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SEPARATIONS Supercritical Fluid Extraction of Toxic Heavy Metals and Uranium from Acidic Solutions with Sulfur-Containing Organophosphorus Reagents Yuehe Lin,* Chongxuan Liu, and Hong Wu Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, Washington 99352
H. K. Yak Department of Chemistry, Chung Yuan Christian University, ChungLi 320, Taiwan
Chien M. Wai Department of Chemistry, University of Idaho, Moscow, Idaho 83844
The feasibility of using sulfur-containing organophosphorus reagents for the chelationsupercritical fluid extraction (SFE) of toxic heavy metals and uranium from acidic media was investigated. The SFE experiments were conducted in a specially designed flow-through liquid extractor. Effective extraction of the metal ions from various acidic media was demonstrated. The effect of the ligand concentration in supercritical CO2 on the kinetics of metal extraction was studied. A simplified model is used to describe the extraction kinetics, and good agreement of experimental data with the equilibrium-based model is achieved. 1. Introduction Supercritical fluids (SFs) are being used increasingly as extraction solvents because of increased restrictions from environmental legislation on the use of solvents, particularly chlorinated ones. Carbon dioxide (CO2) has been the solvent of choice because of its low toxicity, relatively low cost, convenient critical properties, and ease of recycling. For extraction of organic compounds, SF CO2 has been found to be particularly useful at both analytical and process scales.1 Direct extraction of metal ions by neat supercritical CO2 is, however, highly inefficient because the charge neutralization requirement and the weak solute-solvent interaction limit the metal ion solution in neat SF CO2. When metal ions are chelated with suitable organic ligands, they become quite soluble in SF CO2.2-6 Conversion of metal ions into metal chelates can be performed by two methods. One method is online chelation, in which ligands first dissolve into SF CO2 and then flow with the SF CO2 through the sample matrix. Another method is in situ chelation, where ligands are directly added to a sample matrix prior to the SF extraction (SFE). Both methods have been found to be successful for metal ion extraction using SF CO2.2-14 According to the literature,2-14 the following factors are important for effective extraction of metal species in SF CO2: (1) solubility of the chelating agent, (2) solubility and stability of the metal chelate, (3) density of SFs, (4) chemical form of the metal species, and (5) sample matrix. A variety of organic complexing * Corresponding author. Fax:
[email protected].
509-376-5106. E-mail:
agents have been used in initial studies of SFE of metal ions achieving efficient extraction. The current study focuses on two potential applications of chelation-SFE: industrial wastewater treatment and environmental remediation. It is well recognized that the presence of heavy metals in the environment can be detrimental to a variety of living species, including humans. A variety of industries are responsible for the release of heavy metals into the environment through their wastewater disposal practices.15 These industrial sources include the iron and steel production industry, the nonferrous metal industry, mining and mineral processing operations, the pigment manufacturing industry, the wet phosphoric acid process in the fertilizer industry, battery manufacture, the printing and photographic industries, and metal-working and finishing processes. Many of the wastewaters are acidic. Some of them, such as those generated from mining processing and the wet phosphoric acid process in the fertilizer industry, also contain low concentrations of uranium. In the hydrometallurgical industry, solvent extraction methods have been widely used for the separation and purification of heavy metals from various acidic media.16-19 Conventional solvent extraction processes usually require use of a toxic organic solvent. Handling and disposal of the used solvents are typically environmentally problematic. Some research and development work has been conducted in several laboratories on the removal of heavy metals from wastewater stream with SF containing β-diketone or dithiocarbamate ligands.20 However, none of these reported works investigates the SFE of heavy metal ions from an acidic
10.1021/ie020804i CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003
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Figure 1. SFE apparatus with a flow-through liquid extraction vessel.
