Anal. Chem. 1998, 70, 774-779
Solubility of Organophosphorus Metal Extractants in Supercritical Carbon Dioxide Yoshihiro Meguro,* Shuichi Iso, Takayuki Sasaki, and Zenko Yoshida
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
The solubility (S) of several liquid organophosphorus compounds, tributyl phosphate (TBP), diisodecylphosphoric acid (DIDPA), di-(2-ethylhexyl)phosphoric acid (DEHPA), dihexyl-(N,N-diethylcarbamoyl)methylphosphonate (CMP), and octyl(phenyl)(N,N-diisobutylcarbamoyl)methylphosphine oxide (CMPO), in supercritical CO2 was determined over temperature and pressure ranges of 30-90 °C and 7.5-25 MPa. The solubility of the compound increases with increasing density (G) of CO2 fluid, showing a linear relationship: ln S ) p ln G + q (p and q are constants). The slope p measured at 60 °C is 21.8, 11.4, 13.1, 10.8, or 7.5, and the constant q, given solubility at G ) 1 g/mL, is 26.3, 0.4, 2.7, 5.1, or -0.1 respectively for TBP, DIDPA, DEHPA, CMP, or CMPO. A homogeneous mixture of TBP or CMP with CO2 is readily obtained even at relatively low pressure, where the density of CO2 is relatively low. It is found that all the compounds examined are soluble enough in CO2 to prepare an organophosphorus compound-CO2 mixture which can be used in supercritical CO2 fluid extraction of metal ions. Supercritical fluid extraction (SFE) using an extractantsupercritical CO2 mixture instead of an extractant-organic solvent mixture has recently been recognized to be promising as an advanced method for separation of metals from liquid samples or even from solid samples for the purpose of analytical pretreatment or hydrometallurgy. Hence, an increasing number of studies on the development of SFE of metals1-9 is available. One of several important advantages of SFE is that extraction efficiency and extraction selectivity can be enhanced by tuning the pressure and/ or temperature. Also, SFE can minimize the amount of solvent waste. Examples of SFE applications include, e.g., an SFE method for the separation of metal ions such as uranium(VI) and fission product elements from nitric acid solution into supercritical CO2 (1) Wang, J.; Marshall, W. D. Anal. Chem. 1994, 66, 3900. (2) Furton, K. G.; Chen, L.; Jaffe, R. Anal. Chim. Acta 1995, 304, 203. (3) Lin, Y.; Smart, N. G.; Wai, C. M. Trends Anal. Chem. 1995, 14, 123. (4) Zolotov, Yu. A.; Glazkov, I. N.; Efimov, I. P.; Revel’skii, I. A.; Zirko, B. I.; Yashin, Yu. S.; Shakhpenderyan, E. A. Vestn. Mosk. Univ., Ser. 2: Khim. 1995, 36, 41. (5) Wang, S.; Wai, C. M. Environ. Sci. Technol. 1996, 30, 3111. (6) Toews, K. L.; Smart, N. G.; Wai, C. M. Radiochim. Acta 1996, 75, 179. (7) Laintz, K. E.; Tachikawa, E. Anal. Chem. 1994, 66, 2190. (8) Iso, S.; Meguro, Y.; Yoshida, Z. Chem. Lett. 1995, 365. (9) Meguro, Y.; Iso, S.; Takeishi, H.; Yoshida, Z. Radiochim. Acta 1996, 75, 185.
