Characterization of the Water Transport Properties of the Active Layers of Polyamide Reverse Osmosis and Nanofiltration Membranes

The overall objective of this study was to elucidate which parameter among water partitioning, water diffusion and active layer thickness accounts for the differences in water permeability among polyamide reverse osmosis (RO) and nanofiltration (NF) membranes. To achieve this overall objective, the following specific objectives were pursued: (i) to identify appropriate methods (facile and accurate) for measuring active layer thickness in RO/NF membranes; (ii) to ascertain the existence of voids within the polyamide active layers of RO/NF membranes and develop methods to quantify void volume fraction in active layers; (iii) to measure the partition coefficient of water into membrane active layers, and calculate the diffusion coefficient of water in active layers based on measurements of membrane water permeability (A), active layer thickness (δ), and water partition coefficient (K); and (iv) to correlate membrane water permeability (A) to active layer thickness (δ), water partition coefficient (K) and water diffusion coefficient (D) for a group of polyamide RO/NF membranes with a broad range of performance levels. The following major conclusions were drawn through this dissertation: (i) atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), quartz crystal microbalance (QCM), profilometry and ellipsometry produce consistent active layer thickness results among each other and likely provide the most accurate results of active layer thickness of RO/NF membranes; (ii) scanning electron microscopy (SEM) and transmission electron microscopy (TEM) likely overestimate active layer thickness; (iii) voids with sizes in the ≈10-100 nm range commonly exist in the active layers of RO/NF membranes, occupy a significant volume fraction of the active layers, and are filled with water when the membranes are in contact with water (e.g., when they are being used for water purification); (iii) among active layer thickness, water partition coefficient and water diffusion coefficient, the differences in water permeabilities among RO/NF membranes were mostly accounted for by the differences in the water diffusion coefficients in their active layers; (iv) the presence of the voids in the active layer increases the membrane water permeance; and (v) there was not a clear correlation between void volume fraction and water permeance, water partition coefficient or water diffusion coefficient.Doctor of Philosoph