pubs.acs.org/Langmuir © 2009 American Chemical Society
Ion Flotation of Co2+, Ni2+, and Cu2+ Using Dodecyldiethylenetriamine (Ddien) Zhendong Liu* Dow Advanced Materials, The Dow Chemical Company, 451 Bellevue Road, Newark, Delaware 19713
Fiona M. Doyle Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720 Received January 9, 2009. Revised Manuscript Received May 17, 2009 Ion flotation is a separation process involving the adsorption of a surfactant and counterions at an air/aqueous solution interface. It shows great promise for removing toxic heavy metal ions from dilute aqueous solutions. It was found that a chelating surfactant, dodecyldiethylenetriamine (Ddien), could selectively remove one metal ion over others at different pH values. Selectivity was attributed to the formation of surface-active chelated species at specific pH. Surface tension data show that [M-(Ddien)2]2+ is more surface-active than [M-(Ddien)]2+ and other Ddien species, thus the relative fraction of [M-(Ddien)2]2+ in the solution determined the metal ion flotation efficiency. The ion flotation results were consistent with the surface tension data and the relevant speciation diagrams. Theoretical discussion reveals that ΔG0ads and ΔG0chelation for the Ni(II) and Co(II) ions in the Ddien-Ni(II) and Ddien-Co(II) systems are more negative than those for Cu(II) in the Ddien-Cu(II) system.
1. Introduction Ion flotation is a technique for removing and concentrating metal ions from aqueous solutions. In this process, a surfactant, also called collector, is added to a solution sparged with gas bubbles. The surfactant molecules automatically position themselves on the bubble surfaces, with their polar functional groups facing the bulk solution. Certain non-surface-active counterions in the solution, also called colligends (e.g., metal ions), may also attach to the bubble surfaces, by either electrostatic or chemical interactions with the surfactant functional groups. When the bubbles emerge from the solution, they should form a stable foam phase, either through the intrinsic surface activity of the colligends or because of added surfactant. The collector and colligend adsorbed onto the bubbles report to the foam and can be physically separated from the solution.1-5 Figure 1 schematically shows a typical ion flotation process. In order for ion flotation to have any practical utility, for example, for separating ions of value from a process solution or detoxifying waste solutions prior to discharge, the collector must be capable of interacting selectively with the target species, without interacting with background ions. In general, it is relatively straightforward to separate two types of ions with different charges; the higher the valence of a colligend, the higher the selectivity for that species over other colligend ions with lower valence.6 In contrast, few studies have examined selectivity between ions with the same valence. Jorne *To whom correspondence should be addressed. E-mail: zhendongliu@ rohmhaas.com. (1) Sebba, F. Ion Flotation; Elsevier: New York, 1962. (2) Matis, K. A.; Mavros, P. Sep. Purif. Methods 1991, 20, 1–48. (3) Pinfold, T. A. Ion Flotation. In Adsorptive Bubble Separation Techniques; Lemlich, R., Ed.; Academic Press: New York, 1972. (4) Somasundaran, P. Sep. Sci. 1975, 10, 93–109. (5) Polat, H; Erdogen, D. J. Hazard. Mater. 2007, 148, 267–273. (6) Walling, C.; Ruff, E.; Thornton, J. L. J. Phys. Chem. 1957, 61, 486–489.
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and Rubin modeled the selectivity between ions with different effective aqueous radius, using the Gouy-Chapman electric double layer theory. They estimated a selectivity coefficient between Sr2+ and UO22+ that agreed well with their experimental results.7 Huang and Talbot used Jorne and Rubin’s model to compare the ion flotation of copper, cadmium, and lead with sodium dodecylbenzene sulfonate. They found that the order of increased selectivity was Cu2+Cl- and Cs+>Rb+>K+>Na+> Li+, which are the orders of decreasing crystal ionic radii. They further conducted a theoretical investigation into the adsorption selectivity between many ions with the same charge from a dehydration perspective.9-12 However, the model they developed was fairly qualitative in nature, based on dehydration parameters estimated from their experimental data, which left their theory uncorroborated. More recently, Jurkiewicz compared two flotation techniques for separating cadmium and zinc cations from dilute solutions. One is called adsorptive colloidal particle flotation and uses colloidal particles as carrying media for the targeted ions and surfactants as collectors to remove the colloidal particles from solutions. The other technique is pure ion flotation, in which the metal ions are directly removed by collectors. The author found that the zinc cations were removed more preferentially in (7) Jorne, J.; Rubin, E. Sep. Sci. 1969, 4, 313–324. (8) Huang, R. C. H.; Talbot, F. D. Can. J. Chem. Eng. 1973, 51, 709-713. (9) Schulz, J. C.; Warr, G. G. Ind. Eng. Chem. Res. 1998, 37, 2807-2811. (10) Schulz, J. C.; Warr, G. G. J. Chem. Soc., Faraday Trans. 1998, 94, 253257. (11) Morgan, J. D.; Napper, D. H.; Warr, G. G.; Nicol, S. K. Langmuir 1994, 10, 797-801. (12) Morgan, J. D.; Napper, D. H.; Warr, G. G. J. Phys. Chem. 1995, 99, 94589465.