medium. β-Diketone ligands are usually not suitable for chelation in an acidic medium. Dithiocarbamate ligands are unstable in an acidic solution.21 In this work, we examine the feasibility of extracting heavy metals and uranium from various acidic media with SF CO2 containing different sulfur-containing organophosphorus reagents. The sulfur-containing organophosphorus reagents were commercially available with relatively low cost, were relatively stable in an acidic medium, and have been widely used for solvent extraction of heavy metals in hydrometallurgical processing.16-19 These extraction reagents have been demonstrated to be quite soluble in SF CO2.14,22 A model to describe the dynamic extraction of metal ions by SF was developed and applied to interpret the experimental extraction results of metal ions from aqueous solutions. The material presented here should lay the groundwork for further research in this area and accelerate the development of industrial-scale metal ion extraction systems. 2. Experimental Section 2.1. Reagents and Sample Preparation. Cyanex 301 [bis(2,4,4-trimethylpentyl)dithiophosphinic acid] and Cyanex 302 [bis(2,4,4-trimethylpentyl)monothiophosphinic acid] were supplied by Cyanamid and used without further purification. Bis(ethylhexyl)phosphorothioic acid (D2EHTPA) was donated by AG Bayer Germany. A uranyl ion solution was prepared from analytical-grade uranyl nitrate. Other metal ion solutions were prepared from the ICP standard solution. All other cited chemicals used were of analytical reagent grade. 2.2. Experimental Setup and Procedure. All experiments were performed using a SFE system as shown in Figure 1. The SFE system consists of two ISCO 260D syringe pumps (ISCO, Lincoln, NE) and a laboratory-built extractor including a liquid extraction vessel. SFC-grade CO2 (Scott Specialty Gases, Plumsteadville, PA) was cooled by a coolant circulator and delivered to the SFE system using an ISCO 260D syringe pump. A sulfur-containing organophosphorus reagent diluted with methanol (1:1) was delivered by another ISCO 260D syringe pump, mixed with CO2 with a T-junction, and then flowed into the extractor. The extractor consisted of a temperature-controlled oven, a temperature equilibration device, an inlet valve, a liquid extraction vessel, an outlet valve, a restrictor, and a collection vessel. The temperature equilibration device with a volume of 3.5 mL was purchased from Dionex (Sunnyvale, CA). The liquid extraction vessel was modified from a commercial SFE cell (Dionex; 1.0 cm i.d. and 13 cm length) with a volume of 10 mL (Figure
1). The 1/16 in. stainless steel inlet tubing was extended to the bottom of the vessel cavity, forcing the SF CO2 to flow through the liquid in the vessel before exiting through the outlet tubing at the top. The temperature equilibration device, liquid extraction vessel, and outlet valve were placed in an oven with temperature controlled to (0.1 °C by an Omega (Stamford, CT) BS5001J1-A benchtop temperature controller. A fusedsilica capillary (50 µm i.d. × 375 µm o.d. and 40 cm length; J&W Scientific, Folsom, CA) was used as the pressure restrictor to maintain the extraction pressure. To prevent restrictor plugging from ice formation, the outlet valve was installed inside the heated oven and a portion of the restrictor capillary remained inside the oven. The unheated portion of the restrictor capillary was inserted into a collection vessel immersed in a water bath maintained at room