Macromolecular Research, Vol. 19, No. 5, pp 421-426 (2011) DOI 10.1007/s13233-011-0509-5

www.springer.com/13233

Preparation and Application of Chelating Polymer-Mesoporous Silica Composite for Europium-ion Adsorption Myung-Hee Yun1, Jei-Won Yeon1, Jin Hoe Kim2, Hyung Ik Lee2, Ji Man Kim*,2, Seok Kim3, and Yongju Jung*,4 1

Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea Department of Chemistry, BK21 School of Chemical Materials Science, Department of Energy Science and SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Korea 3 Department of Chemical and Biochemical Engineering, Pusan National University, Busan 609-735, Korea 4 Department of Applied Chemical Engineering, Korea University of Technology and Education, Cheonan 330-708, Korea 2

Received July 3, 2010; Revised December 12, 2010; Accepted December 13, 2010 Abstract: Ordered mesoporous silica (MCM-48) functionalized with a carboxymethylated polyethyleneimine (CMPEI) was prepared and applied to adsorb Eu(III) ions. The samples were characterized by SEM, TEM, XRD and N2 adsorption-desorption experiments. Adsorption studies of Eu(III) onto the MCM-48 and CMPEI-functionalized mesoporous silica, which are denoted as CMPEI/MCM-48, were performed by batch experiments. The adsorption of Eu(III) on the CMPEI/MCM-48 obeyed the Langmuir isotherm model. The CMPEI/MCM-48 exhibited much higher adsorption capacity at pH 3.0 to 5.0 compared to that of the pristine MCM-48. The Eu(III) adsorption of the CMPEI/MCM-48 increased with the pH. This was attributed mainly to a change in the chemical structure of CMPEI, which is strongly dependent on the solution pH. The XPS depth profiles showed that CMPEI was present on the mesopore walls inside the MCM-48 as well as on the surface of the particle, and further showed that a change in the Eu(III) content with etching time was not severe, indicating that the complexation of Eu(III) with CMPEI/MCM-48 occurred uniformly, even inside the CMPEI/MCM-48. The overall results showed that the mesopore surface of MCM-48 was functionalized successfully with CMPEI and the resulting CMPEI/MCM-48 could be used as a proper adsorbent for the recovery of trivalent actinide species. Keywords: chelating polymer, mesoporous silica (MCM-48), carboxymethylated polyethyleneimine (CMPEI), Eu(III), adsorption.

Introduction

improve their adsorption capacity. Recently, hybrid materials consisting of rigid matrices and organic binding sites have been extensively studied for the application to the stationary phase of a high performance liquid chromatography (HPLC).11-15 Polymers with complexing ligands are introduced to the surface of inorganic materials by an impregnating process or by covalent bonding. Here, we present a new hybrid material that consists of carboxymethylated polyethyleneimine (CMPEI) with an excellent chelating property and ordered mesoporous silica (MCM-48) substrate with a large surface area which is necessary for increasing the load of chelating polymers. The structure of MCM-48 consists of two independent and intricately interwoven networks of mesoporous channels.16-18 Mesoporous silicas have received a considerable amount of attention due to their great potential for adsorption, catalysis, and nanoscience.19-21 In this study, we functionalized ordered mesoporous silica (MCM-48) with CMPEI by a simple adsorption method and used it as a Eu(III) sorbent. In general, Eu(III), a representative trivalent lanthanide ion, has

A number of materials have been investigated for complexation study of the lanthanides and actinides, with a final goal of removal of them from waste solutions. They include inorganic oxides such as minerals or surface-modified inorganic materials,1-5 conventional ion-exchange resins,6,7 and water-soluble complexation polymers.8-10 Alumina, silica, kaolinite, and zeolite, etc., have been studied as inorganic oxides. It is known that these materials are chemically and thermally stable when compared with complexation polymers. Especially, inorganic solid supports have advantage in that they can be vitrified or ceramized for a permanent disposal of radioactive species. On the other hand, organic resin and complexation polymers have a significant drawback in the aspects of degradation and disposal. The surface of inorganic materials has been modified with complexing materials to *Corresponding Authors. E-mails: [email protected] or [email protected] The Polymer Society of Korea

