**1.1. Dyes**

The first synthetic dye, Mauveine was discovered by the Englishman, William Henry Perkin by chance in 1856. Since then the dyestuffs industry has matured [6].

A dye or dyestuff is a coloured compound that can be applied on a substrate. With few ex‐ ceptions, all synthetic dyes are aromatic organic compounds. A substrate is the material to which a colorant is applied by one of the various processes of dyeing, printing, surface coat‐ ing, and so on. Generally, the substrate includes textile fibers, polymers, foodstuffs, oils, leather, and many other similar materials [7].

Not all coloured compounds are dyestuffs because a coloured compound may not have suit‐ able application on a substrate. For example, a chemical such as copper sulphate, which is coloured, finds no application on any substrate. If it is applied on a substrate, it will not have retaining power on the substrate and for this reason copper sulphate cannot be termed as a dye. On the other hand, congo red, a typical organic coloured compound. When it ap‐ plied to cotton under suitable conditions can be retained on this natural fibre and due to this finds useful application on this fibre. It is termed as a dyestuff [7].

In the field of chemistry, chromophores and auxochromes are the major component element of dye molecule. Dyes contain an unsaturated group basically responsible for colour and designated it as chromophore ("chroma" means colour and "phore" means bearer) (Table 1). Auxochromes ("Auxo" means augment) are the characteristic groups which intensify colour and/or improve the dye affinity to substrate [7] (Table 1).


**Table 1.** Names of chromophore and auxochrome groups of dyes

selected (either nanofiltration or reverse osmosis for the membrane processes). These treat‐ ments apply only to very dilute dye baths [2]. This is generally not the case of the first dye baths recovered which are the most heavily polluted ones. The wastewater produced by a reactive dyeing contains [2,3]: (i) hydrolyzed reactive dyes not fixed on the substrate, repre‐ senting 20-30% of the reactive dyes applied (on average 2 g/L) (this residual amount is re‐ sponsible for the coloration of the effluents and cannot be recycled); (ii) dyeing auxiliaries or organic substances, which are non-recyclable and responsible for the high BOD/COD of the effluents; (iii) textile fibres, and (iv) 60-100 g/L electrolyte, essentially NaCl and Na2CO3,

which is responsible for the very high saline content of the wastewater.

by chance in 1856. Since then the dyestuffs industry has matured [6].

finds useful application on this fibre. It is termed as a dyestuff [7].

and/or improve the dye affinity to substrate [7] (Table 1).

leather, and many other similar materials [7].

**1.1. Dyes**

178 Eco-Friendly Textile Dyeing and Finishing

In addition, these effluents exhibit a pH of 10-11 and a high temperature (50-70 o

ous materials can be broadly used to limit the concentration of colour in effluents [3].

are becoming increasingly severe, including the limits with respect to salinity.

gal regulations respecting the limit values for the release of wastewater are changing and

A typical effluent treatment is broadly classified into preliminary, primary, secondary, and tertiary stages [4,5]. The preliminary stage includes equalization and neutralization. The pri‐ mary stage involves screening, sedimentation, flotation, and flocculation. The secondary stage reduces the organic load and facilitates the physical/chemical separation (biological oxidation). The tertiary stage is focused on decolorization. In the latter, adsorption onto vari‐

The first synthetic dye, Mauveine was discovered by the Englishman, William Henry Perkin

A dye or dyestuff is a coloured compound that can be applied on a substrate. With few ex‐ ceptions, all synthetic dyes are aromatic organic compounds. A substrate is the material to which a colorant is applied by one of the various processes of dyeing, printing, surface coat‐ ing, and so on. Generally, the substrate includes textile fibers, polymers, foodstuffs, oils,

Not all coloured compounds are dyestuffs because a coloured compound may not have suit‐ able application on a substrate. For example, a chemical such as copper sulphate, which is coloured, finds no application on any substrate. If it is applied on a substrate, it will not have retaining power on the substrate and for this reason copper sulphate cannot be termed as a dye. On the other hand, congo red, a typical organic coloured compound. When it ap‐ plied to cotton under suitable conditions can be retained on this natural fibre and due to this

In the field of chemistry, chromophores and auxochromes are the major component element of dye molecule. Dyes contain an unsaturated group basically responsible for colour and designated it as chromophore ("chroma" means colour and "phore" means bearer) (Table 1). Auxochromes ("Auxo" means augment) are the characteristic groups which intensify colour

