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Permeation of iron(III) by an immobilised liquid membrane using Cyanex 923 as mobile carrier
Journal of Membrane Science 176 (2000) 249–255
Permeation of iron(III) by an immobilised liquid membrane using Cyanex 923 as mobile carrier F.J. Alguacil∗ , S. Mart´ınez Centro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain Received 17 February 2000; received in revised form 28 April 2000; accepted 3 May 2000
Abstract A study of iron(III) transport through an immobilised liquid membrane using the phosphine oxide Cyanex 923 as carrier has been carried out using batch experiments. A model is reported describing the transport mechanism which consists of (i) a diffusion process through a feed aqueous diffusion layer, (ii) a fast interfacial chemical reaction and (iii) a diffusion of HFeCl4 ·2L through the membrane. The mathematical equations describing the rate of transport are derived which correlate the membrane permeability coefficient to diffusional and equilibrium parameters as well as the chemical composition of the system, such as carrier concentration in the membrane phase and hydrochloric acid in the feed phase. The experimental data are explained with the derived equations and the diffusion resistances to mass transfer are evaluated. The influence on the transport of stirring speed in the feed phase and the nature of the diluent have also been discussed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Iron(III) transport; Cyanex 923; Permeability
1. Introduction During the past years, the use of liquid membranes has gained a general interest in the treatment of effluents where solute concentrations are low and large volumes of solutions must be processed, and if possible, without generating any secondary waste. Liquid membrane processes have been proposed as a clean technology owing to their characteristics, i.e. high specificity, low energy utilisation, etc. [1,2]. In the case of supported liquid membranes (SLMs) or immobilised liquid membranes (ILMs), one of the claimed advantages is that the extraction, stripping and regeneration steps are reduced to a single stage. ∗ Corresponding author. Tel.: +34-91-553-89-00; fax: +34-91-534-74-25. E-mail address: [email protected] (F.J. Alguacil)
The control of iron is of concern in the production of many metals, facing the world’s hydrometallurgical industry with up to 18 million tons per year of iron in its processing operations [3] and also in the production of waste chloride pickle liquors from steel plants, however, a survey of the literature had shown that little information, if any, is available on iron rejection using ILM [4–6]. Iron(III) can be extracted from this medium using various reagents: D2EHPA, TBP, MIBK, etc., but only recently the possibilities of using neutral phosphine oxides, such as Cyanex 923, has been proposed [7]. In the present investigation, the transport of Fe(III) from hydrochloric acid solutions through an ILM using Cyanex 923 as carrier has been studied. A permeation model describing the transport mechanism consisting of a diffusion process through a feed aqueous diffusion layer, a fast interfacial chemical reaction
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F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255
and a diffusion of HFeCl4 ·2L species through the membrane has been reported.
2. Experimental 2.1. Reagents and solutions The phosphine oxide Cyanex 923 (CYTEC Ind, Canada) was used as received, and its composition and main properties have been described elsewhere [8]. Its general structure is R3 P=O, where R represented alkyl chains (n-hexyl and/or n-octyl). A 0.5 g l−1 stock solution of Fe(III) was prepared from FeCl3 (A.R. Fluka) by dilution with HCl. Xylene, toluene, n-decane and carbon tetrachloride (A.R. Fluka) were used as diluents.
2.0 with NaCl. The aqueous receiving solution contained 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl. The permeation of iron was monitored by periodically sampling the feed phase, and iron was analyzed after appropriate dilution by atomic absorption spectrophotometry using a Perkin-Elmer 1100 B spectrophotometer. For all the permeation experiments performed, the permeability, P, was evaluated using the equation P =
1 d[Fe(III)] V J =− (1) [Fe(III)]TOT dt A [Fe(III)]TOT
where V is the volume of the aqueous feed solution, A is the effective membrane area, t the time and [Fe(III)]TOT is the total concentration of iron in the feed phase at time 0. Integrating Eq. (1), assuming that permeability is time-independent, the following equation is obtained: A [Fe(III)]t = −P t [Fe(III)]0 V
2.2. Membrane
ln
The flat-sheet membrane used was Millipore Durapore GVHP 4700 of 12.5×10−3 cm thick microporous polyvinylidenedifluoride (PVDF) film with nominal porosity of 75%, effective pore size of 0.22 m and tortuosity 1.67. The membrane was impregnated with carrier solution containing the extractant dissolved in the corresponding diluent by immersion for 24 h, then leaving it to drip for a few seconds before being placed in the transport cell.
then, the permeability can be calculated from the slope of ln[Fe(III)]t /[Fe(III)]0 versus time.
