CET 97

DOI: 10.3303/CET2297012

Paper Received: 31 May 2022; Revised: 2 August 2022; Accepted: 1 September 2022
Please cite this article as: Pham C.D., Le T.M., Nguyen C.T.X., Vo N., Do N.H.N., Le K.A., Mai T.P., Le P.T.K., 2022, Review of The Role of
Pretreatment Step in Nanocellulose Production from Rice Straw, Chemical Engineering Transactions, 97, 67-72 DOI:10.3303/CET2297012

CHEMICAL ENGINEERING TRANSACTIONS

VOL. 97, 2022

A publication of

The Italian Association

of Chemical Engineering
Online at www.cetjournal.it

Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš
Copyright © 2022, AIDIC Servizi S.r.l.
ISBN 978-88-95608-96-9; ISSN 2283-9216

Review of The Role of Pretreatment Step in Nanocellulose
Production from Rice Straw

Co Dang Phama, Tan M. Lea, Chi Thi Xuan Nguyena, Nhi Voa, Nga Hoang Nguyen
Doa, Kien Anh Lec, Thanh Phong Maib, Phung Thi Kim Lea,*
aRefinery and Petrochemicals Technology Research Center (RPTC), Ho Chi Minh City University of Technology (HCMUT),
268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam
bVietnam National University Ho Chi Minh City (VNU-HCM), Linh Trung ward, Thu Duc City, Ho Chi Minh City, Vietnam
cInstitute for Tropical Technology and Environmental Protection, 57A Truong Quoc Dung Street, Phu Nhuan District, Ho Chi
Minh City, Vietnam
phungle@hcmut.edu.vn

Nanocellulose is one of the most valuable biomass-derived materials, possesses outstanding properties, and is
widely used in numerous applications for biomedical fields, packaging, and environmental waste treatment. Rice
straw is an abundant by-product from the rice industry and among cellulose-rich feedstocks. The pristine
structural network of this raw material is complicated composing cellulose, hemicellulose, and lignin. Therefore,
the pretreatment step is necessary to facilitate further stages in the biomass conversion process. The effect of
such methods on the characteristics of nanocellulose products from rice straw has not been widely investigated
in comparison with other types of biomass. This review summarized the common methods for rice straw
pretreatment and the effects of distinct methods on the obtained nanocellulose. Alkaline pretreatment is
considered as one of the most effective method for the extraction of cellulose from lignocellulosic complex.
Based on a comprehensive summary, this review also shows that cellulose nanocrystals (CNCs) which is usually
isolated by acid hydrolysis, has a high crystallinity index due to the removal of amorphous region. Cellulose
nanofibrils (CNFs) is obtained by employing mechanical methods to reduce the particle size of cellulose fibers.

1. Introduction
In recent years, biomass feedstocks have been considered to be a renewable source to synthesize a wide
variety of profitable materials. The production of lignocellulosic biomass, which is an abundant organic polymer,
reached the point of 200 trillion kg/y (Mankar et al., 2021). This indicates that the demand for biomass-based
products has been growing sharply. According to the report of Food and Agriculture Organization (FAO),
Vietnam is among the five largest rice producers in the world with an annual output of over 40 trillion kg
(Minamikawa et al., 2021), meaning that this country possesses an enormous amount of rice straw. The majority
of rice straws are currently being burned to generate electricity, compost, or biochar. These processes not only
lead to a critical impact on the environment due to the emission of greenhouse gases including CO2, N2O, and
CH4 but also do not fully exploit the potential of rice straw materials (Le et al., 2022a). Thus, the utilization of
this biomass feedstock for the preparation of more valuable products, such as nanocellulose, is a significant
progress in the sustainable development campaign. This contributes to the effort to protect the environment as
well as takes full advantage of these by-products from the rice industry.
Rice straw is mainly composed of cellulose, hemicellulose, and lignin which are bound together to form a
complex network (Do et al., 2020). Due to the high content of cellulose, rice straw is a common raw material in
biogas or bioethanol manufacture, and advanced materials synthesis (Le et al., 2022b). Among these, the
production of nanocellulose from rice straw in a facile manner exhibits a huge potential. Generally, nanocellulose
(NC) is a natural fiber that has a dimension of less than 100 nm in diameter and several micrometers in length
(Nguyen et al., 2021). In fact, nanocellulose can be categorized into three main types including cellulose
nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial nanocellulose (BNC) based on the differences

in particle sizes, crystallinity, and morphology. All these types possess a similar chemical composition (Peng et
al., 2011). Besides superior properties of a typical porous material, the existence of numerous hydroxyl groups
in nanocellulose networks induces a great affinity towards certain species as well as the opportunity for surface
modification, making it a potential candidate in many applications such as food packing (Cuong et al., 2021).
The production of nanocellulose from lignocellulosic feedstock in general and from rice straw, in particular,
comprises two stages: The isolation of cellulose from the raw material which is also known as the pretreatment
step, and the breakdown of obtained cellulose fibers into the nanoparticles (Pradhan et al., 2022). Although the
employment of different pretreatment methods leads to a variety in product properties, the number of researches
summarizing the effects of extraction methods from rice straw feedstock is still limited. This mini-review aims to
evaluate the effects of the lignocellulosic complex in rice straw on nanocellulose production. The appropriate
methods for the extraction of distinct types of nanocellulose were also investigated.

