New Amidoxime-Based Material TMP-g-AO for Uranium Adsorption

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New Amidoxime Based Material TMP-g-AO for Uranium Adsorption under Seawater Conditions Jiayun Zeng, Hui Zhang, Yang Sui, Nan Hu, Dexin Ding, Fang Wang, Jinhua Xue, and Yongdong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b05006 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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Industrial & Engineering Chemistry Research

1

New Amidoxime Based Material TMP-g-AO for Uranium Adsorption

2

under Seawater Conditions

3

Jiayun Zeng1, Hui Zhang1, Yang Sui2, Nan Hu1, Dexin Ding1,*, Fang Wang1, Jinhua

4

Xue1, Yongdong Wang1

5

1 Key Discipline Laboratory for National Defense for Biotechnology in Uranium

6

Mining and Hydrometallurgy, University of South China, Hengyang 421001, China

7

2 Hunan Taohuajiang Nuclear Power Co., Ltd, Yiyang, 413000, China

8 9 10 11 12 13 14 15 16 17 18

*Corresponding author: Dexin Ding

19

Key Discipline Laboratory for National Defense for Biotechnology in Uranium

20

Mining and Hydrometallurgy, University of South China, 28 West Changsheng

21

Road, Hengyang, Hunan 421001, People’s Republic of China.

22

E-mail: [email protected] (Dexin Ding);

23

Jiayun Zeng and Hui Zhang contributed equally to this work.

24 25

ACS Paragon Plus Environment


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1

ABSTRACT:

2

molybdopyrophosphate-g-amidoxime (TMP-g-AO) was prepared by chemical

3

co-precipitation and subsequent chemical modifications. The successful grafting of

4

acrylonitrile group and the subsequent conversion of acrylonitrile group to amidoxime

5

groups were characterized by Scanning Electron Microscopy-Energy Disperse

6

Spectroscopy, Fourier Transform Infrared Spectroscopy, X-ray Photoelectron

7

Spectroscopy, N2-BET and Thermal analysis. The adsorption behavior of uranium(VI)

8

on TMP-g-AO was investigated for low concentration uranium solution by batch

9

experiments at a fixed pH 8.2±0.1. It is found that the adsorption rate of uranium from

10

solution was 99.77% when the uranium concentration was 42.3 µg/L, pH, 8.2±0.1,

11

temperature, 298.15 K, and the adsorbent dosage, 0.05 g. The kinetic data follow the

12

pseudo-second-order model and adsorption equilibrium data fit the Langmuir model

13

well. The thermodynamics parameters (∆S, ∆H and ∆G) indicate that the adsorption

14

process is spontaneous and endothermic. The functional TMP-g-AO adsorbent

15

exhibits good selectivity and affinity for uranium ions under coexisting multi-metal

16

ions except for Fe3+ and Co2+. Desorption was performed and the adsorption rate of

17

uranium

18

adsorption-desorption cycles. In order to evaluate the potential application of

19

TMP-g-AO for uranium extraction from seawater, the experiments on adsorption of

20

uranium(VI) from natural seawater and the uranium-doped seawater were conducted

21

and the adsorbent exhibited high adsorption rate of uranium(VI). The results show

22

that the TMP-g-AO could be a very promising adsorbent for uranium extraction from

23

seawater.

24

KEYWORDS: uranium(VI); amidoxime; adsorption; seawater

by

A

novel

TMP-g-AO

amidoxime

only

decrease

based

adsorbent

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1.42%

25 26 27

2


after

of

five

titanium-

consecutive

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1

1. . INTRODUCTION

2

Nowadays, the traditional oil, coal and other nonrenewable fossil fuels are being

3

gradually consumed, which brings out various social and environmental problems.1

4

There has been a consensus throughout the world that people should try their best to

5

adjust the energy structure, improve the efficiency of energy, develop green energy and

6

achieve the sustainable development of economic and environment. Uranium is used as

7

a nuclear fuel to produce nuclear energy and the nuclear energy is considered as one of

8

the most environmentally friendly energies. According to a recent estimate, terrestrial

9

uranium reserves can only guarantee uranium supply for nuclear power production for

10

100 years. To ensure the long-term development of nuclear power production, it is

11

crucial to exploit nonconventional uranium resources, such as uranium in seawater.

12

There areapproximately 4.5 billion tons of uranium in the oceans, nearly 1000 times

13

greater than the terrestrial uranium reserves.2 However, efficient and selective

14

extraction of uranium from seawater is particularly challenging because of high

15

salinity, high carbonate concentration, basic pH (7.9 ~8.4), low uranium concentration

16

(∼3.3 ppb) and other metal ions at similar or higher concentrations in seawater.3,4

17

Up to now, a variety of approaches, such as solvent extraction, ion-exchange,

18

membrane separation, nano-filtration and adsorption, have been developed to

19

concentrate uranium from seawater.5-8 Solvent extraction has been successfully used to

20

separate uranium from seawater. However, it has a number of drawbacks such as long

21

time operation and large organic solvent consumption. For ion exchange method, the

22

ion exchange capacity and cycle efficiency are very low. Furthermore, the exchangers

23

are susceptible to saturation and regeneration is difficult. Membrane separation

24

processes have been used for several years to concentrate or fractionate suspended

25

particles and dissolved species,9 which are prone to membrane fouling and membrane

26

specificity needs to be further studied. Nano-filtration is influenced by the size of

27

separated species and pressures which may limit its application in the field of uranium

28

extraction from seawater. Adsorption, due to its high efficiency and ease of handling,

29

has been employed for the removal of uranium from nuclear industrial effluents, mine 3



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1

water and seawater10 and it is considered as one of the most promising techniques for

2

uranium pre-concentration from seawater.

