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SEPARATIONS Mechanism of Photoreductive Extraction of Vanadium in a Liquid-Liquid Extraction System Using Bis(2-ethylhexyl)phosphoric Acid Syouhei Nishihama,† Takayuki Hirai,*,† and Isao Komasawa†,‡ Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Photoenergetics of Organic Materials, Osaka University, Machikaneyama-cho 1-3, Toyonaka, Osaka 560-8531, Japan
The mechanism for the extraction of V, when effected by the photochemical reduction of V(V) to V(IV), was investigated, employing bis(2-ethylhexyl)phosphoric acid as the extractant, n-dodecane as the diluent, and a xenon lamp as the light source. The photochemical reduction of V(V) progresses at a rate that is first order with respect to the concentration of V(V) in both aqueous and organic solutions. Reduction also occurs, in the extraction system, at the interface between the aqueous and organic phases. The observed time-course variation in the distribution ratio with respect to photoirradiation is expressed successfully by a proposed scheme, in which the photochemical reduction of V(V) is a rate-determining step. The organic solution obtained following photoirradiation can be reused for repeated photoreductive extraction-stripping processing. 1. Introduction Liquid-liquid extraction is a major method employed for the separation of rare metals on the industrial scale. Such extractive separation processes are based on differences in the ability for complex formation between the various rare metals and their extractants. However, the separation for some metals is difficult when using a conventional extraction system. Generally, the ions of different valences for a given metal behave like different elements with respect to their extractability, such that the distribution ratio for metal ions having a larger valence is larger than that for ions with a smaller valence.1 The valence adjustment for metals in liquidliquid extraction systems has, therefore, been the subject of investigations in both nuclear reprocessing1-3 and the separation of rare metals.4 Much attention has been paid recently to the use of a photochemical reaction for valence adjustment,5,6 and consequently separation processes for rare metals with liquid-liquid extraction combined with a photochemical redox reaction have been investigated.7-10 There are, however, relatively few reports concerning the mechanism for extraction, when combined with a photochemical reaction. The mechanism of photoreductive stripping of iron(III) in a liquid-liquid extraction system with acidic organophosphorus compounds has been investigated in a previous work.10 This showed that the photoreductive * To whom correspondence should be addressed. Tel: +81-6-6850-6272. Fax: +81-6-6850-6273. E-mail: hirai@ cheng.es.osaka-u.ac.jp. † Graduate School of Engineering Science, Osaka University. ‡ Research Center for Photoenergetics of Organic Materials, Osaka University.
stripping of iron(III) progressed, following an initial induction period caused by dissolved oxygen in the extraction system. The iron(III)-extractant complex was photoexcited in the organic phase and was photoreduced at the aqueous/organic interface to the iron(II) complex by electron donation from water. A kinetic study revealed the photochemical reduction of iron(III) to be the rate-determining step. In the present work, the mechanism of the photoreductive extraction of vanadium, when bis(2-ethylhexyl)phosphoric acid (D2EHPA) is employed as an extractant, is investigated. The distribution ratio for V is expected to increase by combining reduction from V(V) to V(IV), because, in an acidic solution, the positive charge for V(IV) is greater than that for V(V). The kinetics for the photochemical reduction of V(V) in a separate aqueous or organic solution were, therefore, investigated first. The kinetic study was then extended to the extraction system to include the effects of the aqueous/organic interface. Finally, the effect of the wavelength of the irradiated light and the possibility for reuse of the organic phase were investigated. 2. Experiment 2.1. Reagents. D2EHPA (marketed as DP-8R by Daihachi Chemical Industry Co., Ltd., purity 93.7 wt %) was used as an extractant, without further purification. The inorganic chemicals and n-dodecane were supplied by Wako Pure Chemical Industries as analytical-grade reagents. Deionized water was purified by simple distillation, prior to use. The aqueous feed solution was prepared by dissolving either NaVO3 or VOSO‚nH2O into the diluted sulfuric acid. The structures for V(V) and V(IV) in such an acidic solution with
