Copper(II) Removal from Aqueous Solutions by Chelation inSupercritical Carbon Dioxide Using Fluorinated â-Diketones

Jennifer M. Murphy† and Can Erkey*,‡

Departments of Civil and Environmental Engineering and of Chemical Engineering, EnvironmentalEngineering Program, University of Connecticut, Storrs, Connecticut 06269

Copper removal from aqueous solutions by chelation in supercritical carbon dioxide wasinvestigated. The chelating agents were 1,1,1-trifluoroacetylacetone (TFA) and 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (FOD). Extraction efficiencies were determined for asingle equilibrium stage in the temperature range 308-328 K and the pressure range 9.9-24.0MPa and ranged from 4% to 84%. The extraction efficiencies increased with increasing theinitial amount of chelating agent and with decreasing the initial copper concentration. Nosignificant changes in extraction efficiencies were observed with temperature and pressure ata fixed initial amount of chelating agent. A thermodynamic model based on combined reactionand phase equilibria was also developed for prediction of extraction efficiencies.

Introduction

Solvent extraction is a well-established process withinthe inorganic chemical and hydrometallurgical indus-tries for transfer of cations or anions from an aqueousphase into an organic phase (Cox, 1992). As directextraction of ionic species is energetically very unfavor-able, transfer is accomplished by forming organic-soluble neutral complexes in the aqueous phase byreactions between the ionic species of interest and anappropriate organic compound (reagent). However,since the viscosities of many reagents are much too highfor direct use in solvent extraction equipment and forfacilitation of phase separation, it is common practiceto 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 evapora-tion loss and the risk of fire) and a density of about 800kg/m3 to aid phase separation. In general, the solventsused contain a mixture of paraffinic, aromatic, andnaphthenic hydrocarbons. The major oil companieshave also developed solvent mixtures specifically for usein solvent extraction of metals (Flett et al., 1983).Solvent selection for a particular application is basedon many factors such as solvent strength, selectivity,loading, ease of stripping, rates of extraction and strip-ping, chemical stability, aqueous phase solubility ofsolvent components, volatility and flammability of thesolvent, 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 phaseduring loading, the ratio of the aqueous to the organicvolume cannot usually be more than about 10. Thisleads to the use of large volumes of solvent, particularlywhen the feed is lean. The fact that significant quanti-ties of solvent are lost by entrainment reinforces thislimit. Consequently, one of the significant operatingcosts in hydrometallurgical separations involving sol-vent extraction is solvent recovery cost due to largevolumes of solvent that need to be processed (Pratt,

1983). This adverse effect can possibly be eliminatedby 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 comparedto mass-transfer processes and, depending on hydrody-namic conditions in the extraction vessel, the observedkinetics of removal may be controlled by mass-transfer.Since the mass-transfer characteristics of SCFs areexcellent compared to those of organic solvents due theirrelatively low viscosities and high solute diffusivities,the use of SCFs in place of organic solvents mayenhance rates of extraction and stripping. The nontoxicnature and relatively low critical temperature (304 K)and pressure (7.38 MPa) of carbon dioxide make it anattractive choice for this application. Other notableadvantages of supercritical carbon dioxide (SCCO2) arethat it is nonflammable, relatively inexpensive, andreadily available. In addition, the fact that the solvencycharacteristics of supercritical fluids can be varied withsmall changes in temperature and pressure may beexploited in the development of selective extractionschemes.As a result of these favorable solvency properties of

