Shuwen Goh, Singapore Membrane Technology Center (SMTC), Nanyang Environment and Water Research I... more Shuwen Goh, Singapore Membrane Technology Center (SMTC), Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University (NTU), Singapore gohsw@ntu.edu.sg Chun Heng Loh, SMTC, NEWRI, NTU Bibianna J.L. Yeo, SMTC, NEWRI, and Interdisciplinary Graduate School, NTU Andrew G. Livingston, Department of Chemical Engineering, Imperial College London Anthony G. Fane, SMTC, NEWRI, NTU Rong Wang, SMTC, NEWRI, and School of Civil and Environmental Engineering (CEE), NTU
Pressure retarded osmosis (PRO) demonstrates great potential in energy harvesting when combining ... more Pressure retarded osmosis (PRO) demonstrates great potential in energy harvesting when combining with seawater reverse osmosis. However, the lack of suitable membrane modules and the issue caused by the membrane fouling greatly impede the practical application of PRO to a larger scale. In this study, two-inch thin film composite hollow fiber modules were fabricated by using in-house developed PRO membranes. The produced PRO modules have a maximum effective area of 0.5 m 2. By assessing the PRO performances of the modules with different sizes, external concentration polarization (ECP) was found to have significant impact on the flux reduction during module scale-up. Different module designs, including fiber bundles, distribution baffles and distribution tubes, were thus adopted as an attempt to boost the membrane performance. A power density of 8.9 W/m 2 at 15 bar was obtained using tap water as feed and 1 M NaCl solution as draw solution. PRO performance tests were also carried out using the developed two-inch modules on a pilot-scale setup with actual wastewater retentate as feed solution. Low pressure nanofiltration was selected as the pretreatment of the wastewater retentate to mitigate fouling. A power density of larger than 8 W/m 2 was obtained when pretreated wastewater retentate was used as the feed solution, implying high potential of PRO in the pilot scale. Nevertheless, full potential of PRO can only be realized by mitigating ECP, which could be achieved by improving the module design in the further endeavor.
Gas-liquid membrane contactor (GLMC) is a promising method to attain high efficiency for CO2 capt... more Gas-liquid membrane contactor (GLMC) is a promising method to attain high efficiency for CO2 capture from flue gas, biogas and natural gas. However, membranes used in GLMC are prone to pore wetting due to insufficient hydrophobicity and low chemical resistance, resulting in significant increase in mass transfer resistance. To mitigate this issue, inorganic-organic fluorinated titania/polyvinylidene fluoride (fTiO2/PVDF) composite hollow fiber (HF) membranes was prepared via facile in-situ vapor induced hydrolyzation method, followed by hydrophobic modification. The proposed composite membranes were expected to couple the superb chemical stability of inorganic and high permeability/low cost of organic materials. The continuous fTiO2 layer deposited on top of PVDF substrate was found to possess a tighter microstructure and better hydrophobicity, which effectively prevented the membrane from wetting and lead to a high CO2 absorption flux (12.7 × 10-3 mol•m-2 •s-1). In a stability test with 21-day operation of GLMC using 1M monoethanolamine (MEA) as the absorbent, the fTiO2/PVDF membrane remained to be intact with a CO2 absorption flux decline of ~16%, while the pristine PVDF membrane suffered from a flux decline of ~80% due to membrane damage. Overall, this work provides an insight into the preparation of high-quality inorganic/organic composite HF membranes for CO2 capture in GLMC application.
Polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) hollow fiber membranes were develop... more Polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) hollow fiber membranes were developed via thermally induced phase separation (TIPS) method for direct contact membrane distillation (DCMD). The effects of PTFE addition on the thermal behavior of the dope mixtures and membrane formation were investigated. It was found that the crystallization of PVDF was significantly enhanced with increased nucleation sites provided by PTFE particles, leading to promoted formation of smaller spherulites in a greater density. Furthermore, the improved uniformity and increased amount of cavity between the spherical crystallites coherently facilitated the formation of smaller pores ranging from 0.08 to 0.12 µm. With certain PTFE loading, the membranes exhibited improved porosity, water permeability and hydrophobicity as well as enhanced tensile strength of 9.4 ± 0.3 MPa. To examine the DCMD performance, the membranes were tested under various conditions using 3.5 wt.% NaCl solution. A stable permeation flux of 28.3 kg m-2 h-1 at the feed temperature of 60 ºC with 99.99 % NaCl rejection for over 50 hours of operation was achieved, which is comparable with similar type of PVDF membranes while the newly developed membrane exhibited better mechanical strength. This study suggests that the asspun PVDF/PTFE hollow fiber membranes have potential for DCMD applications.
