Copper(II) Removal from Aqueous Solutions by Chelation in

Ind. Eng. Chem. Res. 1997, 36, 5371-5376

5371

Copper(II) Removal from Aqueous Solutions by Chelation in Supercritical Carbon Dioxide Using Fluorinated β-Diketones Jennifer M. Murphy† and Can Erkey*,‡ Departments of Civil and Environmental Engineering and of Chemical Engineering, Environmental Engineering Program, University of Connecticut, Storrs, Connecticut 06269

Copper removal from aqueous solutions by chelation in supercritical carbon dioxide was investigated. The chelating agents were 1,1,1-trifluoroacetylacetone (TFA) and 2,2-dimethyl6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (FOD). Extraction efficiencies were determined for a single equilibrium stage in the temperature range 308-328 K and the pressure range 9.9-24.0 MPa and ranged from 4% to 84%. The extraction efficiencies increased with increasing the initial amount of chelating agent and with decreasing the initial copper concentration. No significant changes in extraction efficiencies were observed with temperature and pressure at a fixed initial amount of chelating agent. A thermodynamic model based on combined reaction and phase equilibria was also developed for prediction of extraction efficiencies. Introduction Solvent extraction is a well-established process within the inorganic chemical and hydrometallurgical industries for transfer of cations or anions from an aqueous phase into an organic phase (Cox, 1992). As direct extraction of ionic species is energetically very unfavorable, transfer is accomplished by forming organicsoluble neutral complexes in the aqueous phase by reactions between the ionic species of interest and an appropriate organic compound (reagent). However, since the viscosities of many reagents are much too high for direct use in solvent extraction equipment and for facilitation of phase separation, it is common practice to dissolve the reagents in an organic solvent. The solvents are usually hydrocarbons selected on the basis of a flash point above 333 K (to minimize evaporation loss and the risk of fire) and a density of about 800 kg/m3 to aid phase separation. In general, the solvents used contain a mixture of paraffinic, aromatic, and naphthenic hydrocarbons. The major oil companies have also developed solvent mixtures specifically for use in solvent extraction of metals (Flett et al., 1983). Solvent selection for a particular application is based on many factors such as solvent strength, selectivity, loading, ease of stripping, rates of extraction and stripping, chemical stability, aqueous phase solubility of solvent components, volatility and flammability of the solvent, toxicity of the solvent within the working area, and solvent cost. In conventional solvent extraction, since the target material must be accumulated in the organic phase during loading, the ratio of the aqueous to the organic volume cannot usually be more than about 10. This leads to the use of large volumes of solvent, particularly when the feed is lean. The fact that significant quantities of solvent are lost by entrainment reinforces this limit. Consequently, one of the significant operating costs in hydrometallurgical separations involving solvent extraction is solvent recovery cost due to large volumes of solvent that need to be processed (Pratt, * Author to whom all correspondence should be addressed. E-mail: [email protected]. Telephone: (860) 486-4601. Fax: (860) 486-2959. † Department of Civil and Environmental Engineering. ‡ Department of Chemical Engineering. S0888-5885(97)00458-2 CCC: $14.00

1983). This adverse effect can possibly be eliminated by the substitution of nontoxic supercritical fluids (SCFs) for organic solvents. In solvent extraction of metals, the chemical reactions occurring at the interfacial plane may be fast compared to mass-transfer processes and, depending on hydrodynamic conditions in the extraction vessel, the observed kinetics of removal may be controlled by mass-transfer. Since the mass-transfer characteristics of SCFs are excellent compared to those of organic solvents due their relatively low viscosities and high solute diffusivities, the use of SCFs in place of organic solvents may enhance rates of extraction and stripping. The nontoxic nature and relatively low critical temperature (304 K) and pressure (7.38 MPa) of carbon dioxide make it an attractive choice for this application. Other notable advantages of supercritical carbon dioxide (SCCO2) are that it is nonflammable, relatively inexpensive, and readily available. In addition, the fact that the solvency characteristics of supercritical fluids can be varied with small changes in temperature and pressure may be exploited in the development of selective extraction schemes. As a result of these favorable solvency properties of SCFs, some research and development work has been conducted in various laboratories on the removal of heavy metals from aqueous solutions by chelation in SCCO2. These studies began with the pioneering work of Laintz et al. (1991, 1992), who investigated the extraction of copper(II) from an aqueous solution by chelation with bis(trifluoroethyl)dithiocarbamate using a dynamic extraction scheme. Nearly 100% of the metal was removed from the aqueous sample after 1 h at a CO2 density of 500 kg/m3 and a temperature of 308 K. The authors indicated that the use of the fluorinated chelating agent yielded much better extraction results than the use of a nonfluorinated analogue, diethyldithiocarbamate. This was attributed to enhanced solubility of the complexes in SCCO2 due to the CO2-philic fluoroalkyl groups. Wang and Marshall (1994) investigated removal of zinc, cadmium, and lead from aqueous solutions using tetrabutylammonium dibutyldithiocarbamate as the chelating agent. Nearly complete metal extractions were achieved within 60 min at 323 K and 24.3 MPa. Lin et al. (1994) studied the extraction of U(VI) and Th(IV) ions from synthetic aqueous solutions and of U(VI) from mine waters at 333 K and 15.2 © 1997 American Chemical Society

