Uptake of Uranium from Seawater by Amidoxime-Based Polymeric

Mar 11, 2014 - The results were used to update an assessment of the cost of large-scale recovery of uranium from seawater using the ORNL adsorbent. Th...
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Uptake of Uranium from Seawater by Amidoxime-Based Polymeric Adsorbent: Field Experiments, Modeling, and Updated Economic Assessment Jungseung Kim,† Costas Tsouris,*,† Yatsandra Oyola,† Christopher J. Janke,† Richard T. Mayes,† Sheng Dai,† Gary Gill,*,‡ Li-Jung Kuo,‡ Jordana Wood,‡ Key-Young Choe,‡ Erich Schneider,*,§ and Harry Lindner§ †

Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6181, United States Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, United States § Nuclear and Radiation Engineering Program, The University of Texas at Austin, University Station C2200, Austin Texas 78712, United States ‡

ABSTRACT: Uranium recovery from seawater has been investigated for several decades for the purpose of securing nuclear fuel for energy production. In this study, field column experiments have been performed at the Marine Sciences Laboratory of the Pacific Northwest National Laboratory (PNNL) using a laboratory-proven, amidoxime-based polymeric adsorbent developed at the Oak Ridge National Laboratory (ORNL). The adsorbent was packed either in in-line filters or in flow-through columns. The maximum amount of uranium uptake from seawater was 3.3 mg of U/g of adsorbent after 8 weeks of contact between the adsorbent and seawater. This uranium adsorption amount was about 3 times higher than the maximum amount achieved in this study by a leading adsorbent developed at the Japan Atomic Energy Agency (JAEA). Both adsorbents were tested under similar conditions. The results were used to update an assessment of the cost of large-scale recovery of uranium from seawater using the ORNL adsorbent. The updated uranium production cost was estimated to be reduced to $610/kg of U, approximately half the cost estimated for the JAEA technology.

1. INTRODUCTION At a concentration of 3.3 μg of uranium/L, the estimated 4.5 billion metric tons (t) of uranium present in seawater offer a sizable potential source of nuclear fuel.1 The challenge is to develop an economically viable approach for uranium recovery from seawater should conventional resources become too scarce, expensive, or environmentally impactful to sustain a competitive nuclear power industry. Researchers from several countries2−7 have made substantial efforts to develop materials and methods suitable for the recovery of dissolved uranium from seawater. Japanese researchers performed a significant amount of the research and development work in this direction.1,3,4,8 After amidoxime-based polymeric materials were developed for uranium adsorption through γ-ray radiation grafting, field experiments were conducted to evaluate the technical feasibility of uranium uptake from seawater at locations off the coast of Japan. Field tests were performed using either a stack of adsorbent sheets or a braid-type adsorbent, resulting in a demonstrated capture of 1 kg of uranium yellow cake after a 240-day campaign.9 Their adsorbent was able to capture 1.5 mg of U/g of adsorbent in 30 days contact with seawater.10 Following the effort of Japanese researchers, investigators from India synthesized amidoxime-based adsorbent and carried out marine tests.4 Nonwoven polypropylene fibers were used as a supporting material for electron beam radiation induced graft polymerization. It was reported that an amount of 0.8 g of uranium was collected from four campaigns by utilizing a tidal wave motion of seawater for 54 days.4 A process flow sheet was © 2014 American Chemical Society

also prepared and different contact methods were tested to extract 100 g of uranium/year in a larger-scale facility. On the basis of the results of the Japanese field tests with braid-type adsorbent,10 which exhibited the best performance, the amount of uranium uptake for the duration of the test was reported as the main component determining the overall cost of uranium recovery from seawater. In general, it is important to know the uranium uptake behavior, i.e., uptake rate and amount of uranium adsorbed per unit mass of adsorbent, to be able to predict the cost of uranium recovery per unit mass of the fuel. The Japanese group proposed a design for a large-scale (1200 t of U/year) seawater-uranium-production system. Assuming recovery of 2 mg of U/g of adsorbent after 60 days of contact with seawater, and six uses of the adsorbent before it must be replaced, the Japanese group in 2006 estimated a uranium production cost of 90,000 yen/kg.11 An independent cost analysis of this system carried out in the United States led to a production cost estimate of $1230/kg of U (year 2011 U.S. dollars) with a 95% confidence interval of [$1030, $1440].12 The confidence interval reflects uncertainties in system costs. In this study, a novel amidoxime-based polymeric adsorbent that was developed using high-surface-area polyethylene fibers to extract uranium from seawater has been characterized using Received: Revised: Accepted: Published: 6076

