PVA cross-linked hydrogel

Hydrometallurgy 104 (2010) 150–155

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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Selective adsorption of uranyl ion on ion-imprinted chitosan/PVA cross-linked hydrogel Yunhai Liu a,b,c,⁎, Xiaohong Cao a,c, Rong Hua c, Youqun Wang c, Yating Liu c, Cui Pang c, Yong Wang c a b c

State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, 330013, PR China Key Laboratory of Radioactive Geology and Exploration Technology Fundamental Science for National Defense, East China Institute of Technology, Fuzhou 344000, PR China Chemical, Biological and Materials Sciences Department, East China Institute of Technology, Fuzhou 344000, PR China

a r t i c l e

i n f o

Article history: Received 25 March 2010 Received in revised form 23 May 2010 Accepted 24 May 2010 Available online 1 June 2010 Keywords: Uranyl ions Adsorption isotherms Ion-imprinted hydrogel Chitosan

a b s t r a c t An interpenetration network (IPN) ion-imprinting hydrogel (IIH) was synthesized using uranyl ions as template for adsorption and removal of uranyl ions from aqueous solutions. The IIH was obtained via crosslinking of blended chitosan/polyvinyl alcohol (PVA) using ethylene glycol diglycidyl ether (EGDE). The ability of the IIH to adsorb and remove uranyl ions from aqueous solutions was assessed using a batch adsorption technique. The maximum adsorption capacity was observed in the pH range of 5.0–6.0. The adsorption process could be well described by both the Langmuir and Freundlich isotherms and the maximum adsorption capacity calculated from Langmuir equation was 156 mg/g. Equilibrium was achieved within 2 h. The kinetic data, obtained at optimum pH 5.0 could be fitted with to a pseudo-second order equation. The selectivity coefficient of uranyl ion and other metal cations on IIH indicated an overall preference for uranyl ions which was much higher compared with the non-imprinted hydrogel. This suggests that the IIH is a promising sorbent material for the selective removal of uranyl ions from aqueous solutions. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Uranium is a toxic and weakly radioactive heavy metal that exists ubiquitously in the environment such as rocks, soils and waters (Jackson et al., 2005). Therefore, the removal and recovery of uranium from contaminated surface and ground water, as a result of nuclear industries, has attracted more and more attention. (Donia et al., 2009; Smith et al., 2009; Xie et al., 2009). In the past decades, several techniques, such as chemical precipitation (Djedidi et al., 2009), solvent extraction (Domanska and Rekawek, 2009), micellar ultrafiltration (Cojocaru et al., 2009), organic and inorganic ion exchange (Rafati et al., 2010) have been employed for the removal of heavy metal ions from aqueous solutions. Most of these techniques suffered from technical, economic, environmental and health problems related to low efficiency, long time of processing, high energy consumption and the large quantity of hazardous materials used. However, adsorption is an attractive method because of its high efficiency, ease of handling, and the availability of different adsorbents (Guo et al., 2009). Various kinds of new adsorbents for removing and recovering uranium have been reported (Ortiz-Oliveros et al., 2009; Choi et al., 2009). Among them, ⁎ Corresponding author. State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, 330013, PR China. Tel.: + 86 794 8258861. E-mail address: [email protected] (Y. Liu). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.05.009

polymeric and biopolymeric materials are considered to be particularly effective because of their chemical stability and high selectivity (Solpan and Torun, 2009; Chauhan and Kumar, 2009). Chitosan is a partially deacetylated polymer of acetyl-glucosamine and is a poly(amino-saccharide) containing mainly of poly(1→4)-2amino-2-deoxy-D-glucose units (Gerente et al., 2007). Due to the presence of amine groups, chitosan is unique among biopolymer in its high affinity for heavy metal ions. Chitosan has demonstrated the potential to adsorb significant amounts of metal ions, such as Cu2+ (Kannamba et al., 2010), Cr(VI) (Elwakeel, 2010), Pb2+, Hg2+ (Cao et al., 2010), Ag+ (Shawky, 2009) etc. However, chitosan-based adsorbents have weak mechanical properties and poor chemical resistances. For these reasons, much research interest has been drawn to improve the physical properties of chitosan-based adsorbents. In particular, much effort has been focused on the interpenetration network (IPN), prepared by the introduction of a second hydrophilic polymer into the chitosan matrix, because it has a durable mechanic strength (Bayramoglu et al., 2007). Poly(vinyl alcohol) (PVA) is a promising candidate as a second hydrophilic polymer, because of its high hydrophilicity, good chemical resistance and mechanical property (Quintero et al., 2010). Molecularly imprinted polymers (MIP) represent a new class of materials possessing high selectivity and good affinity for the target molecule (Claude et al., 2010). As a branch of MIP, ion-imprinted polymer has shown considerable promise as a method for preparing materials which are capable of ion recognition (Ahmadi et al., 2010).