2. Structure of rice straw
Besides the common components of lignocellulosic biomass based-material, rice straw additionally holds a
substantial amount of silica due to the polymerization of silicic acid absorbed from the soil. The strong interaction
of this compound with cellulose and lignin leads to the difference in the characterization of rice straw in
comparison with others (Bhattacharya et al., 2018). Specifically, rice straw is known as one of the most cellulose-
rich biomass (more than 45 %) with low content of lignin and hemicellulose (about 20 % each) (Nguyen et al.,
2018). Meanwhile, in other types of biomass such as pineapple leaf, although the proportions of lignin and
hemicellulose are similar to rice straw, the percentage of cellulose is just over 35. The gummy matter in the
plant cell wall of the pineapple leaf is composed of lignin, pectin, and pentosane. This is one of the main
influential factors leading to the low efficiency of the biomass conversion process (Song et al., 2021). In other
circumstances, coconut coir possesses a high content of cellulose (more than 40%). The lignin quantity is even
greater than this number (Sari et al., 2021). Hence, rice straw is an appropriate feedstock for cellulose recovery
and the production of nanocellulose, in particular.

3. Extraction of nanocellulose from rice straw
Various approaches are being developed to isolate cellulose from rice straw such as chemical (alkali, dilute
acid, or organic solvent), mechanical (ball milling, homogenization), and biochemical methods (enzymatic
extraction). Mechanical methods are among the most effective procedures to increase surface area and reduce
the particle sizes of lignocellulosic material without the requirement of toxic chemicals. The huge energy
consumption and poor productivity in deconstructing biomass networks have hindered the employment of this
approach (Harun et al., 2011). Bio-based methods involve both elevated prices and long periods of time during
pretreatment. To effectively remove non-cellulosic contents while preserving the materials for further conversion,
chemical approaches have emerged as the most appropriate ones due to their efficiency and reasonable cost.
There are several typical groups of treatment in this category including acid hydrolysis, alkaline hydrolysis,
organosolv, and treatments that utilize oxidation agents or ionic liquids, each of which undergoes a distinct
pathway to break down the biomass network (Figure 1) (Lee et al., 2014).

Figure 1: The attack of different agents on the lignocellulosic structure.

3.1 Acid hydrolysis

This treatment involves the attack of hydronium ions from acid molecules to induce the breakdown of
intermolecular and intramolecular bonds between cellulose and hemicellulose. The concentration of utilized
acids is a decisive factor in determining the severity of the fractioning process. Acids with high concentrations
are toxic and corrosive, leading to high capital and maintenance costs (Lee et al., 2014).

3.2 Alkaline and oxidation treatment

The main strategy of alkaline treatment is to separate lignin from the remainder of the biomass structure
(cellulose and hemicellulose). This process includes the saponification of the ester bonds crosslinking xylan
(hemicellulose) and lignin, leading to the removal of lignin from the original network. The frequently used alkali
such as hydroxides and hydrazines also functions as a swelling agent for cellulose, resulting in an increase in
internal surface area. Besides, an oxidation agent is also employed in order to remove lignin from the
lignocellulose structure. Compounds with oxidative properties such as organic peroxides, ozone, and oxygen
have the ability to catalyze the delignification process, resulting in the selective decomposition of lignin and part
of hemicellulose in biomass structure (Lee et al., 2014).

3.3 Organosolv treatment and ion liquid utilization

Some studies on extracting CNCs and CNFs are detailed in Table 1 (Lee et al., 2014).

Table 1: Examples of the extraction of nanocellulose from rice straw with different pretreatment methods

Type
of NC

Pretreatment Size modification Average
diameter

Ref

CNC Dewaxing: Hot water, 1 h
Delignification: NaOH 12 wt%, 121 °C, 1 h
Bleaching: NaClO2 5 wt%, 75 °C, 90 mins

H2SO4 75 wt%, 30 °C, 5 h
followed by sonication.