Published on Web 07/10/2009
DOI: 10.1021/la900098g
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Figure 2. Molecular structure of dodecyldiethylenetriamine (Ddien).
used as a chelating collector to test this hypothesis. The molecular structure of Ddien is schematically shown in Figure 2. The metal ions used were Cu2+, Co2+, and Ni2+, which have similar crystal ionic radii (0.72, 0.74, and 0.72 A˚, respectively21). The Gibbs free energy terms such as ΔG0electric, ΔG0dehydration, and ΔG0hydrophobic would be very similar for these ions or their Ddien complexes. Hence, this is a good model system for studying the effect of ΔG0chelation.
2. Experimental Materials and Methods 2.1. Materials. Dodecyldiethylenetriamine (Ddien, 90%),
Figure 1. Typical ion flotation process.
the adsorptive colloidal particle flotation than in the pure ion flotation.13,14 On the basis of previous theoretical work, Liu and Doyle modeled the selectivity of adsorption of different ions at the air/solution interface using the Grahame equation. Terms were introduced to account for the Gibbs free energy of different types of interactions that might influence the cosorption of a colligend species with surfactant at the interface. These terms included ΔGelectric, ΔGdehydration, ΔGhydrophobic, and ΔGchelation, representing the electrical, dehydration, hydrophobic interaction, and chelating contributions, respectively.15-18 The equation took the form ΓMn þ ¼ δCb exp½ -ðΔG0 electric þ ΔG0 hydrophobic þ ΔG0 chelation þ ΔG0 dehydration Þ=RT
ð1Þ
where ΓMn+ is the adsorption density of a metal ion with a charge of n on the air/solution interface; δ and Cb are the thickness of the adsorbed species layer and the bulk metal ion concentration. The effects of ΔG0electric, ΔG0dehydration, and ΔG0hydrophobic have been studied by the authors and reported in several publications,15-18 but the effect of ΔG0chelation has not been closely examined. Girek et al. and Ulewicz et al. studied the effect of some complexing agents (a cyclodextrin polymer and crown ethers) on the competitive flotation of copper, zinc, and cadmium ions, but their studies were mainly experimental observations rather than theoretical development.19,20 Equation 1 suggests that introducing a negative ΔG0chelation could increase the adsorption density of metal ions compared with a system in which the target species are not chelated by the collector. Furthermore, differences in ΔG0chelation for a given chelating collector interacting with different metal ions could give selectivity between the metal ions, provided the other Gibbs free energy terms are similar for all metal ions. In this study, a nonionic surfactant, dodecyldiethylenetriamine (Ddien), was (13) Jurkiewicz, K. J. Colloids Interface Sci. 2005, 286, 559-563. (14) Jurkiewicz, K. Colloids Surf. A 2006, 276, 207-212. (15) Liu, Z.; Doyle, F. M. Colloids Surf. A 2000, 178, 93-103. (16) Liu, Z.; Doyle, F. M. Miner. Metall. Process. 2001, 18, 167-171. (17) Doyle, F. M.; Liu, Z. J. Colloid Interface Sci. 2003, 258, 396-403. (18) Liu, Z. Removal of Metal Ions from Dilute Solutions; Ph.D. Dissertation, University of California, Berkeley, 2001. (19) Girek, T.; Kozlowski, C. A.; Koziol, J. J.; Walkowiak, W.; Korus, I. Carbohydr. Polym. 2005, 59, 211-215. (20) Ulewcz, M.; Walkowiak, W.; Bartsch, R. A. Sep. Purif. Technol. 2006, 48, 264-269.
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purchased from Fisher Acros, was used as a collector without further purification (the impurities were believed to be homologues of Ddien that would not significantly differ from Ddien in their interaction with the metal ions). Analytical grade cupric chloride (dihydrate) and nickel chloride (dihydrate) were obtained from Aldrich. Cobalt chloride (hexahydrate, 99%) was purchased from Fisher Acros. The solution pH was adjusted using HNO3 and NaOH (Fisher). The ionic strength of some solutions was adjusted by NaNO3 (Fisher). All solutions were prepared by gravimetric methods using double distilled water.
