Separation of Uranium from Rare Earth Elements using Modified Polymeric Resin

International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 www.ijm‐me.org doi: 10.14355/ijmme.2016.05.003

Separation of Uranium from Rare Earth Elements using Modified Polymeric Resin Emad H. Borai1 and Mahmoud G. Hamed2 1) Analytical chemistry Department, Hot Laboratories Center, Atomic Energy Authority, 13759, Egypt 2) Waste Management Department, Hot Laboratories Center, Atomic Energy Authority, 13759, Egypt Abstract Poly (Acrylamide‐ Maleic Acid‐ Amidoxime) P (AM‐MA‐AO) resin has been successfully prepared by modification of polymeric resin Poly (Acrylamide‐ Maleic Acid‐ Acrylonitrile) prepared by γ radiation grafting polymerization. Conversion of nitrile groups into the amidoxime was carried out by treatment of the polymer with hydroxylamine in alkaline solution.

Identification of amidoxime group was qualitatively performed. The structure of the modified resin was confirmed using Fourier transform infrared (FTIR), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). The modified polymer was tested for separation of U4+ from some selected rare earth elements (REEs) such as La3+ and Eu3+ metal ions from aqueous solutions. U4+ ion was selected to represent the actinides while La3+ and Eu3+ were selected to represent the light and intermediate REEs respectively. Different key parameters such as contact time, pH, and initial metal ion concentration have been tested to optimize the sorption capacity as well as the separation efficiency of U4+ metal ions. Results demonstrated that the new prepared sorbent has higher sorption uptake for U4+ than that for La3+, Eu3+. The results also revealed that the new P(AM‐ MA‐AO) resin has relatively high selectivity and high capacity for U4+ reaching to 123 mg/g at pH 5. The separation factor of the resin for U4+ between La3+ and Eu3+ being maximum at pH 5, reaching to 34.5 and 21.9 respectively.

Keywords Gamma Radiation; Polymerization; Separation; Lanthanides; Uranium

Introduction Modification of polymeric materials through chemical reaction improves the properties of participating function groups. Grafting is known to improve the characteristic properties of the backbones. Modification of grafting polymerization is a method of addition of new and desirable properties to backbone without drastically affecting the basic properties of resin used. Large number of the previous studies have been reported for the grafting polymerization of different polymers onto monomers obtained from different sources [1‐2]. Grafting of co‐ monomers on to the backbone of polymer improves their self‐life against thermal stability and metal ion binding ability [3]. Grafting copolymerization initiated by gamma irradiation may be one of the most effective methods than convenient chemical catalytic method because it is very easy to create radical sites on the polymer matrix and introduce various useful functional groups onto these sites. It has been founded to possess potential to simplify the grafting process and reduce the production cost [4].

Adsorption by using superabsorbent materials is the key to remove metal ions from natural resources and waste water [5]. Superabsorbent resin may offer the desired properties and potential applications in this research area [6].

The adsorption capability of metal ions by the polymer is very important for the selection of a suitable adsorbent not only for metal removal but also for metals separation from aqueous solutions [7]. In other fields several attempts have been made to modify the properties of superabsorbent polymeric materials, from macro to micro to nano particles. Much more attention has been focused on the improvement of the swelling ability, gel strength, mechanical and thermal stability of super‐absorbents. Graft copolymerization of acrylamide and acrylacyloxyethyltrimethyl ammonium chloride onto starch was carried out by Song [8], using a complex initiation system of urea and ammonium persulfate. It has been reported that treatment of two kinds of industrial waste water by the grafted starch was better than those treated with cationic polyacrylamide.

In the present study, new absorbent P(AM‐MA‐AO) resin was synthesized by modification of Poly( Acrylamide‐ Maleic Acid‐ Acrylonitrile) resin which prepared by γ ‐ray irradiation‐induced graft polymerization. The new resin

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was characterized and used as an adsorbent material for the removal and separation of uranium ion from rare earth metal ions in aqueous solution.