10.1021/ie0002778 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/06/2000
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a pH value of 0-2 are reported as VO2+ and VO2+, respectively.11 The organic solution was prepared by diluting D2EHPA with n-dodecane, and the resultant extractant concentration was determined by automatic potentiometric titration (Hiranuma Comtite-550). 2.2. Procedure. 2.2.1. Extraction Equilibrium. The organic and aqueous solutions were shaken at 298 K for 5 h at an organic/aqueous (O/A) volume ratio of 1. The metal-loaded organic solutions were stripped with 3 mol/L of HCl for the case of V(V) and 6 mol/L of HCl for the case of V(IV), with an O/A volume ratio of 0.5. The resultant aqueous samples were analyzed using an inductively coupled argon plasma atomic emission spectrometer (ICP-AES, Nippon Jarrell-Ash ICAP-575 Mark II). The equilibrium pH value was measured using an Orion 920A pH meter equipped with a glass combination electrode. 2.2.2. Photochemical Reduction of V(V) in a Separate Aqueous or Organic Solution. The photochemical reduction of V(V) in the single aqueous or organic solution was carried out using a beaker-type glass bottle (32 mm in diameter), following Ar bubbling to purge dissolved oxygen. A 500 W xenon lamp (Ushio, UXL-500D-0) was used as the light source, such that the wavelength of the irradiated light was greater than 300 nm by virtue of the Pyrex glass material of the reaction bottle. Absorption spectra for both aqueous and organic solutions were measured using an UV-vis spectrophotometer (Hewlett-Packard HP 8452A). The concentration of V(IV) was determined spectroscopically at an absorbance of 768 nm. The water concentration in the organic phase was determined by Karl Fischer titration using a Kyoto Electronics MKS-1. 2.2.3. Photoreductive Extraction of V. This was also carried out using the beaker-type glass bottle. An aqueous V(V) solution (10 mL) and an organic solution (10 mL) were agitated together and bubbled with Ar gas for 1 h prior to irradiation. Without Ar bubbling, the reaction started after an initial induction period, caused by the dissolved oxygen in the extraction system, as was also found in the photoreductive stripping of iron(III).10 The resultant mixture was then agitated at over 4000 rpm by a magnetic stirrer and irradiated at an irradiation intensity of 5.80 × 105 W/m2. The photointensity was measured by thermopile (The Eppley Laboratory, Inc.). Appropriate cutoff filters were used during photoirradiation, when studying the effect of the wavelength of the irradiation. 3. Results and Discussion 3.1. Extraction Equilibrium Formulations. The slope analysis method was applied to determine the overall extraction equilibrium formulations for V(V) and V(IV) with D2EHPA. It has been shown by vapor-phase osmometric study that the majority of the organophosphorus acids are dimerized in nonpolar diluent.12 Figure 1 shows the effect of the equilibrium pH value on the distribution ratio, with straight lines having a slope of 1 for V(V) and a slope of 2 for V(IV) being obtained. The effect of dimeric extractant concentration ([(RH)2]) on the two normalized distribution ratios, D[H+] and D[H+]2, was then investigated. Again straight line relationships with slopes of 1 and 2 were obtained for V(V) and V(IV), respectively. The extraction equilibria for V(V) and V(IV) with D2EHPA extractant can therefore be expressed as eqs 1 and 2, respectively.
Figure 1. Effect of the aqueous-phase pH on the distribution ratio. [(RH)2]feed ) 0.5 mol/L and [V]feed ) 0.01 mol/L.
