Abstract
Asian countries, despite being the largest producers and yielding a significant proportion of the world’s rice, have poor disposal facilities for the harvested rice straw (stubble). Due to higher costs in their handling relative to their value, local farmers prefer the burning of stubble fields, thus creating environmental problems. Even though the government has taken initiatives, no effective solution has been discovered to handle this large agro-waste problem efficiently. In this regard, the utilization of rice straw to develop nanocellulose (NC) products is of interest. Renewability and biodegradability, along with suitable mechanical and thermal properties required for the packaging functions, are key advantages of NC. The bio-nanocomposites prepared using NC and other bio-based polymers are also being widely considered for sustainable food packaging applications due to the reinforcement provided by NC and alternative petroleum-based packaging materials. This review provides an overview of process utilization for preparing NC products using rice straw, pulping methods, and isolation to produce bio-nanocomposites for sustainable food packaging applications. The challenges and future aspects covering the utilization of rice straw for producing NC and eventually producing active packaging materials are also discussed.
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Rice Straw as a Source of Nanocellulose for Sustainable Food Packaging Materials: A Review
Makdud Islam,a Praveen Saini,a Rahul Das,a Shubhra Shekhar,b Akhouri Sanjay Kumar Sinha,c,* and Kamlesh Prasad a,*
Asian countries, despite being the largest producers and yielding a significant proportion of the world’s rice, have poor disposal facilities for the harvested rice straw (stubble). Due to higher costs in their handling relative to their value, local farmers prefer the burning of stubble fields, thus creating environmental problems. Even though the government has taken initiatives, no effective solution has been discovered to handle this large agro-waste problem efficiently. In this regard, the utilization of rice straw to develop nanocellulose (NC) products is of interest. Renewability and biodegradability, along with suitable mechanical and thermal properties required for the packaging functions, are key advantages of NC. The bio-nanocomposites prepared using NC and other bio-based polymers are also being widely considered for sustainable food packaging applications due to the reinforcement provided by NC and alternative petroleum-based packaging materials. This review provides an overview of process utilization for preparing NC products using rice straw, pulping methods, and isolation to produce bio-nanocomposites for sustainable food packaging applications. The challenges and future aspects covering the utilization of rice straw for producing NC and eventually producing active packaging materials are also discussed.
DOI: 10.15376/biores.18.1.Islam
Keywords: Rice straw; Nanocellulose; Bio-nanocomposite; Sustainable food packaging
Contact information: a: Department of Food Engineering and Technology, Sant Longowal Institute of Engineering and Technology, Longowal-148106, Punjab, India; b: Department of Food Process Engineering, National Institute of Technology, Raurkela – 769008, Odisha, India; c: Department of Chemical Engineering, Sant Longowal Institute of Engineering and Technology, Longowal-148106, Punjab, India; *Corresponding authors: akhouri_sanjay@yahoo.com; profkprasad@gmail.com
GRAPHICAL ABSTRACT
INTRODUCTION
Rice (Oryza sativa) is one of the world’s most significant agronomic food crops, accounting for around 20% of the worldwide population’s daily energy intake (FAOSTAT 2020). Considering the area under production, it is the world’s third-largest cereal crop, after wheat and maize. The annual rice production in 2019 was reported to be around 765 million metric tons (MMT). Among these, Asian farmers contribute 89.6% of rice production (677 MMT). The contribution of India and China during 2019 was 178 MMT (23.5%) and 211 MMT (28.0%), respectively. A total of more than 50 countries worldwide contribute to 100,000 tons of rice annually (FAOSTAT 2020). Mainly two types of residues, husk and straw, are left over after rice cultivation. Rice straw is made up of stems, leaves, and spikelets that are left over after the grain is harvested. Rice straw is produced predominantly in Asian countries due to the popularity of combine harvesters. In light of the vast scale of cultivation of rice, the byproduct, rice straw, is generated in large quantities, accounting for 23% of total agricultural crop residue in India (IARI 2012). For every ton of rice harvested, 1.40 tons of rice straw is left over in the field. Rice straw has long been utilized in several applications, including animal feed, organic