- Email: [email protected]
Chemical treatment for improved performance of reverse osmosis membranes
DESALINATION ELSEVIER
Desalination 104 (I 996) 239-249
Chemical treatment for improved performance of reverse osmosis membranes Debabrata Mukherjee*, Ashish Kulkarni, William N. Gill'* Isermann Department of Chemical Engineering, Rennselaer Polytechnic Institute, Troy, NY 12180-3590, USA
Received 14 May 1995; accepted 31 July 1995
Abstract
A novel method of chemical treatment is described which causes a simultaneous improvement of flux and rejection of thin-film composite (TFC) reverse osmosis (RO) membranes. The water flux of CPA2 and SWC1 which are commercially important aromatic polyamide based TFC RO membranes has been shown to be increased significantly, without any loss of ion rejection, on short-term treatment with acids such as hydrofluoric and fluosilicic and alcohols like isopropanol. In most cases simultaneous increases in flux and rejection have been observed. Surface characterization studies by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and contact angle measurements reveal that limited hydrolysis and fluorination of the membrane skin seem to be responsible for such improvement in the membrane flux and rejection on treatment with hydrofluoric and fluosilicic acids. With isopropyl alcohol, partial dissolution coupled with surface tension driven collapse of pores in the membrane skin seem to be key factors which cause a simultaneous increase in flux and rejection. Therefore enhancement by short-term chemical treatment may be a very useful approach for the future development of RO membranes having higher flux and rejection. Keywords:Thin-film composite reverse osmosis membranes; Flux enhancement; Hydrofluoric acid; lsopropyl alcohol; Surface modification
1. Introduction The development of the wide array of polymers for membranes [1] has given rise to a large
range of applications from waste water treatment to gas separations. However, relatively few of these polymers are useful for RO applications [1,2]. The polymers that have shown promise for RO separations include cellulose
*Present address: Schoeller Technical Papers I n c . , Pulaski, NY 13142, USA. **Corresponding author,
acetate, polyethylenimine, polyepiamine, polyureas, aliphatic and aromatic polyamides, and polyhydrazide among others. O f these, cross-
0011-9164/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PII S001 1-9164(96)00047-1
240
D. Mukherjee et al. / Desalination 104 (1996) 239-249
linked aromatic polyamides have found extensive commercial usage as the barrier layers of RO membranes. TFCL (UOP), CPA2 ( Hydranautics), FT-30 (Filmtec) and ACM1 (Tricep) are important commercially for RO, and they are based on aromatic polyamide chemistry [2]. The barrier layers of these membranes are formed from interfacial polymerization of 1,3benzene diamines with either isophthaloyl or trimesoyl chloride (or their blends) [2-6]. Continuing efforts have been made to improve the performance of TFC membranes which currently are state-of-the-art. No new dramatically better polymeric materials for the RO membrane skin have appeared in the last decade [2,7] since the development of the aromatic polyamide based FT-30 TFC membranes, The major emphasis now seems to be on the surface modification of the polymeric structure of the skin which plays the most important role in determining the flux and rejection of composite membranes. This has given rise to an increasing number of patents and papers on surface modification of the membranes which enhance their performance. However, most of the work on RO membrane surface modification has reported an increase of either flux with a loss in rejection or vice versa. Efforts to improve the flux with no loss, or an increase, in rejection of the aromatic polyamide membranes have failed mainly due to the conflicting surface requirements of the membrane skin in maintaining both high flux and rejection, 2. Surface modification of membranes Much literature has been published recently on the surface modification of microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and gas separation membranes [8-21]. Plasma reatment [8,9], grafting [ 10,11 ], surface coating and curing [12] and UV irradiation [13] are among the important techniques used for surface modification of MF, UF and RO membranes, Chemical post-treatment by hydrophilicity in-
creasing agents [13,14], protic acids [14-17] and inorganic salts [17] have also been reported for MF, UF and RO membranes. Dimov et. al. [14] have reported hydrophilization with a temporary increase in flux in polyethylene microfiltration membranes by using a ternary mixture of ethanol-water-inorganic acids. It was found that the hydrophilization efficiency of the inorganic acid decreases in the order HNO3 >H2SO4>H3PO 4. Similar flux enhancements by chlorosulfonic acid for polyethylene microfiltration also have been reported [15]. It was concluded that a low concentration of the sulfonating agent and low reaction temperature should be used to perform selectively the sulfonation of the membrane surface and the inner walls of the pore and minimize the destruction of the bulk of the membrane. Higuchi et. al. [16] also found that immersing modified polysulfone UF hollow fiber membranes in HC1 caused an increase in rejection with no change in flux. However, the results of Sano et. al. [ 17] indicate that acrylonitrile-based RO membranes exhibit an increase in flux with no change in rejection properties on dipping such membranes in protic acids (HC1, HNO3, H2SO 4, etc.) and/or inorganic salts (CuSO4, etc.). Literature also exists on surface modification of gas separation membranes and a few methods deserve special mention in the context of the present work [18-21]. Fluorination of gas separation membranes by elemental fluorine gas has been shown to cause an increase in selectivity with reduced permeation [18-20]. Also studied was the effect of mild solvent posttreatment with a variety of vapors and liquids (dichloromethane, cyclohexane and water) on the transport behavior of various gas separation membranes like dry/wet phase inverted asymmetric polysulfone, spin-coated poly(phenylene oxide)-ceramic composites and solution-deposited polyimide-ceramic composite membranes [21 ]. It was observed again that such treatments caused a permanent increase in selectivity of the defective, asymmetric membranes but at the expense of a decrease in permeation.
241
D. Mukherjee et al. /Desalination 104 (1996) 239-249
Our present work shows that it is possible to simultaneously increase both the flux and rejec-
AromaticPolylmin* + PolyfuacUoail Acyl HIIMe O,34anzenedblmine) (BlendofI~ahthtla# and Trlmetmyl chl~ide)
tion of the fully aromatic polyamide-based RO membranes, which are the most important membranes available commercially, after treatment with solutions of hydrofluoric acid (HF), fluosilicic acid (FSA) and isopropyl alcohol (IPA). Both HF and IPA are used extensively in the microelectronics industry where HF etches silicon dioxide based films and IPA cleans wafers. Our efforts to reprocess HF with RO membranes [22,23] led to the discovery of a very large improvement in the transport rates obtained by SW30HR membranes [24]. This work shows that the technique of short-term chemical treatment using these chemicals can be effectively used for the permanent simultaneous improvement of both the membrane flux and ion rejection of other commercially important RO membranes,
+ Aml,,S,lt (q. Trlmethylamine slit of camphorsutfonlc°rid)
3. Experimental 3.1. Membrane CPA2 and SWC1 are two types of the commercially available thin film composite (TFC) RO membranes manufactured by Hydranautics which are suitable for low and high pressure operation, respectively. The formation of the barrier layer of these membranes is shown schematically in Fig 1 [2,4] where one sees that it is essentially a copolyamide derived from the reaction of 1,3-benzenediamine with a blend of isothaloyl and trimesoyl chlorides. The chemistry of the membrane is thus very similar to the FT-30 manufactured by Filmtec except for the presence of the amine salt as an additive in the formulation. One purpose of the amine salt is to reduce the loss in membrane flux due to heatannealing of the membrane skin during menufacture [2].