2.3. Transport experiments The batch transport experiments were carried out in a permeation cell consisting of two compartments made of methacrylate and separated by the microporous membrane. The effective geometrical membrane area was 11.33 cm2 and the volume of the feed and receiving solutions were 200 cm3 . The experiments were performed at the temperature of 30◦ C and using a mechanical stirring speed of 1200 rpm in the feed phase, except in the experiments where the stirring speed was varied, and 1600 rpm in the receiving phase. The aqueous feed solutions contained various concentrations of Fe(III) and 0.8–2.0 mol l−1 HCl concentrations range. The ionic strength was kept constant at
(2)
3. Permeation model of Fe(III) across the ILM The mass transfer of Fe(III) across the membrane is described by considering only diffusional parameters. The interfacial flux due to the chemical reaction is neglected, as the chemical reactions seem to take place at the interface aqueous feed solution/membrane and membrane/receiving aqueous-solution interfaces, and previous studies have suggested that chemical reactions can be considered as occurring instantaneously relative to the diffusion process [9]. Therefore, to model the mass transfer of Fe(III), it is necessary to consider diffusion of the solute through the aqueous feed boundary layer, the reversible chemical reaction at the interface and diffusion of the metal complex species in the membrane. 3.1. Extraction equilibrium The extraction of Fe(III) by Cyanex 923 in xylene has been studied previously [5], however, no information is available on the value of the extraction constant, thus, in the present work experimental
F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255
data were treated numerically using the program LETAGROP-DISTR [10] to assess the composition of the extracted species and its extraction equilibrium constant. The program is based on the minimization of the error-square sum, U X (log Dcal − log Dexp )2 (3) U= N
where Dexp are the experimental values of the distribution coefficient and Dcal are the corresponding values calculated from the relevant mass-balance equations for a proposed model and N is the total number of experimental points. The results of numerical analysis indicate that the extraction of Fe(III) by Cyanex 923 is best represented by the equilibrium and extraction constant given below: FeCl4 aq − + Haq + + 2Lorg = HFeCl4 · 2Lorg Kext
[HFeCl4 · 2L]org = [FeCl4 − ]aq [H+ ]aq [L]2org
(4) (5)
where L represent the ligand. The extraction constant of Cyanex 923 dissolved in xylene, log Kext , at 2 M hydrochloric acid concentration, has been determined to be 3.06±0.052 (σ (log Kext )=0.017) with U=0.088 (σ =0.042, defined as σ =(U/(N−NK ))1/2 where NK is the number of constants to be adjusted and N the number of experimental points).
251
lower than that between the feed phase and the membrane, thus, the concentration of the metal-extracted complex in the membrane phase at the receiving solution side may be negligible compared with that at the feed solution side, then, Eq. (7) can be re-written as Jorg = 1−1 org ([HFeCl4 · 2L]i,f )
(8)
If the chemical reaction expressed by Eq. (4) is assumed to be fast compared to the diffusion rate, local equilibrium at the interface is reached and concentrations at the interface are related by Eq. (5). At steady state, Jaq =Jorg =J and combining Eqs. (5), (6) and (8), the following expression is obtained: J =
Kext [L]2org [H+ ]aq [Fe(III)]TOT 1org + 1aq Kext [L]2org [H+ ]aq
(9)
Being the permeability coefficient, P P =
Kext [L]2org [H+ ]aq 1org + 1aq Kext [L]2org [H+ ]aq
(10)
This expression combines in one equation the equilibrium and diffusion parameters involved in the Fe(III) transport process from hydrochloric acid solutions through immobilized liquid membrane containing Cyanex 923 in xylene as carrier.
4. Results and discussion 3.2. Permeation model in ILM 4.1. Influence of the stirring speed in the feed phase The Fe(III) transport rate is determined by the rate of diffusion of iron-containing species through the feed diffusion layer and the rate of diffusion of HFeCl4 ·2L through the membrane. Then, the flux of Fe(III) crossing the membrane may be derived by applying Fick’s first diffusion law to the diffusion layer in the feed side and to the membrane. The diffusional fluxes at the aqueous feed boundary layer Jaq and at the membrane phase Jorg can be expressed by the equations given below: Jaq = 1−1 aq ([Fe(III)]TOT − [Fe(III)]i,TOT )
(6)
Jorg = 1−1 org ([HFeCl4 · 2L]i,f − [HFeCl4 · 2L]i,r ) (7) The distribution coefficient of Fe(III) between the membrane phase and the receiving phase is much
The influence of stirring speed was studied in order to optimise uniform mixing of both aqueous phases and to minimise thickness of aqueous boundary layer with feed and receiving conditions being maintained as: 3.6×10−4 mol l−1 Fe(III) in 2 mol l−1 HCl and 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl, respectively. The carrier concentration was 5.04×10−1 mol l−1 in xylene immobilised on the Durapore microporous support. The permeability coefficient becomes independent of the stirring speed above 600 rpm. Consequently, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized and the diffusion contribution of the aqueous species to the mass transfer process is assumed to be constant.
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F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255 Table 2 Influence of extractant concentration in the membrane phase on the iron transport
Fig. 1. Influence of the diluent in the membrane phase on iron transport. Feed phase: 3.6×10−4 mol l−1 Fe(III) in 2 mol l−1 HCl. Membrane phase: 2.52×10−1 mol l−1 Cyanex 923 in different diluents. Receiving phase: 2 mol l−1 NaCl.
4.2. Influence of the diluent In Fig. 1, the influence of the diluent on iron transport is shown, xylene is the diluent which gives the best permeability values. 4.3. Evaluation of the mass transfer resistances To study the influence of the hydrochloric acid concentration in the feed phase, experiments were performed at various HCl concentrations, keeping the carrier concentration in the membrane constant. The results are shown in Table 1. As can be seen from the table, iron(III) permeability increases when the hydrochloric acid concentration in the feed phase is Table 1 Influence of HCl concentration in the feed phase on iron transporta HCl (mol l−1 )
P (cm s−1 )
0.80 1.00 1.25 1.50 1.75 2.00
1.12×10−3 1.60×10−3 1.97×10−3 2.36×10−3 2.60×10−3 2.93×10−3
a Feed phase: 1.8×10−4 mol l−1 Fe(III) at different HCl concentrations; membrane phase: 5.04×10−1 mol l−1 Cyanex 923 in xylene; receiving phase: 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl.
Cyanex 923 (mol l−1 )
P (cm s−1 )
1.26×10−1 2.52×10−1 5.04×10−1 7.56×10−1
1.64×10−3 2.01×10−3 1.96×10−3 1.92×10−3
increased up to 2 mol l−1 . Similar trend was observed in liquid–liquid extraction tests [7]. The results concerning transport of iron(III) from the source phase containing 3.6 mol l−1 Fe(III) in 2 mol l−1 HCl and the receiving phase 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl and varying concentrations of Cyanex 923 in the range 1.26×10−1 –7.56×10−1 mol l−1 dissolved in xylene (Table 2) revealed no significant change in P at higher carrier concentrations. This constant permeability value Plim (Plim =1/1aq =2.0×10−3 cm s−1 ), known as limiting permeability, could be explained by assuming that diffusion in the organic membrane (1org ) is negligible compared with the term accounting for aqueous diffusion (1aq Kext [L]2org [H+ ]aq ) in Eq. (10) and the permeation process is controlled by the diffusion in the stagnant film of the aqueous feed phase. To determine the value of the resistances to the mass transfer, Eq. (10) is used and the following expression is obtained: 1org 1 = 1aq + P Kext [L]2org [H+ ]aq
(11)
By plotting 1/P as a function of 1/Kext [L]2org [H+ ]aq , for various carrier concentrations in xylene and aqueous feed phase of 2 mol l−1 HCl, one should obtain a straight line with slope 1org and ordinate to calculate 1aq . The value of 1org and 1aq calculated from the proposed model are 3784.5 and 499.5 s cm−1 , respectively. The calculated value of the diffusion coefficient (1org =dorg /Dorg ) was Dorg =3.3×10−6 cm2 s−1 . Table 3 presents the values of diffusion coefficients determined for other metal-carrier–support systems, which are of the same order of magnitude as the Cyanex 923. The mass transfer coefficient was calcula−3 cm s−1 . In addition, assuming ted as 1−1 aq =2.0×10 −5 2 −1 Daq =10 cm s [16], the thickness of the aqueous boundary layer was calculated to be 5.0×10−3 cm.
F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255
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Table 3 Diffusion coefficients of the metal carrier complexes using different ionophores across ILMs System
Diluent
Cd–TDACl Co–D2EHPA Sr–DC18C6 Au–DMPL Ag–TCTH Fe–Cyanex 923
Triethylbenzene Kerosene n-Hexylbenzene Cumene Cumene Xylene
The diffusion coefficient of the iron complex in the bulk organic phase Db,org can be evaluated from the diffusivity in the membrane, Dorg , from the following ratio [17]: Dorg =
ε Db,org τ2
(12)
The value of Db,org was calculated to be 1.2×10−5 cm2 s−1 . 4.4. Influence of metal concentration on permeability of iron(III)
Support
Dorg (cm2 s−1 )
Ref.
Celgard 2500 Durapore Celgard 2500 Durapore Durapore Durapore
3.2×10−6
[11] [12] [13] [14] [15] This work
6.7×10−6 3.7×10−6 1.0×10−6 9.8×10−6 3.3×10−6
flux is a strong function of the initial metal concentration in the source phase. However, beyond a certain limiting iron concentration (1×10−3 M), J value tends to be independent of the metal concentration, being this probably for two reasons: firstly, to membrane saturation and lower effective membrane area of the ILM, and secondly, maximization due to saturation of the membrane pores with metal-carrier species and in addition, the build-up of a carrier layer on the membrane interface which assists the retention of the separating constituent on the entry side and leads to a constant permeability flux [18].
Fig. 2 shows a plot of the initial iron flux (J) versus the concentration of iron ranging from 9.0×10−5 to 1.4×10−3 mol l−1 in the source phase solution. It is observed that at low iron concentrations, the initial
4.5. Selectivity of iron(III)–Cyanex 923 system
Fig. 2. The influence of initial concentration of Fe(III) on permeability flux (J). Feed phase: iron(III) in 2 mol l−1 HCl. Membrane phase: 5.04×10−1 mol l−1 Cyanex 923 in xylene. Receiving phase: 2 mol l−1 NaCl.