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been used as a surrogate for Am(III) because Eu(III) shows chemical behaviors similar to those of Am(III).10 We characterized the CMPEI/MCM-48 by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and nitrogen sorption. The Eu(III) incorporated CMPEI/MCM-48 was examined using XPS depth-profiling to get information on the location and distribution of the CMPEI and incorporated Eu(III). The Eu(III) adsorption behaviors of the resulting material, designated as CMPEI/MCM-48, were investigated at pH 3.0 to 5.0 as a part of a final goal to develop an optimum material for the recovery of removal of actinide species from dilute aqueous solutions.

Experimental Preparation of CMPEI/MCM-48. Ordered mesoporous silica (MCM-48) with a cubic space-group symmetry of Ia3d, used as a solid matrix of CMPEI, was synthesized as described elsewhere.16-18 The pore surface of the MCM-48 was coated with CMPEI (Mav = ca. 50,000) by mixing 0.5 g of MCM-48 and 1 g of CMPEI at pH 3.0 ± 0.2 for 24 h. After the reaction was completed, the mixture solution was filtered and the resulting silica was washed with a large amount of water under stirring for 24 h at room temperature. The polymer content within the CMPEI-functionalized silica composite, referred to as CMPEI/MCM-48, was analyzed by a thermal gravimetric analysis (TGA), and was found to be 18.2% (Supplementary Information, Figure S1). To evaluate the stability of the CMPEI/MCM-48, the amount of CMPEI released from the composite was examined with time using a TOC analyzer at a pH range from 3 to 9 under vigorous agitation. A general trend that the releasing CMPEI amount slightly increased with the solution pH was observed (Supplementary Information, Figure S2). When a solution is in the pH range of 3 to 5, however, less than 1% of CMPEI was released from MCM-48 matrix for 10 days, indicating that the CMPEI/MCM-48 is very stable in the pH 3 to 5. Scanning electron microscopic (SEM) images of the MCM-48 were collected using a Hitachi 4300SE field emission scanning electron microscope (FE-SEM) with an accelerating voltage of 15 kV. Transmission electron microscopic (TEM) images of the MCM-48 were collected using a JEOL 2100F field emission transmission electron microscope (FE-SEM) with an accelerating voltage of 200 kV. Powder XRD patterns for the MCM-48 and CMPEI/MCM48 were taken in the 2θ range of 0.7 ~ 5.0° by a Rigaku D/ MAX-2200 Ultima instrument equipped with Cu Kα radiation at 30 kV and 40 mA. The nitrogen adsorption-desorption isotherms for the MCM-48 and CMPEI/MCM-48 were obtained using a Micromeritics ASAP 2000 at liquid N2 temperature. Specific surface area and pore volume were estimated by the Brunauer-Emmett-Teller (BET) method. Pore size distribution was calculated by the BJH (BarrettJoyner-Halenda) method on the basis of the adsorption 422