C). The le‐

To further examine the interactions between dyes and substrates, the classification of dyes is required. It is very important to know the chemistry of the dyes in dyeing effluents, in order to synthesize a suitable adsorbent with the appropriate functional group. Hunger et al [7] mentioned that dyes are classified in two methods. The main classification is related to the chemical structure of dyes and particularly considering the chromophoric structure present‐ ed in dye molecules. Another type of classification is based on their usage or applying. The classification of dyes by usage or application is the most important system adopted by the Colour Index (CI). Briefly [8]:

*Reactive dyes*. These dyes form a covalent chemical bond with fiber is ether or ester linkage under suitable conditions. Majority of reactive dyes contains azo that includes metallized azo, triphendioxazine, phthalocyanine, formazan, and anthraquinone. The molecular struc‐ tures of these dyes are much simpler than direct dyes. They also produce brighter shades than direct dyes. Reactive dyes are primarily used for dyeing and printing of cotton fibers.

*Direct dyes*. In the presence of electrolytes, these anionic dyes are water-soluble in aqueous solution. They have high affinity to cellulose fibers. Most of the dyes in this class are polya‐ zo compounds, along with some stilbenes, phthalocyanines, and oxazines. To improve wash fastness, frequently chelations with metal (such as copper and chromium) salts are applied to the dyestuff. Also, their treatment with formaldehyde or a cationic dye-complexing resin.

*Disperse dyes*. These are substantially water insoluble nonionic dyes applied to hydrophobic fibers from microfine aqueous dispersion. They are used predominantly on polyester, polya‐ mide, polyacrylonitrile, polypropylene fibers to a lesser, it is used to dye nylon, cellulose acetate, and acrylic fibers. Chemical structures of dyes are mainly consisted of azo and an‐ thraquinonoid groups, having low molecular weight and containing groups which aid in forming stable aqueous dispersions.

**Class Fiber type Chemistry Characteristics**

anthraquinone

azo, phthalocyanine, stilbene, nitro, benzodifuranone

azo, anthraquinone, styryl, nitro, benzodifuranone

azo, anthraquinone, phthalocyanine, formazan

Methods of dye wastewater treatment have been reported [11-14]. Also, fungal and bacterial decolorization methods have been reviewed [15-18]. There are several reported methods for the removal of pollutants from effluents. The technologies can be divided into three catego‐ ries: biological, chemical and physical [11]. All of them have advantages and drawbacks. Be‐ cause of the high cost and disposal problems, many of these conventional methods for treating dye wastewater have not been widely applied at large scale in the textile and paper industries. At the present time, there is no single process capable of adequate treatment, mainly due to the complex nature of the effluents [19,20]. In practice, a combination of dif‐ ferent processes is often used to achieve the desired water quality in the most economical way. A literature survey shows that research has been and continues to be conducted in the areas of combined adsorption-biological treatments in order to improve the biodegradation

Biological treatment is often the most economical alternative when compared with other physical and chemical processes. Biodegradation methods such as fungal decolorization, mi‐ crobial degradation, adsorption by (living or dead) microbial biomass and bioremediation systems are commonly applied to the treatment of industrial effluents because many micro‐ organisms such as bacteria, yeasts, algae and fungi are able to accumulate and degrade dif‐ ferent pollutants [14,16,17]. However, their application is often restricted because of technical constraints. Biological treatment requires a large land area and is constrained by sensitivity toward diurnal variation as well as toxicity of some chemicals, and less flexibility in design and operation [21]. Biological treatment is incapable of obtaining satisfactory color elimination with current conventional biodegradation processes [11]. Moreover, although

azo, anthraquinone - Anionic compounds

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polyester, inks acridine, oxazine,

Direct cotton, paper, rayon, leather, nylon

Disperse polyester, polyamide,

Mordant wool, leather,

Reactive cotton, wool,

silk, nylon

**1.2. Decolorization techniques**

**Table 2.** Classification of dyes and their properties

of dyestuffs and minimize the sludge production.

acetate, acrylic, plastics

anodized aluminium

*Vat dyes*. These dyes are water insoluble and can apply mainly to cellulose fiber by convert‐ ing them to their leuco compounds. The latter was carried out by reduction and solubiliza‐ tion with sodium hydrosulphite and sodium hydroxide solution, which is called "vatting process". The main chemical/structural groups of vat dyes are anthraquinone and indigoid.

