Anal. Chem. 2003, 75, 2276-2279
Ion Flotation of Cadmium(II) and Zinc(II) in the Presence of Proton-Ionizable Lariat Ethers Malgorzata Ulewicz,† Wladyslaw Walkowiak,‡ Youngchan Jang,§ Jong Seung Kim,§ and Richard A. Bartsch*,§
Department of Chemistry, Technical University of Czestochowa, 42-2000 Czestochowa, Poland, Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wroclaw University of Technology, 50-370 Wroclaw, Poland, and Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79490-1061
Competitive flotation of Cd(II) and Zn(II) from very dilute aqueous solutions by proton-ionizable lariat ethers in the presence of nonylphenol nona(ethylene glycol) ether as a nonionic foaming agent is reported. Influences of structural variation within the collector (identity of the pendent acidic group and lipophilicity), concentration of the collector, and pH of the aqueous solution are assessed. A monoethyl lariat ether phosphonic acid collector is found to exhibit high Cd(II)/Zn(II) flotation selectivity under certain conditions. Since their introduction more than three decades ago, the design, synthesis, and evaluation of macrocyclic ligands for metal ion recognition have received ever-increasing attention.1 Crown ethers and other macrocyclic multidentate ligands have been utilized successfully in metal ion separations in solvent extraction, artificial membrane transport, and ion-exchange systems.2,3 However, these separation methodologies are ineffective for removing metal ion species from very dilute aqueous solutions. Ion flotation, a separation method known since the early 1960s, can effectively remove ionic species from such dilute aqueous solutions;4 however, the selectivity of ordinary ionizable surfactants toward cations and anions is limited. A combination of the ion selectivity of macrocyclic carriers with sufficient water solubility and surface activity could provide a new generation of collectors for ion flotation. In a very limited number of studies, macrocyclic compounds have been utilized in metal species separations by ion flotation. †
Technical University of Czestochowa. Wroclaw University of Technology. § Texas Tech University. (1) For recent reviews on crown ether and related systems, see: (a) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1991, 91, 1721. (b) Izatt, R. M.; Bradshaw, J. S.; Pawlak, K.; Bruening, R. L.; Tarbet, B. J. Chem. Rev. 1992, 92, 1261. (c) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1995, 95, 2529. (d) Gokel, G. W. Crown Ethers and Cryptands; Royal Society of Chemistry: Cambridge, 1991. (e) Zhang, X. X.; Bradshaw, J. S.; Izatt, R. M. Chem. Rev. 1997, 97, 3313. (f) Balzani, V.; Credi, A.; Rammo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3349. (2) Brown, P. R.; Bartsch, R. A. In Inclusion Aspects of Membrane Chemistry; Osa, T., Atwood, J. L., Eds.; Kluwer Academic Publishers: Boston, 1991; pp 1-57. (3) Metal-Ion Separation and Preconcentration. Progress and Opportunities, ACS Symposium Series 716; Bond, A. H., Dietz, M. L., Rogers, R. D., Eds.; American Chemical Society: Washington, DC, 1999. (4) Sebba, F. Ion Flotation; Elsevier: Amsterdam, 1962. ‡
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Koide and Oka investigated resorcinol-type calix[4]arenes with alkyl side chains as collectors in competitive flotation of alkali metal cations and report selective flotation of Cs+ from dilute aqueous solutions.5 In a subsequent study, Koide and co-workers separated UO22+ from seawater with C-undecylcalix[4]resorcinarene phosphate collectors.6 Shulz and Warr studied alkali metal cation separations from dilute aqueous solutions by neutral macrocyclic complexing agents of a crown ether (18-crown-6) or a cryptand (cryptand 222) in combination with the anionic surfactant bis(2,2′)ethylhexylsulfosuccinate (AOT).7 In a different approach, Charewicz et al. employed crown ethers with pendent acidic groups (proton-ionizable lariat ethers) in combination with octylphenol deca(ethylene glycol) ether (Triton X-100), a neutral foaming agent and obtained efficient and selective floation of Sr(II) from dilute aqueous solutions.8 Both zinc and cadmium are present in nature as sulfides as a result of their “calcophilic” character.9 