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Dec 10, 2015 - ABSTRACT: Seawater contains a large amount of uranium (∼4.5 billion tons) which can serve as a nearly limitless supply for an energy ...
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Uranium Adsorbent Fibers Prepared by Atom-Transfer Radical Polymerization from Chlorinated Polypropylene and Polyethylene Trunk Fibers Suree Brown,*,† Sabornie Chatterjee,‡ Meijun Li,† Yanfeng Yue,‡ Costas Tsouris,‡ Christopher J. Janke,§ Tomonori Saito,*,‡ and Sheng Dai*,†,‡ †

Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: Seawater contains a large amount of uranium (∼4.5 billion tons) which can serve as a nearly limitless supply for an energy source. However, to make the recovery of uranium from seawater economically feasible, lower manufacturing and deployment costs are desirable, and good solid adsorbents must have high uranium uptake, reusability, and high selectivity toward uranium. In this study, atom-transfer radical polymerization (ATRP), without the high-cost radiation-induced graft polymerization, was used for grafting acrylonitrile and tert-butyl acrylate from a new class of trunk fibers, forming adsorbents in a readily deployable form. The new class of trunk fibers was prepared by the chlorination of polypropylene (PP) round fiber, hollow-gear PP fiber, and hollow-gear polyethylene fiber. During ATRP, degrees of grafting (d.g.) varied according to the structure of active chlorine sites on trunk fibers and ATRP conditions, and the d.g. as high as 2570% was obtained. Resulting adsorbent fibers were evaluated in U-spiked simulated seawater, and the maximum adsorption capacity of 146.6 g U/kg, much higher than that of a standard adsorbent Japan Atomic Energy Agency fiber (75.1 g/kg), was obtained. This new type of trunk fiber can be used for grafting a variety of uranium-interacting ligands, including designed ligands that are highly selective toward uranium.

1. INTRODUCTION Seawater, with its large volume, contains large amounts of elements, even those that are considered minor constituents and trace elements. Some of these elements, including uranium, can be used for future energy resources.1 The amount of uranium in seawater is estimated to be 4.5 billion tons, almost 1000 times larger than terrestrial sources.1,2 However, to make the recovery of uranium from seawater economically feasible, solid adsorbents must have the following properties: high uranium uptake, reusability (resulting from good mechanical stability and chemical recyclability), and high selectivity toward uranium which exist as complexes of uranyl ion (UO22+) in seawater.1 Among many solid adsorbents, poly(acrylamidoximes) (PAOs), containing amidoxime-type ligands, are commonly used because of their ability to bind uranyl ion (UO22+) at the pH of seawater (pH 8.0−8.3).3 Because oxime groups are known to quench free radicals,4 PAOs are normally prepared in two steps: free-radical polymerization of acrylonitrile (AN), followed by amidoximation to convert nitriles (−CN) to amidoxime-type ligands. A hydrophilic comonomer (e.g., methacrylic acid, acrylic acid) is normally used during the polymerization with AN to increase the uranium uptake.5 To increase the mechanical strength of adsorbents, both the functional ligands and the hydrophilic-rendering groups are usually attached to a chemically inert and mechanically robust trunk polymer by graft polymerization. The graft method conventionally employed is radiation-induced graft polymer© XXXX American Chemical Society

ization (RIGP), and various forms (e.g., stacked unwoven fabrics and braided fibers) of adsorbents were prepared.6 Despite many advantages of the conventional RIGP method, certain limitations, including high manufacturing cost and the inability to tune the polymer composition, degree of grafting (d.g.), conformation, and resulting morphology, exist. This is due to the ill-controlled polymerization mechanism of RIGP. On the other hand, controlled radical polymerization, including atom-transfer radical polymerization (ATRP), enables the control of molecular weight and its distribution and the synthesis of block and graft copolymers with different architecture (e.g., dendritic polymers).7,8 In contrast to RIGP, only negligible homopolymer formation is observed when ATRP is employed in graft polymerization.9,10 With a variety of activated-halogen initiators,7 available on many types of trunk materials,11,12 ATRP can be used to graft functional polymers without the need to irradiate the trunk materials. There are a few reports on using ATRP to prepare adsorbents containing amidoxime groups, including the work done by Zong et al.13−15 and Liu et al.16 In the aforementioned reports, adsorbents for mercury(II) ion13−15 and for silver ion16 were prepared in Special Issue: Uranium in Seawater Received: September 30, 2015 Revised: December 10, 2015 Accepted: December 10, 2015

