Scientific paper
Nano-Assisted Extraction of Alkali Metals using Emulsion
Liquid Membranes
Bahram Mokhtari and Kobra Pourabdollah*
Razi Chemistry Research Center (RCRC), Shahreza Branch, Islamic Azad University, Shahreza, Iran
* Corresponding author: E-mail: pourabdollah@iaush.ac.ir Mobile: +98 912 5143351; Fax: +98 321 3213103
Received: 04-01-2012
Abstract
Nano-assisted inclusion separation of alkali metals from basic solutions was reported by inclusion-facilitated emulsion liquid membrane process. The novelty of this study is application of nano-baskets of calixcrown in the selective and efficient separation of alkali metals as both the carrier and the surfactant. For this aim, four derivatives of diacid calix[4]-1,2-crowns were synthesized, and their inclusion-extraction parameters were optimized including the calixcrown scaffold (13, 4 wt%) as the carrier/demulsifier, the commercial kerosene as diluent in membrane, sulphonic acid (0.2 M) and ammonium carbonate (0.4 M) as the strip and the feed phases, the phase and the treat ratios of 0.8 and 0.3, mixing speed (300 rpm), and initial solute concentration (100 mg/L). The selectivity of membrane over more than ten interfering cations was examined and the results reveled that under the optimized operating condition, the degree of inclusion-extraction of alkali metals was as high as 98-99%.
Keywords: Nano-baskets. Inclusion, Calixcrown, Emulsion Liquid Membrane, Alkali Metals
1. Introduction
Emulsion liquid membrane (ELM) was invented by Li1 in 1968 and is known as one of the most promising separation methods for trace extraction of metal contami-nants2-4 and hydrocarbons5'6 owing to the high mass transfer rate, high selectively, low solvent inventory and low equipment cost. Frankenfeld et al.7 reported that the ELM could be up to 40% cheaper than that of other solvent extraction methods. This process combines both extraction and stripping stage to perform a simultaneous purification and concentration. However, this method has been limited by the emulsion instability8-14.
The lack of emulsion stability will decrease the extraction efficiency. In the ELM process, three steps are followed including an emulsification, extraction, and demulsification. In the first step, the emulsions are prepared by mixing the membrane and the internal phases as water-in-oil (W/O) droplets. In this step, water is dispersed into the oil phase as fine globules. The second step followed by permeation of solutes from the feed phase, through the liquid membrane, to the receiving phase. In the third step, the emulsions are settled and demulsified to release the internal phase containing the concentrated
solutes. This step is associated with the recovery of the membrane phase. Some of the ELM's applications include separation of sugars,15 organic acids,1617 amino acids,18-21 proteins22 and antibiotics.23,24 Application of macrocycles in solvent extraction have been reported in literatures,25-27 however they are not expanded to the ELM's applications. Hence, this work is focused on this area.
Nano-baskets of calixarenes are a versatile class of macrocycles, which have been subject to extensive researches and extractions,28,29 stationary phases,30 trans-porters31 and optical and electrochemical sensors32 over the past years. Baeyer, in the nineteenth century, synthesized the calixarenes by reaction of ^-substituted phenols with formaldehyde in basic or acidic environment.33 However, the limited analytical instrumental techniques at that time were unable to interpret the structure of the synthesized products.
Zinke and Ziegler,34 in the 1940s, discovered that the products possessed cyclic tetrameric structures. Gutsche,35 in 1975, introduced the presently accepted name of calixarene. After that, new advances in the field of metal extraction by calixarenes led to introducing new groups such as the ionizable moieties36-38 and crown
ethers39-41 in their scaffolds. The ionizable moieties not only participate in cooperative metal ion complexation, but also eliminate the need to transfer the anions from the aqueous phase into the organic phase by acting in a cation-exchange mode with the metal cation.42-45 Introducing the crown ether ring on the lower-rims, not only increased the cation binding ability of the calixarenic scaffolds but also enhanced their selectivity.46-48 Other applications and investigations by calixarene scaffolds have been cited in the literatures.49-55
In this study, four nano-baskets of calixcrown were used as bi-functional surfactant/carrier and the method of »one-at-a-time« was used to study of the influences of different factors on ELM performance. In this approach, the experiments are designed to study the effect of a tuned variable at a time while keeping all other independent factors constant. By the method of one-at-a-time, the ELM process for selective extraction of alkali metals was investigated. The process factors such as calixcrown type and concentration (as surfactant and carrier), strip phase type and concentration, base type and concentration in feed, phase and treat ratios, membrane type and selectivity, mixing speed, and solute concentration in feed were investigated and optimized.
