Ultrasonics Sonochemistry xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Short Communication
Increasing stability and transport efficiency of supported liquid membranes through a novel ultrasound-assisted preparation method. Its application to cobalt(II) removal Gerardo León a,⇑, Guillermo Martínez a, María Amelia Guzmán b, Juan Ignacio Moreno a, Beatriz Miguel a, José Antonio Fernández-López a a b
Departamento de Ingeniería Química y Ambiental, Universidad Politécnica de Cartagena, Paseo Alfonso XIII, 44, 30176 Cartagena, Spain Dirección Territorial de Comercio y Delegación del Instituto de Comercio Exterior, Paseo Alfonso X, 6, 30008 Murcia, Spain
a r t i c l e
i n f o
Article history: Received 10 April 2012 Received in revised form 1 October 2012 Accepted 3 October 2012 Available online xxxx Keywords: Supported liquid membranes Ultrasound Cobalt(II) Facilitated transport CYANEX 272
a b s t r a c t A novel ultrasound assisted method for preparing supported liquid membranes is described in this paper. The stability and efficiency of the supported liquid membrane obtained was tested by removing cobalt(II) from aqueous solutions through a facilitated countertransport mechanism using CYANEX 272 as carrier and protons as counterions. The results are compared with those obtained using supported liquid membranes prepared by soaking the polymeric material in the organic solution of the carrier at atmospheric pressure and under vacuum, both for 24 h. Higher transport efficiency (>5%), flux (18%), permeability (20%) and stability (>6% in the second run and 10% in the third run) were obtained by the supported liquid membrane prepared using the ultrasound assisted method. These findings can be explained by the effects of cavitation and acoustical streaming – which result from the ultrasound passing through the organic solution of the extractant – on the porous structure of the polymer support and on the pore filling. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Increasing interest in new chemical separation techniques is reflected by the large number of contributions on this subject that has appeared in the scientific literature during the last two decades, largely due to increasing concerns about environmental problems, the potential for energy saving and the optimization of industrial separation problems [1]. To this end, membrane technology may supply interesting solutions, because it has been successfully used in separation processes of a diverse nature. Among different membrane processes, liquid membranes have shown great potential, especially in cases where solute concentrations are relatively low and other techniques cannot be applied efficiently. Liquid membranes offer a potentially attractive alternative in that they combine the process of extraction and stripping in a single unit operation [2]. Three kinds of liquid membranes have been described: bulk, supported and emulsion liquid membranes [2]. Supported liquid membranes (SLM) are obtained when a rigid porous substrate is filled with an organic solution of an extractant. Such liquid membranes have been widely used for the recovery of metal ions from aqueous solutions, the removal of contaminants ⇑ Corresponding author. Tel.: +34 868 071002; fax: +34 968 32 55 55. E-mail address:
[email protected] (G. León).
from industrial effluents and for the recovery of fermentation products [2]. Although these membranes have several advantages, such as their high selectivity, operational simplicity, low solvent inventory and low energy consumption [3], their industrial application is still rare due to their instability and short lifetime [4]. This instability depends very much on the type of solvent used, the molecular structure and composition of the carrier, the kind of polymeric support and the pore dimensions [4,5]. Different studies have analyzed the influence of these parameters on the stability of the membrane, but very few have analyzed the influence of the way in which the pores of the rigid substrate are filled by the organic solution of the extractant [5]. Two methods are usually used: soaking the polymeric material in the organic solution of the carrier at atmospheric pressure [5,6] or under vacuum [5,7]. Ultrasound has proved to be effective in many processes common in the chemical industry to improve dewatering and drying materials, enhance filtration, to assist heat transfer, to degas liquids, to accelerate extraction processes, to degrade chemical contaminants in water and to enhance processes where diffusion takes place [8,9]. The use of ultrasound to prepare emulsion liquid membranes has also been described [10,11]. The benefits of ultrasound arise from its chemical effects on products, or its mechanical and physical effects on the processes involved [8,9].
