Pei Sean Goh

Inorganic Nanomaterials in Polymeric Ultrafiltration Membranes for Water Treatment

Due to the rapid expansion of nanotechnology and the increasing range of nanomaterials under production and development, a significant amount of research interest has been dedicated to the innovative exploitations of various inorganic nanomaterials in environmental applications. The incorporation of inorganic nanomaterials as fillers within a polymeric matrix has expanded opportunities to produce a multifunctional nanocomposite membrane that is capable of performing tasks beyond separation alone. The architectures and performances of these nanocomposite membranes have triumphed over polymeric membranes to overcome the underlying conspicuous drawbacks. This review aims to shed more light on the roles of inorganic nanomaterials in advancing the characteristics and separation performance of polymeric ultrafiltration membranes. Inorganic nanofillers such as metal oxides, metals and carbon-based materials are incorporated into polymeric membranes to render the desired properties for ultrafiltration separations. Novel strategies using a wide range of these inorganic nanomaterials have been well explored and established for the manufacturing of membrane with significantly enhanced properties that are highly desired to heighten the separation performances. With the renaissance of this emerging innovative technology, many possible solutions and valuable options can be offered to serve as the primary driver in excelling ultrafiltration membrane technology.

A review of purification techniques for carbon nanotubes

Purification of carbon nanotubes (CNTs) is a very actively discussed topic in contemporary CNT literature. To a large extent, impurities embedded in CNTs influence the physical and chemical characteristics of the CNTs. Different purification methods yield different CNT characteristics and may be suitable for the production of different types of CNTs. Developments in the purification methods of CNTs are reviewed, and the production methods are briefly discussed and summarized. This is followed by a detailed description of the three major purification methods, viz. chemical, physical, and multi-step purification.

Characterization Methods of Thin Film Composite Nanofiltration Membranes

This review provides a comprehensive survey on the characterization methods used to study the properties and performances of thin film composite nanofiltration (NF) membranes in water separation processes. In general, the intrinsic properties of NF membranes are characterized with respect to chemical properties and physical properties using various analytical instruments. For instance, FTIR spectroscope, zeta potential analyzer, XPS, XRD, and NMR spectroscope are some of the widely used instruments to evaluate the membrane chemical properties while SEM, FESEM, TEM, AFM, contact angle goniometer, and positron annihilation spectroscopy are those that have been commonly employed to study the membrane structural properties. With respect to permeability and selectivity, NF membranes are generally characterized through filtration experiments using appropriate feed properties under specific testing conditions. Furthermore, this review also covers the characterization techniques used to study the resistance of NF membranes against chlorination, solvent, thermal, and fouling. These resistance tests can be individually conducted, depending on the practical applications of the membranes. Overall, this review attempts to give readers insights into the experimental planning and subsequent interpretation of the above-mentioned characterization techniques. It is also expected to help new researchers in the field of NF to quickly learn various NF characterization methods.

Aqueous room temperature synthesis of zeolitic imidazole framework 8 (ZIF-8) with various concentrations of triethylamine

In this study, a zeolitic imidazole framework (ZIF-8) was prepared, and physico-chemical characterizations were performed to investigate the influences of triethylamine (TEA) on the crystallinity, particle size, morphology, and defects of ZIF-8 synthesized via an aqueous room temperature approach with a relatively low molar ratio of ligands to metal salts. Increasing the concentration of TEA by raising the TEA/total molar ratio from 0.004 to 0.007 prompted the formation of pure phase ZIF-8, whereas a TEA/total molar ratio below 0.004 did not result in a yield of ZIF-8. The particle size of ZIF-8 decreased from approximately 288 to 133 nm with increasing TEA/total molar ratio, except at a TEA/total molar ratio of 0.007. However, an excessive TEA/total molar ratio alters the 2-MeIM chemistry, and partially forms hydroxylated 2-MeIM. Thus, this study shows that at relatively low molar ratios of ligands to metal salts, up to a maximum TEA/total molar ratio of 0.006, the TEA concentration plays an important role in assisting crystal formation and in controlling the particle size of ZIF-8. The proposed procedure enables pure phase nanoscale ZIF-8 to be synthesized with a high product yield and with minimal chemical usage.

Pre-treatment of multi-walled carbon nanotubes for polyetherimide mixed matrix hollow fiber membranes.

Mixed matrix hollow fibers composed of multi-walled carbon nanotubes (MWCNTs) and polyetherimide (PEI) were fabricated. Pre-treatment of MWCNTs was carried out prior to the incorporation into the polymer matrix using a simple and feasible two stages approach that involved dry air oxidation and surfactant dispersion. The characterizations of the surface treated MWCNTs using TEM and Raman spectroscopy have evidenced the effectiveness of dry air oxidation in eliminating undesired amorphous carbon and metal catalyst while surfactant dispersion using Triton X100 has suppressed the agglomeration of MWCNTs. The resultant mixed matrix hollow fibers were applied for O(2)/N(2) pure gas separation. Interestingly, it was found that removal of disordered amorphous carbons and metal particles has allowed the hollow structures to be more accessible for the fast and smooth transport of gas molecules, hence resulted in noticeable improvement in the gas separation properties. The composite hollow fibers embedded with the surface modified MWCNTs showed increase in permeability as much as 60% while maintaining the selectivity of the O(2)/N(2) gas pair. This study highlights the necessity to establish an appropriate pre-treatment approach for MWCNTs in order to fully utilize the beneficial transport properties of this material in mixed matrix polymer nanocomposite for gas separation.

