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Structure and transport properties of polymer inclusion membranes for Pb(II) separation
Desalination 271 (2011) 132–138
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Structure and transport properties of polymer inclusion membranes for Pb(II) separation Andrzej Oberta a,⁎, Janusz Wasilewski b, Romuald Wódzki a a b
Nicolaus Copernicus University, Gagarin Str. 7, 87-100 Toruń, Poland University of Warmia and Mazury, Department of Biochemistry, Oczapowski Str. 1A, 10-719 Olsztyn, Poland
a r t i c l e
i n f o
Article history: Received 19 October 2010 Received in revised form 6 December 2010 Accepted 7 December 2010 Available online 8 January 2011 Keywords: Polymer inclusion membrane Structure Transport Pb(II) Percolation
a b s t r a c t The structure and properties of polymer inclusion membranes for Pb(II) transport from solutions containing Pb(II), Ca(II), K(I) nitrates, and Na(I) acetate as a buffer were examined. The membranes were prepared from cellulose triacetate (support), dioctyl phthalate (plasticizer) and 2-(10-carboxydecylsulfanyl)benzoic acid methyl monoester as a new, Pb(II) selective ionophore (LSI). The effect of membrane composition upon transport rates was discussed in terms of percolation theory. The membrane structure was found inhomogeneous with a gradient-type distribution of the active phase (carrier and plasticizer). The presence of the overall transport limiting skin layer was confirmed by the application of scanning electron microscopy. The percolation parameters, i.e. critical volume fraction of the active phase ΦAP,c and the critical exponent τ, were found as equal to 0.62 and 1.11±0.07, respectively. However, the local parameters calculated for transport limiting layer were estimated as Φap,c equal to 0.10 and τ equal to 1.6±0.2. The following order of membrane selectivity was observed: Pb(II)≫K(I)≈Ca(II)≈Na(I) with the overall Pb(II) separation factors reaching the values up to 48,000 and Pb(II) fluxes up to 1.725×10− 10 mol cm− 2 s− 1. The calculated percolation parameters suggest the porous structure of the attended membranes. The transport mechanism was found as limited by the diffusion processes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer inclusion membranes (PIM), classified earlier as solventpolymer membranes (SPM), were first developed by Bloch et al. [1,2] in the early 1960s and applied for a selective removal of metal ions from technological waste waters. The overall preparation procedure of contemporary polymer inclusion membranes was proposed by Sugiura et al. [3,4] who optimized the composition of cellulose triacetate (CTA) based membranes. These membranes are now regarded as an alternative to typical liquid membrane systems because of their improved properties such as durability and long-term operation stability. Because of their high selectivity, the PIM membranes were reported to be a practical tool in an efficient removal or separation of the desired species [5]. The transport and separation of metal cations still remain the main purpose of PIMs application; however, the transport experiments with other chemical species, e.g. organic and inorganic anions were also described [6]. The investigations are frequently focused on the transport of transition and post-transition metal ions, such as copper(II), zinc(II), cadmium(II), cobalt (II) or lead(II) with the use of a great variety of ionophores. To compare, some results presented by different authors are collected in Table 1. It was reported that some minimum amount of carrier and plasticizer (the active phase) is needed for the flux initiation across PIM membrane. This non-zero amount of the mentioned components, called the ⁎ Corresponding author. Tel.: + 48 566114548. E-mail address: [email protected] (A. Oberta). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.12.023
percolation threshold, was observed recently in PIMs investigations [25–27]. It has been proved that, as long as the amount of a carrier remains above the percolation threshold, PIM systems exhibit carrier-mediated metal ion transport very similar to that observed in supported liquid membranes (SLMs) [25]. Moreover, the proper rescalation of the respective fluxes results in more or less the same transport efficiencies for PIMs and SLMs. However, the presence of the threshold value of the amount of a carrier still maintains the transport mechanism of attended polymer inclusion membranes unclear. The threshold effect can be discussed in terms of the percolation theory, which was applied to describe the influence of the membrane structure on the permeability or conductivity [27,28]. This theory was developed to deal in a quantitative way with transport in disordered media (heterogeneous systems), where the disorder is defined by a random variation in the degree of connectivity [28]. Due to specific topology, the system can be described with one of the lattice types and percolation models (bond percolation BP, site percolation SP, ‘Swiss-cheese’ percolation SCP, etc.). The fundamental thesis of percolation theory assumes a critical amount of the conductor (i.e. the carrier), below which no conduction (transport) is observed. Above this percolation threshold the permeability/conductivity of the system grows rapidly, according to the following power law equation: τ
J = J0 ðΦ−Φc Þ
ð1Þ
where J is the stripping flux, J0 is a constant pre-factor, τ is a critical exponent and Φ and Φc are the volume fraction of the active components
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Table 1 Comparison of CTA-based PIM systems. Carrier
Ions
Flux [mol cm− 2 s− 1]
Selectivity order
Ref.
Cyanex 301 D2EHPA D2EHPA Kelex 100 Aliquat 336⁎) TOPO Cyanex 301 DB14C4⁎)
Pb(II), Cd(II), Zn(II) Pb(II), Cd(II), Zn(II) Pb(II) Cd(II), Pb(II) Cd(II), Cu(II) Pb(II) 60 Co(II), 90Sr(II), 137Cs(I) alkali, alkaline earth K(I), Na(I), Rb(I) Cr(IV) Ag(I), Cu(II), Au(III) Zn(II), Cd(II), Pb(II) Pb(II), transition metals Cd(II), Pb(II), Hg(II) Pb(II), Zn(II), Cu(II) Zn(II), Cd(II), Pb(II)
7.01 × 10− 10 (Pb(II)) 1.34 × 10− 10 (Pb(II)) 3.5 × 10− 10 no data no data no data 3.17 × 10− 13(Co(II)) no data 3.96 × 10− 11(K(I)) 4.5 × 10− 9 5.62 × 10− 10(Ag(I)) 1.1 × 10− 11(Pb(II)) no data 7.4 × 10− 11(Hg(II)) 1.22 × 10− 10(Pb(II)) 3.55 × 10− 10(Pb(II))
Pb(II) N Cd(II) N Zn(II) Pb(II) N Cd(II) ≈ Zn(II)
[7] [8] [9] [10] [11] [12] [13–15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
DC18C6 TOA DB18C6 PNP-16C6 LAPDA calix[4]arene β-CD deriv. DB16C5 deriv.
Pb(II) N Cd(II) Cd(II) N Cu(II) Co(II) N Cs(I) N Sr(II) Li(I) N … K(I) N Na(I) ≈ Rb(I) Ag(I) N Cu(II) N Au(III) Pb(II) N Zn(II) N Cd(II) Pb(II) N Cd(II) ≈ … Hg(II) N Pb(II) N Cd(II) Pb(II) N Zn(II) ≈ Cu(II) Pb(II) N Cd(II) N Zn(II)
Cyanex 301: bis(2,4,4-trimethylpentyl)dithiophosphinic acid.; D2EHPA: di(2-ethylhexyl)phosphoric acid; Kelex 100: 7-(4-ethyl-1-methylocty)-8-hydroxyquinoline; Aliquat 336: trioctylmethylammonium chloride; TOPO: trioctylphosphine oxide; DB14C4: dibenzo-14-crown-4; DB18C6: dibenzo-18-crown-6; TOA: trioctyloamine; PNP-16C6: PNP-16-crown-6 lariat ether; LAPDA: lipophilic acyclic polyether dicarboxylic acid; β-CD deriv.: β-cyclodextrin derivatives; DB16C5 deriv.: dibenzo-16-crown-5 derivatives. ⁎) – PVC membrane.
