Two-Stage Optimization of Nb2O5 Content and Thermal Stabilization in Electrospun PAN/PVP/Nb2O5 Nanofiber Membranes for Cationic Dye Adsorption

Composite nanofiber membranes are one of the most investigated yet promising adsorbent structures for wastewater treatment, where oxide-type nanoparticles are typically used as a functional filler. The impact of this filler on membrane adsorption performance is determined by its material quality and fabrication parameters. Here, we develop electrospun polyacrylonitrile/polyvinylpyrrolidone/niobium pentoxide (PAN/PVP/Nb 2 O 5 ) nanofiber membranes and enhance their performance for cationic dye adsorption by optimizing the Nb 2 O 5 content and post-fabrication thermal stabilization temperature. The best-performing membrane is obtained at 0.5 g Nb 2 O 5 loading and 150°C treatment, retains the largest fraction of polymer functional groups and Nb 2 O 5 cation-exchange sites, while higher temperatures progressively eliminate the surface PVP (N–C=O) groups and the exchangeable NH 4 + . This membrane achieves a pseudo-second-order equilibrium capacity of 26.93 mg/g for methylene blue (MB), with Langmuir monolayer capacities of 23.8 mg/g (MB) and 20.6 mg/g (crystal violet), consistent with a specific cation-exchange interaction. The adsorption process is endothermic (Δ H ° = +48.27 kJ/mol) and entropy-driven (Δ S ° = +234.7 J/(K·mol)), and spontaneous across the studied temperature range (ΔG° = −22.8 to −27.5 kJ/mol). Higher treatment temperatures (≥250°C) cause PVP decomposition, PAN cyclization-induced densification, and thermal release of the exchangeable NH₄⁺, reducing the adsorption capacity to 11–17 mg/g. These findings provide a transferable framework for thermally tuning composite nanofiber adsorbents toward efficient cationic dye remediation in textile wastewater. However, reusability is limited, in which under thermal regeneration the removal efficiency can only retain around 10% of its initial value after five cycles, whereas an ion-based (NaCl/ultrasonic) regeneration is more durable, retaining around 25% despite its only partial capacity recovery. Oxime-grafted PAN nanofiber