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Uranium adsorbent fibers prepared by ATRP from PVC-co-CPVC fiber Suree Brown, Yanfeng Yue, Li-Jung Kuo, Nada Mehio, Meijun Li, Gary A. Gill, Costas Tsouris, Richard T Mayes, Tomonori Saito, and Sheng Dai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03355 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

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Industrial & Engineering Chemistry Research

Uranium Adsorbent Fibers Prepared by ATRP from PVC-co-CPVC Fiber Suree Brown,*,† Yanfeng Yue,‡ Li-Jung Kuo,§ Nada Mehio,† Meijun Li,† Gary Gill,§ Costas Tsouris,‡ Richard T. Mayes,‡ Tomonori Saito,*,‡ Sheng Dai*,†,‡ †

‡

§

Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge TN 37831, USA

Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, WA, 98382, USA

ABSTRACT. The need to secure future supplies of energy attracts researchers in several countries to a vast resource of a nuclear energy fuel, uranium in seawater (estimated at 4.5 billion tons in seawater). In this study, we developed effective adsorbent fibers for the recovery of uranium from seawater via atom-transfer radical polymerization (ATRP) from a poly(vinyl chloride)-co-chlorinated poly(vinyl chloride) (PVC-co-CPVC) fiber. ATRP was employed in the surface graft polymerization of acrylonitrile (AN) and tert-butyl acrylate (tBA), precursors for uranium-interacting functional groups, from PVC-co-CPVC fiber. The [tBA]/[AN] was systematically varied to identify the optimal ratio between hydrophilic groups (from tBA) and uranyl-binding ligands (from AN). The best performing adsorbent fiber, the one with the optimal [tBA]/[AN] ratio and a high degree of grafting (1390%), demonstrated uranium adsorption capacities that are significantly greater than those of the JAEA reference fiber in natural seawater

ACS Paragon Plus Environment

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tests (2.42−3.24 g/kg in 42 days of seawater exposure and 5.22 g/kg in 49 days of seawater exposure; JAEA: 1.66 g/kg in 42 days of seawater exposure and 1.71 g/kg in 49 days of seawater exposure). Adsorption of other metal ions from seawater and their corresponding kinetics were also studied. The grafting of alternative monomers for the recovery of uranium from seawater is now under development by this versatile technique of ATRP.

1. INTRODUCTION The amount of uranium in seawater is estimated to be 4.5 billion tons, nearly 1000 times larger than terrestrial sources1,2 which will eventually be expended. In order to secure an energy supply for the future, this virtually limitless resource should be tapped into via mining uranium from seawater. However, challenges remain in making this technology economically feasible, including the low concentration of uranium (~3.3 ppb) in seawater and the presence of many competing metal ions at much higher concentrations.2,3 Over several decades, important progress has been made on this topic. First of all, poly(acrylamidoximes) (PAOs) were identified as effective uranyl ion (UO22+) sequestering agents at the pH of seawater (pH 8.0−8.3).4 PAOs are normally prepared from the amidoximation of nitrile groups on polyacrylonitrile (PAN), which are synthesized by the freeradical polymerization of AN. Secondly, the incorporation of a hydrophilic comonomer (e.g., methacrylic acid) with AN at an optimal ratio was found to increase the uranium adsorption capacity.5 This could be due to the increased ionophilicity (specifically, a negatively charged environment with an affinity for UO22+)6 or the increased hydrophilicity of the fiber. Thirdly, due to the swelling of PAOs in aqueous solutions, PAN and copolymers are normally grafted from robust backbone polymers, imparting improved mechanical properties on the adsorbents. Traditionally, radiation-induced graft polymerization (RIGP) is used for grafting PAN and PMA


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from backbone polymers, which come in various forms (e.g., stacked unwoven fabrics and braided fibers).7 Compared to other forms, adsorbents in the fiber form have several advantages, in particular they are: 1) readily deployable, 2) light weight, and 3) easy to fabricate into various shapes and lengths. However, the conventional RIGP method has certain limitations, such as the inability to tune the composition, degree of grafting (d.g.), conformation, and morphology of the resulting adsorbent, due to its ill-controlled polymerization mechanism. On the other hand, controlled radical polymerization (e.g., ATRP) allows for the control of molecular weight and molecular weight distributions, well-defined end groups, and the synthesis of block and graft copolymers with different architectures.8,9 Another advantage is that only negligible homopolymer formation is observed when ATRP is employed in graft polymerization.10,11 In addition, recent advances in ATRP have opened up opportunities for grafting various functional polymers from different surfaces.12,13 Moreover, ATRP also has the advantage of utilizing a wide range of available activated-halogen initiators.8 For the preparation of uranium adsorbents, ATRP has been applied to the grafting of PAN from mesoporous cross-linked poly(vinylbenzyl chloride) (PVBC),14 and to the grafting of PAN and poly(tert-butyl acrylate) (PtBA) from PVBC that is grafted onto a polyethylene (PE) fiber.15 The former work14 produced a non-fibrous form of polymer, which might have challenging deployment issues for uranium recovery in seawater. The latter work,15 although it produces adsorbent fibers, it still depends on RIGP for the grafting of ATRP initiation sites (i.e., PVBC) to the PE fiber in the first step, and the crowded brushes on the brush structure have impeded uranium adsorption in seawater experiments. The importance of polymer morphology has also been demonstrated in the literature.15 Specifically, it was recently reported that a short,


