Extraction of Molybdenum from Supersaturated Solutions in Nitric Acid

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Abstract—Data were obtained on the composition of saturated Mo solutions in ... The extractability of Mo in the supersaturation region sharply decreases with.
ISSN 1066-3622, Radiochemistry, 2010, Vol. 52, No. 2, pp. 180–188. © Pleiades Publishing, Inc., 2010. Original Russian Text © N.D. Goletskii, L.G. Mashirov, B.Ya. Zilberman, Yu.S. Fedorov, D.V. Ryabkov, M.N. Makarychev-Mikhailov, E.A. Puzikov, I.V. Blazheva, 2010, published in Radiokhimiya, 2010, Vol. 52, No. 2, pp. 155–161.

Extraction of Molybdenum from Supersaturated Solutions in Nitric Acid with Tributyl Phosphate Solutions N. D. Goletskii*, L. G. Mashirov†, B. Ya. Zilberman, Yu. S. Fedorov, D. V. Ryabkov, M. N. Makarychev-Mikhailov, E. A. Puzikov, and I. V. Blazheva Khlopin Radium Institute, Research and Production Association, Federal State Unitary Enterprise, 2-i Murinskii pr. 28, St. Petersburg, 194021 Russia; * e-mail: [email protected] Received June 23, 2009

Abstract—Data were obtained on the composition of saturated Mo solutions in HNO3 solutions of various concentrations at various temperatures, with a maximum at 5 M HNO3. Extraction of Mo from such solutions of low acidity with concentrated solutions of TBP in xylene at room temperature was studied, and the IR spectra of the extract were measured. The extractability of Mo in the supersaturation region sharply decreases with a decrease in the TBP concentration and with an increase in the acidity of the aqueous phase. The kinetics of the Mo extraction is described by a first-order rate equation. The process apparently involves formation of molybdic acid in the aqueous phase by the reaction МоО2(ОН)+ + H2O = Н2МоО4 + H+. It is assumed that Н2МоО4 dissolves in water present in TBP, instead of precipitating from the aqueous phase. Key words: molybdenum, extraction, nitric acid, tributyl phosphate DOI: 10.1134/S1066362210020116

Additional information on the mechanism of the Mo extraction from its concentrated nitric acid solutions and on the composition of the extractable complexes could be derived from the IR spectra of the Mo extracts. Such information, however, does not concern its TBP extracts obtained from nitrate solutions. Only one paper is known [5] in which it was shown that, in unsaturated extracts of Mo from dilute HNO3 in HDEHP diluted with CCl4 (without TBP), Mo occurs in the form of molybdenyl (absorption bands at 918 and 958 cm–1). The spectrum of Mo in HDBP diluted with xylene has a similar shape [6]. In the extraction of Mo from 4–9 M HCl into ketones or 1–10% TBP in the form of HMoO2Cl3(H2O)3Sx, the same bands are observed [7].

Molybdenum is poorly extracted with dilute TBP from nitrate solutions [1] with a low concentration of Мо (of the order of several g l–1), in contrast to chloride media [2] from which it readily passes into 1–10% TBP in saturated or aromatic hydrocarbon diluents in the forms of molybdenyl chloride MoO2Cl2(H2O)3· (TBP)2 and the complex acid HMoO2Cl3(H2O)3(TBP)2. In extraction of Mo from nitric acid solutions, TBP is usually introduced into the complex extractant based on dialkyl hydrogen phosphates as a solubilizer and/or specific synergistic additive [3, 4]. A UV study [3] showed that, in the extraction of Mo from 6–10 M HNO3 with solutions of HDEHP in dearomatized kerosene containing 3% TBP, the nitrate ion or HNO3 is coextracted with Mo. However, there were no clear evidences that this effect was not due to the presence of TBP. At the same time, in extraction of Mo with a solution of dibutyl hydrogen phosphate (HDBP) in diluted TBP [4], under certain conditions, TBP is apparently capable not only to increase the capacity of the extractant with HDBP over the level corresponding to the formation of МоO2(DBP)2, but also to extract Mo independently in appreciable amounts from nitrate solutions with high Mo content. †

