mixture sonicated in a sonicating bath for 10 minutes. The mixture was then stirred and heated to 75 °C, 12 mL of 2M NaOH was added, and heating was continued for 2 hours at 60 °C. The clusters were then aged at 80 °C for 30 minutes before collecting and washing with water and ethanol using magnetic decantation. The clusters were rinsed with acetone and dried in an oven at 80 °C. The prepared clusters were characterized using SEM/EDS, BET, and VSM. Synthesis of Mn-doped iron oxides. The Mn-doped iron oxides (8 nm, 30 nm, and clusters) were prepared using a method previously described36, 38 where the iron oxide material was stirred in MnSO4H2O solution and KMnO4 added. After a 1 hour stir, the particles were washed with water using magnetic decantation to remove excess salts and MnO2, and dried in an oven at 110 °C. The Mn-doped magnetic nanoparticles (Mn-Fe3O4 MNP) were characterized using TEM, BET, and VSM. Synthesis of MnO2-coated mesoporous silica. The MnO2-coated silica (MnO2 NP composite) was prepared using a technique that results in deposition of MnO2 nanoparticles on the silica surface. The silica, Davisil grade 635 (1 g) was stirred in MnSO4 for 30 minutes. Next KMnO4 was added and the 5



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mixture stirred for 1 hour. The mixture was filtered and dried in air. The particles were resuspended in methanol to allow for settling of larger particles and decantation and removal of “fine” suspended particles. The remaining material was sieved to ensure a uniform particle size. The same method was used to prepare MnO2 on glass beads. The Fe-MnO2 NP composite was prepared by adding FeSO4-7H2O to the aqueous MnSO4-H2O solution (Fe:Mn mole ratio = 1:3 ) during the first stirring step with the silica support. The remainder of the synthesis was identical to that for the MnO2 NP composite. The final product in both cases was heated at 200 °C for 2 hours. The MnO2 NP composite material was characterized using SEM/EDS and BET. 2.2 Kd and percent uptake measurements The solid phase distribution coefficient (Kd) is simply a mass-weighted partition coefficient between solid phase and liquid supernatant phase as shown in Equation 1. It was obtained through batch sorption experiments and calculated from the actual concentrations of uranium detected by ICPMS as well as from the percent (%) uptake of uranium as shown in Equation 2.

=

( )

(1)

% = 100 ×

( )

(2)

Where Co and Cf are the initial and final concentrations of uranium, respectively (at equilibrium), V is the volume of solution, and M is the mass of sorbent used. Kd values are given as mL/g sorbent). The uranium solutions were prepared from ICP standard solutions, purchased from Aldrich, spiked into seawater collected from Washington State’s Sequim Bay, which is connected to the Pacific Ocean via the Strait of Juan de Fuca. This is the same water being used to evaluate many other uranium sorbents being reported in literature.39-43 In some experiments seawater was spiked with uranium to approximately 50 ppb. Then 4.9 mL of the uranium-spiked seawater was placed in a polypropylene bottle and spiked with 0.1 mL sorbent suspended in DI water at a liquid-to-solid ratio (L/S in the unit of mL/g throughout) of 103 and this resulted in the final L/S ratio of 5 × 104. The tubes were shaken for 2 hours at 200 rpm on an orbital shaker. Then the magnetic nanoparticles were separated from the solution using a 1.2 T NdFeB magnet. The nonmagnetic sorbents were collected by filtering the solution 6


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through 0.45 µm syringe Nylon-membrane filters. The removed supernatants were kept in 2 vol. % HNO3 before prior to metal analysis using ICP-MS (Agilent 7500ce, Agilent Technologies, California). The uranium concentration in the control (no sorbent) was treated the same as the test solutions; however, the control without filtration was also analyzed to check for precipitation of uranium ions. The precipitation of uranium in seawater was not significantly detected at the pH range observed in this study. All batch experiments were performed in triplicate, and the average values were reported. The pH of the seawater after adding uranium decreased from 7.9 to between 6 and 7 as a result of stabilizing nitric acid in the stock solution. To avoid further disruption of seawater chemistry, solutions were used without further adjustment during initial sorbent screening efforts. The uranium-spiked seawater (pH ~6-7) was also used for performance testing for uptake kinetics, uptake capacity, and variable liquid-tosolid ratio. 2.3 Effect of liquid –to-solid ratio The effect of L/S ratio for uranium uptake was determined using Sequim Bay seawater spiked with uranium solution and using natural seawater (no uranium added). The L/S ratio of each sorbent was varied to obtain the final L/S ratio in the range of 103–107. The experiments were performed in the same condition as the Kd measurements, through batch sorption, but the contact time, continually shaking at 200 rpm, was extend to 4 hours. 2.4 Effect of stripping solutions generally used for uranium Four common stripping solutions (0.01M HNO3, 0.01M HCl, 4M NaCl/0.01M HNO3, and 1M Na2CO3) were tested as potential stripping agents. The experiments were carried out in the same batch sorption conditions as the Kd measurements through batch contacting. The final concentration of uranium in each solution was compared to the initial concentration and presented as percent remaining uranium. 2.5 Uranium stripping The nonmagnetic and magnetic sorbents that demonstrated the high performance for uranium collection (Kd > 1.8 × 105 mL/g sorbent, or 80% uptake) were further tested with several uranium stripping solutions. The uranium stripping was tested in batch sorption experiments. The sorbents were initially contacted with 5 mL of ~ 50 ppb of uranium in seawater at L/S of 104 and then continually shaken for 1 hour. The magnetic sorbent was separated from the uranium solution using a 1.2 T NdFeB 7



