Structure, Mechanism And Separation Properties Of New Nf Membranes

Common nanocomposite NF membranes can be classified as surface coated NF membranes (SLN), polyamide layer nano-doped membranes (TFN) and nanomaterial hybrid matrix membranes (TFCn). The structure and main properties of these three types of membranes are shown in Table 2. SLN membranes can be obtained by grafting or deposition of nanomaterials on the surface of polyamide layers. The membrane surface modified by nanomaterials can substantially improve the membrane's resistance to contamination or antibacterial properties, thus increasing the membrane cleaning efficiency and extending the membrane's service life". 

However, the additional surface coating inevitably increases the resistance of the membrane, thereby reducing the water flux through the membrane and increasing the operating pressure and energy consumption. liu et al. used amphoteric polymer brushes to modify the surface of commercial membranes, substantially improving the hydrophilicity of the membrane surface. The adhesion of the modified TFC membranes to bovine serum proteins was significantly reduced, substantially improving the membrane's resistance to contamination. In addition, due to the increased hydrophilicity of the membrane surface, the modified composite membranes also significantly reduced the dissolution-diffusion process of hydrophobic contaminants such as endocrine disruptors or pesticides, improving the removal of hydrophobic contaminants. guo et al. used dopamine coating to modify NF90 commercial NF membranes and found that the increased hydrophilicity of the membrane surface contributed to the removal of endocrine disruptors.

In the future, TFC membrane design should not only consider the removal of salt but also focus on the selective removal of contaminants to ensure safe effluent reuse. The amide bonds in the polyamide layer of conventional NF membranes are susceptible to chlorine attack and degradation, resulting in reduced removal rates. Surface coated or grafted TFC membranes avoid direct contact between the polyamide layer and chlorine, thus improving the chlorine resistance of the membrane.

In addition to grafting/coating nanomaterials onto the membrane surface, high performance nano-doped TFN membranes can also be prepared by doping the particles within the polyamide layer.JEONG et al. incorporated zeolite nanomaterials (pore diameter of 0.4 nm) into the polyamide layer during interfacial polymerisation to improve the water flux of the thin film composite membrane. 

The water flux of the zeolite-modified TFN membranes was twice that of the control group, but the NaCl retention rate of the TFN membranes was not significantly reduced. Subsequently, researchers carried out research on the preparation and application of TFN membranes and successfully commercialised TFN membranes (e.g. LG's NanoH2O desalination membrane and AQUAPORIN's Aquaporin A/S1 forward osmosis membrane based on protein water channels). In addition to desalination applications, TFN membranes also have great potential in water treatment and water reuse applications. In terms of membrane selectivity, the modified membranes showed some decrease in salt removal rate, but their retention of endocrine disruptors was significantly improved, demonstrating the great potential of MOF-doped TFN NF membranes in water reuse. However, the TFN membranes also have some obvious shortcomings, for example, their long-term stability after doping with nanoparticles and the assessment of their toxicological properties. 

Also, it was found that after 10 years of use TFN membranes show limited improvement in performance (e.g. limited improvement in water permeability) and a decrease in selectivity (possibly due to the agglomeration of nanomaterials creating defects within the PA layer). Another common nano-doped NF membrane is the addition of nanomaterials to the casting solution to produce a nano-doped substrate by phase conversion, followed by interfacial polymerisation to produce a nanomaterial hybrid substrate composite membrane (TFCn).JEONG et al. first added zeolite to a polyalum substrate to produce a zeolite-based TFCn membrane, and the modified TFCn membrane had higher water flux and better compression resistance ( compaction). 

Since then, TFNn membranes have been used extensively in forward osmosis, where the increased hydrophilicity and porosity of the substrate reduces the intra-membrane concentration polarisation (ICP) and increases the water flux of the membrane. However, some studies have shown that nano-doped substrate materials have a greater impact on the interfacial polymerisation process, leading to a decrease in membrane selectivity.