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Affinity Extraction into CO2. 2. Extraction of Heavy Metals into CO2 from Low-pH Aqueous Solutions J. Li and E. J. Beckman* Chemical Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
Because it is an environmentally benign solvent, CO2 is increasingly being evaluated as a medium with which to perform liquid-liquid extraction from water (unlike conventional organic solvents, no contamination of the aqueous phase results). A number of researchers have explored the design of highly CO2-soluble chelating agents to allow extraction of metal ions, yet the low pH of water in contact with CO2 (2.8-3.0) can lead to poor extraction efficiencies with many common chelating agents. In this paper, we describe the generation of a CO2-soluble analogue to ammonium pyrrolidinedithiocarbamate, an agent which has been shown to effectively extract a variety of metal ions at pH’s as low as 1.0. We have found that the CO2-soluble analogue can extract high fractions of copper, nickel, and cadmium, although, like conventional APDC, extraction of chromium was less successful. Introduction Carbon dioxide has elicited significant scientific interest over the past 15 years because it is considered a “green” alternative to conventional organic liquids. CO2 is inexpensive (approximately $80/ton, 1-2 orders of magnitude less than conventional solvents), is nonflammable, and is relatively nontoxic. Carbon dioxide is not currently regulated as a volatile organic chemical by the U.S. EPA nor is its use restricted in food or pharmaceutical applications by the U.S. FDA. CO2’s inherent “green” properties make it particularly well suited for use in liquid-liquid extraction from water. In such a situation, any organic solvent will contaminate the water to a certain degree, yet in the case of CO2, this “contamination” obviously does not require remediation. A significant obstacle to application of CO2 to conventional chemical processes is its low solvent power. Although its solvent power was once suggested to be comparable to that of liquid alkanes, recent research has shown that this generalization is in error.1,2 Calculation using the heat of vaporization and the molar volume produces solubility parameters for CO2 of 4-5 cal/cm3 in the liquid state at temperatures between 0 and 25 °C, similar to that for fluorinated materials and slightly lower than that for silicones. It is accepted today that CO2 will not solubilize significant quantities of polar, high molecular weight, or ionic compounds. Low solubilities of compounds of interest require large volumes of CO2 in a potential process, and thus, the chance for favorable economics diminishes. This is particularly true for metals and metal chelates. It is not surprising that metal ions exhibit negligible solubility in a low dielectric medium such as CO2, but indeed, previous work has shown that alkyl-functional metal chelates also exhibit low solubilities in CO2 at moderate pressure.3 Research during the 1990s by a number of groups has clearly shown that use of so-called CO2-philic functional groups in the design of molecules allows high substrate * To whom correspondence should be addressed. Telephone: 412-624-9641. Fax: 412-624-9639. E-mail: beckman@ vms.cis.pitt.edu.