The largest number of bands belonging to molybdenum compounds is observed in the spectrum of the solution prepared at high loading of the organic phase with Mo, containing HDBP in 30% TBP with xylene. In this spectrum, additional bands at 805 and 877 cm–1 are observed along with the absorption bands at 918 and 958 cm–1 [8]. There are also data on the IR spectra of solid hydrated H2MoO4 [9] and of the MoO42– anion [10]. Here we report on a more detailed study of the phe-

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nomenon that we discovered previously: extraction of Mo from its supersaturated solutions in dilute HNO3 with mixtures of TBP with nonpolar diluents. Our goal was to elucidate the nature and evaluate the boundaries of manifestation of this phenomenon. EXPERIMENTAL The initial 40–60 g l–1 solution of Мо was prepared by dissolving Mo foil or powder in 6 M HNO3 at 20– 50°С. The resulting solution was allowed to stand for 3–4 days and separated from the precipitate, after which it was kept for no less than 1 day more. This colorless solution did not react with permanganate and hence did not contain Mo(V) [11]. Most probably, it was an acidic solution of molybdenyl oxynitrate [12]. Data on the Mo extraction into 60–100% TBP were obtained by successive contacts of the extractant with the initial Mo solution for 2 h. As diluents we used xylene, Isopar L, and CCl4. The acidity was preliminarily set by quick repeated extraction of HNO3 into 30% TBP in appropriate diluent. As a result, a supersaturated molybdenum solution was obtained, with up to fivefold supersaturation. The kinetics of the Mo extraction from the solution prepared was studied by taking samples at 0.5-h intervals. The total time of most of the experiments was 4 h. Data on the limited saturation of the extract with molybdenum and on its residual content in the aqueous phase were obtained in experiments that lasted for 40 h. The concentration of the nitrate ion in the extract and in the aqueous phase was determined with Fe(II) [13], and the HNO3 concentration in the aqueous phase, by titration of the equilibrium extract in 30% TBP (see below), with the subsequent calculation using the extraction isotherm and neglecting the possible salting-out effect of the hydrolyzed molybdenyl nitrate. Attempts of direct potentiometric titration or titration in a NaF medium gave ambiguous results. The amount of Mo in the solutions was determined colorimetrically with thiocyanate [14] in a sulfuric acid solution. At a low molybdenum concentration and/or high acidity, aqueous samples were preliminarily evaporated until H2SO4 vapor appeared. To determine Mo in the organic phase, it was preliminarily stripped into a 0.1 M NaOH solution. The IR spectra of the extract were recorded with an RADIOCHEMISTRY Vol. 52 No. 2 2010

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Table 1. Residual concentration of Mo [Mo]res in solution in precipitation from supersaturated solutions in relation to the HNO3 concentration and temperature T, °С

1 20 15.2 55 3.2 75 2.5 90 1.3

[Mo]res, g l–1, at indicated [HNO3], M 2 3 4 5 6 8 9 – 58 – 70 65 44 20.4 17.4 22.6 23.5 22.3 – 5.2 4.0 7.8 15.2 17.0 16.5 9.9 5.2 2.6 3.0 4.3 4.8 4.6 4.0 2.3 –

11 – 1.8 1.4 1.0

FTIR-8700 device with digital data processing, using 50-μm-thick cells with CaF2 or KRS (for the range below 1100 cm–1) windows. In some cases, we additionally used cells of smaller thickness (10 μm and less). We also recorded the IR spectra of freshly precipitated molybdic acid obtained either by heating of Mo concentrate after removing HNO3 by extraction with diluted TBP to a residual nitric acid concentration of 1 M, or by evaporation of this solution to 8 M HNO3 with the subsequent storage of the residue. RESULTS AND DISCUSSION Extraction Behavior of Molybdenum In the course of our studies we found that Mo in the cold readily forms supersaturated solutions in dilute HNO3, stable for several days. Data on the steady-state residual concentration of Mo in HNO3 solution (after partial precipitation) after prolonged standing at 20– 90°С are given in Table 1. This composition cannot be considered as equilibrium, because it is attained only from the side of supersaturated solutions and cannot be attained by prolonged dissolution of molybdic acid. The dependences of such “solubility” of Mo on the HNO3 concentration at all the examined temperatures are curves with a maximum at an HNO3 concentration of 5 M. These values are considerably higher than those obtained for the corresponding ammonium molybdate solutions and given in [1]. At 20°С, stable concentration of Мо in 1 M HNO3, equal to ~15 g l–1, is attained in 7 days; in a maximum in 5 M HNO3 it reaches ~70 g l–1. At 55°С, the steady-state composition is attained in 1 day, and at 75 and 90°С, in 8 h. For the extraction with tributyl phosphate, we used Mo solutions of the concentration a fortiori exceeding the steady-state level indicated in Table 1. In contact of