fertilizer, thatching, poultry litter, mushroom production, and as a source of feedstock for bio-refinery on a small scale. However, the disposal of rice straw is a challenging task due to higher costs involved relative to its value (Abraham et al. 2016). Burning it in open fields is considered a cheap disposal method, so it has become popular among farmers. Currently, the primary proposition of rice straw, i.e., nearly three-fourths of total crop residue, equivalent to 70 to 80 million tons of rice straw, is burnt in India every year (Pandey and Sujatha 2011). Open-field burning of rice straw leads to air pollution, causing critical health hazards due to the release of various harmful substances such as volatile organic compounds (VOC), sulfur oxides (SOX), nitrogen oxides (NOX), particulate matter (PM10), and silica (Delivand et al. 2011). Silica is a significant constituent in paddy (Oryza sativa) crops, which belong to the grasses family. This silica is found in the form of nano silica in the epidermal cell of plants. It is very important as filler material in different types of paper and paperboards due to its inert nature. The silica also can be added to a pulp fiber mixture to modulate the antimicrobial and controlled vapor/gas permeability of paperboard, making it more suitable for food packaging application (Bernardos et al. 2019). Open-field burning of rice straw adds 0.05% of total greenhouse gas emissions (Bhuvaneshwari et al. 2019).
Scientists have felt compelled to find biodegradable packaging materials due to the massive accumulation of non-biodegradable packaging materials and carrying bags made of petroleum-based polymers. Synthetic plastic, which is used in various food packaging applications, is a severe global problem because it pollutes the environment and poses serious health risks to humans and other living organisms (Yang et al. 2019). Due to the low cost of plastic polymer and its versatile application, its manufacturing has witnessed a tremendous increase since the 1950s (Schmaltz et al. 2020). Food packaging materials such as plastic bottles, trays, cups, films, and sheets are derived from 40% of total plastic resources. The non-biodegradable property and poor management of these materials result in a severe ecological imbalance due to their negative effect on soil fertility, oceans, animals, and other living organisms (Jambeck et al. 2015). Out of various emerging solutions to replace synthetic plastic. Nanocellulose is an eco-friendly biomaterial that has the potential to manufacture functional food packaging material. In this regard, the abundant availability of rice straw draws attention to the possibility of rice straw-based nanocellulose materials, which can be used as reinforcement fillers for bio-nanocomposite materials (Mamat Razali et al. 2021; El-Wakil et al. 2016). These biodegradable nanocomposite materials are in massive demand by the food packaging and paper industry as an alternative source of plastic materials. Thus, the research community has sought it as a sustainable solution for both straw management and alternative synthetics polymer.
Production of nanocellulose from different types of non-wood lignocellulosic biomass (wheat straw, bagasse) and nanocomposite preparation has been described (Motaung and Mochane 2018; Mohammed et al. 2021). However, it is difficult to find any relevant review work on using rice straw-based nanocellulose containing nano silica and their use in manufacturing biodegradable packaging materials. In view of this gap, there is a need for compiling research on preparing different bio-nanocomposites from rice straw cellulose. Thus, this review article presents collected information from the currently available statistics, the composition of rice straw, and other pulping methods, including bleaching, isolation of NC, and production of nanocomposites from rice straw to develop sustainable packaging materials.
RICE STRAW AVAILABILITY
Since 6500 BC, rice has been widely cultivated in various parts of the world and more dominantly in Asian countries. China, India, Indonesia, Bangladesh, Vietnam, Thailand, Myanmar, Philippines, Brazil, and Japan are among the top 10 rice-cultivating countries in the world (FAOSTAT 2020). Rice straw, which is sometimes regarded as solid waste from rice cultivation, is generated in huge quantities worldwide. The overall biomass of the rice straw is determined by several factors, including rice variety, fertilizer management, and soil and climatic conditions (Satlewal et al. 2018). The harvesting method also determines how much rice straw is left behind in the field, as the height of crop cutting varies with harvesting methods. The paddy to straw ratio lies in the range of 0.74 to 4.3 (Zafar 2018; Kainthola et al. 2019). Total straw biomass yield ranges between 7.5 to 8 t/ha.