Y. Liu et al. / Hydrometallurgy 104 (2010) 150–155

Ion-imprinting chitosan adsorbents have been reported to show good chemical and physical stability as well as high selectivity toward thorium and uranyl ions (Birlik et al., 2006; Singh and Mishra, 2009). During the last decade, some interest has been shown for the preparation of various novel uranyl-imprinted polymers. Sadeghi and Akbarzadeh have reported a new imprinted sorbent for preparation and pre-concentration of UO2+ (Sadeghi and Akbarzadeh Mofrad, 2 2007) Ahmadi and co-workers have reported UO2+ ion-imprinted 2 polymer materials used for solid-phase extraction that were prepared by copolymerization of a ternary complex of uranyl ions with styrene and divinyl benzene in the presence of 2,2-azo-bis-isobutyro-nitrile (Ahmadi et al., 2010). James et al. (2009) have proposed the preparation of a novel meso-porous uranyl ion-imprinted polymer material via trapping of amidoxime functionality either alone or in presence of 4-vinyl-pyridine inside a cross-linked polymer matrix. In view of the above, our laboratory has used imprinted modified chitosan for the separation and enrichment of uranium(VI) from aqueous solutions (Liu et al., 2010). Here, an interpenetration network (IPN) ion-imprinting hydrogel (IIH), composed of chitosan and PVA, was synthesized using uranyl ions as template. Ethylene glycol diglycidyl ether (EGDE) was used as cross-linker for formation of UO2+ 2 -imprinting hydrogel. At the same time, the non-imprinting hydrogel (NIH) was synthesized as a control. The adsorption of uranyl ions from aqueous solutions using the hydrogels has been investigated through equilibrium studies and batch kinetics. The experimental equilibrium adsorption data were tested for the Langmuir and Freundlich equations. The experimental kinetic data were analyzed using a pseudo-second order kinetic model. In order to elucidate its practical application as a selective adsorbent for uranyl ions, the selective adsorption studies were carried out under the optimum conditions.

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20 mL of a uranyl solution (20 mg/mL uranium) was dropwise added to the mixture and kept under stirring for 5 h. The cross-linking and ionimprinting proceeded by the addition of 0.5 g of EGDE and the reaction was carried out over 4 h at 50 °C. The hydrogel thus synthesized was cut into small pieces of equal dimensions and stripped with 0.1 mol/L HNO3 to remove the template ions. Finally, it was washed with distilled water for removal of any unreacted fraction and dried in an oven at 55 °C to get leached IIH. The non-imprinting hydrogel (NIH) was similarly synthesized in the absence of templates. 2.4. Uranyl ions adsorption studies Adsorption of uranyl ions on the hydrogels from aqueous solutions was investigated in a batch-wise method. Aqueous solutions (50 mL) containing different amounts of uranyl ions were incubated with the hydrogels at different initial pHs, (adjusted with 0.1 mol/L HNO3 and 0.1 mol/L NaOH) and allowed to equilibrate for different conditions while being shaken continuously. Aqueous solutions were separated from the hydrogels at desired intervals and the residual concentrations of uranyl ions were determined by the spectro-photometric method. The amount of uranyl ions adsorbed per unit mass of the hydrogels was calculated by using the following expression: qe = ðC0 −Ce ÞV = W where, qe is the adsorption capacity of the hydrogels (mg/g); C0 and Ce are the concentrations of uranium in the initial and equilibrium solution (mg/L), respectively, V is the volume of the aqueous solution (L) and W is the mass of dry hydrogels (g). 2.5. Selective adsorption studies

2. Experimental

The selectivity of the IIH and NIH for uranyl ions over other heavy metals were evaluated from the selectivity coefficient (βUO2+ = Mn+),