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Table 1. Summary of Experimental Conditions for Field Experiments at the Marine Sciences Laboratory of PNNL

a

expt

adsorbent

1 2 3 4 5 6 7

ORNL ORNL ORNL ORNL ORNL JAEA JAEA

temp [°C] 20 20 20 20 20 20 20

± ± ± ± ± ± ±

2 2 2 2 2 2 2

flow rate [mL/min]

test system

holder type

note

± ± ± ± ± ± ±

parallel series/parallel parallel parallel parallel parallel parallel

column in-line filter column column in-line filter in-line filter column

− − − duplicatea − − duplicate

250 500 500 500 500 500 500

25 50 50 50 50 50 50

PNNL independent verification.

Figure 1. Schematic diagrams of flow-through experiments: (A) series/parallel configuration and (B) parallel configuration of field tests at the Marine Sciences Laboratory of PNNL.

batch and flow-through laboratory studies.2 The maximum adsorption capacity was 4 mg of U/g of adsorbent in batch experiments.2 The degree of grafting of the polyethylene fibers used to prepare the adsorbent varied between 250 and 600% by weight. The objective was to obtain kinetic and equilibrium information on uranium uptake from seawater, and to use this information in an economic viability study to assess the applicability of the process and establish goals for future work. Batch experiments with synthetic seawater were conducted at Oak Ridge National Laboratory, and flow-through exposure uptake experiments with natural seawater were conducted at the Marine Sciences Laboratory (MSL) of the Pacific Northwest National Laboratory (PNNL). The polymeric adsorbent that showed the best laboratory performance in batch experiments, with respect to the amount of uranium adsorbed per unit mass of adsorbent,2 was employed in the field column experiments with natural seawater. A kinetic uptake model has been developed to provide a better understanding of the mechanisms controlling the rate of uranium uptake from seawater onto the adsorbent and to aid in the economic assessment. The previous cost analysis12 is updated to

incorporate the experimental and kinetic modeling results reported within.

2. EXPERIMENTAL METHODS 2.1. Adsorbent Preparation. Amidoxime-grafted polymeric adsorbents were prepared at the Oak Ridge National Laboratory (ORNL) following a radiation-induced graft polymerization (RIGP) method, which has been described in detail in previous work.1,2 RIGP includes (i) production of radicals on polyethylene fibers through radiation, (ii) grafting of monomers, (iii) ligand conversion, and (iv) conditioning with alkaline treatment. Various polyethylene fiber geometries have been utilized, and through laboratory screening tests, the best adsorbent was selected for field testing. The adsorbent with the highest uranium capacity (up to 4 mg of U/g of adsorbent), determined from 5-gal batch seawater experiments,2 was based on porous polyethylene fibers as the support material, to provide a high surface area (1.35 m2/g). The polyethylene fibers used in this study had a relatively high surface area which could increase the degree of grafting of the functional groups and promote the transport of various metal ions through the fibers. The average length and density of the fibers prior to 6077

