Novel nanocomposite ultrafiltration polysulfone membranes incorporated with halloysite nanotubes

Abstract

Ultrafiltration (UF) membrane technology arose as an effective technique for water treatment. UF offers many advantages for water treatment, such as low cost and energy consumption, high flux, and efficiency in the removal of organic pollutants, microorganisms, and viruses, low operation pressure, and small footprint. Polymeric-based UF membranes are most commonly used commercially; however, there are several challenges related to fouling, such as the chemical, mechanical, and thermal stability of the membranes. Therefore, the need for novel UF membranes with enhanced performance and properties remains a key objective in the field of membrane development. To that end, selections of polysulfone (PSF) membranes incorporating halloysite nanotubes (HNTs) were synthesized and extensively characterized by means of advanced physisco-chemical methods. HNTs nanoparticles are a cheap, naturally existing clay-type nano-additive that possesses a combination of unique characteristics such as tubular structure, high aspect ratio, availability of functionalities, high mechanical strength, and excellent biocompatibility. In this study, for the first time HNTs, as a sole pore-forming additive, were used in the casting of PSF UF membranes to provide a clear understanding of HNTs role in the formation of the porous membrane structure and the membrane properties. Characterization of the prepared PSF/HNTs membranes by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM) revealed that the composite membranes showed notable changes in the morphology, roughness, and distribution of HNTs within the membrane matrix. SEM, pore size, and porosity estimations indicated that the incorporation of low to medium HNTs loadings resulted in PSF/HNTs membranes characterized by their smaller surface pore size, lower porosity, and thicker top skin layer due to the strong interfacial interactions between HNTs and PSF matrix. The addition of 5 wt.% HNTs reduces the composite membrane's contact angle by 14o from the 78.7o for the pure membrane. The incorporation of 0.2 wt% HNTs resulted in a 140% increase in the top surface Young’s modulus of the composite membrane and notably reduced adhesion of hydrophobic foulants compared to the pure PSF membrane. Besides, PSF/HNTs membrane possessed the optimal bulk mechanical properties in terms of elastic modulus and yield stress The prepared composite membranes showed superior surface hydrophilicity, increased surface charge, and improved mechanical properties compared to their respective pure membrane. These features were implicated in enhanced anti-fouling and compaction resistance of the composite PSF/HNTs membranes during filtration of bovine serum albumin (BSA) and milk solutions. Importantly, the PSF membrane incorporated with 0.2 % HNTs showed excellent rejection (99%) and flux recovery (98%) at BSA filtration. In the second part of this study, to facilitate the incorporation of the modified nano-additive into the PSF membrane matrix, a novel and robust method of HNTs modification by poly(ethyleneimine) (PEI) was developed. The successful modification of HNTs by PEI (P-HNTs) was confirmed with TEM, EDS, and XPS analysis techniques. SEM and EDS analysis revealed that using P-HNTs notably improves the dispersibility of the modified HNTs within the PSF polymer matrix and increases the P-HNTs loading (by 150%) compare to raw HNTs. At 0.5 wt% loading, PSF/P-HNT membranes demonstrated high mechanical properties, high flux, and rejection of heavy metal ions like molybdenum. These findings showed that HNTs could be used as an efficient additive for developing the advanced UF membranes by tuning their structural and physicochemical characteristics, including hydrophilicity, porosity, membrane charge, and mechanical stability. HNTs could also play a key role in introducing additional sorption capabilities to meet specific water treatment applications.

Date of Award	2020
Original language	American English
Awarding Institution	HBKU College of Science and Engineering