Chlorine-resistant composite membranes with high organic rejection

A method for making a chlorine-resistant composite polyamide membrane having high organic rejection, the essential step of which comprises treating a conventional composite membrane with an acyl halide. The novel membrane is especially suitable for the treatment of water containing chlorine or lower molecular weight organic compounds.

The government has certain nonexclusive rights in this invention, which was 
made with government support under Contract Nos. NAS9-18477, 
DE-FG03-92ER81422, and N00167-88-C-0023 awarded by NASA, the Department of 
Energy, and the Department of the Navy, respectively. 
BACKGROUND OF THE INVENTION 
Composite polyamide reverse-osmosis (RO) membranes are known. See, for 
example, U.S. Pat. Nos. 4,259,183, 4,529,646, and 4,277,344. The treatment 
of such membranes to increase rejection has been described, for example, 
in U.S. Pat. No. 4,960,517, which discloses a process whereby a 
cross-linked polyamide RO membrane is treated with a solution of an 
amine-reactive reagent group such as the sodium salt of chloracetic acid, 
a carboxylic acid anhydride, a carboxylic acid ester, a 1,3-heterocyclic 
sultone, and an amine-reactive ethylenically unsaturated compound such as 
acrylic acid. The resulting membrane has an improved rejection for solutes 
such as NaCl, NaNO.sub.3, H.sub.2 SO.sub.4, NaOH, and isopropyl alcohol 
(IPA). U.S. Pat. No. 4,960,518 describes a method of treating a polyamide 
RO membrane with an oxidizing agent such as peroxycarboxylic acid, 
periodic acid, chloramine compounds, and N-bromoamine to improve its 
rejection of sulfuric acid and IPA. U.S. Pat. No. 4,964,998 describes a 
process for separating water from organics such as IPA wherein the 
permselective layer of the RO membrane is a cross-linked polyamide that 
has been treated with an amine-reactive reagent such as sultone or nitrous 
acid. Because such membranes are often used in the treatment of 
chlorinated water, there has been a general recognition in the art of a 
need for such membranes to have a high resistance to attack by chlorine. 
Various methods have been proposed to improve the chlorine resistance of 
composite RO membranes. See, for example, U.S. Pat. No. 4,812,238. 
Thus, although the prior art has utilized various treatments of polyamide 
RO membranes to improve their chlorine-resistance and their rejection of 
organics, there has been no recognition of the use of an acyl halide 
treatment to so improve such membranes. There is therefore a need in the 
art for membranes capable of simple fabrication that have high rejections 
for organic compounds and improved chlorine resistance. 
SUMMARY OF THE INVENTION 
The present invention lies in the discovery that a high rejection, 
chlorine-resistant membrane can be obtained by treating a conventional 
composite polyamide RO membrane with an acyl halide, resulting in the 
production of a membrane having significant increases in 
chlorine-resistance and in the rejection of low molecular weight organics. 
In a most preferred embodiment, fabrication of such a novel composite 
membrane comprises the steps of (a) forming a polyamide composite 
membrane, (b) swelling the membrane, (c) drying the swollen membrane and 
(d) contacting the dried swollen membrane with a solution of an acyl 
halide. Excellent chlorine-resistance and a modest improvement in organics 
rejection may also be obtained by omitting steps (b) and (c).

DETAILED DESCRIPTION OF THE INVENTION 
The preparation of composite polyamide RO membranes is well-known. In the 
present invention, it is preferred that the polyamide membrane be a 
multilayered composite membrane having a polyamide selective layer made by 
interfacial polymerization. Accordingly, the polyamide membrane is 
preferably formed by reacting reactants comprising (1) a compound bearing 
at least two amine groups and (2) a compound bearing at least two acyl 
halide groups, whereby the amine and acyl halide groups react to form the 
polyamide Examples of the preparation of such membranes are given in U.S. 
Pat. Nos. 4,277,344, 4,853,122, 4,978,455, and 4,876,009, the disclosures 
of which are incorporated herein by reference. It is especially preferred 
that the polyamide membrane be made by reaction of an aromatic diamine 
with a trifunctional acyl halide, such as is disclosed in U.S. Pat. Nos. 
4,277,344, 4,828,708, 4,872,984, and 4,830,885, the disclosures of which 
are incorporated herein by reference. 
The membrane support layer (as opposed to the selective layer) that 
provides mechanical strength to the composite should give as little 
resistance to the transport of the permeating species through the 
selective layer as is technically feasible. Additionally, the membrane 
support should be chemically resistant, allowing for operation on feed 
streams containing various chemical constituents. Materials suitable for 
the membrane support include, but are not limited to, organic polymers 
such as polypropylene, polyacrylonitrile, poly(vinylidenefluorides), 
poly(etherimides), polyimides, polysulfones, poly(ethersulfones), 
poly(arylsulfones), poly(phenylquinoxalines), polybenzimidazoles, and 
copolymers and blends of these materials; and inorganic materials such as 
porous glass, carbon, ceramics, and metals. 