2.2. Methods.
2.2.1. Surface Tension Measurements.
Surface tensions were measured at 25 °C with a Fisher Surface Tensiomat (Model 21) using the De No€ uy ring detachment method.22 The surface tension, γ, is given by γ¼
F -Wring f 4πR
ð2Þ
where F is the force that just detaches the ring from the solution, Wring is the weight of the ring, R is the radius of the ring, and f is an empirical factor correcting for R and the radius of the wire. Measurements were made in 100 mL beakers containing about 60 mL of the sample solutions. All glassware for the measurements was thoroughly cleaned using Nochromix-concentrated sulfuric acid solution and rinsed eight times with double distilled water. Between measurements, the ring was heated in a flame to remove any organic contaminants, then rinsed three times in double distilled water.18 For the surface tension measurements reported in Figure 11, the ionic strengths of solutions were kept constant at 8 mM, using appropriate additions of NaNO3. 2.2.2. Metal Ion Analysis. Metal ion concentrations were determined by a Perkin-Elmer 3110 Flame atomic absorption spectrophotometer (FAAS). All sample and standard solutions for atomic absorption analysis were acidified in HDPE plastic bottles to a final pH of 1.18 The standard solutions did not contain Ddien. The concentrations of metal ions in solutions prepared for ion flotation tests were also analyzed by FAAS and were consistent with the concentrations expected gravimetrically; this suggests no significant interference from Ddien. 2.2.3. Ion Flotation. Aqueous solutions for ion flotation were prepared in a 1000 mL beaker by adding appropriate amounts of surfactant and metal chloride salt. The final pH of the solution was then adjusted using 0.1 M HNO3 or 0.1 M NaOH. A Plexiglas column (0.854 m long and 0.044 m internal diameter) was used for ion flotation experiments; this was equipped with a 10-15 μm gas (21) Dean, J. A., Ed. Lange’s Handbook of Chemistry, 13th ed.; McGraw-Hill Inc.: New York, 1985. (22) Zhou, Z.; Gu, X.; Ma, J. Foundation of Colloidal Chemistry; Peking University: Peking, China, 1991.
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Table 1. Stability Constants Used for the Dien-Cu(II)-Ni(II) and Dien-Cu(II)-Co(II) Speciation Diagrams21,24 ligand Dien
hydroxide (OH-)
ions +
H Cu2+ Ni2+ Co2+ Cu2+ Ni2+ Co2+
log β1
log β2 +
9.8 ([HDien] ) 16.02 ([CuDien]2+) 10.9 ([NiDien]2+) 8.4 ([CoDien]2+) 7.0 (Cu(OH)+) 4.97 (Ni(OH)+) 4.3 (Co(OH)+)
log β3 2+
18.76 ([H2Dien] ) 20.88 ([Cu(Dien)2]2+) 19.1 ([Ni(Dien)2]2+) 14.5 ([Co(Dien)2]2+) 13.68 (Cu(OH)2) 8.55 (Ni(OH)2) 8.4 (Co(OH)2)
log β4 3+
22.96 ([H3Dien] )
17.0 (Cu(OH)3-) 11.33 (Ni(OH)3-) 9.7 (Co(OH)3-)
18.5 (Cu(OH)42-) 10.2 (Co(OH)42-)
sparger at the air inlet and a stopcock at the base for drainage. The column was operated in batch mode, using an initial solution volume of 650 mL. Airflow was measured by a rotometer flowmeter that had been calibrated by water displacement and was held around 20 mL/min. A port 0.30 m above the base was used for periodic sampling. About 3 mL of solution was drained from the port before withdrawing each sample, and samples were withdrawn very slowly to minimize entrainment of air bubbles. During experiments, any foam that rose above the top of the column was removed mechanically. Between experiments, the column was cleaned using 1 M HNO3, followed by three rinses with double distilled water.18
3. Results and Discussion 3.1. Selective Ion Flotation in Ddien-M1(II)-M2(II) Systems and the Corresponding Dien-M1(II)-M2(II) Speciation Diagrams. Species distribution diagrams provide valuable insight into the effect of chemical parameters on the behavior of a system. The stability constants of Ddien complexes with Cu2+, Co2+, and Ni2+ are not all available in the literature. However, Allison and Angelici measured the stability constants of methyl- and ethyl-substituted diethylenetriamine (Dien) complexes of copper and concluded that substitution at the central nitrogen does not affect the stability constants for the reaction of the Rdien ligands with Cu(II) to form Cu(Rdien)2+, although substitution on the terminal nitrogen atoms does destabilize the Cu(Rdien)2+ complexes.23 Furthermore, since the pKb values for the primary alkylamines are nearly identical,21 extending the chain length of the alkyl group substituted on the central nitrogen of Dien would not be expected to alter the stability constant of its transition metal complexes. Hence, all evidence suggests that the stability constants for metal-Dien complexes are similar to those of corresponding metal-Ddien complexes, such that metal-Dien speciation diagrams would provide relevant insight into the ion flotation behavior of metal-Ddien systems. Accordingly, speciation diagrams for the Dien-Cu(II)-Ni(II) and Dien-Co(II)-Cu(II) systems, analogues of the Ddien systems studied in the ion flotation tests, were constructed using the stability constants listed in Table 1. The counterion of the metal salts was not considered when calculating the speciation diagrams because of its much lower affinity for the metal ions. Figures 3a, 4a, 5a, and 6a show the residual metal concentrations as a function of time during the ion flotation tests at specified pH values for different metal ion-Ddien systems and different concentrations. The corresponding speciation diagrams for Dien-Cu(II)-Ni(II) and Dien-Cu(II)-Co(II) are shown in Figures 3b, 4b, 5b, and 6b. It is noteworthy that Ni(II) and Co(II) were removed much more preferentially than Cu(II) at around pH = 9.0, under conditions where the Ddien/Mtotal ratio was 2:1. At around pH = 6.0, under conditions where the Ddien/ Mtotal ratio was 1:2, there was no significant removal of any (23) Allison, J. W.; Angelici, R. J. Inorg. Chem. 1971, 10, 2233-2238. (24) Smith, R. M., Martell, A. E., Eds. Critical Stability Constants; Plenum Press: New York, 1976.
