ISSN 1066-3622, Radiochemistry, 2014, Vol. 56, No. 3, pp. 262–266. © Pleiades Publishing, Inc., 2014. Original Russian Text © V.V. Milyutin, V.M. Gelis, N.A. Nekrasova, I.V. Melnyk, O.A. Dudarko, V.V. Sliesarenko, Yu.L. Zub, 2014, published in Radiokhimiya, 2014, Vol. 56, No. 3, pp. 223–226.

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Sorption of Actinide Ions onto Mesoporous Phosphorus-Containing Silicas

V. V. Milyutin*а, V. M. Gelisа, N. A. Nekrasovaа, I. V. Melnykb, O. A. Dudarkob, V. V. Sliesarenkob, and Yu. L. Zubb

а Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, korp. 4, Moscow, 119991 Russia

* e-mail: vmilyutin@mail.ru b Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine,

vul. Henerala Naumova 17, Kyiv, 03164 Ukraine

Received July 12, 2013

Abstract—Equilibrium and kinetic characteristics of template mesoporous silicas containing phosphonic acid residues in sorption of various actinide ions were studied. The sorption equilibrium involving these sorbents is attained within 20 min after introducing the sorbent into the solution. The calculated values of the internal dif-fusion coefficient (D̄) and half-exchange time (τ0.5) in sorption of uranium were ~3.5 × 10–16 m2 s–1 and ~390 s, respectively. Mesoporous phosphorus-containing silicas efficiently sorb from acid solutions uranyl ions, Th(IV), and Pu(IV). In sorption of uranium from sulfuric acid solutions, the capacity of the sorbents is 125– 132 mg g–1, and in sorption from nitric acid solutions (0.5–3.0 M HNO3), 276–299 mg g–1. In sorption of Th(IV) from nitric acid solutions, the capacity of the sorbents is 60–66 mg g–1. In sorption of microamounts of 239Pu(IV), the distribution coefficient reaches 4500 cm3 g–1. Phosphorus-containing silicas in nitric acid solu-tions do not noticeably sorb 241Am, which allows using them for efficient separation of the Pu/Am pair with the separation factor of no less than 2 × 103.

Keywords: mesoporous silicas, phosphonic acid groups, actinide ions, sorption

Ion-exchange processes are widely used for recov-ering actinide ions from solution in uranium recovery from ores, in reprocessing of spent nuclear fuel (SNF), in utilization of liquid radioactive waste, etc. For ex-ample, vinylpyridine anion exchangers are used for recovering actinides from nitric acid solutions formed in SNF reprocessing [1]. Along with advantages, these sorbents have a significant drawback: They take up actinide ions only from solutions with high (>7 M) HNO3 concentration.

Sorbents with phosphorus-containing functional groups exhibit high selectivity to actinide ions in vari-ous oxidation states. In [2, 3], we demonstrated the possibility of recovering actinides from nitric acid so-lutions with various phosphorus-containing sorbents. As we showed, S-957 sorbent with phosphonic and sulfonic acid groups, produced by Purolite, is the most promising for this purpose. The main limiting factor in using organic ion-exchange resins in radiochemical practice is their relatively low radiation-chemical sta-bility.

DOI: 10.1134/S1066362214030072

EXPERIMENTAL

Therefore, growing researchers’ attention is at-tracted today by so-called hybrid organo-inorganic ma-terials in which various complexing groups are fixed on an inorganic matrix [4–9]. For example, Dabrowski et al. [10] showed that hydrolytic polycondensation of tetraethoxysilane with the trifunctional silane (С2Н5O)3 · Si(CH2)2P(O)(OС2Н5)2 yielded xerogels whose treat-ment with boiling hydrochloric acid gave sorption ma-terials with the functional group [≡Si(CH2)2P(O)(OH)2]. Introduction of various templating agents in the course of hydrolytic polycondensation, e.g., of Рluronic 123, allows preparation of mesoporous silicas containing the above groups (up to 2 mmol g–1) with high specific surface area [11].

In this study we examined the ability of mesopor-ous phosphorus-containing silicas prepared by the tem-plate method to sorb ions of various actinides.

In our study we used samples prepared by the tem-plate method using various surfactants. In synthesis of

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MK sample, we took as template Pluronic 123 (poly-ethylene glycol–polypropylene glycol–polyethylene glycol triblock copolymer, ЕО20РО70ЕО20), which is used for preparing mesoporous silicas of type SBA-15 [11], and in synthesis of MS sample we took octade-cyltrimethylammonium bromide [12]. In the latter case, the sample was synthesized by spraying an iso-propanol solution of tetraethoxysilane and functional-izing and templating agents with simultaneous drying at 100°С (spray-drying). The template was removed from the resulting mesophases by refluxing in acidified ethanol. After that, the samples were dried, the ester groups at the P atom were hydrolyzed by boiling in concentrated hydrochloric acid, and the samples were thoroughly washed and again dried [11, 12].

The samples were white powders with the particle size of 1–10 (MS) and up to 1 μm (MK) (Fig. 1).

