- Email: [email protected]
Characterization of ion transport in the active layer of RO and NF polyamide membranes
Desalination 199 (2006) 31–33
Characterization of ion transport in the active layer of RO and NF polyamide membranes Sarit Bason, Adi Ben-David, Yoram Oren, Viatcheslav Freger* Unit of Environmental Engineering and Zuckerberg Institute for Water Research, Ben Gurion University of the Negev, P.O.B. 653, Beer Sheva, 84105, Israel email: [email protected] Received 26 October 2005; accepted 2 March 2006
1. Introduction The classic methods for characterizing transport of individual ions in membranes, such as steady-state streaming potential and membrane potential, apply in a straightforward way only to thick single-layer membranes. Application of these techniques to industrial composite RO and NF membranes has been largely unsuccessful, since these membranes have a multilayer structure, in which the in situ-prepared active layer is only a tiny fraction of the total diffusion resistance [1,2]. As a result of the polarization in the supporting layers, the “salt” phenomenological parameter, e.g., the permeability ws and reflection coefficient s deducible from filtration experiments, cannot be split into individual parameters of ions. Still more difficult is splitting the ionic permeabilities into the partitioning and diffusional factors. Such information could be however highly useful, both from the fundamental viewpoint of understanding the ion exclusion in membranes and from a practical viewpoint of developing predictive models of electrolyte separation for multicomponent salt mixtures.
It has been pointed out that non-steady-state techniques, e.g., potential transients after rapidly changing the concentration or pressure gradient across the membrane, should have advantages over steady-state measurements [2,3]. The use of transients allows observation of different phenomena at various timescales and getting separate information on the partitioning and diffusion components of the permeability (cf. the popular time lag method). A novel technique presented in this study combines the use of a single layer in absence of external polarization with the use of a non-steady-state technique. The measurements employ a free-standing single layer of polyamide isolated from a genuine commercial membrane and placed on a solid electrode, which is characterized by means of electrochemical impedance spectroscopy (EIS) [4]. This approach effectively eleiminates both the polarization in support and ion coupling, which has been the main problems so far, and is capable of producing a comprehensive picture of ion transport in the polyamide layer. 2. Experimental
*Corresponding author.
The membranes used were ESPA1 (Hydranautics) and NF200 (Dow-Filmtec) kindly supplied
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.137
32
S. Bason et al. / Desalination 199 (2006) 31–33
by the manufacturer. The polyamide layer on the electrode was prepared in two ways: (i) isolation of an active layer from commercial composite membranes by removing the nonwoven and dissolving the porous polysulfone in organics solvents and (ii) direct synthesis on the electrode surface. EIS spectra were recorded using a system including a 3-electrode cell with a rotating disc glassy carbon working electrode covered with a film, an Ag/AgCl reference and a Pt wire as a counter electrode. The electrodes were connected to a Gamry PC/300 potentiostat/ ZRA analyzer equipped with software for data acquisition and analysis. The solution contained an equimolar mixture of Fe(CN)43– and Fe(CN)44– in a supporting buffer electrolyte. Data analysis and parameter estimation were based on a general equivalent circuit (EC) recently proposed for this system [5]. 3. Results and discussion Two types of experiments with the film may be performed: – experiments with a solution containing only a supporting buffer; – experiments with a solution containing both 4– redox-active ions, such as Fe(CN)3– 4 /Fe(CN)4 , and the buffer. The first type of experiments yields the high frequency electrical resistance of the film Rm, which is related to the sum of the individual permeablities (w±) of all ions present in solution. Given Rm and ws, the individual permeablities of ions of supporting electrolyte may be retrieved. In addition, these measurements offer a unique opportunity of estimating the effective thickness of the active layer d, which is in general significantly smaller than the total superficial thickness of the polyamide layer, yet only indirect estimates of this parameter have been possible so far. This thickness is easily deduced from the dielectric capacitance of the film, which is observed in the EIS spectrum and whose value
is estimated using an EC analysis. Knowledge of the thickness allows conversion of the permeabilities w to intrinsic permeabilities P = wd, which, unlike w, are true material properties under given conditions. The experiments of the second type offer an additional opportunity for splitting the intrinsic permeabilities P = KD to the partitioning (K) and diffusion (D) coefficients, provided d is known. Although this kind of experiment is only possible for redox-active ions, direct information on the partitioning coefficient is of high interest for understanding the mechanism of ion exclusion. An additional advantage of such tests is that P, K and D may be directly measured for an individual ion. The analysis of EIS spectra and retrieval of parameters is illustrated by experiments with 3 different films: an in situ-prepared fully aromatic polyamide film of the RO type, a polyamide film isolated from an ESPA1 RO membrane and a polyamide film isolated from a NF200 membrane. The possible sources of errors and artifacts are also discussed. 4. Conclusions The results of experiments of both types yields thus a comprehensive picture of ion transport in a polyamide film including structural parameters of the film. The parameters obtained by the proposed technique can be used as input for theoretical transport models in simulations of desalination, softening and removal heavy metals from multicomponent solutions. The results ultimately help quantify and understand the relative roles of diffusional and convectional transport in pressure-driven separation of electrolytes using thin-film composite membranes. References [1]
J. Benavente and G. Jonsson, Influence of the external conditions on salt retention and pressure-induced electrical potential measured across a composite membrane, Coll. Surf., 159 (1999) 431–437.
S. Bason et al. / Desalination 199 (2006) 31–33 [2]
[3]
A.E. Yaroshchuk, A.L. Makovetskiy, Y.P. Boiko and E.W. Galinker, Non-steady-state membrane potential: theory and measurements by a novel technique to determine the ion transport numbers in active layers of nanofiltration membranes, J. Membr. Sci., 172 (2000) 203–221. A.E. Yaroshchuk, Y.P. Boiko and A.L. Makovetskiy, Some properties of electrolyte solutions
[4] [5]
33
in nanoconfinement revealed by the measurement of transient filtration potential after pressure switch off, Langmuir, 21 (2005) 7680–7690. A.J. Bard and L.F. Faulkner, Electrochemical Methods, New York, John Wiley & Sons, 1980. V. Freger, Diffusion impedance and equivalent circuit of a multilayer film, Electrochem. Com., 7 (2005) 957–961.
Characterization of ion transport in the active layer of RO and NF polyamide membranes Sarit Bason, Adi Ben-David, Yoram Oren, Viatcheslav Freger* Unit of Environmental Engineering and Zuckerberg Institute for Water Research, Ben Gurion University of the Negev, P.O.B. 653, Beer Sheva, 84105, Israel email: [email protected] Received 26 October 2005; accepted 2 March 2006
1. Introduction The classic methods for characterizing transport of individual ions in membranes, such as steady-state streaming potential and membrane potential, apply in a straightforward way only to thick single-layer membranes. Application of these techniques to industrial composite RO and NF membranes has been largely unsuccessful, since these membranes have a multilayer structure, in which the in situ-prepared active layer is only a tiny fraction of the total diffusion resistance [1,2]. As a result of the polarization in the supporting layers, the “salt” phenomenological parameter, e.g., the permeability ws and reflection coefficient s deducible from filtration experiments, cannot be split into individual parameters of ions. Still more difficult is splitting the ionic permeabilities into the partitioning and diffusional factors. Such information could be however highly useful, both from the fundamental viewpoint of understanding the ion exclusion in membranes and from a practical viewpoint of developing predictive models of electrolyte separation for multicomponent salt mixtures.
It has been pointed out that non-steady-state techniques, e.g., potential transients after rapidly changing the concentration or pressure gradient across the membrane, should have advantages over steady-state measurements [2,3]. The use of transients allows observation of different phenomena at various timescales and getting separate information on the partitioning and diffusion components of the permeability (cf. the popular time lag method). A novel technique presented in this study combines the use of a single layer in absence of external polarization with the use of a non-steady-state technique. The measurements employ a free-standing single layer of polyamide isolated from a genuine commercial membrane and placed on a solid electrode, which is characterized by means of electrochemical impedance spectroscopy (EIS) [4]. This approach effectively eleiminates both the polarization in support and ion coupling, which has been the main problems so far, and is capable of producing a comprehensive picture of ion transport in the polyamide layer. 2. Experimental
*Corresponding author.
The membranes used were ESPA1 (Hydranautics) and NF200 (Dow-Filmtec) kindly supplied
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.137
32
S. Bason et al. / Desalination 199 (2006) 31–33
by the manufacturer. The polyamide layer on the electrode was prepared in two ways: (i) isolation of an active layer from commercial composite membranes by removing the nonwoven and dissolving the porous polysulfone in organics solvents and (ii) direct synthesis on the electrode surface. EIS spectra were recorded using a system including a 3-electrode cell with a rotating disc glassy carbon working electrode covered with a film, an Ag/AgCl reference and a Pt wire as a counter electrode. The electrodes were connected to a Gamry PC/300 potentiostat/ ZRA analyzer equipped with software for data acquisition and analysis. The solution contained an equimolar mixture of Fe(CN)43– and Fe(CN)44– in a supporting buffer electrolyte. Data analysis and parameter estimation were based on a general equivalent circuit (EC) recently proposed for this system [5]. 3. Results and discussion Two types of experiments with the film may be performed: – experiments with a solution containing only a supporting buffer; – experiments with a solution containing both 4– redox-active ions, such as Fe(CN)3– 4 /Fe(CN)4 , and the buffer. The first type of experiments yields the high frequency electrical resistance of the film Rm, which is related to the sum of the individual permeablities (w±) of all ions present in solution. Given Rm and ws, the individual permeablities of ions of supporting electrolyte may be retrieved. In addition, these measurements offer a unique opportunity of estimating the effective thickness of the active layer d, which is in general significantly smaller than the total superficial thickness of the polyamide layer, yet only indirect estimates of this parameter have been possible so far. This thickness is easily deduced from the dielectric capacitance of the film, which is observed in the EIS spectrum and whose value
is estimated using an EC analysis. Knowledge of the thickness allows conversion of the permeabilities w to intrinsic permeabilities P = wd, which, unlike w, are true material properties under given conditions. The experiments of the second type offer an additional opportunity for splitting the intrinsic permeabilities P = KD to the partitioning (K) and diffusion (D) coefficients, provided d is known. Although this kind of experiment is only possible for redox-active ions, direct information on the partitioning coefficient is of high interest for understanding the mechanism of ion exclusion. An additional advantage of such tests is that P, K and D may be directly measured for an individual ion. The analysis of EIS spectra and retrieval of parameters is illustrated by experiments with 3 different films: an in situ-prepared fully aromatic polyamide film of the RO type, a polyamide film isolated from an ESPA1 RO membrane and a polyamide film isolated from a NF200 membrane. The possible sources of errors and artifacts are also discussed. 4. Conclusions The results of experiments of both types yields thus a comprehensive picture of ion transport in a polyamide film including structural parameters of the film. The parameters obtained by the proposed technique can be used as input for theoretical transport models in simulations of desalination, softening and removal heavy metals from multicomponent solutions. The results ultimately help quantify and understand the relative roles of diffusional and convectional transport in pressure-driven separation of electrolytes using thin-film composite membranes. References [1]
J. Benavente and G. Jonsson, Influence of the external conditions on salt retention and pressure-induced electrical potential measured across a composite membrane, Coll. Surf., 159 (1999) 431–437.
S. Bason et al. / Desalination 199 (2006) 31–33 [2]
[3]
A.E. Yaroshchuk, A.L. Makovetskiy, Y.P. Boiko and E.W. Galinker, Non-steady-state membrane potential: theory and measurements by a novel technique to determine the ion transport numbers in active layers of nanofiltration membranes, J. Membr. Sci., 172 (2000) 203–221. A.E. Yaroshchuk, Y.P. Boiko and A.L. Makovetskiy, Some properties of electrolyte solutions
[4] [5]
33
in nanoconfinement revealed by the measurement of transient filtration potential after pressure switch off, Langmuir, 21 (2005) 7680–7690. A.J. Bard and L.F. Faulkner, Electrochemical Methods, New York, John Wiley & Sons, 1980. V. Freger, Diffusion impedance and equivalent circuit of a multilayer film, Electrochem. Com., 7 (2005) 957–961.