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hydrophilic polymer block at the tip of random poly(acrylamidoxime-co-acrylic acid) grafted chains nearly doubled the adsorption capacities of the random copolymer adsorbent in seawater.15 In this current work, PVC-co-CPVC fiber is used as a backbone to graft AN and tBA via ATRP copolymerization for the preparation of uranium adsorbent fibers. Previous works on ATRP from a similar backbone polymer, PVC, included the grafting of various monomers (e.g., AN),16,17

hydroethyl

methacrylate,20,22

acrylate,18

tert-butyl

butyl

acrylate,19,20

methacrylate,23

glycidyl

2-ethyl

hexylacrylate,19,21

methacrylate,24

methyl

poly(oxyethylene

methacrylate),25-29 acrylamide,30 N-vinyl pyrrolidone,31 4-vinylpyridine,32 styrene,33 and styrene sulfonic acid.34 All the PVC, partially dehydrochlorinated PVC, or poly(vinyl chloride-co-vinyl acetate) polymers used in the aforementioned study were in the form of powders, beads, films, sheets, or suspensions, but not fibers. To our knowledge, the work presented in this manuscript is the first reported work on ATRP grafting from a PVC copolymer in the fiber form and with an unprecedentedly high d.g. 2. EXPERIMENTAL SECTION 2.1. Materials and Characterization Methods. The PVC-co-CPVC fiber used in this study was RhovylTM’s ZCS tow fibre provided by Whitin Yarns and Fibers (Westport, MA). Acrylonitrile (AN, 99+%, Acros) and tert-butyl acrylate (tBA, 99%, Alfa) were passed through an activated alumina column prior to use. Copper(II) chloride (CuCl2·2H2O, 99+%, Acros), copper(I) chloride (CuCl, 99.99%, Acros), tris(2-(dimethylamino)ethyl)amine (Me6TREN, 99+%, Alfa), ethylene carbonate (EC, 99+%, Acros), dimethyl sulfoxide (DMSO, 99.95%, Fisher), methanol (99.9%, Fisher), and potassium hydroxide (88.4%, Fisher) were used as received.

Hydroxylamine solution (HA, 50 wt % in water, Aldrich) was used during the


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amidoximation. Deionized water was freshly collected prior to the usage from a Milli-Q Gradient water deionizer. Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O, B&A Quality), sodium bicarbonate (NaHCO3, ACS Reagent, Aldrich), and sodium chloride (> 99%, Aldrich) were used for the preparation of U-spiked brine. Elemental analyses (EA) for C, H, N, O, and Cl were performed by Galbraith Laboratories, Inc. (Knoxville TN). Solid-state

13

C CP/MAS (cross-

polarization/magic-angle spinning) NMR spectra were acquired on a Varian Inova 400 MHz spectrometer and referenced to an external standard, hexamethylbenzene, at 17.17 ppm. Fourier transform infrared (FTIR) spectra were acquired on an Excalibur FTIR with an MVP-Pro ATR Accessory. Scanning electron microscope (SEM) imaging was performed on the Zeiss Auriga microscope with an electron beam operation of 3keV, which is a dual beam FIB (Focused Ion Beam) with a field emission electron column for high resolution electron imaging and a Canion Ga+ column for precision ion beam milling. 2.2. Typical ATRP Procedures. First, CuCl2·2H2O (3.3 mg, 1.9 × 10-5 mol), PVC-co-CPVC fiber (150 mg, 2.08 × 10-3 mol vinyl chloride repeating units), EC (40.6 mL, 50 vol %), tBA (varied, e.g., 27.9 mL, 0.190 mol, for 624 tBA: 494 AN in feed), AN (varied, e.g., 12.7 mL, 0.190 mol, for 624 tBA: 494 AN in feed), and Me6TREN (106 mg, 4.55 × 10-4 mol) were added to a Schlenk flask equipped with a magnetic stirring bar. The flask was subjected to three freeze−pump−thaw (FPT) cycles. Then, CuCl (38.0 mg, 3.84 × 10-4 mol) was added to the flask, under an argon flow, while the contents were at a solid state. The reaction mixture was subjected to another FPT cycle. The flask was placed in an oil bath with the temperature equilibrated at 65 °C and the reaction was pursued under a sealed argon atmosphere for 24 h. The reaction was terminated by exposure to air. The fiber product was washed with DMSO until the supernatant was colorless, rinsed three times with methanol, and dried under vacuum at 40 °C overnight or


