Characterization and Testing of Amidoxime-Based Adsorbent

Nov 19, 2015 - The AF1 formulation had the best uranium adsorption performance, with a 56 day capacity of 3.9 g U/kg adsorbent, a saturation capacity ...
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Characterization and Testing of Amidoxime-Based Adsorbent Materials to Extract Uranium from Natural Seawater Li-Jung Kuo,*,† Christopher J. Janke,‡ Jordana R. Wood,† Jonathan E. Strivens,† Sadananda Das,‡ Yatsandra Oyola,‡ Richard T. Mayes,‡ and Gary A. Gill† †

Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, United States Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6053, United States

‡

ABSTRACT: Extraction of uranium (U) from seawater for use as a nuclear fuel is a significant challenge due to the low concentration of U in seawater (∼3.3 ppb) and difficulties to selectively extract U from the background of major and trace elements in seawater. The Pacific Northwest National Laboratory (PNNL)’s Marine Sciences Laboratory (MSL) has been serving as a marine test site for determining performance characteristics (adsorption capacity, adsorption kinetics, and selectivity) of novel amidoxime-based polymeric adsorbents developed at Oak Ridge National Laboratory (ORNL) under natural seawater exposure conditions. This manuscript describes the performance of three formulations (38H, AF1, AI8) of amidoxime-based polymeric adsorbents produced at ORNL in MSL’s ambient seawater testing facility. The adsorbents were produced in two forms, fibrous material (40−100 mg samples) and braided material (5−10 g samples), and exposed to natural seawater using flow-through columns and recirculating flumes. All three formulations demonstrated high 56 day uranium adsorption capacity (>3 g U/kg adsorbent). The AF1 formulation had the best uranium adsorption performance, with a 56 day capacity of 3.9 g U/ kg adsorbent, a saturation capacity of 5.4 g U/kg adsorbent, and ∼25 days half-saturation time. The two exposure methods, flowthrough columns and flumes, were demonstrated to produce similar performance results, providing confidence that the test methods were reliable, that scaling up from 10’s of mg quantities of exposure in flow-through columns to gram quantities in flumes produced similar results, and confirm that the manufacturing process produces a homogeneous adsorbent. Adsorption kinetics appear to be element specific, with half-saturation times ranging from minutes for the major cations in seawater, to 8−10 weeks for V and Fe. Reducing the exposure time provides a potential pathway to improve the adsorption capacity of U by reducing the V/U ratio on the adsorbent.

1. INTRODUCTION Conventional land-based uranium sources may be depleted by the end of the century, resulting in concerns about availability and the cost of uranium to fuel nuclear reactors.1−3 Exploitation of alternative uranium sources to ensure the long-term availability of this nuclear fuel is thus of great importance to the U.S. Department of Energy.4 The world’s oceans represent a vast and as yet untapped source of uranium. Although the uranium concentration in seawater is low (∼3.3 ppb), it is estimated that the abundance of uranium in seawater is approximately 4.5 billion metric tons, nearly 1000 times larger than the reserves from terrestrial ores.5 Amidoxime-based polymeric adsorbents are among the most widely described and considered the most promising materials to extract uranium from seawater.3 Japanese scientists introduced the use of braided-type amidoxime-based polymeric adsorbents for passive harvesting of oceanic uranium.6,7 Polymeric adsorbents have advantages including good mechanical strength, the ability to be produced in large quantity, and easy handling (lightweight)/deployment in the ocean.3,5 The Fuel Resources Program at the U.S. Department of Energy’s Office of Nuclear Energy is developing adsorbent technology to extract uranium from seawater. Scientists at Oak Ridge National Laboratory (ORNL) have built upon the pioneering work of the Japanese scientists and have developed several novel amidoxime-based polymeric adsorbents.8,9 A major effort in the development of this technology at PNNL © XXXX American Chemical Society

is to test the performance of the uranium adsorption materials being developed in natural seawater under realistic marine conditions. Detailed adsorption testing from in situ, natural seawater experiments are an important component for advancing adsorbent development for marine deployment. Due to the complexity of natural seawater (numerous dissolved ions, high ionic strength, speciation of trace elements, the presence of natural organic matter, etc.), adsorption performance of adsorbents tested in the laboratory using simple salt solutions or artificial seawater may not be able to translate to the actual performance in natural seawater. This paper summarizes the marine testing activities at PNNL to determine the performance of three novel amidoxime-based polymeric adsorbents developed by ORNL. Additional details about the marine testing program at PNNL are given in a companion paper in this volume.10 Of particular interest here is information obtained in natural seawater on the adsorption capacity and adsorption kinetics of uranium and other trace elements, the efficacy of approaches used for testing (flowthrough columns and flumes), the selectivity of the three Special Issue: Uranium in Seawater Received: September 2, 2015 Revised: October 21, 2015 Accepted: November 19, 2015

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DOI: 10.1021/acs.iecr.5b03267 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Layout and components of seawater manifold system for exposing uranium adsorbents to ambient seawater.