Zinc occurs widely, but as a result of their similarity, cadmium is present by isomorphous replacement in almost all zinc ores. Separation of Cd(II) and Zn(II) is important from industrial and environmental viewpoints because of the well-known toxicity of the former.10-12 Removal of Cd(II) from very dilute aqueous solutions is a particularly important problem. We now present results for competitive ion flotation of Cd(II) and Zn(II) from dilute aqueous solutions (cmetal ) 1.0 × 10-5 M) by lipophilic proton-ionizable lariat ethers 1-5 in combination with nonylphenol nona(ethylene glycol) ether (6). Effects of structural variation within the proton-ionizable dibenzo-16-crown-5 ether collector (identity of acidic groups and lipophilicity) and other experimental variables on Cd(II) and Zn(II) flotation are probed. (5) Koide, Y.; Oka, T. Bull. Chem. Soc. Jpn. 1993, 66, 2137. (6) Koide, Y.; Terasaki, H.; Hato, H.; Shosenji, J.; Yamada, K. Bull. Chem. Soc. Jpn. 1996, 69, 785. (7) Shulz, J. C.; Warr, G. G. Ind. Eng. Chem. Res. 1998, 37, 2807. (8) Charewicz, W. A.; Grabowska, J.; Bartsch, R. A. Sep. Sci. Technol. 2001, 36, 1479. (9) Greenwood, N. N.; Earshaw, E. Chemistry of the Elements, 2nd ed.; Butterworth/Heinemann: Oxford, 1997; p 1202. (10) Jarup, L.; Berglund, M.; Elinder, C.; Nordberg, G.; Vahter, M. Scand. J. Work. Environ. Health 1998, 24 (supplement), 1. (11) Staessen, J. A.; Buchet, J. P.; Ginucchio, G.; Lauweryss, R. R.; Lijnen, P.; Roels, H.; Fagard, R. J. Cardiovasc. Risk 1996, 3, 26. (12) Satarug, S.; Baker, J. R.; Reilly, P. E. B.; Moore, M. R.; Williams, D. J. Arch. Environ. Health 2002, 57, 69. 10.1021/ac026322y CCC: $25.00
© 2003 American Chemical Society Published on Web 04/08/2003
EXPERIMENTAL SECTION Reagents. The aqueous solutions were prepared with doubly distilled water of 0.1 µS/m conductivity at 25 °C. Reagent-grade NaOH, HCl, CdCl2, and ZnCl2 were obtained from POCh (Gliwice, Poland). Nonylphenol nona(ethylene glycol) ether (6), a nonionic foaming agent, and sodium dodecylbenzenesulfonate (7), an anionic surfactant, were purchased from Rokita (Poland) and BHD Reagents (England), respectively. The γ-radioactive isotopes of Cd-115m and Zn-65 were obtained from the Atomic Energy Institute (Swierk/Otwock, Poland). They were of sufficiently high specific activity to neglect the effect of carrier concentration (2.3 GBq/g for Cd-115m and 9.2 GBq/g for Zn-65. Ethyl iodomethylphosphonic acid and sym(R)(hydroxy)dibenzo-16-crown-5 compounds with R ) H, C3H7, C4H9, and C10H21 were prepared according to the literature methods.13,14 sym-(Decyl)dibenzo-16crown-5-oxyacetic acid (4) was prepared by the reported method.15 General Procedure for Synthesis of Sodium 3-[sym(R)dibenzo-16-crown-5-oxy]propanesulfonates. After removal of the protecting mineral oil from NaH (0.69 g, 60% dispersion in mineral oil, 17.3 mmol) by washing with dry pentane under nitrogen, 5.77 mmol of the lariat ether alcohol in dry THF (100 mL) was added slowly, and the mixture was stirred for 2 h at room temperature. A solution of 0.85 g (6.9 mmol) of 1,3propanesultone in dry THF (10 mL) was added with a syringe pump during a 1-h period, and the mixture was stirred for 6 h. After cooling to 0 °C, ice water was carefully added to destroy the excess NaH, and the THF was evaporated in vacuo. To the residue, CH2Cl2 was added, and the mixture was filtered. The filtrate was evaporated in vacuo to give an oil or solid that was crystallized or recrystallized from CH2Cl2-Et2O. Benzene was added to the solid, followed by azeotropic distillation with a Dean Stark apparatus to remove water. The benzene was removed in vacuo to give white crystals. Sodium 3-(sym-Dibenzo-16-crown-5-oxy)propanesulfonate (1). 80% yield, mp 158-159 °C. IR (deposit from CH2Cl2 solution (13) Ford-Moore, A. H.; Williams, J. H. J. Am. Chem. Soc. 1947, 69, 1465. (14) Bartsch, R. A.; Bitalac, L. P.; Cowey, C. L.; Elshani, S.; Goo, M.-J.; Huber, V. J.; Ivy, S. N.; Jang, Y.; Johnson, R. J.; Kim, J. S.; Luboch, E.; McDonough, J. A.; Pugia, M. J.; Son, B.; Zhao, Q. J. Heterocycl. Chem. 2000, 37, 1337. (15) Brown, P. R.; Hallman, J. L.; Whaley, L. W.; Desai, D. H.; Pugia, M. J.; Bartsch, R. A. J. Membr. Sci. 1991, 56, 195.