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longer until constant weights were obtained. Degrees of grafting (d.g.) were calculated from 100 × weight increase from grafting/weight of PVC-co-CPVC fiber. The d.g. presented in subsequent tables are averaged values from at least four repeated experiments. 2.3. Amidoximation and KOH Treatment. The next step involves the amidoximation (AO) of AN on polymer brushes, followed by KOH treatment. Since cyclic glutarimidedioxime was reported to form complexes with uranyl ion with high stability constants,35 complete conversions from nitrile to amidoxime, and from adjacent amidoximes to cyclic imide dioxime were sought after during the course of this study. For this reason, AO was performed twice over long reaction periods (i.e., 2 d, followed by 1 d). A volume of 6.0 mL of HA mixture (10 wt % in 1:1 (w/w) methanol/water) was added to 15 ± 1 mg of the adsorbent. The first AO was performed for 48 h at 80 °C. The reaction mixture was then replaced by a fresh HA mixture (6.0 mL), followed by the second AO (80 °C, 24 h). The fibers were then washed with deionized water until neutral pH was achieved and dried in a vacuum oven at 40 °C overnight, yielding constant weights. Prior to each U adsorption test, KOH treatment of the fibers was performed by adding 15.0 mL of 2.5 wt % KOH solution to an accurate weight of a dry amidoximated fiber (15.0 ± 1.0 mg). The mixture was placed in a heat block with the temperature equilibrated at 80 °C, and the KOH treatment was pursued for 3 h. Fibers were then filtered and washed with deionized water until neutral pH was observed. Care was taken to ensure that the fibers did not become dry, and no loss of fibers occurred at any step of the procedure. 2.4. Uranium Uptake in U-spiked Brine. U adsorption tests were performed on amidoximated, KOH-treated adsorbent fibers, which were kept wet until the test. Testing conditions in summary are: 15 mg adsorbent in 250 or 750 mL of 5–7 ppm U, 1.01 × 104 ppm


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+

−

Na , 1.55 × 104 ppm Cl , 140 ppm HCO3−, pH 8, 20–25 °C, 24 h; ICP-OES (inductively coupled plasma optical emission spectrometry) at λU 367.007 nm. To prepare U-spiked (5–7 ppm U) brine, 17 mg UO2(NO3)2·6H2O, 25.6 g NaCl, and 193 mg NaHCO3 were dissolved in DI water to make a 1 L solution.36,37 A mass of 15.0 ± 1.0 mg fibers was shaken in 750 mL of brine solution (pH ~8) for 24 h at room temperature. The amount of uranyl ion uptake was determined from the concentration difference between the beginning and the end of the test on a Perkin–Elmer Optima 2000 DV ICP-OES at 367.007 nm. The following formulas were used: uranium adsorption capacity (mg/g or g/kg) = [(Ci – Cf), mg/L] × [volume of solution, L]/[mass of adsorbent, g], % adsorbed = [(Ci – Cf)/Ci] × 100, and Kd (L/g) = [(Ci – Cf), mg/L] × [volume of solution, L]/{[Cf, mg/L] × [mass of adsorbent, g]}, where Ci and Cf represent the initial and final solution concentrations. Repeated U uptake experiments were performed with reproducible results, and averaged values were presented in tables. 2.5. Uranium Uptake in Seawater. The performance of the adsorbents was assessed in continuous-flow experiments with natural seawater pumped from Sequim Bay, WA at the Marine Sciences Laboratory of the Pacific Northwest National Laboratory. A mass of ~60 mg of adsorbent was packed in a flow-through column of 1-inch diameter and 6-inch height. The adsorbent was uniformly distributed in the column volume and held in place by adding glass beads of 3-mm diameter. Marine testing was performed using filtered (0.45 µm) seawater at a controlled temperature of 20 ± 2 °C and at flow rate of 250–300 mL/min using an active pumping through a multi-channel flow system. A detailed description of the experimental setup and analytical methods are described elsewhere.38

3. RESULTS AND DISCUSSION


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3.1. ATRP Grafting and Uranium Uptake in U-spiked Brine. Synthesis of uranium adsorbent fibers were performed in three steps: 1) Simultaneous ATRP grafting of a ligand-forming monomer, AN, and a hydrophilicity-yielding monomer, tBA, from active chlorine sites on PVCco-CPVC fiber, 2) AO to convert nitriles on grafted PAN to amidoximes, and 3) KOH treatment to hydrolyze tBA and unreacted AN, if any, on the grafted fibers to carboxylates, rendering hydrophilicity onto the adsorbent fibers (Figure 1). After AO, two possible structures of amidoxime-type ligands are possible: 1) acyclic amidoxime, and 2) cyclic imide dioxime converted from two adjacent amidoximes (Figure 1, final product).39 It is also important to clarify that ATRP is sensitive to the presence of acids, and the solution to this problem has been to polymerize protected monomers (e.g., tBA), followed by a deprotection step (e.g., hydrolysis by KOH).40

Figure 1. Synthesis steps of uranium adsorbent fibers from PVC-co-CPVC fibers. The PVC-co-CPVC fiber used in this study is RhovylTM’s ZCS tow fiber. It is a copolymer between PVC and CPVC, processed without any plasticizer to the round fiber form (average diameter: 15.4 ± 2.8 µm, in Supporting Information, Figure S1) and without any pores that can add extra surface area to it. The measured wt % Cl from elemental analysis (EA) is 49.16%,


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which is lower than expected even for PVC (56.73%). The experimental value of 49.16% was used in the calculation of moles of “alkyl chlorides” (RCl). For example, for each 150-mg RhovylTM’s fiber, (0.150 g × 0.4916)/(35.453 g/mol of Cl) = 2.08 × 10-3 moles RCl were present. Its

