Vertically-aligned carbon nanotube membranes for water desalination and ultrafiltration
Date of Award
Doctor of Philosophy
School of Science
Professor Kamal Alameh
Even though the earth is 70% covered with water, there is a severe shortage of fresh water. Modern water filtration techniques includes Reverse Osmosis (RO), Forward Osmosis (FO), and Solar Distillation etc. Due to its high separation performance and comparatively low prices, membrane technology is widely applied in many fields such as medical science, environmental science and many more. Despite its benefits such as energy efficiency and high performance, they are susceptible to fouling. Such drawbacks encourage the invention of new type of membranes. There are many research groups who are working on different membrane materials, surface modification or incorporation of nanoparticles in existing membranes. Types of nanomaterials include nanoparticles (quantum dots), different polymers or novel carbon structures like Carbon Nanotube (CNTs) or Graphene. CNTs doped in polymer is the best choice for fabrication of these novel membranes. A combination of the CNTs and hydrophobic polymer is the best combination for the mixed matrix class of membrane fabrication. But dispersion of CNTs in polymer to achieve best results is the biggest challenge.
The first phase of the research undertaken throughout the PhD project involved the development of a novel membrane fabrication process, where various densities of Vertically Aligned Carbon Nanotubes (VACNTs) were developed, and, through a spin coating process, spaces between adjacent VACNTs were filled with Poly(dimethylsiloxane) commonly known as PDMS. A prepared mixed matrix block was sliced into 25μm thick slices using the microtome machine. Sliced membranes were transferred onto PVDF support membranes which acted as support layer only. The prepared membranes were tested for water flux and salt rejection capabilities. The performance of VACNT membranes of densities 5×109, 1010, 5×1010 and 1011 cm-2 were 918, 1008, 1112 and 1202 LMH at 1 bar respectively. The permeance of 1202 LMH was around 30 times higher than traditional Polyethersulfone (PES) membrane. The salt rejection was initially 99% but later on it was reduced and stabilized at 96% due to concentration polarization in used setup of modified dead-end filtration cell. Experimental results confirmed that the permeability of VACNT membranes increases with the density of the VACNTs, while the salt rejection is almost independent of the VACNT density.
The second phase of the PhD project focused on membrane fabrication and characterization. The prepared membranes were first tested for different polar and non-polar liquids. Later on those membranes were treated with dry Reactive Ion Etching (RIE) and their surfaces were modified. Through dry RIE, the membrane surfaces were made more hydrophilic compare to the original hydrophobic surface. This hydrophilicity was increased due to hydroxyl ion addition on the membrane surfaces. The filtration of polar and non-polar liquids was again measured. The hydrophilicity gave the advantage of selectivity of polar or non-polar liquids through specific membranes. Experimental results, which were compared with results of a previously reported process of oxygen plasma treated membranes, demonstrated the improved liquid filtration performance of RIE-treated membranes. The improvement factors attained for water, ethanol, IPA, hexane and gasoline permeance were 100%, 37.2%, 11.8%, 20.9% and 3.8% respectively, with respect to oxygen-plasma treated VACNT membranes.
In the last phase of the project, the biofouling capability of the prepared membranes were tested for. The membranes were tested for static and dynamic protein fouling. Bovine Serum Albumin (BSA) was used as a protein source. The static and dynamic protein adsorption experiments at neutral pH did not show any effect on prepared membranes. For static BSA test, Experimental results showed 32 μg/cm2 protein adsorption for the membrane with 1×1011 cm-2 VACNT density, whereas the membrane with lowest density 5×109 cm-2 showed approximately 71 μg/cm2 protein adsorption. Experimental results show that higher flux recovery is achieved with dry etched membranes compared to virgin membranes, and that comparatively low irreversible fouling ratio is achieved with dry etched membranes of low VACNT densities. For the dynamic protein adsorption test, results showed that a cross-flow cell is more suitable compared to a modified dead-end cell which is confirmed with FRR of 90% to 80% from cross-flow cell to modified dead-end cell.
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Trivedi, S. (2017). Vertically-aligned carbon nanotube membranes for water desalination and ultrafiltration. Retrieved from https://ro.ecu.edu.au/theses/2035