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GOLETSKII et al. Table 2. Influence of the Mo concentration on its extraction with TBP in Isopar L (20°С, contact time 2 h, n = 1)

Fig. 1. Extraction of Mo with 80% TBP in xylene as a function of time. (1, 2) DMo at n = 2 and 0.2, respectively; (3) CMo(a) at n = 2.

Fig. 2. Influence of the TBP concentration in xylene on the Mo extraction as a function of time. Initial solution: 50– 54 g l–1 Мо + 1 M HNO3, 20°С; n = 2. [TBP], M: (1) 2.85, (2) 1.82, and (3) 1.09.

such supersaturated solutions with 60–100% TBP, Mo slowly passes into the extract [2]. The data obtained for 80% TBP in xylene (Fig. 1) showed that, at the volume ratio of the organic and aqueous phases n = O : A = 2, its residual concentration in 1 M HNO3 was ~15 g l–1, which corresponds to its solubility under these conditions (Table 1). The distribution ratios of Mo in such a system increase with the progress of its recovery. The limiting saturation of the extract, attained at n = 0.2, appeared to be ~24 g l–1 Мо at its concentration in the aqueous phase of 49 g l–1. The system with n = 2 appeared to be stable with time, whereas in the system with n = 0.2 a precipitate formed on standing. However, as shown in Table 2, the transfer of Mo to the extract drastically decreases with the TBP dilution, with this trend observed in a narrow range of HNO3 concentrations. Molybdenum is extracted most efficiently at [HNO3] ≤ 1 M, whereas at [HNO3] ≥ 2 M the Mo ex-

[TBP], [HNO3], mol % M 100 0.5 100 0.5 100 1 100 2 100 3 100 4 80 0.6* 80 1 80 1 80 1 80 1 80 1 80 1* 80 2 80 3 80 3 80 3 80 6 60 0.8 50* 1.0 30* 1.0 20* 1.0

Mo concentration, g l–1 initial (a) (о) 22 2.8 19.3 45 14,6 30.2 40 26.7 13.3 43 41.0 2.0 45 44.5 0.5 46 45.8 0.2 52 18 34 1.0 0.83 0.045 5.0 4.5 0.25 10 8.4 1.55 26 14.2 12.1 52 35 17.2 52 25 26 16 14 1.7 2.2 2.1 0.015 15 14.5 0.092 32 30.2 0.16 33 32.2 0.06 59 52.0 6.1 58 55 2.5 56 50 0.40 58 57 0.21

DMo 7 2.1 0.5 0.05 0.012 0.004 1.9 0.055 0.055 0.18 0.85 0.49 1.0 0.11 0.007 0.006 0.005 0.002 0.12 0.043 0.008 0.003

Note: Here and in Tables 3 and 4, (a) refers to aqueous, and (o), to organic phase. * Contact time 6 h.

traction regularly decreases. At the same time, it is very difficult to obtain stable supersaturated solutions, from which the extraction was performed, at HNO3 concentrations lower than 0.5–0.7 M because of rapid precipitation of molybdic acid. In addition, the extractability of Mo becomes weaker with a decrease in the TBP concentration, and with 30% TBP in 2 h Mo practically is not extracted at all (Fig. 2). More detailed data on the initial period of the Mo extraction, without reaching the steady state, are given in Table 3. As can be seen, the distribution ratio increases with time irrespective of the diluent, phase ratio, and initial concentration of Mo in the supersaturated solution. Table 4 shows that the presence of U in the extract decreases both the limiting solubility of Mo in the organic phase and the mass transfer rate, but does not affect the minimal content of Mo in the aqueous phase. RADIOCHEMISTRY Vol. 52 No. 2 2010