3

Many types of adsorbents have been studied for the recovery of uranium from

4

seawater, such as metal oxide,11-16 chitosan resin,17-19 metal-organic framework,20

5

layered metal sulfides21,22 and amidoxime based adsorbents.23-25 Amidoxime based

6

adsorbents have attracted intensive attention since the 1960s due to the high

7

selectivity and affinity to uranium. Amidoxime based adsorbents adsorb metal ions

8

owing to lone-pair electrons in electron donating groups (–NH2, HNCH3, and N(CH3)2),

9

which form coordination bond and stable structure with metal ions.26 In many cases,

10

those amidoxime-functionalized materials can be synthesized by introducing

11

acrylonitrile groups (–CH2–CH–C≡N) into solid structures and then converting these

12

groups to amidoxime groups (–CH2–CH–C(NH2)＝NOH)3. For instance, Xu et al.

13

prepared

14

precipitation graft copolymerization of chitosan and acrylonitrile, and then converted

15

the acrylonitrile groups into amidoxime ones using hydroxylamine hydrochloride.27

16

These combinations of chitosan and amidoxime groups had their respective advantages

17

complementary to each other. Das et al. synthesized a new series of amidoxime-based

18

polymer adsorbents using electron beam induced grafting of acrylonitrile and itaconic

19

acid onto polyethylene fiber.28 And the synthetic DMSO-heat-treated sorbents

20

adsorbed uranium as high as 4.48 g-U/kg-ads from seawater. Shen et al. reported that

21

the

22

polymerization, and the uranium adsorption capacity reached up 3.06 mg/g at pH 7.54

23

Lu et al. prepared the C8A-AO adsorbent which exhibited excellent selective

24

adsorption capacity of 98.425 mg/g at pH 7 for uranium ions in simulated seawater.55

25

These studies demonstrated that the amidoxime group has strong affinity and can

26

chelates effectively the uranyl tricarbonate complexes [UO2(CO3)34－] in neutral or

27

weakly alkaline solution. Therefore, the amidoxime based adsorbents are considered

28

as the most promising materials for adsorption of uranium from seawater. In addition,

29

some pyrophosphate adsorbents exhibit a large adsorption capacity for uranium ions

30

in aqueous solution. The maximum adsorption capacity of ZMPP-TBP for

amidoximed

chitosan-grafted

PAN/MMT nanocomposite

was

poly-acrylonitrile

prepared

through

4


(CTS-g-PAO)

in-situ

via

intercalation

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1

uranium(VI) was 196.08 mg/g at 293 K and pH 6.52 Likewise, Wang et al.53 prepared

2

pyrophosphate Zr1-xTixP2O7 and TiP2O7 by microwave-induction, and the maximum

3

amount of the uranium on TiP2O7 reached up to 309.8 mg/g under the experimental

4

conditions (pH=5, t=60 min and T=303 K). These pyrophosphate adsorbents generally

5

have a large specific surface area and may be used for immobilization and long-term

6

storage of uranium ions due to the properties of near zero or negative thermal

7

expansion. In summary, the inorganic bimetallic pyrophosphate, as a base material,

8

grafts with amidoxime group which can compensate for the shortcoming of instability,

9

and the synthetic adsorbents tend to have a good surface reactivity.

10

In

this

work,

a

new

amidoxime

based

adsorbent

Titanium-

11

molybdopyrophosphate-g-amidoxime (TMP-g-AO) was fabricated by chemical

12

coprecipitation and subsequent chemical modifications. The successful grafting of

13

acrylonitrile groups on the surface of TMP-g and the subsequent converting of

14

acrylonitrile groups to amidoxime groups were characterized by Scanning Electron

15

Microscopy-Energy Disperse Spectroscopy (SEM-EDS), Fourier Transform Infrared

16

Spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), N2-BET and Thermal

17

analysis. The uranium adsorption behavior of TMP-g-AO was investigated for the first time.

18

The influences of different factors, such as adsorbent dosage, initial uranium

19

concentration, contact time, temperature and coexisting ions, on uranium(VI)

20

adsorption were investigated by batch experiment at fixed pH 8.2±0.1. Sorption

21

isotherms of uranium(VI) were illustrated by Langmuir and Freundlich patterns. The

22

kinetic

23

pseudo-second-order models and the kinetic parameters were determined. In order to

24

evaluate the potential application of TMP-g-AO for uranium extraction from seawater,

25

the adsorption of uranium(VI) from natural seawater (3.65µg/L) and the

26

uranium-doped seawater (61.02µg/L) was conducted.

27

2. EXPERIMENTS

28

2.1. Materials. Acrylonitrile (AN), hydroxylamine hydrochloride (NH2OH·HCl),

29

N,N-dimethylformamide

experimental

data

were

(DMF),

simulated

silane

by

coupling

5


pseudo-first-order

agent

and

(KH-570),


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1

2,2-Azobisisobutyronitrile (AIBN), titanium tetrachloride (TiCl4), phosphormolybdic

2

acid (H3PO4·12MoO3), potassium pyrophosphate trihydrate (K4P2O7·3H2O),

3

ethanol, ammonia (25%), sodium carbonate and hydrochloric acid (36%) were

4

purchased from Tianjin Kermel Chemical Reagents Development Center (Tianjin,

5

China). All of these chemicals were analytic grade reagents without further

6

purification.

7

The stock solutions of uranium(VI) (1.0g/L) were prepared in the following

8

procedure: 1.1792 g U3O8 powder was first dissolved in hydrochloric acid (10 mL),

9

hydrogen peroxide (3 mL) and two drops of nitric acid by heating in the sand bath; and

10

the mixture was then cooled to room temperature and diluted with ultrapure water to

11

1000 mL, which was the standard solution of uranium. The solutions with different

12

uranium(VI) concentrations for experiments were prepared by diluting the stock

13

solution using ultrapure water.

