Facile synthesis of gelatin modified attapulgite for the uptake of uranium from aqueous solution

Journal of Molecular Liquids 234 (2017) 172–178

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Facile synthesis of gelatin modified attapulgite for the uptake of uranium from aqueous solution Huaxuan Han a,b, Cheng Cheng a, Shuheng Hu b, Xiaolong Li c, Wenjuan Wang c, Chengjian Xiao c,⁎, Zimu Xu b,⁎, Dadong Shao a,c,⁎⁎ a b c

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China

a r t i c l e

i n f o

Article history: Received 23 January 2017 Received in revised form 7 March 2017 Accepted 9 March 2017 Available online 22 March 2017 Keywords: Uranium Adsorption Attapulgite Gelatin Plasma technique

a b s t r a c t Gelatin modified attapulgite (ATT@Gel) with high adsorption capability for U(VI) was synthesized by plasma technique. The ATT and ATT@Gel composites were characterized by scanning electron microscope (SEM), Xray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The characterization results indicated that ATT@Gel was synthesized successfully. The adsorption capability of ATT@Gel for U(VI) was studied by batch adsorption technique. Experiment results indicate that the modified gelatin on ATT surface can markedly improves the adsorption efficiency of ATT@Gel for U(VI). © 2017 Published by Elsevier B.V.

1. Introduction The behaviors of long-lived actinides in environmental has aroused great interest in radioactive waste management [1,2]. The migration and diffusion of actinides on the surface of minerals are of primary importance for its environmental behavior [3]. The fate of actinides in environment solution is mainly dominated by its adsorption and complexation on mineral surfaces [4–7]. Uranium, a typical actinide elements, exists as hexavalent uranyl (U(VI)) in environment solution. U(VI) can be retained and enriched in environment for a long time due to its long half-life (such as 238Ut1/2 = 4.51 × 109 years) [8–10]. Therefore, it is very important to uptake uranium ions from environment solution. Attapulgite, a number of sepiolite families in mineralogy, its basic structural unit is 2: 1 layer type which is two layers of silicon oxide tetrahedraclip a layer of magnesium oxide octahedron. Si4+ is replaced by Al3+ on the surface of attapulgite and thereby its appears negative charge, which ensures it presents adsorption capability for trace heavy metal ions [11,12] and radionuclides (such as U(VI)) in environment solution [13]. However, the low adsorption capability of attapulgite greatly limits its application in the management of

radioactive pollution. Amino groups are widely considered as effective chelating functional groups to separate various contaminants from solution because of their high reactivity [14–16]. Gelatin, a heterogeneous mixture of animal origin protein, contains abundant amounts of amino groups which can form strong complex with metal ions in solution. To further enhance the adsorption capability of attapulgite, attapulgite was modified with gelatin (denoted as ATT@Gel) in this work. Plasma technique was used in synthesis process because it is an environmental friendly and effective technique which can modify material surface without altering its bulk properties [17]. The energy of activated particles formed in plasma is much greater than chemical bond energy and can break chemical bonds to form active species on ATT surface. The active species would then react with the functional groups in gelatin, which result in the modification of gelatin on ATT surface. The prepared ATT@Gel composites were applied to uptake U(VI) from solution to evaluate its potential application in radioactive pollution management. 2. Experimental 2.1. Chemicals

⁎ Corresponding authors. ⁎⁎ Correspondence author at: Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China. E-mail addresses: [email protected] (C. Xiao), [email protected] (Z. Xu), [email protected] (D. Shao).

http://dx.doi.org/10.1016/j.molliq.2017.03.076 0167-7322/© 2017 Published by Elsevier B.V.

All chemical reagents used in this paper were in analytic purity. The attapulgite (ATT) was received from Kai-Xi Co. China. ATT@Gel was synthesized by plasma technique. Briefly, 1.0 g attapulgite was treated by nitrogen plasma (100 W) for 30 min the

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Fig. 1. The XRD patterns (A) and TGA curves (B) of ATT and ATT@Gel.

treated ATT was added into 100 mL 10.0 g/L gelatin solution and reacted at 50 °C for 3 h under magnetic stirring, and then washed with Milli-Q water and dried at 50 °C. The resulted materials were denoted as ATT@Gel. 2.2. Adsorption experiment The adsorption experiment was performed by batch adsorption technique. After the adsorbent (ATT and ATT@Gel) was pre-equilibrated with NaCl for 12 h in polyethylene tubes, U(VI) stock solution and MilliQ water were added to obtain the desired composition, and the pH values were adjusted by diluted nitric acid or sodium hydroxide. After shaken for requested time, the suspensions were centrifuged at 18,000 rpm for 10 min (Beckman Coulter 64R) at the temperature in adsorption experiments. The residual U(VI) concentration in supernatant was measured by arsenazo III spectrophotometric method. All adsorption were repeated for three times, and the uncertainties of adsorption data were b5%.