5372 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

Figure 1. Schematic diagram of the experimental apparatus.

MPa using thenoyltrifluoroacetone (TTA) as the chelating agent. The extraction efficiencies ranged from 38 to 90% and were significantly enhanced by the addition of tributyl phosphate (TBP) to the system. TTA was also utilized to extract trivalent lanthanides from acidic solutions at 35.5 MPa and 333 K with extraction efficiencies ranging from 17 to 92% depending on the type of metal and the percentage of TBP in the SCCO2 phase (Laintz and Tachikawa, 1994). For all eight metals extracted (La3+, Ce3+, Sm3+, Eu3+, Gd3+, Dy3+, Yb3+, and Lu3+), an increase in the percentage of TBP resulted in a corresponding increase in the extraction efficiency. Lin and Wai (1994) also reported on extraction of La3+, Eu3+, and Lu3+ from aqueous solutions at 333 K and 15.2 MPa, using TTA and TBP. They also studied the synergistic effects of using mixed ligands during extraction and found enhancement of extraction efficiencies up to 48% on the addition of TBP to the system containing TTA. Recently, we have also reported on thermodynamics of extraction of copper from aqueous solutions using hexafluoroacetylacetone as the chelating agent (Murphy and Erkey, 1997). The objectives of the present study were 2-fold. The first one was to investigate the applicability of fluorinated β-diketones to extraction of copper from aqueous streams. The study was focused on copper since there are many solvent extraction plants in operation throughout the world producing about 700 tons of copper/day (Cox, 1988). SCCO2 may be an alternative to commonly used organic solvents in these plants. The second objective was to investigate the applicability of our previously developed thermodynamic model for extraction of copper using hexafluoroacetylacetone to other fluorinated β-diketones. Such thermodynamic models are necessary for successful implementation of this technology on an industrial scale. The chelating agents used in the study were 1,1,1-trifluoroacetylacetone (TFA) and 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5octanedione (FOD). The effects of temperature, pressure, initial metal concentration, and initial chelating agent amount on extraction efficiencies in a single equilibrium stage were investigated. Experimental Section Extraction runs were performed in a batch extraction vessel which is a part of the apparatus shown in Figure 1. The extractor was a 0.3 L stainless steel autoclave (Autoclave Engineers Inc.) equipped with a pressure gauge and a Magnedrive II mixing device. Liquid carbon dioxide was charged into the vessel using a highpressure syringe pump (ISCO, 260D) equipped with a

Figure 2. Path of approach to equilibrium for the Cu2+/TFA system ([Cu2+]0 ) 100ppm; [TFA]0 ) 3.81 × 10-3 kg; T ) 328 K; p ) 15.8 MPa) and for the Cu2+/FOD system ([Cu2+]0 ) 100ppm; [FOD]0 ) 2.55 × 10-3 kg; T ) 318 K; p ) 13.4 MPa).