and its threshold value, respectively. The exponent τ is a universal constant dependent only on the system spatial dimensions. The value of the pre-factor J0 is dependent on the physical and chemical properties of the pure active phase [29], i.e. its value corresponds to the ideal system composed only of the “conductor”. According to the present state of knowledge about the influence of PIM structure and composition on its transport ability, the following aims will be considered in this paper: (i) explanation of the presence of the carrier and plasticizer content threshold in terms of percolation theory, and (ii) recognition of the transport mechanism. Additionally, the Pb(II) removal ability of a newly synthesized ionophore will be evaluated. 2. Experimental 2.1. Chemicals Cellulose triacetate (Fluka, Mw = 72,000 to 74,000 g mol− 1) and dioctyl phthalate (Aldrich, 99%) were used for PIMs preparation. Reagent grade lead(II), calcium(II), and K(I) nitrates as well as nitric acid used for solutions preparation were purchased from POCh (Gliwice, Poland). Buffer solutions were prepared using anhydrous sodium acetate supplied by Sigma-Aldrich. 2.2. Synthesis of the ionophore The synthesis of 2-(10-carboxydecylsulfanyl)benzoic acid methyl monoester thereafter referred to as LSI (lead selective ionophore), was accomplished in two steps. First, the ionophore precursor, i.e. 2-[(10carboxydecyl)sulfanyl]benzoic acid, (LSIprec, Fig. 1) was synthesized according to a general procedure described elsewhere [30]. For this purpose, a mixture of 15.42 g (0.1 mol) of thiosalicylic acid, 26.52 g (0.1 mol) of 11-bromoundecanoic acid, 12 g (0.3 mol) of finely powdered sodium hydroxide and 400 cm3 of isopropyl alcohol was placed in a 1 dm3, two-necked, round-bottom flask, equipped with a mechanical stirrer and a water condenser. The mixture was stirred and refluxed over 3 h under nitrogen atmosphere, and thereafter, the solvent was evacuated under reduced pressure. The residue was dissolved in water and acidified with dilute sulfuric acid. The white precipitate formed was filtered, washed several times with distilled water until neutral pH of the filtrate, and air dried. After crystallization from acetone, 26.1 g of dicarboxylic acid (m.p. 120–121 °C, 77.1% yield) was obtained. To improve the solubility of LSIprec in common solvents, such as chloroform, toluene or hexane, and to obtain the final product (LSI), i.e. 2(10-carboxydecylsulfanyl)benzoic acid methyl monoester, one of the
carboxylic groups was blocked with the methyl group. Selective monoesterification of dicarboxylic acid LSIprec was carried out by heating with methanol under acidic conditions [31]. A solution of 4.15 g (0.0123 mol) of LSIprec and 1 cm3 of concentrated hydrochloric acid in 150 cm3 of methanol was refluxed for 15 min. Thereafter, the volume of the mixture was reduced to 30 cm3 by evaporating the solvent under reduced pressure, and cooled. The white crystals formed were filtered, washed several times with cold methanol and air dried. The yield was 3.58 g (82.9 %) of desired monoester (m. p. 87.5–89 °C). The product was additionally twice crystallized from acetone and dried in vacuum for 5 h at room temperature before characterization. The NMR characteristics for LSI are as follows: 1
H-NMR (200 MHz, CDCl3, δ): 1.19–1.82 [m, 16 H, –(CH2)8–)], 2.30 (t, J=7.4 Hz, 2 H, –CH2C(O)–), 2.92 (t, J=7.3 Hz, 2 H, –SCH2–), 3.66 (s, 3 H, –OCH3), 7.12–7.53 (m, 3 H, aromatic), 8.12 (dd, J =7.8, 1.5 Hz, 1 H, aromatic). 13 C-NMR (50 MHz, CDCl3, δ): 24.90, 28.12, 29.08, 29.13, 29.30, 29.34, 32.20, 30.09 [–(CH2)10–], 51.45 (–CH3), 123.78, 125.72, 126.33, 132.49, 133.02, 143.10 (aromatic), 171.19 (–COOH), 174.41 (– COOCH3). After the methyl group introduction, an increase in solubility of LSI as compared to LSIprec was significant.
2.3. Preparation of polymer inclusion membranes The membranes were prepared according to a general procedure proposed by Sugiura [3,4]. 2 cm3 of CTA solution in dichloromethane (1.25 g/100 cm3), known volumes of dioctyl phthalate (DOP) solution in CH2Cl2 (10% v/v) and LSI solution in CH2Cl2 (0.025 mol dm− 3) were mixed and vigorously shaken for 5 min. The resulting mixture was poured into fine-leveled Petri dish (5.6 cm diameter) and covered with a watch glass. The solution was left overnight for the evaporation of
Fig. 1. Structure of the carrier precursor LSIprec (R=H) and the final product LSI (R=CH3).
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dichloromethane. A few drops of water were put on the surface of the prepared membrane to facilitate its peeling off from a glass support. 2.4. Transport experiments The transport experiments were performed in a permeation device constructed according to the scheme presented in Fig. 2. The aqueous feed and stripping solutions were pumped from the reservoirs to the cell, using a peristaltic pump (Gilson Minipuls 3), at the velocity of 30 cm3 min− 1. The feed, 50 cm3, was composed of 0.001 mol dm− 3 of appropriate metal nitrates and 0.01 mol dm− 3 of sodium acetate as a buffering agent. Nitric acid (1 mol dm− 3, 50 cm3) was used as the stripping solution. The both aqueous phases were thermostated (25 °C) using a laboratory thermostat (Laboplay T108-N). The solutions were periodically sampled, and the concentrations of cations were determined using atomic absorption spectroscopy method (AAS, Varian SpectrAA-20). In the transport experiments performed under batch regime, the membrane side exposed to air during its preparation was always contacted with the feed solution. 2.5. SEM/EDX imaging Scanning electron microscope (SEM, 1430VP, LEO Electron Microscopy Ltd, England) coupled with an energy dispersive X-Ray spectrometer (EDX, Quantax 200, Bruker, Germany) was used for imaging and quantitative membrane cross-section examination. For that purpose, to obtain a reliable image of the membrane cross-section, the membrane was broken after immersion in liquid nitrogen. The observed profiles of S, C and O mass content along the cross-section were recalculated to achieve the local volume contents of the active phase (i.e. the carrier and plasticizer). 2.6. Calculations The results of transport experiments were presented as the cumulation curves, QM =f(t), i.e. the amount of moles of the specified cation M permeated across 1 cm2 of the membrane into the stripping solution in time t (Eq. (2)). QM =
i CM V h −2 mol cm : A
ð2Þ
In Eq. (2), CM denotes the concentration of the ion M in the stripping phase, respectively. V is the volume of the stripping solution, and A denotes the working area of the membrane. The experimental cumulation curves for the pertraction in the membrane system with the feed and stripping solution of limited
volumes exhibit sigmoid shape which corresponds to the process occurring with time dependent fluxes. Thus, to calculate the maximum stripping flux (JM,max), each curve was approximated with the Gompertz equation (Eq. (3)) and the JM,max value was calculated as the time derivative of QM in the inflexion point. −e−bðt−tc Þ
QM = ae
:
ð3Þ
In Eq. (3), a denotes the function amplitude (the difference in the upper and the lower asymptote), t is time (s), tc is the location of the inflexion point along the time coordinate, and b is an empirical function parameter. The value of the maximum stripping flux was calculated analytically, according to Eq. (4). −1
JM; max = a b e
h i −2 −1 : mol cm s
ð4Þ
For the comparison of the results obtained for the membranes of various thickness l, the values of the ionic fluxes were appropriately rescaled with respect to the mean membrane thickness ( l = 0.015 cm), (Eq. (5)). h i −2 −1 JM;r = JM;max l mol cm s : l
ð5Þ
To characterize the membrane separation ability, the overall separation factors (αM ΣM) were calculated (Eq. (6)) [32]. M
αΣM =
zMk ½Mk;s ∑j≠k zMj ½Mj;s
!