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DOI: 10.1021/acs.iecr.5b03667 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research nonfibrous forms. As for uranium adsorbents prepared by ATRP, previous works were reported by Yue et al.17 and Saito et al.18 Yue et al. employed mesoporous cross-linked poly(4vinylbenzyl chloride) (PVBC) as an ATRP initiator,19 thus forming nonfibrous adsorbents which might have challenging deployment and recovery issues in seawater. Saito et al. employed RIGP in grafting PVBC as an ATRP initiator on polyethylene (PE) fiber, thus producing adsorbent fibers.18 However, RIGP was still required and the resulting brushes on a brush structure impeded uranium adsorption in seawater experiments. Grafting a block of hydrophilic-rendering polymer, resulting in a changed polymer morphology, enhanced the uranium uptake in seawater.18 In this current work, new trunk fibers, namely chlorinated PE (CPE) and chlorinated polypropylene (CPP) fibers were used for grafting AN and tBA via ATRP copolymerization for the preparation of uranium adsorbent fibers. To our knowledge, ATRP from halogenated PE has not been reported, while there are a few previous reports on ATRP from halogenated PP, all producing grafted polymers in nonfibrous forms.20,21 The former work involves the bromination of PP film and ATRP grafting of N-isopropylacrylamide.20 The weight of brominated PP film increased after ATRP from 79.12 to 87.5 mg (after 24 h) and 98.9 mg (after 48 h), corresponding to the calculated degrees of grafting of 10.6% and 25.0%, respectively. In the latter work, a commercial CPP in the powder form was used to graft 2-ethylhexyl acrylate, resulting in the d.g. of up to 170% after 1−24 h of ATRP.21 This current work is focused on adsorbents in the fiber form because of the following advantages: (1) lower cost to deploy, (2) lightweight, and (3) ease of processing to various shapes and lengths. Round PP, hollow-gear PP, and hollow-gear PE fibers were chlorinated, with controllable Cl content, by in situ generated Cl2 gas. The resulting fibers are stronger and less soluble than commercial poly(vinyl chloride) fibers. The chlorination can be scaled up. The ATRP grafting of functional polymers yielded high d.g., and resulting grafted polymers were effective uranium adsorbents.

(30%, Fisher) were used for the preparation of eluting solution for uranium. Elemental analyses (EA) for C, H, N, O, and Cl were performed by Galbraith Laboratories, Inc. (Knoxville TN). Solid-state 13C 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 instrument with an MVP-Pro ATR accessory. Scanning electron microscopy (SEM) imaging was performed on a Zeiss Auriga microscope with an electron beam operation of 3 keV, 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. Halogenation Procedures. 2.2.1. Chlorination. Prior to the chlorination, PLA was removed from HGPE fiber by submerging the fiber in excess THF at 60 °C overnight. This process was repeated three times, followed by drying at 50 °C under vacuum. Chlorination by in situ generated Cl2 was modified from a procedure reported by Shah et al.22 A HCl solution (15%) was kept under a nitrogen atmosphere. To a Schlenk flask, a selected fiber trunk material (PP, HGPP, or HGPE, 0.60 g) and 10% NaOCl (30.7 mL) were added, evacuated, and refilled with nitrogen three times. Under a nitrogen atmosphere, the 15% HCl solution (6.10 mL) was added dropwise, and the reaction vessel was sealed. The chlorination was continued either under sunlight or under a UV lamp (source, Ace mercury arc, model 7825-36; main λ, 366 nm) in a quartz Schlenk at a 1.5 cm distance (ca. 46 °C), for the amount of time listed in Table S1. The reaction was terminated by exposure to air. The fiber was then washed with deionized (DI) water until neutral pH, rinsed with methanol, and dried under vacuum at 40 °C at least overnight until constant weights were obtained. Weight percent Cl was calculated from 100 × weight increase from grafting/weight of PP fiber. 2.2.2. Bromination. PP fiber (0.050 g) in a quartz Schlenk flask was evacuated and refilled with nitrogen three times. Under a nitrogen atmosphere, 0.10 mL of liquid Br2 (bp 58.8 °C) was injected and the quartz Schlenk flask was sealed. The bromination was continued under a UV lamp at a 7 cm distance (ca. 46 °C) for the amount of time listed in Table S1. The reaction was terminated by exposure to air. The fiber was then washed with DI water until neutral pH, rinsed with methanol, and dried under vacuum at 40 °C at least overnight until constant weights were obtained. Weight percent Br was calculated from 100 × weight increase from grafting/weight of PP fiber. 2.3. Typical ATRP Procedures. First, CuCl2 (4.7 mg, 3.5 × 10−5 mol), halogenated fiber (e.g., 84 mg of CPP with 29.4 wt % Cl (gravimetric), 7.0 × 10−4 mol Cl), EC (63.5 mL, 50 vol %), tBA (varied, e.g., 40.4 mL, 0.278 mol, for [tBA]/[AN] of 400:500 in feed), AN (varied, e.g., 23.1 mL, 0.348 mol, for [tBA]/[AN] of 400:500 in feed), and Me6TREN (195 mg, 8.38 × 10−4 mol) were added to a 250 mL Schlenk flask equipped with a magnetic stirring bar. The reaction mixture was purged with argon for 30 min. Then, CuCl (69.0 mg, 6.97 × 10−4 mol) was added to the flask, under an argon flow. The flask was placed in an oil bath with 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

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization Methods. The round PP fiber has an average diameter of 11.6 ± 1.9 μm, measured by scanning electron microscopy. High-surface-area, hollow-gear PP and PE fibers (HGPP and HGPE, respectively) were prepared by melt-spinning at Hills, Inc. (Melbourne, FL). HGPE used in this study was Hills’ fiber no. 17, prepared using polylactic acid (PLA) as a coextrusion polymer. Concentrated HCl (35%, Fisher) and sodium hypochlorite solution (NaOCl, 13% w/v, Fisher) were used for the preparation of 15% HCl and 10% NaOCl, respectively. Acrylonitrile (AN, 99+%, Acros) and tert-butyl acrylate (tBA, 99%, Alfa) were passed through an activated alumina column prior to use. Anhydrous copper(II) chloride (CuCl2, ≥ 99.995%, Aldrich), copper(I) chloride (CuCl, 99.999%, Alfa), 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 amidoximation. 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 Uspiked simulated seawater. Na2CO3 (99.9%, Fisher) and H2O2 B

