Dear Dziban,

Nanofiltration is a membrane filtration-based method that uses nanometer sized cylindrical through-pores that pass through the membrane at 90°. Nanofiltration membranes have pore sizes from 1-10 nanometers, smaller than that used in microfiltration and ultrafiltration, but just larger than that in reverse osmosis. Membranes used are predominantly created from polymer thin films. Materials that are commonly use include polyethylene terephthalate or metals such as aluminum.Pore dimensions are controlled by pH, temperature and time during development with pore densities ranging from 1 to 106 pores per cm2. Membranes made from polyethylene terephthalate and other similar materials, are referred to as “track-etch” membranes, named after the way the pores on the membranes are made. “Tracking” involves bombarding the polymer thin film with high energy particles. This results in making tracks that are chemically developed into the membrane, or “etched” into the membrane, which are the pores. Membranes created from metal such as alumina membranes, are made by electrochemically growing a thin layer of aluminum oxide from aluminum metal in an acidic medium.

Keeping in mind that NF is usually part of a composite system for purification, a single unit is chosen based off the design specifications for the NF unit. For drinking water purification many commercial membranes exist, coming from different chemical families, having different structures, chemical tolerances and salt rejections and so the characterization must be chosen based on the chemical composition and concentration of the feed stream.

NF units in drinking water purification range from extremely low salt rejection (

 The traditional NF membrane is a thin-film composite formed from piperazine (PIP) and tri-mesoyl chloride, typically on top of a polysulfone or polyethersulfone support.  Such a membrane yields >97% rejection of MgSO4.

A commercial example is the NF-270 from Dow: http://www.dow.com/en-us/markets-and-solutions/products/dowfilmtecnanofiltration8elements/dowfilmtecnf27040034i

Fabrication methodology: http://pubs.acs.org/doi/abs/10.1021/es303673p

Dear Dziban,

You can take PSF as the base polymer and DMF as solvent for membrane formulation. Now to get  a membrane in NF range you may dope Zinc chloride in the casting solution. The membrane shows good rejection of various salts. you may refer to the article for further details!

Dear Mr Naufal

Your question is not clear , do you  have a water problem to remove a high contamination by Cr ions or a manufacturing problem of a NF membranes ? the two fields of research are quite different ,usually we don't go to membrane manufacturing for each water problem that we face.

First, for or a polyvalent heavy metal  cation as Chromium  ( 3+) , all NF membranes will show good rejection. It remains to select the optimum one for your Cr concentration in the wide available range of rejection as mentioned above by Mr Karaman ,  who I am sure that he knows that  polymer RO  membranes ,after the obsolete CA one,  are non- porous.

As an example from our results the NF-270 proposed by Mr Werber above , whch is - as the whole series of Fimtech Dow membranes- a Polyamide Thin Film membrane,is perfect in such appication.

You may find useful to go through our book chapter "NF and Low Energy RO for Advanced WastewatersTreatment", Expanding Issues In Desalination ,Edited by Robert y. Ning,(2011).

Prof Dr Gamal Khedr