compositions in CO2 at relatively moderate pressures. At the present time, the list of CO2-philic functional groups includes fluoroethers, fluoroacrylates, fluoroalkyls, silicones, and certain phosphazenes, although it is not always clear why some functional groups are CO2philic and others are not. Over the past 5 years, several research groups have employed or generated CO2-philic chelating agents to allow extraction of metals into carbon dioxide. Laintz et al.3 have employed fluorinated β-diketones (with and without tributyl phosphate) to extract a variety of metals from both solid and liquid matrixes into carbon dioxide. Laintz et al.4 have also synthesized bis(trifluoroethyl) dithiocarbamate, which exhibits significantly higher CO2 solubility (of the order of 10-4 mol/L) than its nonfluorinated counterpart (of the order of 10-6-10-7 mol/L). Subsequent extractions in CO2 showed that the resulting chelating agent binds a variety of metals (copper, arsenic, antimony, and some lanthanides and actinides) in concentration ranges of the order of 10-4 mol/L of CO2. More recent work has shown that a fluorinated dithiocarbamate (a trifluoroethyl-based material) chelating agent as well as several fluorinated β-diketones can be used to extract heavy metals such as Cu2+, Co2+, Pb2+, Cd2+, and Zn2+ from water and soil samples by using a 1000-10000 molar ratio of chelate to metal with efficiencies as high as 98%. Dehghani and co-workers,5 as well as Smart et al.,29 have also employed organophosphates to extract metals into CO2, while Erkey’s group6 has employed fluorinated β-diketones for this purpose. In our previous work,7,8 we generated fluoroether-functional analogues to conventional dithiocarbamates, dithiols, and picolylamines and extracted arsenic, mercury, and lead from solid matrixes at yields over 90% using 1.5 mol of agent per mole of metal. However, in some extractions from water, we found that the low pH8 generated upon contacting water with high-pressure CO2 led to lower extraction yields from aqueous solution than from, for example, a solid matrix like sand. In the case of arsenic extractions, the efficiency of extraction from water was always less than that from sand, for all of the chelate types investigated. Interestingly, Lagalante and colleagues9 showed that proper design of an alkyl-functional β-diketone to effectively screen the unfavorable
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diaminetetraacetic acid) and examined its ability to extract metals at low pH’s as well. Figure 1. Structure of perfluoropolyether piperazinedithiocarbamate.
metal-CO2 and polar group-CO2 interactions allows reasonable solubility without resorting to use of CO2philic groups. Given previous difficulties in extractions from water, the object of this work was to synthesize a CO2-soluble chelating agent through which the metal ions Cu2+, Ni2+, Zn2+, Cd2+, and Cr6+/3+ could be efficiently extracted into CO2 from water at low pH’s ( Pd2+ > Cu2+ > Ti3+ > Ni2+ > Bi3+ > Co3+ > Co2+ > Ti+ > Zn2+ > In2+ > Sb3+ > Fe3+ > Te4+ >Mn3+ > Mn2+. It has been shown that APDC is much more stable than ammonium diethyldithiocarbamate (ADDC) in forming chelates under acidic conditions.20 Previous work has shown that ADDC decomposes under strongly acidic conditions.21 APDC has been used to extract metal ions (Co, Cu, Ni, Zn, Cd, Fe, Pb, Ag) into organic solvent under acidic conditions (pH ) 1.0-6.0).22 Further, Fe, Co, Cu, and Ni were quantitatively extracted into diisobutyl ketone using APDC as the chelating agent at the optimum pH of 2.3 for trace metal analysis in ZBLAN fluoride glasses.13 APDC was also employed in the extraction of trace elements (Sb, In, Cu, Co, Cd, Hg, Zn) into organic solvent at high extraction efficiencies (>90%) for ultratrace elemental characterization of high-purity materials.11 The APDC-MIBK (methyl isobutyl ketone) system enabled the determination