# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Synthetic Polymer Adsorbents
In the meantime, the extraction efficiencies reached 81.0% for lead (C0 = 3.825 ppm), 79.3% for cadmium (C0 = 6.075 ppm), 83.4% for copper (C0 = 9.325 ppm), 29.8% for zinc (C0 = 188.6 ppm), 22.3% for manganese (C0 = 17.05 ppm), 96.2% for chromium (C0 = 0.25 ppm) and 99.8% for iron (C0 = 9.75 ppm) in melting wastewater using 1.0 g/L sorbent in two hours. Unique polyurethane (PU) was reported for the extraction of Pb\({}^{2+}\) and Ni\({}^{2+}\) ions from a synthetic mixture. The maximum amount of toxic metal ions adsorbed by polyurethane for Pb\({}^{2+}\) and Ni\({}^{2+}\) ions were obtained through the Langmuir isotherm was 236.5 and 217.5 mg/g, respectively. The adsorption competence of the polyurethane was retained even after five cycles [167]. The adsorption mechanism is shown in Figure 10 where a co-ordinate bond is created among the donor atoms of the binding sites of the polyurethane with steel ions. This behaviour is further verified in the desorption process. [PERSON] et al. reported porous attapulgite (ATP)/polyethersulfone beads were synthesised by the phase-inversion method [168]. The adsorption capacity of such beads was 25.3 mg/g for Cu (II) and 32.7 mg/g for Cd (II). [PERSON] and co-workers reported modified nylon fibers for the removal of Pb\({}^{2+}\) and Cr\({}^{6+}\) having 93.8% and 25.6% adsorption capacities [225]. Their results showed a high capacity of the adsorbent (qmqx = 48 mg/g and 45 mg/g) for Pb\({}^{2+}\) and Cr\({}^{6+}\) at pH 5 and 3. It was also revealed that after functionalisation, the adsorption capacity of modified nylon fibers increased by 90% and 20% for Pb\({}^{2+}\) and Cr\({}^{6+}\), respectively. [PERSON] et al. synthesised oak sawdust, naturally occurring materials as an adsorbent [226]. Sawdust was modified with an HCl solution appropriate for the  
Figure 10: Adsorption mechanism of PU. Reused with permission from [167]. extraction of toxic metals from metal solutions. This adsorbent mainly adsorbed three metals ions Cu\({}^{2+}\), Ni\({}^{2+}\), and Cr\({}^{6+}\) of concentrations in the range 1 to 100 mg/L. According to the Langmuir adsorption model, sorption values were found to be 3.22, 3.29, 1.70 mg/g for Cu\({}^{2+}\) Ni\({}^{2+}\) and Cr\({}^{6+}\) particularly. The maximum amount of Cu (II) ions were removed at pH 4.0(93%), while Ni (II) and Cr (VI) showed their maximum efficiencies (82%) at pH 8.0, (84%), and at pH 3.0 for chromium. [PERSON] et al. synthesised polyamine-type chelating resin by using the method of combining atom transfer radical polymerisation (ATRP) technology and hyperbranched polymer principle to synthesise the high-efficiency chelating resin [227]. This resin effectively removed As\({}^{5+}\) and Cr\({}^{6+}\) in acidic conditions while Cu\({}^{2+}\), Pb\({}^{2+}\), Cd\({}^{2+}\), and Cr\({}^{3+}\) were adsorbed effectively at high pH values. The highest sorption values of polyamine resin for As\({}^{5+}\), Cr\({}^{6+}\), Cu\({}^{2+}\), Pb\({}^{2+}\), Cd\({}^{2+}\) and Cr\({}^{3+}\) were 5.75, 4.44, 2.75, 1.40, 1.21 and 0.57 mmol/g, respectively. [PERSON] et al. reported amine-modified glycidyl methacrylate terpolymer for the extraction of mercury and lead ions [209]. The sorption effectiveness of Hg\({}^{2+}\) and Pb\({}^{2+}\) ions were determined by the use of the batch method. The highest uptake of lead ions was found to be 4.74, 4.76, and 4.73 mmol/g, and the highest mercury ions sorption was obtained at 4.76, 4.80, and 4.21 mmol/g respectively for PGMA-EDA, PGMA-DETA, and PGMA-TEPA polymers at their original pH. The sorption of mercury ions by the resulting resin was increased with increased time, and equilibrium was obtained within 20 h while the adsorption of lead ions by functionalised resins was achieved within 45 min. [PERSON] et al. reported disulphide functionalised covalently organic polymers (COP-63) for the extraction of toxic ions mainly Cd\({}^{2+}\), Cu\({}^{2+}\) and Zn\({}^{2+}\) even in the presence of calcium ions from the aquatic system [228].