Preparation and characterization of polyvinylidene fluoride hollow fiber membranes for ultrafiltration

Development of Novel Electrospun Fibers Based on Cyclic Olefin Polymer

For the first time, a systematic study to investigate the electrospinnability of cyclic olefin polymer (COP) was performed. Different solvents and mixtures were tested together with different electrospinning parameters and post-treatment types to prepare bead-free fibers without defects. These were successfully obtained using a chloroform/chlorobenzene (40/60 wt.%) solvent mixture with a 15 wt.% COP polymer, a 1 mL/h polymer solution flow rate, a 15 cm distance between the needle and collector, and a 12 kV electric voltage. COP fibers were in the micron range and the hot-press post-treatment (5 MPa, 5 min and 120 °C) induced an integrated fibrous structure along with more junctions between fibers, reducing the mean and maximum inter-fiber space. When the temperature of the press post-treatment was increased (from 25 °C to 120 °C), better strength and less elongation at break of COP fibers were achieved. However, when applying a temperature above the COP glass temperature (Tg = 138 °C) the fibers coalesced, showing a mechanical behavior similar to a plastic film and a low elongation at break with a high strength. The addition of a high dielectric constant non-solvent, N,N-dimethylacetamide (DMAc), resulted in a considerable reduction in the COP fiber diameter. Based on the cloud point approach, it was found that the use of DMAc and the solvent chloroform or chlorobenzene improved the electrospinnability of COP polymer solution.

Carbon Black/Polyvinylidene Fluoride Nanocomposite Membranes for Direct Solar Distillation

Water reclamation is becoming a growing need, in particular in developing countries where harvesting the required energy can be a challenging problem. In this context, exploiting solar energy in a specifically tailored membrane distillation (MD) process can be a viable solution. Traditional MD guarantees a complete retention of non-volatile compounds and does not require high feed water temperatures. In this work, a suitable amount of carbon black (CB) was incorporated into the whole matrix of a polymeric porous membrane in order to absorb light and directly heat the feed. The mixed matrix membranes were prepared forming a uniform CB dispersion in the PVDF dope solution and then using a non-solvent induced phase separation process, which is a well-established technique for membrane manufacturing. CB addition was found to be beneficial on both the membrane structure, as it increased the pore size and porosity, and on the photothermal properties of the matrix. In fact, temperatures as high as 60 °C were reached on the irradiated membrane surface. These improvements led to satisfactory distillate flux (up to 2.3 L/m2h) during the direct solar membrane distillation tests performed with artificial light sources and make this membrane type a promising candidate for practical applications in the field of water purification.

Study on the modification of polyvinylidene fluoride with polyurethane to achieve excellent hydrophilic property

Polyvinylidene fluoride (PVDF) is a promising membrane material in ultrafiltration (UF) applications; its extensive application however is limited due to the disadvantage in hydrophilicity and low surface energy. Herein, a sort of TPU-modified PVDF membrane is prepared by blending method and its hydrophilicity is compared with a series of pure/modified PVDF membranes. The contact angle and pure water flux (PWF) results demonstrate that the hydrophilicity of the TPU-modified PVDF membrane is enhanced, and the performance is not inferior to that of traditional pore-modified PVDF membranes. SEM image shows that the TPU-modified PVDF membrane maintains morphology of the pure PVDF membrane, indicating that TPU molecules have excellent compatibility with PVDF molecules and can maintain the mechanical property of PVDF membrane to a certain extent. Finally, we explore the effects of TPU molecules and PVDF molecules on water molecules, respectively, from a microscopic perspective involving first principles. This investigation not only establishes that PVDF membrane has been prepared with enhanced hydrophilicity, but also provides a novel avenue for the modification of membrane properties.

Three-Dimensional Membrane Imaging with X-ray Ptychography: Determination of Membrane Transport Properties for Membrane Distillation

Abstract

Membrane distillation (MD) is a desalination technique that uses a membrane to thermally separate potable water from sea or brackish water. The mass transport processes through the membrane are commonly described by the dusty gas model. These processes are modeled assuming uniform, ideally cylindrical capillaries and are adjusted for the membrane geometry by including porosity and tortuosity. The tortuosity is usually set to 2 or is used as an adjusting parameter to fit theoretical models to experimentally measured data. In this work, ptychographic X-ray computed tomography is employed to map the three-dimensional (3D) structure of three commercial state-of-the-art PTFE membranes in MD. The porosity, tortuosity and permeability (viscous flow coefficient) of the samples are computed using the lattice Boltzmann method. The intrinsic permeability is compared to the dusty gas model and an apparent permeability is proposed which is corrected for Knudsen slip effects at the membrane structure.

