Stripping Uranium from Seawater-Loaded Sorbents with the Ionic

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Stripping Uranium from Seawater-Loaded Sorbents with the Ionic Liquid Hydroxylammonium Acetate in Acetic Acid for Efficient Reuse Paula Berton, Steven Paul Kelley, and Robin D. Rogers Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03996 • Publication Date (Web): 06 Dec 2015 Downloaded from http://pubs.acs.org on December 8, 2015

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Industrial & Engineering Chemistry Research

Stripping Uranium from Seawater-Loaded Sorbents with the Ionic Liquid Hydroxylammonium Acetate in Acetic Acid for Efficient Reuse Paula Berton,† Steven P. Kelley,† Robin D. Rogers†,‡,* †

Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487, USA.

‡

Department of Chemistry, McGill University, 801 Sherbrooke St. West, Montreal, QC H3A 0B8, Canada.

KEYWORDS. Uranium; hydroxylammonium acetate; acetic acid; ionic liquid; seawater; stripping agent; shrimp shell mineralization

ABSTRACT. A new stripping and recovery process was developed to harvest the uranium recovered from seawater with amidoxime-functionalized polyethylene fiber sorbents and allow reuse of the sorbent without loss of capacity and without the need to recondition the sorbent before reuse. Hydroxylammonium acetate ([NH3OH][OAc])/aqueous acetic acid (AcOH) solutions were used as weakly acidic stripping agents and the stripped uranium as a soluble acetate was further immobilized on shrimp shells. These solutions also stripped the vanadium and other metal ions coadsorbed, which reduce capacity through competition with uranium for sorbent binding and can resist stripping even by strong acids. [NH3OH][OAc]/AcOH was found to allow recovery of more

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than 85% of the uranium although at a substantial longer time than the current 0.5 M HCl-stripping solutions, (less than 12 h vs. 48-72 h, respectively), however the use of HCl severely compromises the capacity of the sorbent in subsequent reuse (a 50% lost was observed on the third re-use of the fiber). Both [NH3OH][OAc] and the acetic acid were necessary to achieve high uranium recovery without sacrificing the sorbent’s capacity on re-use. The ability to reuse the sorbent without pretreatment and with minimal capacity loss could be an important step towards making the extraction of uranium from seawater energy-efficient and economically viable.


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1. INTRODUCTION The extraction of uranium from seawater has been a long-investigated source of potential nuclear fuel and has received a renewed burst of attention.1 The most successful and heavily investigated approach is currently sorption onto functionalized resins, which has been demonstrated in the marine environment at a pilot scale.2 Resins functionalized with poly(acrylamidoxime) (RC(NH2)=NOH) were identified in a large screening study of materials for uranium uptake in natural seawater as those capable of concentrating uranium at naturally occurring pH and with high selectivity and fast uptake rate.3 However, the same authors also reported that the capacity of the sorbent was irreversibly lost upon acidic stripping.4 This problem is exacerbated by the fact that another metal, vanadium, is adsorbed even more strongly that uranium, and its removal requires still greater acid concentrations and higher temperatures.5 Furthermore, it was found that elution using HCl required re-conditioning of the absorbents with KOH solutions to regenerate the active functional groups before reuse.6 Improving the recyclability of the sorbents is considered an important requirement towards making the extraction of uranium from seawater energy-efficient and economically viable.7 An approach based on the use of a weakly acidic stripping system composed of hydrogen peroxide and carbonate was recently investigated for stripping uranium from amidoxime sorbents without capacity loss.8 This approach was based on a historic method for leaching uranium from mineral samples,9 and had been proposed for polyamidoxime resins in 1982.3 The approach took advantage of the ability of uranyl to form soluble carbonate and peroxide complexes, which is unusual for the other metals.8


