Marianne Nyström

Fouling and retention of nanofiltration membranes

Abstract Nanofiltration membranes retain substances with molar masses higher than ∼300 g/mol and multivalent ions. The retention characteristics depend much on how much free volume there is in the membranes, which can for some membranes be related to the flux. In this study, fouling and retention of four different nanofiltration membranes (NF40, NTR-7450, NTR-7410 and NTR-7250) were followed using different model substances. It could be seen that multivalent salts were retained better than small organic substances, but retention was much depending on the pH. As nanofiltration membranes have characteristics of both ultrafiltration as well as reverse osmosis membranes, their fouling characteristics are also rather individual. The tighter membranes foul less. Multivalent salts foul mostly only when together with organic compounds, e.g., lignosulfonate containing calcium fouls more than if it contains sodium as counterion. Maize starch containing small amounts of protein fouls more than potato starch. The NTR-7250 membranes could be modified by chloride ions at high pH to give better flux without loss of retention. Anionic model substances were retained better and fouled less due to charge repulsion effects especially at high pH

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.

Streaming potential as a tool in the characterization of ultrafiltration membranes

Abstract An apparatus for the measurement of streaming potential versus pressure relationships of ultrafiltration membranes is presented. Results for three types of membranes of various pore sizes at different pH, and ionic strengths are reported. The data have been used to calculate apparent values for zeta potentials of the membranes and the possibility to compare these values to the true zeta potentials is discussed. The influence of modification, the presence of small charged ions and charged polyelectrolytes on the apparent zeta potentials of the membranes has been measured. Finally the effects of the surface charges of the membranes, characterized by their apparent zeta potentials, are illustrated by ultrafiltration experiments performed with chlorolignin and whey protein.

Fouling effects of polysaccharides and humic acid in nanofiltration

Abstract Fouling in nanofiltration was studied using model substances similar to those in the pulp and paper mill waters, such as vanillin, humic acid, locust bean gum, and karaya gum. The parameters studied were the concentration of foulants, pH, cross-flow velocity and membrane hydrophobicity. The substances were nanofiltered alone and in their binary mixtures. The nanofiltration membranes (NTR-7450 and Desal-5 DK) differed by their cut-off values and their material properties, such as hydrophobicity. Various causes of fouling were observed to exist with the model substances studied. The electrostatic attraction markedly influenced the flux of charged substances, but had an insignificant effect on the flux of uncharged substances. Hydrophobic interactions overcame the electrostatic repulsion and caused more fouling of the more hydrophobic membrane. The prediction of filtration properties of the binary mixtures of model substances from the filtration results of individual substances is extremely difficult. Both the interactions between two model substances as well as the interactions between the model substances and the membrane determined the flux and fouling in the filtration of the binary mixtures of the model substances. Humic acid improved both the permeate flux and the pure water flux at neutral pH, but slightly fouled the membranes under acidic conditions. Humic acid might increase both the membrane hydrophilicity and its negative charge when being adsorbed by hydrophobic attraction.

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.

Effects of surface modification on antifouling and performance properties of reverse osmosis membranes

Abstract The effects of surface modification of commercial reverse osmosis (RO) polyamide membranes were studied in terms of flux/rejection and surface properties. Selected RO membranes, spanning ultra-low pressure to seawater membranes, were modified by in-situ graft polymerization using redox generation of monomer radicals. Modification of the membrane significantly reduced both receding and advancing contact angles of some of the membranes as measured by a modified Wilhelmy plate method. Lower contact angles indicate more hydrophilic membranes and such membranes will potentially show better resistance to fouling by hydrophobic foulants. This was confirmed by adsorption measurements carried out on modified and unmodified membranes with a highly fouling surface water. Modified membranes adsorbed less organic material and were more easily cleaned than unmodified membranes. Specific fluxes were not changed by more than 0–25% and NaCl rejection was unchanged or increased slightly.

Modification of polysulfone ultrafiltration membranes with UV irradiation and hydrophilicity increasing agents

Abstract Hydrophobic polysulfone UF membranes were modified with UV irradiation and hydrophilicity increasing agents. The modifications were tested with 0.5% whey-protein solution and 0.05% lysozyme solution at pH 6 and with 0.05% bovine serum albumin solution at various pH values. UV irradiation increased flux and the hydrophilicity of the membranes. The flux increases obtained varied with pH and modification agents used and could be more than 400% compared to unmodified conditions without any loss in retention. The best retentions were obtained at pH values, where both the protein and the membrane had the same charge, and a strong electrostatic repulsion was obtained. The pores enlarged to fixed sizes, which depended on the sizes of the proteins and the range of double layer forces between proteins and membranes at different states of charge density.

