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Alternative Alkaline Conditioning of Amidoxime based Adsorbent for Uranium Extraction from Seawater Sadananda Das, Wei-Po Liao, Margaret Flicker Byers, Costas Tsouris, Chris J. Janke, Richard T Mayes, Erich A. Schneider, Li-Jung Kuo, Jordana Wood, Gary A. Gill, and Sheng Dai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03210 • Publication Date (Web): 18 Oct 2015 Downloaded from http://pubs.acs.org on October 24, 2015

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Industrial & Engineering Chemistry Research

Alternative Alkaline Conditioning of Amidoxime based Adsorbent for Uranium Extraction from Seawater

S. Das,† W-P. Liao,† M. Flicker Byers,$ C. Tsouris,*† C.J. Janke,† R.T. Mayes,† E. Schneider,*$ L.-J. Kuo,‡ J.R. Wood,‡ G.A. Gill,‡ S. Dai† †

Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6053, USA ‡ Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, USA $ Nuclear and Radiation Engineering Program, The University of Texas at Austin, University Station C2200, Austin, Texas 78712, USA

Submitted for publication in Special Issue of Industrial and Engineering Chemistry Research

Invited to submit a paper; Topic: Uranium in Seawater

Submitted: August 2015 Revised: October 2015

Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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Alternative Alkaline Conditioning of Amidoxime based Adsorbent for Uranium Extraction from Seawater S. Das,† W-P. Liao,† M. Flicker Byers,$ C. Tsouris,*† C.J. Janke,† R.T. Mayes,† E. Schneider,*$ L.-J. Kuo,‡ J.R. Wood,‡ G.A. Gill,‡ S. Dai† †

Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6053, USA ‡ Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, USA $ Nuclear and Radiation Engineering Program, The University of Texas at Austin, University Station C2200, Austin, Texas 78712, USA

ABSTRACT Alkaline conditioning of the amidoxime based adsorbents is a significant step in the preparation of the adsorbent for uranium uptake from seawater. The effects of various alkaline conditioning parameters such as the type of alkaline reagent, reaction temperature, and reaction time were investigated with respect to uranium adsorption capacity from simulated seawater (spiked with 8ppm uranium) and natural seawater (from Sequim Bay, WA). An adsorbent (AF1) was prepared at the Oak Ridge National Laboratory by radiation-induced graft polymerization (RIGP) with acrylonitrile and itaconic acid onto high-surface-area polyethylene fibers. For the AF1 adsorbent, sodium hydroxide emerged as a better reagent for alkaline conditioning over potassium hydroxide, which has typically been used in previous studies, because of higher uranium uptake capacity and lower cost over the other candidate alkaline reagents investigated in this study. Use of sodium hydroxide in place of potassium hydroxide is shown to result in a 21-30% decrease in the cost of uranium recovery.

Keywords: Uranium from seawater, uranium adsorbent, amidoxime, alkaline conditioning

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INTRODUCTION A 2011 study by the Organization for Economic Co-operation and Development estimated that, at the current consumption rate, the global conventional reserves of uranium (7.1 million tonnes) could be depleted in roughly a century.1 Therefore it is of significant interest to look for resources of uraniun other than the conventional terrestrial uranium ores. Seawater is a potential source of a vast amount uranium (around 4.5 billion tonnes) with a concentration of 3 ppb.2 The extremely low uranium concentration, relative to other elements, makes the extraction very challenging. Poly(acrylamidoxime) (pAO) has been found to be chemically suitable for uranium recovery from seawater during the last few decades. The proposed deployment system uses braided textile adsorbents with an amidoxime ligand; the positively-buoyant adsorbent is moored to the ocean floor to passively collect uranium. After their soaking campaign has ended, the braids are winched out of the water so that the uranium may be eluted and the braids regenerated with an alkaline solution before being redeployed. The adsorbent may then be reused multiple times before its ultimate disposal, although the uptake performance degrades slightly with each reuse. While the estimated cost of this recovery system continues to evolve as the technology matures, the major factors affecting the economic viability of uranium extraction from seawater are: uranium adsorption capacity, rate of adsorption, cost of adsorbent production and reusability of adsorbent.3 All these factors are currently investigated at the Oak Ridge National Laboratory (ORNL),3-10 Pacific Northwest National Laboratory (PNNL),11,12 Lawrence Berkeley National Laboratory (LBNL),13-16 and universities17-23 in the U.S. and other countries. For example, using polyethylene fibers of relatively high surface area, researchers at ORNL have demonstrated a 3fold increase in uranium capacity over adsorbent prepared with low surface-area polyethylene fibers.3 The ORNL adsorbent capacity for uranium and other metals is evaluated at PNNL using flow-through columns and flumes.11,12,24 Increased uranium capacity has improved the economic evaluation of the process,3 however, additional improvements are needed not only in uranium capacity but also in the volume and cost of chemicals used for adsorbent preparation and regeneration. It is well documented that alkaline conditioning of pAO adsorbent is necessary before its deployment for uranium extraction,4,5 Potassium hydroxide (KOH) has been used extensively as the alkaline reagent in most of the reported works.5, 25-31 In our previous work, the optimal KOH 3 ACS Paragon Plus Environment