Research on membrane technologies has grown exponentially to treat wastewater, recycle polluted w... more Research on membrane technologies has grown exponentially to treat wastewater, recycle polluted water and provide more freshwater. Electrospun nanofibrous membranes (ENMs) exhibit great potential to be applied in membrane processes due to their distinctive features such as high porosity of up to 90% and large specific surface area. Compared with other nanofiber fabrication techniques, electrospinning is capable of developing unique architectures of nanofibrous scaffolds by designing special assemblies, and it is facile in functionalizing nanofibers by incorporating multi-functional materials. This review summarizes the state-of-the-art progress on fabrication and modification of electrospun polymeric membranes with a particular emphasis on their advances, challenges and future improvement in water treatment applications. First, we briefly describe the complex process governing electrospinning, illustrate the effects of intrinsic properties of polymer solutions, operational parameters and surrounding environment conditions on the formation of nanofibers and resultant nanofibrous membranes, and summarize various designs of electrospinning apparatus. That is followed by reviewing the methods to prepare multifunctional composite ENMs, assorted into three categories, including modification in nanofibers, loading target molecules onto nanofibers surface, and implementing selective layers on the ENM surface. Comprehensive discussion about
N-methyl-2-pyrrolidone (NMP) is widely used as a solvent in polymeric membrane fabrication proces... more N-methyl-2-pyrrolidone (NMP) is widely used as a solvent in polymeric membrane fabrication process, its elimination from the process wastewater (normally at a high concentration > 1000 mg/L) prior to discharge is essential because of environmental concern. This study investigated the feasibility of treating high-strength NMP-containing process wastewater in a sequencing batch reactor (SBR; i.e., batch feeding and intermittent aerobic/anoxic condition) and a membrane bioreactor (MBR; i.e., continuous feeding and aeration), respectively. The results showed that the SBR with the acclimated sludge was capable of removing >90% of dissolved organic carbon (DOC) and almost 98% of NMP within 2 h. In contrast, the MBR with the acclimated sludge showed a decreasing NMP removal efficiency from 100% to 40% over 15-day operation. The HPLC and LC-MS/MS analytical results showed that NMP degradation in SBR and MBR could undergo different pathways. This may be attributed to the dissimilar bac...
The preparation of inorganic/organic composite membranes has been demonstrated to bring together ... more The preparation of inorganic/organic composite membranes has been demonstrated to bring together the advantages of ceramic and polymeric materials. However, the fabrication of inorganic/organic thin-film composite membranes in hollow fiber configuration has rarely been reported due to the complexity of existing methods. A facile and economic method, in-situ vapor induced hydrolyzation process, was proposed for preparation of Al 2 O 3 /Polyethersulfone (PES) thin-film composite hollow fiber ultrafiltration (UF) membranes. The amphiphilic copolymer of PEO-PPO-PEO was introduced to bridge the Al 2 O 3 nanoparticles and PES substrate, resulting in a more stable deposition of Al 2 O 3 on the substrate. The surface morphology and pore size of Al 2 O 3 /PES membranes could be precisely tuned by controlling the addition of aluminum precursors. The resultant membrane presented a MWCO of 22 kDa and a high pure water permeability (PWP) of 280 L m −2 h −1 bar −1 due to the completely coated surface hydrophilic Al 2 O 3 ceramic layer. In addition, the as-prepared thin film composite membrane exhibited a lower membrane contact angle than most other mixed matrix inorganic/organic composite membranes. Due to the higher surface hydrophilicity, the composite membranes showed improved antifouling properties to humic acid (HA). This in-situ vapor induced hydrolyzation process was demonstrated to be promising for fabricating thin-film inorganic/organic composite hollow fiber membranes with high performance in separation processes.
Increasing demand for oil and gas leads to the generation of substantial amount of produced water... more Increasing demand for oil and gas leads to the generation of substantial amount of produced water, bringing about deleterious impacts on the environment. Direct-contact membrane distillation (DCMD) could be a possible option for dewatering oil-in-water (O/W) emulsions because of many benefits brought by the DCMD process. However, these low surface tension solutions pose some difficult issues such as membrane fouling and pore wetting. The mechanisms involved are not fully understood due to the lack of study of the interaction between the emulsions and the membrane surface in the DCMD domain. To address the challenges, this study aims at developing a fundamental understanding of the relationship between surfactant-stabilized O/W emulsions and polyvinylidene fluoride (PVDF) membrane surface in DCMD operations. Effects of surfactant types (Span 20, Tween 20, and sodium dodecyl sulfate), oil concentration, and oil types (petroleum and vacuum pump oil) were systematically studied to better understand the fouling and wetting mechanisms involved. The results reveal that surfactant concentration and hydrophobicity had an influence on the membrane fouling and wetting behaviors. Surfactants with a lower hydrophilic-lipophilic balance (HLB) value could make the PVDF membrane surface less hydrophobic and cause less severe fouling by restraining the adsorption of oil droplets on the membrane surface. These findings suggest that membrane surface modification is required to achieve anti-fouling and anti-wetting properties to make DCMD an energy-efficient and effective technology for treating produced water.
In present study, we explored the fabrication of a high performance thin-film composite (TFC) for... more In present study, we explored the fabrication of a high performance thin-film composite (TFC) forward osmosis (FO) membrane by minimizing the structural parameter of hollow fiber substrates and enhancing the water permeability of selective layer. A theoretical analysis specified that a minimized structural parameter of substrate combined with an enhanced water permeability of selective layer could generate a significantly high water flux in FO process. The experimental results showed that the addition of LiCl into polyetherimide (PEI) polymer dope made the membrane less tortuous and thereby significantly reduced the structural parameter. The elevated take-up speed reduced the substrate thickness but also made the substrate more tortuous. Further study showed that a robust hollow fiber with low structural parameter can be obtained by reducing the substrate wall thickness and dimension simultaneously. After optimization, the structural parameter of the substrate declined from 308 µm to 172 µm. After interfacial polymerization, the thin-film composite membrane constructed on the optimized substrate showed an improved water permeability (from 2.85 L m −2 h −1 bar −1 to 3.66 L m −2 h −1 bar −1) and a comparably low salt permeability (0.36 L m −2 h −1 versus 0.31 L m −2 h −1). The low structural parameter, in combination with the high water permeability and low salt permeability, contributed to significantly increased water flux (J v) from 25.4 L m −2 h −1 to 38.5 L m −2 h −1 when 1 M NaCl was used as draw and deionised water as feed in a configuration of active layer facing feed solution (AL-FS). Furthermore, aquaporins were incorporated into the polyamide selective layer to enhance the water permeability, which remarkably reached up to 7.6 L m −2 h −1 bar −1. The aquaporin-incorporated FO membrane demonstrated a J v of 49.1 L m −2 h −1 and a J s /J v (J s indicates the salt flux) of 0.10 g/L with 1 M NaCl as draw and deionised water as feed in the AL-FS configuration, which is favourable to mitigate membrane fouling in practical application. The AQP-FO membrane reported in this study outperforms most of other reported FO membranes.
