Uranium Adsorption from Aqueous Nitric Acid Solution by Solvent

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using the extraction chromatography technique (solvents impregnated material), ... relevant factors affecting uranium adsorption onto the prepared adsorbent of.

The Egyptian

A. E. M. Hussein, et al.

Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017) Arab Journal of Nuclear Sciences and Applications Society of Nuclear Vol 50, 4, (156-170) 2017

ISSN 1110-0451

Web site: esnsa-eg.com

Sciences and Applications

(ESNSA)

Uranium Adsorption from Aqueous Nitric Acid Solution by Solvent Impregnated Polypropylene A. E. M. Hussein, W. M. Youssef, M. H. Taha and M. M. El-Maadawy Nuclear Materials Authority, Cairo, Egypt

Received: 28/11/2016

Accepted: 12/4/2017 ABSTRACT1

The present work studies the adsorption of uranium from nitric acid solution using the extraction chromatography technique (solvents impregnated material), where Di-2ethyl hexyl phosphoric acid (D2EHPA) organic solvent was impregnated on polypropylene polymer as a suitable adsorbent. The influence of various factors affecting the impregnation process including solvent concentration, temperature, impregnation time, mass/volume ratio and diluents type were optimized. The relevant factors affecting uranium adsorption onto the prepared adsorbent of D2EHPA impregnated polypropylene (SIPP) including initial uranium concentration, contact time, solution pH, mass/volume ratio and temperature were investigated. From the obtained adsorption results different isotherms were fitted. The calculated theoretical capacity of prepared adsorbent was 1.0 g U/g polypropylene impregnated D2EHPA. Keywords: Uranium, adsorption, Nitric acid solution, Solvent Impregnated Polypropylene INTRODUCTION Radioactive waste management is having an increasing attention around the world because of the widespread applications of nuclear energy (1-3). A number of techniques are presently used in the management of radioactive wastes. These are ranging from direct release to the environment to complicated techniques for immobilization of the radionuclides in well-designed disposal amenities. Solvent impregnated supports (SIS) or extraction chromatography (EXC) are now at the forefront of these waste management techniques compared with other techniques because of the development of solid adsorbents including chelating polymeric supports, and the advantages in the use of these adsorbent in metal ions removal. Solvent impregnated support offer several important advantages such as(4-9) : i- Higher extraction capacity ii- Absence of emulsion iii- Safety of hazardous samples iv- Low operation costs v- Flexibility vi- Ease of mechanization. Many materials have been developed in which extractant ligands used in solvent extraction processes are incorporated into solid-phase materials in order to produce sorbent materials for uranium separation from radioactive waste solutions. The extractant ligand is impregnated or coated onto solid support structures such as polymer resins, porous silica, polymer membranes, metal oxide particles, clays, carbon-based materials or magnetic nanoparticles(10- 12).

1 Corresponding author E-mail: [email protected]

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

On the other hand, solvent impregnated materials (8,9) gave high adsorption efficiencies, where this technique is providing the best use advantages of both liquid-liquid extraction and solid-liquid techniques and are characterized by their high binding capacity, selectivity and improves mobility of the extractant on the solid surface. In this study, the uranium removal from nitric acid solution using (D2EHPA) impregnated polypropylene has been studied by batch experiments. EXPERIMENTAL Materials and Reagents All reagents used were of analytical reagent grade except for D2EHPA, manufactured by Riedel-deHaen, was of a commercial grade and was used without purification. The working 100% pure polypropylene (pp) (with other names Polypropene, Propene polymers, Propylene polymers) sample was provided from polypropylene water cartridge filters of Suzhou industrial Park Topology Environmental Protection and Purification Co., Ltd. China. The physical description of the obtained sample is: white color with surface area of 450.9 m²/g, density as 0.35 g/cm3, melting point from 130°C to 171 °C, and chemical formula (C3H6). Methyl group improves mechanical properties and thermal resistance, while the chemical resistance decreases (13). The working polypropylene cutoff with average weight = 0.0100 ± 0.002 g, were cut from sheets. Each adsorbent plug was squeezed in 2 M HCl for 1 h, washed with distilled water until free of HCl, again squeezed, and air-dried overnight before using. A synthetic uranium stock standard solution assaying 1000 mg/l was prepared by dissolving 1.664 g uranyl nitrate [ UO2(NO3)2] of BDH Chemicals Ltd. Poole, England in 1 liter. Uranium concentration was determined by colorimetric method (UV/VIS spectrophotometer Perkin-Elmer, USA) using Arsenazo III(14). Preparation of the Impregnated polypropylene In order to study the factors affecting the impregnation process, several series of impregnation experiments have been performed by shaking 0.05g of dry clean samples of polypropylene with the properly prepared impregnation solutions (D2EHPA in benzene) by magnetic stirrers. After the proper time, polypropylene was dried of in oven for one hour at 80 ºC to evaporate the diluent and leave the diffused solvent into the polypropylene pores. The amounts of solvent impregnated on the polypropylene samples were calculated by the difference between the samples weight before and after the process. Experiments and Procedures Uranium Adsorption Batch adsorption experiments were performed by shaking 0.05 g of the impregnated polypropylene sample with 20 ml of the uranium synthetic solution (80 ppm) using a magnetic stirrer at (25 ± 1oC). The amount of the uranium adsorbed was calculated by the difference between the equilibrium concentration and the initial concentration. The amount of uranium retained in the solid phase qe (mg/g) was calculated using the following relation:

qe  Co  Ce  

V m

(1)

Where Co and Ce are the initial and equilibrium concentrations of the uranium (mg/l), respectively, V is the volume of the aqueous phase (l), and m is the weight of the D2EHPA impregnated polypropylene used (g). The removal percent of ions from the aqueous phase is calculated from the following relation: Uranium adsorption % =

C o  Ce  100 Co

157

(2)

A. E. M. Hussein, et al.

Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

The distribution coefficient (Kd) of uranium between the aqueous bulk phase and the solid phase was calculated from the following relation: Kd 

C o  Ce V (3)  Co m

Equilibration Calculation All uranium speciation in this study were performed with Hydra-MEDUSA, a chemical equilibrium calculation program(15,16). RESULTS AND DISCUSSION

1. Results of Polypropylene Impregnation Factors 1.1. Effect of Solvent Concentration To study the effect of solvent concentration upon the amount of D2EHPA impregnated on polypropylene, a series of impregnation experiments were performed at different D2EHPA concentrations ranging from 0.1 up to 1.5 M in benzene, while the other parameters were room temperature, impregnation time, of 1 h, and liquid/ mass ratio, ml/ g, of 30/1. The results obtained in Fig. (1) Shows that the amount of solvent loaded onto the polypropylene increased from 0.14 to 2.7 mmol/ g pp with increasing the solvent concentration from 0.1 to 1 M. Further increase in the D2EHPA concentration has a slight effect on the amount of D2EHPA impregnated on polypropylene. Accordingly, a solvent concentration of 1 M was applied in further experiments as the preferred a solvent concentration.

Fig. (1): Effect of D2EHPA concentration upon the impregnated amount onto polypropylene (time 60 min; temperature 25 ◦C, liquid/ mass ratio, ml/ g 30/1; benzene as diluent) 1.2. Effect of Impregnation Time The effect of impregnation time ranging from 5 to 60 min on D2EHPA impregnated amount was investigated at an impregnation temperature of about 25 ºC, 1 M D2EHPA concentration in benzene, and liquid/ mass ratio, ml/ g, of 30/1. The experimental results are given in Fig. (2) shows that the amount of solvent on the polypropylene increased from 1.7 to 2.7 mmol/ g pp with increasing the impregnation time form 5 up to 30 min. Further increase in impregnation time from 30 to 60 min has rare effect on the amount of the impregnated solvent. Thus, 30 min is the choice of impregnation time used for the other experiments of the D2EHPA impregnation.

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

Fig. (2): Effect of impregnation time upon the amount of D2EHPA impregnated onto polypropylene ([D2EHPA]: 1 M; temperature 25 ◦C, liquid/ mass ratio, ml/ g 30/1; benzene as diluent) 1.3. Effect of Liquid/ Mass Ratio In order to study the effect of liquid/ mass ratio upon the amount of D2EHPA impregnated on polypropylene, several experiments were performed at different liquid/ mass, ml/ g, ratio from 10/1 up to 50/1 at room temperature for 30 min impregnation time using solvent concentration of 1 M and benzene as diluent. The experimental results given in Fig.( 3)reveals that the amount of solvent loaded onto the polypropylene increased from 1.72 to 2.93 mmole/ g pp with increasing the liquid/ solid mass ratio, ml/ g, to 40/1. Further increase in the liquid/ solid ratio, to 50/ 1 has almost no effect on the amount of the impregnated solvent therefore, 40/1 was chosen as preferred liquid/ mass ratio for the experiments.