temperature to prevent the collection solvent temperature from dropping below 0 °C. In this manner, restrictor plugging was minimized. The flow rate of the SF CO2 was about 1.5 mL/min at 60 °C and 200 atm. The SFE system used in this study allows static (extraction vessel pressurized with SF CO2 having no flow through the cell) and dynamic (SF CO2 flow through the cell continuously) extraction to be carried out by use of the outlet and inlet valves. SF CO2 containing a 1-3% (v/v) sulfur-containing organophosphorus reagent continuously flowed through the liquid extraction vessel containing 6.0 mL of an acid solution of metal ions. The chelation and extraction processes were allowed to occur under dynamic SFE conditions for 30-80 min. During the dynamic extraction period, metal chelates were extracted from the sample and flushed out of the extraction vessel with SF CO2. A collection vessel containing 6 mL of a chloroform solvent was used to trap metal chelates from the restrictor. After extraction, the sample was removed from the extraction vessel and analyzed by ICP-MS. The amount of metal remaining in the cell was used to determine the “extraction efficiency” (% extraction), as follows:
% extraction ) metal spike/µg - metal remaining/µg × 100% metal spike/µg The “collection efficiency” was determined by evaporation of the samples collected in chloroform and the residue treated with 1 mL of hot concentrated nitric acid and then diluted to 20 mL with deionized water. The concentration was then determined by ICP-MS. The collection efficiency (% collected) was determined as follows:
% collection )
metal collected/µg × 100% metal spike/µg
The mass balance of extracted and collected metals was checked by comparing the extraction and collection efficiencies. It was observed that quantitative recovery of the metals was possible. For the kinetic experiments, metal chelates were collected in chloroform at the exit of the restrictor at varying time intervals. Chloroform was allowed to evaporate, and the residue was treated with 1 mL of hot concentrated nitric acid and then diluted to 20 mL with deionized water. The concentration was then determined by ICP-MS. The total collection efficiency (total collected) was determined by the amount of the
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Table 1. Extraction Efficiency (%) of Toxic Metal Ions from Acidic Solutions with SF CO2 Containing 3% (v/v) Cyanex 301 at 300 atm and 60 °Ca sample matrix
Hg
Cu
Cd
0.1 M HCl 1.0 M HCl 0.1 M HNO3 1.0 M HNO3 0.05 M H2SO4 0.5 M H2SO4
100 100 100 100 100 100
98.4 99.7 99.3 90.8 99.6 100
100 100 100 88.8 100 99.7
a
Pb
elementb Zn
As3+
Sb3+
Se4+
U
100 100 99.9 90.7 100 100
93.0 91.1 93.7 89.8 94.9 91.8
95.8 99.0 94.1 0 94.4 95.8
100 100 100 0 100 99.7
100 95.1 99.1 99.4 99.3 99.
100 99.0 100 99.0 100 50.3
30 min dynamic extraction. b Acidic solution containing 10 µg/mL of each metal ion.
Table 2. Extraction Efficiency (%) of Toxic Metal Ions from Acidic Solutions with SF CO2 Containing 3% (v/v) Cyanex 302 at 300 atm and 60 °Ca sample matrix
Hg
Cu
Cd
Pb
0.1 M HCl 1.0 M HCl 0.1 M HNO3 1.0 M HNO3 0.05 M H2SO4 0.5 M H2SO4
100 99.8 100 100 100 100
94.4 94.6 99.1 90.8 99.9 99.6
100 62.5 100 99.4 100 100
100 51.5 98.7 53.1 99.7 98.5
a
elementb Zn 93.8 63.8 92.0 69.0 95.6 89.0
As3+
Sb3+
Se4+
U6+
97.0 74.0 0 0 0 0
100 93.0 12.6 0 10.0 0
99.4 97.4 99.1 99.0 99.3 99.5
99.3 97.3 98.8 99.8 98.7 46.8
30 min dynamic extraction. b Acid solution containing 10 µg/mL of each metal ion.
