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Cationically Charged Poly(amidoxime)-Grafted Polypropylene Nonwoven Fabric for Potential Uranium Extraction from Seawater Zehua Zeng,†,§ Yanqi Wei,†,∥ Liang Shen,†,⊥ and Daoben Hua*,†,‡ †

College of Chemistry, Chemical Engineering and Materials Science & School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Suzhou 215123, China ‡ Collaborative Innovation Center of Radiological Medicine of Jiangsu Higher Education Institutions, Suzhou 215123, China

Downloaded by UNIV OF CAMBRIDGE on September 10, 2015 | http://pubs.acs.org Publication Date (Web): August 26, 2015 | doi: 10.1021/acs.iecr.5b01852

S Supporting Information *

ABSTRACT: The development of sorbent for uranium seawater extraction is critical for guaranteeing future uranium resources for nuclear energy. An amidoxime-based cationically charged fibrous sorbent is reported here for potential uranium extraction from seawater. Specially, poly(1-vinylimidazole) was grafted onto polypropylene nonwoven fabric under γ-ray irradiation and was followed by quaternization and amidoximation to give the fibrous sorbent. The sorption could reach equilibrium with a capacity of 119.76 mg U/g within 50 h at pH 8.0 and 298.15 K. Compared with the uncharged sorbent, the cationic sorbent showed faster kinetics, higher selectivity, and larger capacity for uranium extraction. This work indicates that the cationically charged poly(amidoxime)-grafted nonwoven fabric may be a potential promising sorbent to extract uranium from seawater. protein29 were also investigated as new promising efficient sorbents for uranyl extraction. Although a great progress has been achieved in the past decades, separating uranium at trace level from other more abundant metal ions is still a challenge,3,4 and it is significant to develop new materials to extract uranium with higher sorption rate and selectivity. In this study, we report a new sorbent, cationically charged poly(amidoxime)-grafted polypropylene nonwoven fabric for potential uranium extraction from seawater. Specifically, poly(1-vinylimidazole) (PVIm) was grafted onto polypropylene nonwoven fabric under γ-ray irradiation and was then quaternized with 4-bromobutyronitrile and followed by amidoximation to give the fibrous sorbent (Scheme 1). We

1. INTRODUCTION Nowadays increasing energy demands and uncertainty in fossil fuel promote the development of nuclear energy. Uranium supply has received more and more attention for nuclear energy before uranium resource becomes scarce on land. The oceans, where uranium is found at about 4.5 billion tons (i.e., 1000 times of uranium on land), present an important alternative source of uranium in the future.1−3 Therefore, the development of sorbents for uranium seawater extraction is critical for guaranteeing future uranium resources.4 Over several decades, continuous progress has been made to recover uranium from seawater, such as inorganic adsorbents,1,5−7 chelating polymers,8−11 and mesoporous materials.12−14 Among them, inorganic adsorbents were used for uranium extraction by electrostatic interaction,1 which showed good affinity but poor mechanical strength and selectivity. Considering simplicity of operation, operating cost, and environmental risk, chelating synthetic polymers appear to be the promising sorbents for uranium extraction from seawater. Especially poly(amidoxime)-based fibrous sorbents are most widely studied for uranium recovery from seawater by coordinating interaction due to high capacity and affinity in chelating uranyl ions.15−23 For instance, amidoxime-based polymeric adsorbent developed at the Oak Ridge National Laboratory has an adsorption capacity up to 3.3 mg/g.24 It is noticed that through radiation-induced graft polymerization, poly(amidoxime) can be easily grafted onto various fibrous materials for uranium extraction. For example, Li et al.25grafted poly(amidoxime) onto polyethylene nonwoven fabric by preirradiation-induced emulsion graft polymerization for adsorption of the uranyl ions; and Kawai et al.26 prepared hydrophilic poly(amidoxime)-based fibers by the radiationinduced cografting of methacrylic acid with acrylonitrile onto polypropylene fibers for uranium extraction from seawater. Recently mesoporous materials,13,14 metal organic frameworks (MOFs),27 layered metal sulfides,28 and engineered © 2015 American Chemical Society

