Surface modification of ultrafiltration membranes by low temperature plasma. I. Treatment of polyacrylonitrile (original) (raw)
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Ion adsorbing ultrafiltration membranes provide an interesting possibility to remove toxic ions from water. Furthermore, it is also possible to recover valuable elements. In this work, we demonstrate two easy strategies to modify polyacrylonitrile membranes with anion and cation adsorbing groups. The membranes were modified to have positively charged amine groups or negatively charged carboxyl groups. The success of the reactions was confirmed using IR spectroscopy and zeta-potential measurements. The membranes carrying negatively charged groups provided a negative zeta-potential and had an isoelectric point at pH 3.6, while the membranes carrying positively charged groups had a positive zeta-potential in the analyzed pH range. Since only the surface of the polymer was modified, the pore size and permeance of the membranes were not drastically affected. The membranes prepared by both modification strategies had a pure water permeance higher than 1000 L/(m2 h bar) and a water contact...
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The effect of the addition of triblock copolymer polyethylene glycol (PEG)-polypropylene glycol (PPG)-polyethylene glycol (PEG) (Pluronic F127) and polyethylene glycol (PEG-4000, M n = 4000 g mol −1) to the polysulfone (PSF) casting solution on the membrane structure and performance was studied. The phase state, viscosity and turbidity of PSF solutions in N,N-dimethylacetamide (DMAc) with the addition of block copolymer Pluronic F127 were investigated. It was found that 18-22 wt.% PSF solutions in DMAc with Pluronic F127 content ≥ 5 wt.% feature a lower critical solution temperature (LCST). Membrane structure was investigated using scanning electron and atomic force microcopies. It was revealed that average pore size and pore amount on the surface of the membrane selective layer increase and pore size distribution becomes wider with an increase in Pluronic F127 content in the casting solution. It was found that the average surface roughness parameters of the membrane selective layer for PSF/Pluronic F127 membranes significantly exceed those for PSF/PEG-4000 membranes. It was shown that the increase in the membrane flux and the decrease in polyvinylpyrrolidone (PVP K-30, M n = 40,000 g mol −1) rejection are a result of the addition of both Pluronic F127 and PEG-4000 into the casting solution. It was revealed that PSF/Pluronic F127 membranes are characterized by higher pressure resistance in ultrafiltration process, a lower total flux decrease during ultrafiltration of bovine serum albumin solutions. The antifouling performance of PSF/Pluronic F127 membranes was found to exceed significantly the antifouling performance of PSF/PEG-4000 membranes.
Surface Modification of Polyacrylonitrile Based Ultrafiltration Membrane
Ultrafiltration membrane based on polyacry-lonitrile prepared by phase inversion method using zinc chloride as an additive showed more than 90% rejection for BSA and 90 –110 lm 2 h 1 water flux. The surface modification of this membrane was studied using ethanolamine, triethylamine, sodium hydroxide, and potassium hydroxide solutions. The effect of base treatment time and temperature on water flux and rejection was investigated. The membranes exhibited swelling by NaOH treatment followed by deswelling by HCl post-treatment, similar to pH responsive membranes. The treatment by organic as well as inorganic bases improved water flux with a slight lowering in BSA rejection by dead-end mode type treatment. A 230% increase in water flux was achieved by sodium hydroxide treatment in crossflow mode without a noticeable pore swelling by SEM. The contact angle of the modified membranes was decreased as compared to the unmodified one indicating appreciable surface modification. As the treatment time or temperature increased, the ESCA analysis showed increased population of Na-carboxylate groups.
Influence of processing conditions on the properties of ultrafiltration membranes
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The effect of four processing variables on the final properties of ultrafiltration (UF) membranes was investigated. Flat-sheet polyethersulfone (PES) membranes were prepared by wet phase inversion. The variables of interest were: the concentration of base polymer in the casting solution (dope solution), the solvent evaporation time, the addition of surface modifying macromolecules (SMMs), and the use of the additive polyvinylpyrrolidone (PVP). The membranes were characterized by solute transport and scanning electron microscopy (SEM). No appreciable difference between the structures of surface-modified and unmodified membranes was observed by SEM. Membranes prepared with PVP had pure water permeation rates (PWP) significantly higher than membranes prepared without the PVP additive. However, the average polyethylene glycol (PEG 35 ku (35 kDa)) separation by membranes with PVP was approximately 15% lower than the tightest membranes prepared without PVP. The effect of surface modification with fluorinated additives and solvent evaporation time was observed only with high molecular weight cutoff (MWCO) membranes prepared with 12 wt.% PES casting solution.