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Table 3. Distribution ratio of Мо as a function of the phase contact time t in extraction from supersaturated aqueous solutions (HNO3 in the aqueous phase 1 M, 20°С) into 80% TBP with various diluents at various values of n Xylene, n = 2 [Мо], g l–1

t, h (о)

(a)

CCl4, n = 0.7 [Мо], g l–1

D

ΔС(a)

(о)

(a)

(о)

D

53

0.5

3.1

46.8

6.2

0.066

2.2

28.5

1.54

0.08

14.0

43.0

7.0

0.32

1

4.9

43.2

3.6

0.113

3.65

27.4

1.02

0.13

19.0

40.6

2.4

0.47

38.8

1.8

0.57

37.1

1.7

0.69 0.86 0.97 1.09

2

8.0

39.8 37.0

3.4 2.4

0.165

4.9

0.22

0

ΔС(a)

0

6.6

30

(a)

0

1.5

0

D

ΔС(a)

Isopar L, n = 0.5 [Мо], g l–1

26.6

6.1

0.87

25.7

0.18

0.85

0.24

50

22.3 25.7

2.5

9.1

34.8

2.2

0.26

7.0

25.1

0.61

0.28

30.0

35.4

1.7

3 3.5

10.3 11.0

32.3 30.9

2.5 1.4

0.32 0.36

7.6 8.3

24.7 24.2

0.42 0.51

0.31 0.34

32.6 35.2

33.8 32.4

1.6 1.4

Table 4. Extraction of Mo from supersaturated solutions in 1 M HNO3 into 80% TBP in the presence of uranyl nitrate or sodium nitrate, as a function of time [Mo], g l–1, at indicated n and [U] in extract, g l–1 n=4

Time, h 0 2 5 8 18 33 0 8 a

0 (a) 54 38 33 – 13.7 14.2 – –

30 (о) 0 4.1 7.5 9.8 9.8 9.7 – –

(a) 54 45 39 – 14.1 14.0 – –

n = 0,5 130

(о) 0 2.2 3.5 5.4 9.5 9.5 – –

(a) 54 47 46 – 39 31 – –

192 (о) 0 1.7 2.7 3.3 4.1 6.2 – –

(a) 54 48 47 42 41b 40b 6.0 5.4

0 (о) 0 1.6 2.4 3.5 0.91 0.46 0 0.16

(a) 54 – 51 – – 49 – –

a

0 (о) 0 2.2 5.3 5.8 7.2 7.5 – –

(a) 54 46 44 – 42 40 6.0 5.5

30 (о) 0 11.2 13 16.4 22.0 23.8 0 1.1

(a) 54 49 47 – 43 44

120 (о) 0 8.8 10.4 11.9 17.5 17.5

(a) 52 – 50 – – 48

(о) 0 2.8 4.7 6.1 6.4 6.3

1 M NaNO3 added. b A precipitate formed on the phase boundary.

Without initial supersaturation of the system with respect to Mo, Mo is not coextracted with U. In the system with very high extract loading with uranium (192 g l–1 U), precipitation was observed in both phases. It was also found that an increase in the HNO3 concentration in the extract from 0.70 to 1.30 due to the salting-out agent (1 M NaNO3 in the aqueous phase) drastically decreases the extractability of Мо. To evaluate the role of “free” TBP and of the HNO3 concentration, we performed an additional series of prolonged experiments on the extraction of Mo from 1 M HNO3. In these experiments, uranyl nitrate was extracted along with Mo, or the acid concentration in the extract was increased by adding a salting-out agent, 1 M NaNO3, into the aqueous phase. The experiments RADIOCHEMISTRY Vol. 52 No. 2 2010

were performed at various ratios of the aqueous and organic phases, so that the nature of the continuous phase was different. The results of this series of experiments are given in Table 4. For comparison, we performed under the same conditions experiments on Mo extraction with TBP solutions in xylene at a lower TBP concentration, correlating with the concentration of uranium-free TBP in the series with 80% TBP. These data are presented in Table 2 and Fig. 3. As can be seen, the limiting solubility of Mo in the TBP–xylene system is proportional to the TBP concentration to the power ~3.5. Figure 3 is discussed below. In this series of experiments, slow kinetics of Mo extraction was observed only in the case of 50% TBP at n = 2; these

