Kinetic and equilibrium of U(Ⅵ) adsorption onto magnetic amidoxime-functionalized chitosan beads

Journal of Cleaner Production 188 (2018) 655e661

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Kinetic and equilibrium of U(Ⅵ) adsorption onto magnetic amidoxime-functionalized chitosan beads Shuting Zhuang a, Rong Cheng b, Mi Kang b, Jianlong Wang a, c, * a

Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China School of Environment & Natural Resources, Renmin University of China, Beijing 100872, PR China c Beijing Key Laboratory of Radioactive Waste Treatment, INET, Tsinghua University, Beijing 100084, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 6 April 2018

Uranium is used to make nuclear fuels for power generation. Uranium-containing wastewater is produced from fuel fabrication, fuel reprocessing, which resulted in the soil and water pollution. In this study, a cleaner and environmental friendly adsorption material, chitosan, was used to remove uranium from aqueous solution. Magnetic amidoxime functionalized chitosan beads were synthesized, characterized and applied for adsorption of uranium. The adsorption capacity of 117.65 mg/g was achieved at pH ¼ 6, mainly due to coordination ability of amidoxime groups. With an Ms of 21.7 emu/g, the magnetic adsorbents could be fast separated from aqueous solution by the magnetic field. Additionally, the results showed that the adsorption process could be well described by the pseudo-second order kinetic (R2 ¼ 0.9842) and Langmuir models (R2 ¼ 0.9995). Finally, an adsorption mechanism was tentatively proposed to explain the adsorption process. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Chitosan Uranium Adsorption Amidoxime

1. Introduction The removal/recovery of uranium is of great importance. With the development of nuclear power plants, the uranium-containing radioactive wastewater is a big problem remaining to be solved. So far, various kinds of technologies have been applied, including chemical precipitation (Mellah et al., 2007), solvent extraction (Beltrami et al., 2013), and adsorption (Sharma et al., 2017; Yu et al., 2017; Chen and Wang, 2016; Yu and Wang, 2016; Ma et al., 2015). Among them, the application of adsorbents for uranium removal or recovery has received much attention since 1960s due to its ease of operation, low cost and low emissions. Chitosan is an abundant and cheap biomaterial obtained from nature (Shi et al., 2017; Wang and Chen, 2014). It has been widely regarded as an environmental friendly material used in drug delivery, cosmetic, and environmental remediation. Due to its large quantities of amino and hydroxyl groups, chitosan has been extensively studied as a biosorbentfor the removal of heavy metal ions, such as Pb2þ, Cu2þ, and Cd2þ, and radionuclides, such as Co2þ (Zhuang et al., 2018), Sr2þ, Csþ and UO2þ 2 (Wang and Zhuang, 2017).

* Corresponding author. Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China. E-mail address: [email protected] (J. Wang). https://doi.org/10.1016/j.jclepro.2018.04.047 0959-6526/© 2018 Elsevier Ltd. All rights reserved.

Besides good adsorption performance, this environmental friendly material as an adsorbent also shows advantages of low cost, facile functionalization, and versatile application forms. However, the native chitosan has a low adsorption capacity towards radionuclides. To increase its adsorption performance, especially its adsorption capacity, great efforts have been made to modified chitosan. Previously, various kinds of modified chitosan have been reported for radionuclides removal. Functional groups, such as carboxyl (Zhuang et al., 2017), thiol (Chen and Wang, 2012) and cyanoethyl groups, could enrich chitosan with better adsorption capacities for Co2þ, Sr2þ, and Csþ. As for chitosan-based adsorbents, the research of modification methods for uranium removal/recovery has not yet satisfying. Previously, German researchers have done a systematic screening study of about 200 adsorbents and found that the amidoxime functional adsorbent was the most promising candidate for uranium recovery from seawater. Additionally, the countable large scale tests of the uranium recovery from the seawater have all involved the amidoxime functional adsorbents, showing its unique adsorption affinity for uranium. So far, the grafting of amidoxime functional groups into various materials, including fibers (Ladshaw et al., 2016; Yin et al., 2016), MWCNTs, graphene oxides (Wang et al., 2016), and metal organic frames (Liu et al., 2017), has been widely reported. The research of substantial amidoxime functional

