Forward osmosis membranes tailored by hydrogel coatings

Dissertação para obtenção do Grau de Mestre em Engenharia Química e Bioquímica

Forward osmosis (FO) is a promising process to substitute reverse osmosis (RO), as a lower cost and more environmentally friendly desalination process. However, FO still presents some drawbacks, in particular the several internal concentration polarization (CP) effects and insufficient salt selectivity. In order to overcome these disadvantages, this study focuses on the use of hydrogel surface-coated FO membranes to minimize internal CP effect in water purification, and also to improve membrane salt rejection. For this, a series of crosslinked poly(ethylene glycol) (PEG)-based hydrogels were synthesized, by the photopolymerization of poly(ethylene glycol) diacrylate (PEGDA) and the monomer (PEG) in the presence of a photoinitiator. The water uptake and salt permeability of the resulting films were controlled by manipulating the composition ratio of PEGDA and the monomer PEG, by varying the water content in the prepolymerization mixture and the UV-exposure time. High water uptake and low salt permeability values were observed for the films prepared with 50wt% of water content(50%PEGDA). The hydrogels were applied using different techniques (pressure, soaking and coating) to a cellulose acetate (CA) membrane prepared by phase inversion. However, only one technique was effective, surface coating. The CA membranes coated with these hydrogels materials showed an improvement in NaCl rejection (≅100%) and in some cases an enhancement of 100 and 120% of the original water flux (50% PEGDA coating on the active layer and on the porous support, respectively; in PRO mode). The 50%PEGDA coated membrane (with a coating on the porous support) has also shown reduction of the internal CP effects.