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The extraction of vanadium(IV) from sulphuric acid solutions by di-(2-ethylhexyl)-phosphoric acid
J. inorg,nucl.Chem.,1970,VoL32,pp. 3387to 3396. PergamonPress. Printedin Great Britain
THE EXTRACTION OF VANADIUM(IV) FROM SULPHURIC ACID SOLUTIONS BY DI-(2-ETHYLHEXYL)-PHOSPHORIC ACID T A I C H I SATO and T A K E H I K O T A K E D A Department of Applied Chemistry, Faculty of Engineering, Shizuoka University, Hamamatsu, Japan
(Received 2 February 1970) A b s t r a c t - T h e partition of vanadium(IV) between sulphuric acid solutions and solutions of di-(2ethylhexyl)-phosphoric acid (DEHPA) in kerosene has been investigated under different conditions. Both the aqueous and organic phases have been studied by spectrophotometry, and i.r. and N M R spectral studies were carried out for the organic phase. The effects of organic solvent and of the addition of tri-n-butyl phosphate (TBP) on this extraction system have also been examined. The mechanism of the extraction is discussed on the basis of the results obtained.
INTRODUCTION
THE EXTRACTION of vanadium(IV) from sulphuric acid solutions by dialkylphosphoric acid has been studied by earlier workers[1 ], but observations on the extraction mechanism are limited. The present author has previously investigated the extraction of uranium(VI) and thorium from sulphuric acid solutions by di(2-ethylhexyl)-phosphoric acid (DEHPA) [2, 3]. This work has now been expanded to obtain further information on the extraction of vanadium(IV). EXPERIMENTAL D E H P A , as used previously [2], was diluted with a purified kerosene[3] and other organic solvents. Vanadyl sulphate solutions were prepared by dissolving vanadyl sulphate dihydrate in sulphuric acid solutions. The other chemicals were analytical reagent grade. The procedure for obtaining the partition coefficient (the ratio of the equilibrium concentration of vanadium in the organic phase to that in the aqueous phase) was as described for uranium(Vl) extraction [2], except that vanadium in the organic phase was back-extracted with 1 N HCI. The vanadium concentration was determined by back-titration of the aqueous solution, adding an excess of EDTA, with thorium nitrate solution, using xylenol orange as indicator at pH 2-3 [4]. The chloride ion concentration and the water content in the organic phase were determined by Volhard's method [5] and by Karl Fisher titration respectively. Absorption spectra were obtained on a Hitachi EPS-2U recording spectrophotometer. 1.R. spectra of the organic extracts were measured with thallium halide windows on a Japan Spectroscopic Co. Ltd. Model IR-S recording i.r. spectrophotometer equipped with potassium chloride prisms. The spectrum of the complex prepared by drying the organic phase saturated with vanadium from the extraction of vanadyl sulphate solution with a solution of D E H P A in n-hexane was measured 1. C. A. Blake, Jr., C. F. Baes, Jr., K. B. Brown, C. F. Coleman and J. C. White, Proc. Second Int. Conf. Peaceful Uses A tom. Energy, Geneva (1958), Vol. 28, p. 289. United Nations (1958): D. J. Crouse and K. B. Brown, USAEC. Rep. ORN L-2820 (1959). 2. T. Sato,J. inorg, nucl. Chem. 24, 699 (1962). 3. T. Sato, J. inorg, nucl. Chem. 27, 1395 (1965). 4. J. Kinnunen and B. Wennerstrand, Chemist-Analyst46, 92 (1957). 5. T. Sato,J. inorg, nucl. Chem. 27, 1853 (1965). 3387
3388
T. SATO and T. TAKEDA
as a capillary film between thallium halide plates. Nuclear magnetic resonance (NMR) spectra were obtained in carbon tetrachloride solution on a Hitachi Perkin-Elmer Model R-20 high-resolution NMR spectrometer. Tetramethylsilane was used as an internal reference. RESULTS AND DISCUSSION Dependence on acid and solvent concentrations T h e extraction of vanadium(IV) from a 0.006 M solution in sulphuric acid by D E H P A in kerosene at 20°C was examined at different acid concentrations (Fig. 1). If we assume that the decrease in the partition coefficient is dominated b y an ion-exchange reaction similar to that for extraction of uranium(VI)[2] and thorium [3], viz. VO2+(a) + 2(HX)2(o) ~ VO2X4H2(o) ÷ 2H+(a)
(1)
