**3.4 Metal-organic frameworks**

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

The mobilization of metal oxides into carbon nanofibers by several methods has been described in the literature to improve the adsorption performance. The traditional one is to disperse precursors of metal oxides in the PAN polymer solution before electrospinning and carbonization. Nevertheless, the metal oxide nanoparticles are usually located inside the CNFs, which resulted in low adsorption efficiency. Besides, the agglomeration is also of concern because it is detrimental to dye removal efficiency and mechanical properties of the CNFs. The ultrasonic decoration of CNFs with TiO2 was a straightforward technique to achieve uniform distribution of nanoparticles and yield higher efficiency of dye uptake [30]. TiO2@carbon composite nanofibers can be prepared by electrospinning technology, followed by a hydrothermal method to acquire the nanoarray structure [31]. The high adsorption performance was explained as the decoration of TiO2 nanoarray induced the specific surface area enlargement, the tunable wettability from hydrophobicity to the hydrophilicity of the carbon nanofibers, and considerable negative Zeta potential value. Furthermore, the addition of TiCl4 in the electrospinning solution increased the macroscopic flexibility and the adsorption performance of CNF from 9.92 to 24.77% for methylene blue (MB),

Zeolite, an aluminosilicate framework obtained from nature, can be chosen as a suitable filler material in the polymeric nanofibrous matrix due to its porous structure and exchangeable cation feature [32]. Its 3D structure with negatively charged lattice, high specific surface area, and competitive price makes zeolite an appealing choice for dye adsorption. The adsorptive sites in zeolites can be controlled by adjusting the ratio between silicon and aluminum. With its strong adsorption capacity for waste products and toxins, zeolite has been reported to show affinity toward methyl orange (MO) [33, 34], MB, and MG [35], with high adsorption capacity and reusability feature. The adsorption mechanisms are complicated, including porous structure, charged surfaces, heterogeneity, and other imperfections. Lee et al. reported that PMMA/zeolite nanofibers exhibit high

were fitted well with the Langmuir model, which indicated that the dye molecules were adsorbed onto the homogeneous surface and monolayer adsorption existed

Recently, graphene and GO have been studied extensively in the field of catalysis and adsorption as a result of their massive surface area, delocalized π network, and inertness to be used in a wide pH range. Graphene has features of chemical stability, low toxicity, and hydrophobicity. The oxidation of graphene provides an excellent hydrophilic surface; at the same time, it compromises the π electron structure, resulting in poorer attraction to aromatic hazards [36]. The reduction of GO, which forms rGO, is a process to recover the adsorption capacity for GO by giving it back the π-delocalized electron structure and hydrophobic property. Graphene-based materials tend to aggregate due to strong van der Waals and π-π interactions; thus, incorporating them into polymeric nanofibers is a way to overcome the aggregation [7]. Composite GO/PVDF nanofibrous membrane was prepared by ultrasonic treatment for the use of organic dye removal. The facial treatment technique, with the support of ultrasonication, was implemented. The adsorption capacity is mainly dependent on GO contents of the composite membranes, and the

. The isotherm adsorption results

removal efficiency up to 93% for MO at 30 mg L<sup>−</sup><sup>1</sup>

during the process [34].

**3.3 Graphene and graphene oxide**

**124**

respectively.

**3.2 Zeolite**

MOF is an excellent porous media with a multitude of applications in biomedical engineering, photocatalysis, CO2 separation, and dye removal. With the properties of chemical and physical stability, effective surface area, excellent adsorption capacity, and nontoxicity, it has been widely used as an essential material for environmental remediation [39]. However, its poor processability hinders the fabrication into filtration devices. Many researchers have successfully applied MOF-based composite nanofibers for contaminant removal from wastewater. Li et al. reported co-electrospun anionic MOF nanofibrous membranes, which displayed synergistic action of PAN and bio-MOF-1 in the adsorption process for MB [8]. The resulting filter could sustain a constantly high adsorption capacity because of the stable nanofibrous structure and no leaching effects. Desorption was conducted in a saturated Na<sup>+</sup> solution based on the ion exchange equilibrium. The ion exchange process happened to settle the dynamic equilibrium between ions of different species. The high adsorption performance of MOF embedded in the polymeric nanofibers could be explained as the diffusion of dye molecules to the surface and internal channels of MOF, which is governed by a multilayered adsorption process associated with the transportation of Gaussian energy into a heterogeneous structure [39].