Due to its toxicity to ecosystem, phenol removal from industrial wastewater before discharge is a... more Due to its toxicity to ecosystem, phenol removal from industrial wastewater before discharge is a priority concern. Extractive membrane bioreactor (EMBR), a novel wastewater treatment process combining aqueous-aqueous extractive membrane process and biodegradation, has shown potential in treating phenol in wastewater. In this paper, composite hollow fiber membranes with different levels of poly (dimethylsiloxane) (PDMS) intrusion were prepared by coating a layer of PDMS on a Polyetherimide (PEI) hollow fiber substrate. Their applicability to EMBR for phenol removal was studied. The prepared membranes were characterized by microscopy and gas permeation test, and their performances were evaluated in aqueous-aqueous extractive membrane processes and EMBR process. The overall mass transfer coefficient for phenol, or k 0 , was found to be significantly affected by the level of PDMS intrusion in the composite membranes. This is because the penetration of PDMS into the porous substrate results in a denser membrane structure, which consequently increases the membrane resistance. A slight penetration of PDMS into the substrate was found to be necessary for the composite membranes to achieve high k 0 while maintaining low inorganic flux across the membranes. Wilson-plot analysis suggests that membrane resistance dominated over liquid boundary layer resistances. After more than 250 h of EMBR operation, significant biofilm growth was observed on the composite membranes and the k 0 was dropped but stabilized at around 7.5 Â 10 À 7 m/s. This k 0 was 7.5 times higher than commercial PDMS tubular membranes (without biofilm development) reported in previous studies, confirming the superiority of thin film composite membranes prepared in this work. It was also found that process optimization to control biofilm thickness is important in order to enhance phenol removal rate in EMBR.
With modest temperature demand, low operating pressure, and high solute rejection, membrane disti... more With modest temperature demand, low operating pressure, and high solute rejection, membrane distillation (MD) is an attractive option for desalination, waste treatment, and food and pharmaceutical processing. However, large-scale practical applications of MD are still hindered by the absence of effective membranes with high hydrophobicity, high porosity, and adequate mechanical strength, which are important properties for MD permeation fluxes, stable long-term performance, and effective packing in modules without damage. This study describes novel design strategies for highly robust superhydrophobic dual-layer membranes for MD via electrospinning. One of the newly developed membranes comprises a durable and ultrathin 3-dimensional (3D) superhydrophobic skin and porous nanofibrous support whereas another was fabricated by electrospinning 3D superhydrophobic layers on a nonwoven support. These membranes exhibit superhydrophobicity toward distilled water, salty water, oil-in-water emul...
A novel catalytic membrane contactor (CMC) has been designed and constructed by incorporating pol... more A novel catalytic membrane contactor (CMC) has been designed and constructed by incorporating polyoxometalates (POMs) onto polymeric polyvinylidene fluoride (PVDF) micro-porous hollow fiber membranes which were fabricated by a dry-jet wet spinning process. A simple chemical deposition method was utilized to anchor the Keggin-type polyoxometalate H 5 [PV 2 Mo 10 O 40 ] (PV 2 Mo 10) on the outer surface of the PVDF hollow fiber, which was confirmed by the measurements of field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR). The degradation of phenol in synthetic wastewater was carried out using air as green oxidant under room conditions using the CMC. A gas-catalyst-liquid interface was successfully built up to enhance the catalytic efficiency. It was observed that the pressure of the gas flow played an important role and a low pressure is preferable. The long-term stability of the CMC was also preliminarily studied. This novel and facile CMC with PV 2 Mo 10 as catalyst potentially provides a feasible pathway for wastewater treatment under mild conditions.
