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

## **3.1 Toxicity of chromium**

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 causes lung cancer [28, 29].

#### **3.2 Removal of hexavalent chromium**

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 the adsorption approach preferred [23, 30].

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 treatment and the removal efficiency should be taken into consideration.

The pH of the aqueous chromium influences the surface charge of the modified adsorbent, the degree of ionization and the adsorbate species. Depending on the current pH, the Cr (VI) waste solution can be found in dichromate (Cr2O7 2−), chromate (CrO4 2−) and hydrogen chromate (HCrO4 − ) forms. According to the pH of the Cr (VI) in aqueous solution varying ion types are given (**Figure 7**). HCrO4 − and Cr2O7 2− are the dominant species on the experimental concentration and these are the dominant components at a low pH. When the pH is excessive, CrO4 2− is the dominant component of Cr (VI), so the initial solution pH is an important parameter for the adsorption of chromium. Electrostatic attraction forces between positively charged adsorbent surface groups and anionic Cr (VI) types occurs. This is particularly more common because Cr (VI) are present as oxyanions (HCrO4 − , Cr2O7 2− and CrO4 2−) in solution phase. Positively charged adsorbent surface groups are formed as a result of the formation of both protonation or quaternization of the groups on the adsorbent matrix.

The equilibrium among the various Cr (VI) species may be represented by using Eqs (1), (2) and (3).

$$\mathrm{H\_2CrO\_4}^- \rightleftharpoons \mathrm{H^+} + \mathrm{HCrO\_4}^- \tag{1}$$

$$\text{HCrO}\_4^- \rightleftharpoons \text{H}^+ + \text{CrO}\_4^{2-} \tag{2}$$

$$\text{2HClCrO}\_4^- \rightleftharpoons \text{Cr}\_2\text{O}\_7^{2-} + \text{H}\_2\text{O} \tag{3}$$

A few other parameters affecting the adsorbent capacity; solution pH, equilibration time, temperature of solution, coexisting ions and adsorbent dosage concentration.

## **3.3 Application of various chitosan form for Cr (VI) removal**

Cts modifications can be by physical methods or by chemical methods in which crosslinking or gridding of functional groups is performed. Cts can form perfect gels in the membrane, bead, capsule or different forms thanks to its ability to dissolve in organic acids and to combine with polyionic compounds. Four different methods are used to create Cts gels. These are solvent evaporation method, neutralization method, cross-linking method and ionotropic gelation method. Among these methods, the neutralization method is the main method used in the preparation of spherical Cts beads of different sizes and pore properties. It is carried out by

**253**

**Figure 8.**

*Adsorption mechanism of chromium hexavalent [28].*

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

chemical reactions inclusive of O-acetylation [31].

positively charged surface and negatively charged anion (HCrO4

this could motive the increase of adsorption of Cr (VI).

dropping Cts solution into NaOH and ethanol solution drop by drop. Since Cts is not soluble at high pH values, the drops become solid due to polymer precipitation. The chelating property of Cts enables it to retain various metal ions. Cts is a polymer that has a wide range of uses in the removal of heavy metals from wastewater systems, in the food industry, in the encapsulation of drugs that need to be released slowly or overtime, in cosmetics, agriculture, pulp and paper industry to have all these properties. Cts, a poly N acetyl glucosamine, contains cationic amino and polar hydroxyl groups, which are chemically reactive groups attached to the glucosamine chain it carries. These groups support immobilization methods such as adsorption and covalent bonding. Amino capability qualifies Cts for a lot of chemical reactions inclusive of quaternization, grafting, alkylation and reaction with carbonyl compounds. The presence of the hydroxyl group lets in Cts to form hydrogen bonding together with polar atoms, grafting, crosslinking, and some

Recently, more composite materials containing modified Cts used in chromium removal have been reported. Eliodório et al. investigated the synthesis and characterization of Cts functionalized with three different ionic liquids and their application in Cr (VI) waste treatment [28]. Amino groups in Cts are significantly effective in the adsorption mechanism, especially when protonated at acidic pHs. Besides, intermolecular interaction is also effective. This phenomenon will increase the cationic ability on the Cts surface, facilitating the adsorption of negative groups via electrostatic interaction and improving the elimination of Cr (VI) (**Figure 8**). At acidic pHs, there has been a strong protonation of the amine groups that gave the Cts surface a positive character. Thus, the electrostatic attraction force between this

