Modification of polyamide reverse osmosis membranes for the separation of urea

Abstract

Reverse osmosis (RO) membranes are the gold standard for water desalination and have been in use for over three decades. Even though RO membranes exhibit excellent performance rejecting monovalent and divalent salt ions, they do not reject small, neutral, and uncharged molecules, such as urea, to a level to produce potable water, especially at near-neutral pH. Due to the fast, uncontrolled nature of the interfacial polymerization reaction, the polyamide layer contains both network and aggregate free volume holes (pores). Because urea rejection is dominated by the size exclusion mechanism, reducing the free volume to reduce the passage of the urea through the membrane is needed. 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, we modified polyamide RO membrane separation layers of commercial membranes (Dupont XLE and BW30XFR) using the carbodiimide chemistry followed by the application of m-phenylenediamine (MPD) and heat treatment in the post-modification stage. 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. Compared to the control membranes, the modified XLE membranes and modified BW30XFR membranes improved the urea rejection from 16.8% to 54.9% and from 48.4% to 64.6%, but a reduction in water permeance by a factor of up to 4.7 and 2.7 respectively. The results show that combining the application of MPD and heat treatment can enhance the urea rejection of the membranes significantly.