2.2. Ambient Seawater Exposure Systems. 2.2.1. Column Flow-Through System. Marine testing was conducted using ambient seawater from Sequim Bay, WA. Briefly, ambient seawater was drawn by pump from a depth of ∼10 m from Sequim Bay through a PVC pipe and is passed through an Arkal Spin Klin filter system (nominal pore size 40 μm) to remove large particles before being stored in a large volume reservoir tank. Seawater in the tank was then distributed through PVC piping to the adsorbent testing facility, where seawater was filtered sequentially through 5 μm followed by 1 μm cellulose filter cartridges and then collected in a 180 L fiberglass head tank. Seawater in the head tank was heated to the desired testing temperature (20 °C) using a titanium immersion heater. The temperature-controlled seawater was drawn from the head tank with a pump (nonmetallic pump head), passed through a 0.35−0.45 μm poly(ether sulfone) (Memtrex MP, GE Power and Water) or cellulose membrane cartridge filter and into a 24-port PVC manifold. Pressure in the manifold was controlled with a gate valve at the outlet of the manifold. MSL has four separate 24-port manifolds, linked to three separate head tanks, permitting testing of 96 adsorbent materials in flow-through columns simultaneously. A conceptual diagram of the manifold system used for seawater exposure of adsorbent materials in flow-through columns is shown in Figure 1. Adsorbent beds for flow-through experiments were prepared using 1 in. internal diameter by 6 in. long columns fabricated from plastic components, mostly PVC and polypropylene. The conditioned adsorbent (50−100 mg) was dispersed and packed in the column, where it was held in place by acid-cleaned glass wool and 5 mm glass beads that were used to fill the void space of the columns. The flow rate was monitored using an in-line turbine-style flow sensor (model DFS-2W, Digiflow Systems). Seawater salinity and pH were monitored daily using a handheld salinometer (YSI model 30), and pH meter calibrated with NIST buffers, respectively. Water temperature was monitored every 10 min using a temperature logger equipped with a flexible hermetic sealed RTD sensor probe (OMEGA Engineering, Stamford, CT).

adsorbent formulations to metal uptake, and an examination of the efficiency of HCl elution on desorption of uranium and other trace elements from seawater-exposed adsorbents.

2. MATERIALS AND METHODS 2.1. Adsorbent Materials. All adsorbent materials tested in the present study are amidoxime-based polymeric adsorbents developed at ORNL. Adsorbents were prepared using hollowgear-shaped, high surface area polyethylene (PE) fibers by a radiation-induced graft polymerization (RIGP) method with details described in previous work.3,9,11−13 The average length and density of the fibers prior to grafting were 25 mm and 0.941 g/cm3, respectively. The diameter of wet fibers was approximately 153 ± 15 μm.9 Based on the different comonomer grafted, the tested adsorbents were categorized into three formulations: 38H (methacrylic acid), AF1 (itaconic acid), and AI8 (vinylphosphonic acid) formulations.12,13 Among these adsorbents, the AF1 adsorbent was further produced in larger mass (∼10 g) as braided adsorbent materials. These braided adsorbent materials consist of a central supporting spine (∼5−10 cm) and numerous “fibrous feathers” (∼10−15 cm) extended from the core. During the production processes, the braided adsorbent materials were functionalized with amidoxime and hydrophilic ligands as a whole using the RIGP method. All adsorbents were received at PNNL in dry form. Before seawater testing, the adsorbents were pretreated by immersing in a 2.5% KOH solution at 80 °C for 1−3 h at a ratio of 1 mL KOH per mg of adsorbent. This alkaline pretreatment step can enhance the hydrophilicity of the adsorbents and may also induce the swelling of the fibers, which is beneficial for facilitating uranium adsorption.14,15 After the KOH pretreatment, the adsorbents were immediately washed with deionized water until the pH of the rinsewater was neutral. The adsorbents were kept submerged in deionized water before being packed into a flow-through column or deployed in the flume system. B