Effect of PVP Molecular Weights on the Properties of PVDF-TiO2 Composite Membrane for Oily Wastewater Treatment Process

Polyvinylidene fluoride (PVDF) hollow fiber ultrafiltration membranes consisted of TiO2 and different molecular weight (Mw) of polyvinylpyrrolidone (PVP) (i.e., 10, 24, 40, and 360 kDa) were prepared to treat synthesized oily wastewater. The membrane performances were characterized in terms of pure water flux, permeate flux, and oil rejection while their morphological properties were studied using SEM, AFM, and tensile tester. Results show that the PVDF-TiO2 composite membrane prepared from PVP40k was the best performing membrane owing to its promising water flux (72.2 L/m2.h) coupled with good rejection of oil (94%) when tested with 250 ppm oily solution under submerged condition. It is also found that with increasing PVP Mw, the membrane tended to exhibit higher PVP and protein rejection, greater mechanical strength, smaller porosity, and a smoother surface layer. Regarding the effect of pH, the permeate flux of the PVDF-PVP40k membrane was reported to increase with increasing pH from 4 to 7, followed by decrease when the pH was further increased to 10. Increasing oil concentration in the feed solution was reported to negatively affect the water flux of PVDF-PVP40k membrane, owing to the formation of thicker oil layer on the membrane surface which increased water transport resistance. A simple backflushing process on the other hand could retrieve approximately 60% of the membrane original flux without affecting the oil separation efficiency. Based on the findings, the PVDF-TiO2 membrane prepared from PVP40k can be potentially considered for oily wastewater treatment process due to its good combination of permeability and selectivity and reasonably high water recovery rate.

Thin film nanocomposite: the next generation selective membrane for CO2 removal

Recent innovations in nano-enabled membranes, particularly thin film nanocomposites (TFN), are anticipated to accelerate the adoption by industry of this green and energy-efficient technology. TFN properties are highly tunable through meticulous material selection and membrane design while the flexibility of interfacial polymerization (IP) enables easy scale-up of TFN. This review discusses the contemporary advancements in TFN for CO2 separation, where the important aspects involved in the material development are critically reviewed. The challenges and future prospects of TFN to heighten the performance of membrane-based CO2 removal are also highlighted.

Effect of Dispersed Multi-Walled Carbon Nanotubes on Mixed Matrix Membrane for O2/N2 Separation

Mixed matrix membranes (MMMs) consisting of multiwalled carbon nanotubes (MWCNTs) embedded within polyetherimide were prepared. Surfactants of different charges were utilized to disperse the nanotubes through a simple non-covalent approach. The characterization results suggest that proper selection of the dispersing agent contributed to better dispersion of nanotubes. The MMMs exhibited improved thermal stability and mechanical strength, which indicate the improvement of dispersion and compatibility within the polymer matrix. The resulting membrane exhibited permeance improvement of O2 and N2 as much as 87.7% and 120% respectively compared to that of neat polyetherimide. The results implied that Triton-X100 treated MWCNTs is a promising filler to enhance gas permeability.

Graphene oxide/polysulfone hollow fiber mixed matrix membranes for gas separation

Hollow fiber mixed matrix membranes (MMM) were fabricated by embedding graphene oxide (GO) into a polysulfone (PSf) polymer matrix to investigate the gas permeation properties of CO2, N2, and CH4 in the membrane. The properties of the neat membrane and MMM loaded with 0.25% of GO were investigated. The transmission electron microscopy (TEM) and atomic force microscopy (AFM) analysis confirmed the synthesized GO was in the nanosheet form. The prepared MMM exhibited an improvement in mechanical and thermal stability. Field emission scanning electron microscopy (FESEM) analysis showed that the addition of GO into the polymer developed a well integrated skin layer as well as change in the formation of a membrane substructure layer which lead to better gas separation performance properties. The unique characteristics of GO which showed a strong affinity towards CO2 has contributed to the increment of CO2 permeance by 14%. Besides, GO created a path for small molecule gases (CO2) and restricted large molecule gases (N2 and CH4) to pass through the membrane. Both CO2/N2 and CO2/CH4 selectivity of MMM were enhanced by 158% and 74% respectively as compared to that of the neat PSf membrane.

Antifouling polyethersulfone hemodialysis membranes incorporated with poly (citric acid) polymerized multi-walled carbon nanotubes

Poly (citric acid)-grafted-MWCNT (PCA-g-MWCNT) was incorporated as nanofiller in polyethersulfone (PES) to produce hemodialysis mixed matrix membrane (MMM). Citric acid monohydrate was polymerized onto the surface of MWCNTs by polycondensation. Neat PES membrane and PES/MWCNTs MMMs were fabricated by dry-wet spinning technique. The membranes were characterized in terms of morphology, pure water flux (PWF) and bovine serum albumin (BSA) protein rejection. The grafting yield of PCA onto MWCNTs was calculated as 149.2%. The decrease of contact angle from 77.56° to 56.06° for PES/PCA-g-MWCNTs membrane indicated the increase in surface hydrophilicity, which rendered positive impacts on the PWF and BSA rejection of the membrane. The PWF increased from 15.8Lm(-2)h(-1) to 95.36Lm(-2)h(-1) upon the incorporation of PCA-g-MWCNTs due to the attachment of abundant hydrophilic groups that present on the MWCNTs, which have improved the affinity of membrane towards the water molecules. For protein rejection, the PES/PCA-g-MWCNTs MMM rejected 95.2% of BSA whereas neat PES membrane demonstrated protein rejection of 90.2%. Compared to commercial PES hemodialysis membrane, the PES/PCA-g-MWCNTs MMMs showed less flux decline behavior and better PWF recovery ratio, suggesting that the membrane antifouling performance was improved. The incorporation of PCA-g-MWCNTs enhanced the separation features and antifouling capabilities of the PES membrane for hemodialysis application.