**3. Polysaccharide adsorbents for the removal of organic dye in aqueous solution**

Polysaccharides are hydrophilic, non-toxic, and relatively cheap polymers consisting of repeating subunits of sugar linked with glycosidic bonds. The composition and sources of polysaccharides that are generally investigated for their potential to act as adsorbents in aqueous media are presented in **Table 1** [22–25]. The adsorption ability of these biomaterials is due to the occurrence of functionalities such as hydroxyl (-OH), sulphonic acid (-SO3H), the carboxylic acid (-COOH), amino (-NH2) and amide (-CONH2) groups which can serve as binding sites [26]. This feature, complemented with the porous nature, make polysaccharides good candidates for water treatment applications. Some of the commercially available polysaccharides include cellulose, starch, guar gum, chitosan, xanthan gum and carrageenan. Among these, eco-friendly xanthan gum and carrageenan (**Figure 2**) with the ability to form gel have been gaining considerable attractions recently [27, 28]. So far, these have been described to be among the most effective adsorbents for the removal of toxic dyes in aqueous solution due to their tunable surface chemistry and feasible regeneration [29, 30].

Among these, eco-friendly xanthan gum and carrageenan (**Figure 2**) with the ability to form gel have been gaining considerable attractions recently [27, 28]. So far, these have been described to be among the most effective adsorbents for the removal of toxic dyes in aqueous solution due to their tunable surface chemistry and feasible regeneration [29, 30].

*Novel Nanomaterials*

systems.

necessitates concurrent consideration of several societal sectors that compete for limited resources [2]. For that reason, a vast majority are dependent on surface water and groundwater for drinking purpose. The WHO report also highlights the fact that more than two million people die every year from the use of unsafe drinking water. This is well-reported to spread sickness and waterborne diseases such as typhoid and cholera. This situation emerges from the occurrence of pollutants as a result of the environmental disposal of untreated effluent released by human activities [3]. Indeed, it is inconceivable to achieve progress in human civilisation without industrialisation. However, its exponential development in a competitive era and an increasing global population has seriously impaired the quality of freshwater

Among an assortment of environmental pollution, waterbody contamination owing to the discharge of untreated water-containing organic species has attracted significant consideration in recent years [4, 5]. Organic dye-containing wastewater from industries such as textiles, petrochemical, cosmetics, papers and plastics, for example, has been described for their carcinogenic and mutagenic nature [6]. Also, the organic dyes are oxygen-sequestering agents capable of reducing light penetration in the water systems and thereby restraining the photosynthesis of aquatic vegetation [7, 8]. **Figure 1** illustrates the fate of organic dyes in the natural environment. Thought the textile industry had played an enormous role in the development of the South African economy [9], this has also significantly impacted the water resources. The dye-containing effluents disposed to the environment without proper treatment can be highly toxic even at a concentration lower than one ppm [10]. Therefore, the removal of toxic organic dyes from contaminated effluents before being discharged into the environment has evoked considerable attention. From the commercial and environmental viewpoint, the focus of this chapter is to provide a comprehensive discussion on the ability of natural polymers to perform as adsorbents for industrial wastewater remediation. The hybridisation of these

**298**

**Figure 1.**

*The fate of the dye-contaminated effluent in the aquatic environment.*


#### **Table 1.**

*Polysaccharides used as adsorbents for wastewater treatment.*

**Figure 2.**

*The molecular structure of xanthan gum (XG) and* kappa*-carrageenan (*k*C) polysaccharides.*

Xanthan gum (XG) is an anionic polymer obtained by *Xanthmonous campestris* bacterial fermentation of carbohydrate source. This biopolymer is recovered through precipitation in ethanol, isopropyl alcohol or *tert*-butanol [31]. It is a high molecular weight polysaccharide, which provides suspension, thickness, and stabilisation of the combined material. The negative charge of XG biopolymer is mainly ascribed to the presence of carboxylic acid group within its backbone, thereby indicating its affinity for cationic species. XG main chain is made of repeating cellulosic units with side chains involving an α-D-mannose with an acetyl group, a β-D-glucuronic residues and a terminal β-D mannose bearing the pyruvate substituent [32]. The physicochemical characteristics of XG include higher stability

**301**

*Nanoengineered Polysaccharide-Based Adsorbents as Green Alternatives for Dye Removal…*

in solution under a wide range of pH (2–12), higher viscosity at low concentration (1% or less), and higher shelf life. The presence of hydroxyl and carboxylic groups that are pH tunable also confers a polyionic character to XG. Thus this adsorbent can be used for binding various ions through electrostatic interaction. Moreover, the –OH groups are also capable of interacting with the electron clouds of the aromatic rings of organic dye molecules through hydrogen-bonding. Thus, adsorption of toxic dye onto the biodegradable polymeric adsorbent can be examined using Fourier transformed infrared spectroscopy (FTIR). For example, the as-described mechanism was reported by Lozano-Álvarez et al. during adsorption of disperse

