Arto Pihlajamäki

Characterization of ultrafiltration membranes by simultaneous streaming potential and flux measurements

Abstract A new apparatus was developed where streaming potentials and permeate fluxes of membranes could be measured simultaneously. In this way the effect of addition of a protein, bovine serum albumin, on the potential and flux could also be studied. On addition of protein the calculated zeta potential of the membrane changed so as to be close to the potential of the protein at that pH. At very low or high pH, where the protein and the membrane had the same sign of charge, adsorption decreased and the potential of the membrane did not change fully to that of the protein. The point of zero charge of the protein-covered membrane was slightly higher than the isoelectric point of the protein.

Evaluation of electroosmosis and streaming potential for measurement of electric charges of polymeric membranes

Abstract Electrical properties for a range of commercial membranes have been determined by electroosmosis and streaming potential measurements under identical conditions. For both techniques, the pH range was 3–7 and the electrolyte used was 10 −3 M KCl at 25°C, as the charges on the membrane are strongly dependent upon the pH and the ionic strength of the solution passing through the membrane. For both ultrafiltration (UF) and microfiltration (MF) membranes the absolute values of the apparent zeta potential determined from electroosmosis were generally greater than those from streaming potential measurements with a greater difference at pH ≤ 4. The apparent zeta potentials obtained from electroosmosis were negative for all membranes studied, even at pH below the isoelectric point where streaming potentials were positive. A substantial increase of the negative zeta potential at low pH (∼ pH 4) was interpreted in terms of the effect of the applied electric field on distribution of ions and differences in mobility of ions, particularly H + compared to the other ions. The magnitude of the zeta potentials determined from electroosmosis exhibited an apparent difference due to change in orientation of the membrane for asymmetric UF membranes, whereas the streaming potential measurements showed less directional-effect for both UF and MF membranes.

Effect of temperature and membrane pre-treatment by pressure on the filtration properties of nanofiltration membranes

Abstract The effect of temperature on the retention in nanofiltration of model substances (glucose) and substances in industrial streams has been studied in the temperature interval 25°C to 65°C. An increase in temperature decreased the retention until a critical temperature of the membrane was exceeded. After that temperature the flux even decreased and the retention increased. The retention of uncharged substances normally decreased due to alkaline cleaning and the flux of most membranes increased. In the filtration of mechanical pulp mill effluents an increase of temperature decreased the retention of organic substances but did not affect the retention of conductivity. Only few nanofiltration membranes withstood a temperature of 65°C without a significant change in the filtration efficiency. A membrane pre-treatment, such as pressurization with pure water affected dramatically the flux and the flux reduction and somewhat the retention. Experiments made with incompletely wetted membranes underestimate fouling and the retention of the membrane. Most nanofiltration membranes seemed to be almost completely wetted at a pressure of 25 bar.

Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect

Abstract The possibility of measuring the zeta-potentials of porous membranes using the electroviscous effect was investigated. The zeta-potential of Membralox® ceramic microfiltration membranes was determined both with the newly developed electroviscous technique and by streaming potential measurements. It was found that the electroviscous technique provided a simple means of obtaining accurate values of zeta-potential, especially for higher zeta-potentials. The streaming potential measurements were found to be more suitable for the determination of the iso-electric point, i.e. the pH at which the zeta-potential is zero. The iso-electric points of new α-alumina, zirconia, and titania membranes were found to be 8.5, 8.0, and 6.3, respectively. Upon using the membranes and cleaning them with a detergent, the iso-electric point of the α-alumina membrane decreased to 6.5, and that of the zirconia membrane decreased to 5.2, while the iso-electric point of the titania membrane stayed virtually constant. Cleaning these membranes with a strong acid or base could not reverse the observed decreases in iso-electric point.

Chemical and electrical characterization of virgin and protein-fouled polycarbonate track-etched membranes by FTIR and streaming-potential measurements

Abstract Electrical properties for a range of virgin and protein-fouled polycarbonate track-etched (PCTE) membranes have been determined by streaming-potential measurements at pH 4–7 and 10 −3 M KCl at 25°C. The apparent zeta potentials generally increased with increasing pore size above pH 5 and decreased with pore size below pH 5. For membranes with smaller pores (PCTE 0.01, 0.03 and 0.05 μm) the zeta potentials became constant (the surface is fully charged) at about −6 mV at pH ≥ 5, whereas those of larger pores (PCTE 0.1 and 0.2 μm) showed constant zeta potentials of ca. −10 to −13 mV at pH > 6. In contrast, the protein-fouled membranes showed very similar apparent zeta potential vs. pH profiles for all pore sizes. This suggests that the measured zeta potentials are determined by the protein properties. The loss of strong pore-size dependence on zeta potentials upon protein fouling of membranes implies that membrane zeta potentials could be controlled by the conditions at pore entry. This ‘model’ requires protein deposition around the pore openings for all pore sizes; this was in agreement with electron microscope observations. While the isoelectric points (IEP) of the virgin membranes determined from the pH dependence of streaming-potential measurements were between pH 3.7 – 4.4 with a trend to higher IEP for the larger pore membrane, all protein-fouled membranes showed IEP values at pH 4.6 – 4.8, which is close to the IEP of the BSA protein used. The chemical properties of the PCTE membranes investigated had FTIR spectra similar to those of polycarbonate from bis(4-hydroxyphenyl)-2-propane, bisphenol A polycarbonate. No BSA peak was revealed in the FTIR spectra of the small-pore membranes fouled with protein, possibly due to the insufficient instrument detection limit. For the membranes with larger pores, however, the BSA peaks increased with increasing pore size. The depth profiling of the fouled membranes indicated a predominant BSA adsorption on the surface or near the pore entries rather than throughout the membrane thickness, confirming the zeta potential ‘model’ and electron microscope observations.

Comparison of nanofiltration and tight ultrafiltration membranes in the filtration of paper mill process water

Several nanofiltration and tight ultrafiltration membranes were tested by mill-site filtrations using a DSS-Labstak M20-filter. Retention, flux and fouling were analysed. Membranes were also characterised by measuring contact angles and pure water permeability. These data were used to explain the experimental results with effluents and to choose the possible membranes for the paper mill process waters. Comparison of different membranes in the constant pressure filtrations showed that usually the membranes with high pure water permeability or contact angle had worse fouling (fouling that had to be removed by alkaline cleaning). In addition, the fouling was stronger in filtrations made at acidic pH than at neutral conditions. The organic substances were retained significantly better at neutral pH than at acidic pH. Generally, when the hydrophilicity of the membrane increased the effect of pH on the retention properties increased as well. However, an individual membrane may deviate remarkably from the general trends. The results showed that some commercially available membranes have both high permeability and good retention properties at the same time.

Characterization of clean and fouled polymeric ultrafiltration membranes by Fourier transform IR spectroscopy–attenuated total reflection

Abstract Fourier transform IR spectroscopy was successfully used to study clean and fouled polysulphone ultrafiltration membranes of different pore sizes and from different manufacturers. The attenuated total reflection method was found to be very useful as a surface-characterizing method for membranes. The material of the GR polysulphone membranes of different pore sizes was found to be different. The GR 40 and the GR 51 membranes appear to be made of Udel polysulphone or poly(arylenesulphone ether) while the material of the GR 61 and the GR 81 is made of the more hydrophilic Victrex polyethersulphone and bisphenol A as is the PM 10 membrane. In the structure of the PM 10 membrane some unsubstituted OH groups have been left producing an even more hydrophilic surface. The depth profiling of the clean and the bovine serum albumin fouled GR 51 membranes demonstrated that the protein molecules adsorb more inside the pores of the membrane and less on the surface. This was also confirmed with further analysis of the penetration depth of radiation. The intensity of the peaks caused by the adsorbed protein in the spectrum of the fouled membrane increased when the radiation penetrated deeper into the sample, indicating that more protein was present.

Influence of process conditions and membrane/particle interaction in NF of wastewaters

The clean environment concept and the demand of decreasing the use of fresh water make it important to reuse wastewaters from industry. One of the industries producing great amounts of wastewater containing fairly low amounts of COD is the pulp and paper industry. These produced effluent waters can be cleaned by membrane filtration so that they can be reused on site. By using ultrafiltration, a water clean enough for most places in the mill can be produced. But for the more demanding places nanofiltration has to be used in order to get a micro-particle free water. Since the volumes of water to be filtered can be in the order of cubic meters per second a high flux is important. The wastewaters in the pulp and paper industry, like surface waters, contain organic degradation products from trees that are of a hydrophobic nature. These molecules easily aggregate and foul the membranes if the membranes are hydrophobic or if the process conditions are favourable to fouling. In the studies reported here the process conditions, the membrane materials and the interactions of hydrophobic molecules in the wastewater or surface water with different kinds of nanofiltration membranes were investigated. The influence of high shear on critical flux was investigated. Adsorption was to be minimised by choosing the least fouling hydrophilic membranes (characterisation by contact angle). Different measurements were made on the waste-waters like TOC, UV-absorption, conductivity, hardness, SUVA index, etc. The results showed that the best nanofiltration fluxes for long periods of time were achieved using very high shear cross-flow modules. Besides on high shear, the study focused on finding hydrophilic membranes and on trying either to use pretreatment to remove the foulants before filtration or on using some kind of adsorbing filtration aid during the filtration. In some cases fibers naturally formed the filtration aid.

Effects of raw natural gas on the aging of high-voltage electrical machine mainwall insulation

The total recovery of natural gas from subsea wells can be significantly increased with a compressor installed near the wellheads. The compressor is powered by a high-speed induction motor integrated in the same casing. The process gas flows through the motor and acts as a cooling medium. The insulation system of the motor is in direct contact with the gas and must be resistant to it. The gas mixture contains hydrocarbons, water, and monoethylene glycol. The effects of the gas mixture and its individual components on the properties of a high-voltage machine mainwall insulation consisting of mica, glass, and epoxy are obtained by experimental tests with raw natural gas at accelerated conditions. The tests at high pressures and temperatures indicate that heavy hydrocarbon compounds cause similar effects to plasticizers inside the bisphenol A epoxy resin, but such compounds do not penetrate easily into epoxy novolac resin. The plasticizing effect is seen as increased weight and volume, decreased mechanical strength and E-modulus, and reduced glass transition temperature. The polymers did not decompose chemically. The mainwall insulation is vulnerable to delamination, which is initiated by the detachment of glass strains and epoxy resin. Water causes dielectric loss peak at very low frequencies, while the heavy hydrocarbons produce a loss peak in higher frequency range.

Effect of Precipitation Temperature on the Properties of Cellulose Ultrafiltration Membranes Prepared via Immersion Precipitation with Ionic Liquid as Solvent

Supported cellulose ultrafiltration membranes are cast from a cellulose-ionic liquid solution by the immersion precipitation technique. The effects of coagulation bath temperature and polymer concentration in the casting solution on the membrane morphology, wettability, pure water flux, molecular weight cut-off, and fouling resistance are studied. Scanning electron microscopy, contact angle measurements, atomic force microscopy, and filtration experiments are carried out in order to characterise the obtained ultrafiltration cellulose membranes. The results show the effect of coagulation bath temperature and polymer concentration on the surface morphology and properties of cellulose ultrafiltration membranes. Optimisation of the two parameters leads to the creation of dense membranes with good pure water fluxes and proven fouling resistance towards humic acid water solutions.