In this PhD thesis, seawater desalination by Membrane Distillation (MD) has been explored from the perspective of process and membrane. Regarding the process, an innovative, energy efficient, and environmentally friendly Zero Thermal Input Membrane Distillation (ZTIMD) process was proposed. ZTIMD uses thermal energy stored in seawater, which makes the process sustainable by being independent of the external sources of thermal energy, which is one of the major contributors to the cost and energy consumption of conventional MD desalination processes. Economic feasibility study was carried out for the ZTIMD process, and it was demonstrated that drinking water could be produced with a cost of $0.28/m3, which is approximately half of the cost of conventional desalination processes. Regarding the membrane, novel MD membranes were developed through incorporation of nanomaterials in polyvinylidene fluoride (PVDF). Different nanomaterials including superhydrophobic SiO2, amine modified hydrophilic SiO2, CuO, and CaCO3 were used for this purpose. It was shown that membrane structure and consequently its performance could be affected by the nanoparticle properties, concentration, presence of backing material, PVDF blend ratio, and penetration time. In a best membrane developed in this work, almost 2500% increase was observed in the Vacuum Membrane Distillation (VMD) flux over that of the neat PVDF membrane at a feed temperature of 27.5 °C and vacuum pressure of 1.2 kPa, when 7.0 wt.% hydrophilic SiO2 nanoparticles were added into a PVDF membrane supported with Non-Woven Fabric (NWF) polyester. The membrane possessed near perfect selectivity.

Cost-effective purification and desalination of water is a global challenge. Reverse osmosis (RO), the current method of choice, requires high pressure drops across the membranes in order to achieve acceptably high flow rates. Conventional polymer membranes are limited in their performance by a trade-off between water permeability and water/salt selectivity. Biofilm fouling is another critical problem in RO applications. Recent simulations and experiments suggest that properly functionalized carbon nanotubes (CNTs) can be used to construct RO membranes that have high permeation flux as well as complete ion rejection, and that are resistant to biofilm formation. The objective of this research was to combine zwitterion-functionalized carbon nanotubes with traditional thin film polyamide (PA) to fabricate a novel desalination membrane which has both high permeability as well as selectivity. Zwitterion functional groups in CNTs act as molecular gatekeepers at the entrance of the nanotubes to enhance blockage for salt ions. Functionalized CNTs were oriented on a membrane support by high vacuum filtration. These oriented CNTs were sealed by a polyamide film via interfacial polymerization. Cross-sectional image of the nanocomposite membrane taken by scanning electron microscopy (SEM) showed semi-aligned zwitterion-CNTs on top of a porous support covered by a thin PA film with an overall thickness of approximately 250 nm. When the concentration of zwitterion-CNTs in the membrane increased, the nanocomposite membranes experienced significant improvement in permeation flux while the ion rejection increases slightly or remains unchanged. This indicated that the increased water flux is not due to an increase in nonspecific pores in the membrane, but rather due to an additional transport mechanism resulting from the presence of the functionalized CNTs. Significant increase of flux was also observed in separating cations other than sodium. The separation of the PA skin layer dominated the ion rejection mechanism by size exclusion even when the carbon nanotubes were introduced into the polyamide coating. The zwitterion functional groups exposed at the membrane surface also interacted with the feed water to form a strong hydration layer, which results in improved surface biofouling resistance. The adsorption rate of protein foulants on the nanocomposite membrane surface was significantly reduced compared to the control membrane without CNTs, and the adsorbed fouling layer could be easily removed by flushing with water. After washing, the nanocomposite membrane recovered 100% of the decreased water flux whereas the control membrane only recovered 10% of the decreased flux resulting in a permanent loss of 30% in water permeation. We have therefore demonstrated that advanced materials like CNTs can be synthesized with desired functional groups, and can be embedded into traditional RO membranes to simultaneously resolve the challenge of low flux and surface fouling in the current desalination process. / Ph. D.

Zero-valent metals such as bulk iron and zinc are known to dechlorinate toxicorganic compounds. Enhancement in reaction rates has been achieved through bimetallicnanosized particles such as nickel/iron (Ni/Fe) and palladium/iron (Pd/Fe). Batchdegradation of model compounds, trichlroethylene (TCE) and 2,2'-dichlorobiphenyls(DCB), were conducted using bimetallic Ni/Fe and Pd/Fe nanoparticles. Completedegradation of TCE and DCB is achieved at room temperature. Zero-valent iron, as themajor element, undergoes corrosion to provide hydrogen and electrons for the reductivecatalytic hydrodechlorination reaction. The second dopant metals of nickel and palladium(in nanoscale) act as catalyst for hydrogenation through metal hydride formation thatproduces completely dechlorinated final product. Different compositions of bimetallicNi/Fe and Pd/Fe nanoparticles were synthesized and their reactivity was characterized interms of reaction rate constants, hydrogen generation through iron corrosion, andproducts formation. The observed TCE degradation rate constant was two orders ofmagnitude higher than the bulk iron and nanoiron, indicating that the bimetallicnanoparticles are better materials compared to the monometallic iron systems. Longevitystudy through repeated cycle experiments showed minimum loss of activity. The surfacearea-normalized rate constant was found to have a strong correlation with the hydrogengeneration by iron corrosion reaction. A mathematical model was derived thatincorporates the reaction and Langmuirian-type sorption terms to estimate the intrinsicreaction rate constant and rate-limiting step in the degradation process. Bimetallicnanoparticles were also immobilized into the chitosan matrix for the synthesis of ananocomposite membrane reactor to achieve membrane-phase destruction of chlorinatedorganics under convective flow condition. Formation of uniformly distributed nanosizedparticles is confirmed by high resolution transmission electron microscopy. Themembrane-phase degradation results demonstrated similar trends with the previoussolution phase analysis with the observed enhanced reaction rates. The advantage of themembrane system is its ability to prevent the agglomeration of the nanoparticles in themembrane matrix, to minimize the loss of precious metals into the bulk solution phase,and to prevent the formation of precipitated Fe(III) hydroxide. These are due to thechelating effect of the amine and hydroxyl functional groups in the chitosan backbones.

In this study, effects of nanoparticles, including 7 nm TiO2, 200 nm TiO2, and hydrophilic and hydrophobic SiO2 with mean diameter in the range of 15–20 nm and their concentration on the membrane properties and vacuum membrane distillation (VMD) performance were evaluated. The effect of membrane thickness and support materials were also investigated. The membranes were characterised extensively in terms of morphology (SEM), water contact angle, water liquid entrance pressure (LEPw), surface roughness, and pore size. While the best nanocomposite membranes with 200 nm TiO2 Nanoparticles(NPs) were obtained at 2% particle concentration, the optimal particle concentration was 5% when 7 nm TiO2 was integrated. Using nanocomposite membrane containing 2 wt% TiO2 – 200 nm nanoparticles, VMD flux of 2.1 kg/m2h and LEPw of 34 PSI was obtained with 99% salt rejection. Furthermore, it was observed that decreasing the membrane thickness would increase the portion of finger-like layer in membrane and reduce the spongy-like layer when hydrophilic nanoparticles were used. Using continuous flow VMD, a flux of 3.1 kg/m2h was obtained with neat PVDF membranes, which was 600% higher than the flux obtained by the static flow VMD with the same membrane at the same temperature and vacuum pressure. The fluxes of both static and flow-cell VMD increased with temperature. Furthermore, it was evident that the continuous flow VMD at 2 LPM yielded 300% or higher flux than static VMD at any given temperature, indicating strong effects of turbulence provided in the flow-cell VMD.

Polymeric membranes for gas separation are limited in their performance by a trade-off between permeability and selectivity. New methods of design are necessary in making membranes, which can show both high permeability and selectivity. A mixed matrix membrane is one such particular design, which brings in the superior gas separation performance of inorganic membranes together with the easy processability and price of the polymers. In a mixed matrix membrane, the inorganic phase is dispersed in the polymeric continuous phase. Nanocomposite membranes have a more sophisticated design with a thin separation layer on top of a porous support. The objective of this research was to fabricate thin SWNT nanocomposite membranes for gas separation, which have both high permeability and selectivity. SWNT/polyacrylic nanocomposite membranes were fabricated by orienting the SWNTs by high vacuum filtration. The orientation of SWNTs on top of the porous support was sealed by UV polymerization. For making these membranes, the CNTs were purified and cut into small open tubes simultaneously functionalizing them with COOH groups. Gas sorption of CO2 in COOH functionalized SWNTs was lower than in purified SWNTs. Permeabilities in etched membrane were higher than Knudsen permeabilities by a factor of 8, and selectivities were similar to Knudsen selectivities. In order to increase the selectivities, SWNTs were functionalized with zwitterionic functional groups. Gas sorption in zwitterion functionalized SWNTs was very low compared to in COOH functionalized SWNTs. This result showed that the zwitterionic functional groups are kinetically blocking the gas molecules from entering the pore of the CNT. SWNT/polyamide nanocomposite membranes were fabricated using the zwitterion functionalized SWNTs by interfacial polymerization. The thickness of the separation layer was around 500nm. Gas permeabilities in the CNT membranes increased with increasing weight percentage of the SWNTs. Gas permeabilities were higher in COOH SWNT membrane than in zwitterion SWNT membrane. Gas selectivities were similar to the Knudsen selectivities, and also to the intrinsic selectivities in the pure polyamide membrane. The water flux in SWNT-polyamide membranes increased with increasing weight percentage of zwitterion functionalized SWNTs, along with a slight increase in the salt rejection. Membranes exhibited less than 1% variability in its performance over three days. / Ph. D.

Membrane-based filtration and separation processes are widely used in various fields, such as for clean drinking water, food, dairy industry, medical, biotechnology, and environmental applications. Providing clean and safe drinking water has been regarded as one of the global biggest challenges because of increasing world population, impacts of climate changes, increased wastewater production, and increased contamination of surface and groundwater. The most important technologies contributing substantially in this field are based on pressure-driven membrane technologies such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Recently, in situ synthesis of nanoparticles based on molecular level design and tailoring in the membrane matrix have been reported to prepare next generation nano-enhanced membranes. In this work similar technique was utilized to construct PSF-Silica nanocomposite membranes in which hydrolysis and condensation of silica precursor (TEOS, APTES, and TPAPS) and phase inversion of polymer film was achieved simultaneously in one step under acidic condition. This unique process has led to the formation of extremely small silica nanoparticles with high dispersion in every region of the membrane. Such type of distribution of silica nanoparticles is very difficult to achieve using conventional silica nanoparticle blending with polymer solution. The prepared membranes were extensively characterized for their morphology, surface properties, nanoparticle distribution, fouling and permeation properties. Finally, the membranes were tested with rejection experiments with protein and dye solutions to assess their usefulness for water filtration and separation applications. The silica nanoparticles were mostly generated during the phase inversion under acidic condition by hydrolysis and polycondensation reaction of silica precursors mixed with PSF solution. The properties and structure of membranes were characterized by different analytic and physicochemical techniques. The prepared membranes exhibit an asymmetric nature with a dense skin layer, followed by finger-like porous structure at the bottom. The microscopic and elemental analysis confirmed the presence and homogeneous distribution of nanoscopic small silica particles throughout the membrane matrix. Hydrophilic SiO2, SiO2-NH2, SO2-NH-SO3H nanoparticles were respectively formed in situ within PSF membrane matrix during the phase inversion under acidic condition by hydrolysis and polycondensation reaction of corresponding silica precursors mixed with PSF solution. Because the nanosize and good distribution of the pores, the presence and well distribution of silica nanoparticles, the presence and exposure of the hydrophilic charged functional groups form a hydrate layer on the membranes and provide hydration repulsion and electrostatic repulsion against solutes from feed under aqueous medium, the PSF-TEOS, PSF-APTES, PSF-TPAPS membranes respectively with SiO2, SiO2-NH2, and SO2-NH-SO3H nanoparticles exhibited high hydrophilicity, stability, water permeation, rejection, and antifouling performance as compared to the neat PSF membrane for water purification application. The overall membrane properties were highly dependent on the concentration of the silica nanoparticles and can be tuned by adjusting the concentration of initial silica precursors during membrane formation. The low protein fouling, high water permeation, protein rejection, and flux recovery results make those membranes attractive for separation and filtration applications. Thus, the prepared membranes can be used in different applications ranging from separation of biomolecules, desalination/purification of water, and for other charge and size-based separation processes.