Improved performance of thin-film nanofiltration membranes fabricated with the intervention of surfactants having different structures for water treatment

Abstract

We fabricated thin-film composite nanofiltration polyamide membranes through the interfacial polymerization of two monomers: polyethyleneimine (PEI) and trimesoyl chloride (TMC). Three different types of surfactants with varied chemical structures were added to aqueous PEI solutions: sodium dodecyl sulfate (SDS), sodium octyl sulfate, and sodium dodecylbenzenesulfonate; the aim was to improve the rejection of divalent ions. Adding the surfactants led to increased isoelectric point, and, in turn, enhanced the membrane hydrophilic properties, heightened the membrane surface charge, and augmented the rejection of MgCl2. Among the three surfactants, SDS effected the most favorable conditions: the resultant composite membrane delivered the highest pure water flux (44.87 ± 3.22 L∙m−2∙h−1), and exhibited high rejections of divalent salts (RNa2SO4 = 92.26 ± 3.30%; RMgSO4 = 93.83 ± 3.22%; RMgCl2 = 94.15 ± 2.52%). SDS had long alkane chains and highly polar heads, which were conducive to bringing about an effective interaction between PEI and the polysulfone (PSf) support, thereby increasing the sorption of PEI in PSf. Accordingly, the reaction of PEI with TMC produced more amide and amine functional groups that were instrumental in raising the rejections of divalent salts and enhancing the membrane surface properties.