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adsorbents for uranium removal can provide further insights into the adsorption mechanisms, which could provide valuable information for the synthesis of adsorbents in turn. Additionally, the study of amidoxime functional chitosan can provide more alternative adsorbents for seawater uranium recovery. To synthesize amidoxime functional adsorbents, the grafting of eCN groups, followed by the conversation of -CN into the amidoxime groups through the treatment of hydroxylamine, is the most popular way (Abney et al., 2017).. Inspired by the great potential of amidoxime functional adsorbent, as well as its synthetic method, the synthesis of magnetic amidoxime functional chitosan (MAO-chitosan) for uranium was come up. In this study, MAO-chitosan was developed for uranium removal. It was characterized by FTIR, VSM, SEM, and its uranium adsorption performances, involving the adsorption kinetics, isotherms and mechanisms, were studied. 2. Experimental section 2.1. Materials Chitosan with a molecular weight of 179.17 and deacetylation of 95%, were obtained from Sinopharm Chem. Reag. Co., Ltd. (China). Acrylonitrile was purchased from Aladdin. The other chemicals, including ferrous sulfate, sodium hydroxide, ferric chloride hexahydrate, acetic acid, anhydrous ethanol, and hydroxylamine hydrochloride were all obtained from Sinopharm Chem. Reag. Co., Ltd. (China). 2.2. Preparation of amidoxime functional chitosan beads The preparation of MAO-chitosan was obtained in the order of grafting cyanoethyl into chitosan (MCNCS) and turning the functional groups into amidoxime groups. The preparation of MCNCS beads was as follows: In details, chitosan (4 g) was dissolved by the acetic acid solution (3% v/v, 100 mL), followed by an addition of 10 mL mix solution containing Fe3þ (0.02 mol) and Fe2þ (0.01 mol). Then the alkali magnetic chitosan beads were obtained by injecting the above mix into a sodium hydroxide solution (25% w/v, 300 mL) via a needle of 10 mL syringe. Later, the freeze-drying beads were reacted with acrylonitrile (80 mL) for 6 h at room temperature. To eliminate the remaining reactants, the obtained MCNCS beads were washed by distilled water for later use. In details, the preparation of MAO-chitosan beads was conducted with the following conditions: the hydroxylamine hydrochloride solution (50 mL, 10%, pH ¼ 8.59) was added into the MCNCS beads and stirred for 8 h at 353 K. Then the beads were washed by distilled water to eliminate any remaining reactants. Finally, the MAO-chitosan beads were kept in distilled water for later use. 2.3. Characterization methods

solutions were adjusted by adding NaOH or HCl solutions and measured by pH meter. Generally, with the control of other conditions (pH ¼ 6, T ¼ 298 K), a certain amount of adsorbent was added into a U(Ⅵ)-containing solution, shaking at the rate of 150 rpm in a shaker (ZD85, China). For adsorption kinetic experiments, 20 mg adsorbents were added into a 20 mL U(Ⅵ)-containing solution, and the effect of the contact time (0e180 min) was studied. For adsorption isothermal experiments, 10 mg adsorbents were added into a 10 mL solution containing various initial concentrations of U(Ⅵ) ions (10e600 mg/L), respectively. The contact time of 5 h was fixed for ensuring adsorption equilibrium. After adsorption, the adsorbents were separated from the solutions via magnetic separation methods. The concentrations of the U(Ⅵ) were measured by a visible spectrophotometer (Thermo Evolution300, USA) at 651 nm with 1 mL arsenazo (III) (0.1%) as the complex agent and 5 mL chloroacetic acid/sodium acetate buffer solution (pH 2.5) (Khan et al., 2006). Therefore, the adsorption capacity could be calculated by the follow equation:

qt ¼

ðC0  Ct Þ  V m

where C0 and Ct stand for the concentrations of U(Ⅵ) at the beginning and at time t, respectively; V is the solution volume; and m represents the mass of the adsorbents. All the experiences were repeated three times. 3. Results and discussion 3.1. Characterization 3.1.1. Formation of MAO-chitosan beads As showed in Fig. 1, the formation of MAO-chitosan beads was via the graft of eCN groups and the conversation of eCN groups into amidoxime groups. The magnetic Fe3O4 was formed during the injection of the mix containing Fe3þ and Fe2þ (0.02 mol: 0.01 mol) into an alkali solution. Its magnetic property and functional groups were further identified by FTIR and VSM. 3.1.2. FTIR The FTIR spectra of MCNCS and MAO-chitosan beads were presented in Fig. 2. The characteristic band at 627 cm1, which should be attributed to the FeeO vibration, indicated the presence of Fe3O4. (Wang et al., 2012). Additionally, the band at 2250 cm1, which was assigned to be the vibration of eCN groups of MCNCS, disappeared after the reaction with hydroxylamine, suggesting the successful reaction. Furth more, the band at around 1666 cm1 corresponding to the C]N was strengthen (Mahdavinia and Shokri, 2017), indicating the successful grafting of amidoxime groups. 3.1.3. Magnetic property The magnetic property was identify by VSM, as shown in Fig. 3. Results showed that the MAO-chitosan beads possessed good