774 Analytical Chemistry, Vol. 70, No. 4, February 15, 1998
containing an organophosphorus extractant such as tributyl phosphate (TBP).8,9 There have been studies on the solubility and/or phase behavior of organic compounds such as aromatic hydrocarbons10,11 and pesticides12 in supercritical CO2 in order to establish a suitable condition for the extraction of these substances from a sample. Several empirical or theoretical equations to clarify the solubility behavior have been proposed.10-16 Recent work is directed to the solubility of extractants such as diethyl dithiocarbamates,3,17 crown ethers,18 and organophosphine oxides19,20 in supercritical CO2 for evaluating the applicability of extractant in SFE and for designing a new extractant feasible to SFE of a metal ion. Solubilities of extractants and metal-containing compounds were reviewed recently.21 Determination of extractant solubility into supercritical CO2 media is indispensable from both fundamental and practical viewpoints: (i) For elucidating and formulating an extraction reaction, in which the distribution equilibrium of an extractant can be expressed by the ratio of extractant solubilities into both phases, the distribution equilibrium of the extractant itself between aqueous and supercritical CO2 phases should be taken into account. (ii) Solubility of an extractant in supercritical CO2 is, in general, fairly lower than that in a conventional organic solvent, which may restrict the preparation of supercritical CO2 media containing the extractant of sufficiently high concentration. Understanding the solubility of the extractant, therefore, is a topic of importance for further development of SFE. In the present study, the solubilities of five organophosphorus compounds which are liquid at an ambient temperature, tributyl phosphate (TBP),22 diisodecylphosphoric acid (DIDPA),23 di(2ethylhexyl)phosphoric acid (DEHPA),24 dihexyl(N,N-diethylcar(10) Bartle, K. D.; Clifford, A. A.; Jafar, S. A.; Shilstone, G. F. J. Phys. Chem. Ref. Data 1991, 20, 713. (11) Chen, P.-C.; Tang, M.; Chen, Y.-P. Ind. Eng. Chem. Res. 1995, 34, 332. (12) Macnaughton, S. J.; Kikic, I.; Rovedo, G.; Foster, N. R.; Alessi, P. J. Chem. Eng. Data 1995, 40, 593. (13) Liu, G.-T.; Nagahama, K. J. Supercrit. Fluids 1996, 9, 152. (14) Chen, J.-W.; Tsai, F.-N. Fluid Phase Equilibria 1995, 107, 189. (15) Chrastil, J. J. Phys. Chem. 1982, 86, 3016. (16) Yakoumis, I. V.; Vlachos, K.; Kontogeorgis, G. M.; Coutsikos, P.; Kalospiros, N. S.; Tassios, D.; Kolisis, F. N. J. Supercrit. Fluids 1996, 9, 88. (17) Wang, J.; Marshall, W. D. Anal. Chem. 1994, 66, 1658. (18) Wang, S.; Elshani, S.; Wai, C. M. Anal. Chem. 1995, 67, 919. (19) Schmitt, W. J.; Reid, R. C. Chem. Eng. Commun. 1988, 64, 155. (20) Lin, Y.; Smart, N. G.; Wai, C. M. Environ. Sci. Technol. 1995, 29, 2706. (21) Smart, N. G.; Carleson, T.; Kast, T.; Clifford, A. A.; Burford, M. D.; Wai, C. M. Talanta 1997, 44, 137. (22) Schulz, W. W., Burger, L. L., Navratil, J. D., Eds. Science and Technology of Tributylphosphate, Vol. III; CRC Press: Boca Raton, FL, 1990. S0003-2700(97)00739-7 CCC: $15.00
© 1998 American Chemical Society Published on Web 02/15/1998
Figure 1. Apparatus for measuring solubility of the liquid organic compound. 1,2, equilibrium cells; 3, collection vessel; 4, quartz wool; 5, syringe pump; 6, prewarming coil; 7, filter; 8, restrictor; 9, thermostat oven; 10, CO2 cylinder; 11, plunger-type pump; 12, liquid compound.