cooling jacket. For temperature control, the vessel was immersed in a water bath equipped with an immersion circulator (Julabo) having an accuracy of (0.1 K. Temperature was monitored using a thermocouple meter (DP41-TC-MDSS, Omega Engineering), accurate to (0.1 K, and a T-Type thermocouple (Omega Engineering) inserted into a thermowell which extended deep into the extraction vessel. Two sampling lines were installed on the extractor. One sampling line extended deep into the extractor for sampling the aqueous phase, and a shorter one was used for sampling the less dense supercritical phase. For removal of the organic chelating agent, an activated carbon bed was installed on the CO2 vent line which was connected to a fume hood. For each extraction run 100 mL (∼100 g) of a cupric nitrate solution, having a desired Cu2+ concentration, and a measured amount of chelating agent were placed into the vessel. The reactor was sealed, charged with CO2, and stirred at constant temperature and pressure for an equilibration period that was unique to each chelating agent. The stirrer was then shut off, and the phases were allowed to separate for 1 h. The separate phases were sampled using the two sampling lines. Aqueous phase samples were analyzed directly for copper ion concentration using a Milton Roy Spectronic 601 spectrophotometer and the Bathocuproine Method, 3500-Cu E (Greenberg et al., 1992). Supercritical phase samples required a few preliminary steps before they could be analyzed for copper concentration. The contents of the supercritical phase sample loop were first washed with ethanol, which dissolved the copper complex and the chelating agent. The ethanol was then evaporated from the sample vial on a hot plate. The dry vial was filled with 10 g of deionized water, and 200 µL of 1:1 HCl was added. The low pH of the solution caused reversal of the copper complex formation reaction. Nitrogen was then bubbled through the samples to remove the dissolved chelating agent before addition of the 100 µL portions of sodium citrate, hydroxylamine hydrochloride, and disodium bathocuproine disulfonate solutions and analysis by the Bathocuproine Method. Using the aqueous concentrations obtained, the supercritical phase concentrations were backcalculated using a measured sample loop volume of 1200 µL. Mass balance closures were better than 90%. Determination of the necessary equilibration time was done by trial. For each chelating agent, samples were taken periodically during an extraction run. The data were then analyzed to determine the path of approach to equilibrium. Figure 2 shows the evolution of copper-

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5373 Table 1. Variation of Extraction Efficiency with Temperature, Pressure, and Initial Amount of TFA for [Cu2+]0 ) 100 ppm temp pressure density 103[TFA]0 stoichiometric % (K) (MPa) (kg‚m-3) (kg) excess extraction 318 318 318 318 318 318 308 308 318 318 328 328

13.4 13.4 13.4 13.4 13.4 13.4 9.9 13.9 13.4 19.3 17.1 24.0

700 700 700 700 700 700 700 800 700 800 700 800

3.81 2.54 1.27 0.64 0.32 0.13 0.64 0.64 0.64 0.64 0.64 0.64

77.6 51.4 25.2 12.2 5.6 1.7 12.2 12.2 12.2 12.2 12.2 12.2

70 70 46 31 11 4 25 31 32 33 37 35

(II) concentration as a function of time for both of the chelating agents. The experiments indicated that the equilibration time for TFA at 328 K and 15.8 MPa was around 40 min. Therefore, all extraction runs using TFA were performed for an equilibration period of at least 1 h. For FOD, the equilibration time at 318 K and 13.4 MPa was about 20 min, so the extractions with FOD were conducted using equilibration times between 30 and 45 min. The faster equilibration observed with FOD can possibly be attributed to its lower solubility in water. Aqueous copper solutions were prepared gravimetrically by dissolving copper nitrate (Aldrich Chemical Co.) in deionized water. Carbon dioxide (99%) was purchased from Connecticut Airgas Inc. TFA and FOD were obtained from Acros Organics. The analytical chemicals (hydrochloric acid, hydroxylamine hydrochloride, sodium citrate, and disodium bathocuproine disulfonate) as well as the Cu(TFA)2 complex were received from Aldrich Chemical Co. The Cu(FOD)2 complex was purchased from Gelest, Inc. All chemicals were used as received without further purification. Results and Discussion The amount of chelating agent (HA) placed in the extractor (volume: 0.3 L) for each run ranged from 0.13 to 3.81 g for TFA and from 0.26 to 3.81 g for FOD at 318 K and 13.8 MPa. Each chelating agent amount corresponds to a stoichiometric excess which is given, along with the experimental results, in Tables 1 and 2. The stoichiometric excess, E, was defined as

E)

Table 2. Variation of Extraction Efficiency with Temperature, Pressure, and Initial Amount of FOD temp pressure density 103[FOD]0 stoichiometric % (K) (MPa) (kg‚m-3) (kg) excess extraction 318 318 318 318 318 308 318 328