of Cd2+, Co2+, Cu2+, Ni2+, Pb2+, and Zn2+ at the parts-per-billion level by flame atomic absorption spectroscopy over the pH range of 0 < pH < 12 without the interferences associated with high total dissolved solids.19 Mok et al.23 demonstrated effective extraction of As, Cd, Cu, Fe, Mn, and Zn in the pH range of 1-1.5 with the APDCchloroform system for analysis of groundwater. It is concluded that the aqueous solutions of diethyl dithiocarbamates are not stable at acid conditions and decompose rapidly at pH < 2 into carbon disulfide and diethylamine, but APDC is more stable and can generally be used for the extraction of trace metals from acidic solutions.20 In this work, we describe the results of the synthesis and evaluation of a CO2-soluble analogue to APDC, a perfluoropolyether piperazine dithiocarbamate (see Figure 1) and its efficiency in the extraction of Cu2+, Ni2+, Zn2+, Cd2+, and Cr3+/6+ into supercritical CO2 from water at low pH’s. For comparison, we also generated a fluoroether-functional analogue to EDTA (ethylene-
Experimental Section Synthesis. The perfluoropolyether acid (Krytox functional oil, 7500 molecular weight) was received from Du Pont. Unless specified, all other chemicals were received from Aldrich Chemical Co. Except for the drying of solvents with molecular sieves, no further purification was performed on the raw materials. Analytical characterization was performed using a Mattson Polaris Fourier transform infrared (FT-IR) spectrometer and a Bruker MSL 300 nuclear magnetic resonance (NMR) 300-MHz spectrometer. Synthesis of Fluoroether Acid Chloride. The acid chloride of the fluoroether carboxylic acid was prepared as the precursor in the synthesis of the chelating agent. The oligomer of hexafluoropropylene oxide, capped at one end with a carboxylic acid group (7500 molecular weight, Krytox FSH), was transformed to the acid chloride via reaction with thionyl chloride. In a typical reaction, 30 g of 7500 molecular weight fluoroether (4 mmol) and 50 mL of previously dried perfluoro-1,3-dimethylcyclohexane were added to a reaction flask equipped with a condenser. Subsequently, 0.95 g of thionyl chloride (8 mmol) and 0.58 g of dimethylformamide (8 mmol) were added, and the mixture was heated at reflux under a blanket of nitrogen for 4 h. The residual reactants and DMF were removed via extraction in ether, and the solvents were removed under vacuum at 75-80 °C. The product is characterized by the disappearance of the carboxylic acid peak at 1777 cm-1 and the appearance of the acid chloride peak at 1810 cm-1 on the FT-IR spectrum and also by the disappearance of the peak representing the COOH proton at 9.6 ppm on the 1H NMR spectrum. Synthesis of Fluoroether Piperazine. In a typical synthesis, 0.688 g of piperazine (8 mmol) was dissolved in a mixture of 100 mL of 1,1,2-trichloro-1,2,2-trifluoroethane (TCTFE) and 30 mL of chloroform (previously dried using 4-Å molecular sieves), and 0.632 g of pyridine (8 mmol) was then added to the solution to scavenge HCl. After the solution was stirred for 30 min at 0 °C, 30 g of the fluoroether acid chloride (4 mmol) in TCTFE was added dropwise into the cold solution. The mixture was stirred for 8 h at 0 °C. After residual piperazine, pyridine, and pyridine-HCl salt were removed by washing with water, the solvents were removed under vacuum. On the FT-IR spectrum, the peak corresponding to the acid chloride at 1810 cm-1 disappears and an amide peak at 1710 cm-1 appears. 1H NMR δ 2.82 (-CH NH, 4H), 3.63 (-CH NC(O), 4H). 2 2 Synthesis of Fluoroether Piperazine Dithiocarbamate. In a typical synthesis, 30.3 g of fluoroether piperazine (4 mmol) was dissolved in 200 mL of TCTFE. While the contents were kept under a blanket of nitrogen, the reaction flask was cooled to -10 °C using an aqueous NaCl solution-dry ice bath. At this point, 16 mL of 0.5 M sodium methoxide (8 mmol) solution in ethyl ether was added to the reaction solution dropwise under vigorous stirring. Subsequently, 0.912 g of carbon disulfide (10 mmol) was added to the solution. Stirring at -10 °C was continued for another 1 h, and then the temperature was allowed to slowly rise to 0 °C, where it is kept for 30 min before it was allowed to rise to room temperature. The solvents and unreacted portions of the reactants were removed under vacuum at 80 °C. The
