Adsorption of the Uranyl Ions on an Amidoxime-Based Polyethylene

Oct 31, 2012 - The adsorption data obtained under the corresponding conditions are correlated with ... Materials for the Recovery of Uranium from Seaw...
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Adsorption of the Uranyl Ions on an Amidoxime-Based Polyethylene Nonwoven Fabric Prepared by Preirradiation-Induced Emulsion Graft Polymerization Xiyan Liu,†,‡ Hanzhou Liu,†,‡ Hongjuan Ma,† Changqing Cao,† Ming Yu,† Ziqiang Wang,† Bo Deng,† Min Wang,*,† and Jingye Li*,† †

TMSR Research Center and CAS Key Lab of Nuclear Analysis and Energy Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, People’s Republic of China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: A polyethylene (PE) nonwoven fabric adsorbent with amidoxime (AO) groups denoted as PE-g-PAO is prepared by preirradiation-induced emulsion graft polymerization of acrylonitrile (AN) and subsequent amidoximation of the nitrile groups of the poly(acetonitrile) (PAN) grafting chains with hydroxylamine (NH2OH). The chemical structure, thermal stability, and mechanical intensity are evaluated by means of Fourier transform infrared, thermogravimetric analysis, differential scanning calorimetry, and tensile tests, respectively. The adsorption behavior of the uranyl ions in low initial concentrations over a range of 3−50 μg/L on the PE-g-PAO nonwoven fabric is studied by the batch technique at a pH value of 7.5 at different temperatures. It can be found that the temperature induces a positive effect on the adsorption process and the adsorption capacity increases with an increase of the uranyl ion initial concentration. The adsorption data obtained under the corresponding conditions are correlated with the pseudo-first-order and pseudo-second-order kinetic and Freundlich isotherm models, respectively.

1. INTRODUCTION Uranium is the essential element for the operation of the atomic power plant, which mainly exists in two forms in nature, dissolved in seawater and deposited in terrestrial ores. The total amount of uranium in seawater reaches 4.5 billion tons, which is about a thousand times that in the terrestrial ores. Although a huge amount of uranium is dissolved in seawater mainly in the form of the anionic uranyl tricarbonate complex [UO2(CO3)3]4−, its average concentration is only 3.3 μg/L.1,2 Thus, there are grand challenges to make use of this source economically. Many approaches, such as solvent extraction,3 ion exchange,4 and flotation,5 have been proposed to exploit this resource since the 1950s. Among them, an adsorption technique is considered to be a promising approach to recover significant quantities of uranium from seawater. Also, many types of adsorbents, such as hydrous titanium oxide,6 chitosan resin,7 and biomass,8 have been studied. Among the adsorbents reported, those with amidoxime [AO; −C(NOH)NH2] groups have been chosen as promising candidates for their high selectivity and capacity for the uranyl ions since the 1980s.9−14 Compared to the chemical modification method, radiationinduced graft polymerization (RIGP) is prospective for the synthesis of functional adsorbents.15 Those AO adsorbents prepared by the RIGP method have sufficient chemical and mechanical stability because the initial strength of the base material is hardly changed during the synthesis process.16 Much work about uranium adsorption with polymer adsorbents containing the AO groups was reported during the last 2 decades.17,18 Kawai et al. reported the preparation of hydrophilic AO fibers by the cografting of methacrylic acid (MAA) with AN onto polypropylene (PP) fibers and found that the © 2012 American Chemical Society

weight ratio of AN to MAA of 60/40 was the optimized monomer composition for uranium recovery from seawater.19 A polyethylene (PE) membrane with AO groups was prepared via radiation-induced grafting of AN on PE hollow fibers and subsequent amidoximation of the nitrile groups by Chi et al., and the uranium adsorption performance was studied.20 They also reported other PE adsorbents with AO together with carboxyl groups.21 Besides those works, studies from the development of the AO adsorbents in the laboratory to the kinetics of the uranium uptake from seawater have been widely conducted.16,22 Earlier uranium recovery activities mainly focused on the screening tests of the adsorbents through laboratory studies and marine experiments. Most laboratory studies reported in the literature involve the adsorption kinetics of adsorbents with a high uranium initial concentration (mg/L) at an optimal pH value (e.g., 5). There are only a few articles published for the uranium adsorption on the amidoximated nonwoven fabric in a low initial concentration (μg/L) at a neutral pH value. In this paper, the adsorption characteristics of the uranyl ions on the PE-g-PAO nonwoven fabric in low uranyl ion initial concentrations (3−50 μg/L) at a pH value of 7.5 by the batch technique are presented. The influence of experimental conditions such as the contact time, temperature, and uranyl ion initial concentration are studied. The main objective of this work is to identify the adsorption ability of the uranyl ions on an amidoximated nonwoven fabric prepared by the RIGP Received: Revised: Accepted: Published: 15089