2.3. Characterization The ATT and ATT@Gel samples were characterized by SEM, XRD, TGA, and XPS. The SEM images were obtained on a FEI Sirion 200 FEG scanning electron microscope. The XRD patterns were measured on a D/MAX-2500 V equipped with Cu Kα radiation. The TGA measurements were performed on a Shimadzu TGA-50 thermogravimetric analyzer. The heating rate and air flow rate were 10 °C /min and 40 mL/min, respectively. XPS measurements were performed on an ESCALab220i-XL surface microanalysis system equipped with an Al Kα source.

3. Results and discussion Fig. 1A shows the XRD patterns of ATT and ATT@Gel samples. The XRD peaks at 2θ = 8.4°, 13.8°, 19.7°, 21°, 27.5°, 35.0° and 42.6° indicate the crystal structure of ATT [18]. Quartz and montmorillonite are also found in the attapulgite samples. The XRD peak positions of ATT and

Fig. 2. SEM images of ATT (A, B) and ATT@Gel (C, D).

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Fig. 3. XPS C 1s spectra (A) and O 1s spectra (B) of ATT and ATT@Gel.

ATT@Gel are fairly similar, which reveals that the crystal structure of ATT was not destroyed during plasma indiced modification process. Fig. 1B shows the TGA patterns of ATT and ATT@Gel. The first weight losses below 150 °C can be assigned to the releasing of surface adsorbed water, which were estimated to be 6.63% and 6.37% in ATT and ATT@ Gel, respectively. The second weight losses at 150–350 °C can be attributed to the releasing of structure water, which were detected to be 3.02% and 2.44% in ATT and ATT@Gel, respectively. The third weight loss at N 350 °C can be due to other impurity minerals and the decomposition of gelatin, which were estimated to be 5.30% and 17.8% in ATT and ATT@Gel, respectively. According to the difference weight losses, the weight percent of gelatin in dry ATT@Gel is calculated to be ~13.6%. Fig. 2 depicted SEM images of ATT and ATT@Gel. One can see that ATT (Fig. 2A, B) shows a fibrous morphology and ATT fibers also form straight parallel aggregations. It reveals the weak cohesive force between raw ATT can be neglected, and the very low content of organic materials in raw ATT. However, ATT@Gel particles (Fig. 2C, D) are agglomerated and covered by organic films, which can be due to the cohesive force generated from functional groups in modified gelatin. It is well known, gelatin is composed of many kinds of amino acids, and

Table 1 Curve fitting results of XPS C 1 s spectra.

ATT

ATT@Gel

a b

Peak

BEa (eV)

FWHMb (eV)

%

C_C C\ \C C\ \OH NC_O \ \COOH C_C C\ \C C\ \OH NC_O \ \COOH

284.42 285.24 286.50 287.30 289.20 284.26 285.47 286.10 287.44 288.80

1.73 1.77 1.55 0.93 2.29 1.75 1.27 1.25 1.75 0.82

43.1 38.8 7.23 2.57 8.26 51.1 16.4 8.70 22.9 0.85

various interaction forces, such as Van der Waals forces, π-π interactions, hydrophobic interaction, and Lewis acid–base interaction, are exist among the amino acids in gelatin. Thereby, the SEM images indicate that gelatin was successfully modified on to the surface of ATT. Shao et al. [19] reported a similar microstructure transformation of carboxymethyl cellulose modified multiwalled carbon nanotubes. Fig. 3A show the XPS C 1 s spectra of ATT and ATT@Gel. The peak fractions of hydroxyl groups (C-OH) and sp2- hybridized (C = C) increase from 7.23% to 8.70% and from 43.1% to 51.1%, respectively (Table 1), which can be assigned to effect of modified gelatin on ATT surfaces because there gelatin contains abundance aromatic organics. Meanwhile, the XPS O 1 s spectra of ATT and ATT@Gel (Fig. 3B and Table 2) show that hydroxyl groups (C\\OH) and carbonyl groups (C_O) are the dominant oxygen species on the surface of ATT and ATT@Gel, respectively. It further prove the successfully modification of gelatin on ATT surface. 3.1. Adsorption experiment The adsorption of U(VI) on ATT and on ATT@Gel surface as a function of contact time is depicted in Fig. 4. The adsorption of U(VI) was rapid increased at initial contact time, and reached equilibrium in 12 h. Thereby, 24 h was selected as contact time to study the adsorption of U(VI) on ATT and ATT@Gel in following adsorption experiment. The adsorption of U(VI) on ATT@Gel is much higher than that on ATT, which can be explained by the forming binding between modified gelatin and U(VI).

