Removal of uranium(VI) from aqueous solutions by nanoporous ...

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May 27, 2010 - C e. / q e. Ce (mg/L). (b). Fig. 3 Sorption isotherms (a) of U(VI) on the CMK-3 ... G, Jérôme C, Jérôme R (2003) Complexation of uranyl ions by.

J Radioanal Nucl Chem (2010) 286:129–133 DOI 10.1007/s10967-010-0624-3

Removal of uranium(VI) from aqueous solutions by nanoporous carbon and its chelating polymer composite Jin Hoe Kim • Hyung Ik Lee • Jei-Won Yeon Yongju Jung • Ji Man Kim



Received: 7 May 2010 / Published online: 27 May 2010 Ó Akade´miai Kiado´, Budapest, Hungary 2010

Abstract High surface area, ordered nanoporous carbon (CMK-3) and its chelating polymer composites were successfully prepared and utilized for the removal of U(VI) from aqueous solutions. Carboxymethylated polyethyleneimine (CMPEI) with a strong chelating property was introduced to the pore surface and inner pores of CMK-3 substrate. CMPEI-modified CMK-3 composite (CMPEI/ CMK-3) was characterized by scanning electron microscopy and nitrogen sorption. U(VI) sorption capacity was significantly improved by the surface modification of CMK-3 by CMPEI. The CMPEI/CMK-3 showed enormously increased sorption capacities, compared with those of previous sorbents (e.g., surface-functionalized silicas). In particular, the CMPEI/CMK-3 showed a significantly high uranium retention property while allowing only about 1% U(VI) to leach out over a 4 month time period, when treated with polyacrylic acid. Keywords U(VI)  Nanoporous carbon (CMK-3)  Chelating polymer  Hybrid sorbent

J. H. Kim  H. I. Lee  J. M. Kim (&) 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 e-mail: [email protected] J.-W. Yeon Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea Y. Jung (&) Department of Applied Chemical Engineering, Korea University of Technology and Education, Cheonan 330-708, Korea e-mail: [email protected]

Introduction Hybrid sorbents have been developed for use in chromatography columns or in suspension based separations of hazardous heavy metals or actinides [1–5]. Water-insoluble solid materials including metal oxides and polymer resins have been used as substrates, and a variety of complexing materials have been introduced on the surface of a solid substrate. Among these material, polyethyleneimine (PEI) and its derivatives have been used most widely for the surface modification of metal oxides (e.g., silica, zirconia) [1, 4, 6], fungal biomass [7] and polymer resins (e.g., polypyrrole) [8]. In particular, PEI-based silica sorbents with core-shell structures have attracted much attention from researchers, since they exhibited faster sorption kinetics and higher sorption capacity compared with those of conventional bead-form resins [1]. Previously, we reported preliminary results for U(VI) sorption behaviors on a nanoporous carbon functionalized with a carboxymethylated polyethyleneimine (CMPEI) [9]. In this work, the CMPEI-modified nanoporous carbon has been extensively examined in terms of pore structure and U(VI) sorption characteristics. It is well known that it is not easy to practically use sorbents containing chelating agents, since radioactive wastes disposed without a treatment to eliminate chelating agents may cause waste disposal problems such as radionuclide migration from the disposal sites [10]. For this reason, chelating agents such as ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and diethylenetriaminepentaacetic acid (DTPA) have been strictly regulated. For any waste container containing more than 0.1% chelating agents by weight, the shipper of radioactive wastes must provide information on the concentration and principal chelating agent. So, it is of significant importance to completely immobilize radionuclide species within

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sorbents for permanent disposal. In this viewpoint, in the present study, a nanoporous carbon-based hybrid sorbent has been treated with polyacrylic acid (PAA) and the uranium release behavior of the sorbent has been evaluated under vigorous stirring condition over a time period of 4 months.

Experimental

J. H. Kim et al.

polyacrylic acid to remove uranium weakly bound to the sorbent, and washed with a large amount of water and then dried at 100 °C for 24 h. It was found that 48% of uranium finally remains in the sorbent. Next, the resulting uraniumloaded CMPEI/CMK-3 (uranium content: ca. 8%) was dispersed in aqueous solution of pH 4.0. Finally, the amount of uranium released out of the CMPEI/CMK-3 was monitored for 4 months. All the experiments were performed at 22 °C.