In contrast, the finely collected straw quantity (harvested with remaining grains) ranges between 2.7 to 8 t/ha, which corresponds to 50 to 100% of total straw biomass. Estimated data showed that an average of 5.75 t/ha of rice straw was generated during the year 2020 (Verma and Verma 2020). The maximum and minimum estimates for rice straw availability around the globe are shown in Fig. 1. The maximum annual rice straw production in Southeast Asia (SEA), the whole of Asia, and over the world is 140, 470, and 520 million t/yr, respectively.
Fig. 1. Production of rice straw around the world (Van Hung et al. 2020, CC BY 2.0)
COMPOSITION OF RICE STRAW
Rice straw is a tenacious part of the rice plant whose cell walls are naturally designed to meet various growth needs, including protection from extreme weather conditions, insects, and viruses. Rice straw consists of structural components such as cellulose (33 to 47%), hemicellulose (19 to 27%), lignin (5 to 27%), silica (both acid-soluble and insoluble), and other non-silica oxides. Rice straw also has other organic components including proteins (approximately 3%), pectins (2.8%), free sugars, chlorophyll, lipids, oils, and waxes (Harun and Geok 2016). Proximate analysis of rice straw, wheat straw, sugarcane bagasse, and corn stover reveals that ash percentages are 7.8 to 20.3%, 5.2 to 10.5%, 0.9 to 11.5%, and 2.65 to 10.5%, respectively. Based on these amounts, rice straw has higher ash percentage compared to other agricultural residues (Lizotte and Champlain 2015; Reza and Coronella 2015; Aristizábal et al. 2016; Shariff et al. 2016; Yao et al. 2016; Zanatta et al. 2016; Mukherjee and Halder 2017; Mensah and Awudza 2021; Zhong et al. 2021). The silica content in ash obtained from rice straw is nearby 75% (Jha and Sinha 2011). Cellulose is the most abundant polymer in rice straw. It is made up of 1,4 D-glucose units linked together by inter-and solid intra-chain hydrogen bonds to form microfibrils. The mechanical strength of the cellulose fibers depends upon the degree of polymerization and the average length of the glucose chain in the cellulose (Li and Xia 2017). It has been reported that rice straw cellulose has a slightly higher degree of polymerization (weight average molecular weight, Mw: 1820) measured by viscometry method compared to other agricultural residues such as wheat straw (Mw: 1045) and bagasse (Mw: 925) cellulose, but it is much lower than wood lignocellulosic biomass cellulose (Mw: 4000 to 5500) (Hallac and Ragauskas 2011). Moreover, rice straw contains hemicelluloses with modest molecular weights (average molecular weight 18800 to 48700) and degrees of polymerization ranging from 80 to 200 (Kausar et al. 2016). The arabinose/xylose ratio is commonly used to represent the degree of branching in hemicellulose. The lower the ratio, the higher the degree of polymerization. Rice straw has an arabinose/xylose ratio of 0.17, but bagasse has a ratio of 0.2 which indicates that rice straw has a higher degree of polymerization than bagasse (Bezerra and Ragauskas 2016). After cellulose and hemicellulose, lignin is the most abundant biopolymer in rice straw. Lignin is a heterogeneous polymer consisting of three types of monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) connected by different bonds including β-O-4, 4-O-5, β-β, β-1, and β-5. The corresponding phenylpropanoid units are p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S). Among these, rice straw lignin has about 40 to 60% β-O-4 linkage ether bonds (Wang and Cheng 2009). Lignin is connected with hemicellulose via covalent bonds whereas hemicellulose and cellulose are linked with hydrogen bonds (Bezerra and Ragauskas 2016). Due to these chemical bonding and high-strength physical bonds, removing lignin in its natural state is a highly challenging task (She et al. 2012; Ghaffar and Fan 2013).