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conditioning parameters were demonstrated. It was realized that the adsorbent should be treated with mild KOH concentration for a lesser time-period, compared to conditions employed in previous studies,3 for multiple use without loss of uranium adsorption capacity in each cycle. It was also found that a fraction of the amidoxime groups is converted into carboxylate upon conditioning with KOH, and a particular ratio between the two must be maintained to achieve a moderate uranium adsorption capacity. In the current work, investigation for alternate alkaline reagents, other than KOH, was carried out, and an economic assessment was performed on the basis of altered uranium adsorption from seawater and/or lower chemical cost. Sodium hydroxide (NaOH) was also tested as the alkaline reagent in some of the available literatures.32-37 The pAO adsorbents were conditioned with 0.44 M of alkaline solution at different temperatures for different time-periods before they were explored for their uranium uptake capability by: (i) uranium uptake capacity from exposure to simulated seawater screening solution, spiked with 8ppm uranium in the form of nitrate for 24 hours at ORNL, and (ii) uptake of uranium and other metal ions from real seawater exposure for 56 days, at the Marine Sciences Laboratory (MSL) of PNNL, in Sequim, WA. The details of comparison of uranium uptake capacity from seawater by pAO adsorbent after conditioning with KOH, NaOH and ammonium hydroxides, followed by economic assessment, are described in this paper.

MATERIALS AND EXPERIMENTAL METHODS Materials Analytical reagent grade chemicals were used throughout the present study. Acrylonitrile (AN), itaconic acid (ITA), tetrahydrofuran (THF), methanol, dimethylsulfoxide (DMSO), N,Ndimethylformamide (DMF), hydroxylamine hydrochloride (HA-HCl), potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), ammonium hydroxide (AOH), tetramethylammonium hydroxide (TMAOH), tetraethylammonium hydroxide (TEAOH), triethylmethylammonium hydroxide (TEMAOH), tetrapropylammonium hydroxide (TPAOH) and tetrabutylammonium hydroxide (TBAOH) were obtained from Sigma-Aldrich. Ultrapure water (Thermo scientific Nanopore) was used in the preparation of HA-HCl and alkaline solutions. Hollow-gear, high-surface-area polyethylene fibers (PE) were prepared by meltspinning at Hills, Inc. (Melbourne, FL), using polylactic acid (PLA) as the co-extrusion polymer. 4 ACS Paragon Plus Environment

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Uranyl nitrate hexahydrate (UO2(NO3)2.6H2O, B&A Quality), sodium bicarbonate (NaHCO3, ACS Reagent, Aldrich) and sodium chloride (>99%, Aldrich) were used to prepare the simulated seawater, and a 1000 ppm uranium (U) standard solution (High Purity Standards, North Charleston, USA) was used to prepare the standards for inductively coupled plasma (ICP) analysis. Adsorbent Preparation The PE based adsorbent fibers were prepared by radiation-induced graft polymerization (RIGP) at the NEO Beam Electron Beam Cross-linking Facility (Middlefield, OH). The co-extruded PLA was completely removed by submerging the fibers in excess THF at 65-70 °C overnight before RIGP. The pre-weighed dry fiber samples were placed inside double-layered plastic bags within a plastic glove bag and sealed under nitrogen. The fibers were irradiated in the presence of dry ice using a translation table cycling the fibers for 16 passes under the electron beam to a dose of approximately 200 ± 10 kGy using 4.4-4.8 MeV electrons for approximately 22 minutes. After irradiation, the fibers were immersed in 300-mL pressure bottle containing previously degassed grafting solutions of AN and ITA in DMSO. The pressure bottles were then placed in an oven at 64 °C for 18 hours for grafting. After grafting, the fibers were drained from the solution and washed with DMF to remove unreacted monomers and homo polymers, followed by rinsing with methanol, and dried at 40 °C under vacuum. The grafted polyacrylonitrile (pAN) fibers were weighed to calculate the percentage degree of grafting (%DOG), determined gravimetrically from pre-irradiation and post-grafting weights of co-polymers onto the trunk polymer (Eq 1):