=
zMk ½Mk;f ∑j≠k zMj ½Mj;f
! :
ð6Þ
In Eq. (6) [M]k, f and [M]k, s denote the concentrations of metal M in the feed or stripping phase, respectively, and zM is the valence of the specified k (k or j) cation. In general, the value of separation factor αM ΣM indicates how much the composition of stripping solution is different from the feeding k phase. If the value of αM ΣM is greater than 1, the separation is reached and k the cations Mzk + are transported across the membrane. The αM ΣM value lower than 1 shows that the Mzk + cations remain in the feed. The maximum values of the overall separation factors, observed at the final stage of particular transport run, were taken for the comparison of membrane separations. The volume fractions of the ionophore in the PIM membranes, necessary for the application of percolation theory, were calculated according to the Eq. (7), where V0 and VI denote the volume of the blank membrane (with no carrier inside) and the volume of the investigated membrane, respectively. Because the casting solution was always poured into a Petri-dish of the same size, the volume of the investigated membranes was calculated only from the measurements of the particular membrane thickness. For that purpose, a digital micrometer (SYLVAC S229) was used. ΦI = 1−
V0 : VI
ð7Þ
Once the fractional volumes were calculated, the logarithms of the maximum stripping fluxes were plotted as a function of the log(ΦI −ΦI,c) variable and a linear regression analysis of the data was accomplished. The critical exponent τ was found as the slope of the given line and the log(J0) value was found as the absolute term. 3. Results and discussion 3.1. Volume fractions of the carrier and the carrier-plasticizer solution Fig. 2. Scheme of the permeation system used in the experiments: permeation cell (1), membrane (2), stripping phase compartment (3), feed phase compartment (4), peristaltic pump (5), feed phase reservoir (6), stripping phase reservoir (7), and thermostating bath (8).
The volume fractions of the carrier in the membranes were calculated from their overall volumes. The volumes VI were measured for five
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Fig. 3. The membrane volume (VI [cm3], ●) and the volume fraction of the carrier (ΦI, ○) as the function of the number of moles of the carrier added during membrane preparation. Points — measured data, lines — evaluated models.
different contents of the carrier, as shown in Fig. 3. The experimental VI data exhibit linear correlation with the amount of moles of the added carrier. The linear model was then described by Eq. (8), where VI [cm3] and nI [mol] are the membrane volume and the amount of moles of the carrier, respectively. VI = VM nI + V0 :
ð8Þ
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Fig. 5. The membrane with the highest content (~ 5∙10− 5 mol) of the carrier. The overload effect is visible as the LSI crystalline forms (60× magnitude).
membrane. The volume fractions of the active phase ΦAP as dependent on the volume of the added mixture VAP [cm3] are presented in Fig. 4. The effect of LSI overload was observed for the highest contents of the carrier. The crystalline forms of LSI were slightly visible in the membrane with ~ 4 × 10− 5 mol of the carrier and very well observable where ~5 × 10− 5 mol were added. The membrane surface coated with the LSI crystals is presented in Fig. 5.
The function parameter denoted as VM is the estimated molar volume of the ionophore [cm3 mol− 1] and V0 is the volume of the blank membrane (composed only of CTA and DOP) [cm3]. The V0 parameter was measured as equal to 0.0989 cm3 and the molar volume VM was found to be 1184 ± 42 cm3 mol− 1 (r2 N 0.99). The volumes of the other membranes under study were calculated using the evaluated model. The volume fractions of the carrier, ΦI, were calculated according to Eq. (7) and thereafter approximated using the rational-type curve resulting from the combination of Eqs. (7) and (8). The respective curve presented in Fig. 3 exhibits asymptotic course with the upper boundary equal to 1, which remains in accordance with the expected theoretical value. Moreover, the volume fractions of the carrierplasticizer mixture (active phase) were also calculated by the described method. Six membranes containing various amounts of the active phase composed of 0.5 mol dm− 3 solution of the carrier in the plasticizer were prepared and examined. Additionally, a membrane composed only of cellulose triacetate was used as the blank
The typical QPb(II) and JPb(II),r vs. time courses corresponding with the membrane of the highest content of LSI are shown in Fig. 6 as an example of the collected experimental data. The QPb(II) curve exhibits a sigmoid shape with maximum increment at about the third hour of transport run, which corresponds to the highest output flux. Similar curves were observed in all our pertraction experiments. Consequently, the JPb(II),r curves exhibited maximum values corresponding to the maximum stripping fluxes, characteristic for the particular membranes. To compare the permeabilities of various membranes studied, the JPb(II),r values as the function of the carrier volume fraction are presented in Fig. 7. The influence of the crystallines presence on the transport efficiency is noticeable when the highest value of ΦI. is reached. The last point (Fig. 7, filled point), excluded from the fitting curve, represents the process with the overloaded membrane. Thus, in
Fig. 4. The volume fractions of the active phase in the membrane as a function of the added volume of plasticizer and ionophore mixture.
Fig. 6. The lead(II) ion cumulation QPb(II) (points, solid line) and rescaled flux JPb(II),r (dashed line) vs. time. The membrane composition: DOP 2 cm3·g− 1 of CTA, the carrier 1 mol·dm− 3 of DOP.
3.2. Transport of ions
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Fig. 7. Maximum stripping fluxes of Pb(II) as a function of the volume fraction of carrier. Points and line represent experimental data and fitting curve, respectively. The filled point corresponds to the overloaded membrane.
this case, the decrease of the maximum stripping flux is caused by increased viscosity of the membrane as well as by deactivation of some amount of the carrier. The carrier efficiency was additionally characterized by a turnover number TN, i.e. the maximum stripping flux of Pb(II) ions, produced by 1 mol of the carrier operating under conditions determined by particular membrane features (viscosity, size of the active phase clusters). h i −2 −1 TN = JPbðIIÞ;r = nI cm s :
ð9Þ
In Eq. (9), JPb(II),r is the calculated and rescaled (Eq. (5)) Pb(II) flux [mol cm− 2 s− 1], and nI denotes the number of moles of the carrier in the membrane. The dependence of TN on ΦI presented in Fig. 8 indicates that the carrier is most efficient when its volume fraction varies in the range from 0.03 to 0.07. A further increase in the amount of the carrier slightly decreases the turnover number. It is consistent with a general observation for liquid membranes, that the addition of the higher amounts of ionophore causes enlargement of membrane (or the active phase in the case of PIMs) viscosity, and substantially moderates the carrier diffusion. Moreover, the percolation threshold ΦI,c for ionophore volume fraction was not observed in the plot of JM,r vs. ΦI (Fig. 7). These results seem typical for usual diffusion transport mechanism, in opposition to the hopping mechanism of transport (fixed-site transport mechanism) across PIMs as postulated by some authors [23,33].
Fig. 8. Turnover numbers for Pb(II) transport.
Fig. 9. The percolation data of the volume fraction of the active phase. Points — calculated values, line — fitting model (r2 = 0.98).