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FT-IR spectrum showed both the amide peak (1710 cm-1) and that representing the NCdS group at 1500 cm-1. 1H NMR δ 3.37 (-CH2NC(S), 4H), 3.79 (-CH2NC(O), 4H). In the final step, 30 g of the above product was dissolved in 200 mL of TCTFE and stirred vigorously with 100 mL of 1% ammonium chloride solution for 1 h at room temperature. The organic phase was retained and the solvent removed under vacuum at room temperature to recover the fluoroether-functional APDC analogue. Synthesis of Fluoroether Iminodiacetic Acid. In a typical synthesis, 1.135 g of diethyl iminodiacetate (6 mmol) was dissolved in 50 mL of TCTFE (previously dried using 4-Å molecular sieves), after which 0.632 g of pyridine (8 mmol) was then added to the solution to scavenge HCl. After the solution was stirred for 30 min at 0 °C, 16 g of the fluoroether acid chloride (4 mmol) in TCTFE was added dropwise into the cold solution. The mixture was stirred for 8 h at 0 °C. After the solvents were removed under vacuum, the residual diethyl iminodiacetate was removed by washing with chloroform and then pyridine and pyridine-HCl salt were removed by washing with water. On the FT-IR spectrum, the peak corresponding to the acid chloride at 1810 cm-1 disappears and peaks responding to the carbonyl groups at 1756 and 1698 cm-1 appear. 1H NMR δ 1.23 (-CH3, 6H), 3.31 (-OCH2-, 4H), 4.11 (N(CH2COO)2, 4H). To generate the product, 16.68 g of fluoroether diethyl iminodiacetate (4 mmol) was added to a mixture of 50 mL of THF, 50 mL of methanol, 20 mL of water, and 1.6 g of sodium hydroxide powder (40 mmol). The mixture was heated at reflux for 20 min, the organic solvents were removed, and the product was washed with water. On the FT-IR spectrum, a broad peak at 1650-1750 cm-1 appears. Phase Behavior Studies. Phase behavior studies of the chelating agents in carbon dioxide were conducted using a high-pressure, variable-volume view cell (D. B. Robinson and Associates) as shown by Yazdi and Beckman.7 Typically, a known amount of sample (0.31.0 g) is added to the top of the quartz tube sample cell as well as a number of glass or steel ball bearings to provide mixing. The tube is then sealed inside the steel housing, and a known volume of carbon dioxide is injected into the cell using one of the two Ruska syringe pumps. The quartz sample tube contains a floating piston that separates the sample from the pressuretransmitting fluid, in this study, a silicone oil. The pressure on the sample is raised (via the movement of the piston due to injection of silicone oil by the second Ruska pump) to a point where a single phase is present. Mixing is accomplished by the motion of the ball bearings upon rocking of the entire cell. The pressure is then lowered via slow withdrawal of silicone oil from beneath the piston until the first sign of turbidity appear, which is indicative of a phase separation. This procedure is repeated until the point of turbidity is known to within 20-30 psi. The corresponding point on the pressure vs concentration curve is identified as a cloud point. Following identification of the cloud point, an additional amount of measured carbon dioxide is injected into the cell to obtain a new concentration of the chelating agent. The cloud point for this new concentration is measured, and this procedure is repeated until the entire cloud point curve is completed.
Figure 2. Schematic diagram of the extraction system.