2. Experimental
2. 1. Chemicals and Reagents
The liquid membrane consists of a diluent and a cal-ixcrown (as surfactant and extractant). The calixcrowns were synthesized as described below. Commercial kerosene (Shell, USA) was used as diluent, which was a complex mixture of aliphatics and aromatics. Sulphuric acid, hydrochloric acid and nitric acid were purchased from Fluka. Sodium chloride, sodium carbonate and potassium chloride (99%) were purchased from Mallinckrodt, cesium chloride and ammonium carbonate (99%) were obtained from Alfa Aesar and ithium chloride, rubidium chloride. 1.0 N hydrochloric acid was purchased from J. T. Baker, chloroform from EM Science, Lithium hydroxide and sodium hydroxide from Fisher Scientific, n-Deca-ne from Sigma-Aldrich, and 2.0 N sulfuric acid from Mallinckrodt. The chloroform was shaken with deionized
water to remove the stabilizing ethanol and was stored in a dark position.
The experiments carried out using four derivatives of diacid calix[4]-1,2-crowns41 and their chemical structures are presented in Figure 1.
2. 2. Analytical Instruments
Determinations of alkali metals were accomplished by Dionex DX-120 ion chromatographs with a CS12A column, a conductivity detection and membrane suppression. The eluent was 0.011 M sulfuric acid after filtration through a Millipore 0.22 pm filtration membrane, while the pump flow rate at 1700 psi was about 1 mL/min. Nitrogen pressure for the eluent was set at 50 psi. To obtain a stable baseline, the eluent was flowed through the column for 1 h and then, 2.0 mL of standard solutions were injected and they were repeated two other times. PeakNet software was used to manipulate the outputs from the Dionex ion chromatograph. The pH meter was equipped with a Corning 476157 combination pH electrode.
2. 3. Preparation of ELM
The specific amounts of calixcrown were solved in the specific amount of kerosene and thus membrane solutions were prepared. (NH4)2CO3 solution (25 mL, 0.5 M) was used as stripping solution. In 100-mL beaker, stripping solution was added dropwise to the stirred membrane solution and the two-phase system was stirred continuously for 30 min at mixing speed of 1500 rpm by a variable speed mixer equipped with a turbine-type Teflon impeller. The mixture of the membrane and the stripping solution was emulsified.
2. 4. Characterization of ELM
The size, size distribution and stability of emulsions were characterized to examine the method. Size and size distribution of (w/o) droplets obtained by optic microscopy (Mettler FP). The digital format of captured micrographs were analyzed by means of image analyzer software (Digital Micrograph TM, Gatan Inc.). Using a Neubauer camera, the volume of analyzed samples were
Figure 1. Chemical structure of derivatives.
controlled. By size distribution changes at constant times, the stability of w/o droplets was monitored and evaluated by image analyses from photographs obtained during the diafiltration experiments.
2. 5. Batch ELM Experiment
In 500-mL beaker, the ELM prepared was added to some volumes of the feed solution and were stirred by a variable speed mixer equipped with a turbine-type impeller at speed of 500 rpm for extraction time of 30 min. The speed of the mixer was regulated by a voltage regulator. To determine the important variables governing the permeation and separation of alkali metals, calixcrown's type and concentration, strip phase's type and concentration, base type and concentration in feed, the phase and the treat ratios, membrane's diluent type and selectivity, mixing speed, initial solute concentration in the feed phase were varied to observe their effects on the extraction and separation. The samples were taken from the stirred cell periodically during the course of the run. The feed phase of the samples was separated from the emulsions by filtration using a filter paper. The emulsion was demulsified by the freezing. The concentration of alkali metals was analyzed using ion chromatography.
2. 6. The Method of One-at-a-time
Process optimization was performed using the single method of one-at-a-time, which finds the optimum conditions for each criterion one by one. In this optimization method, one variable at a time is searched under the conditions that previously searched variables were at their approximate optimum and variables to be searched later are constant. The method involves searching each variable separately with other variables either constant or varied so that they remain at their estimated optimum for the given conditions. This procedure provides useful information on the effects of independent variables as well as locating the overall optimum and a number of partial optimums and requires a comparable number of trials. Contrasting the strong warnings in most modern texts, there are a few advocates of a role for one-at-a-time plans. For example, Friedman and Savage56 argued for one-at-a-time plans as an effective means of seeking maxima. Daniel suggested minimal augmentations to the one-at-a-time plan to make it more effective and emphasizing the advantages.57 Qu and Wu58 presented two classes of one-at-a-time approaches, while Koita59 presented a one-at-a-time strategy as a part of an overall experimentation plan.