VO2+ + (RH)2 T VO2R(RH) + H+;
For V(V):
Kex,V ) 1.12 × 10-1 (1) For V(IV):
VO2+ + 2(RH)2 T VOR2(RH)2 + 2H+; Kex,IV ) 3.79 × 10-1 (2)
The distribution ratio, D, for V(IV) is much higher than that for V(V), at a high pH region, because V(V) exists in the monovalent state whereas V(IV) exists in the divalent state. This result indicates that the extraction of V into the organic phase should therefore be enhanced, during the extraction process, by the reduction of V from V(V) to V(IV). 3.2. Photochemical Reduction of V(V) in an Aqueous Solution. The photochemical reduction of V(V) in an aqueous solution was also performed, following Ar bubbling in order to purge dissolved oxygen. V(V) is photochemically reduced by irradiation with the xenon lamp, and an absorption at around 768 nm appears, which is attributable to V(IV).7 Figure 2 shows a pseudo-first-order kinetic plot for the variation in the aqueous concentration of V(V), with respect to irradiation time. A linear relationship is observed between the ordinate value of ln([V(V)]feed/[V(V)]) and photoirradiation time and which is also independent of both the pH and sulfuric ion concentration. The rate of photochemical reduction of V(V) in an aqueous solution is therefore first order with respect to V(V) concentration and can be expressed as eq 3, where ka ) 3.58 × 10-4 min-1.
ra ) -
d[V(V)] ) ka[V(V)] dt
(3)
The aqueous-phase photochemical reduction of V(V) is likely to occur by electron donation from water, as indicated in eqs 4-6. The overall reaction is thus shown
VO2(H2O)4+ + H2O f VO2(H2O)4 + H+ + •OH (4) VO2(H2O)4 + 2H+ f VO(H2O)52+ •
OH f
1 1 H2O + O2 2 4
(5) (6)
by eq 7, which is identical with that found by Jeliazkowa
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ro ) -
d[V(V)] ) ko[V(V)] dt
(8)
dissolved water versus the concentration of the extracted V(V) is shown in Figure 4. The quantity of water in the organic solution is sufficient for the V(V) to be photoreduced, because the concentration of water in the organic phase is more than 10 times greater than that of the V that is extracted. The overall reaction is shown by eqs 9 and 10. If the reduction of V(V) as shown by eq
VO2R(RH) + H2O f VO2R(RH)- + H+ + •OH (9) VO2R(RH)- + (RH)2 + H+ f VOR2(RH)2 + H2O (10) Figure 2. Pseudo-first-order plot for the concentration of V(V) in the aqueous solution during photoirradiation. [NaVO3]feed ) 0.01 mol/L.
Figure 3. Pseudo-first-order plot for the concentration of V(V) in the organic solution during photoirradiation.
et al.13 If the reduction of V(V) is the rate-determining
VO2(H2O)4+ +
1 1 H O + H+ f VO(H2O)52+ + O2 (7) 2 2 4
step as shown by eq 4, then the overall reaction rate may also be expressed by means of eq 3. 3.3. Photochemical Reduction of V(V) in an Organic Solution. The photochemical reduction of V(V) in an organic solution was also performed following Ar bubbling. V(V) is photochemically reduced in the organic solution, and again an absorption at around 768 nm, attributable to V(IV), appears. Figure 3 shows the corresponding pseudo-first-order plot for the concentration of V(V) in an organic solution as a function of irradiation time. A linear relationship is again observed between ln([V(V)]feed/[V(V)]) and the photoirradiation time, which is independent of the extractant concentration, thus indicating that the rate of photochemical reduction of V(V) in an organic solution is first order with respect to the V(V) concentration. The rate of photochemical reduction is therefore expressed as eq 8, where ko ) 9.84 × 10-2 min-1. The photochemical reduction of V(V) in an organic solution is also likely to occur via electron donation from water dissolved in the organic solution. A plot of the concentration of the
9 is the rate-determining step, the overall rate equation can be expressed simply by means of eq 8. The hydroxyl radical produced in the reaction is likely to be scavenged by the diluent, as was found previously in the case of iron(III).10 The photochemical reduction of V in the extraction system is expected to progress mainly in the organic phase, because the rate constant for photochemical reduction in the organic phase is much greater than that for the aqueous phase (k0/ka ) 275). 