Article Highlights

Preparation and Application of ZIF-8 Thin Layers

Herein we compare various preparation methods for thin ZIF-8 layers on a Cu substrate for application as a host material for omniphobic lubricant-infused surfaces. Such omniphobic surfaces can be used in thermal engineering applications, for example to achieve dropwise condensation or anti-fouling and anti-icing surface properties. For these applications, a thin, conformal, homogeneous, mechanically and chemically stable coating is essential. In this study, thin ZIF-8 layers were deposited on a Cu substrate by different routes, such as (i) electrochemical anodic deposition on a Zn-covered Cu substrate, (ii) doctor blade technique for preparation of a composite layer containing PVDF binder and ZIF-8, as well as (iii) doctor blade technique for preparation of a two-layer composite on the Cu substrate containing a PVDF-film and a ZIF-8 layer. The morphology and topography of the coatings were compared by using profilometry, XRD, SEM and TEM techniques. After infusion with a perfluorinated oil, the wettability of the surfaces was assessed by contact angle measurements, and advantages of each preparation method were discussed.

Preparation, characterization and dyeing wastewater treatment of a new PVDF/PMMA five-bore UF membrane with β-cyclodextrin and additive combinations

A new type of polyvinylidene fluoride (PVDF)/polymethyl methacrylate (PMMA) hollow fiber membrane (HFM) with five bores was prepared. The effects of polyvinylpyrrolidone (PVP), β-cyclodextrine (β-CD), polyethylene glycol (PEG) and polysorbate 80 (Tween 80) and their combinations on the PVDF/PMMA five-bore HFMs were investigated. The performance and fouling characteristics of five-bore HFMs for dyeing wastewater treatment were evaluated. Results indicated that adding 5 wt.% PVP increased the porosity and water flux of the membrane but decreased the bovine serum albumin (BSA) rejection rate. Adding 5 wt.% β-CD significantly improved the tensile strength and rejection of the HFMs with no effect on the increase of water flux. The characteristic of the HFMs with different additive combinations proved that the mixture of 5 wt.% PVP and 1 wt.% β-CD gave the best membrane performance, with a pure water flux of 427.9 L/ m²·h, a contact angle of 25°, and a rejection of BSA of 89.7%. The COD_cr and UV₂₅₄ removal rates of dyeing wastewater treatment were 61.10% and 50.41%, respectively. No breakage or leakage points were found after 120 days of operation, showing their reliable mechanical properties. We set the operating flux to 55 L/m²·h and cross-flow rate to 10%, which can effectively control membrane fouling.

Influence of bore fluid composition on the physiochemical properties and performance of hollow fiber membranes for ultrafiltration

Porous hollow fiber polysulfone (PSf) membranes were fabricated via a phase-inversion process and their performance during ultrafiltration (UF) was evaluated. The effects of the composition and concentration (0-50%) of different bore fluid mixtures, including N-methyl-2-pyrrolidone (NMP)/water, glycerol (G)/water, and ethylene glycol (EG)/water (in comparison with pure deionized water), on the structure, physicochemical properties, and performance of the fabricated membranes was investigated. Using these various bore fluid mixtures altered the thermodynamic and kinetic properties of the phase inversion system, and changed the morphology and structure of the fabricated membranes, especially on the lumen side. Increasing concentrations of NMP, G, and EG in the bore fluid resulted in increased pore size, porosity, and hydrophilicity. These bore fluid mixtures exhibited a strong influence on the perm-selectivity of the as-spun hollow fiber membranes. The membrane fabricated using 50% NMP/water as the bore fluid mixture exhibited the highest water flux of 166.98 LMH with a bovine serum albumin rejection rate of more than 97%. Overall, this study introduces an easy and effective way to control the structure of the membrane through bore fluid modification and shows how the inner skin layer properties can have a remarkable effect on water permeance, even in the out-in filtration test.

Macrovoid-Inhibited PVDF Hollow Fiber Membranes via Spinning Process Delay for Direct Contact Membrane Distillation

A polyvinylidene fluoride (PVDF) hollow fiber membrane was fabricated through water-induced dope crystallization by allowing a facile spinning process delay (SPD) in the nonsolvent-induced phase separation (NIPS) process for direct contact membrane distillation (DCMD). The SPD was achieved by the addition of a small amount of water to the PVDF dope solution that was held in a closed container for a particular time. The crystalline property of the PVDF dope solution was investigated by differential scanning calorimetry. The obtained PVDF hollow fiber membranes were characterized with different techniques, including field emission scanning electron microscopy, X-ray diffraction, and the mechanical strength. Both the formation mechanism and properties were studied for the membranes with different SPD times. The results showed that macrovoid-inhibited PVDF membranes were obtained from 12 days of the SPD via the crystallization-dominated membrane formation process. The obtained membrane 4D-12 exhibited desirable membrane structure and properties for DCMD, which includes an improved liquid entry pressure of 2.25 bar, a surface water contact angle of 129°, a maximum pore size of 0.40 μm, and a mean pore size of 0.34 μm. The membrane 4D-12 possessed a twofold increase in both energy efficiency and permeate water flux in DCMD and stable permeate water flux and salt rejection through 224 h of continuous desalination operation. Compared to the commonly used approach by adding chemicals to the external coagulant, the SPD method provided a low-cost and environmentally friendly alternative to pursuing the macrovoid-free PVDF membranes for DCMD.

