# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Synthetic Polymer Adsorbents  
There are a large number of synthetic adsorbents synthesised for different heavy metal ions removal and the sorption values of few adsorbents are presented in Table 3. Because of improvements in the field of polymer, new classes of adsorbents have come up, specifically the synthetic permeable polymeric adsorbents which have proper porosity, sufficient surface area, better dimensional stability, and a fast rate of adsorption. Polymeric adsorbents were permeable, porous spherical beads taking into account crosslinked poly(styrene-DVB) or phenol-formaldehyde condensate polymers [154]. Their high internal surface areas can sorb and desorb different types of ions dependent on the functionality and structure of the adsorbent. Polymeric adsorbents might have a few points of interest, including: (a) they can be easily available in a variety of shapes, for example, beads, membranes and fibers, (b) the surface can be changed by different techniques such as polymer brush arrangement, functionalisation, and molecular imprinting to enhance the removal ability of adsorbates, (c) specific sorption can be accomplished by means of functionalising the polymer substrate with the desirable ligands that have selectivity. They are likewise used for the extraction of poisonous toxic metals from contaminated water. Styrene-DVB copolymer-based macro reticular materials were studied by copolymerising styrene and DVB in the presence of organic diluents that result in polymers with high surface area and high porosity [161]. Furthermore, these beads have been functionalised and a new class of cation exchange resins with remarkable metal ion exchange capacity has been synthesised. Crosslinked permeable copolymers of methacrylic acid-N, N' methylene bis acrylamide or ethylene glycol dimethylacrylates were reported in the literature [162; 163]. Polymethacrylic copolymers were assessed for the sorption of Ni and Cu ions from spiked metal particle arrangement in static and dynamic conditions. Poly (ethylene glycol dimethacrylate-N-vinyl imidazole) beads for the extraction of toxic metal ions have additionally been reported by [PERSON] et al. [26]. The uptake capacity was found to be Hg > Cd > Pb and the sorption value was 45.6 milligram for cadmium, 92.5 mg for lead per gram of resin individually. [PERSON] et. al, synthesised poly (ethylene glycol dimethacrylate-acrylamide) copolymer for extraction of (lead mercury and cadmium) metal particles in aqueous solution, and metal uptake results were observed to be in the order lead > cadmium> mercury [100]. They have also checked the desorption capacities, which was found to be up to 98%, and observed that it did not prominently alter during the repeated cycles (Figure 9). It has also been observed that the desorption tests were not affected by the type of eluent. Figure 9: Desorption cycles of the poly (EGDMA-co-AAm) beads for Pb (II) metal ion. Reused with permission from [100]. [PERSON] and [PERSON] reported water dissolvable polyvinyl pyrrolidone (PVP) for extraction and pre-concentration of Hg (II) ions. PVP contains a pendant lact ring having oxygen and nitrogen groups which was used to bind toxic metal ions [164]. [PERSON] et al., researched the sorption characteristics of metal ions by poly(N,N-dimethyl aminoethyl methacrylate) hydrogels. The sorption value of toxic ions by hydrogel was found in the order Cu > Zn = Co > Pb >> Ni > Cd and all gel systems followed the Langmuir adsorption isotherm [170]. Polypyrrole was utilised for the removal of arsenic from wastewater as reported by [PERSON] et al. [171]. Arsenic was removed from the aquatic system by the diethylenetriamine functionalised copolymer glycidyl methacrylate (GMA) on chloromethylated cross-linked styrene-divinylbenzene, synthesised through the atom transfer radical polymerisation technique on the surface of the polymer. The amine group was introduced through the reaction of an amino moiety of diethylenetriamine and epoxy group of GMA.