Article pubs.acs.org/IECR
Extracting Uranium from Seawater: Promising AI Series Adsorbents S. Das,*,† Y. Oyola,*,† R. T. Mayes,† C. J. Janke,† L.-J. Kuo,‡ G. Gill,‡ J. R. Wood,‡ and S. Dai*,† †
Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6053, United States Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, United States
‡
ABSTRACT: A new series of adsorbents (AI10 through AI17) were successfully developed at ORNL by radiation induced graft polymerization (RIGP) of acrylonitrile (AN) and vinylphosphonic acid (VPA) (at different mole to mole ratios) onto high surface area polyethylene fiber, with high degrees of grafting (DOG) varying from 110 to 300%. The grafted nitrile groups were converted to amidoxime groups by reaction with 5 wt % hydroxylamine at 80 °C for 72 h. The amidoximated adsorbents were then conditioned with 0.44 M KOH at 80 °C followed by screening at ORNL with prescreening brine spiked with 8 ppm uranium. Uranium adsorption capacities in prescreening ranged from 171 to 187 g-U/kg-ads irrespective of percent DOG. The performance of the adsorbents with respect to uranium adsorption in natural seawater was also investigated using flow-throughcolumn testing at the Pacific Northwest National Laboratory (PNNL). Three hours of KOH conditioning led to higher uranium uptake than 1 h of conditioning. The adsorbent AI11, containing AN and VPA at the mole ratio of 3.52, emerged as the potential candidate for the highest uranium adsorption (3.35 g-U/kg-ads.) after 56 days of exposure in seawater flow-through-columns. The rate of vanadium adsorption over uranium linearly increased throughout the 56 days of exposure. The total mass of vanadium uptake was ∼5 times greater than uranium after 56 days.
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INTRODUCTION Seawater is considered to be a major source of uranium with an estimated quantity of around 4.5 billion metric tons, which is nearly 1000 times higher than that available in terrestrial ores.1,2 The extraction of uranium from seawater has been explored since the first half of the last century, but it is still far from being an economically viable method of uranium production because of the fact that the concentration of uranium in seawater is of the order of 1.4 × 10−8 mol/L (3.3 ppb).3,4 Functionalized polymeric adsorbents in the forms of resins, beads, gels, or membranes have potential applications in selective preconcentration or separation of target metal ions from multicomponent environmental aqueous samples from groundwater, seawater, process waste streams, etc.5,6 Development of polymeric sorbents having different functional groups for the removal of heavy metal ions is of great importance due to their potentially high ion selectivity and easy handling.7,8 A variety of hydrogels with special characteristics have received increasing importance because of their wide ranging applications.9−11 The extraction of uranyl ions by structured hydrogels composed of acrylamide-maleic acid and acrylamideacrylic systems has been studied by Kavali et al. and Chauhan et al.12,13 Mun et al. and Pekel et al. carried out adsorption studies of the uranyl ions by hydrogels based on polyethylene glycol and methacrylic acid copolymers and N-vinyl 2-pyrrolidone/ acrylonitrile copolymers containing amidoxime groups.14,15 Acrylic acid-co-2-acrylamido-2-methylpropane-1-sulfonic acid hydrogels were evaluated for uranium recovery by Atta et al.16 Over the past few decades, polyamidoxime (PAO) based adsorbents have been demonstrated to be the most promising candidates for uranium extraction from seawater.17−22 Marine tests by Japanese researchers with PAO-braided fiber adsorbents showed 1.5 g-U/kg-adsorbents after 30 days of exposure in the Okinawa area.23 Inspired by this preliminary success, a number of research works have been carried out to © XXXX American Chemical Society
provide a better understanding of the effects of different factors and parameters on the uranium adsorption capacity of PAO from seawater.24−30 Adsorption of U(VI) from seawater by the PAO adsorbent has been found to be highly dependent on the physical parameters of the sorbent matrix, such as free volume, pore structure, tortuosity, hydrophilicity, etc.22,31−33 The presence of an acidic comonomer with appropriate pKa value in the active binding sites may trigger the adsorption kinetics of U(VI) from seawater in the PAO adsorbent.19,31,32 In this present work, a series of adsorbents (AI series) containing acrylonitrile (AN) along with vinylphosphonic acid (VPA) at different mole ratios were prepared by radiation induced graft polymerization (RIGP) onto high surface area polyethylene (PE) trunk fiber.34,35 Conversion of poly(acrylonitrile) (PAN) to PAO in the grafted precursor fibers was done by reacting with hydroxylamine. The AI series adsorbents were conditioned with 0.44 M KOH at 80 °C for two time-periods, i.e., 1 and 3 h before they were explored for their uranium uptake capability by (i) uranium uptake capacity after 24 h contact with high uranyl concentration screening brine of similar sodium, chloride, and bicarbonate concentration as seawater (spiked with uranium nitrate) at ORNL and (ii) uptake of uranium and other metal ions from natural seawater from Sequim Bay, WA, after exposure for 56 days, at the Marine Science Laboratory (MSL) of the Pacific Northwest National Laboratory (PNNL) located in Sequim, WA. Special Issue: Uranium in Seawater Received: August 28, 2015 Revised: October 23, 2015 Accepted: November 10, 2015
A
DOI: 10.1021/acs.iecr.5b03135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Representative functional groups present in the amidoximated AI series adsorbents.
