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      6.2.5 Biofuels

      As carbon-neutral and sustainable, biofuels (i.e., ethanol, biodiesel, and bio-jet fuel) are produced through contemporary biological processes, rather than geological processes [190, 191]. Because of the need for increased energy security, increased petroleum price, and negative impacts of fossil fuels on the environment, biofuels have been gaining increased public and scientific attention [191].

      Biofuels are classified into first, second and third-generation biofuels depending on the carbon source of feedstock. The first-generation feedstock is based on the starch obtained from wheat, corn, barley, etc. The second-generation feedstock includes biomass rich in lignin and cellulose such as rice wheat straw, straw, and sugarcane bagasse. Finally, the third-generation feedstock comprises polysaccharides, such as starch, laminarin, and floridean starch [190]. In the extracellular cell wall of plant cells, cellulose is generally embedded in a lignin matrix. This structure is called lignocellulose and it constitutes the essential part of the woody cell walls of plants. In addition to their other biological functions, such as providing resistance against the penetration of microorganisms or degrading enzymes, cellulose and lignin together provide the plant structure. Cellulose can be fermented by many microorganisms to produce biofuels, such as bioethanol. Lignin is a large 3D polymer of phenylpropanoid molecules and it is an abundant source of high energy because of having a high C/O ratio [192, 193]. Lignocellulose is not a part of the human diet as a food since it cannot be readily digested by the microorganisms reside in the human gut, so lignocellulose has been proposed to be used as a renewable source because of its availability and structural characteristics. However, it has been noticed that the production of lower-cost cellulosic biofuels was challenging because lignocellulosic residues are a complex of carbohydrates and polyphenol polymers usually associated with proteins. Consequently, the necessity of the use of several steps including pre-treatments and enzymatic digestions to extract fermentable carbohydrates from this complex network increases the cost of ethanol production drastically [193, 194].

The world’s energy demand is increasing and so, industrial and scientific communities try to introduce new, cost-efficient, and sustainable energy production approaches to the global energy market. More recently, green algae and cyanobacteria have been also gained attention as sources for biofuel production because of their high polysaccharide and lipid composition, but still this “third-generation bioethanol production technology” needs to be improved [195]. Plant genetic engineering is one of the new approaches in the field. Genetic manipulations of plants to increase polysaccharide content and overall biomass, and to produce cellulases and hemicellulases for reducing the need for pretreatment processes seem promising for the fabrication of biofuels [196]. To obtain a higher monomeric sugar release from plants, increasing cellulose content, reducing cellulose crystallinity, altering the amount or composition of noncellulosic polysaccharides and/or lignin seems to be key options. Modification of chemical linkages within and between lignocellulosic biomass components has also been suggested to improve the ease of deconstruction [197]. Besides, some bacteria and fungi can produce and secrete relatively higher extracellular cellulase which can solubilize crystalline cellulose and cellulase enzymes secreted by these microorganisms are feasible for large scale production. For instance, the marine macroalgal cell wall is predominantly comprised of cellulose with the complex chain of glycosidic linkages. Bioethanol production from macroalgae relies on breaking this complex chain into a simple glucose molecule by using cellulases. To obtain cellulases, cellulose-degrading bacteria can be isolated from wide-ranging sources from marine habitats to herbivore residues and gastrointestinal region [190]. In addition to investigation and characterization of new cellulose/lignocellulose-degrading enzymes with metagenomics approach, improvement of bacterial strains for extraction and fermentation steps is another area of much interest to produce optimal yields of biofuel from lignocellulosic biomass [198, 199].

      6.2.6 Wastewater Treatment

      Over the last years, the use of biocompatible and biodegradable renewable resources for the retaining of different pollutants from wastewaters has attracted the attention of many researchers [200]. Biosorbent abilities of natural polysaccharide matrices have been under investigation for an efficient and low-cost wastewater treatment.

Chitosan, chitosan derivatives, and chitosan composites have been extensively referred in the literature with their biosorbent properties for the desired pollutants, such as heavy metal ions, organochloride pesticides, suspended solids, organic pollutants such as organochloride pesticides, organic oxidized or fatty and oil impurities or textile wastewater dyes [200, 201]. Lessa et al. investigated the adsorption capacity of the composite beads obtained from cell wall polysaccharides pectin and cellulose for removal of multi-metal ions from water [202]. In another study, Elbedwehy et al. designed super adsorbent polymers for the removal of heavy metals Pb²⁺, Cd²⁺, and Cu²⁺ from wastewater by chemically modifying gum arabic [203]. Zhang and Chen were used crosslinked-starch graft copolymers containing amine groups as the sorbents for Pb²⁺ and Cu²⁺. In another study, starch crosslinked with POCl₃ and carboxymethylation has been proposed as sorbents for the removal of Cu²⁺, Pb²⁺, Cd²⁺ and Hg²⁺. Additionally, several crosslinked cyclodextrins gels were investigated for the removal of chloro and nitro phenols, benzoic acid derivatives, and dyes from aqueous solutions. For instance, cyclodextrin-epichlorohydrin copolymer was found to be successful in the removal of Bisphenol A [204]. In addition, carboxylated cellulose nanocrystal sodium alginate hydrogel beads have been investigated for the removal of pharmaceutical and personal care products from wastewater or other water bodies. Recently, another advanced polysaccharide matrix, the porous β-cyclodextrin polymers, claimed as an excellent adsorbent for the removal of pharmaceutical and personal care products from wastewater [201].

      The use of polysaccharide-based materials as sorbents offers several advantages including low cost, versatility, abundance, high capacity and high rate of adsorption, and ease of modification. However, there are also limitations of using polysaccharides in wastewater treatment. For instance, the choice of the adsorbent depends on the nature of the pollutant, thus the extreme variability of industrial wastewater must be taken into account in the design of any polysaccharide-based platform [204]. Still, polysaccharide-based materials one of the most attractive biosorbents for wastewater treatment. Considerable efforts are now being made in the investigation and development of polysaccharides and their derivatives as the natural materials for the removal of pollutants with different structures.

      6.2.7 Textiles

      The natural fibers obtained from wood, hemp, cotton, jute, and flax contain a high amount of cellulose (more than 85%) and serve as raw materials for the paper and textile industries [207]. Cellulose accounts for more than half of all the carbon found in the vegetable kingdom; thus, it is the most abundant of all carbohydrates. About 95% of cotton fibrils is cellulose. In the field of textile production, although the use of noncellulose synthetic fibers has been increasing, cotton and rayon still account for more than 70% of the fabrication [208].

      Cotton (Gossypium sp.) fiber has been widely studied as the major natural fiber used in the textile industry. The fiber primary cell wall is mainly composed of cellulose, hemicelluloses, and pectins. Several of the non-cellulosic polysaccharides, such as xylan, xyloglucan and callose, appeared to be partially retained during textile processing, though they were still largely impacted by both bleaching/scouring and mercerization processes. These hemicelluloses can resist harsh treatments, possibly because they reside within the fiber structure during the textile processing treatments. Such investigations would be of the greatest value since the determination of the fates of non-cellulosic polysaccharides during the textile processing offers useful information for future functionalization studies of cotton by specifically targeting these non-cellulosic polysaccharides and for optimization of cotton treatments [209].

Textiles have undergone surface modifications to enhance their softness, absorbance, dyeability, and wettability. The latest advances in textile chemistry include surface modifications to achieve antimicrobial activity, prevent skin irritation, and even enhance the fragrance [205]. Textile fabrics offer an ideal environment for

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