# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Synthetic Polymer Adsorbents
The maximum and minimum metal adsorption values were found to be 5.2 mmol/g for Ni\({}^{2+}\) at pH 5.0 and 1.5 mmol/g for Co\({}^{2+}\) at pH 1.0 respectively [194]. [PERSON] et al. synthesised new functional resins by reactingpoly (vinyl benzyl chloride-acrylonitrile-divinyl benzene) matrix with ethylenediamine, bis(aminopropyl) amine, bis(aminohexyl) triamine, tris(2-amino ethyl) amine. Amine functionalised resins with 2.8-5.0 mmol of the amino group per gram of any resin were treated with 5-ethylthiourea. Adsorption capacities of resin for gold, platinum, and palladium chloro complex were found to be 190, 245, and 280 mg/g, respectively [195]. The sorptions of metal onto the amidoxime improvised polyacrylonitrile (PAN) nanofibers were investigated by [PERSON] et al. [169]. The equilibrium adsorption values for Cu\({}^{2+}\) and Pb\({}^{2+}\) were 52.70 and 263.45 mg/g demonstrating unilayer sorption on the nanofiber. [PERSON] et al. examined the extraction of cadmium (Cd) from aqueous media by using 2,2\({}^{\prime}\) bipyridine modified polystyrene foam [196]. Maximum 90% sorption was obtained at pH 7 after 30 min of mixing of adsorbent with metal ions, solution and adsorbent can be regenerated with HNO\({}_{3}\). Removal of nickel from wastewater using phenol-formaldehyde resin (Duolite XAD -761) with anionic surfactant sodium dioctyl sulphosuccinate and EDTA-disodium salt was reported [197]. The pseudo-second-order kinetic model was used to explain the sorption of Ni on modified XAD-761 resin. They have also reported extraction of lead (Pb) from synthetic solution by means of dioctyl sodium sulphosuccinate-EDTA tailored amberlite XAD-7 HP resin (acrylic ester-based resin [197]. Similarly, fluoride ions in drinking water were also removed by amine-modified amberlite XAD-4 poly (Styrene-DVB) based resin [198]. The uptake value of fluoride ions by the modified amberlite XAD-4 resin was 5.04\(\times\)10-3 mol/g and took place through an ion exchange adsorption mechanism. Polypropylene fiber was grafted with acrylonitrile and then treated with diethylene triamine to enhance the sorption capacity of grafted polymer for mercury ions [166]. Aminated chelating fiber has a high adsorption capacity for Hg 657.9 mg/g and this was due to the strong soft-soft interaction between mercury and amine. The extraction isotherm indicated that novel foam presents a high affinity for Cd and Pb due to the presence of good extraction sites (S, N, and O) in PU foam material. Chloro methylated polystyrene modified with 2-mercapto benzothiazole as a sorbent was also reported [199]. The modified polymer has a maximum sorption capacity of around 0.493 mmol/g for Ag\({}^{+}\) at pH 2 and desorption was effective with HNO\({}_{3}\). [PERSON] et al. reported new coordinating polymers by reacting ethylenediamine to crosslinked polystyrene through the sulphide linkage [200]. An adsorption study indicated that both the adsorbents had the best absorption capability for mercury, copper, and silver ions especially PSM-EDA which had more affinity for Hg ions. Among all the sorbents, amine-functionalised poly(glycidyl methacrylate)-based polymers had greater capacity to remove toxic metal ions and some of which are mentioned in Table 4. [PERSON] et al. investigated the triethylene tetra amine-functionalised poly (glycidyl methacrylate) sorbents for copper, zinc, cobalt, nickel ions sorption from seawater at the pH of water [201]. [PERSON] et al. studied the removal capacity of copper and lead by ethylene diamine-functionalised poly (glycidyl methacrylate) sorbents and confirmed that sorption behavior depends upon the pH and surface properties of the sorbents [202]. [PERSON] et al. researched poly (glycidyl methacrylate) sorbents with six different amine functional groups for palladium ions sorption and maximum removal of palladium occurred with diethylenetriamine, although desorption phenomena were not studied for palladium ions [203].