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KOH Conditioning of AO’d AI Series Fiber Adsorbents. The AI series adsorbents were conditioned with 0.44 M KOH at 80 °C for two time-periods i.e., 1 and 3 h prior to exposing them in the uranyl screening brine solution, as well as natural seawater for testing the uranium uptake capacity and kinetics. Prescreening for Uranium Adsorption Determination. Uranium adsorption in screening brine was carried out at ORNL. A synthetic solution resembling seawater in ionic strength and pH 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 test solution was approximately 8. The concentrations of sodium, chlorine, and bicarbonate were selected to be similar to those of 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 uranyl screening brine solution for 24 h at room temperature with constant shaking at 400 rpm. After contacting for 24 h, an aliquot was collected and the initial and final solutions were analyzed by inductively coupled plasma-optical emission spectroscopy (PerkinElmer 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 to 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).
MATERIALS AND METHODS Materials. All chemicals were reagent-grade or higher. Acrylonitrile (AN), vinylphosphonic acid (VPA), tetrahydrofuran (THF), methanol, dimethyl sulfoxide (DMSO), N,Ndimethylformamide (DMF), hydroxylamine hydrochloride (HA-HCl), and potassium hydroxide (KOH) were obtained from Sigma-Aldrich. Ultrapure water (18 MΩ cm−1, Thermo scientific Nanopore) was used in the preparation of HA-HCl and KOH solutions. Hollow-gear, high-surface-area polyethylene fibers (PE) were prepared by melt-spinning at Hills, Inc. (Melbourne, FL), using polylactic acid (PLA) as the coextrusion polymer. 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 uranyl screening brine, and a 1000 ppm uranium (U) standard solution (High Purity Standards, North Charleston, USA) was used to prepare the ICP standards. Preparation of Adsorbent. The adsorbent fibers were prepared by RIGP at the NEO Beam Electron Beam Crosslinking Facility (Middlefield, OH). Prior to irradiation, the PLA was removed by submerging the fibers in excess THF under reflux at 65−70 °C overnight followed by drying at 40 °C under vacuum. The preweighed dry fiber samples were placed inside double-layered plastic bags within a plastic glovebag 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 and 1 mA current from an RDI Dynamitron electron beam machine. The total irradiation time was approximately 22 min. After irradiation, the fibers were immersed in 300 mL flask containing previously degassed grafting solutions consisting of AN and VPA in DMSO. The flasks were then placed in an oven at 64 °C for 18 h for grafting. After grafting, the fibers were drained from the solution and washed with DMF to remove unreacted monomers and homopolymers followed by methanol and dried at 40 °C under vacuum. The grafted fibers were weighed to gravimetrically determine the percent degree of grafting (% DOG) of copolymers onto the trunk polymer from preirradiation and postgrafting weights by using eq 1: (wtAG − wtBG) % DOG = × 100 wtBG
uranium (U) adsorption capacity ⎡ initial[U] mg − final[U] L ⎢ =⎢ gof dry adsorbent ⎣
( )
( mgL ) ⎤⎥ × soln vol (L) ⎥ ⎦
(2)
Field Adsorption Tests. The adsorbent performance in the natural seawater was carried out at the Marine Sciences Laboratory (MSL), Pacific Northwest National Laboratory (PNNL), for 56 days in a flow-through column system, to assess and characterize in terms of kinetics and adsorbent capacity within 8 weeks by varying KOH-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 salinity and uranium concentration observed in this study were approximately 12% lower than the normal salinity of 35 practical salinity units (psu) and
(1)
where wtBG = dry weight before grafting and wtAG = dry weight after grafting. The nitriles in the grafted fiber samples were converted to amidoxime (AO) 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 samples were then washed under vacuum filtration with deionized water, followed by a methanol rinse and allowed to dry at 40 °C under vacuum. B
