Ligand-accelerated liquid membrane extraction of metal ions

Journal of Membrane Science, 26 (1986) 129-142 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

129

LIGAND-ACCELERATED LIQUID MEMBRANE EXTRACTION OF METAL IONS

Z.M. GU*, D.T. WASAN

Department of Chemical Engineering, Illinois Znstitute of Technology, Chicago, IL 60616 (U.S.A.)

and N.N. LI

Signal Research Center, Inc., 50 East Algonquin Road, Des Plaines, IL 60017 (U.S.A.)

(Received August 9, 1984; accepted in revised form July 25, 1985)

Summary

A method has been developed to enhance the liquid membrane extraction of heavy metals such as cobalt, copper and nickel. The method consists of introducing anion ligands, such as acetate, to the aqueous solution containing metal ions. In the absence of a ligand in the aqueous phase, it takes about 15 min for a 80% cobalt recovery, while only 2 min are needed for a 95% recovery with the addition of 0.1 M acetate in the feed solution. The Iigand effects on liquid membrane extraction are rationalized in terms of the labile nature of the ligand-metal complexes, the distribution coefficients of the metal ions, the interfacial and surface tensions, and by the nuclear magnetic iesonance (NMR) spectra of the metal-organic complexes.

Introduction

Since the discovery of the effectiveness of liquid surfactant membranes for separating hydrocarbons over a decade ago [l], this novel separation technique has been widely studied. It appears that liquid membranes may become the effective tools for the separation and purification of many sub- stances. In recent years, many authors have reported their studies on the recovery and enrichment of valuable heavy metals [2--81 and the removal of trace contaminants from waste water [9, 101. The formation of liquid surfactant membranes and the general separation process were described thoroughly elsewhere [ 111.

The process of metal ion extraction in the liquid membrane system can be facilitated by utilizing the mechanism of carrier-mediated transport [ 12, 131. In this type of facilitation, an ion exchange reagent is incorporated into the membrane phase to carry the diffusing species across the membrane to the receiving phase.

*Permanent address: Institute of Atomic Energy, Academia Sinica, Beijing, China

0376-7388/86/$03.50 0 1986 Elsevier Science Publishers B.V.

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An example of this process is the extraction of cobalt, achieved according to the following chemical reactions

Extraction: 2 HR + Co*’ + CoR, + 2H’ (1) (ore) (as) (org) (as)

Stripping: 2 H’ + CoR, + Co2’ + 2 HR (2) (as) (org) (as) (org)

where HR represents the protonated form of a liquid exchange agent which is used as the carrier or “transport facilitator”. In this system, extraction (eqn. 1) occurs at the membrane-external aqueous phase interface while stripping (eqn. 2) occurs at the membrane-internal aqueous phase interface. The cobalt is effectively concentrated in the encapsulated phase of the emulsion by the continuous permeation of hydrogen ions from the encapsulated phase to the external phase.

In a well-stirred system of liquid surfactant membranes in which carrier- mediated facilitation is utilized, the mass transfer resistance arises from the following steps: (1) eddy diffusion of metal ions outside emulsion globules; (2) extraction reaction occurring at the interface between the external aqueous phase and the membrane phase; (3) molecular diffusion of the metal-extra&ant complex across the membrane (including surfactant layers) where eddy diffusion is assumed negligible; (4) stripping reaction occurring at the interface between the membrane and internal aqueous droplets; and (5) molecular diffusion of the stripped metal ions inside the droplets. Since the system is well stirred, the resistance of eddy diffusion outside the emulsion globules may be regarded as negligible. The droplet size is very small (about 1 pm). This implies that the resistance due to molecular diffusion inside the droplets may also be neglected. It is very instructive to note here that the interfacial area for extraction is quite different from that of stripping. For a typical liquid membrane system in this study, the size of the dispersed emulsion globules is 0.5 mm, the size of the internal droplets of the emulsion is 1 pm, and the volume ratio of internal aqueous phase to the oil phase of the emulsion is 1. Calculation shows that the ratio of the interfacial area of stripping to that of extraction is more than 250. Therefore, the resistance of stripping may be neglected when compared with that of extraction, and the extraction reaction will become the rate-controlling step if the extraction reaction is very slow. From the above analysis, we theorized that accelerating the extraction reaction would enhance the overall mass transfer process of the liquid membrane system.