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Figure 1. Synthesis steps of uranium adsorbent fibers from trunk fibers.

under vacuum at 40 °C at least overnight until constant weights were obtained. Degrees of grafting (d.g.) were calculated from 100 × weight increase from grafting/weight of trunk fiber. 2.4. Amidoximation and KOH Treatment. The next step involves the amidoximation (AO) of AN groups in the polymer brush, followed by KOH treatment. AO was performed twice to ensure complete conversion of AN groups in grafted polymer chains, and a large excess of hydroxylamine (HA) was used each time. 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 reaction mixture was then placed in a heat block (or in a digitally controlled oven, in the case of a large-scale amidoximation) with temperature equilibrated at 80 °C, and the reaction was pursued for 48 h. 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 room temperature 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 aqueous solution to an accurate weight of a dry amidoximated fiber (15.0 ± 1.0 mg). The mixture was placed in a heat block with 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. 2.5. Uranium Uptake in U-Spiked Simulated Seawater. U adsorption tests were performed on amidoximated, KOHtreated adsorbent fibers, which were kept wet until the test. Testing conditions in summary are 15 mg of adsorbent in 250 or 750 mL of 5−7 ppm U, 1.01 × 104 ppm of Na+, 1.55 × 104 ppm of Cl−, 140 ppm of HCO3−, pH 8, 20−25 °C, 24 h; ICPOES (inductively coupled plasma optical emission spectrometry) at λU 367.007 nm. To prepare U-spiked (5−7 ppm U) simulated seawater, 17 mg of UO2(NO3)2·6H2O, 25.6 g of NaCl, and 193 mg of NaHCO3 were dissolved in DI water to make a 1 L solution.23,24 A mass of 15.0 ± 1.0 mg fibers was shaken in 250 or 750 mL of simulated seawater 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 instrument 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, respectively. Repeated U uptake experiments were performed with reproducible results, and averaged values are presented in the tables. 2.6. Reusability Study in U-Spiked Simulated Seawater. After each cycle of uranium adsorption, uranium was eluted from the fibers according to a procedure reported by Pan et al.25 During the elution, the fiber was stirred at room temperature for 1 h in 10.0 mL of a H2O2 (0.1 M), Na2CO3 (1 M) solution.25 After the elution, the fiber was rinsed with deionized water until neutral pH and used directly in the next adsorption cycle.

3. RESULTS AND DISCUSSION Three types of trunk polymers were used in this study, namely, round PP fiber, hollow-gear PP (HGPP) fiber, and hollow-gear polyethylene (HGPE) fiber. In general, hollow-gear and nonround shaped fibers (0.24−30 μm diameter) have 2−60 times higher surface area than the 20 μm diameter round fiber. Four reaction steps were used for the preparation of adsorbent fibers from trunk polymers: (1) chlorination of PP fibers, forming active chlorine sites; (2) simultaneous ATRP grafting of a ligand-forming monomer (i.e., AN) and a hydrophilicityyielding monomer (i.e., tBA) from active chlorine sites; (3) amidoximation (AO) to convert nitriles on grafted PAN to amidoximes; and (4) KOH treatment to hydrolyze PtBA and unreacted PAN, if any, on grafted fibers to carboxylates, rendering hydrophilicity in adsorbent fibers (see Figure 1). It is 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).26 During chlorination, chlorine gas was generated in situ from the reaction of NaOCl and HCl. Under UV irradiation or sunlight, chlorine free radicals, capable of homolytically cleaving and substituting C−H bonds in PP fiber, were generated. The C

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Industrial & Engineering Chemistry Research incorporation of chlorine is necessary for ATRP, and the chemistry eliminates the RIGP step. The active, chlorinated sites then acted as ATRP initiation sites for grafting functional polymers in the next step. 3.1. Uranium Adsorbents from Round PP Fiber. Chlorination on round PP fiber was performed for various lengths of time under either UV irradiation or sunlight, resulting in chlorinated PP (CPP) round fiber. Overall, chlorination under UV yielded more weight percent Cl (gravimetric weight percent) than chlorination under sunlight (Figure 2). As expected, longer chlorination time resulted in

Figure 3. 100 MHz 13C CP/MAS NMR spectra of CPP round fibers (spinning speed, 7.5 kHz).