The magnetic structure of the as-prepared adsorbents was identified by the vibrating sample magnetometer (VSM) spectra recorded on the PPMS-9 (Quantum, USA). The FTIR spectra were recorded on a VERTEX 70 FT-IR (Bruker) with a scanning range of 400 cm1 to 4000 cm1 and a resolution of 2 cm1. The SEM data were obtained from Merlin SIGMA 300 (Germen). 2.4. Batch adsorption experiments Batch adsorption experiments were performed to study the adsorption performance of MAO-chitosan. The pH of the initial

(1)

Fig. 1. Schematic formation of MAO-chitosan bead.

S. Zhuang et al. / Journal of Cleaner Production 188 (2018) 655e661

657

Fig. 2. The FTIR spectra of (a) MCNCS and (b) MAO-chitosan.

MomentMass (emu/g)

20

10

0

-10

-20 -8

-6

-4

-2

0

2

4

6

8

Fig. 4. Sem and EDS of MAO-chitosan.

Field (kOe) Fig. 3. VSM spectra of MAO-chitosan.

super-paramagnetic characteristic without magnetic hysteresis loop. A saturation magnetization (Ms) of 21.7 emu/g was achieved, enabling the efficient magnetic separation of the AO-chitosan beads after adsorption. 3.1.4. SEM The morphological characteristics and element distribution of the MAO-chitosan beads were analyzed by SEM-EDS. The SEM image of the MAO-chitosan bead was presented in Fig. 4, showing a spherical structure with a diameter of about 1 mm and porous surface. The EDS analysis showed the presence of C, N, O, and Fe,

owing to the presence of AO-chitosan and Fe3O4. 3.2. Adsorption kinetics Adsorption kinetics is important for the evaluation of the adsorption efficiency. To begin with, the effect of contact time, as well as the fitting of various models, were studied to identify the adsorption rate. In all experiments, the pH of the solution was fixed at about 6. pH is a very important parameter, which can affect the adsorption capability and the species of U in the solutions. It was found that pH 5e6 was the optimal value of the highest uranium uptake by many adsorbents (Mishra et al., 2017; Zhao et al., 2014; Fasfous and Dawoud, 2012). At this pH range, UO2þ 2 is the main

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500 120

C (mg/L)

C qm

60

400

qm (mg/g)

90

450

30

0

350 0

50

100

150

200

t (min) Fig. 5. Effect of contact time on adsorption C0(U(Ⅵ)) ¼ 480 mg/L, pH ¼ 6, and T ¼ 298 K.

preference,

m/V ¼ 1 mg/mL,

species in aqueous solutions, which is good for the complexation reaction between the amidoxime groups and uranyl ions (Katsoyiannis and Zouboulis, 2013). The effect of the contact time from 0 to 180 min on adsorption performance was studied and presented in Fig. 5. The adsorption rate increased very fast within the first 20 min due to the substantial adsorption sites and great concentration gradient. Owing to the decreasing of concentration gradient and unoccupied adsorption sites, the adsorption rate slowed down until equilibrium (Feng and Zhu, 2017; Ihsanullah et al., 2016). Although 3 h was enough to achieve equilibrium, 5 h was picked up in the later experience for adsorption equilibrium. The adsorption equilibrium capacity at this given condition was about 113 mg/g. Four classic adsorption kinetic models, including the pseudofirst order, pseudo-second order, intraparticle diffusion, and Elovich models, were applied to fit the data, and their mathematic expression were shown in Table 1. The fitting results of these models were presented in Fig. 6 and Table 1. According to the correlation coefficients (R2) of the linear fitting, the pseudo-second order model fitted the date best (R2 ¼ 0.9842), followed by the intraparticle diffusion model (R2 ¼ 0.9751), Elovich model (R2 ¼ 0.9644), and pseudo-first order model (R2 ¼ 0.9310). Additionally, the calculated maximum adsorption capacity (129.87 mg/g) obtained from the pseudosecond order model, was close to the experiment one (about 113 mg/g). It indicated that chemical adsorption may dominate the