Polyvinylidene fluoride-co-hexafluropropylene (PVDF-HFP) has received much attention recently as ... more Polyvinylidene fluoride-co-hexafluropropylene (PVDF-HFP) has received much attention recently as a promising membrane material for membrane contactor application. A systematic study has been carried out to investigate the effects of polyethylene glycol (PEG) with different molecular weights and different loadings as an additive on the fabrication of PVDF-HFP asymmetric microporous hollow fiber membranes. Moreover, the synergetic effects of coagulation temperature and the second additive (lithium chloride: LiCl) with PEG are also evaluated. Experiments revealed that the addition of PEG into the PVDF-HFP/NMP solution resulted in the system thermodynamically less stable in reaction with water, promoting rapid phase demixing in the phase inversion process. When the same 3 wt% PEG was added into the dope solution, the dimension of fingerlike macrovoids of the resultant membrane increased in parallel with the increase of PEG molecular weight from 200 to 600 and 6000 kDa, and pure water permeability (PWP) also increased accordingly. An increase in PWP was also observed when PEG-200 loading in the dope solution was increased from 3 to 5 and 10 wt%, corresponding to the morphology change of resultant membranes. As a synergetic effect of coagulation temperature with PEG, the finger-like pores occurred in the membrane at room temperature expanded to much larger macrovoids using 10 • C water as the coagulant, and the big finger-like pores almost disappeared when the coagulation bath temperature was increased to 40 • C because of delayed phase demixing. The big macrovoid size can also be suppressed by adding the second small molecule additive, LiCl, due to its strong interactions with NMP and PVDF-HFP to delay the dope precipitation. The irregular inner contour of the membrane can be eliminated by the increase of coagulation bath temperature to 40 • C. The hollow fiber membrane made by a dope of PVDF-HFP/PEG-6000/LiCl/NMP (15/3/3/79 in weight) using 40 • C water as the coagulant exhibited a high PWP of 117 L/m 2 h atm and reasonably good MWCO of 150 kDa. An improvement has been made in the current work as compared to previous PVDF-HFP hollow fiber membranes reported in literatures.
The amphiphilic Pluronic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxi... more The amphiphilic Pluronic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) have received much attention recently as both the surface modifier and pore former in membrane fabrication. In this study, a systematical study has been carried out to investigate the effect of Pluronics with different molecular architectures and contents as a pore-forming additive on the fabrication of polyethersulfone (PES) ultrafiltration (UF) hollow fibers, and to identify the most preferential features of Pluronic for making high performance membranes. The as-spun hollow fibers were characterized in terms of cross-sectional membrane morphology, membrane surface chemistry, mechanical properties, pure water permeation (PWP), molecular weight cutoff (MWCO), and pore size distribution. It was observed that the PWP and MWCO of the as-spun hollow fibers are dependent on the structure of additives. Among all the membranes spun with 5 wt% additives, the hollow fibers spun using Pluronic F127 and F108 as the additives possess the highest PWP, the lowest MWCO, and the narrowest pore size distribution. It is possible that the PEO brush layer formed on the internal pore surface by Pluronic F127 and F108 might reduce the apparent pore size and hence improved the solute rejection of the resultant membranes. A comparison between Pluronic and poly(ethylene glycol) (PEG) as additives also confirmed the importance of the presence of PPO chain in Pluronic in the formation of high-performance membranes. When Pluronic F127 concentration was 10 wt%, the as-spun hollow fiber exhibited the highest PWP of 113.8 L/m 2 h bar and the lowest MWCO of 9 kDa.
Due to the unique properties of amphiphilic Pluronic block copolymers, they have been blended wit... more Due to the unique properties of amphiphilic Pluronic block copolymers, they have been blended with other polymers such as PVDF to prepare membranes. Considering the stability issue, the strong pore-forming ability of Pluronic F127, and the mechanical strength of the resultant membranes, Pluronic/LiCl mixed additive consisting of a very low concentration of Pluronic has been used in this study. The effects of spinning conditions on PVDF hollow fiber fabrication have also been studied. It is found that the use of merely 0.2 wt% of Pluronic is sufficient to impose significant impacts on the morphology, filtration performance, and mechanical properties of the resultant fibers. Hollow fibers spun using higher coagulant temperature, higher take-up speed, and water as the bore fluid exhibit better mechanical properties and filtration performance. PVDF hollow fiber membranes with PWP of 1180 L/m 2 h MPa and MWCO of 5 kDa were obtained when spun with optimized spinning conditions and using 3 wt% of LiCl and 0.2 wt% of Pluronic as the mixed additive. The fibers can withstand a pressure of 0.55 MPa from the lumen.
Poly(vinylidene fluoride) (PVDF) has become one of the most popular materials for membrane prepar... more Poly(vinylidene fluoride) (PVDF) has become one of the most popular materials for membrane preparation via nonsolvent induced phase separation (NIPS) process. In this study, an amphiphilic block copolymer, Pluronic F127, has been used as both a pore-former and a surface-modifier in the fabrication of PVDF hollow fiber membranes to enhance the membrane permeability and hydrophilicity. The effects of 2nd additive and coagulant temperature on the formation of PVDF/Pluronic F127 membranes have also been investigated. The as-spun hollow fibers were characterized in terms of cross-sectional morphology, pure water permeation (PWP), relative molecular mass cutoff (MWCO), membrane chemistry, and hydrophilicity. It was observed that the addition of Pluronic F127 significantly increased the PWP of as-spun fibers, while the membrane contact angle was reduced. However, the size of macrovoids in the membranes was undesirably large. The addition of a 2nd additive, including lithium chloride (LiCl) and water, or an increase in coagulant temperature was found to effectively suppress the macrovoid formation in the Pluronic-containing membranes. In addition, the use of LiCl as a 2nd additive also further enhanced the PWP and hydrophilicity of the membranes, while the surface pore size became smaller. PVDF hollow fiber with a PWP as high as 2530 L•m −2 •h −1 •MPa −1 , a MWCO of 53000 and a contact angle of 71° was successfully fabricated with 3% (by mass) of Pluronic F127 and 3% (by mass) of LiCl at a coagulant temperature of 25 °C, which shows better performance as compared with most of PVDF hollow fiber membranes made by NIPS method.