centered at 75−81 ppm, ascribed to −C(CH3)Cl− moieties was also observed. Very likely the chlorination occurred on 3° carbons first, resulting in −C(CH3)Cl−, followed by the monochlorination on 2° carbons, resulting in −CHCl− (66 ppm). Finally, when another chlorine radical attacked −CHCl−, −CCl2− (105 ppm) formed. In comparison between sunlight and UV-chlorination, UV was found to be more effective in chlorinating PP, consistent with weight percent Cl results. Under UV irradiation, the signal from −CCl2− was observed after 2 h. Under sunlight, it took 3 h of chlorination before the −CCl2− signal appeared (see inset of Figure 3). However, spectra of CPP from UV-chlorination also showed broad signals at more downfield positions, ca. 120−140 ppm, assignable to terminal vinyl groups (e.g., allyl chloride, CH2CH−CHCl−) also found in poly(vinyl chloride).29 These groups plausibly formed during the fiber degradation by chain scission,28 especially after a prolonged UV exposure (i.e., 2 h). In all conditions studied, no signal from monochlorination of 1° carbon (i.e., −CH2Cl at 50 ppm)30 was observed. In our previous study, ATRP grafting from PE-g-PVBC, where PVBC is poly(4-vinylbenzyl chloride), simultaneous and block copolymerization of AN and tBA were studied in comparison. Except in a special circumstance (i.e., when a short hydrophilic block was added to the graft chain terminus prepared by a simultaneous grafting of AN and tBA), the uranium uptakes from simultaneous copolymerization were much higher than those from block copolymerization. Therefore, in this study, simultaneous ATRP grafting of AN and tBA from the trunk fibers was performed (Supporting Information, Table S1). ATRP offers several advantages, including the prevention of homopolymerization, which normally occurs in radiation-induced graft polymerization (RIGP). After each ATRP reaction, a copious amount of 50 vol % methanol in water was added to the reaction mixture to precipitate any homopolymer present. No formation of free PAN or free PtBA (i.e., homopolymer) in the reaction mixture was observed. Under identical ATRP conditions (i.e., the same [tBA]/ [AN]/[RCl] ratios) from the CPP fibers, grafting from fibers chlorinated under UV yielded d.g. (49−255%) lower than those from fibers chlorinated under sunlight (127−2570%), despite higher weight percent Cl (see Table S1). Among the CPP fibers chlorinated under UV, the 1 h twice-chlorinated fiber yielded d.g. (49−255%) higher than or comparable to those from a prolonged UV exposure (55−66%). This is consistent with the

Figure 2. Weight percent Cl in CPP round fibers prepared under different chlorination conditions.

higher weight percent Cl in both cases, UV and sunlight. The maximum weight percent Cl obtained was 44.7%. When the total chlorination time was kept constant (i.e., 2 h) under UV, weight percent Cl were found to be about the same whether the sample was chlorinated twice at 1 h/each time (UV 1 h 2×) or one time for 2 h (UV 2 h). However, the latter condition produced brittle fibers, showing signs of degradation under continuous UV exposure. Elemental analysis (EA) results of CPP prepared under 3 h of sunlight (gravimetric weight percent Cl = 29.4%) showed 19.86 wt % Cl, corresponding to the chemical formula C3H5.92Cl0.33 (i.e., an average of one Cl atom per three repeating units of propylene (C3H6) in PP). Because many alkyl bromides were reported to be several times more active than the corresponding alkyl chlorides as ATRP initiators,27 brominated fiber was also examined. Brominated fiber was prepared by the bromination of round PP fiber with an excess amount of liquid Br2 (20 equiv). However, only 11.6 wt % Br was obtained, even after a 2 h bromination under UV. The unexpectedly low bromine content was plausibly caused by a simultaneous dehydrobromination during bromination.20 Further study on brominated fiber was not pursued because a low d.g. (133%) from an ATRP grafting ([tBA]/[AN]/[CuCl]/[Me6TREN]/[CuCl2]/[RBr] ratio of 1000:1000:1:1.2:0.05:1, 50 vol % EC, 65 °C, 24 h) and a low U adsorption capacity (22.8 g U/kg in 250 mL U-spiked test) were obtained from its grafted fiber. Low degrees of grafting of N-isopropylacrylamide from similar ATRP initiators, brominated isotactic PP films, were also reported (i.e., 10.6−25.0% from ATRP at 90 °C after 24−48 h).20 CPP round fibers were also characterized by 13C CP/MAS NMR which revealed that they were mostly composed of nonchlorinated PP structure (Figure 3, NMR: δ 46 (CH2), 26 (CH), and 22 (CH3)). The result in 13C NMR spectra is in agreement with the limited weight percent Cl obtained gravimetrically and with the chemical formula calculated from EA. Chlorination on PP was reported to begin at 3°, then 2°, and 1° carbons, respectively.28 In all fibers, a broad signal D

