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Anal. Chem. 1987, 5 9 , 2405-2409
Separation of Bivalent Cadmium, Mercury, and Zinc in a Neutral Macrocycle-Mediated Emulsion Liquid Membrane System Reed M. Izatt,* Ronald L. Bruening, Wu Geng, Moon H. Cho, and James J. Christensen Departments of Chemistry a n d Chemical Engineering, Brigham Young University, Provo, Utah 84602
The Preferential transport of neutral catlon-anion moieties In neutral macrocycle-facllltated emuislon liquid membranes Is described. The separations of CdA, from Zn2+ and HgAtwhere A- = SCN-, I-, Br-, or Ci-; of Hg(SCN), from Cd2+ and Zn2+; and of Cd( SCN), from several cation-containing moleties are used to illustrate the principle. Equliibrlum constants for catlon-source phase coanlon, catlon-macrocycle, and catlon-recelvlng phase reagent Interaction as well as the free energy of hydration of the transporting moiety are examlned as parameters for the prediction of catlon transport selectlvItles. An emulsion liquid membrane consisting of an aqueous source phase, 0.02 M dlcyclohexano-18-crown4 In toluene membrane, and an aqueous receiving phase containing MgS,O, or Mg(NO,), is used to obtain the transport data. Span 80 (sorbitan monooleate) Is used as the surfactant (3% (v/v)).
Macrocyclic ligands of the crown ether and cryptand types have been shown to be selective carrier reagents for particular metal cations, M(m), where m = the oxidation state of the metal, in artificial membranes (1-8). This selectivity has prompted interest in the possible use of macrocycle-containing membranes in analytical as well as commercial separations. Of particular interest has been the incorporation of macrocycles into both polymer and liquid membrane ion-selective electrodes (9-15). Macrocycle-containing membrane separations can also be used to concentrate a dilute sample (2-4) and/or to purify a sample from interfering M(m) (1-8) before analysis. Behr et al. (16) and Fyles (17) have shown that macrocycle-mediated M(m) transport in many relatively thin membrane systems, including those studied by us (18),is diffusion limited. The parameters affecting diffusion-limited transport are the diffusion and distribution coefficients of the transported moiety. The diffusion coefficients of different M(m)-macrocycle complexes should be similar due to their similar size and structure. Hence, selective M(m) transport in these membrane systems is basically a function of the factors affecting the distribution coefficients of the M(m) complexes involved in the transport. Most of the macrocycles which have been studied are neutral ligands. When neutral ligands are used to mediate M(m) transport, anion(s) (Aa-) must accompnay the M(m)macrocycle complex to maintain electrical neutrality. The energy required to dehydrate A"- affects the ease of distribution of an Aa--M(m)-macrocycle moiety to an organic membrane from an aqueous source phase and, hence, is a factor in M(m) transport. The largest M(m) transport rates have been found to occur when A"- has a small negative free energy of hydration and/or if the transported M(m) interacts with A"- to form an ion pair (18,19). Transport of particular M(m) will occur a t different rates (18, 19) from samples containing equivalent M(m) concentrations but different types and/or concentrations of A"-. Thus, a basis is laid for effecting
M(m) separations by manipulating these parameters. Recently, we showed that the rate of Cd(I1) transport is maximized, with A"- = SCN- and dicyclohexano-18-crown-6 (DC18C6) used as carrier, when [SCN-] is adjusted such that
Dicyciohexano-I 8-Crown4 (DCISCS)
a maximum amount of Cd(I1) is present as Cd(SCN)2in the source phase of an emulsion liquid membrane system (20). I t was also shown that Cd(I1) is transported selectively over Ag(1) under these conditions, where Ag(1) is present almost The presence of the macrocycle quantitatively as Ag(SCN):-. was shown to be necessary for transport to occur in this system. The discovery of this ability to influence M(m) selectivity, as well as M(m) transport rates, with proper choice of A"- and [Aa-] led us to further study the use of A"- type and concentration for the design of M(m) separation systems. In the present paper, the separation of Cd(II), Hg(II), and Zn(I1) from each other and from other M(m) is presented to illustrate how the proper selection of A"- type and concentration can be used to design macrocycle-mediated membrane separations. Besides the effect of A"- type and concentration, two additional factors affect the distribution coefficient of the transporting moiety and must be considered in predicting selectivities. First, a macrocycle carrier can be designed to interact preferentially with particular M(m). Preference of the macrocycle for a particular M(m) will enhance the transport rate of that M(m) relative to others (1,2,4). Second, reagents can be chosen for incorporation into the receiving phase that allow preferential interaction with a particular M(m) a t the boundary of that phase with the membrane. Thus, the MA,,, concentration in the receiving phase for that cation will be decreased allowing for a greater concentration gradient in MA,,, to be maintained in the membrane. Hence, receiving phase interactions also affect transport rates and selectivities (3). These parameters are also considered in the present study.