Scheme 1. Schematic for Synthesis of Cationically Charged Poly(amidoxime)-Grafted Nonwoven Fabric (PP-gPVIm+Br−AO)

Received: Revised: Accepted: Published: 8699

May 19, 2015 July 14, 2015 July 24, 2015 August 26, 2015 DOI: 10.1021/acs.iecr.5b01852 Ind. Eng. Chem. Res. 2015, 54, 8699−8705

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Industrial & Engineering Chemistry Research

the nonwoven fabrics with different degree of grafting were obtained. 2.4. Synthesis of PP-g-PVIm+Br−AO. After graft polymerization, PP-g-PVIm was quaternized by refluxing the mixture of PP-g-PVIm and 4-bromobutyronitrile according to the literature method.34 Specifically, the PP-g-PVIm (3.6 g, DG = 147.0%) was reacted with 4-bromobutyronitrile (6.75 g, 45.6 mmol) in 50 mL of DMF for 20 h at 60 °C. The product was collected by filtration and washed with DMF (80 mL) and acetone (80 mL) three times and dried in a vacuum oven at 40 °C to give PP-g-PVIm+Br−. PP-g-PVIm+Br− was then reacted with hydroxylamine to convert acrylonitrile group to amidoxime (AO) group.35 Specifically, PP-g-PVIm+Br− (3.6 g, DG = 147.0%) was reacted with NH2OH·HCl (4.75 g, 68.4 mmol) in 160 mL of methanol/water (v:v, 1:1) for 12 h at 60 °C. The nonwoven fabric was then washed with deionized water (80 mL) and subsequently with methanol (80 mL) three times and finally dried in a vacuum oven at 40 °C to a constant weight. The sorbents (20.0 mg) were immersed in NaOH solution (0.625 mol/L, 10 mL) at room temperature for 1.5 h and repeatedly washed three times with deionized water (10 mL) prior to uranium sorption. 2.5. Uranium Sorption Experiments. Sorption experiments were carried out in wide-mouth plastic bottles. Every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3 to simulate seawater.36 The sorption procedure was described as follows: the sorbent (20.0 mg) and uranyl ion solution (0.03 mmol/L, 500 mL) were placed in bottles and shaken for enough time to achieve sorption equilibrium. The pH of solution was adjusted by HCl and Na2CO3 solutions to guarantee that uranium(VI) is presented as the uranyl tricarbonate complex ([UO2(CO3)3]4−).37 The concentrations of uranyl ions before and after sorption were determined by ICP-MS. The sorption equilibrium amount (qe) and the percentage removal (AE) of uranyl ions were calculated according to eq 2 and eq 3, respectively:


noticed that the chelating hydrogel was developed by random copolymerization of 1-vinylimidazole and acrylonitrile under γray irradiation and then by amidoximation of the −CN group;30 however, the hydrogel was obtained with amidoxime groups but without positive charges. On the other hand, it is known that uranium(VI) in seawater is present principally as the uranyl tricarbonate complex ([UO2(CO3)3]4−).31,32 During the sorption process, the anionic complex might be pulled to coordinate with amidoxime group by Coulombic interaction of positive charges while the other cations are repelled. Therefore, the cationically charged amidoxime-based sorbent was expected to extract uranium with fast kinetics and high selectivity.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. 1-Vinylimidazole (VIm, Sinopharm Chemical Reagent Co., Ltd., CP) was distilled under reduced pressure to eliminate the inhibitor and stored at −20 °C prior to use. 4-Bromobutyronitrile (97%) and hydroxylamine hydrochloride (NH2OH·HCl, 99%) were purchased from J&K Chemical Co., Ltd. Acrylonitrile (AN, Sinopharm Chemical Reagent Co., Ltd., CP) was purified by vacuum distillation before use. PP nonwoven fabric (85 g/m2) was kindly provided by Kingway Complex Material Co., Ltd. (Nantong, China). Uranium(VI) solution was prepared with dissolving uranyl nitrate (UO2(NO3)2·6H2O, Fluka, AR). All other reagents and solvents were used as received. 2.2. Characterization Methods. Fourier transform infrared (FT-IR) spectra were recorded on a Varian-1000 spectrometer. Malvern Zetasizer (632.8 nm, He−Ne laser) was used to determine the ζ potential of the cationically charged sorbent. X-ray photoelectron spectroscopy (XPS) was carried out by ESCALAB 250Xi XPS. Field-emitting scanning electron microscopy (FE-SEM) images were performed by using a HITACHI S-570 microscope with an accelerating voltage of 15 kV, and energy-dispersive X-ray (EDX) analysis was carried out with an EDAX-PV 9100 energy dispersion Xray fluorescence analyzer. The concentration of uranyl ions was determined by thermo high resolution inductively coupled plasma mass spectrometry (ICP-MS, Element II). 2.3. Synthesis of PP-g-PVIm. PP-g-PVIm was synthesized through radiation-induced graft polymerization.33 Specifically, PP nonwoven fabric (0.2 g, 0.025 g/mL) was immersed in 8.0 mL of acetone solvent including VIm (0.8 g, 0.1 g/mL) in a 10 mL ampule. After the contents were purged with argon for 20 min to eliminate oxygen, the ampules were flame-sealed. Then the ampules were placed in an insulated room with a 60Co source at the dose rate of 15 Gy/min, which was determined by Fricke dosimeter accurately. After the graft polymerization, the nonwoven fabric was washed with N,N′-dimethylformamide (DMF, 50 mL) and subsequently with acetone (50 mL) three times to remove homopolymer and finally dried in a vacuum oven at 40 °C to a constant weight. The degree of grafting (DG, %) of PVIm was determined by the increase in the weight of nonwoven fabric after graft polymerization, as described in eq 1: DG (%) =

Wg − W0 W0

× 100

qe = (C0 − Ce) AE (%) =

V M

C0 − Ce × 100 C0

(2)

(3)

where V (L) is the volume of solution and M (g) is the weight of the dry sorbent. C0 and Ce (mg/L) are the uranium concentrations in solution before and after sorption, respectively. The effects of contact time and coexisting ions on uranium(VI) sorption were studied. Kinetic studies were conducted with 0.03 mmol/L of uranium(VI) at pH 8.0 and 298.15 K, and the isotherm experiment was performed with different uranium concentrations at pH 8.0 and 298.15 K. Considering the concentration of uranium in seawater is very low, we also test the cationic sorbent with low concentration of uranium solution (1.7 ppb). 2.6. Desorption and Regeneration Studies. To evaluate the stability and reusability, the sorption of uranium(VI) and regeneration of the sorbent was performed in five consecutive cycles. In every cycle, the sorbent (20.0 mg) was shaken with 500 mL of uranyl ion solution (0.03 mmol/L). After efficient sorption, the sorbent was eluted by HCl solution (0.1 mol/L, 45 mL) for 2.0 h and then washed with deionized water (50 mL) three times for reusing.

(1)

where W0 and Wg (g) are the weights of the fibers before and after graft polymerization, respectively. The influences of radiation dose and monomer concentration on degree of grafting were studied in graft polymerization, and 8700

DOI: 10.1021/acs.iecr.5b01852 Ind. Eng. Chem. Res. 2015, 54, 8699−8705

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3. RESULT AND DISCUSSION 3.1. Characterization of PP-g-PVIm+Br−AO. In order to prepare PP-g-PVIm+Br−AO, PVIm was first grafted onto nonwoven fabric through radiation-induced graft polymerization (Scheme 1). The effects of radiation dose and monomer concentrations on the polymerization were investigated (Figure S1): the higher radiation dose and monomer concentration are, the larger is degree of grafting. The PP-g-PVIm samples with different degrees of grafting (such as 90.0% and 147.0%) were chosen for the following experiments. PP-g-PVIm was then quanternized with 4-bromobutyronitrile and followed by amidoximation to give the cationic sorbent. The morphologies of the outcomes in this process were characterized by FE-SEM images. As can be seen from Figure 1,


Figure 2. FTIR spectra of (a) PP nonwoven, (b) PP-g-PVIm, (c) PPg-PVIm+Br−, and (d) PP-g-PVIm+Br−AO.

