Water/Salt Transport Properties of Polyamide Reverse Osmosis Membranes Using Quartz Crystal Microbalance

Thin-film composite (TFC) reverse osmosis (RO) membranes prepared by interfacial polymerization of M-phenylene diamine (MPD) and trimesoyl chloride (TMC) are the most widely used membranes in the current desalination market. However, how to accurately and quantitatively describe the permeation process that quantifies the water/salt partition coefficient (K) and the diffusivity coefficient (D) of the polyamide matrix in the polyamide active layer remains a challenge. Herein, the water and salt transport properties of polyamide (PA) TFC membranes are systematically investigated for the first time based on the quartz crystal microbalance with dissipation (QCM-D), and the relationships between structure and transport properties are investigated. The change of water/salt participation (K-w and K-s) with the TMC concentration is much more sensitive than that with the MPD concentration increase. The K-w slightly changes with increasing monomer concentrations, while the K-s increases first and then remains almost unchanged with an increase in the TMC concentration. There is a significant reduction of the D-w and D-s of the polyamide matrix by enhancing the cross-linking degree as the TMC concentration increased from 0.05 to 0.125%. Enhancing the cross-linking degree of the polyamide matrix by adjusting the monomer concentration is beneficial to improve water/salt diffusivity selectivity. The evolving trend of permeability is consistent with the diffusivity, indicating that the permeability of the polyamide TFC membrane is mainly determined by the diffusivity of the polyamide matrix. In conclusion, varying the TMC concentration is more likely to change the physicochemical properties of the polyamide active layers, including water/salt transport properties.