# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Types of Adsorbent
#### 6.6.8 Ion Exchange Resins  
The resins derived from polyhydric phenol and formaldehyde (HCHO) were cation exchange resins, and anion exchange resins were obtained from m-phenylene diamine, HCHO. These resins were exchanged with ions in solution with other ions of similar charges by an ion-exchange method in which ions in the solution were replaced with ions in the polymer matrix. Cationic exchange resins were mainly responsible for the removal of the cationic hardness of the water (Ca\({}^{2+}\), Mg\({}^{2+}\)). However, chemically these were not very stable and tended to dissolve in basic solution. This prompted the quest for new ion exchange materials and cross-linked styrene-divinylbenzene resins have been produced [154]. These tars have the high synthetic and physical stability form of small beads/spheres. A further sulphonated group was attached to the styrene-DVB skeleton by reacting it with fuming sulphuric acid or chlorosulfonic acid. Sulphonated styrene-DVB resins have a higher exchange capacity at all pH and higher chemical stability, synthesis of their styrene-divinylbenzene copolymer matrices, anion, and cation exchange membranes have been patented by a number of organisations in the USA and UK. Ion exchange resin containing -COOH groups has been synthesised by the reaction of methacrylic acid with crosslinking agents and patented by numerous examiners. It is naturally opaque in appearance, greatly impervious to osmotic shock, and demonstrates no breakage on rehashed swelling (approximately 100%). Examinations have been undertaken in order to remove follow trace metal particles Ca, Mg, Cu, Zn, Fe, Mn, and purified water might be used for developing plants and in commercial enterprises. Styrene-DVB-based amber lite XAD resins were extremely prevalent for the extraction of organic impurities and color bodies from wastewater. Sulphonated styrene-DVB-based ion exchange resins were also additionally utilised for the extraction of poisonous metal particles. For the adsorption isotherm studies, the tested conditions were as follows: initial ion concentrations 0.5-20 mg/L, adsorbent dosage 1.5 g/L, temperature 30 \({}^{\circ}\)C, and pH 7. Cyshtcc-Fe\({}_{3}\)O\({}_{4}\) had relatively high adsorption capacities for all the tested ions, especially for Pb2+ (235.63 mg/g), As (III) (66.3 mg/g) and As (V) (66.7 mg/g). Lead ions showed a high sorption capacity among the other metal ions for the resultant adsorbent (Figure 8) [155]. A bio ion exchanger was synthesised by using medicinal waste herb chicory for the extraction of lead and cadmium from water [156]. This herb chicory was then treated with calcium chloride to enhance the efficiency of the cation exchanger as well as the capacity of Pb (II) and Cd (II) ions removal. The utmost sorption efficiencies were found to be 103.1 and 53.8 mg/g for raw chicory waste and sorption by functionalised chicory was increased to 123.5 and 64.5 mg/g for the respective metal ions i.e., Pb (II) and Cd (II). Regeneration of water and heavy metal ions was undertaken by calcium chloride, sodium chloride, and nitric acid. A new nanosized ultrafiltration membrane was prepared by polyacrylamide as well as goethite nanoparticles. This membrane was used for the extraction of Cu (II) ions from aqueous solutions. Firstly, goethite nanoparticles having a diameter of 50 nm were synthesised. In the next step, these nanoparticles were embedded in the nanocomposite membrane. Response surface methodology based on Box-Behnken design was used to optimise the compositions of membranes. The impact of membrane composition on the extraction rate of the metal from the water was reviewed under constant parameters. The working parameters for the removal of metal ions and water penetration were 15%, 1.3%, and 1.0% for polyacrylamide, polyvinylpyrrolidone, and nanoparticle loading. The maximum copper ions removal was found to be 49.1% by hydrous metal oxide that contained polyacrylamide-based nanocomposite membranes [157].