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In 2014, our group investigated the use of hydroxylammonium acetate in acetic acid ([NH3OH][OAc]/AcOH) as a solvent for the immobilization of uranium through an unusual biomimetic mineralization triggered by reaction with shrimp shell.10 The solution used in that study was based on a historical method for mineral sample processing11 and the use of acetic acid and hydroxylamine (NH2OH) to extract uranium oxides from soils.12 The reported crystallization of uranium (as the uranyl ion) dissolved in a solution of [NH3OH][OAc] in 4.0 M AcOH onto shrimp shell indicated a cation exchange reaction with calcium ions from the shrimp shell, which led us to further investigate [NH3OH][OAc] as an IL for isolating chitin from shrimp shell by reactive dissolution of the less recalcitrant fractions.13 The moderate acidic nature of the system, caused by the presence of the weak acid AcOH; its ability to promote ion exchange with a sorbent, and its high affinity for normally insoluble uranium in soil led us to consider it as an alternative stripping solution for amidoxime-functionalized polyethylene sorbents. Amidoximes are not alone in their high affinity for [UO2]2+; exceptionally strong ligand-to-metal electron donation has been observed in other N-OH containing ligands, such as hydroxamic acid, and often manifests as a deep red color that is unusual in [UO2]2+ complexes.14 Hydroxylamine can coordinate metal ions such as uranyl (UO22+)15,16 and vanadium(V).17 [NH3OH][OAc] might be expected to exploit the affinity of both of these elements for this particular functional group in order to break the strong bonds between the ion and the sorbent. Additionally, hydroxylamine itself is used as a reagent during the functionalization of the polymers,18 and thus its use as a stripping agent should be harmless at worst and possibly even regenerative. As a final advantage, it should be possible to back extract uranium from the loaded solutions with powdered shrimp shell, since they are essentially the same as those previously


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investigated by our group,10 which would allow the immobilization of the recovered uranium for storage. 2. RESULTS AND DISCUSSION 2.1 Efficacy of [NH3OH][OAc] stripping solutions in metal removal. The effect of the nature of the stripping solutions on the uranium recovery was evaluated using a well-characterized benchmark sorbent, amidoxime-functionalized polyethylene fibers7 (donated by Oak Ridge National Laboratory (Oak Ridge, TN, USA). The fibers had been previously exposed to filtered, natural seawater for metal loading at Pacific Northwest National Laboratory (Sequim, WA, USA).7 The fibers as received were first radiolabeled with 233

233

U by soaking them in a dilute solution of

UO2Cl2 in simulated seawater so that we could use the 233U γ activity to quantify the amount of

uranium removed. After equilibration (3 days), the radiolabeled sorbent was cut into pieces and weighed, and the stripping of uranium from each piece was evaluated by batch contact with the leaching solution. The amount of uranium leached was quantified by measuring the specific γ activity of the solutions at varying times. Since HCl is known from the literature to completely strip uranium from amidoxime sorbents,4 the ratio of solution activity to mass for a piece of sorbent stripped by 6.0 M HCl was measured and used as a reference for total elution. In order to maintain constant ionic strength, stripping of uranium was evaluated using a fixed concentration of [NH3OH][OAc] (0.3 M), selected due to its demonstrated effectiveness for the stripping uranium from soil in our previous study.10 Varying concentrations of AcOH (from 0.5 M to 4.0 M) were used in order to first determine the role, if any, of AcOH. For comparison, 4.0 M AcOH alone, 0.5 M HCl in deionized (DI) water, and pure DI water were also evaluated. DI water did not remove detectable amounts of uranium and will not be discussed further.


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A clear, direct relationship between AcOH concentration and stripping efficacy was observed (Figure 1, left), with 0.3 M [NH3OH][OAc] in DI water showing minimal uranium removal. The best system was 0.3 M [NH3OH][OAc] in 4.0 M AcOH, which significantly outperformed 4.0 M AcOH and reached ca. 85% of the efficacy of HCl, albeit at a much longer time. The importance of AcOH might be related to the formation of [UO2(OAc)3]- anions,19 making this approach similar with the one previously reported, which relies on the formation of soluble uranyl complex anions.8 [NH3OH][OAc] may act synergistically to help break the uranium-amidoxime interaction, even if [NH3OH]+ itself does not act as a ligand in solution.