DOI: 10.1021/acs.iecr.5b03135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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3230, 2247, 1376, and 1094 cm−1 (Figure 2b) correspond to O−H, CN, free PO, and P−O−H groups,19 respectively, and thus confirm grafting of acrylonitrile and vinylphosphonic acid onto the polyethylene. The disappearance of the nitrile stretch and appearance of N−H (3390 cm−1), CN (1645 cm−1), and N−O (930 cm−1) clearly indicates the conversion of the nitrile to amidoxime (AO). The appearance of the stretch at 1561 cm−1 (representative of COO− group) represents degradation products from the amidoxime degrading into carboxylate upon treatment with 0.44 M KOH at 80 °C. While the degradation will undoubtedly impact the capacity, the base treatment is required for high capacity.37 This is believed to be due to the base facilitating the formation of a hydrogel from the graft polymer, which enhances transport through the graft polymer. Therefore, the degradation is one aspect of the sorbent’s “conditioning” step that must be optimized during development. Uranyl Prescreening. Uranium adsorption studies with the AI series samples was performed by contacting the sorbent for 24 h with the prescreening brine solution spiked with 8 ppm uranium after conditioning with 0.44 M KOH at 80 °C for 3 h. As illustrated in Figure 3, the uranium adsorption capacity of the AI series adsorbents is higher than that in the adsorbent containing only amidoxime (PAO) and gradually increased with increasing AN/VPA mole ratio with a maximum at 3.52 (i.e., AI11). The uranium adsorption gradually decreased beyond this ratio. The role of acid comonomer is yet to be fully understood. The intra- and intermolecular hydrogen bonding in PAO would render significant hydrophobicity, resulting into low uranium uptake. The addition of an acid comonomer (i.e., VPA) along with AN could be beneficial due to (i) the addition of hydrophilicity while facilitating hydrogel formation, thereby enhancing the accessibility of the amidoxime functional groups toward uranium resulting in higher uranium adsorption and (ii) manipulation of the polymer graft conformation. Conformational changes can impact many
uranium concentration of 3.3 ppb in seawater. Marine testing was performed using filtered (0.45 μm) seawater at a temperature of 20 ± 2 °C and at a flow rate of 250 mL/min, using an active pumping system. The details of the natural seawater adsorption test at MSL have been described elsewhere.25,36
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RESULTS AND DISCUSSION The AI series adsorbents containing different mole ratios of AN and VPA were successfully prepared by RIGP. A representative cartoon description of the functional groups present in the random copolymer after conversion of the nitriles to amidoximes resulting in the AI series adsorbents is shown in Figure 1. A summary of the monomer ratios used for preparation of the AI series adsorbents and their degree of grafting (calculated using eq 1) is presented in Table 1. Table 1. Chemical Compositions and Degree of Grafting of the AI Series Adsorbents adsorbent ID
AN/VPA mol/mol
% DOG
AI12 AI13 AI14 AI10 AI11 AI15 AI16 AI17
1.91 2.33 2.86 3.21 3.52 4.41 5.62 7.39
110 205 173 143 269 235 191 300
Fourier Transform Infrared (FTIR) Spectrometry. The FTIR spectra of a representative AI series adsorbent, AI11, fiber samples were recorded on a PerkinElmer Frontier FTIR with a single-bounce diamond attenuated total reflectance (ATR) accessory at 2 cm−1 resolution and averaged over 16 scans. The spectra are shown in Figure 2. The stretching frequencies at
Figure 2. FTIR spectroscopy of trunk PE fiber (dotted line); AI11 adsorbent after grafting (solid line), amidoximation (blue line), and conditioning with 0.44 M KOH at 80 C for 1 (green line) and 3 h (orange line), respectively. C
DOI: 10.1021/acs.iecr.5b03135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
hydrogel formation facilitated by the degradation of some of the amidoximes during the extended base treatment. Irrespective of the KOH treatment time, the uranium adsorption gradually increased with increasing AN/VPA molar ratio in the adsorbents and reached a maximum at 3.52. It is also interesting to note that 3.52 (i.e., AI11 adsorbent), as predicted from the prescreening study, was the optimum AN/VPA molar ratio providing the highest uranium adsorption capacity, including 3.35 and 3.25 g-U/kg-ads after 3 and 1 h of KOH conditioning, respectively. This represents the optimum balance between the amidoxime content and the acid content of the sorbent.