Fig. (3): Effect of impregnation solution (D2EHPA) volume/ polypropylene mass ratio on the amount of loaded solvent ([D2EHPA] 1 M; temperature 25 ˚C; time: 30 min; benzene as diluent)

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

1.4. Effect of Diluent Type To prepare the impregnation solution it is necessary to use a diluent in order to reduce the solvent viscosity, thus increasing its extension on the surface of the dry polypropylene as well as improving the ability of the solvent to reach the interior polypropylene pores. Several impregnation experiments were carried out using different types of diluents for impregnation solution preparation namely, benzene, kerosene, toluene, and cyclohexane. These experiments were performed under fixed conditions namely, impregnation solution concentration of 1M for 30 min impregnation time , liquid/ mass ratio of 40/1 at room temperature. From the obtained results cited in Table (1) it is clear that benzene gives the best impregnation results, consequently it was preferred as the best diluent. Table (1): Type of diluents affecting the impregnated D2EHPA on the working polypropylene sample ([D2EHPA] 1 M; room temperature; time 30 min; liquid/ mass ratio, ml/ g 40/1) Type of diluent

Impregnated solvent, mmol/g polypropylene.

Benzene Kerosene Toluene Cyclohexane

2.930 2.360 2.115 2.590

1.5. Effect of Impregnation Temperature The effect of temperature on the solvent impregnated amount on polypropylene was investigated for the temperatures ranging from 25 up to 60 ±1 ◦C at D2EHPA concentration of 1 M for 30 min impregnation time using liquid/ mass ratio of 40/1 and benzene as solvent diluent. The obtained results were graphically presented in Fig. (4)where it is noticed that the amount of solvent loaded onto the polypropylene slightly increased from 2.91 to 3.21 mmole/ g pp with increasing the impregnation temperature from 25 ºC to 50 ºC while further increase in the temperature to 60 ºC, led to a decrease in the amount of solvent loaded onto the polypropylene. This behavior may be attributed to the shortage of the polypropylene surface at temperature above 50 ºC. Therefore, the room temperature was preferred for the impregnation investigation. Impregnated solvent, mmol/g pp

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 20

25

30

35

40

45

50

55

60

65

Temp., ºC

Fig. (4): Effect of impregnation temperature upon the amount of D2EHPA impregnated onto polypropylene ([D2EHPA] 1 M; time: 30 min; liquid/ mass ratio, ml/ g 40/1; benzene as diluent) 1.6. Choice of the Preferred Conditions The obtained results show that the preferred impregnation conditions are D2EHPA concentration of 1 M in benzene, solvent/ polypropylene (ml/g) ratio 40/1, impregnation time 30 min, at room temperature. It is suggested that adsorbent D2EHPA on the polypropylene is mainly due to a

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

combination of pore filling as well as surface (polypropylene surface area is 450.9 m²/g) adsorption. This suggestion is supported by SEM micrographs of the polypropylene surface before and after impregnation process as shown in Fig. (5.A, 5.B).

Fig. (5.A, 5.B): SEM Micrographs of the polypropylene surface before and after impregnation with D2EHPA ([D2EHPA] 1 M; time 60 min; liquid/ mass ratio (ml/ g) 40/1; benzene as diluent) The impregnation method of the D2EHPA on inert support is mostly owing to two phenomena, i.e. pour filling( Figure 5) and surface adsorption. This suggestion of filling the D2EHPA solvent of polymer pores is confirmed by a detailed investigation of qualitative FT-IR Spectroscopic description of the polypropylene before and after the impregnation. The spectrum of untreated polypropylene is shown in Figure 6. A, and that after filling with D2EHPA is shown in Fig. 6.B. Figure 6. A shows the typical conjugated -CH3 asymmetric group bands at 2946.7, 2336.34, 1623, 1456.96 cm-1 and CH3, CH2, CH, CH2, CH3,CC symmetric group bands at 1377.89 cm-1 . Weak group bands of CH, CH2, CH3 at 1250.6 cm-1 and medium group band of CC, CH3, CH at 1163.8 cm-1. Medium group band of CH3, CH CH2 at 995 cm-1. Medium group band of CH2, CC, C-CH3, CH3 at 842.7 cm-1 and -(CH2CH)n- at 704.8 cm-1 (17). After the impregnation process, the characteristic vibration bands of D2EHPA(18) are P=O, PO-C or P-O-H and O-H are observed on the polymer surface. Based on the results shown in Fig. 6. B, these typical vibration bands are indexed as 1234, 1030 and6187 cm-6 respectively. After uranium adsorbed on the prepared adsorbent the characteristic vibrational band of D2EHPA (P-O-H, 1033 cm-6 in loaded spectra) was extended (Fig. 6. C).