total metal collected from multiple fractions divided by the amount of the metal ion spiked in the aqueous sample. 3. Results and Discussion 3.1. Extraction Efficiency of Toxic Metal from Acidic Solutions. Extraction efficiencies of toxic metal ions from different sample matrixes by mono- or dithioorganophosphorus acids are summarized in Tables 1-3. The metal ions were complexed and extracted under dynamic conditions for 30 min. As shown in Table 1, Hg2+, Cu2+, Cd2+, Pb2+, and Se4+ can be extracted quantitatively by Cyanex 301 from HCl, H2SO4, and HNO3 solutions at 0.1 and 1.0 N concentrations, respectively. U6+ can be extracted completely from different acidic solutions except for 0.5 M H2SO4. Zn2+, As3+, and Sb3+ can be extracted quantitatively from HCl and H2SO4 solutions. However, their extraction efficiencies decrease to 90, 0, and 0, respectively, in a 1.0 M nitric acid solution. Cyanex 301 is a reducing agent itself, and nitric acid is an oxidizing agent. The decrease in extraction efficiencies of Zn2+, As3+, and Sb3+ from 1.0 M HNO3 is probably due to the partial oxidation of the extractant. The oxidation product, probably (R)2P(S)SS(S)P(R)2, tends to reduce the available extractant concentration. Because of mutual competition for the ligand (soft base), the metal ions, which are relatively hard acids23 (As3+ and Sb3+), may have weaker interaction with the ligand and end up with lower extraction efficiencies. The extraction efficiencies of toxic metal ions with SF CO2 containing 3% (v/v) Cyanex 302 are given in Table 2. The extraction efficiencies of Hg2+, Cu2+, Se4+, and UO22+ are similar to those in Cyanex 301 (Table 1). However, the extraction efficiencies were slightly lower for Cd2+, Pb2+, Zn2+, As3+, and Sb3+. When monothiophosphinic acid, Cyanex 302, is utilized as the chelating agent, it appears that As3+ and Sb3+ may only be extracted from HCl solutions. The extraction of Hg, Cu, and Se exhibited excellent efficiencies in all acidic media tested. The extraction efficiency of Cd was extremely
Table 3. Extraction Efficiency (%) of Toxic Metal Ions from Acidic Solution with SF CO2 Containing 3% (v/v) D2EHTPA at 300 atm and 60 °Ca sample matrix
Hg
Cu
Cd
elementb Pb Zn As3+ Sb3+ Se4+ U6+
0.1 M HCl 100 98.8 99.1 98.9 97.5 1.0 M HNO3 100 97.2 90.8 63.2 88.1
0 0
4.4 0
99.2 99.8 77.0 97.5
a 30 min dynamic extraction. b Acid solution containing 10 µg/ mL of each metal ion.
high, although the extraction efficiency from a 1.0 M HCl solution was lower, at about 62.5%. The extraction of U6+ resulted in excellent extraction efficiency in all acidic media tested, although the extraction efficiency in 0.5 M H2SO4 decreased to about 46.8%. Extraction efficiencies for Pb were nearly 100% in every acidic solution tested with the exception of 1 M HCl and 1 M HNO3, wherein the efficiencies were 51.5% and 53.1%, respectively. As3+ and Sb3+ were extracted at nearly 90% extraction efficiencies from 0.1 M HCl utilizing the monothioohosphinic acid, with somewhat lower extraction efficiencies occurring from the 1.0 M HCl solution. Because Cyanex 301 (pKa ) 2.61) is a stronger acid than Cyanex 302 (pKa ) 5.63), it appears that Cyanex 301 shows a higher extraction efficiency for toxic metals from acidic solutions, especially at higher acid concentrations. In a manner similar to that discussed above, Hg2+, Cu2+, Cd2+, Pb2+, Zn2+, As3+, Sb3+, Se4+, and UO22+ were extracted from 0.1 M HCl and 1.0 M HNO3 solutions utilizing SF CO2 containing 3% (v/v) D2EHTPA as the chelating agent. D2EHTPA showed quite high extraction efficiencies for Hg2+, Cu2+, Cd2+, Pb2+, Zn2+, Se4+, and UO22+ (Table 3) but almost no extraction for As3+ and Sb3+ even at 0.1 M HCl. Because D2EHTPA has a lower affinity for As3+ and Sb3+ than Cyanex 301 and Cyanex 302, no further experiments were performed for extraction of toxic metals from other acidic solutions. 3.2. Dynamic Extraction of Heavy Metals and Modeling. Complexation of a metal ion from an aqueous solution with a sulfur-containing organophosphorus
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reagent carried by SF may be described by the following general reaction:
Mu+ + xH+ + yLv- + zAm- ) MHxLyAzu+x-yv-zm
(1)
where Mu+ is the metal with a valence of +u, Lv- is the ligand with a valence of -v, Am- is the ionized acid with a valence of -m, and MHxLyAzu+x-yv-zm is the complex species with a valence of u + x - yv - zm; x, y, and z are the stoichiometric coefficients. The conditional stability constant (βxyz) of complex species MHxLyAzu+x-yv-zm may be written as
[MHxLyAzu+x-yv-zm] [Mu+][H+]x[Lv-]y[Am-]z
) βxyz
(2)
Figure 2. Experimental and modeling results of metal extraction with SF CO2 containing 1% (v/v) Cyanex 302 at 300 atm and 60 °C.
