**2.1 Electrospinning technique as the fabrication method**

Electrospinning is one of the several well-developed techniques to fabricate fibers at micro- or nanoscale (**Figure 2**). The electrospinning with versatility allows excellent controls over the fiber diameters, nanostructures, and morphology to enhance catalytic, mechanical, electrical, biomedical, optical, and adsorptive properties. With a wide selection of polymers and the facilities for additive incorporation, the electrospinning process can manufacture nanofibers into different fascinating structures for varied applications [14]. With recent advancements in the electrospinning technique, fascinating nanostructures could be obtained with inspiration from objects in nature and can be applied in improving pollutant removal. The tree-like structure is composed of trunk fibers and branch fibers. The trunk fibers with the support role can improve the mechanical property, and the thin branches play the role of connection, decrease the pore size of the membranes, and increase the surface area [15]. The spider web-like structure was

#### **Figure 2.**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

intra-particle diffusion because step (iv) usually happens rapidly [5].

Most dyes are water-soluble and can be classified as cationic, anionic, and nonionic; the names are derived from the charging states when being dissolved into an aqueous medium. Depending on the chemical structures of dyes, the approaches and adsorption conditions can vary accordingly, which include material selection, adsorption or photocatalytic degradation, pH, time, and temperature. A spectrum of organic and inorganic materials such as transitional metal oxide, graphene and GO, carbon nanotubes, zeolites, and MOFs have been used for treating colored waters. These materials are suited for separating dyes from wastewater owing to abundance, low cost, ease of being employed, adsorptive selectivity, and biocom-

The photocatalysis process has emerged as a newly developed technique for wastewater remediation. Photocatalysts with a particular bandgap can be activated by different light sources to generate electron-hole pairs, which either recombine or migrate to the surface and initiate photocatalytic reactions. After that, the holes oxidize H2O to produce hydroxyl radical OH•, whereas electrons react with absorbed O2 to produce oxygen radicals O2• and other intermediate forms [9]. The hydroxyl

*Scheme for photocatalysis of metal oxide nanoparticle-decorated nanofibers under UV or visible light sources.*

be greatly improved [2–4].

patibility [6–8].

capacity with the convenience of recovery and recycling. By engineering various functional groups (carboxylate, amino, acid, and hydroxyl groups) or the integration of adsorbents, including metal oxides, graphene, graphene oxide (GO), and metal-organic frameworks (MOFs) in the nanofibers, the separation capacity can

Among various systems, which have been developed for the removal of dyes in wastewater, namely, adsorption, ion exchange, membrane filtration, and coagulation, adsorption is the most effective and versatile strategy to remove dyes at high concentrations with high removal percentage. The adsorption process involves several stages: (i) dissolving dyes into the solution, (ii) the external diffusion of dyes to the surroundings of the adsorbents, (iii) internal or intra-particle diffusion which fills nanoparticle pores with dye molecules, and (iv) adsorption or desorption on the interior sites. If the amounts of dye uptake are correlated with the square root of time in a linear relation, the adsorption process is significantly influenced by

**120**

**Figure 1.**

*Electrospinning technique to fabricate nanofibers with different morphology (a) poly(ε-caprolactone)-poly (l-lactic acid) nanofiber tubes [10], (b) cellulose acetate nanofibers with morphology control [11], (c) porous carbon nanofibers [12], and (d) cellulose acetate nanofibers with honeycomb-like surface structure [13].*

fabricated by growing zeolitic imidazolate framework-8 (ZIF-8) nanocrystals on the nanofibers. The nanofibers showed high removal efficiency for incense smoke, formaldehyde, and PM particles, which was attributed to the improved surface area and electrostatic interaction between ZIF-8 and particles [16]. Hierarchical bioinspired composite nanofibers comprised of PVA, PAA, GO-COOH, and polydopamine demonstrated the eco-friendly and controllable fabricating process with efficient adsorption capacity for dye removal [17]. The excellent adsorption was due to the strong electrostatic field of carbonyl group modified GO and the unique structure of polydopamine. The membrane exhibited excellent reusability with the potentially large-scale application.
