Understanding the Structural Parameter in Engineered Osmosis

Engineered osmosis (EO) is a platform of membrane technologies for various applications including water desalination (forward osmosis, FO), power production (pressure-retarded osmosis, PRO), dewatering etc. Internal concentration polarization (ICP), a phenomenon identified as a significant impediment towards obtaining high water fluxes in anisotropic membranes, is largely influenced by the structure of the support layer. The structural parameter (a function of the support layer’s porosity, tortuosity and thickness) has been designated to describe the propensity of the particular support layer’s to cause ICP.  However this parameter has so far only been indirectly calculated from experimental flux measurements due to the lack of suitable structure characterization techniques for soft materials.  This method has certain limitations as it assumes some of the associated transport phenomena to be negligible which are therefore unaccounted for during the analysis.  With high performance membranes entering the field, these assumptions are no longer reasonable and new methods of membrane characterization must be considered to measure the true structural parameter of a membrane so that the resistances to osmotic flux performance can be elucidated.  In a recently reported study on round-robin testing methods for membrane characterization, it was shown that structural parameters calculated using the conventional method reported varied values between different research groups for a commercial FO membrane. In this study, the asymmetric structures of two commercially available thin film composite reverse osmosis (RO) membranes, the BW30 and SW30-XLE from Dow® Water and Process Solutions, have been studied using an analytical (mercury intrusion porosimetry) and a 3D imaging (x-ray computed tomography) technique. A multi-length scale approach was used for the imaging technique in order to study both the bulk properties as well as visualize the microporous structure of the support layer. Structural metrics (viz. porosity, tortuosity and thickness) of the membranes were then calculated and structural parameters were directly measured for the membranes. Further, surface porosities at the selective-support layer interface could be measured which is very useful information for EO membranes. Large variations were observed between the “actual” structural parameters and the values obtained from osmotic flux tests. The proposed method to characterize membranes can further be used to individually study the effects of the different structural characteristics (i.e. porosity, tortuosity and thickness) on the severity of the ICP phenomenon. The results of the study indicate a need to revise traditional approaches of characterizing membrane structures overall for any application.