Binding energy. Full width at half-maximum.

Table 2 Curve fitting results of XPS O 1 s spectra.

ATT

ATT@Gel

peak

BE (eV)

FWHM (eV)

%

C\ \OH C_O COOH C\ \OH C_O COOH

531.30 532.40 533.80 531.25 532.40 533.72

1.71 1.78 2.17 2.18 1.00 1.67

29.3 58.0 12.7 95.9 0.00 4.07

Fig. 4. Effect of contact time on U(VI) adsorption on ATT and on ATT@Gel. C[U(VI)]initial = 6.25 mg/L, I = 0.01 M NaCl, m/V = 0.5 g/L, pH = 5.0 ± 0.1, T = 293 ± 1 K.

H. Han et al. / Journal of Molecular Liquids 234 (2017) 172–178 Table 3 Kinetic parameters of pseudo-first-order and pseudo-second-order kinetic models. Pseudo–first–order model

ATT ATT@Gel

t 1 t ¼ þ t qt 2k2 q2e qe

Eq: ð2Þ

Pseudo–second–order model

qe (mg/g)

k1 (1/h)

R2

qe (mg/g)

k2 (mg/g h)

R2

5.76 7.53

1.11 1.06

0.796 0.716

5.92 7.76

0.267 0.188

0.999 0.999

where k1 and k2 are the pseudo-first and pseudo-second order kinetic constants, respectively. qt and qe are the amount of U(VI) adsorbed at time t and at equilibrium. According to the correlation coefficients (R2) [23] in Table 3, the experimental data can be fitted very well by the pseudo-second-order rate model than by the pseudo-second-order rate model, which implies that the adsorption of U(VI) on ATT and on ATT@Gel surfaces is limited by a chemisorption mechanism. Fig. 5 shows the U(VI) adsorption onto ATT@Gel as a function of adsorbent content. The adsorption of U(VI) on ATT@Gel increases with increasing adsorbent content. At same experimental conditions, more adsorption sites can be obtained at higher adsorbent content, which in favor of U(VI) adsorption. Distribution coefficient (Kd) is obtained from Eq. 3 [24] and shown in Fig. 5.

Kd ¼

Fig. 5. Effect of solid content on U(VI) adsorption to ATT@Gel. C[U(VI)]initial = 6.25 mg/L, I = 0.01 M NaCl, pH = 5.0 ± 0.1, T = 293 ± 1 K.

Pseudo-first-order kinetic equation (Eq. 1) and pseudo-second-order kinetic equation (Eq. 2) is used to simulate experimental data [20–22]. ln ðqe −qt Þ ¼ lnqe −k1 t

175

Eq: ð1Þ

Co −Ce V þ Ce m

Eq: ð3Þ

where V and m are the volume of suspension and the mass of adsorbent, respectively. It is obviously that the Kd values are decrease slightly with increasing adsorbent content, which could be due to the competition among the adsorption sites on the surface of ATT@Gel. Meanwhile, adsorbent particle would aggregate at high solid content that can decrease the efficiency of adsorbent, and thereby suppress the increasing of adsorption at high solid content [25,26]. The effect of solution pH on U(VI) adsorption on ATT and ATT@Gel was studied in NaCl solution (0.001, 0.01 and 0.1 M) and depicted in Fig. 6A. The adsorption of U(VI) is strongly dependent on pH, which

Fig. 6. Adsorption of U (VI) on ATT@Gel in different NaCl solutions as a function of pH values (A). CU(initial) = 6.25 mg/L, m/V = 0.5 g/L, T = 293 ± 1 K. The zeta potentials of ATT and ATT@ Gel (B). The relative species of 6.25 mg/L U(VI) as a function of pH in aqueous solutions (C).

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Table 4 Aqueous complexation reactions of U(VI).

Table 5 The relative parameters for Langmuir and Freundlich isotherms at different temperatures.