*Sulfur dyes*. They are water insoluble and are applied to cotton in the form of sodium salts by the reduction process using sodium sulphide as the reducing agent under alkaline condi‐ tions. The low cost and good wash fastness properties of dyeing makes these dyes economic attractive.

*Cationic (Basic dyes)*. These dyes are cationic and water soluble. They are applied on paper, polyacrylonitrile, modified nylons, and modified polyesters. In addition, they are used to apply with silk, wool, and tannin–mordant cotton when brightness shade was more necessa‐ ry than fastness to light and washing.

*Acid dyes*. They are water soluble anionic dyes and are applied on nylon, wool, silk, and modified acrylics. Moreover, they are used to dye paper, leather, inkjet printing, food, and cosmetics.

*Solvent dyes*. They are water insoluble, but solvent soluble, dyes having deficient polar solu‐ bility group for example sulfonic acid, carboxylic acid or quaternary ammonium. They are used for colouring plastics, gasoline, oils, and waxes.

*Mordant dyes*. These dyes have mordant dyeing properties with good quality in the presence of certain groups in the dye molecule. These groups are capable to hold metal residuals by formation of covalent and coordinate bonds involving a chelate compound. The salts of alu‐ minium, chromium, copper, cobalt, nickel, iron, and tin are used as mordant that their met‐ allic salts.

Aside from mentioned above, there are azoic dyes, ingrain dyes, pigment [6,7,9]. Compara‐ tive analysis of dye classes are presented in Table 2:



**Table 2.** Classification of dyes and their properties

#### **1.2. Decolorization techniques**

thraquinonoid groups, having low molecular weight and containing groups which aid in

*Vat dyes*. These dyes are water insoluble and can apply mainly to cellulose fiber by convert‐ ing them to their leuco compounds. The latter was carried out by reduction and solubiliza‐ tion with sodium hydrosulphite and sodium hydroxide solution, which is called "vatting process". The main chemical/structural groups of vat dyes are anthraquinone and indigoid. *Sulfur dyes*. They are water insoluble and are applied to cotton in the form of sodium salts by the reduction process using sodium sulphide as the reducing agent under alkaline condi‐ tions. The low cost and good wash fastness properties of dyeing makes these dyes economic

*Cationic (Basic dyes)*. These dyes are cationic and water soluble. They are applied on paper, polyacrylonitrile, modified nylons, and modified polyesters. In addition, they are used to apply with silk, wool, and tannin–mordant cotton when brightness shade was more necessa‐

*Acid dyes*. They are water soluble anionic dyes and are applied on nylon, wool, silk, and modified acrylics. Moreover, they are used to dye paper, leather, inkjet printing, food,

*Solvent dyes*. They are water insoluble, but solvent soluble, dyes having deficient polar solu‐ bility group for example sulfonic acid, carboxylic acid or quaternary ammonium. They are

*Mordant dyes*. These dyes have mordant dyeing properties with good quality in the presence of certain groups in the dye molecule. These groups are capable to hold metal residuals by formation of covalent and coordinate bonds involving a chelate compound. The salts of alu‐ minium, chromium, copper, cobalt, nickel, iron, and tin are used as mordant that their met‐

Aside from mentioned above, there are azoic dyes, ingrain dyes, pigment [6,7,9]. Compara‐

**Class Fiber type Chemistry Characteristics**

Azo, anthraquinone, azine, xanthene, nitro, nitroso

cyanine, azo, azine, triarylmethane, xanthen,

azo - Contain azo group





forming stable aqueous dispersions.

180 Eco-Friendly Textile Dyeing and Finishing

ry than fastness to light and washing.

used for colouring plastics, gasoline, oils, and waxes.

tive analysis of dye classes are presented in Table 2:

attractive.

and cosmetics.

allic salts.

Acid nylon, wool, silk,

Azoic cotton, rayon,

paper ink, leather

cellulose acetate, polyester

Basic leather, wool, silk paper, modified nylon, polyacrylonitrile,

Methods of dye wastewater treatment have been reported [11-14]. Also, fungal and bacterial decolorization methods have been reviewed [15-18]. There are several reported methods for the removal of pollutants from effluents. The technologies can be divided into three catego‐ ries: biological, chemical and physical [11]. All of them have advantages and drawbacks. Be‐ cause of the high cost and disposal problems, many of these conventional methods for treating dye wastewater have not been widely applied at large scale in the textile and paper industries. At the present time, there is no single process capable of adequate treatment, mainly due to the complex nature of the effluents [19,20]. In practice, a combination of dif‐ ferent processes is often used to achieve the desired water quality in the most economical way. A literature survey shows that research has been and continues to be conducted in the areas of combined adsorption-biological treatments in order to improve the biodegradation of dyestuffs and minimize the sludge production.

