Studies on the properties of RO membranes for salt and boron removal: Influence of thermal treatment methods and rinsing treatments

The physicochemical properties and performance of thin film composite (TFC) membranes depend on its synthesis conditions. In this study, TFC membranes were fabricated using interfacial polymerization (IP) method and the effects of heat treatment methods and post-IP rinsing (prior to heat curing step) on the membrane performances were investigated. Keeping the substrate in minimal heat exposure could prevent substrate pore annealing that potentially reduces the membrane water permeability. Evidently, the membranes with only polyamide (PA) layer being heat-treated exhibited > 250% enhancement in pure water flux (PWF) compared to the membranes where both PA and substrate layer were heat-treated. Also, the membranes rinsed with pure n-hexane tended to display higher PWF without significantly decreasing solute rejection, possibly due to the reduced membrane cross-linking following the removal of unreacted monomers and excess solvent from the membrane surface. The membrane performance became practically the same after post-IP rinsing, regardless of the solvent used in the IP reaction, suggesting that the disparity in the membrane performance is mainly caused by the difference in solvent evaporation rate during heat treatment. The variations in solvent surface tension and viscosity during the IP reaction meanwhile did not play a key role affecting PA layer chemistry and performance.