branch of the nitrogen sorption isotherms. All the samples were completely dried under a vacuum at 100 oC for 24 h before each measurement. Fluorophotometer (FS-900CD, Edinburgh) was used for the measurement of fluorescence spectra, and Eu(III) excitation was set at 422 nm. Adsorption Tests of Eu(III) onto the CMPEI/MCM-48. A stock solution of 1,000 ppm Eu(III) was prepared by dissolving 2.817 g of Eu(NO3)3·5H2O (Aldrich) in distilled water. The complex formation between Eu(III) and CMPEI in an aqueous solution was examined by fluorescence spectra for three types of different solutions: a 5.0×10-3 mM CMPEI solution, a 5.0×10-2 mM Eu(III) solution, and mixture solutions of 5.0×10-3 ~ 5.0×10-2 mM of Eu(III) and 5.0×10-3 mM of the CMPEI. Fluorescence spectra were taken at a position perpendicular to the incident beam of 384 nm by a FL900 (Edinburgh Analytical Instrument). To determine a proper reaction time, amount of Eu(III) adsorbed on to the CMPEI/ MCM-48 was measured at pH 5 with agitation time in a solution containing 5 mg of Eu(III) ions and 30.6 mg of the CMPEI/ MCM-48 material. Adsorption of Eu(III) reached equilibrium less than 6 h (Supplementary Information, Figure S3). Adsorption isotherm tests of Eu(III) on the CMPEI/MCM48 (CMPEI = 18.2%) were investigated at final pHs of 3.0, 4.0, and 5.0 by agitating 80 mL of various mixture solutions consisting of 0.1 to 15 mg of Eu(III) and 30.6 mg of CMPEI/ MCM-48 at 22 oC for 24 h, long enough to obtain equilibrium data. The final solution pHs were adjusted with 1 M HCl or 1 M NaOH. Similar adsorption tests for the pristine MCM-48 and polyethyleneimine-coated silica gel (PEI-SG) were carried out in order to compare their adsorption performances. The polyethyleneimine-coated silica gel (40~200 mesh, Aldrich) was used as received. After the reaction was completed, the mixture was filtered and the filtrate was analyzed by an inductively coupled plasma atomic emission spectroscopy (ICP-AES, Horiba Ultima 2). Equilibrium isotherm data was analyzed by the Langmuir isotherm models. In order to examine the distribution of CMPEI and Eu within the Eu-incorporated CMPEI/MCM-48 particles, XPS depth profile analysis for major elements (C, O, N, Eu) was performed by using a Thermo VG Scientific ESCALAB 250 spectrometer. The etching rate by an argon ion sputtering was about 4.8 nm min-1, which was estimated from a Ta2O5 reference sample.

Results and Discussion Characterization of the MCM-48 and CMPEI/MCM48. The SEM images (Figure 1) showed that the shape of the prepared MCM-48 particles was irregular and the size of the individual MCM-48 particles was less than a few micrometers. The TEM images (Figure 2) revealed that the MCM-48 has highly ordered meso-structure and well developed mesopores with a pore diameter of about 3 nm. In addition, surface morphology of the MCM-48 functionMacromol. Res., Vol. 19, No. 5, 2011

Preparation and Application of Chelating Polymer-Mesoporous Silica Composite for Europium-ion Adsorption

Figure 1. SEM images of the MCM-48: at low magnification (a) and at high magnification (b).

Figure 3. XRD patterns of the MCM-48, CMPEI/MCM-48, and PEI-SG samples. Table I. Structural Characteristics of the MCM-48, CMPEI/ MCM-48, and PEI-SG Sample

Figure 2. TEM images of the MCM-48: scale bars are 100 and 20 nm at low magnification (a) and at high magnification (b), respectively.

BET Surface Total Pore Pore Sizec Areaa (m2 g-1) Volumeb (cm3 g-1) (nm)

MCM-48

949

0.935

2.71

CMPEI/MCM-48

652

0.582

2.57

PEI-SG

105

0.126

4.76

a

alized with CMPEI was examined by SEM and TEM. Any CMPEI agglomerates on the external surface of the MCM48 were not seen in both SEM and TEM images of CMPEI/MCM-48, indicating successful incorporation of CMPEI into the interior of MCM-48 particles (Supplementary Information, Figure S4). Figure 3(a) shows the powder XRD patterns for the MCM48 silica, the CMPEI/MCM-48 and the polyethyleneiminecoated silica gel (PEI-SG), which were taken below 2θ = 5°. The XRD pattern of the MCM-48 showed typical characteristic peaks of MCM-48 mesoporous silica, as reported elsewhere.18 The XRD pattern of the CMPEI/MCM-48 was very similar to that of the MCM-48 except for the intensity loss, especially in the 2θ range of 3 to 5°, which should be attributed to a mesopore filling of the MCM-48 with CMPEI. On the other hand, the PEI-SG showed no peaks at 2θ < 5°. Figure 3(b) shows the XRD patterns for the three materials, which were taken in the 2θ region of 5 to 80°. They exhibited very similar XRD patterns with broad bandwidth, indicating that they are simply disordered silica materials. In order to further investigate the structural information for the MCM-48 silica, the CMPEI/MCM-48 and the PEI-SG, we performed N2 adsorption/desorption experiments. Figure 4 show the N2 sorption isotherms and the corresponding pore size distribution curves, calculated from the adsorption branch by the BJH method, respectively. The values for the BET surface area, total pore volume and pore size of these materials are listed in Table I. The isotherm for the MCM48 showed type IV isotherms, which is a typical characteristic Macromol. Res., Vol. 19, No. 5, 2011