bamoyl)methylphosphonate (CMP),25 and octyl(phenyl)(N,Ndiisobutylcarbamoyl)methylphosphine oxide (CMPO),26 were measured over temperature and pressure ranges of 30-90 °C and 7.5-25 MPa. These liquid compounds have been widely employed in solvent extraction of metal ions, particularly in the field of nuclear technology, because of their high extractability toward metal ions from highly acid aqueous solution and high radiochemical stability. The relation between the solubility and density of CO2 was investigated, and the feasibility of these extractants for SFE was evaluated from a viewpoint of the preparation of a homogeneous mixture of the extractant and the supercritical CO2. EXPERIMENTAL SECTION Apparatus. The apparatus used for solubility measurements is shown in Figure 1. The main part of the apparatus consisted of twin equilibrium cells (1 and 2) of stainless steel and a collecting glass vessel containing 1.5 g of quartz wool (1-5 µm), both of which were kept in a thermostated oven at a defined temperature. The syringe pump (Isco Co. Ltd., 260D), plunger-type pump (JEOL Co. Ltd., CAP-L02), prewarming coil, filter (2 µm in pore size), restrictor (Isco Co. Ltd., stainless steel capillary of 57 µm i.d. × 27 cm in length, 71 µm × 48 cm, or 71 µm × 110 cm), and thermostat oven were identical with those employed in previous work.8,9 Extra heating of the restrictor part to avoid a plugging problem was not necessary in the present work to measure the solubility of such liquid organophosphorus compounds. Gas chromatographic analysis was conducted with a Hitachi type 260-30 gas chromatograph, equipped with a temperatureprogramming device and a flame ionization detector. The column employed was made from stainless steel tubing of 3 mm i.d. and 1000 mm in length, packed with 1.5% OV-101 on Chromosorb G-HP substrate. Helium of 0.6 kgf/cm2 (at 100 °C) was used as carrier gas. (23) Kubota, M.; Dojiri, S.; Yamaguchi, I.; Morita, Y.; Yamagishi, I.; Kobayashi, T.; Tani, S. In High-Level Radioactive Waste and Spent Fuel Management; Slate, S. C., Kohout, R., Duzuki, A., Eds.; The American Society of Mechanical Engineers: Fairfield, NJ, 1989; p 537. (24) Ceccaroli, B.; Alstad, J. J. Inorg. Nucl. Chem. 1981, 43, 1881. (25) Horwitz, E. P.; Muscatello, A. C.; Kalina, D. G.; Kaplan, L. Sep. Sci. Technol. 1981, 16, 417. (26) Reddy, M. L. P.; Damodaran, A. D.; Mathur, J. N.; Murali, M. S.; Krishna, M. V. B.; Iyer, R. H. Solv. Extr. Ion Exch. 1996, 14, 793.
Procedure. An appropriate volume of organophosphorus liquid compound was taken in the equilibrium cells 1 and 2. The syringe pump was filled with liquid CO2 at 25 °C. Carbon dioxide fluid was allowed to flow through the system at constant pressure. The supercritical CO2 flow, after being warmed in the prewarming coil, was introduced into the thermostated equilibrium cells. The effluent from the equilibrium cells was introduced into the collecting vessel, where the pressure was reduced to atmospheric by the aid of a restrictor. Effluents were discarded in the first 2 min of an experiment, and then a solute in the effluent was collected in the vessel over a definite time. The amount of the solute collected was determined by gravimetry and/or gas chromatography. Gravimetric weighing of the collection vessel before and after the solubility measurement procedure is sufficiently precise