13

C CP/MAS NMR spectrum (Figure S2) showed the expected monochloro and dichloro

carbons at 58 and 97 to 91 ppm, respectively. Its methylene carbons collectively appeared at 47 ppm. Also, a broad signal ca. 200 to 100 ppm, assignable to vinyl carbons in allylic chlorides, was observed. These allylic chlorides, reportedly found in PVC,41 are formed as structural defects during the radical polymerization of vinyl chloride. Allylic chlorides and dichlorides were reported as actual ATRP initiation sites in PVC.16,19 Due to the solubility of PVC fiber in various solvents and monomers, especially at elevated temperatures, the ATRP conditions were limited to reactions in ethylene carbonate (EC) at 65 ºC which allowed for reasonable polymer growth rates. Since Cu complexes formed with Me6TREN constitute some of the most active and reducing catalysts that were successfully employed in ATRP42, we utilized Me6TREN as a ligand in this study. Likewise, in our recent study, the Cu– Me6TREN catalyst system gave high d.g., 595–2818%, of AN and tBA from poly(vinylbenzyl chloride) initiation sites.15 In order to identify the optimal amount of catalyst, the ATRP grafting of AN under various amounts of CuCl, from 0.75 [CuCl]/993 [AN] to 2.0 [CuCl]/993 [AN], was performed along with uranium uptake tests on corresponding amidoximated fibers (Table 1). The d.g. of the resulting fibers did not drastically differ with varying concentrations of catalyst. However, due to the higher d.g. and enhanced U uptake performance (i.e., higher U adsorption capacity and distribution coefficient, Kd), of fibers obtained with an [AN]/[RCl]/[CuCl]/[Me6TREN]/[CuCl2] ratio of 993:5.4:1.0:1.2:0.050 (Table 1, no. 1.2), this reactant to catalyst ratio was used as a


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guideline for the rest of this study. It is also worth mentioning that homopolymerization was not observed under the reaction conditions studied (i.e., precipitate was not formed when the reaction mixture was added into 50% aqueous methanol solution). Table 1. ATRP of AN and U Uptake in 750-mL U-spiked Brine. no.

[AN]/[RCl]/[CuCl]/

d.g., % adsorption capacity, % adsorption Kd, L/g

[Me6TREN]/[CuCl2]a

g/kg

1.1 993:5.4:0.75:0.90:0.038

408

94.1

30.0

22.2

1.2 993:5.4:1.0:1.2:0.050

437

116.5

37.4

30.7

1.3 993:5.4:1.5:1.8:0.075

362

63.5

26.0

17.7

1.4 993:5.4:2.0:2.4:0.10

356

106.6

35.9

28.2

a

Constant ratio between CuCl, Me6TREN, and CuCl2, in 50 vol % EC, 65 ºC, 24 h.

With the catalyst concentration held constant (Table 1, no. 1.2), the [tBA]/[AN] feed ratio was varied in simultaneous copolymerization (Table 2). The elemental analysis (EA) of the N and O content of the grafted fibers permitted the calculation of the [PtBA]/[PAN] ratios (4th column). Overall, as expected, the [PtBA]/[PAN] ratios of the grafted fibers decreased with the decreasing [tBA]/[AN] feed ratio. At low [PtBA]/[PAN] ratios (nos. 2.1 and 2.2), the [PtBA]/[PAN] ratios of the fibers were almost identical to the [tBA]/[AN] feed ratios. On the other hand, at higher [tBA]/[AN] molar ratios (nos. 2.3−2.6), the [PtBA]/[PAN] ratios were significantly lower than [tBA]/[AN] feed ratios, indicating a more efficient grafting of PAN than PtBA. This is in agreement with a higher reactivity ratio of AN than that of tBA (rAN = 1.62, rtBA = 0.88).15 Table 2. Simultaneous Copolymerization of AN and tBA and U Uptake in 750-mL U-spiked Brine.


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no. [tBA]/[AN]a

[tBA]/[AN] feed ratio

[PtBA]/[PAN] from EA

d.g., %

adsorption capacity, g/kgb

% adsorption

Kd , L/g

1.2

0:993

0.00

0.047

437

116.5

37.4

30.7

2.1

50:494

0.101

0.166

162

90.0

28.3

20.0

2.2

124:494

0.251

0.256

256

81.6

25.6

17.6

2.3

249:494

0.504

0.356

1390

174.7

60.3

75.1

2.4

371:494

0.751

0.405

1012

150.5

49.5

48.7

2.5

496:494

1.00

0.645

754

154.0

51.9

52.8

2.6

624:494

1.26

0.937

1527

118.0

40.3

32.7

a

[tBA]/[AN]/[RCl]/[CuCl]/[Me6TREN]/[CuCl2] = the above monomer ratios:5.4:1.0:1.2:0.05, in 50 vol % EC, 65 ºC, 24 h. b

Blank experiment: Amidoximated and KOH treated PVC-co-CPVC fiber showed 0.49 g U/kg, 0.16% adsorption, and Kd of 0.08 L/g. High d.g. values (≥ 754%) were obtained under the conditions studied, especially at higher [tBA]/[AN] ratios (nos. 2.3−2.6). These d.g. values were much higher than values normally obtained from RIGP grafting of functional monomers from backbone fibers.43 The much lower d.g. values observed at low [tBA]/[AN] ratios (nos. 1.2, 2.1, and 2.2), might have been the result of termination or side reactions of AN radicals that were present at high concentrations under these conditions. Termination of side reactions of AN radicals might also account for the suppressed reactivity of AN, resulting in almost identical [PtBA]/[PAN] and [tBA]/[AN] ratios (nos. 2.1 and 2.2). Screening tests were conducted in a uranyl brine consisting of seawater relevant concentrations of sodium, chloride, and bicarbonate (pH ~ 8) spiked with 6-ppm U to identify adsorbents with high U adsorption capacities (last three columns). A blank experiment on amidoximated and KOH-treated PVC-co-CPVC fiber showed a negligible U adsorption capacity (i.e., 0.49 g U/kg). This indicates that high U adsorption capacities in grafted adsorbents were originated from


