26 A Mechanistic Study of Halogen Interaction with Polyamide Reverse-Osmosis Membranes JULIUS GLATER and MICHAEL R. ZACHARIAH
Downloaded by CORNELL UNIV on July 2, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch026
School of Engineering and Applied Science, University of California, Los Angeles, CA 90024 Degradation and subsequent failure of polyamide reverse osmosis membranes in response to aqueous chlorine and bromine species has been reported in the literature and observed in the field. The process is clearly pH dependent showing a different pattern of sensitivity related to membrane type. Work described in this study has been directed toward better understanding the nature of halogen attack on membrane polymer. Experiments were conducted with du Pont Aramid B-9 membranes. Halogen exposed samples were examined by infra-red and NMR spectroscopy. Studies with benzanilide, a model compound, are also reported. Experimental evidence shows aromatic ring halogenation to be the dominant process. Membrane failure appears to result from alteration in hydrogen bonding modes within the polymer. Polyamide reverse osmosis membranes are known to be chlorine sensitive as reported from both field and laboratory data (.l*^*^) • Sensitivity varies not only with membrane type but is also related to feed water pH. Considerable test data on membrane halogen interaction has been published by the UCLA Group (4>^5>£) . During the course of this work, we became interested in studying the nature of chemical interaction at the molecular level. These studies were undertaken for academic interest and, in addition, with the intent of providing guidelines for polymer synthesis of halogen resistant membranes. Initial experiments were carried out by attempting to obtain infra-red spectra of various membrane types. Most of this work with thin film composite membranes was abandoned due to insolubility of these polymers in organic solvents. Our investigation focused on du Pont B-9 Aramid membrane. This material is sensitive to chlorine at low pH and to bromine over a wide pH range (6). Both unexposed and halogen exposed membrane are readily soluble in dimethyl sulfoxide. In addition, this patented membrane has a well-defined polymeric structure (_7). Certain experiments were also conducted with benzanilide, a model compound, which simulates the monomeric B-9 unit.
0097-6156/85/0281-O345$06.00/0 © 1985 American Chemical Society
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
346
REVERSE OSMOSIS AND ULTRAFILTRATION
Results reported in this paper suggest mechanisms for both halogen attack and subsequent membrane failure. These chemical models were developed from analysis of infra-red (IR) and nuclear magnetic resonance (NMR) spectra of halogen exposed membrane samples under conditions of accelerated testing.
Downloaded by CORNELL UNIV on July 2, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch026
Experimental Procedures All membrane exposures were conducted at fixed halogen concentrations and pH levels for varying time periods. Following exposure, membrane samples were dissolved in dimethyl solfuxide (DMSO) and re-cast into films of optical quality, on the order of 25 microns thick. IR spectra were taken by direct transmission using a Perkin-Elmer 521 double beam grating spectrophotometer. NMR spectra were taken with membrane solutions in deuterated DMSO solvent using a Bruker WP-200 operating at 200 MHz. Details of experimental procedures and equipment have been described in previous publications (6^.8) • Results and Discussion Several structures for aromatic polyamide membranes are described in the previously mentioned du Pont Patent (7). One listed polymer, the commercial B-9 Aramid membrane consists of a chain of six membered aromatic rings connected by peptide linkages in the meta [1,3] position. The polymer is linear with some hydrogen bonding between chains and with alternate rings bonded on each side to either carbonyl or NH groups. The hydrophilicity of the polymer has been improved by addition of sulfonic acid groups to approximately 10% of the aromatic rings. A general structure for du Pont B-9 membrane is shown in Figure 1. Figure 2 shows an IR spectrum of unexposed polymer. This trace clearly verifies aromatic character, amide linkages and 1,3 aromatic ring substitution. Evidence for hydrogen bonding is also established from N-H and C-N stretch absorptions. Assigned frequencies used in this analysis are given in Table I (2>i2.'ii»ii»il^ • I R spectra of chlorine exposed membrane were obtained at various time intervals. Figure 3 is a typical spectrum of B-9 membrane following exposure to 30 ppm chlorine at pH 3.0 for 112 hours. Note the pronounced changes, with appearance of new bands along with shifts in previously existing spectral details. Since spectral interpretation of polymeric materials is difficult, it was decided to select a simple pure compound which would model B-9 polymer chemistry and thus simplify spectral details. The compound chosen was benzanilide (