in which X is the anion (CsH~70)2PO22-, (HX)2 the dimeric solvent, and (a) and (o) are aqueous and organic phases respectively, the following relationship must hold: log 4Ea ° = log K + 2 log (Cs -- 4Cv)/CH
(2)
where Ea ° is the partition coefficient, K the equilibrium constant, Cs the total D E H P A concentration, and CH the aqueous acidity. When log Ea ° is plotted against log ( C s - 4Cv)/Cn at constant acid concentration, it was found that Equation (2) is satisfied in 0.05, 0.1, 0.2 and 0.4 M H2504, but not when the D E H P A concentration is < 0.07 M in 0.015 M H2504. We therefore postulate that extraction under the latter condition involves the formation of a polymeric v a n a d i u m - D E H P A complex: VOX4H.,(o) + VO2+(a) ÷ (HX)2(o) ~ (VO)~X6 H2(o) ÷ 2H+(a)
(3)
T h e overall reaction is then 2VO2+(a) + 3(HX)2(o) ~- (VO)2X6Hz(o) + 4H+(a)
(4)
leading to the following relationship: log 4 E , ° = log KI + 2 log (Cs -- 2Cv)/fH
(5)
where K1 is constant. A log-log plot o f E a °, vs. (Cs--2Cv)/Cn in 0.015 M H2SO4 shows that Equation (5) is satisfied. We therefore conclude that when D E H P A is present in excess the monomeric species is formed. H e n c e the following equilibrium equation, expressed as an ion-exchange reaction governed by the formation of polymeric species, may be written for the extraction of vanadium(IV): nVO2+(a) + (n + 1)(HX)2(o) ~ (VO)nX2(n+l)H2(o) ÷ 2nH+(a) where n i> 1.
(6)
The extraction of vanadium(I V)
3389
IO
o
8
O.
0.1
0.01
0.I Initial aqueous $ulphuric acid conc., M
I
Fig. 1. Variation of partition coefficient with sulphuric acid concentration for the extraction of vanadium(IV) by solutions of DEHPA in kerosene (numerals on curves are DEHPA concentrations, M).
Extraction in the presence of sodium sulphate The extraction of vanadium(IV) from a 0-006 M solution in sulphuric acid and sodium sulphate by 0-15 M D E H P A in kerosene at 20°(2 shows that the partition coefficient is not much influenced by the sulphate ion concentration, but decreases with increasing hydrogen ion concentration (Fig. 2). This suggests that the species
3390
T. SATO and T. TAKEDA I0
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\
~
0.02
~
o
\
\ \ \ \ \ \ I
--
.e_
8 4--
~
o-,
C~
~x "~\ \ \
0.1
\ \ \ \ \ \ n
n
t
L
0.01
n nLtl
I
\
0.1 Initiol o q u e o u s totol
sulphote c o n c . ,
M
Fig. 2. Extraction of vanadium(IV) from sulphuric acid solutions containing sodium sulphate by 0"15 M DEHPA in kerosene (numerals on curves are initial aqueous sulphuric acid concentrations, M). containing sulphate is inextractable. This was supported by the fact that the i.r. spectra of the extracted organic species do not show an absorption band due to the sulphate group. I f D E H P A extracts v a n a d i u m ( I V ) from sulphuric acid solution according to Equation (6), the partition coefficient should be inversely dependent upon the second p o w e r of the aqueous hydrogen ion concentration. L o g - l o g plots of Ea ° vs. the initial aqueous sulphuric acid concentration have a slope o f - 2 at a fixed total sulphate concentration of 0.2 M for 0.1 M D E H P A in kerosene. D e p e n d e n c e on vanadium concentration T h e variation in the vanadium concentration of the organic phase with the initial aqueous vanadium concentration at 0.05 M aqueous acidity with 0.2 M D E H P A in kerosene at 20°C are shown in Fig. 3. T h e v a n a d i u m concentration of the organic phase increases linearly with increasing initial aqueous vanadium
The extraction of vanadium(IV)
3391
o.i
u "F
o.o~
g
0.001
,
t
iJ,tI
,
,
,
i
,
,,,l
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,
,
,
O,t
,
Ji,,l
,
,
I
Initial aqueous vanadium conc. , M
Fig. 3. Variation in organic vanadium concentration in the extraction of vanadium(IV) from 0.05 M sulphuric acid solution by 0.2 M D E H P A in kerosene.
concentration below - 0 . 0 5 M, but above this concentration it approaches a limiting value of 0.1 M, indicating that two molecules of D E H P A combine with each vanadium ion. This implies that a 1 : 4 vanadium-DEHPA complex is formed at low vanadium concentration, and a 1:2 vanadium-DEHPA complex at a higher concentration.