Figure 3. (A) XPS spectra of (a) PP nonwoven, (b) PP-g-PVIm, (c) PP-g-PVIm+Br−, and (d) PP-g-PVIm+Br−AO and XPS spectra of (B) C 1s and (C) N 1s of PP-g-PVIm+Br−AO.

Figure 1. FE-SEM images of (A) PP nonwoven, (B) PP-g-PVIm (DG = 147.0%), (C) PP-g-PVIm+Br−, and (D) PP-g-PVIm+Br−AO. Scale bar: 50.0 μm.

to show elements present in PP nonwoven, PP-g-PVIm, PP-gPVIm+Br−, and PP-g-PVIm+Br−AO, respectively. In comparison with bare nonwoven fabric (Figure 3A, trace a), PP-g-PVIm showed the new peak for N (1s) element which indicated the successful grafting of PVIm on the surface of nonwoven fabric. It was observed that Br (3p) and Br (3d) peaks appeared on the XPS survey scan for PP-g-PVIm+Br− (Figure 3A, trace c), and the new peak for O (1s) element was clearly shown for PP-gPVIm+Br−AO (Figure 3A, trace d), indicating the successful quaternization and amidoximation reactions. For PP-gPVIm+Br−AO, XPS spectrum of C 1s could be curve-fitted with four peak components attributed to CN, C−N, C−C, and CC species (Figure 3B) while N 1s to C−N, CN, and N−O species (Figure 3C), which further demonstrated that the sorbent PP-g-PVIm+Br−AO was successfully prepared. 3.2. Effects of Positive Charge on Uranium Sorption. The cationic sorbent with amidoxime groups was used for uranium(VI) sorption. It is expected that [UO2(CO3)3]4− would be pulled by Coulombic interaction to coordinate with the ligand group while the other metal cations were repelled, thereby leading to a highly fast and selective sorption. To investigate the role that positive charge played in uranium sorption, the sorption kinetic experiments were performed at pH 8.0 and every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3. The results were shown in Figure 4. For comparison, the sorbent without positive charge (PP-gPAO) was also prepared for the sorption (section “Synthesis of PP-g-PAN and PP-g-PAO”, Supporting Information), and the

the PP fibers were not damaged during the reaction process. It was also noticed that the average diameters of fibers increased markedly after graft polymerization (Figure 1B) and quaternization (Figure 1C) in comparison with bare nonwoven fabric (Figure 1A). The result was related to the increasing degree of grafting (such as PVIm and 4-bromobutyronitrile) onto nonwoven fabric. The chemical structures of the grafted polymers were confirmed by FT-IR spectra (Figure 2). In comparison with bare nonwoven fabric (Figure 2, trace a), the characteristic bands occurred for PVIm of PP-g-PVIm (Figure 2, trace b). For example, the stretching vibrations of CC (1650 cm−1), C N (1500 cm−1), C−N (1290 and 1228 cm−1), and imidazolyl ring (1103 and 915 cm−1) suggested the successful graft of PVIm onto nonwoven fabric.38 For PP-g-PVIm+Br−, the stretching vibrations of CN, C−N, and imidazolyl ring shifted to 1552, 1453 and 1158 cm−1 (Figure 2, trace c) because of the lower electron density of imidazolyl cation.39 After reaction with hydroxylamine, the stretching vibrations of CN and N−O occurred for PP-g-PVIm+Br−AO at 1656 and 938 cm−1, respectively,35 while the characteristic band of CN almost disappeared (Figure 2, trace d) in comparison with PPg-PVIm+Br− (Figure 2, trace c). All the results indicated the successful synthesis of PP-g-PVIm+Br−AO. The chemical bond states on the surfaces further demonstrated this point. Figure 3A is the XPS survey scans 8701


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concentrations in seawater,34 and every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3. Distribution ratio (Kd) of imprinted and nonimprinted composites was calculated by eq 4: Kd =