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grafted functional groups, and not the backbone fiber. Among grafted adsorbents, the adsorbent with the highest grafting yield and an optimal hydrophilic group to ligand ratio showed the highest U adsorption capacity and Kd (See no. 2.3). It is worth mentioning that when the hydrophilic group to ligand ratio was not optimized, the U adsorption capacity and Kd significantly dropped, even with a high d.g. (no. 2.6). The effect of chain length or d.g. was further studied to ensure the optimized chain length was employed (Table 3). With a constant [tBA]/[AN] feed ratio of 494:249, ATRP reaction times were varied, allowing living polymer chains to grow to different lengths. It was found that a period of at least 18-h ATRP reaction was needed for long polymer chains (i.e., high d.g.) and reasonably high U adsorption capacities. After 24 h, the d.g. did not increase significantly, indicating that some polymer chains were no longer reactive. The highest U adsorption capacity in U-spiked brine was found with adsorbent fibers grown for 24 h. Table 3. Effect of Chain Length and U Uptake in 750-mL U-spiked Brine. no. ATRP time,a h d.g., % adsorption capacity, g/kg % U adsorption Kd, L/g 3.1

6

458

75.7

20.7

13.4

3.2

12

392

112.1

35.2

27.1

3.3

18

1487

147.1

41.1

38.9

3.4

24

1390

174.7

60.3

75.1

3.5

48

1436

161.9

49.6

50.2

a

[tBA]/[AN]/[RCl]/[CuCl]/[Me6TREN]/[CuCl2] = 249:494:5.4:1.0:1.2:0.05, in 50 vol % EC, 65 ºC. Reusability of the adsorbent fiber no. 2.3 was also examined in repeated usage in 24-h adsorption in 750 mL of U-spiked brine (Figure 2). After each uranium adsorption, uranium was eluted from the adsorbent with 0.1 M H2O2 in 1 M Na2CO3 according to the method reported by Pan et al.44 The recovery in U adsorption capacity after each usage was calculated relative to the


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adsorption capacity of the first usage. After the first and second usage, the adsorption capacities decreased by approximately 10% per usage. After the third usage, the adsorption capacity further dropped by ca. 20%, leaving the fiber with about 60% of the initial adsorption capacity. The decrease in the adsorption capacity of fiber no. 2.3 was much higher than that in the literature, ca. 3% decrease per usage, up to four usage.44

Figure 2. % Recovery of uranium adsorption capacities in fiber no. 2.3. 3.2. Characterization of Adsorbent Fibers. Adsorbent fibers were also characterized by solid-state

13

C NMR (Figure 3). On the grafted fiber (a), signals from −COOBut in PtBA (173

ppm), −CN in PAN (121 ppm), and 4° carbon in PtBA (83 ppm) were observed as expected. After AO at 80 °C (trace b), a new signal (149 ppm) assigned to cyclic imide dioxime appeared, accompanied by a relatively small shoulder (157 ppm) assigned to acyclic amidoxime,43,45 indicating an incomplete cyclization even after long amidoximation time. In agreement with literature reports, the cyclic imide dioxime is expected to be the major product obtained from AO reactions in aqueous solutions at elevated temperatures.35,39,43,45,46 The formation of cyclic imide dioxime also indicates that simultaneous grafting of AN and tBA did not yield completely random copolymers. Blocks of grafted PAN occurred, in agreement with a higher reactivity ratio of AN than that of tBA (rAN = 1.62, rtBA = 0.88).15 Around 121 ppm, the –CN signal was no


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longer observed, indicating the completion of the AO reaction. In the carbonyl region, the signal from −COOBut in PtBA also slightly shifted downfield to 177 ppm and broadened, very likely indicating partial hydrolysis of PtBA. Signal from 4° carbon in PtBA (83 ppm) was still observed.

Figure 3. 100-MHz

13

C CP/MAS NMR spectra of fibers as (a) grafted (fiber no. 2.3, Table 2,

spinning speed 7.5 kHz), (b) amidoximated, and (c) KOH-treated. * denotes spinning sideband (spinning speed 6.0 kHz, unless noted otherwise). After a 3-h KOH treatment at 80 °C (c), both signals from COO− (184 ppm) and −COOBut (177 ppm) were observed, along with 4º carbon in PtBA (82 ppm), indicating an incomplete hydrolysis of PtBA. Along with the cyclic imide dioxime signal, the small shoulder from the acyclic amidoxime (158 ppm, see Inset) became more pronounced, indicating that some decomposition of cyclic imide dioxime occurred during the KOH treatment. The identity of functional groups on fibers was also confirmed by FTIR (Figure 4). After AO (trace b), the C≡N stretch from PAN (2242 cm–1) completely disappeared and the C=O stretch in COOBut shifted from 1719 to 1700 cm–1, in agreement with the NMR result. This C=O stretch also decreased in intensity and located as a small shoulder on a prominent iminic C=N stretching band. The presence of C=N, C−N, and N−O stretches at 1642, 1391, and 931 cm–1, respectively,


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confirmed the formation of amidoxime-type functional groups. During AO, COO− also formed (1556 cm–1) by the hydrolysis of COOBut.

