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Membranes and membrane fouling: Surface charge analysis in filtration

Membranes are used for selective separation of particles within a feed solution in a variety of filtration processes. Membrane types are as diverse as their application: Reverse osmosis membranes are used, e.g., in seawater desalination, nanofiltration membranes are applied for hardness removal in water purification, ultrafiltration membranes are used in blood dialysis, and microfiltration membranes are used for cold sterilization of beverages.

Depending on the separation process, membranes are available in different geometries: as tubular, flat-sheet, spiral-wound, or hollow-fiber configurations. Polymer-based membranes are utilized for the most common applications, whereas ceramic filters are used for specific filtration tasks under more aggressive conditions.

The main goal of all membrane applications is to achieve optimum separation and productivity. Filter material and process conditions need to be selected carefully in order to achieve the highest rejection and the highest flux – while keeping the filtration process as economical as possible.

Importance of surface charge and zeta potential in filtration

Electrostatic interactions between membrane surface and feedwater compounds can be predicted by membrane surface charges, a key parameter describing the interface between the membrane and its environment.

In order to favor or suppress specific interactions with feedwater components, membrane surface charges can be modified. For this reason, surface charge determination provides important information for optimizing and monitoring membrane filtration processes.

Since surface charges are related to the zeta potential at the solid/liquid interface, zeta potential measurements can be used to analyze membrane surfaces.

The SurPASS 3 instrument determines the surface zeta potential by means of streaming potential or streaming current measurements and allows a fully automated surface charge analysis over a wide pH range.

It provides surface charge analysis from reverse osmosis to nano-, ultra-, and microfiltration membranes, made of different materials ranging from polymers to ceramics. Different membrane types ranging from flat-sheet and tubular to hollow-fiber membranes can be investigated by employing measurement cells that are designed to accommodate specific sample geometries. Furthermore, the instrument provides access to information on membrane surface characteristics under environmental conditions, thus simulating the behavior of a membrane in the technical process.

SurPASS 3 instrument

Adjustable gap cell:

Tangential analysis of flat-sheet membranes

Cylindrical cell:

Transmembrane analysis of flat-sheet membranes

Measurement cell for ceramic membranes:

Transmembrane analysis of ceramic microfiltration membranes

Measurement cell for hollow-fiber membranes:

Inner surface characterization of hollow-fiber membranes

Membrane fouling

Special attention is paid to membrane fouling, an effect strongly influencing productivity and operational costs. Membrane fouling occurs when particles deposit on the membrane surface or in the membrane pores – reducing membrane performance, and affecting membrane selectivity as well as the permeate flux through the membrane pores. In order to maintain membrane productivity, pretreatment of the feedwater and chemical cleaning of the membrane are often required. In case of irreversible membrane fouling or membrane degradation after aggressive cleaning, membranes need to be replaced frequently, which further increases the operational costs. As a consequence, extensive research is required for characterization and optimization of membrane surfaces in order to prevent membrane fouling. The SurPASS 3 instrument determines minimal changes in the membrane zeta potential and therefore enables early detection of membrane fouling and precise analysis of membrane surface properties. Besides zeta potential values in a defined aqueous solution within a certain pH range, the isoelectric point (IEP) is a meaningful parameter to analyze. The IEP is the pH value where the zeta potential is 0 mV and a charge reversal of the surface takes place. It is sensitive to surface modifications and gives strong indications for surface chemistry changes.

Membrane cleaning

In order to diminish membrane fouling, cleaning processes are established. Especially in large filtration plants, it is desirable to reduce the frequency of cleaning cycles. For ecological and economic reasons, the consumption of chemicals for cleaning of fouled membranes must be minimized. Monitoring membrane fouling and cleaning allows optimization of membrane material, cleaning procedures and the filtering process itself. Membrane ageing and degradation due to cleaning will be detected in early stages.

pH-dependent zeta potential of pristine, fouled and cleaned microfiltration membranes (MF-2)

During the filtration process, membranes were fouled with a protein layer (red, FGN-fibrinogen). The shift of IEP is caused by the deposition of the FGN layer on the membrane surface. In the next step, the membrane was cleaned (blue) and analyzed again. The zeta potential of the cleaned membrane is not the same as for the pristine one. The zeta potential result reveals residues of fibrinogen on the membrane surface, which were not removed by the cleaning process.

Membrane fouling by municipal wastewater

Efficient removal of trace contaminants from feed water is indispensable for wastewater filtration processes. Knowledge about membrane surface charges is of paramount importance in the selection of appropriate nanofiltration membranes and prediction of their performance. At the same time, filtration performance should be maximized and interactions between membrane surface and the feed solutes should be minimized to prevent membrane fouling. SurPASS 3 detects even early stages of membrane fouling due to zeta potential changes, thus permitting optimization of wastewater filtration processes. 

Membrane fouling through wastewater

Effect of effluents of different municipal wastewater plants (P1, P2) on the surface properties of a polyamide thin-film composite membrane for reverse osmosis. In all cases, the IEP was shifted towards a lower pH, indicating the adsorption of acidic compounds, such as humic acid, from the feedwater.

Membrane coatings to prevent fouling

Modifying membrane surface properties alters interactions between membrane surfaces and components within the feedwater. For this reason, membrane fouling effects change. A remedy to prevent fouling, especially of polymer membranes, is to equip the membrane surface with functional groups or with a hydrogel layer, which changes the membrane surface from hydrophobic to hydrophilic. Hydrophilic membrane surfaces repel hydrophilic feed components with the same surface charge, so feed components will stick less to the membrane and membrane fouling will be reduced. Moreover, modification of surface-localized functional groups regarding surface charges increases the “toolbox” for membrane surface optimization. Defined surface modifications create distinct membrane surfaces to attract or retract certain feed components.

Membrane zeta potential determinations help find the optimal coating in order to adapt membrane surface properties to specific application needs.

pH dependence of zeta potential for composite membrane coated with poly(ethylene glycol) diacrylate (PEGDA).

PEGDA shifts the IEP of the membrane from pH 3.8 to pH 3.3. The increasing zeta potential at a higher pH is a strong indicator for the enhanced surface hydrophilicity of the PEGDA coating.