Experimental Part Hydroxylamine hydrochloride, salts of lanthanium, eurobium and uranium were obtained from Merck.

Hdrochloric acid and NH4OH are used to adjust pH which were purchased from Fluka. During all our experiments bi‐distilled water was used for dissolution, dilution and other analytical purposes. De‐oxygenated water was used for preparation process [9‐10].

The instrumental tools were used in the present work such as a digital analytical balance, a laboratory oven, a programmable shaker thermostat and thermo orion manufacturing. Computerized UV/Vis double beam spectrophotometer of model T80, pG Instruments Ltd. (England), was used for spectrophotometric determination of individual metal ions. The cell which used for initiation reaction is Cobalt‐60 gamma cell of type MC–20 (Russia) was used as an irradiation source. For characterization of the polymer, FTIR (Fourier transformed infrared) spectra in the range of 4000‐400 cm‐1, TGA thermograms, Scanning electron microscope SEM at accelerating voltage of 20 kV were used.

Preparation of Modified Polymeric Resin 1)

Preparation of Hydroxylamine Solution

Hydroxylamine hydrochloride (NH2OH.HCl) was dissolved in methanolic solution with the percentage 6:1.

Methanolic solution prepared at the ratio methanol to bi‐distilled water of 5:1. The HCl of NH2OH.HCl was neutralized by NaOH solution and the precipitate of NaCl was removed by filtration. The pH of the reaction solution was adjusted to 10.0 using NaOH solution[11].

2)

Preparation of P(AM‐MA‐AO) Resin

In order to obtain chelating resin with various macroreticular structure, the resin was prepared through amidoximation reaction by conversion of acrylonitrile function groups present in P(AM–MA–AN) resin. The reaction is based on treatment of acrylonitrile with hydroxylamine [12]. About 20 g of the prepared P(AM–MA– AN) resin was placed into the two‐neck flask, which was equipped with a mechanical stirrer, condenser and thermostat water bath. Then 40 ml of hydroxylamine solution was added to the flask with 300 ml methanolic solution (methanol/bi‐distilled water 5:1) and the reaction was carried out at 70°C for 2 h duration. The resin was separated from the solution by filtration and washed several times with methanolic solution solution (0.1 N HCl) for at least 5 min. Finally, the resin was filtered, washed several times with methanolic solution (methanol/bi‐distilled water 5:1) and then dried at 50°C to a constant weight.

Adsorption Studies Effect of shaking time was carried out at the range from 5 min to 24 hrs, pH = 4, metal ion concentration 100 mg/l.

The effect of pH on the adsorption of La3+, Eu3+ and U4+ metal ions was tested at different pH values ranging from 1.0 to 5.0. The effect of initial metal concentration on the adsorption process onto the resin was studied at various initial metal concentration ranged from 100 to 2000 mg/l for all studied metal ion. In batch experiments, 0.05 g of the resin was immersed with 10 ml of aqueous solution containing different metal concentration. The mixture was shaken at room temperature. The polymeric resin was separated from the aqueous solution by centrifugation and then filtration using 0.45μm whatman membrane filter to ensure the removal of suspended material in solution.

Finally, the concentration of metal ions was measured. A sensitive coloring reagent 4–(Pyridyl–2–azo) resorcinol (PAR) was used as a coloring reagent of the sample. The sample was then identified by UV/Vis double beam spectrophotometer. The percent uptake, the capacity and the distribution coefficient were determined as shown:‐

  V q(mg / g )  C i C e m % uptake 

20

(1)

(C i  C e ) 100 (2) Ci

International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 www.ijm‐me.org

Kd 

(C i  C e ) V * 1000 (3) Ce m

Where, Ci and Ce are the initial and equilibrium concentrations (mg/l) of metal ion, respectively. (q) is the maximum capacity (mg/g), V is the volume of solution in liter and, m is the weight of the resin in gram.