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Figure 3. (a) Selective ion flotation in the Ddien-Cu(II)-Ni(II) system. Initial feed solution composition: [Ddien] = 0.6 mM, [Cu(II)] = [Ni(II)] = 0.15 mM, pH = 8.86. Air flow rate = 18 mL/min. Feed solution volume = 650 mL. (b) Speciation diagram for the Dien(L)-Cu(II)-Ni(II) system (only the Dien-containing species are shown). Solution composition: [Dien]total = 0.6 mM, [Cu(II)]total = [Ni(II)]total = 0.15 mM. NiL22+ line is bold. CuL22+ line is broken. The dashed, vertical line denotes pH=8.86.
metal ions, either from a relatively concentrated solution ([Cu(II)]total = [Ni(II)]total = 0.6 mM, Figure 5a) or from a very dilute solution ([Cu(II)]=[Co(II)]=0.1 mM, Figure 6a). The conditions used to generate Figures 5 and 6 were selected with reference to the corresponding Dien speciation diagrams to ensure that there was a single, predominant ligand-bearing DOI: 10.1021/la900098g
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Figure 4. (a) Selective ion flotation in the Ddien-Cu(II)-Co(II) system. Initial feed solution composition: [Ddien] = 0.6 mM, [Cu(II)] = [Co(II)] = 0.15 mM, pH = 9.20. Air flow rate = 18 mL/min. Feed solution volume = 650 mL. (b) Speciation diagram for the Dien(L)-Cu(II)-Co(II) system (only the Diencontaining species are shown). Solution composition: [Dien]total= 0.6 mM, [Cu(II)]total = [Co(II)]total = 0.15 mM. CoL22+ line is bold. CuL22+ line is broken. The dashed, vertical line denotes pH=9.20.
species (CuL2+) and also to specifically test whether concentration affected ion flotation. Comparing the Ddien ion flotation results with the corresponding Dien speciation diagrams, it appears that the removal of a given ion correlates well with the fraction of the ion present as the [M-(Ddien)2]2+ species, whereas the [M-(Ddien)]2+ species was not appreciably surface-active. Ananthpadmanabhan and Somasundaran demonstrated that, in the oleate system, the R2Hspecies (R represents an oleate ion) is the most surface-active species.25,26 This surface activity results from the configuration of the dimer with a single charged functional group, which allows the two hydrophobic hydrocarbon chains to interact fully, with the polar head groups residing on the same end. Hence, this dimer can (25) Ananthpadmanabhan, K.; Somasundaran, P.; Healy, T. W. Trans. Inst. Min. Metall. 1979, 266, 2003-2009. (26) Ananthpadmanabhan, K.; Somasundaran, P. Oleate Chemistry and Hematite Flotation. In Interfacial Phenomena in Mineral Processing; Engineering Foundation, 1981.