A kinetics study showed that the extraction process of cobalt by di(2- ethylhexyl)phosphoric acid (D2EHPA) is slow [14]. This was also shown by the published kinetics data of cobalt extraction by a liquid surfactant membrane [15].

Attention has recently been drawn to the use of sulfonic acids, in combi- nation with other extractants, to catalyze the extraction of Co(II), Ni(II),

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Cu( II) and other metal ions in order to speed up the extraction process [ 16-

191. In our recent study of liquid membrane extraction of cobalt 1201, we

found that the introduction of certain anion ligands (such as acetate) to the aqueous solution containing Co2 + greatly accelerated the extraction rate. We suggest that the water molecules in the hexaaqueous cobalt(H) complex which were inert kinetically, were replaced by the ligand, and the ligand- cobalt(I1) complex which was labile kinetically reacted quickly with the extractant, thus enhancing the reaction rate. This phenomenon coincides with the effect found by some other authors [lS, 19, 211. Furthermore, the organic ligand has a hydrophobic-hydrophilic molecular structure; therefore it shows a surface active property. Thus the ligand-metal complex tends to populate at the aqueous-organic interface more than the hydrated metal ions do. In this way the metal ions are relatively concentrated at the membrane-external aqueous phase interface. This is favorable for the membrane extraction process.

The ligand effect, combined with the previously discussed mechanism of carrier-mediated transport, becomes the mechanism of ligand-accelerated liquid membrane extraction. This may be illustrated with Fig. 1.

CONTINUOUS PHASE

MEMBRANE ENCAPSULATED PHASE

Fig. 1. Mechanism of ligand accelerated liquid membrane extraction (M - metal, L -

ligand, HR - extractant).

Experimental

Batch experiments were performed. The liquid membrane phase was com- posed of 20-40 g/l ECA 4360, 2--5% (v/v) DBEHPA, and LOPS. ECA 4360, a nonionic polyamine made by Exxon, was added as a surfactant, DBEHPA (di(2ethylhexyl)phosphoric acid manufactured by Sigma Co. j was used as an extractant, LOPS (Low Odor Paraffin Solvent made by Exxon) was used as membrane solvent. LOPS has an average molecular weight of 180, a specific gravity of 0.799 and a viscosity of 2.6 cSt at 60°F.

The internal phase of the liquid membrane was 50-200 g/l H,SO, solution which served as a stripping agent. The external aqueous phase of the

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liquid membrane system was a CoSO, solution containing 1000 ppm of Co”. The initial pH value was adjusted to 5.0.

The w/o emulsion was prepared by mixing a 50 ml solution of the internal aqueous phase with a 50 ml solution of the oil membrane phase in a

Waring Blender for 2 min at the ambient temperature. The prepared emulsion was examined microscopically and the internal droplets were found to be less than 1 pm in diameter. The emulsion (40 ml) was then added to a 400 ml vessel containing 200 ml CoS04 solution to be extracted. The system was stirred by a variable speed mixer equipped with a marine-type im- peller. Mixing speed was 200 rpm. Samples of the raffinate were taken periodically and analyzed with a UV-spectrophotometer (Beckman Co.) for cobalt concentration.

During the experiments, the feed solution was preconditioned with various ligands such as acetate, tartrate, salicylate, succinate and formate, to in- vestigate their effects on the mass transfer rate.

The surface and interfacial tensions were measured with a Cenco-Du Noiiy interfacial tensiometer (Central Scientific Co,).

Results and Discussion

Several ligands were tested. Their effects on the liquid membrane extraction kinetics of cobalt are shown in Fig. 2. Among these ligands, acetate was found to produce the strongest ligand effect. In the case without ligand in aqueous feed solution, it took about 15 min for 80% cobalt recovery, while only 2 min were needed for 95% recovery with the presence of 0.1 M acetate in the continuous aqueous phase.