The material used to swell the composite polyamide membrane is preferably 
an aqueous solution. A swelling solution comprising merely water is 
sufficient for the treatment process to improve the organic rejection 
characteristics of the membrane, and so mere use of the composite 
polyamide membrane for RO treatment of water, followed by drying, will 
suffice. In a preferred embodiment, additives are added to the swelling 
solution to enhance the swelling treatment. Examples of such additives 
include a surfactant; alcohols such as methanol, ethanol, ethylene glycol, 
phenol, cresols, and glycerol; amines such as triethylamine (TEA), 
monoethanol amine (MEA), and diethanol amine (DEA); amides such as 
dimethylacetamide (DMAC), dimethylformamide (DMF), and formamide (FA); 
pyrrolidinones such as N-methylpyrrolidinone (NMP); ketones such as 
acetone, methylethyl ketone (MEK), and methyl-isobutyl ketone (MIBK); 
esters, such as ethyl acetate; and organic acids such as acetic acid and 
formic acid. The surfactant selected may be anionic, cationic, nonionic, 
or amphoteric. Especially preferred is the surfactant sodium lauryl 
sulfate (SLS). The concentration of surfactant used can range from 1 to 
10,000 ppm or, more preferably, 100 to 5000 ppm. The swelling solution may 
also contain an acid scavenger or base. Examples include sodium hydroxide, 
triethylamine (TEA), and sodium carbonate. TEA is especially preferred. 
The concentration of acid scavenger used may range from 10 to 10,000 ppm 
or, more preferably, 100 to 1000 ppm. 
It has been found that the length of swelling time is particularly 
important in obtaining treated membranes with high fluxes. Generally, the 
swelling time must be greater than one-half hour for the treatment to be 
effective. However, longer swelling times, from 24 to 48 hours, result in 
membranes with higher fluxes, without any consequential loss in organic 
rejection. 
Once the membrane has been swollen, it is dried. Drying may be accomplished 
by simply exposing the membrane to the atmosphere, or by directing a 
stream of dry gas past the membrane surface. Generally, the drying time 
should be less than about 30 minutes. 
The swollen, dried composite polyamide membrane is then contacted with a 
solution of an acyl halide in a water-immiscible solvent. The acyl halide 
is preferably monofunctional in the sense that it has only one acyl halide 
group. Exemplary acyl halides include benzoyl chloride, acetyl chloride, 
furoyl chloride, napthoyl chloride, nitrobenzoyl chloride, pipernylic acid 
chloride, coumalic acid chloride, and 5-oxo-2-tetrahydrofuran carboxylic 
acid chloride. The concentration of acyl halide used in the treatment 
solution may be between 0.01 and 10 wt %, but is most preferably between 
0.5 and 2 wt %. Any water-immiscible liquid may be used as the solvent for 
the acyl halide, provided it does not react with the acyl halide or the 
polyamide RO membrane, and provided the acyl halide dissolves to the 
desired concentration in the solvent. Exemplary solvents include hexane, 
heptane, dodecane, petroleum distillates, napthas, chlorinated 
hydrocarbons, freons, and aromatics. The length of acyl halide treatment 
is also important in determining the final performance of the treated 
membrane. Treatment time may range from 5 seconds to 30 minutes, with 30 
to 60 seconds being preferred. Following the acyl halide treatment, the 
membrane may be dried a second time. The second drying step may also be 
accomplished by simply exposing the membrane to the atmosphere, or by 
directing a stream of dry gas past the membrane surface. Generally, the 
second drying step should be conducted for less than about 30 minutes. 
Following the second drying time, the membrane may be rinsed in an 
appropriate rinse solution such as water. 
Although the most preferred embodiment of the present invention includes 
the swelling and drying steps prior to acyl halide treatment, improved 
chlorine-resistance and modest entrancement of organics rejection are also 
obtained when the swelling and drying steps are omitted. 
The so-fabricated membrane may be used in the form of a flat sheet, a 
hollow fiber or a tube. All of the above treatment process steps may be 
performed either before or after the membrane is placed into a module. The 
resulting treated membrane exhibits an improved rejection for organic 
compounds relative to that of the untreated membrane, allowing the treated 
membrane to produce a significantly "cleaner" permeate, and reducing or 
eliminating the need for a second RO membrane separation to further purify 
the permeate from the first RO membrane. The resulting treated membrane 
also has an improved resistance to chlorinated feed solutions. 