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grafting were 25 mm and 0.941 g/cm3, respectively. The diameter of wet fibers was approximately153 ± 15 μm.2 2.2. Preparation and Setup for Field Adsorption Tests. The adsorbent performance was assessed and characterized in terms of kinetics and adsorbent capacity equilibrium using natural seawater by varying experimental parameters such as temperature, type of flow-through column and experimental configuration (i.e., columns in series versus parallel), and linear velocity. The quality of seawater was quantitatively monitored for pH, temperature, salinity, and trace-metal concentrations over the experimental period, as shown in Table 1. The average uranium concentration observed in this study was slightly lower than the 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 ± 2 °C and at two different flow rates (250 and 500 mL/min) using actively pumping systems. Passive, current-driven systems1 that are less energy intensive will be investigated in future work. The amidoxime-based ORNL adsorbent was tested side by side with adsorbent supplied by researchers from the Japan Atomic Energy Agency (JAEA).13 This approach provided a reference point under similar experimental conditions, allowing a direct comparison in terms of amount and rate of uranium uptake versus time. The difference between the JAEA adsorbent and the ORNL adsorbent is in the form of the polyethylene substrate used for grafting amidoxime functional groups. ORNL fibers have higher surface area than the nonwoven polyethylene sheets used in the synthesis of the JAEA adsorbent. Adsorbent beds were prepared using either 47 mm diameter PFA (perfluoroalkoxy) in-line filter holders (Cole-Parmer, IL, USA) or 1 in. internal diameter (i.d.) by 4 and 6 in. long columns fabricated from all plastic components, mostly PVC and polypropylene. The adsorbent was dispersed and packed in the column, where it was held in place by glass wool and/or 3 mm glass beads that were used to fill up the empty space of the columns. Schematic diagrams of the physical layout used for adsorption exposure experiments are presented in Figure 1. For exposures conducted using four lines of three cartridges in series (Figure 1A), each cartridge was connected through a feed line (Swagelok Teflon tubing) directly to a PTFE diaphragm pump (Cole-Parmer, IL, USA), which provided up to 0.8 L/ min maximum flow rate. For exposures conducted in the parallel configuration (Figure 1 B), a 12-port, all PVC manifold system was used. Water was drawn from a reservoir and forced through the manifold using a pump with all plastic components in the pump head and PVC tubing feed lines. Prior to initial use, the adsorbent exposure cartridges and columns, feed lines, and fittings were cleaned with a laboratory soap and weak acid (10% hydrochloric acid) solution to minimize contamination. Samples of seawater delivered by the system were monitored for trace elements to document any contamination introduced by the exposure system (discussed later). Natural seawater was continuously provided to each adsorbent bed from reservoirs of filtered (0.45 μm polypropylene membrane) seawater collected from Sequim Bay, Sequim, WA. The temperature in the seawater reservoir was controlled with an all-titanium immersion-heating element. To characterize the adsorption of uranium from seawater, seven experiments were conducted using the exposure systems illustrated in Figure 1. Kinetic data on the amount of uranium adsorbed as a function of time were obtained by removing one cartridge of each adsorbent material periodically over a 6−8