onto a NaCl plate): 1326, 1214 (SO3); 1252, 1127 (CO) cm-1. 1H NMR (CDCl3): δ 2.14-2.19 (m, 2H), 2.99-3.05 (t, 2H), 3.774.38 (m, 13H), 4.62-4.74 (m, 2H), 6.84-7.01 (m, 8H). Anal. Calcd for C22H27NaO9S‚0.7CH2Cl2: C, 49.58; H, 5.20. Found: C, 49.54; H, 5.38. Sodium 3-[sym-(Propyl)dibenzo-16-crown-5-oxy)propanesulfonate (2). 64% yield, mp 132-136 °C (foaming) and 149 °C (melting). IR (deposit CH2Cl2 solution onto a NaCl plate): 1325, 1214 (SO3); 1254, 1128 (CO) cm-1. 1H NMR (CDCl3): δ 0.981.05 (t, 3H), 1.46-1.66 (m, 4H), 2.08 (m, 2H), 3.07 (t, 2H), 3.734.30 (m, 12H), 4.72 (t, 2H), 6.65-6.99 (m, 8H). Anal. Calcd for C25H33NaO9S‚0.5H2O: C, 55.45; H, 6.28. Found: C, 55.64; H, 6.08. Sodium 3-[sym-(Decyl)dibenzo-16-crown-5-oxy)propanesulfonate (3). 90% yield, mp 117-118 °C. IR (deposit from CH2Cl2 solution onto a NaCl plate): 1325, 1214 (SO3); 1255, 1128 (CO) cm-1. 1H NMR (CDCl3): δ 0.85-0.92 (t, 3H), 1.21-1.28 (m, 16H), 1.61 (m, 2H), 2.08 (m, 2H), 3.06-3.11 (m, 2H), 3.75-3.91 (m, 4H), 4.07-4.12 (m, 4H), 4.22-4.33 (m, 4H), 4.76-4.77 (m, 2H), 6.867.00 (m, 8H). Anal. Calcd for C32H47Na O9S: C, 60.94; H, 7.51. Found: C, 60.60; H, 7.47. Synthesis of Monoethyl sym-[(Butyl)dibenzo-16-crown5-oxy]methylphosphonic Acid (5). After removal of the protecting mineral oil from NaH (0.20 g, 8.80 mmol) by washing with pentane under nitrogen, a solution of sym-(butyl)(hydroxy)dibenzo16-crown-5 in dry THF (100 mL) was added, and the mixture was stirred for 2 h at room temperature. A solution of ethyl iodomethylphosphonic acid (0.66 g, 2.64 mmol) in 10 mL of dry THF was added, and the mixture was stirred for 10 h at room temperature. After careful addition of water to destroy the excess NaH, the THF was evaporated in vacuo. To the residue, ethyl acetate and water (100 mL of each) were added. The aqueous later was separated, washed with ethyl acetate (3 × 100 mL), and acidified to pH 1 with 6 N HCl and CH2Cl2 (100 mL). The organic layer was separated, washed with 5% aqueous HCl (3 × 50 mL) and brine (2 × 50 mL), dried over MgSO4, and evaporated in vacuo to give a white solid with mp 123-125 °C in 90% yield. IR (deposit from CH2Cl2 solution onto a NaCl plate): 3600-1600 (POH, with broad maxima at 2220 and 1650), 1254 (PO), and 1123 (CO) cm-1. 1H NMR (CDCl ): δ 0.92-0.98 (t, 3H), 1.15-1.23 (t, 3H), 1.41 3 (br s, 4H), 1.91 (br s, 2H), 3.85-4.45 (m, 16H), 6.79-6.97 (m, 8H), 7.42 (br s, 1H). Anal. Calcd for C26H37O9P‚0.6CH2Cl2: C, 56.79; H, 7.00. Found: C, 56.82; H, 6.81. Competitive Metal Ion Flotation. The flotation experiments were conducted in a 45.7 cm × 2.4 cm glass column (Figure 1). The flow rate of water-saturated nitrogen was maintained at 12 mL/min through a sintered glass sparger of 20-30 µm, nominal porosity. The initial volume of the aqueous solution was 100 mL. The temperature was maintained at 20 ( 0.2 °C. The timedependence of the concentration of Cd(II) and Zn(II) in the bulk solution was recorded continuously during the ion flotation experiment by means of radioactive analytical tracers and γ radiation spectrometry following a procedure described by Charewicz and Niemiec16 and more recently by Walkowiak and Ulewicz.17 A ZSG-1 single channel, γ radiation spectrometer (Polon, Poland) with a SS-3S scintillation probe (Polon, Poland) was used as the detector (16) Charewicz, W.; Niemiec, J. Wiad. Chem. 1966, 20, 693. (17) Walkowiak, W.; Ulewicz, M. Physicochem. Prob. Miner. Processes 1999, 33, 201.