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following order: UV < sun 2 h < sun 3 h. Compared with adsorption capacities obtained from identical 250 mL U-spiked tests on conventional adsorbents, including adsorbent fiber provided by the Japan Atomic Energy Agency (JAEA) prepared by RIGP (∼20 g/kg), METSORB 16/60 (25.2 g/kg), METSORB STP (12.3 g/kg), METSORB HMRP 50 (46.3 g/kg), and Dyna Aqua (21.2 g/kg),18,19 adsorbent fibers prepared from ATRP in this study gave comparable or higher U adsorption capacities, 19.8−84.3 g U/kg. Adsorbent fibers with high U adsorption capacities in the 250 mL spiked test (i.e., adsorbents prepared from sunlightirradiated CPP fibers, nos. 6 and 8−14) were selected for the 750 mL spiked test. All adsorbent fibers tested gave U adsorption capacities higher than that of the JAEA fiber prepared by RIGP (71.5 g/kg, tested under identical conditions). Among fibers tested, the fiber grafted from sun 2 h CPP (no. 6) did not show as high adsorption capacity as the fiber grafted under identical ATRP conditions from the sun 3 h CPP fiber (no. 12). This further confirmed the result from the 250 mL spiked test. When monomer ratio ([tBA]/[AN]) was varied (see nos. 8− 14), the interaction between two factors affecting U uptake, namely, the U-binding ligand density and the hydrophilicity of the graft chains, was evident. As a result, two distinctive trends were observed in U uptake from 750 mL of spiked simulated seawater. At lower [tBA]/[AN] ratios, more AN was incorporated in the graft chains, resulting in higher U-binding ligand densities. Therefore, samples 9 and 10, with higher ligand densities, showed higher U adsorption capacities than did sample 11. On the other hand, when more tBA was incorporated, at higher [tBA]/[AN] ratios, increased hydrophilicity was rendered upon hydrolysis of tBA during KOH treatment. As a result, among samples 11−14, sample 12 showed the highest U adsorption capacity. The increase in hydrophilicity enhanced U uptake from samples 11 to 12. Further increase in hydrophilicity from samples 12−14 had an adverse effect on U adsorption capacity due to their decreased ligand density. The effect of the composition ratio on U uptake is consistent with the results from our previous work.18 Four adsorbent fibers with relatively high adsorption capacities were sent for elemental analysis (EA). All [PtBA]/ [PAN] ratios calculated from EA results were about half or lower than half that of their feed ratios, reflecting the higher reactivity ratio of AN than that of tBA, consistent with our previous work.18 The [PtBA]/[PAN] ratio optimized for highest U adsorption capacity in grafted fibers from CPP was 0.362, very close to that of grafted fibers from PVC-co-CPVC fiber that we studied (0.356). Reusability of the adsorbent fibers, nos. 9 and 10, was also examined in repeated usage in 750 mL of U-spiked simulated seawater (Figure 4). In order to enable the comparison between U adsorption capacities from two fibers with different initial adsorption capacities, the initial adsorption capacities were normalized to 100%. Then, the recovery in U adsorption capacities after each cycle was calculated, relative to the initial adsorption capacity. After each cycle of uranium adsorption, uranium was eluted with 0.1 M H2O2 in 1 M Na2CO3 according to the method reported by Pan et al.25 After the first cycle, the adsorption capacities decreased by 20−25% in both fibers. After the second and third cycles, the adsorption capacities further dropped by ca. 10% per cycle, leaving both fibers with only about 60% of their initial adsorption capacities. The decrease in adsorption capacities of these fibers was much

greater extent of degradation of chlorinated sites under a prolonged UV exposure, observed in 13C NMR spectra. A similar ATRP macroinitiator, poly(vinyl chloride) (PVC) was reported to differ from small-molecule alkyl halides, in that the chlorine atoms of PVC do not readily undergo substitution reactions under ordinary conditions.31 Similarly, Percec and coworkers reported that allyl and 3° chlorides are the only active sites capable of initiating metal-catalyzed radical polymerization from PVC and that allyl chlorides are more active initiators than the 3° chlorides.32,33 In the current study, the signal assignable to allyl chlorides was observed only in those CPP fibers prepared under UV. Furthermore, CPP fibers prepared under UV showed more intense signals of 3° chlorides (i.e., −CCl2− and −C(CH3)Cl−) than did CPP fibers prepared under sunlight. As a result, the d.g. from CPP prepared under UV are expected to be much higher than those from sunlight chlorination. It is possible that other mechanisms, including some degradative chain transfer reported on ATRP from CPP films,21 caused the unexpectedly low d.g. Among ATRP grafting from fibers chlorinated under sunlight, 3 h of chlorination gave d.g. (425−2570%) higher than those from 2 h of chlorination (127−439%). This could be due to not only the higher weight percent Cl but also the difference in their chemical structures. Neither type of CPP fibers prepared under sunlight showed any signal from allyl chlorides. Two types of 3° chlorides, −C(CH3)Cl− and −CCl2−, were observed in the spectrum of CPP from 3 h of sunlight. On the other hand, only the signal from −C(CH3)Cl− at a low intensity was observed in CPP from 2 h of sunlight. It is plausible that the 3° chlorides were the only active ATRP initiators and that their amounts played a major role in the d.g. from the CPP fibers prepared under sunlight. Percent conversions (%C) of tBA and AN were calculated from the EA results of grafted fibers (Supporting Information, Table S2; nos. correspond to sample nos. in Table S1). From the grafted fibers from CPP prepared under sunlight (nos. 10− 12), percent conversions of tBA (1.23−1.38%) and AN (2.55− 3.04%) were low, despite their high d.g. (822−1255%). From the grafted fiber from CPP prepared under UV (no. 6), % conversions of tBA (0.39%) and AN (1.23%) were even lower. A low conversion (3%) was also reported in a 24 h ATRP grafting of a similar monomer, 2-ethylhexyl acrylate (70 °C), from commercial CPP powder.21 Within the same sample, the observed percent conversion of AN was more than that of tBA, in agreement with the higher reactivity ratio of AN.18 As can be seen from the low percent conversions of tBA and AN, the initiation efficiency (IE) under these conditions was very likely low. To estimate the IE of initiation sites, which could not be directly measured on the grafted fibers, [PtBA]/ [Cl], [PAN]/[Cl], and wt graft chain/mol Cl were calculated (Table S2). The molar ratios of grafted monomers per Cl and weight of graft chain per mol of Cl were low, indicating that the majority of Cl sites were not active as initiation sites. In general, the wt graft chain/mol Cl obtained, 513−3099 g/mol, from CPP fibers were lower than those reported for P(AN-co-tBA) grafted from PVBC (i.e., 1850−8760 g/mol),18 consistent with the high initiation efficiency reported for PVBC.34 Uranium uptake of amidoximated and KOH-treated adsorbent fibers was measured in U-spiked simulated seawater (Table S1). When the adsorbent fibers prepared under identical ATRP conditions are compared (i.e., the same [tBA]/[AN]/ [RCl] ratio), U adsorption capacities in the 250 mL spiked test also followed the same trend as the d.g., increasing in the E

