Doctor of Philosophy
School of Chemistry
Alshahrani, Ahmed Ali, Developing composite membranes for desalination, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2015. https://ro.uow.edu.au/theses/4330
The survival of living things largely depends on unpolluted pure and palatable water. A major portion (about 97.5%) of Earth is covered with oceans containing mostly saline water, with the remaining 2.5% comprising fresh water. Unfortunately, fresh water is being depleted from these sources with the growth of the world's population, enormous climate changes and contamination by agricultural and industrial effluents. As a result, people are constantly in search of fresh water sources. Many physical-chemical methods have been developed, such as chemical and heat treatments, to obtain fresh water. Sadly, these methods are typically uneconomical, time-consuming and environmentally hazardous or unfriendly. However, one of these techniques, the filtration of ocean water through membranes of different types, shows promise as a sustainable method. The present work focuses on developing a suitable chemical membrane with carbon nanotubes (CNT) incorporating chitosan surfactants and other dispersant additives. Such a CNT membrane (buckypaper [BP]) could be used in the desalination process because it has a uniquely high internal mechanical strength, thermal stability, uniform pore distribution and is easily handled.
In this study, CNT buckypaper membranes were successfully fabricated by a vacuumfiltration method using well-dispersed multi-walled (MWNTs) CNTs and carboxyl functionalised multi-walled carbon nanotubes (MWNT-COOH and MWNT-NH2) MWNTs in the presence of different surfactants, such as chitosan (both non-functionalised and functionalised) and Triton X-100 (Trix). The optimal sonification time, stability and homogeneity of different CNTs (i.e. MWNT, MWNT-COOH and MWNT-NH2) dispersion solutions were first evaluated. The synthesised CNT BP membrane was then characterised using different tools. The functionalised MWNT-COOH and MWNT-NH2 membranes were highly dispersed in solution but possessed relatively decreased electric conductivity (7 ± 1 S/cm and 1 ± 0.3 S/cm respectively) compared to that of the BP membrane made with unfunctionalised MWNTs (70 ± 1 S/cm). Improved mechanical properties were obtained with the BP membranes prepared with unfunctionalised and functionalised MWNTs in the presence of chitosan and crosslinked chitosan. The value of these properties of the BP membranes significantly decreased when Trix dispersant was used instead of chitosan and crosslinked chitosan. Moreover, scanning electron microscope (SEM) and Brunauer-Emmett- Teller (BET) analysis showed that the surface morphologies, internal pore structures and porosities of several BP membranes were improved when both unfunctionalised and functionalised MWNTs were used with a low concentration of chitosan 0.1% (w/v) and Trix (range 114 ± 2 m2/g and 89 ± 6 m2/g). The value of these properties decreased when a concentration of chitosan > 0.1% (w/v) and crosslinked chitosan were used (range 0.07 ± 0.01 m2/g and 45 ± 5 m2/g). Water permeability and salt-rejection capacity of the synthesised BP membranes were investigated using laboratory-scale tests with crossflow-cell and dead-end stirred-cell filtration techniques. The water permeability of BP membranes was mostly related to other physicochemical properties, such as hydrophobicity, surface area and pore size. However, water permeability of BP membranes made with MWNT-COOH/chitosan was considerably higher than that of BP membranes made with MWNT-NH2/chitosan. The rejection of salts by BP membranes was dominated by adsorption mechanisms, and both electrostatic repulsion and size exclusion were related to the rejection of positively charged salts. The BP membranes with lower surface area and porosity were more capable of rejecting lower than higher molecular weight salts. In addition, an acidic feed solution (i.e. decrease in pH from 10 to 3) resulted in an increased rejection of salt molecules. Further research is required, particularly in selectivity studies, as these experiments showed that buckypaper membranes successfully rejected the salt from solution and are very promising in the area of desalination.