Electrodriven Transport of Cs+ through Polymer Inclusion Membrane

Mar 7, 2016 - In continuation with that work, the mechanism of electrodriven transport of Cs+ through HCCD loaded polymer inclusion membranes has been...
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Electrodriven Transport of Cs+ through Polymer Inclusion Membrane as “Solvent Separated Ions” Sanhita Chaudhury, Arunasis Bhattacharyya,* and Asok Goswami Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India S Supporting Information *

ABSTRACT: In our earlier work, we reported [Env. Sci. Technol. 2014, 48, 12994] a novel electrodialysis based selective separation of Cs+ from nuclear waste solution using chlorinated cobalt dicarbollide (HCCD) loaded polymer inclusion membrane (PIM). In continuation with that work, the mechanism of electrodriven transport of Cs+ through HCCD loaded polymer inclusion membranes has been explored. PIMs containing fixed amount of cellulose triacetate and nitrophenyl octyl ether (NPOE) but different concentrations of the carrier have been prepared. The experimental flux of Cs+ across the PIMs, for two different concentrations of the metal ion in the initial feed solution, has been measured using the radiotracer technique. On the basis of the Nernst−Planck equation, an attempt has been made to calculate the time dependence of concentration changes of the metal ion in the feed compartment. The experimental parameters of the membrane., viz., length, self-diffusion coefficient, distribution ratio, electrical resistance, and current, have been used in the calculation. The experimental results indicate that the transport of Cs+ by mobile carrier diffusion or fixed site jumping is not possible. It has been proposed that, under applied electric field, Cs+ is mostly transported as “solvent separated ions” through the polar lipophilic solvent NPOE. The proposed mechanism has been substantiated by comparing the experimental and the calculated results.

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Aliquat-3365 transport ions as ion-pairs and require a counter transport of ion from the receiving to feed phase.2,6 In both the cases, the carrier in the membrane forms a complex at the feed−membrane interface and decomplexes at the receiver− membrane interface. Uphill transport occurs due to the concentration gradient of the complex in the membrane. In spite of extensive research in the field of facilitated transport using PIMs, the mechanism of transport is still not clear.7 In most of the literature reports, dealing with the diffusion mechanism of metal ions in PIMs,2,7−9 the transport process is either driven by a concentration gradient only or facilitated by using stripping agent in the receiver side. To the best of our knowledge, to date, there is limited literature10,11,13 on carrier facilitated electrodialysis for selective transport of metal ions. In our earlier work,11 we reported a novel electrodialysis based selective transport of Cs+ through chlorinated cobalt dicarbollide (HCCD) loaded cellulose triacetate (CTA) or polyvinyl chloride (PVC) based PIM, where the anionic moiety (CCD−) itself is selective for the ion of interest. On application of applied electric field, the anion (Cl−) in the feed compartment is oxidized at the cathode, and thus the cation is forced to move to the receiver compartment through the membrane. In the anode, H2 is released. In this study,11 a high decontamination factor for Cs+ over Na+ and other metal ions present in the nuclear waste solution has been obtained, and

INTRODUCTION Membranes are widely used in separation processes, such as filtration, reverse osmosis, Donnan, and electrodialysis, mainly for purification of water. The reverse osmosis and filtration processes are based on the property of size exclusion selectivity of the membrane, while the dialysis process depends on the property of Donnan exclusion of co-ions in the ion-exchange membrane.1 However, large scale membrane based separations of metal ions and neutral organic molecules from industrial waste solutions have not been employed successfully due to poor selectivity and permeability of membranes in general for the target species to be separated. Supported liquid membranes (SLM) and polymer inclusion membranes (PIM) are a promising choice for this purpose. They mimic natural membrane systems, which show high selectivity for metal ion transport. Poor stability of the SLMs limits their use in large scale industrial applications.2 Over the years, PIMs, because of their superior stability over SLMs, have emerged as a promising alternative for the facilitated transport of organic molecules and metal ions. PIMs are prepared using a mixture of a base polymer, a suitable extractant, and a plasticizer. The extractant provides selectivity to the target analyte, while the plasticizer provides flexibility to the membrane and also acts as a solvent, providing transport pathways for the analyte−extractant complex. Popularity of PIMs as an ion selective electrode is well-known. However, in recent years, a large number of scientific investigations have also been devoted to the facilitated transport of metal ions and different inorganic species using these membranes.2−12 Neutral carrier or solvating carriers3,4 transport metal ions as their salt, while the ionic carriers like © XXXX American Chemical Society

