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Techniques for characterization of polyamide thin film composite membranes
Desalination 282 (2011) 78–86
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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
Techniques for characterization of polyamide thin film composite membranes P.S. Singh ⁎, A.P. Rao, P. Ray, A. Bhattacharya, K. Singh, N.K. Saha, A.V.R. Reddy RO Membrane Division, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial Research, Bhavnagar 364002, India
a r t i c l e
i n f o
Article history: Received 16 March 2011 Accepted 13 April 2011 Available online 11 May 2011 Keywords: TFC RO Polyamide Large-scale Preparation Characterization Performance
a b s t r a c t Thin film composite reverse osmosis (TFC RO) membranes (1 m breadth × 90 m length) were prepared by coating polyamide active layer over a porous polysulfone membrane using indigenous membrane casting and coating devices. The TFC membranes exhibited 56 ± 9 l/m2 h water flux and 96.7 ± 0.6% salt rejection when tested with 2000 ppm NaCl feed at 250 psi pressure. The membrane structural information obtained using SEM, AFM, SANS, PALS, QENS, permeation experiments with organic markers, ATR-IR, and zeta-potential measurements was correlated with the variance in membrane flux and selectivity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The characterization of membranes involves the study of both membrane materials and membrane processes. The fundamental understanding of the membrane materials are required to assess the membrane processes. While the characterization techniques based on the conventional analytical instruments are relatively simpler approaches, the observations of membrane process and transport phenomena are rather complex. Particularly, the processes of thinfilm composite (TFC) ‘reverse osmosis’ (RO) polyamide membrane deal with species of sub-nanometer size ranges and therefore one has to look in the scale of sub-nanometer to nanometer size range. This means that the scale of resolution of the observations should be in angstroms. Therefore, it has been very difficult to observe real-time experiments such as transport process of a permeating species moving in the subnanometer pores of the TFC RO membrane. Model-based approaches are the substitutes to understand the transport phenomena which however, often lead to contrasting views as the models are primarily based on assumptions and hypothesis. Therefore, other complementary techniques and methods are required to overcome the unwanted errors of analysis. Electrons, positrons, X-rays and neutrons are of angstrom scales and therefore the techniques using them can give resolutions of the observation in the angstrom scale. Characterization using these techniques is required to understand the polymer chain nanostructure as well as the pore structure characteristics of the membrane in order to improve the membrane performance.
⁎ Corresponding author. Tel.: + 91 278 2566511; fax: + 91 278 2567562. E-mail address: [email protected] (P.S. Singh). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.04.039
The TFC RO membrane is comprised of top ultra-thin skin polyamide layer, middle polysulfone porous support and bottom non-woven fabric and is extensively used for water desalination and other industrial applications [1–4]. The top polyamide layer is the selective active layer which is responsible for permeability and selectivity. The membrane porosity may be detrimental by the polymer chain nanostructure of the polyamide layer as the pores are arises from the spaces between the chain segments constituting the polymer network within each supramolecular polymer aggregate as well as the spaces between neighboring supramolecular polymer aggregates themselves [5–7]. The size of polymer aggregates are in nanometer length scale and highly compacted but can be loosened by solvent treatment [8]. The pores of the polyamide membranes are irregular in nature and are found to be non-interconnecting [9–10]. A change in membrane pore size is resulted when the polymer chain nanostructure is modified by the introduction of nanoparticles [11–12]. Various techniques have been used to characterize the TFC RO membrane. Attenuated total reflectance infrared (ATR-IR) spectroscopy [13–15] and atomic force microscopic (AFM) techniques [16–18] are non-destructive techniques which can measure the samples in their native form and therefore particularly suited to study the surfaces of the RO polyamide membranes. The surface roughness observed by AFM is an important parameter which is found to be directly related with the permeability of the membrane [19]. The nanoscale heterogeneity which is responsible for surface roughness (hill–valley nodular morphology) in the polyamide is clearly observed by transmission electron microscopy (TEM) studies [20]. 1H NMR and CP-MAS 13C NMR have been used to measure spin-relaxation time in the rotating frame for comparing the polymer chain mobility in the membrane structure that also has a direct relationship with the membrane flux [18,21]. The modification of molecular structure of the polyamide by incorporation of chemical functional groups [22–24]
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resulting to a change in the polyamide chain nanostructure also led to a change in the membrane performances. Studies of electron spin resonance (ESR) and theoretical computer calculation methods [25,26] corroborate that the polymer chain nanostructure is required to be of a uniform network structure for good membrane performances. The preparation of high-quality TFC RO membranes requires a systematic approach and characterization since the polymer film formation is rapid and delicate process with the film thickness is of submicron range. The lab-scale results are subject to various errors leading to difficulty in quantitative comparison and scale-up process [27,28]. Because of the difficulties involved in the membrane preparation only few companies are commercially successful and the detailed information of large-scale preparations is not disclosed. In this paper, we present characterization studies performed on the membranes for large-scale development of TFC RO membrane (1 m breadth × 90 m length) using our indigenous membrane casting and coating units [29–33].
Data analysis was carried out using PATFIT program [34]. Quasi-elastic neutron scattering measurements were carried out using the QENS spectrometer at Dhruva reactor, Trombay [35]. The spectrometer was used in multi angle reflecting crystal (MARX) mode, which essentially uses a combination of a large analyzer crystal and a position sensitive detector. This instrument has an energy resolution of 200 μeV with incident neutron energy of 5 meV. The quasielastic (QE) data were recorded in the wave vector transfer (Q) range of 0.67–1.8 Å− 1 at 300 K. QENS measurement was first carried out on the dehydrated membrane. The sample was then soaked in water and allowed to equilibrate. QENS data were then recorded from the water-sorbed sample. The SANS measurements from the membrane samples over the wave vector range (Q) of 0.018–0.35 Å− 1 were taken at room temperature or higher on the SANS instrument [36] at the Dhruva reactor, BARC, Mumbai, India.
2. Experimental
3.1. Scanning electron microscopy
Polysulfone casting solution was prepared by dissolving 15 wt.% Udel P-3500 PS (Solvay, USA) in A.R. grade N,N-dimethylformamide (DMF). The resultant polymer solution was cast on a non-woven fabric using a phase inversion process from a water bath consisting a fixed amount of DMF and 0.1 wt.% sodium lauryl sulfate (SLS) surfactant. The resulting polysulfone membrane was washed thoroughly with distilled water. Typically, TFC membrane was prepared by initially immersing the polysulfone membrane support in 2% (w/v) solution of mphenylenediamine (MPD, Lancaster Chemical Co.) in water, followed by immersing into n-hexane solution of 0.1% (w/v) trimesoyl chloride (TMC, Lancaster Chemical Co.) which resulted in the lamination of an ultra-thin polyamide film over the surface of polysulfone support. The nascent composite membrane was subsequently cured at around 70 °C and then washed thoroughly to remove unreacted diamine. SEM pictures of the membrane samples were taken on LEO 1430VP environmental scanning electron microscope with 5–15 kV accelerating voltage. AFM images of the samples were acquired using NT-MDT AFM instrument. Samples were freeze-fractured to view crosssectional image. The ATR-IR instrument used consisted of a Barnes model 300 continuously variable ATR accessory interfaced to a Nicolet 5DX Fourier transform infrared spectrometer. For ATR-IR studies of the membrane samples, germanium crystal was fixed at 45 °C angle of incidence, that gave a probing depth of about 0.4 μm in the infrared region of interest. The amount of contact between the sample and crystal is same in all samples. Dry specimens of membrane samples were precisely cut to ATR crystal size and mounted on both faces of germanium crystal with the active layer facing the crystal surface. Zeta potentials of the membrane surfaces were measured by ZETA-CAD instrument using 0.01 M aqueous KCl solution. TFC membrane sample test coupons were evaluated for permeate flux and selectivity on a batch type RO test kit comprising four cells in series. Testing was done with NaCl solution of 2000 ppm concentration at a pressure of 250 psi. A standardized conductivity meter was used to measure the salt (NaCl) concentrations in the feed and product water for determining membrane selectivity. In order to check the permeability and selectivity of the entire membrane sheet, numerous test coupons (each coupon of 4.8 cm diameter) across the length and breadth of the membrane were tested in RO test kits. For PALS measurement, 22Na positron source in the form of aqueous solution of NaCl in kepton foil was sandwiched between the layers of polymer films and kept in-between two scintillation detectors. Positron annihilation lifetime measurements were carried out at room temperature and normal pressure using plastic scintillators coupled to fast–fast coincidence system with resolving time of ~ 230 ps as measured with 60Co source in 22Na energy window settings.
