Oxidative Degradation of Polyamide Reverse Osmosis Membranes: Studies of Molecular Model Compounds and Selected Membranes Neil P. Soice, 1 Adrian C. Maladono, 2 Doreen Y. Takigawa, 2 Arlan D. Norman, 1 William B. Krantz, 1 Alan R. Greenberg 1 1 Departments of Chemistry and Biochemistry, Chemical Engineering, Mechanical Engineering; and the NSF I/U CRC for Membrane Applied Science and Technology, University of Colorado, Boulder, Colorado 80309 2 Koch/Fluid Systems Corporation, 10054 Old Grove Road, San Diego, California 92131 Received 25 February 2002; accepted 20 October 2002 ABSTRACT: Selected aromatic amides were used to model the chemical reactivity of aromatic polyamides found in thin-film composite reverse osmosis (RO) membranes. Chlorination and possible amide bond cleavage of aromatic amides upon exposure to aqueous chlorine, which can lead to membrane failure, were investigated. Correlations are made of the available chlorine concentration, pH, and expo- sure time with chemical changes in the model compounds. From the observed reactivity trends, insights are obtained into the mechanism of RO membrane performance loss upon chlorine exposure. Two chemical pathways for degra- dation are shown, one at constant pH and another that is pH-history dependent. An alternative strategy is presented for the design of chlorine-resistant RO membranes, and an initial performance study of RO membranes incorporating this strategy is reported. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1173–1184, 2003 Key words: polyamides; reverse osmosis; membranes; chlo- rination; degradation INTRODUCTION Thin-film composite membranes that use aromatic polyamides as the barrier layer are widely employed in the purification of water. 1 It is well documented that the performance of such membranes declines 1,2 [i.e. flux increases and salt rejection drops below re- verse osmosis (RO) specifications] after exposure to an oxidizing environment (e.g., 500 –2000 ppmhr of chlo- rine exposure), 3–7 usually from sodium hypochlorite that is used to clean the membrane module or found in residual amounts as a disinfecting agent. Because this situation raises both basic and practical questions, it has been of interest to understand the mechanism of membrane degradation and find ways to increase the lifetime membrane materials by making them resis- tant to oxidizing agents. 1 The polyamide membrane degradation effect is known to be pH dependent 8 ; several groups have studied the mechanism and sought ways to prevent the degradation. 9 –17 Glater et al. 2 published a review of previous efforts in 1994. In particular, there are several cases where insights into a polymer’s chemical behavior have been generated from model com- pounds. Lowell et al. 18 found they could predict the chlorine sensitivity of several types of interfacially polymerized materials by analyzing the extent of chlo- rination in analogous model systems. Khanna and coworkers 19 showed that the mechanism of thermal decomposition in aromatic polyamides could be de- termined using amide model compounds. Finally, Kawaguchi and Tamura 17 reported that two chemical processes occur in polyamides and amide molecular model compounds upon chlorine exposure, a revers- ible N[sbond[H bond chlorination and an irreversible aromatic ring chlorination (Fig. 1). Some also believe these two processes precede cleavage of the polymer chain and interchain crosslinks, most likely at the amide linkage. 10,16 The previous model compound studies have sparked efforts to modify the polyamides used as barrier layers, to increase chlorine resistance. Strate- gies have involved both the removal of the NOH functionality and adding electron-withdrawing groups to the amino aromatic rings. Singh 9 reported that membranes soaked in nitric acid showed im- proved chlorine resistance, presumably because of the addition of nitro groups to aromatic rings in the poly- amide. These electron-withdrawing groups deactivate the aromatic ring to electrophilic chlorine addition. Unfortunately, along with increased chlorine resis- Correspondence to: A. Norman (arlan.norman@colorado.edu). Contract grant sponsor: National Science Foundation/ UCRC Center for Membrane Applied Science and Technol- ogy. Journal of Applied Polymer Science, Vol. 90, 1173–1184 (2003) © 2003 Wiley Periodicals, Inc.