%DOG =

( wt AG − wtBG ) x100 wtBG

(1)

where wtBG = dry weight before grafting and wtAG = dry weight after grafting. The grafted AN groups in pAN fibers samples were converted to pAO groups by treating with 10 wt % hydroxylamine hydrochloride in 50/50 (w/w) water/methanol (previously neutralized with KOH) at 80 °C for 72 h. The resulting adsorbent was named as AF1. The adsorbent samples were then washed under vacuum filtration with deionized water, followed by a methanol rinse, and allowed to dry at 40 °C under vacuum. 5 ACS Paragon Plus Environment


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Alkaline conditioning of amidoximated fibers The amidoximated AF1 fibers were treated with 0.44M KOH and 0.44M NaOH at different temparatures of 60 ºC, 70 ºC, and 80 ºC, and for different time-periods of 1 h, 2 h, and 3 h. The optimum parameters realized from these studies were applied for conditioning the AF1 fibers with sodium carbonate and ammonium hydroxides. The alkaline conditioned samples were immediately filtered and washed with DI water until the pH was neutral, followed by uranium adsorption studies. Simulated seawater screening Uranium adsorption in simulated seawater was carried out at ORNL. The simulated seawater screening solution was prepared by dissolving 193 ppm sodium bicarbonate; 25,600 ppm sodium chloride; and 8 ppm uranium from uranyl nitrate hexahydrate in 18.2 MΩ cm-1 water. The pH of the screening solution was approximately 8. The concentrations of sodium and bicarbonate were selected to be similar to those in seawater. An aliquot was taken before addition of the adsorbent to measure the initial uranium concentration. Each of the alkaline-conditioned adsorbent samples was equilibrated with 750 mL of simulated seawater solution for 24 hours at room temperature with constant shaking at 400 rpm. After contacting for 24 hours, an aliquot was collected, and the initial and final solutions were analyzed by inductively coupled plasma-optical emission spectroscopy (Perkin Elmer Optima 2100DV ICP-OES) at 367 nm. The uranium adsorption capacity was calculated from the difference in uranium concentration in the aliquot samples using Eq 2. The ICP-OES was calibrated using 6 standard solutions ranging from 0-10 ppm, which were prepared from 1000 ppm uranium in 5 wt % nitric acid stock solution, and a linear calibration curve was obtained. A blank solution of 2–3 wt % nitric acid was also prepared, and washouts were monitored between samples to ensure no uranium was carried over into the next analysis. In addition, a solution of 5 ppm yttrium in 2 wt % nitric acid was used as an internal standard, which was prepared from 1000 ppm stock solution (High-Purity Standards, North Charleston, USA).

− Uranium (U) adsorption capacity= × Soln. vol. (L) (2) 6 ACS Paragon Plus Environment

