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Polypropylene Modified with Amidoxime/Carboxyl Groups in Separating Uranium(VI) from Thorium(IV) in Aqueous Solutions Jie Xiong, Sheng Hu, Yi Liu, Jie Yu, Haizhu Yu, Lei Xie, Jun Wen, and Xiaolin Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02663 • Publication Date (Web): 17 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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ACS Sustainable Chemistry & Engineering

Polypropylene Modified with Amidoxime/Carboxyl Groups in Separating Uranium(VI) from Thorium(IV) in Aqueous Solutions Jie Xiong,†,‡ Sheng Hu,† Yi Liu,† Jie Yu,§ Haizhu Yu,§ Lei Xie,† Jun Wen,† Xiaolin Wang†,‡,* †

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, No. 64

Mianshan Road, Mianyang 621999, China ‡

College of Nuclear Science and Technology, University of Science and Technology of China,

No. 96 Jinzhai Road, Hefei 230026, China §

Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui

University, No. 111 Jiulong Road, Hefei 230601, China

*To whom correspondence should be addressed: Prof. Xiaolin Wang, E-mail address: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT The majority of U/Th reprocessing studies is hampered in the production of large amounts of radioactive waste during extraction with organic complexes from highly acidic solutions. Polypropylene grafted with amidoxime/carboxyl groups (PPAC) which exhibits selective binding towards U(VI), can help reduce organic waste. Batch studies on PPAC separating U(VI) from U(VI)/Th(IV) mixture in acidic to weak alkaline solution with complexing agent EDTA were carried out. The result suggested a much better affinity of PPAC to U(VI) over Th(IV). The pH effect study suggested the largest U/Th mass ratio at pH ~8. FTIR spectra at different pHs indicated most of the carboxyl and amidoxime functional groups were protonated at lower pH, while increasing pH, deprotonating degree increased correspondingly. X-ray diffraction analysis showed U(VI) and Th(IV) could both enter the crystal lattice, resulting in smaller diffraction angle which was associated with U(VI) and Th(IV) adsorption capacity. This work revealed polymer or other material with amidoxime/carboxyl groups is a promising candidate for deployment as a uranium adsorbent in U/Th reprocessing.

KEYWORDS: PPAC fiber, uranium extraction, uranium/thorium separation, chelating functional group, amidoxime ligands

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TOC/Abstract Graphic

Synopsis Polypropylene grafted with amidoxime/carboxyl groups combined with EDTA can help reduce the organic waste and solution acidity in U/Th separation, which provides a safer and greener method than traditional techniques.

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INTRODUCTION Radionuclides produced in nuclear waste repositories contain actinides such as uranium, thorium and other fission products.1 Selective separation of thorium and uranium from high level waste would be advantageous and desirable as a means of recycling or removing these radioactive actinides, as well as decreasing potential human health and environmental risks. Organic extractants have been playing an important role in separating uranium from thorium for a long time due to their chelating properties. Especially, phosphates/phosphonates for U(VI)/Th(IV) purification are widely utilized in the extraction and recovery of hexavalent uranyl (UO22+) and tetravalent thorium from effluents of nuclear power plant.2-6 However, a solution of phosphates/phosphonates (like tri-n-butyl phosphate, TBP) in dodecane or other inert aliphatics is often used in practice, as pure TBP alone is too viscous to operate with.7 This may lead to a massive volume of organic wastes and introduce additional issue to deal with. It is a challenging task not only extracting valuable metals from mixed solutions, but also reducing the quantity of disposal waste. Besides, the aforementioned liquid-liquid extracting system only works in highly acidic environment. To ensure operational safety, it is of great importance to separate uranium from thorium in mild conditions. Consequently, solid-phase extraction is proposed as a promising separation method for advanced nuclear fuel reprocessing.8-15 Polymer adsorbents decorated with specific functional groups exhibit a great potential for removal of heavy metal ions selectively and easy disposal at solid state. In the past few years, new adsorption materials/methods are exploited to detect or enrich uranyl from 4