Results and Discussion Preparation of P(AM‐MA‐AO) Resin Amidoximation reaction was carried out for P(AM‐MA‐AN) grafted polymer prepared by gamma‐radiation technique. The conversion of nitrile group to amidoxime one was carried out by treatment of the prepared resin P(AM‐MA‐AN) with hydroxylamine hydrochloride in an alkaline medium at optimum experimental conditions of copolymer‐hydroxylamine ratio 1:1, reaction time 2 h, temperature 70°C, reaction medium methanol to water ratio of 5:1 and pH 10.0. To insure the complete conversion reaction, amidoxime groups in the resin were confirmed by vanadium ion test in the formation of a purple complex [13]. Therefore, 0.2 g of wet resin was shaken with solution of vanadium(V) ions in dilute hydrochloric acid where, a purple coloured complex on the resin beads was observed [14]. The proposed structure and the reaction mechanism for synthesis of P(AM‐MA‐AO) resin is shown on Sch. (1).

H cH

C

cH

H2C

O

C

O

O

H

OH

C

N H2O H

CN NH2 C H2C

NH2 C

O

CH

H2C

O

CH

P (A M - M A - A N )

n

H

H

H

H

C

C

C

cH

H

O C

H2C

O

O

H

C

OH

NOH

H2N

O

C

NH2

NH2

C

C

CH

H2C

CH

O n

P (A M - M A - A O )

SCH. (1): SYNTHESIS OF P(AM-MA-AO) RESIN.

Characterization of the Prepared Adsorbents 1)

FTIR Analysis

FTIR spectrum of the prepared P(AM‐MA‐AO) resin is presented in Fig 1. The stretching frequency of >C=O for P(AM) was observed by the absorption peak at 1650 cm‐1, free >NH stretching was observed by the absorption peak at 3445 cm‐1 that belong to amide (–CONH2) group of PAM. From the literature the bands appearing at 3058 corresponding to the adsorbed water molecule [15‐16]. The absorption peak around 3500 cm‐1 can be ascribed to the C–N group. The absorption peak at 1729 cm‐1 for C=O of carboxylate group. The new band of amidoxime groups >C=N stretching vibration formed at 1648 cm‐1 and the amide II band of NH formed at 1570 cm‐1. The presence of these peaks verifies that the conversion of cyano groups to amidoxime has been successfully occurred.

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100

% Transmitance

95 90 85 80 75 70 4000

3500

3000

2500

2000

1500

1000

500

W a v e n u m b e r C m -1 FIG. 1 FTIR SPECTRA OF P(AM‐MA‐AO) RESIN.

2)

Scanning Electron Microscope

The morphology of the synthesized modified polymeric resin P(AM‐MA‐AO) was investigated by scanning electron microscopy (SEM) as shown in Fig. 2. It can clearly observe that the surface is uniform, flake structure and the layers become thin. This is mainly due to the conversion into AO. This structure is expected to increase the adsorption capacity.

FIG. 2 SCANNING ELECTRON MICROGRAPH OF P(AM‐MA‐AO) RESIN.

3)

Thermogravimetric Analysis (TGA) TGA mg

DTA uV -20.00

-17.560 % -108.765 %

283.31 C 334.80 C

-13.051 %

10.00

104.60 C

-40.00

565.49 C -38.705 %

-60.00

5.00 -13.668 % -25.618

-80.00 -0.00 -0.00

200.00

400.00

600.00 Temp [C]

800.00

1000.00

FIG. 3. DTA AND TGA CURVES FOR P(AM‐MA‐AO) RESIN.