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Uranium adsorption kinetics in batch reactors Five-gallon plastic containers with coastal seawater were used for batch adsorption experiments. AF1 adsorbent (5-10 mg), conditioned with 0.44 M of KOH or NaOH at 70 ºC for 1h, were added into the containers, where adsorbent fibers were freely suspended in the seawater. An initial seawater sample of 1 mL was collected prior to addition of adsorbent. The containers were shaken constantly at 100 rpm at room temperature. This agitation speed was proven sufficient for fluidized fibers to adsorb uranium in the reaction-limited regime, in the presence of bicarbonate ions.4 One-mL samples were collected periodically using a pipette, over the duration of the experiment. The quantitative analysis of the aliquots was carried out using inductively coupled plasma mass spectroscopy (ICP-MS, Thermo Scientific X-Series II). Sample aspiration was performed at 100 µL/min with a Teflon SP nebulizer coupled to an Elemental Scientific Inc. PC3 and Fast combination spray chamber. An internal standard solution containing Bi, In, Sc, Tb, Y (High Purity Standards ICP-MS-IS-2, Perkin-Elmer) was added to eliminate the matrix effect. High-purity nitric acid (2%, Optima, Fisher Scientific) was used as the sample diluent and carrier phase, and for washing out the samples. Wash out of the instrument was monitored between samples. An average of six replicate measurements per sample was used to quantify uranium-238 against a 6-point calibration curve. NASS-5 (seawater) and CASS-6 (seawater) standards supplied by the National Research Council of Canada were used for seawater quality-control experiments. Uranium adsorption: Flow-through column test with natural seawater The adsorbent performance, carried out at PNNL-MSL using natural seawater with flow-through columns for 56 days of exposure,3 was assessed and characterized in terms of adsorbent capacity and kinetics by varying alkaline conditioning parameters, such as temperature and time. The quality of seawater was quantitatively monitored for pH, temperature, salinity, and trace-metal concentrations over the experimental period. The average uranium concentration observed in this study was slightly lower than the normal uranium concentration in seawater of 3.3 ppb [for a salinity of 35 practical salinity units (psu)]. Marine testing was performed using filtered (0.45 µm) seawater at a temperature of 20 ± 1 °C and at a flow rate of 250 mL/min, using an active pumping system.3



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Approximately 50 mg of AF1 adsorbent samples were treated with 0.44 M of respective alkaline solution at various temperatures, e.g., 60, 70, and 80 ºC for different time periods, e.g., 1, 2, and 3 hours prior to packing them into columns of 1-inch diameter and 6-inches length, fabricated from all-plastic components, mostly polyvinylchloride and polypropylene. Glass wool and 5-mm glass beads were used to make sure that the adsorbent fibers remained fixed in place inside the column. Seawater pumped from Sequim Bay, WA, was filtered through 0.45-µm polypropylene cartridges before entering the columns for the continuous-flow adsorption experiments. The temperature of the incoming seawater was maintained at 20±1 ºC using an alltitanium immersion heater. A temperature logger equipped with a flexible hermetically sealed RTD sensor probe (OMEGA Engineering, Stamford, CT, USA) and an in-line turbine-style flow sensor (Model DFS-2W, Digiflow Systems) were used to monitor the temperature and flow rate, respectively, with a 10-minute interval between measurements. The salinity and pH of the seawater were monitored daily using a hand-held salinometer (Model 30, YSI) and pH meter (Orion 3 STAR, Thermo). The adsorbents were removed from the columns after a certain period of exposure and desalted by thoroughly rinsing with DI water, followed by drying in a heating block. The dried and weighted adsorbents were digested with a 50% aqua regia solution at 85 °C for 3 hours. Samples were further diluted with DI water to be in the desired concentration range for analysis. The concentrations of uranium and metals were analysed using a Perkin-Elmer Optima 4300DV ICP-OES, with quantification based on standard calibration curves.

RESULTS AND DISCUSSION Fourier Transform Infrared (FTIR) spectrometry The AN and ITA monomers were grafted onto PE trunk at an 85:15 weight ratio, as discussed elsewhere in this issue.38 FTIR spectra of the AF1 fiber samples were collected on a Perkin Elmer Frontier FTIR with a single-bounce diamond attenuated total reflectance (ATR) accessory at 4 cm-1 resolution and averaged over 16 scans. The stretching frequencies at 2244 cm-1 and 1720 cm-1 represent the C≡N and carbonyl groups, respectively, and thus can be used to determine the grafting of acrylonitrile and itaconic acid onto the polyethylene. The disappearance of nitrile stretch and appearance of N-H (3390, 3265 cm-1), C=N (1640 cm-1) and N-O (930 cm-1) can also verify conversion of the nitrile to amidoxime (AO). Figure 1 describes the effect of conditioning with 0.44M NaOH at 70 ºC for different time-periods. The gradual 8 ACS Paragon Plus Environment

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decrease in intensity, or disappearance, of N-O stretch at 930 cm-1, and appearance of the new band at 1560 cm-1 are due to conversion of amidoxime into carboxylate with increasing NaOH conditioning time-period. A scheme of the reaction mechanism for alkaline conditioning is proposed elsewhere in this issue.38

90 85 80 75 % Transmittance

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70 65 60

AO'd AF1 AF1-NaOH-1h AF1-NaOH-2h AF1-NaOH-3h AF1-NaOH-24h

55 50 45 40

COO- stretch 1560 cm-1 C=N stretch 1640 cm-1

1700

1550

1400 1250 1100 Wavenumber (cm-1)

