**2.3 Resolution of chitosan**

Chitin is insoluble in dilute acids and many organic solvents. Because it is a semi-crystalline polymer and has a large amount of intramolecular and intermolecular hydrogen bonds in its structure. The effect of changes in the N-acetyl-Dglucosamine and D-glucosamine groups on solubility is shown in **Figure 4**.

The solubility concerning chitosan in a range of solvents and its polycationic properties are essential for the advent of chitosan derivatives and the use of them in many different applications. Many different factors are taken into account to control the solubility of Cts in aqueous solutions. These are the result of applying certain parameters such as temperature, alkali concentration, type and

**249**

two purposes:

*Modified Chitosan Forms for Cr (VI) Removal DOI: http://dx.doi.org/10.5772/intechopen.96737*

+

polyol is generally very excellent [15, 16].

**2.4 Cross-linkers used for chitosan cross-linking**

desorption is performed in acidic solutions,

increase in adsorption selectivity).

formation of (NH3)

*Nature of Cts below its pKa.*

**Figure 4.**

concentration of acid, pH, deacetylation time, previous treatments applied to chitin isolation and chitin particle size, etc. Cts is regarded as a strong base as it has amino groups with a pKa value equal to 6.3. Cts dissolves easily in dilute acid solutions below pH 6.0. It is a reality that the amino groups in the chitosan structure have significantly changed its charged state and its properties. H<sup>+</sup>

present in low pH solutions protonate the NH2 groups of Cts and causes the

situation turns Cts into a very easily soluble polyelectrolyte in acidic medium. The most functional amine groups of Cts are protonated in low pH buffer solution. On the other hand, Cts amines are transformed to deprotonated form as the pH rises above 6 and the polymer loses its charge and becomes insoluble in basic solutions [12, 13]. The soluble–insoluble transition forms at its pKa value between pH 6.0 and 6.5. The soluble and insoluble situation of Cts in aqueous media occurs pKa valued around pH between 6 and 6.5. pKa is considered to be significantly dependent on the degree of N-acetylation, hence Cts solubility varies depending on the deacetylation method applied [14]. Acetic formic, formic acid, lactic acids and organic acids are the most commonly used solvents. They can easily dissolve Cts as it is easily converted into quaternary nitrogen salts in aqueous solution at low pH values [15]. Cts, on the other hand, can be easily dissolved in some solutions (formic acid, acetic acid, etc.) at pH <6 due to its cationic structure. Considering the best solvents, it has been found that it is the formic acid from which solutions are obtained in aqueous systems containing 0.2–100% formic acid. 1% acetic acid solution around pH 4.0 has been applied as the most utilized solvent in the Cts applications [15]. Likewise, Cts is very easily soluble in hydrochloric acid and dilute nitric acid solutions, however insoluble in sulfuric and phosphoric acids. These acids are not shown since they can break the chitosan polymeric chains promoting depolymerization. Cts is very difficult to break up in organic solvents, for example, dimethylformamide and dimethyl sulfoxide. Its solubility in the acidified

Cts is soluble in both organic and mineral acids. This is a limiting parameter for industrial applications in wastewater treatments. By using different chemical reagents in the cross-linking process, the structure of Cts is strengthened mechanically and chemically (**Figure 5**). The chemical modification of Cts can be done for

a.To prevent the dissolution of the polymer when metal adsorption or metal

b.To increase the metal adsorption properties (increase in adsorption capacity or

. Thus, Cts has a polycationic biopolymer structure. This

ions

**Figure 3.** *General steps for chitin and Cts production.*

*Modified Chitosan Forms for Cr (VI) Removal DOI: http://dx.doi.org/10.5772/intechopen.96737*

**Figure 4.** *Nature of Cts below its pKa.*

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

4.Decoloration (removal of pigments). These stages are given in **Figure 3**.

Biological methods are processes that can be examined under several headings. Cell cultures of various organisms such as Mucor rouxii, Phycomyces blakesleeanus are used in the microbial synthesis of Cts. The product obtained is developed by adding Aspergillus niger to the culture medium. Thus, this production mechanism also leads to the deacetylation of the Cts. Cts is obtained at the end of the 96-hour incubation period [10]. As a biological method, the use of proteolytic enzymes such as pepsin, trypsin, protease, proteinase and papain in protein removal can be counted [11]. Today, many other polymers used in industry are produced synthetically. Synthetic polymers have limited biocompatibility and biodegradability

Chitin is insoluble in dilute acids and many organic solvents. Because it is a semi-crystalline polymer and has a large amount of intramolecular and intermolecular hydrogen bonds in its structure. The effect of changes in the N-acetyl-Dglucosamine and D-glucosamine groups on solubility is shown in **Figure 4**.