NANOCELLULOSE: DEFINITION AND CLASSIFICATION
Nanotechnology is an interdisciplinary science that combines mathematics, physics, and chemistry to create tiny particles having at least one dimension in the nanoscale range (1 to 100 nm) (Du et al. 2017). Cellulose is a biopolymer with linear homo-polysaccharide composed of D-glucopyranose units joined together by α (1–4) linkages with a repeating unit of cellobiose (Khalil et al. 2014). It can be used as a substitute for petroleum-based packaging materials (Ma and Ramakrishna 2005). Nanocellulose is composed of two regions, crystalline and amorphous, of which the crystalline region is responsible for strength and stiffness, while the amorphous region provides good flexibility to the plant cells (Fig. 2). Nanocellulose has excellent physical and mechanical qualities due to the strong hydrogen bond networking among the numerous hydroxyl groups (Dufresne 2012). They are biodegradable, light in weight, and strong, with an intrinsic density of 1.6 gm/cc and tensile strength (TS) of 10 GPa, which is roughly comparable to cast iron (Sultana et al. 2020). Nanocellulose hydroxyl groups, making them well suited for surface functionalization in various applications. Nanocellulose materials can be classified into three major categories, which are nanofibrillated cellulose (NFC), cellulose nanocrystals (CNCs), and bacterial nanocellulose (BNC) (Table 1). Due to their unique characteristics, including high mechanical strength, customizable surface chemistry, high aspect ratio, crystallinity, non-toxicity, and renewability, they are gaining attention as a green substrate for coatings, fillers in composites, and a variety of other food packaging applications (Urbina et al. 2021). All this has helped nano cellulose materials to achieve a substantial impact on commercial markets (Ferrer and Hubbe 2017).
Fig. 2. Crystalline and amorphous region of cellulose fiber
Table 1. Types of Nanocellulose Fibers (Klemm and Dorris 2011; Khalil et al. 2014; Ferrer and Hubbe 2017)
PRODUCTION OF NANOCELLULOSE FROM RICE STRAW
A significant number of publications have documented the isolation of cellulosic nanomaterials from a wide range of lignocellulosic biomass and their use in the production of value-added goods (Siqueira et al. 2010; Jonoobi et al. 2015; Oun and Rhim 2016; Xu et al. 2018; Djafari Petroudy et al. 2021). NC can be separated from different non-wood lignocellulose biomass such as cotton, kenaf, banana, bamboo, wheat straw, rice straw, bagasse, etc. However, the vast availability and economic viability of rice straws make them a potential candidate for the preparation of natural NC.
Production of nanocellulose from rice straw includes a variety of pre-treatment methods including separation of lignin and hemicellulose. Stepwise procedures for their preparation, as adopted and presented in past studies, includes: (i) separation of cellulose fiber from rice straw, (ii) pretreatment of separated cellulose fibers, and (iii) mechanical isolation of NC through an energy-efficient approach (Khalil et al. 2014), are discussed in the following sections. The production and separation of nanocellulose and nanocrystals from rice straw are shown in detail in Figs. 3 and 5, respectively.
Fig. 3. Production of NC and CNCs from rice straw
Separation of Cellulose Fiber
Separation of rice straw cellulose
The primary step of nanocellulose preparation involves the pulping treatment desired to collect cellulose and remove lignin from the rice straw biomass. Different alkaline and organosolv pulping methods are used for separating cellulosic fibers from rice straw. The detailed process conditions, characteristics of obtained pulp, and paper properties of obtained pulp are summarized in Table 2.