3.2. Experimental set-up and design Our experimental set-up was a single cell, flat sheet, closed loop recycle system as shown
_ A,~,tl~P,~°m~, (a|rrler Layer)
Po,t-~,~tl.~ o,y~ng Fig. 1. Schematic representation of barrier layer formation for CPA2 membranes [2]. in Fig 2. The piping and the cell were made of SS316 and a diaphragm pump (Pulsafeeder 7660) was used for recycling the salt (NaC1) solution through the system. The experiments were done in the pressure range of 200-400 psi and the temperature of the solution in the feed tank was maintained constant at 24°C. The conductivities of the retentate and the permeate samples were measured with a conductivity meter (Cole Parmer). A factorial experiment with multiple replications was designed with chemical type and time of exposure as factors. Fresh sheets of flat CPA2 and SWC1 membranes were cut into circular sections of approximately 38.5 cm 2. All experiments involving a specific chemical type were done with membranes cut from a single sheet of membrane. Thus the membrane sheet was a blocking factor and served to remove any sources of intermembrahe (between-sheet) variability. Multiple replications (with randomization) helped to remove intra-membrane (within sheet)variability, and so the fresh membrane performance results can be compared directly with the exposed membrane (different time periods) performance. CPA2 and SWC1 membranes were soaked under controlled conditions of temperature in solutions of hydrofluoric acid (HF), fluosilicic acid (FSA) and isopropyl alcohol (IPA) in water. The entire circular cut sections were wholly immersed in the solutions in polypropylene containers with no masking of any face since the porous support plays almost no role in determining the separation characteristics of the membranes. The membranes were taken out of the solution after various intervals of time,
242
D. Mukherjee et al. / Desalination 104 (1996) 239-249
~c~[ -'~~EET0~K ~ ~.EF,L~.~
4. Results
_~_
4. I. NaCI permeability and rejection
I ,EV~,CELL ~OSMO~S PERMEATE
=~--E-~-] .~cT ~
~o.......
REJECT
Fig. 2. Schematic diagram of the reverse osmosis experimental set-up,
rinsed with deionized water and then their performance in terms of their flux of purified water and rejection of salt (NaCI) was determined in the system shown in Fig 2 using a salt (NaCI) solution for testing. The concentrations of NaCI used were 0.5 wt% for experiments with the SWC1 (high pressure) membrane and 0.25 wt% for experiments with the CPA2 (low pressure) membrane. The performance of the treated membranes was compared with that of merebranes before treatment . SEM (JOEL- JSM840) and XPS (Perkin Elmer,5500 series) and contact angle studies of the fresh and chemically treated membranes also were done to evaluate the morphological and chemical changes that occured. The rejection values reported in this study are based on the following definition: R = 1 -Ce/C n
(1)
Here C e is the concentration of the permeate and CR is the concentration of the retentate. Cp and CR thus represent the steady state bulk concentration of the permeate and retentate streams leaving the low and the high pressure chambers of the RO test cell respectively,
Figs. 3-8 show the effect of chemical treatment by HF , FSA and IPA on the flux and rejection of the CPA2 and SWC1 TFC membranes. All of the data points were determined by multiple replications, and the standard deviations of the observations were determined to be 5% for flux and 0.5% for rejection. Fig 3 shows the treatment effect of 5 wt% HF on the flux and rejection of CPA2 membrane. The flux increases by about 20% with a slight increase in rejection for an optimal treatment period of around 7 d. Figs. 4 and 5 show the effect of treatment with 15% HF on the flux and rejection properties of the CPA2 membrane. The flux of CPA2 membranes increases about 50% on chemical treatment for around 3 d with 15 wt% HF which appears to be optimum (Figs. 4 and 5). The rejection surprisingly also increases to about 98% at 250 psi after exposure for that period. A similar result also is obtained on exposing the CPA2 membrane to 1 wt% FSA (Fig. 6). Chemical treatment with 15 wt% HF of SWC1 membranes (Fig. 7) also yields a flux increase of about 70% with an optimum treatment time of 3 d. The rejection increases slightly during the treatment period. The flux increase of SWC1 with 40 wt% IPA is slightly less (around 30%) with an optimum treatment time of around 12 h (Fig. 8). 4.2. Surface studies
The different methods that have been used for physicochemical investigation of the changes on the membrane surface on chemical treatment include attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy, electron microscopy for chemical analysis (ESCA, XPS), secondary ion mass spectrometry (SIMS), Rutherford backscattering spectroscopy (RBS), contact angle measurements and scanning electron microscopy (SEM) [25-33]. In this study we have employed the XPS, SEM and contact
243
D. Mukherjee et aL /Desalination 104 (1996) 239-249 --
35
0.99
• -
0.96
3O
"--t)'--
200
psi
200
psi
' 0.g7
i "
.0.96 g
2S
~" /
~
O-
,o.os
-0.94 a0 0
,
'
10
i
I
,
20
I
i
30
I
,
40
I.
i..
SO
|
60
70
Time o f e x p o s u r e (days)
Fig. 3. Variation of the flux and ion (NaCI) rejection for CPA2 membranes with time of treatment of 5 wt% HF. Note the increase in flux by 20% along with the increase in rejection.
40
Ig 2o '
o
'
2
.... Time of
, 4 ezlmsure
,
, e
Fig. 4. Variation of flux of CPA2 membranes on treatment with 15 wt% HF for different time periods. Note the large increase in flux by about 50%.
(Days)
angle method to investigate physical/chemical changes in the surface skin o f the treated membrane. XPS is particularly well suited to examine the skin layer o f the TFC membrane since it probes only a short distance ( 1 0 - 9 0 A) into the surface o f the solids [34-36]. The quantitative elemental composition o f the topmost layer o f the sample can be calculated from the spectra and information as to the binding states o f specific elements can be obtained via the chemical
shift. ESCA thus is an extremely useful method to study structure and bonding in polymeric systems [36]. Differences in the membrane m o r p h o l o g y have been followed by means o f SEM and o f wettability o f the membrane surface by the contact angle technique. The objective is to elucidate, as far as possible, the complex changes governing the change in transport properties o f the membranes on chemical treatment.
D. Mukherjee et al. / Desalination 104 (1996) 239-249
244
0.98 g ~
Fig. 5. Variation of ion (NaC1) rejection of CPA2 membranes on treatment with 15 wt% HF for different time periods. Note the slight increase in rejection.
20o~i
o.~
o.95
i
i
I
o
2
I
I
I
~ . . . . . . . .
4
e
Timeofexposure(Days)
~
.0.99
45
~ ,0.98
---O'-- 250psi • ~50psi a
s
0.97
~ ~-
=
.. ~ ~
Fig. 6. Variation of flux and ioin (NaC1) rejection for CPA2 membrane with time of treatment with 1 wt% FSA. Note the simultaneous increase in flux and rejection.
o.gs
z s |~ o
,
~
,
,
, 40
20
60
T i m e o f exposure (days) 0.99
•
50'
.0,97
$ ft
g 40
-o.ge
-o.9s
,
-----o-aso p~ ~. 850 psi
20
1
,
, 1
,
Timeo f
, 2 treatment
,
(Days)
~ a
,
O.g4
•o.ga
0.92
Fig. 7. Variation of flux and rejection of SWC1 mem-branes on treatment with 15 w t % HF for different time periods. Note the slight increase in rejection.
245
D. Mukherjee et al. /Desalination 104 (1996) 239-249
!
~
"0.99 -0.98 0.97
k. .~
---0"--
E 35
---41..--
350 psi
350psi
r
,0.96~m 3o
j
-g
o
g
~
.0.95~ •0.~
2oc 0
'
'
'
1
Time
'
2
of
Treetment
'
(days)
4.3. Scanning electron microscopy studies
The difference in the surface morphology of the fresh and chemically treated membranes is shown in Figs. 9 andl0. The ridge and valley network structure of the fresh membrane is evident in Fig 9. Thus the surface morphology seems to be similar to that observed for S W 3 0 H R [27]. Fig 10 shows the photomicrograph of the skin of the chemically treated membrane (15 wt% HF for 1 day). Some evidence of surface etching of the active skin surface by HF is shown in the micrograph. However, the microstructure of the surface remains undamaged. So the structural integrity of CPA2 membranes remains intact after exposure to HF, as one would expect because the level of rejection either improved or remained the same. 4.4. XPS studies
The XPS (also known as electron spectroscopy for chemical analysis, i.e., ESCA) spectra were obtained with a Perkin Elmer 5500 multitechnique system using MgKc¢ exciting radiation (1253.6 eV). Typically the X-ray gun was operated at 15 kV and 14 mA and the sample chambet was evacuated to less than 10 -1° Torr. The
'
3
'
4
Fig. 8. Variation of flux and rejection of SWC1 membranes on treatment with 40 wt% isopropyl alcohol for different time periods.Note the increase in flux (30% with no loss in rejection.