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Figure 5. (a) Ion flotation in the Ddien-Cu(II)-Ni(II) system. Initial feed solution composition: [Ddien] = 0.6 mM, [Cu(II)] = [Ni(II)] = 0.6 mM, pH = 6.17. Air flow rate= 18 mL/min. Feed solution volume = 650 mL. (b) Speciation diagram for the Dien (L)-Cu(II)-Ni(II) system (only the Dien-containing species are shown). Solution composition: [Dien]total = 0.6 mM, [Cu(II)]total = [Ni(II)]total =0.6 mM. NiL2+ line is bold. CuL2+ line is broken. The dashed, vertical line denotes pH = 6.17.
reside at a solution vapor interface with the polar groups residing in the polar solvent and the hydrophobic chains residing in the nonpolar vapor. In contrast, if two oleate ions are associated as R22-, the electrical repulsion between the charged polar head groups drives these head groups to opposite ends of the dimer. Such a species has little surface activity. In metal-Ddien systems, metal ions associate with neutral Ddien ligands to form [M-(Ddien)]2+ and [M-(Ddien)2]2+ complexes. The polar head group(s) of the Ddien will interact with the metal ion, and the hydrophobic dodecyl chain(s) will not. The configurations are schematically shown in Figure 7. Since the [M-(Ddien)2]2+ complex has one more Ddien than the [M-Ddien]2+ complex, it should be more surface-active. The experimental results were quite consistent with this hypothesis. At pH near 9.0, the proportion of [Ni(Dien)2]2+ or [Co(Dien)2]2+ species present is much larger than that of [Cu(Dien)2]2+. Assuming similar speciation in the metal-Ddien systems, this would account for the preferential removal of Ni(II) and Co(II) over Cu(II). At pH around 6.0, with a much lower M/Dien ratio, however, there is essentially no [M-(Dien)2]2+ present. None of the ions was appreciably removed by Ddien at this pH, not even copper, with which almost all the Ddien would have been associated. Langmuir 2009, 25(16), 8927–8934
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Figure 7. Schematic configurations of [M-(Ddien)2]2+ and [M-
Ddien]2+ complexes.
Figure 6. (a) Ion flotation in the Ddien-Cu(II)-Co(II) system. Initial feed solution composition: [Ddien] = 0.1 mM, [Cu(II)] = [Co(II)]=0.1 mM, pH = 6.42. Air flow rate=18 mL/min. Feed solution volume = 650 mL. (b) Speciation diagram for the Dien(L)-Cu(II)-Co(II) system (only Dien-containing species are shown). Solution composition: [Dien]total = 0.1 mM, [Cu(II)]total = [Co(II)]total=0.1 mM. CoL2+ line is bold. CuL2+ line is broken. The dashed, vertical line denotes pH=6.42.
For a more quantitative analysis, assume that [M-(Ddien)2]2+ is the primary species removed during the ion flotation process, and that its partition coefficient between the gas and aqueous phases, ΓM(Ddien)2/CM(Ddien)2,b, is independent of the nature of the metal ion included in the complex . This gives ΓNiðDdienÞ2 ΓCoðDdienÞ2 ΓCuðDdienÞ2 ¼ ¼ CNiðDdienÞ2 , b CCoðDdienÞ2 , b CCuðDdienÞ2 , b
ð3Þ
and ΓNiðDdienÞ2 CNiðDdienÞ2 , b ΓCoðDdienÞ2 CCoðDdienÞ2 , b ¼ and ¼ ð4Þ ΓCuðDdienÞ2 CCuðDdienÞ2 , b ΓCuðDdienÞ2 CCuðDdienÞ2 , b where ΓM(Ddien)2 and CM(Ddien)2,b refer to the adsorption density and bulk concentration of [M-(Ddien)2]2+, where M is Ni(II), Co(II), and Cu(II). Assuming that the distribution of species in the M-Ddien system was identical to that in the M-Dien system, the Dien-M(II)-Cu(II) speciation diagrams (Figures 3b and 4b) give CNi(Ddien)2,b/CCu(Ddien)2,b and CCo(Ddien)2,b/CCu(Ddien)2,b as 3.86 Langmuir 2009, 25(16), 8927–8934
and 2.00 (at pH 8.86 for Ni and 9.2 for Co). From eq 4, ΓNi(Ddien)2/ ΓCu(Ddien)2 and ΓCo(Ddien)2/ΓCu(Ddien)2 should also be 3.86 and 2.00 at these pH values. Theoretically, these adsorption density ratios, based on the initial solution composition, should be the same as the ratios between the initial removal rates of M(II) and Cu(II) in the Ddien-M(II)-Cu(II) ion flotation systems, provided that M(Ddien)2 was the only surface-active species containing the metal ion and the assumption of a constant partition coefficient is valid. In fact, however, the initial metal removal rate ratios appeared larger than the predicted adsorption density ratios; the ratio of the initial removal rates of Ni(II) and Cu(II) estimated from Figure 3a was 11.0, and the ratio of the initial removal rates of Co(II) and Cu(II) estimated from Figure 4a was 3.73. The differences may