Ind. Eng. Chem. Res. 1998, 37, 2807-2811
2807
Selective Flotation of Ions by Macrocyclic Complexation Jamie C. Schulz and Gregory G. Warr* School of Chemistry, F11, University of Sydney, Sydney NSW 2006, Australia
Foam fractionation is used to selectively extract one alkali-metal ion from a mixed electrolyte through a combination of preferential complexation by an added macrocycle and selective binding of the macrocyclic complex to a surfactant film. Solution and interfacial equilibrium considerations are used to determine optimal conditions for selective extraction, and the predictions tested using two typical macrocycles, Cryptand 222 and 18-crown-6. Proof of principle of tunable extraction is illustrated by the change in selectivity of alkali-metal ions in the presence of these two ligands. Introduction Ion flotation has often been proposed and investigated as a method for the selective removal of dissolved solutes from mixed aqueous systems (Sebba, 1962). Potential applications include mineral beneficiation, as in the recovery of silver and gold leached as cyano complexes from mine tailings (Galvin et al., 1992), in the extraction of heavy metals from wastewater streams, and in the removal of organic contaminants (Thalody and Warr, 1995, 1997). Widespread industrial use of ion flotation is often hindered by the lack of specificity observed in the interactions between ion and surfactant: Changes in surfactant structure often yield little change in selectivity (Kellaway and Warr, 1997; Schulz and Warr, 1998). There has over the past several years been significant activity surrounding the preparation and use of surfactants with hydrophilic headgroups containing macrocyclic ligands (Arleth, 1995; Koide et al., 1980; Moroi et al., 1979; Ozeki et al., 1989, 1990a,b; Xie et al., 1994). These compounds hold great interest and potential as ion-selective membranes and collectors for foam fractionation and have novel interfacial characteristics highly sensitive to cation type. As a means to achieve ion selectivity, the synthesis of macrocyclic surfactants is highly reliable; complexed ions are necessarily attached to the surfactant. However, the inclusion of an ion into the surfactant transforms it from a nonionic into an ionic surfactant, thereby dramatically increasing its water solubility and critical micelle concentration and lowering its surface activity. Micelles and adsorbed films of these species will be mixtures which are perforce enriched in the more surface-active component. Lunkenheimer has recently demonstrated the critical effect that even trace amounts of surface-active impurities can have on the behavior of interfacial films (Lunkenheimer and Miller, 1987). These factors complicate the understanding of adsorbed films of macrocyclic surfactants and limit their potential application. Any separations process based on micelles or adsorbed films of macrocyclic surfactants will necessarily contain some fraction of surfactants with uncomplexed headgroups. In this work we propose and investigate an alternative strategy in the spirit of macrocycle/surfactant * Author to whom correspondence should be addressed. E-mail:
[email protected].
supramolecular assembly. Surfactants and watersoluble macrocycles self-assemble to create an interfacial film enriched in a particular cation to the exclusion of uncomplexed ions. As a separation process this has the economic advantages of not requiring specialized synthesis of either surfactant or macrocycle. As there is no macrocyclic surfactant present in solution, there can be none at the interface (or in micelles) to complicate analysis. In the experiments described a separation process is examined directly. A mixed film of surfactant, macrocycle, and cations is generated in situ in a foam flotation column. Alkali-metal cations are used because of their low selectivities and hence poor native separation in foam fractionation experiments. The composition of the film formed at the air/solution interface is characterized by an ion-exchange or selectivity coefficient. We relate this to the properties of the complex, particularly the stability constant and the specificity with which a macrocycle complexes a particular cation. Other separations, such as micellar-enhanced ultrafiltration (Scamehorn et al., 1989), follow from the similar co-assembly behavior of macrocyclic ligands in micellar aggregates (D’Aprano et al., 1994; Evans et al., 1986, 1988; Miller et al., 1987; Quintela et al., 1987; Sesta et al., 1992, 1995). First, we define the experimental quantities used to describe the adsorbed film and flotation system, briefly analyze the expected behavior, and then proceed to the experiments. Selectivity Equilibria Selective ion binding at an anionic surfactant film may be characterized by an ion-exchange equilibrium of the type
X+(ads) + Y+(aq) h X+(aq) + Y+(ads)
(1)