Shuwen Goh, Singapore Membrane Technology Center (SMTC), Nanyang Environment and Water Research I... more Shuwen Goh, Singapore Membrane Technology Center (SMTC), Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University (NTU), Singapore gohsw@ntu.edu.sg Chun Heng Loh, SMTC, NEWRI, NTU Bibianna J.L. Yeo, SMTC, NEWRI, and Interdisciplinary Graduate School, NTU Andrew G. Livingston, Department of Chemical Engineering, Imperial College London Anthony G. Fane, SMTC, NEWRI, NTU Rong Wang, SMTC, NEWRI, and School of Civil and Environmental Engineering (CEE), NTU
Pressure retarded osmosis (PRO) demonstrates great potential in energy harvesting when combining ... more Pressure retarded osmosis (PRO) demonstrates great potential in energy harvesting when combining with seawater reverse osmosis. However, the lack of suitable membrane modules and the issue caused by the membrane fouling greatly impede the practical application of PRO to a larger scale. In this study, two-inch thin film composite hollow fiber modules were fabricated by using in-house developed PRO membranes. The produced PRO modules have a maximum effective area of 0.5 m 2. By assessing the PRO performances of the modules with different sizes, external concentration polarization (ECP) was found to have significant impact on the flux reduction during module scale-up. Different module designs, including fiber bundles, distribution baffles and distribution tubes, were thus adopted as an attempt to boost the membrane performance. A power density of 8.9 W/m 2 at 15 bar was obtained using tap water as feed and 1 M NaCl solution as draw solution. PRO performance tests were also carried out using the developed two-inch modules on a pilot-scale setup with actual wastewater retentate as feed solution. Low pressure nanofiltration was selected as the pretreatment of the wastewater retentate to mitigate fouling. A power density of larger than 8 W/m 2 was obtained when pretreated wastewater retentate was used as the feed solution, implying high potential of PRO in the pilot scale. Nevertheless, full potential of PRO can only be realized by mitigating ECP, which could be achieved by improving the module design in the further endeavor.
Gas-liquid membrane contactor (GLMC) is a promising method to attain high efficiency for CO2 capt... more Gas-liquid membrane contactor (GLMC) is a promising method to attain high efficiency for CO2 capture from flue gas, biogas and natural gas. However, membranes used in GLMC are prone to pore wetting due to insufficient hydrophobicity and low chemical resistance, resulting in significant increase in mass transfer resistance. To mitigate this issue, inorganic-organic fluorinated titania/polyvinylidene fluoride (fTiO2/PVDF) composite hollow fiber (HF) membranes was prepared via facile in-situ vapor induced hydrolyzation method, followed by hydrophobic modification. The proposed composite membranes were expected to couple the superb chemical stability of inorganic and high permeability/low cost of organic materials. The continuous fTiO2 layer deposited on top of PVDF substrate was found to possess a tighter microstructure and better hydrophobicity, which effectively prevented the membrane from wetting and lead to a high CO2 absorption flux (12.7 × 10-3 mol•m-2 •s-1). In a stability test with 21-day operation of GLMC using 1M monoethanolamine (MEA) as the absorbent, the fTiO2/PVDF membrane remained to be intact with a CO2 absorption flux decline of ~16%, while the pristine PVDF membrane suffered from a flux decline of ~80% due to membrane damage. Overall, this work provides an insight into the preparation of high-quality inorganic/organic composite HF membranes for CO2 capture in GLMC application.
Polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) hollow fiber membranes were develop... more Polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) hollow fiber membranes were developed via thermally induced phase separation (TIPS) method for direct contact membrane distillation (DCMD). The effects of PTFE addition on the thermal behavior of the dope mixtures and membrane formation were investigated. It was found that the crystallization of PVDF was significantly enhanced with increased nucleation sites provided by PTFE particles, leading to promoted formation of smaller spherulites in a greater density. Furthermore, the improved uniformity and increased amount of cavity between the spherical crystallites coherently facilitated the formation of smaller pores ranging from 0.08 to 0.12 µm. With certain PTFE loading, the membranes exhibited improved porosity, water permeability and hydrophobicity as well as enhanced tensile strength of 9.4 ± 0.3 MPa. To examine the DCMD performance, the membranes were tested under various conditions using 3.5 wt.% NaCl solution. A stable permeation flux of 28.3 kg m-2 h-1 at the feed temperature of 60 ºC with 99.99 % NaCl rejection for over 50 hours of operation was achieved, which is comparable with similar type of PVDF membranes while the newly developed membrane exhibited better mechanical strength. This study suggests that the asspun PVDF/PTFE hollow fiber membranes have potential for DCMD applications.