Fig. (6. A): IR Spectra of polypropylene before impregnation by D2EHPA

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

Fig. (6. B): IR Spectra of polypropylene after impregnation by D2EHPA

Fig. (6. C): IR Spectra of polypropylene after uranium loaded on D2EHPA impregnated polypropylene polymer 2. Results of Uranium Adsorption Studies To investigate the factors affecting uranium adsorption onto the prepared solvent impregnated polypropylene,a suitable amount (2 g) of polypropylene impregnated with D2EHPAis prepared and then divided into suitable portions for further studies. These factors including, solution pH, contact time, volume/mass ratio and initial uranium concentration. 2.1. Effect of Solution pH To study pH value effect of the working solution for uranium adsorption onto the prepared impregnated polypropylene, batch equilibrium experiments were performed in the initial pH range of 0.1− 3 to determine the effect of pH on the adsorption of uranium ions by D2EHPA impregnated polypropylene. The experiments were performed under constant initial uranium concentrations of 80 mg/l at room temperature (≈ 25 ºC) for 60 min. contact time and prepared adsorbent/ uranium solution ratio of 2.5 g/l. The experimental results are plotted in Fig. (7). From the obtained data, it is clear the uranium adsorption efficiency almost does not change with the change in solution pH under the investigation conditions. This means that the adsorption of uranium is highly independent on the surface characteristics of the adsorbents at various pH values but may be correlated to the permanent surface negative charge of the prepared adsorbent and the chemical species forms of uranium in the aqueous phase.

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

Fig. (7): Effect of pH on uranium adsorption efficiency onto D2EHPA impregnated polypropylene (uranium concentration: 80 ppm; time 60 min; liquid/ mass ratio 2.5 g/ l; room temperature) Aqueous speciation distribution of uranium was identified by applying Hydra- Medusa chemical equilibrium database and plotting software(15) and represented in Fig. 8. The results showed that the complexes of UO2(NO3)+ and UO22+ were the predominant species at the pH range from 0 - 4 with mean total percent of about 18% and 82 % respectively. At near neutral and alkaline pH conditions, Uhydroxide complexes start to dominate the aqueous phase. At pH 7, the UO2(OH)2.H2O became the major species with about 100% of total concentration at pH range from 4.5 to 10.5 while at pH 12, UO2(OH)3- became the predominant species within a total percent close to 70% of the total concentration. The formation of UO2(OH)42- species started to grow after pH 10(19, 20).

Fig. (8): Predicted aqueous speciation of U (VI) as a function of pH in 2.4 M HNO 3 (pH of 0.38) using Hydra-Medusa program Based on this identification, the adsorption mechanism of uranium onto D2EHPA impregnated polypropylene was mainly pure ion surface complexation reaction. This conclusion was

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

supported by the structural characteristics of D2EHPA that have ion exchange properties, where the adsorption mechanism could be proposed by following equations (21). 2[𝑈𝑂2 (𝑁𝑂3 )]+ (𝑎𝑞) + (𝐻𝐷)2 (𝑜𝑟𝑔) ↔ 2[𝑈𝑂2 (𝑁𝑂3 )𝐷](𝑜𝑟𝑔) + 2(𝐻)+ (𝑎𝑞) … (1) [𝑈𝑂2 ]2+ (𝑎𝑞) + (𝐻𝐷)2 (𝑜𝑟𝑔) ↔ [𝑈𝑂2 . 𝐷2 ](𝑜𝑟𝑔) + 2(𝐻)+ (𝑎𝑞) … (2) Where (HD) is D2EHPA. 2.2. Effect of Temperature on Adsorption To study the effect of temperature on uranium adsorption efficiency from nitrate solution having uranium initial concentration of 80 mg/l, by D2EHPA impregnated polypropylene, several experiments were carried at different temperatures ranging from 25 to 60 oC at adsorption time of 60 min, solution pH of 0.1 and D2EHPA impregnated polypropylene/ solution ratio of 2.5 g/l. The experimental results are presented in Fig. (9 ) . The obtained results show that, uranium adsorption efficiency was affected by the increase in temperature from 25 to 60 oC where, uranium adsorption efficiency decreased from about 60.5 to 37.5 respectively. This behavior indicates that the uranium adsorption from the nitrate solution by D2EHPA impregnated polypropylene is an exothermic process. Accordingly, the room temperature represents the preferred reaction temperature.