Extraction Studies. Except for Zn2+ solution, which was prepared from Zn metal dissolved in aqueous HCl, all metal solutions were made from their salts as supplied (CuSO4‚5H2O, NiCl2‚6H2O, CdCl2‚21/2H2O, K2CrO4, Cr(NO2)3‚6H2O) in deionized water with HCl or H2SO4 used to adjust the pH. The initial aqueous metal loadings were 1000 ppm for all CO2 extractions. Extractions were carried out using our model chelating agents in both 1,1,2-trichloro-1,2,2-trifluoroethane (TCTFE, Aldrich Chemical Co.) and CO2 for comparison. In the case of TCTFE, 2 mol of chelating agent per mole of metal was dissolved in 50 mL of TCTFE. Equal volumes of the aqueous and organic solutions were then added to a flask and stirred vigorously for 16 h. The phases were then separated, and the aqueous layer was retained for metal analysis. We have observed, not surprisingly, that acidic solutions in contact with stainless steel (316 SS) will extract both iron and chromium. Consequently, extractions into carbon dioxide were conducted using the apparatus shown in Figure 2, where the extraction vessel consists of a glass container (20-mL volume) inside a 316 SS housing (35-mL total volume). The inlet and the outlet tubing connected to the top of the stainless housing are for injecting and releasing CO2. A metal solution (10 mL) and the chelating agent were added to the extraction vessel, after which the system was pressurized via the Eldex piston pump (25 mL of CO2). Upon reaching the operating pressure, the pump and the inlet valve were shut off, and the biphasic mixture was stirred for a desired time. Liquid CO2 at 2800 psi was then used to flush the system (5 mL/min for 1 h), after which the system was slowly depressurized and the aqueous phase recovered for analysis. After extraction, the aqueous phase was analyzed using a UV-visible method (Perkin-Elmer Model Lambda 3B); the procedures for the analysis of Cu, Ni, and Cd are from Sandell.24 Cu was analyzed by a dithiocarbamate method (procedure two; Sandell, p 449), Ni by the dimethylglyoxime-oxidizing agent method (procedure A; Sandell, p 671), and Cd by the method of Fischer and Leopoldi (Sandell, p 356), while those for Zn and Cr are from Marczenko,25 where Zn was analyzed by the dithizone method (Marczenko, p 602), Cr6+ by the diphenylcarbazide method (Marczenko, p 217) and Cr3+ by the EDTA method (Marczenko, p 219). Dilutions with deionized water or acid solutions (for matching pH values required the by the analytical procedure) were made when necessary. The concentration of samples was determined by comparing their absorbance readings against a calibration curve constructed using known standards. Standard deviations were (4-5% generally.
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Figure 3. Cloud-point pressures vs chelating agent concentration in CO2 at 22 °C.
Figure 4. Metal extractions in TCTFE: 1 ) Cu; 2 ) Ni; 3 ) Zn; 4 ) Cd; 5 ) Cr3+; 6 ) Cr6+ at pH 1.6 and 22 °C.
Results and Discussion Phase Behavior. It was previously observed7,8 that increases to the molecular weight of the CO2-philic tail in the functionalized chelating agent can dramatically lower the pressure required for solubilization at a given concentration. Consequently, the model dithiocarbamate used in this study was synthesized with a high molecular weight perfluoropolyether tail (MW 7500). It can be seen from Figure 3 that the cloud-point curve of the chelating agent synthesized for this study is more concentration dependent than those investigated in Yazdi’s work and also exists at significantly higher pressures. In Yazdi’s work, fluoroether-functional picolylamine, dithiol, bis(picolylamine), and dithiocarbamate chelating agents were synthesized. The length of the fluoroether tail in these materials was varied from 2500 to 7500. In all cases, cloud-point pressures rarely exceeded 2000 psi, and the curves were relatively invariant with concentration. Thus, the results in Figure 3 show that it is much more difficult to solubilize the APDC functional group, despite use of the highly CO2-philic fluoroether tail. We suggest that the highly hydrophilic headgroup in this agent primarily contributes to this effect. Metal Extractions. Extractions into 1,1,2-Trichloro-1,2,2-trifluoroethane (TCTFE). To test the ability of the fluoroether-functional piperazinedithiocarbamate to bind metals at low pH’s, extractions were performed using the agent in TCTFE (aqueous pH ) 1.6; agentto-metal ratio ) 2.0). The results from the extraction of metals by TCTFE are shown in Figure 4. A variety of metals can be extracted at relatively high efficiency (chromium is the obvious exception) in one stage under our experimental conditions. These extraction results are comparable to those from organic solvent extractions using APDC in the literature.4,11-19 The extraction of chromium has been investigated previously by a number of groups.14-18,26-28 In most cases, Cr3+ was converted into Cr6+ by preoxidation before the complexing procedure. APDC, either alone or in combination with other chelating agents, was employed in chromium extraction at pH’s ranging from 1 to 7, but most of them could not achieve effective extraction. The extraction efficiencies of Cr3+ and Cr6+ by the fluoroether-functional material into TCTFE are similar under acidic conditions to those in the literature.18
Figure 5. Extraction of metal ions vs pH at 2800 psi (16 h of equilibration time) and 22 °C.