where the ions are denoted X+ and Y+, and (ads) denotes surface species. Electrical neutrality of the interface guarantees a 1:1 exchange between the bulk and the surface. The selectivity coefficient for this process is + KXY+
)
ΓY+[X+] ΓX+[Y+]
(2)
where Γ denotes surface excess and [X+] and [Y+] are
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2808 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
bulk concentrations. In the absence of any added macrocycle, we will refer to this as the native selectivity, + KXY+(native). For practical applications the recovery, selectivity, or upgrade ratio of a specific ion is the quantity of interest (Evans et al., 1995; Warr and Prud’homme, 1995). These, however, generally depend on system parameters such as surfactant concentration, gas flow rate, column geometry, and foam wetness. On the other hand, the selectivity coefficient is independent of such conditions and hence provides a robust measure of selective binding to an interface (Morgan et al., 1994). In a continuous flow process such as a flotation column, bubbles entering the column are coated with surfactant and an exactly neutralizing layer of counterions as they rise and are transported into the foam. In a perfectly mixed flotation vessel the rate of removal of one ion, say X+, is therefore
d[X+] )
ΓX+ dA V
(3)
where dA is the flux of area through the column, which has volume V. The relative rates of removal of two bound species thus satisfies
d[Y+] +
)
d[X ]
ΓY+ ΓX+
+ + [Y ] ) KXY+ + [X ]
(4)
The selectivity coefficient is obtained by integrating eq 4. +
ln[Y+] ) KXY+ ln[X+] + C
(5)
C is a constant of integration which depends on initial solution conditions. By monitoring the relative depletion rates of competing ions X+ and Y+, we obtain the selectivity coefficient for their competitive uptake as the gradient of a log-log plot of eq 5. In the mixed system containing two ions and a macrocyclic ligand, we analyze for total ion concentrations, and hence the apparent selectivity coefficient, + KXY+(overall), may be written most generally as +
KXY+(overall) )
(ΓY+ + ΓYL+)([X+] + [XL+]) (ΓX+ + ΓXL+)([Y+] + [YL+])
KXL+ )
[XL+] [X+][L]
These equilibria lead to the overall selectivity being expressed as +
KXY+(overall) ) +
+ KXY+(native)
(1 + KYL+KYYL+ [L]) (1 + KXL+[L]) +
(1 + KXL+KXXL+ [L]) (1 + KYL+[L])
(7)
At the interface, macrocyclic complexes participate in ion-exchange equilibria according to, for example,
(9)
+
Although KXY+(overall) (eq 6) is not an equilibrium constant, eq 9 shows it to be a constant at a particular concentration of a ligand. In a flotation experiment the ions are removed by a continuous flow of gas bubbles; however, the ligand concentration in solution should remain constant to a good approximation. Hence, we may use eq 5 to describe the variation of total concen+ trations of competing ions and derive KXY+(overall). For the case of competitive binding of macrocyclic complexes, the ions are completely enclosed by the ligand. Their interactions with the surfactant in the foam should be virtually independent of the enclosed YL+ ion, and the selectivity should be unity, KXL + ) 1. We expect this to be a good approximation for bicyclic ligands, but perhaps less so for crown compounds where the ion is still somewhat exposed. Selectivity enhancement is optimized when one ion is fully complexed while the other is not. Macrocyclic ligands are expected to bind strongly at surfactant interfaces compared with bare metal ions, primarily through hydrophobic interactions (D’Aprano et al., 1994). Optimum selectivity is thus defined as the selectivity of the complex YL+ over the bare second ion + + X+, KXY+(overall) e KXYL+ . This limit is obtained from eq 9 by considering the case KYL+[L] . 1 . KXL+[L], which yields +
+
+
KXY+(overall) ) KXY+(native)KYYL+ +
) KXYL+
(10)
+
KXYL+ thus provides a loose upper bound for the in+ + crease in KXY+(overall) over KXY+(native). At higher concentrations of ligand (KXL+[L] .1), eq 9 thus becomes
(6)
This represents the overall selectivity between ions X+ and Y+, whether complexed or free in solution. Here L is the ligand: a crown ether or cryptand in this work. The aqueous solution concentrations are related through the stability constant for the macrocyclic complex, for example,
X+(aq) + L(aq) h XL+(aq),
X+(ads) + XL+(aq) h X+(aq) + XL+(ads), ΓXL+[X+] XL+ (8) KX+ ) ΓX+[XL+]
+
+ KXY+(overall)
≈
+ KXY+(native)
KYYL+
+
KXXL+
≈1 +
(11)
When no ligand is present, KXY+(overall) f Y+ KX+(native). The general situation is illustrated in Figure 1 for some typical equilibrium constants with a native selectivity of 1. Selective binding is enhanced over a broad concentration range. Selectivity is immediately enhanced by the addition of a macrocycle, reaching a + maximum near KXYL+ . Further ligand addition reduces discrimination as the second ion also begins to be complexed.
