Influence of colloidal particles with bimodal size distributions on retention and pressure drop in ultrafiltration membranes

Global advent in technology has seen an increase in the manufacture and incorporation nanomaterials. Nanomaterials are chemical substances that are synthesized and used at a very small scale. Nanotechnology has long been seen as the key to the future of humanity. Needless to say, shortfalls are inevitable when new materials are introduced in the natural environment. Case and point, there has been unintentional excessive release of nano-waste into the environment, particularly water bodies. To counteract this mishap, microfiltration and ultrafiltration using membranes have offered a means of removing well-stabilized nanoparticles in aqueous environments. Technically, the membrane process has two operating modes: cross-flow (feed solution flows tangentially) and dead-end (flow is orthogonal to the membrane surface) filtrations, where the latter more suitable owing to obvious advantages, such as high production recovery, simple operation and less minimum feed volume. Regardless of the flow orientation, membrane fouling is inevitable. Ideally, fouling from ongoing deposition of particles is the major issue for membrane failure and thus many researches on fouling characteristics have been conducted. A review of existing literature reveals that majority of researches have been performed with monodisperse particle systems to clarify effects of particle size on the deposition and fouling mechanisms. Yet, particle systems in nature have a wide size distribution with polydispersity.

Therefore, there is a definite need for the comprehensive understanding of loading and fouling characteristics on membrane filters. To address this, researchers from the Particle Technology Laboratory, Mechanical Engineering at University of Minnesota: Dr. Handol Lee, Dong-Bin Kwak (PhD candidate), Dr. Seong Chan Kim, Dr. Qisheng Ou and Professor David Pui, proposed to use an electrospray-scanning mobility particle sizer method, which is suitable for characterizing the concentration of each component in mixtures, to assess the membrane fouling process. One of their goals was to introduce a characterization method for particles with multimodal size distribution. Their work is currently published in the research journal, Separation and Purification Technology.

Their approach entailed performing a series of filtration tests for 0.03 µm rated Polyethersulfone (PES) membrane against mixtures of different sized polystyrene latex (PSL) particles. In other words, the influence of polydisperse colloidal particles in mixtures on filtration performance, i.e., retention efficiency and pressure drop, was investigated using commercially available PSL nanoparticles with three sizes of 60, 100 and 150 nm.

The authors reported that there was significant change in pressure drop according to particle retention mechanisms. In addition, the monodisperse particle filtration experiments showed that the 60 nm particles were captured by the depth filtration on the 0.03 µm rated PES membrane, but the larger particles with a size of 150 nm showed the cake filtration by clogging the surface openings. For the cases of mixtures of 60 + 150 nm and 100 + 150 nm, the researchers found much more pressure drop increases compared to the summation of the pressure drop increase for each monodisperse case.

In summary, the effects of particle polydispersity on retention efficiency and pressure drop were investigated using 60, 100 and 150nm PSL particles and 0.03µm rated PES membranes by a series of systematic filtration tests. It was seen that there was enhanced retention of 60 nm particles in a mixture with larger particles. Also, transition of retention mechanism from depth to surface filtration by clogged pores was also witnessed. In a statement to Advances in Engineering, Dr. Qisheng Ou highlighted that the systematical filtration experiment and proper measurement method introduced in their study will continuously provide important guidelines for evaluating overall filtration performance in membrane processes.