Fig. 3. ln([M]t /[M]0 ) vs. time for iron(III) and chromium(VI). Feed phase: 1.8×10−4 mol l−1 of each Fe(III) and Cr(VI) in 2 mol l−1 HCl. Membrane phase: 5.04×10−1 mol l−1 Cyanex 923 in xylene. Receiving phase: 2 mol l−1 NaCl.
According to literature data [5], chromium(VI) is extracted preferably to iron(III) at HCl concentra-
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tions up to 2.0 mol l−1 and at this concentration both metals are quantitatively extracted by Cyanex 923 dissolved in xylene. Thus the behaviour of the carrier has also been investigated. Transport experiments have been carried out using a liquid membrane of 5.04×10−1 mol l−1 Cyanex 923 dissolved in xylene and a feed phase which contained 1.8×10−4 mol l−1 of each metal. The results are shown in Fig. 3, it can be observed that a good separation of iron(III) over chromium(VI) is achieved. The selectivity factor, β Fe,Cr , defined as the ratio of permeabilities is 11.9, which indicates that, against liquid–liquid extraction results, iron is transported preferably to chromium(VI). 5. Conclusions
1org =dorg /Dorg τ Abbreviations D2EHPA DC18C6 DMPL MIBK TBP TCTH TDACl [Fe(III)]TOT [Fe(III)]i,TOT
A mechanism of iron(III) transport using Cyanex 923 considering the aqueous film diffusion of metal ions, fast chemical reaction at the interface and diffusion of HFeCl4 ·2L through the membrane is proposed. At high Cyanex 923 concentrations, a limiting value of 2.0×10−3 cm s−1 for permeability is obtained and the transport process is controlled by the diffusion in the aqueous stagnant film. Mass transfer coefficients in the membrane and in the aqueous phase are found to be 2.6×10−4 and 2.0×10−3 cm s−1 , respectively. Iron(III) can be selectively transported from 2 mol l−1 HCl solutions using ILM with Cyanex 923 over chromium(VI).
[HFeCl4 ·2L]i,f [HFeCl4 ·2L]i,r
transport resistance due to diffusion through the membrane tortuosity of the membrane
di (2-ethylhexyl) phosphoric acid dicyclohexano-18-crown-6 1-(dodecyloxy)-3-methyl-1-oxo-13 phospholen isobutyl methylketone tri-n-butyl phosphate 1,6-diethylcarbamoyl imino-1,6diphenyl-2,5-dithiahexane tridodecylammonium chloride total iron concentration in the feed phase total iron concentration at the feed phase/membrane interface HFeCl4 ·2L organic concentration at the feed phase/membrane interface HFeCl4 ·2L organic concentration at the membrane/receiving phase interface
Acknowledgements To the Comunidad de Madrid (CAM, Spain) for Project 07M/0053/1998. References
6. Nomenclature daq dorg Daq
Dorg Db,org 1aq =daq /Daq
thickness of the aqueous feed boundary layer thickness of the membrane average aqueous diffusion coefficient of the iron-containing species membrane diffusion coefficient of the iron-containing species diffusion coefficient of the ironcontaining species in the bulk phase transport resistance due to diffusion by the aqueous feed boundary layer
[1] P.J. Pickering, C.R. Southern, Clean-up to chirality — liquid membranes as a facilitated technology? J. Chem. Technol. Biotechnol. 68 (1997) 417. [2] K. Scott, Handbook of Industrial Membranes, Elsevier Advanced Technology, Kidlington, 1997, p. 64. [3] N. Ocaña, F.J. Alguacil, Solvent extraction of iron(III) by MOC-55TD: experimental equilibrium study and demonstration of lack of influence on copper(II) extraction from sulphate solutions, Hydrometallurgy 48 (1998) 239. [4] K. Scott, Handbook of Industrial Membranes, Elsevier Advanced Technology, Kidlington, 1997, p. 643. [5] A.M. Sastre, A. Kumar, J.P. Shukla, R.K. Singh, Improved techniques in liquid membrane separations: an overview, Sep. Purif. Methods 27 (1998) 213. [6] T. Nonaka, K. Fujita, Transport of ferric ions through 2,3-epithiopropyl methacrylate-dodecyl methacrylate-methacrylamide propyl trimethyl ammonium chloride terpolymer membranes, J. Membr. Sci. 144 (1998) 187.