Improved Performance of Ultrafiltration Membranes after Surface Modification
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Commercial polyethersulfone (PES) membrane was modified and evaluated for the removal of metal ions from an aqueous solution. Modifications were carried out by a radical polymerization technique, using acrylic acid as monomer. The grafting polymerization on the membrane was developed in an aqueous medium at 60°C. Two molar initiator/monomer (3 and 7%) ratios were used to study PES-grafted-polyaceylic acid membranes. Both unmodified and modified membranes were analyzed by attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) in order to look into their morphologies. In addition, contact angle measurements were used for membrane hydrophilicity. The ATR-FTIR spectra, SEM and AFM images confirmed that the modification on the PES membrane surface was carried out by grafting polymer of acrylic acid monomers. The results obtained show that acrylic acid may be used for the preparation of a selective membrane functionalized with carboxylic groups. Water permeability was evaluated by varying the feed pressures (4-10 bars). The flux water of unmodified membrane decreased from 10 to 6 L/h m 2 bar for modified membranes. The rejections of sodium chloride, copper chloride, and aluminum chloride were also studied for each sample. Among the metal ions tested, thevmaximum removal was reported for aluminium for grafted membrane at 7% ratio. The rejection values for grafted membranes (3 and 7%) were higher than those obtained for unmodified membranes and increased with the grafting rate.
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Surface modification of polysulfone ultrafiltration membranes was performed via addition of an anionic polymer flocculant based on acrylamide and sodium acrylate (PASA) to the coagulation bath upon membrane preparation by non-solvent induced phase separation (NIPS). The effect of PASA concentration in the coagulant at different coagulation bath temperatures on membrane formation time, membrane structure, surface roughness, hydrophilic-hydrophobic balance of the skin layer, surface charge, as well as separation and antifouling performance was studied. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, contact angle and zeta potential measurements were utilized for membrane characterization. Membrane barrier and antifouling properties were evaluated in ultrafiltration of model solutions containing human serum albumin and humic acids as well as with real surface water. PASA addition was found to affect the kinetics of phas...
Chemical characterization of ultrafiltration membranes by spectroscopic techniques
Journal of Membrane Science, 1988
In relation to the complicated problem of membrane fouling, a study of the adsorption mechanism of some foulants on polysulfone ultrafiltration membranes is being made. This report deals with an analysis of the bulk and surface compositions of two of these membranes. Attenuated total reflection Fourier transform infrared (ATR FTIR) measurements can elucidate the type of polysulfone in the bulk of the membranes. DDS GR 61 PP appears to consist of Udel (polyethersulfone + bisphenol A) and Dorr Oliver SlO of Victrex (polyethersulfone). Nevertheless, secondary ion mass spectrometry (SIMS) analysis indicates a fragmentation product of bisphenol A in the surface of the SlO membrane. The detailed atomic composition of both membrane surfaces, as analysed by X-ray photoelectron spectroscopy (XPS) , shows deviations from the calculated bulk composition. This may be due to preferential location of some functional groups at the membrane surface or to an applied coating. Moreover, in contrast to the presumed chemical structure, the element N is detected. By combination with SIMS and Kjeldahl analysis the presence of a C-N product in the surface is demonstrated. Further research with these techniques will be directed towards the mechanisms of interaction of membrane and foulant.
Adsorptive fouling of modified and unmodified commercial polymeric ultrafiltration membranes
Journal of Membrane Science, 1999
The fouling tendency, due to adsorption on the pore walls, of two pairs of modi®ed and unmodi®ed ultra®ltration membranes, with similar observed retentions determined by dextran and gel permeation chromatography, was studied. The membranes investigated were made of modi®ed and unmodi®ed polyaramide (PA) and modi®ed and unmodi®ed polyvinylidene¯uoride (PVDF). The PVDF membrane was surface-modi®ed and the PA membrane was made from a modi®ed polymer solution. Membrane modi®cation was used to reduce fouling by adsorption. Octanoic acid was used as the fouling substance, representing a large number of small, hydrophobic compounds. It is demonstrated in this investigation that membrane modi®cation is not always successful. It was determined that at lower concentrations of octanoic acid, the modi®ed PA membrane exhibits a smaller fouling tendency than the unmodi®ed PA membrane, while the result is reversed for concentrations above 60% of the saturation concentration. The fouling tendency of the unmodi®ed PVDF membrane is much lower than that of the modi®ed PVDF membrane at all concentrations. The cross-sections of the membranes were visually examined with scanning electron microscopy, but no difference could be observed between the modi®ed and unmodi®ed membranes. The membranes were also examined with Fourier transform infrared spectroscopy. The spectra of the two PA membranes were different, while no difference was observed for the unmodi®ed and surface-modi®ed PVDF membranes. Remains of octanoic acid were found in the membranes, although they had been thoroughly rinsed with deionized water and the initial pure water¯ux was recovered.