Received: January 11, 2016 Revised: March 4, 2016 Accepted: March 7, 2016

A

DOI: 10.1021/acs.iecr.6b00125 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research the current efficiency for Cs+ transport has been found to be ∼100%. Selective removal of Cs+ is of interest for simpler radioactive waste management. In this present work, we have further investigated the mechanism of Cs+ transport through the HCCD loaded PIM, under the influence of an applied electric field. PIMs containing a fixed amount of cellulose triacetate (CTA) and nitrophenyl octyl ether (NPOE) but different weight percentages of HCCD have been prepared. Electrodriven transport experiments, using the radiotracer method, have been carried out to measure the Cs+ flux across the membranes. The potential drop across the membrane (dV) has been obtained from the current (I), monitored during the transport experiments, and the membrane resistance (Rm) was measured by impedance spectroscopy. The self-diffusion coefficients (SDC) and distribution coefficient (Kd) of Cs+ in the PIMs have also been measured using a standard radiotracer based method. On the basis of the Nernst−Planck equation, these measured parameters (dV, D, Kd) have further been used to calculate the time dependent concentration changes of Cs+ in the feed compartment. This has been compared with the experimental transport profiles, and a plausible mechanism of Cs+ transport has been proposed.

is again placed in the equilibrating solution after counting. This process is continued until equilibrium. The method of calculation of Dtracer from the isotopic exchange profile (Figure S1) is given in detail in the Supporting Information. Impedance Measurement. The resistances (Rm) of the PIMs containing different weight percentages of HCCD have been obtained from electrochemical impedance measurements within the frequency range of 1 MHz to 100 Hz using an Autolab PGSTAT 302 voltammetric analyzer in conjuction with the 663 VA stand multimode electrode.11,15 The synthesized membrane samples (2 cm × 2 cm) are kept in 0.01 N CsCl solution for ∼3 h. After attainment of equilibrium, the membrane samples have been repeatedly washed with deionized water to ensure that there are no sorbed electrolytes. The membrane sample, in appropriate ionic form, has been clamped between two-compartments of a cell. Both the compartments have been filled with mercury (with one platinum electrode in each compartment), and the EIS spectrum has been acquired.11,15 The EIS spectra of PIMs containing different carrier concentrations have been given in the Supporting Information (Figure S2). The impedance measurements have been repeated twice to verify the consistency of the data. The method of extraction of Rm from the EIS spectra is also given in detail in the Supporting Information. Distribution Ratios (Kd). The distribution studies have been performed by equilibrating ∼20 mg of the membrane in 1 mL of aqueous solution containing 137Cs tracer. The Kd values have been determined using the following formula.

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EXPERIMENTAL SECTION Reagents and Chemicals. CsCl (99.9%, Analytical Reagent grade) and deionized water (18 MΩ/cm, Gradient A-10 model, Milli-Q USA) have been used in the present study. HCCD has been procured from Katchem, Czech Republic. 137 Cs, used as a radiotracer in the present study, has been obtained from Board of Radiation and Isotope Technology, Mumbai, India. Preparation of Membrane. The membrane containing 0.08 g of CTA, 0.2 mL of NPOE, and a variable amount of HCCD has been prepared using the standard method.11The carrier concentration in the membrane has been varied from 1.71 to 0.17 wt %. Due to limited solubility of HCCD in NPOE, a membrane with higher wt % of HCCD could not be prepared. The thicknesses (L) of the synthesized CTA based membranes have been measured using a digimatic micrometer (Mitutoyo Corporation, Japan) and are given in Table 1. Electrodriven Transport Studies. Electrodriven Cs+ transport experiments using the PIMs containing different concentrations of HCCD have been carried out following the method as described in ref 11. A digital multimeter has been used to monitor the current (I). The feed compartment contained 32 mL of 0.005 N CsCl solution, and the receiver compartment contained deionized water spiked with NaOH (0.004 mmol). The Cs+ in the feed compartment has been tagged with 137Cs tracer to monitor its transport rate. The same experiments have also been repeated for trace concentrations (≤10−12 N) of metal ion in the feed. Measurement of Self-Diffusion Coefficient (SDC). The self-diffusion coefficient (Dtracer) of Cs+ in the membranes containing different weight percentages of HCCD have been measured by standard nonstationary radiotracer methods as described elsewhere.14 In brief, for measurement of selfdiffusion, a 1 cm × 2 cm membrane sample, preequilibrated with CsCl, is placed in 30 mL of 137Cs tagged 0.1 N CsCl solution. In order to avoid film controlled diffusion, the solution is vigorously (∼300 rpm) stirred. At regular time intervals, the membrane is taken out, washed thoroughly with deionized water, and counted for 137Cs activity. The membrane