for determination of the amount of solute recovered, provided the sample compound is pure enough. When the sample reagent contains appreciable quantities of an impurity, in particular, if the impurity has preferential solubility into the CO2 phase, gas chromatographic analysis of the solute is necessary. For gas chromatography, the solute recovered in the collecting vessel was quantitatively dissolved with ethanol (for TBP, CMP, or CMPO) or hexane (for DIDPA), and an aliquot of the solution was taken and subjected to analysis. Initial temperature and rate of temperature increase in temperature-programmed gas chromatography were 80 °C + 15 °C/min, 80 °C + 20 °C/min, 100 °C + 20 °C/min, or 150 °C + 10 °C/min for the determination of TBP, CMP, CMPO or DIDPA, respectively. The concentration (C; in moles per liter) of a compound dissolved in CO2 was calculated using eq 1,
C)
w/M × 1000 fCO2(25,P)t (FCO2(25,P)/FCO2(T,P)) + w/Fp
(1)
where w is the amount in grams of compound collected, M is the molecular weight of the compound, fCO2(25,P) is the flow rate in milliliters per minute of CO2 at the syringe pump (at 25 °C and pressure P), t gives the collection time in minutes, FCO2(T,P) is the density of CO2 at temperature T and pressure P, and Fp is the density of a compound measured at 25 °C. Here, Fp can be assumed to be independent of temperature and pressure in the range investigated in the present work. The flow rate of CO2 fluid was controlled using restrictors of various inner diameter and length. Materials. TBP (Koso Chemicals, g98% purity guaranteed by the company), DIDPA (Daihachi Chemical, 92.9% as analyzed by the company), CMP (Occidental Chemical, g95% guaranteed), DEHPA (Aldrich, g97% guaranteed), and CMPO (M&T Chemicals, 98.6% analyzed) were used without further purification. A liquid CO2 cylinder of ∼6 MPa and 99.99% pure, supplied by Shin Tokyo Teisan Co. Ltd., was used. RESULTS A solubility equilibrium of CO2 flow with solute should be achieved during the residence time of the flow in the twin equilibrium cells (cf. Figure 1). With 8 and 5 mL of compound being placed in cells 1 and 2, respectively, the concentration of compound determined was independent within the error, (5%, Analytical Chemistry, Vol. 70, No. 4, February 15, 1998
775
Table 1. Solubility of Tributyl Phosphate (TBP) in CO2 temp (°C)
pressure (MPa)
density of CO2 (g/mL)a
30
8.5 10.0 15.0 20.0 25.0
0.726 0.772 0.847 0.891 0.923
1.3 1.4 1.4 1.5 1.4
40
8.5 10.0 15.0 20.0 25.0
0.335 0.625 0.781 0.840 0.880
0.76 1.2 1.3 1.4 1.4
7.5 8.5 9.0 10.0 15.0 20.0 25.0
0.194 0.249 0.284 0.371 0.701 0.785 0.835
0.0094 0.07 0.17 0.73 1.3 1.4 1.5
8.5 9.0 9.3 9.6 10.0 12.0 15.0 20.0 25.0
0.212 0.235 0.250 0.266 0.289 0.429 0.605 0.724 0.787
0.00064 0.0045 0.011 0.13 0.42 0.71 1.1 1.2 1.3
50
60
a
concn of TBP (mol/L)
temp (°C)
pressure (MPa)
density of CO2 (g/mL)a
concn of TBP (mol/L)
65
9.0 10.0 11.0 12.0 15.0 20.0 25.0
0.220 0.266 0.320 0.381 0.553 0.692 0.762
0.00084 0.020 0.24 0.33 1.0 1.3 1.4
70
10.0 10.5 11.0 11.5 15.0 20.0 25.0
0.248 0.270 0.293 0.319 0.505 0.660 0.737
0.00092 0.0070 0.14 0.31 1.0 1.2 1.3
80
8.5 10.0 11.0 12.0 15.0 20.0 25.0
0.175 0.222 0.257 0.297 0.427 0.595 0.687
0.00066 0.0017 0.0035 0.049 0.65 1.2 1.3
90
10.0 12.0 15.0 20.0 25.0
0.203 0.265 0.372 0.534 0.637
0.0016 0.0089 0.43 1.2 1.2
Taken from ref 27 (given for neat CO2).