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groundwater and seawater as a uranium source for nuclear energy, of which amidoxime functional material have been proved to perform excellently in selectively mining uranyl from complicated systems.16,17 The existences of thorium and uranium in solutions are complicated. For example, in aqueous solution of pH 2, Th4+ is hydrolyzed to Th(OH)3+, Th(OH)22+, Th(OH)3+, Th2(OH)44+ etc. When pH is increased to ~4, thorium precipitates as Th(OH)4.18-20 For uranium in aqueous solutions, the dimer (UO2)2(OH)22+ exists mostly in weak acidic (2 < pH < 5) media with total uranium concentrations above 10−4 mol/L, while UO2(OH)2 is the predominant species in pH range 5.1–9.7.18-20 In pH range 6–9, schoepite precipitates when the total uranium concentration exceeds 10−5.5 mol/L. In this study, ethylenediaminetetraacetate (EDTA, abbreviated as H4Y) was added into each solution for metal ion stablization, and thorium may exist as complexes of Th(HY)+, (Th(OH)Y)22-, Th(OH)Y-, Th(OH)2Y21 and uranium as UO22+, UO2H4Y2+, UO2H3Y+, UO2H2Y, UO2HY-, (UO2)2Y, UO2(OH)HY2-, (UO2)2(OH)2H2Y4-.22,23 Though a number of research on amidoxime type adsorbent has been carried out to investigate the factors that can influence the selective uranium adsorption and various opinions on the mechanism of amidoxime groups coordinated with U(VI) under different conditions are proposed,24-27 it is still not clear how amidoxime groups and other auxiliary groups work in separating U(VI)/Th(IV) in aqueous solution containing EDTA. In the present work, polypropylene unwoven fiber modified with amidoxime/carboxyl groups (PPAC) was synthesized successfully. In order to mimic the application in separation of 5



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U(VI)/Th(IV) in reality, other than most other adsorption experiments, the conditions set in this work was restricted with excess of PPAC and shortage of U(VI)/Th(IV), in the hope of getting a reasonable explanation on the behavior of amidoxime materials adsorbing Th(IV) and U(VI).

EXPERIMENTAL Materials. Polypropylene was purchased from WEIJUN nonwoven fiber company in Dongguan, China. Hydroxylamine hydrochloride, sodium hydroxide, 65% nitric acid, acrylonitrile (AN), methacrylic acid (MAA), EDTA and thorium nitrate [Th(NO3)4] all of analytical reagent grade and uranyl nitrate [UO2((NO3)2] of high purity grade were used without further purification. All aqueous solutions were prepared with deionized water with a typical resistivity >18.2 MΩ·cm (Millipore Elix5, Millipore, Billerica, MA, U.S.A.). Pure and mixed solutions of U(VI) and Th(IV) were obtained by dissolving proper amount of metal nitrates with EDTA (0.06 mmol/L) in 0.1 mol/L HNO3. Synthesis of PPAC. Polypropylene-grafted polyamidoxime/methacrylic acid (PPAC): Polypropylene (PP) was placed in the γ-ray field of

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Co, and irradiated in air

atmosphere with 70 kGy dose. The material was stored in a refrigerator (-5 ℃) for spare after irradiation. The grafting copolymerization of PP and AN/ MAA (4/1 in mass ratio) was carried out at 80 ℃ for 6 hours. Copolymerization products were washed by acetone (3 times), ethanol (3 times) and deionized water (many times) successively. After cleaning, the grafted acrylonitrile-co-methacrylic acid copolymer was converted to 6


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amidoxime-co-methacrylic acid by reacting with hydroxylamine in a near neutral aqueous solution (by mixing equal mole of NaOH and NH2OH·HCl) for 6 hours at 80 ℃. The final product PPAC was washed with a large amount of deionized water many times and dried up. The chemical structure of PPAC fiber is depicted in Fig. 1.

