## 1 Introduction  
Chitosan (CS) is a natural polysaccharide composed of 2-amino-2-deoxy-D-glucose residues connected via 1,4-glycosidic bonds, which is obtained by deacetylation of chitin (Figure 1)--the main component of the exoskeleton of crustaceans, insects and the cell walls of fungi [1, 2, 3]. Deacetylation is carried out with concentrated aqueous solutions of NaOH (from 35% up to 50 wt.%) at temperatures of 100-180 \({}^{\circ}\)C for 0.5-10 days. To obtain highly deacetylated CS, an at least 10-fold molar excess of NaOH is required [4]. During the process of chitin deacetylation, the overall ordering of the molecule structure strongly decreases (the degree of crystallinity decreases to 40-50%). Deacetylation is also accompanied by hydrolysis of glycosidic bonds, which leads to the production of a polydisperse CS by molecular weight [5]. Deacetylation degree (DD), which is the ratio of the number of glucosamine units to the total number of monomer units in a polysaccharide molecule, is typically higher than 50% [6]. It is noteworthy that both chitin and CS are polysaccharides of variable composition representing a mixture of polymers that differ in molecular weight, the DD,  
Figure 1: Deacetylation of chitin. and the distribution of monomers units within the molecule [7]. The molecular weight of commercial CS varies in a wide range from 100 kDa up to 1200 kDa depending on the sources of raw materials and production method [1,8]. CS is a unique biopolymer that contains primary amines as well as hydroxyl groups and has exceptional properties and wide application. The high adsorption capacity of CS is used for the capture of cations, anions, organic dyes and pharmaceutical ingredients from wastewater. It is noteworthy that CS-based materials are also used as carriers for drug delivery. In addition, various CS-based metal-containing nanocomposite materials have been effectively used in catalytic reactions. CS can be also used for the development of promising inexpensive materials for the agricultural industry. The constant demand for effective adsorbents for the removal of pollutants can be met by fine-tuning the structural properties of CS using appropriate crosslinking agents or additives. Thus, the development of CS-based materials is a constantly growing field requiring continuous research [1,2,9], which also causes the corresponding growth of the CS market [10]. There is a large number of synthetic polyamines, such as polyaniline (PANI) [11,12,13,14], polyalkylamines [15,16], polylysine [17,18] and polyornithine [19], which can be applied in adsorption, catalysis or medicine. Synthetic materials reveal excellent antibacterial and adsorption properties. However, CS-based materials are still promising due to the low cost, biocompatibility and biodegradability. Muchresearch on chitin and CS isfocused on the search for new and improved production methods. Key areas of interest include enzymatic conversion; chemical or physical modifications of polysaccharides to expand their applicability, investigation of the mechanisms of biological activity of these polymers and products of their physical, chemical or enzymatic degradation; as well as biochemical and molecular characteristics of chitosanolytic and chitinolytic enzymes [20]. CS can be divided into the following types: technical, industrial, food and medical. This division depends on the CS solubility in diluted solutions of organic acids (acetic, lactic and malic acids) and its characteristics such as viscosity, molecular weight and DD. The most valuable for practical use is CS with high DD, low molecular weight and high solubility, which make it a structure-forming agent and effective sorbent. Lowmolecular weight CS can be produced using variety of methods, among which are oxidative depolymerization, plasma process, ultrasound treatment, acid hydrolysis and enzymatic hydrolysis [21,22,23,24,25,26,27]. The pKa of CS' amino groups is 6.3-6.5; hence, below this value, amino groups are protonated, and CS is highly soluble.