Estimated market of products from lignocellulosic waste.
In recent years, alternatives have been sought for the reuse of lignocellulosic waste generated by agricultural and other industries because it is biodegradable and renewable. Lignocellulosic waste can be used for a wide variety of applications, depending on their composition and physical properties. In this chapter, we focus on the different treatments that are used for the extraction of natural cellulose fibers (chemical, physical, biological methods) for more sophisticated applications such as reinforcement in biocomposites. Due to the different morphologies that the cellulose can present, depending from sources, it is possible to obtain cellulose nanocrystals (CNCs), micro- nanofibrillated cellulose (MFC/NFC), and bacterial nanocellulose (BNC) with different applications in the industry. Among the different cellulose nanomaterials highlighted characteristics, we can find improved barrier properties for sound and moisture, the fact that they are environmentally friendly, increased tensile strength and decreased weight. These materials have the ability to replace metallic components, petroleum products, and nonrenewable materials. Potential applications of cellulose nanomaterials are present in the automotive, construction, aerospace industries, etc. Also, this chapter exhibits global market predictions of these new materials or products. In summary, lignocellulosic residues are a rich source of cellulose that can be extracted to obtain products with high value-added and eco-friendly characteristics.
- lignocellulosic waste
- surface treatments
The comprehensive use of lignocellulosic waste coincides with the concept of circular economy because these wastes are renewable, abundant in nature, and generated in large volumes. In addition, they are a main source of natural fibers, chemical compounds, and other industrial products. Lignocellulosic residues are used in various applications depending on their composition and physical properties. Generally, lignocellulosic residues are constituted of cellulose, hemicellulose, lignin, pectin, waxes, and ash . One of its main applications is the production of biofuels, where cellulose is subjected to various physical (mechanical, ozonolysis, pyrolysis), chemical (acid, alkali, organosolv), and biological (commonly used white-rot fungi) pretreatments. However, this review is focused on the different treatments used on the surface of natural fibers in order to improve their compatibility with a polymeric matrix and thus obtain materials with ecological, lightweight, and excellent mechanical properties, called biocomposites. It is important to mention that when carrying out some of these treatments, residues are generated, which can be processed to recover some high value-added compounds (antioxidants, sugars, bioactive phenols, organic acids, polysaccharides, and polyphenolics). Furthermore, the different types of biomaterials that can be obtained from cellulose (MCF, NFC, CNC, BNC) are described. Finally, an investigation of the market size of some of the products derived from lignocellulosic residues was carried out.
The biocomposites are materials formed by a polymer matrix and natural fibers, which act as reinforcements. Among their main advantages, we can highlight the following: low density, low cost, high resistance, and they are eco-friendly as well. However, they have a disadvantage, incompatibility between polymer matrix and natural fibers, because polymers are hydrophobic and natural fibers have a hydrophilic nature. This is reflected in the mechanical performance of biocomposites. Because of this, chemical and physical treatments have been developed to promote interfacial adhesion between polymer and natural fibers, in addition, to improve dimensional stability and water absorption capacity of biocomposites . In comparison to chemical and physical treatments, biological treatments are considered efficient and environmentally friendly processes. In nature, a great variety of microorganisms capable of degrading lignin, cellulose, and hemicellulose are found . Inside these microorganisms, we can find out fungi that have the enzymatic structure necessary to degrade this type of polymers . The main applications of biocomposites are automotive parts (door panel/inserts, seatbacks, spare tyre covers, interior panels, etc.), circuit boards, aerospace industry, building materials, etc.
2.1 Chemical treatment
As mentioned above, the main objective of the chemical treatment is to improve the adhesion between the natural fibers and the polymer matrix, in addition, it is possible to reduce the absorption of moisture, therefore the mechanical properties are improved. Chemical treatments including alkali, silane, acetylation, benzoylation, acrylation, maleated coupling agents, isocyanates, and others are commonly used.
2.2 Physical treatment
There are different types of physical treatments used to modify only the surface of natural fibers without changing their chemical composition. Physical treatments promote the separation of the fiber bundle into individual fibrils and thus increase the surface area of the fibers and the compatibility with the polymer matrix. According to Ahmed et al. , these physical treatments can be classified as follows: mechanical treatment (stretching, calendaring, or rolling), solvent extraction treatment, and electric discharge (plasma treatment, corona treatment, ionized air treatment, thermal treatment, steam explosion, electron radiation, dielectric barrier, and ultraviolet). The
2.3 Biological pretreatment
Biological pretreatment is based in the extracellular enzymes released by microorganisms in which enzymes degrade the noncellulosic components of the fiber surface. Biological pretreatment of fiber offers relevant advantages, such as low chemical and energy use that make it eco-friendly . A great variety of microorganisms exists in nature, they are able to hydrolyze lignin, being the fungi the most studied . Basidiomycetes white-rot fungi are responsible for lignin degradation in nature; they can break down not only lignin but also hemicellulose and cellulose. It has been reported that these microorganisms degrade lignin in a selective way that is able to offer potential biotechnological application . However, recent studies have shown that many bacteria are able to break down lignin . Likewise, enzymes have an enormous potential to be used for lignin valorization.
2.3.1 Fungal lignin degradation
The breaking down of lignin by fungi has been reported mainly for white-rot fungi due to their highly efficient enzymatic system. White-rot fungi are able to degrade lignin in such an efficiently and selectively way that gives them utility in the industry. These fungi have been applied by different industries such as paper, biofuels, and biorefinery for delignifying biomass . According to the selected strain, it is possible to obtain 20–100% for lignin removal. Black liquor from a pulp and paper mill, treated with the fungi
Laccases use molecular oxygen to oxidize aromatic and nonaromatic compounds, such as phenols, arylamines, anilines, thiols, and lignins . The oxidation leads to the constitution of free radicals that act as intermediate for the enzymatic reactions. Likewise, these mediators can react with others high redox potential compounds and mediate nonenzymatic reactions . White-Rot fungi are mainly reported to produce laccases such as,
LMPs belong to class II peroxidases, named plant, and fungal peroxidases, which contain protoporphyrin IX as a prosthetic group . LiP enzymes oxidize different phenolic aromatic compounds and nonphenolic lignin compounds due to the fact that they are not very specific to their substrates . LiP enzymes have been found only in a few white-rot fungi such as the genera:
2.3.2 Bacterial lignin degradation
It has been reported that bacteria are able to degrade lignin through a complex of enzymes, such as extracellular peroxidases, Dye-decolorizing peroxidases (DyPs), and laccases. Among the reported bacterial genus, we found
2.3.3 Lignin-derived aromatic compounds breaking down by microorganisms
Low molecular weight aromatic compounds are obtained after fungal lignin depolymerization, such as guaiacol, coniferyl alcohol, p-coumarate, ferulate, protocatechuate, p-hydroxybenzoate, and vanillate . Bacteria have the enzymatic machinery to metabolize-derived aromatic compounds that could allow the generation of value-added products such as flavors, polymer building blocks, and energy storage compounds (Figure 1).
Fungal and bacterial lignin degraders (BLD) depolymerize the lignocellulosic residues, thus obtaining hemicellulose and cellulose that can be used to produce biocomposites or biofuels and lignin-derived aromatic compounds which can be transformed by bacteria to value-added bioproducts.
2.3.4 Challenges in microbial lignin degradation
Biological lignin degradation process does not involve high temperatures and pressures and does not generate any undesirable products. However, it is a time-consuming process, and there is not an accurate control on it . Long time is necessary to achieve microbial lignin degradation that can range from 10 to 100 days, which is not suitable for commercial applications . Several efforts have been made to engineer microorganisms in order to be more efficient to metabolize lignin-derived compounds with remarkable biotechnological applications, such as pretreatment of lignocellulosics, pulping and bleaching in the paper industry, and decolorization in the textile industry .
2.3.5 Purified enzymes
The application of enzymes is an attractive alternative due to its shortened time, improved yield, and simple processing . The most common enzymes used to break down lignin are peroxidase and laccase, catalyzing lignin oxidation. Among the most studied peroxidases are lignin peroxidases and manganese-dependent peroxidases. These enzymes degrade lignin randomly converting the phenolic group to free radicals, which lead to lignin depolymerization . Fungal peroxidase from
3. New biomaterials
Cellulose is the most abundant polymer in the world. It is a linear polymer of β-d-glucose molecules linked by β(1 → 4) bonds. Due to this bond, each molecule has the ability to rotate 180° with regard to the previous one, forming long linear chains that are stabilized by the presence of hydrogen bonds and join chains to others. The cellulose micelle is made up approximately from 60 to 70 cellulose chains, and the union of 20 or 30 cellulose micelles achieves a semicrystalline packing and the formation of microfibrils. However, the morphology, size, and other characteristics depend on the cellulose origin, and according to the above, cellulose microfibrils (MFC)/nanofibrils (NFC), cellulose nanocrystals (CNCs), and bacterial nanocellulose (BNC) can be obtained .
