Evidence of membrane fouling monitored by AFM and zeta potential

The suppression of membrane fouling is key for a successful application of pressure-driven membrane processes. Based on protein adsorption on a microfiltration membrane, we demonstrate the application of a combination of atomic force microscopy and surface zeta potential analysis as a sensitive technique to confirm the initial steps of membrane fouling.

A flat sheet cellulose acetate (CA) membrane for microfiltration (MF, pore size 0.45 μm) was characterized by means of atomic force microscopy and zeta potential analysis. Bovine serum albumine (BSA) was employed as a model protein to study the effect of protein adsorption on the MF membrane surface.

The AFM measurements were performed in ambient atmosphere. The instrument was mechanically isolated by an active vibration table and an acoustic enclosure. For the topographic investigations, tapping mode was used applying an AP-NCR cantilever with a specified resonance frequency of 285 kHz, a force constant of 42 N/m and a typical tip radius of < 10 nm. The images were acquired with a scan speed between 7.5 μm/s for 15 x 15 μm2 scans and 30 μm/s for the scan size of 60 x 60 μm2. The resolution of the images was set to 400 x 400 pixels.

The zeta potential of the pristine MF membrane was determined in aqueous solutions of KCl at different ionic strengths and of a dilute phosphate buffer (PBS, ionic strength 0.004 mol/l) with SurPASS 3. The MF membrane was mounted in an adjustable gap cell and the pH was automatically adjusted by dispensing either 0.05 mol/l HCl or 0.05 mol/l KOH. The kinetics of protein adsorption was determined by recording the temporal changes of the streaming potential after addition of 50 and 200 μg/ml BSA, respectively.

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