BET surface areas calculated in the range of p/p0=0.05–0.20. bTotal pore volume calculated at P/Po=0.990. cMaximum values in pore size distribution curve calculated by the BJH method on the basis of the adsorption branch of the nitrogen isotherms.

of mesoporous materials according to IUPAC nomenclature.16 The CMPEI/MCM-48 showed somewhat different isotherms to that of MCM-48, as shown in Figure 4(a). The BET surface area was reduced from 949 to 652 m2 g-1, and the total pore volume was decreased from 0.935 to 0.582 cm3 g-1, after the CMPEI coating process (Table I). Both the MCM-48 and the CMPEI/MCM-48 showed basically narrow pore size distribution curves with similar features. The CMPEI/MCM-48 showed a pore size distribution curve different from the MCM-48 in the following two aspects. The value of the dV/dD was greatly reduced for the pore size of 2-3 nm, and the maximum peak was changed from 2.71 to 2.57 nm (Figure 4(b)) after the CMPEI coating process. Overall results indicate that the mesopores of MCM-48 were uniformly coated with CMPEI in the CMPEI/MCM48. On the other hand, the PEI-SG showed much lower BET surface area of 105 m2 g-1 and a smaller pore volume of 0.126 cm3 g-1, when compared with those of the CMPEI/ MCM-48. It was found that the PEI-SG has few or no mesopores below 7 nm in pore diameter. Adsorption Characteristics of Eu(III) onto the CMPEI/ MCM-48. The complex formation between Eu(III) and CMPEI in an aqueous solution was confirmed by the fluorescence spectra. It is well known that Eu(III) complexes show strong peaks associated with 5D0→7F1 and 5D0→7F2 transitions due to a strong absorption of a ligand (e.g, mac423

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Figure 5. Fluorescence emission spectra of 5.0×10-3 mM of CMPEI solution, 5.0×10-3 mM of Eu(III) solution, and the mixture solutions of 5.0×10-3~5.0×10-2 mM of Eu(III) and 5.0×10-3 mM of CMPEI for an excitation wavelength of 422 nm.

Figure 4. N2 adsorption-desorption isotherms (a) and the corresponding pore size distributions (b) for the MCM-48, CMPEI/MCM48, and PEI-SG samples.

rocyclic compounds, carboxylic acid derivatives, cryptands, heterobiaryl compounds) and an efficient energy transfer from a ligand to the Eu(III), even though the absorption coefficients of Eu(III) without a ligand are very small.22-25 Eu(III)-CMPEI complex showed two intense bands with a narrow bandwidth at 594 and 617 nm and a relatively small band at 580 nm, which are the typical fluorescence spectra of Eu(III) complexes, as shown in Figure 5.22-25 It is noteworthy that Eu(III) showed two small bands centered at ca. 594 and ca. 617 nm and the strongest band was changed from 5D0→7F1 to 5D0→7F2 transition after forming a complex with CMPEI. Figure 6(a) shows the adsorption isotherms of Eu(III) on pristine MCM-48, CMPEI/MCM-48, and PEI-SG at pH 3.0, 4.0, and 5.0. The CMPEI/MCM-48 exhibited much higher adsorption capacities when compared with those of the MCM-48 and the PEI-SG, indicating that the surface of the MCM-48 was successfully functionalized with CMPEI. In addition, it was observed that the adsorption capacity increased with the pH. In general, the chemical structure of CMPEI is dependent on the pH due to protonation-deprotonation of the functional groups (i.e., imine and carboxylate group). The complex formation efficiency increases as the functional groups are deprotonated, which explains the increased adsorption capacity of the CMPEI/MCM-48 with the pH. Eu(III) adsorption behaviors of the polymer-modified silica are summarized in Table II. In a dilute Eu(III) solution of 0.04 mM (6.2 mg L-1), the CMPEI/MCM-48 showed 424