Figure 1. Schematic depiction of the functional groups (carboxyl, amidoxime and cyclic imidedioxime) on the modified PPAC fiber.

Separation. The PPAC adsorbent was immersed in a mixed U(VI)/Th(IV) aqueous solution of 35 mL under desired pHs in a vibrating tube. The tube was then put in a constant temperature vibrating box (35 ℃) for 44 hours at 100 r/min. Accurate metal concentration measurement by inductively coupled plasma-mass spectroscopy (ICP-MS) (Agilent 7700x, Agilent Technologies, Santa Clara, CA, U.S.A.). Characterization. Samples of proper quantities were prepared to be detected by various techniques. Fourier transform infrared (FTIR) spectra were acquired by using a Nicolet 5700 spectrometer (Thermo Electron, U.S.A.); the crystallizable microstructure 7



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was examined with X-ray diffraction (XRD) using an X’Pert PRO diffractometer (PANalytical, Netherland); the surface was observed by scanning electron microscopy (S440, Leica Microsystems Cambridge Ltd., U.K.) and transmission electron microscopy (Libra 200FE, Carl Zeise SMT Pte, Germany); energy dispersive X-ray spectroscopy (EDS) was done by an energy dispersive X-ray spectrometer (Oxford IE450x-Max80, Oxford, U.K.); X-ray photoelectron spectroscopy (XPS) was obtained with an ESCALAB250 X system (Thermo, U.K.); the radicals engendered through γ-ray irradiation were measured by electron-spin resonance (E500, Bruker, Karlsruhe, Germany); the specific surface area by BET method and pore size distribution by BJH method were determined on an auto-adsorption system (Autosorb-iQ, Quantachrome, U.S.A.); elemental analysis for C, H, N were performed by Vario EL CUBE (Elementa, Germany). DFT Methods. All density-functional calculations were implemented in the Gaussian 09 program28 using the exchange-correlation hybrid functional B3LYP.29,30 Geometry optimization were performed in aqueous phase in conjunction with the relativistic effective core potential (ECP) of the SDD pseudo-potential basis set for uranium atom31 and 6-311G(d,p) basis set for the light atoms.32 Frequency analysis was performed at the same level of geometry optimization to ensure no imaginary frequency. The most stable configuration is confirmed according to the calculated Gibbs free energies.

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RESULTS AND DISCUSSION Analysis of PPAC preparation. A photograph of prepared PPAC polymer is shown in Fig. S1. During the pre-irradiation of polypropylene, peroxy radicals were generated in ambient atmosphere and the characteristic ESR signal of such radicals33,34 were observed (Fig. S2). FTIR spectra of PP/PPAC were recorded before and after grafting as illustrated in Fig. S3. PPAC spectrum with new stretches of N−H (3390 cm-1), C=N (1650 cm-1), N−O (940 cm-1), and −COO- (1564 cm-1 and 1396 cm-1) that are absent in PP spectrum demonstrates amidoxime and carboxyl groups are grafted onto polypropylene successfully.35,36 SEM and TEM images of PP/PPAC further confirm the functional groups are conglomerated on the surface in side-chains (Fig. S4) since the surface of PP is rather smooth and PPAC surface has viewable bulges. Elemental analysis gives the percentage content of carbon in PPAC as 66.448%, nitrogen as 5.006% and hydrogen as 10.125%. XPS (Fig. S5) and EDS (Fig. S6) can help determine the chemical composition of PP and PPAC qualitatively. The specific surface area (BET method) of PPAC (0.7773 m2/g) is higher than that of PP (0.3302 m2/g), mainly due to the functional groups grafted on the surface (also reflected on the pore size distribution shown in Fig. S7). U(VI)/Th(IV) Adsorption on PPAC. Uranium and thorium will be chelated with EDTA and form multiple complexes. In our study, concentrations of metals were controlled below 11 ppm to ensure their complexed existence in aqueous solutions instead of precipitates or micelles. Note thorium without EDTA was precipitated as 9



