# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Commercially Available Adsorbents
#### 6.9.3 Activated Carbon  
Activated carbon (AC) has been recognised as an essential sorbent for a long time. These adsorbents were synthesised by heat treatment of carbon substances followed by steam or chemical methods that drastically enhance the specific surface area and permeable character of the adsorbent [271]. Several adsorbents of activated carbons of various permeable sizes were obtained using various methods and were used for various applications in the extraction process. Coarse-activated carbon has also been used for the expulsion of harmful metals from groundwater. Walnut shell-based granular enacted carbon has additionally been discovered for evacuation and adsorption of color dissolved organic, heavy metals from wastewater [272]. ACs were synthesised by pyrolyzing polymer networks at high temperature followed by their physical or chemical activation (Figure 14). Many polymers such as polyacrylonitrile, poly(styrene-DVB) sulphonate, polyvinyl chloride, polyvinylidene chloride, and phenolic resins have been utilised as starting materials for the formation of ACs [272; 273]. These resins were potential adsorbents for harmful gases, fluids, and chemical warfare agents because of their permeable nature and high surface area. A unique adsorbent was synthesised by functionalising activated carbon with manganese oxide and tin oxide nanoparticles using formaldehyde as a crosslinker to bind them [274]. The maximum extraction efficiency of Pb\({}^{2+}\), Cd\({}^{2+}\) and Cu\({}^{2+}\) ions on SnO\({}_{2}\)-NPs-AC-MnO2-NPs adsorbent were 2600, 2500 and 4100, \(\mu\) mol/g at an initial concentration of Pb\({}^{2+}\), Cd\({}^{2+}\), and Cu\({}^{2+}\) ions were 0.2 mol/L, time of sorption 30 min at pH 7 of the reaction medium and amount of adsorbent 5.0 mg of SnO\({}_{2}\)-NPs-AC-MnO\({}_{2}\)-NPs used. The batch method was used to find the extraction effectiveness Unwanted plants have been used to synthesise multi-pore activated carbons through the easy gradient heating method having a large area of the resultant sorbent [275]. The obtained sorbent has a high surface area containing multiple functionalities; this sorbent executes excellent adsorption properties for toxic heavy metal ions. The adsorbent was used to extract lead, cadmium, copper, nickel, and zinc metal ions from wastewater and sorption equilibrium was obtained within 150 min. [PERSON] et al. studied the removal of polycyclic aromatic hydrocarbons such as acenaphthylene, phenanthrene, and heavy metals like cadmium, copper, zinc by granular activated carbon [276]. Polycyclic aromatic hydrocarbons PAHs (initial concentration 1.0 ppm) showed better removal efficiency in comparison to toxic metal ions (initial concentration 20 ppm) at pH 5 with a sorbent quantity of 0.3 g per liter. The Langmuir adsorption value for both the PAHs was reduced with an enhanced concentration of metals, and the adsorption was found in the following pattern copper > zinc > cadmium. The activated sewage sludge was used as an adsorbent for dissolved pollutants. Three different kinds of sludge-based adsorbent were synthesised from the dried sludge: biochar/carbonised sludge (CS), sewage sludge-based activated carbon SBAC, and three modified MSBACs (MSBAC0, MSBAC4, and MSBAC10). They investigated the capacity of toxic ions to be extracted from carbonised sewage sludge produced by pyrolysis at 500 \({}^{\circ}\)C. The equilibrium adsorption values for sewage sludge-based activated carbon, MSBAC0, MSBAC4, and MSBAC10 obtained after 24 h were 1.25 mg/g, 3.04 mg/g, 3.4 mg/g, and 4.03 mg/g, respectively. Adsorption values of roughly 0.74 mg/g, 83% for SBAC adsorbent, were obtained after 5 min. With a sorption value of 1.1 mg/g, MSBAC0 had reached equilibrium after 1 min. The sorbents MSBAC4 and MSBAC10 demonstrated a higher sorption value for Pb (II) around 85% and 94.7% within 1 min [277]. Activated carbon derivative obtained through coconut buttons was used as a sorbent for extraction of lead, mercury, and copper from industrial waste material [278].