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Figure 1. Apparatus for ion flotation experiments (1 ) gas flow and pressure meter, 2 ) humidifier, 3 ) flotation column, 4 ) foam receiver, 5 ) spectrometric scintillation probe, 6 ) lead screen, 7 ) one-channel γ spectrometer, 8 ) recorder, 9 ) analog/digital converter, and 10 ) PC computer).
Figure 2. Competitive flotation kinetic curves for Cd(II) and Zn(II) with (a) sodium dodecylbenzenesulfonate (7) and (b) sodium lariat ether sulfonate 3 from dilute aqueous solutions at pH 4.0. [Cd(II)] ) [Zn(II)] ) 1.0 × 10-5 M, [7] ) 2.0 × 10-4 M, [3] ) 1.0 × 10-4 M.
of radiation intensity at characteristic energy bands of 179.4 and 80.2 fJ for Zn-65 and Cd-115m, respectively. RESULTS AND DISCUSSION Several reports have appeared in the literature in which neutral crown ethers are utilized to separate Cd(II) and Zn(II) by solvent extraction18,19 and transport across artificial membranes,20-26 but not by ion flotation. Because of the very dilute metal ion solutions involved in the following competitive metal ion flotation experiments, the preferred analytical method was radioactive analytical tracers and γ radiation spectrometry. To determine the efficiency and selectivity of metal ion flotation in the absence of a proton-ionizable lariat ether, competitive flotation of Cd(II) and Zn(II) from dilute aqueous solutions (1.0 × 10-5 M in each) at pH 4.0 by sodium dodecylbenzenesulfonate (7), an anionic surfactant, was performed. As can be seen from the results presented in Figure 2a, these two metal ion species are floated with almost the same efficiency. Of the proton-ionizable lariat ethers, only sodium 3-[sym-(decyl)dibenzo-16-crown-5-oxy]propanesulfonate (3) had sufficient foaming ability to be used in the absence of a neutral surfactant. Results for the competitive flotation of Cd(II) and Zn(II) by 3 under the same conditions as used for 7 are shown in Figure 2b. Although effective metal ion flotation was achieved with 3, both Cd(II) and Zn(II) were collected with similar efficiencies, so no appreciable separation was achieved. (18) Katsuta, S.; Tsuchiya, F.; Takeda, Y. Talanta 2000, 51, 637. (19) Billah, M.; Honjo, T. Fresenius’ J. Anal. Chem. 1997, 357, 61. (20) Izatt, R. M.; Bonald, R. L.; Geng, W.; Cho, M. H.; Christensen, J. J. Anal. Chem. 1987, 59, 2405. (21) Izatt, R. M.; LindH, G. C.; Bruening, R. L.; Bradshaw, J. S.; Lamb, J. D.; Christensen, J. J. Pure Appl. Chem. 1986, 58, 1453. (22) Cho, M. H.; Seon-Woo, K. H.; Heo, M. Y.; Lee, I.a C.; Yoon, C. J.; Kim, S. J. Bull. Korean Chem. Soc. 1988, 9, 292. (23) Cho, M. H.; Chun, H. S.; Kim, J. H.; Rhee, Ch. H.; Kim, S. J. Bull. Korean Chem. Soc. 1991, 12, 474. (24) Cho, M. H.; Shin, S. Ch. Bull. Korean Chem. Soc. 1995, 16, 33. (25) Dadfarnia, S.; Shamsipur, M. Bull. Chem. Soc. Jpn. 1992, 65, 2779. (26) Gupta, V, K.; Kumar, P. Anal. Chem. Acta 1999, 389, 205.