2.1. Chemicals and materials

which was determined by incubating 100 mg of hydrogels with 100 μg of each individual heavy metal ions present in 0.5 L of distilled water under identical conditions. The selectivity coefficient (Singh and Mishra, 2009) is defined as:

2

Chitosan (degree of deacetylation = 0.85 and Mw = 100,000– 300,000) was supplied by Shanghai Chemical Reagent Co., Ltd. China. Poly(vinyl alcohol) (PVA, 2000 polymerization degrees) was purchased from Guangzhou Chemical Reagent Factory, China. Ethylene glycol diglycidyl ether (EGDE, 98%) was purchased from Aldrich Co. and used without further purification. UO2(NO3)2·6H2O supplied by Chemical Factory in Beijing was used to provide imprinted ions and a stock solution of uranyl ion. All other reagents were of AR grade.

βUO2 += Mn + = 2

D=

C0 −Ce V × W Ce

where C0 and Ce are the concentrations of metal ions in the initial solution and equilibrium solution (mg/L), respectively, V is the volume of the aqueous solution (L) and W is the mass of dry hydrogel (g). The effect of imprinting on selective adsorption was evaluated with the relative selectivity coefficient βr, which can be defined as the following expression:

2.3. Synthesis of IPN ion-imprinting hydrogel The IIH was synthesized by interpenetrating cross-linked chitosan/ PVA with EGDE as a cross-linker and uranyl ions as the template. The detailed synthesis route is described as follows. Firstly, 2 g of chitosan was dispersed in 50 mL of 2% aqueous solution of acetic acid and stirred for 2 h until complete dissolution. 2 g of PVA was dispersed in 50 mL of 2% aqueous solution of acetic acid and stirred for 4 h at 60 °C to allow dissolution. Then, a mixture of 30 mL chitosan solution (4%, w/v) and 10 mL PVA solution (4%, w/v) was stirred overnight at 60 °C to form a homogeneous solution with 3/1 (w/w) chitosan/PVA. Afterwards,

2

DMn +

where, DUO2 + and DMn + are the distribution ratios of the uranyl ions 2 and other coexistent heavy metal ions, respectively. The distribution ratio (D) was calculated by using the following expression:

2.2. Instruments and apparatus PHS-3C precision pH meter, 721-type Spectrophotometer, SHZ82A reciprocating thermostatted air bath shaker, and 78HW-1type thermostatted magnetic stirrer were used in the experiments. FTIR spectra were taken on Nicollet 510. The BET surface area was measured using a F-Sorb 2400 gas adsorption surface analyzer (China). The other heavy metal ions were detected by Atomscan 16 ICP-AES (TJA, USA).

DUO2 +

βr =

β imprinted β nonimprinted

3. Results and discussion Fig. 1 shows a schematic representation of chitosan, PVA and blend chitosan/PVA cross-linked by EGDE. The BET surface areas of the hydrogels were determined to be 0.82 and 0.87 m2/g for NIH and IIH, respectively. Fig. 2 represents the FTIR spectra of chitosan, PVA and

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Y. Liu et al. / Hydrometallurgy 104 (2010) 150–155

Fig. 1. Schematic representation of (a) chitosan; (b) blend chitosan/PVA; (c) PVA with EGDE.

the obtained hydrogels. There were a few changes in the adsorption intensity at wave numbers 1250–1050 cm− 1 (Fig. 2c), and this can be attributed to C–O–C stretching vibration. This implies that the crosslinking reaction occurred between EGDE and chitosan/PVA. Moreover, the adsorption intensity of 1650 cm− 1 (–NH stretching vibration) had no significant change, indicating most of the cross-linkages were formed between EGDE and the hydroxyl groups of chitosan but not amino groups. 3.1. Dependence of pH on adsorption The pH of the aqueous solution, the most important parameter on adsorption studies, strongly affects the adsorption properties of hydrogels for heavy metal ions. As discussed by Salih (Salih, 2002), metal ions are pH-dependently adsorbed onto non-specific and specific sorbents. Fig. 3 shows the effect of pH on the adsorption of uranyl ions by imprinted and non-imprinted hydrogels. It can be seen that the adsorption capacities of uranyl ions on both hydrogels were increased significantly with the increase in pH below pH 5, but then leveled off at around pH 5.0–6.0. The maximum adsorption values for uranyl ions onto IIH and NIH were 132 mg/g and 83 mg/g dry hydrogel respectively, from which the imprinting effect is clearly

