Nanofiltration (NF) membranes have been developed recently and adept for the separation of small neutral and charged solutes in aqueous solutions. NF membranes have two important features in their actual applications . One is the intermediate molecular weight cut-offs (MWCO) between reverse osmosis (RO) membranes and ultrafiltration (UF) membranes, which ranges from 200 to 2,000; the other is salt rejection caused by the charge effect due to their materials. They can be identified into the sieving (steric-hindrance) effect and the charge (electrostatic) effect from viewpoint of membrane separation mechanism. Solutes having larger molecular weight (Mw) than the MWCO of a membrane are rejected almost by the membrane and the ones having lower Mw than the MWCO of a membrane will permeate easily through the membrane. That is so called the sieving effect. Thus solutes having different Mws can be separated basing on sieving effect. The charge effect of a membrane refers to the electrostatic interactions between ions and the membrane. The membrane is charged and mostly negatively charged since the thin films of NF membranes are made of polyelectrolytes. Ions having the same sign of charge as the membrane charged are excluded, and ions having the opposite sign of charge can be attracted. Separation of electrolytes' ions having different signs and valences can be manipulated according to the rejection differences by the membrane. Researches on NF membranes are categorized into two types. One is on NF separation mechanism and the modeling; and the other is on the application of NF membranes and the process design. In this paper, we will focus the application of NF membranes and mention in detail the two interesting application examples of NF membranes, that is, the separation of amino acids and peptides and the separation and purification of oligosaccharides.
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