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the partial hydrolysis of PtBA, as observed in our study on grafted P(AN-co-tBA) from PVC-co-CPVC. After 3 h of KOH treatment at 80 °C (trace c), both signals from COO− (184 ppm) and −COOBut (176 ppm) were observed, along with 4° carbon in PtBA (82 ppm), indicating an incomplete hydrolysis of PtBA. Along with the cyclic imide dioxime signal (149 ppm), the broad shoulder from open-chain amidoxime (ca. 157 ppm) remained. This result is in agreement with the previous finding by Kang et al. that only a trace amount, if any, of cyclic imide dioxime formed from bisamidoxime after 3 h of alkaline treatment at 80 °C.38 Figure 6 shows the FTIR spectra of fiber no. 10 and resulting adsorbent fibers. As expected, the grafted fiber showed CN

Figure 4. Percent recovery of uranium adsorption capacities in fibers 9 and 10.

higher than that in the literature, ca. 3% decrease per cycle, up to four cycles.25 13 C CP/MAS NMR spectra of representative fibers (no. 13, and corresponding AO and KOH-treated fibers) are shown in Figure 5. In the grafted fiber (trace a) signals from −COOBut

Figure 6. FTIR spectra of fibers as (a) grafted, (b) amidoximated, (c) KOH-treated, and (d) fibers after four cycles of adsorption.

(2243 cm−1) and CO stretches (1719 cm−1) from grafted PAN and grafted PtBA, respectively. After AO, the CN stretch disappeared, indicating the completion of the AO reaction, and new bands assigned to imine CN (1640 cm−1) and N−O (923 cm−1) stretches of the oxime were observed. Also, the CO stretch shifted to 1701 cm−1, plausibly reflecting the change in its adjacent groups in polymer grafted chains which were converted from nitriles to amidoximes. After KOH treatment, a new band ascribed to CO in COO− (1555 cm−1) was observed, confirming the result from 13C NMR that the hydrolysis of PtBA occurred. The CO stretch (1701 cm−1) in PtBA remained, indicating some PtBA was not hydrolyzed, which agrees well with the result in 13C NMR. More importantly, CN (1645 cm−1) and N−O (922 cm−1) stretches of the oxime were still observed. The preservation of N−O stretch during KOH is critical because this bond has been reported to chelate the uranyl ion in the η2 fashion, which may lead to selective binding. After four cycles of adsorption, the CO stretch from PtBA was no longer observed. The CN, CO in COO−, and N− O stretches were still observed, although the relative intensity of the N−O stretch slightly decreased. This may indicate a partial decomposition of the amidoxime-type ligands, which may account for the decrease in U adsorption capacities after four cycles of usage. SEM images of the PP fiber, and fiber no. 10 after AO, after KOH, and after four cycles of U adsorption, were taken to ensure the integrity of the fibers (Supporting Information, Figure S1). In comparison to the smooth surface of the PP fiber, rough surfaces were observed on the fibers after AO, KOH, and four cycles of U adsorption, indicating the presence

Figure 5. 100 MHz 13C CP/MAS NMR spectra of (a) grafted, (b) amidoximated, and (c) KOH-treated fibers (* denotes spinning sideband; spinning speed, 7.5 kHz).

in PtBA (173 ppm), −CN in PAN (120 ppm), 4° carbon in PtBA (82 ppm), −CH2− in grafted chains and in CPP (42 ppm), and −CH3 in PtBA, overlapping with CH and −CH3 in CPP (29 ppm), were observed. After AO (trace b), a new signal (149 ppm) assigned to cyclic imide dioxime appeared, accompanied by a broad and obscure shoulder (ca. 157 ppm) assigned to open-chain amidoxime, formed in a small amount.35,36 In agreement with literature reports, the cyclic imide dioxime is expected as the major product from AO reaction in aqueous solutions at elevated temperatures.35−39 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, due to the high reactivity ratio of AN (compared to tBA), occurred. Around 120 ppm, the −CN signal was no longer observed, indicating the completion of AO reaction. Other signals, except that of CH and −CH3, were broader, indicating a more heterogeneous environment in the fiber. Accompanied with the downfield shift of the −COOBut signal to 176 ppm, this very likely indicated F

