Shortage and contamination issues of water resources induced by population explosion and industrialization, make the global economic and social sustainable development face huge challenges. Membrane technology, characterized by high energy efficiency and technological maturity energy efficient and technologically mature, has evolved to be a mainstream for desalination process. However, the permeation-selectivity upper-bound line of current polyamide composite membranes is limited by the intrinsic morphology and structure of the dense separation layer, resulting in a great energy consumption during real application. It has been confirmed that when the permeability of the current polyamide membrane is increased by 3 times, the number of pressure vessels of seawater reverse osmosis membrane (SWRO) can be reduced by 44%, and the number of pressure vessels of brackish water reverse osmosis membrane (BWRO) can be reduced by 63% 1. The corresponding consumption is also reduced by 15% and 46%. Therefore, innovating the structure and morphology of polyamide nanofilm to improve permeability and selectivity, and thus reduce the investment cost and operating energy consumption of RO and NF membranes, is a critical issue.
Recently we have made good progress in the design of novel thin-film composite (TFC) membranes. Based on PSF ultrafiltration substrate, we prepared an asymmetric polyamide nanofilm having a two-layer structure, of which the lower is a polyamide dendrimer porous layer, and the upper is a polyamide dense layer fabricated through conventional interfacial polymerization reaction by TMC and PIP. The polyamide dendrimer is a three-dimensional rigid macromolecule with abundant terminal amino groups and intramolecular pores. A large number of dendrimers were covalently assembled in situ on the surface of the polysulfone (PSF) support by diazotization-coupling reaction to form a uniform and stable hydrophilic layer. Then the surface polarity of the substrate has changed greatly compared with that of the pristine substrate. We further performed interfacial polymerization thereon (Figure 1), and obtained a thinner polyamide layer with highly ordered nanovoids (Figure 2), compared with the conventional one. In addition, we speculate that the morphology of the novel morphology may be related to the change of substrate polarity. The resulting membrane exhibits excellent water permeance and selectivity. We attribute the increase in permeance to the following. (i) The enhancement in pure water flux of the support caused by abundant intramolecular pores and improved hydrophilicity, facilitates the transmission of water molecules; (ii) The thinner polyamide dense layer aims to lower the resistance of water transport; (iii) The highly uniform, ordered hollow nano-stripes structure inside the polyamide dense layer can increase the effective water-permeable area, further improving the water permeance.
Figure 1 Formation of dendrimer porous layer and asymmetric polyamide nanofilm. a Structure of the 32-amines terminated polyamide dendrimer, and the as-prepared dendrimer porous layer assembled by diazotization-coupling reaction. b Schematic of asymmetric polyamide nanofilm prepared on the dendrimer porous layer.
Figure 2 a TEM image of the asymmetric polyamide nanofilm. b SEM cross-section image of the asymmetric polyamide nanofilm. The asymmetric polyamide nanofilm was prepared on the PSF-G4D-1 substrate, reaction time: 60s.
In conclusion, we break through the limitation of trade-off by tuning the structure and morphology of TFC membrane. We believe that this work will inspire the preparation of highly permeable and selective reverse osmosis (RO), organic solvent nanofiltration (OSNF), and pervaporation (PV) membranes.
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 Cohen-Tanugi, D., McGovern, R. K., Dave, S. H., Lienhard, J. H., Grossman, J. C. Quantifying the potential of ultra-permeable membranes for water desalination. Energ. Environ. Sci. 7, 1134–1141 (2014).