Et 0.1 M ACETATE

A O.lM SUCCINATE

0 O.lM FORMATE

l O.lM TARTRATE

0.0 I

2.0 4.0 6.0 8.0 10.0 TIME, MINUTE

Fig. 2. Effects on kinetics of different ligands in external phase.

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An examination of the ligand effect reveals that the selected ligands may act as the phase transfer catalysts which accelerate the transfer process of the species concerned from one phase to another.

Some authors [l&19] have pointed out that some hexaaquo-metal complexes are very inert kinetically, so the extraction reaction of metal ions with extractant from aqueous phase to organic phase is limited by the slow step of water molecule release. However, the introduction of auxiliary anionic ligands in the aqueous phase accelerates extraction rates by forming an intermediate complex in which at least one coordinated water molecule has been replaced. The intermediate ligand-metal complex reacts rapidly with the organic extractant; the whole extraction process is thus accelerated.

Our recent study on the interfacial mass transfer of ligand accelerated metal extraction using a modified stirred Lewis cell has supported the above assumption. These results were presented elsewhere [ 221.

The detailed mechanism of this ligand effect still needs to be established. Hammes and Steinfeld [23 ] gave an electrostatic explanation of this phenomenon. They assumed that replacing some of the coordinated water by anion ligand reduced the net positive charge on the metal ions resulting in the loosening of water molecules on the metal ions. Freiser et al. [ 181 argued that although the rate of water release could be expected to be increased by the lowered net charge on the metal ion complexed by an anion ligand, the rate of reaction with another negatively charged species should also decrease.

0 with AC- in oq. phase

X no AC- in aq. phase

PH

Fig. 3. Relationship between distribution coefficient D and pH.

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Evidence from several sources indicates that the addition of ligand to the aqueous phase does not affect the thermodynamic equilibrium of the extraction. Let us use acetate as representative ligand to elucidate this phenomenon.

The first indicator is the result of our equilibrium extraction experiment. For the extraction of cobalt with DZEHPA accelerated by acetate, if the thermodynamic equilibrium is changed, the equilibrium curve should also be changed with the addition of acetate to the aqueous solution. In our experiment, distribution coefficients of Co(I1) as a function of hydrogen ion concentration with and without acetate in the aqueous phase were measured and plotted in Fig. 3. We found the data of the two cases positioned along the same curve.

Our Nuclear Magnetic Resonance (NMR) measurement of the organic complex yields further evidence. The NMR spectra of Co-D2EHPA and Co-D2EHPA-HAc complexes are shown in Figs. 4a and 4b, respectively.

f”“l”“l”“l”“l”“l”“l”’ 4 3 2 1 0 -1 -2 PPM

Fig. 4a. NMR spectrum of Co-D2EHPA.

r”“l”“l”“l”“l”“l”“1”‘1 4 3 2 1 0 -1 -2 PPM

Fig. 4b. NMR spectrum of Co-D2EHPA-HAc

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The NMR spectra of cobalt complexes with and without acetate in the aqueous phase are exactly the same as shown in Fig. 4. This reveals that no acetate enters into the organic phase to form a Co-D2EHPA-HAc complex, and therefore the thermodynamic equilibrium has not been changed with the addition of acetate to the aqueous phase. Our most recent measurements using UV-absorption spectroscopy also confirm these observations.

Table 1 shows the stability constants (log K,) and the corresponding percentage recovery of Co2+ with 0.1 M ligand in the aqueous phase. Table 1 shows a trend in which the ligand effect decreases with increasing K, of the ligand--cobalt(II) complex. It should be pointed out that no general correla- tion exists between the stability constants and the extraction rate data. How- ever, it is clear that the selected ligand should be a weak complexing agent for the metal ions rather than a strong complexing agent. If the anionic ligand is a very strong complexing agent, it will not only expel the water molecules surrounding it but also prevent the chelation process. EDTA, for example, is such a strong complexing ligand with Co*+ (K, = 1016) that the extraction of cobalt with D2EHPA is actually inhibited with the presence of EDTA in the aqueous phase.