Figure 4. FTIR spectra of fibers as (a) grafted, (b) amidoximated, (c) KOH-treated, and (d) after four usage. After a 3-h, 80 °C KOH treatment (trace c), the majority of COOBut was converted to COO–, as indicated by the diminishing of the C=O stretch of COOBut, and the increased intensity of the C=O stretch of COO–. After four usage and exposure to 0.1 M H2O2 and 1 M Na2CO3 during three elutions (trace d), the C=O stretch from COO– further increased in intensity, while the C=N and N−O stretching bands decreased in intensity, indicating the decomposition of the amidoxime-type functional groups to COO–. This plausibly accounted for the decrease in U adsorption capacities after four usage. SEM microscopy was used to assess the structural integrity of the adsorbent fibers (Supporting Information, Figure S1). As expected, after grafting, the diameter of fibers ranged from 23.0 to 39.1 µm (averaged 27.6 µm), showing an increase from the diameter of the backbone fiber (15.4 ± 2.8 µm). After AO, the fiber expanded drastically, with the diameter ranging from 48.9 to 108.0 µm. The morphology was no longer round fiber, but rough surface on round fiber with


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small fibrils sticking out. Unfortunately, the KOH treatment at 80 °C for 3 h was found to be harsh on the fiber. Deterioration of the fiber with some separation between the backbone fiber and grafted chains was observed. After four usage, SEM images did not reveal much further physical deterioration on the fiber. Overall, SEM images of the fibers revealed the importance of fiber treatment conditions. Currently, other hydrophilic-yielding comonomers that are more easily hydrolyzed than tBA, thus not requiring harsh KOH treatment, are studied in our group. 3.3. Uranium Uptake in Seawater. Adsorbent fibers showing U adsorption capacities greater than 150 g/kg in the U-spiked brine (Table 2, nos. 2.3−2.5) were selected for continuous-flow column experiments in seawater at the Marine Sciences Laboratory of Pacific Northwest National Laboratory (PNNL) at Sequim Bay, WA. After a 49-day exposure to seawater, fiber no. 2.3, which showed the highest U adsorption capacity in brine (Table 2), also showed the highest salinity-normalized adsorption capacity, 5.22 g/kg, in environmental seawater (Table 4). Fiber nos. 2.4 and 2.5 showed lower salinity-normalized adsorption capacities of 2.96 g/kg (49 days) and 2.55 g/kg (42 days), respectively, as also observed in Table 2 for the brine tests. Adsorption capacities obtained with these fibers were much higher than those observed for the JAEA reference fibers, 1.71 g/kg (49 days) and 1.66 g/kg (42 days). Table 4. Uranium Uptake in Seawater at Sequim Bay, WA. no. exposure time, d salinity-normalized adsorption capacity (35 psu), g/kg 2.3

49

5.22a

2.4

49

2.96a

2.5

42

2.55b

a

Compared with a JAEA reference fiber measured simultaneously: 49-day, salinity normalized adsorption capacity (35 psu) 1.71 g/kg. b

Compared with a JAEA reference fiber measured simultaneously, 42-day, salinity normalized adsorption capacity (35 psu) 1.66 g/kg.


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In order to confirm the reproducibility of the best performing adsorbent (i.e., fiber no. 2.3), three batches of adsorbent fibers were prepared by repeating the same experiments, from ATRP through KOH treatment and exposure time with natural seawater. From kinetic plots of U uptake (salinity normalized to 35 psu) and one site ligand saturation modeling (Figure 5), the uranium saturation capacities from the three batches ranged from 3.94 ± 0.40 to 6.91 ± 1.06 g/kg, which are significantly higher than the U uptake of JAEA reference fibers (up to 2.50 g/kg at 56-d exposure). The half saturation time ranged from 13.2 ± 5.6 to 24.7 ± 5.0 days.

Figure 5. Kinetic plots of U uptake of 3 batches of fiber no. 2.3 in Sequim Bay, WA, with one site ligand saturation modelling, along with U uptake of JAEA reference fibers measured simultaneously with each batch (salinity normalized to 35 psu). The elemental analysis of the adsorbents was compared to study the reproducibility of the adsorbent synthesis, i.e. determine the actual compositions. [PtBA]/[PAN] ratios of 0.356, 0.265, and 0.536, were obtained for Batch 1, 2, and 3 fibers, respectively. From Tables 2 and 4, it can also be pointed out that a slight change in [PtBA]/[PAN] ratios in a critical range drastically affected the U uptake, both in brine and in real seawater. In brine (Table 2, nos. 2.2–2.4), when the [PtBA]/[PAN] ratios varied from 0.256, to 0.356, and 0.405, the fibers showed U adsorption


