Performance Optimization of Membranes via Physically-enhanced Spacers and Skeleton-type Structural Reinforcements for Water Filtration and Fouling Mitigation

Abstract

This doctoral thesis will be focusing on the development of novel approaches for enhancing the performance of pressure driven membranes for water filtration and fouling mitigation. It will have three main chapters and one supplementary chapter. It is centered upon the development of reinforcements such as supports and spacers. It first focuses on the development of a new type of support (spacer) by investigating the distribution of shear stress through spacers, then further introduces a newly developed spacer which can have superior performances by getting benefit of electrostatic forces. For the development of these spacers/supports commercial membranes are used for filtrations while 3D printing is used for the fabrication. Finally, the thesis investigates skeleton-type structural reinforcements for pressure driven membranes fabricated through electrospinning technology. In the supplementary chapter, it introduces internal reinforcement of nanofiber membranes through uniform welding of nanofibers. Firstly, sustainable fabrication method: 3D printing was utilized, and a bio-mimetically inspired a novel support composed of honeycombs that has full contact with the membrane surface and creates a shear web to enhance shear distribution in order to decrease fouling and reverse the solute flux of a forward osmosis system is introduced. The results showed that well-distributed shear highly reduced foulant adhesion on the membrane surface and reverse solute diffusion. As the orientations of hexagons also affect shear distribution, they were oriented both vertically and horizontally, and compared with the support of commercially available membrane spacers. The vertically oriented hexagonal-type shear distributing support (V-HEX) showed a better performance in reducing reverse solute flux and in reducing foulant adhesion to 50% compared to the effect of a commercially available spacer (COM). V-HEX was also tested as a draw support and as a feed support combined with the commercially available spacer, and it was still successful in reducing reverse solute diffusion. Secondly, an electrically-polarized graphene-polylactic acid (E-GRP) spacer is introduced for the first time by a novel fabrication method, which consists of 3D printing followed by electrical polarization under a high voltage electric field (1.5 kV/cm). The fabricated E-GRP was tested in an osmotic-driven process (forward osmosis system) to evaluate its performance in terms of water flux, reverse solute flux, and ion attraction compared to a 3D printed non-polarized graphene-polylactic acid (GRP) spacer and a polylactic acid (PLA) spacer. The use of the developed E-GRP as a draw spacer showed > 50% water flux enhancement (32.4  2 Liter/m2/h (LMH)) compared to the system employing the GRP (20.5  2.3 LMH) or PLA (20.8  2.1 LMH) spacer. This increased water flux was attributed to the increased local osmotic pressure across the membrane surface due to the ions adsorbed by the polarized (E-GRP) spacer. As a feed spacer, the E-GRP also retarded the gypsum scaling on the membrane compared to the GRP spacer due to the dispersion effect of electrostatic forces between the gypsum aggregation and negatively charged surfaces. The electric polarization of the E-GRP spacer was shown to be maintained for > 100 h by observing its salt adsorption properties (in a 3 M NaCl solution). At the final chapter, nylon 6 membranes with ultrathin thickness (~4 µm for each layer) and double filtration mechanism (100-200 nm distance between two layers) are fabricated by electrospun nanofiber coating on conductive graphene PLA skeletons fabricated through 3D printing. Optimal skeletoned sample SK-2H showed 1449 LMH water flux with 99.8% kaolin rejection. This performance outranged commercial ultrafiltration membranes with 30 times higher water flux and ordinary nylon nanofiber membranes with 5 times higher flux values. Physical bonding between PLA and nylon 6 successfully worked for long-term maintenance of membranes without any detachment. SK-2H also showed two times better fouling mitigation performance than skeletonless Nylon-6 nanofiber membrane by depositing most of the humic acid foulant on the membrane areas near skeleton edges. In the supplementary chapter, selectivity of nylon 6 membranes is studied. A novel type of UF membrane is fabricated by using a nylon 6 membrane as base polymer. Electrospun nylon 6 membrane is semi-dissolved by spraying diluted formic acid solvent, and re-casted by thermal rolling. In the end, ultra-strong (53 MPa tensile strength) tight UF membrane: SP6 was obtained. SP6 showed 98% humic acid and 92% organic dye rejection, while solely thermally rolled nylon 6 membrane (HPA6) showed 72% and 0 rejections, respectively for humic acid and rose bengal organic dye. Furthermore, 19 times less foulant observed on SP6 than HPA. Both membranes in chapter 3 and supplementary chapter can also be considered for the support layers of forward osmosis (FO) membranes.