(4)

where C0 and Ce (mg/L) are the concentrations of uranium(VI) in solution before and after sorption, respectively, V (L) is the volume of solution, and M (g) is the weight of the dry sorbent. The uranium uptake and the selective sorption parameters are summarized for PP-g-PVIm+Br−AO (DG = 147.0%) and PP-g-AO (DG = 48.7%) in Table 2. The distribution ratios (Kd) of PP-g-PVIm+Br−AO are far larger than that of the sorbent without positive charge, which suggests that the sorbent with positive charge has higher sorption selectivity. The result may be attributed tot positive charge that makes the sorbents repel the other metal cations, which improves the selectivity of PP-g-PVIm+Br−AO on uranium extraction. 3.3. Sorption Isotherm. The maximum sorption capacity is one of the most important factors of the sorbent. To evaluate the capacity of the cationic sorbent, the experiments of sorption isotherm were conducted at 298.15 K and pH 8.0 with uranium(VI) concentration over the range of 6.62 × 10−6 mol/ L to 1.155 × 10−4 mol/L, and every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3. As shown in Figure 5, the sorption capacities of PP-g-PVIm+Br−AO (DG = 147.0%) was much higher than PP-g-PAO (DG = 48.7%). The equilibrium data were applied to Langmuir and Freundlich isotherm models (Figure S5, Support Information). The isotherm parameters are summarized in Table 3. The Langmuir model fits the experimental data better than Freundlich model for the two kinds of sorbent. It is noted that the sorption capacity comes to 119.76 mg U/g with PP-gPVIm+Br−AO (DG = 147.0%), which is much larger than that of PP-g-PAO (28.93 mg U/g). The result may be attributed to the cooperative interaction between positive charge and amidoxime group. 3.4. Test with Low Concentration Uranium Solution. Considering the concentration of uranium in seawater is very low, we tried the sorption experiments in simulated seawater with low uranium concentration. The experiments were conducted at 298.15 K and pH 8.0 with uranium(VI) concentration of 1.7 ppb, and every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3. The result was depicted in Figure 6. It can be found that even at low uranium concentration (1.7 ppb), the cationic sorbent also shows higher sorption capacity and higher sorption rate in comparison with uncharged sorbent. The results showed the cationic sorbent

Figure 4. (A) Effect of contact time on the sorption of uranium(VI) by (a) PP-g-PVIm+Br−AO (DG = 147.0%), (b) PP-g-PVIm+Br−AO (DG = 90.0%), and (c) PP-g-PAO (DG = 48.7%). Experiment conditions: 20.0 mg sorbent dose, 500 mL solution, 0.03 mmol/L uranium(VI), pH 8.0, and 298.15 K. Every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3. (B) FE-SEM image PP-gPVIm+Br−AO after uranium extraction.


C0 − Ce V × Ce M

functional group densities (FGD) of two materials are similar by calculation (Supporting Information, page S3). And the ζ potential of PP-g-PAO was −9.51 mV, while that of PP-gPVIm+Br−AO was 14.5 mV (Figures S2 and S3, Supporting Information). The sorption amount (qt) of PP-g-PVIm+Br−AO could reach equilibrium within 50 h (Figure 4A, traces a and b), while it is more than 80 h for PP-g-PAO (Figure 4A, trace c). And the sorption amount of PP-g-PVIm+Br−AO was much higher than that of PP-g-PAO. From Figure 4A (a and b), we could find that the larger degree of grafting, the higher sorption capacity. The sorbent−uranium complex was characterized by FE-SEM image and EDX spectrum. The crystal appeared on the surface of the PP nonwoven sorbent (Figure 4B), suggesting uranium(VI) was combined onto PP-gPVIm+Br−AO. The sorption kinetics of the sorbents above was simulated with two common semiempirical kinetic models: the pseudofirst-order40 and pseudo-second-order models41,42 (Figure S4, Supporting Information). The kinetics parameters such as kinetic rate constants, correlation coefficients, sorption equilibrium amount (qe) are shown in Table 1. Through comparison of the correlation coefficients, the pseudo-firstorder and pseudo-second-order model are both suitable to describe the sorption kinetic profiles of uranium(VI) on PP-gPVIm+Br−AO, while pseudo-first-order model is suitable for PP-g-PAO. Importantly, the kinetic constants of PP-gPVIm+Br−AO are larger than PP-g-PAO, suggesting that PPg-PVIm+Br−AO has faster kinetics and rapid sorption, which may be attributed to the synergistic interaction: the negative uranyl complex can be pulled to coordinate with amidoxime group by Coulombic interaction of positive charges. In order to test the selectivity of cationic sorbent, the experiments were conducted with the other coexisting ions (such as Mg2+, Ca2+, K+, SO42−, Br−, and BO33−) at the same