Moreover, the influence of the active phase volume fraction on membrane permeability was studied, because it was recently found, that some CTA-based PIM membranes can exhibit threshold value of the content of carrier-plasticizer mixture [34]. For this purpose a series of experiments was performed with the membranes of various contents of the active phase (ΦAP) and the sharp percolation threshold for flux was found at the critical volume fraction ΦAP,c equal to 0.62. The experimental results, presented in Fig. 9, exhibit a linear dependence of log JM,r on log (ΦAP–ΦAP,c) according to the basic law of percolation theory (Eq. (1)) which enabled us to estimate the parameters of percolation in the studied PIMs system. For that purpose, the critical exponent τ was found as the slope of the presented line and equal to 1.11± 0.07. The pre-factor J0 was calculated from the absolute term as equal to 1.10 ± 0.01 × 10− 9 mol cm− 2 s− 1. It should be noted that the high value of ΦAP,c observed for the PIMs studied is inconsistent with typical values reported for three-dimensional systems [29]. In our opinion, this is caused by possible nonuniform distribution of the membrane components. To verify this hypothesis, the distribution of the active phase was additionally investigated by means of the SEM-EDX analysis. For that purpose, the membrane containing the active phase amount corresponding to ΦAP equal to 0.56, i.e. slightly below the threshold, was taken for imaging. It was found that the active phase exhibits a nonuniform distribution with a maximum saturation next to the membrane surface exposed to air during preparation (Fig. 10).
Fig. 10. The distribution of the active phase and polymer support in the membrane containing ΦAP = 0.56. The dashed line corresponds to CTA, and the solid line denotes the active phase. The zero value of the thickness indicates the surface exposed to air during the membrane preparation.
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Fig. 11. The calculated distribution curves of the active phase along the membranes cross-sections. ΦAP = 0.78, 0.74, 0.71, 0.67, 0.63, 0.54. The membranes thicknesses were unified for clarity.
Moreover, the opposite membrane surface was found as composed almost only of the polymer matrix. Thus, it can be concluded that the CTA-enriched layer constitutes a non-permeable barrier for carrier diffusion, and thus, the whole membrane exhibits the properties of an “isolator” in spite of the relatively high content of the active phase. Consequently, the high value of percolation threshold is necessary to prevent the formation of the transport-limiting skin layer. As pointed above, these conditions are reached only just for ΦAP,c equal to 0.62 of the active phase. For further membrane characterization, the volume fractions of the active phase contained in the transport-limiting layer were taken into consideration. For each examined PIM membrane, the local volume fraction was calculated as ΦAP for the glass-side membrane surface, thereafter referred to as Φap. For that purpose, the active phase distribution function was calculated using the parabolic equation. The calculated distribution curves are presented in Fig. 11 as the ΦAP vs. normalized distance plots. The local volume fractions Φap, i.e. corresponding with the lowest value of ΦAP in the distribution curves, are collected in Table 2. These volume fractions of the active phase were used for the determination of the new percolation parameters (Φap,c and τ). The linear regression analysis of a new dataset was performed. The local critical volume fraction of the active phase Φap,c and the critical exponent τ were found equal to 0.10 and 1.6 ± 0.2, respectively. The calculated values are consistent with percolation parameters of bicontinuum three-dimensional systems. Thus, it can be concluded that the active phase exists in the polymer matrix as nonuniformly distributed overlapping microdroplets which are able to form infinite clusters above some critical volume fraction. The distribution of the active phase in the membrane described with ΦAP =0.54 (non-permeable membrane) was imagined by the numerical simulation technique (Fig. 12). For that purpose, the inversed ‘Swisscheese’ percolation model was applied [35] using the 100×100×100 3D cubic lattice. The lattice was filled with the active phase (marked as the dark points) with accordance to the respective distribution curve. In
137
Fig. 12. The results of membrane modeling using the inversed ‘Swiss-cheese’ percolation method. The black areas denote the active phase, white areas correspond to polymer support.
Fig. 12, the lattice cross-section along transport direction (from the feed to the stripping side) is presented. The results confirm the gradient type distribution of the active phase, with high enrichment of one of the membrane surfaces. The content of the active phase near the opposite surface is poor, and thus, no permeating clusters are formed. 3.3. System selectivity The separation ability of LSI for Pb(II) in the presence of competing Na(I), K(I), and Ca(II) was examined. The competing cations were chosen as the representatives of common mono and bivalent cation present in an environment. The investigated system was expected to exhibit selectivity against Pb(II) ions. The typical results, demonstrating the time dependent evolution of the separation factors (Eq. (6)), corresponding to the membrane of the highest content of the carrier (the overloaded membrane) are shown in Fig. 13. As presented, the high separation against Pb(II) cations is reached as contrasted with the other cations which are not transported and remain in the feeding phase. For comparison among the membranes with various content of LSI, the maximum values of the overall separation factors (observed in the nearequilibrium stage of the process) were taken into consideration (Fig. 14). It was concluded, that all the studied membranes exhibit satisfactory Pb(II) separation ability, regardless of the amount of the carrier used. As presented, the highest separation is reached when the membrane is nearly saturated with LSI. The overload effect decreases the membrane separation ability due to slower transport of Pb(II) ions occurring inside the hydrophobic phase of high viscosity.
Table 2 Overall vs. local volume fraction of the active phase in PIM membrane. Volume fraction Overall, ΦAP
Local, Φap
0.78 0.74 0.71 0.67 0.63 0.54
0.26 0.21 0.18 0.15 0.13 0.06
Fig. 13. Overall separation factors vs. time. The membrane composition: dioctyl phthalate (DOP) 2 cm3/g CTA, the carrier 1 mol/dm3 of DOP. △ – Na(I), □ – K(I), ○ – Ca(II), ◊ – Pb(II).
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active PIMs of the overall content of the active phase lower than 0.62 by changing the conditions of preparation. This conclusion comes from percolation theory which was found applicable for a description of the investigated PIMs properties, i.e. the calculations based on ‘Swiss-cheese’ model allowed the visualization of the membrane morphology. Acknowledgements This work was financial program of EU and the Administration (the Marshall) of KP Voivodeship. References
Fig. 14. Maximum overall separation factors of Pb(II) as a function of the carrier volume fraction. The filled point corresponds to the overloaded membrane.