F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255 [7] J. Saji, T. Prasada Rao, C.S.P. Iyer, M.L.P. Reddy, Extraction of iron(III) from acidic chloride solutions by Cyanex 923, Hydrometallurgy 49 (1998) 289. [8] F.J. Alguacil, C. Caravaca, S. Mart´ınez, A. Cobo, The phosphine oxides Cyanex 923 and Cyanex 925 as extractants for gold(I) cyanide aqueous solutions, Hydrometallurgy 36 (1994) 369. [9] G. Zuo, S. Orechho, M. Muhammed, Facilitated transport of gold through a membrane via complexation to thiourea-based reagents, Sep. Sci. Technol. 31 (1996) 1597. [10] D.H. Liem, High-speed computers as a supplement to graphical methods, Acta Chem. Scand. 25 (1971) 1521. [11] B.C. Garret, D.G. Truhlar, R.S. Grev, A.W. Magnuson, Improved treatment of threshold contributions in variational transition-state theory, J. Phys. Chem. 84 (1980) 1730. [12] T.C. Huang, R.S. Juang, Rate and mechanism of divalent metal transport through supported liquid membrane containing di(2-ethylhexyl) phosphoric acid as a mobile carrier, J. Chem. Technol. Biotechnol. 42 (1988) 3. [13] J.F. Dozol, J. Casas, A.M. Sastre, Influence of the extractant on strontium transport from reprocessing concentrate solutions
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through flat-sheet supported liquid membranes, Sep. Sci. Technol. 29 (1994) 1999. A. Sastre, A. Madi, J.L. Cortina, N. Miralles, Modelling of mass transfer in facilitated supported liquid membrane transport of gold(III) using phospholene derivatives as carriers, J. Membr. Sci. 139 (1998) 57. F.Z. El Aamrani, A. Kumar, L. Beyer, A. Florido, A.M. Sastre, Mechanistic study of active transport of silver(I) using sulfur containing novel carriers across a liquid membrane, J. Membr. Sci. 152 (1999) 263. M. Rovira, A.M. Sastre, Modelling of mass transfer in facilitated supported liquid-membrane transport of palladium (II) using di-(2-ethylhexyl) phosphoric acid, J. Membr. Sci. 149 (1998) 241. T. Imato, H. Ogawa, S. Mooroka, Y. Kato, Transport of copper(II) through supported liquid membrane containing LIX 65N, J. Chem. Eng. Jpn. 14 (1981) 289. J.P. Shukla, J.V. Sonawane, A. Kumar, R.K. Singh, Amine facilitated up-hill transport of plutonium(IV) cations across an immobilized liquid membrane, Indian J. Chem. Technol. 3 (1996) 145.
Permeation of iron(III) by an immobilised liquid membrane using Cyanex 923 as mobile carrier F.J. Alguacil∗ , S. Mart´ınez Centro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain Received 17 February 2000; received in revised form 28 April 2000; accepted 3 May 2000
Abstract A study of iron(III) transport through an immobilised liquid membrane using the phosphine oxide Cyanex 923 as carrier has been carried out using batch experiments. A model is reported describing the transport mechanism which consists of (i) a diffusion process through a feed aqueous diffusion layer, (ii) a fast interfacial chemical reaction and (iii) a diffusion of HFeCl4 ·2L through the membrane. The mathematical equations describing the rate of transport are derived which correlate the membrane permeability coefficient to diffusional and equilibrium parameters as well as the chemical composition of the system, such as carrier concentration in the membrane phase and hydrochloric acid in the feed phase. The experimental data are explained with the derived equations and the diffusion resistances to mass transfer are evaluated. The influence on the transport of stirring speed in the feed phase and the nature of the diluent have also been discussed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Iron(III) transport; Cyanex 923; Permeability
1. Introduction During the past years, the use of liquid membranes has gained a general interest in the treatment of effluents where solute concentrations are low and large volumes of solutions must be processed, and if possible, without generating any secondary waste. Liquid membrane processes have been proposed as a clean technology owing to their characteristics, i.e. high specificity, low energy utilisation, etc. [1,2]. In the case of supported liquid membranes (SLMs) or immobilised liquid membranes (ILMs), one of the claimed advantages is that the extraction, stripping and regeneration steps are reduced to a single stage. ∗ Corresponding author. Tel.: +34-91-553-89-00; fax: +34-91-534-74-25. E-mail address: [email protected] (F.J. Alguacil)
The control of iron is of concern in the production of many metals, facing the world’s hydrometallurgical industry with up to 18 million tons per year of iron in its processing operations [3] and also in the production of waste chloride pickle liquors from steel plants, however, a survey of the literature had shown that little information, if any, is available on iron rejection using ILM [4–6]. Iron(III) can be extracted from this medium using various reagents: D2EHPA, TBP, MIBK, etc., but only recently the possibilities of using neutral phosphine oxides, such as Cyanex 923, has been proposed [7]. In the present investigation, the transport of Fe(III) from hydrochloric acid solutions through an ILM using Cyanex 923 as carrier has been studied. A permeation model describing the transport mechanism consisting of a diffusion process through a feed aqueous diffusion layer, a fast interfacial chemical reaction
0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 4 4 2 - 7
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F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255
and a diffusion of HFeCl4 ·2L species through the membrane has been reported.
2. Experimental 2.1. Reagents and solutions The phosphine oxide Cyanex 923 (CYTEC Ind, Canada) was used as received, and its composition and main properties have been described elsewhere [8]. Its general structure is R3 P=O, where R represented alkyl chains (n-hexyl and/or n-octyl). A 0.5 g l−1 stock solution of Fe(III) was prepared from FeCl3 (A.R. Fluka) by dilution with HCl. Xylene, toluene, n-decane and carbon tetrachloride (A.R. Fluka) were used as diluents.