3.1 Micro/nanofibrillated cellulose (MFC/NFC)
Microfibrillated cellulose (MFC) is obtained with the longitudinally disintegration of cellulose fibers by multiple mechanical shearing actions; in this way, a three-dimensional network of cellulose microfibrils (10–100 nm) is achieved, which has a higher surface area than conventional cellulose fibers. Due to its structure, MFC has the ability to form gels. Different mechanical treatment procedures have been reported to obtain MFC (high-pressure homogenization and grinding for example) and various pretreatments to facilitate the mechanical treatment (enzymatic, acid hydrolysis, mechanical cutting pretreatments, etc.) . The mechanical properties of MFCs are higher compared to lignocellulosic fibers because they have a more homogeneous structure. The main application of MFCs is in the packaging industry due to its excellent mechanical and barrier properties, which are required in this sector . Adel et al.  obtained micro/nanofibrillated cellulose from lignocellulosic residues (rice straw, sugarcane bagasse, cotton stalk) and botnia softwood Kraft pulp. First, the lignocellulosic residues were subjected to an alkaline pretreatment to eliminate the lignin, and later, the mechanical treatment was applied to them using a mill. According to their results, the crystallinity index of MFC increased and the length of the fibers that correspond to lignocellulosic residues decreased compared to the fibers of the pulp. And they concluded that the MFC obtained have optimal mechanical and optical properties; therefore, they can be used as reinforcement in the paper-making industry. Nanofibrillated cellulose (NFC) is obtained by delamination of wood pulp (wood, sugar beet, potato tuber, hemp, flax, etc.) by mechanical pressure before and/or after chemical enzymatic treatment with a diameter between 5 and 60 nm and its length in several micrometers. It exhibits amorphous and crystalline domains and high specific surface area. Nanofibrillated cellulose (NFC)/polyvinyl alcohol (PVA) nanocomposites are prepared by dispersion of nanofibers obtained from several biomass sources, normally at low contents (1–10%), into PVA aqueous solutions typically followed by solvent casting. Frone et al.  also used cellulose nanofibers obtained from microcrystalline cellulose by ultrasonic treatment as reinforcement (at lower 1–5 wt%) dispersed in PVA. In summary, these materials exhibit a high aspect ratio and specific surface area, excellent flexibility and strength, low thermal expansion, high optical transparency, and barrier properties. Consequently, they can be used to form strong transparent films and aerogels, as a rheology modifier and strength additive in the paper-making industry, like a constituent of food packaging and in different biomedical applications (drug delivery) .
3.2 Bacterial cellulose (BC)
Bacterial cellulose is produced by bacteria such as
3.3 Cellulose nanocrystals (CNCs)
Cellulose nanocrystals are obtained by enzymatic hydrolysis and have the following characteristics: elongated, less flexible, cylindrical, and rod-like nanoparticles with 4–70 nm in width, 100–6000 nm in length, and 54–88% crystallinity index . Gopi et al.  used hydrochloric acid to carry out the hydrolysis of cellulose and obtained an improvement in the thermal stability of the CNCs but with a significant agglomeration of the crystals. Park et al.  demonstrated a facile and green method of CNC extraction that uses only an high-pressure homogenization (HPH). The obtained CNCs presented rod-like shapes with a size distribution of 4–14 nm for width and 60–20 nm for length. Nanocrystalline cellulose (CNC) was dispersed in an alginate matrix for film application by Huq et al. . They observed that with a small amount of CNC (approximately 5% wt), the mechanical and barrier properties of the films made were improved by comparing with an alginate film. According to the results obtained by infrared spectroscopy (FTIR), they concluded that there was a molecular interaction between the CNC and the alginate through hydrogen bonds. In summary, the morphology and size of cellulose nanocrystals vary according to the kind of lignocellulosic biomass, extraction method, and manufacturing conditions. Nanocellulosic materials can be characterized by employing a variety of techniques : X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), helium pycnometer, differential scanning calorimetry (DSC), thermogravimetric analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and atomic force (AFM) among others. On the other hand, cellulose nanocrystals not only consist of primary reactive sites (i.e., hydroxyl groups) but also they possess higher surface area to volume ratio, making CNC highly reactive and easy to be functionalized. The most common surface modifications of CNCs are sulfonation, TEMPO-mediated oxidation, esterification, etherification, silylation, urethanization, amidation, polymer grafting, etc. The applications having the greatest potential due to the high available amount of volume on cellulose nanomaterials are placed in the following industries: automotive (body components, interiors), construction (air and water filtration, insulation, and soundproofing), packaging (fiber/plastic replacement, filler, coating, film), paper (filler, coatings), personal care (cosmetics), textiles (clothing), aerogels, aerospace (structural, interiors), industrial (viscosity modifiers, water purification), paint, sensors (medical, environmental, and industrial), electronics, photonic structures, etc. .