be due to the pH of the weakly buffered solution decreasing slightly with time due to the uptake of CO2 from the air being sparged through the solution. From inspection of Figures 3b and 4b, this would increase the initial metal ion removal rate ratio over that predicted on the basis of the original pH. Although the overall pH depression would not be expected to be significant (particularly for initial removal rates), the flux of CO2 from the air bubbles into the solution could have created an interfacial pH somewhat lower than the bulk pH, and the ion flotation behavior would be dominated by the speciation at this localized pH. Alternatively, the discrepancy could reflect the limitation of using the speciation diagrams for the DienM(II)-Cu(II) systems to predict the behavior of the DdienM(II)-Cu(II) systems. Consideration of Figures 3b and 4b shows that even a tiny shift of the [M(Ddien)2]2+ lines to higher pH would markedly increase the ratio between [M-(Ddien)2]2+ and [Cu(Ddien)2]2+. Finally, the inconsistency might be due to the invalidity of the assumption that the partition coefficient of [M-(Ddien)2]2+ is independent of the nature of the metal ion in the complex. The speciation diagram would better predict the ratios of the initial metal ion removal rates if the partition coefficients for [Ni(Ddien)2]2+ and [Co(Ddien)2]2+ were higher than that of [Cu(Ddien)2]2+. Subtle geometric differences between the complexes, for example, Jahn-Teller distortion of [Cu(Ddien)2]2+, could affect the partition coefficients. 3.2. Surface Tensions of Ddien-Metal Ion Systems and Corresponding Dien-Metal Ion Speciation Diagrams. To test the hypothesis that the species [M-(Ddien)2]2+ is surfaceactive and responsible for the removal of metal ions during ion flotation, whereas the species [M-Ddien]2+ is not, the surface tensions of Ddien-Ni(II), Ddien-Cu(II), and Ddien-Co(II) solutions were measured as a function of pH and are shown in Figures 8a, 9a, and 10a. These solutions had a Ddien/M concentration ratio of 2:1. The corresponding speciation diagrams for the metal ion-Dien systems are shown in Figures 8b, 9b, and 10b. DOI: 10.1021/la900098g
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Figure 8. (a) Surface tensions of Ddien-Ni(II) solutions at various pH (no ionic strength adjustment). Solution composition: [Ddien]=0.3 mM, [Ni(II)]=0.15 mM. (b) Speciation diagram for the Dien-Ni(II) system (only the Dien-containing species are shown). Solution composition: [Dien] = 0.3 mM, [Ni(II)] = 0.15 mM. The [Ni(Dien)2]2+ line is bold and broken.
Figure 8b shows that [NiDien]2+ reaches a maximum concentration around pH 6.6. Therefore, if [NiDdien]2+ were the most surface-active Ni-bearing species, the surface tension would be expected to drop near pH 6.6 in the Ddien-Ni(II) system, then increase at higher pH. From Figure 8a, this was evidently not the case. Instead, the surface tension decreased sharply, from 41.9 to 25.5 mN/m between pH 7.7 and 9.3, then remained constant at higher pH. This implies that a strongly surface-active species started to be present in appreciable concentrations around pH 7.7, and that the concentration increased with increasing pH, reaching a plateau at pH 9.3. The speciation diagram (Figure 8b) confirms that [Ni(Dien)2]2+ (the bold, broken line) fits this description well. It starts to be present in appreciable concentrations around pH 7.0, then the concentration increases from pH 7.0 to 9.6, reaching a plateau at pH g 9.6. Thus, it is most probable that [Ni(Ddien)2]2+ was the most surface-active species in the DdienNi(II) system. The surface tensions of Ddien-Cu(II) solutions (Figure 9a) show a dependence on pH that is qualitatively similar to that of Ddien-Ni(II) solutions. The surface tensions were insensitive to pH between 3.0 and 8.0, decreased sharply from 56.2 to 27.1 mN/m between pH 8.2 and 9.2, then were again insensitive to pH above 8932 DOI: 10.1021/la900098g
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Figure 9. (a) Surface tensions of Ddien-Cu(II) solutions at various pH (no ionic strength adjustment). Solution composition: [Ddien]=0.3 mM, [Cu(II)]=0.15 mM. (b) Speciation diagram for the Dien-Cu(II) system (only the Dien-containing species are shown). Solution composition: [Dien] = 0.3 mM, [Cu(II)] = 0.15 mM. The [Cu(Dien)2]2+ line is bold and broken.