week time period (a time series). Details associated with the exposure conditions and configurations for the experiments are given in Table 1. The initial experiments were conducted using the in-series configuration (Figure 1A). We judged this exposure approach to be acceptable based on the low adsorption rate obtained from batch experiments.2 Evidence that this exposure approach is viable is given under Results and Discussion. Later experiments were conducted using the parallel, once-through manifold system (Figure 1B), which exposes adsorbent samples to identical seawater exposure conditions simultaneously. Flow rate, salinity, and temperature of the seawater were monitored at the outlet of the exposure columns/cartridges at least twice daily for all experiments. Adjustments were made when the temperature was more than 2 °C outside of the target temperature and the flow rate was more than 10% above or below the target. The flow rate was determined using an in-line turbine-style flow sensor (Model DFS-2W) manufactured by Digiflow Systems connected to either a hand-held digital readout or a multiport data acquisition system (Measurement Computing, MicroDAQ, Contoocook, NH, USA). Flow rate adjustments were made by adjusting the flow at the peristaltic pump (Exposure System 1A) or by a 10-turn needle valve placed on the outlet of the columns/cartridges in the manifold system (Figure 1B). Salinity and temperature were determined using a hand-held salinometer (YSI, Model 30). In later experiments, the temperature was determined using a temperature logger (RDXL4SD, OMEGA Engineering, Stamford, CT, USA). The logger was set to record the outlet temperature of seawater at least every 30 min. Discrete samples of seawater were collected periodically during the exposure period of the feedwater and the water passing through the cartridges/columns for measurement of uranium and a suite of trace elements (e.g., V, Cu, Ni, Zn, Fe, and Mn). 2.3. Sample Handling and Analytical Procedures. Seawater exposed adsorbent fibers were collected from adsorbent beds, washed with deionized (DI) water to remove salts, and dried under vacuum filtration using a nylon membrane filter with a pore size of 200 nm (Pall Life Sciences, Port Washington, NY, USA). The dried fibers (100−200 mg) were then digested with high-purity (Optima, Fisher Scientific) concentrated aqua regia acid mixture (3:1; i.e., 15 mL:5 mL hydrochloric acid:nitric acid) for 24 h at room temperature with 500 rpm shaking frequency. High-purity DI water (Optima, Fisher Scientific) was then added to make up a 100-mL dilute acid solution and to have the desired concentration range of uranium for the analysis. An additional period of 3 days was allowed, at room temperature and 500 rpm shaking frequency, to complete the digestion process. Inductively coupled plasma mass spectroscopy (ICP-MS, Thermo Scientific X-Series II) was used for quantitative analysis. The average of six replicate measurements per sample was used to quantify uranium-238 against a six-point calibration curve. Samples of spent glass wool and glass beads used for packing were also analyzed to investigate whether uranium was retained on these packing materials. No appreciable uranium was observed to be adsorbed on these materials. The recovery and analysis of fibers from the columns for experiment 4 were conducted at PNNL using similar approaches and analysis methods with the following exceptions. Drying of DI washed fibers was conducted at room temperature on a class-100 clean air bench. Digestion of the adsorbent fibers 6078

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was conducted with a 50% aqua regia solution, for 3 h at a temperature of 85 °C. Analysis of uranium and other trace elements was conducted using a Perkin-Elmer 6100 inductively coupled plasma mass spectrometer (ICP-MS), with quantification based on standard calibration curves. Determination of uranium in natural seawater samples was conducted at PNNL using ICP-MS and the method of standard addition calibrations. Addition calibration is a variant of the standard additions method and is often used when all samples have a similar matrix. Instrumental calibration curves were prepared in Sequim Bay seawater that was diluted 20-fold with high-purity DI water and then spiked at four different concentration levels of 0.1, 0.2, 0.3, and 0.4 μg/L, along with a 2% nitric acid blank in diluted seawater. The seawater samples were then analyzed at 20-fold dilution with high-purity DI water and then quantified using the matrix matched additions calibration curve. The standard reference material CASS-5 (Nearshore seawater reference material for trace metals) available from the National Research Council Canada, which is certified for uranium (3.18 ± 0.10 μg/L), was also analyzed at a 20-fold dilution every 10 samples to verify the analytical results. The uranium recovery for the analysis of CASS-5 ranged from 93 to 99% (n = 15). Duplicate analyses and matrix spikes were conducted with each batch of samples. The relative percent difference for duplicates ranged from 1 to 5%, and the recovery of matrix spikes ranged from 93 to 109% (n = 7). Analysis of trace elements in seawater samples was also conducted by ICP-MS at PNNL following sample preconcentration and seawater matrix elimination. Seawater samples of 40 mL were preconcentrated by sodium borohydride reductive precipitation (5% solution, w/v) of an iron and palladium mixture (∼0.5 mL of 1:1 solutions of 1000 μg/L) spiked into the samples. A 0.25 mL volume of a 2% (w/v) ammonium pyrrolidine-dithio-carbamate (APDC) solution was also added to the samples prior to reductive precipitation to complex trace elements and facilitate interaction with the Fe/Pd precipitate. Following precipitation of the Fe/Pd mixture, the samples were centrifuged at 2500 rpm for 30 min, and the overlying water was carefully decanted off the precipitate. The precipitate was dissolved with 0.1 mL of concentrated high-purity nitric acid (Optima grade, Fisher Scientific) and diluted to a suitable volume (∼5 mL) for analysis with DI water. This scheme produced a sample preconcentration of approximately 8-fold, depending on the exact starting and final solution volumes. This analytical method for seawater is based on the reductive precipitation preconcentration techniques described by Nakashima et al.,16 Sella et al.,14 and Skogerboe et al.15