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Figure 3. Influence of R group variation in sodium 3-[sym(R)dibenzo-16-crown-5-oxy]propanesulfonate on competitive flotation of Cd(II) and Zn(II) from dilute aqueous solutions at pH 4.0: (a) R ) H, (b) R ) propyl, and (c) R ) decyl. [Cd(II)] ) [Zn(II)] ) 1.0 × 10-5 M, [1] ) [2] ) [3] ) 1.0 × 10-4 M, [6] ) 2.0 × 10-4 M.
Compounds 1-5 provide a series of proton-ionizable dibenzo16-crown-5 compounds in which the acidic function is varied from sulfonic acid to carboxylic acid to phosphonic acid monoethyl ester. For sodium lariat ether sulfonates 1-3, the lipophilicity increases as the group geminal to the acidic sidearm is varied from hydrogen to propyl to decyl. For this series of protonionizable lariat ethers, only 3 had sufficient foaming ability to be used in the absence of a neutral surfactant. In the following comparisons of proton-ionizable lariat ether structure upon the efficiency and selectivity of Cd(II) and Zn(II) flotation, all experiments were conducted in the presence of the neutral surfactant, nonylphenol nona(ethylene glycol) ether (6). To access the influence of varying the lipophilicity of the proton-ionizable lariat ether collector, sodium 3-[sym-(R)dibenzo16-crown-5-oxy]propanesulfonates with geminal R groups of hydrogen (in 1), propyl (in 2), and decyl (in 3) were examined. The effect of this structural variation for competitive flotation of Cd(II) and Zn(II) (1.0 × 10-5 M in each) from aqueous solution at pH 4.0 by 1.0 × 10-4 M collector and 2.0 × 10-4 M surfactant 6 for 60 min is shown in Figure 3. The flotation efficiency is seen
Figure 4. Influence of collector concentration for competitive flotation of Cd(II) and Zn(II) from dilute aqueous solution by (a) 1 (pH 4.0), (b) 2 (pH 4.0), (c) 3 (pH 4.0), (d) 4 (pH 8.0), and (e) 5 (pH 4.0). Cd(II)] ) [Zn(II)] ) 1.0 × 10-5 M, [1] ) [2] ) [3] ) [4] ) [5] ) 1.0 × 10-4 M, [6] ) 2.0 × 10-4 M.