2.0 with NaCl. The aqueous receiving solution contained 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl. The permeation of iron was monitored by periodically sampling the feed phase, and iron was analyzed after appropriate dilution by atomic absorption spectrophotometry using a Perkin-Elmer 1100 B spectrophotometer. For all the permeation experiments performed, the permeability, P, was evaluated using the equation P =
1 d[Fe(III)] V J =− (1) [Fe(III)]TOT dt A [Fe(III)]TOT
where V is the volume of the aqueous feed solution, A is the effective membrane area, t the time and [Fe(III)]TOT is the total concentration of iron in the feed phase at time 0. Integrating Eq. (1), assuming that permeability is time-independent, the following equation is obtained: A [Fe(III)]t = −P t [Fe(III)]0 V
2.2. Membrane
ln
The flat-sheet membrane used was Millipore Durapore GVHP 4700 of 12.5×10−3 cm thick microporous polyvinylidenedifluoride (PVDF) film with nominal porosity of 75%, effective pore size of 0.22 m and tortuosity 1.67. The membrane was impregnated with carrier solution containing the extractant dissolved in the corresponding diluent by immersion for 24 h, then leaving it to drip for a few seconds before being placed in the transport cell.
then, the permeability can be calculated from the slope of ln[Fe(III)]t /[Fe(III)]0 versus time.
2.3. Transport experiments The batch transport experiments were carried out in a permeation cell consisting of two compartments made of methacrylate and separated by the microporous membrane. The effective geometrical membrane area was 11.33 cm2 and the volume of the feed and receiving solutions were 200 cm3 . The experiments were performed at the temperature of 30◦ C and using a mechanical stirring speed of 1200 rpm in the feed phase, except in the experiments where the stirring speed was varied, and 1600 rpm in the receiving phase. The aqueous feed solutions contained various concentrations of Fe(III) and 0.8–2.0 mol l−1 HCl concentrations range. The ionic strength was kept constant at
(2)
3. Permeation model of Fe(III) across the ILM The mass transfer of Fe(III) across the membrane is described by considering only diffusional parameters. The interfacial flux due to the chemical reaction is neglected, as the chemical reactions seem to take place at the interface aqueous feed solution/membrane and membrane/receiving aqueous-solution interfaces, and previous studies have suggested that chemical reactions can be considered as occurring instantaneously relative to the diffusion process [9]. Therefore, to model the mass transfer of Fe(III), it is necessary to consider diffusion of the solute through the aqueous feed boundary layer, the reversible chemical reaction at the interface and diffusion of the metal complex species in the membrane. 3.1. Extraction equilibrium The extraction of Fe(III) by Cyanex 923 in xylene has been studied previously [5], however, no information is available on the value of the extraction constant, thus, in the present work experimental
F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255
data were treated numerically using the program LETAGROP-DISTR [10] to assess the composition of the extracted species and its extraction equilibrium constant. The program is based on the minimization of the error-square sum, U X (log Dcal − log Dexp )2 (3) U= N
where Dexp are the experimental values of the distribution coefficient and Dcal are the corresponding values calculated from the relevant mass-balance equations for a proposed model and N is the total number of experimental points. The results of numerical analysis indicate that the extraction of Fe(III) by Cyanex 923 is best represented by the equilibrium and extraction constant given below: FeCl4 aq − + Haq + + 2Lorg = HFeCl4 · 2Lorg Kext
[HFeCl4 · 2L]org = [FeCl4 − ]aq [H+ ]aq [L]2org
(4) (5)
where L represent the ligand. The extraction constant of Cyanex 923 dissolved in xylene, log Kext , at 2 M hydrochloric acid concentration, has been determined to be 3.06±0.052 (σ (log Kext )=0.017) with U=0.088 (σ =0.042, defined as σ =(U/(N−NK ))1/2 where NK is the number of constants to be adjusted and N the number of experimental points).
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lower than that between the feed phase and the membrane, thus, the concentration of the metal-extracted complex in the membrane phase at the receiving solution side may be negligible compared with that at the feed solution side, then, Eq. (7) can be re-written as Jorg = 1−1 org ([HFeCl4 · 2L]i,f )
(8)
If the chemical reaction expressed by Eq. (4) is assumed to be fast compared to the diffusion rate, local equilibrium at the interface is reached and concentrations at the interface are related by Eq. (5). At steady state, Jaq =Jorg =J and combining Eqs. (5), (6) and (8), the following expression is obtained: J =
Kext [L]2org [H+ ]aq [Fe(III)]TOT 1org + 1aq Kext [L]2org [H+ ]aq
(9)
Being the permeability coefficient, P P =
Kext [L]2org [H+ ]aq 1org + 1aq Kext [L]2org [H+ ]aq
(10)
This expression combines in one equation the equilibrium and diffusion parameters involved in the Fe(III) transport process from hydrochloric acid solutions through immobilized liquid membrane containing Cyanex 923 in xylene as carrier.