where [ ] denotes the concentration. Under the assumption that species MHxLyAzu+x-yv-zm is a dominant complex for a specific metal, an equilibrium partitioning coefficient (Kc) of the metal between extracting and solution phases can be derived from eq 2.
Kc ) βxyz[H+]x[Lv-]y[Am-]z
(3)
Equation 3 indicated that the equilibrium partitioning coefficient is generally a function of aqueous compositions of the SF and solution. Considering a well-mixed system where a metal in solution is in equilibrium with the SF through reaction (1), a governing equation describing the dynamic metal concentrations could be derived from the overall mass balance in the system
Vscf
dCscf dCs + Vs ) -FCscf dt dt
(4)
where Vscf and Vs are the volumes of the SF and solution in the extracting vessel, respectively; F is the flow rate of the SF, Cscf and Cs are the total metal concentrations in the SF and solution phases, respectively. Using Kc (eq 3) to relate the concentrations of Cscf and Cs, integrating eq 4, and after some algebraic manipulation, we obtain an expression for the metal concentration in the solution
Cs(t) ) C0 exp[-KcFt/(VscfKc + Vs)]
(5)
where C0 is the initial metal concentration in the solution phase. The mass fraction that is extracted by the SF as a function of time can be calculated from eq 5 and has the following expression:
Mex(t)/M0 ) 1 - exp[-KcFt/(VscfKc + Vs)]
(6)
Equation 5 or 6 was derived under the assumption of metal equilibrium partitioning between the SF and solution phases with a constant partitioning coefficient. Rigorously, the metal concentrations in both solution and extracting phases were affected by the transport of the SF and metal solutions and localized mass transfer within these phases and between them. The pressure difference between the influent and effluent forces the convection of the SF in the extracting vessel. The momentum exchange through collision and viscous dragging between the SF and metal solutions induced their hydrodynamic dispersion.24 These processes trans-
Figure 3. Experimental and modeling results of metal extraction with SF CO2 containing 2% (v/v) Cyanex 302 at 300 atm and 60 °C. Table 4. Partition Coefficient (Kc) for the Metal Extraction by a SFa Cyanex 302
Cd
Cu
Pb
Zn
1% (v/v) 2% (v/v)
0.30(0.01) 0.60(0.01)
0.31(0.01) 1.04(0.02)
0.05(0.00) 0.50(0.01)
0.03(0.00) 0.18(0.01)
a
The values in parentheses are the standard deviation.
