(280g) Post-Synthesis Modification of Polyamide Reverse Osmosis Membranes Via Diamines for Enhanced Urea Rejection

Reverse osmosis (RO) membranes are one of the most efficient membrane filtration technologies because of their strong separation capabilities and great potential for the treatment of water worldwide. Even though RO membranes exhibit excellent performance in salt rejection, they do not reject small, neutral, and uncharged molecules (SNUs), such as urea, to a level to produce potable water. Because of the rapid and uncontrolled nature of the interfacial polymerization reaction, the polyamide separation layer contains both network and aggregate free volume holes (pores). As SNU rejection is governed mainly by the size exclusion mechanism, decreasing the free volume to reduce the passage of the urea through the membrane is required. In this regard, the modification of RO membranes to increase the degree of cross-linking and/or decrease the free volume hole size is an ideal approach. We hypothesize that if the polyamide layer can be modified with a diamine, then the urea rejection will be increased. In this work, a method was developed for combining chemical modification and heat treatment of commercial DuPont XLE membranes to enhance the rejection of urea. For the chemical modification, carbodiimide chemistry was used followed by the coupling of various diamine. The modification of the commercial membrane was performed at room temperature (~22 °C) and 63 °C. The modified membranes were characterized using ATR-FTIR, XPS, SEM, contact angle goniometry, and electrokinetic analyzer. Membranes were performance tested for water permeance, NaCl rejection, and urea rejection using a dead-end stirred cell. Results show that both heat treatment and amine modification increase the urea rejection of the membranes.