Parlayıcı and Pehlivan inspect the adsorption of chromium ions on Cts doped

with multiwalled carbon nanotubes (Cht-MWCNT) [32]. Under optimum

−

) will boom and

**Figure 7.** *Chromium speciation as a function of pH.*

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

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

2−) and hydrogen chromate (HCrO4

chromate (CrO4

2− and CrO4

Eqs (1), (2) and (3).

concentration.

groups on the adsorbent matrix.

and Cr2O7

Cr2O7

the current pH, the Cr (VI) waste solution can be found in dichromate (Cr2O7

are the dominant components at a low pH. When the pH is excessive, CrO4

the dominant component of Cr (VI), so the initial solution pH is an important parameter for the adsorption of chromium. Electrostatic attraction forces between positively charged adsorbent surface groups and anionic Cr (VI) types occurs. This is particularly more common because Cr (VI) are present as oxyanions (HCrO4

are formed as a result of the formation of both protonation or quaternization of the

A few other parameters affecting the adsorbent capacity; solution pH, equilibration time, temperature of solution, coexisting ions and adsorbent dosage

Cts modifications can be by physical methods or by chemical methods in which crosslinking or gridding of functional groups is performed. Cts can form perfect gels in the membrane, bead, capsule or different forms thanks to its ability to dissolve in organic acids and to combine with polyionic compounds. Four different methods are used to create Cts gels. These are solvent evaporation method, neutralization method, cross-linking method and ionotropic gelation method. Among these methods, the neutralization method is the main method used in the preparation of spherical Cts beads of different sizes and pore properties. It is carried out by

**3.3 Application of various chitosan form for Cr (VI) removal**

The equilibrium among the various Cr (VI) species may be represented by using

of the Cr (VI) in aqueous solution varying ion types are given (**Figure 7**). HCrO4

2− are the dominant species on the experimental concentration and these

−

2−) in solution phase. Positively charged adsorbent surface groups

−+ − H CrO H HCrO 2 4 + <sup>4</sup> (1)

−+ − + <sup>2</sup> HCrO H CrO 4 4 (2)

− − + <sup>2</sup> 2HCrO Cr O H O 4 27 2 (3)

2−),

− ,

2− is

−

) forms. According to the pH

**252**

**Figure 7.**

*Chromium speciation as a function of pH.*

dropping Cts solution into NaOH and ethanol solution drop by drop. Since Cts is not soluble at high pH values, the drops become solid due to polymer precipitation.

The chelating property of Cts enables it to retain various metal ions. Cts is a polymer that has a wide range of uses in the removal of heavy metals from wastewater systems, in the food industry, in the encapsulation of drugs that need to be released slowly or overtime, in cosmetics, agriculture, pulp and paper industry to have all these properties. Cts, a poly N acetyl glucosamine, contains cationic amino and polar hydroxyl groups, which are chemically reactive groups attached to the glucosamine chain it carries. These groups support immobilization methods such as adsorption and covalent bonding. Amino capability qualifies Cts for a lot of chemical reactions inclusive of quaternization, grafting, alkylation and reaction with carbonyl compounds. The presence of the hydroxyl group lets in Cts to form hydrogen bonding together with polar atoms, grafting, crosslinking, and some chemical reactions inclusive of O-acetylation [31].

Recently, more composite materials containing modified Cts used in chromium removal have been reported. Eliodório et al. investigated the synthesis and characterization of Cts functionalized with three different ionic liquids and their application in Cr (VI) waste treatment [28]. Amino groups in Cts are significantly effective in the adsorption mechanism, especially when protonated at acidic pHs. Besides, intermolecular interaction is also effective. This phenomenon will increase the cationic ability on the Cts surface, facilitating the adsorption of negative groups via electrostatic interaction and improving the elimination of Cr (VI) (**Figure 8**). At acidic pHs, there has been a strong protonation of the amine groups that gave the Cts surface a positive character. Thus, the electrostatic attraction force between this positively charged surface and negatively charged anion (HCrO4 − ) will boom and this could motive the increase of adsorption of Cr (VI).

