pH-responsive nanofiltration membrane containing chitosan for dye separation

Nanofiltration (NF) membranes with adjustable permeability and selectivity are highly desired for the separation of small molecules. A novel pH-responsive NF membrane was synthesized via interfacial polymerization with a mixture of piperazine (PIP) and chitosan (CS) as the aqueous phase and trimesoyl chloride (TMC) in n-hexane as the organic phase. The effects of pH on the physicochemical structure and performance of the PIP-CS/TMC NF membrane obtained were studied in detail. The results showed that the surface physicochemical structure and performance of the membrane could be changed by adjusting the pH of feed solution. Specifically, the surface roughness, hydrophilicity and pore size of the NF membrane were increased at high pH values. The pure water flux through the NF membrane remained stable in the pH range 2–8 but increased exponentially from 248.2 to 643.8 L m−2·h−1 as the pH was increased from 8 to 12. Furthermore, the PIP-CS/TMC NF membrane demonstrated more than 99.0% rejection for Congo red, methyl blue, coomassie brilliant blue G250 in the pH range 2–12, while low rejection was observed for methyl orange and inorganic salts (Na2SO4, MgCl2) at pH 12. The NF membrane exhibited excellent separation performance of the mixtures of anionic dye/salt or dye/dye with different molecular weights at pH 12. The reversibility of pH response with a high responsive coefficient of the NF membrane was also proved. This work provides a new route for NF membrane in the separation of some dye/salt and mixed dyes systems.