Study On Polypiperazine-Amide/Polysulfone Composite Nanofiltration Membrane

A thin film composite (TFC) in the form of hollow fiber was prepared by interfacial polymerization using trimesoyl chloride (TMC) and piperazine(PIP) as two monomers onto a polysulfone ultrafiltration substrate. Its structure and retention performance of inorganic salt, low molecular organic matter were investigated. Desalination of dyes by the membrane was also studied.The polyamide membrane was made from hollow fiber ultrafiltration membrane with the molecular weight cutoff (MWCO) of 20000. The addition of triethylamine shortened the pre-press time and membranes with good and stable performance were obtained. The optimum conditions to produce high performance membrane are: concentration of piperazine: 2%; concentration of trimesoyl chloride (TMC): 0.5%; concentration of triethylamine:1%; interfacial polymerization time:30s. Using 0.1% MgSO₄ as testing solution, the salt rejection of the membrane prepared under this condition reaches 99%, and the permeate flux is as high as 30 Lm^-2/h under the operating pressure of 0.4MPa.The morphology of the composite membranes was visualized and analyzed using attenuated total reflection-Fourier transform infrared spectroscopy, scanning electrical microscopy and atomic force microscopy. The results showed that the composite membrane consisted of two distinctive layers with a dense layer coated on top of the porous layer, which could be accompanied by a change of the surface morphology. Higher thickness of the top layer implied higher salt rejection flux but lower permeate flux. AFM images reflected the true surface morphology of the membrane and higher average roughness implied higher permeating flux.It was concluded from the experiments that (1) the membrane showed different rejections toward different inorganic salts (NaCl, Na₂SO₄, MgSO₄), which characterized nanofiltration membrane and conformed the fact the polyamide membrane we made was negatively charged; (2) the salt concentration and operating pressure played great role in the separation process, the salt rejection went up as the operating pressure increased and became lower as the salt concentration increased; (3) as for uncharged solute, the molecular weight related to the retention.For most experiments conducted, permeate samples were colorless and dyestuff rejections of greater than 95% were achieved indicating that the acidic dyestuffs were effectively rejected. Apparent salt rejection decreased with the increase of salt concentration and even dropped down under zero depending on the salt concentration, which suggested that NF membranes could be applied not only to wastewater treatment but also to dye production due to the amazingly effective purification effect by negative salt rejection.