Biological treatment is often the most economical alternative when compared with other physical and chemical processes. Biodegradation methods such as fungal decolorization, mi‐ crobial degradation, adsorption by (living or dead) microbial biomass and bioremediation systems are commonly applied to the treatment of industrial effluents because many micro‐ organisms such as bacteria, yeasts, algae and fungi are able to accumulate and degrade dif‐ ferent pollutants [14,16,17]. However, their application is often restricted because of technical constraints. Biological treatment requires a large land area and is constrained by sensitivity toward diurnal variation as well as toxicity of some chemicals, and less flexibility in design and operation [21]. Biological treatment is incapable of obtaining satisfactory color elimination with current conventional biodegradation processes [11]. Moreover, although many organic molecules are degraded, many others are recalcitrant due to their complex chemical structure and synthetic organic origin [22]. In particular, due to their xenobiotic na‐ ture, azo dyes are not totally degraded.

that are useful to be developed are "chemisorption" and "physisorption". Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the absor‐ bate. New chemical bonds are generated at the adsorbent surface. Examples include macro‐ scopic phenomena that can be very obvious, like corrosion, and subtler effects associated with heterogeneous catalysis. The strong interaction between the adsorbate and the sub‐ strate surface creates new types of electronic bonds. In contrast with chemisorption is physi‐ sorption, which leaves the chemical species of the adsorbate and surface intact. It is conventionally accepted that the energetic threshold separating the binding energy of "physisorption" from that of "chemisorptions" is about 0.5 eV per adsorbed species. Chemi‐ sorption is deemed to be irreversible in the majority of cases [24]. Suzuki [25] covers the role of adsorption in water environmental processes and also the development of newer adsorb‐ ents to modernise the treatment systems. Most adsorbents are highly porous materials. As the pores are generally very small, the internal surface area is orders of magnitude greater

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Separation occurs because either the differences in molecular mass, shape or polarity causes some molecules to be held more strongly on the surface than others or the pores are too small to admit large molecules [25]. However, amongst all the adsorbent materials pro‐ posed, activated carbon is the most popular for the removal of pollutants from wastewater [26,27]. In particular, the effectiveness of adsorption on commercial activated carbons (CAC) for removal of a wide variety of dyes from wastewaters has made it an ideal alternative to other expensive treatment options [26]. Because of their great capacity to adsorb dyes, CAC are the most effective adsorbents. This capacity is mainly due to their structural characteris‐ tics and their porous texture which gives them a large surface area, and their chemical na‐ ture which can be easily modified by chemical treatment in order to increase their properties. However, activated carbon presents several disadvantages [27]. It is quite expen‐ sive, the higher the quality, the greater the cost, non-selective and ineffective against dis‐ perse and vat dyes. The regeneration of saturated carbon is also expensive, not straightforward, and results in loss of the adsorbent. The use of carbons based on relatively expensive starting materials is also unjustified for most pollution control applications [28].

The majority of commercial polymers and ion exchange resins are derived from petroleumbased raw materials using chemical processes that are not always safe or environmental friendly. Today, there is growing interest in developing natural low-cost alternatives to syn‐ thetic polymers [29]. Chitin (Figure 1), found in the exoskeleton of crustaceans, the cuticles of insects, and the cells walls of fungi, is the most abundant aminopolysaccharide in nature [30]. This low-cost material is a linear homopolymer composed of b(1-4)-linked N-acetyl glu‐ cosamine. It is structurally similar to cellulose, but it is an aminopolymer and has acetamide groups at the C-2 positions in place of the hydroxyl groups. The presence of these groups is highly advantageous, providing distinctive adsorption functions and conducting modifica‐ tion reactions. The raw polymer is only commercially extracted from marine crustaceans pri‐

This has led many workers to search for more economic adsorbents.

*1.3.1. Polymeric adsorbents (chitosan)*

than the external area.