SCFs, some research and development work has beenconducted in various laboratories on the removal ofheavy metals from aqueous solutions by chelation inSCCO2. These studies began with the pioneering workof Laintz et al. (1991, 1992), who investigated theextraction of copper(II) from an aqueous solution bychelation with bis(trifluoroethyl)dithiocarbamate usinga dynamic extraction scheme. Nearly 100% of the metalwas removed from the aqueous sample after 1 h at aCO2 density of 500 kg/m3 and a temperature of 308 K.The authors indicated that the use of the fluorinatedchelating agent yielded much better extraction resultsthan the use of a nonfluorinated analogue, diethyldi-thiocarbamate. This was attributed to enhanced solu-bility of the complexes in SCCO2 due to the CO2-philicfluoroalkyl groups. Wang and Marshall (1994) inves-tigated removal of zinc, cadmium, and lead from aque-ous solutions using tetrabutylammonium dibutyldithio-carbamate as the chelating agent. Nearly completemetal extractions were achieved within 60 min at 323K and 24.3 MPa. Lin et al. (1994) studied the extractionof U(VI) and Th(IV) ions from synthetic aqueous solu-tions and of U(VI) from mine waters at 333 K and 15.2

* Author to whom all correspondence should be addressed.E-mail: cerkey@eng2.uconn.edu. Telephone: (860) 486-4601.Fax: (860) 486-2959.

† Department of Civil and Environmental Engineering.‡ Department of Chemical Engineering.

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

S0888-5885(97)00458-2 CCC: $14.00 © 1997 American Chemical Society

MPa using thenoyltrifluoroacetone (TTA) as the chelat-ing agent. The extraction efficiencies ranged from 38to 90% and were significantly enhanced by the additionof tributyl phosphate (TBP) to the system. TTA wasalso utilized to extract trivalent lanthanides from acidicsolutions at 35.5 MPa and 333 K with extractionefficiencies ranging from 17 to 92% depending on thetype of metal and the percentage of TBP in the SCCO2phase (Laintz and Tachikawa, 1994). For all eightmetals extracted (La3+, Ce3+, Sm3+, Eu3+, Gd3+, Dy3+,Yb3+, and Lu3+), an increase in the percentage of TBPresulted in a corresponding increase in the extractionefficiency. Lin and Wai (1994) also reported on extrac-tion of La3+, Eu3+, and Lu3+ from aqueous solutions at333 K and 15.2 MPa, using TTA and TBP. They alsostudied the synergistic effects of using mixed ligandsduring extraction and found enhancement of extractionefficiencies up to 48% on the addition of TBP to thesystem containing TTA. Recently, we have also re-ported on thermodynamics of extraction of copper fromaqueous solutions using hexafluoroacetylacetone as thechelating agent (Murphy and Erkey, 1997).The objectives of the present study were 2-fold. The

first one was to investigate the applicability of fluori-nated â-diketones to extraction of copper from aqueousstreams. The study was focused on copper since thereare many solvent extraction plants in operation through-out the world producing about 700 tons of copper/day(Cox, 1988). SCCO2 may be an alternative to commonlyused organic solvents in these plants. The secondobjective was to investigate the applicability of ourpreviously developed thermodynamic model for extrac-tion of copper using hexafluoroacetylacetone to otherfluorinated â-diketones. Such thermodynamic modelsare necessary for successful implementation of thistechnology on an industrial scale. The chelating agentsused in the study were 1,1,1-trifluoroacetylacetone(TFA) and 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (FOD). The effects of temperature, pres-sure, initial metal concentration, and initial chelatingagent amount on extraction efficiencies in a singleequilibrium stage were investigated.

Experimental Section

Extraction runs were performed in a batch extractionvessel which is a part of the apparatus shown in Figure1. The extractor was a 0.3 L stainless steel autoclave(Autoclave Engineers Inc.) equipped with a pressuregauge and a Magnedrive II mixing device. Liquidcarbon dioxide was charged into the vessel using a high-pressure syringe pump (ISCO, 260D) equipped with a