100

Uranium Stripped (% relative to 6.0 M HCl)

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80 60 40 20 0 0

15

30

45

60

75

Stripping Time (h) Figure 1. Left - Uranium recovered after stripping with solutions of 0.3 M [NH3OH][OAc] in DI water (●),0.3 M [NH3OH][OAc] in 0.5 M AcOH (■),0.3 M [NH3OH][OAc] in 1.5 M AcOH (▲),0.3 M [NH3OH][OAc] in 4.0 M AcOH (♦), 4.0 M AcOH (□), and 0.5 M HCl (○). Error bars represent the standard deviation from the mean value (n = 3). Right - Appearance of fibers stripped by 0.3 M [NH3OH][OAc] in 4.0 M AcOH (top) and by 0.3 M [NH3OH][OAc] in DI water (bottom).


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When the fibers were exposed to [NH3OH][OAc] in 4.0 M AcOH, no noticeable change on their color was evidenced (Figure 1, right, top). However, the change in appearance of the fibers stripped by [NH3OH][OAc] in the absence of AcOH is also notable (Figure 1, right, bottom). Despite removing significantly lower amounts of uranium, the red color of the fibers (which is a welldocumented effect of their exposure to seawater)1 is lost. This change in the color might be indicative of a reaction of hydroxylamine with the compounds from the sorbent surface which is suppressed at lower pH due to the presence of AcOH. The effect of AcOH on the removal of other elements was also evaluated to determine how stripping selectivity might be controlled by analyzing the leachates from 0.3 M [NH3OH][OAc]/4.0 M AcOH and from 0.3 M [NH3OH][OAc]/DI water using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Figure 2). Twenty three elements were detected in the leachate using AcOH of which only 17 were detectable in the leachate without AcOH. In addition, all but three of them (Na, Si, and Zn) were extracted more effectively when AcOH was present. For most of the detected metals, the role of AcOH in stripping is likely either as a complexing agent (most metal acetate complexes are soluble) or as a source of H+ ions. For instance, the group IIa ions, Mg2+, Ca2+, and Sr2+, show a minor improvement upon the addition of AcOH to the stripping solution which is likely due to the increased concentration of [OAc] -. The tested transition metals, on the other hand, experience dramatic increases in their extraction, which suggest that cation exchange, rather than complexation, is the driving force for their removal. The apparent inhibition of Na+ stripping by the addition of AcOH to the solution is anomalous but might be explained by the formation of insoluble salts, anionic metal complexes such as polyoxovanadates. The improvement in stripping of many metals by [NH3OH][OAc] in


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AcOH vs. aq. AcOH alone suggests that this solution may have utility in other applications, such

Concentration (mg/g sorbent)

as recycling sorbents used to collect transition metals from industrial waste or desalination brine.

Concentration (mg/g sorbent)

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12000 10000 8000 6000 4000 2000 0 Mg Ca

V

Na

Si

B

Al

K

Fe

500

400

300

200

100

0 Se Tl Ni Zn Sn Sr As CuMnCo Cr Ba Ti Be

Element

Element

Figure 2. Metal ions stripped by 0.3 M [NH3OH][OAc] in a) DI water (black) and b) in 4.0 M AcOH (gray). Note the change in scales from right to left. Since the elements are adsorbed in different amounts, the absolute amount of metal ions stripped does not necessarily indicate differences in the efficiencies. We therefore normalized the amounts of each metal stripped (in mg/g sorbent) against the amount of each metal reported to be adsorbed on the amidoxime-functionalized polyethylene fibers,7 with the exception of uranium, which was normalized based on the radiochemical method described earlier. The results for all the elements for which benchmarks were available are presented in Figure 3. As the capacities published7 are not necessarily equal to the unmeasured amount on the sorbents, the fractions in Figure 3 are not quantitative. However, the fact that the values range from 0 to 1 for most metals suggest that the published capacities are a good estimate. Tin appears to be an outlier, with the amount stripped