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Th(OH)4 when pH goes above 4, therefore pH ~4 was chosen as a starting point. Experimental data for thorium and uranium adsorption onto PPAC respectively (QTh and QU in mg per gram of PPAC) at pH ~4 are listed in Table 1, with adsorbent solid(g)/liquid(mL) ratio ~1:1500. The affiliation of PPAC to U(VI) is ~2 times higher than that to Th(IV). As the temperature rises, the adsorption capacities for both metals increase simultaneously, while the ratio of QU/QTh remains ~2 though the initial concentration of thorium is 1.5 times higher than uranium.

Table 1. Adsorptive Behaviors of U(VI) and Th(IV) on PPAC at pH ~4a QU (mg/g)

QTh (mg/g)

Temperature

a

QU/QTh pH=4.09

pH=4.02

298 K

5.879 ± 0.002

2.797 ± 0.004

2.102

308 K

7.953 ± 0.006

3.256 ± 0.007

2.443

318 K

8.818 ± 0.008

3.730 ± 0.003

2.364

Initial concentration of thorium(IV): 10.25 ppm and uranium(VI): 6.73 ppm

U(VI)/Th(IV) Separation by PPAC. The above tests only represent single element adsorption behaviors onto PPAC without competition adsorption or interference between metal ions. To testify the separation efficiency, additional experiments of PPAC contacted with U(VI)/Th(IV) mixed solution were conducted. Another concern of this work is

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avoidance of high acidity in typical phosphates/phosphonates extraction of uranium; hence the separation experiments of U(VI)/Th(IV) mixture were performed in weak acidic (pH ~4) to near neutral environment (pH ~6). The solid/liquid ratio was controlled at ~1:500, and the ratio of initial uranium and thorium concentration was controlled at 1:1 (both approximate 7 ppm in each temperature test). The extraction results of mixed thorium and uranium solutions are given in Table 2. In the temperature range of 298 to 318 K, the adsorption amount of U(VI) remains almost unchanged (about 3.7−3.8 mg per gram adsorbent). Meanwhile, the adsorption of thorium rises when the temperature goes up at both pHs, though always lower than uranium (the same trend in the pure solution assays). Note the adsorption capacity of thorium at pH 3.97 and 318 K is slightly lower than that at 308 K. This unexpected Th(IV) dot and the nearly constant U(VI) adsorption amount may suggest different adsorption mechanisms of the two metals on PPAC. Adsorption of uranium can be regarded as chiefly chemisorption at both pHs since temperature hasn’t any obvious desorption effect and almost all the initial uranium is adsorbed onto PPAC confirmed by ICP-MS data. For the case of thorium, there seems to be no chemisorption behavior at pH ~4 (desorption occurs at 318 K). At pH ~6, the adsorption amount is increasing concomitantly with temperature (all higher than those at pH ~4), hinting the majority of thorium adsorption on PPAC changes from physisorption to chemisorption. It suggests chemical structures (functional sites) of PPAC are altered by pH and in turn influence the adsorption mechanisms of the metals to some extent. 11



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Table 2. Comparison of Adsorption of U(VI) and Th(IV) on PPAC at pH 3.97 and 6.19a pH 3.97

pH 6.19

Temperature

a

QU (mg/g)

QTh (mg/g)

QU (mg/g)

QTh (mg/g)

298 K

3.299 ± 0.001

0.053 ± 0.001

3.421 ± 0.003

1.196 ± 0.001

308 K

3.362 ± 0.001

0.744 ± 0.001

3.476 ± 0.001

2.161± 0.001

318 K

3.458 ± 0.001

0.585 ± 0.001

3.525 ± 0.002

2.546 ± 0.001

Initial concentration of thorium(IV) and uranium(VI): ~7 ppm

We further investigated the effect of pH. The temperature was set at 308 K since it indicated the transition point from physisorption to chemisorption. From the results (Fig. 2), it can be learned at the lowest pH studied (~2), the functional groups do not work well; nonetheless at pH 4, 6 and 8 the adsorption ability of uranium increases greatly, reaching a plateau of ~3.7 mg/g. The affinity of thorium to PPAC displays a different trend: first raising from pH 2 to 6, then going downwards. The selectivity of PPAC for U(VI) over Th(IV) follows the order: pH 6 < pH 4 < pH 8 (the maximum U(VI)/Th(IV) ratio is 8.98 at pH ~8). Note at pH ~2, no thorium is adsorbed but uranium binds to PPAC weakly (the adsorption quantity is only ~20% of that at higher pHs) so it cannot serve as an ideal separation pH. From all the information, pH ~8 is deducted to be the best condition for U(VI)/Th(IV) separation by PPAC.

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Figure 2. Adsorption Capacity of U(VI) and Th(IV) on PPAC under different pHs.

One possible explanation for weak uranium binding and scarcely thorium adsorption at pH ~2 is the formation of protonated carboxyl groups (–COOH) that cannot provide sufficient coordination sites for competence against EDTA binding with metals. At pH 4, the carboxyl groups are partially deprotonated to generate carboxylate –COO- (pKa of carboxylate monomer is ~4.0).37 At this moment thorium mainly adsorbs physically on PPAC surfaces; at pH≈6, most carboxyl groups exist in deprotonated form, promoting thorium to interact chemically and form Th(IV)-PPAC complex. One interesting phenomenon is thorium adsorption weakens once pH further increases to ~8. This may be caused by –NH2 in the amidoxime group turning into –NH-, whose binding affinity to thorium is less than that of EDTA at this pH and fails in competition for thorium adsorption. This opinion of the role of –NH2 group is first proposed in all of the studies of 13



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amidoxime materials for adsorbing Th(IV) and U(VI). The amine group in amidoxime is different from amine, because amidoximation is an addition reaction with –C≡N and NH2OH, during which the bond between –NH2 and –OH is broken and attack carbon and nitrogen separately (no charge transfer during this addition reaction). The electron of nitrogen in –NH2 is conjugated with C=N bond in amidoxime hence the activity of this nitrogen is higher than that in the ordinary amine.38 The PPAC fibers adsorbed with U(VI)/Th(IV) at different pHs were characterized systematically using FTIR (Fig. 3) aiming at providing evidence for the proposed mechanism. The peaks at 2870 cm−1 and 2962 cm−1 belong to –CH3 symmetric and asymmetric stretches; 2850 cm−1, 2925 cm−1 and 1462 cm−1 peaks are attributed to stretching and bending modes of backbone –CH2.39 These aliphatic groups’ intensities are nearly invariable in all cases, indicating the backbone carbon chain of PPAC keeps its structure undisturbed from introduction of actinide metals, protons or hydroxyl ions. After interacting with U(VI)/Th(IV) solution at pH ~2 (Fig. 3, red line), N−H, C=N and –COO- stretches diminish in regards to that of untreated PPAC (Fig. 3, black line), demonstrating hydrolyzed amidoxime groups and protonated carboxyl groups to form –COOH partially, which causes 1700 cm-1 peak for –COOH to be enhanced and broadened. At this pH, thorium cannot be adsorbed, but uranium can bind to amidoxime’s =N–OH through η2 complexation24 or tetra-complexation composed of two amidoxime groups and one uranyl ion. As pH goes up, –COO- peak appears and becomes stronger gradually. Thorium is adsorbed onto PPAC from physisorption to chemisorption with 14


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increasing binding sites. In the meantime, uranium may not only coordinate with carboxyl group but also with amidoxime’s =N–OH/=N–O- and –NH2/–NH-, as can be seen from the huge increase of N−H stretch (3390 cm−1) and C=N peak.