Research on membrane technologies has grown exponentially to treat wastewater, recycle polluted w... more Research on membrane technologies has grown exponentially to treat wastewater, recycle polluted water and provide more freshwater. Electrospun nanofibrous membranes (ENMs) exhibit great potential to be applied in membrane processes due to their distinctive features such as high porosity of up to 90% and large specific surface area. Compared with other nanofiber fabrication techniques, electrospinning is capable of developing unique architectures of nanofibrous scaffolds by designing special assemblies, and it is facile in functionalizing nanofibers by incorporating multi-functional materials. This review summarizes the state-of-the-art progress on fabrication and modification of electrospun polymeric membranes with a particular emphasis on their advances, challenges and future improvement in water treatment applications. First, we briefly describe the complex process governing electrospinning, illustrate the effects of intrinsic properties of polymer solutions, operational parameters and surrounding environment conditions on the formation of nanofibers and resultant nanofibrous membranes, and summarize various designs of electrospinning apparatus. That is followed by reviewing the methods to prepare multifunctional composite ENMs, assorted into three categories, including modification in nanofibers, loading target molecules onto nanofibers surface, and implementing selective layers on the ENM surface. Comprehensive discussion about
N-methyl-2-pyrrolidone (NMP) is widely used as a solvent in polymeric membrane fabrication proces... more N-methyl-2-pyrrolidone (NMP) is widely used as a solvent in polymeric membrane fabrication process, its elimination from the process wastewater (normally at a high concentration > 1000 mg/L) prior to discharge is essential because of environmental concern. This study investigated the feasibility of treating high-strength NMP-containing process wastewater in a sequencing batch reactor (SBR; i.e., batch feeding and intermittent aerobic/anoxic condition) and a membrane bioreactor (MBR; i.e., continuous feeding and aeration), respectively. The results showed that the SBR with the acclimated sludge was capable of removing >90% of dissolved organic carbon (DOC) and almost 98% of NMP within 2 h. In contrast, the MBR with the acclimated sludge showed a decreasing NMP removal efficiency from 100% to 40% over 15-day operation. The HPLC and LC-MS/MS analytical results showed that NMP degradation in SBR and MBR could undergo different pathways. This may be attributed to the dissimilar bac...
The preparation of inorganic/organic composite membranes has been demonstrated to bring together ... more The preparation of inorganic/organic composite membranes has been demonstrated to bring together the advantages of ceramic and polymeric materials. However, the fabrication of inorganic/organic thin-film composite membranes in hollow fiber configuration has rarely been reported due to the complexity of existing methods. A facile and economic method, in-situ vapor induced hydrolyzation process, was proposed for preparation of Al 2 O 3 /Polyethersulfone (PES) thin-film composite hollow fiber ultrafiltration (UF) membranes. The amphiphilic copolymer of PEO-PPO-PEO was introduced to bridge the Al 2 O 3 nanoparticles and PES substrate, resulting in a more stable deposition of Al 2 O 3 on the substrate. The surface morphology and pore size of Al 2 O 3 /PES membranes could be precisely tuned by controlling the addition of aluminum precursors. The resultant membrane presented a MWCO of 22 kDa and a high pure water permeability (PWP) of 280 L m −2 h −1 bar −1 due to the completely coated surface hydrophilic Al 2 O 3 ceramic layer. In addition, the as-prepared thin film composite membrane exhibited a lower membrane contact angle than most other mixed matrix inorganic/organic composite membranes. Due to the higher surface hydrophilicity, the composite membranes showed improved antifouling properties to humic acid (HA). This in-situ vapor induced hydrolyzation process was demonstrated to be promising for fabricating thin-film inorganic/organic composite hollow fiber membranes with high performance in separation processes.
Increasing demand for oil and gas leads to the generation of substantial amount of produced water... more Increasing demand for oil and gas leads to the generation of substantial amount of produced water, bringing about deleterious impacts on the environment. Direct-contact membrane distillation (DCMD) could be a possible option for dewatering oil-in-water (O/W) emulsions because of many benefits brought by the DCMD process. However, these low surface tension solutions pose some difficult issues such as membrane fouling and pore wetting. The mechanisms involved are not fully understood due to the lack of study of the interaction between the emulsions and the membrane surface in the DCMD domain. To address the challenges, this study aims at developing a fundamental understanding of the relationship between surfactant-stabilized O/W emulsions and polyvinylidene fluoride (PVDF) membrane surface in DCMD operations. Effects of surfactant types (Span 20, Tween 20, and sodium dodecyl sulfate), oil concentration, and oil types (petroleum and vacuum pump oil) were systematically studied to better understand the fouling and wetting mechanisms involved. The results reveal that surfactant concentration and hydrophobicity had an influence on the membrane fouling and wetting behaviors. Surfactants with a lower hydrophilic-lipophilic balance (HLB) value could make the PVDF membrane surface less hydrophobic and cause less severe fouling by restraining the adsorption of oil droplets on the membrane surface. These findings suggest that membrane surface modification is required to achieve anti-fouling and anti-wetting properties to make DCMD an energy-efficient and effective technology for treating produced water.
In present study, we explored the fabrication of a high performance thin-film composite (TFC) for... more In present study, we explored the fabrication of a high performance thin-film composite (TFC) forward osmosis (FO) membrane by minimizing the structural parameter of hollow fiber substrates and enhancing the water permeability of selective layer. A theoretical analysis specified that a minimized structural parameter of substrate combined with an enhanced water permeability of selective layer could generate a significantly high water flux in FO process. The experimental results showed that the addition of LiCl into polyetherimide (PEI) polymer dope made the membrane less tortuous and thereby significantly reduced the structural parameter. The elevated take-up speed reduced the substrate thickness but also made the substrate more tortuous. Further study showed that a robust hollow fiber with low structural parameter can be obtained by reducing the substrate wall thickness and dimension simultaneously. After optimization, the structural parameter of the substrate declined from 308 µm to 172 µm. After interfacial polymerization, the thin-film composite membrane constructed on the optimized substrate showed an improved water permeability (from 2.85 L m −2 h −1 bar −1 to 3.66 L m −2 h −1 bar −1) and a comparably low salt permeability (0.36 L m −2 h −1 versus 0.31 L m −2 h −1). The low structural parameter, in combination with the high water permeability and low salt permeability, contributed to significantly increased water flux (J v) from 25.4 L m −2 h −1 to 38.5 L m −2 h −1 when 1 M NaCl was used as draw and deionised water as feed in a configuration of active layer facing feed solution (AL-FS). Furthermore, aquaporins were incorporated into the polyamide selective layer to enhance the water permeability, which remarkably reached up to 7.6 L m −2 h −1 bar −1. The aquaporin-incorporated FO membrane demonstrated a J v of 49.1 L m −2 h −1 and a J s /J v (J s indicates the salt flux) of 0.10 g/L with 1 M NaCl as draw and deionised water as feed in the AL-FS configuration, which is favourable to mitigate membrane fouling in practical application. The AQP-FO membrane reported in this study outperforms most of other reported FO membranes.