SEM images of the TFC membrane were taken to analyze microscopic structure of the membrane. Cross-section images were obtained after the cutting with a sharp razor blade or freeze-fracturing of the samples. Fig. 1(A) shows cross-sectional images at two different magnifications from the sample cut with a sharp razor blade. Clearly, the rough edge of uppermost polyamide layer is seen at different magnifications but clear distinction between the polyamide and underneath layer of polysulfone is not revealed. The pore structure of asymmetric polysulfone layer is distorted due to mechanical stress caused by the blade cutting. Fig. 1(B) shows the asymmetry of polysulfone — less porous near the surface formed by nodular dendritic structure and large macrovoids underneath. The polysulfone nodules are sub-micron sized and are tightly compacted near the surface. These polysulfone nodules near the surface are seen to be covered by the polyamide layer. The polyamide film penetration up to about 2–3 μm deep from the surface is visible, as shown by the sharp boundary between the polyamide– polysulfone intermixed layer and uncoated polysulfone dendritic sublayer. It was possible that initially the polyamide film penetrates between the polysulfone nodules and fills the voids of nodular polysulfone structure. In turn, the continuous polyamide layer was formed over this polyamide–polysulfone intermixed composite. As shown in Fig. 1(C), a closer SEM examination of the polyamide– polysulfone layer reveals that there is a layer of about 1 μm thickness at the top. At further higher magnification as shown in Fig. 1 4(D), it was revealed that this top layer of 1 μm in thickness was indeed comprised of outermost polyamide continuous film of about 0.2 μm in thickness and underneath polysulfone sub-micron sized nodules. The microscopic features of cross-sectional images among the samples taken from different areas of the membrane were found to be similar but a quantitative estimation of polyamide thickness variance among the samples was not possible by the SEM used.
3. Results and discussion
3.2. Atomic force microscopy Fig. 2(A) shows a surface AFM image [20 μm × 20 μm frame] of the RO TFC membrane. The surface morphology of the sample as revealed by the AFM image shows a typical nodular morphology of reverse osmosis polyamide membrane [18]. Analysis of the surface roughness by taking a simple section horizontally as shown in Fig. 2(B) shows a peak to peak distance in the range of 1–2 μm. It appears that there are smaller peaks within the micrometer sized peaks since the peaks are broad. The peak heights are in the range of 0.1 to 1 μm with maximum distribution of peaks of about 0.5 μm as shown in the peak height histogram (Fig. 2(C)). In order to further resolve the surface roughness parameters, the surface image is taken in 5 μm × 5 μm frame as shown in Fig. 3(A). It clearly shows the intergrown structures formed by the
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(D-1)
(D-2)
(C-1)
(B-1)
(A-1)
(C-2)
(B-2)
(B-3)
(A-2)
Fig. 1. SEM images of the membrane sample at various magnifications. (A) are cross-sectional images at two different magnifications from the sample cut with a sharp razor blade; (B), (C) and (D) are images from the samples prepared by freeze fracturing in liquid nitrogen.
individual peaks. The size of the individual peaks is about 1 μm as measured by the roughness analysis using the simple section horizontally (Fig. 3(B)). The peaks are broad and the numerous smaller peaks within the broad peak are seen clearer in the peak height histogram (Fig. 3(C)). In order to visualize the small peaks and their distribution in the broad peak of 1 μm, a scanning image in 1 μm × 1 μm
frame was taken as shown in Fig. 4(A). The roughness analysis using the simple section horizontally shows numerous peaks of sizes in the range of 5–150 nm (Fig. 4(B)) with majority of peaks in the range of 60–120 nm as evident from peak histogram shown in Fig. 4(C). The surface roughness analysis of this section clearly indicates that the broad peak indeed is compacted units comprised of numerous small
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C
C
B
B
A
A
Fig. 2. AFM image [20 μm × 20 μm frame] of the membrane surface. (A) 3-dimensional surface image; (B) analysis of the surface roughness by taking a simple section horizontally shown as a straight line in the image; (C) the peak height histogram of the image frame.
peaks of about 5–20 nm heights. The following statistical characteristics for the surfaces were estimated by roughness analysis of the AFM surface images. Zij = Z(Xi,Yj) is the source discrete function defined in the XY plane; Nx and Ny are X and Y dimensions. The mean value of peak to peak distance, the first moment of the distribution is expressed as N
μ=
Nx y 1 ∑ ∑ Zij : Nx :Ny j = 1 i = 1
ð1Þ
The average arithmetic roughness (average roughness) as Ny Nx 1 ∑ ∑ Zij −μ : Nx :Ny j = 1 i = 1 The root mean square roughness as
Sa =
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 u 1 Ny Nx ∑ ∑ Zij −μ Sq = t Nx :Ny j = 1 i = 1
81
Fig. 3. AFM image [5 μm × 5 μm frame] of the membrane surface. (A) 3-dimensional surface image; (B) analysis of the surface roughness by taking a simple section horizontally shown as a straight line in the image; (C) the peak height histogram of the image frame.
intergrown micrometer sized domains wherein each domain is comprised of nanometer sized blocks. 3.3. Small-angle neutron scattering The SANS measurements from the membrane film samples over the wave vector range (Q) of 0.018–0.35 Å− 1 were taken at room temperature over the period of 6–8 h to collect the statistically meaningful data for deriving structural information of polyamide units of RO TFC membrane. Throughout the data analysis, corrections were made for instrumental smearing. The scattered intensity is given by the general equation.
ð2Þ IðQ Þ = ϕP ðQ Þ:SðQ Þ
ð3Þ
These results of statistical analysis in three different image frames are summarized in Table 1. It clearly reveals that surface roughness parameter varies when the image analysis is taken at different scales. As discussed above, this implies that the membrane surface is of
ð4Þ
ϕ is the number density of scattering particle and Q is defined as Q = (4π/λ) sin θ, where λ is the wavelength and 2θ is scattering angle. The form factor P(Q) reflects the distribution of scattering material in the scattering particle, and the structure factor, S(Q) is related to the spatial distribution of the scattering particles in the surrounding medium such as solvent. The intensity of scattering from the top polyamide layer of RO TFC over the Q range 0.018–0.030 Å− 1 collected from the SANS instrument [36] at Dhruva reactor, Mumbai, is shown in Fig. 5. As shown in the
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C -1 Q = 0.057 Å
Intensity (cm-1)
d = 11 nm
B
1 9
polyamide of RO-TFC model fit
8 2
3
4
5
6 7 8 9
2
3
0.1 Q, Å-1
A Fig. 5. The intensity of neutron scattering from the top polyamide layer of RO TFC over the Q range 0.018–0.030 Å− 1.