R=

dq = k(qe − q) dt

(2)

q = qe(1 − e−kt )

(3)

d(CV ) = Q FC F − QC − Mkqee−kt dt

(4)

τ=

V Q

(5)

Mkqe −kt dC 1 e = (C F − C[t ]) − dt Qτ τ C[t ] = C F + (C0 − C F)e−t / τ +

(6)

Mkqe Q (kτ − 1)

(e−kt − e−t / τ ) (7)

Here, k is the reaction rate constant for the linear-drivingforce model [day−1], while qe is the amount of uranium adsorbed at equilibrium [mg of U/g of adsorbent], τ is the residence time of uranyl species in an adsorption bed [days], and C0 is the initial uranium concentration of the solution in the adsorbent bed [ppb]. qe is determined by the final uranium uptake amount after 8 weeks. The linear-driving-force reaction model (eq 2), which lumps transport and reaction effects, was considered instead of more complicated models, such as the Azizian kinetic model,7 to derive a simple analytical solution (eq 7) that can be easily employed to quantitatively predict the adsorption uptake performance.

4. RESULTS AND DISCUSSION 4.1. Uranium Uptake Amount. A compilation of all the uranium-adsorption-kinetics experiments is shown in Figure 2,

3. MODELING The following mass balance equation can be derived for each adsorbent bed: (1)

Figure 2. Experimental data from field tests at the Marine Sciences Laboratory of PNNL. The corresponding experimental conditions are summarized in Table 1

CF and C are the concentrations of uranium in the feed solution and in the adsorbent bed at any time, respectively [ppb]. V is the volume of seawater in each adsorbent bed [L], and t is time [days]. QF and Q are the feed and exit flow rates of seawater [L/day] and can be considered identical. M is the mass of the adsorbent [g], and R is the rate of uranium uptake from seawater [μg of U/g of adsorbent·day]. Using a linear-drivingforce approximation, the following equations apply:

and the corresponding experimental conditions are summarized in Table 1. Experiments with natural seawater demonstrate that the ORNL adsorbent has a capacity (mg of U/g of adsorbent) 3 times higher than that of the JAEA adsorbent (Figure 2). The uranium uptake was near 3.3 mg of U/g of adsorbent, after 8 weeks of exposure, which is the highest adsorption capacity reported in the literature to date.1 Based on the uptake amount

d(CV ) = Q FC F − QC − MR dt

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after 8 weeks of operation presented in Figure 2, the distribution coefficient (Kd) for uranium between natural seawater and the ORNL adsorbent was estimated to be between 880 and 1140 L/g. This range in the distribution coefficient is consistent with the value reported in Table 2. Table 2. Concentrations and Uptake Amounts of Various Elements in a Field Test (Experiment 3 in Figure 2)a

elem

filtered Sequim Bay seawater [ng/kg]

(a) filtered ambient seawater concn in test syst [ng/kg]

(b) amt of metal uptake [mg of metal/g of adsorbent]

Kdb [L/g]

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

1500 2850 2490 190 320 285 − 135 1000 3 0.02 −

1480 2840 2200 540 560 2100 − 180 1200 25 0.01 −

5.7 2.7 1.9 1.3 0.7 0.7 0.3 0.2 0.1 0.1 0.08 0.03

3775 932 − 2360 1225 326 − 1089 − 3921 7.8 × 106 −

Figure 3. Amount adsorbed and rate of uranium uptake vs time.

8 days of exposure, suggesting that the adsorption rate of the JAEA adsorbent is slightly faster. However, the uptake rate of the ORNL adsorbent was higher than that of the JAEA adsorbent for the duration of the test (60 days). The uranium uptake for the JAEA adsorbent reached a plateau after approximately 4 weeks of contact with seawater, while the ORNL adsorbent exhibited no plateau after 60 days of contact with seawater. 4.2. Effect of Experimental Conditions. The adsorbent performance was similar for the flow rates of 250 mL/min in 47 mm diameter filter holders (experiment 1 in Figure 2) and 500 mL/min in 47 mm diameter filter holders (experiment 2) or 1 in. diameter columns (experiments 3 and 4). These conditions correspond to linear velocities of 1 and 2 cm/s, respectively. To compare the effect of flow rate in different size columns, the linear velocity was estimated by eq 8 with a typical packing porosity of 0.4.

a

Density of seawater: 1.02 g/mL. bThe distribution coefficient, Kd, is determined by the ratio of the corresponding values of columns (b) and (a).

