Magnetic-Field-Induced Surface Enrichment of Magnetic Carboxylated Carbon Nanotubes in Poly(vinylidene fluoride) Membranes for Antifouling Ultrafiltration

Poly(vinylidene fluoride) (PVDF) membranes suffer from severe fouling because of their intrinsic hydrophobicity, while the random distribution of functional nanofillers in conventional blend membranes often limits their modification efficiency. In this work, a magnetically responsive nanocomposite, consisting of Fe3O4 nanoparticles anchored onto carboxylated multiwalled carbon nanotubes (c-MWCNTs@Fe3O4), was developed and incorporated into a PVDF matrix. During the nonsolvent induced phase separation process, an external magnetic field was employed to direct the migration of the c-MWCNTs@Fe3O4 nanocomposite toward the membrane surface, promoting its preferential localization at the membrane interface and thereby enhancing the interfacial exposure of hydrophilic functional groups. Consequently, the membrane surface hydrophilicity and pore structure were simultaneously improved, leading to a reduction in the water contact angle from 131° to 50.93° and an approximately 6-fold increase in pure water flux (518.55 L·m–2·h–1) while maintaining high rejection toward bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA). Moreover, the magnetically regulated membrane exhibited excellent antifouling performance, with flux recovery ratios of 88.30% for SA and above 92% for both BSA and HA, together with markedly reduced irreversible fouling and filtration resistance. Resistance and adsorption analyses further demonstrated that the interfacially enriched c-MWCNTs@Fe3O4 effectively alleviated pore blocking and cake-layer formation by weakening foulant–membrane interactions. This work demonstrates that magnetic-field-induced interfacial regulation provides an effective strategy for controlling the spatial localization of functional nanofillers in blended membranes, thereby maximizing their interfacial functionality and simultaneously enhancing membrane permeability and antifouling performance.