14

2.2. Preparation of the TMP-g-AO adsorbent. TMP-g-AO adsorbent was prepared

15

by chemical co-precipitation and subsequent chemical modifications. The preparation

16

process of TMP-g-AO consists of four steps and is illustrated in detail in Figure S1.

17

(i) Synthesis of TMP by chemical co-precipitation: Firstly, equimolar amounts of

18

potassium pyrophosphate trihydrate and phosphomolybdic acid solutions were mixed

19

with a ratio of 10:1 (v/v), and the pH of the mixture was adjusted to 2~3. Then, the

20

solution of titanium tetrachloride was added dropwise to the flask under stirring until no

21

more white precipitation appeared. After centrifuging and discarding the supernatant

22

liquor, the sediment obtained was washed with ethanol several times and then dried in a

23

vacuum oven overnight at 333.15 K.

24

(ii) Modifying TMP with silane coupling agent KH-570 (TMP-g): 6.0 g of the

25

dried TMP was immersed in ethanol (120 mL) and deionized water (6.5 mL), and then

26

4 mL of 25 wt% aqueous ammonia and 2 mL of silane coupling agent KH-570 were

27

added. The mixture was stirred continuously at 328.15 K for 48 h. After filtration, the

28

sediment was repeatedly washed with ethanol to remove residual KH-570 and dried in

29

a vacuum oven overnight at 333.15 K.

30

(iii) Grafting acrylonitrile on the surface of TMP-g: First, 1.5 g of TMP-g and 100 6


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1

mL solvent of N,N-dimethylformamide (DMF) were added in three-necked round

2

bottom flask (with reflux condenser and thermometer), and the mixture was heated in a

3

water bath to 343.15 K under N2 flow and magnetic stirring of 1000 r/min. Then, 12 mL

4

of acrylonitrile (including 0.15 g of 2,2-azobisisobutyronitrile) was added to the

5

mixture by syringe, the mixture was kept in N2 atmosphere at 343.15 K for 5 h, and the

6

product of TMP-g-AN was collected by centrifugation and then washed with ethanol

7

several times to remove the residual DMF. Finally, the product was dried overnight at

8

333.15 K.

9

(iv) Converting acrylonitrile to amidoxime (TMP-g-AO): First, 1.5 g of

10

TMP-g-AN, together with 160 mL of water/ethanol (3v/1v) and 1.0 g of sodium

11

carbonate, was added in four-necked round bottom flask (with reflux condenser and

12

thermometer), and the mixture was heated in a water bath to 343.15 K under N2 flow

13

and magnetic stirring of 1000 r/min. Then, 1.39 g of hydroxylamine hydrochloride

14

(dissolved in 5 mL of water) was slowly added to the mixture by syringe, the mixture

15

kept in N2 atmosphere at 343.15 K for 5 h, and the yellow product of TMP-g-AO was

16

collected by centrifugation and then washed with ethanol three times. Finally, the

17

product was dried overnight at 333.15 K.

18

2.3. Instruments. A field emission scanning electron microscope equipped with

19

Energy Disperse Spectroscopy (SEM-EDS, Raith ELPHY Quantum Electron

20

Lithography (kit), Raith, USA) was used to characterize the morphologies of the

21

synthetic products. Fourier Transform Infrared spectroscopy (FT-IR, NicoletiS10,

22

Thermo Scientific, USA) was used to evaluate the change in the structure of the

23

synthetic products. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi,

24

Thermofisher-VG Scientific, USA) of the TMP-g-AO and TMP-g-AO –U powders

25

were measured using a Thermo Scientific ESCALAB 250 Xi spectrometer equipped

26

with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 300 W. The

27

concentrations of uranium (VI) in solutions were measured by Inductively Coupled

28

Plasma Mass Spectrometry (ICP-MS, 7700X, Agilent, USA).

29

2.4. Batch Adsorption Experiments. The TMP material is dissolvable in solution, and

30

the materials of TMP-g, TMP-g-AN, and TMP-g-AO are insoluble and stable in 7



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1

distilled water and hydrochloric acid (0.1 M). The adsorption experiment of uranium

2

ions was respectively carried out by TMP-g, TMP-g-AN, and TMP-g-AO (m=0.05 g,

3

C0=53.83 µg/L, V=50 mL, pH=8.2±0.1, T=298.15 K, t=120 min), and the adsorption

4

rates of TMP-g, TMP-g-AN, and TMP-g-AO are respectively 58.37%, 69.16% and

5

98.94% which have a big difference. Therefore, we only studied the adsorption

6

behavior of TMP-g-AO by batch adsorption in the next experiment.

7

Batch adsorption experiments were carried out to investigate the effects of

8

adsorbent dosage, initial uranium concentration, contact time, temperature and

9

coexisting ions on uranium(VI) adsorption. Considering the practical application of

10

uranium adsorption from seawater, the pH of uranium(VI) solution was adjusted to

11

8.2±0.1 with 0.1 M HCl and Na2CO3 solutions. All experiments were conducted by

12

mixing 50 mL of uranium(VI) ion solutions with 0.05 g of TMP-g-AO adsorbent in

13

conical flasks, followed by shaking in a constant temperature shaker at 180 rpm for a

14

given time at 288.15~333.15 K. After that, 5 mL of the mixture was filtered by 0.22 µm

15

syringe filter and then soured by concentrated nitric acid. The mixture was used to

16

analyze the residual concentration of uranium(VI) by ICP-MS. All the experiments

17

were performed in duplicates and a blank sample was set at the same time to minimize

18

experimental error. Finally, the data were analyzed by origin software (Version 8.0,

19

USA).