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being several times larger than the amount reportedly adsorbed which may be a consequence of the small (and therefore less precisely determined) concentrations in both studies. 1.0

Fraction Removed

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0.8

0.6

0.4

0.2

0.0 V U Fe Ni Sr Co Mn Zn Cr Ti Cu Pb Sn

Element Figure 3. Ratios of metal ions stripped by 0.3 M [NH3OH][OAc] in a) DI water (black) and b) in 4.0 M AcOH (gray) to reported amounts adsorbed.7 Notably, Figure 3 reveals that the V and U are stripped especially well. This supports our hypothesis that elements with a high affinity for the sorbent may have an especially high affinity for [NH3OH][OAc]. However, there is no obvious correlation between the fractions of each element stripped and their corresponding dissociation constant (Kd) for the sorbent.7 When looking at stripping efficacy, it appears that there are roughly three groups corresponding to a) effective stripping (V, U), b) moderate to poor stripping (Ni, Sr, Co, and Mn), and c) very poor stripping (Cr, Ti, Cu, and Pb), with Fe and Zn as borderline elements. No obvious trends are detected that would allow a direct link to the elements in a particular group on the basis of oxidation state, sizeto-charge ratio, or Lewis acidity. Given the wide range of elements accumulated from seawater


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and the propensity of hydroxylamine to engage in redox chemistry, there are almost certainly multiple stripping mechanisms at work. 2.2. Effect of [NH3OH][OAc] on capacity. Recycling experiments in enriched, simulated seawater were conducted to measure how stripping with [NH3OH][OAc] affected the uranium uptake capacity when compared to HCl. After the first stripping test, the fibers were washed with DI water and simulated seawater, and their capacity to resorb uranium was tested by batch contact with simulated seawater spiked with 10 ppm uranium. As the solutions are much more concentrated than the natural seawater under which the fibers were originally loaded, the sorbents would be expected to sorb much more uranium. We therefore repeated the sorption and stripping in simulated seawater three times, using a spectrophotometric method to quantify the amount of uranium removed, and compared the amounts of uranium stripped in each cycle by 0.5 M HCl, 4.0 M AcOH, and 0.3 M [NH3OH][OAc] in 4.0 M AcOH (Figure 4). 3.5

Uranium Concentration (mg U/g sorbent )

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3.0 2.5 2.0 1.5 1.0 0.5 0.0 1

2

3

Stripping Cycle # Figure 4. Three cycles of uranium loading and stripping with solutions of 0.3 M [NH3OH][OAc] in 4.0 M AcOH (■), 0.5 M HCl (▼), and 4.0 M AcOH (●).


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The amount of uranium stripped using 0.5 M HCl decreased each cycle, which is consistent with the capacity loss previously reported.6 Regeneration of the fibers exposed to HCl using KOH pretreatment might have increased the adsorption capacity of the fibers, but this step was avoided in order to follow the same steps for all the stripping solutions. No capacity loss was observed for the other two solutions upon recycling. The 0.3 M [NH3OH][OAc] in 4.0 M AcOH outperformed 4.0 M AcOH by approximately the same magnitude as observed when stripping the fibers loaded with seawater metal ions. Interestingly, the reuse of fibers for uranium extraction did not require a regeneration step using KOH or any other reagent. 2.3 Immobilization on Shrimp Shell. The immobilization of uranium onto shrimp shells was evaluated as a means of back-extracting the uranium with no additional hazardous chemicals. This approach converts the uranium into the form of an insoluble phosphate mineral,10 allowing it to be stored for later fabrication of the fuel (for instance, by incinerating the shrimp shell to leave behind uranium oxides). Following the previously reported technique,10 ground shrimp shells were added to the stripping solution of 0.3 M [NH3OH][OAc]/4.0 M AcOH (pH = 3.5) containing stripped metals (see Experimental). As expected, the aqueous complex [NH3OH][UO2(OAc)3] reacted immediately with phosphate and calcium carbonate present in the shrimp shells to form the complex NH4[UO2(PO4)]·H2O and release CO2(g) (ESI, Figure S1).10 After extraction, the solution was filtered and an aliquot was used to determine the remaining uranium in the solution. Unlike the previous study,10 when the uranium present in the solution was removed from the solution in less than 1 h, here a minimum of 3 days was needed to guarantee the maximum recovery of U from the solution (Figure 5, left). The time difference might be explained by the low concentrations of the metal present in the stripping solutions in this study.