Due to its toxicity to ecosystem, phenol removal from industrial wastewater before discharge is a... more Due to its toxicity to ecosystem, phenol removal from industrial wastewater before discharge is a priority concern. Extractive membrane bioreactor (EMBR), a novel wastewater treatment process combining aqueous-aqueous extractive membrane process and biodegradation, has shown potential in treating phenol in wastewater. In this paper, composite hollow fiber membranes with different levels of poly (dimethylsiloxane) (PDMS) intrusion were prepared by coating a layer of PDMS on a Polyetherimide (PEI) hollow fiber substrate. Their applicability to EMBR for phenol removal was studied. The prepared membranes were characterized by microscopy and gas permeation test, and their performances were evaluated in aqueous-aqueous extractive membrane processes and EMBR process. The overall mass transfer coefficient for phenol, or k 0 , was found to be significantly affected by the level of PDMS intrusion in the composite membranes. This is because the penetration of PDMS into the porous substrate results in a denser membrane structure, which consequently increases the membrane resistance. A slight penetration of PDMS into the substrate was found to be necessary for the composite membranes to achieve high k 0 while maintaining low inorganic flux across the membranes. Wilson-plot analysis suggests that membrane resistance dominated over liquid boundary layer resistances. After more than 250 h of EMBR operation, significant biofilm growth was observed on the composite membranes and the k 0 was dropped but stabilized at around 7.5 Â 10 À 7 m/s. This k 0 was 7.5 times higher than commercial PDMS tubular membranes (without biofilm development) reported in previous studies, confirming the superiority of thin film composite membranes prepared in this work. It was also found that process optimization to control biofilm thickness is important in order to enhance phenol removal rate in EMBR.
With modest temperature demand, low operating pressure, and high solute rejection, membrane disti... more With modest temperature demand, low operating pressure, and high solute rejection, membrane distillation (MD) is an attractive option for desalination, waste treatment, and food and pharmaceutical processing. However, large-scale practical applications of MD are still hindered by the absence of effective membranes with high hydrophobicity, high porosity, and adequate mechanical strength, which are important properties for MD permeation fluxes, stable long-term performance, and effective packing in modules without damage. This study describes novel design strategies for highly robust superhydrophobic dual-layer membranes for MD via electrospinning. One of the newly developed membranes comprises a durable and ultrathin 3-dimensional (3D) superhydrophobic skin and porous nanofibrous support whereas another was fabricated by electrospinning 3D superhydrophobic layers on a nonwoven support. These membranes exhibit superhydrophobicity toward distilled water, salty water, oil-in-water emul...
A novel catalytic membrane contactor (CMC) has been designed and constructed by incorporating pol... more A novel catalytic membrane contactor (CMC) has been designed and constructed by incorporating polyoxometalates (POMs) onto polymeric polyvinylidene fluoride (PVDF) micro-porous hollow fiber membranes which were fabricated by a dry-jet wet spinning process. A simple chemical deposition method was utilized to anchor the Keggin-type polyoxometalate H 5 [PV 2 Mo 10 O 40 ] (PV 2 Mo 10) on the outer surface of the PVDF hollow fiber, which was confirmed by the measurements of field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR). The degradation of phenol in synthetic wastewater was carried out using air as green oxidant under room conditions using the CMC. A gas-catalyst-liquid interface was successfully built up to enhance the catalytic efficiency. It was observed that the pressure of the gas flow played an important role and a low pressure is preferable. The long-term stability of the CMC was also preliminarily studied. This novel and facile CMC with PV 2 Mo 10 as catalyst potentially provides a feasible pathway for wastewater treatment under mild conditions.