Kd =

(C0 − Ce)/W Ce/V ′

(1)

where C0 is the initial concentration of metal ion, Ce the concentration of metal ion at equilibrium, W the weight of membrane taken in grams, and V′ the volume of the aqueous phase in milliliters. Calculation. The plots of ln(Ct/C0) vs time, obtained for the PIMs containing different carrier concentrations, are shown in Figure 1. Here C0 (0.005 N) denotes the initial metal ion concentration in feed solution and Ct denotes the concentration of Cs+ in the feed solution at time “t”. It is seen from the figure that all the curves show an initial lag time during which

Figure 1. Transport kinetics of Cs+ through HCCD-PIM containing different wt % of the carrier. The plots are for 0.005 N CsCl in initial feed solution. The solid lines represent the fit to experimental data points. B

DOI: 10.1021/acs.iecr.6b00125 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Results of Various Measurements and Calculations for PIMs Containing Different Carrier Concentrationsa % wt of HCCD

L (μm)

I (mA)

Rm (Ω)

dV (V)

Kd (mL/g)

1.71

33

0.43

1180 ± 11

0.634

9.40 ± 0.02

1.37

40

0.37

1869 ± 13

0.882

5.56 ± 0.01

1.03

39

0.15

3009 ± 22

0.630

4.70 ± 0.01

0.69

50

0.07

7351 ± 62

0.487

3.23 ± 0.01

a

Dtracer (cm /s)

calc. slope, C (s−1)

expt. slope, E (s−1)

1.03 × 10−8 ± 9.47 × 10−10 8.52 × 10−9 ± 3.76 × 10−10 6.99 × 10−9 ± 3.56 × 10−10 3.56 × 10−9 ± 2.64 × 10−10

3.22 × 10−5 ± 3.06 × 10−6 1.76 × 10−5 ± 7.80 × 10−7 8.08 × 10−6 ± 4.12 × 10−7 2.35 × 10−6 ± 1.74 × 10−7

3.82 × 10−5 ± 1.68 × 10−6 2.22 × 10−5 ± 2.87 × 10−7 1.20 × 10−5 ± 9.54 × 10−8 1.49 × 10−6 ± 1.66 × 10−8

2

ratio

distance between two carrier molecules (nm)

1.18 ± 0.12

3.44

1.26 ± 0.06

3.76

1.49 ± 0.08

4.20

0.64 ± 0.05

4.91

The data, which have been obtained from transport experiments, are for 0.005 N metal ion concentrations in feed solution.

hardly any transport of Cs+ occurs. Post-lag time, the curves are linear and the slope of the curves decrease with decrease in carrier concentration in the membrane. Similar plots have also been obtained for trace concentrations of the metal ion in initial feed solution and are given in the Supporting Information (Figure S4). The linear plots indicate exponential dependence of Ct on time which is commonly observed for diffusion dialysis experiments. In the case of diffusion dialysis, the slope of the line is dependent on the permeability of the diffusing species. However, in the present case, involving electrodialysis, the physical significance of the slope is different as the transport is driven by potential gradient, rather than the concentration gradient. The zero time flux (J0) of Cs+ is obtained from the slope (Sexp) of the linear part of the plots (Figure 1) using the following relation J0 = SexpXC0

Vol dC S F dV = −zDtracerC̅ RT dx A dt

Again, C̅ = KdC S

Vol dC S F dV = −zDtracerKdC S A dt RT dx

∫C ln

Vol dC S A dt

S 0

dC S A F dV = −zDtracerKd S V RT dx C

∫0

t

dt

(9)

C tS C0S

= −zDtracerKd

A F dV t Vol RT dx

(10)

predicts linear variation of ln(CtS/C0S) A F dV parameter (Scal) is −zDtracer Kd Vol RT dx .

