United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Cincinnati OH 45268
EPA-600/2-78-040
March 1978
Research and Development
&EPA
FBI Reverse Osmosis
Membrane for
Chromium Plating
Rinse Water
Environmental Protection
Technology Series
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-040
March 1978
FBI REVERSE OSMOSIS MEMBRANE
FOR CHROMIUM PLATING RINSE WATER
by
Howard J. Davis
Frank S. Model
Joseph R. Leal
Celanese Research Company
Summit, New Jersey 07091
for
THE AMERICAN ELECTROPLATERS1 SOCIETY
Winter Park, Florida 32789
Grant No. R-803620
Project Officer
Mary K. Stinson
Industrial Pollution Control Division
Industrial Environmental Research Laboratory
Edison, New Jersey 08817
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
-------
DISCLAIMER
This report has been reviewed by the Industrial Environmental
Research Laboratory - Cincinnati, U. S. Environmental Protection Agency
and approved for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
11
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FOREWORD
When energy and material resources are extracted, processed, con-
verted, and used, the related pollutional impacts on our environment and
even on our health often require that new and increasingly more efficient
pollution control methods be used. The Industrial Environmental Research
Laboratory - Cincinnati (IERL-CI) assists in developing and demonstrating
new and improved methodologies that will meet these needs both efficiently
and economically.
This report is a product of the above efforts. It was a laboratory
scale research study to assess the potential utility of recently developed
polybenzimidazole membranes in a reverse osmosis system for the treatment
of chromium plating rinse waters. Feasibility studies had demonstrated
that FBI possessed the requisite chemical stability to withstand long-term
contact with chromic acid waste streams where other, earlier membranes,
such as cellulose acetate and polyamides, have failed. The results of the
report are of value to R & D programs concerned with the treatment of
wastewaters from various metal finishing, nonferrous metal, steel, in-
organic and other industries. Further information concerning the subject
can be obtained by contacting the Metals and Inorganic Chemicals Branch
of the Industrial Pollution Control Division.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
111
-------
ABSTRACT
The objective of this laboratory research study was to develop a high
temperature, oxidation-resistant reverse osmosis (RO) membrane for the
treatment of metal finishing effluents. Specifically, the study was directed
to the selection and optimization of a flat polybenzimidazole (PBI) mem-
brane to treat chromium plating rinse water.
Important variables in casting PBI films and subsequent heat treatment
were investigated to assess their effects on membrane RO properties in
this application. Membranes were tested on a high pressure circulating
flow system in flat film test cells. Membrane screening tests were carried
out by pretesting them with sodium chloride feed solution at 5,000 ppm con-
centration. Membranes with optimum properties were then tested with a
simulated rinse water containing 200 ppm chromic acid or with authentic
plating bath solution diluted to 200 ppm chromic acid.
PBI RO membranes were found to be resistant to attack by chromic
acid. After pretesting with dilute sodium chloride solution, the membranes
showed a short-term effective level of rejection with chromic acid. Upon
retesting with sodium chloride, the same membranes exhibited high salt
rejections, thereby demonstrating that they had not been chemically
attacked by chromic acid.
A significant improvement in long-term rejection of chromic acid was
obtained when the membranes were pretreated with 5, 000 ppm sodium
tungstate solution. The continuous addition of sodium tungstate to the feed
solution further improved chromic acid rejection. However, this approach
to long-term high level rejection of chromic acid is not acceptable in
practice because of the possible contamination of the plating bath by
tungstate ions.
Another approach to long-term high level chromic acid rejection in-
volves pH adjustment. In this case, no pretreatment or preconditioning
of the membrane is required. When the pH of the feed was raised from
2.9 to 4. 0 with sodium hydroxide, an 80% chromic acid rejection resulted.
Adjustment of the pH to 5. 7 with ammonium hydroxide increased the
chromium rejection level to 95%. Again, this approach turned out to be un-
acceptable in practice because of the possible introduction of extraneous
ions into the working bath. The project failed to develop an acceptable PBI
RO membrane for treating chromium plating rinse water.
This report was submitted in fulfillment of Grant No. R-803620 by the
American Electroplaters' Society under the sponsorship of the U.S. Envir-
onmental Protection Agency. The report covers work performed in the
period between March 15, 1975 and December 15, 1976.
iv
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CONTENTS
Foreword jlii
Abstract iv
Figures vi
Tables vi
Acknowledgment .vii
I Introduction 1
II Conclusions 4
III Recommendations 5
IV Experimental 6
V Results and Discussion 9
VI References 27
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FIGURES
Number
1
2
Experimental Reverse Osmosis Test System 7
RO Properties of PBI Membranes in Sequential
Feed Changes 16
Effects of Different Membrane Treatments and Feed
pH on Chromic Acid Rejection 26
TABLES
Number Page
1 Effect of Casting ancl Annealing Conditions on Membrane
RO Properties 10
2 Long-term RO Properties with Sodium Chloride and
Chromic Acid Feed Solutions 12
3 RO Properties of Unconditioned Membranes 13
4 Effect of Sequential Feed Changes on Membrane RO
Properties. . 14
5 RO Properties of Tung state-Treated Membranes 17
6 RO Properties of Molybdate-Treated Membranes 19
7 RO Properties of Tung state-Treated Membranes 20
8 RO Properties of Membranes Coagulated in Tungstate
Solution 21
9 Effect of Tungstate Addition to Chromic Acid Feed
Solution on Membrane RO Properties 23
10 RO Properties of Fluo silicate-Treated Membranes 24
11 RO Properties of Crosslinked Membranes 25
VI
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ACKNOWLEDGMENT
Throughout this laboratory study, valuable guidance and suggestions were
offered by the members of the American Electroplaters1 Society Project 37
Committee. Particular recognition is given to Mssrs. Lawrence J. Durney
and O. Gardner Faulke, AES. The EPA project officer, Mary K. Stinson,
also contributed significantly to the program.
Financial support for this laboratory research study from the Office of
Research and Development of EPA and from the American Electroplaters'
Society is gratefully acknowledged.