Figure 6. Adsorption isotherm data of Eu(III) on the MCM-48, CMPEI/MCM-48 and PEI-SG (a) and a Langmuir isotherm plot for the Eu(III) adsorption on the CMPEI/MCM-48 (b). Solid lines represent the fitting results by a linear regression analysis.

almost complete removal of the Eu(III) from the solution at pH 5.0, resulting in a high distribution coefficient (Kd) value of 6.28 × 104. This suggests that the CMPEI/MCM-48 could be a promising adsorbent for removing lanthanide(III) ions from solutions containing low concentrations of lanthanide ions. On the other hand, at the higher Eu(III) concentration of 0.40 mM (62 mg L-1), the CMPEI/MCM-48 showed a small removal percentage (ca. 35%) and greatly reduced Kd values even at pH 5.0. The adsorption isotherms were analyzed by the Langmuir isotherm model with the assumption that the adsorption Macromol. Res., Vol. 19, No. 5, 2011

Preparation and Application of Chelating Polymer-Mesoporous Silica Composite for Europium-ion Adsorption

Table II. Eu(III) Adsorption Behaviors of the CMPEI/MCM-48 pH Weight (mg) Initial Eu Concentration (mg L-1) Final Eu Concentration (mg L-1) Capacity (mg-Eu g-1) 30.6

3 4

6.20

12.7

7.72 × 10

1.25

2

3

30.6

62.0

49.3

32.9

6.67 × 10

3.19

30.6

6.20

0.436

15.2

3.49 × 104

1.50

48.6

1.15 × 10

4.63

4

30.6

5

1.58

Eu Content (wt%)b

Kda

62.0

42.2

3

30.6

6.20

0.250

15.7

6.28 × 10

1.55

30.6

62.0

40.5

56.3

1.39 × 103

5.33

a

-1

b

Kd = the amount of adsorbed metal (µg) per gram of adsorbent / metal concentration (µg mL ) remaining in the solution. Eu content means the weight percentage of Eu(III) within the Eu(III)-adsorbed CMPEI/MCM-48.

energy is independent of the surface coverage. The Langmuir isotherm equation is given as follows: Ce / qe = 1 / (Q0 KL) + Ce / Q0

(1)

where qe is the amount of adsorbed adsorbates per unit weight of adsorbents at an equilibrium (mg g-1), Ce is the concentration of an adsorbate in a solution at an equilibrium (mg L-1), Q0 is the maximum adsorption capacity per 1 g of adsorbent (mg g-1), and KL is a constant related to the adsorption energy (L mol-1).26-28 Figure 6(b) shows the Langmuir plot for the adsorption data of Eu(III) on CMPEI/MCM-48 at pH 3.0, 4.0, and 5.0 and the fitting result (solid line) by a linear regression analysis. The Langmuir isotherms show excellent fits to the experimental adsorption data with very high correlation coefficients (R2 > 0.990). This strongly suggests that the adsorption of Eu(III) on CMPEI/MCM-48 followed the Langmuir model. The Q0 and KL, calculated from the slope and intercept of the regression line, respectively, are presented in Table III. The adsorption of Eu(III) on CMPEI/MCM-48 was significant at the pH values, increasing Q0 value from 34.4 mg g-1 at pH 3.0 to 64.9 mg g-1 at pH 5.0, which were estimated from the Langmuir isotherm equation. Distribution of Eu(III) Inside CMPEI-MCM-48. It is well-known that XPS technique is fairly useful for a qualitative analysis of the elements on the surface of samples, but depth profiling can help to prove the distribution of elements with depth from the surface of materials. Figure 7(a) shows the XPS survey spectrum of the CMPEI/MCM-48 with Eu(III). This spectrum revealed the presence of Eu, C, N, O, and Si, which are the main components of the CMPEI/MCM-48

with Eu(III). XPS depth profile analysis for major elements (Eu, C, N, O, and Si) was performed in order to examine the distribution of CMPEI and Eu(III) with the depth of the CMPEI/MCM-48 particles containing Eu(III). Figure 7(b) shows the depth profile for the major elements (C, O, N, Eu) in the CMPEI/MCM-48 particles. The atomic percentages of C and N, originated from CMPEI, decreased with an etching time of less than 10 min, but reached almost constant values