EXPERIMENTAL SECTION The emulsion liquid membrane (Figure 1)was prepared from an organic membrane phase and from aqueous source and receiving phases as described (3,ZO). The organic phase consisted of a 0.02 M DC18C6 (mixtureof cis-syn-cisand cis-anti-cisisomers (21),Parish Chemical Co.) solution in toluene (Fisher), which was 3% (v/v) in the nonionic surfactant sorbitan monooleate (Span 80, IC1 Americas). Blank experiments were performed with DC18C6 absent from the toluene phase. One set of source phases contained either 0.2 M Mg(SCNI2, 0.28 M MgClz (J. T. Baker), 0.15 M MgBr, (MCB), or 0.0185 M MgIz (ICN). These source phases also contained either 0.001 M Cd(N03)2(Baker and Adamson) for Cd(I1) single transport ex-
0003-2700/87/0359-2405$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
Table I. Fraction of Cd(II), Zn(II), and Hg(I1) Present as MA2 in the Source Phase as a Function of Aazo
A-
[A-1, M
Cd(I1)
Zn(I1)
Hg(I1)
SCNIBr-
0.4 0.037 0.3 0.56
0.47 0.26 0.303 0.44
0.26 0 0 0.23
9 X 10-5 9 x 10-4
c1-
Source Phase
0.001 0.02
"The fraction of M(m) present as MA2. The az values were calculated from the log P,(H,O) values for formation of MAn2-"(28).
Receiving Phsse
Organic Phsre
Flgure 1. Formation of an emulsion liquid membrane.
periments or 0.001 M Cd(N03),, 0.001 M Zn(N03)z(Mallinckrodt), and 0.001 M Hg(N03)2(G. Frederick Smith) for competitive transport experiments. For other experiments, the source phase contained 0.2 M Mg(SCN)2along with 0.001 M Cd(N03),and/or either Hg(N03)2,Zn(NOJ2, TlN03 (Fisher), NaN03 (J.T. Baker), KNO, (MCB),Ca(N03)2(Baker and Adamson), Sr(N03)2(Fisher), Ni(NO& (Spectrum),Pb(N03)z(J. T. Baker), or AgNO, (Sargent-Welch) at a concentration of 0.001 M. Experiments were performed with all of the above source phases both with and without 0.282 M MgSZO3(Fluka-Garantie) in the receiving phase. Sufficient Mg(NOJ2 (Baker and Adamson) was added to the source or receiving phase, as needed, to balance the ionic strengths of these phases (20). The Mg(SCN)2solutions were prepared from Ba(SCN), (ICN) and MgS04 (Spectrum) as reported previously (20). The magnesium salts were used for the additional reagents because DC18C6 has been shown to form weaker complexes with Mg2+ than with the other cations studied (21). The concentrations of MgA,, where A- = SCN-, Cl-, Br-, or I-, were chosen to maximize formation of Hg(SCN)z(aq) for A- = SCN- and [A-] = 0.004 M and to maximize formation of CdAz for all other source phase conditions. In each experiment (ZO), the emulsion (1.8 mL consisting of the membrane and receiving phases) and the source phase (9 mL) were stirred together continuously at 25 "C and 600 rpm with an inserted glass propeller in a weighing bottle for a period of time. Thirty seconds was allowed for the membrane and the source phases to separate upon cessation of stirring before sampling the source phase. Samples of the original source phase solutions were also taken. All samples were anaylzed for cation concentration by atomic absorption spectrophotometry (Perkin-Elmer Model 603). The experiments were done in triplicate with fresh emulsions prepared each time. The percent uncertainty in the data presented was calculated as the standard deviation as a percentage of the mean. The uncertainty in the