A new insight into roughness structure of polyamide RO membranes

Abstract

Water scarcity is one of the major global challenges of our time. Reverse osmosis technology plays a crucial role in providing alternative sources of water through seawater desalination and water reclamation. Thin film composite polyamide membranes are the gold standard for reverse osmosis technology, but tuning their permeability and selectivity remains a major challenge because of the inherent permeability-selectivity trade-off. The polyamide rejection layer of a typical reverse osmosis membrane contains a large number of discrete nano-sized voids occupying as much as 30% of its total volume. Despite the common belief that these nanovoids regulate the membrane water permeability and salt rejection, little is known about their formation mechanism. In this presentation, we show that these nanovoids are formed due to the degassing of nanobubbles from the monomer solution during the interfacial polymerization process to fabricate the polyamide selective film. Simple nano-foaming methods, such as addition of sodium bicarbonate and ultrasound application, can result in remarkable improvement in both membrane water permeability and salt rejection, thus overcoming the long-standing permeability-selectivity trade-off of desalination membranes. Our study further establishes a framework for the fundamental understanding of the structure-property-performance relationship of polyamide desalination membranes.