Anti-fouling thin film composite (TFC) polyamide reverse osmosis (RO) membranes based on piezoelectric polyvinylidene fluoride (PVDF) substrates

• Main Author: Fu, Jia Shen
• Other Authors: Chong Tzyy Haur
• Format: Thesis-Master by Research
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering
• Online Access: https://hdl.handle.net/10356/174234
• Institution: Nanyang Technological University

Reverse osmosis (RO) technology, which exhibits advantages such as lower energy consumption, reduced footprint and higher water quality as compared to thermal-based technologies, has become one of the main technologies in desalination industry. Despite the attractive advantages of RO, the inevitable problem significantly affecting RO processes is membrane fouling. Fouling can cause the reduction in water production, produced water quality and even membrane lifespan. Recently, piezoelectric polyvinylidene (PVDF) membrane as the source of vibration in ultrafiltration (UF) field provided an innovative way to mitigate RO fouling. However, to fabricate anti-fouling RO membrane based on piezoelectric PVDF substrates, there were three problems to be solved. Firstly, hydrophobic nature of PVDF has hindered its application in thin film composite (TFC) RO membrane fabrication as highly hydrophobic substrate may cause the formation of defects in the PA layer. Secondly, TFC RO membrane typically consists of a non-woven fabric as a supporting material for mechanical strength enhancement. This non- woven layer has poor conductance in nature. As such, the effectiveness of piezoelectricity enhancement by electrical poling for PVDF substrate with the presence of non-woven fabric is unknown. Thirdly, unlike PVDF UF membrane, PVDF layer is sandwiched between the selective layer and a non-woven fabric. PVDF layer which is the vibration source does not have direct contact with the feed solution or foulant layer. As such, the effectiveness of its piezoelectric effect in fouling mitigation was still unclear. These problems and challenges were explored in this thesis. In the first part of the study, two solvents (triethyl phosphate (TEP) and N,Ndimethylformamide (DMF)) were selected to fabricate PVDF substrates via nonsolvent phase separation (NIPS). PVDF substrates were modified by electrical poling to enhance their piezoelectric properties. The properties of these PVDF substrates were analyzed. It was found PVDF-TEP substrate had larger mean pore size (around 33 nm) and higher pure water permeability (254±8 LMH/bar), making it more suitable to be substrate for RO membrane fabrication. For both PVDF-TEP and PVDF-DMF, the mean pore size became smaller and the pore size distribution narrowed after electrical poling. With the unmodified RO membrane fabrication protocol, the RO membrane with salt rejection less than 40% were fabricated. In the second part of the study, electrical poling was performed to PVDF-TEP substrate, RO membrane fabrication protocol was modified, and unpoled and poled PVDF-TEP substrates were used to fabricate RO membranes. Piezoelectric coefficient d33 was used to represent piezoelectric property of substrate sample. Electrical poling greatly enhanced piezoelectric property (d33) of such PVDFTEP substrate the d33 value increased 72%. The combination of substrate wetting by ethanol solution and adding sodium dodecly sulphate (SDS) into mphenylenediamine (MPD) solution was used to decrease interfacial tension between PVDF substrate and MPD solution. Fowkes method was introduced to confirm such modification could decrease interfacial tension between PVDF substrate and MPD solution (52-66% decrease in interfacial tension). By such modification, the MPD solution amount in substrate pores increased (35% increase in MPD solution uptake), and RO membrane with reasonable salt rejection (more than 97.5%) was obtained. The fouling test clearly demonstrated that it was feasible to fabricate anti-fouling RO membrane based on piezoelectric PVDF substrates. In the third part of the study, PVDF polymer solution was cast onto non-woven fabric (NWF) to fabricate PVDF-TEP-NWF substrate via NIPS, and electrical poling was also performed to such substrate. Due to membrane shrinkage prevention during NIPS (membrane shrinkage around 2%), PVDF-TEP-NWF substrate with larger mean pore size (around 41 nm) and much higher permeability ((469±23 LMH/bar)) was obtained. It was found the effectiveness of electrical poling in piezoelectric property (d33) enhancement for such substrate was insignificant (d33 value improvement was less than 5%). Besides, it was confirmed piezoelectric property was able to be induced during NIPS. Based on such substrate and modified RO membrane fabrication protocol, RO membranes with higher permeability (~ 2 LMH/bar) and reasonable salt rejection (~98.9%) were obtained. The fouling tests clearly demonstrated that RO membranes based on both unpoled and poled PVDF-TEP-NWF substrates exhibited fouling mitigation behavior when electric signal was applied. The findings of this study would provide the method of using hydrophobic PVDF membrane as substrate for RO membrane fabrication and come up with an innovative way to mitigate RO membrane fouling.