Hexane Treatment to Facilely Tailor Polyamide Nanofiltration Membrane Performance: The Critical Role of Treatment Duration

Abstract

Facile methods for simultaneously tailoring various properties of nanofiltration (NF) membranes are of great significance for fit-for-purpose membrane design and thus achieving more efficient NF applications. This study revealed that, when preparing polyamide (PA) thin-film composite (TFC) NF membranes, a simple hexane treatment right after the interfacial polymerization reaction could substantially alter membrane properties and enhance separation performance for drinking water treatment, with the degree particularly dependent on the treatment time even in the magnitude of tens of seconds. Hexane rinsing for 5 s could lead to a thinner active layer and achieve more improvement (>60%) of water permeance, while a more effective reduction (∼70%) of membrane surface negative charge density could be realized by hexane soaking for a prolonged time. These property variations rendered the hexane-treated membranes more competitive than representative commercial and lab-made conventional PA membranes for hardness removal. Moreover, the membrane rejection performance for organic micropollutants could be maintained after hexane treatments of varying durations due to the barely unaltered membrane pore sizes. The involved mechanisms include removal of loosely polymerized PA segments and unreacted monomers from the membrane surface and the provision of a solvent environment for the further cross-linking of residual monomers. This study provides a facile strategy and mechanistic insights for the development of TFC membranes with tailored properties for efficient drinking water treatment.

Evaluating Post-Treatment Effects on Electrospun Nanofiber as a Support for Polyamide Thin-Film Formation

“…It is revealed that the pressure-normalized flux was greatly enhanced while retaining high salt rejection at about >95% ( Figure 15 ) compared to TFNC without an n-hexane rinse. The n-hexane rinse is beneficial to residual TMC monomers, creating a thinner PA-selective layer [ 67 ]. Based on XPS analysis ( Figure S5 ), after the n-hexane rinse, the O/N ratio dropped while the amide/carboxyl ratio increased ( Table S3 ).…”

“…compared to TFNC without an n-hexane rinse. The n-hexane rinse is beneficial to residual TMC monomers, creating a thinner PA-selective layer [67]. Based on XPS analysis (Figure S5), after the n-hexane rinse, the O/N ratio dropped while the amide/carboxyl ratio increased (Table S3).…”

Distinct Efficacies of Interlayers in Tailoring Polyamide Nanofiltration Membrane Performance for Organic Micropollutant Removal: Dependent on Substrate Characteristics

“…Nanofiltration (NF) has great promise for water treatment applications but is still challenged by the emergence of a variety of organic micropollutants (OMPs) in water. − Conventional thin-film composite (TFC) nanofiltration membranes with a common molecular weight cutoff (MWCO) of 200–300 Da have high rejections of natural organic matters and divalent ions but are not effective enough at removing small-molecule OMPs with different physicochemical properties. , The intrinsic broad free volume distribution of polyamide (PA) selective layers also leads to a trade-off effect between permeability and selectivity of membranes. , To address this issue, extensive research has been conducted to optimize the structure and enhance the performance of TFC PA membranes by various approaches such as exploring novel reactive monomers, regulating the interfacial polymerization (IP) conditions, and introducing functional nanomaterials. − Among them, the construction of an interlayer structure between the polyamide film and the porous substrate has shown advantages in tailoring membrane properties and performance for targeted water treatment applications. , …”