Extractions were also conducted in TCTFE to gauge the effectiveness of the fluoroether-functional diacetic acid in binding several of the metals ions of interest (Cu, Ni, Cr3+, and Cr6+). If extractions were performed from 0.1 N HCl, the amount of metal extracted was negligible. However, extractions at pH ) 3.0 and 2 mol of F-EDTA to 1 mol of metal (5 mL of 1000 ppm metal solution, 50 mL of TCTFE), after stirring for 24 h at room temperature, gave the following results: metal percent extracted; Cu, 48; Ni, 34.5; Cr3+, 25; Cr6+, 32.8. Thus, the performance of the fluoroether-functional iminodiacetic acid at pH ) 3.0 is not significantly better than what we have observed for other agents we have tested previously, while those from very acidic solution were significantly worse than the fluoroether-functional APDC analogue. Consequently, extractions in CO2 were limited to F-APDC. Extractions into CO2. The extraction results in Figure 4 suggest that, like APDC, the fluoroetherfunctional analogue can extract metals at relatively low pH’s. Consequently, a series of extractions were performed using CO2 as the organic phase (2800 psi, 16 h of equilibration time, pH range of 1.2-7.0, 2 mol of chelating agent per mole of metal). As shown in Figure 5, Cu is extracted most effectively into CO2 among the target metals over the entire pH range, and up to 85% Cu can be extracted at pH 1.2. As is the case for
4772 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
tions,20 and thus, it appears that the fluoroether piperazinedithiocarbamate used in this study is more stable than the conventional materials (as is the case with APDC) at low pH values under our experimental conditions. It should be noted that Cr(III) has not been included in the temporal study; it has been shown previously that Cr(III) forms complexes relatively slowly, and thus, rapid equilibration can only be assumed for those metals evaluated in Figure 7. Conclusions
Figure 6. Extraction of Cr6+ vs number of equilibrium stages at pH ) 1.6, 2800 psi at 22 °C (16 h of equilibration time).
A CO2-soluble chelating agent, fluoroether piperazinedithiocarbamate, was synthesized for the extraction of heavy metals from acidic solutions. The phase behavior study shows that the chelating agent is miscible with CO2 at moderate pressure, although its cloud-point curve is very dependent on concentration due to the highly polar chelating headgroup. The fluoroether piperazinedithiocarbamate prepared for this study efficiently extracts copper, zinc, nickel, and cadmium at pH’s down to 1.2 in a one-stage extraction. The results for chromium are not as good, with extraction yields of less than 50%. However, multiple-stage extractions are demonstrated to be effective for high extraction efficiencies of Cr6+ at pH 1.6 under our conditions. This specific dithiocarbamate chelating agent shows good stability during extraction at low pH’s under our experimental conditions in this study. Acknowledgment We thank Normex International (Houston, TX) for its generous support of this work. Literature Cited
Figure 7. Time dependence of extraction efficiency (2800 psi, 22 °C).