Fig. 2. FTIR of (a) chitosan; (b) PVA and (c) the obtained hydrogels.

observed. Due to the ionization of amine groups of chitosan at acidic pH values, at low pH values (≤pH 5), the decrease of the adsorption capacities can be attributed to the competitive binding of H3O+ and uranyl ions to the amine groups (Alakhras et al., 2005). In addition, it can also be explained by the presence of different mononuclear and polynuclear uranium(VI) hydrolysis products in the form [(UO2)p (OH)q](2p − q)+ at different pH values and metal concentrations in the solution (Baes and Mesmer, 1976). At higher pH values (≥ 6), dissolved carbonate and bicarbonate anions also increased, which form complex anions with uranium(VI) so the adsorption capacities decreased as the concentration of dissolved carbonate and bicarbonate anions increased (Han et al., 2007). 3.2. Evaluation of adsorption isotherm models Adsorption isotherms are the basic requirements for the design of an adsorber by providing fundamental physic-chemical features (Wang and Chen, 2009). The most frequently used isotherms for adsorption experimental data correlations are the Langmuir and Freundlich models because of their simplicity. In order to evaluate the applicability of adsorption processes as a unit operation, the initial uranium (VI) concentrations in the range of 30–120 mg/L have been used for investigation of the adsorption

Fig. 3. Effect of pH on the adsorption.

Y. Liu et al. / Hydrometallurgy 104 (2010) 150–155

Fig. 4. Effect of uranyl ions concentration on the adsorption of uranyl ions.

isotherm. The adsorption isotherms of uranyl ions from aqueous solutions onto the hydrogels at an ambient temperature are shown in Fig. 4. In the range of the studied concentration, the adsorption capacity of the hydrogels rises with increasing concentration of uranyl ions until it reaches saturation. It should be noted that the adsorption capacity of IIH is greater than that of NIH at the identical initial concentration of uranyl ions. In our work, two models, both Langmuir and Freundlich, are employed to analyze the data. The Langmuir model is based on several assumptions, including homogeneous surface, localized adsorption on the surface and solo molecule accommodated active site. The Langmuir isotherm equation may be written as:

qe = qm

bCe 1 + bCe

where qe is the adsorption capacity of the hydrogels (mg/g) at equilibrium, Ce are the concentration of uranium in the equilibrium solution (mg/L), and qm is the Langmuir constant, which is equal to the monolayer adsorption capacity (mg/g). The parameter b is the Langmuir sorption equilibrium constant (L/mg) related to the free energy of adsorption.

153

Fig. 6. Freundlich isotherms.

The Freundlich equation is an empirical equation, which is among the earliest empirical equations applied to predict adsorption equilibrium data. The Freundlich equation can be written in the following form: 1

qe = KF C ne where KF is the constant (mg/g), which indicates the relative adsorption capacity of the adsorbent. The parameter n characterizes the heterogeneity of the system. The Langmuir and Freundlich models for the adsorption of uranyl ions on the hydrogels are presented in Fig. 5 and 6. The straight lines of plots indicate that the adsorption processes of all cases could be well described by both Langmuir and Freundlich models. The parameters and the correlation coefficients calculated from corresponding models were given in Table 1, which indicated that the adsorption of the hydrogels is a complex process containing physical adsorption and chemical adsorption, therefore the adsorption belongs to multilayer sorption. The greater qm value of IIH compared with that of NIH suggests that the IIH has the greater affinity to uranyl ions than the NIH, due to the development of a large number of cavities matching with uranyl ions by ion-imprinting. The lower b value of IIH compared with that of NIH reflects in the steep initial slope of an adsorption isotherm, indicating a desirable high affinity (Kratochvil and Volesky, 1998). The values of 1/n of the hydrogels were all less than 1 which indicates that in the whole adsorption process the adsorption is easy. 3.3. Kinetics of uranyl ions adsorption In order to achieve the proper design of an adsorber, the adsorption equilibrium needs to be supplemented with adsorption kinetics, which offers information on the rate of metal adsorption (Lesmana et al., 2009). The time required to achieve adsorption equilibrium for uranyl ions from aqueous solutions was determined. The relationship between adsorption capacity and adsorption time was described in Fig. 8. As seen, the initial adsorption rate of the hydrogels was very fast, due to smaller mass transfer resistance on the Table 1 Langmuir and Freundlich parameters for uranyl ions adsorption. Hydrogels

Fig. 5. Langmuir isotherms.

