## 3 Types of Adsorbents Made from Polyacrylonitrile
### Polyacrylonitrile as Fiber  
Polyacrylonitrile fiber has many favorable characteristics, such as elasticity, huge surface area, and excellent toughness, making it a super adsorbent [64]. The polyacrylonitrile structure contains semi-crystalline and amorphous phases. Polyacrylonitrile contains a large number of carbon-carbon in its formula, which is responsible for its stability and excellent toughness. It is used to produce carbon nanofibers owing to its high carbon content (56%), and thus, stable products are formed through the reaction of the nitrile group [65]. Many research papers have discussed the techniques for forming nanofibers such as vapor growth, arc discharge, laser ablation, and chemical vapor deposition. However, these methods are very costly and the product yield is very poor. Electrospinning is a new method employed to produce fibers from polymer solutions under electrostatic forces. The components used in the electrospinning process are the high-voltage power supply, syringe pump, spinneret, and collector (Figure 4). In this process, high voltage is applied to a polymer viscous solution obtained by dissolving polymer into a suitable solvent and spinning it by applying high voltage current. Many researchers have reported different modified fiber adsorbents of polyacrylamide, as discussed below. [PERSON] and [PERSON] (2004) utilized aminated polyacrylamidefibers (APANFs) as adsorbents for the evacuation of Cr\({}^{3+}\) and Cr\({}^{6+}\) species from wastewater at various values of pH [66]. The adsorption values demonstrated that large quantities of Cr\({}^{3+}\) and Cr\({}^{6+}\) species are absorbed using APANFs, even though the adsorption values depend on the pH. As per the results, the amount of adsorbed of Cr\({}^{3+}\) ions increased with the increase in pH in the range of 1.5 to 6.5, and adsorption of Cr\({}^{6+}\) was decreased with the rise in pH of the solution in the range of 2.4 to 12. The adsorption performances of Cr\({}^{3+}\) and Cr\({}^{6+}\) on aminated polyacrylamidefibers varied according to the forces between metal and adsorbent. A kinetic study revealed that adsorption equilibrium was obtained within one to two hours. Recovery of Cr\({}^{6+}\) ions from the adsorbent APANFs was done in basic conditions and reutilized without loss of adsorption value, while Cr\({}^{3+}\) ions recovery in acidic conditions was not very effective. The adsorption of metal on oxime-modified polyacrylamide nanofibers was studied by [PERSON] et al. (2008). They obtained functionalized polyacrylamide nanofibers by reacting hydroxylamine hydrochloride with the cyano group of polyacrylamidefibers [67]. The maximal adsorption uptake values for Cu\({}^{2+}\)and Pb\({}^{2+}\) were 52.70 mg/g and 263.45 mg/g, respectively, obtained from the Langmuir adsorption isotherm, indicating that the adsorption was a single layer on nanofiber. The desorption of metal stacked on the above adsorbent could be done by 1.0 M HNO\({}_{3}\) acid. Acrylonitrile was introduced into polypropylene fiber by \(\gamma\)-rays irradiation followed by reaction with  
Figure 4: Schematic diagram for preparation of nanofibers through the electrospinning method. diethylene triamine, resulting in an aminated chelating fiber with a high adsorption value for Hg\({}^{2+}\) ions [68]. The functionalized chelating fibers were found to be much more suitable adsorbents for mercury ions, and the maximum adsorption value for mercury ions was 657.9 mg/g. The maximum adsorption value of mercury ions was due to attraction between the NH\({}_{2}\) moiety and Hg\({}^{2+}\) ions. Hydrolyzed polyacrylamide fibers have also been successful in the extraction of copper ions [69]. Polyacrylonitrile and PAN fibers functionalized with amines were used to remove lead and nickel individually from the aqueous medium by the hexamethylenediamine-modified polyacrylamide nanofibers and utilized for the extraction of toxic ions of lead, copper, and nickel from the artificial solution of metal ions [21,54,70,71].