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Chitosan, a partly deacetylated form of chitin obtained by heating it with a heavy alkaline, as sodium or potassium hydroxide is a poly-β (1–4)-2-amino 2-deoxy-d-glucopyranose. Chitin is the main structural component in crustacean (shrimps, lobsters, crabs) scales and exoskeleton; it is produced as waste from the seafood industry from where it is collected for producing analytical grade chitin and chitosan. After cellulose, it’s the second richest natural polysaccharide on the earth [13] and is a major component of fungal cell walls. Chitin is a N-acetyl glucosamine polymer (GlcNAc), chemically aligned with β (1→4) binding. The alkali treatment partially removes some of the acetyl groups in chitin, producing chitosan. Chitosan is a polymer with residues connected by β (1→4), of N-acetylglucosamine (GlcNAc) and glucosamine (GlcN). The reaction of deacetylation leads to shift in molecular weight and its extent is calculated by the deacetylation degree (DD) definition, described as the molar fraction of glucosamine in chitosan (composition of N-acetyl glucosamine and glucosamine). Both chitin and chitosan belong to the aminoglucopyran family of polysaccharides [14, 15].

It is noteworthy that, due to the strong hydrogen bonding, the chitin-based components are stable in nature [16], while, in the presence of other cross-linking molecules, the chitosan-based substances are relatively easier to treat or handle [17]. Chitosan is processed in various ways, such as powder dust, glue paste, fabric and therefore is extensively used in the industrial applications [18]. The presence of various polar functional groups (–OH, –NH2 and C–O–C) implies that these polysaccharides have a high water retentivity capacity. Primary amine groups and primary alcoholic groups are present at C-2 and C-6 position along with secondary hydroxyl group [14]. The hydrogen bond insolubility of chitosan in bulk is insoluble at neutral and alkaline pH, the extent of which is determined by the DD (~60–100%). When in contact with anions, chitosan starts gelling, forming bead like structures under mild conditions [14, 19].

Chitosan are derived and processed from chitin either by chemical or enzymatic treatment as the glycosidic bonds are sensitive to these treatments. Enzyme treatment of chitin leads to the production of oligomers and comparatively, a more stable, well-defined chitosan is formed [20]. The mean chitosan molecular weight varies ~3.5–20 kDa (66–95% deacetylated) [21].

Use of chitosan as an effective flocculant due to factors like high ionic strength, abundantly available compound, biodegradable in nature and low rate of toxicity compared to any chemical flocculant is well established [22]. Chitosan consists of positive charges due to its high charge density ratio and microalgal cells are negative charge, therefore it is easily absorbed by the cells resulting in destabilizing the microalgae. Firstly, it neutralizes charges on microalgae, electrostatic charges are weakened and thus, reducing the interparticle repulsion leading to charge neutralization. The prices of biopolymer (chitosan) are substantial and expensive, at almost $21 per kg (Qingdao Yunzhou Biochemistry Co., Ltd) [23]. Although the cost of chitosan compared to the chemical inorganic flocculants are higher, it is a preferred methodology for harvesting the microalgal biomass. Chitosan, due to its divergent ability and properties like biodegradability, biocompatibility, bioactivity, etc., is employed in various applications like wastewater treatments, food processing, biomedical engineering, drug delivery, etc. [24, 25].