TABLE 1

Stability constants of ligand-cobalt(I1) complexes and the corresponding percentage recovery of Co’+ with 0.1 M ligand in the aqueous phase

Ligand log K,a Co’+ recovery (X)

Acetate 0.61 98 Propionate 0.70 89 Formate 0.73 89 Succinate 0.99 91 Tartrate 3.53 55 Salicylate 6.72 45 EDTA 16.31 0

aSource: stability Constants of Metal Complexes: Part B, Organic Ligands, compiled by D.D. Perrin, Pergamon Press, 1979.

Our interfacial tension study of the system shows that the addition of ligands in the aqueous phase slightly decreases the interfacial tension (IFT) between the continuous aqueous phase and the organic membrane phase as shown in Table 2. This implies that the selected ligands exhibit weak surface active properties. Each ligand has a hydrophobic-hydrophilic molecular structure with the hydrophilic portion exposed to the aqueous phase and the hydrophobic end directed towards the organic phase of the interface. Thus the ligand-metal complex tends to populate at the aqueous-organic interface more than do the hydrated metal ions. In this way the metal ions are relatively concentrated at the membrane--external aqueous phase interface. This is favorable for the membrane extraction process,

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TABLE 2

Interfacial tension between the membrane phasea and continuous aqueous phase containing 0 .l M ligand

Ligand 7m/w (dyne/cm)

- 9.3 Formate 6.8 Acetate 4.4 Propionate 3.9 Succinate 3.0 Tartrate 4.4 Salicylate 4.1

aMembrane phase consists of LOPS containing 2% (w/w) ECA 4360 and 5% (v/v) DZEHPA.

Another effect resulting from the weak surface active property of the ligmds is the easy dispersion of w/o emulsion in the continuous aqueous phase in the liquid membrane extraction system.

The introduction of ligands to the continuous aqueous phase decreased the globule size of emulsion dispersed in the continuous aqueous phase under identical agitation conditions. For instance, at agitation speeds under 200 rpm, the mean diameters of the emulsion globules with and without acetate addition in the aqueous phase were 0.4 mm and 0.8 mm, respectively. This tendency toward easy dispersion of the w/o emulsion in aqueous phase allows us to reduce the energy requirements for mixing the emulsion and continuous aqueous phase during liquid membrane mass transfer.

The above phenomena may be explained with the data of spreading coefficients (S) calculated from the surface tension and interfacial tension values. These are shown in Table 3. The spreading coefficient S of liquid 1 on liquid 2 may be expressed as follows [ 241

s = w, - wc (4)

where WA is the work of adhesion and WC is the work of cohesion for liquid 1. When WA > WC (S > 0), spreading occurs. The more positive value of S is related to the stronger tendency to spreading.

From Table 3 one can see that the presence of ligands in the continuous aqueous phase decreases the final spreading coefficient SF.

Ross and his co-workers [25] pointed out that emulsion stability was related to the mutual spreading between the two phases. Lowering the S

value was favorable for emulsion formation. The maximum stability of the emulsion should be related to the negative S value.

In our liquid membrane system, the dispersion of w/o type emulsion in continuous aqueous phase forms a w-o-w type emulsion. One can easily imagine that lowering the SF value of the oil phase of the w/o emulsion on another aqueous phase will increase the tendency to form a w-o-w type

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TABLE 3

Values of final spreading coefficientsa of oil membrane phase on continuous aqueous

phase containing 0.1 M ligand and the corresponding globule size of dispersed emulsion

Ligand

Salicylate Formate Tartrate Propionate Succinate Acetate Lauryl sulphateC

SF (dyne/cm) d (mm)b

9.6 0.8 8.8 0.8 3.4 0.6 3.9 0.5 4.6 0.5 3.4 0.4 3.4 0.4 0.15

aThe final spreading coefficient (SF) refers to the mutually saturated state between the membrane phase and the continuous aqueous phase after 2 min mixing of emulsion and continuous aqueous phase. bGlobule size was determined under 200 rpm. CConcentration of sodium lauryl sulfate is 1% (w/w).

emulsion (i.e., the tendency to disperse the w/o emulsion in continuous aqueous phase). In other words, smaller globules of emulsion can be ob- tamed for lower S, values under the same agitation condition. Therefore, the addition of ligand in aqueous phase (this corresponds to the lower SF values of oil phase of w/o emulsion) makes emulsion globules dispersed in continuous aqueous phase smaller.