scattering profile of Fig. 5, there is a correlation peak over the Q range 0.0505–0.0555 Å− 1 corresponding to the length scale (2π/Q) of 11– 12 nm (110–120 Å) as a result of somewhat orderly arrangement of repeating units of aggregated polyamide chains. The scattering data was treated using fits for a population of polyamide aggregated units, including structure factor arising from interactions between the units. There was a good agreement between the SANS data with the fits which suggests that these correlation peaks are due to the interparticle structure factor S(Q) generated by the interacting polyamide aggregated nanosized units which is in corroboration with the numerous small peaks of about 5–20 nm heights observed in AFM surface image of the sample. 3.4. Attenuated total reflectance infra-red spectroscopy The ATR-IR spectra of neat polysulfone and polyamide-polysulfone are given in Fig. 6. The spectra of polysulfone–polyamide reveal not only
Table 1 The peak to peak maximum distance (Sy), the mean value of peak to peak distance (first moment of distribution) (μ), the average arithmetic roughness (Sa), and the root mean square roughness (Sq) for the RO membrane. Image scan area
Sy (nm)
μ (nm)
Sa (nm)
Sq (nm)
20 μm × 20 μm 5 μm × 5 μm 1 μm × 1 μm
1267 864 146
507 387 89
138 116 20
177 146 25
Neat polysulfoone Polyamide-polysulfone
40x10-3
IR intensities (absorbance)
Fig. 4. AFM image [1 μm × 1 μm frame] of the membrane surface. (A) 3-dimensional surface image; (B) analysis of the surface roughness by taking a simple section horizontally shown as a straight line in the image; (C) the peak height histogram of the image frame.
bands ascribable to polyamide but also those due to the polysulfone sublayer. The polyamide bands at 1660 cm− 1 due to amide I (C= O stretch), the 1547 cm− 1 due to amide II (C–N stretch) and the 1609 cm− 1 due to polyamide aromatic ring breathing are easily distinguishable from the polysulfone bands in the IR spectra. This is understandable since the beam penetration depth exceeds the thickness of polyamide skin layer. Because of this top polyamide layer, the IR intensities of bands related to polysulfone sub-layer are relatively less intense in the spectrum of polyamide–polysulfone than that of neat polysulfone. From the intensity of characteristic IR 1660 cm− 1 band of polyamide group the thickness of polyamide layer deposited over polysulfone surface is estimated using Eqs. (2) and (3), as reported previously [13,15]. Numerous samples along the length of the membrane were taken for the estimation of film thickness by ATR-IR analysis. Fig. 7 shows ATR-IR spectra of these samples. Based on the absorbance values of the 1660 cm− 1 band, film thicknesses are computed as about 0.2 μm for majority of samples. We further observed in the ATR-IR spectra, that the absorbance values of 1660 cm− 1 do not correlate with the absorbance values of 1488 cm− 1 band. We have reported earlier [13] that in a system where there is significant penetration of polyamide deep inside the pores of polysulfone, then the laminated polyamide layer on top of
30
20
10
0 1000
1200
1400
1600
1800
2000
Frequency (cm-1) Fig. 6. The ATR-IR spectra of neat polysulfone and polyamide–polysulfone membranes.
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Table 2 The RO-TFC [11] membrane parameters given by the fit to the isopropanol and dioxane permeation data along with the diffusivities and Stokes radii of the isopropanol and dioxane.
IR intensities (absorbance)
80x10-3
60 MW (g/mol) Diffusivity (10− 10 m2/s) Stokes radius (nm) Solute permeability, P [10− 6 m s− 1] Reflection coefficient, σ Pore radius (nm) Pore number density (no. of pores per m3)
40
20
0 1200
1400
1600
Frequency (cm-1) Fig. 7. The ATR-IR spectra of numerous samples taken along the length of the membrane.
polysulfone is computed from the difference of 1488 cm− 1 band intensities of the neat polysulfone and polyamide coated polysulfone. In such system, the samples may have similar absorbance values of 1660 cm− 1 polyamide band, but different absorbance values of 1488 cm− 1 polysulfone band. The thickness of laminated polyamide layers deposited on top of polysulfone is computed as ranging from 0.13 to 0.20 μm whereas the penetrated continuous-polyamide layers in the pores of polysulfone sub-layer may be up to 0.11 μm. The variance in polyamide thickness in different areas of the membrane may lead to different salt/water permeation rates of sample test coupons. 3.5. Porosity measuring techniques The selectivity in pressure-driven transport process of the membranes can be related to the average volume flux over the membrane surface (Jv) and solute permeability (P) by the general equation derived by Spiegler and Kedem [37] as given below 1−σ Jv σ 1−exp − P ; R= 1−σ Jv 1−σ exp − P
n oi 16 2 h 2 2 η : ð1−ηÞ 2−ð1−ηÞ σ = 1− 1 + ; 9
ð5Þ
ð6Þ
r where η = s , rs is the Stokes radius and rp is the pore radius. The rp 16 2 η is the steric parameter related to wall correction term 1 + 9 factor and the term [(1 − η)2{2 − (1 − η)2}] is the distribution coefficient of solute by the steric-hindrance effect under convection condition [38]. The solute permeability is related to the membrane porosity (Ak, the total pore area divided by membrane surface area) and thickness (Δx) by the following expression. Ak Δx
Isopropanol
Dioxane
60 10.2 0.241 0.22 ± 0.02 0.80 ± 0.05 ~ 0.38 ~ 1022
88 9.1 0.234 0.20 ± 0.02 0.80 ± 0.05 ~ 0.38 ~ 1022
ments using aqueous solution containing dioxane or isopropanol of 500 ppm concentration at pressures ranging from 50 to 400 psig were performed to measure the pore characteristics of the RO TFC membrane. The membrane parameters (reflection coefficient, σ and solute permeability, P) are obtained by fitting the permeation data with Spiegler–Kadem model [37] with taking into account the steric parameter of wall correction factor and the distribution coefficient by the steric-hindrance effect [38–40]. These membrane parameters given by the fit to the isopropanol and dioxane permeation data along with the diffusivities and Stokes radii of the isopropanol and dioxane employed are tabulated in Table 2. The pore radius of the membrane is then calculated using the given solute radius (Stokes) and the σ value given by the fit to the model. The average pore radius for silica-free neat polyamide membrane is calculated as about 0.35 nm which is comparable with the radii for commercial polyamide membranes obtained by other pore analysis experiments [41]. From the solute permeability, P values obtained by the model fit to the organic permeation data, the ratio of membrane porosity to active layer thickness (Ak/Δx) is calculated. From the obtained ratio, the pore number density of the membrane is calculated using the estimated average pore radius and active layer thickness of 0.2 μm approximated by ATR-IR. The pore number densities of the samples are in the order of magnitude 1022 per m3 which is similar to the reported value for commercial polyamide thin film composite and cellulose acetate reverse osmosis membranes [7,42]. 3.6. Positron annihilation lifetime spectroscopy (PALS)
where σ is the reflection coefficient and expressed as
P = DS HD SD
83
ð7Þ
DS is the solute diffusivity; HD is the steric parameter related to wall correction factors under diffusion and taken as 1 and SD is the distribution coefficient of solute by the steric-hindrance effect under diffusion and expressed as SD = (1− η)2. Liquid permeation experi-
The PALS technique was also used to measure the pore size of the RO-TFC membrane. The rate of o-Ps pick-off annihilation as a consequence of the liberated photons from open spaces such as holes or voids for the sample is studied to estimate the size of the pores in the sample. So by measuring o-Ps pick-off lifetime (τp) one can find out the void size according to the Tao [43] and Eldrup et al. [44] semiempirical equation.
τp =
−1 1 R 1 2πR 1− + sin 2 R + ΔR 2π R + ΔR
ð8Þ
where an infinite spherical potential well of radius R0 = R + ΔR is considered wherein ΔR = 1.66 Å which is the thickness of the homogeneous electron layer inside the wall of free-volume void. The positrons and o-Ps are localized in pre-existing voids and free volumes in polymers. Therefore a change in measuring conditions may result into a small change in the pore size values of the samples. Particularly the RO polyamide membrane structure is of disordered network pores formed by the inter- and intra-chain spacing of the polymer that can be varied under the influence of temperature, pressure, aging, etc. The average void radius, R0 which is the total of electron layer thickness and free-hole radius obtained for the RO-TFC membrane [9] is 0.386 nm which is similar to the values obtained in the case of commercial FT-30 of FilmTec Corp [45].