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magnet. The nonmagnetic sorbent was separated from the uranium solution utilizing centrifugation at 3000 rpm for 10 minutes. Then, 5 mL of stripping solution (1M Na2CO3) was added into the bottle (to maintain a L/S ratio of 104). The stripping solutions were separated from sorbents after they were continually shaken for 1 hour. The seawater and stripping solutions were collected and stored in 2 vol. % HNO3 for ICP-MS analysis. The % stripping was calculated by comparing to the different in uranium adsorbed on the sorbents. 2.6 Uptake kinetics The uptake kinetics of uranium sorbent materials were assessed by the same method as the Kd measurements, except that the sample volume was increased to 50 mL to minimize the change in L/S ratio due to the frequent sampling. A 1 mL sample was collected from each well-mixed aliquot over time from 1 minute through 24 hours. The same solution without contact with sorbents served as the zero time point. 2.7 Uptake capacity The uptake capacity of sorbents for uranium was measured using the same method as the Kd measurements. The initial uranium concentration was varied until the maximum uptake capacity was obtained. The batch contact was carried out for 2 hours. This was accomplished by using a large molar excess of uranium relative to the number of sorbent to the binding sites on the sorbent material at a L/S ratio of 104 mL/g.

3. Results and Discussion 3.1 Screening evaluations of sorbents for collection of uranium from seawater An initial screening evaluation of sorbent materials for uranium collection from seawater was conducted using batch contact studies in uranium-spiked seawater. The performance of selected sorbents is presented as collection efficiency (percent uptake) as well as the solid phase distribution coefficient (Kd). The Kd (mL/g) is simply a mass-weighted partition coefficient between the liquid phase and solid phase. It represents a direct measurement of chemical affinity in the specific conditions of an experiment and is a key parameter for evaluation of sorbent performance for trace collection.

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Uranium uptake from seawater by the selected metal oxides sorbents and commercial sorbents is shown in Tables 2 and 3. These initial screening tests were done by batch contacting in seawater minimally spiked with uranium (59 ppb) at a relativity high L/S ratio of 5 × 104 mL/g sorbent. Spiking of the batch solutions with uranium and the deprotonation of the sorbent as it complexes with the metal ion results in unavoidable sample acidification. The final equilibrium pH was monitored and determined to be 6–7 (down from 7.9). Similar to natural seawater, the dominant uranium species (in these spiked batch contact samples) are uranyl carbonate and uranyl hydroxide complexes, and these species are proposed to adsorb onto the surfaces of metal oxides with both monodentate and bidentate active hydroxyl sites.5, 15, 32, 44 A review of the data in Tables 2 and 3 clearly shows two principal sorbent performance trends. First, the sorbents with higher surface areas were more effective. Second, the Fe-Mn-O surface chemistry was the best performing oxide surfaces. Table 2 has data on the nonmagnetic sorbent materials, and Table 3 has data on the magnetic sorbent materials.

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Table 2. Selected inorganic sorbents evaluated for the collection of uranium from seawater Particle Mean Surface Kd Uptake size Sorbent Material Pore Area (mL/g (%) Size (Å) (m2 g-1) sorbent) (µ µm) Custom Sorbents Fe-MnO2 NP composite MnO2 NP composite MnO2 on glass bead

63–106 63–106 105–150

71 116 331

330 193 70

94 87 75

827 000 324 000 151 000

150 >150 >150 63–250 75–150 75–150 ~45 75–150

Nanostructured Metal Oxide Sorbents for the Collection and Recovery

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