Effect of temperature The partition coefficient for the extraction of vanadyl sulphate solutions (0.006 M) containing sulphuric acid (0-02, 0,05 and 0-1 M) with 0.2 M D E H P A in kerosene at 10 ° and 50°C increases with rising temperature (Table 1). This dependence on temperature is contrary to that for uranium(VI) or thorium [2, 3]. The heat of reaction (change in enthaipy, - A H ) in Equation (1) was estimated Table 1. Temperature dependence of partition coefficient on the extraction of vanadium(IV) from sulphuric acid solutions by 0.2 M D E H P A in kerosene Initial aqueous HzSO 4 concn.
Partition coefficient
(M)
(lO°C) * (20°C) * (30°C) * (40°C)* (50°C) *
0.02 0-05 0.1
5-29 1-30 0.239
6-01 1-39 0.365
*Temperature of extraction.
6.46 1.48 0.402
6.99 1.60 0.430
7.42 1.69 0.461
3392
T. SATO and T. TAKEDA
to be approximately - 1-5 kcal/mole ( - 1.57, - 1.49 and - 1.55 kcal/mole in 0.02, 0.05 and 0.1 M H2SO4 respectively).
A bsorption spectra The visible absorption spectra of both the aqueous and organic phases from t h e e x t r a c t i o n o f v a n a d y l s u l p h a t e s o l u t i o n s (0.03 M ) c o n t a i n i n g s u l p h u r i c a c i d at v a r i o u s c o n c e n t r a t i o n s w i t h 0.2 M D E H P A in k e r o s e n e at 20°C a r e g i v e n in Fig. 4. T h e s p e c t r a o f v a n a d y l s u l p h a t e s o l u t i o n s e x h i b i t c h a r a c t e r i s t i c a b s o r p t i o n
0-4
¢ / / / /
0"3
==
g
.o ~[ 0"2
I I I
o ' ~ ~
0.1
\ \ 600
700
800
900
I000
I100
W o v e l e n g t h , rn/~
Fig. 4. Absorption spectra of aqueous vanadium sulphate solutions in the presence of sulphuric acid at different concentrations and of the organic solutions from the extraction with 0.2 M DEHPA in kerosene (numerals on curves are initial aqueous sulphuric acid concentrations, M; continuous and broken lines represent organic and aqueous solutions, respectively; thickness of cell, 1.00 cm).
The extraction of vanadium(IV)
3393
bands at 625 and 770 m/z due to the aquo-ion VO(H20)52+ [6, 7] for 0.02-1.0 M acid as illustrated in the case of 0.05 M H2SO4. In contrast, the spectra of the organic phases show largely absorptions at 690 and 820 m/z, although a progressive decrease in intensity is observed as the aqueous acidity increases. In the organic phases from the extraction of aqueous solutions containing vanadyl sulphate at 0-15, 0-6 and 2.4 M in 0.05 M sulphuric acid, however, those absorptions become more intense and are accompanied by a band at 990 m/x (Fig. 5). I.R. and N M R spectra
The organic phases from the extraction of vanadyl sulphate solutions (0.03, 0.24 and 1.2 M) in 0.05 M sulphuric acid by 0.1 M D E H P A in kerosene at 20°C were examined by i.r. spectroscopy. Figure 6 compares the spectra with that of the complex prepared by drying the organic phase. As the vanadium concentration of the organic phase increases, the band at 930 cm -1, assigned to the V-O stretching frequency[8], becomes stronger. Simultaneously, the intensities of the OH stretching bands at 2680 and 2350 cm -1 (which arise from hydrogen bonding in the dimer) and of the OH bending band at 1690 cm -1 decrease, while the P-O stretching band at 1230cm -1 splits into two peaks at 1235 and 1210cm -1. A significant change in the [P-O]-C stretching vibration is observed: an absorption at 1055 cm -~ appears in addition to the peak at 1030 cm -x in the D E H P A spectrum, that at 1085 cm -a with slightly lower intensity than that of the former, and that at 985 cm -~ as a shallow shoulder. This is observed more clearly in the spectrum of the complex freed from organic solvent. Similar phenomena have been found in other instances[2,3,9-11], and it is therefore presumed that the change in the absorption due to [P-O]-C stretching probably results from the effect of the interaction between the oxygen atom and the extracted vanadyl ion on the formation of the polymeric species. The i.r. results thus confirm that vanadium extracted into D E H P A by cation exchange is bonded to the phosphoryl oxygen atom. The decrease in intensity of the OH bands as the extracted vanadium concentration increases is supported by the determination of the water content of the organic phase. The organic phases from the extraction of vanadyl sulphate solutions (0.03 and 0.15 M) containing 0.05 M sulphuric acid by 0.2 M D E H P A in carbon tetrachloride at 20°C were also examined by NMR spectroscopy. The N M R spectrum for water-saturated D E H P A shows a sharp triplet at ~"= 9.09 due to the methyl protons, a strong doublet at 8.66 assigned to methylenic protons, a triplet at 6.12 from the methylenic protons attached to the carbon atoms immediately adjacent to phosphorus atoms, and in addition a broad hydroxyl proton signal. However, the spectra for the organic solutions from the extractions of vanadyl sulphate solutions show that the hydroxyl proton signal almost disappears with increasing initial aqueous vanadium concentration, in agreement with the i.r. result. 6. 7. 8. 9. 10. 11.