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Fig. 3. Limiting saturation of the extract with molybdenum as a function of the concentration of (a) TBP not involved in bonding with U and (b) water dissolved in the extractant. HNO3 concentration in the aqueous phase 1 M. (1) Without U and (2) in the presence of U. For U(VI) concentrations, see Table 4.

HNO3 (proton) in the molybdenum extract and in the aqueous phase in comparison with the data obtained in the blank experiment without Mo. The effective HNO3 concentration in the equilibrium aqueous phase after the Mo extraction at n = 0.2 was determined by its extraction into 30% TBP, into which Mo is not noticeably extracted (Table 2), because potentiometric titration in the aqueous phase in the presence of 60 g l–1 Mo did not give a clear result. Simultaneously in the aqueous and organic phases we determined the concentration of the nitrate ion and obtained the following results: the organic phase contained 34 g l–1 Мо and, in total, 0.67 M nitrate ion; the aqueous phase contained 60 g l–1 Мо, ~0.65 M HNO3 (0.13 M HNO3 in 30% TBP, 0.14 M nitrate ion), and in total 1.15 M NO–3; 80% TBP in xylene contained 0.5 M HNO3 (0.54 M NO3–) in equilibrium with 0.7 M HNO3 (blank test) and 0.7 M HNO3 in equilibrium with 1.0 M HNO3. Thus, extraction of Mo is not accompanied to a noticeable extent by the displacement of nitric acid from the extract. IR Spectra of Molybdenum in the Organic Phase

Fig. 4. Kinetic dependences of the Mo extraction into 80% TBP. Initial solution: 50 g l–1 Мо + 1 M HNO3, 20°С. Diluent: (1) Isopar L, (2) xylene, and (3) CCl4.

A study of the IR spectra of Mo solutions in 80% TBP (Fig. 5), prepared from its supersaturated solutions in 1 M HNO3, did not reveal the presence of coordinated or ionic nitrate. Only well-known absorption bands of “free” HNO3 at 1300 and 1650 cm–1 were observed. In the presence of 34 g l–1 Mo, their intensity is somewhat lower than in the reference sample. In the region of absorption of the OH group (~3000 cm–1), there are no apparent changes in the spectra, which indicates that the concentration of hydroxy groups in both extracts is close. At the same time, the spectra contain bands at 806 and 960 cm–1. The latter band, as noted above, is characteristic of all Mo extracts. In addition, as can be readily seen in the differential spectrum, the intensity of the –P=O absorption at 1031 cm–1 somewhat increases without changes in the band shape.

Рис. 5. (1, 2) Initial and (3) differential IR spectra of the Mo extract in 80% TBP in xylene from 1 M HNO3 solution. Mo extract (1): 34 g l–1 Мо + 0.67 M NO–3. Reference extract (2): 0.5 M HNO3.

This fact indicates that the most probable extractable species is a certain form of molybdic acid. We recorded the spectra of solid molybdic acid samples of various origins (Fig. 6). The spectrum of commercial (crystalline) molybdic acid (Fig. 6a) is similar in many details to the previously studied [9] spectrum, which was interpreted on the basis of vibrations of the distorted MoO6 octahedron and H2O. The band assignment is given in Table 5. We also recorded the spec-

data are shown in Figs. 2 and 4 and are also discussed below. In view of the data obtained, it was interesting to determine the content of the nitrate ion and of free

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Рис. 6. Spectra of (a) crystalline and (b) amorphous forms of molybdic acid in xylene.

trum of molybdic acid precipitated from hot 8 M HNO3. We do not present here this spectrum, because it appeared to be similar to the spectrum of commercial H2MoO4 with the only difference that the splitting of molybdenum bands was not observed and strong bands of the captured HNO3 were present.