adsorption process. However, it should be noted that it was not enough to identify the rate-limiting step. Additionally, the C value of the intraparticle diffusion model is higher than zero, indicating that the adsorption process is not controlled by single factors only. Furthermore, according to the Elovich model, a, which indicates the initial adsorption rate, is rather high (11.4503 (mg/(g min))), whilst b, which stands for the desorption constant, is lower (0.0397 g/mg). That shows the MAO-chitosan has good affinity for uranyl ions. 3.3. Adsorption isotherms Fig. 7(a) depicted the qe variation as a function of Ce. It was found that the qe increased gradually with the increasing Ce, gradually reaching its maximum adsorption capacity. To study the adsorption isotherms, the adsorption data were subjected to different isotherm models, including the Langmuir, Freundlich, and Temkin models (Crini and Badot, 2008). The Langmuir model, which is based on the assumptions of monolayer surface adsorption, is an ideal model. Whilst, the Freundlich model is an empirical model. The smaller its constant, n, is, the more easily the adsorption can occur (Xu and Wang, 2017). While the Temkin model is a proper model for the chemical adsorption based on strong electrostatic interaction. Their mathematic linear formulas can be given as shown in Table 2. The adsorption isotherms for uranyl ions on MAO-chitosan beads were presented in Fig. 7(b)e(d) and Table 2. With an R2 of 0.995, the Langmuir model fit best with the data than the other two models. According to the assumption of the Langmuir model, the adsorption process could be well explained by monolayer adsorption. The maximum adsorption capacity was calculated to be 117.65 mg/g. Table 3 shows the adsorption capacity of uranyl ions by various adsorbents. As shown in Table 3, except for the amidoxime-grafting adsorbents, most adsorbents exhibit uranyl adsorption capacity of lower than 60 mg/g. Compared to chitosan-based adsorbents, our synthetic adsorbent shows higher adsorption capacity than that of the reported crosslinked chitosan (72.64 mg/g) (Wang et al., 2009) and graphene oxide-chitosan (50.51 mg/g) (Yang et al., 2017). It is also higher than MWCNTs (24.9 mg/g) (Fasfous and Dawoud, 2012), oxidized MWCNTs (33.32 mg/g) (Sun et al., 2012) and biochar derived from eucalyptus wood (27.2 mg/g) (Mishra et al., 2017). Compared with other amidoxime-grafting adsorbents, MAOchitosan is still very competitive in uranium removal capacity, which is lower than AO-g-MWCNTs (145 mg/g) (Wang et al., 2014), but higher than Fe3O4@SiO2-AO (104.96 mg/g) (Zhao et al., 2014). Additionally, MAO-chitosan can be magnetic separated from aqueous solutions, which enriches it more advantages in practice application.

Table 1 Kinetic parameters for uranium adsorption by MAOechitosan. Kinetic models

Linearized forms

The pseudo-first order model

logðqe  qt Þ ¼ logqe 

The pseudo-second order model

t q

The intraparticle diffusion model

qt ¼ ki t 0:5 þ C

Elovich model

qt ¼ 1b lnðabÞ þ 1b ln t

¼

1 k2 q2e2

þ

t qe2

k1 2:303 t

Parameters

Nomenclatures

References

Values

k1 (1/min) qe1 (mg/g) R2 k2 (g/(mg min)) qe2 (mg/g) R2 ki (mg/g min1/2) C (mg/g) R2 a (mg/(g min)) b (g/mg) R2

k1 is equilibrium rate constant of pseudo-first order; qe1 is equilibrium adsorption capacity. k2 is the pseudo-second-order model constant; qe2 is equilibrium adsorption capacity. ki is the film diffusion rate coefficient; C provides information about the thickness of the boundary layer a is the initial adsorption rate; b is the desorption constant;

(Lagergren, 1898)

0.0269 120.06 0.9310 0.0002 129.87 0.9842 8.5128 8.7299 0.9751 11.4503 0.0397 0.9644

(Ho and McKay, 1998)

(Weber and Morris, 1963)

(Aharoni and Tompkins, 1970)

S. Zhuang et al. / Journal of Cleaner Production 188 (2018) 655e661

(a)

659

(b)

2

t/qt

log(qe-qt)

2

1

1

0

0 0

50

100

150

200

0

50

100

150

200

t (min)

t (min) 120

(c)

120

(d)

90

qt

qt

80

60

40 30 0 0

5

10

t

15

1

2

3

4

5

6

ln t

0.5

Fig. 6. Linearized (a) pseudo-first order (b) pseudo-second order (c) intra-particle diffusion, and (d) Elovich kinetic of uranium adsorption onto MAO-chitosan beads.

3.4. Adsorption mechanism

Fig. 7. (a) Effect of initial concentration on the adsorption of uranyl ions (m/V ¼ 1 mg/ mL, pH ¼ 6, T ¼ 298 K, t ¼ 5 h, and C0¼(10e600) mg/L) the fitting results of (b) the Langmuir model, (c) the Freundlich model, and (d) the Temkin model.