Critical flux in NF of high molar mass polysaccharides and effluents from the paper industry

Abstract High molar mass polysaccharides (locust bean gum and karaya gum) and effluents from a mechanical pulp mill and a paper mill were nanofiltered with commercially available nanofiltration (NF) membranes. The effect of the filtration conditions on the flux (critical flux), retention, and the fouling of the membranes was studied. The experiments were conducted by increasing and decreasing the pressure and measuring the flux thus obtained. The critical flux was observed to increase with increasing flow velocity and decreasing concentration. An increase in pH increased the electrostatic repulsion between the membrane and the dissociated (charged) components in the paper mill effluents. As a result, a higher critical flux was obtained and also the retentions of the charged substances improved. Only a weak form of the critical flux was observed with the mill effluents. The permeate flux deviated from the pure water flux even at the lowest pressure, but increased linearly with pressure until the weak form of the critical flux was exceeded. The small decrease in flux immediately after filtration was started was probably caused by the plugging of the free spaces in the membranes or by the adsorption of foulants onto the membrane surface. In the filtrations with the high molar mass polysaccharides, a strong form of the critical flux as well as a weak form were observed. The significant irreversible fouling of the most hydrophobic membrane was due to adsorption of the model substances by hydrophobic interaction. A precleaning of the membranes with an alkaline cleaning agent improved the pure water fluxes by up to 30%, but it had only a small effect on the critical or the limiting flux. The pure water fluxes of precleaned membranes after filtration were still higher than the pure water fluxes of the untreated membranes before filtration.

Humic acid as a fouling agent in filtration

Humic acid is a degradation product of lignin, carbohydrate and protein. It is contained in soils and therefore it is often present in surface waters in small amounts, which vary with the seasons. Besides giving the waters a yellowish to brownish color, the humic acid can also cause fouling problems in filtration of surface waters. In the present study humic acid was studied in different types of filtration processes because it seemed to be a problem at many mill sites using humic acid containing process waters. Humic acid was studied as a model substance in pressure and membrane filtration at different low concentrations. In the pressure filtration experiments it was added to a copper concentrate, containing as much iron and sulfur as copper. In membrane filtration it was filtered together with iron chloride. Also means to measure humic acid were investigated. In pressure filtration either capillary disc filters made of aluminum and silica oxides or filters made of different types of woven cloth were used. In membrane filtration the experiments were made with organic nano- and ultrafiltration membranes and with inorganic ultrafiltration membranes. The experiments were made on the laboratory scale. The humic acid could be analyzed using SEM-EDAX methods or with UV/VIS spectrophotometric methods. The studies showed that humic acid was most harmful in capillary pressure filtration. Even at small concentrations the filter was blocked and the cake did not form properly when added to the ore concentrates to be filtered. The flux decreased greatly with an addition of 10 ppm, and the filter was almost completely blocked when 100 ppm were added. In pressure filtration using cloth the harmful effect was not as remarkable. In membrane filtration humic acid did not disturb filtration as much as in pressure filtration even though it was used together with iron chloride. The effect of humic acid was a bit larger in ultrafiltration than in nanofiltration. The reason for the fouling tendency of humic acid seems to be its binding tendency to multivalent salts. The capillary filters used in pressure filtration are positively charged containing Al and Si. The humic acid is either not charged or somewhat negatively charged and probably forms chelates with the metals. It can be seen as a gel-like layer on the filter, which disturbs the formation of the cake and blocks the pores of the filter.

Fouling of unmodified and modified polysulfone ultrafiltration membranes by ovalbumin

Abstract Dilute ovalbumin solutions were ultrafiltered with unmodified and modified polysulfone membranes. With unmodified membranes, flux reduction was greatest at the isoelectric point (IP) of the protein and decreased on both the acidic and the basic side of the IP. Modification of the membrane with the polyelectrolyte polyethylenimine decreased flux reduction when coulombic repulsion between the protein molecules and the membrane surface was achieved. High ionic strength enhanced flux, as it increased repulsion by increasing the electric charge of the protein molecule or by lowering the electroviscous effects in the pores. However, the hydrophilicity of the membrane seemed to be a more important factor in flux reduction than its actual charge.