Improvements to Polyamide Reverse Osmosis Membranes for Removal of Small, Neutral, Hydrophilic Solutes

Abstract

Potable water supplies around the world are under stress. Since 1950 the world population has nearly tripled, while current evidence suggests that conventional potable water supplies are shrinking. One technology with the ability to tap into new potable water sources, such as municipal wastewater, is reverse osmosis (RO). RO uses permselective membranes to remove solutes from source water while allowing clean permeate to pass through. While RO facilities treating municipal wastewater for potable purposes are already in operation, selectivity limitations to state-of-the-art RO membranes force these facilities to employ costly post-treatment technologies. Specifically, RO membranes are limited in their rejection of small, neutral, hydrophilic solutes (SNHs). The goal of this research was to develop an RO membrane with improved selectivity to SNHs, particularly the carcinogen N-nitrosodimethylamine (NDMA). To do this, commercial RO membranes were modified using a variety of methods, the most successful of which was surface bonding of graphene oxide (GO) nanosheets. On average, membranes modified with GO nanosheets reduced NDMA permeability by 31% when compared to unmodified membranes, increasing average NDMA rejection from 76.5% to 82.7%. Improvement to NDMA rejection came at a 15% loss of water permeability through the GO modified membranes, but this is moderate when compared to losses experienced from other modifications found in literature. We hypothesize that the mechanism for improved NDMA rejection is increased steric hinderance due to the layer of GO nanosheets, which we believe help to cover natural free-volume defects in the membrane structure.