4. Conclusions
[1] [2] [3] [4] [5] [6] [7] [8] [9]
It was found that 2-(10-carboxydecylsulfanyl)benzoic acid methyl monoester (LSI), when applied as the ionophore in PIM membranes, exhibits excellent selectivity against Pb(II) ions in the presence of cations such as: Na(I), K(I), Ca(II). Thus, the investigated PIMs can be applied for a selective removal of Pb(II) cations from aqueous multication mixtures of pH in the range of 5–7. The results of the performed experiments, interpreted with application of percolation theory, allow us to exclude the hopping mechanism as a transport mode in the PIM membranes studied. The variation of the carrier volume fraction does not result in the percolation threshold for the carrier content. Moreover, the maximum efficiency (maximum turnover number) of LSI was reached for ΦI equal to 0.05. This indicates that usual diffusion phenomena are responsible for Pb(II) transport in the examined PIMs. However, some interdiffusion processes (cation-exchange between loaded and free form of the ionophore) occurring in the bulk volume of the membrane cannot be neglected but their influence on the overall transport efficiency seems insignificant. It was observed that the LSI exhibits rather limited solubility in the plasticizer (~ 0.17 g cm− 3) and, above some amount of the carrier, the stripping fluxes decrease rapidly due to the precipitation of the ionophore. Thus, an optimum JPb(II),r with LSI can be reached when the volume fraction of the carrier ΦI equals to ~0.33. Exceeding this value does not increase the overall Pb(II) flux, and is pointless. The membranes exhibit quantitatively diverse compositions along their cross-sections which is caused probably by a membrane preparation method. Below some critical amount of the active phase (overall critical volume fraction equal to 0.62) no transport was observed because of the formation of a skin layer composed almost only of the polymer matrix. However, the local critical volume fraction of the active phase estimated for the transport limiting layer is much lower and equals to 0.10. Thus, it seems possible to synthesize the
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Structure and transport properties of polymer inclusion membranes for Pb(II) separation Andrzej Oberta a,⁎, Janusz Wasilewski b, Romuald Wódzki a a b
Nicolaus Copernicus University, Gagarin Str. 7, 87-100 Toruń, Poland University of Warmia and Mazury, Department of Biochemistry, Oczapowski Str. 1A, 10-719 Olsztyn, Poland
a r t i c l e
i n f o
Article history: Received 19 October 2010 Received in revised form 6 December 2010 Accepted 7 December 2010 Available online 8 January 2011 Keywords: Polymer inclusion membrane Structure Transport Pb(II) Percolation
a b s t r a c t The structure and properties of polymer inclusion membranes for Pb(II) transport from solutions containing Pb(II), Ca(II), K(I) nitrates, and Na(I) acetate as a buffer were examined. The membranes were prepared from cellulose triacetate (support), dioctyl phthalate (plasticizer) and 2-(10-carboxydecylsulfanyl)benzoic acid methyl monoester as a new, Pb(II) selective ionophore (LSI). The effect of membrane composition upon transport rates was discussed in terms of percolation theory. The membrane structure was found inhomogeneous with a gradient-type distribution of the active phase (carrier and plasticizer). The presence of the overall transport limiting skin layer was confirmed by the application of scanning electron microscopy. The percolation parameters, i.e. critical volume fraction of the active phase ΦAP,c and the critical exponent τ, were found as equal to 0.62 and 1.11±0.07, respectively. However, the local parameters calculated for transport limiting layer were estimated as Φap,c equal to 0.10 and τ equal to 1.6±0.2. The following order of membrane selectivity was observed: Pb(II)≫K(I)≈Ca(II)≈Na(I) with the overall Pb(II) separation factors reaching the values up to 48,000 and Pb(II) fluxes up to 1.725×10− 10 mol cm− 2 s− 1. The calculated percolation parameters suggest the porous structure of the attended membranes. The transport mechanism was found as limited by the diffusion processes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer inclusion membranes (PIM), classified earlier as solventpolymer membranes (SPM), were first developed by Bloch et al. [1,2] in the early 1960s and applied for a selective removal of metal ions from technological waste waters. The overall preparation procedure of contemporary polymer inclusion membranes was proposed by Sugiura et al. [3,4] who optimized the composition of cellulose triacetate (CTA) based membranes. These membranes are now regarded as an alternative to typical liquid membrane systems because of their improved properties such as durability and long-term operation stability. Because of their high selectivity, the PIM membranes were reported to be a practical tool in an efficient removal or separation of the desired species [5]. The transport and separation of metal cations still remain the main purpose of PIMs application; however, the transport experiments with other chemical species, e.g. organic and inorganic anions were also described [6]. The investigations are frequently focused on the transport of transition and post-transition metal ions, such as copper(II), zinc(II), cadmium(II), cobalt (II) or lead(II) with the use of a great variety of ionophores. To compare, some results presented by different authors are collected in Table 1. It was reported that some minimum amount of carrier and plasticizer (the active phase) is needed for the flux initiation across PIM membrane. This non-zero amount of the mentioned components, called the ⁎ Corresponding author. Tel.: + 48 566114548. E-mail address: [email protected] (A. Oberta). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.12.023
percolation threshold, was observed recently in PIMs investigations [25–27]. It has been proved that, as long as the amount of a carrier remains above the percolation threshold, PIM systems exhibit carrier-mediated metal ion transport very similar to that observed in supported liquid membranes (SLMs) [25]. Moreover, the proper rescalation of the respective fluxes results in more or less the same transport efficiencies for PIMs and SLMs. However, the presence of the threshold value of the amount of a carrier still maintains the transport mechanism of attended polymer inclusion membranes unclear. The threshold effect can be discussed in terms of the percolation theory, which was applied to describe the influence of the membrane structure on the permeability or conductivity [27,28]. This theory was developed to deal in a quantitative way with transport in disordered media (heterogeneous systems), where the disorder is defined by a random variation in the degree of connectivity [28]. Due to specific topology, the system can be described with one of the lattice types and percolation models (bond percolation BP, site percolation SP, ‘Swiss-cheese’ percolation SCP, etc.). The fundamental thesis of percolation theory assumes a critical amount of the conductor (i.e. the carrier), below which no conduction (transport) is observed. Above this percolation threshold the permeability/conductivity of the system grows rapidly, according to the following power law equation: τ
J = J0 ðΦ−Φc Þ
ð1Þ
where J is the stripping flux, J0 is a constant pre-factor, τ is a critical exponent and Φ and Φc are the volume fraction of the active components
A. Oberta et al. / Desalination 271 (2011) 132–138
133
Table 1 Comparison of CTA-based PIM systems. Carrier
Ions
Flux [mol cm− 2 s− 1]
Selectivity order
Ref.
Cyanex 301 D2EHPA D2EHPA Kelex 100 Aliquat 336⁎) TOPO Cyanex 301 DB14C4⁎)
Pb(II), Cd(II), Zn(II) Pb(II), Cd(II), Zn(II) Pb(II) Cd(II), Pb(II) Cd(II), Cu(II) Pb(II) 60 Co(II), 90Sr(II), 137Cs(I) alkali, alkaline earth K(I), Na(I), Rb(I) Cr(IV) Ag(I), Cu(II), Au(III) Zn(II), Cd(II), Pb(II) Pb(II), transition metals Cd(II), Pb(II), Hg(II) Pb(II), Zn(II), Cu(II) Zn(II), Cd(II), Pb(II)
7.01 × 10− 10 (Pb(II)) 1.34 × 10− 10 (Pb(II)) 3.5 × 10− 10 no data no data no data 3.17 × 10− 13(Co(II)) no data 3.96 × 10− 11(K(I)) 4.5 × 10− 9 5.62 × 10− 10(Ag(I)) 1.1 × 10− 11(Pb(II)) no data 7.4 × 10− 11(Hg(II)) 1.22 × 10− 10(Pb(II)) 3.55 × 10− 10(Pb(II))
Pb(II) N Cd(II) N Zn(II) Pb(II) N Cd(II) ≈ Zn(II)
[7] [8] [9] [10] [11] [12] [13–15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
DC18C6 TOA DB18C6 PNP-16C6 LAPDA calix[4]arene β-CD deriv. DB16C5 deriv.
Pb(II) N Cd(II) Cd(II) N Cu(II) Co(II) N Cs(I) N Sr(II) Li(I) N … K(I) N Na(I) ≈ Rb(I) Ag(I) N Cu(II) N Au(III) Pb(II) N Zn(II) N Cd(II) Pb(II) N Cd(II) ≈ … Hg(II) N Pb(II) N Cd(II) Pb(II) N Zn(II) ≈ Cu(II) Pb(II) N Cd(II) N Zn(II)
Cyanex 301: bis(2,4,4-trimethylpentyl)dithiophosphinic acid.; D2EHPA: di(2-ethylhexyl)phosphoric acid; Kelex 100: 7-(4-ethyl-1-methylocty)-8-hydroxyquinoline; Aliquat 336: trioctylmethylammonium chloride; TOPO: trioctylphosphine oxide; DB14C4: dibenzo-14-crown-4; DB18C6: dibenzo-18-crown-6; TOA: trioctyloamine; PNP-16C6: PNP-16-crown-6 lariat ether; LAPDA: lipophilic acyclic polyether dicarboxylic acid; β-CD deriv.: β-cyclodextrin derivatives; DB16C5 deriv.: dibenzo-16-crown-5 derivatives. ⁎) – PVC membrane.
