Forward Osmosis Membrane Characterization By Microscopic Methods – a Comparative Study of Optical Microscopy (OM), Clsm, SEM and TEM

Osmotically driven membrane processes including forwards osmosis (FO) and pressure retarded osmosis (PRO) have received increasing attention in recent decades. Internal concentration polarization (ICP), which is a strong function of membrane support layer porosity, is one of the most important factors that limit the membrane performance. This study adopted four microscopic methods that are optical microscopy (OM), confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to quantitatively/semi-quantitatively characterize the pore structure of commercially available cellulose triacetate (CTA) FO membranes. As non-invasive methods, OM and CLSM were used to observe membrane active surface and support layer side, respectively. Superior to OM, the optical sectioning function of CLSM makes it possible to distinguish pores at each scanned depth and thus the porosity depth profile and pore connectivity can be obtained. The porosity results (over the membrane thickness) by CLSM method were further compared to that obtained from TEM and SEM cross-sectional images, and reasonably good agreement was observed. The structural parameter (S) which is straightly linked to ICP can be also estimated by using the porosity depth profile. In addition, the two non-invasive methods can be further used for in-situ FO fouling characterization - foulant deposition on surface by OM and pore blocking by CLSM.