Heavy metals, known for their toxic nature and ability to accumulate and magnify in the food chain, are a major environmental concern. The use of environmentally friendly adsorbents, such as chitosan (CS)—a biodegradable cationic polysaccharide, has gained attention for removing heavy metals from water.
1. Introduction
Various materials, including natural contaminants, fluoride, chloride, nitrate, iron, calcium, magnesium, and other contaminants from byproducts of agriculture and industries such as heavy metals, organic dyes, insecticides and fertilizers, spilled oils, batteries, diesel fuel, household chemicals such as synthetic detergents, and some pathogenic microbes can contaminate the ocean, groundwater and surface water bodies (rivers, lakes, ponds, reservoirs)
[1][2][3][4][5][6]. Contaminated water will not be suitable for drinking, habitat, irrigation, recreation, and other industrial activities
[4][7].
Heavy metal has gained significant importance in ecotoxicology due to their long persistence, bioaccumulation, and bio-magnification in the food chain
[8]. Heavy metal ions are commonly discharged by various industrial activities, including electroplating, battery manufacturing, pesticide production, mining, nuclear power, textile manufacturing, and other similar industries
[9][10][11]. The global average content of Cr, Mn, Co, Ni, As and Cd on surface water bodies exceeded the permitted level suggested by WHO and USEPA Guidelines
[8]. Heavy metal ions, such as As, Cr, Ni, Co, Hg, Pb, U, and Cd, have become a major threat as they are toxic in nature and accumulate in food chains due to their non-biodegradability and cause various health-related issues even at trace levels. Therefore, in order to safeguard human health and the environment, it is crucial to eliminate heavy metals prior to their release into the surroundings
[10][12][13][14][15].
2. Chitin Sources and Composition
To address environmental concerns, the industrial production of chitin and chitosan (CS) must be carried out on a large scale and at a competitive cost
[16]. Chitin can be found in various sources, including crustacean exoskeletons (lobsters, shrimps, prawns, crabs, krill, crayfish), mollusks (octopus, cuttlefish, clams, oysters, squids, snails), algae (diatoms, brown algae, green algae), insects cuticles, and fungal cell walls
[17]. Presently, crustacean waste, such as shrimp, crabs, prawns, and lobsters, is the primary source of industrial chitin
[17][18]. Though, chitin/chitosan from crustaceans has limitations, such as limited raw material supply, a seasonal, higher concentration of CaCO
3, requirement of chemical treatment, the possibility of heavy metal contamination, and consume long time. In contrast, chitin/chitosan fungal sources have seasonal independence, superior particle size, uniformity, lower molecular weight, and are free from heavy metals
[19].
Table 1 outlines the proximate composition of several chitin sources, with shells comprising chitin, protein, minerals, and pigments
[20]. Crustacean (crab, shrimp, and lobster) shell wastes consist of 15–40% chitin, 20–50% CaCO
3, and 20–40% protein, along with lipids, pigments, and other minerals in small amounts
[21]. Therefore, removing proteins, minerals, and pigments is necessary for chitin production
[18].
Table 1. Proximate composition of different chitin sources
[22].
Sources |
Protein (%) |
Ash (%) |
Chitin (%) |
Moisture (%) |
Lipid (%) |
Shrimp shells |
32.77 |
32.46 |
36.43 |
45.65 |
- |
Shrimp shells (P. longirostris) |
29.23 |
25.06 |
26.98 |
3.25 |
15.48 |
Shrimp shells (Penaeus durarum) |
34.02 |
42.26 |
23.72 |
- |
- |
Insect cuticles (Cicada sloughs) |
39.8 |
11.7 |
36.6 |
8.7 |
2.7 |
Crabs’ shells |
16.68 |
66.58 |
16.73 |
- |
- |
Mussel shells |
9.99 |
23.25 |
23.25 |
- |
- |
Squid gladius (L. vulgaris) |
36.52 |
2.57 |
31.2 |
- |
0..32 |