Table 1. Kinetic Parameters for the Sorption of Uranium(VI) by (a) PP-g-PVIm+Br−AO (DG = 147.0%), (b) PP-gPVIm+Br−AO (DG = 90.0%), and (c) PP-g-PAO (DG = 48.7%)a pseudo-first-order sorbent a b c

qe,exp (mg/g) 86.743 60.820 18.661

k1 (h−1) 0.0473 0.0472 0.0198

pseudo-second-order R2

qe,cal (mg/g) 78.03 61.13 18.64

0.9896 0.9950 0.9876

k2 (g mg−1 h−1) −4

5.669 × 10 4.279 × 10−4 3.592 × 10−4

qe,cal (mg/g)

R2

104.82 83.612 32.938

0.9951 0.9797 0.8440

a

Experiment conditions: 20.0 mg sorbent dose, 500 mL solution, 0.03 mmol/L uranium(VI), pH 8.0, and 298.15K. Every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3. 8702


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Industrial & Engineering Chemistry Research Table 2. Selective Sorption of Uranium(VI) on PP-g-PVIm+Br−AO (DG = 147.0%) and PP-g-AO (DG = 48.7%)a uranium uptake (mg/g) sample no.

salt added

1 2 3 4 5 6 7

MgCl2 Na2SO4 CaCl2 KCl KBr H3BO3

concn (mol/L) 5.2 2.7 9.9 9.7 8.0 4.0

× × × × × ×

distribution ratio (Kd), L/g

C0U (mg/L)

PP-g-PVIm+Br−AO

PP-g-AO

PP-g-PVIm+Br−AO

PP-g-AO

6.614 7.462 6.851 7.214 7.320 6.720 6.831

75.002 45.479 62.121 49.515 67.722 43.894 67.856

12.931 7.633 4.710 1.703 10.326 11.386 11.780

11.396 6.095 8.976 6.898 9.296 6.532 9.934

1.955 1.023 0.681 0.236 1.411 1.694 1.725

10−2 10−2 10−3 10−3 10−4 10−4

a

Experiment conditions: 500 mL solution, 0.03 mmol/L uranium(VI), a certain amount of various ions, 20 mg sorbent dose, pH 8.0, and 298.15 K. Every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3.


(0.1 mol/L) was used as the desorbing agent, and the reusability of the composites was evaluated in five sorption/ desorption cycles. As shown in Figure 7A, the sorption capacity

Figure 5. Sorption isotherm plots for the sorption of uranium(VI) by (a) PP-g-PVIm+Br−AO (DG = 147.0%) and (b) PP-g-PAO (DG = 48.7%). The sorbents had similar functional group density (6.2 × 10−3 mmol/g). Experiment conditions: 20.0 mg sorbent dose, 500 mL solution, pH 8.0, and 298.15 K. Every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3.

Figure 7. (A) Recycling of PP-g-PVIm+Br−AO (DG = 147.0%) in the uranium sorption and (B) the FE-SEM image of the sorbent regenerated after five cycles. Experiment conditions: 20.0 mg sorbent dose, 500 mL solution, 0.03 mmol/L uranium(VI), pH 8.0, and 298.15K.