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capacities ranging from 81.6, to 174.7, and 150.5 g/kg, respectively. In real seawater (Table 4, nos. 2.3–2.4), when the [PtBA]/[PAN] ratios varied from 0.356 to 0.405, fibers showed U adsorption capacities ranging from 5.22 to 2.96 g/kg, respectively. In order to prepare a fast and highly efficient adsorbent fiber (i.e., Batch 1 fiber), extra care must be taken to ensure the ratio of [PtBA]/[PAN] is as close as possible to 0.356. It is worth mentioning that 1-h KOH treatment was also tested with the Batch 2 fiber. However, U adsorption capacities, 1.52 g/kg (20-d) and 2.59 g/kg (56-d, averaged value), were much lower than those obtained from the 3-h KOH treatment, 2.42 g/kg (20-d) and 3.65 g/kg (56-d, averaged value). This is in accordance with the incomplete hydrolysis of PtBA observed in 13C CP/MAS NMR spectra even after the 3-h KOH treatment (Figure 3). Multi-element ICP-OES analysis of the uptake was also performed. Figure 6 depicts metal ion concentrations in fibers (mmol metal/kg fiber), plotted according to the descending order of their molar abundance in seawater.47 Due to much higher uptakes of Na+, Mg2+, and Ca2+, their concentrations on the fibers (Figure 6, Batches 1, 2, and 3, Na−Ca) were depicted on separate graphs from those of much less adsorbed metal ions (see Batches 1, 2, and 3, Sr−Cr). Compared between batches, all metal ions showed the same trend in their uptakes as the uranium uptake, i.e., uptakes in Batch 1 > Batch 2 > Batch 3.


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Figure 6. Molar adsorption capacities of metal ions on three batches of fiber no. 2.3 and on JAEA reference fibers in Sequim Bay, WA at various exposure time.


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As expected, major constituents in seawater, Na+, Mg2+, and Ca2+, were rapidly adsorbed on all three batches of fiber. Their adsorption capacities, especially those of Mg2+, in all three batches remained consistently high throughout all exposure times, up to 56 days. Among minor constituents and trace elements in seawater (see Batches 1, 2, and 3, Sr−Cr), V and Fe were dominant competing elements. Vanadium was adsorbed at about 10 times (molar ratio) that of uranium. Next, Zn, Ni, and Cu were shown to compete with U in various ratios in three batches of the fiber. Other elements, except Mn in the Batch 3 fiber, were adsorbed at much lower amounts than that of U. Overall, the adsorption of competing elements increased with the exposure time. Previous studies on polyamidoxime fiber prepared by RIGP48 and on polyamidoxime macroporous monoliths49 also reported the adsorption of several metal ions from seawater, with V and Fe as dominant competing elements. Figure 7 demonstrated molar adsorption ratios for the uptake of uranium against the uptake of four leading competing ions. In general, the selectivity of all batches toward metal ions decreased in the following order: Ni > Zn > Fe > V >> U. Even though the U adsorption capacities of these fibers were higher than those of JAEA reference fibers (Figure 5), their selectivity toward U, against Ni, Zn, and Fe, in most cases, was less than that of JAEA fibers. The U/V selectivity of the three batches was approximately the same or slightly less than that of JAEA fibers. The complexation between V(v) and cyclic imide dioximes (single molecules) have been reported,50,51 which may explain the low U/V selectivity observed in this study. It is also interesting that after approximately 21 days, the U/Fe and U/V selectivity did not increase anymore, hinting that it may be beneficial to harvest uranyl ion at 21 days or less.


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Figure 7. Molar adsorption ratios on three batches of fiber no. 2.3 and on JAEA reference fibers in Sequim Bay, WA at various exposure times. (Inset: expanded view). Due to harsh treatments required for the removal of V from adsorbents (e.g., 3–5 M HCl, ≥ 50 ºC),43 increasing U/V selectivity is the goal for our continuing work on this new type of adsorbent fibers. Adsorbents with high selectivity toward U and fast kinetics for U adsorption are needed to further improve the economic feasibility of this technology. With ATRP grafting, however, new functional groups, including designed ligands with high selectivity toward U and hydrophilic groups with less affinity for other metal ions, can be grafted in high yields from PVC-co-CPVC or other chlorinated fibers.


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4. CONCLUSION PVC-co-CPVC fiber was proven to be an active ATRP initiator for simultaneous grafting of AN and tBA, resulting in functional fibers with high grafting yields. Uranium adsorption experiments in brine and in natural seawater at Sequim Bay, WA, showed that adsorbent fiber with the highest adsorption capacity possessed an optimal hydrophilic group to ligand ratio [tBA]/[AN] of 0.356 and a high d.g. Uranium adsorption capacities of 2.42−3.24 g/kg at 42-days contact with seawater and 5.22 g/kg at 49-days exposure in seawater, much higher than those obtained simultaneously with the JAEA reference fibers, were measured. Adsorbent fibers also adsorbed other metal ions, especially V and Fe. In this work, ATRP was successfully demonstrated in the preparation of U adsorbents in the fiber form, without RIGP. This method is applicable

to

the

grafting

of

other

functional

monomers,

including

1-vinyl-3-(3-

cyanopropyl)imidazolium bromide, 4-vinylpyridine, 1-vinylimidazole, N-vinylformamide, sodium acrylate, and dimethyl itaconate, that were grafted in our group from halogenated solid adsorbents. ASSOCIATED CONTENT Supporting Information. SEM images.

