# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Synthetic Polymer Adsorbents
The chelating properties of the synthesised resins were studied by the formation of an acetate buffer of pH 3.7 and 5 for copper, cadmium, nickel, and zinc metal ions. Sorption of the following cations on the improved resin was superior to the adsorption on the original polymer containing N-substituted amides of monocarboxylic acid. New chelating hydrogels were synthesised by the reaction of N-vinyl imidazole and acrylonitrile [177]. These gels were used in heavy metal ion adsorptions which were synthesised by irradiation of binary monomer mixture followed by reaction with hydroxylamine hydrochloride. In another study, poly(hydroxyethyl methacrylate) (PHEMA) beads containing thiazoline group (0.318 mmol/g) were prepared by [PERSON] et al. and used for the elimination of Pb\({}^{2+}\) and Cd\({}^{2+}\) metal ions from synthetic solution in different acidic conditions (pH = 0.3-7.0). Adsorptions of Pb\({}^{2+}\) and Cd\({}^{2+}\) onto polymer microbeads were 0.336 and 0.397 mmol/g correspondingly. However, in the case of a mixture, the polymer was more selective for Pb ions. PHEMA microbeads having thiazolline as functionality can be regenerated by washing with HCl acid (0.05 M). The obtained resins can be used repetitively with the same adsorption capacity [178]. [PERSON] and co-workers reported chelating resin synthesised by crosslinked polystyrene functionalised with 2, 5 dimercapto-1, 3, 4-thiadiazole for the elimination of mercury ions [105], and adsorption results were explained by the Langmuir adsorption isotherm model. Regeneration of chelating resins could be undertaken by 2% thiourea in 0.1 mol/L HCl. [179]. The sorption behavior of amidoxime poly(N,N'-dipropionitrileacrylamide) [Poly (DPAAm) and non-woven fabric grafted with the same UO\({}_{2}\)\({}^{2+}\), Pb\({}^{2+}\), Cu\({}^{2+}\), and Co\({}^{2+}\) at high concentrations were investigated for metal ion adsorption studies [180]. particulate amidoximated poly(DPAAm) has shown a better sorption value in comparison to the amidoximated non-woven fabrics for all toxic ions mainly for UO\({}_{2}\)\({}^{2+}\) ions. Sorption values were obtained such as 400 mg UO\({}_{2}\)\({}^{2+}\)/g of dry amidoximated poly (DPAAm) and 250 mg UO\({}_{2}\)\({}^{2+}\)/g of dry amidoximated graft polymer. [PERSON] and [PERSON] synthesised dithiosemicarbazone functionalised polymer, namely, poly(styrene-divinylbenzene) and poly (hydroxyethyl methacrylate-divinyl benzene) beads having definite porosity for adsorption of toxic metal ions [181]. Improved crosslinked polyacrylamide containing various functional moieties have been synthesised by transamidation reaction in the aquatic and non-aquatic medium by Hofmann's reaction. These functionalised resins were used for the elimination of copper, cadmium, and lead ions from synthetic solutions at various pH [182]. The metal ion adsorption value was obtained at around 0.11-1.71 mmol/g of polymer. The above-synthesised polymer containing secondary amine moieties was more specific for copper ions (99.4%), while the polymer with secondary amide and carboxylate moiety was more selective towards lead ions (99.5%). With a decrease in the pH of the solution, the resin's specificity for copper ions decreased while it increased for lead ions. [PERSON] et al. reported macroporous poly(4-vinylridine-DVB) with a large surface area (130 m\({}^{2}\)/g) was synthesised by suspension copolymerisation of 4-vinyl pyridine and divinylbenzene. This resin has a pyridine group that was able to adsorb 94% chromium ion at pH 6.5 [183]. [PERSON] and co-workers also removed 95% of chromium from wastewater solution having concentration 4-500 ppm by using poly(4-vinyl pyridine-DVB) copolymers, functionalised with N-oxide groups [184]. Poly(acrylamide-4 vinyl pyridine) was investigated by [PERSON] and co-workers to adsorb metal ions namely Mn\({}^{2+}\), Co\({}^{2+}\), Ni\({}^{2+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\)[185].