July 24, 2012 October 28, 2012 October 31, 2012 October 31, 2012 dx.doi.org/10.1021/ie301965g | Ind. Eng. Chem. Res. 2012, 51, 15089−15095

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Scheme 1. Preparation of AO Adsorbent by Preirradiation-Induced Emulsion Graft Polymerization of AN and Subsequent Amidoximation

microscope (JEOL, Japan). These samples are attached on a carbon tape and sputtered with gold to enhance the electronic conductivity under a vacuum prior to observation. The FT-IR spectra of the pristine PE, PE-g-PAN, and PE-gPAO nonwoven fabrics are performed on a Nicolet Avatar FTIR spectrometer (Thermo Nicolet Co., USA) in the range of 4000−500 cm−1 at a resolution of 4 cm−1 and 32 scans. TGA of pristine PE, PE-g-PAN, and PE-g-PAO nonwoven fabrics is taken on a TG 209 F3 Tarsus analyzer (Netzsch, Germany), under a nitrogen atmosphere at a 10 °C/min heating rate. The DSC curves of the pristine PE, PE-g-PAN, and PE-gPAO nonwoven fabrics are taken on a DSC Mettler Toledo 822e apparatus (Mettler-Toledo International Inc., Zurich, Switzerland) with a heating rate of 10 °C/min from 50 to 300 °C in an aluminum holder with a nitrogen gas flow of 30 mL/ min. The mechanical property tests of the pristine PE, PE-g-PAN, and PE-g-PAO nonwoven fabrics are performed on a QJ210 electronic tensile testing machine (Shanghai Tilting Technology Science and Technology Instrument Co., Ltd.) at room temperature. The elongated samples are stretched at a speed of 10 mm/min. The thickness of each sample is about 0.18 mm, the width 5 mm, and the length 18.6 mm. The final mechanical properties are evaluated from 10 measurements. 2.4. Uranyl Ion Adsorption. The uranyl ion adsorption experiments are performed by the batch technique in 5-L glass bottles. The pH value of the solutions is adjusted to 7.5 prior to the adsorption experiments by the addition of a small amount of Na2CO3/HNO3. About 0.35 g of dried and weighed PE-gPAO nonwoven fabric samples are placed in bottles that contain 3.5 L of uranyl ion solution with initial concentrations from 3 to 50 μg/L. The adsorption solutions are shaken at the same rate of 100 rpm at different temperatures for various contact times in a thermostatic oscillator. The uranyl ion concentration in solution is determined by a trace uranium analyzer (WJG-III). The adsorption amount of the uranyl ions is calculated using eq 2.

technique. The Langmuir and Freundlich equations are used to fit the equilibrium isotherm, while the pseudo-first-order and pseudo-second-order models are chosen to determine the adsorption rates. For this study, the successful grafting of AN onto the pristine PE nonwoven fabric and the subsequent conversion of AN to AO groups are confirmed by Fourier transform infrared (FT-IR) spectra. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are used to compare the thermal properties of the nonwoven fabrics, while the mechanical properties are tested by tensile tests.