on the flow rate of the CO2 in the range from 0.02 to 1.8 mL/min. Thus, we hereafter define the concentration of compound being measured in this flow rate range as an equilibrium solubility. It was observed that the dissolution reaction did not attain equilibrium when a single cell was used with CO2 of relatively high flow rate. For example, the solubility of TBP measured using a single cell containing 5 mL of TBP remarkably decreased with increasing the flow rate in the range between 0.02 and 1.0 mL/min. The recovery efficiency of solute in the collection vessel was examined by the following procedure. After allowing the CO2 fluid to flow through the apparatus with empty cells 1 and 2, a known amount of sample compound was added to the CO2 stream at a known flow rate using a plunger-type pump (11 in Figure 1). By analyzing the solute in CO2 after collection, a recovery of g98% was confirmed. Three repeated experiments at given pressure and temperature, e.g., 15 MPa and 60 °C, showed the reproducibility of the measurements to be (5%, irrespective of the amount of the solute collected in the range 0.15-5 g. It has been well-known28 that the presence of modifier enhances the solubility of a solute when an appreciable amount of modifier coexists and the amount of the modifier is in excess of the amount of an objective solute in CO2. The modifier effect due to impurities contained in organophosphorus compounds employed in the present work is estimated to be negligible within (27) Angus, S., Armstrong, B., de Reuck, K. M., Eds. IUPAC International Thermodynamic Tables of The Fluid State. Vol. 3, Carbon Dioxide; Pergamon Press: New York, 1976. (28) Taylor, L. T. Supercritical Fluid Extraction; John Wiley & Sons: New York, 1996; Chapter 3.
776 Analytical Chemistry, Vol. 70, No. 4, February 15, 1998
the experimental error of (5%, since total concentration of impurities is low enough, except for DIDPA, and the concentration of impurities dissolved in CO2 is lower than that of the organophosphorus compound. The modifier effect on the DIDPA solubility due to impurity will be discussed below. Solubility of Tributyl Phosphate. In the gas chromatogram of reagent grade TBP, whose purity was given as g98%, no peak indicating the impurity could be observed. If there were an impurity with preferential solubility relative to TBP, this impurity should be concentrated in the solute collected after the solubility measurement procedure. No peak due to the impurity was detected in the gas chromatogram, even for the solute collected by the solubility measurements at 80 °C and 10 MPa (amount of the solute collected, 70 mg) or at 90 °C and 10 MPa (solute, 100 mg). These results suggest that gravimetry is precise enough to determine the solubility of TBP. The results of concentration (C) of TBP dissolved in CO2 at 30-90 °C and 7.5-25 MPa are summarized in Table 1. The TBP concentration remarkably increases with an increase of pressure at temperature higher than 50 °C and in a relatively lower pressure region. The TBP concentration in this range, where a strong pressure dependence of C is observed, corresponds to a pressuredependent solubility of TBP in supercritical CO2. The concentration of TBP in CO2 measured at g50 °C becomes less dependent on the pressure when the pressure is considerably higher and the TBP concentration ranges from 1 to 1.5 mol/L. TBP solubilities measured at 30-40 °C were found in the range 1-1.5 mol/L and showed relatively weak pressure dependence. Under these dissolution conditions, the mixture of TBP and CO2
Table 2. Ratio of Peak Areas, R, of Impurities and Pure Compound in the Gas Chromatogram of Organophosphorus Compounds condition of solubility measurement compd DIDPA
temp (°C)
pressure (MPa)
amount of solute collected (g)
40 50 60 70
12.5 12.5 15 17.5
1.80 0.74 0.93 1.44
DIDPA reagent as received CMP
60
10 11 12 15
60
10 12 15 20 25
CMPO reagent as received a
temp (°C)
pressure (MPa)
density (g/mL)
concn (mol/L)
40
12.5 15 25
0.732 0.781 0.880
0.015 0.032 0.12
50
12.5 15 25
0.612 0.701 0.835
0.0032 0.017 0.12
60
15 20 25
0.605 0.724 0.787
0.0046 0.041 0.090
70
17.5 20 25
0.599 0.660 0.737
0.0059 0.019 0.057
Ra 0.07 0.19 0.17 0.11 0.02
0.039 0.039 1.48 2.22
0.02 0.02 0.02 0.02 0.02 ( 0.01
CMP reagent as received CMPO
Table 3. Solubility of Diisodecylphosphoric Acid (DIDPA) in CO2
0.016 0.30 0.48 0.89 1.36
∼0 0.16 0.01 ∼0 ∼0 ∼0
Table 4. Solubility of DEHPA, CMP, and CMPO in CO2 at 60 °C organophosphorus compound
pressure (MPa)
density (g/mL)
DEHPA
15 20 25
0.605 0.724 0.787
0.022 0.16 0.81
CMP
10 11 12 15
0.289 0.355 0.429 0.605
0.00023 0.0010 0.078 0.41
CMPO
10 12 15 20
0.289 0.429 0.605 0.724
0.000082 0.0015 0.017 0.089
R ) (∑ peak area for impurities)/(peak area for pure compound).