N-O stretch 930 cm-1

950

800

650

Figure 1. FTIR spectra of amidoximated AFI fiber after conditioning with 0.44 M NaOH at 70 ºC for different time periods. A summary of the FTIR spectra of the AF1 samples after conditioning with 0.44M of different alkaline solutions at 70 ºC for 1h is shown in Figure 2. The stretching frequency at 1560 cm-1 appears as a result of formation of carboxylate due to hydrolysis of the amidoxime (AO) upon alkaline conditioning with 0.44 M of KOH, NaOH, TMAOH (TEAOH, TEMAOH, TPAOH and TBAOH were also used, but are not included in the figure), except for Na2CO3 and AOH. It is believed that formation of carboxylate is very important for the adsorbent ability to adsorb



uranium. AF1 adsorbent, therefore, was not expected to adsorb uranium after alkaline conditioning with Na2CO3 and AOH.

90 85 80 75 % Transmittance

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70 65 1560 cm-1

60 AO'd fiber KOH NaOH Na2CO3 AOH TMAOH

55 50 45

930 cm-1

1640 cm-1

40 1650

1450 1250 Wavenumber (cm-1)

1050

850

Figure 2. FTIR spectra of amidoximated AFI fiber after conditioning with 0.44 M of different alkaline solution at 70 ºC for 1h. SIMULATED SEAWATER SCREENING TEST The uranium adsorption efficiency of AF1 adsorbent in simulated screening solution spiked with 8 ppm uranium was carried out after conditioning with 0.44 M of different alkaline solutions at 70 ºC for 1h. The uranium adsorption capacities of the AF1 samples after contact with the screening solution for 24 hours are shown in Figure 3. The adsorption capacity for uranium was virtually zero without any alkaline conditioning. The uranium adsorption capacity was found to be comparable upon conditioning with KOH and NaOH. As discussed in the previous section, conditioning with Na2CO3 and AOH indeed resulted into practically zero uranium adsorption capacity, probably due to the fact that deprotonation is important for the preparation of the 10 ACS Paragon Plus Environment

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adsorbent and conversion of a portion of the amidoxime group into carboxylate is also necessary to increase the hydrophilicity of the adsorbent. However, other ammonium hydroxide solutions (i.e., TMAOH, TEMAOH, TEAOH, TPAOH and TBAOH) were also proved to be potential reagents for alkaline conditioning towards higher uranium adsorption capacity.

180 160 140 g-U/Kg-ads

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120 100 80 60 40 20 0

Figure 3. Uranium adsorption capacities of AF1 after 24 h in screening solution, spiked with 8 ppm uranyl ions, after conditioning with 15 mL of 0.44 M of different alkaline solutions at 70 ºC, for 1h. URANIUM ADSORPTION KINETICS IN BATCH REACTORS Two AF1 adsorbent samples of approximately 8 mg each were treated with 0.44 M KOH and NaOH, at 70 ºC for 1 h. The samples were then placed in 5-gallon seawater containers, shaken at 100 rpm, to study the kinetics of uranium uptake from coastal seawater obtained from Charleston, SC. One-mL solution samples were drawn periodically for ICP-MS analysis of uranium. From the solution uranium concentrations, the amount of uranium adsorbed was 11 ACS Paragon Plus Environment


estimated through a mass balance. Results of uranium adsorption versus time are presented in Figure 4, where it is shown that the uranium uptake kinetics is faster for the KOH-conditioned sample than for the NaOH-conditioned sample. This result probably explains the reason behind selection of KOH over NaOH for alkaline conditioning by the Japanese scientists, even though it is more expensive than NaOH.

3.0 KOH-1 hr treatment

2.5 mg-U/g-ads.

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NaOH-1 hr treatment

2.0 1.5 1.0 0.5 0.0 0

10

20 30 40 Exposure Time (days)

50

60

Figure 4. Adsorption kinetics of uranium by same-batch AF1 samples after 56 days contact with coastal seawater in 5-gallon batch reactors. The adsorbent samples were treated with 0.44 M of KOH and NaOH at 70 ºC for 1 h. FIELD SEAWATER FLOW THROUGH COLUMN TESTING The performance of the AF1 adsorbent for uranium adsorption from natural seawater was tested at PNNL-MSL with filtered seawater from Sequim Bay, for 56 days in flow-through columns. The amidoximated AF1 adsorbent samples were treated with 0.44 M KOH or NaOH at three different temperatures, i.e., 60 ºC, 70 ºC, and 80 ºC for three different time periods, i.e., 1 h, 2 h, and 3 h at each temperature. The adsorption capacity of uranium along with vanadium and iron by AF1 after conditioning with 0.44 M of NaOH and KOH is shown in Figure 5. The uptake of


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other metal ions from seawater was lower. The uranium uptake seems to be optimum after alkaline conditioning at 80 ºC for 1h and 70 ºC for 1 h with KOH and NaOH, respectively. (a) 0.44M KOH

(b) 0.44M NaOH

Figure 5. Adsorption of U, V and Fe by AF1 after 56 days contact with seawater in flow-through columns. The adsorbent samples were treated with 0.44 M of KOH and NaOH at 60 ºC, 70 ºC, and 80 ºC for three time periods i.e., 1 h, 2 h, and 3h at each temperature. Reported adsorption capacities were normalized to 35 psu seawater.



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The uranium adsorption, in general, decreases gradually with increasing conditioning time at 70 ºC and 80 ºC. It is also interesting to note that uranium adsorption is higher for conditioning at 60 ºC with 0.44 M NaOH as compared to that with 0.44 M KOH. This particular observation may be significant in efforts to decrease the cost of uranium extraction from seawater. Vanadium is being considered as the main competitive ion for uranium extraction from seawater.39 The overall vanadium uptake by AF1 is slightly less for adsorbent conditioned with NaOH as compared to the capacity of adsorbent conditioned with KOH. These flow-through-column studies were carried out using two different AF1 samples prepared at different batches, and the difference observed is within the range of variability observed from different batches. Using AF1 adsorbent samples from the same batch, flow-through-column studies were conducted to determine the effects of conditioning with 0.44 M of KOH, NaOH, CsOH, TEMAOH and TMAOH at 70 ºC for 1 h. CsOH was used mainly for comparison with NaOH and KOH, specifically, with respect to the diffusion of Na+, K+, and Cs+ ions through the adsorbent polymer. A summary of uranium uptake after an exposure time of 56 days with seawater in flowthrough columns is shown in Figure 6. It can be seen that the AF1 has the highest uranium adsorption capacity (3.89 g-U/Kg-ads) after conditioning with 0.44 M of NaOH at 70 ºC for 1 h as compared to the other alkaline reagents. Thus, the same-batch AF1 adsorbent resulted in a 3.2% enhancement of uranium uptake for conditioning with NaOH over KOH. However, previous column experiments from PNNL-MSL, for a different batch of AF1 samples conditioned for 1 and 3 h at 70 °C, showed a decrease in uptake for fibers conditioned with NaOH as compared to KOH, as seen in Figure 7.


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3.90 3.70 3.50 g-U/Kg-ads

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3.30 3.10 2.90 2.70 2.50 KOH

NaOH

CsOH

TEMAOH TMAOH

Figure 6. Adsorption of uranium by same-batch AF1 samples after 56 days contact with seawater in flow-through columns. The adsorbent samples were treated with 0.44 M of KOH, NaOH, TMAOH and TEMAOH at 70 ºC for 1 h.



3.9 3.7 3.5 g-U/Kg-ads

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3.3 3.1 2.9 2.7 2.5 KOH 1h

NaOH 1h

KOH 3h

NaOH 3h

Figure 7. Adsorption of uranium by a different batch of AF1 samples after 56 days contact with seawater in flow-through columns. The adsorbent samples were treated with 0.44 M of KOH and NaOH at 70 ºC for 1 h and 3 h.

The uptake of uranium and other metals by AF1 adsorbent was monitored as a function of exposure time. The adsorption history of uranium, vanadium and iron by the AF1 adsorbent conditioned with 0.44 M of KOH, NaOH, and CsOH, respectively, over a total period of 56 days is shown in Figure 8. Uranium adsorption gradually increased over 56 days by adsorbent conditioned with NaOH, whereas it reached a plateau after 42 days for adsorbent conditioned with KOH or CsOH. This is probably due to the kinetic effect observed in batch experiments. It is also interesting to note that the vanadium was continuously adsorbed over 56 days by adsorbent conditioned with KOH or CsOH, but adsorption was completed after 42 days for adsorbent conditioned with NaOH. The uptake of iron was gradually increased over the period of 56 days and was higher for adsorbent conditioned with KOH or CsOH as compared to iron uptake by adsorbent conditioned with NaOH. Thus, NaOH emerged as a better reagent for alkaline conditioning of the AF1 adsorbent because of higher uranium uptake capacity; higher 16 ACS Paragon Plus Environment

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uranium uptake selectivity over vanadium; and lower cost (discussed in the following sections) over the other candidate alkaline reagents. The cost savings per unit mass of uranium produced by using NaOH versus KOH as the alkaline reagent are discussed in the next section.

