Antifouling membrane based on sodium alginate coated polyamide thin film composite for desalination of brackish water

Polyvinyl alcohol and sodium alginate hydrogel coating with different crosslinking procedures on a PSf support for fabricating high-flux NF membranes

Polyvinyl alcohol (PVA) and sodium alginate (SA) hydrogel-coated nanofiltration (NF) membranes with high-flux and permselectivity were prepared. The coating of PVA and SA hydrogel selective layer on a porous polysulfone (PSf)/non-woven fabric ultrafiltration substrate membrane was conducted through different three procedures including pre-crosslinking, in-situ crosslinking, and immersing crosslinking and the use of glutaraldehyde as a crosslinking agent. The properties and performances of all types of the prepared membranes were evaluated through ATR-FTIR spectroscopy, AFM, SEM, zeta potential, contact angle, and cross-flow permeation tests. The immersing technique resulted in the formation of TFC membranes with higher hydrophilicity, smoother surface layer, higher negative charge, higher permeation flux, higher salt rejection and better anti-fouling performance. Also, the higher negative surface charge of the immersing coated TFC membranes due to dissociation of hydrophilic functional groups of the PVA and SA hydrogel selective layer resulted in higher As(III) rejection. SA coated NF membrane through immersing method exhibited a higher pure water permeability of 11.2 L m^-2 h^-1 bar^-1, NaCl, MgSO₄, and Na₂SO₄ rejection of 38.2%, 55.1%, and 70.4%, respectively with As(III) rejection of 60.6%. All types of the PVA and SA hydrogel-coated PSf membranes possessed improved fouling resistance to BSA aqueous solution, superior anti-fouling performance was obtained with SA hydrogel coating through immersing method. Such optimum membranes indicated high stability in the long-term experiments. This study showed that the coating of the SA hydrogel layer on a PSf support through immersing method could be a promising candidate for fabricating high-flux NF membranes.

Surface modification of polyamide reverse osmosis membranes with small-molecule zwitterions for enhanced fouling resistance: a molecular simulation study

Surface modification with small-molecule zwitterions is experimentally proved to be an effective solution to improve the antifouling performance of polyamide membranes. However, there is no comprehensive understanding of their microscopic mechanism. In order to address this issue, in this work we constructed two atomistic models, PA (a pure polyamide membrane) and QDAP-PA (a polyamide membrane surface-modified with QDAP), where QDAP was a zwitterion that was prepared by 2,6-daaminopyridine quaternized with 3-bromopropionic acid experimentally. Density functional theory was adopted to elucidate the variations in the electrostatic potential before and after modification. Then, equilibrium molecular dynamics (EMD) simulations were conducted to investigate the structure and hydrophobic/hydrophilic nature of the membrane surface in the two models. Finally, we introduced two typical organic foulants, sodium dodecyl sulfonate (SDS) and dodecyl trimethyl ammonium chloride (DTAC), to evaluate the antifouling performance of the membranes with the umbrella sampling method. The analyses of the membrane structure and properties show that surface modification with small-molecule zwitterions can densify the membrane surface as well as enlarge the distribution of electrostatic potential on the membrane surface. Water molecules tend to have more interactions with the membrane and more hydrogen bonds near the membrane surface are observed in QDAP-PA. The antifouling test supports that QDAP-PA shows a better antifouling performance, as the surface-modified membrane exhibits a stronger resistance to SDS and DTAC. Even if the foulant is adsorbed to the membrane surface, the denser interface region can prevent a further pollution of the foulant. Also, the free energy needed during the process for QDAP-PA to desorb a foulant is relatively small, indicating that this kind of membrane is easy to clean. The current work might provide a comprehensive understanding of the enhanced fouling resistance of polyamide membranes after surface modification with small-molecule zwitterions.