Table III. Summary of Parameters Calculated from the Fitting Results of Adsorption Isotherm of Eu(III) on the CMPEI/ MCM-48 pH 3 4 5

Langmuir Isotherm -1

Q0 (mg g )

KL (L mg-1)

R2

34.4

5.83 × 104

0.9917

49.8

7.02 × 10

0.9971

64.9

5.03 × 10

0.9957

4

Macromol. Res., Vol. 19, No. 5, 2011

4

Figure 7. XPS survey spectrum (a) and depth profiles (b) for the Eu(III)-incorporated CMPEI/MCM-48. 425

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in 50 min. This shows that the concentration of CMPEI was relatively higher on the surface of the particles than inside the particles but a large amount of CMPEI was present inside the MCM-48 particles. Interestingly, it was observed that the atomic percentages of Eu(III) did not change greatly with the etching time, indicating that Eu(III) was evenly present at least to the depth etched by the argon ion sputtering (3,000 Å).

Conclusions Ordered mesoporous silica (MCM-48) was functionalized with carboxymethylated polyethyleneimine (CMPEI) by a simple adsorption method and characterized by SEM, TEM, XRD, and N2 adsorption-desorption experiments. It was proved from N2 adsorption-desorption isotherms that the CMPEIfunctionalized mesoporous silicas (CMPEI/MCM-48) still maintained mesoporous structure with reduced pore volume and pore size when compared to those of MCM-48. The formation of the Eu(III) complex with CMPEI in an aqueous solution was confirmed by the fluorescence spectra. The fluorescence spectra of Eu(III)-CMPEI complex showed two intense bands with a narrow bandwidth at 594 and 617 nm, associated with 5D0→7F1 and 5D0→7F2 transitions, respectively, which are the characteristic bands for Eu(III) complexes. Equilibrium adsorption tests of Eu(III) onto the pristine MCM-48 and the CMPEI/MCM-48, were carried at pH 3.0 to 5.0. It was observed that the adsorption of Eu(III) onto the CMPEI/MCM-48 obeyed the Langmuir isotherm model. It is noteworthy that the CMPEI/MCM-48 exhibited much higher adsorption capacities when compared with those of the MCM-48 and commercially available PEI-coated silica. The adsorption of Eu(III) on CMPEI/MCM-48 was significant at the pH values, increasing Q0 value from 34.4 mg g-1 at pH 3.0 to 64.9 mg g-1 at pH 5.0. XPS depth profiles showed that CMPEI existed on mesopore walls inside the MCM-48 particles as well as on the surface of the particles and that Eu(III) concentration with etching time did not change greatly, indicating that Eu(III) is uniformly present with depth. Overall results strongly suggest that the mesopores of MCM-48 were successfully functionalized with CMPEI and that Eu(III)-CMPEI complex was formed within the silica mesopores. Due to its simplicity for making chelating polymer-mesoporous silica, this approach could be used as a practical adsorbent for the removal of actinide species. Acknowledgements. This work was supported by the Education and Research Promotion Program of KUT and the Basic Science Research program (NRF, 2009-0076903). J. M. Kim also thanks to the WCU (World Class University, MEST, R31-2008-000-10029-0) program. Electronic Supplementary Information (ESI) is available: TGA results, the effect of solution pH on stability of the CMPEI/MCM-48, the adsorption amount of Eu(III) on the CMPEI/MCM-48 with time, SEM and TEM images of the 426

CMPEI/MCM-48. The materials are available via the Internet at http://www.springer.com/13233.

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