Figure 8. Adsorption of uranium (A), vanadium (B) and iron (C) AF1 samples as a function of exposure time over 56 days contact with seawater in flow-through columns. The adsorbent samples were treated with 0.44 M of KOH, NaOH and CsOH at 70 ºC for 1 h.



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ECONOMIC BENEFITS OF NaOH VERSUS KOH ALKALINE CONDITIONING A cost analysis of the textile adsorbent technology for recovery of uranium from seawater uses discounted cash flow techniques to follow the life cycle of a unit mass of adsorbent through fabrication, deployment and elution cycles, and disposal40. While the cost analysis evolves in tandem with the technology, the majority of the methodology and data sets for the existing analysis mirror those set forth in reference with more recent updates reflected in subsequent analyses.5,41 The cost of uranium recovery from seawater is predominantly driven by chemical and raw material expenses. The breakdown shown in Figure 9 is dominated by the production cost of the adsorbent, which includes substrate fabrication and radiation-induced graft polymerization, as well as the elution and regeneration cost. The regeneration cost is in turn dominated by the alkaline solution, which in the previous reference case was KOH. The highlighted regions illustrate the contribution of KOH to the total system cost.

The rest of this section will

demonstrate the economic benefits of replacing KOH with NaOH in these parts of the procedure.

Figure 9. Cost breakdown of uranium production. 18 ACS Paragon Plus Environment

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The production cost displayed in Figure 9 results from the base case system parameters shown in Table 1. These parameters and the associated system cost do not necessarily reflect the best available technology, which remains under active development. Table 1. Base case parameters for uranium production system Degradation Rate (% loss in uptake per re-use) Ocean Temperature (°C) Alkaline Solution Number of Uses of adsorbent prior to disposal Length of Campaign (days)

5 20 KOH 10 81

The degradation rate has significant impact on the final cost of recovery. Although re-using the adsorbent circumvents the costly production step, the decreased mass of uranium recovered with each additional use will eventually become low enough to cease outweighing the cost of another deployment. The degradation rate used in this analysis comes from experiments conducted in Japan42 on a similarly structured adsorbent. The ocean temperature is known to affect uranium uptake, and cost benefits could follow from deployment in warmer waters.43 The remaining two properties, length of campaign and number of adsorbent uses, are optimized to result in the lowest possible uranium production cost for the given scenario. A comparison of the two alkaline solutions under consideration can be found in Table 2. The unit costs of NaOH and KOH are obtained from historical data sets44 as well as vendor quotes, specifically for bulk purchases on the order of tens of metric tons, obtained using Alibaba, a Chinese e-commerce company. Bulk purchases were considered for both commodities to ensure similar economics of scale. Sodium hydroxide is available at roughly half the cost of potassium hydroxide. Since alkaline consumption is calculated on a stoichiometric basis, the lower molar mass of NaOH results in reduced consumption rates on a unit mass of salt per unit mass adsorbent basis. Given the purpose of deprotonation, 2 moles of potassium or sodium hydroxide are consumed



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per mole of itaconic acid and 1 mole of base per mole of acrylonitrile present on the adsorbent. Like all other chemicals in the economic analysis, the alkaline solutions are assumed to be used with negligible losses. Table 2. Comparison of commercially available salts Sodium Hydroxide

Potassium Hydroxide

Cost ($/tonne)

$480

$1,050

Molar Mass (g/mol)

40

56

Since the use of sodium hydroxide has been shown to result in a range of effects, ranging from a modest decrease to a small enhancement in capacity, two scenarios will be considered. One case considers the adsorbent performance seen in Figure 6, which incorporates a 3% increase in capacity to the cost model. The second case averages the effects on adsorbent uptake seen in Figure 7 to result in a 10% decrease in capacity. The changes in adsorbent performance and the production cost benefits of using NaOH combine to decrease the cost of producing uranium as seen in Table 3. Table 3. Cost impacts of using sodium vs potassium hydroxide Potassium Hydroxide