Specifically, when exposed to a chlorinated feed, the water flux does not 
decline as severely as it does for an untreated membrane. The treated 
membrane will therefore find considerable application for the treatment of 
waste-waters containing either chlorine or low molecular weight organics 
or both. 
EXAMPLE 1 
A commercially available flat sheet composite polyamide membrane ("FT-30" 
from FilmTec Corp. of Minneapolis, Minn.) was swollen in an aqueous 
solution of 500 ppm TEA for 30 minutes. After blotting excess solution 
from the membrane surface, the swollen membrane was allowed to air-dry for 
10 minutes. The swollen, dried membrane was then treated for 10 minutes in 
a solution of 1 wt % benzoyl chloride in hexane. The membrane was then 
rinsed in water for 5 minutes and allowed to air-dry for 2 hours. The 
resulting membrane was then tested for organic passage (100% minus % 
organic rejection) in an RO test using a feed solution containing 100 ppm 
of the organic benzyl alcohol in water at 55 atm, 25.degree. C., and pH 6. 
The so-treated membrane exhibited a rejection for benzyl alcohol of 90% 
corresponding to a benzyl alcohol passage of 10%, and had a water flux of 
25 L/m.sup.2 -hr. 
COMATIVE EXAMPLE 1 
An untreated FT-30 membrane was operated on an identical feed solution and 
under identical conditions as in Example 1. This membrane exhibited a 
benzyl alcohol passage of 20%, and had a water flux of 50 L/m.sup.2 -hr. 
Thus, the benzyl alcohol passage in the membrane was twice that of the 
treated membrane of Example 1. 
EXAMPLES 2 to 6 
The treated FT-b 30 membrane of Example 1 was tested on feed solutions 
containing 100 ppm of various organic solutes and compared with tests of 
the same feed solutions by untreated FT-30 membranes. The results of these 
tests are presented in Table 1. These data show that the organic rejection 
of the treated membrane was significantly higher than that of the 
untreated membrane. 
EXAMPLE 7 
A sample of the commercially available FT-30 membrane was swollen in an 
aqueous solution of 500 ppm TEA and 1000 ppm sodium lauryl sulfate (SLS) 
in water for 48 hours. After blotting excess solution from the membrane 
surface, the swollen membrane was allowed to air-dry for 10 minutes. The 
swollen, dried membrane was then treated for 30 seconds in a solution of 1 
wt % furoyl chloride in hexane. The membrane was again allowed to air-dry 
for 10 minutes prior to rinsing in water. The resulting membrane was then 
tested in an RO test using a feed solution of 100 ppm phenol in water at 
55 atm, 25.degree. C., and pH 6. The same membrane, untreated, was used on 
the same feed under the same conditions and the results are shown in Table 
1. As is apparent from Table 1, the phenol passage rate through the 
treated membrane (2%) was reduced by a factor of 3.5, when compared to 
that for the untreated membrane (7%). 
EXAMPLES 8 to 10 
The treated membrane of Example 7 was tested in the same manner as in 
Example 7 on feed solutions containing 100 ppm of various solutes. The 
results of these tests are also presented in Table 1, along with the 
results of treatment of the same feeds under identical conditions by 
untreated FT-30 membranes. 
TABLE 1 
______________________________________ 
Water Flux* 
Ex. Organic Rejection 
Treated/ 
No. Organic Treated Untreated 
Untreated 
______________________________________ 
2 benzaldehyde 
95% 89% 25/50 
3 ethanol 80% 60% " 
4 2-butoxyethanol 
96% 93% " 
5 cresol 96% 89% " 
6 urea 98% 60% " 
7 phenol 98% 93% 25/55 
8 IPA 96% 82% " 
9 MEK 96% 91% " 
10 TCE 73% 68% " 
______________________________________ 
*in units of L/m.sup.2hr 
EXAMPLES 11 to 13 
Samples of the commercially available FT-30 membrane were swollen in a 
solution of 1000 ppm TEA and 100 ppm SLS in water for times ranging from 
30 minutes to 48 hours. After blotting excess solution from the membrane 
surface, the swollen membrane was allowed to air-dry for 10 minutes. The 
swollen, dried membrane was then treated for 10 minutes in a solution of 1 
wt % furoyl chloride in hexane. The membrane was again allowed to air-dry 
for 10 minutes prior to rinsing in water. The resulting treated membranes 
were then tested in an RO test using a feed solution of 100 ppm of the 
organic phenol in water at 55 atm, 25.degree. C., and pH 6. The results of 
these tests, presented in Table 2, indicate that longer swelling times 
result in higher phenol rejections and higher water fluxes. 
TABLE 2 
______________________________________ 
Example Swelling Phenol 
No. Time (hr) Rejection 
Water Flux* 
______________________________________ 
11 0.5 94% 14 
12 22 97% 28 
13 48 97% 25 
______________________________________ 
*in units of L/m.sup.2hr 
EXAMPLES 14 to 17 
Samples of the commercially available FT-30 membrane were swollen in an 
aqueous solution of 1000 ppm TEA and 100 ppm SLS in water for 22 hours. 
After blotting excess solution from the membrane surface, the swollen 
membrane was allowed to air-dry for times ranging from 0 to 30 minutes. 
The swollen, dried membrane was then treated for 2 minutes in a solution 
of 1 wt % furoyl chloride in hexane. The membrane was again allowed to 
air-dry for 10 minutes, prior to rinsing in water. The resulting treated 
membranes were then tested in an RO test using a feed solution of 100 ppm 
phenol in water at 55 atm, 25.degree. C., and pH 6. The results of these 
tests, presented in Table 3, indicate that shorter air-dry times lead to 
higher water fluxes with substantially the same improvement in the rate of 
phenol rejection. 
TABLE 3 
______________________________________ 
Example First Dry Time 
Phenol 
No. (min) Rejection 
Water Flux* 
______________________________________ 
14 0 97% 39 
15 2 96% 34 
16 10 97% 28 
17 30 97% 26 
______________________________________ 
*in units of L/m.sup.2hr 
EXAMPLES 18 to 21 
Treated membranes were prepared using the same procedures outlined in 
Examples 14 to 17, except that the first air-dry time was set at 10 
minutes and the concentration of furoyl chloride was varied from 0.5 to 2 
wt %. The resulting membranes were then tested in an RO test using a feed 
solution of 100 ppm phenol in water at 55 atm, 25.degree. C., and pH 6. 
The results of these tests, presented in Table 4, indicate that lower acyl 
halide concentrations lead to higher water fluxes with no change in the 
high rate of phenol rejection. 
TABLE 4 
______________________________________ 
Example Acid Chloride Conc. 
Phenol 
No. (wt %) Rejection Water Flux* 
______________________________________ 
18 0.5 97% 34 
19 1.0 97% 35 
20 1.5 97% 23 
21 2.0 97% 28 
______________________________________ 
*in units of L/m.sup.2hr 
EXAMPLES 22 to 25 
Treated membranes were prepared using the same procedures outlined in 
Examples 14 to 17, except that the swelling time was set at 48 hours, the 
first air-dry time was set at 10 minutes and the acyl halide treatment 
time was varied from 0.5 to 30 minutes. The resulting membranes were then 
tested in an RO test using a feed solution of 100 ppm phenol in water at 
55 atm, 25.degree. C., and pH 6. The results of these tests, presented in 
Table 5, indicate that shorter acyl halide treatment times lead to higher 
water fluxes with substantially no change in the high rate of phenol 
rejection. 
TABLE 5 
______________________________________ 
Example Treatment Time 
Phenol 
No. (min) Rejection 
Water Flux* 
______________________________________ 
22 0.5 98% 25 
23 2 97% 25 
24 10 97% 23 
25 30 97% 18 
______________________________________ 
*in units of L/m.sup.2hr 
EXAMPLE 26 
A composite polyamide membrane was prepared by interfacial polymerization 
of isophthaloyl chloride and tri-tetrakis(aminomethyl)methane [(NH.sub.2 
CH.sub.2).sub.3 -C-O-C(CH.sub.2 NH.sub.2).sub.2 -O-C(CH.sub.2 
NH.sub.2).sub.3 ] on the surface of a microporous polysulfone support. 
This membrane was swelled by immersing in water for 10 minutes, and then 
air-dried overnight. The membrane was then treated for 30 seconds with a 
solution of 0.5 wt % oxalyl chloride in hexane. The treated membrane was 
then rinsed in hexane and allowed to again air-dry overnight prior to 
evaluating in an RO test. The resulting treated membrane had a water flux 
of 39 L/m.sup.2 -hr in the RO test. This same membrane was then tested on 
a feed solution containing 20 ppm chlorine. After one hour of operation, 
the water flux was 32 L/m.sup.2 -hr, representing a water flux decline of 
17%. 
For comparison, an untreated membrane of the same composition suffered a 
73% loss in flux after exposure to a second feed stream containing 
one-half the chlorine concentration (10 ppm) for 1 hour, demonstrating 
that the post-treatment technique of the present invention substantially 
improves the chlorine-resistance of the composite membrane. 
The terms and expressions employed in the foregoing specification are used 
therein as terms of description and not of limitation, and there is no 
intention, in the use of such terms and expressions, of excluding 
equivalents of the features shown and described or portions thereof, it 
being recognized that the scope of the invention is defined and limited 
only by the claims which follow.