and its threshold value, respectively. The exponent τ is a universal constant dependent only on the system spatial dimensions. The value of the pre-factor J0 is dependent on the physical and chemical properties of the pure active phase [29], i.e. its value corresponds to the ideal system composed only of the “conductor”. According to the present state of knowledge about the influence of PIM structure and composition on its transport ability, the following aims will be considered in this paper: (i) explanation of the presence of the carrier and plasticizer content threshold in terms of percolation theory, and (ii) recognition of the transport mechanism. Additionally, the Pb(II) removal ability of a newly synthesized ionophore will be evaluated. 2. Experimental 2.1. Chemicals Cellulose triacetate (Fluka, Mw = 72,000 to 74,000 g mol− 1) and dioctyl phthalate (Aldrich, 99%) were used for PIMs preparation. Reagent grade lead(II), calcium(II), and K(I) nitrates as well as nitric acid used for solutions preparation were purchased from POCh (Gliwice, Poland). Buffer solutions were prepared using anhydrous sodium acetate supplied by Sigma-Aldrich. 2.2. Synthesis of the ionophore The synthesis of 2-(10-carboxydecylsulfanyl)benzoic acid methyl monoester thereafter referred to as LSI (lead selective ionophore), was accomplished in two steps. First, the ionophore precursor, i.e. 2-[(10carboxydecyl)sulfanyl]benzoic acid, (LSIprec, Fig. 1) was synthesized according to a general procedure described elsewhere [30]. For this purpose, a mixture of 15.42 g (0.1 mol) of thiosalicylic acid, 26.52 g (0.1 mol) of 11-bromoundecanoic acid, 12 g (0.3 mol) of finely powdered sodium hydroxide and 400 cm3 of isopropyl alcohol was placed in a 1 dm3, two-necked, round-bottom flask, equipped with a mechanical stirrer and a water condenser. The mixture was stirred and refluxed over 3 h under nitrogen atmosphere, and thereafter, the solvent was evacuated under reduced pressure. The residue was dissolved in water and acidified with dilute sulfuric acid. The white precipitate formed was filtered, washed several times with distilled water until neutral pH of the filtrate, and air dried. After crystallization from acetone, 26.1 g of dicarboxylic acid (m.p. 120–121 °C, 77.1% yield) was obtained. To improve the solubility of LSIprec in common solvents, such as chloroform, toluene or hexane, and to obtain the final product (LSI), i.e. 2(10-carboxydecylsulfanyl)benzoic acid methyl monoester, one of the
carboxylic groups was blocked with the methyl group. Selective monoesterification of dicarboxylic acid LSIprec was carried out by heating with methanol under acidic conditions [31]. A solution of 4.15 g (0.0123 mol) of LSIprec and 1 cm3 of concentrated hydrochloric acid in 150 cm3 of methanol was refluxed for 15 min. Thereafter, the volume of the mixture was reduced to 30 cm3 by evaporating the solvent under reduced pressure, and cooled. The white crystals formed were filtered, washed several times with cold methanol and air dried. The yield was 3.58 g (82.9 %) of desired monoester (m. p. 87.5–89 °C). The product was additionally twice crystallized from acetone and dried in vacuum for 5 h at room temperature before characterization. The NMR characteristics for LSI are as follows: 1
H-NMR (200 MHz, CDCl3, δ): 1.19–1.82 [m, 16 H, –(CH2)8–)], 2.30 (t, J=7.4 Hz, 2 H, –CH2C(O)–), 2.92 (t, J=7.3 Hz, 2 H, –SCH2–), 3.66 (s, 3 H, –OCH3), 7.12–7.53 (m, 3 H, aromatic), 8.12 (dd, J =7.8, 1.5 Hz, 1 H, aromatic). 13 C-NMR (50 MHz, CDCl3, δ): 24.90, 28.12, 29.08, 29.13, 29.30, 29.34, 32.20, 30.09 [–(CH2)10–], 51.45 (–CH3), 123.78, 125.72, 126.33, 132.49, 133.02, 143.10 (aromatic), 171.19 (–COOH), 174.41 (– COOCH3). After the methyl group introduction, an increase in solubility of LSI as compared to LSIprec was significant.
2.3. Preparation of polymer inclusion membranes The membranes were prepared according to a general procedure proposed by Sugiura [3,4]. 2 cm3 of CTA solution in dichloromethane (1.25 g/100 cm3), known volumes of dioctyl phthalate (DOP) solution in CH2Cl2 (10% v/v) and LSI solution in CH2Cl2 (0.025 mol dm− 3) were mixed and vigorously shaken for 5 min. The resulting mixture was poured into fine-leveled Petri dish (5.6 cm diameter) and covered with a watch glass. The solution was left overnight for the evaporation of
Fig. 1. Structure of the carrier precursor LSIprec (R=H) and the final product LSI (R=CH3).
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dichloromethane. A few drops of water were put on the surface of the prepared membrane to facilitate its peeling off from a glass support. 2.4. Transport experiments The transport experiments were performed in a permeation device constructed according to the scheme presented in Fig. 2. The aqueous feed and stripping solutions were pumped from the reservoirs to the cell, using a peristaltic pump (Gilson Minipuls 3), at the velocity of 30 cm3 min− 1. The feed, 50 cm3, was composed of 0.001 mol dm− 3 of appropriate metal nitrates and 0.01 mol dm− 3 of sodium acetate as a buffering agent. Nitric acid (1 mol dm− 3, 50 cm3) was used as the stripping solution. The both aqueous phases were thermostated (25 °C) using a laboratory thermostat (Laboplay T108-N). The solutions were periodically sampled, and the concentrations of cations were determined using atomic absorption spectroscopy method (AAS, Varian SpectrAA-20). In the transport experiments performed under batch regime, the membrane side exposed to air during its preparation was always contacted with the feed solution. 2.5. SEM/EDX imaging Scanning electron microscope (SEM, 1430VP, LEO Electron Microscopy Ltd, England) coupled with an energy dispersive X-Ray spectrometer (EDX, Quantax 200, Bruker, Germany) was used for imaging and quantitative membrane cross-section examination. For that purpose, to obtain a reliable image of the membrane cross-section, the membrane was broken after immersion in liquid nitrogen. The observed profiles of S, C and O mass content along the cross-section were recalculated to achieve the local volume contents of the active phase (i.e. the carrier and plasticizer). 2.6. Calculations The results of transport experiments were presented as the cumulation curves, QM =f(t), i.e. the amount of moles of the specified cation M permeated across 1 cm2 of the membrane into the stripping solution in time t (Eq. (2)). QM =
i CM V h −2 mol cm : A
ð2Þ
In Eq. (2), CM denotes the concentration of the ion M in the stripping phase, respectively. V is the volume of the stripping solution, and A denotes the working area of the membrane. The experimental cumulation curves for the pertraction in the membrane system with the feed and stripping solution of limited
volumes exhibit sigmoid shape which corresponds to the process occurring with time dependent fluxes. Thus, to calculate the maximum stripping flux (JM,max), each curve was approximated with the Gompertz equation (Eq. (3)) and the JM,max value was calculated as the time derivative of QM in the inflexion point. −e−bðt−tc Þ
QM = ae
:
ð3Þ
In Eq. (3), a denotes the function amplitude (the difference in the upper and the lower asymptote), t is time (s), tc is the location of the inflexion point along the time coordinate, and b is an empirical function parameter. The value of the maximum stripping flux was calculated analytically, according to Eq. (4). −1
JM; max = a b e
h i −2 −1 : mol cm s
ð4Þ
For the comparison of the results obtained for the membranes of various thickness l, the values of the ionic fluxes were appropriately rescaled with respect to the mean membrane thickness ( l = 0.015 cm), (Eq. (5)). h i −2 −1 JM;r = JM;max l mol cm s : l
ð5Þ
To characterize the membrane separation ability, the overall separation factors (αM ΣM) were calculated (Eq. (6)) [32]. M
αΣM =
zMk ½Mk;s ∑j≠k zMj ½Mj;s
!