DOI: 10.1021/acs.iecr.5b03267 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 2.2.2. Flow-Through Channel (Flume) System. Flowthrough channel (flume) systems were designed for conducting seawater exposure studies with braided adsorbent materials under controlled temperature and flow-rate conditions. Flow rate (linear velocity) can be manipulated by using pumps with varying capacity and different flume dimensions. Three flumes constructed from darkened Plexiglas with different dimensions are available for flume studies.10 In the present study, we used a 63 L flume (length 122 cm, width 20.3 cm). The target linear velocity was 2 cm/s, which was approximately the linear velocity being used for column flow-through testing described above. Figure 2 shows a cross sectional view of the flume illustrating the recirculation system, fresh seawater input, and positioning

temperature for 2 h. The 2 h exposure was determined to be sufficient in a separate test. After the elution, adsorbents were removed from the HCl solutions by filtration and the HCl solutions were submitted for trace element analysis. 2.4. Analyses. Adsorbent materials exposed to seawater were washed with deionized water to remove salts followed by drying overnight using a heating block at 80 °C. The dried fibers (50−100 mg) were weighed and then digested with 10 mLs of a high-purity (Trace element grade, Fisher Scientific) 50% aqua regia acid mixture (3:1; hydrochloric: nitric acid) for 3 h at 85 °C on a hot block. Analysis of uranium and other trace elements was conducted using either a PerkinElmer 4300 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) or a Thermo Scientific iCAP Q Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Quantification with both instruments was based on standard calibration curves.

3. RESULTS AND DISCUSSION 3.1. Adsorbent Performance in Column Flow-through Tests. A summary of the time-dependent measurements of

Figure 2. Conceptual side-view of a flume system showing fresh seawater inlet and recirculation system.

of braided adsorbent materials. Temperature controlled, fresh filtered seawater was fed into the system from the head tank at flow-rates up to 8 L/min. The height of water level in the flume was controlled by the height of the overflow standpipe, which can be varied between approximately 6 and 10 in. Controlled water flow within the flume was accomplished by recirculating water using a centrifugal water pump with a nonmetallic pump head (Finish Thompson pump, model DB6H) to minimize contamination concerns. Precise control of flow-rate/linear velocity was further achieved by putting a flow restriction (globe valve) at the outlet of the pump. The flow-rate in the recirculating water was continuously monitored by placing a flow meter (Omega Engineering Inc.) in the line between the flume outlet and pump inlet. Braided adsorbent materials with weight 5−10 g were mounted in the flume for seawater exposure. Over the course of seawater exposure (42−56 days), a small portion or “snips” (∼100 mg) was taken from braided adsorbents using titaniumcoated scissor at selected time points. This allowed us to study the adsorption kinetics of uranium and other trace elements. 2.3. Adsorbent Elution Test. A single seawater exposed (56 days) AF1 braided adsorbent was used to determine the effectiveness of various concentrations of HCl to strip uranium and selected trace elements from the adsorbent. The elution test was performed by immersing individual adsorbent snips into HCl solutions ranging from 0.001 to 0.5 M. The adsorbent/HCl solution volume ratio was kept at 1 mg adsorbent per mL of HCl solution. The test solutions with adsorbent were placed on an orbital shaker at room

Figure 3. Time-dependent measurements of uranium adsorption capacity (g U/kg adsorbent) for the ORNL adsorbent formulations 38H, AF1, and AI8. The experiment was conducted at temperature 20 ± 1.5 °C and linear velocity in flow-through column experiments ∼2 cm/s. Uranium adsorption capacity was normalized to a salinity of 35 psu. Curves drawn through the data represent fitting to a one-site ligand saturation model (eq 1).

Table 1. One-Site Ligand Saturation Modeling of TimeDependent Measurements of Uranium with Three Adsorbent Formulations Obtained from ORNL ORNL adsorbent

56-day adsorption capacitya,b,c (g U/kg adsorbent)

saturation capacitya,b,c (g U/ kg adsorbent)

half-saturation timea,b,c (days)

38H AF1 AI8

3.37 ± 0.26 3.86 ± 0.18 3.54 ± 0.17

4.49 ± 0.27 5.43 ± 0.19 5.17 ± 0.18

18.6 ± 3.00 22.8 ± 1.90 25.8 ± 2.14

a

Adsorption capacity is expressed in units of g U/kg adsorbent and has been normalized to a salinity of 35 psu. bExposure temperature was controlled at 20 ± 1.5 °C. cPredicted using one-site ligand saturation modeling.