Reactions

Log K (I = 0)

+ H2O = UO2(OH)+ + H+ UO2+ 2 + 2H2O = UO2(OH)02 + 2H+ UO2+ 2 + UO2+ 2 + 3H2O = UO2(OH)3− + 3H + 2− UO2+ 2 + 4H2O = UO2(OH)4 + 4H 2UO2+ + H2O = (UO2)2(OH)3+ + H+ 2 + 2H2O = (UO2)2(OH)2+ + 2H+ 2UO2+ 2 2 + 5H2O = (UO2)3(OH)5+ + 5H+ 3UO2+ 2 3UO22+ + 7H2O = (UO2)3(OH)7− + 7H+ + 7H2O = (UO2)4(OH)7+ + 7H+ 4UO2+ 2

−5.25 −12.15 −20.25 −32.4 −2.70 −5.62 −15.55 −32.20 −21.90

increases quickly and slowly at pH b 6.0 and at pH = 6.0–7.5, respectively. The adsorption of U(VI) was then decreased with further increasing pH, which can be due to electrostatic repulsion. As can see from Fig. 6B, the zeta potentials of ATT and ATT@Gel becomes more negative with the increase of pH value in pH 3–10. Meanwhile, there have no significant differences of the zeta potentials between ATT and ATT@Gel. Zhu et al. [23] also reported similar U(VI) adsorption behavior on attapulgite and they contributed it to the different existing forms of U(VI) at different pH values. To further study U(VI) adsorption behavior, the distribution of U(VI) species as a function of pH were calculated according the thermodynamic data (Table 4) and showed in Fig. 6C. The distribution of U(VI) species are fairly affected by pH values. Free uranyl ion (UO22 +) and U(VI) hydrolysis complexes are dominant species at pH b 5 and pH N 5, respectively. Moreover, U(VI) species are positively charged at pH b 8, and then change gradually from positively charge to negatively charge. According to the adsorption isotherms of U(VI) on ATT and ATT@Gel are depicted in Fig. 7A, the adsorption on ATT@Gel are significantly higher than on ATT. Meanwhile, the adsorption isotherms are increased with increasing temperature, which illustrate that high temperature is

ATT

ATT@Gel

T (K)

Langmuir model Cs max (mg/g)

b (L mg−1)

R2

Freundlich model KF (mg1−nLng−1)

n

R2

293 313 333 293 313 333

9.35 10.4 11.2 23.9 24.5 25.3

0.422 0.520 0.093 0.402 0.519 0.626

0.996 0.995 0.999 0.990 0.992 0.995

3.79 4.91 5.97 7.59 8.79 9.75

0.269 0.222 0.207 0.381 0.351 0.338

0.977 0.992 0.916 0.968 0.969 0.967

in favor of U(VI) adsorption on ATT@Gel and on ATT. These can be interpreted by the fact that U(VI) is hydrated in solution and the losing of U(VI) hydration shell need energy during adsorption [27]. The Langmuir and Freundlich isotherm models are applied to analysis adsorption data to get a better understanding of the adsorption mechanism of U(VI) on ATT@Gel. The Langmuir isotherm model is often applied to simulate the adsorption in a monolayer. The model can be described by Eq. 4: Ce Ce 1 þ ¼ Cs Cs max bCs max

Eq: ð4Þ

where Ce is the concentration in supernatant after centrifugation, Cs is the equilibrium concentration adsorbed on adsorbent, Cs max and b are constants related to maximum adsorption capacity and adsorption energy, respectively. Freundlich isotherm model [28] is often used to illustrate the adsorption on heterogeneous adsorption surface, which usually expressed as Eq. 5: LnCs ¼ nLnCe þ LnK F

Eq: ð5Þ

Fig. 7. Adsorption isotherms of U(VI) adsorption (A). Liner plots of lnKd versus Ce of U(VI) adsorption (B). Langmuir isotherm model (C) Freundlich isotherm model (D) onto ATT@Gel at three different temperatures.

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Table 6 Thermodynamic parameters for U(VI) adsorption on ATT and on ATT@Gel.