=
zMk ½Mk;f ∑j≠k zMj ½Mj;f
! :
ð6Þ
In Eq. (6) [M]k, f and [M]k, s denote the concentrations of metal M in the feed or stripping phase, respectively, and zM is the valence of the specified k (k or j) cation. In general, the value of separation factor αM ΣM indicates how much the composition of stripping solution is different from the feeding k phase. If the value of αM ΣM is greater than 1, the separation is reached and k the cations Mzk + are transported across the membrane. The αM ΣM value lower than 1 shows that the Mzk + cations remain in the feed. The maximum values of the overall separation factors, observed at the final stage of particular transport run, were taken for the comparison of membrane separations. The volume fractions of the ionophore in the PIM membranes, necessary for the application of percolation theory, were calculated according to the Eq. (7), where V0 and VI denote the volume of the blank membrane (with no carrier inside) and the volume of the investigated membrane, respectively. Because the casting solution was always poured into a Petri-dish of the same size, the volume of the investigated membranes was calculated only from the measurements of the particular membrane thickness. For that purpose, a digital micrometer (SYLVAC S229) was used. ΦI = 1−
V0 : VI
ð7Þ
Once the fractional volumes were calculated, the logarithms of the maximum stripping fluxes were plotted as a function of the log(ΦI −ΦI,c) variable and a linear regression analysis of the data was accomplished. The critical exponent τ was found as the slope of the given line and the log(J0) value was found as the absolute term. 3. Results and discussion 3.1. Volume fractions of the carrier and the carrier-plasticizer solution Fig. 2. Scheme of the permeation system used in the experiments: permeation cell (1), membrane (2), stripping phase compartment (3), feed phase compartment (4), peristaltic pump (5), feed phase reservoir (6), stripping phase reservoir (7), and thermostating bath (8).
The volume fractions of the carrier in the membranes were calculated from their overall volumes. The volumes VI were measured for five
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Fig. 3. The membrane volume (VI [cm3], ●) and the volume fraction of the carrier (ΦI, ○) as the function of the number of moles of the carrier added during membrane preparation. Points — measured data, lines — evaluated models.
different contents of the carrier, as shown in Fig. 3. The experimental VI data exhibit linear correlation with the amount of moles of the added carrier. The linear model was then described by Eq. (8), where VI [cm3] and nI [mol] are the membrane volume and the amount of moles of the carrier, respectively. VI = VM nI + V0 :
ð8Þ
135
Fig. 5. The membrane with the highest content (~ 5∙10− 5 mol) of the carrier. The overload effect is visible as the LSI crystalline forms (60× magnitude).
membrane. The volume fractions of the active phase ΦAP as dependent on the volume of the added mixture VAP [cm3] are presented in Fig. 4. The effect of LSI overload was observed for the highest contents of the carrier. The crystalline forms of LSI were slightly visible in the membrane with ~ 4 × 10− 5 mol of the carrier and very well observable where ~5 × 10− 5 mol were added. The membrane surface coated with the LSI crystals is presented in Fig. 5.
The function parameter denoted as VM is the estimated molar volume of the ionophore [cm3 mol− 1] and V0 is the volume of the blank membrane (composed only of CTA and DOP) [cm3]. The V0 parameter was measured as equal to 0.0989 cm3 and the molar volume VM was found to be 1184 ± 42 cm3 mol− 1 (r2 N 0.99). The volumes of the other membranes under study were calculated using the evaluated model. The volume fractions of the carrier, ΦI, were calculated according to Eq. (7) and thereafter approximated using the rational-type curve resulting from the combination of Eqs. (7) and (8). The respective curve presented in Fig. 3 exhibits asymptotic course with the upper boundary equal to 1, which remains in accordance with the expected theoretical value. Moreover, the volume fractions of the carrierplasticizer mixture (active phase) were also calculated by the described method. Six membranes containing various amounts of the active phase composed of 0.5 mol dm− 3 solution of the carrier in the plasticizer were prepared and examined. Additionally, a membrane composed only of cellulose triacetate was used as the blank
The typical QPb(II) and JPb(II),r vs. time courses corresponding with the membrane of the highest content of LSI are shown in Fig. 6 as an example of the collected experimental data. The QPb(II) curve exhibits a sigmoid shape with maximum increment at about the third hour of transport run, which corresponds to the highest output flux. Similar curves were observed in all our pertraction experiments. Consequently, the JPb(II),r curves exhibited maximum values corresponding to the maximum stripping fluxes, characteristic for the particular membranes. To compare the permeabilities of various membranes studied, the JPb(II),r values as the function of the carrier volume fraction are presented in Fig. 7. The influence of the crystallines presence on the transport efficiency is noticeable when the highest value of ΦI. is reached. The last point (Fig. 7, filled point), excluded from the fitting curve, represents the process with the overloaded membrane. Thus, in
Fig. 4. The volume fractions of the active phase in the membrane as a function of the added volume of plasticizer and ionophore mixture.
Fig. 6. The lead(II) ion cumulation QPb(II) (points, solid line) and rescaled flux JPb(II),r (dashed line) vs. time. The membrane composition: DOP 2 cm3·g− 1 of CTA, the carrier 1 mol·dm− 3 of DOP.
3.2. Transport of ions
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Fig. 7. Maximum stripping fluxes of Pb(II) as a function of the volume fraction of carrier. Points and line represent experimental data and fitting curve, respectively. The filled point corresponds to the overloaded membrane.
this case, the decrease of the maximum stripping flux is caused by increased viscosity of the membrane as well as by deactivation of some amount of the carrier. The carrier efficiency was additionally characterized by a turnover number TN, i.e. the maximum stripping flux of Pb(II) ions, produced by 1 mol of the carrier operating under conditions determined by particular membrane features (viscosity, size of the active phase clusters). h i −2 −1 TN = JPbðIIÞ;r = nI cm s :
ð9Þ
In Eq. (9), JPb(II),r is the calculated and rescaled (Eq. (5)) Pb(II) flux [mol cm− 2 s− 1], and nI denotes the number of moles of the carrier in the membrane. The dependence of TN on ΦI presented in Fig. 8 indicates that the carrier is most efficient when its volume fraction varies in the range from 0.03 to 0.07. A further increase in the amount of the carrier slightly decreases the turnover number. It is consistent with a general observation for liquid membranes, that the addition of the higher amounts of ionophore causes enlargement of membrane (or the active phase in the case of PIMs) viscosity, and substantially moderates the carrier diffusion. Moreover, the percolation threshold ΦI,c for ionophore volume fraction was not observed in the plot of JM,r vs. ΦI (Fig. 7). These results seem typical for usual diffusion transport mechanism, in opposition to the hopping mechanism of transport (fixed-site transport mechanism) across PIMs as postulated by some authors [23,33].
Fig. 8. Turnover numbers for Pb(II) transport.
Fig. 9. The percolation data of the volume fraction of the active phase. Points — calculated values, line — fitting model (r2 = 0.98).
