Preparation, Characterization and Salt Rejection of Negatively Charged Polyamide Nanofiltration Membranes

Four thin-film composite (TFC) polyamide negatively charged nanofiltration (NF) membranes (NF-PS-1, NF-PS-2, NF-PS-3 and NF-PS-4) were prepared via interfacial polymerization technique which involved the formation of a polyamide thin film at the interfacial polysulfone (PS) microporous substrate between two immiscible liquid phases; an aqueous phase containing piperazine and an organic phase containing trimesoyl chloride (TMC = 0.25, 0.50, 1.00 and 2.00 wt%). Correlations of the intrinsic properties of these four NF filtration membranes, in regarding to the physical structures, surface negative charge density, pure water flux, pore size and the NF filtration performances, on the composition of the weight-% of TMC in the organic phase were investigated. Experimental results clearly demonstrated that NF membrane prepared from using 1.0 wt-% of TMC in hexane phase (i.e., NF-PS-3) gave a much higher salt rejections on salt containing negative divalent salts like Na2SO4 and MgSO4; the maximum rejection of Na2SO4 and MgSO4 were both to be 99.6 % for NF-PS-3 (TMC = 1.0 wt-%) while the rejections of Na2SO4 and MgSO4 for the rest of the three NF membranes were in the order of NF-PS-2 (TMC = 0.50 wt-%) > NF-PS-1 (TMC = 0.25 wt-%) > NF-PS-4 (TMC = 2.00 wt-%). Effects of the pH of the feed and the applied filtration pressure on the MgSO4 rejection were also investigated for this series of nanofiltration membranes. Resumen. Se preprararon cuatro membranas de nanofiltración (NFPS-1, NF-PS-2, NF-PS-3, NF-PS-4) de película delgada (TFC) de poliamida cargadas negativamente, mediante la técnica de polimerización interfacial, la cual implica la formación de una película delgada en la interfase de un sustrato microporoso de polisulfona (PS) entre dos fases líquidas inmiscibles; una fase acuosa que contiene piperazina y una fase orgánica que contiene tricloruro de mesoilo (TMC) a diferentes concentraciones (0.25, 0.50. 1.00 y 2.00 % peso). Se investigaron las correlaciones entre las propiedades intrínsecas de las cuatro membranas de NF y la composición de la fase orgánica (% peso de TMC). Las propiedades estudiadas fueron; estructura física, densidad de carga superficial negativa, flujo a través de la membrana de agua pura, tamaño de poro y capacidad de filtración de las membranas. Los resultados experimentales demostraron claramente que la membrana de NF preparada con la solución que contiene 1.0 % de TMC (NF-PS-3) presenta mayor remoción de sales cuando tienen iones divale