13.4 13.4 13.4 13.4 13.4 13.9 19.3 24.0

700 700 700 700 700 800 800 800

[Cu2+]0 ) 100 ppm 3.82 2.55 1.27 0.64 0.26 1.27 1.27 1.27

40.0 26.4 12.6 5.8 1.7 12.2 12.2 12.2

84 78 50 33 10 56 55 56

35 45 55

13.9 19.3 24.0

800 800 800

[Cu2+]0 ) 70 ppm 1.27 1.27 1.27

12.2 12.2 12.2

68 68 67

chelating agent (HA) is given in Figure 3. The distribution of the metal between the two phases is governed by the equilibrium constants of the aqueous phase reactions and the distribution coefficients of the molecular species. Both the reaction equilibrium constants and the distribution coefficients are dependent on temperature, pressure, and composition. Extraction with SCCO2 is more complicated than extraction using organic solvents due to the formation of carbonic acid and its derivatives. A thermodynamic model based on combined phase and reaction equilibria was developed for prediction of extraction efficiencies. In the model, the following aqueous phase reactions and three phase equilibrium relations for molecular species were considered:

aqueous phase reactions K1

(mol of HA)0 - 2(mol of Cu2+)0 2+

Figure 3. Chelate extraction equilibria.

)

CO2(aq) + H2O(l) 9 7 8 H2CO3

(mol of HA)0

H2CO3 798 HCO3- + H+

2(mol of Cu )0 2+

2(mol of Cu )0

K2

- 1 (1)

The extraction efficiency increased with increasing the initial amount of chelating agent. The increase occurred rapidly at low initial chelating agent amounts but leveled off toward some apparent maximum efficiency for each chelating agent. No significant changes in extraction efficiencies were observed with temperature and pressure at a fixed initial amount of chelating agent. The extraction efficiencies increased with decreasing the initial copper concentration. Using acidic extractants, extraction is achieved by compound formation, which is a complex process. A schematic diagram of the equilibria involved in SCCO2 extraction of a divalent copper ion (Cu2+) using an acidic

K3

HA(aq) 798 H+ + AK4

2A- + Cu2+ 798 CuA2(aq)

(2) (3) (4) (5)

phase equilibrium relations KCO

2

7 8 CO2(aq) CO2(sc) 9 KHA

HA(sc) 9 7 8 HA(aq) KCuA

2

CuA2(sc) 9 7 8 CuA2(aq)

(6) (7) (8)

where K1, K2, K3, and K4 are reaction equilibrium

5374 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Table 3. Equilibrium Constants and Distribution Coefficients for a Model of the Cu2+/TFA System temp (K)

pressure (MPa)

K1

107K2

107K3

10-9K4

105yCu(TFA)2

KCO2

KTFA

KCu(TFA)2

308 308 318 318 328 328

9.9 13.9 13.4 19.3 17.1 24.0

1.0 1.0 1.0 1.0 1.0 1.0

4.7 4.7 5.1 5.1 5.6 5.6

3.4 3.4 4.4 4.4 5.8 5.8

3.5 3.5 4.8 4.8 6.6 6.6

23.4 26.2 39.1 43.3 58.5 64.0

42.8 41.5 44.4 42.2 46.1 42.9

1.7 1.9 2.8 3.1 4.2 4.6

7.8 8.7 13.0 14.4 19.5 21.3

Table 4. Equilibrium Constants and Distribution Coefficients for a Model of the Cu2+/FOD System temp (K)

pressure (MPa)

K1

107K2

107K3

10-10K4

105yCu(FOD)2

KCO2

KFOD

KCu(FOD)2

318

13.4

1.0

5.1

3.5

6.3

141

44.4

100

2342

Figure 4. Cu2+/TFA: Comparison of experimental data and model predictions for variation of the initial TFA amount (T ) 318 K; p ) 13.4 MPa; [Cu2+]0 ) 100 ppm).