20 21

The adsorption capacity Q (µg/g), adsorption rate Ads% and distribution coefficient K (mL/g) were calculated using the following formulas. − ) × 1) − ) % = × 100 2) − 1000 = × 3) =

22

Where C and C are the initial and equilibrium concentrations of uranium(VI) (µg/L),

23

respectively; V is the volume of testing solution (L); and m is the amount of the

24

adsorbent (g).

25

2.5. Desorption and Regeneration Studies. To evaluate the stability and reusability of

26

TMP-g-AO material, the adsorption and regeneration were performed in five 8


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1

consecutive cycles. In each cycle, 0.05 g of adsorbent was shaken with 50 mL of

2

uranium(VI) solution for a given time at 298.15 K. After that, adsorbent loaded with

3

uranium(VI) was eluted using desorption solution (50 mL 0.1 M HCl), and washed

4

with large amount of ultrapure water. Then, it was reused for uranium adsorption as

5

before. The desorption rate DE% was calculated using the following formula. #$% =

× 100 4) −

6

Where C (µg/L) is the concentration of uranium(VI) in solution after desorption;

7

and C (µg/L)and C (µg/L) are the initial and equilibrium concentrations of uranium

8

(VI), respectively.

9 10

3. RESULTS AND DISCUSSION

11

3.1. Characterization

12

3.1.1. Scanning Electron Microscopy and Energy Disperse Spectroscopy. The

13

scanning electron microscopy is widely used to investigate the morphological features

14

and surface characteristics of the adsorbent materials.29 SEM was used to observe

15

morphologies of TMP (a), TMP-g (b), TMP-g-AN (c), TMP-g-AO (d) and

16

TMP-g-AO-U (e), and to compare their differences. As is shown in Figure 1, it is

17

clear that the diameters of samples are of nanoscale. In Figure 1(a), the TMP sample

18

presents a spherical shape and a smooth surface. After TMP-g is modified with

19

KH-570, the surface of TMP-g particles becomes rough. Furthermore, a dense

20

structure can be seen and much irregular matter has been generated between spherical

21

particles in Figure 1(c, d). These changes in the surface appearance of TMP-g-AN and

22

TMP-g-AO samples are due to the grafting and oximation. The surface of

23

TMP-g-AO-U are also rough and dense, as shown in Figure 1(e), and the surface of

24

TMP-g-AO is covered with many small particles which may be due to the adsorption

25

of uranium ions. The chemical compositions of the TMP-g-AO and TMP-g-AO-U

26

obtained by the EDS analysis are shown in Figure 1(f, g) and Table S1. In comparison,

27

uranium ion is adsorbed on the surface of the material and the atomic percentage of U 9



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1

is 1.75% after adsorption. However, the atomic percentages of both N and O elements

2

decrease, which may be attributed to the reaction between amidoxime groups and

3

uranium ions.

4

3.1.2. Fourier Transform Infrared Spectroscopy. The Fourier Transform Infrared

5

Spectroscopy of TMP (a), TMP-g (b), TMP-g-AN (c), TMP-g-AO (d) and

6

TMP-g-AO-U (e) materials are shown in Figure 2, and the characteristic peaks of –

7

OH and –P–O at 3582 and 1564 cm-1 can be observed in the curve of TMP. After

8

surface modification of TMP by KH-570, the peak of –OH disappears and the new

9

characteristic peaks at 3161, 1406, 1178 and 885 cm-1occur respectively in the curve of

10

TMP-g, which is attributed to the groups of –CH2–,–Si–O–,–C–O– and –C=C–.

11

Additionally, in the curve of TMP-g-AN, a sharp peak at 2242 cm-1 is assigned to –

12

C≡N– and the peak of –C=C– at 885 cm-1 disappears owing to the polymerization

13

between –C=C– and acrylonitrile.30 In the curve of TMP-g-AO, the new characteristic

14

peaks for –NH2 (or –OH), –C=N–, –C–N– and –N–O– appear respectively with

15

wavenumbers at 3194, 1649, 1269 and 920 cm-1.31-33 Meanwhile, the peak at 2242 cm-1

16

related to –C≡N– disappears and the peak of –C=N– increases sharply, which

17

indicates that the nitrile group was converted completely into the amidoxime group

18

after reaction with hydroxylamine hydrochloride. In addition, the peak of –N–O–

19

moves from 920 cm-1 to 913 cm-1, and changes in peak positions and intensity around

20

550-1000 cm-1 region in the curve of TMP-g-AO-U can be assigned to asymmetric

21

stretching vibration of uranyl ion and stretching vibrations of weakly bonded oxygen

22

ligand with uranium.50,51

23

3.1.3. X-ray Photoelectron Spectroscopy. The chemical bonding states on the

24

surface of the samples are further investigated by XPS. By comparing the element

25

contents of TMP-g-AO with those of TMP-g-AO-U in Table S2, it can be found that

26

the contents of C1s and O1s increase from 38.29% to 39.46% and from 33.20% to

27

35.25% after adsorption, respectively. However, the N1s content decreases from 16.77%

28

to 10.85%, the reason of which may be that the surface of the synthetic adsorbent is

29

heterogeneous, and uranium adsorbed on the surface of the TMP-g-AO material

30

affects the elemental analysis since the analysis depth of XPS is generally from 2 to 5 10


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1

nm. Therefore, the C/N ratio increases obviously from 2.28 to 3.64. As shown in

2

Figure 3, the U4f5/2 and U4f7/2 peaks appear with the binding energies at 392.24 and