ported the metal associated with each phase in both the flow and lateral directions. At a specific spatial location within each phase and at the phase interface, metal could further be subjected to localized redistribution (e.g., diffusion) driven by the spatial concentration gradient and by the electrostatic force resulting from differential transport of charged species.25 A complete metal extraction model in the flow-through system will require consideration and characterization of all of these mass-transfer processes, if they are indeed quantifiable, together with reaction (1) at each spatial location. Equation 5 or 6 is, therefore, a significantly simplified model. Because of its simplicity with the minimum number of fitting parameters (only Kc), here we used the model as a test for describing the experimental results. The equilibrium-based model has been previously used to describe metal extraction by the SF in a well-stirred batch system.26 Equation 6 well described the experimental results of all metal extractions (Figures 2 and 3) with values of parameter Kc listed in Table 4, suggesting that the model (5) or (6) is a plausible one that can be used to describe apparent metal extraction in the flow-through system. The estimated Kc values (Table 4) were within the ranges reported in the literature for a flow-through
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system.27 The Kc values of Cd and Cu were relatively higher than those of Pb and Zn. Increasing the concentration of the complex ligand (Cyanex 302) increased the Kc values for all metals. This result is expected from eq 3, which shows that Kc is an increasing function of the ligand concentration. The values of Kc increased about 6-10 times for Pb and Zn and about 2-3 times for Cd and Cu by increasing the volumetric ratio of Cyanex 302 from 1% (v/v) to 2% (v/v). The good agreement of the experimental data with the equilibriumbased model (eq 6) was surprising given the potential complicated mass transport processes in the extracting vessel. This result suggests that either the fluid dispersion within the vessel and the mass transfer between the solution and SF is faster than flow of the SF or the influences of mass-transfer processes were minor and could be effectively lumped into the value of Kc. The experimental results of this study were not allowed to resolve this issue. 4. Conclusion Effective extraction of toxic metal ions from various acidic media by using a chelation-SFE technique with a range of sulfur-containing organophosphorus reagents was demonstrated in a continuous-flow system. The kinetics of the SFE of the metal ions was studied and a simplified equilibrium-based model is developed for the analysis of the kinetic results. The model agrees very well with the experimental results. This indicates that the flow-through system designed for this work appears to result in a fast mass-transfer efficiency between the solution and SF phases in the reaction vessel. Therefore, an equilibrium exchange coupled with flow is sufficient to describe the apparent metal extraction kinetics in effluents. Such a model is the simplest and only involves one parameter (Kc), thus typically reducing the uncertainties of fit parameters. For a large-scale or practical application of the extraction approach proposed in the paper, the simplest model, such as the one used in this paper, is still the best choice as an initial test because of its simplicity. Such a test could reveal or imply whether one needs a model requiring explicit consideration of mass-transfer processes within and between the solution and SF phases as well as more independent characterization of these processes. For a large-scale application, the liquid extraction vessel needs to be redesigned to reduce the dead volume and increase the extraction rate. Online recovery of extracted metals and regeneration/recycle of the ligand and SF CO2 would greatly reduce the operation cost.13 The SFE technology is environmentally friendly, which can be used for direct removal of toxic metal ions from acidic solutions with minimum waste generation. The results obtained from this work form the basis for further development of a green process technology for the treatment of industrial waste streams. Acknowledgment This work is supported in part by the Environmental Management Science Program, Office of Science, of U.S. Department of Energy (U.S. DOE). The work was partially performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility located at Pacific Northwest National Laboratory (PNNL) and sponsored by U.S. DOE’s Office of Biological and Environmental Research. PNNL is operated by Battelle