However, one must bear in mind that in the liquid membrane system the formation of a stable w-o-w emulsion is undesirable. The formation of such an emulsion will make it difficult to separate the w/o emulsion from the continuous aqueous phase after mass transfer, and the vigorous dispersion of the w/o emulsion in the aqueous phase will make the emulsion itself unstable, thus breaking up the liquid membrane system. For example, with the addition of 1% (w/w) sodium lauryl sulfate, a strong surfactant, in the aqueous phase, the SF of the oil membrane phase on the aqueous phase decreases to only 0.15. The liquid membrane system is completely broken under such a condition.

Therefore we conclude that only those ligands with weak surface active properties should be considered,

The kinetics of cobalt extraction as a function of acetate concentration in the continuous aqueous phase is shown in Fig. 5. This can be used as an example to explain the effect of ligand concentration on liquid membrane extraction. From Fig. 5. one can see that there exists an optimum acetate concentration for the maximum recovery of Co” from the aqueous phase. This may be attributed to the following:

There are four factors which govern the overall ligand effect in membrane extraction. First, an increase in the ligand concentration favors the thermo-

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101 I I I I I 0.0 1.0 2.0 3.0 4.0 5.0

TlME,MIN.

Fig. 5. Effect on kinetics of acetate concentration in external phase. Key: x , no acetate; 0,0.05 M; A) 0.075 M; q , 0.10 M; .,0.15 M; A, 0.20 M.

dynamic equilibrium for transforming kinetically inert hydrated metal ions into labile ligand-metal complexes. This is beneficial to the extraction kinetically. Second, an increase in the ligand concentration strengthens the role of the ligand as a buffer (because the selected ligand is a strong base- weak acid type salt). This is favorable for the metal extraction thermodynamically. Third, an increase in the ligand concentration lowers the SF value, as shown in Fig. 6. This results in a strong tendency toward emulsion dispersion in the continuous aqueous phase and causes emulsion instability. Finally, the ligands themselves are electrolytes: their addition to the external aqueous phase can lead to a competitive cation-hydrogen exchange at the interface as observed by Strzelbicki and Charewicz [ 151.

When the ligand concentration is low (<O.l M for acetate), the effect of competitive extraction is not significant, and the SF value is relatively high and does not affect the emulsion stability substantially. Therefore, the first two factors play the predominant role at this stage, and the increase in the ligand concentration favors the liquid membrane extraction.

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With further increase in the ligand concentration, the competitive extraction becomes significant and the emulsion dispersed in the continuous aqueous phase becomes more unstable due to the further decrease of the SF value. For example, in one run of the experiment, when the acetate concentration reached 0.2 M, the SF value dropped to only 0.3, and the liquid membrane system soon became turbid. In this case the third and fourth unfavorable factors counterbalance the above two favorable factors, and the overall effect is to make the extraction rate reach a maximum value and then drop down.

. lk 2.0- x\

I I I

0.0 0.10 0.20 0.30

CONC. OF AC-, [M ]

Fig. 6. SF values of membrane phase on continuous aqueous phase as a function of acetate concentration in continuous aqueous phase.

In the case of cobalt extraction, we found that the optimum concentration of acetate is around 0.1 M.

Similar ligand effects have been found for the liquid membrane extraction of nickel(H) and copper(I1) by D2EHPA. The effect of 0.1 M acetate as a ligand in the external aqueous phase on the extraction kinetics of nickel(I1) and copper (II) is shown in Fig. 7.