2. EXPERIMENTS 2.1. Materials. A polyethylene (PE) nonwoven fabric supplied by DuPont Company is used as the trunk polymer. The weight-average molecular weight of the pristine PE nonwoven fabric is about 4 × 105−6.8 × 105 with an average thickness of 0.18 mm. A uranium standard solution was purchased from Analytical Laboratory, Beijing Research Institute of Uranium Geology. Acrylonitrile (AN) and other chemical agents are all of analytical grade and were purchased from Sinopharm Chemical Reagent Co. Ltd. All of the agents were used without further purification. 2.2. Preparation of the Amidoxime (AO) Adsorbents. The PE nonwoven fabric adsorbents with the AO groups (PEg-PAO) are prepared by preirradiation-induced emulsion graft polymerization of AN and subsequent amidoximation of the PAN grafting chains. The reaction process is outlined in Scheme 1. In a previous paper,23 the emulsion graft polymerization conditions, such as the adsorbed dose, monomer concentration, reaction temperature, and reaction time, are described in detail. The nitrile groups of the grafted nonwoven fabric PE-g-PAN are converted into the AO groups by reacting with hydroxylamine (NH2OH), subsequently. The grafted nonwoven fabric PE-g-PAN with a degree of grafting (DG) of 26.9% is reacted with a 0.1 mol/L NH2OH solution at a pH value of 6.5 at 70 °C for 2 h. Subsequently, the modified nonwoven fabric PE-g-PAO is removed from the solution and washed repeatedly with distilled water. After drying in a vacuum, the content of the AO groups is determined by eq 1. Wa − Wg 1000 AO (mmol/g ) = Wg 33

Q=

1 (C0 − Ct )V 238 1000w

(2)

where Q (mmol/g) is the amount of uranyl ions adsorbed onto nonwoven fabric, C 0 (μg/L) and C t (μg/L) are the concentrations of the uranyl ions in the initial solution and in the aqueous phase after adsorption for a certain time t, respectively, V (L) is the volume of the aqueous phase, and w (g) is the weight of the dry nonwoven fabric.

(1)

where Wa (g) and Wg (g) denote the weights of the amidoximated nonwoven fabric PE-g-PAO and the grafted nonwoven fabric PE-g-PAN, respectively. The molecular weight of NH2OH is 33.24 The content of the AO groups in PE-g-PAO nonwoven fabric is 1.3 mmol/g accordingly. 2.3. Characterizations. Scanning electron microscopy (SEM) images of the pristine PE, PE-g-PAN, and PE-g-PAO nonwoven fabrics are taken on a JSM-6700F scanning electron

3. RESULTS AND DISCUSSION 3.1. Characterization of the PE-g-PAO Adsorbents. 3.1.1. Surface Appearance and Chemical Structure. The SEM images of pristine PE at low (a) and high (b) 15090

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spectrum of these three samples in Figure 2a. In Figure 2b, a sharp peak at 2242 cm−1 is observed because of the adsorption of nitrile groups, which are nearly gone in Figure 2c. The appearance of such a peak shows the successful grafting of AN, while the disappearance of this peak indicates the conversion of the nitrile groups into the amidoximation groups after reaction with NH2OH. In Figure 2c, the new characteristic adsorption bands at 3200−3500, 1647, and 928 cm−1 are attributed to the adsorption of −OH, −CN−, and −N−O− of the AO groups, respectively.20 These results show that PAN is grafted onto the pristine PE nonwoven fabric and the grafted nitrile groups are converted into the AO groups, which means the adsorbent is prepared successfully. 3.1.2. Thermal Property. Thermal analysis techniques are often used for characterization of the polymeric materials.9,25 Figure 3 shows the TGA curves of the pristine PE, PE-g-PAN,

magnification, PE-g-PAN (c), and PE-g-PAO (d) nonwoven fabrics are shown in Figure 1. The SEM image of the pristine

Figure 1. SEM images of pristine PE nonwoven fabric at low (a) and high (b) magnification, (c) PE-g-PAN nonwoven fabric with a DG of 26.9%, and (d) PE-g-PAO nonwoven fabric with an AO density of 1.3 mmol/g.