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3.7. Quasi-elastic neutron scattering
SðQ; ωÞ = AðQ ÞδðωÞ + ½1−AðQ ÞLðω; ΓÞ
ð9Þ
4
liters/m2.h
The elastic component and Quasi-elastic (QE) contribution to the scattering intensity can be written as
2
10 4 2
1
41 meters 100
liters/m2.h
4
30 meters 100 4
0.16
Γ (meV)
0.12
0.08
0.04
1
2
10 4 2
1
15 meters 100
2
3
4
Q2 (Å-2) Fig. 8. The variation of half width at half maximum, Γ(Q) as a function of Q2 for the RO polyamide material.
4
liters/m2.h
The TFC membrane was evaluated for permeate flux and selectivity on a batch type RO test kit with NaCl solution of 2000 ppm concentration at a pressure of 250 psi. The distribution of permeate (salt and water) fluxes in different areas of membrane were done by measuring RO performance of numerous sample test coupons at different membrane lengths. Fig. 9 shows water and salt permeation rates of sample test coupons taken at membrane lengths of 2, 15, 30, and 41 m. Along the membrane lengths significant variances in permeate fluxes and salt rejection efficiencies were observed. The flux and selectivity variance of the different portions of the membrane may be related with the heterogeneity of polyamide film deposited on the polysulfone sub-layer. We further observed that even for the test
0
4 2
3.8. Salt and water permeability
0.00
2
10
1
liters/m2.h
where the first term represents the elastic component and the second term is the QE contribution. L(ω, Γ) is the Lorentzian function having half width at half maximum, Γ(Q) which is inversely proportional to the time scale of the motion. Presence of QE broadening is related to the existence of dynamical motion and the ratio of the elastic to total intensity known as elastic incoherent structure factor (EISF) provides information about the geometry of motion. QENS data from water sorbed in RO polyamide membrane material displayed QE broadening at all the Q values in the range 0.67–1.8 Å− 1 suggesting the presence of stochastic dynamics related to water molecules in the polyamide material [9]. The QE components were separated using Eq. (9) in which the elastic contribution was taken as that from the anhydrous dry polyamide sample. The variation of half width at half maximum, Γ(Q) as a function of Q2 is shown in Fig. 8. The variation of Γ(Q) with Q2 has a non-linear behavior for the water sorbed polyamide membrane material. This is an indication of motion by jumps and is termed as jump diffusion in which a molecule spends some time called residence time on a site before jumping to another site separated from the first by a distance called jump length. In the present case the jump lengths distribution is random as that of Singwi and Sjölander model [46] and the values of diffusion coefficient and the mean jump length for the water are respectively 1.9 ± 0.4 × 10− 5 cm2/s and 1.8 ± 0.5 Å.
water flux salt flux
2
10 4 2
1 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Sample test coupons (2 meters) Fig. 9. The water and salt permeation rates of sample test coupons taken along the membrane length.
sample coupons having same flux, salt rejection efficiency varies from 94% to 98.5% (Fig. 10). This may be due to the combined influence of surface properties and membrane network structure where there are contributions from solution-diffusion transport and pore flow in the permeates fluxes, and wherein the contribution of pore flow transport is quite significant in salt permeation rate but negligible in water permeation rate according to “solution diffusion imperfection” model [47,48]. This is quite possible as the polyamide network structure of the prepared TFC membrane may have a varied range of pore structures as the polyamide layer comprises of a cross-linked network of −CONH– linkages with the presence of some linear polymer chain network having pendant −COOH groups [15]. The linear polymer chain is widely reported to be formed due to partial hydrolysis of trimesoyl chloride during the condensation reaction between trimesoyl chloride and m-phenylene diamine. It has further been reported [49] that a totally linear network polyamide prepared from isophthaloyl chloride and m-phenylene diamine has salt rejection efficiency of only about 20% when tested under a RO testing conditions of 2000 ppm NaCl solution, 440 psi.
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for the samples measured at different pH values and at different KCl concentration are plotted in Fig. 11. As shown in Fig. 11, the membrane test coupon having 97% S/R and 60 LMH (l/m2 h) permeate flux had a higher negative potential (about −6 mV higher) than the test coupon having 92% S/R and 45 LMH. A normal trend observed in both the samples was increase in zeta-potential as pH increases. The isoelectric point for the 97% S/R 60 LMH sample was found to be at 2.8 pH while the 92% S/R and 45 LMH sample was found to be at 3.7 pH. This implies that the higher performing membrane has polyamide network structure with more of pendant COOH groups responsible for higher negative potential. Therefore, the COOH to CONH ratios of the polyamide membrane structure are mainly responsible for varied salt rejection efficiency.
90
4. Conclusions
100
S/R, %
98
96
94
40
50
60
70
80
2
Flux, liters/m .h Fig. 10. A plot of flux vs. salt rejection efficiency of the numerous sample test coupons.
3.9. Zeta-potential measurements of membrane surface The membrane surface properties are related to the membrane performances. In order to examine this, two membrane test coupons having different performances were selected for the zeta-potential measurements of the membrane surfaces. The zeta-potential values
A -10 97% SR, 60 LMH 92% SR, 45 LMH
Zeta potential (mV)
85
-15
-20
-25
pH = 7.86 -30 0.2
0.4
0.6
0.8
1.0
Using an indigenous casting and coating devices developed here at CSMCRI, we prepared a semi-large scale (1 m × 90 m, breadth × length) thin film composite reverse osmosis membrane. SEM images reveal the asymmetry nature of polysulfone—less porous near the surface resulting due to tightly compacted sub-micron sized nodular dendritic structure and large macrovoids underneath. It is evident from the SEM examination that polysulfone nodules near the surface are coated with polyamide film during interfacial polymerization and that the polyamide film initially fills the voids of nodular polysulfone structure. The polyamide film structure is formed by intergrown micrometer sized domains wherein each domain is comprised of nanometer sized blocks. The pore number densities of the samples are in the order of magnitude 1022 per m3 and the average void radius estimated using PALS from infinite spherical potential well of radius, R0 is 0.386 nm. This top polyamide film is of heterogeneous nature as revealed by ATR-IR spectra of samples taken from different areas of the membrane. The variance in polyamide active layer thickness in different areas of the membrane may therefore be correlated with varied product fluxes of sample test coupons, which are ranging from 34 to 82 l/m2 h under standard RO test condition of 2000 ppm NaCl and 250 psi pressure. The polyamide active layer structure may have a range of network structures with varied macromolecular structural units depending on the ratio of linear polymer chain network having pendant −COOH groups and crosslinked networks of −CONH– linkages, and that this structural variance in macromolecular chains mainly results in salt rejection variance from 94% to 98.5%. This result is in consistent with zeta-potential of membrane surface which shows a higher negative potential for the higher performing membrane that has polyamide network structure with more of pendant COOH group.
KCl Concentration, mM Acknowledgements
B 10
Zeta potential (mV)
We acknowledge Mr. V.J. Shah, Dr. P.K. Ghosh, Dr. S.V. Joshi, Dr. J.J. Trivedi, and Dr. C.V. Devmurari for guidance and encouragement of this work; Dr. V.K. Aswal, Dr. R. Mukhopadhyay, Mr. V. K. Sharma, Dr. S. Gautam, Dr. P. Maheshwari, and Dr. D. Dutta, for neutron and PALS experiments and fruitful discussions; Mr. H. Brahmbhatt, Mr. S. Gothwal, Mr. V. K. Agrawal and Mr. C.K. Chandrakant for TOC analysis, IR and SEM measurements of the samples. Financial assistance to carry out this work from the Council of Scientific & Industrial Research, India, Department of Science & Technology, India, and University Grants Commission-Department of Atomic Energy Consortium for Scientific Research, India is also gratefully acknowledged.
97% SR, 60 LMH 92% SR, 45 LMH
3.7 pH 0
-10
2.8 pH
-20
1mM KCl 2
4
6
8
10
References
pH Fig. 11. Zeta-potential values for the samples measured (A) at different pH values and (B) at different KCl concentration.
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Contents lists available at ScienceDirect
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
Techniques for characterization of polyamide thin film composite membranes P.S. Singh ⁎, A.P. Rao, P. Ray, A. Bhattacharya, K. Singh, N.K. Saha, A.V.R. Reddy RO Membrane Division, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial Research, Bhavnagar 364002, India
a r t i c l e
i n f o
Article history: Received 16 March 2011 Accepted 13 April 2011 Available online 11 May 2011 Keywords: TFC RO Polyamide Large-scale Preparation Characterization Performance
a b s t r a c t Thin film composite reverse osmosis (TFC RO) membranes (1 m breadth × 90 m length) were prepared by coating polyamide active layer over a porous polysulfone membrane using indigenous membrane casting and coating devices. The TFC membranes exhibited 56 ± 9 l/m2 h water flux and 96.7 ± 0.6% salt rejection when tested with 2000 ppm NaCl feed at 250 psi pressure. The membrane structural information obtained using SEM, AFM, SANS, PALS, QENS, permeation experiments with organic markers, ATR-IR, and zeta-potential measurements was correlated with the variance in membrane flux and selectivity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The characterization of membranes involves the study of both membrane materials and membrane processes. The fundamental understanding of the membrane materials are required to assess the membrane processes. While the characterization techniques based on the conventional analytical instruments are relatively simpler approaches, the observations of membrane process and transport phenomena are rather complex. Particularly, the processes of thinfilm composite (TFC) ‘reverse osmosis’ (RO) polyamide membrane deal with species of sub-nanometer size ranges and therefore one has to look in the scale of sub-nanometer to nanometer size range. This means that the scale of resolution of the observations should be in angstroms. Therefore, it has been very difficult to observe real-time experiments such as transport process of a permeating species moving in the subnanometer pores of the TFC RO membrane. Model-based approaches are the substitutes to understand the transport phenomena which however, often lead to contrasting views as the models are primarily based on assumptions and hypothesis. Therefore, other complementary techniques and methods are required to overcome the unwanted errors of analysis. Electrons, positrons, X-rays and neutrons are of angstrom scales and therefore the techniques using them can give resolutions of the observation in the angstrom scale. Characterization using these techniques is required to understand the polymer chain nanostructure as well as the pore structure characteristics of the membrane in order to improve the membrane performance.
⁎ Corresponding author. Tel.: + 91 278 2566511; fax: + 91 278 2567562. E-mail address: [email protected] (P.S. Singh). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.04.039
The TFC RO membrane is comprised of top ultra-thin skin polyamide layer, middle polysulfone porous support and bottom non-woven fabric and is extensively used for water desalination and other industrial applications [1–4]. The top polyamide layer is the selective active layer which is responsible for permeability and selectivity. The membrane porosity may be detrimental by the polymer chain nanostructure of the polyamide layer as the pores are arises from the spaces between the chain segments constituting the polymer network within each supramolecular polymer aggregate as well as the spaces between neighboring supramolecular polymer aggregates themselves [5–7]. The size of polymer aggregates are in nanometer length scale and highly compacted but can be loosened by solvent treatment [8]. The pores of the polyamide membranes are irregular in nature and are found to be non-interconnecting [9–10]. A change in membrane pore size is resulted when the polymer chain nanostructure is modified by the introduction of nanoparticles [11–12]. Various techniques have been used to characterize the TFC RO membrane. Attenuated total reflectance infrared (ATR-IR) spectroscopy [13–15] and atomic force microscopic (AFM) techniques [16–18] are non-destructive techniques which can measure the samples in their native form and therefore particularly suited to study the surfaces of the RO polyamide membranes. The surface roughness observed by AFM is an important parameter which is found to be directly related with the permeability of the membrane [19]. The nanoscale heterogeneity which is responsible for surface roughness (hill–valley nodular morphology) in the polyamide is clearly observed by transmission electron microscopy (TEM) studies [20]. 1H NMR and CP-MAS 13C NMR have been used to measure spin-relaxation time in the rotating frame for comparing the polymer chain mobility in the membrane structure that also has a direct relationship with the membrane flux [18,21]. The modification of molecular structure of the polyamide by incorporation of chemical functional groups [22–24]
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resulting to a change in the polyamide chain nanostructure also led to a change in the membrane performances. Studies of electron spin resonance (ESR) and theoretical computer calculation methods [25,26] corroborate that the polymer chain nanostructure is required to be of a uniform network structure for good membrane performances. The preparation of high-quality TFC RO membranes requires a systematic approach and characterization since the polymer film formation is rapid and delicate process with the film thickness is of submicron range. The lab-scale results are subject to various errors leading to difficulty in quantitative comparison and scale-up process [27,28]. Because of the difficulties involved in the membrane preparation only few companies are commercially successful and the detailed information of large-scale preparations is not disclosed. In this paper, we present characterization studies performed on the membranes for large-scale development of TFC RO membrane (1 m breadth × 90 m length) using our indigenous membrane casting and coating units [29–33].
Data analysis was carried out using PATFIT program [34]. Quasi-elastic neutron scattering measurements were carried out using the QENS spectrometer at Dhruva reactor, Trombay [35]. The spectrometer was used in multi angle reflecting crystal (MARX) mode, which essentially uses a combination of a large analyzer crystal and a position sensitive detector. This instrument has an energy resolution of 200 μeV with incident neutron energy of 5 meV. The quasielastic (QE) data were recorded in the wave vector transfer (Q) range of 0.67–1.8 Å− 1 at 300 K. QENS measurement was first carried out on the dehydrated membrane. The sample was then soaked in water and allowed to equilibrate. QENS data were then recorded from the water-sorbed sample. The SANS measurements from the membrane samples over the wave vector range (Q) of 0.018–0.35 Å− 1 were taken at room temperature or higher on the SANS instrument [36] at the Dhruva reactor, BARC, Mumbai, India.
2. Experimental
3.1. Scanning electron microscopy
Polysulfone casting solution was prepared by dissolving 15 wt.% Udel P-3500 PS (Solvay, USA) in A.R. grade N,N-dimethylformamide (DMF). The resultant polymer solution was cast on a non-woven fabric using a phase inversion process from a water bath consisting a fixed amount of DMF and 0.1 wt.% sodium lauryl sulfate (SLS) surfactant. The resulting polysulfone membrane was washed thoroughly with distilled water. Typically, TFC membrane was prepared by initially immersing the polysulfone membrane support in 2% (w/v) solution of mphenylenediamine (MPD, Lancaster Chemical Co.) in water, followed by immersing into n-hexane solution of 0.1% (w/v) trimesoyl chloride (TMC, Lancaster Chemical Co.) which resulted in the lamination of an ultra-thin polyamide film over the surface of polysulfone support. The nascent composite membrane was subsequently cured at around 70 °C and then washed thoroughly to remove unreacted diamine. SEM pictures of the membrane samples were taken on LEO 1430VP environmental scanning electron microscope with 5–15 kV accelerating voltage. AFM images of the samples were acquired using NT-MDT AFM instrument. Samples were freeze-fractured to view crosssectional image. The ATR-IR instrument used consisted of a Barnes model 300 continuously variable ATR accessory interfaced to a Nicolet 5DX Fourier transform infrared spectrometer. For ATR-IR studies of the membrane samples, germanium crystal was fixed at 45 °C angle of incidence, that gave a probing depth of about 0.4 μm in the infrared region of interest. The amount of contact between the sample and crystal is same in all samples. Dry specimens of membrane samples were precisely cut to ATR crystal size and mounted on both faces of germanium crystal with the active layer facing the crystal surface. Zeta potentials of the membrane surfaces were measured by ZETA-CAD instrument using 0.01 M aqueous KCl solution. TFC membrane sample test coupons were evaluated for permeate flux and selectivity on a batch type RO test kit comprising four cells in series. Testing was done with NaCl solution of 2000 ppm concentration at a pressure of 250 psi. A standardized conductivity meter was used to measure the salt (NaCl) concentrations in the feed and product water for determining membrane selectivity. In order to check the permeability and selectivity of the entire membrane sheet, numerous test coupons (each coupon of 4.8 cm diameter) across the length and breadth of the membrane were tested in RO test kits. For PALS measurement, 22Na positron source in the form of aqueous solution of NaCl in kepton foil was sandwiched between the layers of polymer films and kept in-between two scintillation detectors. Positron annihilation lifetime measurements were carried out at room temperature and normal pressure using plastic scintillators coupled to fast–fast coincidence system with resolving time of ~ 230 ps as measured with 60Co source in 22Na energy window settings.
SEM images of the TFC membrane were taken to analyze microscopic structure of the membrane. Cross-section images were obtained after the cutting with a sharp razor blade or freeze-fracturing of the samples. Fig. 1(A) shows cross-sectional images at two different magnifications from the sample cut with a sharp razor blade. Clearly, the rough edge of uppermost polyamide layer is seen at different magnifications but clear distinction between the polyamide and underneath layer of polysulfone is not revealed. The pore structure of asymmetric polysulfone layer is distorted due to mechanical stress caused by the blade cutting. Fig. 1(B) shows the asymmetry of polysulfone — less porous near the surface formed by nodular dendritic structure and large macrovoids underneath. The polysulfone nodules are sub-micron sized and are tightly compacted near the surface. These polysulfone nodules near the surface are seen to be covered by the polyamide layer. The polyamide film penetration up to about 2–3 μm deep from the surface is visible, as shown by the sharp boundary between the polyamide– polysulfone intermixed layer and uncoated polysulfone dendritic sublayer. It was possible that initially the polyamide film penetrates between the polysulfone nodules and fills the voids of nodular polysulfone structure. In turn, the continuous polyamide layer was formed over this polyamide–polysulfone intermixed composite. As shown in Fig. 1(C), a closer SEM examination of the polyamide– polysulfone layer reveals that there is a layer of about 1 μm thickness at the top. At further higher magnification as shown in Fig. 1 4(D), it was revealed that this top layer of 1 μm in thickness was indeed comprised of outermost polyamide continuous film of about 0.2 μm in thickness and underneath polysulfone sub-micron sized nodules. The microscopic features of cross-sectional images among the samples taken from different areas of the membrane were found to be similar but a quantitative estimation of polyamide thickness variance among the samples was not possible by the SEM used.
3. Results and discussion
3.2. Atomic force microscopy Fig. 2(A) shows a surface AFM image [20 μm × 20 μm frame] of the RO TFC membrane. The surface morphology of the sample as revealed by the AFM image shows a typical nodular morphology of reverse osmosis polyamide membrane [18]. Analysis of the surface roughness by taking a simple section horizontally as shown in Fig. 2(B) shows a peak to peak distance in the range of 1–2 μm. It appears that there are smaller peaks within the micrometer sized peaks since the peaks are broad. The peak heights are in the range of 0.1 to 1 μm with maximum distribution of peaks of about 0.5 μm as shown in the peak height histogram (Fig. 2(C)). In order to further resolve the surface roughness parameters, the surface image is taken in 5 μm × 5 μm frame as shown in Fig. 3(A). It clearly shows the intergrown structures formed by the
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(D-1)
(D-2)
(C-1)
(B-1)
(A-1)
(C-2)
(B-2)
(B-3)
(A-2)
Fig. 1. SEM images of the membrane sample at various magnifications. (A) are cross-sectional images at two different magnifications from the sample cut with a sharp razor blade; (B), (C) and (D) are images from the samples prepared by freeze fracturing in liquid nitrogen.
individual peaks. The size of the individual peaks is about 1 μm as measured by the roughness analysis using the simple section horizontally (Fig. 3(B)). The peaks are broad and the numerous smaller peaks within the broad peak are seen clearer in the peak height histogram (Fig. 3(C)). In order to visualize the small peaks and their distribution in the broad peak of 1 μm, a scanning image in 1 μm × 1 μm
frame was taken as shown in Fig. 4(A). The roughness analysis using the simple section horizontally shows numerous peaks of sizes in the range of 5–150 nm (Fig. 4(B)) with majority of peaks in the range of 60–120 nm as evident from peak histogram shown in Fig. 4(C). The surface roughness analysis of this section clearly indicates that the broad peak indeed is compacted units comprised of numerous small
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C
C
B
B
A
A
Fig. 2. AFM image [20 μm × 20 μm frame] of the membrane surface. (A) 3-dimensional surface image; (B) analysis of the surface roughness by taking a simple section horizontally shown as a straight line in the image; (C) the peak height histogram of the image frame.
peaks of about 5–20 nm heights. The following statistical characteristics for the surfaces were estimated by roughness analysis of the AFM surface images. Zij = Z(Xi,Yj) is the source discrete function defined in the XY plane; Nx and Ny are X and Y dimensions. The mean value of peak to peak distance, the first moment of the distribution is expressed as N
μ=
Nx y 1 ∑ ∑ Zij : Nx :Ny j = 1 i = 1
ð1Þ
The average arithmetic roughness (average roughness) as Ny Nx 1 ∑ ∑ Zij −μ : Nx :Ny j = 1 i = 1 The root mean square roughness as
Sa =
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 u 1 Ny Nx ∑ ∑ Zij −μ Sq = t Nx :Ny j = 1 i = 1
81
Fig. 3. AFM image [5 μm × 5 μm frame] of the membrane surface. (A) 3-dimensional surface image; (B) analysis of the surface roughness by taking a simple section horizontally shown as a straight line in the image; (C) the peak height histogram of the image frame.
intergrown micrometer sized domains wherein each domain is comprised of nanometer sized blocks. 3.3. Small-angle neutron scattering The SANS measurements from the membrane film samples over the wave vector range (Q) of 0.018–0.35 Å− 1 were taken at room temperature over the period of 6–8 h to collect the statistically meaningful data for deriving structural information of polyamide units of RO TFC membrane. Throughout the data analysis, corrections were made for instrumental smearing. The scattered intensity is given by the general equation.
ð2Þ IðQ Þ = ϕP ðQ Þ:SðQ Þ
ð3Þ
These results of statistical analysis in three different image frames are summarized in Table 1. It clearly reveals that surface roughness parameter varies when the image analysis is taken at different scales. As discussed above, this implies that the membrane surface is of
ð4Þ
ϕ is the number density of scattering particle and Q is defined as Q = (4π/λ) sin θ, where λ is the wavelength and 2θ is scattering angle. The form factor P(Q) reflects the distribution of scattering material in the scattering particle, and the structure factor, S(Q) is related to the spatial distribution of the scattering particles in the surrounding medium such as solvent. The intensity of scattering from the top polyamide layer of RO TFC over the Q range 0.018–0.030 Å− 1 collected from the SANS instrument [36] at Dhruva reactor, Mumbai, is shown in Fig. 5. As shown in the
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C -1 Q = 0.057 Å
Intensity (cm-1)
d = 11 nm
B
1 9
polyamide of RO-TFC model fit
8 2
3
4
5
6 7 8 9
2
3
0.1 Q, Å-1
A Fig. 5. The intensity of neutron scattering from the top polyamide layer of RO TFC over the Q range 0.018–0.030 Å− 1.
scattering profile of Fig. 5, there is a correlation peak over the Q range 0.0505–0.0555 Å− 1 corresponding to the length scale (2π/Q) of 11– 12 nm (110–120 Å) as a result of somewhat orderly arrangement of repeating units of aggregated polyamide chains. The scattering data was treated using fits for a population of polyamide aggregated units, including structure factor arising from interactions between the units. There was a good agreement between the SANS data with the fits which suggests that these correlation peaks are due to the interparticle structure factor S(Q) generated by the interacting polyamide aggregated nanosized units which is in corroboration with the numerous small peaks of about 5–20 nm heights observed in AFM surface image of the sample. 3.4. Attenuated total reflectance infra-red spectroscopy The ATR-IR spectra of neat polysulfone and polyamide-polysulfone are given in Fig. 6. The spectra of polysulfone–polyamide reveal not only
Table 1 The peak to peak maximum distance (Sy), the mean value of peak to peak distance (first moment of distribution) (μ), the average arithmetic roughness (Sa), and the root mean square roughness (Sq) for the RO membrane. Image scan area
Sy (nm)
μ (nm)
Sa (nm)
Sq (nm)
20 μm × 20 μm 5 μm × 5 μm 1 μm × 1 μm
1267 864 146
507 387 89
138 116 20
177 146 25
Neat polysulfoone Polyamide-polysulfone
40x10-3
IR intensities (absorbance)
Fig. 4. AFM image [1 μm × 1 μm frame] of the membrane surface. (A) 3-dimensional surface image; (B) analysis of the surface roughness by taking a simple section horizontally shown as a straight line in the image; (C) the peak height histogram of the image frame.
bands ascribable to polyamide but also those due to the polysulfone sublayer. The polyamide bands at 1660 cm− 1 due to amide I (C= O stretch), the 1547 cm− 1 due to amide II (C–N stretch) and the 1609 cm− 1 due to polyamide aromatic ring breathing are easily distinguishable from the polysulfone bands in the IR spectra. This is understandable since the beam penetration depth exceeds the thickness of polyamide skin layer. Because of this top polyamide layer, the IR intensities of bands related to polysulfone sub-layer are relatively less intense in the spectrum of polyamide–polysulfone than that of neat polysulfone. From the intensity of characteristic IR 1660 cm− 1 band of polyamide group the thickness of polyamide layer deposited over polysulfone surface is estimated using Eqs. (2) and (3), as reported previously [13,15]. Numerous samples along the length of the membrane were taken for the estimation of film thickness by ATR-IR analysis. Fig. 7 shows ATR-IR spectra of these samples. Based on the absorbance values of the 1660 cm− 1 band, film thicknesses are computed as about 0.2 μm for majority of samples. We further observed in the ATR-IR spectra, that the absorbance values of 1660 cm− 1 do not correlate with the absorbance values of 1488 cm− 1 band. We have reported earlier [13] that in a system where there is significant penetration of polyamide deep inside the pores of polysulfone, then the laminated polyamide layer on top of
30
20
10
0 1000
1200
1400
1600
1800
2000
Frequency (cm-1) Fig. 6. The ATR-IR spectra of neat polysulfone and polyamide–polysulfone membranes.
P.S. Singh et al. / Desalination 282 (2011) 78–86
Table 2 The RO-TFC [11] membrane parameters given by the fit to the isopropanol and dioxane permeation data along with the diffusivities and Stokes radii of the isopropanol and dioxane.
IR intensities (absorbance)
80x10-3
60 MW (g/mol) Diffusivity (10− 10 m2/s) Stokes radius (nm) Solute permeability, P [10− 6 m s− 1] Reflection coefficient, σ Pore radius (nm) Pore number density (no. of pores per m3)
40
20
0 1200
1400
1600
Frequency (cm-1) Fig. 7. The ATR-IR spectra of numerous samples taken along the length of the membrane.
polysulfone is computed from the difference of 1488 cm− 1 band intensities of the neat polysulfone and polyamide coated polysulfone. In such system, the samples may have similar absorbance values of 1660 cm− 1 polyamide band, but different absorbance values of 1488 cm− 1 polysulfone band. The thickness of laminated polyamide layers deposited on top of polysulfone is computed as ranging from 0.13 to 0.20 μm whereas the penetrated continuous-polyamide layers in the pores of polysulfone sub-layer may be up to 0.11 μm. The variance in polyamide thickness in different areas of the membrane may lead to different salt/water permeation rates of sample test coupons. 3.5. Porosity measuring techniques The selectivity in pressure-driven transport process of the membranes can be related to the average volume flux over the membrane surface (Jv) and solute permeability (P) by the general equation derived by Spiegler and Kedem [37] as given below 1−σ Jv σ 1−exp − P ; R= 1−σ Jv 1−σ exp − P
n oi 16 2 h 2 2 η : ð1−ηÞ 2−ð1−ηÞ σ = 1− 1 + ; 9
ð5Þ
ð6Þ
r where η = s , rs is the Stokes radius and rp is the pore radius. The rp 16 2 η is the steric parameter related to wall correction term 1 + 9 factor and the term [(1 − η)2{2 − (1 − η)2}] is the distribution coefficient of solute by the steric-hindrance effect under convection condition [38]. The solute permeability is related to the membrane porosity (Ak, the total pore area divided by membrane surface area) and thickness (Δx) by the following expression. Ak Δx
Isopropanol
Dioxane
60 10.2 0.241 0.22 ± 0.02 0.80 ± 0.05 ~ 0.38 ~ 1022
88 9.1 0.234 0.20 ± 0.02 0.80 ± 0.05 ~ 0.38 ~ 1022
ments using aqueous solution containing dioxane or isopropanol of 500 ppm concentration at pressures ranging from 50 to 400 psig were performed to measure the pore characteristics of the RO TFC membrane. The membrane parameters (reflection coefficient, σ and solute permeability, P) are obtained by fitting the permeation data with Spiegler–Kadem model [37] with taking into account the steric parameter of wall correction factor and the distribution coefficient by the steric-hindrance effect [38–40]. These membrane parameters given by the fit to the isopropanol and dioxane permeation data along with the diffusivities and Stokes radii of the isopropanol and dioxane employed are tabulated in Table 2. The pore radius of the membrane is then calculated using the given solute radius (Stokes) and the σ value given by the fit to the model. The average pore radius for silica-free neat polyamide membrane is calculated as about 0.35 nm which is comparable with the radii for commercial polyamide membranes obtained by other pore analysis experiments [41]. From the solute permeability, P values obtained by the model fit to the organic permeation data, the ratio of membrane porosity to active layer thickness (Ak/Δx) is calculated. From the obtained ratio, the pore number density of the membrane is calculated using the estimated average pore radius and active layer thickness of 0.2 μm approximated by ATR-IR. The pore number densities of the samples are in the order of magnitude 1022 per m3 which is similar to the reported value for commercial polyamide thin film composite and cellulose acetate reverse osmosis membranes [7,42]. 3.6. Positron annihilation lifetime spectroscopy (PALS)
where σ is the reflection coefficient and expressed as
P = DS HD SD
83
ð7Þ
DS is the solute diffusivity; HD is the steric parameter related to wall correction factors under diffusion and taken as 1 and SD is the distribution coefficient of solute by the steric-hindrance effect under diffusion and expressed as SD = (1− η)2. Liquid permeation experi-
The PALS technique was also used to measure the pore size of the RO-TFC membrane. The rate of o-Ps pick-off annihilation as a consequence of the liberated photons from open spaces such as holes or voids for the sample is studied to estimate the size of the pores in the sample. So by measuring o-Ps pick-off lifetime (τp) one can find out the void size according to the Tao [43] and Eldrup et al. [44] semiempirical equation.
τp =
−1 1 R 1 2πR 1− + sin 2 R + ΔR 2π R + ΔR
ð8Þ
where an infinite spherical potential well of radius R0 = R + ΔR is considered wherein ΔR = 1.66 Å which is the thickness of the homogeneous electron layer inside the wall of free-volume void. The positrons and o-Ps are localized in pre-existing voids and free volumes in polymers. Therefore a change in measuring conditions may result into a small change in the pore size values of the samples. Particularly the RO polyamide membrane structure is of disordered network pores formed by the inter- and intra-chain spacing of the polymer that can be varied under the influence of temperature, pressure, aging, etc. The average void radius, R0 which is the total of electron layer thickness and free-hole radius obtained for the RO-TFC membrane [9] is 0.386 nm which is similar to the values obtained in the case of commercial FT-30 of FilmTec Corp [45].
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P.S. Singh et al. / Desalination 282 (2011) 78–86
100
3.7. Quasi-elastic neutron scattering
SðQ; ωÞ = AðQ ÞδðωÞ + ½1−AðQ ÞLðω; ΓÞ
ð9Þ
4
liters/m2.h
The elastic component and Quasi-elastic (QE) contribution to the scattering intensity can be written as
2
10 4 2
1
41 meters 100
liters/m2.h
4
30 meters 100 4
0.16
Γ (meV)
0.12
0.08
0.04
1
2
10 4 2
1
15 meters 100
2
3
4
Q2 (Å-2) Fig. 8. The variation of half width at half maximum, Γ(Q) as a function of Q2 for the RO polyamide material.
4
liters/m2.h
The TFC membrane was evaluated for permeate flux and selectivity on a batch type RO test kit with NaCl solution of 2000 ppm concentration at a pressure of 250 psi. The distribution of permeate (salt and water) fluxes in different areas of membrane were done by measuring RO performance of numerous sample test coupons at different membrane lengths. Fig. 9 shows water and salt permeation rates of sample test coupons taken at membrane lengths of 2, 15, 30, and 41 m. Along the membrane lengths significant variances in permeate fluxes and salt rejection efficiencies were observed. The flux and selectivity variance of the different portions of the membrane may be related with the heterogeneity of polyamide film deposited on the polysulfone sub-layer. We further observed that even for the test
0
4 2
3.8. Salt and water permeability
0.00
2
10
1
liters/m2.h
where the first term represents the elastic component and the second term is the QE contribution. L(ω, Γ) is the Lorentzian function having half width at half maximum, Γ(Q) which is inversely proportional to the time scale of the motion. Presence of QE broadening is related to the existence of dynamical motion and the ratio of the elastic to total intensity known as elastic incoherent structure factor (EISF) provides information about the geometry of motion. QENS data from water sorbed in RO polyamide membrane material displayed QE broadening at all the Q values in the range 0.67–1.8 Å− 1 suggesting the presence of stochastic dynamics related to water molecules in the polyamide material [9]. The QE components were separated using Eq. (9) in which the elastic contribution was taken as that from the anhydrous dry polyamide sample. The variation of half width at half maximum, Γ(Q) as a function of Q2 is shown in Fig. 8. The variation of Γ(Q) with Q2 has a non-linear behavior for the water sorbed polyamide membrane material. This is an indication of motion by jumps and is termed as jump diffusion in which a molecule spends some time called residence time on a site before jumping to another site separated from the first by a distance called jump length. In the present case the jump lengths distribution is random as that of Singwi and Sjölander model [46] and the values of diffusion coefficient and the mean jump length for the water are respectively 1.9 ± 0.4 × 10− 5 cm2/s and 1.8 ± 0.5 Å.
water flux salt flux
2
10 4 2
1 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Sample test coupons (2 meters) Fig. 9. The water and salt permeation rates of sample test coupons taken along the membrane length.
sample coupons having same flux, salt rejection efficiency varies from 94% to 98.5% (Fig. 10). This may be due to the combined influence of surface properties and membrane network structure where there are contributions from solution-diffusion transport and pore flow in the permeates fluxes, and wherein the contribution of pore flow transport is quite significant in salt permeation rate but negligible in water permeation rate according to “solution diffusion imperfection” model [47,48]. This is quite possible as the polyamide network structure of the prepared TFC membrane may have a varied range of pore structures as the polyamide layer comprises of a cross-linked network of −CONH– linkages with the presence of some linear polymer chain network having pendant −COOH groups [15]. The linear polymer chain is widely reported to be formed due to partial hydrolysis of trimesoyl chloride during the condensation reaction between trimesoyl chloride and m-phenylene diamine. It has further been reported [49] that a totally linear network polyamide prepared from isophthaloyl chloride and m-phenylene diamine has salt rejection efficiency of only about 20% when tested under a RO testing conditions of 2000 ppm NaCl solution, 440 psi.
P.S. Singh et al. / Desalination 282 (2011) 78–86
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for the samples measured at different pH values and at different KCl concentration are plotted in Fig. 11. As shown in Fig. 11, the membrane test coupon having 97% S/R and 60 LMH (l/m2 h) permeate flux had a higher negative potential (about −6 mV higher) than the test coupon having 92% S/R and 45 LMH. A normal trend observed in both the samples was increase in zeta-potential as pH increases. The isoelectric point for the 97% S/R 60 LMH sample was found to be at 2.8 pH while the 92% S/R and 45 LMH sample was found to be at 3.7 pH. This implies that the higher performing membrane has polyamide network structure with more of pendant COOH groups responsible for higher negative potential. Therefore, the COOH to CONH ratios of the polyamide membrane structure are mainly responsible for varied salt rejection efficiency.
90
4. Conclusions
100
S/R, %
98
96
94
40
50
60
70
80
2
Flux, liters/m .h Fig. 10. A plot of flux vs. salt rejection efficiency of the numerous sample test coupons.
3.9. Zeta-potential measurements of membrane surface The membrane surface properties are related to the membrane performances. In order to examine this, two membrane test coupons having different performances were selected for the zeta-potential measurements of the membrane surfaces. The zeta-potential values
A -10 97% SR, 60 LMH 92% SR, 45 LMH
Zeta potential (mV)
85
-15
-20
-25
pH = 7.86 -30 0.2
0.4
0.6
0.8
1.0
Using an indigenous casting and coating devices developed here at CSMCRI, we prepared a semi-large scale (1 m × 90 m, breadth × length) thin film composite reverse osmosis membrane. SEM images reveal the asymmetry nature of polysulfone—less porous near the surface resulting due to tightly compacted sub-micron sized nodular dendritic structure and large macrovoids underneath. It is evident from the SEM examination that polysulfone nodules near the surface are coated with polyamide film during interfacial polymerization and that the polyamide film initially fills the voids of nodular polysulfone structure. The polyamide film structure is formed by intergrown micrometer sized domains wherein each domain is comprised of nanometer sized blocks. The pore number densities of the samples are in the order of magnitude 1022 per m3 and the average void radius estimated using PALS from infinite spherical potential well of radius, R0 is 0.386 nm. This top polyamide film is of heterogeneous nature as revealed by ATR-IR spectra of samples taken from different areas of the membrane. The variance in polyamide active layer thickness in different areas of the membrane may therefore be correlated with varied product fluxes of sample test coupons, which are ranging from 34 to 82 l/m2 h under standard RO test condition of 2000 ppm NaCl and 250 psi pressure. The polyamide active layer structure may have a range of network structures with varied macromolecular structural units depending on the ratio of linear polymer chain network having pendant −COOH groups and crosslinked networks of −CONH– linkages, and that this structural variance in macromolecular chains mainly results in salt rejection variance from 94% to 98.5%. This result is in consistent with zeta-potential of membrane surface which shows a higher negative potential for the higher performing membrane that has polyamide network structure with more of pendant COOH group.
KCl Concentration, mM Acknowledgements
B 10
Zeta potential (mV)
We acknowledge Mr. V.J. Shah, Dr. P.K. Ghosh, Dr. S.V. Joshi, Dr. J.J. Trivedi, and Dr. C.V. Devmurari for guidance and encouragement of this work; Dr. V.K. Aswal, Dr. R. Mukhopadhyay, Mr. V. K. Sharma, Dr. S. Gautam, Dr. P. Maheshwari, and Dr. D. Dutta, for neutron and PALS experiments and fruitful discussions; Mr. H. Brahmbhatt, Mr. S. Gothwal, Mr. V. K. Agrawal and Mr. C.K. Chandrakant for TOC analysis, IR and SEM measurements of the samples. Financial assistance to carry out this work from the Council of Scientific & Industrial Research, India, Department of Science & Technology, India, and University Grants Commission-Department of Atomic Energy Consortium for Scientific Research, India is also gratefully acknowledged.
97% SR, 60 LMH 92% SR, 45 LMH
3.7 pH 0
-10
2.8 pH
-20
1mM KCl 2
4
6
8
10
References
pH Fig. 11. Zeta-potential values for the samples measured (A) at different pH values and (B) at different KCl concentration.
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