uranium adsorption kinetics on three different formulations (38H, AF1, AI8) of ORNL adsorbents is shown in Figure 3. These data were compiled from several experimental runs C

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Figure 4. Relative abundance of elements absorbed by the ORNL AF1 adsorbent after 56 day seawater exposure in a column flow-through system: (A) by mass percentage; (B) by molar percentage. The insets are the expanded view for Ni, Ti, Sr, Co, Cr, and Mn.

where u is uranium capacity (g U/kg adsorbent), t is exposure time (days), βmax is the adsorption capacity at saturation (g U/ kg adsorbent), and Kd is the half-saturation time (days). The modeled 56 day exposure capacity, saturation capacity, and halfsaturation time of the three adsorbent formulations produced by ORNL are shown in Table 1. Among the three adsorbent formulations, the AF1 adsorbent had the highest uranium adsorption capacity during a 56 day seawater exposure (3.86 ± 0.18 g U/kg adsorbent), with a predicted saturation capacity of 5.4 g U/kg adsorbent. On both 56 day and saturation capacity, AI8 adsorbent had the second best uranium adsorption performance and the 38H adsorbent

conducted independently across more than two years (four sets of 38H experiments; five sets of AF1 experiments; and one AI8 experiment). Due to the conservative behavior of uranium in seawater,16 we normalized all uranium adsorption capacity data to a salinity of 35 psu in order to correct for the varying salinity of natural seawater observed in different adsorption experiments. Adsorption kinetics and saturation capacity were determined by fitting time course measurements of adsorption capacity using a one-site ligand saturation model (eq 1):

u = βmax t /Kd + t

(1) D

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Figure 6. Time-dependent measurements of uranium adsorption capacity (g U/kg adsorbent) for seven ORNL AF1 adsorbent braids exposed to filtered natural seawater in a flume. The temperature in the flume was held at 20 ± 1.5 °C and the water in the flume was recirculated to mimic the linear velocity in flow-through column experiments (∼2 cm/s). The uranium adsorption capacity was normalized to a salinity of 35 psu. Curves drawn through the data represent fitting to a one-site ligand saturation model (eq 1). One-site ligand saturation modeling of all the data yielded a saturation capacity of 5.92 ± 0.19 g U/kg adsorbent, a 56 day adsorption capacity of 4.05 ± 0.18 g U/kg adsorbent, and a half-saturation time of 25.9 ± 1.78 days.

Table 3. Comparison of Adsorption Capacity, Adsorption Kinetics, and 56 Day Adsorption Capacity for the ORNL AF1 Adsorbent Exposed to Natural Filtered Seawater in Flow-through Column and Flume Exposures at 20°C Figure 5. Time-dependent measurements of adsorption capacities (g, element/kg adsorbent) for several trace elements retained by the ORNL AF1 adsorbent in flow-through column experiments. (A) Mg, Ca, Na, and V; (B) U, Fe, Zn, Cu, and Ni. Experiment conditions: Temperature 20 ± 1.5 °C; linear velocity in flow-through column experiments ∼2 cm/s. Note that these adsorption capacities were not normalized to 35 psu salinity. If a salinity correction were made, it would result in capacities approximately 11% higher based on an average salinity at PNNL of ∼31.5 psu. Curves drawn through the data represent fit to a one-site ligand saturation model (eq 1).

V U Fe Zn Cu Ni

saturation capacity 20.7 4.82 2.66 1.23 1.30 0.49

± ± ± ± ± ±

0.94 0.20 0.37 0.06 0.05 0.02

± ± ± ± ± ±

5 7

3.86 ± 0.18 4.05 ± 0.18

halfsaturation timea,b (days)

5.43 ± 0.19 5.92 ± 0.19

22.8 ± 1.90 25.9 ± 1.78

Adsorption capacity is expressed in units of g U/kg adsorbent and has been normalized to a salinity of 35 psu. bPredicted using one-site ligand saturation modeling.

adsorbent) manufactured by the Japan Atomic Energy Agency (JAEA).9 Comparing to all reported values in the literature, the AF1 adsorbent, with a capacity of ∼4 g U/kg adsorbent in 56day natural seawater exposure, is the best-performing seawater uranium adsorbent to date.3 The selectivity of the AF1 adsorbent to trace elements in natural seawater is shown in Figure 4. Illustrated is the percentage abundance of selected elements from several batches (n = 5) of AF1 adsorbents that were exposed to natural seawater for 42−56 days using the flow-through column exposure system. The results (Figure 4A) show that uranium is the fourth most abundant element by mass (7 ± 1%). Calcium and Mg, two major divalent cations in seawater, account for the majority of the cations adsorbed (∼60% by mass). Vanadium(V) accounts for 21 ± 3% abundance (by mass) and outcompetes uranium on the AF1 adsorbent. When relative abundance is expressed on a molar basis (mol U/kg adsorbent), uranium becomes the seventh most abundant element adsorbed (abundance order = Mg > Ca > V> Na ≫ Fe > Zn ≈ U > Cu > Ni) (Figure 4B). Notable competing ions are V,

half-saturation time (day) 58.2 24.4 69.1 2.55 41.1 6.71

n

column flume

saturation capacitya,b (g U/ kg adsorbent)

a

Table 2. One-Site Ligand Saturation Modeling of TimeDependent Measurements of Selected Elements Adsorbed on the ORNL AF1 Adsorbenta element

exposure method

56-day adsorption capacitya,b (g U/kg adsorbent)

4.53 2.48 15.8 0.93 3.07 1.09

Experiment conditions: Temperature 20 ± 1.5°C; linear velocity in flow-through column experiments ∼2 cm/s. The unit of adsorption capacity is g U/kg adsorbent.

a

was the third (Table 1). Half-saturation times of the uranium adsorption on these three adsorbent formulations are similar, between 20 and 25 days. The 56 day adsorption capacities of 38H, AF1, and AI8 adsorbents are all 3−4 times higher than the reported 56-day capacity of an adsorbent (1.0 g U/kg E

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Figure 7. Comparison of uranium adsorption capacity (g U/kg adsorbent) between flume and flow-through column exposures with four ORNL AF1 adsorbent braids (panels A−D). The uranium adsorption capacity was normalized to a salinity of 35 psu. All experiments were run at a temperature of 20 ± 1.5 °C and a linear velocity was ∼2 cm/s.

but rather primarily to nonspecific sites (e.g., COO−) on the adsorbent, possibly due to their very high concentration in seawater. One-site ligand saturation modeling of several elements, excluding the major ions, was used to estimate the saturation capacity and half-saturation time (Table 2) for the ORNL AF1 adsorbent. Zn and Ni had the most rapid adsorption kinetics among the trace elements investigated, with half-saturation times less than 7 days. V, Fe, and Cu had half-saturation times >40 days, slightly longer than that of uranium at 24 days. Because U has a half-saturation time less than that of V, Fe, and Cu, it offers a means to select for U over these elements by reducing the exposure time. To illustrate, V had a saturation capacity of ∼20 g/kg, 5 times higher than the uranium saturation capacity (4.8 g/kg); while the V/U ratio for a 56 day seawater exposure was ∼3 (Figure 4A). Shortening the exposure time to less than 56 days would offer additional improvements in the V/U ratio on the AF1 adsorbent. This highlights the importance of improved adsorbent design for faster uranium adsorption kinetics and shorter seawater exposure times.21 3.2. Adsorbent Performance in Flume Tests. A scaled up seawater exposure using the flow-through channel (flume) system was conducted with the ORNL AF1 adsorbent in braided form. Seven AF1 adsorbent braids, each weighing

Fe, and Cu that high distribution coefficients (Kd) with amidoxime-based adsorbents were reported.3,5,9 Sun et al.17 observed that stability constants of the complex of Fe (III) with glutarimidedioxime, a cyclic form of amidoxime single molecule, is stronger than U (VI)-glutarimidedioxime complex. Strong complexation of VO43− with glutarimidedioxime was also reported by Wai (2014).18 Among these competing ions the extremely high affinity of vanadium over uranium is one major challenge faced by current amidoxime-based adsorbents.3,9,19−21 Improving the adsorption selectivity of uranium over vanadium is a clear need for the next-generation high uranium capacity adsorbents. Figure 5 shows adsorption kinetics of elements with % abundance (by mass) greater than 0.5% in the ORNL AF1 adsorbent. No salinity correction was attempted here because, unlike uranium, many elements do not behave conservatively in seawater. The adsorption kinetics of the major cations in seawater, Na+, Ca2+, Mg2+, and V, the third most abundant element retained by the AF1 adsorbent, are shown in Figure 5A. The adsorption rate for the major cations was extremely rapid, reaching >90% saturation in the “time zero” sample. We define the “time zero” samples as exposure to a seawater flow of 250−300 mL/min for a very short time period, typically 1−2 min. This extremely fast kinetics suggests that these major seawater cations are not binding solely to amidoxime ligands, F