Carrageenans are sulfated polysaccharides obtained through alkaline extraction from some red marine algae. Depending on the extraction method and the algae species from which this anionic polysaccharide is obtained, there are several types of carrageenans with different solubility. The polymer chains of carrageenans consist of alternate units of D-galactose and 3,6-anhydrogalactose joined by α-1,3- and β-1,4-glycosidic linkages. The main types are lambda (*λ*), kappa (*k*), and iota (*i*). *k*-Carrageenan is less soluble than the others owing to the hydrophobic 3.6-anhydro-D-galactose group, which form part of its repeating unit and the relatively lesser number of sulphate groups (one sulfate functional group for each

Although these renewable materials show unique properties for water treatment in the adsorption process, they exhibit poor specific surface area and mechanical properties which limits their applicability as lasting adsorbents [34, 35]. In general, the effectiveness of an adsorbent strongly depends on its chemical and mechanical stability, which determines the suitability for application under harsh conditions.

**4. Functionalised polysaccharide adsorbents for the efficient removal of** 

The polysaccharide surface modification can improve their physicochemical properties and mechanical characteristics [26]. This strategy has also been reported to avoid leaching of organic substances and improve the adsorption potential of the nanoengineered adsorbent. The polysaccharide surface modification can be accomplished through graft copolymerisation and/or incorporation of specific nanoscale inorganic particles. Deposition of inorganic nanoparticles such as SiO2, Fe3O4, TiO2, and carbon nanotube onto polymeric supports has been reported to enhance their

Graft copolymerisation of vinyl monomers onto polysaccharides is a wellreported and versatile technique that allows for the increased potential applicability of biopolymers. This procedure usually involves the attachment of a vinyl monomer to the polysaccharide backbone. In general, the vinyl monomer will undergo polymerisation in the presence of polysaccharide chains to generate a copolymeric network. The vinyl monomers that are frequently reported for the modification of gum-based polysaccharides involve the acrylamide (AAm), methyl methacrylate (MAA), acrylic acid (AA), *N*-vinyl imidazole (VI), and acrylonitrile (AN). The synthesis of polysaccharide graft copolymers is achieved by amending the biopolymer molecules *via* formation of branches of synthetic polymers. This can be accomplished through "grafting onto" or "grafting from" methodology [38]. The "grafting from" process entails the development of polymer chains from initiating sites

*DOI: http://dx.doi.org/10.5772/intechopen.94883*

yellow 54 dye in aqueous solution onto XG [33].

**organic dye in aqueous solution**

chemical, mechanical and thermal stabilities [36, 37].

**4.1 Polysaccharide functionalised by graft copolymerisation**

disaccharide unit).

#### *Nanoengineered Polysaccharide-Based Adsorbents as Green Alternatives for Dye Removal… DOI: http://dx.doi.org/10.5772/intechopen.94883*

in solution under a wide range of pH (2–12), higher viscosity at low concentration (1% or less), and higher shelf life. The presence of hydroxyl and carboxylic groups that are pH tunable also confers a polyionic character to XG. Thus this adsorbent can be used for binding various ions through electrostatic interaction. Moreover, the –OH groups are also capable of interacting with the electron clouds of the aromatic rings of organic dye molecules through hydrogen-bonding. Thus, adsorption of toxic dye onto the biodegradable polymeric adsorbent can be examined using Fourier transformed infrared spectroscopy (FTIR). For example, the as-described mechanism was reported by Lozano-Álvarez et al. during adsorption of disperse yellow 54 dye in aqueous solution onto XG [33].

Carrageenans are sulfated polysaccharides obtained through alkaline extraction from some red marine algae. Depending on the extraction method and the algae species from which this anionic polysaccharide is obtained, there are several types of carrageenans with different solubility. The polymer chains of carrageenans consist of alternate units of D-galactose and 3,6-anhydrogalactose joined by α-1,3- and β-1,4-glycosidic linkages. The main types are lambda (*λ*), kappa (*k*), and iota (*i*). *k*-Carrageenan is less soluble than the others owing to the hydrophobic 3.6-anhydro-D-galactose group, which form part of its repeating unit and the relatively lesser number of sulphate groups (one sulfate functional group for each disaccharide unit).

Although these renewable materials show unique properties for water treatment in the adsorption process, they exhibit poor specific surface area and mechanical properties which limits their applicability as lasting adsorbents [34, 35]. In general, the effectiveness of an adsorbent strongly depends on its chemical and mechanical stability, which determines the suitability for application under harsh conditions.