PE at low magnification reveals the fiber morphology, crisscrossed by a network of fibers in irregular shape with different diameters. At high magnification, it can be found that the surface of the single fiber in the pristine PE nonwoven fabric has many wrinkles (Figure 1b). For the PE-g-PAN nonwoven fabric with a DG of 26.9%, the morphological characteristics were nearly unchanged at low magnification, while at higher magnification, the wrinkles are smoothed (Figure 1c). Also, the overall surface structure of the amidoximated nonwoven fabric PE-g-PAO is also almost unchanged (Figure 1d), which means no trail of destruction happened during the grafting and modification process. Figure 2 presents the FT-IR spectra of the pristine PE, PE-gPAN, and PE-g-PAO nonwoven fabrics. The adsorption bands of −CH2− antisymmetric stretching (2919 cm−1) and symmetric stretching (2851 cm−1) can be observed in the

Figure 3. TGA curves of (a) pristine PE nonwoven fabric, (b) PE-gPAN nonwoven fabric with a DG of 26.9%, and (c) PE-g-PAO nonwoven fabric with an AO density of 1.3 mmol/g.

and PE-g-PAO nonwoven fabrics. The thermogram of the pristine PE nonwoven fabric gives a clean, single-step degradation with an initial decomposition temperature of 336 °C, which is in accordance with the reported results.26 For PEg-PAN nonwoven fabric, two degradation platforms appear, and the initial decomposition temperatures are around 257 and 336 °C, respectively. The first minor weight loss can be attributed to degradation of the grafting chains PAN. The second major weight loss corresponds to degradation of the main chains PE. Two-stage decomposition is also observed for the degradation progress of PE-g-PAO nonwoven fabric. With respect to PE-gPAO, it presents weight loss behavior similar to that of PE-gPAN, but the first initial degradation temperature declines to 203 °C, showing a little lower thermal stability. DSC curves of the pristine PE, PE-g-PAN, and PE-g-PAO nonwoven fabrics are presented in Figure 4. In the DSC curve of the pristine PE nonwoven fabric, a single strong endothermic peak with a rate maximum at about 145 °C appears. The peak is referred to the melting of the PE. As for PE-g-PAN and PE-gPAO nonwoven fabrics, the peak slightly reduces to 140 and 143 °C, respectively. This phenomenon reflects that grafting and amidoximation processes have little effect on the thermal stability of the pristine PE. The major difference between the curves of the pristine PE and PE-g-PAN nonwoven fabrics is the distinct exothermic peak at 278 °C because of the

Figure 2. FT-IR spectra of (a) pristine PE nonwoven fabric, (b) PE-gPAN nonwoven fabric with a DG of 26.9%, and (c) PE-g-PAO nonwoven fabric with an AO density of 1.3 mmol/g. 15091

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dependent data are obtained by varying the temperature from 10 to 30 °C while the other parameters are kept constant. The influence of the temperature on uranyl ion adsorption is shown in Figure 6a,b. It should be noted that it takes quite a few days

Figure 4. DSC curves of (a) pristine PE nonwoven fabric, (b) PE-gPAN nonwoven fabric with a DG of 26.9%, and (c) PE-g-PAO nonwoven fabric with an AO density of 1.3 mmol/g.

cyclization reaction of PAN graft chains, which moves to 260 °C and becomes extremely weak with respect to the PE-g-PAO nonwoven fabric. 3.1.3. Mechanical Properties. The mechanical properties of the nonwoven fabric adsorbents are quite important in the practical application, which is expected to be maintained to a large extent during the period of their lifetime. The stress− strain behavior of the pristine PE, PE-g-PAN, and PE-g-PAO nonwoven fabrics is shown in Figure 5. The maximum tensile

Figure 6. Relationship between adsorption of the uranyl ions with initial concentrations of (a) 3 μg/L and (b) 50 μg/L and adsorption times at different temperatures.

to research the adsorption equilibrium. Most previous papers referring to the uranium adsorption researches are restricted to a perfect system: a high uranium initial concentration of the ppm level, an optimal pH value, and a relatively small volume of 50 mL.27,28 The equilibration can basically be reached in minutes or hours. This study is carried out with a very low uranyl ion initial concentration of the ppb level in a large-scale volume of 3.5 L and at a neutral environment. Studies of the uranium uptake by various amidoximated adsorbents from seawater have also been conducted. Usually, large-scale cyclic (30 or 50 days) contacts with seawater were conducted for equilibrium adsorption.11,29 The amount of uranyl ions adsorbed on the nonwoven fabric increases for initial concentrations of 3 and 50 μg/L when the temperature is changed from 10 to 30 °C. From the relationship between the rate constant and sorption temperature (Arrhenius relationship, eq 3), the activation energy, Ea, for an initial concentration of 3 μg/L is calculated to be 65.81 kJ/mol. The increasing adsorption amount of uranyl ions with an increase in the temperature and a positive value of Ea suggest