The specific surface area and the pore volume and diameter were determined from the low-temperature nitrogen adsorption–desorption isotherms (Kelvin-1042). The content of functional groups [≡Si(CH2)2P·(O)(OH)2] was determined by pH-potentiometric titra-tion. Some characteristics of the synthesized sorbents are given in Table 1.

For comparing the sorption parameters, along with mesoporous phosphorus-containing silicas we took the following sorbents: S-957 cation exchanger containing phosphonic and sulfonic acid groups (Purolite) and highly basic anion exchangers AMP (Russia) and PFA-600/4740 (Purolite).

Experiments on sorption of macroamounts of UO22+

and Th4+ ions and of microamounts of 239Pu(IV) and 241Am(III) were performed in the batch mode by stir-

(a) (b)

Fig. 1. SEM images of (a) MS and (b) MK samples.

ring a weighed portion of the dry sample with a solu-tion aliquot for 24–48 h. Then, the solid and liquid phases were separated by filtration, and the filtrate was analyzed to determine either the residual concentration of the sorbate ions (in sorption of UO2

2+ and Th4+) or their specific activity [in sorption of 239Pu(IV) and 241Am(III)]. In the former case, we calculated the static exchange capacity (SEC), and in the latter case, the distribution coefficients (Kd) of the corresponding ra-dionuclides by formulas (1) and (2), respectively:

where С0 and Сe are the initial and equilibrium concen-trations of the ions in solution, respectively, g dm–3; А0 and Аe are the initial and equilibrium specific activities of the radionuclide, respectively, Bq dm–3; Vl is the liquid phase volume, cm3; and ms is the sorbent weight, g.

The uranium sorption was performed from model solutions containing 2.3–2.6 g dm–3 UО2

2+ (in terms of U) and 0.5–3.0 M HNO3 at the liquid to solid (L : S) ratio of 200; the phase contact time was 24 h.

The thorium sorption was performed from a model solution containing 0.75 g dm–3 Th4+ and 0.3 M HNO3

SEC = (С0 – Сe)(Vl/ms),

Kd = [(A0 – Ae)/Ae](Vl/ms),

(1)

(2)

Table 1. Main characteristics of the synthesized samples

Characteristic MK MS Specific surface area, m2 g–1 490 747 Pore volume, cm3 g–1 0.82 0.36 Pore diameter, nm 5.4 2.2 Concentration of groups, mmol g–1 0.9 0.9

at L : S = 250; the phase contact time was 24 h. The sorption of microamounts of 239Pu(IV) and

241Am(III) was performed from model solutions con-taining ~1 × 105 and ~2 × 104 Bq dm–3 239Pu(IV) and 241Am(III), respectively, and 3.0 M HNO3 at L : S = 200; the contact time was 48 h.

When constructing the uranium sorption isotherms, we used as liquid phase a model sulfuric acid solution with the initial uranium concentration varied in the range 0.023–0.95 g dm–3, pH 1.63–1.91. The L : S ra-tio was 200, and the phase contact time was 48 h.

We studied the kinetics of uranium sorption onto samples of mesoporous silicas from a sulfuric acid solution with the initial uranium concentration of 0.360 g dm–3, pH 1.63. The procedure of the kinetic experiments is described below.

A temperature-controlled beaker was charged with 100 cm3 of the solution, and a weighed portion (0.177 g) of the air-dry sorbent was added with vigor-ous stirring. Solution samples (2 cm3) were taken at definite time intervals. The samples were filtered, and the concentration of uranyl ions in the filtrate was de-termined. The degree of attainment of the equilibrium (F) at time τ was calculated by formula (3):

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RESULTS AND DISCUSSION

Fig. 2. Degree of attainment of the equilibrium F in ura-nium sorption onto MS sample as a function of time τ.

Fig. 3. Kinetic plot for uranium sorption onto MS sample in F–τ1/2 coordinates.

F(τ) = Еτ/Е∞, (3)

where Еt and Е∞ are the sorbent capacities for uranium at time τ and at complete saturation, respectively, mg g–1. The quantity Е∞ was determined in an inde-pendent experiment after continuous stirring of the mixture for 3 days.

To evaluate the kinetic characteristics of the sor-bent, we calculated the coefficient of internal diffusion (D̄) of the radionuclide inside the sorbent grain and the half-exchange time (τ0.5) by formulas (4) and (5), re-spectively [13]:

F = (6/r0)(D̄τ/π)1/2 at F < 0.4,

τ0.5 = 0.03r02/D̄,

(4) (5)

where r0 is the mean granule radius, m, and τ is the sorption time, s. The calculation was performed by the least-squares treatment of the initial portion of the ki-netic curve in the F–τ1/2 coordinates. The sorbent grain size in the calculations was assumed to be equal to 5 μm (R0 = 2.5 × 10–6 m).

The concentration of uranyl ions in solutions was determined spectrophotometrically with Arsenazo III [14]. The concentration of thorium ions was deter-mined by volumetric complexometric titration [15]. The specific activity of 239Pu in solutions was deter-mined by radiometry with the α-ray radiometer of a Progress 2000 universal spectrometric complex, and that of 241Am, with the γ-ray radiometer of the same complex.

