**2.2 Electrospinning bio-based polymers for water treatment**

Due to concerns about sustainability and environment, bio-based polymers such as cellulose, chitosan, zein, collagen, silk, hyaluronic, alginate, and DNA have been used significantly to fabricate nanofibrous composite membranes [18]. The applications of these polymers in water filtration at the commercialization scale have seen the increase over the past few years due to the beneficial properties of biocompatibility, biodegradability, safety, and nontoxicity. One of the unique features of bio-based polymer is the possession of various functional groups, which can be utilized for pollutant collection (**Figure 3I**). The dye adsorption mechanisms onto polymers can be of chemisorption or physisorption. The former is often related to strong bonding (such as covalent or ionic bonding) or chemical reactions and is irreversible. The latter is a reversible process, thus more preferable. The physisorption is governed by van der Waals forces, hydrogen bonding, hydrophobic interaction, and electrostatic attraction.

Gopakumar and coworkers modified cellulose nanofibers by esterification with Meldrum's acid, which endowed the nanofibers with the affinity toward positively charged dyes [21]; the mechanisms of adsorption were suggested as electrostatic forces between carboxylate groups and the dye molecules. Chitosan/polyamide nanofibers were reported to have excellent adsorption capacity toward anionic dyes, 456.9 mg/g for Reactive Black 5 (RB5) and 502.4 mg/g for Ponceau 4R (P4R), primarily due to the affinity of amino and hydroxyl groups in the chemical structure [22]. With the increase of the ratios of chitosan/polyamide, the adsorption capacities improved, which was assigned to the fact that more reactive sites are present in chitosan than polyamide. Li et al. reported an efficient and facile route to cover electrospun silk nanofibers with MOFs for high removal efficiency toward

#### **Figure 3.**

*Mechanism of dye affinity: (I) RB5 on zein nanofibers based on hydrogen bonding, electrostatic interactions, and hydrophobic interactions [19], (II) MB on CNFs governed by π-π stacking interactions [20], and (III) orange II on titania aerogel via H bonding and electrostatic forces [5].*

**123**

*Composite Nanofibers: Recent Progress in Adsorptive Removal and Photocatalytic Degradation…*

rhodamine B (RB) and malachite green (MG) [23]. The authors successfully loaded the composite nanofibers with high contents of MOFs, and more importantly, the 3D structure of MOFs was well retained within the silk nanofibrous membrane.

Carbon nanofibers with unique and tunable morphology have been used in catalytic, environmental, and energy applications [24]. Polyacrylonitrile (PAN) nanofibers have been used extensively as an efficient precursor for CNF fabrication. CNFs have been employed as an efficient carrier for the loading of catalytic and bioactive materials. A broad spectrum of metal oxide nanoparticles has been immobilized in/onto CNFs for dye removal applications. The main approaches to decorate CNFs with active materials include electrodeposition, chemical synthesis, and dry synthesis. Interestingly, the CNFs also present adsorption capacity (**Figure 3II**), due to the binding between aromatic rings of CNFs and adsorbate based on π-π stacking interaction [20]. The high conductive and chemically inert properties of CNFs enhance its application in dye degradation via photocatalysis and promote reusability. The dye molecules have high chances to be attracted to CNFs, having a π-conjugative structure, before being decomposed by photocatalysts loaded on the

**3. Adsorptive removal by inorganic absorbents incorporated into** 

Transitional metal oxide nanoparticles, including copper oxide, zinc oxide, iron oxide, titanium dioxide, and mixed metal oxide nanocomposites, have been investigated in the dye uptake or dye removal efficiency. Metal oxides have remarkable physical and chemical characteristics, which have been proven useful for water purification. The electrostatic attraction, hydrophobic interactions, and hydrogen linkages between the surface of metal oxides and dye molecules were supposed to dominate the adsorption, controlling the kinetics and isotherm of adsorbent-

Li et al. reported that the maximum adsorptions for Fe, Co, and Ni oxides were found to be at neutral pH and the rise of temperature has a positive impact on the capacity of dye removal. The BET surface areas of these composite nanoparticles were

adsorption capacity at the neutral region is the corrosive destruction of metal oxide nanostructure at high or low pH. At acidic pH, the leaching of metal happens because

attach to the active sites on the metal oxides, which are positively charged, reducing the available number of sites and thereby the attraction between adsorbent surface and dye molecules. Malwal and coworkers reported that the pHPZC values of CuO and ZnO nanoparticles were around 9.4 and 9.5, which means at pH lower than pHPZC, the surface of CuO and ZnO becomes positively charged. The electrostatic attraction is the primary mechanism of anionic dye affinity [27]. Similarly, the pHPZC of MgO is 12.4, and the anionic dye adsorption is driven by electrostatic forces. The pHPZC of TiO2 is around neutral values (6–6.8), which is not so much different from those of iron oxides; the PZC values measured for FeO, Fe3O4, and Fe2O3 were around 6.1–6.5 [28, 29]. The pHPZC of TiO2 and iron oxides is in the neutral range, making them efficient adsorbents for both cationic and anionic dyes in a wide range of pH mediums.

/g [6]. The plausible explanation for the best

. At alkaline pH, the hydroxyl groups

*DOI: http://dx.doi.org/10.5772/intechopen.91201*

surface of the nanofibers [25].

adsorbate interactions [26].

reported to be between 97.26 and 273.5 m2

of the reaction between metal oxides and H+

**nanofibers**

**3.1 Metal oxide**

**2.3 Carbon nanofibers as supporting materials**

rhodamine B (RB) and malachite green (MG) [23]. The authors successfully loaded the composite nanofibers with high contents of MOFs, and more importantly, the 3D structure of MOFs was well retained within the silk nanofibrous membrane.