in the equilibrium cell may form a single TBP-CO2 phase, in which the amount of TBP dissolved in the CO2 phase increases and the amount of CO2 dissolved in the TBP phase also increases, and finally both phases become of identical composition, resulting in formation of the single phase. The single phase formation of TBP and CO2, e.g., at 60 °C and g15 MPa, was confirmed visually in a separate experiment using a sapphire window cell. The minimum pressure at which the single phase formation occurs increases with increasing temperature. Solubility of Diisodecylphosphoric Acid. In the gas chromatogram of DIDPA reagent, a peak corresponding to an impurity was observed with retention time, tR, of 2.9 ( 0.3 min, while tR of the main peak due to DIDPA was 10.4 ( 0.4 min. The gas chromatogram recorded with the solute collected in the solubility measurement showed peaks for both DIDPA and impurity. A ratio of peak areas of both peaks, which is defined as R ) [area of impurity peak(s)]/[area of pure reagent peak], in the gas chromatogram for the solute collected was larger than R for the reagent as received; the results of R measured under various conditions are summarized in Table 2. The larger R for the solute collected implies the preferential dissolution of the impurity into CO2 and, thus, the enrichment of the impurity by the solubility measurement procedure. The enrichment effect is more clearly observed when the amount of the solute collected is smaller, as shown in Table 2. Hence, gravimetry could not be applied to the determination of the solubility of DIDPA, because the gravimetric result includes a fairly large positive error due to the impurity. The solubility data for DIDPA determined by gas chromatography are listed in Table 3. The solubility increases with an increase of the pressure in the range 12.5-25 MPa and with an increase of the temperature in the range 40-70 °C. If the impurity which is enriched in CO2 fluid enhances the solubility of DIDPA through the so-called modifier effect, then the solubility determined by collecting a smaller amount of the solute must be higher than that determined by collecting a larger
concn (mol/L)
amount of the solute. The experimental results showed that the solubility of DIDPA was independent of the amount of the solute collected in the range of 0.15-1.5 g. This implies that the impurity does not enhance the DIDPA solubility appreciably. Solubility of Di-2-ethylhexylphosphoric Acid. The purity of DEHPA used in this study is rather high. To confirm the accuracy of solubility data from gravimetric measurement, 0.202.5 g of solute (at 60 °C and 15 MPa) was collected. Solubilities determined by both experiments were 0.022 ( 0.001 and did not depend on the amount of the solute collected. These results indicate that the enrichment effect of an impurity is insignificant for DEHPA, indicating that interferences by impurity in the gravimetric determination were negligible. The solubility of DEHPA at 60 °C was determined by gravimetry, and the results are summarized in Table 4. Significant pressure dependence of the solubility is observed. Solubility of Dihexyl-(N,N-diethylcarbamoyl)methylphosphonate. In the gas chromatogram of reagent CMP, at least three peaks due to impurities were detected at tR ) 2.9 ( 0.2, 5.8 ( 0.1, and 8.2 ( 0.1 min. The tR of the CMP peak was 6.8 ( 0.3 min. Peak area ratio R was calculated to be 0.02 ( 0.01. Identical peaks of impurities were observed for all solute samples collected. The R values, as shown in Table 2, were determined for the solute at 60 °C and 10-15 MPa and for wide ranges of the amount collected. It is found that R determined for the solute is identical with R for the reagent CMP and is not influenced by the amount of the solute collected ranging from 40 mg to 2 g. This indicates that there is no enrichment effect of the impurity in supercritical Analytical Chemistry, Vol. 70, No. 4, February 15, 1998