cooling jacket. For temperature control, the vessel wasimmersed in a water bath equipped with an immersioncirculator (Julabo) having an accuracy of (0.1 K.Temperature was monitored using a thermocouplemeter (DP41-TC-MDSS, Omega Engineering), accurateto (0.1 K, and a T-Type thermocouple (Omega Engi-neering) inserted into a thermowell which extendeddeep into the extraction vessel. Two sampling lineswere installed on the extractor. One sampling lineextended deep into the extractor for sampling theaqueous phase, and a shorter one was used for samplingthe less dense supercritical phase. For removal of theorganic chelating agent, an activated carbon bed wasinstalled on the CO2 vent line which was connected toa 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 placedinto the vessel. The reactor was sealed, charged withCO2, and stirred at constant temperature and pressurefor an equilibration period that was unique to eachchelating agent. The stirrer was then shut off, and thephases were allowed to separate for 1 h. The separatephases were sampled using the two sampling lines.Aqueous phase samples were analyzed directly forcopper ion concentration using a Milton Roy Spectronic601 spectrophotometer and the Bathocuproine Method,3500-Cu E (Greenberg et al., 1992). Supercritical phasesamples required a few preliminary steps before theycould be analyzed for copper concentration. The con-tents of the supercritical phase sample loop were firstwashed with ethanol, which dissolved the copper com-plex and the chelating agent. The ethanol was thenevaporated from the sample vial on a hot plate. Thedry vial was filled with 10 g of deionized water, and 200µL of 1:1 HCl was added. The low pH of the solutioncaused reversal of the copper complex formation reac-tion. Nitrogen was then bubbled through the samplesto remove the dissolved chelating agent before additionof the 100 µL portions of sodium citrate, hydroxylaminehydrochloride, and disodium bathocuproine disulfonatesolutions and analysis by the Bathocuproine Method.Using the aqueous concentrations obtained, the super-critical phase concentrations were backcalculated usinga measured sample loop volume of 1200 µL. Massbalance closures were better than 90%.Determination of the necessary equilibration time was

done by trial. For each chelating agent, samples weretaken periodically during an extraction run. The datawere then analyzed to determine the path of approachto equilibrium. Figure 2 shows the evolution of copper-

Figure 1. Schematic diagram of the experimental apparatus.

Figure 2. Path of approach to equilibrium for the Cu2+/TFAsystem ([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).

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

(II) concentration as a function of time for both of thechelating agents. The experiments indicated that theequilibration time for TFA at 328 K and 15.8 MPa wasaround 40 min. Therefore, all extraction runs usingTFA were performed for an equilibration period of atleast 1 h. For FOD, the equilibration time at 318 K and13.4 MPa was about 20 min, so the extractions withFOD were conducted using equilibration times between30 and 45 min. The faster equilibration observed withFOD can possibly be attributed to its lower solubilityin water.Aqueous copper solutions were prepared gravimetri-

cally by dissolving copper nitrate (Aldrich Chemical Co.)in deionized water. Carbon dioxide (99%) was pur-chased from Connecticut Airgas Inc. TFA and FODwere obtained from Acros Organics. The analyticalchemicals (hydrochloric acid, hydroxylamine hydrochlo-ride, sodium citrate, and disodium bathocuproine di-sulfonate) as well as the Cu(TFA)2 complex were re-ceived from Aldrich Chemical Co. The Cu(FOD)2 com-plex was purchased from Gelest, Inc. All chemicalswere used as received without further purification.

Results and Discussion

The amount of chelating agent (HA) placed in theextractor (volume: 0.3 L) for each run ranged from 0.13to 3.81 g for TFA and from 0.26 to 3.81 g for FOD at318 K and 13.8 MPa. Each chelating agent amountcorresponds to a stoichiometric excess which is given,along with the experimental results, in Tables 1 and 2.The stoichiometric excess, E, was defined as

The extraction efficiency increased with increasingthe initial amount of chelating agent. The increaseoccurred rapidly at low initial chelating agent amountsbut leveled off toward some apparent maximum ef-ficiency for each chelating agent. No significant changesin extraction efficiencies were observed with tempera-ture and pressure at a fixed initial amount of chelatingagent. The extraction efficiencies increased with de-creasing the initial copper concentration.Using acidic extractants, extraction is achieved by

compound formation, which is a complex process. Aschematic diagram of the equilibria involved in SCCO2extraction of a divalent copper ion (Cu2+) using an acidic

chelating agent (HA) is given in Figure 3. The distribu-tion of the metal between the two phases is governedby the equilibrium constants of the aqueous phase reac-tions and the distribution coefficients of the molecularspecies. Both the reaction equilibrium constants andthe distribution coefficients are dependent on temper-ature, pressure, and composition. Extraction withSCCO2 is more complicated than extraction using or-ganic solvents due to the formation of carbonic acid andits derivatives. A thermodynamic model based on com-bined phase and reaction equilibria was developed forprediction of extraction efficiencies. In the model, thefollowing aqueous phase reactions and three phase equi-librium relations for molecular species were considered:

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

Table 1. Variation of Extraction Efficiency withTemperature, Pressure, and Initial Amount of TFA for[Cu2+]0 ) 100 ppm

temp(K)

pressure(MPa)

density(kg‚m-3)

103[TFA]0(kg)

stoichiometricexcess

%extraction

318 13.4 700 3.81 77.6 70318 13.4 700 2.54 51.4 70318 13.4 700 1.27 25.2 46318 13.4 700 0.64 12.2 31318 13.4 700 0.32 5.6 11318 13.4 700 0.13 1.7 4308 9.9 700 0.64 12.2 25308 13.9 800 0.64 12.2 31318 13.4 700 0.64 12.2 32318 19.3 800 0.64 12.2 33328 17.1 700 0.64 12.2 37328 24.0 800 0.64 12.2 35 Figure 3. Chelate extraction equilibria.

Table 2. Variation of Extraction Efficiency withTemperature, Pressure, and Initial Amount of FOD

temp(K)

pressure(MPa)

density(kg‚m-3)

103[FOD]0(kg)

stoichiometricexcess

%extraction

[Cu2+]0 ) 100 ppm318 13.4 700 3.82 40.0 84318 13.4 700 2.55 26.4 78318 13.4 700 1.27 12.6 50318 13.4 700 0.64 5.8 33318 13.4 700 0.26 1.7 10308 13.9 800 1.27 12.2 56318 19.3 800 1.27 12.2 55328 24.0 800 1.27 12.2 56

[Cu2+]0 ) 70 ppm35 13.9 800 1.27 12.2 6845 19.3 800 1.27 12.2 6855 24.0 800 1.27 12.2 67

aqueous phase reactions

CO2(aq) + H2O(l) 798K1

H2CO3 (2)

H2CO3 798K2

HCO3- + H+ (3)

HA(aq) 798K3

H+ + A- (4)

2A- + Cu2+ 798K4

CuA2(aq) (5)

phase equilibrium relations

CO2(sc) 798KCO2

CO2(aq) (6)

HA(sc) 798KHA

HA(aq) (7)

CuA2(sc) 798KCuA2

CuA2(aq) (8)

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

2(mol of Cu2+)0)

(mol of HA)02(mol of Cu2+)0

- 1 (1)

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5373

constants and KCO2, KHA, and KCuA2 are distributioncoefficients. Reactions (2) and (3) and reactions (4) and(5) were combined as

Following the work of Walas (1985) on combinedreaction and phase equilibria, expressions were derivedfor molalities of aqueous species. Substitution of theexpressions into the law of mass action for reactions (9)and (10) resulted in two equations. A computer pro-gram was developed to solve two equations for reactionextents using a modified Levenberg-Marquardt algo-rithm. Once the aqueous phase species molalities atequilibrium are known, the extraction efficiency can becalculated from

For the model, the equilibrium constants at 25 °C foraqueous phase reactions (2) and (3) were obtained fromSnoeyink and Jenkins (1980) and were adjusted fortemperature using standard enthalpies of formation bythe van’t Hoff equation. The dissociation constant at298 K for reaction (4) for TFA was taken from Scribneret al. (1965) and that for FOD was taken from Sweetand Brengartner (1970). The dissociation constantswere extrapolated to the temperatures used in thisstudy using the van’t Hoff equation. The standardenthalpies of formation of both of the chelating agentswere assumed to be the same as that of acetylacetone,which was calculated by fitting the dissociation con-stants of acetylacetone in the temperature range 283-