DOI: 10.1021/acs.iecr.5b03667 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research of graft chains on the fibers. The fiber after AO showed the largest average diameter, ca. 30.4 ± 4.7 μm. The average diameters of the fibers after KOH and after four cycles of U adsorption were about the same, 18−19 ± 3 μm, still larger than the average diameter of PP fiber (11.6 ± 1.9 μm), confirming the presence of graft chains. The decrease in the diameter of the fibers after KOH may be due to the hydrolysis of PtBA. After four cycles of adsorption, no damage to the surface of the fiber was observed. The decrease in U adsorption capacities after four adsorption cycles was very likely due to the partial decomposition of amidoxime-type ligands and not the physical deterioration of the fiber. Further work to improve the uranyl elution method for this type of adsorbent fibers is in progress. 3.2. Uranium Adsorbents from Hollow-Gear PP (HGPP) Fiber. Despite the higher surface area in HGPP fiber, the maximum weight percent Cl gained after chlorination was only 4.3−5.4 wt % (conditions, UV 1 h). Chlorination of HGPP under a more rigorous condition (i.e., UV 1 h, followed by UV 2 h) resulted in a weight loss of the fiber. ATRP of chlorinated HGPP was not performed because of its limited amount of chlorination. 3.3. Uranium Adsorbents from Hollow-Gear PE (HGPE) Fiber. Chlorination of HGPE was done under UV irradiation, generating chlorinated HGPE (CHGPE) fibers. Weight percent Cl from 1 h and 2 h of chlorination were plotted along with the results from CPP round fibers (from Figure 2) for comparison (see Figure 7). Overall, weight

Figure 8. 100 MHz 13C CP/MAS NMR spectra of CHGPE fibers (spinning speed, 7.5 kHz).

not be clearly distinguished from the spectrum’s baseline. This is consistent with the lower weight percent Cl obtained after 2 h of chlorination, compared to 1 h chlorination. The disappearance of dichlorinated carbon after 2 h of chlorination was very likely due to its decomposition under prolonged UV exposure. Because CHGPE prepared by 1 h of chlorination had higher weight percent Cl than that from 2 h of chlorination, it was used as an ATRP initiator for simultaneous grafting of AN and tBA (Table 1). The d.g. obtained, though reasonably high, were Table 1. Simultaneous ATRP Grafting of AN and tBA from CHGPE Fibers and U Uptake in U-Spiked Simulated Seawater 750 mL Uspiked test chlorination conditions

wt % Cl

solvent

UV 1 h

33.7

EC

UV 1 h

33.7

DMSO

[tBA]/[AN] (feed)a

d.g. (%)

g/kg

% ads

246:492 or 0.500 310:492 or 0.630

545

80.8

22.9

367

64.6

18.5

a

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

not as high as those from CPP round fibers. U adsorption capacities were also lower, in agreement with their lower d.g., than those from CPP fibers. Overall, even though HGPE possesses surface area that is higher than that of PP round fiber, the weight percent Cl in CHGPE was found to be limited, similar to the round CPP. The d.g. and U adsorption capacities of grafted fibers from CHGPE were lower than those from round CPP. The higher surface area of CHGPE did not yield the expected advantage, maybe because of inaccessible surface area and nonoptimized conformation.

Figure 7. Weight precent Cl in chlorinated hollow-gear PE (CHGPE) compared with chlorinated PP (CPP) round fibers.

percent Cl in CHGPE was found to be of the same magnitude with those in CPP fibers. Effective chlorination of HGPE occurred rapidly, within 1 h, yielding 33.7 wt % Cl (gravimetrically), which was confirmed by the elemental analysis result, 31.60 wt % Cl. The EA result corresponds to the chemical formula C2H1.58Cl0.38 (i.e., an average of ca. one Cl atom per two repeating units of ethylene (C2H4) in PE). Unlike the CPP, prolonged chlorination, did not increase the weight percent Cl, very likely due to side reactions (e.g., chain scission). 13 C CP/MAS NMR spectra of CHGPE fibers are shown in Figure 8. The UV 1 h CHGPE showed signals from PE (33 ppm), monochlorinated (66 ppm), and dichlorinated carbons (98 ppm). The UV 2 h CHGPE showed mainly the PE and monochlorinated carbons. The amount of dichlorinated carbon was minimal, if any, and its signal, expected at 98 ppm, could

4. CONCLUSION A new class of trunk fibers were prepared via chlorination of PP and HGPE fibers under the in situ generated chlorine gas. In contrast, the HGPP fiber could not be chlorinated in high degree and was not used as an ATRP initiator. Among CPP and CHGPE trunk fibers, the following are listed in order of resulting U adsorption capacities: sun 3 h CPP > sun 2 h CPP > UV CPP ∼ UV CHGPE. The maximum adsorption capacity of 146.6 g U/kg with 45.5% adsorption (in 750 mL of U-spiked G

