(711c) Novel Reverse Osmosis Membranes for Seawater Desalination

Authors: 
Zhao, L., The Ohio State University
Ho, W. S. W., The Ohio State University



         Reverse
Osmosis (RO) is one of the most promising technologies for water
desalination, which serves as an important solution to the global water shortage.  However, the hydrophilicity of the aromatic
polyamide (PA) thin-film-composite (TFC) RO membrane is
insufficient, which results in a considerably limited water flux.  In addition, traditional RO membrane is
very vulnerable to fouling from large molecules.

         A
hydrophilic additive is defined as a chemical compound which
contains at least one hydrophilic portion and
one reactive portion that can react with
either MPD or TMC during the interfacial polymerization.  Therefore, the additive is chemically
bound to the cross-linking network formed by MPD and TMC.  It is
believed that the hydrophilic additive incorporated in the TFC membrane can
provide an additional pathway for the molecular transport of water.  As a result, the incorporation of hydrophilic additive is able to
improve the water flux of the RO membranes, thereby increasing the water
productivity.

         In
this work, o-aminobenzoic
acid-triethylamine salt was selected as the specific hydrophilic additive to investigate its effects
on membrane separation
performance under
seawater desalination conditions for the first time.  In addition, different membrane
preparation conditions were investigated and optimized to further
improve the water flux of the synthesized membranes.  After optimization, the membrane samples
were also tested
by using
the seawater from Port Hueneme, CA to determine their desalination performance
for the real application.  Furthermore, the
fouling resistance of the high-flux membranes was compared with that of the standard
TFC membranes.

         The
high-flux RO membranes were synthesized through advanced interfacial
polymerization where o-aminobenzoic
acid-triethylamine salt was added in the aqueous MPD solution.  The membrane synthesized with the
hydrophilic additive exhibited significantly improved desalination performance,
in a side-by-side comparison with the membrane synthesized without the
additive.  After optimizing the
concentration of the hydrophilic additive in the MPD solution, the water flux
of the synthesized membrane was enhanced by about 62%, and the salt rejection remained the same.  For instance, the membrane synthesized with
1.0
wt% o-aminobenzoic acid/triethylamine
salt showed 39.2
gfd (gallons/ft2/day) water flux and 99.46% salt rejection.  The incorporation concentration of o-aminobenzoic acid-triethylamine salt was maintained at
1.0 wt% for the optimization of other membrane synthesis conditions to enhance
water flux without compromising salt rejection.  With 40 wt% isopropanol (IPA) in the
aqueous MPD solution, the water flux increased to 44.4 gfd, and salt rejection
was still above 99.4%.  The effects
of additional drying time after soaking polysulfone support in the amine
solution and hydrocarbon removal time after interfacial polymerization were
also investigated.

         The high-flux
membranes synthesized under the optimal conditions were tested by using the
seawater from Port Hueneme, CA with an average turbidity of 0.032 Nephelometric
Turbidity Units (NTU) and a silt density index (SDI) (15 min) of 2.2.  The average water flux was about 41.8
gfd and the average salt rejection was above 99.4%.  The lower water flux was found to result
from the higher solid concentration in the real seawater compared to the
synthetic seawater which contains 3.28% NaCl.  In addition, the synthesized
high-flux RO membrane exhibited an excellent stability in terms of water flux
(average 40
gfd) and salt rejection (average 99.4%) during a 30-day test with the seawater from
Port Hueneme, CA at 25oC and 800 psi applied pressure across the
membrane.  This good stability was
verified by comparing FTIR spectra of the same membrane sample before and after
the 30-day test.  These results are
important for the commercialization of this high-flux RO membrane for seawater
desalination.

       Sodium
alginate was selected as the foulant to investigate membrane fouling, since it
is a common compound in seawater from seaweeds.  The membrane fouling resistance was
significantly improved with the incorporation of the hydrophilic additive.  For example, in the presence of 50 ppm
sodium alginate in the feed solution, the water flux reduction of the membrane
synthesized with the hydrophilic additive was 19.6% compared to 32.0% for the
membrane synthesized without the additive.