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2809 Table 1. Selectivity Coefficients for the Alkali Metals against Sodium Using the Surfactant AOT with and without the Macrocycle C222 (Initial Solution Concentrations Are Also Shown) ion X+ Li+ Na+ K+ Rb+ Cs+ Cs+
+
X KNa [C222]/ [AOT]/ + mM mM (native)a
0.77 1 1.32 1.55 1.62 1.62
[X+]/ mM
+
X KNa log +(overall) with C222 K[XC22]+
0.309
0.302
0.307
0.59 ( 0.03
0.322 0.615 0.300 2.99
0.303 0.301 0.317 0.302
0.307 0.163 0.299 0.312
2.66 ( 0.23 2.69 ( 0.11 0.66 ( 0.05 0.66 ( 0.06
1.0b 3.9c 5.4c 4.4c 0b 0b
a Taken from Schulz and Warr, 1998. b Taken from Izatt et al., 1991. c Taken from Lehn and Sauvage, 1975. +
Figure 1. Calculated apparent selectivity, KXY+(overall) selectivity for the formation of surfactant + macrocyclic complex films at the air/solution interface as a function of equilibrium ligand + concentration, [L], with KXYL+ ) 2.5 and two different native Y+ selectivity coefficients. s, KX+(native) ) 1.0 (no selectivity) and + ---, KXY+ ) 5.0 (selectivity exists). We see for the case where selectivity does exist, selectivity is reduced, and for the case where there is no native selectivity, there is an enhancement in the removal of ion Y+. KYL+ ) 800 and KXL+ ) 1. +
+
If KXY+(native) > KYYL+ , then addition of a ligand only reduces the selectivity (see Figure 1). However, as macrocycles are intended to enhance systems with poor native selectivities, this is not entirely germane. In the alkali-metal ion systems studied here, low native selectivities should readily be modified by macrocycle addition. Experimental Section Materials. The surfactant bis(2,2′)-ethylhexylsulfosuccinate (AOT) was obtained as a sodium salt from Sigma at >99% purity and was used without further purification. The macrocycles 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6 or 18C6) and 4,7,13,16,21,24hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (C222) were obtained from Sigma at >98% purity and were used without further purification. The counterions lithium, potassium, and rubidium (Merck >99%) and cesium (Sigma >99%) were all obtained as chloride salts. Oneliter solutions of 3 × 10-4 M AOT, 3 × 10-4 M alkalimetal salt, and various concentrations of macrocycles were prepared in Milli-Q water for each experiment at 25 °C. All experiments were performed below the critical micelle concentration of AOT (2.5 × 10-3 M) to avoid possible complications in the interpretation. Flotation. Ion flotation was carried out in a waterjacketed and thermostated flotation column at 25 °C, fitted with a porous frit through which nitrogen was passed at a flow rate of 50 mL/min (Morgan et al., 1992). One liter of the solution was placed in the column under these conditions and allowed to foam for approximately 2-3 h. Samples (4 mL) were taken from the bulk solution at regular intervals. These samples were analyzed using ion chromatography coupled with a Waters 431 conductivity detector. The ions in the samples were separated by the column and their concentrations obtained from peak areas. The results obtained thus describe the total concentrations of each of the two competing ions remaining in the column and do not discriminate between the complexed and free states (see eq 6).
Results and Discussion The selectivity coefficients for the uptake of the alkalimetal ions in the absence of any macrocycle are listed in Table 1. These “native” selectivities all lie below 2 and reflect the insensitivity of the surface film to counterion type. The mechanism of association for these and other ions has been discussed elsewhere (Morgan et al., 1994, 1995; Schulz and Warr, 1998). The experimental flotation results used for the determination of selectivity coefficients of the alkali-metal ions over sodium with the macrocycles C222 and 18C6 are shown in Figure 2. The data are all linear when plotted according to eq 5, as has been observed previously for flotation of simple ions (Morgan et al., 1994, 1995; Schulz and Warr, 1998). In the presence of the cryptand C222, selective association with the interface is substantially modified, as Table 1 shows. Native and overall selectivity coefficients are also shown together in Figure 3. With C222 the overall selectivities exhibit a clear peak for potassium and rubidium which parallels the stability constants for C222 complexation by these ions (Table 1). As expected from consideration of the solution and interfacial equilibria, strong preferential complexation of one ion by a hydrophobic macrocycle leads to its preferential uptake into the foam, altering the overall measured selectivity. The effect of 18C6 on selectivity is listed in Table 2. As the stability constants of the macrocyclic complexes are much smaller for 18C6 than for C222, we expect a smaller fraction of ions to be complexed, and hence a much reduced effect. In fact, no change in selectivity coefficients is observed upon the addition of 18C6 for either system beyond the limits of experimental uncertainty. The differences between C222 and 18C6 serve to illustrate the expected result that high-stability constants for complexation are necessary to generate selectivity enhancement at moderate ligand concentrations. Although both macrocycles are optimized for complexation of potassium, the macrobicyclic ligand forms a complex at near stoichiometric amounts even in dilute solution due to its high-stability constants. Under the same conditions the fraction of ions complexed by the crown ether is negligible, and interfacial composition is virtually unaffected. Selectivity Enhancement by C222. Qualitatively, C222 addition allows us to achieve the desired control over composition of the foam. Uptake of potassium and rubidium are strongly enhanced at the expense of sodium, and lithium and cesium are excluded from the interface. Reversal of the cesium/sodium selectivity
2810 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
(a)
(c)
(b)
(d)
+
X + (b) and Figure 2. Determination of KNa +(overall) coefficients for the alkali metals and AOT with the two macrocycles. (a) C222 with Cs Cs+ ([) (C222 is in excess). (b) C222 with K+ (9) and Rb+ (2). (c) C222 with Li+ ([). (d) 18-crown-6 with Li+ ([) and K+ (9).
