containing cellulose to generate cellulose nanoyarn (Quan et al. 2010;
Stylianopoulos et al. 2012; Araki and Miyayama 2020).
3 Extraction of NC
Nanocellulose can be extracted from plant-based feedstocks (An et al. 2020; Gea
et al. 2020; Trilokesh and Uppuluri 2019), certain aerobic bacteria (Vigentini et al.
2019), algae (Zhou et al. 2019), and marine animals (Iwamoto et al. 2011). The green
alga, Cladophora in particular, has become popular for the production of highly
crystalline nanocellulose (Zhou et al. 2019). Its product shows a remarkable inertness toward chemical treatments and therefore finds promising applications in filters
(Metreveli et al. 2014). Indeed, cheaper and more abundant resources, such as
lignocellulose feedstock, are considered the most promising source. However, the
bulk cellulose in plants (stems, leaves, roots, and fruit) and agricultural residues,
such as rice straw, wheat bran, bagasse, fruit peel, coconut husk, etc., exist together
with other polymers and therefore requires chemical hydrolysis or mechanical
defibrillation to produce crystalline nanocellulose and nanocellulosic fibers, respectively. On the other hand, many research laboratories mostly use pure cellulose (that
may be obtained from cotton fibers or other sources) or microcrystalline cellulose or
wood pulp (delignified wood that contains almost pure cellulose); however, the
large-scale production of nanocellulose is based on the continuous supply of raw
materials and consequently uses lignocellulosic starting material, with the extraction
and production of NC, executed in three prime steps: pre-treatment, NC conversion,
and post-treatment.
Pre-treatment, aimed at releasing cellulose from lignin, hemicellulose, wax, or
any other impurities, can be carried out using chemical, mechanical, or biological
methods or a combination of these methods (Baruah et al. 2018; Wan and Li 2012;
Putro et al. 2016; Mahmood et al. 2019). Typically, dozens of pre-treatment methods
are available, and the choice of method depends on the type of NC, scalability,
efficiency, and the cost of the process. Furthermore, pre-treatment of lignocellulosic
feedstocks is often projected as a serious bottleneck in commercialization, and
readers can consult excellent reviews to get a compressive insight into these aspects
(Kumar and Sharma 2017; Ahorsu et al. 2018). Alkali- or mechanically pre-treated
feedstock is usually converted to cellulose at nanoscale by applying strong acids
(Kandhola et al. 2020), oxidative reagents (Isogai and Zhou 2019), ionic acids
(Babicka et al. 2020), or physical (Cherian et al. 2010) or mechanical forces (Piras
et al. 2019). Mineral acid hydrolysis, which is preceded by alkali treatment (NaOH
4%, 80
C) and a bleaching process (usually with NaClO 2 ), is a frequently used
method for the extraction of nanocellulose crystals. The approach mainly cleaves the
glycosidic bonds in noncrystalline regions of cellulose fibrils, yielding needlelike
cellulose crystals with higher crystallinity (over 60%), shorter length (10–100 nm),
and a lower degree of polymerization than CNFs (Mokhena and John 2020). On the
other hand, various mechanical methods are employed for nanocellulose fiber
4 Harnessing the Sustainable Bioresource, Cellulose at the Nanoscale for. . .
69
Stylianopoulos et al. 2012; Araki and Miyayama 2020).
3 Extraction of NC
Nanocellulose can be extracted from plant-based feedstocks (An et al. 2020; Gea
et al. 2020; Trilokesh and Uppuluri 2019), certain aerobic bacteria (Vigentini et al.
2019), algae (Zhou et al. 2019), and marine animals (Iwamoto et al. 2011). The green
alga, Cladophora in particular, has become popular for the production of highly
crystalline nanocellulose (Zhou et al. 2019). Its product shows a remarkable inertness toward chemical treatments and therefore finds promising applications in filters
(Metreveli et al. 2014). Indeed, cheaper and more abundant resources, such as
lignocellulose feedstock, are considered the most promising source. However, the
bulk cellulose in plants (stems, leaves, roots, and fruit) and agricultural residues,
such as rice straw, wheat bran, bagasse, fruit peel, coconut husk, etc., exist together
with other polymers and therefore requires chemical hydrolysis or mechanical
defibrillation to produce crystalline nanocellulose and nanocellulosic fibers, respectively. On the other hand, many research laboratories mostly use pure cellulose (that
may be obtained from cotton fibers or other sources) or microcrystalline cellulose or
wood pulp (delignified wood that contains almost pure cellulose); however, the
large-scale production of nanocellulose is based on the continuous supply of raw
materials and consequently uses lignocellulosic starting material, with the extraction
and production of NC, executed in three prime steps: pre-treatment, NC conversion,
and post-treatment.
Pre-treatment, aimed at releasing cellulose from lignin, hemicellulose, wax, or
any other impurities, can be carried out using chemical, mechanical, or biological
methods or a combination of these methods (Baruah et al. 2018; Wan and Li 2012;
Putro et al. 2016; Mahmood et al. 2019). Typically, dozens of pre-treatment methods
are available, and the choice of method depends on the type of NC, scalability,
efficiency, and the cost of the process. Furthermore, pre-treatment of lignocellulosic
feedstocks is often projected as a serious bottleneck in commercialization, and
readers can consult excellent reviews to get a compressive insight into these aspects
(Kumar and Sharma 2017; Ahorsu et al. 2018). Alkali- or mechanically pre-treated
feedstock is usually converted to cellulose at nanoscale by applying strong acids
(Kandhola et al. 2020), oxidative reagents (Isogai and Zhou 2019), ionic acids
(Babicka et al. 2020), or physical (Cherian et al. 2010) or mechanical forces (Piras
et al. 2019). Mineral acid hydrolysis, which is preceded by alkali treatment (NaOH
4%, 80
C) and a bleaching process (usually with NaClO 2 ), is a frequently used
method for the extraction of nanocellulose crystals. The approach mainly cleaves the
glycosidic bonds in noncrystalline regions of cellulose fibrils, yielding needlelike
cellulose crystals with higher crystallinity (over 60%), shorter length (10–100 nm),
and a lower degree of polymerization than CNFs (Mokhena and John 2020). On the
other hand, various mechanical methods are employed for nanocellulose fiber
4 Harnessing the Sustainable Bioresource, Cellulose at the Nanoscale for. . .
69