J. Selbin and L. Morpurgo, J. inorg, nucl. Chem. 27, 673 (1965). C.J. Ballhausen and H. B. Gray, lnorg. Chem. 1, 111 (1962). J. Selbin, L. H. Holmes, Jr. and S. P. McGlynn, J. inorg, nucl. Chem. 25, 1359 (1963). T. Sato, J. inorg, nucl. Chem. 25, 109 (1963); ibid. 29, 555 (1967). T. Sato, J. appl. Chem. 15, 489 ( 1965); ibid. 16, 53 (1966). T. Sato, J. inorg, nucl. Chem. 26, 311 (1964).
3394
T. SATO and T. T A K E D A
0"5
0.4
0"3 c o .0
3
0.15 0-2
o.C
0.1
60o
7o0
800 900 Wavelength, rap.
I000
i i O0
Fig. 5. Absorption spectra of the organic solutions from the extraction of vanadyl sulphate solutions containing 0.05 M sulphuric acid with 0-2 M D E H P A in kerosene (numerals on curves are initial aqueous vanadyl sulphate concentrations, M; thickness of cell, 0-50 cm).
Effect of the addition of TBP The synergic enhancement of the extraction of uranium(VI) from sulphuric acid solutions by combinations of DEHPA and tri-n-butyl phosphate (TBP) has been investigated previously[l 1]. In the extraction of thorium(IV) by
The extraction of vanadium(IV)
3395
c
2
E= rE
I---
1300
1200 I100 Frequency
I000 ,
900
cm -I
Fig. 6. I.R. spectra of organic solutions from the extraction of vanadyl sulphate solutions containing 0.05 M sulphuric acid with 0-1 M D E H P A in kerosene (A, D E H P A ; B, C and D, vanadyl sulphate solutions at 0.03, 0.24 and 1.2 M, respectively; E, vanadiumD E H P A complex, freed from organic solvent).
D E H P A , however, an antagonistic effect was observed in the presence of TBP [3]. Results for the extraction of vanadium(IV) from 0.006 M solutions in suiphuric acid at 0.01 and 0.05 M with 0.05 M D E H P A + T B P in kerosene at 20°C (Table 2) show that the partition coefficient at first rises slightly with TBP concentration, then passes through a maximum at a molar ratio of [TBP]/[DEHPA] ~- 0.5-1 and then falls gradually. Although the synergic enhancement in the extraction of vanadium(IV) is lower than that for uranium(VI) [l l], it appears that the behaviour is substantially the same for both systems.
Effect of organic solvent Values of the partition coefficient for the extraction of vanadium(IV) at an
3396
T. SATO and T. T A K E D A Table 2. Extraction of vanadium(IV) from sulphuric acid solutions by 0-05 M D E H P A + T B P in kerosene Initial aqueous H2SO 4 concn. (M)
0*
0.01 0.05
2.10 0.111
Partition coefficient 0.5* 1.0" 2.0*
4-0*
2.30 0.132
1.37 0-075
2.31 0.133
1.88 0.102
*Molar ratio of [TBP]/[DEHPA].
initial aqueous concentration of 0.006M in 0.1M sulphuric acid by 0.2M D E H P A in various organic solvents at 20°C are presented in Table 3. The partition coefficient depends on the nature of the diluent and increases in the order: CHCI3 < C6H5C1 < CoHo < o--C6H4C12 < C6HsCH3 < CH2C1 • CH2C1 < CC14 < C6H~NO2 < cyclohexane < kerosene < n-hexane. The order of increasing partition coefficient for vanadium(IV) is basically analogous to those for uranium(VI) and thorium [2, 3]. Table 3. Extraction of vanadium(IV) from 0.1M sulphuric acid solution by 0.2M D E H P A in various organic solvents Diluent n-Hexane Kerosene Cyclohexane Carbon tetrachloride Benzene Toluene Chloroform Chlorobenzene o-Dichlorobenzene
1,2-Dichloroethane Nitrobenzene
Partition coefficient 0.408 0.365 0.326 0-0945 0.0627 0.0875 0.0603 0.0618 0.0692 0.0889 0.106
Acknowledgement- We wish to thank Mr. F. Ozawa for assistance with N M R spectral measurement.
THE EXTRACTION OF VANADIUM(IV) FROM SULPHURIC ACID SOLUTIONS BY DI-(2-ETHYLHEXYL)-PHOSPHORIC ACID T A I C H I SATO and T A K E H I K O T A K E D A Department of Applied Chemistry, Faculty of Engineering, Shizuoka University, Hamamatsu, Japan
(Received 2 February 1970) A b s t r a c t - T h e partition of vanadium(IV) between sulphuric acid solutions and solutions of di-(2ethylhexyl)-phosphoric acid (DEHPA) in kerosene has been investigated under different conditions. Both the aqueous and organic phases have been studied by spectrophotometry, and i.r. and N M R spectral studies were carried out for the organic phase. The effects of organic solvent and of the addition of tri-n-butyl phosphate (TBP) on this extraction system have also been examined. The mechanism of the extraction is discussed on the basis of the results obtained.
INTRODUCTION
THE EXTRACTION of vanadium(IV) from sulphuric acid solutions by dialkylphosphoric acid has been studied by earlier workers[1 ], but observations on the extraction mechanism are limited. The present author has previously investigated the extraction of uranium(VI) and thorium from sulphuric acid solutions by di(2-ethylhexyl)-phosphoric acid (DEHPA) [2, 3]. This work has now been expanded to obtain further information on the extraction of vanadium(IV). EXPERIMENTAL D E H P A , as used previously [2], was diluted with a purified kerosene[3] and other organic solvents. Vanadyl sulphate solutions were prepared by dissolving vanadyl sulphate dihydrate in sulphuric acid solutions. The other chemicals were analytical reagent grade. The procedure for obtaining the partition coefficient (the ratio of the equilibrium concentration of vanadium in the organic phase to that in the aqueous phase) was as described for uranium(Vl) extraction [2], except that vanadium in the organic phase was back-extracted with 1 N HCI. The vanadium concentration was determined by back-titration of the aqueous solution, adding an excess of EDTA, with thorium nitrate solution, using xylenol orange as indicator at pH 2-3 [4]. The chloride ion concentration and the water content in the organic phase were determined by Volhard's method [5] and by Karl Fisher titration respectively. Absorption spectra were obtained on a Hitachi EPS-2U recording spectrophotometer. 1.R. spectra of the organic extracts were measured with thallium halide windows on a Japan Spectroscopic Co. Ltd. Model IR-S recording i.r. spectrophotometer equipped with potassium chloride prisms. The spectrum of the complex prepared by drying the organic phase saturated with vanadium from the extraction of vanadyl sulphate solution with a solution of D E H P A in n-hexane was measured 1. C. A. Blake, Jr., C. F. Baes, Jr., K. B. Brown, C. F. Coleman and J. C. White, Proc. Second Int. Conf. Peaceful Uses A tom. Energy, Geneva (1958), Vol. 28, p. 289. United Nations (1958): D. J. Crouse and K. B. Brown, USAEC. Rep. ORN L-2820 (1959). 2. T. Sato,J. inorg, nucl. Chem. 24, 699 (1962). 3. T. Sato, J. inorg, nucl. Chem. 27, 1395 (1965). 4. J. Kinnunen and B. Wennerstrand, Chemist-Analyst46, 92 (1957). 5. T. Sato,J. inorg, nucl. Chem. 27, 1853 (1965). 3387
3388
T. SATO and T. TAKEDA
as a capillary film between thallium halide plates. Nuclear magnetic resonance (NMR) spectra were obtained in carbon tetrachloride solution on a Hitachi Perkin-Elmer Model R-20 high-resolution NMR spectrometer. Tetramethylsilane was used as an internal reference. RESULTS AND DISCUSSION Dependence on acid and solvent concentrations T h e extraction of vanadium(IV) from a 0.006 M solution in sulphuric acid by D E H P A in kerosene at 20°C was examined at different acid concentrations (Fig. 1). If we assume that the decrease in the partition coefficient is dominated b y an ion-exchange reaction similar to that for extraction of uranium(VI)[2] and thorium [3], viz. VO2+(a) + 2(HX)2(o) ~ VO2X4H2(o) ÷ 2H+(a)
(1)
in which X is the anion (CsH~70)2PO22-, (HX)2 the dimeric solvent, and (a) and (o) are aqueous and organic phases respectively, the following relationship must hold: log 4Ea ° = log K + 2 log (Cs -- 4Cv)/CH
(2)
where Ea ° is the partition coefficient, K the equilibrium constant, Cs the total D E H P A concentration, and CH the aqueous acidity. When log Ea ° is plotted against log ( C s - 4Cv)/Cn at constant acid concentration, it was found that Equation (2) is satisfied in 0.05, 0.1, 0.2 and 0.4 M H2504, but not when the D E H P A concentration is < 0.07 M in 0.015 M H2504. We therefore postulate that extraction under the latter condition involves the formation of a polymeric v a n a d i u m - D E H P A complex: VOX4H.,(o) + VO2+(a) ÷ (HX)2(o) ~ (VO)~X6 H2(o) ÷ 2H+(a)
(3)
T h e overall reaction is then 2VO2+(a) + 3(HX)2(o) ~- (VO)2X6Hz(o) + 4H+(a)
(4)
leading to the following relationship: log 4 E , ° = log KI + 2 log (Cs -- 2Cv)/fH
(5)
where K1 is constant. A log-log plot o f E a °, vs. (Cs--2Cv)/Cn in 0.015 M H2SO4 shows that Equation (5) is satisfied. We therefore conclude that when D E H P A is present in excess the monomeric species is formed. H e n c e the following equilibrium equation, expressed as an ion-exchange reaction governed by the formation of polymeric species, may be written for the extraction of vanadium(IV): nVO2+(a) + (n + 1)(HX)2(o) ~ (VO)nX2(n+l)H2(o) ÷ 2nH+(a) where n i> 1.
(6)
The extraction of vanadium(I V)
3389
IO
o
8
O.
0.1
0.01
0.I Initial aqueous $ulphuric acid conc., M
I
Fig. 1. Variation of partition coefficient with sulphuric acid concentration for the extraction of vanadium(IV) by solutions of DEHPA in kerosene (numerals on curves are DEHPA concentrations, M).
Extraction in the presence of sodium sulphate The extraction of vanadium(IV) from a 0-006 M solution in sulphuric acid and sodium sulphate by 0-15 M D E H P A in kerosene at 20°(2 shows that the partition coefficient is not much influenced by the sulphate ion concentration, but decreases with increasing hydrogen ion concentration (Fig. 2). This suggests that the species
3390
T. SATO and T. TAKEDA I0
\ \ \
\
~
0.02
~
o
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\ \ \ \ \ \ I
--
.e_
8 4--
~
o-,
C~
~x "~\ \ \
0.1
\ \ \ \ \ \ n
n
t
L
0.01
n nLtl
I
\
0.1 Initiol o q u e o u s totol
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M
Fig. 2. Extraction of vanadium(IV) from sulphuric acid solutions containing sodium sulphate by 0"15 M DEHPA in kerosene (numerals on curves are initial aqueous sulphuric acid concentrations, M). containing sulphate is inextractable. This was supported by the fact that the i.r. spectra of the extracted organic species do not show an absorption band due to the sulphate group. I f D E H P A extracts v a n a d i u m ( I V ) from sulphuric acid solution according to Equation (6), the partition coefficient should be inversely dependent upon the second p o w e r of the aqueous hydrogen ion concentration. L o g - l o g plots of Ea ° vs. the initial aqueous sulphuric acid concentration have a slope o f - 2 at a fixed total sulphate concentration of 0.2 M for 0.1 M D E H P A in kerosene. D e p e n d e n c e on vanadium concentration T h e variation in the vanadium concentration of the organic phase with the initial aqueous vanadium concentration at 0.05 M aqueous acidity with 0.2 M D E H P A in kerosene at 20°C are shown in Fig. 3. T h e v a n a d i u m concentration of the organic phase increases linearly with increasing initial aqueous vanadium
The extraction of vanadium(IV)
3391
o.i
u "F
o.o~
g
0.001
,
t
iJ,tI
,
,
,
i
,
,,,l
0-01
,
,
,
O,t
,
Ji,,l
,
,
I
Initial aqueous vanadium conc. , M
Fig. 3. Variation in organic vanadium concentration in the extraction of vanadium(IV) from 0.05 M sulphuric acid solution by 0.2 M D E H P A in kerosene.
concentration below - 0 . 0 5 M, but above this concentration it approaches a limiting value of 0.1 M, indicating that two molecules of D E H P A combine with each vanadium ion. This implies that a 1 : 4 vanadium-DEHPA complex is formed at low vanadium concentration, and a 1:2 vanadium-DEHPA complex at a higher concentration.
Effect of temperature The partition coefficient for the extraction of vanadyl sulphate solutions (0.006 M) containing sulphuric acid (0-02, 0,05 and 0-1 M) with 0.2 M D E H P A in kerosene at 10 ° and 50°C increases with rising temperature (Table 1). This dependence on temperature is contrary to that for uranium(VI) or thorium [2, 3]. The heat of reaction (change in enthaipy, - A H ) in Equation (1) was estimated Table 1. Temperature dependence of partition coefficient on the extraction of vanadium(IV) from sulphuric acid solutions by 0.2 M D E H P A in kerosene Initial aqueous HzSO 4 concn.
Partition coefficient
(M)
(lO°C) * (20°C) * (30°C) * (40°C)* (50°C) *
0.02 0-05 0.1
5-29 1-30 0.239
6-01 1-39 0.365
*Temperature of extraction.
6.46 1.48 0.402
6.99 1.60 0.430
7.42 1.69 0.461
3392
T. SATO and T. TAKEDA
to be approximately - 1-5 kcal/mole ( - 1.57, - 1.49 and - 1.55 kcal/mole in 0.02, 0.05 and 0.1 M H2SO4 respectively).
A bsorption spectra The visible absorption spectra of both the aqueous and organic phases from t h e e x t r a c t i o n o f v a n a d y l s u l p h a t e s o l u t i o n s (0.03 M ) c o n t a i n i n g s u l p h u r i c a c i d at v a r i o u s c o n c e n t r a t i o n s w i t h 0.2 M D E H P A in k e r o s e n e at 20°C a r e g i v e n in Fig. 4. T h e s p e c t r a o f v a n a d y l s u l p h a t e s o l u t i o n s e x h i b i t c h a r a c t e r i s t i c a b s o r p t i o n
0-4
¢ / / / /
0"3
==
g
.o ~[ 0"2
I I I
o ' ~ ~
0.1
\ \ 600
700
800
900
I000
I100
W o v e l e n g t h , rn/~
Fig. 4. Absorption spectra of aqueous vanadium sulphate solutions in the presence of sulphuric acid at different concentrations and of the organic solutions from the extraction with 0.2 M DEHPA in kerosene (numerals on curves are initial aqueous sulphuric acid concentrations, M; continuous and broken lines represent organic and aqueous solutions, respectively; thickness of cell, 1.00 cm).
The extraction of vanadium(IV)
3393
bands at 625 and 770 m/z due to the aquo-ion VO(H20)52+ [6, 7] for 0.02-1.0 M acid as illustrated in the case of 0.05 M H2SO4. In contrast, the spectra of the organic phases show largely absorptions at 690 and 820 m/z, although a progressive decrease in intensity is observed as the aqueous acidity increases. In the organic phases from the extraction of aqueous solutions containing vanadyl sulphate at 0-15, 0-6 and 2.4 M in 0.05 M sulphuric acid, however, those absorptions become more intense and are accompanied by a band at 990 m/x (Fig. 5). I.R. and N M R spectra
The organic phases from the extraction of vanadyl sulphate solutions (0.03, 0.24 and 1.2 M) in 0.05 M sulphuric acid by 0.1 M D E H P A in kerosene at 20°C were examined by i.r. spectroscopy. Figure 6 compares the spectra with that of the complex prepared by drying the organic phase. As the vanadium concentration of the organic phase increases, the band at 930 cm -1, assigned to the V-O stretching frequency[8], becomes stronger. Simultaneously, the intensities of the OH stretching bands at 2680 and 2350 cm -1 (which arise from hydrogen bonding in the dimer) and of the OH bending band at 1690 cm -1 decrease, while the P-O stretching band at 1230cm -1 splits into two peaks at 1235 and 1210cm -1. A significant change in the [P-O]-C stretching vibration is observed: an absorption at 1055 cm -~ appears in addition to the peak at 1030 cm -x in the D E H P A spectrum, that at 1085 cm -a with slightly lower intensity than that of the former, and that at 985 cm -~ as a shallow shoulder. This is observed more clearly in the spectrum of the complex freed from organic solvent. Similar phenomena have been found in other instances[2,3,9-11], and it is therefore presumed that the change in the absorption due to [P-O]-C stretching probably results from the effect of the interaction between the oxygen atom and the extracted vanadyl ion on the formation of the polymeric species. The i.r. results thus confirm that vanadium extracted into D E H P A by cation exchange is bonded to the phosphoryl oxygen atom. The decrease in intensity of the OH bands as the extracted vanadium concentration increases is supported by the determination of the water content of the organic phase. The organic phases from the extraction of vanadyl sulphate solutions (0.03 and 0.15 M) containing 0.05 M sulphuric acid by 0.2 M D E H P A in carbon tetrachloride at 20°C were also examined by NMR spectroscopy. The N M R spectrum for water-saturated D E H P A shows a sharp triplet at ~"= 9.09 due to the methyl protons, a strong doublet at 8.66 assigned to methylenic protons, a triplet at 6.12 from the methylenic protons attached to the carbon atoms immediately adjacent to phosphorus atoms, and in addition a broad hydroxyl proton signal. However, the spectra for the organic solutions from the extractions of vanadyl sulphate solutions show that the hydroxyl proton signal almost disappears with increasing initial aqueous vanadium concentration, in agreement with the i.r. result. 6. 7. 8. 9. 10. 11.