Table 2. Different Pulping Conditions (laboratory scale) for the Separation of Cellulose from Rice Straw
Alkaline pulping
Hugh Burgess and Charles Watt pioneered the alkaline pulping technique in the United Kingdom in 1851. The alkaline pulping method is an entirely chemical method that involves pulping different concentrations of NaOH, KOH, cooking temperatures, time, and pH values. NaOH (caustic soda) is the most frequently used chemical for rice straw pulping. The NaOH concentration, cooking temperature, and time typically range from 0.4 to 16% (w/v), 60 to 160 ℃, and 1 to 4 h, respectively (Sharma and Goswami 2017; Wu et al. 2017; Xu et al. 2018). Moreover, adding anthraquinone (AQ) as a pulping agent prevents carbohydrate breakdown during the cook (Bajpai 2018). The use of AQ (0.05 wt%) as a pulping agent during soda pulping reduces the kappa number (7.5%) and increases the tensile index (14.05%), burst index (8.84%), and viscosity (7.78%) of the pulp solution (Kaur and Lohchab 2018). Different ranges of temperature (150 to 185 °C), time (30 to 90 min), soda (10 to 20%), AQ (0 to 1%), and liquor ratio (6 to 8) were used to optimize the condition of soda-AQ pulping for rice straw. The highest pulp yield and viscosity were achieved at a low soda level, less time, and lower temperature (10%, 30 min, and 155 °C), but lower kappa number and higher brightness was obtained at 20% soda, 90 min, 185 ℃. The process condition at 20% soda, 30 min, 155 ℃, and 1.0% AQ showed the best beating degree, burst index, and tear index (Rodríguez and Jiménez 2008). However, using oxygen in soda pulping is a relatively new development. Zhang et al. (2019) used oxygen in soda pulping with an effective alkaline conventional dose of 18% (w/dry straw) and an overdose of 36% (w/dry straw). They found that overdose soda-oxygen pulping achieved a higher delignification degree (92.4%), crystallinity index (12.4%), and significantly reduced the pulp viscosity (57.2%). However, other researchers reported higher mechanical strength of the paper by using a low level of alkaline dose (2 wt%) for rice straw pulping (Polyium et al. 2019). The main disadvantage of alkaline pulping is the emission of highly alkaline liquid waste, a point of concern for the paper industry and the environment. The level of silica content in black liquor (residual fluid left after removal of rice straw pulp) is often higher in the soda process, which creates problem in the chemical recovery operation due to scaling of silica in pipelines and equipment during processing in the paper industry (Kaur et al. 2017). But, considering the case of energy consumption during fibrillation, less energy consumption was required for the xylanase-treated soda pulp (8%) as compared to the xylanase-treated neutral sulfite pulp (21%) (Hassan et al. 2018).
Organosolv pulping
Organosolv pulping is another essential chemical pulping method in which organic solvents such as ethylene glycol, methanol, ethanol, acetone, and other organic acids (formic acid, acetic acid) are used to separate hemicellulose and lignin from cellulose (Li et al. 2012). Due to their high silica content, the complexity of utilizing rice straw at the industrial level can be handled using the organosolv pulping method. The addition of catalysts (hydrochloric acid, sulphuric acid, and phosphoric acid) even boosts this pretreatment procedure and reduces the reaction temperature (Baruah et al. 2018).
A study involving the pulping of rice straw with several organic solvents (diethanolamine, ethanolamine, diethylene glycol, and ethylene glycol) showed that amine-containing solvent (diethanolamine) was more efficient in producing pulp with lower kappa index (4.42 times) and higher viscosity (1.29 times) and burst index compared to the glycol group (diethanolglycol) (Rodríguez et al. 2008). The effects of process variables of rice straw delignification in the catalyzed acetic acid medium at atmospheric pressure were carried out and found to be suitable for making an average grade of paper. The catalyst used was H2SO4 (Sinha 2008). In different research work, rice straw was treated with different formic acid concentrations and a higher degree of lignification was found with increasing formic acid concentration up to 90%. Further peroxyformic acid treatment reduced the kappa number to 17.6 and pulp yield by 17.5% due to a reduction in the lignin content of the pulp. The combined treatment induced a good TS index but lower tear and burst index (Ferdous et al. 2020).
Another study also reported similar findings for the formic acid pulping, where an increase in formic acid concentration (up to 85%) decreased the pulp yield because it increased the delignification and solubilization of hemicellulose (Sinha 2021). Pulping with 1% catalyst (H2SO4), for 180 min resulted in the best delignification, maximum holocellulose percentage (75%), and minimum kappa number (26). A mixture of acetic acid, formic acid, and water, in the ratio of 40:50:10, was cooked for 4 h to make dissolving grade pulp from rice straw and the ultimate yield of the pulp was 46.2%. It contained 100% inherent silica and with low kappa no 23.9 (Jahan et al. 2015). However, some drawbacks of organosolv treatment include its high cost when used on a large basis, highly flammable, and volatile nature of most of the solvents, necessitating pretreatment in extremely enclosed situations (Borand and Karaosmanoglu 2018).
