The present work investigates novel nanotube materials for application in the field of membrane technology, with the aim of going beyond the well-investigated carbon, by studying nanotubes made of carbon nitride, boron nitride and polystyrene.Carbon nanotubes (CNTs) have long held the promise to revolutionise filtration technology, with orders of magnitude higher fluxes compared to commercial membranes. Nevertheless, during the introduction of CNTs in current membrane technology, several issues were encountered, amongst which the poorly understood dependence of water flow enhancement on the nanotubes' atomic structure, limited rejection potential and difficulties in scaling up. In this thesis, the independent effect of nanotube surface chemistry and structure on the flow of water under nanoscale confinement is first demonstrated via the synthesis of carbon nitride nanotube (CNNT) membranes. Using a combination of experiments and molecular dynamics (MD) simulations, it is shown here that the hydrophilisation of the sp2 carbon structure, induced by the presence of the C-N bonds, decreases the pure water permeance in CNNTs, when compared with CNTs with different degree of defects. The effect on permeance is explained in terms of solid-liquid interactions with increased water viscosity and decreased surface diffusion near the CNNT wall, when compared to CNTs. The effect that different surface properties have on flow enhancement was also studied in polystyrene nanotubes membranes, with polystyrene being one of the commonly used polymers in membrane science. This was achieved by means of a one-factor-at-a-time optimisation of key parameters impacting the formation of nanotubes with well-defined geometries.Moreover, it was previously found that CNTs can only reject particles and ions wider than their internal diameter. Per contra, this work reports the fabrication of aligned boron nitride nanotube (BNNT) membranes, with a 45 % higher permeate flow rate than CNT membranes with similar rejection. The increased permeance is due to a charge-based rejection mechanism in addition to the size-based one, enabled by the BNNT surface structure and chemistry and elucidated here with molecular dynamics and CFD simulations. This phenomena allows using nanotubes with larger diameters and also addresses challenges in the manufacturing of sub-nanometer CNTs in large quantities. Following the aforementioned fundamental studies of nanotubes' permeance and rejection behaviours, the embedment of nanotubes in commercial nanofiltration membranes was investigated. A novel thin film nanocomposite (TFN) membrane was obtained by incorporating BNNTs in a polyamide (PA) thin selective layer prepared via interfacial polymerisation. The addition of just 0.02 wt % of BNNTs led to a 4-fold increase in pure water permeance with no loss in rejection for divalent salts, methylene blue and humic acid compared to the pure PA membrane. Fouling tests with humic acid showed a flux recovery ratio of > 95% with 40-50 % lower flux loss during the fouling cycle compared to the polyamide only membrane. These values represent a significant improvement for both commercial PA membranes but also TFN using CNTs. The work presented in this thesis investigates novel nanomaterials for membrane technology opening the way to tailoring surface chemistry and structure inside nanotube membranes for a wide variety of processes. These range from enhanced transport to improved targeted rejection, in ceramic and polymeric membranes.
|Date of Award||1 Apr 2020|
|Supervisor||Davide Mattia (Supervisor) & John Chew (Supervisor)|