3

381.39 eV,34-37 and the U4f content reaches 2.29%. In addition, the C1s and N1s

4

spectra of the TMP-g-AO sample before and after uranium adsorption are shown in

5

Figure 4. The C1s of TMP-g-AO consist of the CO32-, C-O, H2N-C=N-OH and

6

C-C(C-H) at 288.34, 287.09, 285.89 and 284.50eV, respectively.38 The N1s can be

7

curve-fitted with two peaks at 399.19 and 400.64 eV for H2N-C=N-OH and

8

H2N-C=N-OH.39 After adsorption, the peak comprised of CO32-, C-O, H2N-C=N-OH

9

and C-C(C-H) at 288.35, 287.25, 286.08 and 284.69 eV can fit well with the C1s

10

curve of TMP-g-AO-U. And the N1s can also be fitted with H2N-C=N-OH and

11

H2N-C=N-OH peaks at 399.41 and 401.29 eV.40 In conclusion, the XPS analysis can

12

further confirm the FT-IR and EDS results.

13

3.1.4. N2-BET Analysis. The structural properties of the synthesized TMP-g-AO

14

material can be analyzed by the nitrogen adsorption-desorption isotherm. As shown in

15

Figure 5, the isotherm of TMP-g-AO firstly presents a steep increase in adsorption

16

which attributes to the characteristic of microporous materials, and then there is a

17

smoother hysteresis loops at relative pressures of 0.4~1.0 due to the nitrogen

18

condensation in the mesoporous. Therefore, the isotherm of TMP-g-AO seems to be

19

nearly type IV with a H4 hysteresis loop according to the IUPAC classification. The

20

BET surface area, pore volume and average pore diameter are calculated from the

21

appropriate section of the adsorption-desorption isotherm and the values of them are

22

48.183 m2/g, 0.116 cm3/g and 3.911 nm, respectively. Obviously, the pore volume of

23

TMP-g-AO is very small due to their dense surface, which is also confirmed by the

24

results from SEM.

25

3.1.5. Thermal Analysis. The TGA and DTG characterization was carried out to study

26

the thermal stability of TMP-g-AO adsorbent in an argon atmosphere. As can be seen

27

from Figure 6, the weight loss of 4.84% at ~120 °C can be assigned to the loss of

28

adsorbed moisture. Then, the TMP-g-AO sample has a 13.74% weight loss at

29

120~340 °C, which may be attributed to the decomposition of the amidoxime

30

groups.55 At last, the weight loss is about 11.07% due to the complete degradation and 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

decomposition of the polymerized amidoxime group when the temperature is above

2

340 °C.57 Therefore, the mass ratio of the polymerized amidoxime group to TMP-g

3

and the weight percent of the polymerized amidoxime group in dry TMP-g-AO are

4

0.423:1 and 29.7%, respectively. This result provides further evidence that the

5

TMP-g-AO adsorbent is successfully prepared.

6 7

3.2. Uranium(VI) Adsorption Performance

8

3.2.1. Effect of Adsorbent Dosage. The adsorbent dosage is one of most important

9

factors that influence the adsorption equilibrium. Adsorbent dosages ranging from 0.01

10

to 0.08 g were used to investigate the effect of adsorbent dosage on the adsorption

11

behavior of the TMP-g-AO at a fixed pH value of 8.2±0.1and temperature of 298.15

12

K. As shown in Figure 7, the adsorption rate of uranium(VI) increases rapidly with the

13

increase of TMP-g-AO dosage when it is lower than 0.02 g, and then remains almost

14

constant. However, the adsorption capacity decreases continuously for the reason that

15

the amount of uranium ions adsorbed per unit mass of TMP-g-AO is reduced. With

16

the increase of TMP-g-AO dosage, the available adsorption sites on adsorbent surface

17

increase and thereby the amounts of uranium(VI) ions in aqueous solution will be

18

further decreased.

19

3.2.2. Effect of Coexisting Ions. In order to evaluate the selectivity of the TMP-g-AO

20

adsorbent, the influence of coexisting ions (such as V5+, Fe3+, Ni2+, Cu2+, Pb2+, Zn2+

21

and Co2+) on uranium adsorption was studied according to some related literature.41-43

22

The concentrations of coexisting ions in actual adsorption tests were designed by

23

taking into account of the concentrations of these ions in natural seawater and

24

presented in Table S3. The concentrations of V5+ and Ni2+ are close to those in natural

25

seawater, but the concentrations of other ions are several tens and thousands times

26

higher than those in natural seawater. The selectivity coefficient and distribution

27

coefficient of ions are determined after the adsorption reaches equilibrium. As shown

28

in Figure 8, it is obvious that the selectivity of TMP-g-AO for the coexisting ions is in

29

the order of U6+>Fe3+>Co2+>Pb2+>Ni2+>Zn2+>V5+>Cu2+. Moreover, the values of 12


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1

selectivity coefficient for coexisting ions are greater than 2 except for Fe3+ and Co2+,

2

suggesting that Fe3+ and Co2+ may be adsorbed onto TMP-g-AO with uranium(VI) ion

3

at the same time. Therefore, the functional TMP-g-AO adsorbent still has very good

4

selectivity and affinity for uranium ions under coexisting multi-metal ions except for

5

Fe3+ and Co2+.

6

3.2.3. Effect of Contact Time and Adsorption Kinetics. The effect of contact time on

7

the adsorption of uranium(VI) ions onto the TMP-g-AO was investigated for three

8

different initial uranium concentrations (42.3, 104.8 and 226.5 µg/L) and the results

9

are shown in Figure 9. It is clear that the adsorption of uranium(VI) ions consists of two

10

steps: a relatively rapid step and a subsequent slow step. The adsorption rate of

11

uranium(VI) ions increases rapidly in the first 30 min and then increases gradually until

12

the adsorption process achieves equilibrium after 300 min. The first rapid step may be

13

due to the surface physical sorption and chemical reactive sorption. However, the

14

subsequent slow step may be attributable to the reactive adsorption of the inner

15

polymer chain segments.40

16

In order to study the adsorption kinetics of uranium(VI) on TMP-g-AO in aqueous

17

solutions, two kinetic models (pseudo-first-order and pseudo-second-order kinetic

18

models) were used to investigate the kinetic mechanism of adsorption processes

19

between adsorbent and adsorbate.