Chemical methods include coagulation or flocculation combined with flotation and filtra‐ tion, precipitation-flocculation with Fe(II)/Ca(OH)2, electroflotation, electrokinetic coagula‐ tion, conventional oxidation methods by oxidizing agents (ozone), irradiation or electrochemical processes. These chemical techniques are often expensive, and although the dyes are removed, accumulation of concentrated sludge creates a disposal problem. There is also the possibility that a secondary pollution problem will arise because of excessive chemi‐ cal use. Recently, other emerging techniques, known as advanced oxidation processes, which are based on the generation of very powerful oxidizing agents such as hydroxyl radi‐ cals, have been applied with success for pollutant degradation. Although these methods are efficient for the treatment of waters contaminated with pollutants, they are very costly and commercially unattractive. The high electrical energy demand and the consumption of chemical reagents are common problems.

Different physical methods are also widely used, such as membrane-filtration processes (nanofiltration, reverse osmosis, electrodialysis) and adsorption techniques. The major dis‐ advantage of the membrane processes is that they have a limited lifetime before membrane fouling occurs and the cost of periodic replacement must thus be included in any analysis of their economic viability. In accordance with the very abundant literature data, liquid-phase adsorption is one of the most popular methods for the removal of pollutants from wastewa‐ ter since proper design of the adsorption process will produce a high-quality treated efflu‐ ent. This process provides an attractive alternative for the treatment of contaminated waters, especially if the adsorbent is inexpensive and does not require an additional pre-treatment step before its application. Adsorption is a well known equilibrium separation process and an effective method for water decontamination applications [23]. Adsorption has been found to be superior to other techniques for water re-use in terms of initial cost, flexibility and simplicity of design, ease of operation and insensitivity to toxic pollutants. Adsorption also does not result in the formation of harmful substances.

### **1.3. Adsorption**

Adsorption techniques for wastewater treatment have become more popular in recent years owing to their efficiency in the removal of pollutants, which are difficulty treated with bio‐ logical methods. Adsorption can produce high quality water while also being a process that is economically feasible. Decolourisation is a result of two mechanisms (adsorption and ion exchange) and is influenced by many factors including dye/adsorbent interaction, adsorb‐ ent's surface area, particle size, temperature, pH and contact time.

Physical adsorption occurs when weak interparticle bonds exist between the adsorbate and adsorbent. Examples of such bonds are van der Waals, hydrogen and dipole-dipole. In the majority of cases physical adsorption is easily reversible [24]. Chemical adsorption occurs when strong interparticle bonds are present between the adsorbate and adsorbent due to an exchange of electrons. Examples of such bonds are covalent and ionic bonds. Two means that are useful to be developed are "chemisorption" and "physisorption". Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the absor‐ bate. New chemical bonds are generated at the adsorbent surface. Examples include macro‐ scopic phenomena that can be very obvious, like corrosion, and subtler effects associated with heterogeneous catalysis. The strong interaction between the adsorbate and the sub‐ strate surface creates new types of electronic bonds. In contrast with chemisorption is physi‐ sorption, which leaves the chemical species of the adsorbate and surface intact. It is conventionally accepted that the energetic threshold separating the binding energy of "physisorption" from that of "chemisorptions" is about 0.5 eV per adsorbed species. Chemi‐ sorption is deemed to be irreversible in the majority of cases [24]. Suzuki [25] covers the role of adsorption in water environmental processes and also the development of newer adsorb‐ ents to modernise the treatment systems. Most adsorbents are highly porous materials. As the pores are generally very small, the internal surface area is orders of magnitude greater than the external area.

Separation occurs because either the differences in molecular mass, shape or polarity causes some molecules to be held more strongly on the surface than others or the pores are too small to admit large molecules [25]. However, amongst all the adsorbent materials pro‐ posed, activated carbon is the most popular for the removal of pollutants from wastewater [26,27]. In particular, the effectiveness of adsorption on commercial activated carbons (CAC) for removal of a wide variety of dyes from wastewaters has made it an ideal alternative to other expensive treatment options [26]. Because of their great capacity to adsorb dyes, CAC are the most effective adsorbents. This capacity is mainly due to their structural characteris‐ tics and their porous texture which gives them a large surface area, and their chemical na‐ ture which can be easily modified by chemical treatment in order to increase their properties. However, activated carbon presents several disadvantages [27]. It is quite expen‐ sive, the higher the quality, the greater the cost, non-selective and ineffective against dis‐ perse and vat dyes. The regeneration of saturated carbon is also expensive, not straightforward, and results in loss of the adsorbent. The use of carbons based on relatively expensive starting materials is also unjustified for most pollution control applications [28]. This has led many workers to search for more economic adsorbents.

