Analytica Chimica Acta, 100 (1978) 619-625 @Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands

LIQUID-LIQUID SANDWICH-TRANSFER EXTRACTIQN: OPTIMIZATION AND ANALYTICAL USE

T. BRATIN* and A. B. FARAGE

Institute of Inorganic and Analytical Chemistry, L. EBtvZis University, P.O. Box 123, 1443 Budapest (Hungary)

(Received 5th October 1977)

SUMMARY

The feasibility of using liquid-liquid sandwich-transfer extraction methods as fast analytical tools was examined. The effect of surhctant, acid and base concentrations on the stability of the sandwich membrane was tested. The possibility of using this method in preconcentration processes was investigated for chromium(V1). The rate of transfer extraction of radioactive “Cr from aqueous sulphuric acid to sodium hydrotide solution was found to be rapid at reasonable concentrations (1%) of the sandwich reagent tri-n-octylamine.

Liquid-liquid extraction is a versatile technique which has been applied successfully to the separation and pre-concentration of a wide variety of inorganic and organic species [I]. Although this method has solved many problems in analytical chemistry, continuous attempts are made to modify it in order to solve more complicated separation problems and also to develop new separation systems. The new methods that have resulted from modifications of the conventional liquid-liquid extraction technique can be classified into three main categories:

(1) systems based on immobilization of one of the liquid phases on a solid support (extraction chromatography) 121;

(2) systems based on a third liquid phase in addition to the or+nal phases (triphase or “ternary” liquid-liquid extraction) [3, d] ;

(3) various solvent [5-71 and liquid surfactant membrane [S-IO] systems which have not yet been exploited satisfactorily 21 analytical chemistry. Fig. 1 shows a schematic representation of some of these systems.

The possibility of separations by means of liquid surfactant membranes was first described by Li [ 11]_ This method is based on first making an emulsion of two immiscible phases, i.e. encapsulating an aqueous phase in a surfactant membrane, and then suspending this emulsion in another aqueous or non-aqueous phase. Separation is achieved by diffusion of a particular species from one phase to the other through the relatively high

8 Permanent address: Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt.

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Fig. 1. Various forms of liquid-liquid sandwich-transfer extraction. (A) Theoretical model [5 ]_ (B) Transfer through a supported planar solvent membrane IS]. (C) Transfer through a spherical membrane (idealized representation with one droplet).

surface area of the encapsulating membrane_ Differences in solubilities of the species to be separated in the membrane phase are of capital importance for selective separation.

An important improvement to this method has been introduced by Cussler et al. [ 12,131 who suggested the addition of a complexing reagent (active carrier) to the Iiquid surfactant membrane (sandwich reagent). The reagent molecules react with a specific solute on one side of the membrane, then the complex formed diffuses across the membrane and finally releases the solute on the other side. Thus, the particular solute is selectively and continuously extracted from one liquid phase into the membrane which instantaneously releases it to the other liquid phase. In other words, transfer extraction occurs in the system for a specific species. The term liquid-liquid “sandwich-transfer” extraction is suggested here to designate these methods.

The aim of the present work was to make a first step in evaluating and optimizing liquid-liquid sandwich-transfer extraction for general analytical purposes_ As a first model, the separation of radioactive chromium(V1) from acidic aqueous solutions was investigated.

EXPERIMENTAL

Reagents and materials All reagents used were of analytical purity unless otherwise specified.

Laboratory-grade paraffin oil (Hungarian) with an average molecular weight of 350, a viscosity of 67.5 cSt at 20°C and a specific gravity of 0.869 g ml-’ at 2O”C, was used as solvent in the preparation of the liquid membranes. Span-80 surfactant (sorb&an monooleate; Atlas Chemie, F.R.G.) was used to stabilize the membrane_ Laboratory-reagent grade Amberlite LA-1 (IV-dodecyl (trialkylmethyl)amine; BDH), TOP0 (trioctylphosphine oxide) and TNOA (tri-n-o&&mine) were tested as sandwich reagents. Potassium dichromate solutions were spiked with radioactive chromium-51 and radiometric detection was applied.

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Apparatus and instrumentation Jacketed glass vessels (6-cm i-d.; Fig. 2) were used in the preparation of

the liquid membrmes and also in the separation =d recovery processes. Activities were measured with a NaI(TI) detector and an energy-selective

counting device (Type NK-107/B; Gamma, Budapest).

Construction of a liquid-liquid sandwich transfer extraction system In general, water-in-oil type emulsion was prepared by dropwise addition

of 10 ml of 0.2 M sodium hydroxide solution to 25 ml of light mineral oil containing carbon tetrachloride (20%, v/v), Span-80 (2%, w/w) and TNOA (I%, w/w), in a jacketed glass column (Fig. 2). The mixture was emulsified by vigorous stirring with a glass stirrer at 1500 rpm for 10 min. Then 50 or 100 ml of a solution of 0.05 M sulphuric acid containing various concen- trations of chromate was added and the new mixture was stirred for 10 min at 300 rpm, with the same siZrrer, to disperse the sodium hydroxide micro- bubbles coated with the oil membrane in the acidic dichromate solution. After completing the agitation, the aqueous layer was released through the vessel tap (see Fig. 2) and the radioactivity was measured.

To recover the transferred chromium(W), the emulsion was broken by the addition of 100 ml of n-heptane with hot water (at ca. 80°C) circulating &-rough the column jacket. The aqueous layer thus separated contained the extracted chromium and the solvent could be recovered for further use. Figure 2 shows a schematic representation of the process.

Solvent (oilI surfactant l

sandwich reagent

Water

eous soln.

(outer phase)

(water m 010

Outer phose

Inner phase

, Orgcnic phase (heptane + sandwich)

Water at Ca BOT-

Emulsion aftei

the extraction Aqueous phase (inner phase)

U

IV. v

Fig. 2. Schematic representation of the liquid-liquid sandwich transfer exkaction process.

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RESULTS AND DISCUSSION

Liquid-liquid sandwich-transfer extraction seems to offer an effective and advantageous procedure for analytical separation and pre-concentration of various inorganic species from aqueous solution. In general, the efficiency of any liquid-liquid extraction system depends mainly on the rate of mass transfer between the two phases, and on the chemical reactions, if any, which occur in the system. The first factor is enhanced by increasing the effective interface between the two phases. That is why the rate of separation by conventional liquid-liquid extraction is much faster than that achieved with the so-called static liquid-liquid extraction technique [ 141; in the former method, new surfaces are continuously made available for separation bg shaking. in the present case, the liquid surfactant membrane provides a large area for separation because the system consists of very small droplets of aqueous solution (emulsion size) coated with a very thin film of immiscible liquid. These coated droplets are agitated in’ the other aqueous solution, Obviously, the surface area available with such membrane-coated droplets is very high. Consequently, the rate of extraction in the proposed system should be faster than any other liquid-liquid extraction method.

In fact, liquid-liquid sandwich-transfer extraction can be considered as a double extraction process in which the inorganic species is first extracted from the outer equeous solution into the very thin membrane phase and then leached into the aqueous phase trapped inside the droplet. The se!ec- tivity can be increased by adjusting the conditions in the two aqueous solutions, e-g. pH, ionic strength or addition of a suitable masking agent.

In order to prove and optimize liquid--liquid sandwich-transfer extraction for general analytical applications, the effects of various factors on mem- brane stability and efficiency were investigated. In preliminary experiments, Amberlite LA-l, TOP0 and TNOA were tested as sandwich reagents for the transfer extraction of chromium(VI) from an aqueous acidic solution outside the droplets to an alkaline solution inside the emulsion droplets. TNOA was found to be the best of the reagents examined, and consequentIy was used in all the subsequent experiments.

Effect of surfacfant concentration on membrane stability

The dependence of the membrane stability on surfactant concentration was studied by making an emulsion from 25 Iml of mineral oil--carbon tetrachloride mixture (4: 1) containing various concentrations of Span-80 and 10 ml of aqueous solution spiked with radioactive chromium-51. Stirring was used for 10 min at ca. 1500 rpm. The emulsions produced were separately mixed with 50 ml of distilled water and stirred at 300 rpm for different times (5,20 and 40 min). A Z-ml aliquot was then withdrawn from the outer aqueous phase for activity measurement. The percent break- down of the emuNon is defined as the percent of.radioactivity found outside the emulsion. The curves of Fig. 3 represent the emulsion breakdown as a

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Surfc;tont ccncentrotcn. wt %

Fig. 3. Effect of surfactant concentration on emulsion breakdown. (a) 5- and 20-min agitation time. (b) 40-min agitation time.

function of surfactant concentration at various agitation times. As can be seen, the emulsion breakdown is decreased (i.e. the stability of the membrane increases) as the surfactant concentration is increased. At high surfactant concentration, the stability of the membrane is not affected by the time of agitation. In subsequent experiments a 2% Span-80 solution was used as it was found to give liquid membranes with reasonably good stability.

Effects of the percentage of aqueous phase inside the emulsion and of the volume of the outer aqueous phase on membrane stability

Emulsion breakdown was studied as a function of percentage of water in the emulsion. Aliquots (25 ml) of oil-CC~ mixture (4: 1) containing 2% Span-80 were emulsified with various volumes of water containing “Cr. Then 50 ml of water was added to each emulsion and stirring was continued for 10 min at 300 rpm. Figure 4 shows that stable membranes are obtained with up to 72% water in the emulsion. At higher water per- centages the radioactivity appeared in the outer aqueous phase.

To examine the effect of the volume of the outer aqueous phase, equal portions of water-in-oil emulsion were prepared from 25 ml of oil-CC4

0 10 20 50 40 50 60 0 --%0200303430m600 7cx3EcoScolom

Percent water in em&Ian ,?A H’a:?r m the external phase

Fig. 4. Emulsion breakdown XS. percent water in the emxlsion.

Fig. 5. Emulsion breakdown vs. the volume of water in the externai phase.

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mixture (4:l) and 10 ml of distilled water containing 51Cr. In separate experiments, these aliquots were suspended and agitated in various volumes of distil&d water at ca. 300 rpm for 10 min. Figure 5 shows that the stability of the liquid membrane is not seriously affected by the volume of the outer aqueous phase used at least up to 11.

Effect of acid and base concentmtions outside and inside the droplets on membrane stability

The chemical stability of the sandwich membranes towards various con- centrations of sulphuric acid and sodium hydroxide outside and inside the droplets, respectively, was studied. The emulsion was made from 25 ml of oil-CC& mixture (4:l) containing 2% Span-SO; and 10 ml of aqueous solution containing various concentrations of sodium hydroxide spiked with 51Cr. The emulsions produced were mixed separately with 50 ml of aqueous solution containing different concentrations of sulphuric acid and stilred for 10 min at ca. 300 r-pm. The curves of Fig. 6 shows that the percentage breakdown of the emulsion is increased by increasing both the sodium hydroxide and sulphuric acid concentrations. Sodium hydroxide concen- trations up to 0.2 M inside the droplet and up to 0.05 M sulphuric acid or 0.5 M hydrochloric acid concentrations in the outer aqueous phase, can be used without serious effect on the sandwich membrane stability.

Rate of transfer extraction of Cr(VI) from acidic aqueous solution to an alkaline solution at different TNOA concentrations

In order to estimate the analytical utility of the liquid-liquid sandwich, transfer extraction method, the rate of transfer extraction of chromium(V1) from aqueous sulphuric acid to sodium hydroxide solution was examined. A water-inoil emulsion was prepared from 25 ml of oil-CCL, mixture (4: l), containing 2% Span-30 and various concentrations of TNOA, and 10 ml of aqueous 0.2 M sodium hydroxide solution. In separate experiments, the emulsions produced were mixed with 100 ml of 0.05 M sulphuric acid solution containing 20 mg of chromium(VI). The mixture was stirred at 300 rpm. At various times, stirring was stopped and a 2-ml aliquot was rapidly

:__ fi

L_J~~_._._-.c--’ 0 -

cl

0.3 0.4 - 0.5 M,NoOH ~nsd.? the emuIc!3n

Fig. 6. Emulsion breakup vs. sodium hydroxide concentration inside the emulsion at various sulphuric acid concentrations in the outer phase. (a) 0.25 M H,SO,. (b) 0.05 M H,SO,. (c) 0.005 M H.$O,. (d) water. (e) 0.5 M HCl.

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Fig. 7. Rate of transfer extraction of chromium(V1) from sulphuric acid solution through liquid membranes containing various TNOA concentrations. (a) 0.1% TNOA in membrane phase. (b) 1.0% TNOA. (c) 10% TNOA.

withdrawn from the outer aqueous solution for radiocounting. The concen- tration of chromium(V1) in the outer phase was then determined. The plots of percentages of chromium(VI) vs. stirring time (Fig. 7) for various TNOA concentrations in the sandwich membrane, show that the rate of transfer extraction is increased by increasing the TNOA concentrations, but extraction is more or less complete at 0.1% TNOA concentrations. At 1% and 10% TNOA concentrations, the transfer extraction is complete after only 3-min agitation. Accordingly, a 1% concentration of sandwich reagent was used for the separation of chromium(V1) in subsequent experiments.

The average transfer extraction of various concentrations (50-500 ppm) of chromium(VI) from aqueous sulphuric acid solution by the above- mentioned procedure was found to be higher than 97%.

REFERENCES

1 See, e.g., A. K. De, S. M. Khopkar and R. A. Chalmers, Solvent Extraction of Metals, Van Nostrand-Reinhold, London, 1970.

2 T. Braun and G. Ghersini (Eds.), Extraction Chromatography, Elsevier, Amsterdam, and Akademiai Kiadd, Budapest, 1975.

3 A. I. Busev, IV. P. Sniwopiszew, B. I. Petrow and J. A. Machnew, Talanta, 19 (1972) 173.

4 T. W. Steele and D. J. Nicoias, NIM Report No. 1684, National Institute of Metailurgy, Johannesburg, South Africa, 1974.

5 R. Bloch, A. Finkeistein, 0. Kedem and D. Vofsi, Ind. Eng_ Chem. Process Design Devel., 6 (1967) 231.

6 R. Bloch, 0. Kedem and D. Vofsi, Nature, 199 (1963) 802. 7 D. Vofsi, 0. Kedcm, R. Bloch and S. _Marian, J. Lnorg. Nucl. Chem., 31 (1969) 2631. 8 E. S. Matulevicius and N. N. Li, Separation and Purification Methods, 4 (1975) 73. 9 D. K. Schiffer, A. Hochhauser, D. F. Evans and E. L. Cussler, Nature, 250 (1974) 484.

10 E. L. Cussier and D. F. Evans, Separation and Purification Methods, 3 (1974) 399. 11 N. N. Li, U.S. Patent 3,410,794, November 12,1968; AIChEJ, 17 (1971) 459;

I/EC Proc. Des. Dev., 10 (1971) 215. 12 E. L. Cussier, AIChEJ, 17 (1971) 1300. 13 E. L. Cumler, D. F. Evans and M. A. Matesich, Science, 172 (1971) 377. 14 R. Ko, Anal. Chem., 39 (1967) 1903.