20

The pseudo-first-order kinetic model is expressed as:

21

)* × , 5) 2.303 The pseudo-second-order kinetic model is expressed as: ln − ( ) = ln −

, 1 , = + 6) . ( ). × 22

Where Q (µg/g) and Q1 (µg/g) are the adsorption capacities at equilibrium and time t

23

(h), respectively;k* (h-1) and k . (h-1) are the rate constant of pseudo-first-order and

24

pseudo-second-order models for the adsorption uranium(VI), respectively.

25

The kinetic parameters such as k* , k . , Q and correlation coefficient (R2), can be

26

calculated from the linear form of models in Figures S2a and b and the results are

27

shown in Table 1. The values of Q increase obviously with the increase of the initial 13



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1

uranium concentration. Moreover, the values of Q and R2 for the pseudo-second-order

2

kinetics are higher than those for the pseudo-first-order kinetics. Therefore, the kinetic

3

behavior of uranium adsorption onto TMP-g-AO can be described by the

4

pseudo-second-order kinetic model. This indicates that chemical adsorption might be

5

the controlling step which may involve valence forces through sharing of electrons

6

between uranium(VI) ions and adsorbent.44

7

3.2.4. Effect of Initial Uranium Concentration and Adsorption Isotherm. The initial

8

uranium concentration provides an important driving force to overcome all mass

9

transfer resistance of uranium between the aqueous and solid phases.4 The experiment

10

on adsorption of uranium(VI) on TMP-g-AO adsorbent was carried out in various

11

concentrations and the contact time was long enough (3 days) to ensure that the

12

adsorption of uranium reached equilibrium. The results are shown in Figure 10. The

13

adsorption capacity firstly increases linearly with initial uranium concentration and

14

then it almost tends to equilibrium.

15

The adsorption isotherm reflects on the relation between the adsorption capacity

16

and uranium concentration when the adsorption process reaches equilibrium. The

17

adsorption data have been measured to simulate different adsorption isotherms

18

including Langmuir and Freundlich models. The Langmuir model is based on

19

assumption of homogenous adsorption and its basic equation can be expressed as: Q =

20

3 4 5 = 7) 1 + 5 )

The standard form of linear equation can be derived from equation (7) as follows: 1 = + 8) 4 5 4

21

Where X (µg) is the total amount of uranium in adsorbent at equilibrium, m (g) is the

22

dosage of adsorbent, C μg/L) is the concentration of uranium at equilibrium,

23

Q μg/g) is the adsorption capacity of uranium at equilibrium, Q= and b are

24

Langmuir constants related to adsorption capacity and adsorption energy, respectively.

25

The plot of C /Q against C is shown in Figure S3a.

14


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1

The Freundlich model is widely applied to describe the adsorption of uranium on

2

heterogeneous surfaces as well as multilayer adsorption and it is tested in the following

3

equation: ⁄

= > × * ? (9)

4 5

A new linear equation can be derived from equation (9) as follows: 1 ln = ln> + ln 10) A

6

Where K B and n are the Freundlich constants related to adsorption capacity and

7

adsorption intensity, respectively. The plot of ln Q against ln C is shown in Figure

8

S3b.

9

The isotherm parameters of Q= , b, KF, n and the correlation coefficient R2 are

10

calculated accordingly and summarized in Table2. It can be seen that Langmuir model

11

(R2=0.999) fits the experimental data better than Freundlich model (R2=0.756) and the

12

maximum adsorption capacity is 35.37 mg/g. The fact that the Langmuir model fits

13

the experimental data very well may be due to homogenous distribution of active sites

14

on the adsorbent surface, since the Langmuir equation assumes that the surface is

15

homogeneous.45

16

Considering many experimental conditions to affect the maximum adsorption

17

capacity of adsorbents, such as pH, temperature, contact time and so on, it is difficult

18

to directly compare the adsorption capacities in reports in literature. Table 3 presents a

19

comparison of the maximum adsorption capacities for uranium ions of different

20

adsorbents. The maximum adsorption capacity of uranium on TMP-g-AO reaches

21

35.37 mg U/g at 298.15 K and pH 8.2±0.1, which is much higher than that on other

22

adsorbents except for AO-OMS under very similar experimental conditions.

23

Adsorbents such as UHMWPE, amidoxime-based polymeric, and nanofibrous

24

adsorbent, are investigated for the removal of uranium(VI), but all of them need more

25

time to arrive at equilibrium and their maximum adsorption capacities are lower than

26

that of TMP-g-AO. Although hematite can reach adsorption equilibrium in a shorter

27

time, its maximum adsorption capacity is less than one tenth of that of TMP-g-AO. As

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

a result, the TMP-g-AO adsorbent is suitable for uranium(VI) extraction in weakly

2

alkaline solution or seawater.

3

3.2.5. Effect of Temperature and Thermodynamic Studies.The effect of temperature

4

on the adsorption of uranium(VI) was investigated at nine different temperatures of

5

288.15, 293.15, 298.15, 303.15, 308.15, 313.15, 318.15, 323.15 and 333.15 K. As

6

shown in Figure 11, the adsorption rate of uranium(VI) increases gradually as the

7

temperature increases. In other words, the higher temperature is beneficial to the

8

adsorption of uranium ions, which may be attributed to the fact that increasing

9

temperature can promote the movement of molecules.