transport data was less than 10% when the amount of transport exceeded 20% and less than 20% for lower transport levels. No transport was detected for any of the M(m) studied under any of the source and receiving phase conditions when DC18C6 was absent from the membrane phase. Calorimetric determinations of the log K , AH, and TAS values for the 1:l and 1:2 interactions in methanol of Ni2+and Zn2+with DC18C6 (mixture of isomers) were made at 25 OC by using a precision isoperibol titration calorimeter (22). Each determination was repeated five times. The calorimeter description, general experimental procedure, and method of calculating log K , AH, and TAS values from the calorimetric data are available (23-27). The uncertainty of the log K and AH values was less than 1% .
RESULTS AND DISCUSSION Separation of Cd(I1) from Hg(I1) and Zn(I1). The a, values presented in this paper are defined as the fraction of the total M(m) concentration present as an MA, complex (i.e. a, = [MA,]/&,"[MA,] where the charges are omitted for simplicity). In Table I, the a2value for M(m) (M(m) = Cd(II), Hg(II), and Zn(I1)) is given where A- = SCN-, I-, Br-, or C1and the [A-] is such that a2 for Cd(I1) is maximized. As discussed in the introduction, the source phase neutral M(m)
complex present in the largest amount should have the highest rate of transport. Hence, one can make selectivity predictions for competitive M(m) transport experiments based on the calculated cyj values. Specifically, when A- = SCN-, a large Cd(I1)-Hg(I1) but a poor Cd(I1)-Zn(I1) separation is predicted from the az values in Table I. For A- = I- or Br-, selective transport of Cd(I1) over both Hg(I1) and Zn(I1) should occur, but the rate of Cd(I1) transport should decrease compared to the case where A- = SCN- since a, is less when A- = I- or Br-. When A- = C1-, Cd(I1)-Zn(I1) separation should be poor and there should be less Cd(I1)-Hg(I1) separation than in the other A- cases. As stated in the introduction, M(m)-macrocycle and M(m)-receiving phase reagent interaction also influence selectivity. As part of Table 11, the constants for M(m) interaction with DC18C6 (carrier) in methanol and with Sz02- (receiving phase agent) in water are given where M(m) = Cd(II), Zn(II), or Hg(I1). The Zn(II)-DC18C6 data were determined as part of this study. The AH and TAS values for the 1:l and 1:2 interactions were also determined and are given as a footnote to the table. The relative differences in the log K(CH30H) values for M(m)-macrocycle interaction have been shown to be indicative of the relative differences in M(m)-macrocycle interactions in membranes used in transport experiments such as the one used in this study (16-18). The log K(CH,OH) values are approximately 3 log K units above log K(HzO) values for the same reaction (29-31). The log of the equilibrium constant for the 1:n interaction of a cation with a complexing agent is defined as log @., On the basis of the log 8, values in Table 11, one would expect a slight reduction in the az-based prediction for Cd(I1)-Zn(I1) transport selectivity due to the use of DC18C6 as carrier since M ~ I ) - ( D C ~ ~ C ~ ) ~ interaction occurs only when M(m) = Zn(I1) and M(m)DC18C6 interaction is similar for M(m) = Cd(I1) or Zn(II), but one