pH 9.2. In the corresponding speciation diagram for the DienCu(II) system (Figure 9b), [Cu(Dien)2]2+ (the bold, broken line) and Dien start to be present around pH 8.0. Their concentrations increase from pH 8.0 to 10.5, and finally level off at pH g 10.5. This behavior is roughly consistent with the surface tension curve for the Cu-Ddien system, except that the lowest surface tension was reached at pH around 9.2 rather than 10.5. Assuming that the speciation of the Dien-Cu(II) system is a reasonable, if not perfect, model of the speciation in the Ddien-Cu(II) system, it is unlikely that Ddien is responsible for the lowest surface tensions. As discussed earlier, Ddien would be expected to be less surface-active because it only has one dodecyl chain per molecule, while [Cu(Ddien)2]2+ has two. The difference in the pH values at which the minimum surface tension and the maximum concentration of [Cu(Dien)2]2+ were attained probably reflects subtle differences in the relative stabilities of the species in the Dien-Cu(II) and Ddien-Cu(II) systems or the presence of other unknown surface-active species. Langmuir 2009, 25(16), 8927–8934
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Figure 11. Surface tensions of Ddien-Ni(II), Ddien-Co(II), and Ddien-Cu(II) solutions at different Ddien concentrations: pH= 9.0; ionic strength=8 mM. The ratio of the concentration between Ddien and the metal ion is 2:1. The broken line denotes the DdienCu(II) system. The solid line represents the Ddien-Ni(II) and Ddien-Co(II).
Figure 10. (a) Surface tensions of Ddien-Co(II) solutions at various pH (no ionic strength adjustment). Solution composition: [Ddien] = 0.3 mM, [Co(II)] = 0.15 mM. (b) Speciation diagram for the Dien-Co(II) system (only the Dien-containing species are shown). Solution composition: [Dien] = 0.3 mM, [Co(II)]= 0.15 mM. The [Co(Dien)2]2+ line is bold and broken.
In contrast to the Ddien-Ni(II) and Ddien-Cu(II) systems, the surface tensions of Ddien-Co(II) solutions decreased smoothly with increasing pH (Figure 10a). The magnitude of the surface tension drop was also limited (from 37.7 mN/m at pH 3.0 to 26.8 mN/m at pH 9.9). Again assuming that the speciation in the Dien-Co(II) system parallels that in the Ddien-Co(II) system, it appears that [Co(Ddien)2]2+ is the species with high surface activity. However, the relatively low surface tensions at lower pH values (3.0-7.5) and the smoother transition suggest that there are other fairly surface-active species at lower pH values. Structural considerations indicate that [H2Ddien]2+ might be more surface-active than either [H3Ddien]3+ or [M-Ddien]2+. The diethylenetriamine functional groups in [H3Ddien]3+ or [MDdien]2+ would have to reside in the aqueous phase, with all their nitrogen atoms bonded with aqueous species H3O+ or [M(H2O)n]2+. For [H2Ddien]2+, however, the tertiary amine need not reside in the solution phase. Rather, it would be energetically favorable for this group to reside in the air phase, thereby allowing the two ethylene chains in the functional group to leave the aqueous solution. Such an arrangement of [H2Ddien]2+ at the solution/air interface would increase the effective hydrocarbon chain length of the [H2Ddien]2+ species, thereby making it more surface-active. Close inspection of Figures 8b, 9b, and 10b reveals that the fraction of [H2Dien]2+ at pH 3.0-7.5 decreases in the Langmuir 2009, 25(16), 8927–8934
order Dien-Co(II)>Dien-Ni(II)>Dien-Cu(II) in the DienM(II) systems, which is very consistent with the observed surface tensions over this pH range in these three systems (order of surface tension: Ddien-Co(II) [H3Ddien]3+ = [M-Ddien]2+. At lower pH values (3.0-7.5), [H2Ddien]2+ is the dominant surface-active species whose abundance dominates the surface tensions of Ddien-M(II) solutions. Since the metal-bearing species [M-Ddien]2+ is less surface-active, there was no significant removal of metal from the DdienNi(II)-Cu(II) and Ddien-Co(II)-Cu(II) systems at pH near 6.0 (Figures 5a and 6a). At higher pH values (>8.5), the metalbearing species [M-(Ddien)2]2+ has an overriding effect on the surface tension and allowed appreciable removals of metal ions at pH near 9.0 (Figures 3a and 4a). 3.3. Surface Tensions and Qualitative Evaluation of ΔG0chelation. As noted above, the overall Gibbs free energy for the adsorption of a metal ion from the bulk solution to the air/ water interface can be expressed in terms of different terms: electrostatic, hydrophobic interaction, chelation, and dehydration:15 ΔG0 ads ¼ ΔG0 electric þ ΔG0 hydrophobic þ ΔG0 chelation þ ΔG0 dehydration
ð5Þ
Assuming that the electrostatic, hydrophobic interaction, and dehydration terms are similar for the Ni(II), Co(II), and Cu(II), DOI: 10.1021/la900098g