On the whole, the IR spectra obtained suggest that the species dissolved in TBP is a form of molybdic acid close to its molecular form.

The spectrum of amorphous molybdic acid freshly precipitated from 1 M HNO3 (Fig. 6b, Table 5) is considerably simpler and can be interpreted in the model of isolated MoO42– tetrahedron [10] in which O–Mo–O bridging bonds are absent. The absorption that we assigned to the О–Н···О vibrations is also absent. Therefore, the two bands at 806 and 960 cm–1, observed in the spectrum of the above-mentioned extract of Mo in 80% TBP, can be assigned to vibrations of the distorted Н2MoO4 tetrahedron coordinated to TBP via hydrogen bond, probably with the participation of water molecules. It is characteristic that in all the aboveconsidered cases diffuse bands characteristic of polymeric molybdenum species are absent [15].

In Fig. 3a we plotted data from Table 2, and also data from Table 3 for lower TBP concentrations. The limiting solubility of Mo was related to the concentration of TBP not involved in bonding with U (СTBP – 2CU), neglecting changes in the extract volume. It fol-

It should be noted that the majority of bands in the above-mentioned IR spectrum of Mo in the HDBP– TBP mixture (Fig. 7) [8], not interpreted in our previous studies, are similar to those in the spectrum of crystalline molybdic acid. The difference is in the presence in the former spectrum of a band at 918 cm–1, characteristic of the МоО2+ 2 ion. Thus, this extract contains both a molybdenyl HDBP salt, or, more probably, salt solvates МоO2(DBP)2(HDBP)2 and partially МоO2(DBP)2(TBP)2, and molecular Н2MoO4, as assumed in [4]. RADIOCHEMISTRY Vol. 52 No. 2 2010

DISCUSSION

Table 5. Assignment of bands in the IR spectrum of molybdic acid Crystalline acid ν, cm Assignment 3151 ν3 H2O 3043.5 ν1 H2O 1402.2 O–H···O 954.7 Combination bands 931.6 896.8 ν1 MoO6 881.4 812 ν3 MoO6 785 709 ν5 MoO6 682 578.6 H2O librations 530.4 ν4 MoO6 –1

Amorphous acid ν, cm Assignment 3363 ν3 H2O –1

1622 δ H2O 999.1 ν1 MoO4 871.8 ν3 MoO4

696 Combination bands 640 547.7 H2O librations

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Fig. 9. Initial rate of Mo extraction as a function of TBP concentration.

Рис. 7. IR spectra of Mo extracts in 30% TBP in xylene from 1 M HNO3 in the presence of 0.2 M HDBP (with subtraction of the spectrum of 30% TBP). [Mo], g l–1: (1) 0, (2) 5.5, and (3) 9.

Fig. 8. Calculated content of “free” water in 80% TBP at various saturations with (1) nitric acid and (2) uranium. Aqueous phase acidity 1 M.

lows from these data that the limiting concentrations of Mo in such interpretation appreciably exceed those in the system without uranium (with the change in the organic phase volume taken into account, the difference will be still more significant). Thus, there can be another factor that directly affects the Mo extraction in both systems. One of such factors can be the concentration of free water in the extract, i.e., of water that is not bound to the proton of the extracted acid or to uranium at more than 10% saturation of the extract with uranium relative to the total capacity. To check this assumption, we calculated the content of free water in the extract (Fig. 8) using a mathematical model reported in [16] and reference data [17]. The results of data processing are shown in Fig. 3b. The data for the systems with U

