Development of polymeric membranes for forward osmosis

Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do grau de Mestre em Engenharia Química e Bioquímica

Forward osmosis (FO) using a semi-permeable membrane may be a viable alternative to reverse osmosis (RO) as a lower cost and more environmentally friendly desalination technology. To develop a FO membrane showing high water flux and high salt rejection, P84 Lenzing polyimide flat sheet membranes were fabricated by immersion precipitation from a casting solution containing dimethylformamide (DMF) as the solvent and 1,4-Dioxane (dioxane) as the co-solvent.. Different P84 concentrations as well as different solvent systems compositions were tried. The composition of the solvent system showed to provide control over the molecular weight cut-off and porosity of the resultant membranes. High water flux (2.18 kg.m-2.h-1) and improved salt rejection (95.2%) in FO were achieved by using 18 wt.% P84, DMF:Dioxane, 1:6, crosslinked for 4h by 1,6-Hexanediamine (HDA). Glycerol and Polyethylene glycol (MW 400 g.mol-1) (PEG) were used to store membranes after casting in order to prevent pores collapse. The best membrane fabricated, 18 wt.% P84, DMF:Dioxane, 1:6, showed better performance when stored in PEG than when stored in glycerol. Internal concentration polarisation is thought to be the cause for the experimental FO water fluxes, which were far lower than those theoretically predicted based on bulk osmotic pressure difference and membrane pure water permeability data.