Synthesis, modification and characterization of economical high performance reverse osmosis membranes
Falath, Wail Sulaiman
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Water is becoming increasingly scarce as the demand for fresh water continues to increase in a drastic manner. One potential new water resource is desalination of sea and brackish water. Reverse osmosis (RO) membranes desalination is one of the many processes used to obtain potable water fit for human consumption from seawater. Nevertheless, the membranes used in this process are prone to fouling by microorganisms and Chlorine attack. It has been shown from literature that developing an anti-fouling, Chlorine resistant, highly selective and highly permeable reverse osmosis membrane is a necessity. To develop such a membrane, one should improve the hydrophilicity of the membrane surface, reduce its roughness and make the surface negatively charged. The overall goal of this research was the development of high performance reverse osmosis membranes materials with improved permeability, high salt rejection and superior biofouling and Chlorine resistance. The present research was designed to investigate novel poly (vinyl alcohol) (PVA) RO membranes with various fillers and combinations and their effectiveness as active RO separation layers with improved biofouling and Chlorine resistance. The uniqueness of this work was that the PVA polymer matrix was utilized as an active RO layer without the use of any polymeric or ceramic substrate. The crosslinked PVA RO membranes incorporated with various fillers were fabricated using dissolution casting method. The fabricated membranes were then characterized and analyzed using various techniques like attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), contact angle measurements, X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and mechanical testing. The actual reverse osmosis performance of the membranes, including permeation testing, salt rejection and Chlorine resistance was examined using a reverse osmosis permeation unit. This study showed that the incorporation of Pluronic F127 and MWCNTs into the PVA polymer matrix improved the overall RO performance of the membrane in terms of hydrophilicity, surface roughness, water permeability, salt rejection, Chlorine resistance and biofouling resistance. The membranes that contain 0.08 and 0.1 wt% MWCNTs provided optimal salt rejection, Chlorine and biofouling resistance and mechanical strength. Although the permeation of these two membranes is not the best, they relatively have an excellent water flux. Furthermore, It has been shown that the conjugation of Vanillin and Pluronic F-127 improved the overall RO performance of the membrane in terms of hydrophilicity, surface roughness, salt rejection, Chlorine resistance, biofouling resistance and mechanical strength. Membranes PVA-V4 and PVA-V5 provided optimal salt rejection, Chlorine resistance, mechanical strength and surface hydrophilicity. Although the permeation of these two membranes is not the best, they relatively have an excellent water flux. It has been shown that conjugating PVA with Gum Arabic and Pluronic F127 improved the overall RO performance of the membrane in terms of hydrophilicity, surface roughness, water permeability, salt rejection, Chlorine resistance, biofouling resistance and mechanical strength. The membrane PVA-GA-5 that contains 0.9 wt% Gum Arabic provided optimal salt rejection, Chlorine and biofouling resistance, mechanical strength, permeability, surface roughness and surface hydrophilicity. The incorporation of ZnO-NPs and Pluronic F-127 improved the performance of RO membrane, as well. The improvement was in terms of water permeability, salt rejection and Chlorine resistance by increasing hydrophilicity and affecting the roughness of the membrane. The membrane TFN4, containing 0.08 wt% ZnO-NPs, exhibited superior permeation flux, salt rejection and Chlorine resistance. HPEI-RO membranes having zwitterions were effectively synthesized, as well. FTIR analysis confirmed the HPEI and MA functional group of the membranes. The AFM roughness value of all HPEI-RO membranes decreased with increasing HPEI content in zwitterionic membranes. Both water contact angle and RO performance test measurements demonstrated that HPEI-RO membranes had high permeation flux and could bind high amount of water molecules with zwitterions in membranes. In addition, the HPEI-RO3 exhibited the stability during desalination process and good fouling resistance property. Lastly, the outcomes of this study have shown a great promise for the proposed crosslinked PVA membrane as an active RO separation layer without a substrate. The results of this investigation showed that the fabricated RO membrane overcame PVA drawbacks through appropriate crosslinking and through appropriate selection of fillers. The synthesized membranes had an improved RO performance and an enhanced Chlorine and biofouling resistance.