Morphological control of poly(vinylidene fluoride)@layered double hydroxide composite fibers using metal salt anions and their enhanced performance for dye removal

Abstract

Self-standing membranes of poly(vinylidene fluoride)@layered double hydroxide (PVDF@LDH) composite fibers were fabricated by using metal salts with different anions (NO₃⁻, Cl⁻, SO₄²⁻) as the reactants through a combination of electrospinning and hydrothermal treatment. The anions have a significant effect on the morphology and loading amount of LDHs anchored onto the surface of PVDF nanofibers. In a monovalent anion (such as nitrate) system, small crystal nuclei were formed. Only a small proportion with a large size grew up to well-defined hexagonal LDH layered nanocrystals with large thicknesses. However in a divalent anion sulfate system, large crystal nuclei were generated first and most of them could grow up to thin and curly LDH layered nanocrystals owing to the serious steric hindrance. Well-defined PVDF@CoAl-LDH composite fibers with a hierarchical structure and uniform core/sheath morphology could only be obtained in the sulfate system, which could act as an excellent adsorbent for treating dye solutions. The obtained PVDF@CoAl-LDH core/sheath fibers present a maximum adsorption capacity of 621.17 mg g⁻¹ for methyl orange (MO). Moreover, benefiting from the perfect properties and stable form, the composite fiber membrane could be used directly in membrane filtration with high removal efficiency and cycling stability. The MO removal rate of the membrane remained above 93% with a maximum flux of 140 L (m² h)⁻¹ despite three times of regeneration, indicating its practical application for the efficient treatment of dye pollution.