Tingting Li

A functionalized ethylene–vinyl-alcohol (EVOH) nanofibrous membrane (NFM) was fabricated via co-electrospinning H₄SiW₁₂O₄₀ (SiW₁₂) and EVOH first, and then grafting citric acid (CCA) on the electrospun SiW₁₂@EVOH NFM. Characterization with FT-IR, EDX, and XPS confirmed that CCA was introduced to the surface of SiW₁₂@EVOH NFM and the Keggin structure of SiW₁₂ was maintained well in the composite fibers. Due to a number of carboxyl groups introduced by CCA, the as-prepared SiW₁₂@EVOH-CCA NFM can form a high number of hydrogen bonds with CR, and thus can be used to selectively absorb congo red (CR) in aqueous solutions. More importantly, the CR enriched in the NFM can be rapidly degraded via photocatalysis. SiW₁₂ in the NFM acted as a photocatalyst, and the hydroxyl groups in the NFM acted as an electron donor to accelerate the photodegradation rate of CR. Meanwhile, the SiW₁₂@EVOH-CCA NFM was regenerated and then exhibited a relatively stable adsorption capacity in five cycles of filtration–regeneration. The bifunctional nanofibrous membrane filter showed potential for use in the thorough purification of dye wastewater.

For comprehensive treatment of wastewater, an electrospun H₄SiW₁₂O₄₀ (SiW₁₂)/chitosan (CS)/polycaprolactam (PA6) sandwich nanofibrous membrane (SNM) with dual-functions of adsorption and photocatalysis was prepared to remove Cr(VI) and methyl orange (MO) from aqueous solution. The three layers of the SNM had good compatibility and thus no visible stratification phenomenon was observed in the SNM. Taking advantage of the merits of high porosity, good hydrophilicity, robust mechanical strength and superior water tolerance, the obtained SNM presented an excellent comprehensive performance with high adsorption capacity toward Cr(VI) (78.5 mg g⁻¹), high photodegradation efficiency toward MO (96.7%) and good reusability. Based on the images of the SNM after Cr(VI) adsorption, the SiW₁₂/PA6 layer (upper and bottom layer) was not affected in the adsorption process, which was consistent with our initial design. XPS analysis was carried out to investigate the mechanism of adsorption and photocatalysis. In the adsorption process, Cr(VI) was initially adsorbed on the CS/PA6 layer (intermediate layer), followed by the reduction of Cr(VI) to Cr(III) by the –NH₂ of CS. The photocatalytic process was driven by the reductive pathway with faster degradation rate due to the electron donation from PA6 to SiW₁₂. Although Cr(III) at the layer boundary was involved in the photocatalytic process, the photocatalytic activity of the SiW₁₂/PA6 layer was almost not influenced. This work may provide a new and effective strategy to construct multifunctional nanofibrous membranes for the comprehensive treatment of wastewater.