copper 2P3/2 level at 932.4 eV binding energy was used for calibration and had a full width half maximum of 1.17+ 0.1 eV. The spectra were taken with the electron emission angle at 60 ° to give a sampling depth of approximately 50 A. The analysis times were kept short for minimizing radiation damage to the sample. The sensitivity factors used were: carbon, 0.296; oxygen, 0.711; fluorine, 1.0; nitrogen, 0.477; sulphur, 0.666; calcium, 1.833, and represented the average values provided by the manufacturer (Perkin Elmer). The peak areas of the survey scan were used to determine the atomic concentration for the various elements with the help of a software program (ESCA version 4.0 and multitechnique version 2.0). Table 1 shows the results of the XPS survey scan (0-1100 eV with a sampling depth and diameter around 50 A and 1 mm, respectively). The atomic concentrations of the skin layer of the fresh membrane were determined from the survey scan and are shown in Table 1. The atomic ratios seem to agree well with what is expected from the polyamide structure shown in Fig. 1. Tables 2 and 3 show the atomic concentrations of the skin layer as determined from the survey scan, for the membrane chemically treated with 15% HF for 1 and 7 d, respectively,
246
D. Mukherjee et al. /Desalination 104 (1996) 239-249
Fig. 9, Scanning electron micrograph of fresh (unexposed) CPA2 showing the microstructure of the membrane surface. 5 kV; WD: 8 nma; magnification: 10,000. Bar: 1 ~m.
Fig. 10. Scanning electron micrograpnof chemically treated (15 wt% for 1 d) CPA2 membrane showing etching of the polymeric surface. 5 kV; WD: 9 mm; Magnification: 10,000, Bar: 1 ~m.
Table 1 X-ray photoelectron spectrum of fresh CPA2: atomic concentration table
Table 3 X-ray photoelectron spectrum of exposed (15 wt% HF for 7 d) CPA 2 showing partial surface fluorination with time of exposure: atomic concentration table
Element Carbon (Cls) Oxygen (Ols) Nitrogen (Nls)
Concentration (%)
Atomic ratios (element/carbon)
73.07 18.25 8.675
0.249 0.118
Table 2 X-ray photoelectron spectrum of exposed (15 wt% HF for 1 d) CPA2: atomic concentration table Element
Concentration (%)
Atomic ratios (element/carbon)
Carbon (Cls) Oxygen (Ols) Nitrogen (Nls) Fluorine (Fls)
73.53 16.31 8.1 0.66
0.222 0.11 0.009
w h i c h show evidence o f partial fluorination o f the m e m b r a n e skin. The atomic concentrations g i v e n in Tables 1-3 represent the average atomic concentrations calculated f r o m the XPS
Element
Concentration (%)
Atomic ratios (element/carbon)
Carbon (Cls) Oxygen (Ols) Nitrogen (Nls) Fluorine (Fls)
69.38 20.47 8.605 1.545
0.295 0.124 0.022
Table 4 Contact angle measurements for fresh and HF treated membranes Treatment time (d) 0 0.5 7
Contact angle (octane/ water/membrane) 128 138 141
spectra o f multiple sampling points on the m e m b r a n e surface for a n u m b e r o f m e m b r a n e sampies.
D. Mukherjee et al. / Desalination 104 (1996) 239-249
247
4.5. Contact angle m e a s u r e m e n t
5. Discussion
To provide a measure of the changes in wettability of the membrane sample on treatment with HF, contact angle measurements were performed on the fresh (untreated) membranes which were compared to those measured on the chemically treated membrane. The captive bubble contact angle technique was used in this work to avoid any influence from drying of the membrane which may change the surface properties of the membrane [37,38]. Octane/water (bulk liquid phase)/membrane interfaces are used in our case for measurement of the contact angle [39]. Octane~water~membrane interfaces were formed by immersing small panels of membrane in a glass observation cell containing deionized octane-saturated water and releasing octane drops beneath the membrane surface with a syringe. A Zeiss microscope fitted with a video camera provided a magnified picture of the bubble which was then recorded and used to measure the contact angles. The reported values are the average of at least six independent measurements on the membrane where the contact angle was measured on at least two sites of the octane bubble. Reproducibility of the measurements was better than +2 °. The measurements were taken within 1 min of placing the drop on the membrane surface, Table 4 shows the results of contact-angle measurements on fresh and membranes treated with 15 wt% HF for different time periods. As can be seen, the contact angles increase with the time of treatment and the angles for the treated membranes are greater by more than l0 ° c o m pared to the fresh (untreated) membrane. Taking a simplistic viewpoint in correlating changes in contact angle to properties of the membrane surface, which is morphologically and chemically heterogeneous, this increase in contact angle reflects an increase in hydrophilicity (wettability) on chemical treatment,
The interaction between the polymer (aromatic polyamide) and chemical agents (HF and IPA) seems to be responsible for the complex changes that occur due to the chemical treatment. McDonough et al. [40] and Yao et al. [41] have also shown that strong mineral acids (e.g., HCI) cause hydrolysis of polyamide 6 which can be studied by determining changes in molecular weight. Hydrolysis was shown to be first order with a long half-life of about 250 d [41]. Aromatic polyamides are more resistant to acid than the aliphatic ones but are still susceptible to hydrolytic scission [42,43]. In fact, strong protic acids (e.g., concentrated sulfuric acid) are solvents for the linear aromatic polyamides [44]. Exposing the barrier layers of CPA2 and SWC1, for example, to strong concentratedsolutions of sulfuric acid dissolves the barrier layer which leads to a large increase in flux with a huge loss of rejection for the TFC membrane. So it is postulated that decreasing the concentration of the acids and using low concentrations of the acid will cause limited hydrolysis of the amide network. This limited hydrolysis by acids (e.g., HF) will reduce polymer chain entanglement which in turn would lead to enhanced water flux. The limited hydrolysis manifests itself as surface etching of the polymer network as is clearly shown by the scanning electron micrographs (Figs. 9 and 10). As was shown in our previous work [24], this etching is clearly seen on the chemically treated membrane surface. Also our ESCA results of membranes treated with HF suggest partial fluorination (either through covalent or hydrogen bonding [45,46]) which causes an increase in hydrophilicity due to the unbalanced dipole moments; this incomplete fluorine substitution causes an increase in wettability [25, 47-49] which is also seen from our contact angle study. The acid hydrolysis of the aromatic polyamides skin gives rise to car
248
D. Mukherjee et al. /Desalination 104 (1996) 239-249
boxylic and amidic groups on the membrane skin [50,51] which not only contributes to its increase in hydrophilicity but also helps to increase the rejection [52] of the treated membrane, offsetting the loss in rejection expected due to the higher free-volume created by the partial hydrolysis of the membrane skin [53]. The partial fluorination coupled with the limited hydrolysis of the membrane skin probably causes the enhanced membrane performance (increased flux and rejection) of the barrier layer on treatment with HF. Likewise, the limited solvency power of IPA
The average absolute fluxes observed in our experimental system for the fresh batches of SW30HRand SWC1 (0.5 wt% NaC1)membrane at 350 psi were around 6 and 19 1/m2/h, respectively. After optimal treatment with 15 wt% HF, the SW30HR and SWC1 both gave a flux of around 33 1/m2/h. A low pressure membrane like CPA2, on the other hand, increases in flux on optimal chemical treatment from 26 1/m2/h to around 35 l/m2/h (0.25 wt% NaCI, 250 psi).