13

C CP/MAS NMR spectrum of trunk fiber. This

material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].


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*E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was sponsored by the US Department of Energy, Office of Nuclear Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed by UT-Battelle, LLC. The JAEA sorbent was kindly donated for testing by the Japan Atomic Energy Agency. The PVC-co-CPVC fiber was a sample from RhovylTM fibre, courteously donated by Whitin Fiber and Yarn, Massachusetts, USA. ABBREVIATIONS AN = acrylonitrile AO = amidoximation ATRP = atom-transfer radical polymerization d.g. = degree of grafting CP/MAS = cross-polarization/magic-angle spinning EA = elemental analysis EC = ethylene carbonate PAN = polyacrylonitrile PtBA = poly(tert-butyl acrylate) PVC = poly(vinyl chloride) PVC-co-CPVC = poly(vinyl chloride)-co-chlorinated poly(vinyl chloride) tBA = tert-butyl acrylate REFERENCES 1.

Saito, K.; Uezu, K.; Hori, T.; Furusaki, S.; Sugo, T.; Okamoto, J. Recovery of uranium

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36. Gorka, J.; Mayes, R. T.; Baggetto, L.; Veith, G. M.; Dai, S. Sonochemical functionalization of mesoporous carbon for uranium extraction from seawater. J. Mater. Chem. A 2013, 1, 3016–3026. 37. Yue, Y. F.; Sun, X. G.; Mayes, R. T.; Kim, J.; Fulvio, P. F.; Qiao, Z. A.; Brown, S.; Tsouris, C.; Oyola, Y.; Dai, S. Polymer-coated nanoporous carbons for trace seawater uranium adsorption. Sci. China Chem. 2013, 56, 1510–1515. 38. Kim, J.; Tsouris, C.; Oyola, Y.; Janke, C. J.; Mayes, R. T.; Dai, S.; Gill, G.; Kuo, L. J.; Wood, J.; Choe, K. Y.; Schneider, E.; Lindner, H. Uptake of uranium from seawater by amidoxime-based polymeric adsorbent: Field experiments, modeling, and updated economic assessment. Ind. Eng. Chem. Res. 2014, 53, 6076–6083. 39. Kang, S. O.; Vukovic, S.; Custelcean, R.; Hay, B. P. Cyclic imide dioximes: Formation and hydrolytic stability. Ind. Eng. Chem. Res. 2012, 51, 6619–6624. 40. Davis, K. A.; Matyjaszewski, K. Atom transfer radical polymerization of tert-butyl acrylate and preparation of block copolymers. Macromolecules 2000, 33, 4039–4047. 41. Benedikt, G. M.; Goodall, B. L.; Rhodes, L. F.; Kemball, A. C. NMR-spectroscopy of poly(vinyl chloride) defects. 2. 1H and

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43. Seko, N.; Katakai, A.; Tamada, M.; Sugo, T.; Yoshii, F. Fine fibrous amidoxime adsorbent synthesized by grafting and uranium adsorption-elution cyclic test with seawater. Sep. Sci. Technol. 2004, 39, 3753–3767. 44. Pan, H.-B.; Wai, C. M. W.; Oyola, Y.; Janke, C. J.; Tian, G.; Rao, L. Carbonate–H2O2 leaching for sequestering uranium from seawater. Dalton Trans. 2014, 43, 10713-10718. 45. Kobuke, Y.; Tanaka, H.; Ogoshi, H. Imidedioxime as a significant component in socalled amidoxime resin for uranyl adsorption from seawater. Polym. J. 1990, 22, 179–182. 46. Eloy, F.; Lenaers, R. The chemistry of amidoximes and related compounds. Chem. Rev. 1962, 62, 155–183. 47. Monterey Bay Aquarium Research Institute. Summary Table of Mean Ocean Concentrations and Residence Times. http://www3.mbari.org/chemsensor/summary.html 48. Suzuki, T.; Saito, K.; Sugo, T.; Ogura, H.; Oguma, K. Fractional elution and determination of uranium and vanadium adsorbed on amidoxime fiber from seawater. Anal. Sci. 2000, 16, 429–432. 49. Yue, Y. F.; Mayes, R. T.; Gill, G.; Kuo, L. J.; Wood, J.; Binder, A.; Brown, S.; Dai, S. Macroporous monoliths for trace metal extraction from seawater. RSC Adv. 2015, 5, 50005– 50010. 50. Wai, C. M. Innovative Elution Processes for Recovering Uranium from Seawater; Technical Report for U.S. Department of Energy–Nuclear Energy University Programs, May 2014.


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51. Leggett, C. J.; Parker, B. F.; Teat, S. J.; Zhang, Z.; Dau, P. D.; Lukens, W. W.; Peterson, S. M.; Cardenas, A. J. P.; Warner, M. G.; Gibson, J. K.; Arnold, J.; Rao, L. Structural and spectroscopic studies of a rare non-oxido V(v) complex crystallized from aqueous solution. Chem. Sci. [Online early access]. DOI: 10.1039/C5SC03958D. Published Online: Jan 14, 2016. http://pubs.rsc.org/en/Content/ArticleLanding/2016/SC/C5SC03958D#!divAbstract

(accessed

Mar 4, 2016).


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