The availability of fresh water is directly associated with accessible natural resources. However, 2.5 billion of the world's population (around 40%) does not have access to proper sanitation systems, with 6 to 8 million annual deaths related to inadequate water supply, sanitation and hygiene in 2013. Currently, sea water desalination offers a feasible strategy to face global water challenge. Different water desalination techniques were developed and membrane desalination is currently the highest cost effective technique. Reverse osmosis (RO) system is by far considered the least expensive membrane process. Typically, RO system uses the thin film composite (TFC) membranes. A typical TFC membrane consists of two layers: a top dense polyamide (PA) skin layer (responsible for salt rejection) applied on an underlying support layer (responsible for mechanical support of the thin PA layer). Recently, a new category of membranes has emerged known as thin film nanocomposite membranes (TFNC) where nanoparticles (NPs) are incorporated into the support layer to enhance its properties. The support layer surface pore diameters are quite crucial in supporting and preserving the integrity of the PA layer. Thus, the ideal support layer shall comprise a non-porous to slightly porous top surface. However, a support layer with non-porous surface would resist the water flow. Consequently, the main target of the work represented was to fabricate a highly porous membrane that could still support a PA layer on top of it. Membranes with symmetric cross section have high permeability due to the highly interconnected porous structure. Yet, the surface of the symmetric membranes is also highly porous; and hence, serving as a TFNC support is challenging. Thus, this study focuses on tailoring symmetric TFNC support membranes to effectively support the PA layer. Firstly, we investigated the influence of different fabrication parameters on the support membrane properties. This entailed the understanding of the thermodynamic behavior of the cast solution during fabrication till the final precipitation of the support membrane. TFNC support membranes were prepared using cast solution of Polyethersulfone (PES) polymer in N-methyl-2-pyrrolidone (NMP) as a solvent. Afterwards, the effect of non-solvent addition was investigated using Triethylene Glycol (TEG). Furthermore, PluronicÂ® (Plu) and Titanium dioxide (TiO2) NPs were incorporated in two different sets of experiments to compare the enhancement of support membrane hydrophilicity and mechanical stability. Support membranes were fabricated using two consecutive phase separation processes, namely: Vapor-Induced Phase Separation (VIPS) followed by Liquid-Induced Phase Separation (LIPS). Various conditions were tested during the VIPS process, including relative humidity degree (RH) at exposure, exposure time and the effect of air convection during the exposure period. The cast solutions were prepared under 30% and 80% RH for exposure time ranging from 1 to 5 minutes. Forced convection condition was applied to the cast solutions whereas compressed dry air was introduced to the cast solution during the exposure period. On the other hand, free convection condition was defined in terms of the absence of compressed dry air introduction during VIPS process. Solution composition was systematically changed to further understand its influence on the thermodynamic behavior under VIPS process. This entailed studying the change in PES content ranging from 10 to 15 wt%, as well as the TEG (0 to 60 wt%), Plu (0 to 5 wt%) and TiO2 (0 to 1 wt%). This variability in cast solution composition clarified the influences of the solution viscosity and hygroscopicity on the thermodynamic behavior of the cast solution, which in turns, reflected on the support membrane final morphology. Afterwards, support membranes were characterized for their cross-sectional morphology using scanning electron microscopy, pore size distribution using the capillary flow porometer, hydrophilicity using contact angle method, surface charge using surface charge analyzer and chemical composition using Fourier transform infrared spectroscopy and proton nuclear magnetic resonance. Also, membranes hydraulic permeability and wettability were tested. Membranes fabricated under different conditions showed various structures including asymmetric and symmetric cross section morphologies. The effect of air convection was significantly important and in some cases even switched the cross section structure from asymmetric to completely symmetric. Interestingly, at low RH value (30%) and under free convection condition, membranes with semi-symmetric structure were successfully produced. This novel structure holds the privileges of both symmetric and asymmetric membranes. It showed high water permeability and mechanical stability due to the highly interconnected pores structure, as well as, having a very thin skin surface to support the PA layer on top of it. Furthermore, the semi-symmetric membrane showed higher compaction resistance (91.3%) and recovery (94%) as compared to the asymmetric membrane. As a consequence, the semi-symmetric morphology was considered as the structure of our interest as a TFNC support membrane. Support membrane hydrophilicity, water permeability, mechanical stability and morphology are known to have high contribution to the overall TFNC membrane performance. Thus, the developed semi-symmetric structure was then reproduced using cast solutions containing the hydrophilic additives Plu and TiO2. Results showed that the addition of TiO2 had increased both the membrane hydrophilicity and compaction resistance. However, semi-symmetric supports were only achievable with 0.05 and 0.1 wt% TiO2 concentrations. As a concluding step, polyamide (PA) top skin layer was fabricated on semi-symmetric support membranes of different compositions. The final TFNC showed the higher permeability values when semi-symmetric supports were compared to asymmetric support of same composition. Furthermore, the highest TFC permeability was for support membrane containing 1 wt% Plu and that containing 0.1 wt% TiO2.
MS in Nanotechnology
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Khalil, Ahmed S.G.
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(2016).Synthesis and characterization of efficient polyamide thin film nanocomposite membranes [Master’s thesis, the American University in Cairo]. AUC Knowledge Fountain.
Elleithy, Mohamed M. A.. Synthesis and characterization of efficient polyamide thin film nanocomposite membranes. 2016. American University in Cairo, Master's thesis. AUC Knowledge Fountain.