# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Synthetic Polymer Adsorbents
0.1 g amine-functionalised polymer was used for the extraction of 7 mmol/L As\({}^{5+}\) solutions at various pH values and the mixture was agitated at a speed of 200 rpm for 12 h. Small amounts of metal ions solution around 1.0 ml were taken from the mixture to study the adsorbed concentration. A mixture of arsenic ions 7.0 millimole per liter and other metal ions such as Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), Fe\({}^{3+}\) and Zn\({}^{2+}\) (7 mmol/L) were taken to check the selectivity of 0.1 g of diethylenetriamine functionalised adsorbent for As\({}^{5+}\) over the other ions. The adsorption process from the mixture was explained through the hard-soft acid-base concept. The metal ions like Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\) are hard acids while Fe\({}^{3+}\), Zn\({}^{2+}\) are borderline acids. Fe\({}^{3+}\), Zn\({}^{2+}\) ions being soft acid can interfere with As\({}^{5+}\) in the adsorption process. However, it was found that DETA functionalised polymer effectively adsorb arsenic than iron, zinc metal ions. The adsorption capacity of As\({}^{5+}\) was reached around 5.25 mmol g\({}^{-1}\) at pH 4.0 follow the Langmuir adsorption and pseudo-second-order kinetic equation [172]. (E)-2-[(1H-Imidazole-4-yl)methylidene]-hydraziencarbothamide ligand (EIMH) synthesised by the reaction of thiosemicarbazide and imidazolecarboxaldehyde by using a mixture of ethanol and water in the presence of sulphuric acid as catalysts used to remove heavy metal ions such as lead, copper, and cadmium ions from wastewater. The highest uptake occurred within 10 min with sorption efficiency of 99.80% of Pb (II), 99.25% of Cu (II), and 98.68% of Cd (II) were removed at pH 2-8 [173]. Various polymeric material-based adsorbents used for removal of toxic metal ions is given in Table 3.  
Chelating resins are ion exchange resins that are synthesised to bind anion or groups of ions. Chelating resins contains donor groups with lone pair of electrons such as nitrogen, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**S. No.** & **Adsorbent** & **Adsorption Capacity** & **Reference** \\ \hline
1 & Poly (ethylene glycol dimethylate-n-vinyl imidazole) & 1. Hg = 74.2 mg/g & \\
1 & Poly (ethylene glycol dimethylate-n-vinyl imidazole) & 2. Pb = 92.5 mg/g & [26] \\  & & 3. Cd = 45.6 mg/g & \\
4 & methylacet-methacrylolmidephenylalanine) & 1. Hg = 26.5 mg/g & \\
5 & Acrylonitrile onto polypropylene fiber having diethyene & 2. Pb = 20.2 mg/g & [164] \\
5 & triamine group & 3. Cd = 15.3 mg/g & \\
6 & Polyurethane & 1. Hg = 26.8 mg/g & \\
7 & Porous attapulgite (ATP)/polyethersulfone beads & 1. Cu = 25.3 mg/g & \\
8 & amidoxime improved polyacrylonitrile (PAN) nanofibers & 2. Cd = 32.7 mg/g & [168] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Different Polymeric adsorbents used for removal of various toxic metal ions. oxygen, sulphur and phosphorous. These moieties are able to form a coordinate bond with metal ions selectively. [PERSON] and [PERSON], synthesised poly(acrylamide) chelate resins by redox polymerisation of AN and DVB followed by reaction with hydroxylamine hydrochloride. Complexation properties of resin were observed to be viable for sequestering of La, Ce, and Nd from aqueous solutions [174]. [PERSON] et al. additionally reported the complexation ability of copper ions by utilising chelating resin containing amidoxime groups [28]. [PERSON] and [PERSON] investigated chelating resins containing a cysteine group by reaction of hydrolyzed macroporous polyacrylamide with L-cysteine and 1,6 hexanediol. This resin was very particular for silver, mercury, gold, and platinum in aqueous acidic solutions [175]. [PERSON] reported a novel chelating polymer with an enhanced extraction capacity of metal ions [176]. These copolymers were prepared by the functionalisation of a vinyl benzyl chloride-divinylbenzene copolymer with the sodium salt of diethyl malonate after that reaching the modified resin with amine i.e., ethylene diamines, diethylenetriamine, 2-aminomethyl pyridine, 3-aminopropyl imidazole.