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URANIUM ADSORPTION SELECTIVITY OVER VANADIUM The concentration of vanadium in seawater is quite low with an average level of ∼1.9 ppb. One of the greatest drawbacks of amidoxime-based adsorbents is competition from vanadium.38,39 Adsorption of vanadium reduces the uranium capacity, and it also binds so strongly that eluting this metal may irreversibly damage the adsorbent.26,40 A comparison of the molar ratio of uranium over vanadium adsorption, after 56 days flow-through column testing in natural seawater is shown in Figure 5 after conditioning of the PAO and AI series adsorbent
Figure 3. Uranium uptake capacities of AI series samples after 24 h contact with the prescreening brine solution spiked with uranyl ions at 8 ppm, after conditioning with 15 mL of 0.44 M KOH at 80 °C for 3 h.
factors, including the ability to hydrogen bond, and are currently being studied. The prescreening studies indicated that the adsorbent with AN/VPA molar ratio in the range of 3.21− 3.52 appears to be the optimum chemical composition for the highest uranium adsorption capacity from real seawater therefore the 3.52 ratio was chosen as it optimizes the acrylonitrile content within the polymer thereby increasing the number of chelation sites for uranium. Field Seawater Testing. The performance of AI series adsorbents in natural seawater was tested at the PNNL Marine Sciences Laboratory using filtered seawater from Sequim Bay for 56 days contact in flow-through columns. The sorbents were treated with 0.44 M KOH at 80 °C for 1 and 3 h prior to seawater contact. The results are shown in Figure 4. The AI
Figure 5. Adsorption of uranium over vanadium as a function of AN and VPA molar ratio of the AI series adsorbents after 56 days contact with seawater in flow-through columns. The adsorbent samples were treated with 0.44 M KOH at 80 °C for 1 and 3 h.
with 0.44 M KOH at 80 °C for 1 and 3 h. Three hours of KOH conditioning, in general, favored adsorption of more uranium relative to vanadium as compared to 1 h KOH conditioning. However, the data suggests the presence of the phosphonic acid decreases the selectivity as the selectivity ratio increased with increasing amidoxime content. The adsorbent AI10 (AN:VPA = 3.21) treated with KOH for 3 h, adsorbed more uranium relative to vanadium within the AI series while the AI11 sorbent exhibited similar uranium selectivities to the higher acrylonitrile content sorbents. Given the combination of the higher capacity and moderate selectivity data, the AN/VPA molar ratio of 3.52, thus emerged as the optimum ratio for the highest and better selective adsorption of uranium from seawater.
Figure 4. Uranium adsorption capacities of AI series samples after 56 days of contact with Sequim Bay seawater in flow-through columns. The adsorbent samples were treated with 0.44 M KOH at 80 °C for 1 and 3 h.
series adsorbents had higher uranium adsorption than the adsorbent with PAO only but comparable capacities to the ORNL 38H sorbent based upon amidoxime and methacrylic acid.36 When comparing the base treatment time, it appears that 3 h of KOH contact was slightly better than 1 h of contact with respect to uranium adsorption capacity by all the AI series adsorbents. This could be due to better hydrophilicity through D
DOI: 10.1021/acs.iecr.5b03135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. Adsorption kinetics of uranium, vanadium, iron, calcium, and magnesium by the AI series adsorbents after 56 days of contact with seawater in flow-through columns. The adsorbent samples were treated with 0.44 M KOH at 80 °C for 1 and 3 h.
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KINETICS OF ADSORPTION OF METAL IONS The AI11 adsorbent with a 3 h base treatment time was used to study the adsorption kinetics of uranium, vanadium and other metals from seawater. Approximately 50 mg of KOH-treated AI11 samples were packed in the respective columns through which filtered seawater from Sequim bay was passed at a flow rate of 250−300 mL·min−1, at 20 °C, over fixed durations, i.e., 7, 14, 21, 35, and 56 days. The adsorption kinetics of uranium and some of the other metals adsorbed in large relative quantities (i.e., copper, zinc, iron, vanadium, calcium, and magnesium) from seawater is illustrated in Figure 6. The rate of adsorption of the charge balancing ions of calcium and magnesium was very fast in the first 7 days and reached a near plateau after 3 weeks. Uptake of copper and zinc is very low (