Memorial Institute for U.S. DOE under Contract DEAC06-76RLO 1830. Literature Cited (1) Phelps, C. L.; Smart, N. G.; Wai, C. M. Past, Present, and Possible Future Applications of Supercritical Fluid Extraction Technology. J. Chem. Educ. 1996, 73, 1163-1168 (2) 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. (3) 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. (4) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Supercritical Fluid Extraction of Lanthanides and Actinides from Solid Materials with a Fluorinated β-Diketone. Anal. Chem. 1993, 65, 25492551. (5) Wai, C. M.; Lin, Y.; Brauer, R. D.; Wang, S.; Beckert, W. F. Supercritical fluid extraction of organic and inorganic mercury from solid materials. Talanta 1993, 40, 1325-30. (6) Lin, Y.; Smart, N. G.; Wai, C. M. Supercritical Fluid Extraction and Chromatography of Metal Chelates and Organometallic Compounds. Trends Anal. Chem. 1995, 14 (3), 123-133. (7) Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Supercritical Fluid Extraction of Thorium, Uranium from Solid and Aqueous Materials with β-Diketones and Mixed ligands. Environ. Sci. Technol. 1994, 28, 1190-1193. (8) Lin, Y.; Wai, C. M. Supercritical Fluid Extraction of Lanthanides with β-Diketones and Tributyl Phosphate. Anal. Chem. 1994, 66, 1971-1975. (9) Lin, Y.; Smart, N. G.; Wai, C. M. Supercritical Fluid Extraction of Uranium and Thorium from Nitric Acid Solutions with Organophosphorus Reagents. Environ. Sci. Technol. 1995, 29, 2706-2711. (10) Wai, C. M.; Lin, Y.; Ji, M.; Toews, K. L.; Smart, N. G. Extraction and Separation of Uranium and Lanthanides with Supercritical Fluids. In Metal Ion Separation and Preconcentration: Progress and Opportunities; Bond, A. H., Dietz, M. L., Eds.; ACS Symposium Series 716; American Chemical Society: Washington, DC, 1999; Chapter 23, pp 390-400. (11) Lin, Y.; Wu, H.; Smart, N. G.; Wai, C. M. Investigation of Adducts of Lanthanide and Uranium β-Diketonates with Organophosphorus Lewis Bases by Supercritical Fluid Chromatography. J. Chromatogr. A 1998, 793, 107-113. (12) Lin, Y.; Wu, H.; Smart, N. G.; Wai, C. M. Separation of Divalent Transition Metal β-Diketonates and Their Adducts by Supercritical Fluid Chromatography. Talanta 2000, 52, 695-701. (13) Lin, Y.; Wu, H.; Smart, N. G.; Wai, C. M. System Optimization of In-Situ Chelation/Supercritical Fluid Extraction of Lanthanides and Actinides Using a Radiotracer Technique. Sep. Sci. Technol. 2001, 36, 1149-1162. (14) Elshani, S.; Smart, N.; Lin, Y.; Wai, C. M. Application of Supercritical Fluids to the Reactive Extraction and Analysis of Toxic Heavy Metals from Environmental Matrices. Sep. Sci. Technol. 2001, 36, 1197-1210. (15) Williams, C. J.; Aderhold, D.; Edyvean, R. G. J. Comparison between Biosorbents for the Removal of Metal Ions from Aqueous Solutions. Water Res. 1998, 32, 216-224. (16) Tait, B. K. The Extraction of Some Base-Metal Ions by Cyanex-301, Cyanex-302 and Their Binary Extractant Mixtures with Aliquot-336. Solvent Extr. Ion Exch. 1992, 10, 799. (17) Tait, B. K. Cobalt Nickel Separation-The Extraction of Cobalt(II) and Nickel(II) by Cyanex-301, Cyanex-302 and Cyanex272. Hydrometallurgy 1993, 32, 365-372. (18) Sole, K. C.; Hiskey, J. B. Solvent Extraction of Copper by Cyanex-272, Cyanex-302 and Cyanex-301. Hydrometallurgy 1995, 37, 129-147. (19) Hira, T.; Hashimoto, T.; Tsuboi, I.; Hino, A.; Komasawa, I. Extraction and Separation of Molybdenum and Vanadium Using Bis(2-ethylhexyl) monothiophosphoric Acid and Bis(2-ethylhexyl)phosphoric Acid. J. Chem. Eng. Jpn. 1995, 28, 85-90. (20) Erkey, C. Supercritical carbon dioxide extraction of metals from aqueous solutions: a review. J. Supercrit. Fluids 2000, 17, 259-287. (21) Wai, C. M. In Preconcentration Techniques for Trace Elements; Alfassi, Z. B., Wai, C. M., Eds.; CRC Press: Baca Raton, FL, 1991; p 101.
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Received for review October 11, 2002 Revised manuscript received January 24, 2003 Accepted January 25, 2003 IE020804I