Based on the amount of 2.7 mg of U/g of adsorbent of uranium uptake shown in Table 2 and assuming that equilibrium has been reached with a 2.85 ppb uranium concentration (see Table 2) with lower salinity, a distribution coefficient of 934 L/ g is obtained. In a previous study,2 we observed adsorption capacities of 3.94 mg of U/g of adsorbent for seawater off the coast of Savannah (Georgia, USA) that had a uranium concentration of 3.6 ppb and 3.36 mg of U/g of adsorbent for seawater from the Charleston (South Carolina, USA) harbor that had a uranium concentration of 3.2 ppb. These capacity values suggest that the adsorbent capacity obtained in the field experiments of this study would be higher if the seawater salinity was typical of open ocean conditions (35 psu) instead of 28.5 psu coastal seawater in this study. Using an average Kd value of 1010 L/g, and a seawater uranium concentration of 3.3 ppb (the concentration at a salinity of 35 psu), the predicted uranium uptake by the ORNL adsorbent for an exposure time of 8 weeks is 3.3 mg of U/g of adsorbent. The JAEA adsorbent showed an adsorption capacity of 1.1 mg of U/g of adsorbent (Figure 2), which corresponds to a Kd value of 380 L/g, after 10 weeks of exposure. Normalizing to full strength seawater (35 psu), with a uranium concentration of 3.3 ppb, increases the adsorption capacity of the Japanese adsorbent to 1.3 mg of U/g of adsorbent. This comparison suggests that the affinity of the ORNL adsorbent for uranium is approximately 2.6 times higher than that of the JAEA adsorbent. The initial uptake rate of the ORNL adsorbent was 0.19 mg of U/g of adsorbent/day, which is 2.6 times faster than the initial uptake rate of the JAEA adsorbent at 0.073 mg of U/g of adsorbent/day (Figure 3). The uptake rate of the ORNL adsorbent decreased by 50% after 10 days of exposure, while the uptake rate of the JAEA adsorbent decreased by 50% within

VS (8) ε VL is the linear velocity, VS is the superficial velocity or the empty column velocity, and ε is the mobile phase porosity in a packed column (in this case, ε = 0.4). Results from experiments 1−4 indicate that, above a linear velocity of ∼1 cm/s, the effects of interparticle and film masstransfer resistances were insignificant. These results agree well with those of our previous studies2 where it was concluded that, above a certain stirring rate, mass-transfer effects are less significant than the binding kinetics in batch uranium adsorption experiments. Experiment 5 was run with ORNL adsorbent in a 47 mm diameter filter holder for close to 8 weeks and showed slightly lower performance than experiment 1, which included the same type of adsorbent bed geometry. This behavior suggests that interparticle diffusion limitations may exist at 1 cm/s linear velocity. A comparison of the results presented in Figure 2 also shows that the test configurations, e.g., series/parallel configuration (Figure 1A) for experiment 1 and parallel configuration (Figure 1B) for experiments 2 and 3, had no significant effects on the rate of uranium retention or adsorbent capacity. Results shown in Figure 4 were obtained by using the mathematical model for the flow-through system (eq 7). The uranium concentration dropped 1.5 and 0.7%, respectively, for 250 and 500 mL/min flow rates from entrance to exit of an adsorbent bed containing 100 mg of adsorbent. Thus, after passing through a series of three adsorbent beds containing 100 VL =

