Investigation of novel molecularly tunable thin-film nanocomposite nanofiltration hollow fiber membranes for boron removal

Thin-film nanocomposite (TFN) hollow fibers consisting of unique 3-dimensional (3D) hollow cup-like macrocyclic compounds with different intrinsic size-sieving cavities and functionality have been incorporated into the polyamide layer via interfacial polymerization on the inner surface of polyethersulfone (PES) hollow fiber substrates for boron removal. It is found that the TFN membrane comprising water-soluble sulfocalix [4]arene (SCA4) nanoparticles have the best performance because SCA4 not only possesses a proper pore size of 4.2 Å but also forms ionic bonds with the polyamide network. Comparing with the blank thin-film composite (TFC) membrane, the newly developed TFN membrane incorporated with 0.05 wt% SCA4 has 37.5%, 32.1% and 37.4% higher water fluxes at pH equal to 4, 8 and 10, respectively, while their rejections are comparable of about 66.0%, 87.7% and 97.3% at pH = 4, 8 to 10, respectively. Clearly, the unique SCA4 closed-loop structure provides the TFN membrane with additional free volume for water transport without compromising its molecular sieve capability for boron removal. This study may provide a useful strategy to design next-generation nanofiltration membranes using hollow cup-like macrocyclic compounds for water reuse and boron removal.