1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2012.10.002
Please cite this article in press as: G. León et al., Increasing stability and transport efficiency of supported liquid membranes through a novel ultrasoundassisted preparation method. Its application to cobalt(II) removal, Ultrason. Sonochem. (2012), http://dx.doi.org/10.1016/j.ultsonch.2012.10.002
2
G. León et al. / Ultrasonics Sonochemistry xxx (2012) xxx–xxx
When ultrasound passes through a liquid medium it causes mechanical vibration of the liquid and generates acoustic streaming within it. If the liquid medium contains dissolved gas nuclei, as happens in normal conditions, they may expand and collapse as a result of the action of the ultrasound. This phenomenon of microbubble growth and collapse in an ultrasonic field is known as acoustic cavitation [12]. If a succession of compression and rarefaction phases is produced, the liquid will break down and voids may be generated in the liquid. The created voids are the cavitation bubbles which are responsible for the ultrasonic effect. When these bubbles reach a critical size they collapse and release large amounts of energy. The temperature and the pressure at the moment of collapse have been estimated to be up to 5000 K and 2000 atmospheres, respectively, in an ultrasonic bath at room temperature [9]. This creates hotspots and generates several physical effects, namely shock waves, microjets, turbulence, shear forces, etc., increasing the chemical reactivity of the medium [9,12]. This paper describes a novel method for preparing supported liquid membranes assisted by ultrasound. The stability and efficiency of the resulting supported liquid membrane is tested and compared with the results obtained using supported liquid membranes prepared by the soaking and the vacuum methods. The effect of ultrasound on the porous structure of the membrane is also analyzed. Cobalt(II) was selected as the chemical species to be removed in this comparative study since the discharge of cobalt, and other heavy metal pollutants, into the environment is a serious problem facing numerous industries [13]. Because heavy metals are not biodegradable in natural conditions, they tend to accumulate in living organisms, were they cause a variety of diseases and disorders [14]. Furthermore, the presence of heavy metal ions in wastewater inhibits the biodegradation of organic pollutants, which might be present in wastewater [15]. The effects of acute cobalt poisoning in humans include asthma-like allergy, damage to the heart, often resulting in heart failure, damage to the thyroid and liver and genetic changes in living cells [16]. It is for these reasons that cobalt concentrations in aqueous effluents must be reduced to acceptable levels before they are discharged into the environment. The use of liquid membranes for the removal of cobalt from aqueous solutions has been described by several authors [7,17–19]. In order to improve the cobalt(II) removal process, an ion exchange carrier is added to the organic membrane phase to accelerate and facilitate the transport of cobalt(II) from the feed to the product phase, being this process accompanied by the transport of other chemical specie from the product to the feed phase (facilitated countertransport). This coupled transport mechanism is interesting because it offers the possibility of transporting a component against its own concentration gradient [20]. In this paper CYANEX 272 (bis[2,4,4-trimethylpentyl] phosphinic acid) is used as carrier and protons are used as counterions.
2. Material and methods A microporous hydrophobic PVDF film (Millipore Durapore GVHP 10), geometrical area 20 cm2, was used as solid porous support. According to the specification provided by the supplier, this support has a porosity of 75%, pore dimensions of 0.22 lm and a thickness of 125 lm. The liquid membrane phase was constituted by a 10% solution of CYANEX 272 in commercial paraffin. The pores of the micro porous support were filled with the organic solution of CYANEX 272 by soaking the polymeric support in the organic solution in three ways: at atmospheric pressure for 24 h, under vacuum for 24 h and at atmospheric pressure but assisted by ultrasound (30 kHz, 150 lm) for 30 min (three times, 10 min each time). For the ultrasound assisted method, Labsonic M (Sartorius) ultrasound equipment (titanium probe 10 mm diameter, sound rating density 130 W/cm2), was used. The active layer of the polymeric support was positioned perpendicularly to the direction of the ultrasound and at a distance of 16 mm from the ultrasound probe (Fig. 1a). To study the effect of ultrasound on the solid porous support, non-treated and ultrasound treated solid supports were analyzed by scanning electronic microscopy using a Hitachi S-3500N scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan). The morphology of the non-treated and ultrasound treated membrane solid supports were imaged at 1500, 4000 and 7000 magnification, 5 kV accelerating voltage, 6 mm of working distance, in high vacuum conditions. Transport studies were carried out using a two compartment permeation cell which consisted of a feed phase (250 cm3) separated from the receiving phase chamber (250 cm3) by a supported liquid membrane with an effective area of 15 cm2. A schematic illustration of the experimental cell is shown in Fig. 1b. The feed and receiving phases were mechanically stirred at 200 rpm at room temperature. As feed phase, 0.265 kg/m3 cobalt(II) solutions in 0.2 M acetate buffer, pH 5, were used. Aqueous sulfuric acid solutions (0.2 M) were used as product phase. Samples from the product phase compartment were taken at time intervals and cobalt(II) concentrations were measured by atomic absorption spectrophotometry at 240.7 nm using Shimadzu AA-2600 equipment. Significant differences were obtained for the studied parameters after 120 min, which was selected as the end time. The experiments were carried out in duplicate and the results obtained were showed less than 2% deviation. Membrane fluxes (J) were determined by monitoring cobalt(II) concentration in the product phase as the function of time, based on the following equation [21]:
J¼
V dC A dt
ð1Þ
where V is the volume of the product phase, A is the effective surface area of the membrane (geometrical area multiplied by porosity [7]), C is the cobalt(II) concentration in the product phase and t is the time elapsed. The cobalt(II) fluxes can be calculated from the
Ultrasonic generator
Ultrasonic probe
F Extractant solution
PVDF support
(a)
M
S
P S
(b)
Fig. 1. Schematic representations of the sonication system (a) and of the experimental transport cell (b).