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Based on the estimated average value of Kd of 1010 L/g, the corresponding value of qe for a uranium concentration of 3.3 ppb is 3.3 mg of U/g of adsorbent. This value is very close to the qe value of 3.1 mg of U/g of adsorbent, which has been estimated through regression of the experimental data shown in Figure 5. The comparison in Figure 5 shows that, although it is not based on transport or reaction mechanisms, the lineardriving-force model of the amount of uranium adsorbed as a function of time describes well the experimental data, within ±10%. Until a mechanistic predictive model is developed, this simple model can be used in the economic assessment of the uranium from seawater process. 4.4. Cost Analysis. The uranium uptake measurements and kinetic model presented in sections4.1−4.3 were used to update the uranium production cost model presented previously.12 This assessment was based upon the Japanese adsorbent, but it has been updated to depict the ORNL adsorbent and its fabrication process.17 Table 3 lists the basic system parameters used to prepare the cost estimate. The size of the uranium production system

Figure 4. Uranium concentration vs time at the exit of an adsorption bed packed with 100 mg of adsorbent.

mg of adsorbent each, at a flow rate of 500 mL/min, the maximum drop in the total uranium concentration is 3%. Therefore, due to a negligible decrease in uranium concentration after contacting each adsorbent bed, the two experimental configurations presented in Figure 1 produced very similar results (shown in Figure 2). This behavior is due to the relatively slow kinetics of uranium uptake, which we previously reported was a reaction-limited process.2 It was also difficult to differentiate the uptake amount as a function of flow rate, suggesting that, for the range of flow rates or linear velocities used in this study, the interparticle and liquid-film mass-transfer resistances can be neglected. 4.3. Modeling. Figure 5 compares the data for the ORNL adsorbent at 20 °C from experiments 1−4 with the results from

Table 3. Uranium Production System Parameters for Cost Analysis parameter

value

unit

annual system uranium production length of mooring campaign 60 day uranium uptake adsorbent uses adsorbent degradation rate

1200 60 3.09 6 5

t/year days mg of U/g of adsorbent N/A % per reuse

affects scale economies. For comparability, it was set to match the value chosen by Tamada and co-workers,11 which represents approximately 7% of annual U.S. requirements. The 60-day uranium uptake was determined by regressing the kinetic model onto the ORNL adsorbent data, as illustrated in Figure 2. [Sixty days was shown in a previous report12 to be a near cost-minimizing soaking campaign duration. The optimal duration is affected by the trade-off between the cost of a lower time-averaged adsorption rate and the benefit of a higher capacity per unit mass of adsorbent, but the optimum was shown to be shallow.] To ensure conservatism in the cost analysis, the uranium uptake was left unadjusted for the lower seawater uranium concentration at the PNNL experimental site. While the adsorbent capacity is known to degrade as a result of repeated uranium elution and regeneration cycles, the extent of this degradation is not yet well-established. The degradation rate reported in Table 3 was observed during five adsorbent soaking/elution cycles carried out by Tamada et al.11 Reading from the top of Figure 6, the first bar shows the expected uranium production cost and 95% confidence interval for the Japanese system summarized in section 4.1. The second and third bars show the calculated costs when the model12 is updated with the ORNL adsorbent capacity reported in this paper. They are $760/kg of U for the adsorbent deployment and mooring system design proposed by the Japanese11 and $610/kg of U for a modified design featuring smaller work boats working from an offshore mother ship as well as lighter, stronger polymer anchor ropes. For a quantitative comparison of the two mooring system designs, see the paper by Schneider and Lindner.17 The 95% confidence interval associated with the $610/kg of U expected production cost is [$510, $720]. Returning to Figure 6, three hypothetical adsorbent perform-

Figure 5. Comparison of experimental data and modeling results using the linear-driving-force model for the amount of uranium adsorbed vs time.

the linear-driving-force model, which lumps transport and reaction kinetic effects into a single parameter, k: q = qe(1 − e−kt )

for qe = 3.1 mg of U/g of adsorbent and k = 0.0607 day−1. 6081

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activity in unfiltered seawater. Results from these studies will be used to update the energy-intensity and cost analysis.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was conducted at the Oak Ridge National Laboratory (ORNL) and at the Marine Sciences Laboratory, a part of the Pacific Northwest National Laboratory (PNNL) located in Sequim, WA. Work at ORNL was supported by the U.S. DOE Office of Nuclear Energy, under Contract No. DEAC05-00OR22725 with ORNL, managed by UT−Battelle, LLC. The PNNL effort was also supported by the U.S. DOE Office of Nuclear Energy, under Contract No. DE-AC0576RL01830 to PNNL. The Japan Atomic Energy Agency (JAEA) uranium adsorbent material was kindly donated for testing by Dr. Tamada and Dr. Seko of JAEA, Takasaki, Japan.