The FTIR spectra of MAO-chitosan before and after adsorption of U(Ⅵ) were presented in Fig. 2. Compared with raw MAO-chitosan, the MAO chitosan after contacting with U(Ⅵ) showed weaker peaks of 1666 cm1 (C]N vibrating) and 1574 cm1 (amino vibrating). Additionally, the peak of 3338 cm1 (-OH vibration) was red-shifted into 3443 cm1. These indicated that both O and N atoms of the amidoxime groups may involve in the adsorption process. It was consistent with the previous reports (Choi and Nho, 2000; Das et al., 2009). Therefore, an adsorption mechanism was tentatively proposed based on the above analysis and other research (Choi and Nho, 2000; Das et al., 2009), as shown in Fig. 8. UO2þ 2 was the main species of U(Ⅵ) at the given conditions. If both the -NH2 and eOH of one amidoxime group participated in the coordination, then two amidoxime groups coordinated with one UO2þ 2 , as shown in Fig. 8(a) was likely. Whilst, if only -NH2 or eOH of one amidoxime group involved in the adsorption process, then one possible adsorption process, as shown in Fig. 8(b) was come up.

Table 2 Isotherm parameters for uranium adsorption by MAOechitosan. Adsorption isotherm models

Forms

Freundlich isotherm model

ln qe ¼

Langmuir isotherm model

Ce qe

Temkin model

qe ¼

¼

1 ln n

1 qm KL

þ

Ce þ ln KF

Ce qm

RT lnðaC Þ e b

Parameters

Nomenclatures

Values

References

KF (mg/L) n R2 qm (mg/g) KL (L/mg) R2 b (J/mol) a (L/g) R2

KF is a sorption equilibrium constant, indicative of the sorption capacity; n is a constant representative of sorption intensity qm is adsorption capacity at equilibrium; KL is bonding energy of adsorption

18.24 3.00 0.8459 117.65 0.14 0.9995 148.43 2.84 0.9484

(Freundlich, 1906)

b is the constant of the Temkin model related to heat of the adsorption a is the equilibrium binding constant of the Temkin model

(Langmuir, 1918)

(Temkin and Pyzhev, 1940)

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Table 3 Adsorption capacity of uranium by various adsorbents. Adsorbents

Experimental conditions

Capacity (mg/g)

Ref.erences

Crosslinked chitosan graphene oxide-chitosan MWCNTs Oxidized MWCNTs Biochar (eucalyptus wood) Fe3O4@SiO2-AO AO-g-MWCNTs MAO-chitosan

pH pH pH pH pH pH pH pH

72.64 50.51 24.9 33.32 27.2 104.96 145 117.65

(Wang et al., 2009) (Yang et al., 2017) (Fasfous and Dawoud, 2012) (Sun et al., 2012) (Mishra et al., 2017) (Zhao et al., 2014) (Wang et al., 2014) This study

3, 298 K 5.0, 298 K 5.0, 298 K 5.0, 298 K 5.5, 293 K 5, 298 K 4.5, 298 K 6, 293 K

Fig. 8. Possible adsorption mechanisms of U(Ⅵ) on MAO-chitosan.

4. Conclusion For the removal of uranium from aqueous solution, a chitosanbased adsorbent with the grafting of amidoxime groups and combination of magnetics, was synthesized and it showed a good monolayer adsorption capacity of 117.65 mg/g. The magnetic property (Ms ¼ 21.7 emu/g) and amidoxime groups of the spherical MAO-chitosan were confirmed by VSM and FTIR spectra. Additionally, the pseudo-second order kinetic model (R2 ¼ 0.9842) and Langmuir isotherm model (R2 ¼ 0.9995) fit well with the adsorption data. Whilst, the FTIR spectra of the adsorbents before and after adsorption of uranium showed that the grafting amidoxime groups played a major role for uranium adsorption. Acknowledgements The research was supported by the National Natural Science Foundation of China (51578307), the National Key Research and Development Program (2016YFC1402507), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13026) and the National S&T Major Project (2013ZX06002001). References Abney, C.W., Mayes, R.T., Saito, T., Dai, S., 2017. Materials for the recovery of uranium from seawater. Chem. Rev. 117, 13935e14013. Aharoni, C., Tompkins, F.C., 1970. Kinetics of Adsorption and Desorption and the Elovich Equation, in Advances in Catalysis and Related Subjects, vol. 21. Academic Press, New York, USA, pp. 1e49. Beltrami, D., Chagnes, A., Haddad, M., Varnek, A., Mokhtari, H., Courtaud, B., Cote, G.,

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