would expect that Cd(I1)-Zn(I1) separation would be enhanced by the presence of Sz03" in the receiving phase since M ( T ~ ) - S ~ interaction O~~is greater with M(m) = Cd(I1). On the other hand, the selective transport of Cd(I1) over Hg(I1) predicted by the a2values will be diminished by the use of DC18C6 as carrier and by the presence of S2032-in the receiving phase since the equilibrium constants for both interactions are greater with M(m) = Hg(I1) keeping in mind that the log K value for Hg(I1)- DC18C6 interaction is valid in aqueous solution. The results of the transport experiments are given in Table 111. When NO3- is present in the receiving phase, the relative transport rates of Cd(II), Zn(II), and Hg(I1) match well the a2-based predictions. The effect of NO3- in the receiving phase is minimal since M(II)-N03- interaction is weak for all three cations (31). We expect that the selective transport of Cd(1I) over Hg(I1) would be improved if a macrocycle was used that interacted to the same or greater degree with Cd(I1) as with Hg(I1). The selective transport of Cd(I1) over Zn(I1) could also be improved by proper selection of carrier reagent. The addition of S2032-to the receiving phase enhances the transport of all three metals due to the interaction of all three
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
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Table 11. Parameters for the Prediction of Transport Selectivities for Several M(m) in Membranes' Containing 0.4 M SCNin the Source Phase interaction data DC18C6c 0.47 0.26 9 x 10-5 0.34 0.28 0.42 2 x 10-5 j j j j.
3.0 2.9e
5.0e
3.92 2.35
6.3
0.869 2.42h
29.23 0.729 4.86h
30.6 0.28 6.2h
6.2h
i
j
j
j
2.d
5.2 4.431 2.9' 1.5gf 4.21 5.97 3.86' 2.641
5.0'
8.82k 0.53 0.96 1.98 2.04
13.67k
14.2k
" A 0.001 M M(m), 0.4 M SCN-/O.OQ M DC18C6 in toluene/0.282 M SZO3'- or NO< emulsion liquid membrane. bFraction of M(m) present as a neutral M(SCN), moiety. clog P, for the n:l interaction of a mixture of DC18C6 isomers with M(m) at 25 "C in CH,OH except where indicated (20,30). dlog 8, for the n:l interaction of S202- with M(m) at 25 "C in H20 at zero ionic strength except where indicated (28). eThe AH, and TASi values for the 1:l and 1:2 interactions (corresponding to the P1 and P2 values) in kJ/mol are AHl = -0.18 f 0.02, TAS1 = 16 f 1, AH2 = -0.42 0.06, and TAS2 = 28 f 1. The log P, values have standard deviations of f0.3 and f0.1, respectively. 'Cis-anti-cis isomer of DC18C6 in HzO was used (29). log K(CH30H)values are -3 log X units above log K(HzO)for the same reaction (29-31). gIonic strength = 4.0 M. hIonicstrength = 3.0 M. 'The AHi and TAS, values for the 1:l and 1:2 interactions (corresponding to the PI and PZ values) in kJ/mol are AHl = -0.17 f 0.02, TAS, = 17 f 3, AHz = -0.27 f 0.02, and TAS2 = 29 f 1. The log Pivalues have standard deviations of f0.5 and fO.l, respectively. 'No values reported. 'T = 20 "C. 'Value is for 18C6-Ca(II) interaction, which should be similar to that for DC18C6-Ca(II) interaction (29).
Table 111. Competitive Transport' of Cd(II), Zn(JI), and Hg(I1) with Maximum CdAz in the Source Phase and NO3- or S z O l in the Receiving Phase as a Function of Apercent transport
szo2-
NOC A-
[A-I, M
equilibrium time, min
SCNIBr-
0.4 0.037 0.3 0.56
