SUPPORTED LIQUID MEMBRANE EXTRACTION OF TERBIUM(III) BY CYTOS IL102/D2EHPA (TRIHEXYLTETRADECYLPHOSPHONIUM BROMIDE/DI(2- ETHYLHEXYL) PHOSPHATE) EXTRACTANT MIXTURE

We have developed a membrane impregnated with ionic liquid Cytos IL102/D2EHPA (trihexyltetradecylphosphonium bromide/di(2ethylhexyl) phosphate) for the extraction of Tb(III) from aqueous solutions at different pH values. Various parameters such as the mixing effect Cytos IL102/D2EHPA, the initial terbium concentration, the stirring speed and the extraction time have been studied. The amount of Tb(III) retained per gram of extractant (Cytos IL102/D2EHPA) is 4.37 mg g for a concentration of Tb(III) of 10 M. The optimal yield was obtained at 240 min and stirring speed at 900 rpm.


INTRODUCTION
The supported liquid membrane separation (LMS) technique is an advanced solvent extraction technique that provides a simple and effective method for extracting and separating metal ions. 1 The use of membranes is becoming increasingly important in the separation and recovery of toxic and valuable metals as well as in the treatment of effluents containing low concentrations of solutes in large volumes, without generating secondary waste. 2,3[6][7][8] Terbium is used in alloys and in the production of electronic devices and other magneto mechanical devices.Terbium oxide is used in green phosphors in fluorescent in trichromatic lighting technology of lamps and colour TV tubes.In order to meet the fast-growing demand and to ensure sufficient supply of terbium, it is essential to develop an efficient Terbium recovery process from post-consumer terbium containing products.In the present work, the extraction of Tb(III) from a nitrate solution through a supported liquid membrane impregnated with the new ionic liquid D2EHPA (Scheme 1)/Cytos IL102 (trihexyltetradecylphosphonium bromide) has been studied.
Various parameters have been studied, such as agitation speed, pH of the feed phase and the initial concentration of terbium.The use of the mixture of D2EHPA and Cytos IL 102 as extractants for the extraction of terbium(III) on a supported liquid membrane has not been reported in the literature.

EXPERIMENTAL
Terbium solution at 10 -2 M was prepared by dissolving of terbium(III) nitrate (3.025 g) in 1 L of distilled water (purchased from Sigma-Aldrich).The initial pH of the sample solutions were adjusted by using dil.HNO 3 or NaOH (from Sigma-Aldrich).NaNO 3 (from Merck) was used to study the salt effect.Arsenazo III 10 -3 M (from Alfa-Aesar) was prepared by dissolving 0.0820 g in absolute ethanol.Cytos IL102 (trihexyl (tetradecyl)phosphonium bromide) was obtained from Cytec (www.cytec.com).
The membrane support was a microporous polyvinylidene difluoride (PVDF) film, with nominal porosity of 70 %, an average pore size of 0.1 µm and a total thickness of 125 µm (VVHP04700), procured from Millipore, Germany (Figures 1 and 2).
Samples containing Tb(III) were analyzed by spectrophotometer (Analytik Jena Specord 210 Plus), with Arsenazo III as ligand.The morphology of the hydrophobic support membrane at the surface and in the thickness was determined using a scanning electron microscope (SEM) Carl Zeiss EVO®40 EP. pH measurements were taken on a potentiometer Consort C831.The membrane extraction experiments were carried out a one compartment cell with mechanical stirring throughout the experiments, separated by a microporous membrane, one for feed solution and the other for stripping solution.Initial concentration of Tb in the feed phase was 10 -3 mol L -1 in all the SLM studies.The liquid membrane phase was prepared by dissolving of D2EHPA and Cytos IL102 (Scheme 1) in diethyl ether.The PVDF support was impregnated with the carrier solution for 24 h, SLMs needed more than 12 h, then removed from the solution and wiped carefully with a tissue paper to remove the excess carrier after with water to remove the excess of the organic solvent from the surface of the membrane.After this, each membrane was leaved and dripped for 30 second before being placed in the transport cell, which consists of two identical compartments of 55 mL separated by the impregnated membrane.The effective membrane area was 11.2 cm 2 (see Scheme 2).The extraction of Tb(III) was monitored by taking 100 µL from the compartment at different times for the spectrometer analysis after the addition of a buffer solution (pH = 4.0) and 150 µL 10 -3 M of Arsenazo III.All experiments were performed at 25°C. 9,10heme 2. Illustration of transport of terbium ion in SLM.
The reaction of Arzenazo III with Tb(III) is very fast to form a green complex, which absorbs in the visible range (λmax = 654 nm). 11Three concentrations of Tb(III) variants from 1.10 -4 M to 10 -3 M were prepared to plot the calibration.The percentage of Tb(III) that was extracted by MLS was determined using Eqn.(1). 12) The uptake rate of Tb, qt (mg g -1 ) was determined by Eqn.
(2), (2)   where Ci, Ct and Ce were the initial, at time, t, and equilibrium Tb(III) concentration (mol L -1 ), respectively; V (55 mL) was the volume solution; M molecular weight (g.mol -1 ), and m was the mass of extractant used.The yields are obtained with an error of  0.01 %.

RESULTS AND DISCUSSION
In this study, we have used of the mixture of D2EHPA and Cytos IL102 as extractants for the extraction of terbium on a supported liquid membrane.Cytos IL102 is a commercial phosphorous ionic liquid (trihexyl(tetradecyl)phosphonium bromide).In this study, the hydrophobic membrane support was a microporous polyvinylidene difluoride (PVDF) film with nominal porosity of 70 %, an average pore size of 0.1 µm and a total thickness of 125 µm (VVHP04700), was procured from Millipore (Germany), was used for the extraction of Tb(III) from a solution of Terbium nitrate.A parametric study was conducted to optimize the extraction conditions.

Effect of stirring speed
One of the main resistances in the liquid membrane technique is the extraction of metal ions.To minimize this resistance, the solutions in the two compartments must be kept in agitation.Figure 1 represents the extraction yield of Tb(III) as a function of time by a hydrophobic membrane for two stirring speeds of 180 rpm and 900 rpm.It is observed that the extraction yield of Tb(III) increases with increasing stirring speed.The best yield was obtained for a stirring speed of 900 rpm.Thus, the stirring speed of 900 rpm was used for the other experiments to be carried out (see Figure 3).

Effect of initial pH
The effect of pH in the feeding phase on Tb(III) extraction was studied in a pH range of 2.0 to 5.3 where the predominant species is Tb(III).The solution was adjusted with HNO3 solutions.The initial concentration of Tb(III) in the feeding phase is 10 -3 M, the volume of the solution to be extracted in the feed phase is 55 mL and with a molar ratio of D2EHPA/Cytos IL102 = 1.The results obtained are illustrated in Figure 4.
The curves in Figure 4 show that as the pH decreases from 5.3 to 2.0 in the feed phase, the extraction yield decreases; a maximum yield (80%) was observed at pH 5.3 and for 240 min.

Effect of initial concentration
The influences of the initial concentration of terbium(III) on the extraction yield were carried out in a concentration range between 10 -3 and 10 -4 M. The volume of the solution to be extracted was adjusted to 55 mL and the molar ratio of D2EHPA/Cytos IL 02 was 1:1.The results obtained are presented in Figure 5. From Figure 5 it is observed that the extraction yield decreases with the increase in the initial concentration of terbium(III) in the feeding phase.In addition, a maximum yield of 100 % is obtained in 120 minutes of stirring, when the initial concentration of terbium(III) is 10 -4 M.

CONCLUSION
Our work in this study focuses on the extraction of Tb(III) by a membrane impregnated with the ionic liquid D2EHPA/Cytos IL102.Various parameters, such as the mixing effect D2EHPA/Cytos IL102, the initial terbium concentration, the stirring speed and the extraction time, have been studied.The amount of Tb(III) retained per gram of extractant (D2EHPA/ Cytos IL102) was 4.37 mg g -1 for a concentration 10 -3 M of Tb(III).The optimal yield was obtained at 240 min and stirring speed at 900 rpm.

Figure 1 .
Figure 1.Surface SEM images of a PVDF hydrophobic support membrane and its thickness (Millipore VVHP04700).