#### *1.3.1. Polymeric adsorbents (chitosan)*

many organic molecules are degraded, many others are recalcitrant due to their complex chemical structure and synthetic organic origin [22]. In particular, due to their xenobiotic na‐

Chemical methods include coagulation or flocculation combined with flotation and filtra‐ tion, precipitation-flocculation with Fe(II)/Ca(OH)2, electroflotation, electrokinetic coagula‐ tion, conventional oxidation methods by oxidizing agents (ozone), irradiation or electrochemical processes. These chemical techniques are often expensive, and although the dyes are removed, accumulation of concentrated sludge creates a disposal problem. There is also the possibility that a secondary pollution problem will arise because of excessive chemi‐ cal use. Recently, other emerging techniques, known as advanced oxidation processes, which are based on the generation of very powerful oxidizing agents such as hydroxyl radi‐ cals, have been applied with success for pollutant degradation. Although these methods are efficient for the treatment of waters contaminated with pollutants, they are very costly and commercially unattractive. The high electrical energy demand and the consumption of

Different physical methods are also widely used, such as membrane-filtration processes (nanofiltration, reverse osmosis, electrodialysis) and adsorption techniques. The major dis‐ advantage of the membrane processes is that they have a limited lifetime before membrane fouling occurs and the cost of periodic replacement must thus be included in any analysis of their economic viability. In accordance with the very abundant literature data, liquid-phase adsorption is one of the most popular methods for the removal of pollutants from wastewa‐ ter since proper design of the adsorption process will produce a high-quality treated efflu‐ ent. This process provides an attractive alternative for the treatment of contaminated waters, especially if the adsorbent is inexpensive and does not require an additional pre-treatment step before its application. Adsorption is a well known equilibrium separation process and an effective method for water decontamination applications [23]. Adsorption has been found to be superior to other techniques for water re-use in terms of initial cost, flexibility and simplicity of design, ease of operation and insensitivity to toxic pollutants. Adsorption

Adsorption techniques for wastewater treatment have become more popular in recent years owing to their efficiency in the removal of pollutants, which are difficulty treated with bio‐ logical methods. Adsorption can produce high quality water while also being a process that is economically feasible. Decolourisation is a result of two mechanisms (adsorption and ion exchange) and is influenced by many factors including dye/adsorbent interaction, adsorb‐

Physical adsorption occurs when weak interparticle bonds exist between the adsorbate and adsorbent. Examples of such bonds are van der Waals, hydrogen and dipole-dipole. In the majority of cases physical adsorption is easily reversible [24]. Chemical adsorption occurs when strong interparticle bonds are present between the adsorbate and adsorbent due to an exchange of electrons. Examples of such bonds are covalent and ionic bonds. Two means

ture, azo dyes are not totally degraded.

182 Eco-Friendly Textile Dyeing and Finishing

chemical reagents are common problems.

also does not result in the formation of harmful substances.

ent's surface area, particle size, temperature, pH and contact time.

**1.3. Adsorption**

The majority of commercial polymers and ion exchange resins are derived from petroleumbased raw materials using chemical processes that are not always safe or environmental friendly. Today, there is growing interest in developing natural low-cost alternatives to syn‐ thetic polymers [29]. Chitin (Figure 1), found in the exoskeleton of crustaceans, the cuticles of insects, and the cells walls of fungi, is the most abundant aminopolysaccharide in nature [30]. This low-cost material is a linear homopolymer composed of b(1-4)-linked N-acetyl glu‐ cosamine. It is structurally similar to cellulose, but it is an aminopolymer and has acetamide groups at the C-2 positions in place of the hydroxyl groups. The presence of these groups is highly advantageous, providing distinctive adsorption functions and conducting modifica‐ tion reactions. The raw polymer is only commercially extracted from marine crustaceans pri‐ marily because a large amount of waste is available as a by-product of food processing [30]. Chitin is extracted from crustaceans (shrimps, crabs, squids) by acid treatment to dissolve the calcium carbonate followed by alkaline extraction to dissolve the proteins and by a de‐ colorization step to obtain a colourless product [30].

Partial deacetylation of chitin results in the production of chitosan (Figure 2), which is a pol‐ ysaccharide composed by polymers of glucosamine and N-acetyl glucosamine (Figure 3).