IIH NIH

Langmuir isotherm model

Freundlich isotherm model

qm (mg/g)

b (L/mg)

R2

KF

1/n

R2

156 129

0.0366 0.0405

0.9982 0.9944

15.76 16.63

0.468 0.412

0.9959 0.9987

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Y. Liu et al. / Hydrometallurgy 104 (2010) 150–155 Table 2 Selective adsorption properties of the IIH and NIH. Element

UO2+ 2 4+

Th Ni2+ Fe3+ Zn2+ Mn2+ Cu2+ Co2+

Distribution ratio, L/g

Selectivity coefficient

βUO2 2

IIH

NIH

IIH

NIH

495 81 66 70 52 51 98 43

86 69 63 74 55 46 92 39

– 6.11 7.50 7.07 9.52 9.71 5.05 11.51

– 1.25 1.37 1.16 1.56 1.87 0.93 2.21

+

= Mn

+

Relative selectivity coefficient βr – 4.89 5.47 6.09 6.10 5.19 5.43 5.21

W = 100 mg; V = 0.5 L; C0 = 200 μg/L; T = 25 °C; pH = 5.0.

Fig. 7. Effect of time on the adsorption of uranyl ions.

surface with the continuation of adsorption. After this initial adsorption period, the adsorption equilibrium was gradually achieved within 120 min. The faster adsorption rate compared with other adsorbents reported elsewhere could be attributed to the absence of internal diffusion resistance (Donat, 2009). The adsorption kinetics of uranyl ions by the hydrogels were analyzed on the basis of the pseudo-second order kinetic model, which is expressed as:

t 1 t = 2 + qt qe kqe

where t is the contact time (h), qt and qe are the amount of uranyl ions adsorbed at an arbitrary time t and at equilibrium (mg/g), respectively, and k is the rate constant (g/mg h). From the data of Fig. 7, plots of t/qt versus t for the adsorption of uranyl ions are obtained, as shown in Fig. 8. It was observed that the adsorption on both hydrogels followed the pseudo-second order kinetic model (the correlation coefficients are larger than 0.98). As a result, the qe values for IIH and NIH were calculated to be 160 and 114 mg/g, respectively, indicating an improvement of the adsorption capacity of IIH by ion-imprinting effect. On the other hand, the k values were 1.07× 10− 4 and 1.84× 10− 4 min g/ mg for IIH and NIH, respectively.

3.4. Evaluation of the selective adsorption For the purpose of evaluating the selectivities of the hydrogels, the selective adsorption studies were carried out under the optimum conditions. The distribution ratios and selectivity coefficients with respect to other heavy metal ions (that are likely to co-exist with uranium(VI) in natural sources) using IIH and NIH are shown in Table 2. It can be seen that the distribution ratio of IIH for uranyl ions was 6-fold greater than that of NIH, whereas it was almost equal for other heavy metal ions. Furthermore, the relative selectivity coefficient of IIH for each individual heavy metal ion was far greater than 1. These observations are attributed to the specific recognition cavities for uranyl ions created in IIH unlike NIH, which are developed by ionimprinting. Based on the results shown in Table 2, it is evident that the IIH has a strong ability to selectively adsorb uranyl ions from several heavy metal ions present in aqueous solutions. 4. Conclusions The uptake of uranyl ions was successfully accomplished using an interpenetration network ion-imprinting hydrogel (IIH). Rapid attainment of adsorption equilibrium and high adsorption capacity values were some of the significant features. Adsorption of uranyl ions on IIH followed the Langmuir and Freundlich isotherms. The kinetics of adsorption followed a pseudo-second order rate equation. An overall selectivity for uranyl ions was observed showing that ionimprinted chitosan/PVA cross-linked hydrogel can be used effectively to remove and recover uranyl ions from aqueous solutions. Acknowledgements This work was financially supported by the National Defense Basic Research Foundation of China (Grant No. A3420060146), and Science & Technology Project of Jiangxi Provincial Department of Education (Grant No. GJJ10493, GJJ09528). References

Fig. 8. Plots for pseudo-second order kinetic modeling.

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