10

The thermodynamic parameters of the adsorption process including Gibbs free

11

energy change (∆G), enthalpy change (∆H) and entropy change (∆S) can be calculated

12

using following equations. ∆C = −DE FA G 11)

13

∆C = ∆H − E∆I (12)

14

∆I ∆H − 13) D DE Where K J is the equilibrium constant (Q /C ), T and R are the absolute temperature (K)

15

and the gas constant (8.314 J/mol/K), respectively. The plot of ln K J against 1/T is

16

shown in Figure S4, ∆S and ∆H can be obtained from the intercept and slope, and the

17

results are shown in Table 4.

ln G =

18

The enthalpy change (∆H) and entropy change (∆S) of adsorption are 77.86 kJ/mol

19

and 295.73 J/mol/K. The positive value of ∆H demonstrates that the adsorption of

20

uranium(VI) ions is endothermic in nature, and the positive value of ∆S indicates the

21

stability of adsorption system and reflects an increase on the randomness during the

22

adsorption process. Furthermore, the value of ∆G decreases with increasing

23

temperature, which indicates that the adsorption process of uranium onto TMP-g-AO is

24

spontaneous and higher temperatures are feasible for uranium adsorption.

25

3.2.6. Desorption and Regeneration. Desorption and regeneration processes are very

26

important to evaluate the economic performance of absorbent in actual application. In

27

this study, the hydrochloric acid was used as desorption agent. The adsorption and 16


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1

desorption experiments were carried out five times. Table S4 show that both

2

adsorption and desorption rates slightly decline after five cycles. The declination of

3

adsorption rate may be attributed to the fact that the undesorbed uranium ions occupy

4

the binding sites due to insufficient contact time and the active sites of binding

5

uranium ions decrease on TMP-g-AO as the adsorption-desorption experiments

6

increase. It is reported that the sorption amount of uranium(VI) for AOGONRs

7

decreases slightly from 0.94 to 0.89 mmol/g after five consecutive sorption/desorption

8

cycles.46 Likewise, AO-HTC has a 7.37% decrement of sorption capacity after five

9

cycles.47 Although the adsorption rate of uranium on TMP-g-AO has a 1.42%

10

decrease after five cycles, it is less than that of AOGONRs and AO-HTC. As a result,

11

TMP-g-AO material has the potential of regeneration and reuse, which enhances the

12

economy of the adsorption process to some extent.

13

3.3. Application in Adsorption of Uranium from Seawater. From the experimental

14

results obtained, it can be seen that TMP-g-AO adsorbent has a high adsorption rate

15

and a large adsorption capacity for uranium ions in weakly alkaline solution and the

16

adsorption can reach equilibrium in a very short time. Furthermore, the adsorbent

17

exhibits a very good selectivity and affinity for uranium ions in the presence of

18

coexisting ions. In order to evaluate the potential application of TMP-g-AO for

19

uranium extraction from seawater, we carried out the experiments on adsorption of

20

uranium(VI) from natural seawater and the uranium-doped seawater. The natural

21

seawater used in the adsorption experiments came from near-surface seawater from

22

Hainan Province, China, collected in the tanks. The concentrations of uranium ions in

23

seawater and the uranium-doped seawater are 3.65 and 61.02 µg/L, respectively. After

24

three days of adsorption by the adsorbent (0.05 g) at 298.15 K, the concentrations

25

decreased to 0.66 and 1.34 µg/L, the adsorption rates of uranium(VI) reached to 81.92%

26

and 97.80%, respectively. This demonstrates that the material still has high adsorption

27

rate in high salinity and multi-ionic solutions. Therefore, TMP-g-AO has great

28

potential application in the area of separation and enrichment of uranium from

29

seawater.

30 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

4.CONCLUSIONS

2

A novel TMP-g-AO adsorbent was developed for highly efficient extraction of

3

uranium from seawater. The TMP-g-AO adsorbent was synthesized by chemical

4

co-precipitation and subsequent chemical modifications. According to the batch

5

adsorption experiment, the adsorption rate and capacity are influenced by adsorbent

6

dosage, coexisting ions, initial uranium concentration, contact time and temperature to

7

some extent. In addition, the synthetic process and uranium adsorption were

8

characterized by SEM-EDS, FT-IR, XPS, N2-BET and Thermal analysis. SEM images

9

show that TMP-g-AO is spherical shape particle of nanoscale and the surfaces of the

10

samples become rough and dense during chemical modification processes. A sharp

11

peak at 2242 cm-1 indicates the successful grafting of acrylonitrile and new

12

characteristic peaks at 3194, 1649, 1269 and 920 cm-1 demonstrate the conversion of

13

nitrile group into the amidoxime group. The results of EDS, FT-IR and XPS show that

14

uranium ion is adsorbed on the surface of TMP-g-AO after adsorption. The values of

15

BET surface area, pore volume, and average pore diameter from N2-BET analysis are

16

48.183 m2/g, 0.116 cm3/g and 3.911 nm, respectively, which indicate that the pore

17

volume of TMP-g-AO is very small due to its dense surface. The results of TGA and

18

DTG analyses show that the mass ratio of the polymerized amidoxime group to

19

TMP-g and the weight percent of the polymerized amidoxime group in dry

20

TMP-g-AO are 0.423:1 and 29.7%, respectively.