ATT

ATT@Gel

T (K)

△G° (kJ/moL)

△H° (kJ/moL)

△S° (J/moL/K)

293 303 333 293 303 333

−21.2 −23.7 −26.1 −23.4 −25.1 −28.3

10.8

111

11.5

120

where KF and n represent the adsorption capacity and the degree of adsorption relates to equilibrium concentration, respectively. Fig. 7B and C and Table 5 show that the results of Langmuir and Freundlich models. According to the correlation coefficients (R2) in Table 5, the Langmuir isotherm model fits experiment data better than the Freundlich isotherm model. The Cs,max values of U(VI) on ATT@Gel surface obtained from the Langmuir isotherm model are 23.9, 24.5, and 25.3 mg/g at 293, 313, and 333 K, which are comparable to many adsorbent under similar experimental conditions, such as NaATT (14.42 mg/g) [23], illite (5.27 mg/g) [29], hematite (5.59 mg/g) [30], and hierarchical Fe4(P2O7)3 (14.92 mg/g) [31]. The thermodynamic parameters for U(VI) adsorption on ATT@Gel are obtained from the temperature-dependent adsorption isotherms. The free energy change (ΔG°) is firstly obtained from following relationship (Eq. 6) [32]: ΔG ° ¼ −RTLnK °

Eq: ð6Þ

where R and T are the ideal gas constant (8.314 J mol−1 K−1) and the reaction temperature in Kelvin, respectively. LnK° values are got by plotting LnKd versus Ce at T = 293, 313 and 333 K (Fig. 7D) and extrapolating the Ce values to zero. The standard entropy change (ΔS°) is obtained from following equation (Eq. 7): ∂ΔG ° ΔS ¼ − ∂T

!

°

Eq: ð7Þ p

The average standard enthalpy change (ΔH°) is obtained from the values of ΔG° and ΔS° (Eq. 8): ΔH ° ¼ ΔG ° þ TΔS °

Eq: ð8Þ

The relative thermodynamic parameters in Table 6 give an explanation about the adsorption mechanism of U(VI) on ATT@Gel. The positive

Fig. 9. Recycling of ATT@Gel for the removal of U(VI). C[U(VI)]initial = 6.25 mg/L, I = 0.01 M NaCl, m/V = 0.5 g/L, pH = 5.0 ± 0.1, T = 293 ± 1 K.

ΔH° value reveals that the adsorption is endothermic process. This positive ΔS° can serve as an explanation for how well U(VI) adsorption onto ATT@Gel surface from aqueous solution. High temperature is in favor of the dehydration process that needs energy [33]. The negative ΔG° values are believed to be a spontaneous process. Furthermore, the ΔG° values are also decreased with increasing reaction temperature, which indicates that the adsorption is increases at higher temperature. It is well known that many metal ions, such as Sr(II), Ni(II), Pb(II), Cu(II), and Fe(III), co–exists with U(VI) in radioactive wastewater, and these metal ions can compete with U(VI) for the adsorption sites on adsorption surface. Therefore, the selective adsorption of those metal ions and U(VI) on ATT@Gel surface was studied. As can be seen from Fig. 8, the adsorption of U(VI) on ATT@Gel surface is much higher than other metal ions under same experimental conditions, which reveals the high selectivity of ATT@Gel toward U(VI) in radioactive wastewater. The result highlights the potential application of ATT@Gel in selective uptake of U(VI) from radioactive wastewater. The recycling of ATT@Gel is critical for its usage in potential application. The regeneration of ATT@Gel was achieved by dispersed 0.1 g Uladen ATT@Gel in 100 mL 0.1 M HNO3 solution and rinsed with MilliQ water thoroughly, and dried at 50 °C for 24 h. the regenerated ATT@ Gel was applied in adsorption experiment. As shown in Fig. 9, the adsorption of U(VI) on regenerated ATT@Gel just slightly decreases with increasing rounds, and the regenerated ATT@Gel still presents higher adsorption capability for U(VI) even after (at least) eight cycles of applications. From the industrial point of view, ATT@Gel has excellent stability and the regenerated ATT@Gel still meets the requirement of real application. 4. Conclusions In summary, the modified gelatin on ATT surface can sound improve its adsorption capability for U(VI). The results of XRD pattern show that the crystal structure of ATT was not destroyed during plasma induced modification process. The SEM images indicate that gelatin was successfully modified on ATT surface. The adsorption of U(VI) on ATT@Gel increases with increasing adsorbent content. The adsorption of U(VI) increases at pH b 7.5, and then decrease at pH N 7.5. The adsorption of U(VI) onto ATT@Gel is an endothermic and spontaneous process. It is followed the pseudo-second-order kinetic model and Langmuir isotherm model, and is dependent on solution pH and ionic strength. Acknowledgements

Fig. 8. Selective adsorption comparison of metal ions and U(VI) on ATT@Gel surface. m/V = 0.50 g/L, C0 = 6.25 mg/L, T = 293 ± 1 K, pH = 5.0 ± 0.1, I = 0.1 mol/L NaCl, contact time 24 h.

Financial supports from the NSAF (U1530131), the National Natural Science Foundation of China (11675210), the Radiochemistry 909

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