From the above discussion, we can see that for some cases of metal extraction by means of liquid surfactant membranes, the process which is originally very slow may be drastically speeded up by introducing certain kinds of ligands to the feed solution.

Because of the very fast extraction reaction resulting from the ligand effect, it appears reasonable that a diffusion controlled model could be applied to describe the mass transfer process for the metal extraction in a liquid surfactant membrane system and that, in this model, the resistance to mass transfer due to the extraction reaction could be neglected.

Recently, a mathematical model of diffusion controlled mass transfer for uniform emulsion globules with no internal circulation has been developed in our laboratory. The details of the model are discussed by us elsewhere [261.

0.0 1.0 2.0 3.0 4.0 5.0 6.0

TIME, MINUTE

Fig. 7. Effect of 0.1 M acetate on kinetics of Ni”‘, Cu’+ extraction (- - - , no ligand).

Conclusions

On the basis of the mechanism of carrier-mediated transport we explored a new approach to enhance the liquid membrane mass transfer process by introducing certain organic ligands to the continuous aqueous phase. The selected ligands can form weak complexes with metal ions and are weakly surface active, i.e. the ligand-metal complex should be thermodynamically less stable but kinetically more labile so that it can be concentrated at the organic membrane-aqueous external phase interface. The ligand effect in the liquid membrane extraction brings about the following advantages:

1. The shorter contacting time between phases resulting from the fast chemical reaction and thereby increased mass transfer may reduce or avoid

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the commonly encountered oxidization problem of the complex during the cobalt extraction, When radioactive effluent is treated, the short contacting

time may reduce the radiolysis loss of the organic reagents. 2. The fast separation process may reduce the need for emulsion stability.

This aspect of the ligand effect needs to be studied further. 3. The weakly surface active property of the ligands leads to the easy dis-

persion of the w/o emulsion in the aqueous phase. 4. In the case of waste water treatment by means of liquid membranes,

certain useful ligands may already exist in the waste water. They may be used with little or no conditioning of the effluent to accelerate the separation process, thus reducing the reagent cost.

Therefore we believe that selecting a suitable ligand for accelerating extraction in specific systems offers an important opportunity in the improve- ment of liquid membrane separation.

Acknowledgement

This work was partly supported by an EPA grant awarded to the Industri- al Waste Elimination Research Center at the Illinois Institute of Technology. The authors wish to thank Mr. R.M. Kurzeja for his assistance with the prep- aration of this manuscript.

References

1

9

10

11

N.N. Li, Separation of hydrocarbons with liquid membranes, U.S. Patent 3,410,794, 1968. E.L. Cussler and D.F. Evans, Liquid membranes for separations and reactions, J. Membrane Sci., 6 (1980) 113-121. E.L. Cussler, Membranes which pump, AIChE J., 17 (1971) 405. R.M. Izatt, M.P. Biehl, J.D. Lamb and J.J. Christensen, Rapid separation of Tl’ and PbZ+ from various binary cation mixtures using dicyclohexane-18-crown-6 incorporated into emulsion membranes, Sep. Sci. Technol., 17 (1982) 1351-1360. K.H. Lee, D.F. Evans and E.L. Cussler, Selective copper recovery with two types of liquid membranes, AIChE J., 24 (1978) 860. T.P. Martin and G.A. Davies, The extraction of copper from dilute aqueous solutions using a liquid membrane process, Hydrometallurgy, 2 (1971) 315. J.W. Frankenfeld, R.P. Cahn and N.N. Li, Extraction of copper by liquid membranes, Sep. Sci. Technol., 16 (4) (1981) 385. A. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AIChE Symp. Ser., 71 (1975) 136. N.N. Li and A.L. Shrier, Liquid membrane water treating, in: N.N. Li (Ed.), Recent Developments in Separation Science, Vol. 1, CRC Press, Cleveland, OH, 1972, p. 163. T. Kitagawa, Y. Nichikawa, J.W. Frankenfeld and N.N. Li, Wastewater treatment by liquid membrane process, Environ. Sci. Technol., 11 (1977) 602. R.P. Cahn and N.N. Li, Separation of phenol from waste water by the liquid membrane technique, Sep. Sci., 9 (1974) 505.

142

12 ES. Matulevicins and N.N. Li, Facilitation transport through liquid membranes, Sep.

13

14

15

16

17

Purif. Meth., 4 (1975) 73. N.N. Li, Facilitated transport through liquid membranes - An extended abstract, J. Membrane Sci., 3 (1978) 265. M.L. Brisk and W.J. McManamey, Liquid extraction of metals from sulphate solu-

tions by alkylphosphoric acids: II. Kinetics of the extraction of copper and cobalt

with di(2-ethylhexyl)phosphoric acid, J. Appl. Chem., 19 (1969) 109. J. Strzelbicki and W. Charewicz, Separation of cobalt by liquid surfactant mem-

branes, Sep. Sci. Technol., 13 (2) (1978) 141. K. Osseo-Asare and M.E. Kaeney, Sulfonic acids: Catalysts for the liquid-liquid ex-

traction of metals, Sep. Sci. Technol.. 15 (4) (1980) 999. A. van Dalen, J. Wijkstra and K.W. Gerritsma, Liquid-liquid extraction of com-

nlexed metal ions by inclusion in dinonyl naphthalene sulphonic acid micelles, J. inorg. Nucl. Chem., 40 (1978) 875.

13 P.R. Subbaraman, S.M. &r&s and H. Freiser, Effect Of auxiliary complex? agents

on the rate of extraction of zinc(H) and nickel(I1) with diphenyl thio-carbazone, Anal. Chem., 41 (13) (1969) 1878.

19 H.L. Finston and Y. Inone, The effect of SCN- on the extraction of Fe(II)-TTA, J. Inorg. Nucl. Chem., 29 (1967) 199.

20 D.T. Wasan, Z.M. GU and N.N. Li, Separation of metal ions by ligand accelerated transfer through liquid surfactant membranes, presented at the Royal Society of Chemistry - Faraday Discussion No. 77, University of Hull, England, April 9-11, 1984, published in: Faraday Discuss. Chem. Sot., 77 (6) (1984).

21 H. Eccles, G.J. Lawson and D.J. Rawlence, Single drop kinetic studies of the solvent extraction of copper from synthetic leach liquors, Proc. International Solvent Ex- traction Conference ISEC’77, CIM Special Vol. 21, 1979.

22 Z.M. Gu, D.T. Wasan and N.N. Li, Interfacial mass transfer of ligand accelerated metal extraction by liquid membranes, Paper presented at the AIChE 1983 Spring National Meeting and Petro Expo ‘83, Paper No. 5Oa, Houston, March, 1983, published in: Sep. Sci. Technol., 20 (1985) 599.

23 G.G. Hammes and J.I. Steinfeld, Relaxation spectra of some nickel(I1) and cobalt(II) complexes, J. Amer. Chem. Sot., 84 (24) (1962) 4369.

24 P. Becher, Emulsion, Theory and Practice, Reinhold, New York, 2nd edn., 1965, p. 15.

25

26

27

S. Ross, ES. Chen, P. Becher and H.J. Rananto, Spreading coefficients and hydro- phile-lipophile balance of aqueous solutions of emulsifying agents, J. Phys. Chem., 63 (1959) 1681. Z.M. Gu, H.F. Zhang, D.T. Wasan and N.N. Li, M ass transfer through liquid surfactant membranes: A diffusion controlled model of permeation in an emulsion globule, Paper presented at the AIChE National Meeting, Denver, August, 1983. H.C. Hayworth, W.S. Ho, WA Burns and N.N. Li, Extraction of uranium from wet process phosphoric acid by liquid membranes, presented at AIChE National Meeting Orlando, FL, February 28-March A 1982, (1983) 493.

published in: Sep. Sci. Technol., 18 (6;

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Ligand-accelerated liquid membrane extraction of metal ions