Table 3. Langmuir and Freundlich Parameters for Uranium(VI) Sorption by (a) PP-g-PVIm+Br−AO (DG = 147.0%) and (b) PP-g-PAO (DG = 48.7%)a Langmuir sorbent

qmax (mg/g)

b (L/mg)

a b

119.76 28.93

0.289 0.107

Freundlich R2

KF (mol1−n Ln/g)

n

R2

0.9874 0.9801

26.863 3.265

1.975 1.598

0.8120 0.9227

did not decrease obviously after five cycles and could still reach 75 mg/g. The FE-SEM image showed that the sorbent was stable after five cycles (Figure 7B). The results indicated that the sorbent could be efficiently regenerated and reused with high sorption capacity after five cycles, which may enhance the economy of the sorption process.

The sorbents had similar functional group density (6.2 × 10−3 mmol/ g). Experiment conditions: 20.0 mg sorbent dose, 500 mL solution, pH 8.0, and 298.15K. Every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3. a

4. CONCLUSION In summary, we report a new sorbent, cationically charged poly(amidoxime)-grafted nonwoven fabric, for potential uranium extraction from seawater. Specifically, poly(1-vinylimidazole) was grafted onto polypropylene nonwoven fabric under γ-ray irradiation and then quanternized and amidoximed to give the sorbent. The effects of degree of grafting, contact time, coexisting ions, and initial concentration on sorption of uranium were studied. The sorption capacity was closely related to the degree of grafting of PVIm: the higher the degree of grafting content, the higher is the sorption capacity. Compared with the uncharged sorbent, PP-g-PVIm+Br−AO showed faster kinetics, higher selectivity, and larger sorption capacity for uranium extraction. The sorption could reach equilibrium with a capacity (qmax) of 119.76 mg U/g within 50 h at pH 8.0 and 298.15 K, and PP-g-PVIm+Br−AO could be efficiently regenerated and reused with high sorption capacity after five cycles. To the best of our knowledge, this is the first multifunctional nonwoven fabric with positive charges and ligand groups. The functional material can be used for the extraction of uranium tricarbonate complex with higher selectivity and faster kinetics,

Figure 6. Sorption of uranium(VI) by PP-g-PVIm+Br−AO (DG = 147.0%) and PP-g-PAO (DG = 48.7%). The sorbents had similar functional group density (6.2 × 10−3 mmol/g). Experiment conditions: 20.0 mg sorbent dose, 500 mL solution, C0U = 1.7 ppb, pH 8.0, and 298.15 K. Every sample contained 0.438 mol/L NaCl and 2.297 mmol/L NaHCO3.

may be used as a promising sorbent for potential uranium extraction from seawater. 3.5. Desorption and Regeneration Studies. It is very important to be easily regenerated and reused for an economical and effective sorbent. In this study, HCl solution 8703


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which may be used as a promising sorbent for potential uranium extraction from seawater. The concept should be applicable to the other system (such as fabrics, film, etc.) containing positive charges and other ligand groups for the sorption of negative metal complex from aqueous solution. Therefore, this work may provide a new method to prepare the multifunctional material by graft polymerization and quaternization of 1-vinylimidazole.

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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.5b01852. Sorption kinetics and isotherm; the influence on graft polymerization; FTIR spectra and ζ potentials for sorbent (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel and fax: (+) 86-51265883261. Notes

The authors declare no competing financial interest. § Z.Z.: e-mail, [email protected]. ∥ Y.W.: e-mail, [email protected]. ⊥ L.S.: e-mail, [email protected].

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grants 91326202, 21174100), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Qing-Lan Project of Jiangsu Province, and Jiangsu Key Laboratory of Radiation Medicine and Protection. We thank Prof. Zhifang Chai and Prof. Shuao Wang for helpful suggestions.

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ABBREVIATIONS poly(1-vinylimidazole) = PVIm uranyl tricarbonate complex = [UO2(CO3)3]4− VIm = 1-vinylimidazole NH2OH·HCl = hydroxylamine hydrochloride UO2(NO3)2·6H2O = uranyl nitrate DMF = N,N′-dimethylformamide AN = acrylonitrile AO = amidoxime FT-IR = Fourier transform infrared XPS = X-ray photoelectron spectroscopy FE-SEM = field-emitting scanning electron microscopy EDX = energy-dispersive X-ray ICP-MS = inductively coupled plasma mass spectrometer

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REFERENCES

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