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100

Uranium Recovery (%)

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80 60 40 20 0 0.0

83

1

3

Time (days) Figure 5. Left – Percentage of uranium recovered from 0.3 M [NH3OH][OAc]/4.0 M AcOH stripping solution after exposure to shrimp shell vs. time. Right – PXRD of shrimp shell before (blue) and after (red) exposure to stripping solutions, with PXRD pattern of [NH3OH][OAc] for comparison (black). The remaining shrimp shells after uranium extraction were evaluated using powder X-ray diffraction (PXRD) to determine the crystallinity (Figure 5, right). New PXRD peaks were observed but could not be assigned to either the previously observed crystalline uranium phases10 or to [NH3OH][OAc]. The lower concentration of uranium used here may have prevented the formation of detectable amounts of crystalline uranium-containing phases, and instead other ion exchange products may have been formed. It should be noted that the competitive crystallization of other metals onto shrimp shell was not investigated in our previous study, and research will likely be needed to overcome possible contamination of the recovered uranium by other elements. 3. CONCLUSIONS We have shown that an acidic solution of [NH3OH][OAc] is able to remove uranium from amidoxime-functionalized polyethylene sorbents without causing the same capacity loss caused


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by HCl under the same conditions. While we suspected that hydroxylamine itself might act as the stripping agent, the dependence of uranium stripping on acetic acid concentration suggests that uranyl triacetate complex formation could also play a role. Besides uranium, the presence of acetic acid highly influences the stripping of other elements, including vanadium. The best performing system, 0.3 M [NH3OH][OAc]/4.0 M AcOH, allows the back-extraction of uranium with shrimp shell by a biomineralization mechanism, providing a convenient, low-chemical means of immobilizing the recovered uranium. These preliminary results show that [NH3OH][OAc] is a potentially useful system to add to the arsenal of new stripping methods currently under development for uranium from seawater sorbents. [NH3OH][OAc] reduces the chemical intensity of the stripping and recycling process in a number of ways not yet explored in the literature, which include stripping uranium and vanadium under weakly acidic conditions, removing the KOH reconditioning step, and allowing backextraction to be conducted in a single step with a non-hazardous material. Further studies are ongoing to determine the stripping of other metals, to improve the selectivity, and to design methods for recycling the stripping agent. The effect of [NH3OH][OAc] concentration will also need to be investigated to optimize the efficiency and clarify its important but incompletely understood role in the stripping process. 4. EXPERIMENTAL 4.1. Fibers and Chemicals. The benchmark sorbent, amidoxime-functionalized polyethylene fibers7 were donated by Oak Ridge National Laboratory (Oak Ridge, TN, USA). The fibers had been previously exposed to filtered, natural seawater for metal loading at Pacific Northwest National Laboratory (Sequim, WA, USA).7


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Aqueous acetic acid (AcOH, >99.7% purity), nitric acid (HNO3, 70.4% purity), and Arsenazo (III)