Please cite this article in press as: G. León et al., Increasing stability and transport efficiency of supported liquid membranes through a novel ultrasoundassisted preparation method. Its application to cobalt(II) removal, Ultrason. Sonochem. (2012), http://dx.doi.org/10.1016/j.ultsonch.2012.10.002
3
G. León et al. / Ultrasonics Sonochemistry xxx (2012) xxx–xxx
Membrane phase
Feed phase
Co+2
ln
2H+
CoR2(HR)2
C0 A ¼ Pt C 0 C pt V
Co+2
ET ¼ 100
H+
Cobalt(II) transport from the feed to the product phase by a countertransport mechanism using CYANEX 272 as carrier and protons as counterions is illustrated in Fig. 2 [17]. At the feed/ membrane interface, an interfacial ion exchange reaction takes place, in which D2EHPA releases protons in the feed phase and binds with cobalt(II) to form a complex, which is extracted by the membrane. The complex is transported due to the concentration gradient to the membrane/product interface, where another interfacial ion exchange reaction takes place, thereby regenerating the carrier and releasing cobalt(II) into the product phase. The carrier returns to the feed/membrane interface to begin a new separation cycle. The permeation net result is a cobalt flux from feed to
ð2Þ
where Cft is the concentration of cobalt(II) remaining in the feed solution at a certain time t, C0 is the initial concentration of cobalt(II) ions in the feed solution before extraction and P is the permeability coefficient of the SLM system. Since the amount of Co(II) retained in the supported liquid membrane is negligible, it can be assumed that Cft = C0 Cpt, (being Cpt the concentration of the
100
(a)
80
[Co(II)]p (Kg/m3)
[Co(II)]p (Kg/m3)
100
ð4Þ
3. Results and discussion
slope of the straight line obtained when plotting the metal concentration in the product phase as a function of time. Membrane permeabilities were determined using the following equation [22]:
C ft A ¼ Pt V C0
C pt C0
Liquid membrane stability was determined as the percentage of Co(II) transported in successive runs, taking the first run as reference.
Fig. 2. Diagram of the facilitated transport of Co(II) ions using CYANEX 272 as carrier and H+ as counter ion.
ln
ð3Þ
By plotting ln[C0/(C0 Cpt)] against t, a straight line is obtained. The slope of this line can be used to calculate the permeability coefficient of the SLM system. The transport efficiency or stripping percentage, ET (%), was calculated as the percentage of cobalt(II) transported from the feed to the product phase, measured at 120 min, according to the equation [23]:
Co+2
2(HR)2
2H+
cobalt(II) ions released in the product solution at time t) and Eq. (2) can be written:
Product phase
60 40 20 0
(b)
80 60 40 20 0
0
20
40
60
80
100
120
140
0
20
40
Time (minutes)
[Co(II)]p (Kg/m3)
100
60
80
100
120
140
Time (minutes)
(c)
80 60 40 20 0 0
20
40
60
80
100
120
140
Time (minutes) Fig. 3. Time course of cobalt(II) concentration in product phase in three successive experiments ( first run; j second run; Nthird run) using liquid membranes prepared by different methods (a) soaking; (b) vacuum; and (c) ultrasound.
Please cite this article in press as: G. León et al., Increasing stability and transport efficiency of supported liquid membranes through a novel ultrasoundassisted preparation method. Its application to cobalt(II) removal, Ultrason. Sonochem. (2012), http://dx.doi.org/10.1016/j.ultsonch.2012.10.002
4
G. León et al. / Ultrasonics Sonochemistry xxx (2012) xxx–xxx
Fig. 4. Microscopic images of untreated (a) and ultrasound treated (b) PVDF supported porous material (magnifications: 80,000, above; 400,000, below).