(as can be seen from the closeness of the solubility parameter values of IPA and aromatic polyamides [44,54,55]) causes limited dissolution which normally should cause an increase in flux with a decrease in rejection. But the process of surface defect healing (removal of surface discontinuities) by surface tension driven pore collapsing, which results from the lowering of the modulus of the polymer matrix [21,56] on treatment with IPA causes a simultaneous increase in rejection along with the flux. While the use of treating agents like HF and IPA causes an enhancement of membrane performance, certain limitations must be recognized. Use of very high concentration of HF over extended time periods deteriorates membrane performance as can be expected from increased hydrolysis of the membrane skin. Thus, optimizing the treatment concentration and time is essential to effectively utilize this process for performance improvement. Our present efforts are directed towards other mineral acids and the combination of acid/alcohol for effecting optimized flux enhancement without decreasing rejection. Comparing this work with that of our previ-
The novel chemical treatment shown here
ous work [24] on SW30HR (Filmtec) membranes, one can observe that the more dense, high-pressure, fresh membranes characterized by lower fluxes tend to give much higher flux increases with this method of treatment than do the higher flux, low pressure fresh membranes,
8. Conclusions causes an increase in flux without any loss in rejection for the commercially important, stateof-the-art polyamide-based TFC RO membranes used in this study. In most cases permanent increases in both flux and rejection have been observed. The flux enhancement observed is as much as 70% for the CPA2 and SWC1 membranes. This procedure easily can be used as a post-treatment step to the interfacial polymerization used to manufacture commercial RO membranes. The simultaneous increase in productivity and rejection of the spiral wound membrane modules manufactured from these chemically treated membranes will make them even more cost-effective for use in various desalination, waste-treatment and other process applications. Acknowledgment The authors wish to gratefully acknowledge the financial support of the New York State Energy Research and Development Authority ( N Y S E R D A ) for this work. We also thank Hydranautics for providing samples of their membranes for our research.
References [1] W.J. Koros, G.K. Fleming, S.M. Jordan, T.H.Kim and H.H. Hoehn, Prog. Polym. Sci., 13 (1988) 339.
D. Mukherjee et al. / Desalination 104 (1996) 239-249
[2] R.J.Petersen, J. Memb. Sci., 83 (1993) 81. [3] W.G. Light, H.C. Chu and C.N. Tran, Desalination, 64 (1987) 411. [4] J.E. Tomaschke, U.S. Pats. 4,872,984 ,Oct 10, 1989, and 4,948,507, Aug 14, 1991. [5] J.E. Cadotte, in: D.R. Lloyd, ed., Material Science of Synthetic Membranes, ACS Symposium Series 269, 1985 p. 273. [6] J.E. Cadotte, U.S. Patent 4,277,344, July 7, 1981. [7] H.H. Hoehn, in: D.R.Lloyd, ed., Material Science of Synthetic Membranes, ACS Symposium Series ,269, 1985 p. 81. [8[ K. Asfardjani, Y. Segul, Y. Aurelle and N. Abidine, J. Appl. Polym. Sci., 43 (1993) 271. [9] J.Y. Lai and Y.C. Chao, J. Appl. Polym. Sci., 39 (1990) 2293. [I0] S. Takigami, F. Kobayashi and Y. Nakamura, J. Polym, Sci., Part A: Polym. Chem., 25 (1987) 63. [11] F. Vigo and C. Uliana, J. Appl. Polym. Sci., 38 (1989) 1197. [12] F. F. Stengaard, Desalination, 70 (1988) 207. [13] M. Nystrom and P. Jarvinen, J. Membr. Sci., 60 (1991) 275. [14] A. Dimov and M.A. Islam, J. Membr. Sci., 50 (1990) 97. [15] A. Dimov and M.A. Islam, J. Appl. Polym. Sci., 42 (1991) 1285. [16] A. Higuchi, N. Iwata and T. Nakagawa, J. Appl. Polym. Sci., 40 (1990) 709. [17] T. Sano, T. Shimomura and I. Murase, US Patent No. 4,268,662, 1981. [18] J.M. Mohr, D.R. Paul, T.E. Mlsna and R.J .Lagow, J. Membr. Sci., 55 (1991) 131. [19] J.M. Mohr, D.R. Paul, Y. Taru, T.E. Mlsna and R.J. Lagow, J. of Memb. Sci, 55 (1991) 149. [20] J.M. Mohr, D.R. Paul, I. Pinnau and W.J. Koros, J. Membr. Sci., 56 (1991) 77. [21] M.E. Rezac, J.D. Le Roux, H. Chen, D.R. Paul and W.J. Koros, J. Membr. Sci., 90 (1994) 213. [22] A. Kulkami, D. Mukherjee, D. Mukherjee and W.N. Gill, Chem. Eng. Commun., 129 (1994) 53. [23] D. Mukherjee, A. Kulkami, A, Chawla and W.N. Gill, Chem. Eng. Commun., 130 (1995) 127. [24] D. Mukherjee, A. Kulkami and W.N. Gill, J. Memb. Sci., 97 (1994) 231. [25] R.H. Sedath, D.R. Taylor and N.N. Li, Sci.and Tech., 28(1-3) (1993) 255. [26] M. Oldani and G. Schock, J. Membr. Sci., 43 (1989) 243. [27] J. Lukas and V Tyrackova, J. Membr. Sci., 58 (1991) 49. [28] K. Kim and A.G. Fane, J. Membr. Sci., 88 (1994) 103. [29] K.J. Kim, M.R. Dickson, A.G. Fane and C.J.D. Fell,
249
J. of Microscopy, 162 (1991) 403. [30] C.R. Bartels, J.of Memb. Sci., 45 (1989) 225. [31] J.M. Mohr, D.R. Paul, Y. Tam, T.E. Mlsna and R.J. Lagow, J. of Applied Poly. Sci., 42 (1991) 2509. [32] J.D. LeRoux, D.R. Paul, M.F. Arendt, Y. Yuan and I. Cabasso, J.of Memb. Sci., 90 (1994) 37. [33] M. Fontyn, B.H. Bijsterbosch and K. Van't Riet, J. Membr. Sci., 36 (1987) 141. [34] D.T. Clark, Pure & Applied Chem., 54(2) (1982) 415. [35] D.T. Clark, Adv. Polym. Sci., 24 (1977) 125. [36] D.T. Clark and W.J. Feast, J. Macromol. Sci., -Revs. Macromol. Chem., C12(2) (1975) 191. [37] W. Zhang and B. Hallstrom, Desalination, 79 (1990) I. [38] W. Zhang, M. Wahlgren. and B. Sivik, Desalination, 72 (1989)263. [39] W.C. Hamilton, J. Colloid Interface Sci., 40(2) (1972) 219. [40] R.M. McDonogh, C.J.D. Fell and A.G. Fane, J. Memb. Sci., 31 (1987) 321. [41] C.W. Yao, R.P. Burford, A.G. Fane, C.J.D. Fell and R.M. McDonogh, J. Appl. Polym. Sci., 34 (1987) 399. [42] J.I. Kroschwitz, ed., Encyclopedia of Polym. Sci. and Eng., Vol ll,Wiley-Interscience, 1988, pp 381-409. [43] E.M. Fettes, ed., Chemical Reactions of Polymers, Vol. 19, Interscience, 1964, p. 561. [44] S..M. Aharoni, J. Appl. Polym. Sci., 45 (1992) 813. [45] K.O. Patten Jr. and L. Andrews, J. Phys. Chem., 90 (1986) 1073. [46] R.B. Bohn and L. Andrews, J. Phys. Chem., 93 (1989) 5684. [47] A.H. Ellison and W.A. Zisman, J.Phys. Chem., 58 (1954) 260. [48] A.F.M. Barton, Handbook of Solubility and Other Cohesion Parameters, CRC, Boca Raton, FL,1983. [49] D.K. Owens and R.C. Wendt, J. Appl. Polym. Sci., 13 (1969) 1741. [50] L.J. Broadbelt, S. Dziennik and M.T. Klein, Polym. Degrad. Stab., 44 (1994) 137. [51] L.J. Broadbelt, A. Chu. and M.T. Klein, Polym. Degrad. Stab., 45 (1994) 57. [52] R.F. Fibiger, J. Colucci, D.J. Forgach, R.A. Wessling and D.L. Schmidt, U.S. Patents 4,828,700, 1989, 4,894,165, 1990, 4,909,943, 1990. [53] H. Wood and S. Sourirajan, J. Colloid Interface Sci., 149(1) (1992) 105. [54] E.A. Gmlke, in: Polymer Handbook, 3rd. ed. J. Brandrup and E.H. Immergut, eds., Wiley, N.Y, 1989. [55] R.E. Kesting, Synthetic Polymeric Membranes: A Structural Perspective, Chap. 5 , Wiley 1985. [56] G.L. Brown, J. Polym. Sci., 22 (1956) 423.
