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Introduction to the structure of NF membranes and separation characteristics of commercial NF membranes

Author: Site Editor     Publish Time: 2022-09-09      Origin: Site

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NF membranes originated in the 1970s with the development of thin-layer composite RO membranes and in 1978, Cadotte et al. prepared the first thin-layer composite polyamide NF membrane using piperazine (PIP) and 1,3,5-homophthaloyl chloride (TMC). It has been widely used in water and wastewater treatment, biopharmaceuticals and food engineering because of its high efficiency in retaining multivalent ions, higher water flux, lower NaCl retention rate and low process pressure and energy savings compared to RO. 


Currently, most commercial NF membranes are polyamide composite membranes, consisting of a polyamide separation layer (50-100 nm), a porous support layer (~50 um) and a non-woven layer (~100 um). The non-woven layer at the bottom is used to increase the mechanical strength of the membrane, the porous support layer in the middle is usually made up of poly alum (Psf) or polyether alum (PES), while the polyamide separator layer, as the main retention layer, is mostly a polyamide material obtained by polymerising PIP with TMC.


NF membranes have a separation accuracy between UF and RO membranes, effectively retaining multivalent ions and small organic molecules with molecular weights of 200 to 2000 Da, and partially removing monovalent ions and substances with molecular weights <200. However, due to the differences in pore structure and surface properties of UF, NF and RO membranes, the separation performance of the three membranes differs significantly. 

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The separation mechanism of UF membranes is mainly size exclusion, which retains solutes with diameters larger than the membrane pore size, while RO membranes are generally considered to be dense and non-porous, following the dissolution-diffusion mechanism, which separates solutes and solvents through differences in diffusivity and solubility within the polymer. NF membranes, on the other hand, are generally considered to be charged separation membranes with a nanoscale microporous structure, with a large negative charge on the surface and a pore size mainly around 1 nm, corresponding to a molecular weight of 300-500 Da.


Currently, the separation mechanism of NF membranes is generally considered to be a combination of three factors, namely size sieving, the donnan effect and the dielectric effect. dielectric effect.) The donnan effect, also known as electrostatic repulsion, reveals the difference in the concentration distribution of charged substances in a charged membrane, i.e. an increase in the valence of ions with the same electrical properties as the membrane increases retention, while an increase in the valence of ions with opposite electrical properties to the membrane decreases retention. The dielectric effect is divided into 2 main areas: the solvationenergy barrier and the image forces. Both of these mechanisms arise due to the extreme spatial and nanoscale constraints present in NF membrane separations and are influenced by the repulsion of electrical charges. 


In simple terms, due to the different dielectric constants of water molecules in the nano-membrane pores and in the bulk, the solvation energy of ions entering the NF membrane pores from the bulk solution increases, i.e. an energy barrier is created which affects the partitioning of ions within and outside the membrane pores, which is related to the squared value of the ion valence state. On the other hand, due to the difference in the dielectric constants of water and polymer membrane materials, the two are affected by the ions in solution to produce different degrees of polarisation, which in turn produces a polarised charge at the interface between the two phases, and this phenomenon of ion-induced polarisation acting in turn on the ions themselves is known as the mirror force effect. 


As the dielectric effect is independent of the charged nature of the ions, it is often used to complement the high retention of multivalent ions (especially oppositely charged multivalent ions) by NF membranes.


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