Correlating the Role of Nanofillers with Active Layer Properties and Performance of Thin-Film Nanocomposite Membranes.

Thin-film nanocomposite (TFN) membranes are emerging water-purification membranes that could provide enhanced water permeance with similar solute removal over traditional thin-film composite (TFC) membranes. However, the effects of nanofiller incorporation on active layer physico-chemical properties have not been comprehensively studied. Accordingly, we aimed to understand the correlation between nanofillers, active layer physico-chemical properties, and membrane performance by investigating whether observed performance differences between TFN and control TFC membranes correlated with observed differences in physico-chemical properties. The effects of nanofiller loading, surface area, and size on membrane performance, along with active layer physico-chemical properties, were characterized in TFN membranes incorporated with Linde Type A (LTA) zeolite and zeolitic imidazole framework-8 (ZIF-8). Results show that nanofiller incorporation up to ~0.15 wt% resulted in higher water permeance and unchanged salt rejection, above which salt rejection decreased 0.9-25.6% and 26.1-48.3% for LTA-TFN and ZIF-8-TFN membranes, respectively. Observed changes in active layer physico-chemical properties were generally unsubstantial and did not explain observed changes in TFN membrane performance. Therefore, increased water permeance in TFN membranes could be due to preferential water transport through porous structures of nanofillers or along polymer-nanofiller interfaces. These findings offer new insights into the development of high-performance TFN membranes for water/ion separations.

Accessing greater thickness and new morphology features in polyamide active layers of thin-film composite membranes by reducing restrictions in amine monomer supply.

Polyamide formation, via interfacial polymerization (IP) during thin-film composite (TFC) membrane fabrication, is regarded as self-limiting-in the sense that the polyamide film limits its own growth as it forms. During IP, trimesoyl chloride (TMC) and m-phenylenediamine (MPD) react rapidly to form an incipient polyamide film that densifies and slows the diffusion of the more permeable monomer (MPD), thereby limiting polyamide growth and yielding films that typically exhibit thicknesses <350 nm. The morphology of these polyamide films is characterized by a basal layer of void nodular and leaf-like features that is sometimes overlaid by a secondary layer of overlapping flat features. Here, we present evidence showing that polyamide active layers are substantially permeable to MPD, and that minimizing certain restrictions in the MPD supply conditions during IP can result in polyamide active layers of thicknesses several times greater (>1 µm) than those typically reported in the literature. In addition to the basal layer of void nodular features and secondary layer of overlapping flat features that characterize typical polyamide active layers, the thicker films also exhibited three additional morphological features: blanket-like layers atop the basal layer or other void features, multi-layer void structures, and/or void mega-nodules (up to over a micron in diameter). Overall, the results indicate that reducing restrictions in the MPD supply conditions during IP: (1) overcomes the limited polyamide growth observed in conventional TFC membrane fabrication and (2) leads to film morphologies with a more prominent void structure. This latter observation is consistent with recent literature describing the role of CO2 degassing and nanobubble confinement in the development of polyamide active layer morphology. Future studies could vary MPD supply conditions as a new tool to expand the range of achievable thicknesses in active layer casting, regulate active layer morphology and optimize nanobubble confinement conditions independently of MPD supply. Such capabilities could aid in the development of novel supports and TFC structures.

Effect of Feed Water pH on the Partitioning of Alkali Metal Salts from Aqueous Phase into the Polyamide Active Layers of Reverse Osmosis Membranes.

The partitioning of solutes into the polyamide active layers of reverse osmosis (RO) membranes is a key membrane property determining solute permeation. Quantification of partition coefficients and their dependence on feedwater pH would contribute to the development of predictive transport models of contaminant transport through RO membranes; however, neither solute partitioning nor the effect of feed solution pH on partitioning has been thoroughly characterized in the literature. Accordingly, we characterized the partitioning of all chloride salts of alkali metals (CsCl, RbCl, KCl, NaCl, and LiCl) from the aqueous phase into the polyamide active layers of five polyamide RO membranes, including one prepared in-house and four commercial membranes. We evaluated the effect of pH on the partitioning of alkali metal salts and whether the effect of pH on salt partitioning and rejection is consistent with Donnan theory predictions. Results showed that for all membranes, the partition coefficients of all salts were less than one and did not differ substantially among RO membranes. Results also indicated that for all membranes tested, Donnan theory provided an appropriate theoretical framework to estimate the effect of pH on salt partitioning (evaluated for all chloride salts of alkali metals) and salt rejection (evaluated for NaCl). Thus, we conclude that changes in salt rejection resulting from feed solution pH are primarily driven by changes in salt partitioning with comparatively small changes in salt diffusion coefficients.