Figure 3. FTIR spectra of PPAC before and after U(VI)/Th(IV) adsorption at different pHs. The black line shows PPAC untreated with thorium/uranium solutions; other lines with pH in brackets represent PPAC fiber contacted with aqueous solutions of adsorbates at respective pHs. The characteristic stretch modes of N−H (3390 cm−1), C=N (1650 cm−1), N−O (940 cm−1) and –COO- (1564 cm−1 and 1396 cm−1) clearly indicate amidoxime and carboxyl functional groups in PPAC.

X-ray diffraction (Fig. S8) was conducted to see whether metals were chelated with PPAC by chemical bonding. From XRD Analysis (see Supporting Information), it was 15



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found U(VI) and Th(IV) could both enter the crystal lattice, resulting in smaller diffraction angle which was associated with U(VI) and Th(IV) adsorption capacity. Then DFT calculation on several possible chemical structures of U(VI)-PPAC complex was performed to examine the thermodynamically stable coordination configuration. Here we select two models of most probability for PPAC with U(VI) shown in Fig. 4 with other two modes from literature.16,24,25 From our results, amidoxime has a major role in tetra-coordination to uranyl at lower pH (structure A).16 When pH gets higher (4, 6, 8) and deprotonated carboxyl groups come into existence, the complex with both functional groups (penta-coordination) is more stable (structure B). This finding is in accordance with aforementioned fact that affinity of PPAC to uranium is favorable at pH > 4. The η2 coordination (structure C) at low pH which is energetically unfavorable cannot be ruled out since the XRD data shows an exotic behavior. Moreover, cyclic imidedioxime coordination mode (structure D) is recognized as an important component of U(VI)-PPAC complex with the lowest energy in DFT calculation. However, the formation of this cyclic imidedioxime groups is enhanced in KOH solution at 80 ℃ in the preparation.36 In this work we only performed amidoximation in near neutral environment, therefore the amount of cyclic imidedioxime functional groups is small (shown in the low intensity of IR spectra) and the penta-coordination configuration (structure B) dominates at higher pH. This kind of structure involving dual functional groups has also been studied by others, for example when introducing MAA or itaconic acid (ITA), there exists an optimum molar ratio for AN/MAA at ~2.33 in dimethyl 16


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sulfoxide (DMSO)40 and AN/ITA at 7.57−10.14 in DMSO.41 The calculated bond lengths are listed in Table 3. Compared to the bond lengths in free amidoxime ligand, the N(H)-C bond (1) and C=N bond (2) in all complexes are shortened while N-O(H) (3) prolonged. In the meantime, U-O bond (4) is shorter than U-N(H) bond (5) in all cases, meaning once coordinated, C=N bond is slightly strengthened through complexation with uranium in the five-member ring, and uranium preferably binds to oxygen over nitrogen as evidenced by the changes in lengths of bonds 1, 3, 4 and 5. For carboxyl interaction, choose MAA saturated alkyl form of isobutyric acid as the free ligand, the i and ii bonds in deprotonated carboxyl group are intrinsically the same under higher pH. However, the carboxyl group comes into coordination to uranyl ion (structure B) and i, ii bonds are now discernable, possibly due to one is hydrogen bonded to H atom and another is complexed with U(VI). The U-O bond (iii) length of carboxyl group is much longer than that of U-O bond (4), meaning the amidoxime functional group has a more profound impact on the coordination of PPAC with uranium.