Thermo‐gravimetric analysis (TGA) and differential thermal analysis (DTA) were used to provide an alternative model of the kinetics of the prepared polymer degradation. For P(AM‐MA‐AO) resin, the curves of TGA and DTA are shown on Fig.3. The prepared polymeric resin was heated at the rate of 10°C/min. The degradation

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proceeds in three stages. The first stage from (50 to 200°C) with a maximum endothermic peak at 104.6°C exhibits a weight loss of 17.5% which, is due to the removal of absorbed or coordinated water molecules. The second stage from (201 to 510°C), which shows endothermic peaks at (283–334°C) exhibits a weight loss of 51.8% due to decomposition of uncoordinated amide group[17]. This decomposition was carried out through a cyclization process with the removal of ammonia or water molecules from two amide group. The reaction is similar to the intramolecular or intermolecular de‐amination in the pyrolysis of other amides. The splitting of these molecules occurs either by intermolecular or intramolecular process.

In the third stage from (511 to 1000°C), the weight loss was 38.6% with a maximum endothermic peak at 565.4°C which, may be due to process accompanying by main chain scission, removal of volatile hydrocarbons and complete degradation to the oxide form[18‐19].

Adsorption Ability of the Resin for Metal Ions 1)

Metal Hydrolysis

The sorption processes of the resin for La3+, Eu3+ and U4+ metal ions are mainly affected by the side reaction that may be occurred due to the precipitation at different pHs. In this respect, based on our previous work, series of experiments have been tested separately to evaluate the degree of contribution of each of the two reaction mechanisms (sorption and precipitation). Therefore, the sorption experiments for all the metal ions were tested in the pH range from 1 to 5 to avoid the metal hydrolysis [12, 20].

2)

Effect of Contact Time

The effect of time on the adsorption rate of La3+, Eu3+ and U4+ metal ions is shown in Fig. 4. The adsorption result of the resin shows an initial rapid adsorption rate. It can be attributed to the fact that, at the initial stage, the adsorption sites are more available and the metal ions can interact easily with the sites. Therefore, high adsorption rate takes place. Besides, the driving force for adsorption is the concentration gradient between the bulk solution and the solid‐liquid interface and the concentration gradient is high in the initial period, which results in a high adsorption rate. The sorption uptake percentage was attained to equilibrium after 3 hrs for the investigated metal ions. The percent uptake found to be 91, 46 and 35 for U4+, Eu3+ and La3+metal ions, respectively.

1 00

91

% uptake

80

60

46 35

40

20

L a n th a n u m E u ro b iu m U ra n iu m

0

50

) n i m ( e m i T

0

10 0

15 0

20 0

2 50

FIG. 4 EFFECT OF CONTACT TIME ON THE ADSORPTION OF LA3+, EU3+ AND U4+ METAL IONS ONTO P(AM‐MA‐AO) RESIN AT PH 4.

3)

Effect of pH

The influence of pH on the adsorption capacity on the prepared resin for the adsorption of La3+, Eu3+ and U4+ metal ions are shown in fig. 5. The results revealed that, the percentage uptake increases with increasing pH reached to maximum at pH 5. The hydrolysis of the metal ions begins at pH 6 and various species can be formed. Thus, as pH increases, (over pH 5.0) hydrolysis precipitation most probably starts due to the formation of various hydro‐complexes in aqueous solution.

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The modified polymer contains carboxylate, carboxamide and amidoxime groups which are of anionic type polymer, which ionizes at a high pH values according to calculated dissociation constants. Below pH 5, the carboxylate groups are protonated and the copolymers networks shrinksʹ significantly, therefore the adsorption capacity is lower at low pH due to electroststic repulsion forces between similar charges. It is also found that, the adsorption of (U4+) metal ions is higher than the adsorption of La3+ and Eu3+ metal ions. The investigated ions show different trends of adsorption, probably because they belong to different groups of the periodic table.

The maximum uptake percent at pH 5.0 reached to 96, 52.2 and 41% for U4+, Eu3+ and La3+ respectively. It is clear that, P(AM‐MA‐AO) is more effective for the quantitative removal of (U4+) than trivalent lanthanides. This can be explained by, the dissociation of amide, carboxylic and amidoxime groups increase with the increase of pH providing additional binding function for the adsorption of metal ions. Chemical bonds (hydrogen and co‐ ordinated bonds) may be formed between the function groups present in the resin and metal ions. Their different extents of adsorption may be attributed to their different effective hydrated ionic radii [21].