Figure 5. Stress−strain curves of (a) pristine PE nonwoven fabric, (b) PE-g-PAN nonwoven fabric with a DG of 26.9%, and (c) PE-g-PAO nonwoven fabric with an AO density of 1.3 mmol/g.

strength value is about 33 MPa, and the elongation at the break is about 17% for the pristine PE nonwoven fabric. Compared to the pristine PE nonwoven fabric, the tensile strength value and the elongation at the break both decrease to about 5.5 MPa and 20%, respectively, for the PE-g-PAN and PE-g-PAO nonwoven fabrics. However, there is not much difference between the stress−strain behavior of the PE-g-PAN and PE-g-PAO nonwoven fabrics. It can be determined that the amidoximation reaction has a slight effect on the mechanical properties of the material, and the main influencing factors are irradiation and the grafting process. 3.2. Uranyl Ion Adsorption Performance. 3.2.1. Temperature Effect and Adsorption Kinetics. The temperature15092

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that the sorption of uranyl ions on the PE-g-PAO nonwoven fabric is an endothermic process.30 Arrhenius equation: ln k 2 = ln A −

Ea RT

(3)

where k2 is the pseudo-second-order rate constant, A is the Arrhenius frequency factor, Ea is the activation energy, R is the gas constant, and T is the solution temperature. The adsorption kinetic study is an important way to understand the adsorption process roundly.27,31 The pseudofirst-order and pseudo-second-order models are chosen to fit the experimental data obtained at different temperatures for an initial concentration of 50 μg/L. The corresponding kinetic parameters are shown in Table 1. The pseudo-first-order and pseudo-second-order kinetic models are given as follows in linear form: Table 1. Kinetic Parameters for Adsorption of the Uranyl Ions on the PE-g-PAO Nonwoven Fabric at Different Temperatures (C0 = 50 μg/L) temperature (°C) 10 Qe,exp (10−3 mmol/g) k1 (10−6 min−1) R2 k2 [g/(mmol·min)] R2

0.72 Pseudo-First-Order 2.95 0.9901 Pseudo-Second-Order 0.35 0.9941

20

30

1.01

1.25

3.14 0.9840

2.80 0.9700

0.29 0.9833

0.29 0.9980

Figure 7. Linearized (a) pseudo-first-orderl and (b) pseudo-secondorder kinetic models for uranyl ion adsorption on PE-g-PAO nonwoven fabric at different temperatures (C0 = 50 μg/L; pH = 7.5; time = 10 days).

Pseudo-first-order model: ln(Q e − Q t ) = ln Q e − k1t

Pseudo-second-order model: t 1 t = + 2 Qt Qe k 2Q e

fabric is obtained for five different initial concentrations (3, 6, 12, 25, and 50 μg/L) at a fixed pH value of 7.5 at 30 °C. The plot of the amount of uranyl ions adsorbed on the PE-g-PAO nonwoven fabric versus the initial concentration of the uranyl ions is presented in Figure 8. Figure 8 shows that the adsorption amount of the uranyl ions increases with an increase

(4)

(5)

where Qe (mmol/g) and Qt (mmol/g) are the amounts of uranyl ions at the equilibrium state and at any time t, respectively, and k1 (min−1) and k2 [g/(mmol·min)] are the pseudo-first-order and pseudo-second-order rate constants of adsorption, respectively. ln(Qe − Qt) versus t using eq 4 at different temperatures is plotted in Figure 7a. The first-order model data fall on straight lines with a correlation coefficient (R2) of over 0.97, showing the applicability of this model. When t/Qt is plotted against t for the experimental data, straight lines presented in Figure 7b with R2 > 0.98 are obtained. These second-order model correlation coefficients are always greater than 0.98. It can be suggested that uranyl ion adsorption on the PE-g-PAO nonwoven fabric follows the pseudo-second-order model well, which is in agreement with the chemisorption mechanism being the rate-controlling step. It is more likely to predict that the adsorption behavior may involve valency forces between the uranyl ions and adsorbent.32 3.2.2. Initial Concentration Effect and Adsorption Isotherms. The effect of the uranyl ion initial concentration on the adsorption behavior of the amidoximated nonwoven