and without it are fitted by a common straight line with a slope of ~3.5, in contrast to the dependence on the TBP concentration (Fig. 3a), where the data for the systems with U and without it fall on diverging straight lines. In our opinion, this fact indicates that the water concentration in the extract plays a decisive role in the transfer of Mo to the organic phase. We also plotted the kinetic dependences based on data from Table 3. The formal reaction order with respect to Mo was determined using the standard procedure [18] by plotting the function log(ΔC/Δτ) = F(logCτ), as shown in Fig. 4. The difference is that, instead of Cτ, we used the value of the solution supersaturation (acting concentration) Ca = Cτ – Сl, where Сl is the limiting residual concentration (solubility) of Mo in the aqueous phase (1 M HNO3) after lifting supersaturation, which is equal to 16 g l–1. This concentration difference in the mass transfer theory is also termed concentration head. The data presented above show that, after the initial rapid ordering of the mass transfer, the process follows the first-order law with respect to the acting concentration of Mo in the aqueous phase at invariable reaction rate constant, i.e., the driving force of the process is supersaturation of the solution with molybdenum. Unfortunately, it appeared impossible to treat in a similar manner the data on the kinetics of Mo extraction into 50% and 30% TBP in xylene, shown above in Fig. 2, because of insufficiently strong changes in the Mo concentration in the aqueous phase up to saturation (Tables 3, 4). Therefore, we made an attempt to evaluate the dependence of the initial extraction rates from Mo accumulation in the organic phase at different TBP concentrations (Fig. 9). These data show that the initial extraction rate depends on the TBP concentration raised to the fourth power. However, substitution of the concentration of free water in the system instead of TBP (by analogy with Fig. 3) leads to a conclusion that an ensemble of four to five molecules of water disRADIOCHEMISTRY Vol. 52 No. 2 2010

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solved in TBP participates in the reaction. The latter conclusion is better consistent with the modern concepts and with the results of reported in [16].

should lead to a decrease in the HNO3 concentration by a factor of 5, i.e., from 0.7 to 0.15 M. However, in both cases the actually observed pattern is opposite.

Apparently, an increase in the acidity of the aqueous phase suppresses the driving force of this process, because it leads to an increase in the total solubility of Mo species in aqueous solution and to a decrease in the concentration of “free” TBP. Addition of a salting-out agent and of uranyl nitrate probably acts in the same direction. Finally, an increase in the acidity and/or saturation of the extract with uranium, or a decrease in the total TBP concentration will lead to a sharp decrease in the concentration of water in the organic phase, especially of water that is not involved in binding with acid proton [16]. This factor apparently plays a decisive role in the transfer of Mo to the extract in the form of a hydrate solvate. The same conclusion follows from the above-given data on the extraction of Mo into 80% TBP in the presence of uranium. The influence of the nonpolar diluent at a high TBP concentration and of the phase ratio (i.e., type of emulsion) is probably less significant.

It could also be assumed that the extractable species are certain nitrates of cationic molybdenum species, but this assumption is refuted both by the IR spectra of the extract, in which the nitrate ion bands are not detected and the well-defined doublet peak of the molybdenyl ion is absent, and by the shape of the dependence of the molybdenum extraction on the acid concentration.

The observed pattern is apparently associated with the hydrolysis of Мо [19–21], which is introduced into the system in the form of a strongly acidic aqueous solution, mainly in the cationic form, with the subsequent slow formation of molybdic acid and its transfer into the extract. However, different mechanisms of this transformation, described by the overall equation МоО2(ОН)+ + хH2O + уTBP = TBPу·H2МоО4(H2O)x–1 + H+, are possible.

1. Khimiya i tekhnologiya redkikh i rasseyannykh elementov (Chemistry and Technology of Rare and Trace Elements), Bol’shakov, K.A., Ed., Moscow, 1978, part 3, p. 212. 2. Zelikman, A.N., Molibden (Molybdenum), Moscow: Metallurgiya, 1970. 3. Constantinescu, I., Vladulescu, M., and Constantinescu, I., Fres. Z. Anal. Chem., 1986, vol. 324, part 2, pp. 137–141. 4. Goletskiy, N.D., Zilberman, B.Ya., Fedorov, Yu.S., et al., Czech. J. Phys., 2006, vol. 56, suppl. D, part 2, pp. D509–D517. 5. Zelikman, A.N. and Nerezov, V.M., Zh. Neorg. Khim., 1969, vol. 14, no. 5, pp. 1307–1313. 6. Goletskii, N.D., Zilberman, B.Ya., Mashirov, L.G., et al., Abstracts of Papers, XIII Rossiiskaya konferentsiya po ekstraktsii (ХIII Russian Conf. on Extraction), Moscow: Ross. Akad. Nauk, 2004, part I, pp. 203–205. 7. Zelikman, A.N. and Nerezov, V.M., Zh. Neorg. Khim., 1968, vol. 13, no. 12, pp. 2778–2784. 8. Goletskii, N.D., Zilberman, B.Ya., Fedorov, Yu.S., et al., in Khimicheskaya tekhnologiya: Sbornik tezisov dokladov Mezhdunarodnoi konferentsii po khimicheskoi tekhnologii KhT’07 (Chemical Technology: Coll. of Abstracts of Reports at the Int. Conf. on Chemical Technology CT’07), Moscow, June 17–23, 2007, Moscow: LENAND, 2007, vol. 4, pp. 58–61. 9. Daizy, P., Aruldhas, G., and Ramakrishnan, V., Pramana J. Phys., 1988, vol. 30, no. 2, pp. 129–133.