4. Results and discussion 3.2. Permeation model in ILM 4.1. Influence of the stirring speed in the feed phase The Fe(III) transport rate is determined by the rate of diffusion of iron-containing species through the feed diffusion layer and the rate of diffusion of HFeCl4 ·2L through the membrane. Then, the flux of Fe(III) crossing the membrane may be derived by applying Fick’s first diffusion law to the diffusion layer in the feed side and to the membrane. The diffusional fluxes at the aqueous feed boundary layer Jaq and at the membrane phase Jorg can be expressed by the equations given below: Jaq = 1−1 aq ([Fe(III)]TOT − [Fe(III)]i,TOT )
(6)
Jorg = 1−1 org ([HFeCl4 · 2L]i,f − [HFeCl4 · 2L]i,r ) (7) The distribution coefficient of Fe(III) between the membrane phase and the receiving phase is much
The influence of stirring speed was studied in order to optimise uniform mixing of both aqueous phases and to minimise thickness of aqueous boundary layer with feed and receiving conditions being maintained as: 3.6×10−4 mol l−1 Fe(III) in 2 mol l−1 HCl and 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl, respectively. The carrier concentration was 5.04×10−1 mol l−1 in xylene immobilised on the Durapore microporous support. The permeability coefficient becomes independent of the stirring speed above 600 rpm. Consequently, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized and the diffusion contribution of the aqueous species to the mass transfer process is assumed to be constant.
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Fig. 1. Influence of the diluent in the membrane phase on iron transport. Feed phase: 3.6×10−4 mol l−1 Fe(III) in 2 mol l−1 HCl. Membrane phase: 2.52×10−1 mol l−1 Cyanex 923 in different diluents. Receiving phase: 2 mol l−1 NaCl.
4.2. Influence of the diluent In Fig. 1, the influence of the diluent on iron transport is shown, xylene is the diluent which gives the best permeability values. 4.3. Evaluation of the mass transfer resistances To study the influence of the hydrochloric acid concentration in the feed phase, experiments were performed at various HCl concentrations, keeping the carrier concentration in the membrane constant. The results are shown in Table 1. As can be seen from the table, iron(III) permeability increases when the hydrochloric acid concentration in the feed phase is Table 1 Influence of HCl concentration in the feed phase on iron transporta HCl (mol l−1 )
P (cm s−1 )
0.80 1.00 1.25 1.50 1.75 2.00
1.12×10−3 1.60×10−3 1.97×10−3 2.36×10−3 2.60×10−3 2.93×10−3
a Feed phase: 1.8×10−4 mol l−1 Fe(III) at different HCl concentrations; membrane phase: 5.04×10−1 mol l−1 Cyanex 923 in xylene; receiving phase: 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl.
Cyanex 923 (mol l−1 )
P (cm s−1 )
1.26×10−1 2.52×10−1 5.04×10−1 7.56×10−1
1.64×10−3 2.01×10−3 1.96×10−3 1.92×10−3
increased up to 2 mol l−1 . Similar trend was observed in liquid–liquid extraction tests [7]. The results concerning transport of iron(III) from the source phase containing 3.6 mol l−1 Fe(III) in 2 mol l−1 HCl and the receiving phase 0.4 mol l−1 HCl in 1.6 mol l−1 NaCl and varying concentrations of Cyanex 923 in the range 1.26×10−1 –7.56×10−1 mol l−1 dissolved in xylene (Table 2) revealed no significant change in P at higher carrier concentrations. This constant permeability value Plim (Plim =1/1aq =2.0×10−3 cm s−1 ), known as limiting permeability, could be explained by assuming that diffusion in the organic membrane (1org ) is negligible compared with the term accounting for aqueous diffusion (1aq Kext [L]2org [H+ ]aq ) in Eq. (10) and the permeation process is controlled by the diffusion in the stagnant film of the aqueous feed phase. To determine the value of the resistances to the mass transfer, Eq. (10) is used and the following expression is obtained: 1org 1 = 1aq + P Kext [L]2org [H+ ]aq
(11)
By plotting 1/P as a function of 1/Kext [L]2org [H+ ]aq , for various carrier concentrations in xylene and aqueous feed phase of 2 mol l−1 HCl, one should obtain a straight line with slope 1org and ordinate to calculate 1aq . The value of 1org and 1aq calculated from the proposed model are 3784.5 and 499.5 s cm−1 , respectively. The calculated value of the diffusion coefficient (1org =dorg /Dorg ) was Dorg =3.3×10−6 cm2 s−1 . Table 3 presents the values of diffusion coefficients determined for other metal-carrier–support systems, which are of the same order of magnitude as the Cyanex 923. The mass transfer coefficient was calcula−3 cm s−1 . In addition, assuming ted as 1−1 aq =2.0×10 −5 2 −1 Daq =10 cm s [16], the thickness of the aqueous boundary layer was calculated to be 5.0×10−3 cm.
F.J. Alguacil, S. Mart´ınez / Journal of Membrane Science 176 (2000) 249–255
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Table 3 Diffusion coefficients of the metal carrier complexes using different ionophores across ILMs System
Diluent
Cd–TDACl Co–D2EHPA Sr–DC18C6 Au–DMPL Ag–TCTH Fe–Cyanex 923
Triethylbenzene Kerosene n-Hexylbenzene Cumene Cumene Xylene
The diffusion coefficient of the iron complex in the bulk organic phase Db,org can be evaluated from the diffusivity in the membrane, Dorg , from the following ratio [17]: Dorg =
ε Db,org τ2
(12)
The value of Db,org was calculated to be 1.2×10−5 cm2 s−1 . 4.4. Influence of metal concentration on permeability of iron(III)
Support
Dorg (cm2 s−1 )
Ref.
Celgard 2500 Durapore Celgard 2500 Durapore Durapore Durapore
3.2×10−6
[11] [12] [13] [14] [15] This work
6.7×10−6 3.7×10−6 1.0×10−6 9.8×10−6 3.3×10−6
flux is a strong function of the initial metal concentration in the source phase. However, beyond a certain limiting iron concentration (1×10−3 M), J value tends to be independent of the metal concentration, being this probably for two reasons: firstly, to membrane saturation and lower effective membrane area of the ILM, and secondly, maximization due to saturation of the membrane pores with metal-carrier species and in addition, the build-up of a carrier layer on the membrane interface which assists the retention of the separating constituent on the entry side and leads to a constant permeability flux [18].
Fig. 2 shows a plot of the initial iron flux (J) versus the concentration of iron ranging from 9.0×10−5 to 1.4×10−3 mol l−1 in the source phase solution. It is observed that at low iron concentrations, the initial
4.5. Selectivity of iron(III)–Cyanex 923 system
Fig. 2. The influence of initial concentration of Fe(III) on permeability flux (J). Feed phase: iron(III) in 2 mol l−1 HCl. Membrane phase: 5.04×10−1 mol l−1 Cyanex 923 in xylene. Receiving phase: 2 mol l−1 NaCl.
Fig. 3. ln([M]t /[M]0 ) vs. time for iron(III) and chromium(VI). Feed phase: 1.8×10−4 mol l−1 of each Fe(III) and Cr(VI) in 2 mol l−1 HCl. Membrane phase: 5.04×10−1 mol l−1 Cyanex 923 in xylene. Receiving phase: 2 mol l−1 NaCl.
According to literature data [5], chromium(VI) is extracted preferably to iron(III) at HCl concentra-
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tions up to 2.0 mol l−1 and at this concentration both metals are quantitatively extracted by Cyanex 923 dissolved in xylene. Thus the behaviour of the carrier has also been investigated. Transport experiments have been carried out using a liquid membrane of 5.04×10−1 mol l−1 Cyanex 923 dissolved in xylene and a feed phase which contained 1.8×10−4 mol l−1 of each metal. The results are shown in Fig. 3, it can be observed that a good separation of iron(III) over chromium(VI) is achieved. The selectivity factor, β Fe,Cr , defined as the ratio of permeabilities is 11.9, which indicates that, against liquid–liquid extraction results, iron is transported preferably to chromium(VI). 5. Conclusions
1org =dorg /Dorg τ Abbreviations D2EHPA DC18C6 DMPL MIBK TBP TCTH TDACl [Fe(III)]TOT [Fe(III)]i,TOT
A mechanism of iron(III) transport using Cyanex 923 considering the aqueous film diffusion of metal ions, fast chemical reaction at the interface and diffusion of HFeCl4 ·2L through the membrane is proposed. At high Cyanex 923 concentrations, a limiting value of 2.0×10−3 cm s−1 for permeability is obtained and the transport process is controlled by the diffusion in the aqueous stagnant film. Mass transfer coefficients in the membrane and in the aqueous phase are found to be 2.6×10−4 and 2.0×10−3 cm s−1 , respectively. Iron(III) can be selectively transported from 2 mol l−1 HCl solutions using ILM with Cyanex 923 over chromium(VI).
[HFeCl4 ·2L]i,f [HFeCl4 ·2L]i,r
transport resistance due to diffusion through the membrane tortuosity of the membrane
di (2-ethylhexyl) phosphoric acid dicyclohexano-18-crown-6 1-(dodecyloxy)-3-methyl-1-oxo-13 phospholen isobutyl methylketone tri-n-butyl phosphate 1,6-diethylcarbamoyl imino-1,6diphenyl-2,5-dithiahexane tridodecylammonium chloride total iron concentration in the feed phase total iron concentration at the feed phase/membrane interface HFeCl4 ·2L organic concentration at the feed phase/membrane interface HFeCl4 ·2L organic concentration at the membrane/receiving phase interface
Acknowledgements To the Comunidad de Madrid (CAM, Spain) for Project 07M/0053/1998. References
6. Nomenclature daq dorg Daq
Dorg Db,org 1aq =daq /Daq
thickness of the aqueous feed boundary layer thickness of the membrane average aqueous diffusion coefficient of the iron-containing species membrane diffusion coefficient of the iron-containing species diffusion coefficient of the ironcontaining species in the bulk phase transport resistance due to diffusion by the aqueous feed boundary layer
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