Figure 8. Relationship between the adsorption amount and initial concentration of uranyl ions (pH = 7.5; T = 30 °C; time = 10 days). 15093

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respectively. The appearance of a sharp peak at 2242 cm−1 shows the successful grafting of AN. New characteristic adsorption bands at 3200−3500, 1647, and 928 cm−1 indicate successful amidoximation. SEM images show that no trail of destruction on the surface of the nonwoven fabric happened during the graft polymerization and modification processes. TGA and DSC results show that the graft polymerization and amidoximation processes have little effect on the thermal stability of the pristine PE. The main influence factor of the mechanical properties of PE nonwoven fabric is the irradiation and grafting progress. The adsorption kinetic study suggests that uranyl ion adsorption on the PE-g-PAO nonwoven fabric follows pseudo-first-order and pseudo-second-order kinetic models; however, the correlation coefficient from the latter is higher (R2 > 0.98) than that from the former (R2 > 0.97). The activation energy, Ea, for an initial concentration of 3 μg/L is calculated as 65.81 kJ/mol according to the Arrhenius equation, which indicates that the adsorption of uranyl ions on PE-g-PAO is endothermic. The adsorption capacity increases with an increase of the uranyl ion initial concentration, and the Freundlich isotherm model is available to fit the adsorption data.

of the uranyl ion initial concentration from 3 to 50 μg/L. Kavakli et al. reported similar results in their studies.33 The adsorption isotherm indicates the distribution of adsorbates between the aqueous and solid phases when the adsorption process reaches the equilibrium state. The Langmuir and Freundlich models are often selected to describe the equilibrium adsorption isotherms.34 In the Langmuir equation (eq 6), Qe (mmol/g) is the amount adsorbed at equilibrium, Qmax (mmol/g) is the Langmuir monolayer sorption capacity, Ce (mmol/L) is the equilibrium concentration, and KL (L/ mmol) is the Langmuir equilibrium constant related to the energy of adsorption and affinity of the adsorbent. The empirical Freundlich isotherm is based on a heterogeneous surface with a nonuniform distribution of adsorbates over the surface. In the Freundlich equation (eq 7), Qe (mmol/g) is the amount adsorbed at equilibrium, Ce (mmol/L) is the equilibrium concentration, and KF and n are constants indicative of the adsorption capacity and adsorption density. The linearized plots of the Freundlich adsorption isotherms obtained at 30 °C are given in Figure 9. From the distribution

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.W.), [email protected] (J.L.). Tel. and Fax: 0086-21-39194505. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 10975177, 11175234, and 11105210), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant XDA02040300), the “Knowledge Innovation Program” of the Chinese Academy of Sciences (Grant KJCX2-YW-N49), and Shanghai Municipal Commission for Science and Technology (Grant 11ZR1445400 and 12ZR1453300).

Figure 9. Linearized Freundlich adsorption isotherm of uranyl ion adsorption on PE-g-PAO nonwoven fabric (pH = 7.5; T = 30 °C; time = 10 days).

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of the points, it can be found that the Freundlich isotherm model is suitable for describing the adsorption equilibrium of the uranyl ions on the PE-g-PAO nonwoven fabric under the studied concentration range. The Freundlich empirical parameter n obtained for the present system lies between 1 and 10, indicating favorable adsorption of the uranyl ions.16 Langmuir equation: Ce Ce 1 = + Qe Q max Q maxKL

(6)

Freundlich equation: log Q e = log KF +

1 log Ce n

REFERENCES

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4. CONCLUSIONS The PE-g-PAO nonwoven fabric adsorbent is prepared by preirradiation-induced emulsion graft polymerization of AN onto PE nonwoven fabric and subsequent amidoximation of the grafting chains. Graft polymerization and amidoximation are proven by means of the FT-IR, TGA, DSC, and tensile tests, 15094

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dx.doi.org/10.1021/ie301965g | Ind. Eng. Chem. Res. 2012, 51, 15089−15095