323 K (Liljenzin, 1969) to the van’t Hoff equation. Thecomplexation constant of Cu(TFA)2 was obtained fromSekine and Ihara (1971). The complexation constantof Cu(FOD)2 was treated as an adjustable parameterin the model. The distribution coefficient of carbondioxide, KCO2, was calculated using the solubility datafor CO2 in water by Wiebe and Gaddy (1940). Thedistribution coefficients of copper complexes of TFA andFOD were estimated by a method suggested by Brudiet al. (1996). According to this method, the distributioncoefficient of an organic compound between the SCCO2phase and the aqueous phase can be estimated as theratio of the solubilities of the compound in the SCCO2phase and in the aqueous phase. The solubility of thecopper-TFA complex in SCCO2 was recently reportedby Lagalante et al. (1995) at 313 K and various pres-sures. The data were extrapolated to the conditionsused in this study. The mole fraction solubility of theCu(TFA)2 complex in water was determined in ourlaboratory at 318 K as 3 × 10-5. The mole fractionsolubility of the copper-FOD complex was estimated

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 9.9 1.0 4.7 3.4 3.5 23.4 42.8 1.7 7.8308 13.9 1.0 4.7 3.4 3.5 26.2 41.5 1.9 8.7318 13.4 1.0 5.1 4.4 4.8 39.1 44.4 2.8 13.0318 19.3 1.0 5.1 4.4 4.8 43.3 42.2 3.1 14.4328 17.1 1.0 5.6 5.8 6.6 58.5 46.1 4.2 19.5328 24.0 1.0 5.6 5.8 6.6 64.0 42.9 4.6 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 andmodel predictions for variation of the initial TFA amount (T )318 K; p ) 13.4 MPa; [Cu2+]0 ) 100 ppm).

CO2(aq) + H2O(l) 798KI ) K1K2

H+ + HCO3- (9)

2HA(aq) + Cu2+ 798KII ) K3

2K42H+ + CuA2 (10)

% extraction )mol of CuA2(sc) at equilibrium

initial mol of Cu2+ ×100 (11)

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

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

5374 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

as 91 × 10-5 at 313 K and 23.4 MPa using the linearrelationship given by Lagalante et al. (1995) betweenthe natural log of solubility in SCCO2 and the Fedors’solubility parameter of the deprotonated â-diketoneligands. The solubility parameter of the enolate formof FOD was calculated as 9.32 cal1/2 cm-3/2 using a groupcontribution method (Barton, 1983). The data wereextrapolated to the conditions used in this study. Themole fraction solubility of the Cu(FOD)2 complex inwater at 318 K was determined in our laboratory as 4× 10-7. The distribution coefficients for TFA and FODwere measured at 318 K and 13.4 MPa by charging theextraction vessel with water, CO2, and a certain amountof chelating agent. The system was equilibrated, thephases were allowed to separate, and the water phasewas sampled and analyzed for TFA or FOD concentra-tion. Using the measured water phase concentrationand a mass balance on the chelating agent, the distribu-tion coefficient was calculated. The distribution coef-ficients were extrapolated to other conditions assumingtemperature and pressure dependencies similar to thoseof the distribution coefficients of the copper complexes.The equilibrium constants and distribution coefficientsare given in Tables 3 and 4 as functions of temperatureand pressure.Comparisons of experimental and predicted extraction