Bleaching pretreatment
The presence of lignin in lignocellulosic biomass after pulping may obstruct the isolation of cellulose, as well as leading to poor surface wettability between the different polymer matrix and natural fibers. Lignin is the most difficult chemical component to remove from the lignocellulosic biomass (Kargarzadeh et al. 2018). In such a case, bleaching is an extra treatment that is required to remove the remaining cementing material, primarily lignin, from the pretreated rice straw (Panaitescu et al. 2013). In most paper mills across the world, chlorine dioxide (ClO2) (denoted as D) bleaching is preferred; however, other chemical sequences, such as ozone (denoted as Z), hypochlorite solutions of various strengths (H), oxygen (O), and DED (chlorine dioxide-alkali extraction- chlorine dioxide) and DEDED are mostly used. Utilization of ozone (Z) and H2O2 (P) is slightly more expensive for producing white paper on a large scale (Kaur and Lohchab 2018).
Based on the literature review, the various bleaching treatments for rice straw cellulose are summarized in Table 3. Almost all alkali-treated rice straw fibers are bleached under acidic conditions by boiling them in sodium chlorite (NaClO2) solution (Dilamian and Noroozi 2019; Sharma et al. 2017). During the bleaching process, the NaClO2 is dissolved in an acidic buffer solution and then it decomposes into chlorine dioxide (ClO2) in the presence of buffer salts. Because ClO2 can oxidize lignin that has been left in the fibers by targeting the aromatic ring of the lignin, the loss of lignin could cause the alkali-treated fiber to fibrillate even more (Zainuddin et al. 2013). Bleaching of lignocellulosic materials with chlorine and chlorine derivatives results in very toxic and dangerous pollutants (Tripathi et al. 2019). Bleaching processes involving oxygen bleaching (Kaur and Lohchab 2017), ozone bleaching (Kaur et al. 2018), and the use of peroxy acids (Polyium et al. 2019) have been introduced to decrease chlorinous waste products.
Generally, bleaching methods are mainly divided into two categories: elemental chlorine-free (ECF) and total chlorine-free (TCF). ClO2 is a common bleaching reagent used in the ECF process, and it helps to minimize halogenated residues and avoid the production of chlorinated dioxins and furans. The ECF method has been utilized to improve the pulp strength and reduce the bleaching effluent load in rice straw pulp (Kaur and Lohchab 2018). On the other hand, ozone bleaching (Z) was found to be effective in lowering the effluent load. There were decreases in recalcitrant and carcinogenic chemicals and increases in the brightness to 85% ISO, which was 3.6% higher and also provided higher strength values compared to ECF bleached pulps (Kaur and Lohchab 2018). The DED bleaching sequence of rice straw pulp showed a brightness of 82.3% ISO, and there was a decrease in ash and silica percentage with the increase in bleaching percentage (Sinha et al. 2012). However, the two-stage peroxide alkaline extraction (PE) bleaching sequence for rice straw powder increased the crystalline index by 21%, which confirmed the separation of lignin and hemicellulose from the powder (Weerakkody et al. 2021).
It has been found that an enzyme cocktail (xylanase, pectinase, a-amylase, protease, and lipase) as a bio-bleaching agent for non-wood pulp reduced the chemical usage up to 50% without compromising the brightness. Also, the process increased the TS (23.55%), burst factor (20.3%), tear factor (3.17%), and reduced the kappa number (19.5%) (Sharma et al. 2020).
Pretreatment prior to preparation of nano-fibrillated cellulose
Different pretreatments such as enzymatic and oxidation with the 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) are sometimes required to reduce the energy needed during mechanical fibrillation. These pretreatments are discussed in detail as follows.