Fig. (9): Effect of temperature on uranium adsorption efficiency onto D2EHPA impregnated polypropylene (uranium concentration: 80 ppm; time 60 min; liquid/ mass ratio 2.5 g/ l; solution pH of 0.1) 2.3. Effect of Contact Time The kinetics experiments for uranium (VI) adsorption from nitrate solution on the D2EHPA impregnated polypropylene was performed as a function of shaking time at different time intervals ranging from 1 to 120 min by contacting 2.5 g D2EHPA impregnated PP/ liter of nitrate uranium solution containing an initial concentration of 80 mg/l, solution pH of 0.1 at room temperature (≈ 25 ºC). The results are represented in Fig. (10).that indicates that the adsorption efficiency increases rapidly where 61 % of the uranium (VI) is adsorbed during the first 60 min. and then gradually, tends to remain constant. The equilibrium time considered for the further work has been taken as 60 min.

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

Fig. (10): Effect of contact time on uranium adsorption efficiency onto D2EHPA impregnated polypropylene (uranium concentration 80 ppm; liquid/ mass ratio 2.5 g/ l; solution pH of 0.1; room temperature) 2.4. Effect of Mass/Volume Ratio The effect of D2EHPA impregnated polypropylene/ nitrate solution on the uranium, 80 ppm, adsorption efficiency from nitrate solution was studied in the range from 1 to 5 g/ l at adsorption time of 60 min, solution pH of 0.1 at room temperature was investigated. The experimental results are plotted in table Fig.(11)which shows that the uranium adsorption efficiency percentage increases from 33 to 61.3 as D2EHPA impregnated polypropylene/ nitrate solution increase from 1 to 2.5 g/l. Further increase in the mass/ volume has a slight effect on the uranium adsorption efficiency. Therefore, D2EHPA impregnated polypropylene/ nitrate solution ratio was kept at 2.5 g/l during all the experiments.

Fig. (11): Effect of mass/ volume ratio on uranium adsorption efficiency on D 2EHPA impregnated polypropylene (uranium concentration 80 ppm; time 60 min; solution pH of 0.1; room temperature) 2.5. Effect of Initial Uranium Concentration To investigate the effect of initial uranium concentration on the adsorption efficiency onto D2EHPA impregnated polypropylene, a series of experiments was performed by equilibrating 2.5 g of D2EHPA impregnated polypropylene / litter of nitrate solutions containing various concentrations of the uranium ion ranged from 60 to 10000 mg/l for 60 min at room

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Arab J. Nucl. Sci. Appl, Vol 50, 4, 156-170 (2017)

100

400

90

350

80

300

70

250

60

ad., %

50

200

qe

40

150

30

qe, mg/g

Uranium adsorption efficiency %

temperature (≈ 25 ºC) and solution pH 0.1. The result was presented in Fig. (12)which shows that uranium adsorption efficiency decreased with increasing its initial concentration. The uranium adsorption capacity of impregnated polypropylene was determined (Fig. 12) as about 360 mg U/g polypropylene.

100

20

50

10 0 0

1000

2000

3000

4000

5000

6000

7000

8000

0 9000 10000

Initial uranium conc., mg/l

Fig. (12): Effect of uranium concentrations on adsorption efficiency on uranium adsorption efficiency on D2EHPA impregnated polypropylene (liquid/ mass ratio: 2.5 g/l; time: 60 min; solution pH 0.1 room temperature) 2.6. Adsorption Isotherms Several common adsorption isotherm models such as Langmuir and Freundlich and others were considered to fit the obtained data isotherm under the equilibrium adsorption of the adsorbent. 2.6.1. Langmuir Isotherm According to the Langmuir model, adsorption occurs uniformly on the active sites of the sorbent, and once a sorbate occupies a site, no further sorption can take place at this site. Thus, the Langmuir model was given by the following equation (3) (22). Ce/qe = 1/bQ0 + Ce/Q0

(3)

where Q0 and b, the Langmuir constants, were the saturated monolayer sorption capacity and the sorption equilibrium constant, respectively. A plot of Ce/qe versus Ce would result in a straight line with a slope of (1/bQ0) and intercept of 1/Q0 as seen in Fig. 13 for D2EHPA impregnated polypropylene foam respectively. The Langmuir parameters given in Table 2 can be used to predict the affinity between the sorbate and sorbent using the dimensionless separation factor RL equation number (4) (22) RL = 1/(1 + bC0)

(4)

RL value indicate the type of isotherm to be irreversible (RL = 0), favorable (0

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