## 1 Introduction  
The release of toxic metal cations to the ecosystem through industrial wastewater is an alarming situation. The presence of such metal ions is a threat to animals and humans due to its toxic nature in both soil and water [1]. These metals ions entered the aquatic environment through wastewater coming from different industries such as the fertilizer industry, mining, ore, refining, electroplating, painting, batteries, and tanneries [2]. One of the most toxic metal ions is lead, which is considered as a natural toxin. Lead is mostly released from factories, metal recycling, lead-acid batteries, and pipes [3]. Its exposure can cause reproductive, renal, hematopoietic, and central nervous system disorders in the human body [4]. Acute exposure mostly results in headaches, nausea, emotional disruption, and cognitive changes [5]. Cadmium, a heavy metal ion, has an ample noxiousness and harmful impact on most of the organ systems [6]. Cadmium is considered as one of the toxic substances to humans and can cause death. Exposure to cadmium through soil, water, and air can cause cancer, respiratory, central, and peripheral nervous, urinary, and skeletal disorders [7]. Copper is a crucial component for human body, acting as a cofactor of a variety of enzymes such as cytochrome oxidase, ceruloplasmin, superoxide dismutase, dopamine \(\beta\)-hydroxylase, and tyrosinase. However, excess copper and other toxic metal ions can disturb the capability of the liver for disrupting excess copper contents inside the human body. Thus, eventually affecting the organ system such as nervous, reproductive, and adrenal functions, connective tissues, the learning ability of newborn babies, etc. [8; 9; 10; 11]. Various physical and chemical processes have been developed for the decontamination of these toxic metal ions from industrial effluents such as filtration [12], solvent extraction [13], precipitation, ion exchanger [14], reverse osmosis [15], oxidation/reduction [16], filtration [12], electrochemical techniques [17], and sorption [18; 19]. Currently, sorption is advocated as one the most efficient, low-cost, and simple techniques for eliminating toxic metal ions from aqueous solution. It is currently perceived as a successful and monetary strategy for the expulsion of contaminants from water and wastewater systems. Therefore, there is a need for synthesizing an efficient, low cost, simple, and environmentally friendly sorbent for cations remediation from wastewater [20]. For the decontamination of these toxic metal ions from wastewater, a variety of synthetic as well as natural polymers is being manufactured or modified chemically. Sorbents obtained from biopolymers of natural origin have immensely being used to remove heavy metal ions from industrial wastewater and related aqueous environments [21]. Biopolymers can completely decontaminate metal ions from industrial effluents. The class of said biopolymers subsumes cellulose, chitin, chitosan, lignins, alginate, and carrageenan [22; 23; 24]. After cellulose, chitin is considered the most abundant natural polymer obtained from the arthropod's exoskeleton, mostly crabs (_Malacostraca_) and insects (Insecta) [25]. Chitosan is a linear polysaccharide and deacetylated derivative of chitin [24]. It has been intensively applied in food industries, biomedical, biotechnological, pharmaceutical, cosmetics, agricultural, and water treatment [26]. Chitosan is biocompatible, environmentally friendly, antibacterial, and biodegradable; it also demonstrates high sorption capacity toward metal ions because of the chemical composition of amino and hydroxyl groups [24; 27]. The degree of deacetylation is the main controlling factor for sorption capacity [28]; it also affects the properties of chitosan, which are mainly physical, chemical, and biological. The alteration of natural polymers is an effective technique for the manufacturing of new materials to present extraordinary properties and amplify the field of the plausible uses of those large existing biopolymers [29].