4. Recovery of chemical compounds of industrial interest
Diverse processes can be used to release lignin as the main product for the revaluation of different biomasses with high-value applications. Each process uses respective chemical agents to extract and obtain different materials from lignocellulosic biomass and produces other materials with different compositions and properties. There are distinct chemical processes of biomass hydrolysis, which use acids, bases, or enzymatic hydrolysis and others (other processes can be used, but their description would come out of the focus of this chapter) whose choice mainly depends on the material structure and characteristics desired for the products to be recovered. However, various sources of lignocellulosic materials need to be considered separately since they have different compositions of cellulose, hemicellulose, and lignin. Against all odds, the depolymerization process of the lignocellulosic biomass is a common goal for all different feedstocks for the production of all types of chemicals . In particular, polyphenolic acids are a group of chemical compounds that are widely distributed in plant biomasses. Those compounds are important antioxidants that efficiently interact with biomolecules such as DNA, RNA, lipids, proteins, enzymes, and other cellular molecules to produce desired results. Due to the benefic effects, that can be useful for preventing the oxidation in foods, and therapeutic human disorders , all of them can be used with potential applications in the pharmacy, food, cosmetic, and nutraceutical industries.
4.1 Chemicals derived from alkaline-based methods
Alkaline pretreatment is one of the most intensively studied technologies for biomass delignification , and the application of alkaline liquid with NaOH into the bagasse to obtain a black liquor that contains value-added chemicals has been investigated. This procedure is useful for the releasing of chemical compounds in different biomasses; particularly, this method has been commonly used for the processing of the switchgrass (
4.2 Chemicals derived from acid-based methods
The acidic pretreatment is a contemporaneous method for the processing of different cereal straws. Nowadays, acidic and alkaline methods are used especially with other methods such as enzymatic hydrolysis for the production of fermentable sugar and polyphenols. Dilute sulfuric acid pretreatment was used on corn stover feedstock and storage for 3 months, resulting in nonobservable microbial infestation. The cellulose content was stable while the hemicellulose content exhibited a slight decrease in furfural and oligomers, and the concentration of chemical compounds such as
4.3 Chemicals derived from hot water methods
Hot water, also known as autohydrolysis, hydrolyzes hemicellulose to release acetyl chemical groups and diverse polyphenols and removes lignin, making cellulose fibers more accessible . The hot water method is very extreme, due to the fact that this method uses water at high temperatures usually between 170 and 230°C . The resulting liquor contains different concentrations of sugars and chemical constituents such as polyphenols. Polyphenol compounds are covalently attached to the cell wall constituents such as cellulose, hemicelluloses, lignin, pectin, and structural proteins . For example, hydroxycinnamic and hydroxybenzoic acids form ether linkages with lignin through their hydroxyl groups in the aromatic ring and ester linkages with structural carbohydrates and proteins through their carboxylic group . Therefore, the recovery of the polyphenols can be made by selective extraction with ethyl acetate, purified and cleaned with resins to obtain a high yield of polyphenols with a direct use in food industries . Ares-Péon et al. characterized phenolic compounds from liquors of stems maize (
4.4 Chemicals derived from enzymatic-based methods
Different enzymes have been involved in the lignin break down in order to release value-added chemical compounds, with different uses in the food industries. It is important to note that alkaline and acidic methods can support the delignification of the biomasses residues to support the use of enzymatic digestion and obtain mainly sugars, polyphenols, and organic acids. Biomasses such as sugarcane, maize, agave, and sweet sorghum bagasse are widely used for the sugar and phenol extractions . There are other nonconventional biomasses that can use this type of acidic or alkaline pretreatments for the degradation of hemicellulose and therefore obtain fermentable sugars and release antioxidant molecules. For example, biomasses such as corn cobs, orange, and pomegranate peels produced high yields of glucose and reduced sugars employing alkaline and enzymatic treatments . Pomegranate biomass contains a high concentration of fermentable sugars that can be used in ethanol production and secondary polyphenols derived from the chemical hydrolysis. Pomegranate biomass contains a high concentration of fermentable sugars that can be used in ethanol production and secondary polyphenols derived from the chemical hydrolysis, due to this fact, pomegranate peels were subjected to acidic hydrolysis, and after an enzymatic process with cellulase there were released different fermentable sugars, moreover, bioethanol in presence of ethanol-producing microorganisms was produced. High concentrations of different sugars were released, with acid hydrolysis, such as glucose, xylose, cellobiose, arabinose, and fructose, with a range of ethanol production between 4.2 and 14.3 g/L . Similarly, Talekar et al.  incorporated hydrothermal processing in combination with acid and enzymatic hydrolysis in pomegranate peels to recover pectin, phenols, and bioethanol. They recovered pectin ranges of 19–21% and phenolic compounds between 10.6 and 11.8%.
