A critical review on the techniques used for the synthesis and applications of crystalline cellulose derived from agricultural wastes and forest residues

Highlights

• Potential of agricultural wastes and forest residues towards the production of crystalline cellulose was explored.

• The conventional methods of synthesis of MCC and NCC and the involved mechanisms were summarized.

• Comparison of the conventional thermochemical and biochemical methods was discussed.

• The various methods used for the environmental, industrial and other potential applications of MCC and NCC are summarized.

• Commercial aspects of MCC and NCC production from agricultural wastes and forest residues were discussed.

Abstract

In order to meet the growing energy crisis of the 21st century, the utilization of bio-based materials has become a field of high research endeavour. In view of that, the present review paper is focused on different techniques that are frequently explored for the synthesis of value-added crystalline derivatives of cellulose like MCC and NCC from agricultural wastes and forest residues. Moreover, a comparative analysis between thermochemical and biochemical methods is carried out for such valorization of biomass considering the mechanism involved with various reactions. Further, a critical analysis is performed on various individual techniques specifically used for the applications of MCC and NCC in different fields including environmental, polymer industry, pharmaceutical and other emerging sectors. This article will assist the readers not only to explore new biomass sources but also provides an in-depth insight on various green and cost-effective methods for sustainable production of crystalline cellulose.

Introduction

The rapid growth of human population along with industrial development is increasing the worldwide demand of energy which is presently maintained by the traditional fossil fuels. The estimated increase in energy consumption from 2012 to 2040 is around 48% and it is likely to grow six times in this century (Sagastume Gutiérrez, Cabello Eras, Hens, & Vandecasteele, 2020). Excessive use of energy and the consequent depletion of resources are the major issues in the recent era and hence, the environment friendly sustainable development is of more interest in the research field. One significant way to mitigate the environmental concern is to utilize the naturally available renewable materials and eco-friendly biomass-based products. The valorization of biomass is an encouraging strategy to achieve such objectives (Wang, Mei, & Zhang, 2021). Biomass is generally characterized as the biodegradable organic fraction of various waste materials such as agricultural wastes, forest residues, municipal wastes and industrial wastes. Agricultural waste usually includes plant-based waste materials such as leaves, shoots, cereal straw, grass, vine pruning, roots and fruit peels. Forest residues include trunks of old tree, pruning and roots (Moustakas, Parmaxidou, & Vakalis, 2020). Generally, agricultural wastes are generated at a rapid rate with an average annual increase rate of 5–10% which attributes towards negative environmental impacts because of the random disposal or burning of such wastes lead to pollution of the aquatic environment, soil and air. So, proper utilization of waste biomass residues through conversion and valorization is of high demand to control its harmful effects on the environment (Wang et al., 2016).