Table 1 Transport efficiency, flux, permeability and stability (transport efficiency of second and third run as percentage of first run transport efficiency) of supported liquid membranes prepared by different methods.
Transport efficiency (%) Flux (kg/m2 s) Permeability (m s1)
Soaking
Vacuum
Ultrasound
29.06 1.793 106 8.06 106
28.87 1.805 106 7.22 106
34.91 2.18 106 1.00 105
Stability (%) (percent of transport efficiency of the first run) 2nd run 85.71 90.19 3rd run 77.27 78.43
96.22 88.11
the product phase and a hydrogen ion flux in the opposite direction. Fig. 3 shows variations in the cobalt(II) concentration obtained in the product phase in three successive experiments using the different preparation methods. Table 1 indicates the transport efficiency, flux, permeability and stability (transport efficiency of second and third run as percentage of first run transport efficiency) of the supported liquid membranes prepared by the three methods. Supported liquid membranes prepared by the ultrasound assisted method provided better results than the membranes prepared by the other two methods, as seen from the higher values of transport efficiency (>5%), flux (18%), permeability (20%) and stability (>6% in the second run and 10% in the third run). Acoustical streaming and cavitation produced when ultrasound passes through a liquid medium, can affect porous structure of the polymeric support through microjets impacting on the surface, with a resulting shock wave that can alter the surface of the solid material [8,9,12]. Modifications of the surface of polymeric membrane supports which lead to an increase in pore radius and to an overall increase in pore density and porosity have been described, although these modifications are much less important in the case of PVDF polymeric supports than for other types of support [8]. The effect of ultrasound on the PVDF solid porous support
used in this study is shown in Fig. 4. As can be seen, pore density and porosity are greater in the ultrasound treated support (Fig. 4a) than in the non-treated support (Fig. 4b). No SEM micrographs were obtained for the supported liquid membranes prepared by the soaking and vacuum processes since no change in the porous structure of the polymeric support can be expected in these processes. Moreover, acoustical streaming and cavitation resulting from ultrasound passing through the organic solution of the extractant increase pore filling. This is a result of decreased viscosity of the liquid, due to the increased temperature [8,24], and the high speed liquid jets directed at the support surface, which are formed when cavitation occurs near the surface [24,25]. Accordingly, the better performance of the supported liquid membrane prepared by ultrasound can be explained by the increased pore radius and pore density of the support and the increase in pore filling. 4. Conclusion A novel method of preparing supported liquid membranes assisted by ultrasound is described in this paper. The stability and efficiency of the supported liquid membrane is confirmed by the removal of cobalt(II) from aqueous solutions through a facilitated countertransport mechanism using CYANEX 272 as carrier and protons as counterions. Comparisons are made with supported liquid membranes prepared by soaking the polymeric material in the organic solution of the carrier at atmospheric pressure and under vacuum, both during 24 h. Higher values of transport efficiency (>5%), flux (18%), permeability (20%) and stability (>6% in the second run and 10% in the third run) are obtained by the membrane prepared by the ultrasound assisted method. These results are explained by both the increase of pore radius and pore density of the support and the increase in pore filling as a consequence of the acoustical streaming and cavitation resulting from ultrasound passing through the organic solution of the extractant when the ultrasound assisted liquid membrane preparation method is used.