efficiencies for the systems investigated are given inFigures 4-6. Considering the extrapolations involvedin thermophysical property estimation, the agreementbetween the model results and experimental data isgood, which indicates that such a model captures thechemistry involved in extraction of metals from aqueousstreams 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 in-creases, the amount of chelating agent in the water alsoincreases. This increase in HA is naturally accompa-nied by generation of more A- ions available for com-plexation. Consequently, extraction efficiencies increasewith increasing the amount of chelating agent in thesystem. 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. Conse-quently, there is a decrease in pH with an increase inextraction efficiencies. As the amount of chelating agentdecreases, the pH approaches a value of 3.1 for anaqueous solution in contact with SCCO2. This valuecompares well with the experimental value of 2.9determined by Toews et al. (1995). The leveling ofextraction efficiencies with an increase in the amountof chelating agent is also predicted by the model. Asshown in Figure 6, no significant changes occur withpressure at a constant temperature since increasing thepressure increases the distribution coefficients of boththe chelating agent and the copper chelate complex.Since these two distribution coefficients have oppositeeffects on extraction efficiencies, no significant changesare observed. At a constant density, the slight increaseof extraction efficiencies with temperature can possiblybe attributed to the increase of equilibrium constantsof reactions (3) and (4). For the Cu2+/TFA system, athigh concentrations of TFA, a significant fraction ofcopper exists in the Cu(TFA)2 form. At comparablestochiometric excess amounts, the concentration of FODin the aqueous phase is about 2 orders of magnitudeless than the concentration of TFA in the aqueousphase. Therefore, FODwould be the preferred chelatingagent on a large-scale process due to an insignificantloss to the aqueous phase.

Literature Cited

Barton, A. F. M. CRC Handbook of Solubility Parameters andOther Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL,1983.

Brudi, K.; Dahmen, N.; Schmieder, H. Partition Coefficients ofOrganic Substances in Two-Phase Mixtures of Water andCarbon Dioxide at Pressures of 8 to 30 MPa and Temperaturesof 313 to 333 K. J. Supercrit. Fluids 1996, 9, 146.

Cox, M. Industrial Applications of Solvent Extraction. In Develop-ments in Solvent Extraction; Alegret, S., Ed.; John Wiley &Sons: New York, 1988.

Cox, M. Solvent Extraction in Hydrometallurgy. In Principles andPractices of Solvent Extraction; Rydberg, J., Musikas, C.,Choppin, G. R., Eds.; Marcel Dekker, Inc.: New York, 1992.

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+ 3.0 × 10-2 2.7 × 10-3 2.2 × 10-3 1.7 × 10-3 1.2 × 10-3 9.3 × 10-4

CO2 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100H2CO3 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100HCO3

- 2.1 × 10-4 2.2 × 10-4 2.8 × 10-4 3.7 × 10-4 5.0 × 10-4 6.7 × 10-4

HA 9.1 × 10-2 6.1 × 10-2 3.0 × 10-2 1.5 × 10-2 7.5 × 10-3 3.1 × 10-3

A- 1.4 × 10-5 9.8 × 10-6 6.1 × 10-6 4.1 × 10-6 2.7 × 10-6 1.5 × 10-6

Cu2+ 1.8 × 10-4 3.1 × 10-4 6.1 × 10-4 9.3 × 10-4 1.2 × 10-3 1.4 × 10-3

CuA2 1.6 × 10-4 1.4 × 10-4 1.1 × 10-4 7.3 × 10-5 4.2 × 10-5 1.5 × 10-5

pH 2.52 2.56 2.66 2.78 2.91 3.03% extraction 75.4 68.5 52.2 35.0 20.1 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+ 2.9 × 10-3 2.6 × 10-3 2.0 × 10-3 1.5 × 10-3 1.1 × 10-3

CO2 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100H2CO3 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100 1.2 × 100HCO3

- 2.1 × 10-4 2.4 × 10-4 3.0 × 10-4 4.1 × 10-4 5.8 × 10-4

HA 2.1 × 10-3 1.4 × 10-3 6.8 × 10-4 3.4 × 10-4 1.4 × 10-4

A- 2.6 × 10-7 1.9 × 10-7 1.2 × 10-7 7.9 × 10-8 4.6 × 10-8

Cu2+ 2.3 × 10-4 3.8 × 10-4 7.0 × 10-4 1.0 × 10-3 1.3 × 10-3

CuA2 9.6 × 10-7 8.5 × 10-7 6.2 × 10-7 4.0 × 10-7 1.7 × 10-7

pH 2.54 2.58 2.69 2.82 2.97% extraction 81.9 72.8 53.1 34.2 14.9

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5375

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Received for review June 30, 1997Revised manuscript received September 17, 1997

Accepted September 22, 1997X

IE970458I

X Abstract published in Advance ACS Abstracts, November1, 1997.

5376 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997