Novel porous composite hydrophobic/hydrophilic polysulfone membranes for desalination by direct contact membrane distillation

Abstract

Novel composite membrane distillation membranes were prepared by blending the hydrophilic polysulfone with hydrophobic surface modifying macromolecules (SMMs). Three different types of SMMs were tested. These SMMs were synthesized and characterized for fluorine content, molecular weights and glass transition temperature. Phase inversion method in a single casting step was used to prepare the composite membranes. The membranes were characterized by means of different techniques such as contact angle measurement, gas permeation test, liquid entry pressure of water and scanning electron microscopy. Finally, these membranes were tested for desalination by direct contact membrane distillation (DCMD). Different membrane preparation conditions affecting membrane morphology, structure and DCMD performance were investigated. The parameters studied were the SMMs type, polysulfone concentration, solvent type and non-solvent additive concentration in the casting solution. Attempts linking the membrane morphology to its DCMD performance have been made. it was found that increasing the polymer concentration or the non-solvent additive concentration decreased the permeate flux of the porous composite hydrophobic/hydrophilic membranes since the liquid entry pressure of water increased and the ratio of the membrane pore size time the porosity over the effective pore length (r epsilon/L(p)) decreased. Furthermore, the stoichiometric ratio of the SMMs, type of SMMs, was found to affect considerably the characteristics and permeate flux of the composite membranes. In general, the composite membranes with higher liquid entry pressure of water exhibited smaller permeate fluxes. Moreover, the obtained results were compared to those of a commercial polytetrafluoroethylene membrane and it was observed that some of the SMMs blended polysulfone membranes achieved better DCMD fluxes than those of the commercial membrane. A permeate flux 43% higher than that of the commercial membrane was achieved with 99.9% NaCl separation factor.