Figure 5. Cu2+/FOD: Comparison of experimental data and model predictions for variation of the initial FOD amount (T ) 318 K; p ) 13.4 MPa; [Cu2+]0 ) 100 ppm).

constants and KCO2, KHA, and KCuA2 are distribution coefficients. Reactions (2) and (3) and reactions (4) and (5) were combined as KI ) K1K2

CO2(aq) + H2O(l) 9 7 8 H+ + HCO3-

(9)

KII ) K32K4

2HA(aq) + Cu2+ 798 2H+ + CuA2

(10)

Following the work of Walas (1985) on combined reaction and phase equilibria, expressions were derived for molalities of aqueous species. Substitution of the expressions into the law of mass action for reactions (9) and (10) resulted in two equations. A computer program was developed to solve two equations for reaction extents using a modified Levenberg-Marquardt algorithm. Once the aqueous phase species molalities at equilibrium are known, the extraction efficiency can be calculated from

% extraction )

mol of CuA2(sc) at equilibrium initial mol of Cu2+

×

100 (11) For the model, the equilibrium constants at 25 °C for aqueous phase reactions (2) and (3) were obtained from Snoeyink and Jenkins (1980) and were adjusted for temperature using standard enthalpies of formation by the van’t Hoff equation. The dissociation constant at 298 K for reaction (4) for TFA was taken from Scribner et al. (1965) and that for FOD was taken from Sweet and Brengartner (1970). The dissociation constants were extrapolated to the temperatures used in this study using the van’t Hoff equation. The standard enthalpies of formation of both of the chelating agents were assumed to be the same as that of acetylacetone, which was calculated by fitting the dissociation constants of acetylacetone in the temperature range 283-

Figure 6. Cu2+/TFA: Comparison of experimental data and model predictions for variation of temperature and pressure ([Cu2+]0 ) 100 ppm; [TFA]0 ) 0.64 × 10-3 kg).

323 K (Liljenzin, 1969) to the van’t Hoff equation. The complexation constant of Cu(TFA)2 was obtained from Sekine and Ihara (1971). The complexation constant of Cu(FOD)2 was treated as an adjustable parameter in the model. The distribution coefficient of carbon dioxide, KCO2, was calculated using the solubility data for CO2 in water by Wiebe and Gaddy (1940). The distribution coefficients of copper complexes of TFA and FOD were estimated by a method suggested by Brudi et al. (1996). According to this method, the distribution coefficient of an organic compound between the SCCO2 phase and the aqueous phase can be estimated as the ratio of the solubilities of the compound in the SCCO2 phase and in the aqueous phase. The solubility of the copper-TFA complex in SCCO2 was recently reported by Lagalante et al. (1995) at 313 K and various pressures. The data were extrapolated to the conditions used in this study. The mole fraction solubility of the Cu(TFA)2 complex in water was determined in our laboratory at 318 K as 3 × 10-5. The mole fraction solubility of the copper-FOD complex was estimated

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5375 Table 5. Species Molalities (mol/kg of Solution) at 318 K and 13.4 MPa for Cu2+/TFA 103[TFA]0 (kg) species

3.81

2.54

1.27

0.64

0.32

0.13

H+ CO2 H2CO3 HCO3HA ACu2+ CuA2 pH % extraction

3.0 × 10-2 1.2 × 100 1.2 × 100 2.1 × 10-4 9.1 × 10-2 1.4 × 10-5 1.8 × 10-4 1.6 × 10-4 2.52 75.4

2.7 × 10-3 1.2 × 100 1.2 × 100 2.2 × 10-4 6.1 × 10-2 9.8 × 10-6 3.1 × 10-4 1.4 × 10-4 2.56 68.5

2.2 × 10-3 1.2 × 100 1.2 × 100 2.8 × 10-4 3.0 × 10-2 6.1 × 10-6 6.1 × 10-4 1.1 × 10-4 2.66 52.2

1.7 × 10-3 1.2 × 100 1.2 × 100 3.7 × 10-4 1.5 × 10-2 4.1 × 10-6 9.3 × 10-4 7.3 × 10-5 2.78 35.0

1.2 × 10-3 1.2 × 100 1.2 × 100 5.0 × 10-4 7.5 × 10-3 2.7 × 10-6 1.2 × 10-3 4.2 × 10-5 2.91 20.1

9.3 × 10-4 1.2 × 100 1.2 × 100 6.7 × 10-4 3.1 × 10-3 1.5 × 10-6 1.4 × 10-3 1.5 × 10-5 3.03 7.1

Table 6. Species Molalities (mol/kg of Solution) at 318 K and 13.4 MPa for Cu2+/FOD 103[FOD]0 (kg) species

3.82

2.55

1.27

0.64

0.26

H+ CO2 H2CO3 HCO3HA ACu2+ CuA2 pH % extraction

2.9 × 10-3 1.2 × 100 1.2 × 100 2.1 × 10-4 2.1 × 10-3 2.6 × 10-7 2.3 × 10-4 9.6 × 10-7 2.54 81.9