5. Pellets elaboration
Pellets are a type of biomass fuel, that is made from different agroindustrial biomasses; as an example, pellets are a derivative of forest biomass such as wood, sawdust, fruit shells, and kernels as well as agricultural remains derived from straw, corn stove, rice husk, and additionally from plant species with energetic potential such as
6. Market of eco-friendly and high added-value products derived from lignocellulosic wastes
In recent years, a great number of studies have focused on the use of lignocellulosic waste due to the high volume generated by the agroindustrial sector and the need to manufacture new eco-friendly materials. Through a specialized search in the innovation platform “Lens” and using the keywords “cellulose,” “hemicellulose,” “lignin,” “nanocellulose,” and “novel” between 2006 and 2020, an increase is shown in the production of research papers regarding cellulose, lignin, and nanocellulose. On the other hand, Table 1 shows the estimated market size of some of the major high value-added products from lignocellulosic waste before the COVID-19 pandemic as well as the negative impacts and area of opportunity caused by COVID-19. Based on the report by Global Market Insight , the market size for nanocellulose was close to 146.7 million USD in 2019 and is expected to grow to 418.2 million USD in 2026 because the global nanocellulose market indicates an increase in demand for certain applications by 2026, like paper processing, food and beverage packaging, paint and coatings, among others. It is important to mention that the term “nanocellulose” used in this report includes micro/nanofibrillated cellulose, cellulose nanocrystals, and bacterial nanocellulose. Among the main nanocellulose manufacturing companies , we can mention: Fiberlan technologies (UK), Borregard (Norway), Nippon Paper Industries (Japan), Celluforce (Canada), etc. Due to the COVID-19 pandemic, demand also increased in the pulp and paper industry, mainly in personal hygiene paper products, food packaging products, corrugates packaging materials, and medical specialty papers . Based on the above, we can conclude that the materials obtained from lignocellulosic residues have a wide field of application and have been successfully positioning themselves in the market before and after COVID-19.
|Lignocellulosic waste||Estimated market size before COVID-19||Applications||Negative impact||Opportunities||References|
|Cellulose||$48.37 billion USD by 2025||Textile, paper, fiber-reinforced, and starch foams||Stranded supply chains, breach of contracts, supply chain shortage, and temporary closure of department stores||Increased the digital market, strengthening of the local supply chain, new buying and selling cycle, personal hygiene and protections equipment, made of corrugated paper, demand for toilet paper and sanitizing wipes, and medical materials packaging||[120, 121, 122, 123]|
|Hemicellulose||$1.3 billion USD by 2007||Ethanol and fermentation products||Fuel ethanol consumption decreased||Opportunities in disinfection of medical materials and equipment||[124, 125]|
|Lignin||Lignin market size worth $1.12 billion USD by 2027||Adhesives and binders||Temporary business closure, automotive supply chain, and automotive adhesives||Packaging adhesives and adhesives for medical applications||[126, 127]|
|Nanocellulose||$0.78 billion USD by 2025||Biomedical, personal care, oil gas, paint, coatings, food, paper processing, and composites||Disruption in production and supply chains||Development antimicrobial surfaces and packaging||[128, 129]|
|Biocomposites||$46.30 billion USD by 2025||Transport, construction, and electronics||Temporary closure of assembly plants||Medical applications|||
The use of lignocellulosic waste is an alternative to generate environmentally friendly products with high added value. There is a variety of methods to modify the surface of cellulose fibers both to obtain biofuels and to improve their compatibility with a polymeric matrix and in this way, develop biocomposites with high mechanical performance to be used mainly in the automotive and packaging sectors. Likewise, from the chemical treatment waste, the black liquor is generated, and it can be reused for the generation of high added-value compounds. On the other hand, lignocellulosic residues have had a high growth potential in the market in a wide variety of applications; however, the COVID-19 pandemic has increased the use of some of these products mainly in medical applications and in the packaging industry.
We want to thank to Alejandro Carreón for kind suggestions in the early version of this manuscript. The authors gratefully acknowledge the financial support of CONACYT through project CB A1-31735.
Conflicts of interest
The authors of this chapter do not have potential conflicts of interest.