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amidoxime and imidoxime functional groups. Sep. Sci. Technol. 1983, 18, 307−339. (4) Bordwell, F. G.; Ji, G. Z. Equilibrium acidities and homolytic bond dissociation energies of the H−O bonds in oximes and amidoximes. J. Org. Chem. 1992, 57, 3019−3025. (5) Kawai, T.; Saito, K.; Sugita, K.; Kawakami, T.; Kanno, J.-I.; Katakai, A.; Seko, N.; Sugo, T. Preparation of hydrophilic amidoxime fibers by cografting acrylonitrile and methacrylic acid from an optimized monomer composition. Radiat. Phys. Chem. 2000, 59, 405−411. (6) Tamada, M. Current Status of Technology for Collection of Uranium from Seawater. http://nuclearinfo.net/twiki/pub/Nuclearpower/ WebHomeAvailabilityOfUsableUranium/2009_Tamada.pdf. 2009 (7) Coessens, V.; Pintauer, T.; Matyjaszewski, K. Functional polymers by atom transfer radical polymerization. Prog. Polym. Sci. 2001, 26, 337−377. (8) Shipp, D. A.; Wang, J. L.; Matyjaszewski, K. Synthesis of acrylate and methacrylate block copolymers using atom transfer radical polymerization. Macromolecules 1998, 31, 8005−8008. (9) von Werne, T.; Patten, T. E. Preparation of structurally welldefined polymer−nanoparticle hybrids with controlled/living radical polymerizations. J. Am. Chem. Soc. 1999, 121, 7409−7410. (10) Grubbs, R. B.; Hawker, C. J.; Dao, J.; Frechet, J. M. J. A tandem approach to graft and dendritic graft copolymers based on ’’living’’ free radical polymerizations. Angew. Chem., Int. Ed. Engl. 1997, 36, 270− 272. (11) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Polymer brushes via surface-initiated polymerizations. Chem. Soc. Rev. 2004, 33, 14−22. (12) Zhao, B.; Brittain, W. J. Polymer brushes: surface-immobilized macromolecules. Prog. Polym. Sci. 2000, 25, 677−710. (13) Zong, G. X.; Chen, H.; Qu, R. J.; Wang, C. H.; Ji, N. Y. Synthesis of polyacrylonitrile-grafted cross-linked N-chlorosulfonamidated polystyrene via surface-initiated ARGET ATRP, and use of the resin in mercury removal after modification. J. Hazard. Mater. 2011, 186, 614−621. (14) Zong, G. X.; Chen, H.; Tan, Z.; Wang, C. H.; Qu, R. J. Atom transfer radical polymerization with activators regenerated by electron transfer of acrylonitrile from silica nanoparticles, and adsorption properties of the resin for Hg2+ after amidoximation with hydroxylamine. Polym. Adv. Technol. 2011, 22, 2626−2632. (15) Zong, G. X.; Ma, J.; Chen, H.; Wang, C. H.; Ji, N. Y.; Liu, D. L. Synthesis of crosslinked polyacrylonitrile via atom transfer radical polymerization with activators regenerated by electron transfer and use of the resin in mercury removal after modification. J. Appl. Polym. Sci. 2012, 124, 2179−2186. (16) Liu, D. L.; Chen, H.; Ji, N. Y.; Tan, Z. Living radical polymerization of acrylonitrile catalyzed by copper with a high concentration of radical initiator and its application in removal of Ag(I) after modification. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 340−346. (17) 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. (18) Saito, T.; Brown, S.; Chatterjee, S.; Kim, J.; Tsouris, C.; Mayes, R. T.; Kuo, L.-J.; Gill, G.; Oyola, Y.; Janke, C. J.; Dai, S. Uranium recovery from seawater: Development of fiber adsorbents prepared via atom-transfer radical polymerization. J. Mater. Chem. A 2014, 2, 14674−14681. (19) Yue, Y. F.; Mayes, R. T.; Kim, J.; Fulvio, P. F.; Sun, X. G.; Tsouris, C.; Chen, J. H.; Brown, S.; Dai, S. Seawater uranium sorbents: Preparation from a mesoporous copolymer initiator by atom-transfer radical polymerization. Angew. Chem., Int. Ed. 2013, 52, 13458−13462. (20) Desai, S. M.; Solanky, S. S.; Mandale, A. B.; Rathore, K.; Singh, R. P. Controlled grafting of N-isoproply acrylamide brushes onto selfstanding isotactic polypropylene thin films: Surface initiated atom transfer radical polymerization. Polymer 2003, 44, 7645−7649. (21) Haloi, D. J.; Naskar, K.; Singha, N. K. Modification of chlorinated poly(propylene) via atom transfer radical graft copoly-

simulated seawater) was obtained from the sun 3 h CPP trunk fiber with the optimized [PtBA]/[PAN] ratio (EA) of 0.362. The synthesis procedure and a new class of trunk fibers offer the advantage of a production cost lower than that of RIGP, and reactions can be scaled up. This study demonstrated a novel strategy to graft uranium-adsorbing polymer chains via ATRP on readily available fibers modified by direct chlorination. Future study involves the ATRP grafting of Uselective ligands and various hydrophilic-rendering comonomers from the low-cost CPP round fiber.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03667. Simultaneous ATRP grafting of AN and tBA from CPP round fibers; uranium uptake of fibers in U-spiked simulated seawater; SEM images of PP fiber, fiber no. 10 after AO, after KOH treatment, and after four cycles of adsorption (PDF)

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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 sponsored by the U.S. Department of Energy, Office of Nuclear Energy under Contract DE-AC0500OR22725 with Oak Ridge National Laboratory, managed by UT-Battelle, LLC. The JAEA sorbent was kindly donated for testing by the Japan Atomic Energy Agency.

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ABBREVIATIONS AN = acrylonitrile AO = amidoximation ATRP = atom-transfer radical polymerization CHGPE = chlorinated hollow-gear polyethylene CHGPP = chlorinated hollow-gear polypropylene CPP = chlorinated polypropylene CP/MAS = cross-polarization/magic-angle spinning d.g. = degree of grafting EA = elemental analysis EC = ethylene carbonate PAN = polyacrylonitrile PtBA = poly(tert-butyl acrylate) tBA = tert-butyl acrylate

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REFERENCES

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DOI: 10.1021/acs.iecr.5b03667 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX