Biopolymer-Based Materials from Polysaccharides: Properties, Processing, Characterization and Sorption Applications

1.1 Biopolymers

As the name suggests, biopolymers are polymers synthesized by living organisms. Therefore, they are polymeric biomolecules i.e. long chain biomolecules comprised of covalently linked repeating monomeric units [1]. Living organisms (plants, animals, bacteria, fungi and yeast) synthesize a wide range of biopolymers such as deoxyribonucleic acid (DNA), ribonucleic acid (RNA), proteins, cellulose, chitin, starch, etc. Biopolymers carry out countless number of vital functions, such as storage of energy, preservation and transmittance of genetic information, and cellular construction, in vivo. DNA and RNA are the hereditary materials for the storage and passage of the genetic information in all living organisms, and thus making perpetuation of life possible in the planet. Proteins not only catalyze reactions (e.g. enzymes) and take part in cell signaling (e.g. hemoglobin) but also provide structural support (e.g. collagen). Cellulose is the major structural component of the plant cell walls.

There are three main classes of biopolymers owing to their universal occurrence and abundance: (i) polynucleotides, (ii) polypeptides/poly amino acids, and (iii) polysaccharides. Polynucleotides (DNA and RNA) are long polymers composed of 13 or more nucleotide monomers [1]. Polypeptides are the short polymers comprised of amino acids as monomeric units and amide bonds link the monomeric units together [2]. Polysaccharides are composed of monomeric sugars linked together by O-glycosidic linkages. Among hundreds identified polysaccharides, cellulose, starch, chitin, chitosan are some of the important examples [3, 4, 5]. In contrast to the often simpler and more random structure of the synthetic polymers, biopolymers, in their natural states, are complex molecular assemblies that assume defined 3D structure and shapes, often known as hierarchical levels of structure.

Since biopolymers possess material properties suitable for various industrial and medical applications, they have garnered a great deal of interest both in academia and industry. Primarily, the interest and increasing trend of application of biopolymers as commodity products stems from their renewability, biodegradability and, often, their biocompatibility. Even though traditional plastics (petroleum based polymer products) are still indispensable in our daily life, there is a growing concern about their environmental impacts as they are non-biodegradable and as a consequent, accumulate in the environment. Furthermore, petroleum is not a renewable resource and its reserve is dwindling at a fast pace. Thus, biopolymers derived from renewable resources possess competitive advantage over synthetic non-renewable polymers. However, the cost of production of biopolymers and biopolymer based commodity products is of prime importance if they are to compete with oil-based synthetic polymers in the market.

Exploiting biopolymers, which are abundant and can be obtained from source material at lower cost, as a main component of biopolymeric products can be a smart strategy in the first place to mitigate the cost related issues. To that end, polysaccharides, the biopolymers comprised of monosaccharides (sugars) linked together by O-glycosidic linkages are of obvious choice. Both the most abundant natural resource, cellulose, and the second most abundant biopolymer, chitin, are polysaccharides [3, 6]. Additionally, polysaccharides are widely distributed in nature as they can be derived from plants, animals and microorganisms. Furthermore, variation in physicochemical properties, such as mechanical properties, solubility, viscosity, gelling potential, surface and interfacial properties, governed by monosaccharide composition, chain length (degree of polymerization), linkage types and patterns, provide polysaccharides versatility in preparation of materials with diverse applications. In fact, polysaccharides based materials in different forms including fibers, films, food casing, membranes, hydrogels, aerogels and sponges, with applications in several important commercial areas such as food, pharmaceuticals, biomedical, electronics, and adsorption have been developed [7, 8, 9, 10, 11, 12, 13]. Thus, polysaccharide-based biopolymers are promising candidates in the preparation of materials that can meet the much coveted dual requirements of environmental friendliness and economic sustainability. In this book chapter we focus on three important polysaccharide based biopolymers: cellulose, chitin, and chitosan.

1.2 Cellulose

Cellulose, the most abundant renewable resource in the planet with an annual yield of 1.5 × 10¹² tons, is the major structural component in plant cell walls. Besides plants, some species of bacteria, algae also biosynthesize cellulose. Tunicates are the only known animals capable of biosynthesizing cellulose [3, 14]. Because of its abundance, cellulose can serve as a virtually inexhaustible source of raw material in production of sustainable bioproducts, the so called “green products” [3]. It is a linear homopolymer of β-D glucose monomers covalently linked together by 1→4 glycosidic bonds i.e. the ringed glucose monomers are joined together via polycondensation reaction between hydroxyl groups at C1 of a glucose unit and C4 of the neighboring glucose unit. Since, every glucose unit is rotated 180° with respect to its neighbors along the fiber axis, dimer of glucose, called cellobiose, is considered as a repeating unit of cellulose polymer (Figure 1a) [15]. However, the convention of considering cellobiose as a repeating unit has been challenged and a recent review in this regard makes a cogent argument that glucose is the repeating unit rather than the cellobiose [16]. Cellulose is a long chain molecule and the degree of polymerization (DP) differs with source and the treatments employed during its extraction. The DP of cellulose is reported to be as high as 10,000 and 15,000 in wood and cotton fibers [14, 17].

Cellulose in nature is not found as an isolated molecule, but rather possesses an intriguing multi-level assembly, popularly known as hierarchical structure [18]. It is usually agreed that, during biosynthesis, approximately 36 individual cellulose molecules are assembled together to form elementary fibrils, which have dimensions in nano-scale (a square cross section, 3–5 nm in size). Elementary fibrils further undergo packing into a larger entity called microfibrils with a cross section of ∼20 nm × 8 nm. These microfibrils further assemble into macro sized cellulose fibers (e.g. wood fiber, cotton fiber) [14, 15]. However, specific packing of cellulose may be different based on the source.

Prevalence of hydroxyl groups in cellulose (three hydroxyl groups per glucose monomer) results in extensive intra-and interchain hydrogen bonding in cellulose (Figure 1b). Intra-and interchain hydrogen bonding and the stacking interactions mainly originating from Van der Waals forces facilitate highly ordered arrangement (crystalline regions) of cellulose fibrils although there are also amorphous regions present (where arrangement of cellulose chains is disordered) as depicted in Figure 1c [19]. In general, cellulose, regardless of the source, is highly crystalline and high molecular weight biopolymer and is, usually, a fibrous, tough and hydrophilic but insoluble in water and other common solvents [20]. However, important properties such as DP, crystallinity and fibrillar organization are dependent on the source. For example, cellulose from cotton fiber is more crystalline (∼80% crystallinity) as compared to wood cellulose (∼50% crystallinity). Similarly, quite different from plant derived cellulose, bacterial cellulose has ultrafine nanofibrous network structure, high hydrophilicity, and moldability [21].

1.3 Chitin

Chitin is another important polysaccharide and the most abundant biopolymer on the planet after cellulose. It is a structural biopolymer, contributing in strength and reinforcement, present in the exoskeleton of arthropods (i.e., in the shells of shrimps and crabs), and the cuticle of insects. It is also found in the cell walls of fungi, yeast and other organisms in lower plant and animal kingdoms [4]. Structurally, it is similar to cellulose except the presence of acetamide or amine functional groups at C2 position of the glucose monomer unit. Therefore, chitin is a linear polymer of 2-acetamido-2-deoxy-D-glucopyranose linked together by β(1→4) glycosidic bond [22] (Figure 2). The degree of acetylation (DA) which is the share of nitrogen sites occupied by acetyl group is typically ∼90% in chitin after extraction and purification from source material since alkali treatment applied for the removal of protein also lowers DA [23].

Similar to cellulose, native chitin is arranged in highly crystalline microfibrils because of the extensive hydrogen bonding among the polymer chains. It is tough, hydrophobic, and insoluble in water and other common organic solvents. Chitin is even more recalcitrant to solubilization and consequently harder to process than cellulose [6].

1.4 Chitosan

Chitosan is the most important partially deacetylated derivative of chitin with degree of acetylation (DA) <50%. Typically chitin has DA <35% [24]. Chitin can be deacetylated by treating it with concentrated alkali at elevated temperature. Enzymatic hydrolysis in the presence of chitin deacetylase can also be employed for deacetylation purpose [21]. Therefore, chitosan can be represented by the same chemical structure as chitin. While chitin is insoluble in common solvents, chitosan can be dissolved in weak acidic solutions (aqueous solutions with pH < 6.5). It is soluble in hydrochloric acid and aqueous organic acids such as formic, acetic, oxalic and lactic acids. Protonation of the free glucosamine in acidic condition facilitates chitosan solubility [25, 26]. Chitin and chitosan are reported to have inherent antimicrobial properties.

2.1 Derivatization of cellulose, chitin and chitosan

Because of the abundance of hydroxyl (OH) groups present in cellulose, it can be derivatized through esterification, etherification, and other reactions to produce cellulose derivatives such as acetylated (e.g. cellulose acetate), carboxy methylated (e.g. carboxymethyl cellulose), and phosphorylated (e.g. cellulose diphenylphosphate) and cationized cellulose [27, 28, 29, 30]. Cellulose acetate (CA) is one of the most important commercially available cellulose derivatives with many applications such as fiber, film, and osmotic membrane. CA is a cellulose ester in which hydroxyl groups in cellulose are esterified with acetate groups to varying degrees [31]. Similarly, chitin and chitosan can be derivatized through O- and N-substitution to obtain carboxyalkyl derivatives such as O-carboxymethyl chitin, O-carboxymethyl chitosan and N,N-dicarboxymethyl chitosan [21, 24]. Derivatization not only eases the processing of these biopolymers since derivatives can be solubilized in common solvents and even in water based on degree and type of substitution [21, 29, 32], but also imparts some novel chemical and physical properties to cellulose. For instance, phosphorylation of cellulose can impart enhanced flame retardancy and bioactivity [21, 32]. Similarly, carboxyalkylation of chitosan enhances its affinity towards metal ions and has application in metal ions collection [24].

2.2 Dissolution of cellulose, chitin and chitosan

Due to their high molecular weight and complex biopolymeric network (extended hydrogen bonding and crystallinity), processing of cellulose and chitin into desirable forms, such as films, membranes, fibers, and gels, via dissolution is generally considered as nontrivial task as they are insoluble in water and other common organic solvents [13, 33]. Therefore, although studied for a long time, cellulose and chitin dissolution in cost effective and environmentally friendly manner remains an active area of research. A number of solvents/solvent systems have been developed over the course of time. Derivatizing solvent (carbon disulfide/aqueous sodium hydroxide used in “Viscose” process), and other non-derivatizing solvents such as aqueous alkali solution (e.g. aqueous solution of 7% sodium hydroxide/12% urea), N,N-dimethylacetamide/lithium chloride (DMAC/LiCl), N-methylmorpholine-N-oxide (NNMO) (used in “Lyocell” process), and ionic liquids (ILs) have been some of the important cellulose solvents [34, 35, 36, 37, 38]. Aqueous sodium hydroxide/urea solution, DMAc/LiCl and ILs such as 1-allyl-3-methylimidzolium bromide, 1-butyl-3-methylimidzolium chloride and 1-butyl-3-methylimidazolium acetate have been reported to be capable of dissolving chitin [33]. Although they are yet to find significant industrial breakthrough, ionic liquids are thought to have advantages over other solvents due to their chemical and thermal stability, tunability, non-flammability, negligible low vapor pressure, high efficiency in dissolution of both cellulose and chitin. Regenerated cellulose products such as fibers, film, aerogels and beads have been prepared from biopolymer solutions in different solvents [39, 40, 41, 42].

2.3 Aerogels

Aerogels are broadly defined as sol–gel derived materials, which are highly porous, low density and predominantly mesoporous (pore diameter: 2–50 nm) [43]. They are derived from hydrogels followed by the removal of liquid component using super critical point drying or freeze-drying to avoid damage to the original gel micro-structure prepared from biopolymers solution. Because of their very low density, high porosity and high specific surface area, aerogels have potential applications in separation, adsorption, catalyst, photo-electricity, and biomedicine [13]. High surface area aerogels from cellulose (400–500 m² g⁻¹) [44], chitin (560 m² g⁻¹) [45], and chitosan (248 m² g⁻¹) [46] have been prepared.

2.4 Nanofibrillation/nanoparticles (nanocellulose, nanochitin, nanochitosan)

The term “nanocellulose”/“nanochitin”/“nanochitosan” encompasses various materials derived from respective biopolymers, which possess at least one dimension in the nanometer range [14, 47]. Nanoparticles from cellulose and chitin are usually prepared by destructing the native hierarchical structure of these biopolymers [48]. Cellulose and chitin nanofibers, in general, are obtained by subjecting purified cellulose and chitin substrates to multiple mechanical shearing actions, which disintegrate the native microfibril structure and release enmeshed individual or bundle of fibrils. Chitosan nanofibers are typically produced by electrospinning [49].

Highly crystalline elongated rod like (or needle-like) nanoparticles called nanocrystals can be obtained when cellulose and chitin substrates are subjected to a strong acid hydrolysis treatment due to preferential dissolution of amorphous domains. For acid hydrolysis to produce nanocrystals, sulfuric acid and hydrochloric acid are usually employed [50, 51]. Biopolymer nanoparticles can be directly processed into films and aerogels by drying from the suspension or they can be utilized as reinforcement agents in other polymer matrices.

3.1 Materials

Microcrystalline (MCC) was provided by FMC Biopolymer (Newark, DE). Chitosan (75–85% deacetylated) and chitin from shrimp shells were used as received from Sigma-Aldrich (Saint Louis, MO).

3.2 Instrumentation

4.1 X-ray diffraction (XRD) analysis

Wide-angle X-ray diffraction measurements were collected for raw cellulose, chitin, and chitosan powder samples. Figure 3 shows the X-ray diffraction curves for cellulose, chitin, and chitosan. The XRD pattern of cellulose exhibits five major diffraction peaks at 14.9 (101), 15.8 (10ī), 21.8 (021) 22.5 (002) and 34.6 (004) which are in agreement with the literature values reported for cellulose I_β [52]. The XRD patterns of chitin showed two strong reflections at 9.2° (020) and 19.1° (110) and minor reflections at 12.6° (021), 22.9° (130) and 26.2° (013) corresponding to α-chitin [53]. Chitosan shows two distinct peaks at 10.67 (020) and 19.92° (110) [54]. Our results suggest that chitin has the highest crystallinity as compared to cellulose and chitosan. The low crystallinity of chitosan is the result of the deacetylation process.

4.2 FTIR characterization

FTIR studies were performed on cellulose, chitin and chitosan powders. Figure 4 shows the UATR-FTIR spectra of cellulose, chitin, and chitosan collected in the wavenumber range of 4000–600 cm⁻¹. The peaks of cellulose at 3335 and 3234 cm⁻¹ are attributed to the O-H stretching vibrations and the extensive inter- and intra-molecular hydrogen bonding network [55]. For chitin, the vibrations attributed to O-H and N-H stretching are visible at 3434 and 3256 cm⁻¹ and for chitosan those peaks appear at 3434 and 3256 cm⁻¹ [56, 57]. The bands in the region of 3000–2800 cm⁻¹ correspond to the -CH₂ symmetrical and asymmetrical stretching vibrations of polysaccharides [55, 56, 57]. Chitin exhibits the doublet amide I band at 1658 and 1619 cm⁻¹ representing the presence of H-bonding in a C=O group with the NH group of the adjacent chain and the O-H group of the inter-chain [58]. The amide II band of chitin and chitosan is located at 1553 and 1586 cm⁻¹, respectively [56, 57]. The peak at 1307 cm⁻¹ corresponds to the amide III band (in-plane mode of CONH₂ group) of chitin. Respective, amide I and C-N stretching of amide III of chitosan appear at around 1647 and 1318 cm⁻¹. The vibrations at ∼1152 and ∼1110 cm⁻¹ are attributed to the anti-symmetrical bridge of C-O-C stretching and anti-symmetric in-plane stretching. The peaks at ∼1060 and ∼1025 cm⁻¹ correspond to the C-O stretching of the polysaccharide [55, 56, 57]. The band at ∼895 cm⁻¹ is assigned to β-linkage of the polysaccharide [55, 56, 57].

4.3 TG and DTG analysis

High-resolution thermogravimetry (TG) profiles recorded in flowing nitrogen (N₂) were used to investigate the thermal stability of cellulose, chitin and chitosan, see Figure 5(a). All samples display two main weight loss regions at 35–150 and 250–450°C. In the first region, approximately, 5.1–9.0% of the weight loss was observed, which is attributed to the removal of physically adsorbed water. The second region represents the largest weight loss of 90.4, 86.6 and 63.4% for cellulose (1), chitin (2) and chitosan (3), respectively, corresponding to the degradation of polysaccharide structure of the biopolymer. The differential thermogravimetric (DTG) profiles generated from TG data are shown in Figure 5(b). The DTG curves for cellulose, chitin, and chitosan exhibit the main decomposition peaks at 371, 391 and 300°C, respectively. These results suggest that among three biopolymers, chitin has the highest thermal stability while chitosan has the least thermal stability.

4.4 SEM studies

The morphological characteristic of cellulose, chitin and chitosan was investigated and Figure 6 shows the comparison of typical surface morphologies for cellulose, chitin and chitosan. As it can be seen, all samples exhibited more irregular, flat, rough nanofiber surface with no porosity.

5.1 Dye removal

Industries including textile, leather, paper, printing, and paint utilize large amounts of water for processing have the potential to contaminate waterways through the discharge of organic dyes into natural water resources and water treatment systems. Over 10,000 different textile dyes with an estimated annual production of over 7 × 10⁵ tons are available commercially [59, 60]. Most of these organic dyes are excessively used and 10–20% is directly discharged as aqueous effluents into different water bodies [61]. Most problematic ones are the brightly colored, water-soluble reactive and acid dyes as these dyes cannot be removed through conventional treatment systems. Discharging of organic dyes into water bodies has raised acute and chronic concerns to the ecosystems and human health. For example, the release of those organic dyes can lead to eutrophication, non-esthetic pollution and imbalance in the aquatic biological systems and also causes chronic toxicity, carcinogenicity and neurotoxicity towards humans and animals. Commonly applied technologies for removing organic dyes from wastewater include chemical precipitation and adsorption [62, 63], electrochemical oxidation and reduction [64], aerobic and anaerobic treatment [65, 66], coagulation and flocculation [67, 68], membrane separation [69], ultra-filtration [70], H₂O₂/ultraviolet (UV) and photocatalysis [71, 72, 73], ion exchange [74], sonochemical degradation [71], Fenton and heterogeneous Fenton-like catalysis [75, 76, 77], electrolysis [78], and advanced oxidation processes (AOPs) [56, 79]. At present, there is a critical need for research that enables new means of inexpensive, reusable, environmentally and energetically sustainable wastewater management systems for wastewater treatment. Recently, biopolymer-based materials have attracted interests as emerging alternative techniques for the removal of organic dyes from industrial effluents due to their relative abundance, eco-friendly, high thermal and mechanical stability, low cost, nontoxicity, easy functionalization, and excellent sorption capacities. Here, we report the roles of biopolymer-based materials mainly derived from cellulose, chitin, and chitosan as sorption materials for the removal of organic dyes from industrial wastewater.

Liu et al. investigated the porous cellulose-based bioadsorbent for the removal of anionic dye acid blue 93 (AB93) and cationic dye methylene blue (MB) from single and binary dye solutions by grafting acrylic acid and acrylamide onto cellulose surface [80]. They reported maximum sorption capacity of 1372 mg/g at an initial absorbent concentration of 2500 mg/L for both of AB93 and MB with a reusability up to three consecutive cycles. The adsorption process followed the pseudo-second-order kinetic model and the Freundlich isotherm model. Jiang and coworkers prepared ultralight aerogels from cellulose nanofibrils (CNFs) derived from rice straw [81]. These ultra-lightweight aerogels have been studied as adsorbents for the removal of cationic dye malachite green (MG) dye from aqueous media. The highest adsorption capacity of MG was found to be 212.7 mg/g. They also reported a 100% MG removal via four consecutive adsorptions at a 1:5 mg/mL aerogel/MG ratio and 10 mg/L dye concentration and 92% MG adsorption in a single batch at 10:5 mg/mL aerogel/MG ratio and 100 mg/L dye concentration. The adsorption process obeyed a pseudo-second-order kinetic model and monolayer Langmuir adsorption isotherm. The dye desorption was also achieved by increasing ionic strength, leading to the recovery of both dye and CNF aerogel. Ruan and coworkers prepared porous 2,3-dialdehyde cellulose-chitosan (DAC-CS) beads and their applications in the removal of Congo red dye [82]. DAC-CS beads showed 100% dye removal at a capacity of 200 mg/g at pH. 2 in 100 g/L dye solution. About 93% of the adsorbed Congo red in DAC-CS beads could be desorbed at pH. 12 NaOH solution. A similar trend was observed in the second cycle with the adsorption and desorption of 100 and 89%, respectively. Cross-linked quaternized cellulose nanofibrils (Q-CNFs) have been investigated as adsorbents for removal of three different anionic dyes by Maatar and Boufi [83]. They reported the adsorption capacity of approximately 160, 230, and 560 mg/g for red 180, blue CR19, and orange 142 dye, respectively. The authors also confirmed the main contribution of electrostatic interactions between positive sites and the CNF surface and dye sulfonate groups by zeta potential measurements. The regeneration of the Q-CNF sorbent could be achieved by extraction with KCl solution in ethanol-water mixture.

Kinetics of the adsorption of reactive yellow 2 (RY2) and reactive black 5 (RB5) by chitin was investigated by Akkaya et al. [84]. They concluded that the adsorption of RY2 by chitin from aqueous solution is favorable at low temperature and pH and the adsorption of RB5 by chitin is favorable at high temperature and pH. Cao and coworkers studied the adsorption isotherms and kinetic analysis of methylene blue (MB) onto porous chitin sorbents (PChs) with different content of chitin, ranging from 0.9 to 3.5% [85]. The maximum adsorption capacity of 384 mg/g with 79.8% MB removal was observed for PChs containing 3.5% chitin. The same adsorption capacity with 65% removal ratio was observed after six repetitive cycles. The adsorption equilibrium followed Freundlich isotherm model. Xu and coworkers reported the synthesis and efficient removal of MB by nanocomposite microspheres fabricated from chitin and clay [86]. These chitin/clay microspheres (CCMs) showed a maximum adsorption capability of 156.7 mg/g. A dye removal of 99.99% was observed within 20 at a low MB concentration of 10 mg/g. They also demonstrated a stability over five adsorption/desorption cycles for CCMs. Their kinetic data fitted well with the pseudo-second-order and monolayer Langmuir isotherm models. Gopi et al. prepared a chitin nanowhisker (ChNW)-functionalized polyvinylidene fluoride (PVDF) membrane using electrospinning technique [87]. They reported a maximum adsorption capacity of 72.6 mg/g and 88.9% removal efficiency of indigo carmine (IC) dye with their PVDF/ChNW (15%:1% wt.) composite membrane as compared to neat PVDF.

Xu and coworkers synthesized poly(2-acrylamido-2-methylpropane sulfonic acid grafted magnetic chitosan microspheres (PMCMs) for MB adsorption [88]. They investigated the effects of initial solution pH (1.0–10.0), temperature (30–50°C), contact time (0–660 min) and initial concentration (50–1600 mg/L) on dye adsorption kinetics. At pH. 9, PMCMs exhibited maximum MB sorption capacity of 1000, 1250 and 1428 mg/g at 30, 40 and 50°C, respectively. They also showed that the adsorption capacity increased with increasing of the initial solution pH and temperature. The adsorption kinetic and adsorption equilibrium data followed the pseudo-second-order kinetic model and monolayer Langmuir isotherm model, respectively. PMCMs could also be separated under external magnetic field and regenerated under acidic conditions. Naseeruteen et al. exhibited the removal of Malachite Green (MG) by mesoporous chitosan ionic liquid beads prepared from chitosan and 1-butyl-3-methylimidazolium based ionic liquids [89]. The effects of initial pH, adsorbent dosage, agitation time and initial MG concentration have also been investigated. The optimum conditions reported were pH 4.0, 0.008 g of adsorbent dosage and 20 min of agitation time. The data obtained at pH 4 fitted well to a pseudo-second order kinetic model and followed the Langmuir model. The maximum adsorption capacity obtained for two chitosan beads prepared from 1-butyl-3-methylimidazolium acetate and 1-butyl-3-methylimidazolium was 8.07 and 0.24 mg/g respectively.

5.2 Removal of heavy metals

The surge in energy production and the increasing use of heavy metals in various industrial processes have led to the generation of large quantities of industrial waste containing heavy metals. Therefore, heavy metals are released into the environment including air, water, soil, and biosphere in excessive amounts on daily basis. Heavy metal ions are mainly elements from the fourth period of the periodic table including chromium (Cr), arsenic (As), cobalt (Co), copper (Cu), nickel (Ni), zinc (Zn), lead (Pb), and mercury (Hg) and have a high atomic weight and a density at least 5 times higher than that of water [90]. Heavy metals are typically toxic, persistent, non-biodegradable and bioaccumulative. Due to the high solubility, heavy metals are easily absorbed by plants and aquatic species and subsequently enter the food chains and then the human body. The presence of high levels of heavy metals in the human body may cause various health effects including skin irritations, stomach cramps, vomiting, multiple organ damage, birth defects, nerve system damage and development of autoimmunity. Most commonly used techniques for the removal of heavy metals include membrane filtration, adsorption, coagulation, chemical precipitation, ion-exchange, electrochemical, biological treatments and advanced oxidation processes [91]. However, adsorption technologies are widely considered as cheap and energy efficient solutions for removal of heavy metals. Biopolymers including cellulose, chitin, and chitosan have been widely investigated as sorbents for remediation of heavy metals due to their excellent sorption properties and environmental benignity [92, 93, 94]. Here we present, recently reported applications of biopolymer-based materials for the removal of heavy metals.

d’Halluin et al. prepared a chemically modified cellulose filter paper with ethylenediaminetetraacetic acid (EDTA) for heavy metal remediation in water [95]. Cellulose-EDTA composite showed 90–95% removal efficiency for various heavy metal ions including Ag(I), Pb(II), Cd(II), Ni(II), Zn(II), Sn(II), and Cu(II). They also investigated the adsorption kinetics using Langmuir, Freundlich, and Temkin isotherms and the recyclability of their material. Fakhre and coworkers synthesized a supramolecular polysaccharide composite material from cellulose (CEL) and dibenzo-18-crown 6 (DB18C6) using ceric ammonium nitrate as initiator [96]. They studied the removal of five different heavy metal ions including Cd²⁺, Zn²⁺, Ni²⁺, Pb²⁺ and Cu²⁺ by [CEL + DB18C6] composites. They reported adsorption capacities and percentage of metals removal of (197 mg/g, 98%), (180 mg/g, 94%), (186 mg/g, 93%), (194 mg/g, 97%), and (192 mg/g, 96%) for Cd²⁺, Zn²⁺, Ni²⁺, Pb²⁺ and Cu²⁺, respectively. The regeneration of the material is dependent on the type and concentration of regenerating solution (NH₄Cl, HNO₃, NaCl and CaCl₂). Shao et al. studied the removal of Cu²⁺ ions by a porous waste paper (WP)-chitosan adsorbent (CSA) [97]. WP-CSA exhibited adsorption capacity of 156.3 mg/g for Cu²⁺ and 98.3% desorption efficiency in 0.1 M H₂SO₄ solution. This adsorption equilibrium followed the pseudo-second order model and the Langmuir monolayer model.

Duan and coworkers presented the fabrication of lignin/chitin films from a binary solvent system composed of the ionic liquid 1-butyl-3-methylimidazolium acetate and γ-valerolactone and their application as an adsorbent for Fe(III) and Cu(II) cation uptake from aqueous solutions [98]. Lignin/chitin film showed the adsorption capacity of 84 and 22 wt% for Fe(III) and Cu(II) cations within 48 h. During the regeneration process up to 12 and 46 wt%, respectively, could be desorbed. The adsorption isotherms for both Fe(III) and Cu(II) obeyed the Langmuir model. Ethylenediaminetetraacetic acid (EDTA) modified β-cyclodextrin/chitosan (CDCS-EDTA) composite was prepared and applied for the removal of Pb(II) and anionic dye acid red 73 (AR) by Wu and coworkers [99]. They also investigated the effects of adsorbent dose, contact time and pH value on the CDCS-EDTA adsorption efficiency. The maximum adsorption capacities reported were 114.8 and 754.6 mg g⁻¹ for Pb(II) and AR under optimal conditions with adsorption efficiently of 93.4 and 92.1%, respectively. Both adsorption processes followed the pseudo-second-order adsorption kinetic model and the Langmuir isotherm model. Kwok et al. studied the sorption of anionic species arsenite, As(III) ions and arsenate, AS (V) ions onto chitosan and nanochitosan [100]. The equilibrium adsorption capacity of chitosan and nanochitosan for As(III) and As(V) ions were (0.5 and 8 mg/g) and (6.1 and 13 mg/g), respectively.

5.3 Oil and solvent spillage cleanup

An oil or a solvent spill is the accidental or intentional discharge of petroleum hydrocarbons into the environment, especially the aquatic ecosystem. Oil and solvent spills have become a serious environmental problem due to the increasing use, exploration, production and transportation of oils and solvents worldwide by various petrochemical and chemical industries. Those spillages lead to the pollution of aquatic ecosystems and natural water resources including underground water streams and coastal waters and affect both human and animal health through inhalation, skin, digestion, and eye irritation. Therefore, efficient and rapid removal of oil and solvent spills on the surface of water is imperative for protecting the aquatic environment, water resources, and subsequently human and animal health. Currently available oil and solvent spill techniques include physical diffusion, in-situ burning, bioremediation, and mechanical recovery [101, 102]. Among those technologies, cleaning the spilled oil and solvents by physical adsorbents is an attractive approach because of their ease of use and simplicity. Biopolymer-based adsorbents have been widely investigated for cleanup of oil and solvents due to their relative abundance, sustainability, low-cost, selectivity, fast adsorption kinetics, reusability and environmental benignity. Some recent developments in the use of biopolymer-based materials in oil and solvent spillage cleanup processes are discussed below.

Ao et al. reported the synthesis and separation of different oil/water mixtures using a cellulose hydrogel-coated mesh (CHM) [103]. They reported selective separation of a series of mixtures including hexane, cyclohexane, petroleum ether, liquid paraffin, pump oil and xylene with a high separation efficiency of over 98.9% and a high flux of 12,885 L/ m² h. Their material also showed high reusability, anti-salt properties, and stability after 60 successive cycles of separation with a separation efficiency of >98.2%. Magnetic hydrophobic polyvinyl alcohol (PVA)-cellulose nanofiber (CNF) aerogels (MHPCA) were prepared and used as effective oil absorbents by Xu and coworkers [104]. A series of oils including soybean oil, corn germ oil, pump oil, used pump oil, gasoline, motor oil, ethanol, and dimethylformamide (DMF), were tested and MHPCA showed oil absorption capacity in the range of 59–136 g/g. MHPCA exhibited magnetically driven oil absorption and elasticity after 30 compression-release cycles, showing its reusability and durability. Cheng and coworkers investigated the use of pure cotton and cotton/cellulose aerogels derived from pure cotton and cellulose fiber from paper waste for the removal of oil and organic solvents [105]. The cotton/cellulose aerogels exhibited better performance over pure cotton aerogels because of the synergetic effects of two different cellulose fiber sources. The sorption capacity of the cotton/cellulose composite aerogel was 72.3 and 94.3 g/g for machine oil and dichloromethane, respectively [105].

Duan and coworkers prepared hydrophobic and oleophilic methyltrichlorosilane (MTCS) coated-chitin sponges and showed their ability to absorb a wide range of oils and nonpolar organic solvents including chloroform, gasoline oil, hexane, pump oil, silicon oil and toluene from the surface and bottom of the polluted water [106]. They reported that low viscosity organics such as chloroform, gasoline oil, hexane, and toluene could be absorbed to equilibrium within 8 min, whereas higher viscosity oils, such as pump oil and silicon oil could reach the absorption equilibrium within 13 min. MTCS-coated chitin sponges displayed recyclability with at least 10 times with approximately 93% adsorption capacity. Li et al. prepared porous chitosan-oxidized cellulose aerogel as an oil and organic solvent absorbent [107]. They reported the adsorption capacity in the range of 13.77–28.20 g/g for various oils and organic solvents including carbon tetrachloride, ethylene glycol, silicon oil, ethylene acetate, pump oil, crude oil, acetone, ethanol, diesel, and gasoline. Their material was stable over 50 absorption-desorption cycles following a pseudo-second-order kinetic model. The utilization of N,O-carboxymethyl chitosan for the adsorption of Marine Diesel, Diesel and Marine-2T oil for oil spill treatment has been reported by Doshi and coworkers [108].

5.4 Carbon dioxide adsorption

Carbon dioxide (CO₂) is the primary greenhouse gas (GHG) emitted through human activities and also considered as one of the major contributors to the greenhouse effect and global warming. CO₂ emissions, primarily from fossil fuel based power plants account for over 87% of the global annual emission of CO₂ [109]. According to the Intergovernmental Panel (IPCC) Fifth Assessment Report, the industrial revolution has significantly contributed to the raised atmospheric CO₂ levels from 280 to 400 parts per million (ppm) in the last 150 years [110]. It is estimated that by the end of year 2030, the global GHG emission could rise by 25–90% as compared to the GHG level in 2000, reaching CO₂ atmospheric levels in the range of 600–1550 ppm [111]. Some long-term effects of global warming include higher temperatures, changing rainfall patterns, rising sea levels, severe weather events ranging from flash floods, high intense hurricanes, freezing winters, severe droughts and heat waves. Therefore, many approaches including improved energy efficiency and energy conservation, increased utilization of low carbon fuels such as natural gas, hydrogen or nuclear power, usage of renewable energy such as solar, wind and hydropower and bioenergy, application of geoengineering processes such as afforestation and reforestation and use of carbon dioxide capture, storage and utilization (CCSU) techniques are extensively considered and adopted by various countries to mitigate CO₂ emissions [112, 113]. Among those, CCSU technologies have received a significant interest due to their ability to control in-situ CO₂ emissions from both industrial sources and natural sources. CCSU technologies mainly accompany separation of CO₂, transportation of captured and compressed CO₂, and underground storage of CO₂ for reuse. However, the success of these CCSU technologies relies on the CO₂ adsorption efficiency, manufacturing cost, and ease of handling and regeneration. Solid adsorbents include carbonaceous materials, covalent organic frameworks (COFs), porous organic frameworks (POPs) and metal–organic frameworks (MOFs), metal oxides (alkaline, alkaline earth and transition metals) and mesoporous silica-based materials and amines have been proposed for CCSU technologies [114, 115, 116]. Solid adsorbents derived from biopolymers are attracting interest for CCSU technologies because of their unique properties. Recently reported applications of biopolymer-based materials in capturing CO₂ gas are discussed in this section.

Dassanayake et al. investigated the preparation of activated carbon monoliths derived from cellulose-based aerogel (aerocellulose) and its CO₂ adsorption properties at low and ambient temperatures [117]. The activated carbon prepared from aerocellulose exhibited a specific surface area of 753 m²/g, a total pore volume of 0.72 cm³/g, and a micropore volume of 0.27 cm³/g. The physical CO₂ uptakes for these materials were 5.8 mmol/g of CO₂ at 0°C and 1 atm and 3.7 mmol/g of CO₂ at 25°C and 1.2 atm. They also reported the synthesis and CO₂ adsorption of a series of amidoxime (AO)-functionalized microcrystalline (MCC) and nanocrystalline cellulose (NCC)-mesoporous silica composites at ambient and elevated temperatures [115, 118]. They reported the CO₂ sorption capacities of MCC-AO composites in the range of 0.40–1.27 and 2.84–3.85 mmol/g at ambient (25°C, 1.2 atm) and elevated temperatures (120°C, 1.0 atm), respectively. Whereas, NCC-AO composites which showed highest CO₂ uptakes of 3.30 mmol/g at 25°C (1.2 atm) and 5.54 mmol/g at 120°C (1 atm), respectively. Both MCC-AO and NCC-AO composites displayed a good recyclability and stability after 10 successive adsorption/desorption cycles with negligible losses of the sorption capacity. Shehaqui et al. reported the direct CO₂ capture from air onto nanofibrillated cellulose (NFC)-polyethylenimine (PEI) foams [119]. They demonstrated the impact of both PEI and relative humidity (RH) on the CO₂ capture under atmospheric conditions with CO₂ concentration of ∼400 ppm. At 80% RH and PEI content of 44 wt%, a CO₂ uptake of 2.22 mmol/g was achieved with a stability over five repetitive cycles.

Dassanayake and coworkers also studied the CO₂ adsorption on activated carbon prepared by carbonization and KOH activation of chitin aerogels [39]. Their material showed CO₂ adsorption capacities of 5.02 mmol/g at 0°C and 3.44 mmol/g at 25°C under ambient pressure of 1 atm. Eftaiha et al. investigated the CO₂ adsorption by a chitin-acetate (CA)/dimethyl sulfoxide (DMO) binary system and reported the CO₂ uptake of 3.63 mmol/g at 3.95 atm and 0°C [120]. Fujiki and Yogo investigated CO₂ adsorption capacities of nitrogen-doped activated carbons prepared from chitosan at two different pressure conditions; namely 0.15 and 0.99 atm at 25°C [121]. They reported CO₂ adsorption capacities of 1.6 mmol/g at 0.15 atm and 4.9 mmol/g at 0.99 atm for their material. Alhwaige et al. reported the preparation of montmorillonite (MMT) reinforced bio-based chitosan-polybenzoxazine (CTS-PBZ) composite carbon aerogels and their ability to capture CO₂ [122]. MMT-CTS-PBZ composite carbon aerogels showed a maximum CO₂ adsorption of 5.72 mmol/g with multi-cyclic adsorption-desorption stability.

Table 1 summarizes the main applications of biopolymer-based materials for pollutants removal from water and sorption of CO₂.