Please cite this article in press as: G. León et al., Increasing stability and transport efficiency of supported liquid membranes through a novel ultrasoundassisted preparation method. Its application to cobalt(II) removal, Ultrason. Sonochem. (2012), http://dx.doi.org/10.1016/j.ultsonch.2012.10.002
G. León et al. / Ultrasonics Sonochemistry xxx (2012) xxx–xxx
References [1] J.G. Crespo, K.W. Böddeker, in: Membrane Processes in Separation and Purification, NATO ASI Serie. E vol. 272, Kluwer Academic Publishers Dordrecht 1994. [2] A.M. Sastre, A. Kumar, J.P. Shukla, R.K. Singh, Improved techniques in liquid membrane separations: an overview, Sep. Purif. Meth. 27 (1998) 213–298. [3] N.M. Kocherginsky, Q. Yang, L. Seelam, Recent advances in supported liquid membrane technology, Sep. Purif. Technol. 53 (2007) 171–177. [4] A.M. Neplenbroek, D. Bargeman, C.A. Smolders, Supported liquid membranes: instability effects, J. Membr. Sci. 67 (1992) 121–132. [5] M. Szpakowska, O.B. Nagy, Stability of supported liquid membranes, J. Membr. Sci. 129 (1997) 251–261. [6] H.D. Zheng, B.Y. Wang, Y.X. Wu, Q.L. Ren, Instability mechanisms of supported liquid membranes for copper(II) ion extraction, Colloids Surf. A: Physicochem. Eng. Asp. 351 (2009) 38–45. [7] P.K. Parhi, K. Sarangi, Separation of copper, zinc, cobalt and nickel ions by supported liquid membrane technique using LIX 84I, TOPS-99 and CYANEX 272, Sep. Purif. Technol. 59 (2008) 169–174. [8] I. Masselin, X. Chasseray, L. Durand-Bourlier, J.M. Lainé, P.Y. Syzaret, D. Lemordant, Effect of sonication on polymeric membranes, J. Membr. Sci. 181 (2001) 213–220. [9] F. Chemat, Z. Huma, M.K. Khan, Applications of ultrasound in food technology: processing, preservation and extraction, Ultrason. Sonochem. 18 (2011) 813– 835. [10] R.S. Juang, K.H. Lin, Ultrasound-assisted production of W/O emulsions in liquid surfactant membrane processes, Colloids Surf. A 238 (2004) 43–49. [11] M. Chiha, O. Hamdaoui, F. Ahmedchekkat, C. Pétrier, Study of ultrsonically assisted emulsification and recovery of copper(II) from wastewater using an emulsion liquid membrane process, Ultrason. Sonochem. 17 (2010) 318–325. [12] M. Ashokkumar, The characterization of acoustic cavitation bubbles – An overview, Ultrason. Sonochem. 18 (2011) 864–872.
5
[13] J.E. Ferguson, The Heavy Metal Elements: Chemistry Environmental Impact and Health Effects, Pergamon Press, Oxford, 1990. [14] C.D. Klaassen, Casarett & Doull’s Toxicology: Basic Science of Poisons, McGraw-Hill, New York, 1996. [15] C.M.A. Ademoroti, Model to predict BOD from COD values, Effluent Water Treat. J. 26 (1986) 80–84. [16] M. De Boeek, M. Kirsch-Volders, D. Lison, Cobalt and antimony: genotoxicity and carcinogenicity, Mut. Res. 533 (2003) 135–152. [17] G. León, M.A. Guzmán, Kinetic study of the effect of carrier and stripping agent concentrations on the facilitated transport of cobalt through bulk liquid membranes, Desalination 184 (2005) 79–87. [18] F.J. Alguacil, M. Alonso, Separation of zinc(II) from cobalt(II) solutions using supported liquid membrane with DP-8R (di(2-ethylhexyl) phosphoric acid) as a carrier, Sep. Purif. Technol. 41 (2005) 179–184. [19] R.A. Kumbasar, I. Sahin, Separation and concentration of cobalt from ammoniacal solutions containing cobalt and nickel by emulsion liquid membrane using 5,7-dibromo-8-hydroxyquinoline (DBHQ), J. Membr. Sci. 325 (2008) 712–718. [20] J. Gyves, E. Rodríguez, Metal ion separations by supported liquid membranes, Ind. Eng. Chem. Res. 38 (1999) 2182–2202. [21] M. Hor, A. Riad, A. Benijjar, L. Lebrum, M. Hlaïbi, Technique of supported liquid membranes (SLMs) for the facilitated transport of vanadium ions (VO2+). Parameters and mechanism on the transport, Desalination 255 (2010) 188– 195. [22] G. Muthuraman, K. Palanivelu, Transport of textile dye in vegetable oils based supported liquid membrane, Dyes Pigm. 70 (2006) 99–104. [23] R. Güell, C. Fontàs, E. Anticó, V. Salvadó, J.G. Crespo, S. Velizarov, Transport and separation of arsenate and arsenite from aqueous media by supported liquid and anion-exchange membranes, Sep. Purif. Technol. 80 (2011) 428–434. [24] H.M. Kyllonen, P. Pirkonen, M. Nystrom, Membrane filtration enhanced by ultrasound: a review, Desalination 181 (2005) 319–335. [25] T. Mason, E.D. Cordemans, Ultrasonic intensification of chemical processing and related operations: a review, Trans. IChemE 74A (1996) 511–516.
Please cite this article in press as: G. León et al., Increasing stability and transport efficiency of supported liquid membranes through a novel ultrasoundassisted preparation method. Its application to cobalt(II) removal, Ultrason. Sonochem. (2012), http://dx.doi.org/10.1016/j.ultsonch.2012.10.002