Primarily, agricultural waste and forest residues exhibit numerous environmental benefits due to their natural abundance, renewability, low cost and minimum energy requirement for conversion. Moreover, most of the plants and agricultural residues contain substantial quantity of cellulose as the major structural component. The natural fibres of the residues are generally made up of cellulose, hemicellulose and lignin from which cellulose can be extracted through physical and chemical methods (Reddy, Uma Maheswari, Muzenda, Shukla, & Rajulu, 2016). Further, agricultural wastes such as leftover portions of food crops (straws of rice, wheat, maize, sunflower, corn stover), perennial plants and forest residues like woody biomass are frequently used as the abundant sources of cellulosic derivatives. Utilization of these resources is considered as a sustainable approach as they do not compete with food production (Mohammed, Kabbashi, & Alade, 2018). Both of the agricultural wastes and forest residues can be efficiently used for the extraction of valuable cellulose derivatives such as microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC) (Ventura-Cruz & Tecante, 2021). MCC and NCC are the most common and widely used crystalline derivatives of cellulose. MCC is a white, odorless, crystalline, fine powder having diameter of 50 μm and length in the range of 100–1000 μm. Partial hydrolysis of the amorphous regions present in cellulose results in the formation of MCC. So, both crystalline and amorphous regions may be present in MCC, whereas the presence of amorphous regions is usually not observed in NCC. NCC is characterized by needle or rod-shaped structure having diameter in between 5 and 20 nm and length ranging from 100 nm to several micrometres. Both MCC and NCC have gained immense interest over the last decades for their specific properties such as biodegradability, bio-compatibility, non-toxicity, large surface area and high mechanical strength which make them promising candidates for applications in various fields (Haldar & Purkait, 2020b; Mishra, Kharkar, & Pethe, 2019). Several studies are reported on the potential of different biomass residues for the extraction of MCC and NCC. Crystallinity is one of the most important structural properties of MCC and NCC and it is considered as the key characteristic behind the mechanical and thermal stability of MCC and NCC. Higher crystallinity of MCC and NCC indicates the removal of amorphous regions from the native cellulose of the source biomass. Crystallinity is associated with structural rigidity of MCC and NCC, which contributes high mechanical properties to the final composite materials and thereby, make MCC and NCC suitable candidates as load-bearing material or reinforcing agents in composite structures (Kian, Jawaid, Ariffin, & Karim, 2018; Ventura-Cruz, Flores-Alamo, & Tecante, 2020). Degree of polymerization (DP) is another important structural feature of cellulose derivatives. It is defined as the number of repeating monomer units in the polymer. Degree of polymerization and its distribution indicates the quality of the produced MCC and NCC. DP can affect the efficiency of hydrolysis of biomass for the synthesis of MCC and NCC, solubility of cellulose in a particular solvent and also the mechanical properties of the composites prepared with MCC and NCC (Chang, 2014; Shao et al., 2020). Ventura-Cruz et al. successfully utilized rose stem wastes for the synthesis of MCC possessing high crystallinity and good thermal properties (Ventura-Cruz, Flores-Alamo, & Tecante, 2020). MCC isolated from roselle fibres exhibited higher crystallinity (78%) than that of the C-MCC (commercial-microcrystalline cellulose) (74%) and improved the thermal stability as compared to the roselle pulp (Kian, Jawaid, Ariffin, & Alothman, 2017). Kian et al. reported the synthesis of NCC with high crystallinity (79.5%) from the roselle-derived MCC (Kian, Jawaid, Ariffin, & Karim, 2018). Shao et al. used corncob for the production of MCC with higher economic feasibility as compared to C-MCC (Shao et al., 2020). The potential of agricultural and forest biomass for the synthesis of NCC were also explored by using corncob, pine wood (Ditzel, Prestes, Carvalho, Demiate, & Pinheiro, 2017), cocoa pod husk (Akinjokun, Petrik, Ogunfowokan, Ajao, & Ojumu, 2021), olive fibres (Kian, Saba, Jawaid, Alothman, & Fouad, 2020), bamboo fibres (Rasheed, Jawaid, Parveez, Zuriyati, & Khan, 2020). Therefore, different biomass are employed for the conversion of lignocellulosic residues into the value-added materials (Haldar and Purkait, 2020a, Haldar and Purkait, 2020b).

In the present review article, a comparative analysis on the various techniques that are frequently explored for the synthesis and specific applications of value-added crystalline cellulose from agricultural wastes and forest residues are reviewed together. In view of that, the worldwide availability of these two types of lignocellulosic biomass and their structural compositions are briefly discussed. Thereafter, a comparative study on two different types of synthesis techniques such as thermochemical and biochemical methods is performed to explore the involved mechanism and effectiveness of these methods for the conversion of biomass into value-added products. Further, the advantages of the newly developed modified synthesis techniques over the conventional methods are discussed. Apart from the synthesis, a critical analysis on the individual techniques is also carried out which are mostly explored for the application of biomass derived crystalline cellulose in different sectors like, environmental, polymer industries, pharmaceutical and several other emerging fields. Finally, the challenges faced during the commercialization of MCC and NCC production from agricultural wastes and forest residues are discussed and recommendations are provided for overcoming these limitations. Therefore, the present review will be beneficial to the readers for gaining an in-depth knowledge on different techniques for the synthesis and applications of crystalline cellulose derivatives and also help them to explore new agricultural and forest biomass for eco-friendly and sustainable production of crystalline cellulose.

