Composite Nanofibers: Recent Progress in Adsorptive Removal and Photocatalytic Degradation of Dyes
Fiber Composites [Working Title]
This chapter intends to review the state of the art of a new nanomaterial generation based on electrospun composite nanofibers for dye removal from wastewater. Natural polymer-based nanofibers, nanofibers with unique morphology, and carbon nanofibers were comprehensively reviewed as capable carriers for a broad spectrum of functional materials such as metal oxides, zeolite, graphene and graphene oxide (GO), and metal-organic frameworks (MOFs) in the application of dye removal. The various
... . The various nanostructures, adsorption capacity, advantages, and drawbacks were discussed along with mechanistic actions in the adsorption process and photocatalytic performance that emphasize current research development, opportunities, and challenges. The chapter covers multiple intriguing topics with in-depth discussion and is a valuable reference for researchers who are working on nanomaterials and the treatment of colored waters. 2 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 be greatly improved    . 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 intra-particle diffusion because step (iv) usually happens rapidly  . 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 biocompatibility    . 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 H 2 O to produce hydroxyl radical OH•, whereas electrons react with absorbed O 2 to produce oxygen radicals O 2 • and other intermediate forms  . The hydroxyl Figure 1. Scheme for photocatalysis of metal oxide nanoparticle-decorated nanofibers under UV or visible light sources.