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Figure 4. Two complexing ligands and possible coordination configurations for U(VI)-PPAC complexes (optimized structure A to structure D).16,24,25

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Table 3. Bond Lengths in Proposed Structures of U-PPAC Complex by DFT Calculation (in Å) Amidoxime Ligand

Carboxyl Ligand

U(VI)-PPAC Complex

C-N

C=N

N-O

C=O

C-O

U-O(-N)

N(H)-U

U-O(-C)

N-H(--O)

(N-)H--O

(1 or 1')

(2 or 2')

(3 or 3')

(i)

(ii)

(4 or 4')

(5 or 5')

(iii)

(iv)

(v)

1.465

1.467

1.378

—

—

—

—

—

—

—

—

—

—

1.38

1.384

—

—

—

—

—

1.357

1.312

1.428

—

—

2.230

2.492

—

—

—

1.349

1.309

1.423

2.288

2.479 1.014

2.362

—

—

—

—

—

—

Free

Amidoxime

Ligand

Free

Carboxyl

Ligand

A

B

C

1.242 1.354'

1.308'

1.420'

1.343

1.300

1.383

1.369

1.298

1.393

—

1.367'

1.297'

—

2.455 2.285'

2.503'

2.224

2.331

2.452 —

D

1.281

—

1.395'

2.622 2.434'

19



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CONCLUSIONS Phosphates/phosphonates are popular materials for U(VI)/Th(IV) purification in the extraction and recovery of uranyl ion from effluents of nuclear power plant, but the required high acidity and massive organic waste may limit their feasible application. In this work, PPAC fibers are synthesized by pre-irradiation-copolymerization method with amidoxime/carboxyl functional groups, which have shown a high affinity in extracting uranyl from a mixed solution of U(VI)/Th(IV). The behavior of PPAC adsorbing these actinides has been analyzed by adsorption capacity measurement, FTIR, XRD and DFT calculations. The results conclude that amidoxime together with carboxyl groups play an important role in the formation of uranium-PPAC complex. It is of great interest to find PPAC has a good selectivity for uranium over thorium especially in pH ranges of seawater (~8), promoting the separation and recovery of this valuable element from abundant natural resources in an environmentally compatible manner. This work can provide essential knowledge for rational design of new ligands with enhanced uranyl affinity and selectivity in U(VI)/Th(IV) separation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. A photograph of PPAC; ESR spectrum of PP after 70 kGy γ-irradiation in air; SEM and TEM images of PP and PPAC; FTIR, XPS and EDS spectra of PP and PPAC; XRD 20


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patterns of PPAC before and after adsorption of U(VI)/Th(IV) at four pHs and shifts of diffractive angles for peak I to peak V; BJH pore size distributions from desorption branch for PP and PPAC (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Xiaolin Wang) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was sponsored by State Administration of Science, Technology and Industry for National Defense under nuclear power development project and National Natural Science Foundation of China (Grant No. 91326110, 21401175).

21



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O HO N HO

NH2

O

H2N

HO

N

O

N

OH

OH

OH


N H

N OH


6

Adsorption Capacity (mg/g)

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5

4

Total U(VI)

3

2

1

Th(IV) 0 2

3

4

5

6

pH ACS Paragon Plus Environment

7

8

9

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PPAC (pH 8.24) PPAC (pH 6.19) Relative Absorbance

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PPAC (pH 3.97)

PPAC (pH 2.05) PPAC (untreated)

4000

3500

3000

2500

2000

1500 -1

Wavenumber (cm ) ACS Paragon Plus Environment

1000


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CH3 NH2

1

i

O H

ii OH

2

N

OH 3

Free amidoxime ligand

Free carboxyl ligand

v C2H5

H3CH2C

O

H N

4

U N

O

H

5 N O H 1

N 2

N

2 N

CH2CH3

ii

iv

1

O 3

3 O

i-C3H8

i

O

5 O iii

4' O 3'

U

4

O

O

N

5'

2' HN

1' C2H5

Structure A (pH 2, 4)16

H3CH2C C H2N 1

2

O 5'

N

U

3

O

4 O

Structure B (pH 4, 6, 8)

2N 1 1' 2'

Structure C (pH 2)24

3

O

N

5

N

O

4

O

O

N

N

U 4'

3'

O

O

N

Structure D (pH 4, 6, 8)25


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O

UO22+

U O

Th4+ EDTA

penta-coordination


Carboxyl ... - ACS Publications

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