3. Results and Discussion
In several studies, it was shown that calixcrown are an appropriate carrier for extraction of alkali metals in the organic phase. At the basic internal interface of the mem-
brane phase, alkali metals (as their cations) were stripped by the internal agent and transformed into a new species that cannot penetrate the membrane reversibly. The reversible reactions at both interfaces of the membrane phase with non-ionizable and ionizable calixcrown as surfactant/carrier in an ELM system are depicted in eqs.1 and 2, respectively.
M [ CIO, )„ + Calix
M** + CalixH.
[M : Calix]*".n(ClOt) (1)
[M :Calix] + nHh
(2)
Where Mn+ depicted the alkali cation (n = 1), CalixHn shows the calixcrown scaffold in the molecular form, and M:Calix presents the calixcrown complex with alkali metal.
Calixarenes and di-ionizable calixarenes in the acidic solutions are formed as molecular state, while are hydrolyzed in the basic solutions. The ionic form includes the cationic species, while the molecular form can't capture them. After that, the new uncharged complex state diffuses throughout the organic membrane. In the side of acidic striping phase, the calixcrown complex is dissociated as an uncharged molecular calixcrown and diffuses into the organic membrane again. This transportation is repeated during the extraction until the chemical potentials in both sides be equal. Figure 2 depicts the mechanism of facilitated transport of alkali metals with ELM process.
Figure 2. Facilitated transport mechanism of alkali metals in ELM using ionizable calixarenes.
The optimum conditions for the extraction of alkali metals were determined by the method of one-at-a-time. Table 1 presented all conditions were tested as well as the optimum conditions in bold. The methodology of optimizations is discussed as the following sections, in which the term of "C/C0" in y-axis represents a criterion for examining the extraction performance. Where, C and C0 depict the concentration of alkali metals in the feed-phase after and before the extraction, respectively. Hence, by decreasing this term (C/C0), the extraction performance increases.
3. 1. Effect of Calixcrown Type
Type of calixcrown is the most important factor that influences the selectivity of an inclusion-ELM system, and
Table 1. The experimental and optimum conditions for the extraction of alkali metals.
1	calixcrown type	10
2	calixcrown concentration (wt%)	1
3	acid type in strip phase	h2so4
4	acid concentration in strip (M)	0.1
5	base type in feed	NaOH
6	base concentration in feed (M)	0.1
7	phase ratio	0.4
8	treat ratio	0.1
9	membrane type	kerosene
10	Membrane selectivity	
11	stirring rate (rpm)	100
12	solute concentration in feed (mg/L)	10
11	12	13	-
3	4	5	10
HCl	hno3	-	-
0.2	0.3	0.4	0.5
NH4OH	Na2CO3	(NH4>2CO3	-
0.2	(2.3	0.4	0.5
0.6	0.8	1.0	1.2
0.2	0.3	0.4	-
n-decane	k:d*	-	-
200	300	400	500
100	1000	-	-
The bold items were obtained and used as the optimum conditions, M: Mole/Liter. * kerosene/n-decane 1:1
can often be used in related liquid-liquid extractions. The effect of calixcrown type on the extraction efficiency of alkali metals was studied in the ELM process and the results obtained are shown in Figure 3. According to the results, although calixcrown 13 gives higher rate of extraction in the first 10 min compared to calixcrowns 10-12, it gradually deteriorates with time. Examination of these results indicates that calixcrown 13was more favorable than calix-crowns 10-12 as emulsifier/carrier. Therefore calixcrown 13was selected as among all scaffolds.
1.000
o
y o
0 100 -
0.010-
Û.001
	10 ---11 ---13 .........12
% % \V\ vk v	
	
10	15
Time (min)
20
Figure 3. Effect of calixcrown type on the extraction efficiency of alkali metals in the ELM process.
3. 2. Effect of Calixcrown Concentration
The extraction of alkali metals increased by increasing of calixcrown concentration from 1-5%, while more increase from 5-10% hardly affected the extraction performance. As depicted in Figure 4, further increase of cal-ixcrown concentration decreased the efficiency of extraction, owing to the access of molecular calixcrown in membrane phase. Under the optimum concentration, the molecular form of calixcrown is considered enough for
forward extraction. Increasing of calixcrown concentration up to 5% increased the stability of emulsion liquid membrane, which led to the decrease in the break-up rate, hence the extraction of solutes was increased. Further increase in the concentration of calixcrown leads to the decrease in the rate of capturing and stripping reaction. This is because the metallic cations remain in the complex form (in the membrane) without being stripped. This affects the final recovery by the ELM process.
The excessive calixcrown tend to increase the interface's resistance and increase the viscosity of membrane. This increasing from 5% increased the emulsion stability but the mass transfer was adversely decreased. Similar results have been reported by other researchers.60,61 Hence, there is an optimum in the concentration of calixcrown around 4%. The excess of calixcrown concentration leads to osmotic swelling and membrane breakdown. Hence, the concentration of 4% was accepted as optimum concentration. Another criterion is the financial aspects, in which the calixcrowns are the most expensive agents among the other components of ELM process, and the lower concentrations are preferred.
Figure 4. Effect of calixcrown 13 concentration on the extraction % of alkali metals in the ELM process.
3. 3. Effect of Acid Type in Strip Phase
The stripping agent in the internal aqueous phase is an important factor that influences the selectivity of an ELM system. A suitable stripping agent dissociates the complex of calixcrown:alkali metal to the desired cation directly, and thus shortens the recovery process. The type of the acids used in the acidic solution is a parameter influencing the extractant efficiency. Selection of a mineral acid in the strip phase solution is suitable for the protonation of calixcrown and exchange interaction. The effect of the presence of 0.05 M of different acids; sulfuric acid, hydrochloric acid and nitric acid in the acidic solution on the transport of calixcrown complex was investigated. Figure 5 depicts the results, in which there is a little difference in the extraction efficiency between the acids used. Obviously, the extraction rates of alkali metals up to 10 min followed the order: sulfuric acid < hydrochloric acid < nitric acid. However, at 10-15 min interval, the acidic feed solutions yielded near quantitative extraction and the highest extraction efficiency was obtained with sulfuric acid. Thus, 0.05 sulfuric acid solution was accepted as the best acid and was used as the strip phase solution in the following experiments.
0.001 J-,-r-,-—
o	5	10	15	20
Time (min)
Figure 5. Effect of acid type in the strip phase on the extraction efficiency of alkali metals in the ELM process. 1: nitric acid, 2: hydrochloric acid, 3: sulfuric acid.
3. 4. Effect of Acid Concentration in Strip
The effect of sulfuric acid concentration in the strip phase on the extraction of alkali metals was studied. To determine the influence of sulfuric acid concentration on the extraction of solutes, the experiments were performed with various concentrations of sulfuric acid in the range 0.1-0.5 M. Figure 6 depicted the effect of acid concentration on the extraction of alkali metals. Obviously, below 0.2 M, the extractions decreased with decrease in acid concentration. The decrease in the extraction with the decrease in proton concentration can be explained by the
fact that the protonation rate of calixcrown complexes decrease due to the less availability of protons for the reac-tion62. On the other hand, the extractions were maximum at 0.2 M. Above this concentration, the extraction decreased, since the increase in proton concentration in the strip phase will form species like (CalixHn+m)m+, which may not mobilize to the membrane completely at higher acid concentrations. Hence, the extraction will decrease with the more increase in acid concentration.
0	5	10	15	20
Time (min)
Figure 6. Effect of sulfuric acid concentration in the strip phase on the extraction efficiency of alkali metals in the ELM process.
3. 5. Effect of Base Type in Feed
As the extraction occurs in the interface between the basic solution and the liquid membrane, the transport of metal necessarily requires a simultaneous back-extraction step at the opposite side of the membrane. In the stage of back-extraction, the calixcrown is regenerated and the alkali metal is stripped. As reported in literature, the stability of emulsions is the main factor in ELM. In addition to mixing speed, extractant type and concentration, and surfactant type and concentration, another parameter is the agent's types in the feed phase. Therefore, the selection of suitable feed solution is considered one of the key factors for cation extraction. Hence, NaOH, NH4OH, Na2CO3, and (NH4)2CO3 were used. According to this figure, (NH4)2CO3 solution was more preferable in making the feed solution since it stabled the emulsions during the extraction process. Therefore, the proper concentration of ammonium carbonate was selected as the best base in the feed phase.
3. 6. Effect of Base Concentration in Feed
The literature contains many options for accomplishing the ELM process by cation complex. Among them, solutions of ammonium carbonate, sodium carbonate and sodium hydroxide have been used in the feed phase. From our list, ammonium carbonate solution was used as the
best feed phase. The molarity of ammonium carbonate was varied between 0.1-0.5, in which there is difference in the extraction efficiency in the concentration range aforementioned. Obviously, the extraction rate of solutes up to about 10 min increased with the increase of base concentration in the feed solution. However, at 10 min, the efficiency of extraction decreased with the increase of base concentration in the feed solution owing to instability of emulsion droplets. Therefore, at tenth minute, the highest extraction efficiency was obtained with 0.4 M (NH4)2CO3 solution. Thus, 0.4 M (NH4)2CO3 solution was selected as the best concentration for feed phase.
3. 7. Effect of Phase Ratio (Strip Phase Volume/Membrane Volume)
The phase ratio is defined as the volume of stripping solution to volume of membrane. The phase ratio increased with an increase of phase ratio up to 4:5. At 4:5 phase ratio, the maximum extractions were observed. By increasing the volume of the strip phase, the thickness of film in the emulsion was reduced owing to dispersion of strip phase in the membrane by mixing. This was favorable in extractions and results in an increase in the extraction of alkali metal cations. Beyond 4:5, the further increase in the volume of strip phase caused the instability of globules.
3. 8. Effect of Treat Ratio (Feed Volume/Emulsion Volume)
The treatment ratio, defined as the volume ratio of the emulsion phase to the feed phase, plays an important role in determining the efficiency of ELM process. By increasing the amount of emulsion in the feed phase, the number of available droplets and interfacial surface area per unit volume of the feed solution increases. This leads to increasing the mass transfer of solutes from the feed to the membrane; and more efficiency. Increasing of treat ratio slightly increased the size of emulsion droplets and caused inversely a reduction in interfacial surface area.
The increment in the size of droplets was suppressed by the increment in the number of droplets. The extraction efficiency was improved by increasing the treat ratio from 0.1 to 0.3. Beyond 0.3, the further increase in the ratio caused the instability of globules and less extraction efficiency.
3. 9. Effect of Membrane Type
The most crucial task in all types of LM processes is the choice of the membrane phase. The interactions of membrane toward the carrier as well as its viscosity are two main parameters that is controlled by choosing the membrane type. The membrane phase viscosity determines the rate of transport of carrier or solutes and the residence or contact time of the emulsion with the feed phase. It is important to note that residence time is system specific and varies for each organic phase under the given conditions. In this work the effect of tree organic phases on the extraction performance were investigated. Kerosene, n-decane and their blend 1:1 were investigated as the diluent. According to the results, kerosene was selected as the best diluent in the following experiments.
3. 10. Membrane Selectivity
The selectivity of membrane was examined as the enrichment factor (EF). The Enrichment factors of alkali metals with respect to the other cations that exist in the solutions were determined and the results are given in Table 2. In inclusion separations, the enrichment factor is the factor by which the ratio of the amounts of two compounds in the solution must be multiplied to give their ratio after extraction. Eq. 3 depicted how to calculate the enrichment factor.
Cf C' ~Y = EF-—J-Cü Cß
(3)
Where, CiA and CiB are the initial amounts of species A and B in the feed solution. CfA and CfB depict the final
Table 2. Separation factors of alkali metals over other cations at the optimum conditions.
Intervals		2-6 min					6-	12 min				12-	20 min		
cations	Li	Na	K	Rb	Cs	Li	Na	K	Rb	Cs	Li	Na	K	Rb	Cs
Ca	074	112	134	102	094	076	112	136	102	099	077	114	144	102	102
Ba	218	314	442	208	158	222	306	475	303	196	230	298	480	176	186
Ag	146	180	145	198	223	188	202	209	270	176	190	214	210	283	180
Pb	280	324	166	207	332	334	217	247	319	298	330	220	242	308	290
Mn	304	314	298	323	362	318	315	300	384	311	320	311	301	383	311
Zn	288	319	299	257	296	330	303	288	302	288	334	300	280	308	280
Cd	305	248	313	260	200	240	340	205	243	240	244	338	205	245	241
Cr	428	389	367	360	408	355	369	328	434	370	355	360	325	438	370
Cu	414	376	329	300	310	370	380	289	326	385	375	375	259	320	377
Co	366	325	310	203	213	303	300	244	189	290	300	305	244	188	293
Ni	300	284	309	362	340	202	288	350	322	273	202	285	355	322	270
amounts of them, respectively in the strip solution. The EF factor represents the enrichment factor. At the end of the experiments, except for calcium, at interval 4-10 min, liquid membrane selectivity of alkali metals with respect to other ions were high.
3. 11. Effect of Stirring Rate
The speed of mixing is a key factor in the rate of mass transfer through emulsion liquid membranes. The effect of stirring speed in the basic solution was investigated in the range of 100-500 rpm in order to obtain optimal speed with effective extraction of alkali metal cations in the ELM process. When the mixing speed was increased from 100 to 300 rpm, an increase in extraction rate was observed. Above 300 rpm the extraction rate again reduced. As a result, an increase in the mixing speed would increase the interfacial area, and this was true up to certain level of mixing speed beyond which an increase in the speed was likely to break the emulsions thereby reducing overall enrichment and the efficiency of extraction. As discussed by Thien et al.63, the impact on the wall of a contactor on the emulsion droplets or the shear induced breakage of fragile emulsion droplets near the tip of the impeller imposes upper limit on the speed of agitation. At the same time, swelling was also increased owing to transport of water from feed to strip phase. Some particles are broken owing to shear after reaching larger size. The swollen droplets are breakdown on their own or induced by shear. Therefore, the extraction performance is a tradeoff between two effects of swelling phenomena and mixing speed.
3. 12. Effect of Solute Concentration in Feed
The effect of initial concentration of solutes on the degree of extraction was studied. The concentration of alkali metal cations in the feed solution was varied from 10 to 1000 mg/L. Within 10 min, the concentration of solutes in the feed solution was reduced from 10 to 1.0 mg/L, from 100 to 6.0 mg/L, and from 1000 to 35 mg/L, with the extraction efficiencies of 90, 94, and 96.5 %, respectively.
4. Conclusion
Alkali metals in basic and dilute water can be recovered by an ELM process using nano-baskets of calix-crown. Hence, an ELM using four derivatives of diacid p-ieri-butylcalix[4]arene-1,2-crowns as both the extractant and the demulsifier has been investigated to extract and concentrate alkali metals from the basic solutions. The selectivity of this novel approach was assessed over interfering cations containing Co(II), Ni(II), Cu(II), Zn(II) and Cd(II), etc. From this work the following conclusions can be drawn:
1.	The optimum conditions of inclusion ELM process have been determined experimentally and tabulated in Table 1.
2.	The membrane selectivity of inclusion-extraction of alkali metals from the basic solutions containing interfering cations has been performed by ELM process using calixcrown derivative 13 (4 wt%) and the results are tabulated in Table 2.
3.	The highest efficiency for inclusion-extractions was obtained when the acid type and concentration in the strip solution was sulfuric acid (0.2 M).
4.	The best stirring speed was determined to be 300 rpm and increasing from 300 to 500 rpm resulted in deterioration of emulsion stability the efficiency of inclusion-extractions.
5.	The optimum conditions of both the phase and the treat ratios were determined to be 0.8 and 0.3, respectively.
6.	At the optimum conditions, the extraction of alkali metals has been achieved with an efficiency of about 98.0-99.0% from the basic solution (ammonium carbonate, 0.4 M) within almost 10-20 min.
5. Acknowledgements
This work was supported by Islamic Azad University (Shahreza branch) and Iran Nanotechnology Initiative Council.
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Povzetek
Proučevali smo separacijo alkalijskih kovin iz alkalnih raztopin z uporabo emulzijske tekočinske membrane, sestavljene iz kaliks kronskih etrov. Za pripravo le te smo sintetizirali štiri derivate kaliks[4]-1,2-kronskih etrov. Parametere za inkluzijsko ekstrakcijo smo optimizirali z različnim vključevanjem kaliks kronskih gradnikov (13, 4 wt %, nosilec/de-mulgator), komercialnega kerosena (topilo v membrani), žveplene kisline (0.2 M) in amonijevega karbonata (0.4 M). Za deset različnih kationov smo preverili selektivnost membrane in ugotovili, da pri optimalnih pogojih stopnja inkluzijske ekstrakcije doseže tudi 98-99 % učinkovitost.