to increase as R is varied: H , propyl < decyl. The very low maximal percent removal (M) values for 1 suggest that the collector has insufficient lipophilicity. Incorporation of a propyl group enhances the flotation efficiency. Changing from a propyl to a decyl group produces some additional increase in flotation efficiency. Despite a pronounced effect of R group variation upon flotation efficiency, the selectivity remains constant, with a small preference for flotation of Cd(II) over Zn(II). The effect of varying the collector concentration for competitive flotation of Cd(II) and Zn(II) (1.0 × 10-5 M in each) from aqueous solutions at pH 4.0 by proton-ionizable lariat ethers 1-3 and 5 and neutral surfactant 6 (2.0 × 10-4 M) and at pH 8.0 by collector 4 and neutral surfactant 6 for 60 min is presented in Figure 4. Variation of the collector concentration over a range (0.3-2.0) × 10-4 M had a relatively small influence on flotation efficiency for sodium lariat ether sulfonates 1 and 2 and lariat ether carboxylic acid 4. On the other hand, increasing the collector concentration from 0.3 × 10-4 to 1.0 × 10-4 M for sodium lariat ether sulfonate 3 and monoethyl lariat ether phosphonic acid 5 markedly enhances flotation of both Cd(II) and Zn(II) for the former, but only of Cd(II) for the latter. Further increase of the concentrations of collectors 3 and 5 from 1.0 × 10-4 to 2.0 × 10-4 M had little effect. Although flotation of Cd(II) and Zn(II) by collectors 1-4 takes place with only a small preference for Cd(II), monoethyl lariat ether phosphonic acid 5 provides excellent Cd(II)/Zn(II) selectivity. The influence of varying the pH of the aqueous solution for competitive flotation of Cd(II) and Zn(II) (1.0 × 10-5 M in each) by collectors 3-5 (1.0 × 10-4 M) and neutral surfactant 6 (2.0 × 10-4 M) for 60 min is presented in Figure 5. For lariat ether carboxylic collector 4 (Figure 5a), the flotation efficiency increases as the pH is enhanced in steps from 4 to 8. (As a result of hydrolysis at pH ) 8.0, the Zn(II) exists as Zn2+ (90.0%) and Zn(OH) (10.0%), and Cd(II), as Cd2+ (95.2%) and Cd(OH)+ (4.8%). Presumably, the greater flotation efficiency as the pH increases results from a greater proportion of the collector being in the carboxylate form at the higher pH values. For sodium lariat ether sulfonate collector 3 (Figure 5c), the pH dependence of flotation
Figure 5. Influence of pH for competitive flotation of Cd(II) and Zn(II) from dilute aqueous solution by (a) 4, (b) 5, and (c) 3. Cd(II)] ) [Zn(II)] ) 1.0 × 10-5 M, [3] ) [4] ) [5] ) 1.0 × 10-4 M, [6] ) 2.0 × 10-4 M.
efficiency is complex. Thus, the maximum percent removal (M) values for both Cd(II) and Zn(II) increase in going from pH 2 to pH 4 and then decrease at pH 6 and diminish even further at pH 8. The reason for this complex effect of pH on flotation efficiency is not apparent at the present time. For both collectors 3 and 4, the relative efficiencies for Cd(II) and Zn(II) flotation exhibit only minor variations as the pH of the aqueous solution is changed. The situation is quite different for monoethyl lariat ether phosphonic acid collector 5 with which the Cd(II) and Zn(II) flotation efficiencies respond differently to the pH changes (Figure 5b). Thus, at pH 4.0, the Cd(II)/Zn(II) separation is much greater than at pH 2.0, 6.0, or 8.0. At pH 4, the Cd(II)/Zn(II) flotation selectivity is 41. CONCLUSIONS Proton-ionizable lariat ethers 2-5 provide separation of Cd(II) and Zn(II) from very dilute aqueous solutions by ion flotation with the neutral surfactant nonylphenol nona(ethylene glycol) ether (6) as a foaming agent. The efficiency and selectivity of metal ion flotation are influenced by structural variation within the proton-ionizable lariat ether collector (identity of the acidic group and lipophilicity), the carrier concentration, and the pH of the aqueous solution. Under certain conditions, collector 5, a monoethyl lariat ether phosphonic acid, provides effective Cd(II) flotation with high Cd(II)/Zn(II) selectivity. The results of this study will be utilized in the design of new proton-ionizable lariat ethers for use in metal ion flotation. ACKNOWLEDGMENT This research was supported by the Polish Science Foundation (Grant 4 T09B10722 to M.U. and W.W.) and the Division of Chemical Sciences, Geosciences, and Biosciences of the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG03-94-ER14416 to R.A.B.). Received for review November 18, 2002. Accepted March 13, 2003. AC026322Y
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