2.6 × 10-3 1.2 × 100 1.2 × 100 2.4 × 10-4 1.4 × 10-3 1.9 × 10-7 3.8 × 10-4 8.5 × 10-7 2.58 72.8

2.0 × 10-3 1.2 × 100 1.2 × 100 3.0 × 10-4 6.8 × 10-4 1.2 × 10-7 7.0 × 10-4 6.2 × 10-7 2.69 53.1

1.5 × 10-3 1.2 × 100 1.2 × 100 4.1 × 10-4 3.4 × 10-4 7.9 × 10-8 1.0 × 10-3 4.0 × 10-7 2.82 34.2

1.1 × 10-3 1.2 × 100 1.2 × 100 5.8 × 10-4 1.4 × 10-4 4.6 × 10-8 1.3 × 10-3 1.7 × 10-7 2.97 14.9

as 91 × 10-5 at 313 K and 23.4 MPa using the linear relationship given by Lagalante et al. (1995) between the natural log of solubility in SCCO2 and the Fedors’ solubility parameter of the deprotonated β-diketone ligands. The solubility parameter of the enolate form of FOD was calculated as 9.32 cal1/2 cm-3/2 using a group contribution method (Barton, 1983). The data were extrapolated to the conditions used in this study. The mole fraction solubility of the Cu(FOD)2 complex in water at 318 K was determined in our laboratory as 4 × 10-7. The distribution coefficients for TFA and FOD were measured at 318 K and 13.4 MPa by charging the extraction vessel with water, CO2, and a certain amount of chelating agent. The system was equilibrated, the phases were allowed to separate, and the water phase was sampled and analyzed for TFA or FOD concentration. Using the measured water phase concentration and a mass balance on the chelating agent, the distribution coefficient was calculated. The distribution coefficients were extrapolated to other conditions assuming temperature and pressure dependencies similar to those of the distribution coefficients of the copper complexes. The equilibrium constants and distribution coefficients are given in Tables 3 and 4 as functions of temperature and pressure. Comparisons of experimental and predicted extraction efficiencies for the systems investigated are given in Figures 4-6. Considering the extrapolations involved in thermophysical property estimation, the agreement between the model results and experimental data is good, which indicates that such a model captures the chemistry involved in extraction of metals from aqueous streams by chelation in SCCO2. The species molalilities calculated by the model are given in Table 5 for Cu2+/TFA and Table 6 for Cu2+/ FOD. As the amount of chelating in the system increases, the amount of chelating agent in the water also increases. This increase in HA is naturally accompanied by generation of more A- ions available for complexation. Consequently, extraction efficiencies increase with increasing the amount of chelating agent in the system. As more copper is removed into the SCCO2

phase in the form of CuA2, equilibrium in reaction (10) is shifted to the right, generating H+ ions. Consequently, there is a decrease in pH with an increase in extraction efficiencies. As the amount of chelating agent decreases, the pH approaches a value of 3.1 for an aqueous solution in contact with SCCO2. This value compares well with the experimental value of 2.9 determined by Toews et al. (1995). The leveling of extraction efficiencies with an increase in the amount of chelating agent is also predicted by the model. As shown in Figure 6, no significant changes occur with pressure at a constant temperature since increasing the pressure increases the distribution coefficients of both the chelating agent and the copper chelate complex. Since these two distribution coefficients have opposite effects on extraction efficiencies, no significant changes are observed. At a constant density, the slight increase of extraction efficiencies with temperature can possibly be attributed to the increase of equilibrium constants of reactions (3) and (4). For the Cu2+/TFA system, at high concentrations of TFA, a significant fraction of copper exists in the Cu(TFA)2 form. At comparable stochiometric excess amounts, the concentration of FOD in the aqueous phase is about 2 orders of magnitude less than the concentration of TFA in the aqueous phase. Therefore, FOD would be the preferred chelating agent on a large-scale process due to an insignificant loss to the aqueous phase. Literature Cited Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL, 1983. Brudi, K.; Dahmen, N.; Schmieder, H. Partition Coefficients of Organic Substances in Two-Phase Mixtures of Water and Carbon Dioxide at Pressures of 8 to 30 MPa and Temperatures of 313 to 333 K. J. Supercrit. Fluids 1996, 9, 146. Cox, M. Industrial Applications of Solvent Extraction. In Developments in Solvent Extraction; Alegret, S., Ed.; John Wiley & Sons: New York, 1988. Cox, M. Solvent Extraction in Hydrometallurgy. In Principles and Practices of Solvent Extraction; Rydberg, J., Musikas, C., Choppin, G. R., Eds.; Marcel Dekker, Inc.: New York, 1992.