Preparation of chelating ligand-modified nanoporous carbon sorbents Results and discussion A nanoporous carbon (CMK-3) with a high specific surface area of ca. 1550 m2/g was synthesized using nanoporous silica (MSU-H) as nano-templates [11]. The surface of CMK-3 was modified with CMPEI (Mw = ca. 5.0 9 104) to make CMPEI/CMK-3 hybrid sorbents as described elsewhere [9, 12]. Here, 50 mg of CMK-3 was added to 80 mL of polymer solution with 100 mg of CMPEI. Final CMPEI content within CMPEI/CMK-3, estimated from total organic carbon analysis (TOC), was about 24%. The CMK-3 and CMPEI/CMK-3 hybrid sorbent were characterized by scanning electron microscopy (SEM) and nitrogen sorption. SEM images were taken by a Hitachi 4300SE field emission scanning electron microscope with an accelerating voltage of 15 kV. The nitrogen sorptiondesorption isotherms were obtained using Micromeritics ASAP 2000 at liquid N2 temperature. Specific surface area and total pore volume were calculated by the BrunauerEmmett-Teller (BET) method. Pore size distribution was examined by the BJH (Barrett-Joyner-Halenda) method from the adsorption branch of the nitrogen isotherm data.

Characterization of CMPEI/CMK-3 Figure 1 shows the SEM images of the nanoporous carbon (CMK-3) and CMPEI/CMK-3. It was observed that the CMK-3 exists in an aggregated form of primary particles with a submicron-scale particle size. It seems that the surface morphology of the CMK-3 did not change after

Sorption of U(VI) onto CMPEI/CMK-3 sorbent A 1000 ppm U(VI) stock solution was prepared from uranyl nitrate (Merck) in distilled water for every sorption experiment. A CMPEI/CMK-3 sorbent was used in the U(VI) sorption tests. The U(VI) sorption on the CMK-3 and CMPEI/CMK-3 was performed in several solutions of 0.5 to 20 mg of uranium and 25 mg of CMPEI/CMK-3 at pH 3.0 and 4.0 for 24 h. The final pH of the solutions was adjusted with 0.1 M HCl. The U(VI) amount captured by the CMPEI/CMK-3 was estimated from the concentration of permeate passing through membrane. For the uranium concentration analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES, Horiba Ultima 2) was used. The isotherm data were analyzed by the Langmuir isotherm model. Furthermore, uranium leach characteristics of CMPEI/CMK-3 were examined as a preliminary study on immobilization of radionuclide species within sorbents for permanent disposal. First, 30 mg of uraniumloaded sorbent (uranium content: 15%) was treated with

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Fig. 1 SEM images of the CMK-3 (a) and CMPEI/CMK-3 (b)

Removal of uranium(VI) from aqueous solutions

shows that the value of the dV/dD greatly decreased in the pore size range of 1–10 nm, but the peak maximum shifted slightly to a lower value as CMPEI was incorporated into CMK-3. These results suggest the following two possibilities: (1) the surface of the nanopores of CMK-3 was modified with CMPEI or (2) some part of the nanopores of CMK-3 were filled with CMPEI.

(a)

1000

Volume adsorbed (cm3/g)

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Adsorption Desorption

800

CMK-3

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0

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dV/dD (cm / g nm)

CMK-3

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CMPEI/CMK-3

0

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Pore diameter (nm)

Fig. 2 N2 adsorption-desorption isotherms (a) and the corresponding pore size distribution (b) for the CMK-3 and CMPEI/CMK-3

modification with CMPEI, suggesting that most of CMPEI was introduced into the interior of CMK-3 rather than the outer surface of CMK-3. Figure 2a and b show the N2 sorption isotherms and the corresponding pore size distribution curves of the CMK-3 and the CMPEI/CMK-3 sorbents, respectively. The BET surface area, total pore volume and pore size of these materials are presented in Table 1. It is noteworthy that when CMPEI was introduced into CMK-3 substrates, the BET surface area and total pore volume of CMPEI/CMK-3 significantly decreased to approximately half the level of pristine CMK-3. Figure 2b

Figure 3a shows the U(VI) sorption isotherms on the pristine CMK-3 and CMPEI/CMK-3 at pH 3.0 and 4.0. The CMPEI/CMK-3 presented much higher sorption capacities at all tested pH values, when compared with those of the pristine CMK-3. It is remarkable that the pristine CMK-3 showed a high U(VI) sorption capacity at pH 4.0, suggesting that the CMK-3 itself can be used as a good sorbent at a specific pH. It was observed that the CMPEI/CMK-3 showed a much larger U(VI) sorption capacity at pH 4.0 (222 mg/g-sorbent) than that at pH 3.0 (119 mg/g-sorbent). This can be mainly attributed to the change in the chemical structure of the functional groups (i.e., imine and carboxylate group) of CMPEI which depends on pH. Their acid dissociation constants (Ka) suggest that they become much more deprotonated at pH 4.0 than at pH 3.0, resulting in a better chelating ability. The adsorption isotherms were analyzed by using the Langmuir isotherm model according to methods described elsewhere [9]. Figure 3b shows the Langmuir plots for the U(VI) sorption on CMPEI/CMK-3 at pH 3.0 and 4.0, and their linear regression lines. The Langmuir isotherm model exhibited good fitting results with high correlation coefficients (R2 [ 0.980), as shown in Table 2. This suggests that the U(VI) sorption on the CMPEI/CMK-3 followed the Langmuir isotherm model. It is noteworthy that the CMPEI/CMK-3 showed significantly higher sorption capacity (222 mg/g-sorbent) at pH 4.0, as compared with those of PEI-coated silica (52.4 mg/gsorbent) [1], amine anchored silica gel (35.9 mg/g-sorbent) [5] and MX-80 bentonite (32.4 mg/g-sorbent) [13]. In the aspect of permanent disposal of radioactive wastes, it is significantly important to study uranium leach behavior of a sorbent. In the previous study, uranium loading stability of CMPEI/CMK-3 was examined without