DNA Adsorption Studies of Poly(4,4'-Cychlohexylidene Bisphenol Oxalate)/Silica Nanocomposites

The present study deals with the synthesis, characterization, and DNA extraction of poly(4,4′-cyclohexylidene bisphenol oxalate)/silica (Si) nanocomposites (NCs). The effects of varying the monomer/Si (3.7%, 7%, and 13%) ratio towards the size and morphology of the resulting NC and its DNA extraction capabilities have also been studied. For the NC synthesis, two different methods were followed, including the direct mixing of poly(4,4′-cyclohexylidene bisphenol oxalate) with fumed Si, and in situ polymerization of the 4,4′-cyclohexylidene bisphenol monomer in the presence of fumed silica (11 nm). The formed NCs were thoroughly investigated by using different techniques such as scanning electron microscopy (SEM), fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), powdered X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) analysis where the results supported that there was the successful formation of poly(4,4′-cyclohexylidene bisphenol oxalate)/Si NC. Within the three different NC samples, the one with 13% Si was found to maintain a very high surface area of 12.237 m²/g, as compared to the other two samples consisting of 7% Si (3.362 m²/g) and 3.7% Si (1.788 m²/g). Further, the solid phase DNA extraction studies indicated that the efficiency is strongly influenced by the amount of polymer (0.2 g > 0.1 g > 0.02 g) and the type of binding buffer. Among the three binding buffers tested, the guanidine hydrochloride/EtOH buffer produced the most satisfactory results in terms of yield (1,348,000 ng) and extraction efficiency (3370 ng/mL) as compared to the other two buffers of NaCl (2 M) and phosphate buffered silane. Based on our results, it can be indicated that the developed poly(4,4′-cyclohexylidene bisphenol oxalate)/Si NC can serve as one of the suitable candidates for the extraction of DNA in high amounts as compared to other traditional solid phase approaches.

Parameter Screening of PVDF/PVP Multi-Channel Capillary Membranes

The increasing research in the field of polymeric multi-channel membranes has shown that their mechanical stability is beneficial for a wide range of applications. The more complex interplay of formation process parameters compared to a single-channel geometry makes an investigation using Design of Experiments (DoE) appealing. In this study, seven-channel capillary membranes were fabricated in a steam–dry–wet spinning process, while varying the composition of the polymer solution and the process temperatures in a three-level fractional factorial linear screening design. The polymers polyvinylidene flouride (PVDF) was the chemically resistant main polymer and polyvinylpyrrolidone (PVP) was added as hydrophilic co-polymer. Scanning electron microscopy and atomic force microscopy were applied to study the membrane morphology. Fabrication process conditions were established to yield PVDF/PVP multi-channel membranes, which reached from high flux (permeability P = 321.4L/m2/h/bar, dextran 500 kDa retention R = 18.3%) to high retention (P = 66.8L/m2/h/bar, R = 80.0%). The concentration of the main polymer PVDF and the molecular weight of the co-polymer PVP showed linear relations with both P and R. The permeability could be increased using sodium hypochlorite post-treatment, although retention was slightly compromised. The obtained membranes may be suitable for micro- or ultra-filtration and, at the same time, demonstrate the merits and limitations of DoE for multi-channel membrane screening.

Surface modification of polymeric flat sheet membranes by adding oligomeric fluoroalcohol

The present study attempted to modify the surface of polymeric membranes by mixing 2-(perfluoroalkyl)ethanol with polyethersulfone to improve the hydrophobicity of the membrane. The characteristics of the surface-modified membrane were examined by using the contact angle, liquid entry pressure (LEPw), X-ray photoelectron spectroscopy spectra, and scanning electron microscopy measurements. The modified membrane was tested for SO2 absorption. The surface contact angle and LEPw values increased by the addition of fluoroalcohol. The fluorine/carbon atomic ratio decreased with increasing evaporation period. The modified membrane was utilized for the absorption of SO2 from the simulated flue gas in alkaline medium (0.5 m NaOH solution). The SO2 removal efficiency slightly decreased probably due to the transformation of the finger-like structure into a sponge-like structure, and the increase in the membrane resistance. The SO2 absorption rate also increased as the gas flow rate increased.