Agricultural wastes and forest residues are usually abundant and easily available. The global availability of agricultural lands and forest areas are shown in Fig. 1. Both of these sectors generate enormous quantities of waste products with global annual production of around 140 Gt. Cereal crops are the most important among all the agricultural waste biomass and their leaf, stem, sheath materials contribute the highest quantity about 66% of the total worldwide residual biomass production whereas sugarcane is the second largest contributor. The expected increase in the global yield of cereal is near about 0.9% over the period starting from 2007 up to 2050. China is the largest producer of agricultural residues having a cumulative production potential of 716 Mt. India is in the 3rd position after China and USA, with 605 Mt cumulative generation potential. Forests cover around 4 billion ha area worldwide that is almost 30% of the world's land area, out of which 1 billion ha is wooded land. Forestry sector produces significant amount of woody biomass wastes including leaves, branches, logs, stumps and sawdust. The global annual production of wood biomass is near to 4.6 Gt, 20% of which remains as production loss and left in the field to decay (Tripathi, Hills, Singh, & Atkinson, 2019). According to the main report of global forest resource assessment (FRA) 2020, prepared by the Food and Agriculture Organization (FAO) of the United Nations, about 1/3rd of the global land area is covered by forest. About 30% of the global forests is primarily utilized for production of woody as well as non-wood forest products and multiple other uses (FAO, 2020). During the manufacture of paper from wood, nearly 50% of the wood pulp is lost as waste residues in pulping and bleaching, while in timber production, waste residues are generated as branches, bark, sawdust and other trimmings accounting for 60% of the wooden logs (Stafford et al., 2020). The agriculture sector reported to generate nearly 1 ton of waste residue per ton of the harvested grain (Mohammed, Kabbashi, & Alade, 2018). The annual global production of straws and husk of cereal crop and rice is around 973.9 Mt. China produces the highest amount of rice straw that accounts for 180–270 t annually. The rice straw residues are usually burnt on field. In India, near about 25% leftover of the rice straw are openly burnt in the field which ultimately results in the emission of green-house gases (Atinkut et al., 2020). Rice, wheat, maize and sugarcane are the four major agricultural crops cultivated around the world which collectively yield about 5358.54 Mt of dry lignocellulosic biomass annually (Islam, Kao, Bhattacharya, Gupta, & Choi, 2018). Therefore, special attention needs to be given to use these waste biomass in order to reduce the huge waste load and their negative environmental impacts.

Agricultural wastes are heterogeneous in nature and their moisture content, particle size, distribution and density vary depending on the operational treatment and the geographic location. Chemical composition of the crop residues are usually dependent on many aspects such as species, age of the residue, period of storage and types of harvesting practices (Mohammed, Kabbashi, & Alade, 2018). Agricultural residues usually contain cellulose 37–50%, hemicellulose 25–50% and lignin 5–15% (Fatma et al., 2018; Kumar & Sharma, 2017). Woody biomass or forestry residues are also lignocellulosic consisting of cellulose 34–54%, hemicellulose 19–34%, lignin 11–30% and extractives 3–6% (Stafford et al., 2020).

Cellulose is the most abundant natural biopolymer available on this planet which is generally utilized as a renewable, biodegradable and environment-friendly resource of value-added products. It is a semi-crystalline polysaccharide having high molecular weight and composed of d-glucose monomers connected by β-(1,4)-glycosidic bonds. The structural configuration of cellulose is composed of crystalline and amorphous regions. The presence of –OH group in cellulose facilitates its modification into various derivative products (Haldar & Purkait, 2021; Halder et al., 2019; Salama, 2020). Hemicellulose is a highly branched low molecular weight biopolymer comprising of 5-C pentose monomers (xylose, arabinose), 6-C hexose monomers (galactose, mannose, glucose) and sugar acids (glucuronic, galacturonic, cinnamic and methyl galacturonic acid). Hemicellulose is of amorphous structure with less degree of polymerization, which makes it more susceptible towards disintegration into its monomeric units. Lignin is the 2nd most amply available biopolymer on the earth after cellulose, consisting of aromatic rings of phenylpropane units linked together by different chemical bonds such as β-O-4 ether linkages, some carbon–oxygen and carbon–carbon bonds. Phenolic lignin is connected to the carbohydrate polymer hemicellulose through covalent bonds and forms a layer to provide support to the biomass. The hemicellulose composition and lignin content of lignocellulosic biomass generally depend upon the type and characteristics of the biomass such as soft or hard biomass (Duarah, Haldar, & Purkait, 2020; Haldar & Purkait, 2021; Halder et al., 2019; Zhuang et al., 2016). Agricultural wastes and forest residues, being lignocellulosic in nature, can be successfully used in biorefineries for the production of biofuels (bioethanol, biogas) and also as the source of biochemicals. The lignin separated from agricultural and forest biomass can be used for the preparation of lignin nanoparticles for utilizing in the products of food, biomedical and pharmaceutical industries. The rich cellulose content of this waste biomass makes them suitable for the synthesis of value-added crystalline cellulose with tremendous potential for applications in different industrial fields which are discussed later in the present article.