# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Types of Adsorbent
#### 6.6.1 Bark and Other Resources with Ample Tannin Content  
Agricultural origins contain tannin, lignin, cellulose, hemicellulose, extracts, lipids, proteins, sugars, water, and a variety of other compounds with different functional groups [114; 115]. The bark is obtained as a by-product of the wood manufacturing units. In bark, tannin is present in a large amount and this organic substance is very efficient for the extraction of heavy metal ions. Tannin contains polyhydroxy polyphenol moieties which help in the adsorption of metal ions [18]. [PERSON] et al. [116] were the first to report the removal of cadmium, mercury, and lead using black oak and Douglas fir bark respectively. Cadmium and chromium were removed from wastewater employing coffee, waste tea, and walnut shell [117]. [PERSON] et al. [118] also investigated the elimination of heavy metal ions like cadmium, lead, copper, and mercury by using bark, formaldehyde polymerised bark, and peanut skins. Adsorption capacities for bark have been reported to be in the range of 1.5-27.6 mg/g for various tannin-containing materials. However, the problem of discoloration of water arose due to the presence of soluble phenols. Chemical pre-treatment of bark with acid, base, or acidified formaldehyde has been shown to overcome the problem of discoloration in water without appreciably affecting ion removal capacity. Another by-product that is obtained from the wood industry is sawdust [119]. Adsorptions of Cu, Cr, Ni, Cd, and Pb have been reported by investigators using oak, red fir, and cedar sawdust in the capacities 0.0683-0.0982 meq/g [120]. Thus, the bark is helpful in the elimination of  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**S. No.** & **Adsorbent** & **Metal Ion/Adsorption Capacity** & **Reference** \\ \hline \(1\) & Lignin & \begin{tabular}{c} Pb\({}^{2+}\) = 1865 mg/g, \\ Zn\({}^{2+}\) = 95 mg/g \\ \end{tabular} & [107] \\ \hline \(2\) & Chitosan (powder) & Cd\({}^{2+}\) = 420 mg/g & [108] \\ \hline \(3\) & Chitosan (beads) & Cd\({}^{2+}\) = 518 mg/g & [108] \\ \hline \(4\) & Seaweed brown algae & Cd\({}^{2+}\) = 67 mg/g & [109] \\ \hline \(5\) & _A. nodosum_ seaweed & Cd\({}^{2+}\) = 215 mg/g & [109] \\ \hline \(6\) & \begin{tabular}{c} Formaldehyde cross-linked \\ _A. nodosum_ seaweed \\ \end{tabular} & Cd\({}^{2+}\) = 149 mg/g & [110] \\ \hline \(7\) & \begin{tabular}{c} Starch xanthate \\ \end{tabular} & \begin{tabular}{c} Cd\({}^{2+}\) = 33.3 mg/g \\ Cr\({}^{2+}\) = 17.6 mg/g \\ Hg\({}^{2+}\) = 1.15 mg/g \\ \end{tabular} & [111; 112] \\ \hline \(8\) & Cellulose xanthate & \begin{tabular}{c} Cd\({}^{2+}\) = 19.9 mg/g \\ Cr\({}^{2+}\) = 19.7 mg/g \\ Hg\({}^{2+}\) = 0.64 mg/g \\ Hg\({}^{2+}\) = 30.1 \(\pm\) 40.1 \\ \end{tabular} & [112] \\ \hline \(10\) & \
\begin{tabular}{c} Zeolites \\ \end{tabular} &
\begin{tabular}{c} Pb\({}^{2+}\) = 155.4 mg/g \\ Cd\({}^{2+}\) = 84.3 mg/g \\ Cr\({}^{2+}\) = 26.0 mg/g \\ Hg\({}^{2+}\) = 150.4 mg/g \\ \end{tabular} & [113] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of adsorption capacities of natural adsorbents for removal of toxic metal ions.  
toxic ions from water bodies and is the main possible option due to economics along with its large accessibility. In another study, [PERSON] et al. [101] used Andean Sacha inchi shell biomass (SISB) to expel Pb\({}^{2+}\) and Cu\({}^{2+}\) from wastewater. The better suit isotherms for both Pb\({}^{2+}\) and Cu\({}^{2+}\) were Langmuir > Freundlich, with maximal adsorption capacities of 17.066 and 9.699 mg/g at 323 K, respectively [101].