Table 3. Different Bleaching Methods (laboratory scale) for Rice Straw Cellulose Fibers
Pretreatment with enzymes
Enzymatic hydrolysis is an environment-friendly approach that can be utilized instead of chemical pretreatments to allow fiber refinement and disintegration. This process also provides selective hydrolysis of the fibers, which results in lower degradation of cellulose chains of fibers and less formation of glucose (Karim et al. 2017). Several researchers have documented that enzymatic pretreatment decreased the energy requirement during the fibrillation of NC from different lignocellulose biomass (Djafari Petroudy et al. 2021). A cellulase enzyme complex with five subunits (endocellulases, exocellulases, cellobiases, oxidative cellulases, and cellulose phosphorylases) has been used to hydrolyze the cellulose fiber into the nanoscale. But only endocellulases and cellobiases subunits contribute to the hydrolytic degradation of β (1→4) linkages. Endocellulases stimulate the breaking of internal bonds in cellulose chains’ disorganized regions, resulting in the formation of new chain ends. The cellobiases are enzymes that hydrolyze the oligomeric exocellulase products into separate monosaccharides (Karim et al. 2017). Xylanase treated fibrillation from rice straw required less time and energy. In addition, the paper prepared from xylanase treated NC (82 MPa) showed better mechanical properties than the untreated pulp (66 MPa). Thus, the xylanase enzyme has been used to lower the size of the fibers and pre-defibrillate them, resulting in less frequent equipment blockage (Hassan and Oksman 2018). Apart from rice straw, a tailor-made enzyme treatment has been used to obtain nanocellulose from eucalyptus. A more successful and long-lasting method has been developed in which CNCs with a diameter of 24 nm were isolated from cellulose pulp employing a cellulolytic enzyme complex with intense endoglucanase-specific activity (Squinca et al. 2020). Furthermore, enzymatic pretreatment of bagasse pulp achieved isolation of nanocellulose that was 30 nm in diameter. It was created using a combination of cellulase and a low concentration of cold alkali. The enzymatic pretreatment also reduced alkali consumption and clean water (Tao et al. 2019). However, the combined effect of enzymatic (cocktail cellulite) pretreatment and organosolv pulping of miscanthus biomass showed less effect on the structure and composition of the fiber. Still, enhanced removal of the lignin and hydrolyzability of the cellulose pulp was observed, which was attributed to the applied enzymatic pretreatment (Obama et al. 2012).
Pre-treatment with TEMPO (2,2,6,6-Tetramethylpiperidine-1-oxyl radical) mediated oxidation
Besides the pulping and enzymatic pretreatment process, TEMPO-mediated oxidation has also been recommended for nanocellulose surface modification. Under mild circumstances, TEMPO transforms the primary hydroxyl groups (-OH) of C6 glucose unit of cellulose first into aldehyde product which on further oxidation results into carboxyl derivatives (COO–) (Fig. 4), thus resulting in a mixture of aldehyde and carboxylic groups.
The oxidation process can reduce the energy cost of mechanical fibrillation for the synthesis of NC from lignocellulosic biomass by reducing the inter-fibril hydrogen bonding between NC particles (Peyre et al. 2015). The oxidized carboxylated cellulose fibers prevent clogging and enable defibrillation during mechanical delamination. Thus, the process was found to reduce the number of passes required during the homogenization of cellulose fiber and lowered energy consumption (Tejado et al. 2012). This oxidation treatment is carried out using catalysts like sodium bromide (NaBr) and bleaching agents such as sodium chlorite (NaClO2), which are frequently used in alkaline conditions with a pH range of 9 to 11. NC particles were isolated from rice straw using alkaline pretreatment followed by TEMPO-mediated oxidation, and mechanical blending. It was found that TEMPO-mediated oxidation produced a yield of 36.5%. Again, the yield obtained was higher than NC produced from a three-step process (dewaxing-sodium chloride oxidation- alkali leaching) with a yield of 29.1% (Gu and Hsieh 2017). In terms of energy consumption, it was found that moderate disintegration of TOCN (TEMPO-Oxidized Cellulose Nanofiber) with 90% yield needed 1.94 kWh/kg in an aqueous medium which is very efficient for the production of rice straw NC in comparison to other processes (Isogai et al. 2018).
Fig. 4. TEMPO-mediated oxidation of cellulose primary hydroxyl group to carboxyl groups