5376 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Flett, D. S.; Melling, J.; Cox, M. Commercial Solvent Systems for Inorganic Processes. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.; John Wiley & Sons: New York, 1983. Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the Examination of Water and Wastewater, 18th ed.; American Public Health Association, American Water Works Association, Water Environment Federation: Washington, DC, 1992. Lagalante, A. F.; Hansen, B. N.; Bruno, T. J.; Sievers, R. E. Solubilities of Copper(II) and Chromium(III) β-Diketonates in Supercritical Carbon Dioxide. Inorg. Chem. 1995, 34, 5781. Laintz, K. E.; Tachikawa, E. Extraction of Lanthanides from Acidic Solutions Using Tributyl Phosphate Modified Supercritical Carbon Dioxide. Anal. Chem. 1994, 66, 2190. Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Solubility of Fluorinated Metal Diethyldithiocarbamates in Supercritical Carbon Dioxide. J. Supercrit. Fluids 1991, 4, 194. Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Extraction of Metal Ions from Liquid and Solid Materials by Supercritical Carbon Dioxide. Anal. Chem. 1992, 64, 2875. Liljenzin, J. O. The Dissociation Constant for Acetylacetone in 1 M Sodium Perchlorate Solution at 15-45 °C. Acta Chem. Scand. 1969, 23, 3592. Lin, Y.; Wai, C. M. Supercritical Fluid Extraction of Lanthanides with Fluorinated β-Diketones and Tributyl Phosphate. Anal. Chem. 1994, 66, 1971-1975. Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Supercritical Fluid Extraction of Thorium and Uranium Ions from Solid and Liquid Materials with Fluorinated β-Diketones and Tributyl Phosphate. Environ. Sci. Technol. 1994, 28, 1190. Murphy, J. M.; Erkey, C. Thermodynamics of Extraction of Copper(II) from Aqueous Solutions by Chelation in Supercritical Carbon Dioxide. Environ. Sci. Technol. 1997, 31, 1674. Pratt, M. W. T. Cost of Process. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.; John Wiley & Sons: New York, 1983.

Scribner, W. G.; Treat, W. J.; Weis, J. D.; Moshier, R. W. Solvent Extraction of Metal Ions with Trifluoroacetylacetone. Anal. Chem. 1965, 37, 1137. Sekine, T.; Ihara, N. Studies of the Liquid-Liquid Partition Systems. VIII. Stabilities and Extractabilities of Copper(II) and Zinc(II) Complexes with Acetylacetone, Trifluoroacetylacetone, and Hexafluoroacetylacetone in Aqueous Sodium Perchlorate Solution-Carbon Tetrachloride Systems. Bull. Chem. Soc. Jpn. 1971, 44, 2942. Snoeyink, V. L.; Jenkins, D. Water Chemistry; John Wiley & Sons, Inc.: New York, 1980. Sweet, T. R.; Brengartner, D. The Formation Constants and Solvent Extraction of Lanthanide Complexes of 1,1,1,2,2,3,3Heptafluoro-7,7-Dimethyl-4,6-Octanedione. Anal. Chim. Acta 1970, 52, 173. Toews, K. L.; Shroll, R. M.; Wai, C. M. pH-Defining Equilibrium Between Water and Supercritical CO2. Influence on SFE of Organics and Metal Chelates. Anal. Chem. 1995, 67, 4040. Walas, S. M. Phase Equilibria in Chemical Engineering; Butterworth Publishers: Boston, 1985. Wang, J.; Marshall, W. D. Recovery of Metals from Aqueous Media by Extraction with Supercritical Carbon Dioxide. Anal. Chem. 1994, 66, 1658. Wiebe, R.; Gaddy, V. L. The Solubility of Carbon Dioxide in Water at Various Temperatures from 12 to 40° and Pressures to 500 Atmospheres. Critical Phenomena. J. Am. Chem. Soc. 1940, 62, 815.

Received for review June 30, 1997 Revised manuscript received September 17, 1997 Accepted September 22, 1997X IE970458I

X Abstract published in Advance ACS Abstracts, November 1, 1997.