extractions into TCTFE, the extraction efficiencies of Cr3+ and Cr6+ remain relatively poor. In general, the extraction yield did not vary significantly as the pH changed over the range 1.2-7.0, but it should be noted that pH’s above 3.0 are nominal values only. Exposure to high-pressure CO2 will likely reduce the nominal values to 3.0 relatively quickly, which could explain the relative insensitivity of extraction yield to nominal pH. Given that metal binding is an equilibrium process, it was concluded previously that extraction efficiency could be improved using a multiple-stage extraction.8 In this work, multiple-stage extractions are carried out for Cr6+ at pH 1.6, 2800 psi, and room temperature with 2 mol of chelating agent per mole of metal at each stage. As shown in Figure 6, the yield of Cr6+ extracted was 75% after three stages, which is comparable to the yield achieved using 1000 mol of chelating agent per mole of metal in Subramanian’s work in conventional organic solvents.17 Figure 7 shows the extraction efficiency dependence on extraction time at pH 1.6 and 2800 psi. We and others [ref 8 and references therein] have found that equilibrium is reached very rapidly (approximately 5-6 min) in these systems, so the results in Figure 7 are not surprising. Conventional alkyl dithiocarbamates decompose within a few minutes under acidic condi-
(1) Johnston, K. P.; Sunwook, K.; Combes, J. Spectroscopic Determination of Solvent Strength and Structure in Supercritical Fluid Mixtures: A Review. In Supercritical Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989; p 52. (2) Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C. L.; Eckert, C. A. Specific Intermolecular Interaction of Carbon Dioxide with Polymers. J. Am. Chem. Soc. 1996, 118, 1729-1736. (3) Laintz, K. E.; Shieh, G. M.; Wai, C. M. Simultaneous Determination of Arsenic and Antimony Species in Environmental Samples using Bis (trifluoroethyl) dithiocarbamate chelation and Supercritical Fluid Chromatography. J. Chromatogr. Sci. 1992, 30 (April), 120-123. (4) 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, 2875-2878. (5) Dehghani, F.; Wells, T.; Cotton, N. J.; Foster, N. R. Extraction and separation of Lanthanides Using dense Gas CO2 Modified with Tributyl Phosphate and 2-(diethylhexyl)phosphoric Acid. J. Supercrit. Fluids 1996, 9, 263-272. (6) Murphy, J. M.; Erkey, C. Thermodynamics of Extraction of Copper (II) from Aqueous Solutions by Chelation in Carbon Dioxide. Environ. Sci. Technol. 1997, 31, 1674-1679. (7) Yazdi, A. V.; Beckman, E. J. Design of Highly CO2-soluble Chelating Agents for Carbon Dioxide. J. Mater. Res. 1995, 10 (3), 530-537. (8) (a) Yazdi, A. V.; Beckman, E. J. Design, Synthesis, and Evaluation of Highly CO2 Soluble Chelating Agents for the Removal of Heavy Metals. Ind. Eng. Chem. 1996, 35, 3644. (b) Yazdi, A. V.; Beckman, E. J. Design of Highly CO2-soluble Chelating Agents. 2. Effect of Chelate Structure and Process Parameters on Extraction Efficiency. Ind. Eng. Chem. Res. 1997, 36, 2368-2374. (9) Lagalante, A. F.; Hansen, B. N.; Bruno, T. J.; Sievers, R. E. Solubilities of Copper(II) and Chromium(III) Beta-Diketones in Supercritical Carbon Dioxide. Inorg. Chem. 1995, 34, 23.
