A Novel Characterization Method for Determination of Transport and Structural Parameters of Forward Osmosis Membranes

With growing interest in developing high-performance forward osmosis (FO) membranes, rapid and accurate characterization will be critical to facilitate sharing of data and advances in the field.  We present a characterization protocol to determine the water and solute permeability (A and B, respectively) and the structural parameter (S) of a membrane in a single FO experiment.  In this method, water and salt fluxes are measured at distinct draw solution concentrations and are entered into a regression algorithm that uses the governing FO transport equations to determine A, B, and S.  The method is validated through testing of four different membranes: hand-cast thin-film composite (TFC), commercial FO-TFC, TFC reverse osmosis (RO), and cellulose acetate asymmetric membranes.  Results show that membrane parameters determined with the proposed methodology are significantly different than those obtained with the conventional characterization method involving operation in RO and FO.  Parameters determined using the new FO-based method more accurately predict water and salt fluxes for all types of membranes.  The reliability of this method given error in experimental measurements is thoroughly analyzed.  Additionally, we find that for the three polyamide TFC membranes tested, the permeability-selectivity ratio in FO was consistently 50-70% lower that determined in RO.  This casts doubt on the notion that membrane parameters are conserved regardless of the driving force and requires further investigation by the scientific community.