777
Table 5. Solubility Constants of Organophosphorus Compounds in the Equation ln S ) p ln G + q compd
temp (°C)
p
q
50 60 65 70
6.5 ( 0.6 21.8 ( 2.1 15.2 ( 1.0 24.4 ( 3.4
6.2 ( 0.8 26.3 ( 2.9 16 ( 1.3 27.1 ( 4.3
10-25
40 50 60 70
11.4 ( 0.1 11.6 ( 0.3 11.4 ( 0.6 10.8 ( 0.7
-0.6 ( 0.1 -0.0 ( 0.1 0.4 ( 0.2 0.5 ( 0.3
DEHPA
15-25
60
13.1 ( 2.1
2.7 ( 0.8
CMP
10-15
60
10.8 ( 2.3
5.1 ( 2.2
CMPO
10-20
60
7.5 ( 0.2
-0.1 ( 0.2
TBP
DIDPA
Figure 2. Ln S-ln F plots for organophosphorus compounds in CO2 at 60 °C. Compounds: 1, TBP; 2, CMP; 3, CMPO; 4, DEHPA; 5, DIDPA.
CO2 during solubility measurement procedure. Both gravimetry and gas chromatography give enough precise solubility data, which was ascertained by comparison of results from these two methods. Solubilities given in Table 4 were measured at 60 °C and 10-15 MPa using gas chromatography. The solubility of CMP was found to increase with pressure. Solubility of Octyl(phenyl)(N,N-diisobutylcarbamoyl)methylphosphine Oxide. There was no noticeable peak corresponding to an impurity in the gas chromatogram for reagent CMPO. The tR for CMPO was 8.7 ( 0.2 min. The solute sample collected in the solubility measurement at 60 °C and 10-25 MPa was analyzed by gas chromatography, and the results are shown in Table 2. Four peaks with tR ) 0.7, 1.9, 3.1, and 7.4 min were detected in the gas chromatograms for the solute collected at 12 and 15 MPa, showing R ) 0.16 and 0.01, respectively (Table 2). The solubility data for CMPO obtained by gas chromatography are summarized in Table 4. DISCUSSION The solubilities of the five organophosphorus compounds investigated in the present work increase significantly with an increase of the pressure, except in the case of a formation of a single phase in the equilibrium cells, typically observed in the solubility of TBP at a relatively high pressure region. Such a pressure effect has been explained in terms of an effect of supercritical fluid density on the solubility of a solute. A simple relation, as given by eq 2, between the solubility S and the density F of CO2 was proposed15 for systematizing the solubilities of carboxylic acids, carboxylic esters, and water. A recent review21 suggests the validity of this correlation for the solubility of a variety of organic compounds,
ln S ) p ln F + q
(2)
where F is the density of neat CO2 in grams per milliliter, p is a constant relating to the solvation of the solute in the supercritical fluid, and q is the solubility at F ) 1 g/mL. The solubility data listed in Tables 1, 3, and 4 were plotted against the density of supercritical CO2, some examples of which are illustrated in Figure 2. As shown by plot 1 for the solubility of TBP at 60 °C, the ln S-ln F plot has a clear linear portion. Identical linear relations were observed in ln S-ln F plots 778 Analytical Chemistry, Vol. 70, No. 4, February 15, 1998
pressure range (MPa) 7.5-12
measured at 50-70 °C. The p and q values determined on the basis of eq 2 are listed in Table 5. These values are calculated by linear least-squares regression, and the uncertainty denotes its standard deviation. The slope p at 50 °C is smaller than those at 60-70 °C, and the average p at 60-70 °C is 20.5 ( 4.7. The uncertainty is large but results mostly from the very high slope. The ln S-ln F plots at 80 and 90 °C do not show a linear portion in the wide range of the density of CO2. Density dependences of solubility below 0.2 g/mL are smaller than those at higher density. It is assumed that the contribution of the volatility of TBP to the overall solubility becomes larger at higher temperature and at the lower CO2 density region. A similar behavior was reported for the solubility of menthol in supercritical CO2, indicating a decrease in the slope of the ln S vs ln F plot