For example, the reaction can occur slowly in the bulk of the aqueous phase, but the transfer to the organic phase occurs faster than polymerization of molybdic acid in solution with the subsequent formation of the precipitate. In this case, the extraction rate should depend neither on the TBP concentration nor on the phase ratio, which is inconsistent with the experiment. It can also be assumed that the slow step of the process is extraction involving participation in the mass transfer of four TBP molecules simultaneously. At the same time, even without rigorous studies with a Lewis cell, it can be noted that the reaction rate should decrease with a decrease in the volume ratio of the organic and aqueous phases, because in this case the mass exchange surface area decreases. Furthermore, 0.26 M of Мо in the extract should bind in this case more than 1 M of TBP of the present 2.9 M, which RADIOCHEMISTRY Vol. 52 No. 2 2010

Apparently, Мо passes into the extract in the form of molybdic acid, but the available information is insufficient for making more detailed conclusions. As the simplest model we can suggest that the reaction occurs at the phase boundary and involves water dissolved in TBP. Then the extraction equation formally takes the form МоО2(ОН)+ + [TBP]H2Ox = [TBP]·H2МоО4(H2O)x–1 + H+aq. REFERENCES

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10. Woodward, L.A. and Roberts, L.H., Trans. Farad. Soc., 1956, vol. 52, pp. 615–619. 11. Esbelin, E., Etude de complexation et de precipitation du molybdene(VI) par Zr(IV) en milieu tres acide. Application a la dissolution du combustible nucleaire irradie, Rapport CEA-R-5872, 1999, p. 26. 12. Cruywagen, J.J., Heyns, J.B.B., and Rohwer, E.F.C.H., J. Inorg. Nucl. Chem., 1976, vol. 38, no. 11, pp. 2033– 2036. 13. Leithe, W., Z. Anal. Chem., 1964, vol. 202, no. 1, p. 102. 14. Lazarev, A.I. and Lazareva, V.I., Zavod. Lab., 1958, vol. 28, no. 7, pp. 798–801. 15. Busev, A.I. and Frolkina, V.A., Zh. Neorg. Khim., 1968, vol. 13, no. 5, pp. 1289–1295.

16. Zilberman, B.Ya., Fedorov, Yu.S., Puzikov, E.A., et al., Proc. Int. Conf. ATALANTE 2008, Monpellier (France), May 19–26, 2008, Paris: CEA, paper 01_17. 17. Nikolotova, Z.I. and Kartashova, N.A., Ekstraktsiya neitral’nymi organicheskimi soedineniyami, Rozen, A.M., Ed., Moscow: Atomizdat, 1976, vol. 1. 18. Spravochnik khimika (Chemist’s Handbook), Nikol’skii, B.P. et al., Eds., Moscow: Khimiya, 1965, vol. 3, p. 843. 19. Tytko, K.H. and Crlemser, O., Adv. Inorg. Chem. Radiochem., 1976, vol. 19, pp. 239–315. 20. Cruywagen, J J., Heyns, J.B.B., and Rohwer, E.F.C.H., J. Inorg. Nucl. Chem., 1978, vol. 40, no. 1, pp. 53–59. 21. Ojo, J.F., Taylor, R.S., and Sykes, A.G., J. Chem. Soc., Dalton Trans., 1975, pp. 500–505.

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