The solubility concerning chitosan in a range of solvents and its polycationic properties are essential for the advent of chitosan derivatives and the use of them in many different applications. Many different factors are taken into account to control the solubility of Cts in aqueous solutions. These are the result of applying certain parameters such as temperature, alkali concentration, type and

3.Deproteinization (removal of proteins).

compared to Cts, a natural polymer.

**2.3 Resolution of chitosan**

**248**

**Figure 3.**

*General steps for chitin and Cts production.*

concentration of acid, pH, deacetylation time, previous treatments applied to chitin isolation and chitin particle size, etc. Cts is regarded as a strong base as it has amino groups with a pKa value equal to 6.3. Cts dissolves easily in dilute acid solutions below pH 6.0. It is a reality that the amino groups in the chitosan structure have significantly changed its charged state and its properties. H<sup>+</sup> ions present in low pH solutions protonate the NH2 groups of Cts and causes the formation of (NH3) + . Thus, Cts has a polycationic biopolymer structure. This situation turns Cts into a very easily soluble polyelectrolyte in acidic medium. The most functional amine groups of Cts are protonated in low pH buffer solution. On the other hand, Cts amines are transformed to deprotonated form as the pH rises above 6 and the polymer loses its charge and becomes insoluble in basic solutions [12, 13]. The soluble–insoluble transition forms at its pKa value between pH 6.0 and 6.5. The soluble and insoluble situation of Cts in aqueous media occurs pKa valued around pH between 6 and 6.5. pKa is considered to be significantly dependent on the degree of N-acetylation, hence Cts solubility varies depending on the deacetylation method applied [14]. Acetic formic, formic acid, lactic acids and organic acids are the most commonly used solvents. They can easily dissolve Cts as it is easily converted into quaternary nitrogen salts in aqueous solution at low pH values [15]. Cts, on the other hand, can be easily dissolved in some solutions (formic acid, acetic acid, etc.) at pH <6 due to its cationic structure. Considering the best solvents, it has been found that it is the formic acid from which solutions are obtained in aqueous systems containing 0.2–100% formic acid. 1% acetic acid solution around pH 4.0 has been applied as the most utilized solvent in the Cts applications [15]. Likewise, Cts is very easily soluble in hydrochloric acid and dilute nitric acid solutions, however insoluble in sulfuric and phosphoric acids. These acids are not shown since they can break the chitosan polymeric chains promoting depolymerization. Cts is very difficult to break up in organic solvents, for example, dimethylformamide and dimethyl sulfoxide. Its solubility in the acidified polyol is generally very excellent [15, 16].

## **2.4 Cross-linkers used for chitosan cross-linking**

Cts is soluble in both organic and mineral acids. This is a limiting parameter for industrial applications in wastewater treatments. By using different chemical reagents in the cross-linking process, the structure of Cts is strengthened mechanically and chemically (**Figure 5**). The chemical modification of Cts can be done for two purposes:


The modifications carried out in the structure of Cts are aimed to further improve its biological and chemical properties and to change its solubility in the solvent or aquatic environment to be used. The behavior of Cts chains in diluted acids has been studied and at higher ionic strength or lower pH values, amino groups will undergo a higher protonation. In these cases, the macromolecules have been found to behave more like stiff rods [17, 18]. Cross-linking process can be applied by the reaction of Cts with different crosslinking agents such as glutaraldehyde, epichlorohydrin, polyethyleneimine etc. (**Figure 5**). Today, there are many activating agents and newly developed methods. Glutaraldehyde is widely used crosslinking agent because it is cheap and very effective crosslinker. Glutaraldehyde, a linear 5-carbon dialdehyde, is transparent and colorless [19]. It is an oily liquid with a pungent odor that can be dissolved in water, alcohol and organic solvents in all proportions. Chemical stability may be enhanced through chemical procedures which include crosslinking with glutaraldehyde for utility in a chemically acidic environment (**Figure 6**). By means of developing new bonds between Cts chains, the polymer is proof against dissolution even in strong solutions such as hydrochloric acid solution. In addition, the reality that Cts includes amine groups is an essential factor in being a good adsorbent. It is possible to observe an increase in the adsorption

**251**

*Modified Chitosan Forms for Cr (VI) Removal DOI: http://dx.doi.org/10.5772/intechopen.96737*

**3.1 Toxicity of chromium**

causes lung cancer [28, 29].

**3.2 Removal of hexavalent chromium**

the adsorption approach preferred [23, 30].

the Cts chain. It quickly joins the matrix structure of Cts.

capacity by increasing the amine groups. The most important advantage of glutaraldehyde is the presence of suitable amino groups that perform the binding on Cts surfaces [21, 22]. Adding cross-links to the structure of Cts gives it a solid threedimensional structure. To form cross-linked Cts, chemicals with a wide variety of groups are used to form the cross-link structure. Condensation reaction resulting from the reaction between an aldehyde function and a primary amine group from

There may be the opportunity to apply mono-functional reagents (epichlorohydrin) by opening the ether group for grafting to an amine function through the Schiff base reaction, which is likewise capable of react, at the same time the chloride group then interacts with different functional groups or other amines. Tri-polyphosphate is also selected as a possible cross-linking agent, which may be

The toxicity of chromium compounds varies according to the pH, temperature and oxidation step. Cr (VI) ions are much more toxic than chromium (III) (Cr (III)) in terms of toxicity [23, 24]. EPA's threshold limit value is 10 times lower than Cr (III). When Cr (VI) solution mixes with seawater, it prevents some aquatic plants from photosynthesis, reduces reproduction in fish and can cause fish death. Cr (VI) causes burns in human body, in case of contact, causing irritation, wounds and ulcers on the skin and respiratory tract [25]. Sensitivity to Cr (III) and Cr (VI) causes allergic reactions, redness in the eyes and nose, itching and rash. Taking Cr (VI) with the digestive system can cause ulcers, necrosis and death in the stomach and intestines. The recommended Cr (VI) limit in drinking water is 0.05 ppm [26, 27]. One of the most important effects of Cr (VI), which is a very oxidizing substance, is that it

Chromium pollution is especially caused by chrome plating, automotive, leather and paint industry wastes [18]. Traditional refining methods used for chromium refining are not highly efficient, and these techniques require large amounts of chemicals and energy. Because they are costly, their use is impractical. Adsorption has superior properties in these aspects. In chromium treatment, the ability to use many different sources as adsorbents, such as plants, animal materials and various microorganisms, are easy to obtain, they can be produced by cheap and simple methods, regeneration ease and high removal efficiency are the features that make

Chemical precipitation, microfiltration, ultrafiltration, flotation, reduction, dialysis, membrane technologies, chelating, ion exchange, evaporation, solvent extraction, reverse osmosis, and adsorption can be listed among the methods used for dechroming of industrial wastewater [20]. In the selection of these methods, the acidic or basic character of the wastewater, the target envisaged for removal and recovery, the type and concentration of the chromium compound in the waste, the cost, chemical and energy consumption, the management of the waste generated by

The pH of the aqueous chromium influences the surface charge of the modified adsorbent, the degree of ionization and the adsorbate species. Depending on

treatment and the removal efficiency should be taken into consideration.

used to prepare Cts gel beads by its coagulation and neutralization impact.

**3. Adsorption of Cr (VI) using chitosan-based materials**

**Figure 6.** *Schematic representation of the cross-linking reaction of Cts with glutaraldehyde [20].*

*Modified Chitosan Forms for Cr (VI) Removal DOI: http://dx.doi.org/10.5772/intechopen.96737*

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

The modifications carried out in the structure of Cts are aimed to further improve its biological and chemical properties and to change its solubility in the solvent or aquatic environment to be used. The behavior of Cts chains in diluted acids has been studied and at higher ionic strength or lower pH values, amino groups will undergo a higher protonation. In these cases, the macromolecules have been found to behave more like stiff rods [17, 18]. Cross-linking process can be applied by the reaction of Cts with different crosslinking agents such as glutaraldehyde, epichlorohydrin, polyethyleneimine etc. (**Figure 5**). Today, there are many activating agents and newly developed methods. Glutaraldehyde is widely used crosslinking agent because it is cheap and very effective crosslinker. Glutaraldehyde, a linear 5-carbon dialdehyde, is transparent and colorless [19]. It is an oily liquid with a pungent odor that can be dissolved in water, alcohol and organic solvents in all proportions. Chemical stability may be enhanced through chemical procedures which include crosslinking with glutaraldehyde for utility in a chemically acidic environment (**Figure 6**). By means of developing new bonds between Cts chains, the polymer is proof against dissolution even in strong solutions such as hydrochloric acid solution. In addition, the reality that Cts includes amine groups is an essential factor in being a good adsorbent. It is possible to observe an increase in the adsorption

*Schematic representations of the cross-linking reaction of Cts with cross-linking agents.*

*Schematic representation of the cross-linking reaction of Cts with glutaraldehyde [20].*

**250**

**Figure 6.**

**Figure 5.**

capacity by increasing the amine groups. The most important advantage of glutaraldehyde is the presence of suitable amino groups that perform the binding on Cts surfaces [21, 22]. Adding cross-links to the structure of Cts gives it a solid threedimensional structure. To form cross-linked Cts, chemicals with a wide variety of groups are used to form the cross-link structure. Condensation reaction resulting from the reaction between an aldehyde function and a primary amine group from the Cts chain. It quickly joins the matrix structure of Cts.

There may be the opportunity to apply mono-functional reagents (epichlorohydrin) by opening the ether group for grafting to an amine function through the Schiff base reaction, which is likewise capable of react, at the same time the chloride group then interacts with different functional groups or other amines. Tri-polyphosphate is also selected as a possible cross-linking agent, which may be used to prepare Cts gel beads by its coagulation and neutralization impact.