Figure 6. Seawater uranium production cost progression and potential milestones.

ance scenarios leading to uranium production costs comparable to the peak uranium spot-market price observed during the 2007−2009 uranium price boom are presented. Each scenario assumes that the cost of fabricating, braiding, and grafting the polymer adsorbent material itself is unchanged from that of the current ORNL process. The scenarios show that a very high capacity adsorbent may lead to competitive uranium production costs even if made from a single-use material. Perhaps more likely is the evolutionary performance goal of 6 mg of U/g of adsorbent and 3% average capacity loss per reuse over 10 uses. This reflects a further doubling of capacity from levels reported here along with a modest improvement in durability.

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REFERENCES

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5. CONCLUSIONS AND FUTURE WORK Field experiments at the Marine Sciences Laboratory of PNNL have been conducted using amidoxime-based polymeric adsorbent synthesized at ORNL to investigate uranium uptake from seawater. Experimental data showed a reproducible uranium uptake performance, within ±10% from the value predicted by the linear-driving-force model, with a maximum capacity of 3.3 mg of U/g of adsorbent after 8 weeks of contact with seawater. Both the amount and rate of uranium uptake by the ORNL adsorbent were higher than those found using amidoxime-based adsorbent supplied by JAEA. Specifically, the capacity of the ORNL adsorbent was approximately 3 times higher than that of the JAEA adsorbent (1.1 mg of U/g of adsorbent) and the initial uptake rate of the ORNL adsorbent (0.19 mg of U/g of adsorbent/day) was 2.6 times higher than that of the JAEA adsorbent (0.073 mg of U/g of adsorbent/ day). The ORNL adsorbent is seen to enable the production of uranium from seawater at an expected cost of $610/kg of U. The uncertainty associated with this estimate remains considerable; future experimental data are needed to further reduce sources of uncertainty. The performance of the ORNL amidoxime-based polymeric adsorbents, as well as adsorbents with different functional groups, is currently evaluated in seawater (1) at different geographic regions with varying biogeochemical parameters; (2) at different temperatures and linear velocities; (3) to determine the effects of competing metal ions, such as vanadium; and (4) to determine the effects of biological 6082

dx.doi.org/10.1021/ie4039828 | Ind. Eng. Chem. Res. 2014, 53, 6076−6083

Industrial & Engineering Chemistry Research

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(14) Sella, S.; Sturgeon, R. E.; Willie, S. N.; Campos, R. C. Flow Injection On-line Reductive Precipitation Preconcentration With Magnetic Collection for Electrothermal Atomic Absorption Spectrometry. J. Anal. At. Spectrom. 1997, 12 (11), 1281−1285. (15) Skogerboe, R. K.; Hanagan, W. A.; Taylor, H. E. Concentration of trace elements in water samples by reductive precipitation. Anal. Chem. 1985, 57 (14), 2815−2818. (16) Nakashima, S.; Sturgeon, R. E.; Willie, S. N.; Berman, S. S. Determination of trace elements in sea water by graphite-furnace atomic absorption spectrometry after preconcentration by tetrahydroborate reductive precipitation. Anal. Chim. Acta 1988, 207, 291− 299. (17) Schneider, E.; Lindner, H. Energy Balance of Uranium Recovery from Seawater. In Proceedings of GLOBAL 2013: Nuclear Energy at a Crossroads, American Nuclear Society. 8 pp., October 2013.

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dx.doi.org/10.1021/ie4039828 | Ind. Eng. Chem. Res. 2014, 53, 6076−6083