The equation vs time, and the slope The term (dV/dx) can be obtained from the membrane resistance (Rm), the current (I) across the membrane during the transport process, and the membrane thickness L using the following relation: IR dV = m dx L

(3)

(11) 11

As determined in our earlier work, the current efficiency for Cs+ transport is ∼100%. Hence, the calculated slope can be written as Scal = −Dtracer Kd

A F IR m Vol RT L

(12)

Thus, Scal can be obtained from the experimental parameters (I, Rm, Kd, Dtracer, L, A, Vol) using eq 12.

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(4)

RESULTS AND DISCUSSION Table 1 summarizes the results of various measurements and calculations for PIMs containing different concentrations of HCCD. The data, which have been obtained from transport experiments, are for 0.005 N metal ion concentrations in feed solution. The absolute errors on each number have also been shown in the table. It can be seen from the table that the average thickness of the membrane (L) varies from as low as 31 μm to as high as 50 μm. As seen from the table, the current across the membranes during the electrodriven transport

Using the condition of mass balance, the flux can be related to the change in concentration of the ion in solution by the equation J = no. of moles/cm 2/s =

CtS

Thus,

where dV/dx is the electric potential gradient across the membrane, F is the Faraday constant, R is the gas constant, and T is the absolute temperature. Here we have neglected the contribution from convective flow and electro-osmosis. In electrodialysis, the diffusive flux is also negligible as compared to that from migration.16 Thus, eq 3 reduces to F dV RT dx

(8)

Rearranging and integrating, we get

Assuming that the membrane behaves like a typical ion exchange membrane and the Nernst−Planck (NP) equation is applicable, the slope has been related to the experimentally determined parameters of the membranes, i.e., I, Rm, L, Dtracer, and Kd, as given in the following. According to the Nernst−Plank equation, the flux (J) of an ion (valency z, concentration C̅ in the membrane, and diffusion coefficient Dtracer) is given by

J = (J )el = −zDtracerC̅

(7)

Here, we have assumed instantaneous equilibrium, so the SDC in membrane controls the transport rate. Thus, eq 6 reduces to

(2)

⎛ F dV ⎞⎟ J = (J )diff + (J )el = −Dtracer ⎜grad C̅ + zC̅ ⎝ RT dx ⎠

(6)

(5)

where Vol is the volume of feed compartment and A is the membrane active surface area. The superscript “S” refers to the aqueous phase. Thus, equating eqs 4 and 5 we get C

DOI: 10.1021/acs.iecr.6b00125 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

approximately equal to its diameter.2 This gives rise to a percolation threshold, i.e., minimum carrier concentration for transport to occur. This corresponds to the maximum average distance between two carrier molecules, beyond which fixed site jumping is not possible. The distance between the neighboring carrier molecules decreases as carrier concentration in the membrane increases. In the present work, the absence of percolation threshold indicates that Cs+ diffusion does not occur by fixed site jumping. The distances between two HCCD molecules in different PIMs are also given in Table 1.23 As expected, it increases as the carrier concentration in the membrane decreases. With the radius of one HCCD molecule being ∼0.35 nm, even for the highest carrier concentration in the membrane, the distances between two nearby carrier molecules are too high for a fixed site jumping to occur. This again supports the observations obtained from the zero-time flux measurement. From the absence of threshold carrier concentration for transport, it appears that the transport of Cs+ occurs via mobile carrier diffusion mechanism. However, in our earlier work with the same system,11 it has been shown that Cs+ transport does not occur in the absence of applied electric field, i.e., Donnan dialysis type of counter transport of metal ions between the feed and the receiver phase cannot take place. Thus, from the absence of counterion transport, the possibility of mobile carrier diffusion can be ruled out. However, the current flow across the membrane under an applied electric field indicates the presence of charge carriers in the membrane. It can be assumed that, like ionic liquid, Cs-CCD is dissociated in NPOE, which has a high dielectric constant (εr = 24). Thus, a possible alternative mechanism in the present case can be that, under applied electric field, the Cs+ ions are transported only as a “solvent (NPOE) separated free ion”.6,10,24,25 Reinhoudt and co-workers6,24 have also reported a similar transport mechanism for the crown ether mediated transport of potassium perchlorate through NPOE contained in a Accurel membrane. The experimental flux could only be explained based on the assumption that the K+-crown ether complex is fully separated from the perchlorate anion. They reported24 that depending upon the dielectric constant of the solvent, the cationic and the anionic component will remain as an ion pair or “solvent separated free ions”. Hansen and Fyles10 have also reported a similar mechanism for the valinomycin assisted electrodialysis based transport of K+ ion through supported liquid membrane. In the most recent literature, a similar mechanism for the facilitated transport of Cu(II) has also been proposed by Wang et al.25 Even in solvent extraction, the state of the extracted species in the organic phase is an active area of investigation, and Osakai et al. have explored the coextraction of water molecules by inorganic cations and anions in hydrophilic solvents like nitrobenzene.26 Sieffert and Wipff,27,28 by molecular dynamics simulations, have shown that addition of a polar component assists in the solvent extraction of Cs+ by a supramolecular carrier. They have also shown that Cs+−calix crown complex is better solvated in ionic liquid than comparatively nonpolar chloroform. These observations corroborate our proposition. In our calculation of Scal, we have assumed that, in the membrane phase, the carrier concentration is equal to the metal ion concentration; i.e., Cs+ is fully dissociated in the solvent (NPOE). As seen from Table 1, for all the cases, the ratio of Scal to Sexp is ∼1, indicating that almost the entire Cs+ is transported as a solvent separated ion pair. Moreover, this mechanism can also account for the

experiments decreases as the carrier concentration in the membrane decreases. This directly reflects the effect of reduced HCCD concentration. The increase in the membrane resistance is due to the combined effect of reduced carrier concentration and membrane thickness. Table S1 shows the comparison between the SDCs calculated from radiotracer and EIS for HCCD-PIM containing different weight % of the carrier. As reported in the literature,15 the EIS data deviates substantially from the radiotracer data. The data indicates that, in all the cases, DEIS is higher by a factor ∼2 than the corresponding Dtracer. Thus, the measured Rm, as obtained from EIS, has been multiplied by the corresponding factor and shown in Table 1. The potential drops (dV) across the membrane, as obtained from the product of I and Rm, are nearly constant in the range of 500−800 mV. It can also be seen from the table that the distribution ratio of Cs+ decreases as HCCD concentration in the membrane decreases. The calculated (using eq 12) slope as well as experimental slopes of the time dependence of concentration changes (of Cs+) in the two compartment experiments are also shown in the table. The mechanism of bulk transport in PIMs has been explained on the basis of two models: (i) fixed site jumping17,18 and (ii) mobile carrier diffusion.19,20 It is also reported that the kinetic profiles for transport by carrier-diffusion and fixed-site jumping are generally quite similar and only differ in the dependency of flux on carrier concentration.17 For fixed-site jumping, a percolation threshold in carrier concentration is predicted, i.e., the transport does not occur below a threshold carrier concentration when the distance between fixed sites is too large for solute to jump.2,17,18,21,22 Below the threshold concentration, flux is negligible. Above the threshold concentration, depending on the experimental conditions, the flux may vary linearly or at higher power with carrier concentration. However, no threshold carrier concentration is observed for transport by the mobile carrier-diffusion mechanism, and the flux profile varies linearly with carrier concentration and passes through the origin. The plot of zero time flux (J0) of Cs+ as a function of HCCD concentration, as obtained from the present work, is shown in Figure 2. The plot is for trace concentration of metal ion in the initial feed solution, ensuring that the ligand (HCCD) is always in excess to the metal ion.17 The figure indicates that the flux profile varies linearly with carrier concentration and passes through the origin. For fixed site jumping to occur, the distance between two neighboring carrier molecules should be

Figure 2. Zero time Cs+ flux (J0) through HCCD-PIM having different carrier concentrations. The plot is for trace concentration of metal ion in initial feed solution. D

DOI: 10.1021/acs.iecr.6b00125 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 100% current efficiency for the Cs+ transport, as mentioned in our earlier work.11

(8) Tayeb, R.; Fontas, C.; Dhahbi, M.; Tingry, S.; Seta, P. Cd(II) transport across supported liquid membranes (SLM) and polymeric plasticized membranes (PPM) mediated by Lasalocid A. Sep. Purif. Technol. 2005, 42, 189. (9) Rodríguez de San Miguel, E. R. S.; Aguilar, J. C.; Gyves, J. Structural effect on metal ion migration across polymer inclusion membranes: Dependence of membrane transport properties and transport profiles on nature of active plasticizer. J. Membr. Sci. 2008, 307, 105. (10) Hansen, S. P.; Fyles, T. M. Carrier-mediated electrodialysis. Chem. Commun. 2011, 47, 6428. (11) Chaudhury, S.; Bhattacharyya, A.; Goswami, A. Electrodriven Selective Transport of Cs+ Using Chlorinated Cobalt Dicarbollide in Polymer Inclusion Membrane: A Novel Approach for Cesium Removal from Simulated Nuclear Waste Solution. Environ. Sci. Technol. 2014, 48, 12994. (12) Kumar, R.; Pandey, A. K.; Sharma, M. K.; Panicker, L. V.; Sodaye, S.; Suresh, G.; Ramagiri, S. V.; Bellare, J. R.; Goswami, A. Diffusional Transport of Ions in Plasticized Anion-Exchange Membranes. J. Phys. Chem. B 2011, 115, 5856. (13) Fyles, T. M. Synthetic ion channels in bilayer membranes. Chem. Soc. Rev. 2007, 36, 335. (14) Goswami, A.; Acharya, A.; Pandey, A. K. Study of Self-Diffusion of Monovalent and Divalent Cations in Nafion-117 Ion-Exchange Membrane. J. Phys. Chem. B 2001, 105, 9196. (15) Chaudhury, S.; Agarwal, C.; Pandey, A. K.; Goswami, A. Selfdiffusion of ions in Nafion-117 membrane having mixed ionic composition. J. Phys. Chem. B 2012, 116, 1605. (16) Tanaka, Y. Ion Exchange membranes: Fundamentals and Application, 2nd ed.; Elsevier: 2015. (17) Riggs, J. A.; Smith, B. D. Facilitated transport of small carbohydrates through plasticized cellulose triacetate membranes. Evidence for fixed site jumping transport mechanism. J. Am. Chem. Soc. 1997, 119, 2765. (18) Munro, T. A.; Smith, B. D. Facilitated transport of amino acids by fixedsite jumping. Chem. Commun. 1997, 22, 2167. (19) Lamb, J. D.; Nazarenko, A. Y. Lead(II) ion sorption and transport using polymer inclusion membranes containing trioctylphosphine oxide. J. Membr. Sci. 1997, 134, 255. (20) Schow, A. J.; Peterson, R. T.; Lamb, J. D. Polymer inclusion membranes for use in cation separations. J. Membr. Sci. 1996, 111, 291. (21) Gherrou, A.; Kerdjoudj, H.; Molinari, R.; Seta, P.; Drioli, E. Fixed sites plasticized cellulose triacetate membranes containing crown ethers for silver(I), copper(II) and gold(III) ions transport. J. Membr. Sci. 2004, 228, 149. (22) Cussler, E. L.; Aris, R.; Bhown, A. On the limits of facilitated diffusion. J. Membr. Sci. 1989, 43, 149. (23) Hao, T.; Riman, R. E. Calculation of interparticle spacing in colloidal systems. J. Colloid Interface Sci. 2006, 297, 374. (24) Nijenhuis, W. F.; Buitenhuis, E. G.; Jong, F. de; Sudholter, E. J. R.; Reinhoudt, D. N. Calixcrowns as Selective Potassium Cation Carriers in Supported Liquid Membranes. J. Am. Chem. Soc. 1991, 113, 7963. (25) Wang, D.; Hu, J.; Li, Y.; Fu, M.; Liu, D.; Chen, Q. Evidence on the 2-nitrophenyl octyl ether (NPOE) facilitating Copper (II) transport through polymer inclusion membranes. J. Membr. Sci. 2016, 501, 228. (26) Osakai, T.; Ogata, A.; Ebina, K. Hydration of Ions in Organic Solvent and Its Significance in the Gibbs Energy of Ion Transfer between Two Immiscible Liquids. J. Phys. Chem. B 1997, 101, 8341. (27) Sieffert, N.; Wipff, G. Comparing an Ionic Liquid to a Molecular Solvent in the Cesium Cation Extraction by a Calixarene: A Molecular Dynamics Study of the Aqueous Interfaces. J. Phys. Chem. B 2006, 110, 19497. (28) Sieffert, N.; Wipff, G. The effect of a solvent modifier in the cesium extraction by a calix[4] arene: a molecular dynamics study of the oil phase and the oil−water interface. Phys. Chem. Chem. Phys. 2007, 9, 3763.

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CONCLUSION In the present work, an attempt has been made to predict and validate the mechanism of Cs+ transport through the HCCD loaded PIMs under externally applied electric field. Absence of percolation threshold in the plot of J0 vs carrier concentration for trace concentration of metal ion in the feed solution indicated that diffusion by the fixed site jumping mechanism is not present. Moreover, as the process is electric field driven, movement of Cs+ by mobile carrier movement cannot account for the current through the membrane. Thus, based on the calculation using the NP equation, a third plausible mechanism has been predicted. The model has been validated by comparing with the experimental data. It has been observed that Cs+ is fully dissociated in the polar lipophilic solvent (NPOE) and is transported mostly as solvent separated ions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00125. The isotopic exchange profile and the corresponding method of calculation of SDC of Cs+ and the transport kinetics of Cs+ for trace level metal in initial feed solution. The Nyquist plots obtained from impedance measurement for HCCD-PIM containing different weight % of the carrier. The comparison of the SDCs obtained from EIS and radiotracer measurement (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(A.B.) Fax: +91-22-25505150/25505151. Tel.: +91-2225594099. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Baker, R. W. Membrane technology and applications, 2nd ed.; John Wiley and Sons: 2004. (2) Nghiem, L. D.; Mornane, P.; Potter, I. D.; Perera, J. M.; Cattrall, R. W.; Kolev, S. D. Extraction and transport of metal ions and small organic compounds using polymer inclusion membranes (PIMs). J. Membr. Sci. 2006, 281, 7. (3) Kusumocahyo, S. P.; Kanamori, T.; Sumaru, K.; Aomatsu, S.; Matsuyama, H.; Teramoto, M.; Shinbo, T. Development of polymer inclusion membranes based on cellulose triacetate: carrier-mediated transport of cerium(III). J. Membr. Sci. 2004, 244, 251. (4) Ballinas, M. D.; San Miguel, E. R.; Rodriguez, M. T. D.; Silva, O.; Munoz, M.; Gyves, J. de. Arsenic(V) removal with polymer inclusion membranes from sulfuric acid media using DBBP as carrier. Environ. Sci. Technol. 2004, 38, 886. (5) Kolev, S. D.; Cattrall, R. W.; Paimin, R.; Potter, I. D.; Sakai, Y. Theoretical and experimental study of palladium(II) extraction into Aliquat 336/PVC membranes. Anal. Chim. Acta 2000, 413, 241. (6) Visser, H. C.; Reinhoudt, D. N.; de Jong, F. Carrier-mediated Transport through Liquid Membranes. Chem. Soc. Rev. 1994, 23, 75. (7) Rodríguez de San Miguel, E.; Monroy-Barreto, M.; Aguilar, J. C.; Ocampo, A. L.; Gyves, J. Structural effect on metal ion migration across polymer inclusion membranes: Dependence of membrane transport properties and transport profiles on the weight and volume fractions of the components. J. Membr. Sci. 2011, 379, 416. E

DOI: 10.1021/acs.iecr.6b00125 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX