## 3 Types of Adsorbents Made from Polyacrylonitrile
### Polyacrylonitrile in Hydrogels  
Hydrogels consist of a three-dimensional structure of polymer capable of absorbing large quantities of water and thus can also be used in many biomedical fields. However, hydrogels do not dissolve in water due to chemical and physical crosslinking between the chains of the polymer. Moreover, the functional groups present in hydrogels, such as COOH, -SO\({}_{3}\)H, -NH\({}_{2}\), and -OH, improve the chelating power and increase adsorption value towards the toxic metal ions and dyes [112]. Hydrogels have attracted the attention of many researchers because of their lower cost and more effective properties [113]. Available literature confirms that hydrogels are effective in the removal of water pollutants. In one study, polyacrylamide hydrogel was synthesized using different concentrations of amine via transamidation and Hofmann methods [114]. The obtained products were utilized for the extraction of Cu\({}^{2+}\) ions at pH 5.5 by the batch adsorption method. The maximum extraction value of copper was 2.93 mmol/g, acquired after a recovery cycle. Adsorption rate constants and activation energy of resin for ruthenium and iridium were also reported in this investigation. [PERSON] et al. (2015) reported cobalt-iron bimetallic magnetic nanoparticles prepared through the inverse suspension polymerization method [115]. The amidoxime group was introduced by reacting the cyano group with hydroxylamine hydrochloride, which helped to increase the extraction capacity of the microgel. The magnetic behavior was introduced in the functionalized microgel matrix by in situ synthesis using cobalt-iron nanoparticles. This modified magnetic microgel was an effective adsorbent for the extraction of metal ions such as cadmium, chromium, and organic dyes such as methylene blue, rhodamine 6G, and herbicide paraquat. The adsorption values of amidoxime-poly(methacrylic-co-acrylonitrile) microgels for methylene blue, rhodamine 6G, paraquat, cadmium, and chromium ions were 88.1 ppm, 89.9 ppm, 190.0 ppm, 334.5 ppm, and 166.5 ppm (previous values were 40.2 ppm, 37.4 ppm, 75.3 ppm, 57.4 ppm, and 56.3 ppm). In another study, the ultra-fine CNF membrane was synthesized through a TEMPO (2,2,6,6-Tetramethylpiperidin-1-yl) oxyl) oxidation approach [116]. The dynamic adsorption experiment was performed in dead-end filtration to study the adsorption of lead and chromium metal ions. The pH of the solutions was adjusted by using sodium hydroxide solution and hydrochloric acid. The elimination of bacteria and viruses through filtration membranes was determined via a retention test. For this test, 10 mL of _E. coli_ suspension (106 CFU/mL) or MS2 bacteriophage suspension (106 PFU/mL) was used in a dead-end filtration mode. Different copolymer hydrogels of acrylonitrile-N-vinyl-2-pyrrolidone (NVP/AN) and acrylonitrile-methylmethacrylate (NVP/MMA) were synthesized by subjecting them to \(\gamma\)-irradiation in different dosages. These hydrogels were then modified with a hydroxylamine-hydrochloride solution at pH 7. The amidoxime-containing hydrogels were washed to remove hydroxylamine, desicated at 70 \({}^{\circ}\)C for six hours, and used to remove dyes from industry wastewater. It was found that the adsorption of dyes increased with the increase in concentration of dyes in a synthesized solution up to 200 ppm, after which there was a decrease in adsorption values. Results showed that modified NVP/AN hydrogel showed more attraction for dyes compared with NVP/MMA. Moreover, dye adsorption increased with an increase in temperature because of the heating effect. In another study, a semi-interpenetrating polymer network (semi-IPNs) was prepared by combining chitosan and polyacrylamide hydrogel with glutaraldehyde by a casting solution process [22]. As hydrogels are soft and can be easily distorted, semi-IPN was used to increase the stability and mechanical strength of the gel.