Polyamide thin-film composite (TFC) membranes for water purification are composed of a top polyamide active layer (∼20−200 nm) that constitutes the main barrier to water and solute permeation, an intermediate polysulfone support (~20-50 mm), and a polyester backing (~50-150 mm). Contaminants permeate TFC membranes in a three-step process: (1) partitioning into, (2) diffusion through, and (3) partitioning out of the polyamide active layer. However, given the extreme thinness of the active layer, direct quantification of diffusion coefficients in active layers has remained elusive, and only few studies have quantified solute partition coefficients. Specifically, for inorganic salts and small molecules, we could find only two experimental studies (ES&T, 2007, 41, 3290-3295; Appl. Phys. Lett., 2007, 91, 181904) that quantified partition coefficients. Both studies used Rutherford backscattering spectrometry (RBS) to measure solute concentration in the polyamide layer and, therefore, had as an important limitation that membrane samples were dried between immersion in the solution of interest and RBS analysis. An additional drawback of the RBS-based procedures is that highly-specialized and expensive equipment is required. Therefore the objective of this study was to develop a method to measure partition coefficients into polyamide active layers that is simple, bench scale, and eliminated the need to dry membrane samples before measurement of solute concentration in the polyamide layer. The method developed to measure solute partition coefficients into polyamide active layers uses a quartz crystal microbalance (QCM) to measure solute concentration in the active layer, and polyamide active layers isolated onto QCM sensors as test samples. The results indicate that the partition coefficients of NaCl and boric acid (H3BO3) are lower than one. This finding is inconsistent with the partition coefficients reported in the literature (ES&T, 2007, 41, 3290-3295; Appl. Phys. Lett., 2007, 91, 181904) for inorganic salts (including KCl and arsenious acid (H3AsO3)), which were significantly larger than one (~3-8). The partition coefficients lower than one obtained using the QCM-based method are consistent with the high rejection of salts by polyamide TFC membranes (&gt;99.5% for seawater desalination membranes). In this talk we describe in detail the QCM-based method and corresponding results, and discuss the advantages and disadvantages of each the RBS and QCM based methods.

Partitioning of inorganic contaminants into the polyamide active layers of thin-film composite membranes for water purification

251st National Meeting (2016)

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