21

The kinetic study suggests that uranium adsorption onto TMP-g-AO follows the

22

pseudo-second-order kinetic model. The adsorption equilibrium data fit the Langmuir

23

isotherm model well and the maximum adsorption capacity of uranium is 35.37 mg/g

24

at 298.15 K and pH 8.2±0.1. The results of thermodynamic study show that the

25

adsorption process is spontaneous and endothermic. The functional TMP-g-AO

26

adsorbent exhibits very good selectivity and affinity for uranium ions under

27

coexisting multi-metal ions except for Fe3+ and Co2+. Desorption is performed and the

28

adsorption rate of uranium on TMP-g-AO only has a 1.42% decrease after five

29

consecutive adsorption-desorption cycles. The TMP-g-AO adsorbent also has high 18


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1

adsorption rate for uranium in seawater. As a result, the TMP-g-AO could be a very

2

promising adsorbent for uranium extraction from seawater.

3 4

ACKNOWLEDGMENTS

5

This work was supported by the National Natural Science Foundation of China

6

(91326106, U1401321 and 114055081), the Development Program for Science and

7

Technology for National Defense (B3720132001) and the Research Foundation of

8

Education Bureau of Hunan Province (16C1386).

9 10

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2

U(VI) in low concentration uranium solutions. Rsc. Adv. 2016, 6, 101087-101097.

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25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Figure captions

2

Figure 1. SEM images (TMP (a), TMP-g (b), TMP-g-AN (c), TMP-g-AO (d) and

3

TMP-g-AO-U (e)) and EDS (TMP-g-AO (f) and TMP-g-AO-U (g)).

4

Figure 2. FT-IR of TMP (a), TMP-g (b), TMP-g-AN (c), TMP-g-AO (d) and

5

TMP-g-AO-U (e).

6

Figure 3. XPS full-scan spectra and U4f spectra of TMP-g-AO and TMP-g-AO-U.

7

Figure 4. C1s and N1s spectra of TMP-g-AO and TMP-g-AO-U.

8

Figure 5. Adsorption-desorption isotherms of nitrogen on TMP-g-AO.

9

Figure 6. TGA and DTG analyses of TMP-g-AO.

10

Figure 7. Effect of adsorbent dosage on adsorption rate and capacity of uranium(VI)

11

(V=50 mL, C0=65 µg/L, pH=8.2±0.1, t=120 min, T=298.15 K).

12

Figure 8. Comparison of selectivity and distribution coefficients of coexisting ions on

13

TMP-g-AO (V=50 mL, pH=8.2±0.1, m=0.05 g, T=298.15 K).

14

Figure 9. Effect of contact time on adsorption rate of uranium(VI) (V=100 mL,

15

pH=8.2±0.1, m=0.1 g, T=298.15 K).

16

Figure 10. Effect of initial uranium concentration on adsorption capacity of uranium(VI)

17

(V=50 mL, pH=8.2±0.1, m=0.05 g, T=298.15 K).

18

Figure11. Effect of temperature on adsorption rate of uranium(VI) (V=50 mL, C0=65

19

µg/L, pH=8.2±0.1, t= 120 min, m=0.05 g).

20

26


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1

Table 1. Parameters for kinetic models of uranium(VI) adsorption onto

2

TMP-g-AO Pseudo-first-order kinetics Concentration

Q (exp)

(µg/L)

(µg/g)

k1(min-1)

Q (cal)

R2

Pseudo-second-order kinetics k2(min-1)

(µg/g)

Q (cal)

R2

(µg/g)

42.3

42.26

0.0318

8.53

0.833

0.0058

42.63

0.999

104.8

104.47

0.0316

27.09

0.828

0.0018

105.71

0.999

226.5

211.01

0.0265

85.88

0.990

0.0006

210.97

0.998

3 4

27



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Page 28 of 42

1

Table 2. Constants for adsorption isotherm models for uranium(VI) adsorption

2

onto TMP-g-AO. Langmuir

Freundlich

Q= (103µg/g)

b

R2

KF

n

R2

35.37

3.3

0.999

13.21

2.03

0.756

3 4

28


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1 2

Table 3. Comparison of adsorption capacities for uranium ion of various adsorbents. Experimental

4KL

conditions

(mg/g)

UHMWPE

298.15±1K, pH8.1±0.3, 42 days

1.97

42

pA

296.15 K,pH8,72 hours

14.8

48

Amidoxime-based polymeric

293.15 K, seawater, 8 weeks

3.3

2

57

3

Adsorbent

Reference

ambient temperature, simulated AO-OMS seawater, 24 hours Hematite

293 K, pH7, 6 hours

3.36

56

Nanofibrous adsorbent

298.15±2 K, pH8, 30 days

2.86

49

TMP-g-AO

298.15 K, pH8.2±0.1, 72 hours

35.37

This work

3 4

29



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Page 30 of 42

1

Table 4. Thermodynamic parameters for uranium(VI) adsorption onto

2

TMP-g-AO. ∆H (kJ/mol)

∆S (J/mol·K)

T(K)

∆G (kJ/mol)

R2

77.86

295.73

288.15

-7.35

0.953

293.15

-8.83

298.15

–10.31

303.15

–11.79

308.15

–13.27

313.15

–14.75

318.15

–16.23

323.15

-17.71

333.15

-20.66

3 4

30


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1 2

Figure 1.

3

4

5 6 7

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 2.

3 4 5

32


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1 2 3

Figure 3.

4 5 6 7 8 9 10 11 12

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 4.

3

4 5 6 7

34


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1 2

Figure 5.

3 4

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 6.

3 4

36


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1 2

Figure 7.

3 4 5 6 7 8 9 10

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 8.

3 4 5 6 7 8 9 10 11

38


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1 2

Figure 9.

3 4 5 6 7 8 9 10

39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 10.

3 4 5 6 7 8

40


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1 2

Figure 11.

3 4 5

41



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42


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New Amidoxime-Based Material TMP-g-AO for Uranium Adsorption

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