High-Flux Reverse Osmosis Membranes With Fouling-Resistance for Seawater Desalination

         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 25^oC 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.