J. Selbin and L. Morpurgo, J. inorg, nucl. Chem. 27, 673 (1965). C.J. Ballhausen and H. B. Gray, lnorg. Chem. 1, 111 (1962). J. Selbin, L. H. Holmes, Jr. and S. P. McGlynn, J. inorg, nucl. Chem. 25, 1359 (1963). T. Sato, J. inorg, nucl. Chem. 25, 109 (1963); ibid. 29, 555 (1967). T. Sato, J. appl. Chem. 15, 489 ( 1965); ibid. 16, 53 (1966). T. Sato, J. inorg, nucl. Chem. 26, 311 (1964).
3394
T. SATO and T. T A K E D A
0"5
0.4
0"3 c o .0
3
0.15 0-2
o.C
0.1
60o
7o0
800 900 Wavelength, rap.
I000
i i O0
Fig. 5. Absorption spectra of the organic solutions from the extraction of vanadyl sulphate solutions containing 0.05 M sulphuric acid with 0-2 M D E H P A in kerosene (numerals on curves are initial aqueous vanadyl sulphate concentrations, M; thickness of cell, 0-50 cm).
Effect of the addition of TBP The synergic enhancement of the extraction of uranium(VI) from sulphuric acid solutions by combinations of DEHPA and tri-n-butyl phosphate (TBP) has been investigated previously[l 1]. In the extraction of thorium(IV) by
The extraction of vanadium(IV)
3395
c
2
E= rE
I---
1300
1200 I100 Frequency
I000 ,
900
cm -I
Fig. 6. I.R. spectra of organic solutions from the extraction of vanadyl sulphate solutions containing 0.05 M sulphuric acid with 0-1 M D E H P A in kerosene (A, D E H P A ; B, C and D, vanadyl sulphate solutions at 0.03, 0.24 and 1.2 M, respectively; E, vanadiumD E H P A complex, freed from organic solvent).
D E H P A , however, an antagonistic effect was observed in the presence of TBP [3]. Results for the extraction of vanadium(IV) from 0.006 M solutions in suiphuric acid at 0.01 and 0.05 M with 0.05 M D E H P A + T B P in kerosene at 20°C (Table 2) show that the partition coefficient at first rises slightly with TBP concentration, then passes through a maximum at a molar ratio of [TBP]/[DEHPA] ~- 0.5-1 and then falls gradually. Although the synergic enhancement in the extraction of vanadium(IV) is lower than that for uranium(VI) [l l], it appears that the behaviour is substantially the same for both systems.
Effect of organic solvent Values of the partition coefficient for the extraction of vanadium(IV) at an
3396
T. SATO and T. T A K E D A Table 2. Extraction of vanadium(IV) from sulphuric acid solutions by 0-05 M D E H P A + T B P in kerosene Initial aqueous H2SO 4 concn. (M)
0*
0.01 0.05
2.10 0.111
Partition coefficient 0.5* 1.0" 2.0*
4-0*
2.30 0.132
1.37 0-075
2.31 0.133
1.88 0.102
*Molar ratio of [TBP]/[DEHPA].
initial aqueous concentration of 0.006M in 0.1M sulphuric acid by 0.2M D E H P A in various organic solvents at 20°C are presented in Table 3. The partition coefficient depends on the nature of the diluent and increases in the order: CHCI3 < C6H5C1 < CoHo < o--C6H4C12 < C6HsCH3 < CH2C1 • CH2C1 < CC14 < C6H~NO2 < cyclohexane < kerosene < n-hexane. The order of increasing partition coefficient for vanadium(IV) is basically analogous to those for uranium(VI) and thorium [2, 3]. Table 3. Extraction of vanadium(IV) from 0.1M sulphuric acid solution by 0.2M D E H P A in various organic solvents Diluent n-Hexane Kerosene Cyclohexane Carbon tetrachloride Benzene Toluene Chloroform Chlorobenzene o-Dichlorobenzene
1,2-Dichloroethane Nitrobenzene
Partition coefficient 0.408 0.365 0.326 0-0945 0.0627 0.0875 0.0603 0.0618 0.0692 0.0889 0.106
Acknowledgement- We wish to thank Mr. F. Ozawa for assistance with N M R spectral measurement.