Table 1 Structural characteristics of the CMK-3 and the CMPEI/CMK-3 sorbents Sample

BET surface area (m2 g-1)

CMK-3 CMPEI/CMK-3 a

Total pore volumea (cm3 g-1)

Pore sizeb (nm)

1,546

1.20

*4.0

820

0.66

*3.3

Total pore volume calculated at P/Po = 0.99

b

Pore size at the peak of pore size distribution curve calculated by the BJH method on the basis of the adsorption branch of the nitrogen isotherms

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0 0

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a sorbent particle, the uranium-loaded sorbent was treated with polyacrylic acid (PAA). Figure 4 shows uranium leach data for U-loaded CMPEI/CMK-3 (U: 8% by weight) after the simple treatment with PAA. Surprisingly, it was found that only about 1% uranium was released out of CMPEI/CMK-3 during 4 months. This implies that the PAA-treated CMPEI/CMK-3 can be disposed directly in high integrity containers or in cement-solidified waste forms with a sufficient safety margin forgoing any further treatment.

pH 4

1.0

0.5

0.0 0

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Fig. 4 Uranium leach characteristics of CMPEI/CMK-3 in an aqueous solution (pH 4.0), after the treatment with polyacrylic acid. Ct and Co mean U(VI) content at time t and the initial U(VI) content, respectively

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Ce (mg/L)

Ce / q e

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Ce (mg/L) Fig. 3 Sorption isotherms (a) of U(VI) on the CMK-3 (filled triangle: pH 3.0, filled circle: pH 4.0) and the CMPEI/CMK-3 (open triangle: pH 3.0, open circle: pH 4.0) and Langmuir isotherm plots (b) for the U(VI) adsorption on the CMPEI/CMK-3. Solid lines represent the linear regression line by Langmuir isotherm model

Table 2 Fitting results of the sorption isotherms of U(VI) on the CMPEI/CMK-3 by the Langmuir isotherm model pH

Langmuir isotherm Qa0 (mg g-1)

KbL (L mg-1)

R2

3

125

1.57 9 104

0.985

4

250

2.63 9 104

0.990

a

Maximum sorption capacity predicted by Langmuir model

b

Constant related to adsorption energy

any treatment following U(VI) sorption [9]. More than 30% of uranium was released from CMPEI/CMK-3 sorbent in 3 weeks. This can be attributed to the weakly sorbent bound uranium. To completely immobilize uranium within

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Conclusion Ordered nanoporous silica (CMK-3) was modified with carboxymethylated polyethyleneimine (CMPEI) for the removal of U(VI) from an aqueous solution. It is suggested from the analysis of SEM and nitrogen sorption that CMPEI existed in the pore surface or inner pore of CMK-3 rather than on the outer surface of CMK-3. U(IV) sorption experiments were performed at pH 3.0 and 4.0, considering the hydrolysis of uranyl ions which occurs strongly at pHs higher than 4.0. The CMPEI/CMK-3 showed an enormously increased sorption capacity (222 mg/g-sorbent) at pH 4.0, compared with those of amine anchored silica, PEIcoated silica and MX-80 bentonite. The CMPEI/CMK-3 showed a much higher U(VI) sorption capacity at pH 4 than that at pH 3.0. This is mainly caused by the change in the chelating efficiency of CMPEI, which is linearly proportional to the degree of deprotonation of its functional groups. In particular, the PAA-treated CMPEI/CMK-3 exhibited distinguishing characteristics in terms of uranium immobilization. Only about 1% U(VI) was released out of the hybrid sorbent during 4 months, implying that it can be

Removal of uranium(VI) from aqueous solutions

disposed directly in high integrity containers for permanent disposal without any further treatment. Acknowledgments This work was supported by the start-up funds for new faculty of Korea University of Technology and Education, and the Basic Science Research program (NRF, 2009-0076903). J.M. Kim also thanks to the WCU (World Class University, MEST, R312008-000-10029-0) program.

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