(2,2′-(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo)bisbenzenearsonic

acid)

were

purchased from Sigma-Aldrich (St. Louis, MO, USA). Hydrochloric acid was purchased from EMD Millipore (Billerica, MA, USA). Hydroxylamine (50% aqueous solution) was purchased from Alfa Aesar (Ward Hill, MA, USA). (CAUTION! Hydroxylamine can become explosive if heated and/or concentrated above 50 wt%. Please refer to Material Safety Data Sheet (MSDS) for further details (CAS # 7803-49-8)). All reagents were “solvent grade” and used as received. Deionized (DI) water was acquired from an in-house system (Culligan Water Systems, Rosemont, IL, USA) with a typical resistivity of 17.4 MΩ·cm. The simulated seawater was prepared dissolving a commercially available aquarium salt (Instant Ocean Aquarium Salt) in DI water. The simulated seawater contained the ions Na+ (462 mmol kg-1), K+ (10.2 mmol kg-1), Mg2+ (53 mmol kg-1), Ca2+ (10.3 mmol kg-1), Sr2+ (0.09 mmol kg-1), Cl- (550 mmol kg-1), SO42- (28 mmol kg-1), total CO2 species (1.90 mmol kg-1), and total B species (0.42 mmol kg-1) at pH ca. 8.3.20 Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) was synthesized from uranium trioxide (UO3) using the following procedure: 1 g UO3 (Strem Chemical, Newburyport, MA, USA), 1 g of concentrated HNO3, and 1 g of DI water were combined in a 20 mL borosilicate glass scintillation vial with magnetic stirring to give a yellow solution at room temperature. The solution was heated in an oil bath at 80 °C and ambient pressure to give a yellow powder. The reaction mixture was allowed to reach room temperature and DI water was slowly added to the powder to give a slurry which partially dissolved upon heating in an oil bath to 70 °C. The hot liquid was decanted from the undissolved solids and allowed to cool to room temperature, forming yellow-green prismatic crystals of UO2(NO3)2·6H2O (mp 65 °C).


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A 500 mg L-1 uranium stock standard solution was prepared by dissolving 52.7 mg UO2(NO3)2·6H2O in 100 mL volumetric flask using simulated seawater with a final HNO3 concentration of 0.1 mol L-1. Further dilutions were prepared daily using simulated seawater. For uranium determination, a 0.05 wt% solution of Arsenazo (III) dye was prepared by adding 50 mg Arsenazo (III) into a beaker containing 99.95 g DI water. The mixture was manually stirred until complete dissolution of Arsenazo was achieved. An 8 M HCl solution was prepared by mixing 66.12 g 12.1 M HCl with 33.88 g DI water. Further dilutions were prepared using DI water. All prepared solutions were stored in Nalgene plastic bottles for further use. The IL hydroxylammonium acetate ([NH3OH][OAc]) was synthesized following the previously reported technique.10,13,21 After synthesis and purification, [NH3OH][OAc] appeared as a white crystalline solid, which produces large monolith-type crystals with a mp 87 °C when recrystallized from water. The 1H Nuclear Magnetic Resonance (NMR) spectrum was recorded using a Bruker AV-360 spectrometers (Karlsruhe, Germany) with a Bruker/Magnex UltraShield 500 MHz magnet (Madison, WI, USA) operating at 360 MHz, and matched that previously reported.10 4.2. Biomass source. Dried shrimp shells were received from the Gulf Coast Agricultural and Seafood Cooperative in Bayou La Batre, AL, where the chitinous biomass was dried at a specialized facility by first pressing with a screw press to eliminate the majority of the water (down to ca. 50-85 wt %), followed by heating at up to 160 °C in a fluidized bed dryer until the material had a final moisture content of less than 5 wt%. The material was then shipped to The University of Alabama, where it was ground using an IKA Works Universal Grinding Mill (Ika Labortechnik, Wilmington, NC, USA) and sieved using a brass sieve with wire mesh (Ika Labortechnik) to obtain a powder with a particle size

Stripping Uranium from Seawater-Loaded Sorbents with the Ionic

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