Parlayıcı and Pehlivan inspect the adsorption of chromium ions on Cts doped with multiwalled carbon nanotubes (Cht-MWCNT) [32]. Under optimum

**Figure 8.** *Adsorption mechanism of chromium hexavalent [28].*

situations, maximum adsorption capacity of Cr (VI) determined by using Langmuir model have been improved 26.14 mg/g. When carbon nanotubes (CNT) is examined, carbon atoms are allotropes of carbon that have an aromatic surface while in a sp2 -type hybridization rolled together like a tubular system (1D system) [33]. The structural properties of the CNT's surface permit because of an intense together with solids having organic molecules through non-covalent forces, for example, hydrogen bonding, π–π stacking, electrostatic forces, van der Waals forces and hydrophobic interactions. Moreover, the CNT structure allows the incorporation of one or more chemicals functional groups and they can build selectivity and stability of the subsequent framework. The chemical functional groups may be anchored to CNT surface through functionalization or purification processes [34]. This carbon allotrope structure is an interesting nanostructure that harbors remarkable electronic and mechanical properties that are directly dependent on chirality and diameter. Its excellent properties combined with unusual morphology make it extremely attractive for some many practical applications in wastewater systems; for example, with the improvement of solid composite materials for ideal adsorption, high impact ability, selectivity and productivity [35].

Cts-humic acid-graphene oxide (Cts-HA-GO) composite was produced (**Figure 9**) and the removal of toxic Cr (VI) ion from aqueous solutions was

**255**

**Figure 10.**

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

Cr (VI) ion has occurred at pH 2 value as 83.64 mg g−1.

the Cts biopolymer. After Fenton modification, HCrO4

*The adsorption of Cr (VI) onto the Fenton modified Cts by a sandwich model [24].*

studied by batch equilibration technique [27]. The percentage adsorption was 85.28% by using 200 ppm Cr (VI) and 2 g/L composite. Maximum adsorption of

and outstanding hydrophilic properties owing to hydrogen bonding [36].

Graphene oxide (GO)-based adsorbents are very useful substances because of the presence of functional groups on the surface of imparted for the duration of the oxidation of graphite. The presence of those groups is taken into considered best adsorbent for the use of graphene oxide for metal ion chelation because they show off hydrophilic properties due to hydrogen bond. Some other exciting magnificence adsorbents is GO-based adsorbent, mainly due to the plentiful –OH, –COOH and – C=O functional groups on their surface imparted throughout oxidation of graphite. The presence of such groups makes GO an ideal adsorbent for metal ion chelation

Humic acids (HA) are a principal component of humic substances, which are the significant constituents of soil, peat and coal. It is a perplexing combination of various acids containing carboxyl and phenolate groups so that the mixture acts functionally as a dibasic acid or, occasionally, as a tribasic acid. There are functional groups in the HA structure. These consist of various types of carboxylic (COOH), phenolic hydroxyl and carbonyl (C=O) structures. HAs have an extraordinary effect on metal adsorption as they have carboxyl and phenolic -OH groups that interact with various metal ions [37]. The carboxyl groups, which render the polymers negatively charged at neutral pH values, are in particular effective in complexing metal ions in aqueous solution. Cts consists of a crystal phase and a non-crystalline (amorphous) phase [38]. Crystalline Cts molecules are in the form of layers and are tightly held by intra-layer hydrogen bonds (**Figure 10**). Fenton reaction was used to increase the Cr (VI) adsorption capacity of Cts [24]. With the Fenton modification, Cts both efficiently adsorbed Cr (VI) and converted it to less toxic Cr (III) over a wide pH range as a result of layer formation defined by a sandwich model. The highly reactive HO• destroyed the hydrogen bond in the Cts structure, and Fe3+ ions were chelated with

Cts structure and was adsorbed in the newly formed adsorption sites. The adsorption process of Cr (VI) using the Fenton modified Cts, adsorption isotherms of Freundlich were investigated and the adsorption capacity exceeded 120 mg/g.

Cellulose and lignin-based adsorbents are the most basic biopolymer widely used for toxic metal removal from wastewater. Cellulose exhibits natural qualities such as strength, reward, biodegradability, non-toxicity, and mechanical stability. However, cellulose is an odorless water-soluble linear polymer covalently bonded in its structure by the monomeric units of b-D-anhydroglucopyranose C1-C4 b-glycosidic bonds [39]. Due to its water solubility, it has some drawbacks in terms of being used as an

4−entered the gap in the

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

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

tion, high impact ability, selectivity and productivity [35].

