Molecular sieving through 'layer-by-layer' self-assembly of polyelectrolytes and highly crosslinked graphene oxide

Lack of access to potable water and abating levels of ground water level demands the reuse of unconventional water sources after remediating it in a sustainable way. In this context, purifying brackish, land and sea water seems a feasible solution to the ever-growing population. In this work, a novel composite membrane was fabricated by 'layer-by-layer' self-assembly of poly-dopamine (PDA) and polystyrene sulfonate (PSS) supported on a highly crosslinked graphene oxide (GO) membrane to sieve ions to purify contaminated water as well as enhance the resistance towards chlorine. This GO membrane was sandwiched between layers of various nanoporous polyvinylidene difluoride (PVDF) membranes obtained by selectively etching out the PMMA component from the demixed blends. The blend membranes were designed following the melt-extrusion process and subsequent quenching to facilitate confined crystallization of PVDF and selective etching of PMMA. The membranes with different pore sizes were tuned on varying the composition in blends and a gradient in microstructure was achieved by stitching the membranes. Pure water flux, salt rejection, dye removal, and antibacterial activity were performed to study the membrane's efficiency. The GO membrane was chemically crosslinked with methylenediamine to impart dimensional stability and to enhance rejection efficiency through the nanoslits that GO offers. Besides effective rejection, the sandwiched membrane was modified with ‘layer-by-layer’ self-assembly of polyelectrolytes on the surface to improve the chlorine tolerance performance. This strategy resulted in an excellent salt (about 95% and 97% for monovalent and divalent ion, respectively) and dye rejection (100% for both cationic and anionic dye), besides facilitating excellent chlorine tolerance performance. Moreover, this modified membrane showed superior antifouling properties (flux recovery ratio is more than 90%) and excellent antibacterial performance (near about 3 log reduction). Thus the concept of using layer-by-layer self-assembly of polycations (PDA) and polyanions (PSS) onto a hierarchical chemically modified GO sandwiched PVDF membrane proved to be a productive strategy to purify contaminated water. Thus the membrane can be a potential candidate for domestic as well as industrial application.