Full elemental depth-profiling with nanoscale resolution: The potential of Elastic Recoil Detection (ERD) in membrane science

Fabrication of desalination membranes by interfacial polymerization: history, current efforts, and future directions

Membrane desalination is a promising technology for addressing the global challenge of water scarcity by augmenting fresh water supply. Continuous progress in this technology relies on development of membrane materials. The state-of-the-art membranes used in a wide range of desalination applications are polyamide thin-film composite (TFC) membranes which are formed by interfacial polymerization (IP). Despite the wide use of such membranes in desalination, their real-world application is still hampered by several technical obstacles. These challenges of the TFC membranes largely stem from the inherent limitations of the polyamide chemistry, as well as the IP reaction mechanisms. In the past decade, we have witnessed substantial progress in the understanding of polyamide formation mechanisms and the development of new IP strategies that can potentially lead to the redesign of TFC membranes. In this Tutorial, we first present a brief history of the development of desalination membranes and highlight the major challenges of the existing TFC membranes. We then proceed to discuss the pros and cons of emerging IP-based fabrication strategies aiming at improving the performance of TFC membranes. Next, we present technical obstacles and recent efforts in the characterization of TFC membranes to enable fundamental understanding of relevant mechanisms. We conclude with a discussion of the current gap between industrial needs and academic research in designing high-performance TFC membranes, and provide an outlook on future research directions for advancing IP-based fabrication processes.

Elemental Depth Profiling of Chlorinated Polyamide-Based Thin-Film Composite Membranes with Elastic Recoil Detection

The chlorine resistance of nanofiltration and reverse osmosis membranes is of high importance in the water treatment industry. Elastic recoil detection (ERD) is now presented as a powerful tool to uniquely provide elemental depth profiles, including hydrogen, of NaOCl-treated polyamide-based thin-film composite (TFC) membranes. The influence of pressure, pH, and chlorine feed concentration on the volume-averaged Cl uptake, the location of chlorine throughout the membrane, and the z-gradient in the Cl/N ratio is demonstrated. The results suggest that (i) higher volume-averaged Cl uptakes are achieved at higher chlorine doses and at acidic pH; (ii) chlorination is mostly restricted to the top layer; (iii) a gradient in the Cl/N ratio exists along the membrane depth; and (iv) the shape of this gradient is influenced by the chlorination pH and the applied pressure. Conclusions on the chlorination mechanisms could also be deduced. Conversely, no conclusive relationships between H fractions and Cl uptake could be drawn, even though changes in the H content after chlorination were observed. To corroborate these results and fully exploit the potential of ERD, the exact microstructure of the (chlorinated) TFC membranes should be better understood.