Table 2. Selectivity Coefficients for the Lithium and Potassium against Sodium with the Surfactant AOT and Macrocycle 18-Crown-6 (Initial Solution Concentrations Are Also Shown) ion X+ Li+ Na+ K+
+
X KNa [18C6]/ [AOT]/ + mM mM (native)a
0.77 1 1.32
+
X KNa log +(overall) with 18C6 K[X18C6]+
[X+]/ mM
0.311
0.279
0.318
0.299
0.301
0.299
0.78 ( 0.04 1.45 ( 0.07
0b 0.8c 2.1c
b
a
Taken from Schulz and Warr, 1998. Taken from Izatt et al., 1991. c Taken from Izatt et al., 1976.
+
+
X X Figure 3. Comparison of KNa +(overall) and KNa+(native) coefficients for the alkali metals and AOT (with and without C222) with the theoretical upper bound of 2.45 for this system. No C222 (O); with C222 (b).
followed by a weak diminution of selectivity with further ligand addition (Figure 1). Such strong complexation and high selectivity by C222 allows us to regard sodium ions in the sodium/ cesium mixed solution to be fully complexed, and cesium to be completely uncomplexed. The selectivity of [NaC222]+ over bare Na+ is thus obtained from +
order illustrates this most clearly. With equimolar C222 added, the selectivity order is changed as follows: native: with C222:
Li Li
Na Cs
K Na
Rb K
Cs Rb
Note also the insensitivity of the sodium/cesium result to the addition of excess C222 (Table 1). Even with a 10-fold excess of ligand the selectivity coefficient is unchanged. This is consistent with expectations for equilibrium ion exchange, which predicted an abrupt selectivity enhancement on the addition of a ligand
+
+
[NaC222] [NaC222] Cs KNa ) KCs × KNa + + +(native) +
)
Cs KNa +(native) +
Cs K[NaC222] +
(12)
The resulting enhancement of the removal of sodium [NaC222]+ ) 2.45 ( 0.25. As all complexes by C222 is KNa + are presumed to bind equally to the interface, eq 6 suggests that this should be an upper bound on the selectivity. Within experimental uncertainty this is in accord with our observations (see Figure 3).
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2811
Conclusions We have described a technique for altering the native selective binding of ions to surfactant films by solution complexation. Macrobicyclic ligands have most of the desired properties of high-stability constants and high selectivity for solution complexation. Selectivity is, however, inherently limited by selective binding of the complex over bare ions, and this could be further enhanced by modification of the ligand structure. In particular, increasing ligand hydrophobicity should lead to a large increase in binding strength. Nomenclature L ) macrocyclic ligands, which are 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6 or 18C6) and 4,7,13,16,21,24hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (C222 or cryptand C222) in this study [X+] ) bulk concentration of ion X+ in water, mol L-1 ΓX+ ) surface excess of ion X+ at the air/water interface, mol m-2 KXL+ ) stability constant for the macrocyclic complexation of ion X+ by ligand L + KXYL+ ) selectivity coefficient for the uptake of complex YL+ over bare ion X+ Y+ KX+(native) ) selectivity coefficient of ion Y+ over X+ in the absence of macrocycle + KXY+(overall) ) selectivity coefficient of ion Y+ over X+ in the presence of macrocycle
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Received for review December 12, 1997 Revised manuscript received April 3, 1998 Accepted April 5, 1998 IE970905U
