Development of Nanofiltration Membrane

Population growth and water environment pollution have exacerbated the water crisis, seriously affecting the health and sustainable development of the economy and society, becoming one of the most important issues facing the 21st century. With the improvement of people's living standards, the quality of drinking water has also gradually increased, and obtaining quality drinking water is an urgent demand of the people. Compared with traditional water treatment processes, membrane technology has the advantages of small footprint, high treatment efficiency and good effluent quality, which has attracted much attention in the field of water treatment and water reuse.

Nanofiltration (NF) is highly efficient in retaining polyvalent salts and organic pollutants in water, but has a relatively low retention rate for monovalent salts, thus providing selective separation of monovalent and polyvalent salts in mixed systems, and has the advantage of low energy consumption compared to RO, and has been widely used in desalination, drinking water treatment, water reuse, industrial wastewater treatment, etc. NF mostly uses a thin-film composite (TFC) membrane structure, consisting of a polyamide (PA) desalination layer, an ultrafiltration (UF) layer (often polyalum or polyether alum UF membranes and a non-woven support layer. NF membranes have a pore size between RO and UF membranes and have a high water permeability of 5-20 L/(m3-h-bar)]. Compared to RO membranes, NF membranes have a retention rate of over 90% for divalent and multivalent ions, and 10% to 90% for monovalent salts.

However, existing commercial NF membranes also have very obvious drawbacks. For example, increasing the water flux of a membrane generally reduces its solute retention rate due to the interdependent "trade-off" phenomenon between permeability and selectivity. At the same time, NF membranes have a history of excessive removal (>90%) of divalent ions (such as calcium, magnesium and other mineral ions), however, the excessive removal of these minerals can lead to the emergence of 'unhealthy' drinking water. At the same time, the removal rate of heavy metals, fluoride and trace contaminants, which are harmful to humans, is generally low, further threatening human health. In addition, polyamide-based NF membranes are less resistant to chlorine, and in order to ensure the integrity of the membrane, residual chlorine needs to be removed from the membrane process, but this will increase the corresponding operating costs.

In recent years, with the development of nanotechnology, nano-doped NF membranes have received extensive attention and research. Some of the more typical nano-composite membranes include surface coated NF membranes (SLN), polyamide layer nano-doped membranes (TFN), mixed matrix membranes of nanomaterials (TFCn) and intermediate layer based modified NF membranes (TFNi). These new NF membranes offer significantly enhanced water permeation fluxes and selective removal of salts and specific organic pollutants, for example, due to hydrophobic effects, hydrophilic nano-silver and metal organic framework materials (MOFs) doping can enhance the selective removal of endocrine disruptors by the composite membranes, showing good prospects for applications in water treatment and water reuse.