Author: Site Editor Publish Time: 2023-01-09 Origin: Site
In addition to nanomaterial-doped NF membranes, the preparation of high-performance polyamide NF membranes with an interlayer structure has received increasing attention in recent years. The interlayer structure of this membrane can be uniformly covered/deposited onto the substrate to form a multilayer structure before the formation of a polyamide separation layer by conventional interfacial polymerisation.By introducing the concept of sacrificial layer, Karan et al. used cadmium hydroxide nanowires to modulate the interfacial polymerisation reaction and diffusion rate of m-phenylenediamine monomer to prepare a polyamide membrane with an ultra-thin separation layer, enabling the water flux of the TFNi membrane to exceed that of commercial membranes by about 2 orders of magnitude, while the selectivity of the membrane is not affected, breaking the membrane permeability-selectivity trade-off. in recent years, more and more researchers are constructing composite NF membranes with zero-dimensional nanoparticles, one-dimensional nanowires, two-dimensional nanosheet layers and other interfacial coatings as an intermediate layer through surface coating, covalent bonding, co-deposition, in situ growth and evaporation.
Compared to conventional polyamide NF membranes, composite membranes based on an interlayer (TFNi) offer significant improvements in water flux (typically a 2 to 10-fold increase) and selectivity by the following mechanisms:
1) TFNi membranes have a highly permeable interlayer structure that optimises the water transport pathway (inflow effect) of the polyamide layer. Specifically, for conventional NF membranes without an interlayer, under the assumption that the substrate is impermeable, water molecules passing through the polyamide separation layer need to pass into the pores of the substrate before being collected. As a result, the polyamide layer at a distance from the pores must follow an inclined path through the higher resistance polyamide layer. With the addition of the interlayer, according to the hydraulic minimum resistance model, water molecules will preferentially take a shorter path through the higher resistance polyamide layer and a longer path through the lower resistance loose interlayer, resulting in the actual effective transport path of the TFNi membrane being one order of magnitude lower than that of a conventional NF membrane without an interlayer, thereby significantly increasing water flux.
2) Optimisation of the polyamide separation layer by the interlayer material. The interlayer changes the interfacial polymerisation reaction, eliminating the generation of polyamide within the substrate pores during traditional interfacial polymerisation and reducing the transmembrane membrane resistance. At the same time, the interlayer material also has the advantage of in situ improvement of the interfacial polymerisation reaction (e.g. controlling the monomer reaction rate or hindering the escape of nanobubbles during interfacial polymerisation, thus forming more cavities within the polyamide separator layer), resulting in a better polyamide selective layer (e.g. thinner and higher crosslinking), which further enhances the water flux and selectivity of the TFNi membrane.
Yang et al. show that TFNi membranes with high fluxes can reduce energy consumption by up to 80% in water reuse applications where the raw fluid osmotic pressure is low, while improving the retention of trace contaminants. However, existing research on TFNi membranes has focused on water-salt selectivity, and future research is needed on the removal of trace contaminants, heavy metals, disinfection by-products, etc. The cost of TFNi membranes, their long-term stability and the leaching of doped nanomaterials need to be fully considered. Future research can quantify the water flux and water/solute and solute/solute transfer mechanisms of the interlayer to TFNi membranes through more simulation work, and investigate the membrane contamination and chlorine resistance based on the interlayer to promote its application in water treatment and water reuse.