100 Urannum Lanthanumm Eurobium

90

96

% uptake

80 70 60

52.2 50

41

40 30 20 1

2

3

4

5

pH FIG. 5 EFFECT OF PH ON THE ADSORPTION OF LA3+, EU3+ AND U4+ METAL IONS ONTO P(AM‐MA‐AO) RESIN.

4)

Separation of U4+ from La3+ and Eu3+.

The separation factors between the two metals U4+ from La3+ and U4+ from Eu3+ are calculated based on the ratio of their distribution coefficients under identical experimental conditions as follows:

 SF  KdU / KdEu , or , La  (4) As shown in figure 6 and table 1, the separation factor increased with increasing pH. At equilibrium pH 5.0, the maximum separation factors for U4+ from La3+ and Eu3+ were 34.5, 21.9 respectively. Thus, the selectivity of the resin for U4+ was sufficient to be used for separation light rare earth La3+ more than intermediate rare earth Eu3+metal ions.

35

La Eu

separation factor

30 25 20 15 10 5 0 1

2

3

Equilibrium pH

4

5

FIG. 6. OPTIMIZED SEPARATION CONDITIONS FOR U4+ FROM LA3+ AND EU3+ AT DIFFERENT PH.

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TABLE (1). THE EFFECT OF PH ON SEPARATION FACTOR OF U4+ FROM LA3+ AND EU3+.

Kd(mL/g)

separation factor

pH

5)

U4+

La3+

Eu3+

U4+/ La3+

U4+/ Eu3+

1

216.6

63.5

69.1

3.4

3.13

2

489.6

74.3

93.0

6.5

5.26

3

1466.7

89.8

126.9

16.3

11.55

4

2012.2

107.6

163.6

18.7

12.3

5

4800

138.98

218.4

34.5

21.9

Effect of Initial U4+Concentration on the Adsorption Process of P(AM‐MA‐AO) Resin

Depending on the high resin affinity for U4+, the effect of initial metal concentration on the adsorption capacity has been carefully focused only on U4+ ion as illustrated in figure 7. The experiments was carried out at initial concentrations of U4+ ranged from 50 to 2000 mg/l at pH 5. The surface of the adsorbent is expected to be negatively charged, which facilitates the adsorption of the positively charged uranium ions, consequently, the adsorption capacity of U4+ metal ions increases. The amidoxime groups present on the adsorbent also, increase the adsorption of U4+ metal ions [22, 23]. The adsorption results show an initial rapid adsorption rate and the maximum capacity reached to 123 mg/g.

140

123

capacity (mg/g)

120 100 80 60 40 20 0 0

500

1000

1500

2000

4+

U mg/L FIG. 7. EFFECT OF INITIAL U4+ METAL ION CONCENTRATION ON THE ADSORPTION CAPACITY OF P(AM‐MA‐AO) RESIN.

Conclusion New P(AM‐MA‐AO) polymeric resin is successfully modified by gamma radiation induced graft polymerization.

The identification of amidoxime function groups was confirmed by vanadium ion test. This structure was confirmed by the FTIR, TGA and SEM micrographs. The characteristic data confirmed the grafting of P(AM) remarkably alters the structure of the resin. SEM revealed that the modified polymer has a much uniform surface.

The application area of the resultant modified polymers is not only targeting to improve the selective for U4+ rather than lanthanide metal ions but also improving the corresponding maximum adsorption capacity. The adsorption results showed that the maximum capacity for U4+ reached to 123 mg/g. The results showed satisfactory separation factors 34.5, 21.9 for U4+ from La3+ and Eu3+ respectively.

ACKNOWLEDGMENTS

This project was supported financially by the Science and Technology Development Fund (STDF), Egypt, Grant No

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5021. Also, the authors thank our colleagues in Cyclotron Project, Nuclear Research Center, Atomic Energy Authority, Egypt for their technically supported during the irradiation process.

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