In situ assembly of a graphene oxide quantum dot-based thin-film nanocomposite supported on de-mixed blends for desalination through forward osmosis

In this work, in order to enhance the desalination performance, a unique thin-film composite membrane was designed by in situ assembly of a polyamide (PA)–graphene oxide quantum dot (GQD) framework. This unique assembly was supported on a templated hierarchical porous membrane derived from the de-mixing of a classical UCST (upper critical solution temperature) system consisting of polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA). The de-mixing was achieved by melt processing the blend above the UCST (in the miscible state) and quenching it below UCST. The pore size was controlled by varying the composition in the blends and by etching the PMMA phase. A sandwich architecture was developed by stacking different membranes using polyacrylic acid, as an adhesive, to achieve a gradient in pore size. Pure water flux, dye removal, and desalination experiments were carried out to study the efficacy of this strategy. The stacked membrane (used here as control) showed moderate dye rejection (about 50%) and poor desalination performance. In order to improve the desalination performance, the membranes were suitably modified by depositing a layer of polyamide (PA)–GQD framework obtained using interfacial polymerization. This strategy resulted in efficient salt rejection (more than 94% and 98% for monovalent salt and divalent salt, respectively) when studied through a pressure enhanced osmosis process using a 1000 ppm draw solution, and dye rejection (more than 90% and 85% for methylene blue (MB) and Congo red (CR), respectively) was studied through a cross-flow experimental set up using a 10 ppm feed solution @ 60 psi. Moreover, the antifouling properties of the PA–GQD modified membranes were superior (80%) to those of the control stacked membrane.