Degradation by Free Chlorine of Aromatic Polyamide Active Layers of Thin-Film Composite Membranes
Creators: name: Powell, Joshua A.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2013

description: Polyamide-based thin-film composite membranes are the current technology of choice to meet the growing demand for drinking water desalination applications. One significant drawback to the use of this class of membranes is the high sensitivity of their polyamide active layer to oxidation by free chlorine. The current understanding of the mechanisms of chlorine uptake and eventual polymer degradation that lead to membrane failure has been gained mostly through the quantitative study of the effects of chlorination on molecular model polyamide compounds and membrane surfaces (top~5nm) with X-ray photoelectron spectroscopy (XPS), as well as through qualitative analyses of chlorinated membrane samples with attenuated total reflectance-Fourier transformed infrared (ATR-FTIR)spectroscopy. The physico-chemical changes induced by chlorination within the bulk of the active layer, however, have not been characterized by means other than ATR-FTIR, and therefore it has not been confirmed in the literature that the physico-chemical changes observed at the membrane surface are representative of the volume-averaged changes in the active layer, and that the mechanisms that have been proposed as leading to membrane failure based on studies with model compounds are consistent with observations for the bulk region of the active layer. Accordingly, we exposed a polyamide thin-film composite membrane to free chlorine at a range of concentrations, exposure times and pH values and quantified in situ the volume-averaged kinetics of chlorine uptake and resulting de-polymerization of the polyamide active layer. We performed volume-averaged measurements for the membrane active layer using Rutherford Backscattering Spectrometry (RBS) as an analytical technique. Our results indicate that the trends observed for the kinetics of chlorine uptake into the bulk region of the active layer are mostly consistent with the corresponding trends reported in the literature for chlorine uptake into the active layer surface. Our results also show that consistent with mechanisms proposed in the literature based on studies with model compounds, chlorine uptake into the bulk region of the active layer can be explained by chlorination of the amidic nitrogen by hypochlorous acid at all pH conditions and ring chlorination. Analysis of chlorine uptake results at acidic conditions indicates that ring chlorination occurs by Orton rearrangement, not direct ring chlorination. We also provide the first measurements of the kinetics of chain scissioning in the active layer as a result of exposure to free chlorine. Our results indicate that de-polymerization of the active layer occurs when the membrane is exposed to alkaline conditions following or during chlorination of the amidic nitrogen. Chain scission of the amide linkage also occurs via a hydrolysis mechanism as is dependent on both exposure to hypochlorous acid and hydroxyl ion. By contrast, ring chlorination does not result in polyamide de-polymerization. KeyWords: reverse osmosis, nanofiltration, polyamide, free chlorine, chain scissioning, ion probing, RBS, charge density, amide N-Cl, ring chlorination.