VII
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I. INTRODUCTION
GENERAL
Electroplating and metal finishing waste streams are significant con-
tributors to stream pollution. This occurs either directly, owing to the
content in the effluents of such toxic and corrosive materials as cyanides,
acids and metals, or indirectly, as a result of the deleterious effects these
compounds exert on sewage treatment systems. Effective waste treatment
methods are needed by the metal processing industries to meet the strin-
gent effluent guidelines for 1977 and the more stringent guidelines proposed
for 1983 and 1985. Satisfactory methods exist for the removal and recove-
ry, if warranted, of most metals from plating waste waters, but chromium
remains a problem.
Chromium can be removed from waste or rinse water by a number of
methods -- chemical treatment, evaporation or ion exchange. Chemical
treatments methods are commonly practiced. These include the reduction
of hexavalent chromium to trivalent chromium by reagents such as sulfur
dioxide, sodium bisulfite or ferrous sulfate, followed by neutralization to
precipitate the metal; precipitation of hexavalent chromium by barium
chloride or sodium hydroxide; neutralization by sodium hydroxide, lime-
stone, sodium carbonate, etc. Chemical treatment methods, however,
pose the vexing problem of sludge and liquid waste disposal. Moreover,
there is an economic loss of valuable chromium. It would be preferable
to recover and recycle the chromium as well as the purified water.
PRINCIPLES OF REVERSE OSMOSIS
Reverse osmosis, a relatively new membrane process discovered
about 17 years ago and originally aimed at providing potable water from
brackish water, has aroused great interest in major industries, such as
food, textiles, chemical, metal processing, etc. It involves the pressure-
activated transport of.water through a semipermeable membrane by a
selective solubility process in which water dissolves and is subsequently
transmitted to an extent much greater than that for dissolved solutes.
Pressure, in excess of the solution's osmotic pressure, acts preferentially
to enhance the selective permeation of water; solute-flux is essentially
pressure-insensitive.
-------
The equations describing the transport of water and solute through a
membrane are:
J1= A ( A P - A IT )
J2 = D2K A C
A X
where J^ and J2 are water and solute fluxes. A P is the pressure differ-
ence across the membrane; ATT is osmotic pressure difference and A is
a membrane constant related to the diffusion coefficient and solubility of
water in the membrane. D2 is the solute diffusion coefficient; K is the dis-
tribution coefficient for the solute between membrane and solution; AC is
the concentration difference on the two sides of the membrane; and AX is
the effective membrane thickness.
APPLICATION OF REVERSE OSMOSIS TO METAL FINISHING WASTE
TREATMENT
Reverse osmosis offers the potential for achieving closed loop control
for treating metal finishing waste waters. Previous studies have shown
that RO is both technically and economically feasible for some plating
waste streams. Particular advantages of RO over other recovery proces-
ses include:
Low capital cost. The modular nature of RO units
make them particularly well-suited for small-scale
installations.
Low energy cost. Only power for pumping is required.
Low labor cost. The process is simple to operate
since it involves primarily the pumping of liquids.
Low space requirements. RO equipment is compact
and operates continuously, requiring minimal tankage.
No sludge. There are only two exit streams: purified
water return to the rinse bath, and concentrated me-
tal solution return to the finishing bath.
On the basis of pilot plant investigations sponsored by the Environmen-
tal Protection Agency and the American Electroplaters' Society, reverse
osmosis appears to be attractive for treating rinse waters from Watts
nickel, nickel sulfate (acid copper), copper pyrophosphate, zinc chloride,
and copper-zinc and cadmium cyanide baths, as well as total plant effluents.
However, the technique has not yet been found workable for chromium
wastes which need more chemically stable membranes.
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FBI REVERSE OSMOSIS MEMBRANES
The potential of the polybenzimidazole (FBI) polymer, poly-^2, 2'-
(m-phenylene)-5, 5!-bibenzimidazole, prepared as described by Concia-
tor.i et al* ', as a reverse osmosis membrane was explored several years
ago. RO properties of flat FBI membranes were found to be comparable
to cellulose acetate at ambient conditions and significantly superior at ele-
vated temperature. The polymer is known to possess outstanding thermal,
physical and chemical stability over a wide pH range and a high affinity for
water. Its solubility in N, N' -dimethylacetamide (DMAc) permits easy con-
version of polymer to fiber or flat film.
In a series of research studies concerned with morphological anil
transport characteristics, PBI membranes were found to possess useful,
long-term RO properties in both hollow fiber and flat film configurations
(2, 3, 4, 5). FBI's oxidative resistance to, and stability in, high levels of
chlorination in the seawater desalination study ' ' and its resistance to
strong acid made it an attractive RO candidate for chromium plating rinse
water application where earlier membranes, including cellulose acetate
and polyamides, cannot be used.
In this research project, PBI RO membranes were evaluated in flat
film configuration with a simulated rinse water containing 200 ppm chromic
acid. Major efforts were devoted to the optimization of membrane prop-
erties as influenced by fabrication variables and post treatment techniques,
including those based on reacting the basic -NH- groups of polybenzimi-
dazole with reactive agents to favorably alter transport properties.
-------
II. CONCLUSIONS
Pretreatment of PBI RO membranes with sodium tungstate significant-
ly improves the long-term rejection of chromic acid over that of the Tin-
treated membrane. Newly fabricated, untreated membranes exhibit low
chromic acid rejection levels. The tungstate treatment markedly increases
the rejection which remains effective for at least nine days. Even higher
rejection levels are achieved when 100 ppm sodium tungstate is added to the
chromic acid feed solution. However, the addition of extraneous ions, such
as tungstate, to chromium plating rinse water would not be acceptable in
practice since it would not be feasible to recycle the chromium solution
containing tungstate ions.
Other conditioning or treatment agents tested were not as effective as
sodium tungstate. With sodium chloride, only a short-term beneficial
effect is observed.
Optimum RO membrane properties are obtained from PBI films cast
from a dimethylacetamide solution containing 18% polymer by weight,
coagulated in water at room temperature, and annealed in ethylene glycol
for 10 minutes at 165°.
PBI RO membranes are resistant to attack by chromic acid. The in-
trinsic RO properties are not affected by several weeks exposure to 200
ppm chromic acid feed solution at pH 2. 9-3. 0. This is demonstrated by
the high salt rejection achieved after testing with chromic acid solution.
Adjustment of the feed pH by addition of sodium hydroxide or aqueous
ammonia yields the highest chromic acid rejection levels and an increase
in water flux, without pretreatment or preconditioning of the membrane.
Rejection levels as high as 95% were obtained on adjusting feed pH from
pH 2. 9 to pH 5. 7 with aqueous ammonia. As with the tungstate addition to
the rinse water, pH adjustment is not acceptable in practice because of the
possible buildup of harmful salts in the plating solutions.
Although membrane performance was improved by feed pH adjustment
and membrane treatment, the project failed to develop a PBI membrane
which would be acceptable or satisfactory for the treatment of chromium
plating rinse water.
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IE. RECOMMENDATIONS
In view of the demonstrated improved level of chromic acid rejection
obtained with PBI RO membranes by pretreatment with sodium tungstate
and the demonstrated polymer's resistance to chromic acid, it is recom-
mended that the study be continued on a laboratory scale to develop a
membrane with higher long-term rejection levels without the addition of
agents to the feed solution.
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IV. EXPERIMENTAL
MEMBRANE CHARACTERIZATION
A high pressure, closed loop, circulating flow loop with flat film test
cells taking 2 - inch membrane circles was assembled. The system was
described in an earlier study ' ' and is shown in Figure 1. Feed solution
from a five-gallon feed tank was pumped through the test cells by a positive
displacement Lapp diaphragm pump rated for 1500 psi service at a flow
rate of about 20 gallons per hour. From the pump, the feed passed to the
manifold on which 12 cells were mounted in series. The membrane sample,
1. 9 inches in diameter, was placed in the cell on top of a filter paper cir-
cle and supported underneath by a porous steel disc. Cell diameter was
1. 9 inches and the chamber clearance of 0 . 090 inches over the membrane
sample resulted in a linear feed flow rate of about 65 feet/min. Operating
pressure was controlled by a back-pressure regulator. Feed composition
was maintained constant over an extended test period by returning permeate
to the feed tank by means of a small return pump. The chromic acid feed
solution was changed about every two days because it reacted with parts of
the metal system and slowly changed in concentration. Percent conversion
i.e., the ratio of product volume removed to feed volume, was essentially
zero since negligible volumes of permeate were removed for analyses.
Flux, percent chromic acid rejection (%R) and pH of feed and product
were determined. Flux is the volume rate of flow of permeate per unit
membrane area and was determined by timing the volume of permeate
flowing into a graduated cylinder. It is expressed as gallons per square
foot per day (gfd).
The percent rejection, %R, of solute is defined as:
%R = C£ - Cp x 100
Cf"
where C| = solute concentration in the feed
Co= solute concentrations in the product
Chromium concentration was determined by a spectrophotometric method
based on measuring the absorbance at 540 nanometers of the highly colored
cdiromium-diphenylcarbazide complex. A Beckman recording spectre-
photometer, DB-GT, was used. Atomic absorption analyses were also
6
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FIGURE 1 EXPERIMENTAL REVERSE OSMOSIS TEST SYSTEM
BACK PRESSURE
VALVE
H.R
PRESSURE
GAUGE
O
L.P.
PRESSURE
GAUGE
O
x—
/
FLOW
METER
CHECK
VALVE
FLOW
VOLUME
CELL
u
0*3
RETURN
PUMP
(MEMBRANE
COND.A CELL
CELLV
PERMEATE
LINE
•M-
i-HXJ-
FEED
TANK
L.P.
SURGE
TANK
FILTER
H.P.
SURGE
TANK
DAMPER
VALVE
SOLENOID
-VALVE
BYPASS
VALVE
H.P. SAFETY
VALVE
H.R PUMP
LTJ
HEAT
EXCHANGER
FEED STREAM
PERMEATE STREAM
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carried out on selected samples to verify the accuracy of the spectropho-
metric method.
Measurements of pH on feed and permeate samples were made with a
Beckman SS-2 pH meter.
Concentrations of sodium chloride in sodium chloride feed solutions and
permeate samples were obtained by conductivity measurements with a
Radiometer conductivity meter, Model CDM 2nd (Copenhagen, Denmark.)
MEMBRANE OPTIMIZATION
Important variables in the film casting operating and annealing step
which were studied included (1) polymer concentration in the casting solu-
tion, (2) water coagulation bath temperature, (3) film thickness and (4)
annealing conditions (time and temperature). The 4" x 12" films were cast
on glass plates in a laboratory film casting unit, dried 60 seconds in an
air drying chamber and then immersed in water coagulation baths. As a
result of the operations an asymmetric film is obtained. Polymer solutions
with 16%, 18% and 20% polymer in dimethylacetamide (DMAc) were prepared
by diluting a 24% PBI dope to the target solids concentration with DMAc.
Coagulation temperatures of 25°C and 5°C were employed. Film thickness
ranged from 2 mils to 5 mils. Annealing was carried out in ethylene glycol
at 165°C and 180°C and in polyethylene glycol at 220°C. It is during the
annealing step that the RO properties are developed in the thin, dense sur-
face (the air side of the film as cast onto glass plate) of the asymmetric
membrane. This active layer, about 1 micron thick, is supported by the
open, porous substructure. The films were lightly constrained during
annealing to prevent distortion but not completly constrained because
shrinkage, even though small, was sufficient to cause tears and cracks in
the film.
Post treatment of membranes included treating the membrane with
inorganic salts or with strong acid to prevent or reduce interaction between
the -NH-groups in PBI and chromic acid feed solution.
8
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V. RESULTS AND DISCUSSION
The effects of film casting variables and annealing conditions on RO
properties are summarized in Table 1. These data were obtained with a
5, 000 ppm sodium chloride feed solution at pH 6. 4. Duplicate samples
were tested and the values reported are the averages. Membrane casting
and annealing conditions for each sample are given. These are, in the
order presented: casting solution concentration; film coagulation tempera-
ture; annealing medium, (ethylene glycol and polyethylene glycol); anneal-
ing temperature; and wet film thickness.
These results indicate that the best combination of RO properties was
obtained with films cast from 18% solutions (dopes) and annealed in ethylene
glycol at 165°C. An annealing time of 10 minutes was adequate. Annealing
times as long as 30 minutes are detrimental while annealing times of less
than 5 minutes do not develop as good RO properties. Membrane sample
F-l, 2 is typical of these preparative conditions. It exhibited good flux,
27.6 gfd, and rejection, 83% after 24 hours (this would improve with time)
against 5, 000 ppm sodium chloride feed. Flux decreases and rejection
improves with time due to pressure-induced compaction and tightening up
of the membrane. This effect is noted for samples B, C, G and H. Mem-
branes from the 20% solution exhibited properties equivalent to F-l, 2.
Coagulation temperature does not appear to have any effect on RO proper -
ties; hence, room temperature is preferred as a matter of convenience.
Membrane thickness of 4-5 mils proved to be adequate to withstand
sustained, high pressure operation. Film thickness less than 2. 5 mils
proved to be inadequate. This is shown for membrane samples D and E,
Table 1, both of which failed under pressure testing at 600 psi.
Annealing studies in ethyjene glycol showed no real difference in mem-
brane RO properties between 165° and 180° annealing temperatures. The
lower annealing temperature was adopted as the standard in membrane
fabrication. Membranes annealed at a much higher temperature, e. g.
220°C, in polyethylene glycol, exhibited poor RO properties, as shown by
the very low flux values and low rejection levels for membrane sample
1-A in Table 1.
Films were cast at room temperature from 16%, 18% and 20% solutions
of PBI in dimethylacetamide to assess the effect of polymer concentration
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TABLE 1 EFFECT OF FILM CASTING AND ANNEALING CONDITIONS ON
MEMBRANE RO PROPERTIES
Membrane Casting, Annealing
Samples Conditions*
B-l, 2
C-l, 2
D
E
F-1,2
G-l, 2
H-l, 2
1-A
A-l, 2
16%, RT, EG, 165°, 4 mils
16%, 5° C, EG, 165°
4 rails
18%, RT, EG, 165°,
21/2 mils
18%, 5° C, EG, 165° C,
21/2 mils
18%, RT.EG, 165°C,
4-5 mils
18%,RT,EG, 180° C,
4-5 mils
18%, 5°C, EG,165°C
4-5 mils
18%, RT, PEG, 220° C,
4-5 mils
20%, RT, EG, 165° C,
3 mils
Total Flux
Test gfd
Hours
24
192
24
192
(Too thin;
(Too thin;
24
24
48
24
48
75
92
192
18.8
17.2
16.9
15.2
failed)
failed)
27.6
21.3
19.7
23.0
19.5
0.4
0.3
11.4
Rejection
%
84
89
64
73
83
86
87
86
88
52
53
89
Test Conditions: 600 psig, 25°C, 20 gal/hr, 0% conversion
Feed Solution: Sodium Chloride, 5, 000 ppm, pH 6. 4
#EG-Ethylene glycol
PEG-Polyethylene glycol
RT -Room temperature
10
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on film and membrane properties. Casting solutions with 18% polymer
concentration were found to yield the best combinations of film strength,
solution viscosity, film casting speed, film thickness control and RO
properties. This therefore was adopted as the standard solids concentra-
tion.
Membrane samples from Table 1 were selected for long-term testing
with sodium chloride solution before proceeding to the critical testing with
chromic acid feed solution. Membrane samples A and B performed well
through 420 hours with sodium chloride. This is shown in Table 2. Re-
jection levels were 94% and 88% and flux values were 10 gfd and 17 gfd,
respectively, for samples A and B. Samples F and H also exhibited good
RO properties with the sodium chloride feed solution.
The feed solution was then changed at the 420th hour from dilute sodi-
um chloride to 180-200 ppm chromic acid, as shown in Table 2. Through
the next 72 hours, i. e., from 420 to 490 total test hours of testing with
chromic acid, all four membrane samples were performing effectively in
rejecting chromic acid. The rejection values ranged from 62% to 69% and
the flux values ranged from 5 gfd to 12 gfd. Feed pH was 2. 9-3. 0 and pro-
duct pH was 3. 3-3.4. However, the next 48 hours of testing with chromic
acid revealed a rapid deterioration in RO properties. Chromic acid re-
jection values dropped sharply and after 5 days (120 hours) testing with
chromic acid had declined to 10% and less for all four samples. In addition,
a decline in water flux was observed over this period. Some of the decline
in chromic acid rejection level can be attributed to the lower flux but the
total drop in rejection level far exceeds that attributed to flux decline.
Fresh, unconditioned membranes were then tested against chromic acid
first, followed by sodium chloride feed solution to determine whether or
not the membranes were irreversibly attacked by chromic acid. A pre-
liminary indication that the membranes were not attacked by chromic acid
was available from the flux data in Table 2. The fact that the product
flux did not increase as the chromic acid rejection dropped sharply,
strongly suggested that the membranes were not attacked. Table 3 shows
rejection/flux data for fresh membranes, including one (J) which was
cross-linked with perfluoroglutaric acid in an attempt to prevent the basic
imidazole nitrogens in PBI from reacting with chromic acid. All three
membranes exhibited very low rejections of chromic acid. After 72 hours,
the rejection values for samples J and L were less than 10%. Surprisingly,
Sample K showed a slight increase in rejection during the 72 hours test
period. This behavior is typical of RO membranes in which pressure-
induced compaction results in a tightening of the membrane and a conse-
quent increase in rejection.
Additional long-term testing was carried out to assess the effects of
11
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TABLE 2. LONG-TERM RO PROPERTIES WITH SODIUM CHLORIDE
AND CHROMIC ACID FEED SOLUTIONS
Membrane Sample
A
Test
Feed Hours
NaCl 24
48
145
170
240
392
420
Cr03 440
492
518
542
Reject.
93
94
94
95
95
93
94
62
70
65
21
3
Flux
gfd
15.0
14.5
12.5
12.5
12.0
10.5
10.0
5.5
5.0
5.2
5.0
4.0
B F H
Reject.
84
87
88
88
89
88
88
66
74
62
25
11
Flux Reject.
gfd %
18.5
17.5
16.0
16.0
15.0
17.0 82
17.0 83
11. 5 69
10. 0 77
11. 0 65
9.3 27
7.5 10
Flux Reject. Flux
gfd % gfd
26.5 86 23
89 19-0
17.0 69 13.0
14. 0 75 11. 5
12.0 63 10.3
10.0 29 8.7
9.0 3 10.3
Test Conditions: 600 psig; 25°C; 20 gal/hr; 0% Conversion.
Feed Solutions; NaCl, 5, 000 ppm, pH 6. 4.
CrO3, 180-200 ppm, pH 2. 8-3.0.
12
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TABLE 3. RO PROPERTIES OF UNCONDITIONED
MEMBRANES
Membrane Sample
J
Test Reject. Flux
Feed Hours % gfd
CrO3 20 13 10.5
44 4 6.2
72 4 5.3
Nad 92 61 5. 5
116 64 5.0
140 66 5. 5
230 65 5.0
K
L
Reject. Flux Reject.
% gfd %
15 2.8
15 2.0
22 1.5
83 2.0
87 1.8
88 2.0
93 1.8
9
3
6
54
53
56
55
Flux
gfd
13.5
6.8
4.7
5.0
4.5
5.0
4.5
Test Conditions: 600 psig; 25°C; 20 gal/hr; 0% conversion.
Feed Solutions: NaCl; 5, 000 ppm; pH 6.4.
CrO3; 200 ppm; pH 2.8.
13
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membrane pretreatment, pH adjustment and annealing conditions on RO
properties. Test results are given in Table 4.
It is concluded from these test data that neither the chemical nor phy-
sical integrity of the FBI membrane is affected by long-term exposure to
dilute chromic acid. This is substantiated by the high rejection levels -
80% with chromic acid adjusted to pH 4. 0 and 96% with sodium chloride -
achieved in subsequent testing with these feed solutions. The beneficial
effect of pfl adjustment is striking, with rejection of hexavalent chromium
increasing from 20% to about 80%. This is presumable due to the forma-
tion of sodium dichromate salt upon addition of sodium hydroxide to adjust
the pH.
The membranes in Table 4 were cast from 18% polymer solutions, and
all, except sample 2A, were annealed in ethylene glycol at 165°C. Sample
2A was annealed in polyethylene glycol at 220°C, which resulted in poor
RO properties. Very low flux values, 0.4-0. 5 gfd, were obtained. Mem-
branes IB and 2A were pretreated with 10% sulfuric acid before testing in
an attempt to improve the RO properties. No significant effect was noted,
however.
Following the feed pH adjustment which sharply increased the rejection
of chromium (+6), and changing back to 5, 000 ppm sodium chloride feed,
the membranes exhibited excellent salt rejection. Rejections of 93% and
better were obtained, thereby demonstrating that PBI is resistant to attack
by chromic acid. Upon replacing the sodium chloride test feed with 200
ppm chromic acid feed, these membranes again exhibited a sharp drop in
chromic acid rejection. The sequential test results in Table 4 are pre-
sented in Figure 2 for a convenient overview of RO properties obtained with
the different feed solutions and membrane treatments.
The RO properties of PBI membranes were improved when the mem-
branes were pretreated with sodium tungstate. The objective was to form
a complex ol the anion with the reactive, basic NH groups in the polymer
in order to prevent their interaction with chromic acid. However, to be
effective, the resultant PBI-tungstate complex must be sufficiently stable
and non-leachable when exposed to dilute chromic acid solution. The mem-
brane samples from Table 4, were, therefore, conditioned with sodium
tungstate solution by exposing them to a 5, 000 ppm sodium tungstate feed
solution for 48 hours in the high pressure flow loop. At the end of this per-
iod, the membranes we re then tested again with chromic acid. The results
are presented in Table 5.
A significant improvement in long-term chromic acid rejection was ob-
tained. After 9 days testing against 200 ppm chromic acid feed, the re-
jections levelled off, ranging from 42% to 55% for three samples. This was
14
-------
TABLE 4. EFFECT OF SEQUENTIAL FEED CHANGES ON
MEMBRANE RO PROPERTIES
1
Reject.
Feed
NaCl
CrO3
Adjust
pH to
4.0
NaCl
CrO3
Hours
75
96
112
132
156
225
250
275
300
395
420
448
525
545
582
606
630
707
%
90
92
74
41
19
16
57
73
75
78
78
81
95
68
36
14
22
24
Membrane Sample
IB* 2A*
Flux Reject,
gfd
11.0
10.1
2.3
2.0
2.2
2.0
1.9
1.8
- -
1.6
1.6
1.7
3.1
1.4
1.2
1.3
1.4
1.1
%
86
88
76
43
19
20
58
74
77
81
81
84
93
69
37
18
20
25
3##
. Flux Reject Flux Reject Flux
gfd % gfd
9.5 83 1.6
8.9 85 1.4
2.6 97 0.5
2.4 64 0.4
2.6 30 0.4
2.4 33 0.4
2.1 52 0.5
1. 9 (Removed)
- -
1.7
1.7
1.6
2.8
1.6
1.3
1.2
1.4
1.0
% gfd
76
76 5.1
78 4.4
96 9.2
70 4.8
37 3.9
14 4.1
19 4.5
22 3.4
*Pretreated with sulfuric acid.
**Same membrane as 1, but without NaCl conditioning.
Test Conditions: 600 psig; 25°C; 20 gal/hr; 0% conversion.
Feed Solutions: NaCl; 5, 000 ppm; pH 6.4.
CrO3; 200 ppm; pH 2. 8.
15
-------
100
2 80
a
3 60
OC
40
20
5,000 ppm NaCI
200 ppm Ci
I pH 2.9
15
£ 10
x
ADJUST pH
I TO 4.0
5,000 ppm NaCI
200 ppm 003
100
200
300
400
500
600
700
_L
±
100
200
300 400
HOURS
500
600
700
O
o
o
a
#
M
o
o
hd
w
M
w
O
4
^
W
Q
M
en
H
en
en
M
D
a
w
H
5:
t-
-------
TABLE 5. RO PROPERTIES OF TUNGST ATE-TREATED
MEMBRANES
Membrane Sample
Test
Feed Hours
Na2WO4 24
48
Cr03 48
72
96
125
194
218
242*
266
1
Reject.
98
98
-
90
60
48
44
46
47
45
Flux
gfd
8.2
7.3
-
3.9
1.7
1.3
1.3
1.2
1.4
1.1
2
Reject.
98
99
-
90
77
53
51
50
51
55
Flux
gfd
7.1
6.5
-
?.9
1.6
1.4
1.3
1.4
2.0
1.4
Reject
98
98
-
90
60
50
46
49
45
42
3
Flux
gfd
13
12
-
7.2
2.6
2.6
2.8
2.7
4.1
2.9
Test Conditions: 600 psig, 25°C, 20 gal/hr, 0% Conversion.
Feed Solutions: Sodium tungstate, Na2WO4; 5, 000 ppm, pH 7. 3.
Chromic acid, CrO3; 200 ppm, pH 2.8-3.0.
*Raised pressure to 750 psig for 6 hours.
17
-------
significantly better than the 22%-25% rejections for the same membranes
at the end of the chromic acid test cycle reported in Table 4. Product
fluxes for the three membranes in Table 5 ranged from 1.2 to 2. 9 gfd. It
is worth noting that during the conditioning period with 5, 000 ppm sodium
tungstate (pH 7. 3), rejections as high as 98%-99% were obtained along with
product flux ranging from 6. 5 to 12 gfd. These results again show that the
intrinsic RO properties of PBI membranes are not affected by long-term
exposure to chromic acid rinse water.
Molybdate, another large oxygenated anion, was investigated as a con-
ditioning agent for PBI membranes. Following the procedure used in the
tungstate treatment, membranes were conditioned for 48 hours in the high
pressure (600 psi) test loop with 5, 000 ppm sodium molybdate solution,
after which they were tested with 200 ppm chromic acid feed plus 2 ppm
sulfuric acid. The results are reported in Table 6.
After 234 total test hours - - the last 186 hours or nearly eight days
against chromic acid - - chromic acid rejections for the four membranes
levelled off at about 30%. This is a significantly lower level of rejection
than was achieved with the tungstate-treated membranes. Product flux
values were similar, ranging from 1.4 gfd to 2. 3 gfd. Again, as was ob-
served in the tungstate treatment, very high solute rejections, ranging from
90% to 98%, were obtained in the conditioning period after 48 hours.
In order to obtain test data which more closely reflect actual plating
operation conditions, an authentic chromium plating bath solution
(33 oz/gal) was obtained and diluted down to a rinse water concentration of
200 ppm CrO3. All subsequent membrane test results to be discussed be-
low were obtained with the diluted plating solution. New membranes were
prepared from the parent batch of 18% polymer solution.
The tungstate treatment was repeated and the treated membranes were
tested against the diluted plating bath solution. Test results are given in
Table 7 and very closely approximate those in Table 6 obtained with the
simulated rinse water. Chromic acid rejection values for the four samples
levelled off and after 8 days ranged from 42% to 60%. Product flux ranged
from 1. 3 to 2.1 gfd.
A modification of the tungstate treatment was investigated in an effort
to obtain higher chromic acid rejection. This consisted of coagulating the
freshly cast film in a 5% aqueous solution of sodium tungstate, instead of
water alone, to entrain tungstate in the film. Annealing was then carried
out in the manner previously described. On testing these membranes with
dilute plating bath solution, the chromic acid rejection values were slightly
lower than those obtained with the tungstate treated membranes in Tables
5 and 7. Test results are given in Table 8. The RO properties are
18
-------
TABLE 6. RO PROPERTIES OF MOLYBD ATE-TREATED
MEMBRANES
Membrane Sample
Feed
Na2MoO4
CrO3
Test
Hours
24
48
48
72
96
120
144
168
234
C
Reject .
%
81
90
-
32
23
31
32
24
27
-1
Flux
Kfd
10.0
6.6
-
2.9
2.7
1.8
1.8
2.3
2.3
C-2
Reject.
%
98
98
-
68
57
50
53
22
34
Flux
gfd
10.3
9.2
-
1.6
1.6
1.5
1.5
2.4
1.9
C
Reject .
%
96
96
-
72
52
48
47
24
34
-3
Flux
gfd
8*9
7.6
-
1.4
1.4
1.4
1.3
2.0
1.6
C
Reject.
%
98
98
-
84
67
59
60
22
34
-4
Flux
gfd
8.2
8.1
-
1.1
1.2
1.4
1.1
1.9
1.4
Test Conditions: 600 psig, 25°C, 20 gal/hr, 0% Conversion
Feed Solutions: Sodium molybdate, Na2MoO4> 5, 000 ppm, pH 7. 6
Chromic acid, CrO 200 ppm + 2 ppm sulfuric acid,
pH 2. 9.
19
-------
TABLE 7. RO PROPERTIES OF TUNGST ATE-TREATED
MEMBRANES
Membrane Sample
Feed
Na2WO4
CrO3
Test
Hours
24
48
48
72
96
168
A-l
Reject
%
98
98
-
69
49
45
Flux
gfd
6.8
5.4
-
2.1
1.9
1.1
A-
Reject.
%
99
99
-
87
71
62
2
Flux
gfd
5.6
4.2
-
1.4
1.3
1.2
A-
Reject.
%
97
98
-
74
53
47
3
Flux
gfd
5.1
4.2
-
1.7
1.5
1.6
A-4
Reject.
%
99
99
-
66
48
47
Flux
gfd
6.8
5.6
_
1.8
1.7
1.8
238 42 2.1 60 1.3 43 1.6 43 1.9
Test Conditions: 600 psig, 25°C, 20 gal/hr, 0% Conversion
Feed Solutions: Sodium tungstate, Na2WO4; 5, 000 ppm; pH 6. 8.
Diluted plating bath solution, 175 ppm CrO-,
pH 3.0
20
-------
TABLE.8. RO PROPERTIES OF MEMBRANES COAGULATED
IN TUNGSTATE SOLUTION
Membrane Sample
E-l E-3
Feed
CrO3
Test Reject.
Hour s %
24 32
48 34
72 34
Flux Reject.
gfd %
3.2 40
3.4 43
3.0 42
Flux
gfd
3.2
3.5
2.9
E-4
Reject. Flu
% gfd
35 3.2
39 3. 3
40 2. 7
E-5
Reject. Flux
% gfd
41 2. 5
39 2,8
'41 2. 4
Test Conditions: 600 psig, 25°C, 20 gal/hr, 0% Conversion.
Feed Solutions: Sodium tungstate, Na2WO4; 5, 000 ppm; pH 6. 8.
Diluted plating bath solution, 175 ppm CrOj, pH 3. 0
21
-------
significantly better, however, than those of untreated membranes, attes-
ting to the beneficial effect of the tungstate anion on FBI membrane proper-
ties.
In yet another modification of the tungstate treatment and in a continued
effort to achieve higher chromic acid rejections, 100 ppm sodium tungstate
was added to the chromic acid feed solution. The membranes were first
pretreated in the test loop with 5, 000 ppm sodium tungstate feed solution
as before. Percent rejection and flux values which were obtained in long-
term testing are found in Table 9. These results clearly show that mem-
brane performance improved significantly as a result of adding 100 ppm
sodium tungstate to the feed. At 210 total test hours, 168 hours with the
200 ppm chromic acid plating solution, chromic acid rejection values had
levelled off at 69%-74% for the four samples. The average value of 71%
represents a significant improvement over the approximately 50% rejection
value from tungstate pretreatment alone (Tables 5 and 7). Product flux
values at 210 total test hours ranged from 2. 0 to 2.9 gfd. These fluxes are
higher than those obtained in Tables 5 and 7 for the pretreatment alone.
Thus, the addition of tungstate anion to the feed raised the chromic acid re-
jection level to a useful, sustained level of 70% or more over 7 days, at
which point the feed pH was raised to assess the effect of pH adjustment.
Highest rejections of hexavalent chromium were obtained when the pH
of the feed, containing 200 ppm chromic, acid, 100 ppm sodium tungstate and
about 10 ppm sulfuric acid, was raised to pH 5. 7 by addition of aqueous
ammonia. At 48 hours after pH adjustment, rejections of chromium (+6)
had risen to 95% and product flux increased substantially, ranging from 4.1
gfd to 6. 0 gfd, as seen in Table 9.
Other membrane treatments were investigated but these proved ineffec-
tive in improving chromic acid rejection. One was the pretreatment with
sodium fluosilicate and the other involved ionically crosslinking the poly-
mer with perfluoroglutaric acid, a strong acid. Test results for the fluo-
silicate treatment are given in Table 10 and in Table 11 for the crosslink-
ing case. It is seen that the chromic acid rejections for the fluosilicate-
treated membranes were much lower than that achieved with the tungstate-
treated membranes. Even lower rejections were observed with the cross -
linked membranes.
The effects of the various membrane treatments and feed pH on chro-
mic acid rejection are presented in Figure 3 for convenient comparison.
Although membrane performance was improved by feed pH adjustment and
membrane treatment, the project failed to develop a PBI membrane which
would be acceptable in practice for the treatment of chromium plating
rinse water.
22
-------
TABLE 9. EFFECT OF TUNGSTATE ADDITION TO CHROMIC ACID
FEED SOLUTION ON MEMBRANE RO PROPERTIES
Feed Hours
Na2WO4
CrO3+
Na2W04
t
24
48
48
72
96
120
168
210
234
258
Membrane
F-l F-2
Reject. Flux Reject, Flux
% gfd % gfd
99.0 13.5 99.5 13.4
99.4 11.2 - 11.0
-
84 2.8 86 2.7
78 3.0 81 2.9
74 3.1 75 2.7
74 2.9 77 2.8
74 2.8 74 2.8
91 6.7 91 6.1
95 6.0 95 5.6
Sample
F
Reject.
%
99.4
99.4
-
89
85
80
79
70
90
95
_3
Flux
gfd
12.2
9.7
-
2.5
2.1
2.5
2.4
2.4
5.3
4.8
F
Reject
%
99.0
98.9
-
89
85
80
79
69
94
94
-4
Flux
gfd
11.5
9.3
-
2. 2
2.2
2.1
2.1
2.0
4.7
4.1
Test Conditions: 600 psig, 25°C, 20 gal/hr, O% Conversion
Feed Solutions: Sodium tungstate, 5, 000 ppm, pH 7. 2
Diluted plating bath solution, 200 ppm CrO3+ 100 ppm
sodium tungstate + 10 ppm sulfuric acid to bring to
pH 3.0.
-------
TABLE 10. RO PROPERTIES OF FLUOSILICATE
TREATED MEMBRANES
Membrane Sample
D-2 D-3
Reject. Flux Reject. Flux
Feed Hours % gfd % gfd
Na2SiF6 24 82 1. 7 85 1. 8
48 79 1.6 84 1.3
CrO3 48
72 55 1.1 41 1.6
96 46 0.9 42 1.1
120 39 1.0 38 1.2
192 27 1.1 29 1.2
D-4 D-6
Reject. Flux. Reject. Flux
% gfd % gfd
71 2.1 59 1.6
69 1.5 58 1.2
- - - -
31 2.3 31 2.6
17 2.3 17 2.1
19 2.2 10 2.5
15 2.6 15 2.0
Test Conditions: 600 psig, 25° C, 20 gal/hr, 0% Conversion.
Feed Solutions: Sodium fluo silicate, Na2SiF/, 5, 000 ppm, pH 3. 8.
Chromic acid, CrO^, 200 ppm, pH 3. 0.
24
-------
TABLE 11. RO PROPERTIES OF CROSSLINKED
MEMBRANES
Membrane Sample
G-l G-2 G-3 G-4
Reject. Flux Reject. Flux Reject Flux Reject Flux
Feed Hours % gfd % gfd % gfd % gfd
CrO3 24 10 ' 12 10 7.2 12 5.9 14. 5.0
48 0 10.6 8 5.9 7.9 5.0 12 4.1
Test Conditions: 600 psig, 25°C, 20 gal/hr, 9% Conversion
Feed Solution: Chromic acid, 200 ppm, pH 3. 0.
25
-------
FIGURE 3:
EFFECTS OF DIFFERENT MEMBRANE TREATMENTS AND
FEED pH ON CHROMIC ACID REJECTION
100
UJ
-------
VI. REFERENCES
1. Conciatori, A.B.; E.G. Chenevey; T. C. Bohrer; and A. E. Prince, Jr.
Polymerization and Spinning of PBI. J. Polymer Sci. , Part C.
19: 49-64, 1969-
2. Davis, H. J. ; F.S. Model; M. Jaffe; M. A. Sieminski; A. A. Boom;
and L. A. Lee. An Investigation of Polybenzimidazole Hollow Fiber
Reverse Osmosis Membranes. Final Report; Contract No. 14-30-2810,
U. W. Department of the Interior, Office of Saline Water, Washington,
D.C. July 1972.
3. Davis, J. J. ; J. W. Soehngen; and F. S. Model. Seawater Desalination
with PBI Hollow Fiber Reverse Osmosis Membranes. Final Report,
Contract No. 14-30-3199, U.S. Department of the Interior, Office of
Saline Water, Washington, D. C. , February 1974.
4. Davis, J. J. and F. S. Model. Celanese Research Company. PBI
Reverse Osmosis Membrane Developement for use in Wash Water
Recovery at 165°F. (Presented at 1973 Intersociety Conference on
Environmental Systems, San Diego. July, 1973) 5 p.
5. Poist, J. E. and F. S. Model. Development of Improved PBI
Membrane Systems for Wash Water Recycling at Pasteurization
Temperatures. Final Report, Contract No. 14-30-3112, U.S. Depart-
ment of the Interior, Office of Saline Water, Washington, D. C.
July 1974.
6. Model, F.S. ; H. J. Davis; A. A. Boom; C. Hrlfgott; and L. A. Lee.
The influence of Hydroxyl Ratio on the Performance of Reverse
Osmosis Desalination Membranes. Research and Development
Progress Report No. 657. U.S. Department of the Interior, Office
of Saline Water, Washington, D.C. June 1971
7 Standard Methods for the Examination of Water and Waste Water.
Twelfth Edition. New York, American Public Health Association,
Inc., 1969. p. 123-124.
27
-------
TECHNICAL REPORT DATA
(Please read InsJmctionS on tne reverse before completing)
1. REPORT NO.
EPA-600/2-78-040
4. TITLE AND SUBTITLE
FBI REVERSE OSMOSIS MEMBRANE FOR CHROMIUM PLATING
RINSE WATER
3. RECIPIENT'S ACCESSION"NO.
5. REPORT DATE
March 1978 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Howard J. Davis, Frank S. Model, Joseph R. Leal,
Celanese Research Company, Summit, N.J. 07901
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The American Electroplaters' Society, Inc.
Winter Park, Florida 32789
10. PROGRAM ELEMENT NO.
1BB610
11. CONTRACT/GRANT NO.
R-803620
12. SPONSORING AGENCY NAME AND ADDRESS
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY-Cin., OH
CINCINNATI, OHIO, OFFICE OF RESEARCH AND DEVELOPMENT
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 46268
13. TYPE OF REPORT AND PERIOD COVERED
FINAL
14. SPONSORING AGENCY CODE
EPA-600/12
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A laboratory research study was carried out to select and optimize polybenzimida-
zole(PBI) reverse osmosis(RO) membranes for the treatment of chromium plating rinse
water. The effects of important film casting and annealing variables on RO properties
were investigated. Membranes were tested in high pressure flow cells with dilute
sodium chloride and chromic acid feed solutions.
PBI RO membranes were found to be resistant to attack by chromic acid. After
pretesting with sodium chloride, the membranes showed a short-term effective level
of rejection with chromic acid. Upon retesting with sodium chloride, the same mem-
branes again exhibited high salt rejections, thereby demonstrating they had not been
chemically attacked by chromic acid.
A significant improvement in long-term rejection of chromic acid was obtained when
the membranes were pretreated with sodium tungstate solution. The addition of sodium
tungstate to the feed solution further improved chromic acid rejection. Highest
levels of chromic acid rejection were obtained by adjusting the pH of the feed.
However, neither pH adjustment nor addition of tungstate to the feed are acceptable
in practice because of possible contamination of the plating bath by the extraneous
ions. T^e project failed to develop an acceptable PBI RO membrane for treating
chromium plating rinse water.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Electroplating*
Waste Treatment
Chromium*
8. DISTRIBUTION STATEMENT
Release to Public
b.lDENTIFlERS/OPEN ENDED TERMS
Polybenzimidazole
Polymer(PBI) membrane*
Chrom plating bath*
Reverse Osmosis*
Rinse water
19. SECURITY CLASS (ThisReport}
Unclassified
20. SECURITY CLASS (Thispage)
Unclassified
COSATI Field/Group
68C
68D
21. NO. OF PAGES
35
22. PRICE
EPA Form 2220-1 (9-73)
28
* U.S. GOVERNMENT PRINTING OFFICE: 1978—260-880-41
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