3.4. Photoreductive Extraction of V. The photoreductive extraction of V was then carried out. In this, the extractant D2EHPA in n-dodecane was contacted with the aqueous solution, containing NaVO3, at a volume ratio of O/A ) 1. The mixture was then photoirradiated following Ar bubbling with agitation by a magnetic stirrer. Figure 5 shows the effect of photoirradiation time on the distribution ratio for V under conditions of vigorous stirring at over 4000 rpm. In the extraction system, the time-course variation of the distribution ratio can be predicted, when assuming that the rate of the reaction is dominated by the photochemical reduction of V in the aqueous and organic phases, and the distribution of both V(V) and V(IV) between the two phases is attained immediately. Such predictions, however, lead to much lower values for the distribution ratio than for the experimental results. This difference may be caused by photochemical reduction of V at the interface between the aqueous and organic phases, as was also found in the case for the photoreductive stripping of iron(III).10 The photochemical reduction of V(V) at the interface may be thought to be expressed by the mechanisms indicated in eqs 11-14. If the rate
VO2R(RH) T VO2R(RH); Ki
(11)
VO2R(RH) + H2O f VO2R(RH)- + H+ + •OH; k′i (12) VO2R(RH)- + (RH)2 + H+ f VOR2(RH)2 + H2O (13) VOR2(RH)2 T VOR2(RH)2
(14)
of photochemical reduction at the interface is first order with respect to the concentration of V(V) at the interface, as in the case of reduction in aqueous and organic solutions, then the interfacial surface rate equation can
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Figure 4. Effect of the concentration of V(V) extracted on the concentration of water in the organic phase. [(RH)2]feed ) 0.5 mol/L and [NaVO3]feed ) 0.01 mol/L.
Figure 6. Effect of the photoirradiation time on the distribution ratio and reduction yield in each phase. [(RH)2]feed ) 0.5 mol/L, [NaVO3]feed ) 0.01 mol/L, and pHini ) 1.02.
and ki, as indicated in eqs 17-20. Assuming that an
[V(V)]t+∆t )
[V(V)]t
(17)
eka∆t
[V(IV)]t+∆t ) [V(IV)]t + {[V(V)]t - [V(V)]t+∆t} (18) [V(V)]t+∆t )
(
)
1 1 + - 1 [V(V)]t eko∆t eki∆t
(19)
[V(IV)]t+∆t ) [V(IV)]t + {[V(V)]t - [V(V)]t+∆t} (20)
Figure 5. Effect of the photoirradiation time on the distribution ratio for V. Comparison of the observed data with predicted values, shown by the solid lines. [(RH)2]feed ) 0.5 mol/L and [NaVO3]feed ) 0.01 mol/L.
be expressed by eq 15,
r′i ) -
Q d[V(V)] ) k′i[V(V)] ) Kik′i[V(V)] A dt
(15)
where Q and A are the volume of the organic phase and the area of the interface, respectively. Assuming the interface area to be constant, the overall rate equation at the interface can be expressed as eq 16. The overall
ri )
A r′ ) ki[V(V)] Q i
(16)
rate constant for photochemical reduction of V at the interface was determined as ki ) 1.75 × 10-1 min-1, by a nonlinear least-squares fit to the experimental results. The change in the concentrations of V(V) and V(IV) in the aqueous and organic phases for a time interval ∆t lying between time t and t + ∆t can be calculated from a knowledge of the values of the rate constants, ka, ko,
equilibrium distribution of both V(V) and V(IV) between the phases is attained immediately, the distribution ratio at the new time (t + ∆t) can be calculated using the extraction equilibrium formulations previously determined. Repeated calculation, until the values of the distribution ratios and the proton concentration converge, enables the apparent distribution ratio, D ) ([V(V)] + [V(IV)])/([V(V)] + [V(IV)]), to be determined. These predicted values according to the above method are shown in Figure 5 by means of the solid lines. The experimental data are seen to fall well on the prediction lines, thus indicating that the photochemical reduction of V progresses in the aqueous phase, in the organic phase, and at the interface. Figure 6 shows the timecourse variation in the reduction yield for each phase, Y, defined as the ratio of the concentration of V(IV) reduced in the appropriate phase to that of the feed V(V) concentration. The photoreductive extraction progresses mainly via photochemical reduction at the interface and in the organic phase, with the reduction in the aqueous phase hardly contributing to the overall photoreductive extraction. 3.5. Effect of the Wavelength of the Irradiated Light. Figure 7 shows the effect of the wavelength of the irradiated light, when varied by the use of appropriate cutoff filters, on the time-course variation in the distribution ratio for V by photoirradiation. For wavelengths greater than 300-450 nm, significant photochemical reduction of V(V) occurs, with the distribution ratio increasing with irradiation time. For wavelengths greater than 500-550 nm, however, hardly any reaction occurs. This may be caused by the ability for photoabsorption of the extracted organic species of V(V),
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Figure 7. Effect of the wavelength of the light source on the timecourse variation in the distribution ratio. [(RH)2]feed ) 0.5 mol/L and [NaVO3]feed ) 0.01 mol/L.
Figure 9. Concentration of V in the organic phase following the repeated photoirradiation-stripping processing. Stripping was carried out with 6 mol/L of HCl with an O/A volume ratio of 0.5, following 60 min of photoirradiation. [(RH)2]feed ) 0.5 mol/L, [NaVO3]feed ) 0.01 mol/L, and pHfeed ) 0.86.
4. Conclusion
Figure 8. Absorption spectra for an organic solution containing V(V). [(RH)2]feed ) 0.1 mol/L, and [V(V)] ) (a) 0 mol/L and (b) 0.32 mmol/L. Reference: air.
because, as mentioned in section 3.4, the photochemical reduction of V(V) in the aqueous phase hardly contributes to the total photoreductive extraction. Figure 8 shows the absorption spectrum for the organic solution with and without V(V) and indicates that the absorption band, caused by V(V), appears at 300-500 nm. These results thus show that the photochemical reduction of V in the organic phase and at the interface progresses following photoabsorption at wavelengths of 300-500 nm. 3.6. Reusability of the Organic Phase. The reuse of an organic solution following photoirradiation was also investigated. In these tests, the aqueous and organic solutions were contacted at an O/A volume ratio of 1 and were photoirradiated for 60 min. The resultant organic solution was then stripped using 6 mol/L of HCl, at an O/A volume ratio of 0.5. The stripped organic solution was then used for repeated processing after washing with 3 mol/L of HCl and then water. Figure 9 shows the concentration of V in the organic phase during the repeated photoreductive extraction-stripping processing. The results thus show that the organic phase possesses a sufficient loading and stripping capacity for use in repeated processing and confirm the possible reuse of the organic phase following photoirradiation.
The mechanism for the photoreductive extraction of V has been investigated, with the following results. (1) Extraction equilibria for V(V) and V(IV) with D2EHPA have been formulated, with the respective extracted species VO2R(RH) and VOR2(RH)2 having been found. (2) The photochemical reduction of V occurs in both aqueous and organic solutions, at a rate which is first order with respect to the concentration of V in the respective solutions. The rate constant for photochemical reduction in the organic phase is much greater than that for the aqueous phase. In the two-phase extraction system, reduction also occurs at the interface between the aqueous and organic phases. The photoreductive extraction is mainly contributed by the photochemical reduction at the interface and in the organic phase. The variation in the distribution ratio with respect to time of photoirradiation is expressed successfully by the proposed reaction scheme, in which the photochemical reduction of V is the rate-determining step. (3) The organic solution following photoirradiation is reusable for repeated photoreductive extraction-stripping processing. Acknowledgment The authors are grateful to the Division of Chemical Engineering for the Lend-Lease Laboratory System. S.N. is grateful to the Research Fellowships of the Japan Society for Promotion of Science for Young Scientists and to the Morishita Zintan Scholarship Foundation. Nomenclature A ) interfacial area, dm2 D ) distribution ratio k ) rate constant, min-1 k′ ) rate constant at the interface, dm min-1 Kex ) extraction equilibrium constant Ki ) equilibrium constant for V between the organic phase and the interface Q ) volume of the organic phase, L r ) rate of photochemical reduction for V, mol L-1 min-1 r′ ) rate of photochemical reduction for V at the interface, mol dm-2 min-1 (RH)2 ) dimeric species of D2EHPA Y ) reduction yield
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 3023 Subscripts a ) aqueous phase feed ) aqueous or organic feed solution i ) interface o ) organic phase Superscripts s ) organic-phase species d ) interface species
Literature Cited (1) Benedict, M.; Pigford, T.; Levi, H. Nuclear Chemical Engineering; McGraw-Hill Book Company: New York, 1981. (2) Tsujino, T.; Aochi, T.; Hoshino, T. Reduction Stripping of Plutonium Loaded on TBP with Addition of Nitrous Acid. J. Nucl. Sci. Technol. 1976, 13, 321. (3) Chitnis, R. R.; Wattal, P. K.; Ramanujam, A.; Dhami, P. S.; Gopalakrishman, V.; Mathur, J. N.; Murali, M. S. Separation and Recovery of Uranium, Neptunium, and Plutonium from High Level Waste Using Tributyl Phosphate: Countercurrent Studies with Simulated Waste Solution. Sep. Sci. Technol. 1998, 33, 1877. (4) Hirai, T.; Komasawa, I. Separation of Rare Metals by Solvent Extraction Employing Reductive Stripping. Miner. Process. Extr. Metall. Rev. 1997, 17, 81. (5) Ohno, S.; Kobayashi, Y.; Kimura, T. Application for Photochemical Techniques to Nuclear Fuel Reprocessing and Waste Separations (in Japanese). Nihon Genshiryoku Gakkaishi 1986, 28, 933.
(6) Enokida, Y. Laser-Induced Chemistry for Radioactive Solution (in Japanese). Reza Kenkyu 1990, 18, 75. (7) Hirai, T.; Onoe, N.; Komasawa, I. Photoreductive Stripping of Vanadium in Solvent Extraction Process for Separation of Vanadium and Molybdenum. J. Chem. Eng. Jpn. 1993, 26, 416. (8) Hirai, T.; Komasawa, I. Separation of Eu from Sm/Eu/Gd Mixture by Photoreductive Stripping in Solvent Extraction Process. Ind. Eng. Chem. Res. 1995, 34, 237. (9) Hirai, T.; Komasawa, I. Separation of Ce from La/Ce/Nd Mixture by Photooxidation and Liquid-Liquid Extraction. J. Chem. Eng. Jpn. 1996, 29, 731. (10) Nishihama, S.; Hirai, T.; Komasawa, I. Mechanism of Photoreductive Stripping of Iron(III) in a Liquid-Liquid Extraction System and Its Application for a Hydrometallurgical Process. Ind. Eng. Chem. Res. 1999, 38, 4850. (11) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solutions; Marcel Dekker: New York, 1985. (12) Yuan, C.; Ye, W.; Ma, H.; Wang, G.; Long, H.; Xie, J.; Qin, X.; Zhou, Y. Synthesis of Acidic Phosphates and Phosphonates and Their Structure-Reactivity Studies on The Extraction of Neodymium, Samarium, Ytterbium and Yttrium. Sci. Sin. Ser. B (Engl. Transl.) 1982, 25, 7. (13) Jeliazkowa, B. G.; Nakamura, S.; Fukutomi, H. Photochemical Reduction of Vanadium(V) in Aqueous Perchloric Acid Solutions. Bull. Chem. Soc. Jpn. 1975, 48, 347.
Received for review February 29, 2000 Revised manuscript received May 8, 2000 Accepted May 15, 2000 IE0002778
