GO and surfactant assisted regulation of polyamide nanofiltration membranes for improved separation performance

Fabricating nanofiltration membranes with high water permeance and selectivity by tailoring the structure and morphology is crucial to efficient water purification. In this work, the fabrication of thin film nanocomposite (TFN) nanofiltration membranes by combining two methods, the surfactant-assembly interfacial polymerization (SARIP) and the addition of graphene oxide (GO) or ionic liquid functionalized graphene oxide (GO-IL) for the preparation of the selective PA layer on a novel porous substrate is reported. For the preparation of the porous substrate, mechanically robust, aromatic polyethers containing polar pyridine units (PDSPP) were chosen targeting to improved hydrophilicity and thus enhanced water permeability. The anionic sodium dodecyl sulfate (SDS) surfactant was used to regulate the interfacial polymerization thereby enabling the formation of dense PA layers with high cross-linking degree and consequently high salt rejections. On the other hand, the addition of GO/GO-IL nanosheets could impart hydrophilicity and antifouling properties. The novel porous PDSPP substrate was selected for the fabrication of TFN membranes due to its improved water permeability and superior salt rejection and increased hydrophilicity compared to commercial PSf substrate. The detailed study of the fabricated TFN membranes by ATR-FT-IR, SEM, AFM, XPS, contact angle measurements revealed the formation of polyamide selective layers with rougher surfaces, increased hydrophilicity and high cross-linking degrees (from 44 % to 94 %), resulted from the synergistic effect of SDS and GO incorporation. Moreover, the prepared TFN membranes exhibited high salt rejections while maintaining a moderate water permeability. In particular, the membrane containing the neat GO (TFNGO50) displayed excellent Na₂SO₄ and MgSO₄ rejections (98.8 % and 99.5 %, respectively) while the NaCl rejection was 47.0 % and a water permeability value of 4.2 L m^-2h⁻¹ bar⁻¹. The divalent salt rejections were among the highest values reported in the literature in comparison with commercial NF membranes and TFN membranes containing GO/functionalized GO. When GO-IL nanosheets were embedded in the PA layer, the water permeability was slightly improved while the salt rejections were significantly reduced compared to TFNGO50. Lastly, the incorporation of GO enhanced the anti-fouling properties of the prepared membranes due to increased hydrophilicity of the PA surface.