Atomic force microscopy study of the biofouling and mechanical properties of virgin and industrially fouled reverse osmosis membranes

Sea water desalination using reverse osmosis and membrane separation technology has been applied for the production of ultra-pure water. However, this process is often impeded by membrane fouling which causes considerable technical problems such as high pressure requirement, increased demands for cleaning processes and lower quality water. As a consequence the fouling reduces the lifespan of the membrane. Microbial biofilm formation is a major contributor to the fouling of membranes. A number of strategies can be applied to control and prevent membrane fouling such as chemical washing or back washing, however these processes can age the membrane. New studies are therefore necessary to develop techniques that either control or inhibit biofilm membrane fouling.

In a recent paper published in Desalination Dr. Chris Wright and colleagues at Swansea University in United Kingdom studied the biofouling and mechanical properties of virgin and industrially fouled reverse osmosis membrane using Atomic Force Microscopy. They aimed at gaining greater understanding of the fouling mechanism so as to entirely control it, and guide membrane fabrication and membrane cleaning strategies.

The research team obtained virgin and fouled membranes for the study of the biofouling process. The microbial diversity of the bacteria that caused the biofilm fouling on the fouled industrial membrane had been previously determined as  mainly Proteobacteria although traces of Plantomycetes and Bacterioidetes were also spotted.. The research team then imaged the membranes using AFM tapping mode in air as opposed to the imaging in liquid since the latter is experimentally  demanding and may remove  diffuse biofilm foulant layers. This imaging identified the morphology and extent of the industrial biofouling. The images of the fouled membrane clearly showed the relatively non-uniform distribution of the fouling present on the membrane surface The  Nano Wizard II BioAFM was then used for the measurement of  forces at all membrane surfaces. The interpretation of force data was aided  by the determination of the streaming potentials of the membranes using an Electro Kinetic Analyzer, such measurement of electrostatic potential allow analysis of processes, including biofouling, in terms of charge relationships .

In AFM a value of the force can be  obtained from deflection of the AFM cantilever using Hookes’ law.  From the AFM force-distance data  the team determined for adhesion of fouling particulates at both the fouled and virgin membranes, the energy of adhesion  and the maximum adhesion. They also quantified the mechanical properties of the virgin membrane and the fouling layers by measuring  the Young’s’ modulus using an AFM nanoindentation method.

This study demonstrated that pH control can be applied in a cleaning regime to strengthen membranes against chemical and physical challenges. As observed from Young’s’ modulus measurement at lower pH, this environment proves to be advantageous for membrane cleaning  since mechanical robustness, and thus protection of the membrane is boosted. In contrast,  in certain low pHs, that could be used during cleaning during  the biofouling layer is observed to be at its weakest and at this point the chemical properties of the membrane material are at their least prone to damage.