*Preparation of Cts coated humic acid/graphene oxide composite beads.*

while in a sp2

situations, maximum adsorption capacity of Cr (VI) determined by using

Langmuir model have been improved 26.14 mg/g. When carbon nanotubes (CNT) is examined, carbon atoms are allotropes of carbon that have an aromatic surface

[33]. The structural properties of the CNT's surface permit because of an intense together with solids having organic molecules through non-covalent forces, for example, hydrogen bonding, π–π stacking, electrostatic forces, van der Waals forces and hydrophobic interactions. Moreover, the CNT structure allows the incorporation of one or more chemicals functional groups and they can build selectivity and stability of the subsequent framework. The chemical functional groups may be anchored to CNT surface through functionalization or purification processes [34]. This carbon allotrope structure is an interesting nanostructure that harbors remarkable electronic and mechanical properties that are directly dependent on chirality and diameter. Its excellent properties combined with unusual morphology make it extremely attractive for some many practical applications in wastewater systems; for example, with the improvement of solid composite materials for ideal adsorp-

Cts-humic acid-graphene oxide (Cts-HA-GO) composite was produced (**Figure 9**) and the removal of toxic Cr (VI) ion from aqueous solutions was


**254**

**Figure 9.**

studied by batch equilibration technique [27]. The percentage adsorption was 85.28% by using 200 ppm Cr (VI) and 2 g/L composite. Maximum adsorption of Cr (VI) ion has occurred at pH 2 value as 83.64 mg g−1.

Graphene oxide (GO)-based adsorbents are very useful substances because of the presence of functional groups on the surface of imparted for the duration of the oxidation of graphite. The presence of those groups is taken into considered best adsorbent for the use of graphene oxide for metal ion chelation because they show off hydrophilic properties due to hydrogen bond. Some other exciting magnificence adsorbents is GO-based adsorbent, mainly due to the plentiful –OH, –COOH and – C=O functional groups on their surface imparted throughout oxidation of graphite. The presence of such groups makes GO an ideal adsorbent for metal ion chelation and outstanding hydrophilic properties owing to hydrogen bonding [36].

Humic acids (HA) are a principal component of humic substances, which are the significant constituents of soil, peat and coal. It is a perplexing combination of various acids containing carboxyl and phenolate groups so that the mixture acts functionally as a dibasic acid or, occasionally, as a tribasic acid. There are functional groups in the HA structure. These consist of various types of carboxylic (COOH), phenolic hydroxyl and carbonyl (C=O) structures. HAs have an extraordinary effect on metal adsorption as they have carboxyl and phenolic -OH groups that interact with various metal ions [37]. The carboxyl groups, which render the polymers negatively charged at neutral pH values, are in particular effective in complexing metal ions in aqueous solution.

Cts consists of a crystal phase and a non-crystalline (amorphous) phase [38]. Crystalline Cts molecules are in the form of layers and are tightly held by intra-layer hydrogen bonds (**Figure 10**). Fenton reaction was used to increase the Cr (VI) adsorption capacity of Cts [24]. With the Fenton modification, Cts both efficiently adsorbed Cr (VI) and converted it to less toxic Cr (III) over a wide pH range as a result of layer formation defined by a sandwich model. The highly reactive HO• destroyed the hydrogen bond in the Cts structure, and Fe3+ ions were chelated with the Cts biopolymer. After Fenton modification, HCrO4 4−entered the gap in the Cts structure and was adsorbed in the newly formed adsorption sites. The adsorption process of Cr (VI) using the Fenton modified Cts, adsorption isotherms of Freundlich were investigated and the adsorption capacity exceeded 120 mg/g.

Cellulose and lignin-based adsorbents are the most basic biopolymer widely used for toxic metal removal from wastewater. Cellulose exhibits natural qualities such as strength, reward, biodegradability, non-toxicity, and mechanical stability. However, cellulose is an odorless water-soluble linear polymer covalently bonded in its structure by the monomeric units of b-D-anhydroglucopyranose C1-C4 b-glycosidic bonds [39]. Due to its water solubility, it has some drawbacks in terms of being used as an

**Figure 10.**

*The adsorption of Cr (VI) onto the Fenton modified Cts by a sandwich model [24].*

adsorbent in the raw structure; thus, it is made more suitable for adsorption applications by applying various functionalization techniques. Abundance, low cost and availability of various functions groups in agro-based byproducts (hydroxyl, carboxyl, carbonyl etc.) like shells, barks, straws, stem and seeds, they attract the attention of researchers to explore potential applications in the removal process of toxic metals from wastewater. These substances have the small surface area and their low stage of adsorption performance in wastewater treatment limits their implementation among their ordinary state. Valorization of agricultural waste in adsorption processes is an environmentally friendly approach for wastewater treatment studies. Parlayıcı and Pehlivan reported the preparation of glutaraldehyde crosslinked Cts coated Rosehip (*Rosa canina*) seed shell (Cts/RS) capsules to evaluate the adsorption of Cr (VI) ions from aqueous solution [40]. The data were fitted to the Langmuir adsorption isotherm. The maximum capacity of Cts/RS was determined as 34.13 mg/g.

Black sesame (*Sesamum indicum* L.) seed pulp (BSSP) and Cts (Cts)-coated black sesame seed pulp bead (Cts-BSSP) composite were produced (**Figure 11**) and tested to remove hexavalent chromium (Cr (VI)) during the adsorption process [23]. BSSP is an agricultural waste that has no economic value. After reaching equilibrium time, the Cr (VI) removal capacity was calculated as 31.44 mg/g for Cts-BSSP and 18.32 mg/g for BSSP. The results indicated the feasibility of using Cts-BSSP as adsorbents for the removal of Cr (VI) from aqueous medium.

Let us talk about the synergy in adsorption and reduction during removal of Cr (VI) from aqueous media by natural adsorbents. Anionic Cr (VI) is adsorbed onto protonated solid adsorbent surfaces preferentially by electrostatic bonding or anion exchange. When the strong redox nature of Cr (VI) is examined, it causes the adsorbent surface to oxidize while it is reduced to Cr (III). If the aqueous medium has acidic conditions and the presence of electrons is large, this further catalyzes the reduction. Heteroatoms such as; O, N and S in the adsorbent structure carry these electrons, making Cr (VI) easier to reduce. In the case of Cr (VI) adsorption, the chelation mechanism is very touchy to pH in the attachment of the metal by the adsorbent and it is generally found that no adsorption happens at low pH. Therefore, a simple change in the pH of the solution medium can reverse the adsorption reaction completely.

$$\text{Cr}\_2\text{O}\_7^{2-} + \text{14H}^+ + \text{6e}^- \rightleftharpoons 2\text{Cr}^{3+} + 7\text{H}\_2\text{O} \to \text{1.33 V} \tag{4}$$

$$\rm{Cr\_2O\_7}^{2-} + 4H\_2O + 3e^- \rightleftharpoons \rm{Cr(OH)\_3} + 5OH^- \ E\_0 = -0.13\,\rm{V} \tag{5}$$

It has been determined that there are two feasible reduction mechanisms in the removal of Cr (VI) from aqueous solutions (Reaction 1 and Reaction 2). In mechanism I indicates that Cr (VI) will be reduced to Cr (III) in acidic conditions at some stage in adsorption. Mechanism II is an indirect reduction. Adsorbed Cr (VI) species take electrons from an adjoining location of the adsorbent mass to facilitate the reduction. Besides, reducing agents such as ferrous iron, zerovalent iron, and iron composites have also been used [29].

The physicochemical properties of Cts can be improved by making a stronger structure, especially by making a composite by chemical modification or the inclusion of some strengthening agents. New adsorbents can also be formed by integrating into the Cts matrix with composite materials that allow continuous capture of toxic materials [41]. Clay mineral has a long enchanting history of metal-binding capacity while being used independently or combined with other natural polymers. Clay-biopolymer composites show excellent potential and high efficiency for the removal of chromium from aqueous solution. Parlayıcı prepared Cts coated perlite

**257**

**Figure 12.**

**Figure 11.**

*Preparation of Cts coated perlite beads.*

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

composite beads (**Figures 12** and **13**) and investigated systematically the process

Jiang et al. synthesized magnetic Fe3O4@SiO2–Cts (MFSC) biopolymers successfully by a simple cross-link method assisted and evaluated them for Cr (VI) removal [21]. The maximum adsorption capacities reported were 336.7 mg g−1. Chitosan was successfully coated with inert substrate perlite and prepared as spherical beads. In this modified form of chitosan, it exhibits two types of accessory mechanisms to remove Cr (VI) from the aqueous environment, one of which is electrostatic interaction and the other is a procedure involving reduction (**Figure 14**).

parameters influencing the adsorptions of Cr (VI) ions [30].

*Preparation of Cts-coated black sesame seed pulp beads (Cts-BSSP) [23].*

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

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

adsorbent in the raw structure; thus, it is made more suitable for adsorption applications by applying various functionalization techniques. Abundance, low cost and availability of various functions groups in agro-based byproducts (hydroxyl, carboxyl, carbonyl etc.) like shells, barks, straws, stem and seeds, they attract the attention of researchers to explore potential applications in the removal process of toxic metals from wastewater. These substances have the small surface area and their low stage of adsorption performance in wastewater treatment limits their implementation among their ordinary state. Valorization of agricultural waste in adsorption processes is an environmentally friendly approach for wastewater treatment studies. Parlayıcı and Pehlivan reported the preparation of glutaraldehyde crosslinked Cts coated Rosehip (*Rosa canina*) seed shell (Cts/RS) capsules to evaluate the adsorption of Cr (VI) ions from aqueous solution [40]. The data were fitted to the Langmuir adsorption isotherm. The maximum capacity of Cts/RS was determined as 34.13 mg/g. Black sesame (*Sesamum indicum* L.) seed pulp (BSSP) and Cts (Cts)-coated black sesame seed pulp bead (Cts-BSSP) composite were produced (**Figure 11**) and tested to remove hexavalent chromium (Cr (VI)) during the adsorption process [23]. BSSP is an agricultural waste that has no economic value. After reaching equilibrium time, the Cr (VI) removal capacity was calculated as 31.44 mg/g for Cts-BSSP and 18.32 mg/g for BSSP. The results indicated the feasibility of using Cts-BSSP as adsorbents for the removal of Cr (VI) from aqueous medium.

Let us talk about the synergy in adsorption and reduction during removal of Cr (VI) from aqueous media by natural adsorbents. Anionic Cr (VI) is adsorbed onto protonated solid adsorbent surfaces preferentially by electrostatic bonding or anion exchange. When the strong redox nature of Cr (VI) is examined, it causes the adsorbent surface to oxidize while it is reduced to Cr (III). If the aqueous medium has acidic conditions and the presence of electrons is large, this further catalyzes the reduction. Heteroatoms such as; O, N and S in the adsorbent structure carry these electrons, making Cr (VI) easier to reduce. In the case of Cr (VI) adsorption, the chelation mechanism is very touchy to pH in the attachment of the metal by the adsorbent and it is generally found that no adsorption happens at low pH. Therefore, a simple change in the pH of the solution medium can reverse the adsorption reaction completely.

− −+ <sup>+</sup> ++ + = 2 3 Cr O 14H 6e 2Cr 7H O E 1.33 V 2 7 2 0 (4)

( ) − − <sup>−</sup> + + + =− <sup>2</sup> Cr O 4H O 3e Cr OH 5OH E 0.13 V 27 2 <sup>3</sup> <sup>0</sup> (5)

It has been determined that there are two feasible reduction mechanisms in the removal of Cr (VI) from aqueous solutions (Reaction 1 and Reaction 2). In mechanism I indicates that Cr (VI) will be reduced to Cr (III) in acidic conditions at some stage in adsorption. Mechanism II is an indirect reduction. Adsorbed Cr (VI) species take electrons from an adjoining location of the adsorbent mass to facilitate the reduction. Besides, reducing agents such as ferrous iron, zerovalent iron, and

The physicochemical properties of Cts can be improved by making a stronger structure, especially by making a composite by chemical modification or the inclusion of some strengthening agents. New adsorbents can also be formed by integrating into the Cts matrix with composite materials that allow continuous capture of toxic materials [41]. Clay mineral has a long enchanting history of metal-binding capacity while being used independently or combined with other natural polymers. Clay-biopolymer composites show excellent potential and high efficiency for the removal of chromium from aqueous solution. Parlayıcı prepared Cts coated perlite

iron composites have also been used [29].

**256**

**Figure 11.** *Preparation of Cts-coated black sesame seed pulp beads (Cts-BSSP) [23].*

composite beads (**Figures 12** and **13**) and investigated systematically the process parameters influencing the adsorptions of Cr (VI) ions [30].

Jiang et al. synthesized magnetic Fe3O4@SiO2–Cts (MFSC) biopolymers successfully by a simple cross-link method assisted and evaluated them for Cr (VI) removal [21]. The maximum adsorption capacities reported were 336.7 mg g−1. Chitosan was successfully coated with inert substrate perlite and prepared as spherical beads. In this modified form of chitosan, it exhibits two types of accessory mechanisms to remove Cr (VI) from the aqueous environment, one of which is electrostatic interaction and the other is a procedure involving reduction (**Figure 14**).

**Figure 12.** *Preparation of Cts coated perlite beads.*

**Figure 13.** *Photograph images of Cts coated perlite beads (a) wet beads (b) dry beads.*

#### **Figure 14.**

*Adsorption mechanisms involved in adsorption of hexavalent chromium onto Fe3O4@SiO2–Cts (MFSC) biopolymers [21].*