Desalination 104 (I 996) 239-249
Chemical treatment for improved performance of reverse osmosis membranes Debabrata Mukherjee*, Ashish Kulkarni, William N. Gill'* Isermann Department of Chemical Engineering, Rennselaer Polytechnic Institute, Troy, NY 12180-3590, USA
Received 14 May 1995; accepted 31 July 1995
Abstract
A novel method of chemical treatment is described which causes a simultaneous improvement of flux and rejection of thin-film composite (TFC) reverse osmosis (RO) membranes. The water flux of CPA2 and SWC1 which are commercially important aromatic polyamide based TFC RO membranes has been shown to be increased significantly, without any loss of ion rejection, on short-term treatment with acids such as hydrofluoric and fluosilicic and alcohols like isopropanol. In most cases simultaneous increases in flux and rejection have been observed. Surface characterization studies by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and contact angle measurements reveal that limited hydrolysis and fluorination of the membrane skin seem to be responsible for such improvement in the membrane flux and rejection on treatment with hydrofluoric and fluosilicic acids. With isopropyl alcohol, partial dissolution coupled with surface tension driven collapse of pores in the membrane skin seem to be key factors which cause a simultaneous increase in flux and rejection. Therefore enhancement by short-term chemical treatment may be a very useful approach for the future development of RO membranes having higher flux and rejection. Keywords:Thin-film composite reverse osmosis membranes; Flux enhancement; Hydrofluoric acid; lsopropyl alcohol; Surface modification
1. Introduction The development of the wide array of polymers for membranes [1] has given rise to a large
range of applications from waste water treatment to gas separations. However, relatively few of these polymers are useful for RO applications [1,2]. The polymers that have shown promise for RO separations include cellulose
*Present address: Schoeller Technical Papers I n c . , Pulaski, NY 13142, USA. **Corresponding author,
acetate, polyethylenimine, polyepiamine, polyureas, aliphatic and aromatic polyamides, and polyhydrazide among others. O f these, cross-
0011-9164/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PII S001 1-9164(96)00047-1
240
D. Mukherjee et al. / Desalination 104 (1996) 239-249
linked aromatic polyamides have found extensive commercial usage as the barrier layers of RO membranes. TFCL (UOP), CPA2 ( Hydranautics), FT-30 (Filmtec) and ACM1 (Tricep) are important commercially for RO, and they are based on aromatic polyamide chemistry [2]. The barrier layers of these membranes are formed from interfacial polymerization of 1,3benzene diamines with either isophthaloyl or trimesoyl chloride (or their blends) [2-6]. Continuing efforts have been made to improve the performance of TFC membranes which currently are state-of-the-art. No new dramatically better polymeric materials for the RO membrane skin have appeared in the last decade [2,7] since the development of the aromatic polyamide based FT-30 TFC membranes, The major emphasis now seems to be on the surface modification of the polymeric structure of the skin which plays the most important role in determining the flux and rejection of composite membranes. This has given rise to an increasing number of patents and papers on surface modification of the membranes which enhance their performance. However, most of the work on RO membrane surface modification has reported an increase of either flux with a loss in rejection or vice versa. Efforts to improve the flux with no loss, or an increase, in rejection of the aromatic polyamide membranes have failed mainly due to the conflicting surface requirements of the membrane skin in maintaining both high flux and rejection, 2. Surface modification of membranes Much literature has been published recently on the surface modification of microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and gas separation membranes [8-21]. Plasma reatment [8,9], grafting [ 10,11 ], surface coating and curing [12] and UV irradiation [13] are among the important techniques used for surface modification of MF, UF and RO membranes, Chemical post-treatment by hydrophilicity in-
creasing agents [13,14], protic acids [14-17] and inorganic salts [17] have also been reported for MF, UF and RO membranes. Dimov et. al. [14] have reported hydrophilization with a temporary increase in flux in polyethylene microfiltration membranes by using a ternary mixture of ethanol-water-inorganic acids. It was found that the hydrophilization efficiency of the inorganic acid decreases in the order HNO3 >H2SO4>H3PO 4. Similar flux enhancements by chlorosulfonic acid for polyethylene microfiltration also have been reported [15]. It was concluded that a low concentration of the sulfonating agent and low reaction temperature should be used to perform selectively the sulfonation of the membrane surface and the inner walls of the pore and minimize the destruction of the bulk of the membrane. Higuchi et. al. [16] also found that immersing modified polysulfone UF hollow fiber membranes in HC1 caused an increase in rejection with no change in flux. However, the results of Sano et. al. [ 17] indicate that acrylonitrile-based RO membranes exhibit an increase in flux with no change in rejection properties on dipping such membranes in protic acids (HC1, HNO3, H2SO 4, etc.) and/or inorganic salts (CuSO4, etc.). Literature also exists on surface modification of gas separation membranes and a few methods deserve special mention in the context of the present work [18-21]. Fluorination of gas separation membranes by elemental fluorine gas has been shown to cause an increase in selectivity with reduced permeation [18-20]. Also studied was the effect of mild solvent posttreatment with a variety of vapors and liquids (dichloromethane, cyclohexane and water) on the transport behavior of various gas separation membranes like dry/wet phase inverted asymmetric polysulfone, spin-coated poly(phenylene oxide)-ceramic composites and solution-deposited polyimide-ceramic composite membranes [21 ]. It was observed again that such treatments caused a permanent increase in selectivity of the defective, asymmetric membranes but at the expense of a decrease in permeation.
241
D. Mukherjee et al. /Desalination 104 (1996) 239-249
Our present work shows that it is possible to simultaneously increase both the flux and rejec-
AromaticPolylmin* + PolyfuacUoail Acyl HIIMe O,34anzenedblmine) (BlendofI~ahthtla# and Trlmetmyl chl~ide)
tion of the fully aromatic polyamide-based RO membranes, which are the most important membranes available commercially, after treatment with solutions of hydrofluoric acid (HF), fluosilicic acid (FSA) and isopropyl alcohol (IPA). Both HF and IPA are used extensively in the microelectronics industry where HF etches silicon dioxide based films and IPA cleans wafers. Our efforts to reprocess HF with RO membranes [22,23] led to the discovery of a very large improvement in the transport rates obtained by SW30HR membranes [24]. This work shows that the technique of short-term chemical treatment using these chemicals can be effectively used for the permanent simultaneous improvement of both the membrane flux and ion rejection of other commercially important RO membranes,
+ Aml,,S,lt (q. Trlmethylamine slit of camphorsutfonlc°rid)
3. Experimental 3.1. Membrane CPA2 and SWC1 are two types of the commercially available thin film composite (TFC) RO membranes manufactured by Hydranautics which are suitable for low and high pressure operation, respectively. The formation of the barrier layer of these membranes is shown schematically in Fig 1 [2,4] where one sees that it is essentially a copolyamide derived from the reaction of 1,3-benzenediamine with a blend of isothaloyl and trimesoyl chlorides. The chemistry of the membrane is thus very similar to the FT-30 manufactured by Filmtec except for the presence of the amine salt as an additive in the formulation. One purpose of the amine salt is to reduce the loss in membrane flux due to heatannealing of the membrane skin during menufacture [2].
3.2. Experimental set-up and design Our experimental set-up was a single cell, flat sheet, closed loop recycle system as shown
_ A,~,tl~P,~°m~, (a|rrler Layer)
Po,t-~,~tl.~ o,y~ng Fig. 1. Schematic representation of barrier layer formation for CPA2 membranes [2]. in Fig 2. The piping and the cell were made of SS316 and a diaphragm pump (Pulsafeeder 7660) was used for recycling the salt (NaC1) solution through the system. The experiments were done in the pressure range of 200-400 psi and the temperature of the solution in the feed tank was maintained constant at 24°C. The conductivities of the retentate and the permeate samples were measured with a conductivity meter (Cole Parmer). A factorial experiment with multiple replications was designed with chemical type and time of exposure as factors. Fresh sheets of flat CPA2 and SWC1 membranes were cut into circular sections of approximately 38.5 cm 2. All experiments involving a specific chemical type were done with membranes cut from a single sheet of membrane. Thus the membrane sheet was a blocking factor and served to remove any sources of intermembrahe (between-sheet) variability. Multiple replications (with randomization) helped to remove intra-membrane (within sheet)variability, and so the fresh membrane performance results can be compared directly with the exposed membrane (different time periods) performance. CPA2 and SWC1 membranes were soaked under controlled conditions of temperature in solutions of hydrofluoric acid (HF), fluosilicic acid (FSA) and isopropyl alcohol (IPA) in water. The entire circular cut sections were wholly immersed in the solutions in polypropylene containers with no masking of any face since the porous support plays almost no role in determining the separation characteristics of the membranes. The membranes were taken out of the solution after various intervals of time,
242
D. Mukherjee et al. / Desalination 104 (1996) 239-249
~c~[ -'~~EET0~K ~ ~.EF,L~.~
4. Results
_~_
4. I. NaCI permeability and rejection
I ,EV~,CELL ~OSMO~S PERMEATE
=~--E-~-] .~cT ~
~o.......
REJECT
Fig. 2. Schematic diagram of the reverse osmosis experimental set-up,
rinsed with deionized water and then their performance in terms of their flux of purified water and rejection of salt (NaCI) was determined in the system shown in Fig 2 using a salt (NaCI) solution for testing. The concentrations of NaCI used were 0.5 wt% for experiments with the SWC1 (high pressure) membrane and 0.25 wt% for experiments with the CPA2 (low pressure) membrane. The performance of the treated membranes was compared with that of merebranes before treatment . SEM (JOEL- JSM840) and XPS (Perkin Elmer,5500 series) and contact angle studies of the fresh and chemically treated membranes also were done to evaluate the morphological and chemical changes that occured. The rejection values reported in this study are based on the following definition: R = 1 -Ce/C n
(1)
Here C e is the concentration of the permeate and CR is the concentration of the retentate. Cp and CR thus represent the steady state bulk concentration of the permeate and retentate streams leaving the low and the high pressure chambers of the RO test cell respectively,
Figs. 3-8 show the effect of chemical treatment by HF , FSA and IPA on the flux and rejection of the CPA2 and SWC1 TFC membranes. All of the data points were determined by multiple replications, and the standard deviations of the observations were determined to be 5% for flux and 0.5% for rejection. Fig 3 shows the treatment effect of 5 wt% HF on the flux and rejection of CPA2 membrane. The flux increases by about 20% with a slight increase in rejection for an optimal treatment period of around 7 d. Figs. 4 and 5 show the effect of treatment with 15% HF on the flux and rejection properties of the CPA2 membrane. The flux of CPA2 membranes increases about 50% on chemical treatment for around 3 d with 15 wt% HF which appears to be optimum (Figs. 4 and 5). The rejection surprisingly also increases to about 98% at 250 psi after exposure for that period. A similar result also is obtained on exposing the CPA2 membrane to 1 wt% FSA (Fig. 6). Chemical treatment with 15 wt% HF of SWC1 membranes (Fig. 7) also yields a flux increase of about 70% with an optimum treatment time of 3 d. The rejection increases slightly during the treatment period. The flux increase of SWC1 with 40 wt% IPA is slightly less (around 30%) with an optimum treatment time of around 12 h (Fig. 8). 4.2. Surface studies
The different methods that have been used for physicochemical investigation of the changes on the membrane surface on chemical treatment include attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy, electron microscopy for chemical analysis (ESCA, XPS), secondary ion mass spectrometry (SIMS), Rutherford backscattering spectroscopy (RBS), contact angle measurements and scanning electron microscopy (SEM) [25-33]. In this study we have employed the XPS, SEM and contact
243
D. Mukherjee et aL /Desalination 104 (1996) 239-249 --
35
0.99
• -
0.96
3O
"--t)'--
200
psi
200
psi
' 0.g7
i "
.0.96 g
2S
~" /
~
O-
,o.os
-0.94 a0 0
,
'
10
i
I
,
20
I
i
30
I
,
40
I.
i..
SO
|
60
70
Time o f e x p o s u r e (days)
Fig. 3. Variation of the flux and ion (NaCI) rejection for CPA2 membranes with time of treatment of 5 wt% HF. Note the increase in flux by 20% along with the increase in rejection.
40
Ig 2o '
o
'
2
.... Time of
, 4 ezlmsure
,
, e
Fig. 4. Variation of flux of CPA2 membranes on treatment with 15 wt% HF for different time periods. Note the large increase in flux by about 50%.
(Days)
angle method to investigate physical/chemical changes in the surface skin o f the treated membrane. XPS is particularly well suited to examine the skin layer o f the TFC membrane since it probes only a short distance ( 1 0 - 9 0 A) into the surface o f the solids [34-36]. The quantitative elemental composition o f the topmost layer o f the sample can be calculated from the spectra and information as to the binding states o f specific elements can be obtained via the chemical
shift. ESCA thus is an extremely useful method to study structure and bonding in polymeric systems [36]. Differences in the membrane m o r p h o l o g y have been followed by means o f SEM and o f wettability o f the membrane surface by the contact angle technique. The objective is to elucidate, as far as possible, the complex changes governing the change in transport properties o f the membranes on chemical treatment.
D. Mukherjee et al. / Desalination 104 (1996) 239-249
244
0.98 g ~
Fig. 5. Variation of ion (NaC1) rejection of CPA2 membranes on treatment with 15 wt% HF for different time periods. Note the slight increase in rejection.
20o~i
o.~
o.95
i
i
I
o
2
I
I
I
~ . . . . . . . .
4
e
Timeofexposure(Days)
~
.0.99
45
~ ,0.98
---O'-- 250psi • ~50psi a
s
0.97
~ ~-
=
.. ~ ~
Fig. 6. Variation of flux and ioin (NaC1) rejection for CPA2 membrane with time of treatment with 1 wt% FSA. Note the simultaneous increase in flux and rejection.
o.gs
z s |~ o
,
~
,
,
, 40
20
60
T i m e o f exposure (days) 0.99
•
50'
.0,97
$ ft
g 40
-o.ge
-o.9s
,
-----o-aso p~ ~. 850 psi
20
1
,
, 1
,
Timeo f
, 2 treatment
,
(Days)
~ a
,
O.g4
•o.ga
0.92
Fig. 7. Variation of flux and rejection of SWC1 mem-branes on treatment with 15 w t % HF for different time periods. Note the slight increase in rejection.
245
D. Mukherjee et al. /Desalination 104 (1996) 239-249
!
~
"0.99 -0.98 0.97
k. .~
---0"--
E 35
---41..--
350 psi
350psi
r
,0.96~m 3o
j
-g
o
g
~
.0.95~ •0.~
2oc 0
'
'
'
1
Time
'
2
of
Treetment
'
(days)
4.3. Scanning electron microscopy studies
The difference in the surface morphology of the fresh and chemically treated membranes is shown in Figs. 9 andl0. The ridge and valley network structure of the fresh membrane is evident in Fig 9. Thus the surface morphology seems to be similar to that observed for S W 3 0 H R [27]. Fig 10 shows the photomicrograph of the skin of the chemically treated membrane (15 wt% HF for 1 day). Some evidence of surface etching of the active skin surface by HF is shown in the micrograph. However, the microstructure of the surface remains undamaged. So the structural integrity of CPA2 membranes remains intact after exposure to HF, as one would expect because the level of rejection either improved or remained the same. 4.4. XPS studies
The XPS (also known as electron spectroscopy for chemical analysis, i.e., ESCA) spectra were obtained with a Perkin Elmer 5500 multitechnique system using MgKc¢ exciting radiation (1253.6 eV). Typically the X-ray gun was operated at 15 kV and 14 mA and the sample chambet was evacuated to less than 10 -1° Torr. The
'
3
'
4
Fig. 8. Variation of flux and rejection of SWC1 membranes on treatment with 40 wt% isopropyl alcohol for different time periods.Note the increase in flux (30% with no loss in rejection.
copper 2P3/2 level at 932.4 eV binding energy was used for calibration and had a full width half maximum of 1.17+ 0.1 eV. The spectra were taken with the electron emission angle at 60 ° to give a sampling depth of approximately 50 A. The analysis times were kept short for minimizing radiation damage to the sample. The sensitivity factors used were: carbon, 0.296; oxygen, 0.711; fluorine, 1.0; nitrogen, 0.477; sulphur, 0.666; calcium, 1.833, and represented the average values provided by the manufacturer (Perkin Elmer). The peak areas of the survey scan were used to determine the atomic concentration for the various elements with the help of a software program (ESCA version 4.0 and multitechnique version 2.0). Table 1 shows the results of the XPS survey scan (0-1100 eV with a sampling depth and diameter around 50 A and 1 mm, respectively). The atomic concentrations of the skin layer of the fresh membrane were determined from the survey scan and are shown in Table 1. The atomic ratios seem to agree well with what is expected from the polyamide structure shown in Fig. 1. Tables 2 and 3 show the atomic concentrations of the skin layer as determined from the survey scan, for the membrane chemically treated with 15% HF for 1 and 7 d, respectively,
246
D. Mukherjee et al. /Desalination 104 (1996) 239-249
Fig. 9, Scanning electron micrograph of fresh (unexposed) CPA2 showing the microstructure of the membrane surface. 5 kV; WD: 8 nma; magnification: 10,000. Bar: 1 ~m.
Fig. 10. Scanning electron micrograpnof chemically treated (15 wt% for 1 d) CPA2 membrane showing etching of the polymeric surface. 5 kV; WD: 9 mm; Magnification: 10,000, Bar: 1 ~m.
Table 1 X-ray photoelectron spectrum of fresh CPA2: atomic concentration table
Table 3 X-ray photoelectron spectrum of exposed (15 wt% HF for 7 d) CPA 2 showing partial surface fluorination with time of exposure: atomic concentration table
Element Carbon (Cls) Oxygen (Ols) Nitrogen (Nls)
Concentration (%)
Atomic ratios (element/carbon)
73.07 18.25 8.675
0.249 0.118
Table 2 X-ray photoelectron spectrum of exposed (15 wt% HF for 1 d) CPA2: atomic concentration table Element
Concentration (%)
Atomic ratios (element/carbon)
Carbon (Cls) Oxygen (Ols) Nitrogen (Nls) Fluorine (Fls)
73.53 16.31 8.1 0.66
0.222 0.11 0.009
w h i c h show evidence o f partial fluorination o f the m e m b r a n e skin. The atomic concentrations g i v e n in Tables 1-3 represent the average atomic concentrations calculated f r o m the XPS
Element
Concentration (%)
Atomic ratios (element/carbon)
Carbon (Cls) Oxygen (Ols) Nitrogen (Nls) Fluorine (Fls)
69.38 20.47 8.605 1.545
0.295 0.124 0.022
Table 4 Contact angle measurements for fresh and HF treated membranes Treatment time (d) 0 0.5 7
Contact angle (octane/ water/membrane) 128 138 141
spectra o f multiple sampling points on the m e m b r a n e surface for a n u m b e r o f m e m b r a n e sampies.
D. Mukherjee et al. / Desalination 104 (1996) 239-249
247
4.5. Contact angle m e a s u r e m e n t
5. Discussion
To provide a measure of the changes in wettability of the membrane sample on treatment with HF, contact angle measurements were performed on the fresh (untreated) membranes which were compared to those measured on the chemically treated membrane. The captive bubble contact angle technique was used in this work to avoid any influence from drying of the membrane which may change the surface properties of the membrane [37,38]. Octane/water (bulk liquid phase)/membrane interfaces are used in our case for measurement of the contact angle [39]. Octane~water~membrane interfaces were formed by immersing small panels of membrane in a glass observation cell containing deionized octane-saturated water and releasing octane drops beneath the membrane surface with a syringe. A Zeiss microscope fitted with a video camera provided a magnified picture of the bubble which was then recorded and used to measure the contact angles. The reported values are the average of at least six independent measurements on the membrane where the contact angle was measured on at least two sites of the octane bubble. Reproducibility of the measurements was better than +2 °. The measurements were taken within 1 min of placing the drop on the membrane surface, Table 4 shows the results of contact-angle measurements on fresh and membranes treated with 15 wt% HF for different time periods. As can be seen, the contact angles increase with the time of treatment and the angles for the treated membranes are greater by more than l0 ° c o m pared to the fresh (untreated) membrane. Taking a simplistic viewpoint in correlating changes in contact angle to properties of the membrane surface, which is morphologically and chemically heterogeneous, this increase in contact angle reflects an increase in hydrophilicity (wettability) on chemical treatment,
The interaction between the polymer (aromatic polyamide) and chemical agents (HF and IPA) seems to be responsible for the complex changes that occur due to the chemical treatment. McDonough et al. [40] and Yao et al. [41] have also shown that strong mineral acids (e.g., HCI) cause hydrolysis of polyamide 6 which can be studied by determining changes in molecular weight. Hydrolysis was shown to be first order with a long half-life of about 250 d [41]. Aromatic polyamides are more resistant to acid than the aliphatic ones but are still susceptible to hydrolytic scission [42,43]. In fact, strong protic acids (e.g., concentrated sulfuric acid) are solvents for the linear aromatic polyamides [44]. Exposing the barrier layers of CPA2 and SWC1, for example, to strong concentratedsolutions of sulfuric acid dissolves the barrier layer which leads to a large increase in flux with a huge loss of rejection for the TFC membrane. So it is postulated that decreasing the concentration of the acids and using low concentrations of the acid will cause limited hydrolysis of the amide network. This limited hydrolysis by acids (e.g., HF) will reduce polymer chain entanglement which in turn would lead to enhanced water flux. The limited hydrolysis manifests itself as surface etching of the polymer network as is clearly shown by the scanning electron micrographs (Figs. 9 and 10). As was shown in our previous work [24], this etching is clearly seen on the chemically treated membrane surface. Also our ESCA results of membranes treated with HF suggest partial fluorination (either through covalent or hydrogen bonding [45,46]) which causes an increase in hydrophilicity due to the unbalanced dipole moments; this incomplete fluorine substitution causes an increase in wettability [25, 47-49] which is also seen from our contact angle study. The acid hydrolysis of the aromatic polyamides skin gives rise to car
248
D. Mukherjee et al. /Desalination 104 (1996) 239-249
boxylic and amidic groups on the membrane skin [50,51] which not only contributes to its increase in hydrophilicity but also helps to increase the rejection [52] of the treated membrane, offsetting the loss in rejection expected due to the higher free-volume created by the partial hydrolysis of the membrane skin [53]. The partial fluorination coupled with the limited hydrolysis of the membrane skin probably causes the enhanced membrane performance (increased flux and rejection) of the barrier layer on treatment with HF. Likewise, the limited solvency power of IPA
The average absolute fluxes observed in our experimental system for the fresh batches of SW30HRand SWC1 (0.5 wt% NaC1)membrane at 350 psi were around 6 and 19 1/m2/h, respectively. After optimal treatment with 15 wt% HF, the SW30HR and SWC1 both gave a flux of around 33 1/m2/h. A low pressure membrane like CPA2, on the other hand, increases in flux on optimal chemical treatment from 26 1/m2/h to around 35 l/m2/h (0.25 wt% NaCI, 250 psi).
(as can be seen from the closeness of the solubility parameter values of IPA and aromatic polyamides [44,54,55]) causes limited dissolution which normally should cause an increase in flux with a decrease in rejection. But the process of surface defect healing (removal of surface discontinuities) by surface tension driven pore collapsing, which results from the lowering of the modulus of the polymer matrix [21,56] on treatment with IPA causes a simultaneous increase in rejection along with the flux. While the use of treating agents like HF and IPA causes an enhancement of membrane performance, certain limitations must be recognized. Use of very high concentration of HF over extended time periods deteriorates membrane performance as can be expected from increased hydrolysis of the membrane skin. Thus, optimizing the treatment concentration and time is essential to effectively utilize this process for performance improvement. Our present efforts are directed towards other mineral acids and the combination of acid/alcohol for effecting optimized flux enhancement without decreasing rejection. Comparing this work with that of our previ-
The novel chemical treatment shown here
ous work [24] on SW30HR (Filmtec) membranes, one can observe that the more dense, high-pressure, fresh membranes characterized by lower fluxes tend to give much higher flux increases with this method of treatment than do the higher flux, low pressure fresh membranes,
8. Conclusions causes an increase in flux without any loss in rejection for the commercially important, stateof-the-art polyamide-based TFC RO membranes used in this study. In most cases permanent increases in both flux and rejection have been observed. The flux enhancement observed is as much as 70% for the CPA2 and SWC1 membranes. This procedure easily can be used as a post-treatment step to the interfacial polymerization used to manufacture commercial RO membranes. The simultaneous increase in productivity and rejection of the spiral wound membrane modules manufactured from these chemically treated membranes will make them even more cost-effective for use in various desalination, waste-treatment and other process applications. Acknowledgment The authors wish to gratefully acknowledge the financial support of the New York State Energy Research and Development Authority ( N Y S E R D A ) for this work. We also thank Hydranautics for providing samples of their membranes for our research.
References [1] W.J. Koros, G.K. Fleming, S.M. Jordan, T.H.Kim and H.H. Hoehn, Prog. Polym. Sci., 13 (1988) 339.
D. Mukherjee et al. / Desalination 104 (1996) 239-249
[2] R.J.Petersen, J. Memb. Sci., 83 (1993) 81. [3] W.G. Light, H.C. Chu and C.N. Tran, Desalination, 64 (1987) 411. [4] J.E. Tomaschke, U.S. Pats. 4,872,984 ,Oct 10, 1989, and 4,948,507, Aug 14, 1991. [5] J.E. Cadotte, in: D.R. Lloyd, ed., Material Science of Synthetic Membranes, ACS Symposium Series 269, 1985 p. 273. [6] J.E. Cadotte, U.S. Patent 4,277,344, July 7, 1981. [7] H.H. Hoehn, in: D.R.Lloyd, ed., Material Science of Synthetic Membranes, ACS Symposium Series ,269, 1985 p. 81. [8[ K. Asfardjani, Y. Segul, Y. Aurelle and N. Abidine, J. Appl. Polym. Sci., 43 (1993) 271. [9] J.Y. Lai and Y.C. Chao, J. Appl. Polym. Sci., 39 (1990) 2293. [I0] S. Takigami, F. Kobayashi and Y. Nakamura, J. Polym, Sci., Part A: Polym. Chem., 25 (1987) 63. [11] F. Vigo and C. Uliana, J. Appl. Polym. Sci., 38 (1989) 1197. [12] F. F. Stengaard, Desalination, 70 (1988) 207. [13] M. Nystrom and P. Jarvinen, J. Membr. Sci., 60 (1991) 275. [14] A. Dimov and M.A. Islam, J. Membr. Sci., 50 (1990) 97. [15] A. Dimov and M.A. Islam, J. Appl. Polym. Sci., 42 (1991) 1285. [16] A. Higuchi, N. Iwata and T. Nakagawa, J. Appl. Polym. Sci., 40 (1990) 709. [17] T. Sano, T. Shimomura and I. Murase, US Patent No. 4,268,662, 1981. [18] J.M. Mohr, D.R. Paul, T.E. Mlsna and R.J .Lagow, J. Membr. Sci., 55 (1991) 131. [19] J.M. Mohr, D.R. Paul, Y. Taru, T.E. Mlsna and R.J. Lagow, J. of Memb. Sci, 55 (1991) 149. [20] J.M. Mohr, D.R. Paul, I. Pinnau and W.J. Koros, J. Membr. Sci., 56 (1991) 77. [21] M.E. Rezac, J.D. Le Roux, H. Chen, D.R. Paul and W.J. Koros, J. Membr. Sci., 90 (1994) 213. [22] A. Kulkami, D. Mukherjee, D. Mukherjee and W.N. Gill, Chem. Eng. Commun., 129 (1994) 53. [23] D. Mukherjee, A. Kulkami, A, Chawla and W.N. Gill, Chem. Eng. Commun., 130 (1995) 127. [24] D. Mukherjee, A. Kulkami and W.N. Gill, J. Memb. Sci., 97 (1994) 231. [25] R.H. Sedath, D.R. Taylor and N.N. Li, Sci.and Tech., 28(1-3) (1993) 255. [26] M. Oldani and G. Schock, J. Membr. Sci., 43 (1989) 243. [27] J. Lukas and V Tyrackova, J. Membr. Sci., 58 (1991) 49. [28] K. Kim and A.G. Fane, J. Membr. Sci., 88 (1994) 103. [29] K.J. Kim, M.R. Dickson, A.G. Fane and C.J.D. Fell,
249
J. of Microscopy, 162 (1991) 403. [30] C.R. Bartels, J.of Memb. Sci., 45 (1989) 225. [31] J.M. Mohr, D.R. Paul, Y. Tam, T.E. Mlsna and R.J. Lagow, J. of Applied Poly. Sci., 42 (1991) 2509. [32] J.D. LeRoux, D.R. Paul, M.F. Arendt, Y. Yuan and I. Cabasso, J.of Memb. Sci., 90 (1994) 37. [33] M. Fontyn, B.H. Bijsterbosch and K. Van't Riet, J. Membr. Sci., 36 (1987) 141. [34] D.T. Clark, Pure & Applied Chem., 54(2) (1982) 415. [35] D.T. Clark, Adv. Polym. Sci., 24 (1977) 125. [36] D.T. Clark and W.J. Feast, J. Macromol. Sci., -Revs. Macromol. Chem., C12(2) (1975) 191. [37] W. Zhang and B. Hallstrom, Desalination, 79 (1990) I. [38] W. Zhang, M. Wahlgren. and B. Sivik, Desalination, 72 (1989)263. [39] W.C. Hamilton, J. Colloid Interface Sci., 40(2) (1972) 219. [40] R.M. McDonogh, C.J.D. Fell and A.G. Fane, J. Memb. Sci., 31 (1987) 321. [41] C.W. Yao, R.P. Burford, A.G. Fane, C.J.D. Fell and R.M. McDonogh, J. Appl. Polym. Sci., 34 (1987) 399. [42] J.I. Kroschwitz, ed., Encyclopedia of Polym. Sci. and Eng., Vol ll,Wiley-Interscience, 1988, pp 381-409. [43] E.M. Fettes, ed., Chemical Reactions of Polymers, Vol. 19, Interscience, 1964, p. 561. [44] S..M. Aharoni, J. Appl. Polym. Sci., 45 (1992) 813. [45] K.O. Patten Jr. and L. Andrews, J. Phys. Chem., 90 (1986) 1073. [46] R.B. Bohn and L. Andrews, J. Phys. Chem., 93 (1989) 5684. [47] A.H. Ellison and W.A. Zisman, J.Phys. Chem., 58 (1954) 260. [48] A.F.M. Barton, Handbook of Solubility and Other Cohesion Parameters, CRC, Boca Raton, FL,1983. [49] D.K. Owens and R.C. Wendt, J. Appl. Polym. Sci., 13 (1969) 1741. [50] L.J. Broadbelt, S. Dziennik and M.T. Klein, Polym. Degrad. Stab., 44 (1994) 137. [51] L.J. Broadbelt, A. Chu. and M.T. Klein, Polym. Degrad. Stab., 45 (1994) 57. [52] R.F. Fibiger, J. Colucci, D.J. Forgach, R.A. Wessling and D.L. Schmidt, U.S. Patents 4,828,700, 1989, 4,894,165, 1990, 4,909,943, 1990. [53] H. Wood and S. Sourirajan, J. Colloid Interface Sci., 149(1) (1992) 105. [54] E.A. Gmlke, in: Polymer Handbook, 3rd. ed. J. Brandrup and E.H. Immergut, eds., Wiley, N.Y, 1989. [55] R.E. Kesting, Synthetic Polymeric Membranes: A Structural Perspective, Chap. 5 , Wiley 1985. [56] G.L. Brown, J. Polym. Sci., 22 (1956) 423.