in the lower density region.29 The linear relationship between ln S and ln F is clearly observed for the solubility of DIDPA at 40-70 °C, and the result at 60 °C is shown as plot 5 in Figure 2. As given in Table 5, slope p of DIDPA is not influenced by the temperature in the range examined, and q increases with an increase of temperature. The DIDPA solubility at a given density increases with an increase of temperature, though such a systematic temperature effect cannot be observed for TBP. Plots 2-4 for solubilities of CMP, CMPO, and DEHPA at 60 °C indicate linear relationships between ln S and ln F, explained by eq 2. The constants p and q are summarized in Table 5. Slope p in eq 2 has been considered to be an indication of the solvation of the solute in the supercritical fluid, and there was an attempt to correlate p directly to the number of CO2 molecules coordinated onto the solute.15 Slopes p of five extractants (at 60 °C) are 21.8 (TBP, molecular weight, 266), 13.1 (DEHPA, 322), 10.8 (CMP, 363), 11.4 (DIDPA, 379), and 7.5 (CMPO, 408). It is noteworthy that slope p decreases with an increase in the molecular weight of the extractants, even though a reason cannot easily be given. It is found that all organophosphorus compounds examined have sufficiently high solubility which makes it possible to prepare a homogeneous extractant-CO2 mixture for SFE of metal ions. For example, if the pressure is sufficiently high to obtain CO2 density above 0.3 g/mL for TBP, 0.45 g/mL for CMP, 0.7 g/mL (29) Sovova, H.; Jez, J. J. Chem. Eng. Data 1994, 39, 840.
for DEHPA, 0.75 g/mL for CMPO, or 0.8 g/mL for DIDPA, a 0.1 mol/L extractant-CO2 homogeneous mixture can be prepared. The regions shown by dotted lines for TBP and CMP in Figure 2 correspond to the formation of the single phase of extractantCO2 mixture in the equilibrium cells. This phase behavior reflects the fact that TBP and CMP are more soluble in CO2 than CMPO, DEHPA, and DIDPA. Other organophosphorus compounds, such as trioctylphosphine oxide and triphenyl phosphate,19 were also reported to be less soluble in comparison with TBP and CMP studied in the present work. CONCLUSION Solubilities were determined for five organophosphorus liquid extractants using a twin equilibrium cells method. Results suggest that all extractants examined are sufficiently soluble in CO2 media and can be employed as a metal extractant in SFE. In SFE of metal ions from aqueous solution into the supercritical CO2 phase, the distribution of extractant between aqueous and supercritical phases is defined as the ratio of solubilities of extractant between both phases. It is obvious from the results discussed in this paper that the solubility of the extractant in supercritical CO2 can be controlled by changing the CO2 density, namely by tuning pressure and temperature. It is thus expected that the extraction
equilibrium of a metal can be controlled by tuning the density of CO2 fluid, because the extraction equilibrium of the metal strongly depends on the distribution of the extractant itself.9 Studies of the SFE equilibrium of U(VI), Pu(IV), and lanthanide(III) ions between nitric acid solution and supercritical CO2 using TBP or DIDPA are now in progress. ACKNOWLEDGMENT The authors thank Professor C. M. Wai of University of Idaho, Professor A. A. Clifford of University of Leeds, and Dr. N. G. Smart of British Nuclear Fuels plc. for their helpful discussion and encouragement throughout this work. SUPPORTING INFORMATION AVAILABLE Figures showing ln S vs pressure plots for TBP and ln S vs ln F plots for TBP or DIDPA (3 pages). Ordering and access information is given on any current journal masthead page.
Received for review July 9, 1997. Accepted December 2, 1997. AC9707390
Analytical Chemistry, Vol. 70, No. 4, February 15, 1998
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