Polyvinylidene fluoride-co-hexafluropropylene (PVDF-HFP) has received much attention recently as ... more Polyvinylidene fluoride-co-hexafluropropylene (PVDF-HFP) has received much attention recently as a promising membrane material for membrane contactor application. A systematic study has been carried out to investigate the effects of polyethylene glycol (PEG) with different molecular weights and different loadings as an additive on the fabrication of PVDF-HFP asymmetric microporous hollow fiber membranes. Moreover, the synergetic effects of coagulation temperature and the second additive (lithium chloride: LiCl) with PEG are also evaluated. Experiments revealed that the addition of PEG into the PVDF-HFP/NMP solution resulted in the system thermodynamically less stable in reaction with water, promoting rapid phase demixing in the phase inversion process. When the same 3 wt% PEG was added into the dope solution, the dimension of fingerlike macrovoids of the resultant membrane increased in parallel with the increase of PEG molecular weight from 200 to 600 and 6000 kDa, and pure water permeability (PWP) also increased accordingly. An increase in PWP was also observed when PEG-200 loading in the dope solution was increased from 3 to 5 and 10 wt%, corresponding to the morphology change of resultant membranes. As a synergetic effect of coagulation temperature with PEG, the finger-like pores occurred in the membrane at room temperature expanded to much larger macrovoids using 10 • C water as the coagulant, and the big finger-like pores almost disappeared when the coagulation bath temperature was increased to 40 • C because of delayed phase demixing. The big macrovoid size can also be suppressed by adding the second small molecule additive, LiCl, due to its strong interactions with NMP and PVDF-HFP to delay the dope precipitation. The irregular inner contour of the membrane can be eliminated by the increase of coagulation bath temperature to 40 • C. The hollow fiber membrane made by a dope of PVDF-HFP/PEG-6000/LiCl/NMP (15/3/3/79 in weight) using 40 • C water as the coagulant exhibited a high PWP of 117 L/m 2 h atm and reasonably good MWCO of 150 kDa. An improvement has been made in the current work as compared to previous PVDF-HFP hollow fiber membranes reported in literatures.
The amphiphilic Pluronic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxi... more The amphiphilic Pluronic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) have received much attention recently as both the surface modifier and pore former in membrane fabrication. In this study, a systematical study has been carried out to investigate the effect of Pluronics with different molecular architectures and contents as a pore-forming additive on the fabrication of polyethersulfone (PES) ultrafiltration (UF) hollow fibers, and to identify the most preferential features of Pluronic for making high performance membranes. The as-spun hollow fibers were characterized in terms of cross-sectional membrane morphology, membrane surface chemistry, mechanical properties, pure water permeation (PWP), molecular weight cutoff (MWCO), and pore size distribution. It was observed that the PWP and MWCO of the as-spun hollow fibers are dependent on the structure of additives. Among all the membranes spun with 5 wt% additives, the hollow fibers spun using Pluronic F127 and F108 as the additives possess the highest PWP, the lowest MWCO, and the narrowest pore size distribution. It is possible that the PEO brush layer formed on the internal pore surface by Pluronic F127 and F108 might reduce the apparent pore size and hence improved the solute rejection of the resultant membranes. A comparison between Pluronic and poly(ethylene glycol) (PEG) as additives also confirmed the importance of the presence of PPO chain in Pluronic in the formation of high-performance membranes. When Pluronic F127 concentration was 10 wt%, the as-spun hollow fiber exhibited the highest PWP of 113.8 L/m 2 h bar and the lowest MWCO of 9 kDa.
Due to the unique properties of amphiphilic Pluronic block copolymers, they have been blended wit... more Due to the unique properties of amphiphilic Pluronic block copolymers, they have been blended with other polymers such as PVDF to prepare membranes. Considering the stability issue, the strong pore-forming ability of Pluronic F127, and the mechanical strength of the resultant membranes, Pluronic/LiCl mixed additive consisting of a very low concentration of Pluronic has been used in this study. The effects of spinning conditions on PVDF hollow fiber fabrication have also been studied. It is found that the use of merely 0.2 wt% of Pluronic is sufficient to impose significant impacts on the morphology, filtration performance, and mechanical properties of the resultant fibers. Hollow fibers spun using higher coagulant temperature, higher take-up speed, and water as the bore fluid exhibit better mechanical properties and filtration performance. PVDF hollow fiber membranes with PWP of 1180 L/m 2 h MPa and MWCO of 5 kDa were obtained when spun with optimized spinning conditions and using 3 wt% of LiCl and 0.2 wt% of Pluronic as the mixed additive. The fibers can withstand a pressure of 0.55 MPa from the lumen.
Poly(vinylidene fluoride) (PVDF) has become one of the most popular materials for membrane prepar... more Poly(vinylidene fluoride) (PVDF) has become one of the most popular materials for membrane preparation via nonsolvent induced phase separation (NIPS) process. In this study, an amphiphilic block copolymer, Pluronic F127, has been used as both a pore-former and a surface-modifier in the fabrication of PVDF hollow fiber membranes to enhance the membrane permeability and hydrophilicity. The effects of 2nd additive and coagulant temperature on the formation of PVDF/Pluronic F127 membranes have also been investigated. The as-spun hollow fibers were characterized in terms of cross-sectional morphology, pure water permeation (PWP), relative molecular mass cutoff (MWCO), membrane chemistry, and hydrophilicity. It was observed that the addition of Pluronic F127 significantly increased the PWP of as-spun fibers, while the membrane contact angle was reduced. However, the size of macrovoids in the membranes was undesirably large. The addition of a 2nd additive, including lithium chloride (LiCl) and water, or an increase in coagulant temperature was found to effectively suppress the macrovoid formation in the Pluronic-containing membranes. In addition, the use of LiCl as a 2nd additive also further enhanced the PWP and hydrophilicity of the membranes, while the surface pore size became smaller. PVDF hollow fiber with a PWP as high as 2530 L•m −2 •h −1 •MPa −1 , a MWCO of 53000 and a contact angle of 71° was successfully fabricated with 3% (by mass) of Pluronic F127 and 3% (by mass) of LiCl at a coagulant temperature of 25 °C, which shows better performance as compared with most of PVDF hollow fiber membranes made by NIPS method.
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Papers by Chun Heng Loh