Preparation of Polybenzimidazole Hollow-Fiber Membranes for Reverse Osmosis and Nanofiltration by Changing the Spinning Air Gap

High-performance polybenzimidazole (PBI) hollow-fiber membranes (HFMs) were fabricated through a continuous dry-jet wet spinning process at SRI International. By adjusting the spinning air gap from 4″ (10.2 cm) to 0.5″ (1.3 cm), the HFM pore sizes were enlarged dramatically without any significant change of the fiber dimensional size and barrier layer thickness. When fabricated with an air gap of 2.5″ (6.4 cm) and a surface modified by NaClO solution, the PBI HFM performance was comparable to that of a commercial reverse osmosis (RO) HFM product from Toyobo in terms of salt (NaCl) rejection and water permeability. The PBI RO HFM was positively surface charged in acidic conditions (pH < 7), which enhanced salt rejection via the Donnan effect. With an air gap of 1.5″ (3.8 cm), the PBI HFM rejected MgSO₄ and Na₂SO₄ above 95%, a result that compares favorably with that achieved by nanofiltration. In addition, the PBI HFM has a defect-free structure with an ultra-thin barrier layer and porous sublayer. We believe PBI HFMs are ideal for water purification and can be readily commercialized.

High-performance membranes with full pH-stability

Following current strong demands from, among others, paper, food and mining industries, a novel type of nanofiltration membrane was developed, which displays excellent performance in terms of selectivity/flux with a unique combination of chemical stability over the full (0-14) pH-range and thermal stability up to 120 °C. The membrane consists of polyvinylidene fluoride grafted with polystyrene sulfonic acid. The optimum membrane showed water permeances of 2.4 L h-1 m-2 bar-1 while retaining NaCl, MgSO4 and Rhodamine B (479 Da) for respectively ≈60%, ≈80% and >96%.

Polyetherimide hollow fiber membranes for CO2 absorption and stripping in membrane contactor application

Porous asymmetric polyetherimide (PEI) hollow fiber membranes with various non-solvent additives, e.g. lithium chloride, methanol and phosphoric acid (PA) were prepared for CO₂ absorption and stripping process in a membrane contractor. The PEI membranes were characterized via gas permeation, liquid entry pressure of water (LEPw), contact angle and field emission scanning electronic microscopy analysis. The CO₂ absorption and stripping performance was evaluated via the membrane contactor system. Addition of non-solvent additives increased the LEPw and membrane porosity of the PEI membrane with the formation of various membrane microstructures and contact angles. Absorption test was performed at 40 °C showed that the PEI–PA membrane produced the highest absorption flux of 2.7 × 10⁻² mol m⁻² s⁻¹ at 0.85 m s⁻¹ of liquid velocity. Further testing on PEI–PA membrane was conducted on CO₂ stripping at 60 °C, 70 °C to 80 °C and the results indicated that the stripping flux was lower compared to the absorption flux. Stripping tests at 80 °C produced the highest stripping flux which might due to the increase in equilibrium partial pressure of CO₂ in the liquid absorbent. Modification of PEI membrane via incorporation of additive can enhanced the performance of a membrane contactor via increasing the absorption and stripping flux.

Porous asymmetric polyetherimide (PEI) hollow fiber membranes with various non-solvent additives, e.g. lithium chloride, methanol and phosphoric acid (PA) were prepared for a CO₂ absorption and stripping process in a membrane contractor.

Interlaced CNT Electrodes for Bacterial Fouling Reduction of Microfiltration Membranes

Interlaced carbon nanotube electrodes (ICE) were prepared by vacuum filtering a well-dispersed carbon nanotube-Nafion solution through a laser-cut acrylic stencil onto a commercial polyvinylidene fluoride (PVDF) microfiltration (MF) membrane. Dead-end filtration was carried out using 10⁷ and 10⁸ CFU mL^-1 Pseudomonas fluorescens to study the effects of the electrochemically active ICE on bacterial density and morphology, as well as to evaluate the bacterial fouling trend and backwash (BW) efficacy, respectively. Finally, a simplified COMSOL model of the ICE electric field was used to help elucidate the antifouling mechanism in solution. At 2 V DC and AC (total cell potential), the average bacterial log removal of the ICE-PVDF increased by ∼1 log compared to the control PVDF (3.5-4 log). Bacterial surface density was affected by the presence and polarity of DC electric potential, being 87-90% lower on the ICE cathode and 59-93% lower on the ICE anode than that on the PVDF after filtration, and BW further reduced the density on the cathode significantly. The optimal operating conditions (2 V AC) reduced the fouling rate by 75% versus the control and achieved up to 96% fouling resistance recovery (FRR) during BW at 8 V AC using 155 mM NaCl. The antifouling performance should mainly be due to electrokinetic effects, and the electric field simulation by COMSOL model suggested electrophoresis and dielectrophoresis as likely mechanisms.