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Article
ΔG0chelation cannot be simply interpreted as -RTln K (where K is the stability constants of the M(II)-Ddien complexes) because Table 1 shows that the Cu(II)-Ddien complexes are more stable than those of the Ni(II)-Ddien and Co(II)-Ddien complexes. The impact of chelation on ion flotation depends upon the abundance of the surface-active species. Although the Cu(II)-Dien complexes are more stable than the Ni(II)-Dien and Co(II)-Dien complexes, the greater propensity of Cu(II) to hydrolyze results in less chelated Cu(II) than chelated Ni(II) or Co(II). Figure 11 shows the surface tensions of Ddien-Ni(II), DdienCo(II), and Ddien-Cu(II) solutions as a function of the Ddien concentration (log scale) at pH 9.0. It appears that in some ion flotation experiments (Figures 3a and 4a) Ddien initially existed in feed solutions as micelles (the initial concentration of Ddien is 0.6 mM, marginally above the CMC). As the Ddien concentration in the bulk solution decreased, the micelles dissociated. However, the micelles appear to have no deleterious effects on the removal of colligends ions, as would be expected. It is also seen that the surface tension curves for the DdienNi(II) and Ddien-Co(II) systems lie to the left of that for the Ddien-Cu(II) system over a wide Ddien concentration range. This is similar to the behavior seen in homologous series of surface-active compounds such as alcohols and monocarboxylic acids, where the concentration required to yield a certain surface tension decreases regularly as one ascends the series, as noted by Traube.27 Langmuir showed that this decrease in concentration needed to attain a given (depressed) surface tension is due to a regular decrease in Gibbs free energy for adsorption with increasing hydrocarbon chain length.28 In the case of Figure 11, this clearly shows that ΔG0ads for the surface-active species in the Ddien-Ni(II) and Ddien-Co(II) systems is more negative than that of the surface-active species in the Ddien-Cu(II) system: ΔG0 ads, Ddien-NiðIIÞ or ΔG0 ads, Ddien-CoðIIÞ < ΔG0 ads, Ddien-CuðIIÞ Assuming that ΔG0electric, ΔG0hydrophobic, and ΔG0dehydration are similar for the Ddien complexes with Ni(II), Co(II), and Cu(II), it follows that ΔG0chelation must also be more negative in the Ddien(27) Traube, I. Ann. 1891, 265, 27. (28) Langmuir, I., J. Am. Chem. Soc. 1917 , 39, 1848.
8934 DOI: 10.1021/la900098g
Liu and Doyle
Ni(II) and Ddien-Co(II) systems than in the Ddien-Cu(II) system ΔG0 chelation, Ddien-NiðIIÞ or ΔG0 chelation, Ddien-CoðIIÞ < ΔG0 chelation, Ddien-CuðIIÞ It should be noted that the assumption of similarity of ΔG0electric, ΔG0hydrophobic, and ΔG0dehydration may not be totally valid. Whereas ref 21 gives the crystal ionic radii of Cu2+, Ni2+, and Co2+ ions as 0.72, 0.72, and 0.74 A˚, respectively, ref 29 gives these as 0.73, 0.69, and 0.65. With these values, ΔG0dehydration would be more negative for Cu(II) than for Ni(II) or Co(II). Any differences in the structures of the [M-(Ddien)2+] complexes would affect ΔG0hydrophobic. Regardless, one sees that the use of Ddien as a surface-active chelating agent allows the selective ion flotation of nickel or cobalt over copper.
4. Conclusions The selective ion flotation behavior of Ni(II), Co(II), and Cu(II) using Ddien can be qualitatively explained by speciation diagrams for the Dien M1(II)-M2(II) systems. The hypothesis that [M-(Ddien)2]2+ is the most surface-active species seems consistent with both the ion flotation results and the surface tension data. At pH values around 9, Ni(II) and Co(II) ions were preferentially removed by Ddien over Cu(II) ions, which indicates that the Gibbs free energies for adsorption (ΔG0ads) and most likely for chelation (ΔG0chelation) of Ni(II) and Co(II) by Ddien are more negative than that for Cu(II). At pH around 6, with a lower Ddien/metal ratio, none of the metal ions was appreciably removed by Ddien, suggesting that the predominant metalbearing species were not surface-active. The dependence of the ion flotation results on pH was consistent with the surface tension data. It should be cautioned that the speciation data of the DienM(II)-Cu(II) system did not quantitatively predict the ratio between the initial removal rates of M(II) and Cu(II) in the Ddien-M(II)-Cu(II) ion flotation system, presumably due to weak buffering of the solution pH, slight differences between the formation constants of Dien-M(II) and Ddien-M(II) species, and differences in the partition coefficient of the different M(Ddien)2 complexes. (29) Lide, D. R., Ed. Handbook of Chemistry and Physics, 82nd ed.; CRC Press LLC: Boca Raton, FL, 2001.
Langmuir 2009, 25(16), 8927–8934