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4773 (10) 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. Ref. 1995, 67, 4040. (11) Arunachalam, J.; Gangadharan, S. Preirradiation Separations for the Ultratrace Elemental Characterisation of High Purity Materials by NAA. J. Radioanal. Nucl. Chem. 1993, 168 (2), 471476. (12) Bone, K. M.; Hibbert, W. D. Solvent extraction with ammonium pyrrolidine dithiocarbamate and 2,6-dimethyl-4-heptanone for the determination of trace metals in effluents and natural waters. Anal. Chim. Acta, 1979, 107, 219-229. (13) Bertrand, D.; Guery, J.; Jacoboni, C. Fe, Co, Ni, Cu trace metal analysis in ZBLAN fluoride glasses. J. Non-Crystalline Solids 1993, 161, 32-35. (14) Mullins, T. L. Selective Separation and Determination of Dissolved Chromium Species in Natural Waters by Atomic Absorption Spectrometry. Anal. Chim. Acta 1984, 165, 97-103. (15) Gilbert, T. R.; Clay, A. M. Determination of Chromium in Seawater by Atomic Absorption Spectrometry, Anal. Chim. Acta 1973, 67, 289-295. (16) Sturgeon, R. E.; Berman, S. S.; Desaulniers, A.; Russell, D. S. Preconcentration of Trace Metals from Seawater for Determination by Graphite-Furnace Atomic-Absorption Spectrometry. Talanta 1980, 27, 85-94. (17) Subramanian, K. S. Determination of Chromium (III) and Chromium (VI) by Ammonium Pyrrolidinecarbodithioate-Methyl Isobutyl Ketone Furnace Atomic Absorption Spectrometry. Anal. Chem. 1988, 60, 11-15. (18) Sugiyama, M.; Fujino, O.; Kihara, S.; Matsui, M. Preconcentration by dithiocarbamate extraction for determination of Trace Elements in Natural Waters by Inductively coupled Plasma Atomic Emission Spectrometry. Anal. Chim. Acta 1986, 181, 159168. (19) Williams, M. C.; Cokal, E. J. Masking, Chelation, and Solvent Extraction for the Determination of Sub-parts-per-Million Levels of Trace Elements in High Iron and Salt Matrixes. Anal. Chem. 1986, 58, 1541-1547. (20) Alfassi, Z. A.; Wai, C. M. Preconcentration Techniques for Trace Elements; CRC: Ann Arbor, MI, 1992.
(21) Magnusson, B.; Westerlund, S. Solvent Extraction Procedures Combined with Back-Extraction for Trace Metal Determinations by Atomic Absorption Spectrometry. Anal. Chim. Acta 1981, 131, 63-72. (22) Kinrade, J. D.; Van Loon, J. C. Solvent extraction for use with flame atomic absorption spectrometry. Anal. Chem. 1974, 46 (13), 1894. (23) (a) Mok, W. M.; Riley, J. A.; Wai, C. M. Arsenic speciation and quality of groundwater in a lead-zinc mine, Idaho. Wat. Res. 1988, 22 (6), 769-774. (b) Mok, W. M.; Riley, J. A.; Wai, C. M. Distribution and mobilization of arsenic species in the Creeks around the Blackbird mining district, Idaho. Wat. Res. 1989, 23 (1), 7-13. (24) Sandell, E. B. Colorimetric Determination of Traces of Metals; Interscience: New York, 1959. (25) Marczenko, Z. Spectrophotometric Determination of Elements; Wydawnictwa Naukowo-Techniczne: Warsaw, Poland; Ellis Horwood: Sussex, England, 1976. (26) Kudo, K.; Shigematsu, T.; Kobayashi, K. Substoichiometric Determination of Chromium by Neutron activation Analysis. J. Radioanal. Chem. 1977, 36, 65-75. (27) Tao, H.; Miyazaki, A.; Bansho, K.; Umezaki, Y. Determination of trace level of heavy metals in waters by extraction with ammonium tetramethylenedithiocarbamate into xylene followed by inductively coupled plasma emission spectrometry. Anal. Chim. Acta 1984, 156, 159-168. (28) Hannaker, P.; Hughes, T. C. Multielement Trace Analysis of Geological Materials with Solvent Extraction and Flame Atomic Absorption Spectrometry. Anal. Chem. 1977, 49 (11), 1485-1488. (29) Smart, N. G.; Carleson, T. E.; Elshani, S.; Wang, S.; Wai, C. M. Extraction of Toxic Heavy Metals using Supercritical Fluid Carbon Dioxide Containing Organophosphorus Reagents. Ind. Eng. Chem. Res. 1997, 36, 1819-1826.
Received for review April 30, 1998 Revised manuscript received July 29, 1998 Accepted August 11, 1998 IE9802717