Moreover, the influence of the active phase volume fraction on membrane permeability was studied, because it was recently found, that some CTA-based PIM membranes can exhibit threshold value of the content of carrier-plasticizer mixture [34]. For this purpose a series of experiments was performed with the membranes of various contents of the active phase (ΦAP) and the sharp percolation threshold for flux was found at the critical volume fraction ΦAP,c equal to 0.62. The experimental results, presented in Fig. 9, exhibit a linear dependence of log JM,r on log (ΦAP–ΦAP,c) according to the basic law of percolation theory (Eq. (1)) which enabled us to estimate the parameters of percolation in the studied PIMs system. For that purpose, the critical exponent τ was found as the slope of the presented line and equal to 1.11± 0.07. The pre-factor J0 was calculated from the absolute term as equal to 1.10 ± 0.01 × 10− 9 mol cm− 2 s− 1. It should be noted that the high value of ΦAP,c observed for the PIMs studied is inconsistent with typical values reported for three-dimensional systems [29]. In our opinion, this is caused by possible nonuniform distribution of the membrane components. To verify this hypothesis, the distribution of the active phase was additionally investigated by means of the SEM-EDX analysis. For that purpose, the membrane containing the active phase amount corresponding to ΦAP equal to 0.56, i.e. slightly below the threshold, was taken for imaging. It was found that the active phase exhibits a nonuniform distribution with a maximum saturation next to the membrane surface exposed to air during preparation (Fig. 10).
Fig. 10. The distribution of the active phase and polymer support in the membrane containing ΦAP = 0.56. The dashed line corresponds to CTA, and the solid line denotes the active phase. The zero value of the thickness indicates the surface exposed to air during the membrane preparation.
A. Oberta et al. / Desalination 271 (2011) 132–138
Fig. 11. The calculated distribution curves of the active phase along the membranes cross-sections. ΦAP = 0.78, 0.74, 0.71, 0.67, 0.63, 0.54. The membranes thicknesses were unified for clarity.
Moreover, the opposite membrane surface was found as composed almost only of the polymer matrix. Thus, it can be concluded that the CTA-enriched layer constitutes a non-permeable barrier for carrier diffusion, and thus, the whole membrane exhibits the properties of an “isolator” in spite of the relatively high content of the active phase. Consequently, the high value of percolation threshold is necessary to prevent the formation of the transport-limiting skin layer. As pointed above, these conditions are reached only just for ΦAP,c equal to 0.62 of the active phase. For further membrane characterization, the volume fractions of the active phase contained in the transport-limiting layer were taken into consideration. For each examined PIM membrane, the local volume fraction was calculated as ΦAP for the glass-side membrane surface, thereafter referred to as Φap. For that purpose, the active phase distribution function was calculated using the parabolic equation. The calculated distribution curves are presented in Fig. 11 as the ΦAP vs. normalized distance plots. The local volume fractions Φap, i.e. corresponding with the lowest value of ΦAP in the distribution curves, are collected in Table 2. These volume fractions of the active phase were used for the determination of the new percolation parameters (Φap,c and τ). The linear regression analysis of a new dataset was performed. The local critical volume fraction of the active phase Φap,c and the critical exponent τ were found equal to 0.10 and 1.6 ± 0.2, respectively. The calculated values are consistent with percolation parameters of bicontinuum three-dimensional systems. Thus, it can be concluded that the active phase exists in the polymer matrix as nonuniformly distributed overlapping microdroplets which are able to form infinite clusters above some critical volume fraction. The distribution of the active phase in the membrane described with ΦAP =0.54 (non-permeable membrane) was imagined by the numerical simulation technique (Fig. 12). For that purpose, the inversed ‘Swisscheese’ percolation model was applied [35] using the 100×100×100 3D cubic lattice. The lattice was filled with the active phase (marked as the dark points) with accordance to the respective distribution curve. In
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Fig. 12. The results of membrane modeling using the inversed ‘Swiss-cheese’ percolation method. The black areas denote the active phase, white areas correspond to polymer support.
Fig. 12, the lattice cross-section along transport direction (from the feed to the stripping side) is presented. The results confirm the gradient type distribution of the active phase, with high enrichment of one of the membrane surfaces. The content of the active phase near the opposite surface is poor, and thus, no permeating clusters are formed. 3.3. System selectivity The separation ability of LSI for Pb(II) in the presence of competing Na(I), K(I), and Ca(II) was examined. The competing cations were chosen as the representatives of common mono and bivalent cation present in an environment. The investigated system was expected to exhibit selectivity against Pb(II) ions. The typical results, demonstrating the time dependent evolution of the separation factors (Eq. (6)), corresponding to the membrane of the highest content of the carrier (the overloaded membrane) are shown in Fig. 13. As presented, the high separation against Pb(II) cations is reached as contrasted with the other cations which are not transported and remain in the feeding phase. For comparison among the membranes with various content of LSI, the maximum values of the overall separation factors (observed in the nearequilibrium stage of the process) were taken into consideration (Fig. 14). It was concluded, that all the studied membranes exhibit satisfactory Pb(II) separation ability, regardless of the amount of the carrier used. As presented, the highest separation is reached when the membrane is nearly saturated with LSI. The overload effect decreases the membrane separation ability due to slower transport of Pb(II) ions occurring inside the hydrophobic phase of high viscosity.
Table 2 Overall vs. local volume fraction of the active phase in PIM membrane. Volume fraction Overall, ΦAP
Local, Φap
0.78 0.74 0.71 0.67 0.63 0.54
0.26 0.21 0.18 0.15 0.13 0.06
Fig. 13. Overall separation factors vs. time. The membrane composition: dioctyl phthalate (DOP) 2 cm3/g CTA, the carrier 1 mol/dm3 of DOP. △ – Na(I), □ – K(I), ○ – Ca(II), ◊ – Pb(II).
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active PIMs of the overall content of the active phase lower than 0.62 by changing the conditions of preparation. This conclusion comes from percolation theory which was found applicable for a description of the investigated PIMs properties, i.e. the calculations based on ‘Swiss-cheese’ model allowed the visualization of the membrane morphology. Acknowledgements This work was financial program of EU and the Administration (the Marshall) of KP Voivodeship. References
Fig. 14. Maximum overall separation factors of Pb(II) as a function of the carrier volume fraction. The filled point corresponds to the overloaded membrane.
4. Conclusions
[1] [2] [3] [4] [5] [6] [7] [8] [9]
It was found that 2-(10-carboxydecylsulfanyl)benzoic acid methyl monoester (LSI), when applied as the ionophore in PIM membranes, exhibits excellent selectivity against Pb(II) ions in the presence of cations such as: Na(I), K(I), Ca(II). Thus, the investigated PIMs can be applied for a selective removal of Pb(II) cations from aqueous multication mixtures of pH in the range of 5–7. The results of the performed experiments, interpreted with application of percolation theory, allow us to exclude the hopping mechanism as a transport mode in the PIM membranes studied. The variation of the carrier volume fraction does not result in the percolation threshold for the carrier content. Moreover, the maximum efficiency (maximum turnover number) of LSI was reached for ΦI equal to 0.05. This indicates that usual diffusion phenomena are responsible for Pb(II) transport in the examined PIMs. However, some interdiffusion processes (cation-exchange between loaded and free form of the ionophore) occurring in the bulk volume of the membrane cannot be neglected but their influence on the overall transport efficiency seems insignificant. It was observed that the LSI exhibits rather limited solubility in the plasticizer (~ 0.17 g cm− 3) and, above some amount of the carrier, the stripping fluxes decrease rapidly due to the precipitation of the ionophore. Thus, an optimum JPb(II),r with LSI can be reached when the volume fraction of the carrier ΦI equals to ~0.33. Exceeding this value does not increase the overall Pb(II) flux, and is pointless. The membranes exhibit quantitatively diverse compositions along their cross-sections which is caused probably by a membrane preparation method. Below some critical amount of the active phase (overall critical volume fraction equal to 0.62) no transport was observed because of the formation of a skin layer composed almost only of the polymer matrix. However, the local critical volume fraction of the active phase estimated for the transport limiting layer is much lower and equals to 0.10. Thus, it seems possible to synthesize the
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