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Figure 8. Relative abundance (by mass percentage) of elements absorbed by the ORNL AF1 adsorbents braids after 42 or 56 days of seawater exposure. The data were compiled from the seven AF1 braids shown in Figure 6. The insets are the expanded view for Ni, Cu, Cr, Sr, Ti, Mn, and Co.

Figure 9. Removal of selected elements from the ORNL AF1 adsorbent (exposed to seawater for 56 days) with HCl concentrations between 0.001 and 0.5 M. The adsorbent was extracted on an orbital shaker at room temperature for 2 h. Note that X-axis scale is not linear.

similar results, with the flume exposures having slightly higher modeling results. For example, the relative percentage difference (RPD) of AF1’s modeled 56 day uranium capacity between flow-through column and flume experiments was less than 5%. We also conducted some column flow-through tests on a few snips (∼50 mg each) from AF1 braid no. 1−4 for the comparison between flume and column. As shown in Figures 7A−D, for all four AF1 braid materials, consistent uranium adsorption performance was observed between flume and column exposures, with RPD of 21-day or later uranium adsorption capacity between two experiment settings U > Fe) is consistent with the reported stability constants of glutarimidedioxime with Cu (II), UO22+, and Fe (III) (Cu < U < Fe).17 Only 2% of the V was removed from the adsorbent with 0.5 M HCl, suggesting a very strong complexation with the amidoxime ligand. In a separate study of acid elution with the ORNL 38H adsorbent, V removal from the adsorbent required use of 6 M HCl under elevated temperature (60 °C). This is a very harsh treatment that most likely will render the adsorbent unable to be reused. The difficulty in stripping V from amidoxime-based adsorbents has been reported in the literature.19,25 Finally, the fact that dilute HCl solutions readily removed Ca2+ and Mg2+ support the hypothesis made earlier that adsorption of major seawater cations are not associated with binding with amidoxime, but rather with nonspecific sites.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the U.S. Department of Energy, Office of Nuclear Energy, Fuel Cycle Research and Development Program, Fuel Resources Program (Contract No. DEAC05- 76RL01830). We thank Mr. Nicholas J. Schlafer and Mr. Brett A. Romano for their help with construction, maintenance, and operation of the marine testing facility. We also thank Ms. Carolynn R. Suslick and Ms. Julie K. Snelling for quality assurance and database management support.

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REFERENCES

(1) Lindner, H.; Schneider, E. Review of cost estimates for uranium recovery from seawater. Energy Economics 2015, 49, 9−22. (2) Uranium 2011: Resources, Production and Demand; Organization for Economic Co-operation and Development/Nuclear Energy Agency, International Atomic Energy Agency, 2012. (3) Kim, J.; Tsouris, C.; Mayes, R. T.; Oyola, Y. S.; Saito, T.; Janke, C. J.; D, S.; Schneider, E.; Sachde, D. Recovery of uranium from seawater: A review of current status and future research needs. Sep. Sci. Technol. 2013, 48, 367−387. (4) U.S. Department of Energy (2010) Nuclear Energy Research and Development Roadmap: Report to Congress. Document. http:// energy.gov/ne/downloads/nuclear-energy-research-and-developmentroadmap. (5) Tamada, M. Current status of technology for collection of uranium from seawater. http://wiki.ornl.gov/sites/nfrw/ Shared%20Documents/Uranium%20Extraction%20Seawater/2009_ Tamada%5B1%5D.pdf. (6) Shimizu, T.; Tamada, M.; Seko, N.; Sakaguchi, I. Recovery system for uranium from seawater using braid type adsorbent. Proceedings of Civil Engineering in the Ocean 2002, 18, 737−742.

4. CONCLUSIONS Natural seawater testing of ORNL’s amidoxime-based polymeric adsorbents 38H, AF1, and AI8 formulations demonstrated the high uranium adsorption capacity of all three formulations, with 56 day capacity >3.3 g U/kg adsorbent, modeled saturation capacity >4.5 g U/kg adsorbent, and halfsaturation times ∼20−25 days. Among the three formulations, H

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.5b03267 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX