Fabrication of Advanced Nano-Enabled Membranes for Water Treatment and Desalination Applications

先進納米膜製備並應用於水處理及海水淡化

Student thesis: Doctoral Thesis

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Award date12 Aug 2019

Abstract

Membrane-based technology has offered practical and sustainable solutions for water and energy sustainability. However, membrane fouling, high-energy requirements, short membrane life span and the trade-off between solute rejection and water permeability remain as major unresolved challenges that limit their applicability. Today, membrane-based technology is undergoing a new paradigm shift where the rapid development of nanotechnology has opened new and practical solutions to overcome the limits of existing polymeric membranes. In an attempt to better design and fabricate advanced membranes for water desalination and treatment applications, the work presented in this dissertation explored the potential application of nanostructured materials with exceptional physicochemical properties for synthesizing advanced antifouling and photothermal nano-enabled membranes. In addition, this study employed several new concepts and techniques such as laser induced breakdown spectroscopy (LIBS) to probe foulant-material interactions, optical coherence tomography (OCT) to comprehend foulant-membrane, and foulant-foulant interactions as well as the dynamics of fouling growth on the different membrane surfaces, while a new concept of interfacial heat localization was studied for the photothermal application.

Recognizing the importance of understanding foulant-material interactions for fabricating nano-enabled antibiofouling membranes, the present research first investigated the bactericidal mechanism of graphene oxide (GO) nanosheets against membrane biofouling-causing bacteria using a laser-based spectroscopic technique (LIBS). A detailed experimental investigation revealed that by direct GO-bacterium contact, GO makes intense membrane disruptive interactions with bacteria, simultaneously inducing bacterial membrane and oxidative stress followed by the degradation/release of intracellular organelles, thereby causing bacterial inactivation or cell death. These findings offered sufficient evidence to mechanistically elucidate the bactericidal mechanism of GO nanosheets, which is crucial for developing GO-based biofouling-resistant membranes.

Next, we fabricated a flexible and mechanically stable GO-based membrane and probed its ability to inactivate bacterial growth and subsequent biofilm formation. Here we employed a nondestructive, real-time in-situ biofilm characterization technique (OCT) to study the bacterial inactivation and dynamics of biofilm development on the GO surface. The results of this probe revealed that GO membrane, owing to its distinct physicochemical surface properties (strong negative charge, exposed nanosheet edges, improved surface roughness, and hydrophilicity) significantly restricted bacterial attachment, growth, and subsequent biofilm development on the its surface. These findings highlighted the potential of GO-based antibacterial membranes for practical applications in membrane-based processes.

The research next explored another promising carbon-based nanomaterial i-e carbon nanotubes (CNTs) for its potential application in developing antifouling membranes. Subsequently, we fabricated a CNT-based membrane and investigated its resistance to both organic and biofouling. Results revealed that a thin CNT layer on the membrane surface effectively prevented HA molecules from interacting directly with the porous structure of the membrane, thereby averting the possibility of irreversible fouling. Antibiofouling results based on the ultrastructural examination of cell membrane integrity authenticated the superior bactericidal potential of the CNT-membrane towards bacterial cells. In addition, the CNT-membranes also exhibited excellent fouling reversibility characteristics and better performance in the pretreatment of real seawater involving a complex water matrix.

Beside membrane fouling, the high energy requirement of membrane-based technologies is another major challenge that keeps them out of the reach of many developing countries/regions (the most water stressed areas). In response to such challenge, we further explored the potential of nano-enabled membranes for producing fresh water using sun light as a renewable energy source. Herein, we fabricated titanium nitride (TiN)-based photothermal membranes with the concept of interfacial heat localization for low cost desalination using interfacial solar-driven evaporation. A 3.5 % NaCl solution was used to mimic the average seawater salinity. For solar vapor generation (SVG), when tested in a highly insulated system, the TiN photothermal membrane efficiently generated localized heat at the surface and produced fresh water from saline water at the rate of 6.98 kg m-2 h-1 with 84.3% evaporation efficiency under 5kWm-2 simulated solar irradiation. The superior performance of the TiN photothermal membranes for interfacial solar-driven desalination was attributed to the superior light absorption and outstanding photothermal conversion properties of TiN nanoparticles that enabled efficient localized heating on the membrane. These energy efficient photothermal membranes can provide an inexpensive point-of-use fresh water producing system for remote communities, which may not be able to afford conventional desalination processes but have abundant solar irradiations throughout the year.

In conclusion, this research taking advantage of promising nanomaterials offers practical solutions to overcome major limitations that are associated with conventional membranes and/or membrane-based processes namely, membrane fouling and high cost and energy requirements.

    Research areas

  • Graphene oxide, Nano-enabled membranes, Optical coherence tomography, Laser induced breakdown spectroscopy, Photothermal, Carbon nanotubes, Titanium nitride nanoparticles, Desalination, Wastewater treatment, Antifouling