# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Types of Adsorbent
#### 6.6.3. Chitosan and Seafood Processing Wastes  
Chitin is also present in large quantities in nature and its origin is from the body of some invertebrate animals like crabs, different arthropods, and from various fungi. Chitosan is a deacetylated type of chitin and broadly utilised as an adsorbent particularly for wastewater treatment [127]. [PERSON] and [PERSON] reported that chitosan metal uptake was more than chitin due to the free amino moieties present during deacetylation [128]. Chitosan was found to have extraordinary holding efficiency at 1 mmol metal/g for the majority of metals in comparison to bark-activated sludge, poly(p-amino styrene), and other sorbents [116]. [PERSON] et al. noticed that the sorption capacity of chitosan changes with certain parameters like crystalline nature, hydrophobic nature, percentage of deacetylation, and free amino moiety [129]. Polysaccharides, after 50% deacetylation still have high solvency in water and present trouble for practical use. Therefore, in order to ensure the high stability of deacylated polysaccharides, they are crosslinked with glutaraldehyde, but it may cause a decrease in the sorption capacity of chitosan [130]. The effectiveness of crosslinked chitosan beads was also discussed by [PERSON] et al., (1993). The adsorbent obtained in the form of beads had more surface area than chitosan powder. Adsorption capacities of cadmium on chitosan powder and beads were found to be more significant capacity, a faster rate of adsorption, and enhanced ease of operation over different structures. Instead of being dissolved in an acid medium, chitosan is also non-porous in nature [108]. [PERSON] et al. used chitosan polysaccharides for the extraction of molybate ions from polluted water [131]. Chitosan was a cationic polysaccharide and its hydroxyl group was used to bind the Mo (VI) metal anions. Chitosan has a high affinity to bind Mo(VI) at pH 2.7. The adsorption capacity reached 100% after two cycles and adsorption time was found to be 160 min and the volume of treated water during this time period was 1.441 in the first cycle. The adsorption capacity of these polysaccharides was found to be 265 mg/g at 20 \({}^{\circ}\)C at an of pH 2.7. [PERSON] et al. recommended N-acylation as a method for intensifying permeability within chitosan [132]. Despite the fact that chitosan was excellently adsorbed by the ions in its original state, its adsorption capacity could be enhanced by reacting with different functional groups, for example, organic acids, onto the chitosan backbone [133]. Some functional groups grafted to chitosan were amino acid; esters, pyridyl, substituted pyridine rings, oxo 2-glutaric acid, and polyethyleneimine enhance the adsorption capacity. Chitosan derived from shrimp shells was used to remove Cu\({}^{2+}\) (98.97%), Cr\({}^{4+}\) (37.51%), and Fe\({}^{2+}\) (65.2%) from industrial water. After that, the mixture was rotated with a speed of 7000 revolutions per minute for 5 min. Cu\({}^{2+}\) (98.97%) had a higher affinity than Fe\({}^{2+}\) (65.2%), Cr\({}^{4+}\) (37.51%), and Zn\({}^{2+}\) (37.51%) among the four metal ions studied (86.15%) [134]. In addition, the researchers employed 0.1 g of the aforementioned adsorbent for chromium metal ions removal and obtained 19.28 mg/g removal efficiency by using shrimp chitosan. The removal efficiency was essentially boosted by increasing the adsorbent dosage and centrifuge speed [101]. Because of the presence of different functional groups, chitosan derived from a crustacean's shell was employed to remove hazardous metal ions from  
Figure 4: A schematic process flow diagram illustrating the synthesis of the lignin-based hybrid magnetic nanoparticles. Adapted with permission from Reference [125]. wastewater [135]. Due of the presence of amino groups in the chitosan sample, the amount of adsorption increased with the degree of deacetylation. The amount of adsorption was increased with the degree of deacetylation of chitosan because of the presence of amino groups in the chitosan sample.