Sodium Hydroxide Reduced Uptake

Sodium Hydroxide Increased Uptake

Saturation Capacity of fresh Adsorbent (g U/kg ads)

5.42

4.88

5.59

Cost of Uranium ($/kg)

$694

$545

$486

Number of Uses

10

13

15

Soaking Time (days)

81

61

65

Uptake of Fresh Adsorbent (g U/kg ads)

4.24

3.56

4.15


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Even if a 10% loss in uptake is suffered, the use of NaOH is economically advantageous. The economics offered by the lower cost and molar mass of NaOH lead to benefits sufficient enough to outweigh the small decrease in uptake. This chemical replacement leads to a savings of approximately 21-30%, depending on the effect on uptake. Since the back end cost associated with regenerating adsorbent using NaOH is much lower than that of KOH, the optimal number of adsorbent re-uses increases. The high regeneration cost resulting from KOH use limits the number of economically viable deployment and regeneration cycles, and thus encourages longer optimized soaking times in order to recover more uranium per immersion. In the case of NaOH, a greater number of re-uses is favorable. Subsequently, the production cost does not benefit from these prolonged soaking times, which are associated with decreased rates of uranium uptake and a lowered uranium value due to time value of money effects upon the revenue stream. Note that the uptake of the adsorbent is a function of the saturation capacity and soaking time, so the NaOH-treated adsorbent, which has a shorter optimal soaking time, can exhibit lower uptake than the KOH-treated adsorbent even when it has higher capacity. Figure 10 contrasts the cost breakdown for these two scenarios.



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Figure 10. Comparison of uranium production costs.

Since the conditioning process itself is quite similar regardless of which alkaline solution is used, this change is simple to implement and requires no system or design overhauls. Therefore, NaOH is strictly superior to KOH from a cost benefit standpoint, even if it does result in slightly reduced uranium uptake.

CONCLUSIONS AF1 adsorbent containing acrylonitrile and itaconic acid was successfully prepared by radiationinduced graft polymerization onto high-surface-area polyethylene fiber. Alkaline treatment of the amidoximated adsorbent is important because it leads to the formation of carboxylates, thus 22 ACS Paragon Plus Environment

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affecting hydrophilicity, swelling, and conformation of the adsorbent. Current work is focused on the performance of adsorbent grafted with different mole ratios of acrylonitrile and itaconic acid, starting from 100% acrylonitrile and 0% itaconic acid, which can be used as a reference point. Different mole ratios of acrylonitrile and itaconic acid are expected to yield different ratios of carboxylate over amidoxime upon KOH conditioning. Investigation on the conditioning parameters, such as chemical nature of alkaline, temperature and time has been successfully demonstrated in this paper on the basis of high uranium uptake capacity, selectivity from seawater and its cost effectiveness on the overall process. Inorganic based reagents, such as potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), cesium hydroxide (CsOH); as well as organic based reagents, such as ammonium

hydroxide

(AOH),

tetramethylammonium

hydroxide

(TMAOH),

tetraethylammonium hydroxide (TEAOH), triethylmethylammonium hydroxide (TEMAOH), tetrapropylammonium hydroxide (TPAOH) and tetrabutylammonium hydroxide (TBAOH) were used for alkaline conditioning. NaOH is the best candidate for alkaline treatment among all reagents mentioned above. Conditioning of AF1 adsorbent with 0.44M NaOH at 70 ºC for one hour over conditioning with 0.44M KOH at 80 ºC for 1-3 hours (which was the usual practice) has been proven to be better for higher uranium uptake capacity as well as uptake selectivity from seawater in 56 days exposure in the flow-through-column study. NaOH, thus emerged as a better reagent for alkaline conditioning of the AF1 adsorbent because of: higher uranium uptake capacity; higher uranium uptake selectivity over vanadium; lower cost over the other candidate alkaline reagents. These findings lead to significant economic impacts as the use of NaOH conditioning in place of KOH results in a 21-30% decrease in the cost of uranium production.

Acknowledgment This research was supported by the U.S. DOE Office of Nuclear Energy, under Contract DEAC05-00OR22725 with ORNL, managed by UT-Battelle, LLC. We thank Mr. Jonathan E. Strivens and Brett A. Romano (PNNL) for technical support on the marine testing.



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