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The "chitosan label" generally corresponds to polymers with less than 25% acetyl content. The fully deacetylated product is rarely obtained due to the risks of side reactions and chain depolymerization. Copolymers with various extents of deacetylation and grades are now commercially available. Chitosan and chitin are of commercial interest due to their high per‐ centage of nitrogen compared to synthetically substituted cellulose. Chitosan is soluble in acid solutions and is chemically more versatile than chitin or cellulose. The main reason for this is undoubtedly its appealing intrinsic properties, as documented in a recent review [30], such as biodegradability, biocompatibility, film-forming ability, bioadhesivity, polyfunction‐ ality, hydrophilicity and adsorption properties. Most of the properties of chitosan can be re‐ lated to its cationic nature [30], which is unique among abundant polysaccharides and natural polymers. These numerous properties lead to the recognition of this polyamine as a

The elevated interest in chitin and chitosan is reflected by an increase in the number of arti‐ cles published and talks given on this topic. Currently, these polymers and their numerous derivatives as described in reviews [29,31] are widely used in pharmacy, medicine, biotech‐ nology, chemistry, cosmetics and toiletries, food technology, and the textile, agricultural, pulp and paper industries and other fields such as oenology, dentistry and photography. The potential industrial use of chitosan is widely recognized. These versatile materials are also widely used in clarification and water purification, water and wastewater treatment as coagulating, flocculating and chelating agents. However, despite a large number of publica‐ tion on the use of chitosan for pollutant recovery, the research failed to find practical appli‐

The basic idea of modifications is to make various changes in chitosan structure to enhance its properties (capacity, resistance etc). In particular, several researchers have proposed cer‐

**Figure 3.** Chemical structures of commercial chitosan composed of N-acetyl glucosamine.

promising raw material for adsorption purposes.

**2. Synthesis of adsorbents**

**2.1. Grafting reactions**

cations on the industrial scale: this aspect will be discussed later.

**Figure 1.** Chemical structures of chitin.

Since the biodegradation of chitin is very slow in crustacean shell waste, accumulation of large quantities of discards from processing of crustaceans has become a major concern in the seafood processing industry. So, there is a need to recycle these by-products. Their use for the treatment of wastewater from other industries could be helpful not only to the envi‐ ronment in solving the solid waste disposal problem, but also to the economy. However, chitin is an extremely insoluble material. Its insolubility is a major problem that confronts the development of processes and uses of chitin [30], and so far, very few large-scale indus‐ trial uses have been found. More important than chitin is its derivative, chitosan.

**Figure 2.** Chemical structures of chitosan.

Partial deacetylation of chitin results in the production of chitosan (Figure 2), which is a pol‐ ysaccharide composed by polymers of glucosamine and N-acetyl glucosamine (Figure 3).

**Figure 3.** Chemical structures of commercial chitosan composed of N-acetyl glucosamine.

The "chitosan label" generally corresponds to polymers with less than 25% acetyl content. The fully deacetylated product is rarely obtained due to the risks of side reactions and chain depolymerization. Copolymers with various extents of deacetylation and grades are now commercially available. Chitosan and chitin are of commercial interest due to their high per‐ centage of nitrogen compared to synthetically substituted cellulose. Chitosan is soluble in acid solutions and is chemically more versatile than chitin or cellulose. The main reason for this is undoubtedly its appealing intrinsic properties, as documented in a recent review [30], such as biodegradability, biocompatibility, film-forming ability, bioadhesivity, polyfunction‐ ality, hydrophilicity and adsorption properties. Most of the properties of chitosan can be re‐ lated to its cationic nature [30], which is unique among abundant polysaccharides and natural polymers. These numerous properties lead to the recognition of this polyamine as a promising raw material for adsorption purposes.

The elevated interest in chitin and chitosan is reflected by an increase in the number of arti‐ cles published and talks given on this topic. Currently, these polymers and their numerous derivatives as described in reviews [29,31] are widely used in pharmacy, medicine, biotech‐ nology, chemistry, cosmetics and toiletries, food technology, and the textile, agricultural, pulp and paper industries and other fields such as oenology, dentistry and photography. The potential industrial use of chitosan is widely recognized. These versatile materials are also widely used in clarification and water purification, water and wastewater treatment as coagulating, flocculating and chelating agents. However, despite a large number of publica‐ tion on the use of chitosan for pollutant recovery, the research failed to find practical appli‐ cations on the industrial scale: this aspect will be discussed later.