Figure 2 shows the kinetic curve of the uranium sorption onto MS sample from a sulfuric acid solution. As can be seen, the sorption equilibrium is attained within approximately 20 min after introducing the sor-bent into the solution. To determine the mechanism of the uranium sorption, we plotted the kinetic depend-ence in the F–τ1/2 coordinates (Fig. 3). This depend-ence is well approximated by a straight line, which suggests internal diffusion control of the sorption [16]. The internal diffusion coefficient (D̄) and half-exchange time (τ0.5), calculated by formulas (4) and (5), appeared to be ~3.5 × 10–16 m2 s–1 and ~390 s (6.5 min), respectively.

The uranium sorption isotherms, i.e., the depend-ences of the sorbent capacity (Е) on the equilibrium concentration of uranium in solution (Сe), are given for MS and MK samples in Fig. 4. Linearization of these isotherms in the Сe/Е–Сe coordinates in accordance

τ,

τ

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Fig. 4. Isotherms of uranium sorption from a sulfuric acid solution onto (1) MK and (2) MS samples.

with the Langmuir equation (Fig. 5) shows that the isotherms are well described by the linear relationship with the correlation coefficient of 0.99. By the least-squares treatment of the isotherms obtained, we calcu-lated the limiting distribution coefficients at microcon-centrations (Kd), the exchange constants (Kex), and the limiting capacities (Е0) for sorbent samples MK and MS (Table 2). The values obtained under similar con-ditions for S-957 cation exchanger are given in Table 2 for comparison.

As can be seen, the sorption characteristics of the mesoporous phosphorus-containing silicas in sorption of uranium from sulfuric acid solutions are lower than those of S-957 cation exchanger. It should be noted, however, that the content of complexing groups in the sorbents under consideration is approximately four times lower than in S-957, which is reflected in the Е0 values. Taking into account the Е0 values for MK and MS sorbents (Table 2) and the concentration of phos-phonic acid residues in these samples (Table 1), we can conclude that the [≡Si(CH2)2P(O)(OH)2] : UО2

2+ ratio in the forming complexes is close to 2 : 1.

The values of SEC values for uranium in 0.5 and 3.0 M HNO3 solutions for MK and MS samples, S-957 cation exchanger, and AMP and PFA-600/4740 anion exchangers are given in Table 3.

Fig. 5. Linearized isotherms of uranium sorption onto (1) MK and (2) MS samples.

Table 2. Limiting distribution coefficients (Kd), exchange constants (Kex), and limiting capacities (Е0) in uranium sorp-tion onto various sorbents

Sorbent Kd, cm3 g–1 Kex E0, mg g–1 (mmol g–1) S-957 2.9 × 104 57 517 (2.17) MK 1.1 × 103 8.4 132 (0.554) MS 0.74 × 103 5.7 125 (0.525)

Table 3. Values of SEC of various sorbents for uranium (mg g–1) in nitric acid solutions

Sorbent 0.5 M HNO3 3 M HNO3 AMP 41 25 PFA-600/4740 40 36 MK 227 299 MS 232 276 S-957 363 264

0.3 M HNO3. The results obtained show that SEC for thorium of MK and MS samples is 66 and 60 mg g–1, respectively, which is considerably lower compared to S-957 cation exchanger (380 mg g–1).

It should be noted that the ability of mesoporous silicas (KIT-6 и SBA-15) modified with ≡Si(CH2)2P·(O)(OС2Н5)2 groups from toluene to sorb uranyl [and Th(IV)] ions was also studied by Lebed et al. [17, 18]. These sorbents exhibited high kinetic characteristics, but their SEC for uranium was as low as 55 and 49 mg g–1, respectively, i.e., it was considerably lower than that of MK and MS samples (Table 3).

The distribution coefficients (Kd) of the mi-croamounts of 241Am and 239Pu on MK and MS sam-ples and on S-957 cation exchanger in a 3.0 M HNO3 solution are given in Table 4.

The results obtained show that the mesoporous sil-ica samples, especially MS, exhibit high ability to sorb

As can be seen, in nitric acid solutions the uranium sorption onto MK and MS samples considerably in-creases, and in 3 M HNO3 the SEC values for uranium on these samples exceed that on S-957 cation ex-changer. As compared to highly basic AMP and PFA-600/4740 anion exchangers, the capacity of MK and MS samples in nitric acid solutions is higher by an or-der of magnitude.

The values of SEC for thorium were determined in

ACKNOWLEDGMENTS

REFERENCES

Translated by G. Sidorenko

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Table 4. Distribution coefficients (Kd) of 241Am and 239Pu(IV) on various sorbents in 3.0 M HNO3 solution

Radionuclide МS МK S-957 239Pu 4500 173 1310

241Am <2 <1 9.1

239Pu from nitric acid solutions. At the same time, these sorbents do not noticeably take up 241Am, which allows their use for efficient separation of the Pu/Am pair with the separation factor of no less than 2 × 103.

The study was financially supported by the Federal Target Scientific and Technical Program of Ukraine “Nanotechnologies and Nanomaterials” (project no. 6.22.5.42) and by the Presidium of the National Academy of Sciences of Ukraine (grants for young scientists nos. 3/2011, 2/2012).

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