5–15 nm
CrI: 76 %
Yield: 90.28 %

(Thakur
et al.,
2020)

CNC Dewaxing: Benzene/ethanol [2:1 (v:v)], 90 °C,

12 h
Delignification: NaOH 5 wt%, 60 °C, 3 h
Bleaching: H2O2 and CH3COOH, 50 °C, 6 h

Ultrasonication -20 kHz,
400 W, 30 mins

10-15 nm
CrI: 76.99 %

(Xu et al.,
2018)

CNC Delignification: K2CO3-Glycerol deep eutectic

solvent, (DES) 140 °C, 60 min, mass ratio 1:9
Bleaching: Oxalic acid-Choline chloride DES,
80 °C, 4 h, molar ratio 1:10

H2SO4 64%, 45 °C,
45 mins

12.3-13.3 nm
CrI: 76.7 %

(Lim et
al., 2021)

CNC Delignification and bleaching: H2O2 30 wt%,

90 °C, 5 h

500 mL ammonium
persulfate 1 mol/L, 75 °C,
16 h (for 5 g cellulose
fiber)

8-24 nm
Yield: 28 %

(Oun et
al., 2018)

CNF _ 500 mL ammonium

persulfate 1 mol/L, 75 °C,
16 h (for 5 g rice straw
fiber)

7–21 nm
Yield: 25.6 %

CNF Dewaxing: Toluene/ethanol [2:1 (v/v)], 100 °C,

24 h
Delignification: KOH 5 wt%,
room temperature, 16 h then increase
temperature to 90 °C, 2h
Bleaching: NaClO2 1.25 % with a liquid-to-solid
ratio of 15 mL/g at 75 °C, 1 h

High-shear
homogenization and high
intensity ultrasonication

6–20 nm
CrI: 65 %

(Dilamian
et al.,
2019)

CNF Dewaxing: Ethanol/toluene [2:1 (v/v)], 6 h

Delignification: 5 wt% NaOH at 90 °C, 2 h
Bleaching: NaClO2 at 70 °C, 1 h

High pressure-grinding at
1500 rpm for 5 times

13.3 nm
CrI: 54.4 %

(Zhao et
al., 2019)

Organosolv method utilizes volatile organic solvents (methanol. ethanol, ethylene glycol, ethyl acetate, etc.) as
a dissolving agent for lignin and several hemicellulosic compounds under heat, resulting in the formation of solid
cellulose residue. It was proposed that the hydroxyl groups from the solvents have the ability to promote the
cleavage of the bonds between lignin and hemicellulose (acid-ester bonds), leading to the breakdown in
biomass structure. In other circumstances, ionic liquids (ILs) are able to simultaneously and effectively dissolve
cellulose, hemicellulose, and lignin by homogenizing the reaction medium, making the β-glycosidic bonds
become more accessible for catalysts. Certain adjustments are required to alter the selectivity of ILs towards
different components in the network and promote the fractionating process.
In general, pretreatment is a crucial step prior to NC production, which is usually divided into 3 stages including
dewaxing, delignification, and bleaching. Although the wax content in rice straw is minor, the removal of this
component contributes to the increase in conversion process efficiency (Paulraj Gundupalli et al., 2021). The
delignification step also plays an important role by removing a substantial amount of lignin, leading to the
separation of biomass constituents. And once the crude cellulose is acquired, the bleaching step is necessary
to obtain cellulose with extremely high purity (Hassan et al., 2020). As can be seen, the alkaline-utilized method
is usually used as one of the most effective ways to disrupt the structure of lignin. In addition, the pretreating
solution has the ability to interact with silica in rice straw. Therefore, a substantial amount of non-cellulose
compounds was also eliminated from the raw material. This process is carried out under milder conditions,
especially when compared with acid pretreatment (Kim et al., 2016). After the cellulose isolation stage, various
methods would have been employed to induce the breakdown of cellulose fibers into the nanoscale. Despite
having similar chemical compositions, the characteristics of nanocellulose resulted from each breakdown
process would vary considerably, depending on the utilized method. Examples for the extraction of different
types of nanocellulose would be discussed in the following sections.

3.4 Cellulose nanocrystals (CNCs)

Cellulose nanocrystal is usually isolated from cellulose fibers by acid hydrolysis (i.e., H2SO4, HCl) and
possesses a high content of cellulose. It usually has a high crystallinity index because of the removal of
amorphous zones and extraction of crystalline regions from the raw fibers (Figure 2). Also, due to the breaking
of the glycoside bondings in cellulose, nanocellulose obtained from acid hydrolysis method also has a smaller
size than other methods (Gan et al., 2020).

Figure 2: The acid hydrolysis and mechanical treatment of rice straw fibers

In 2020 Thakur (2020), combined concentrated acid with sonification in the production of CNCs. It was revealed
that the yield of CNCs production of about 90.28 % was reached when the experiment was carried out at 30 °C
for 5 h with H2SO4 75 vol%. In other circumstances, Xu (2018) also utilized acid and ultrasonic waves to reduce
the particle size of cellulose. The results showed that there was the appearance of rod-like shaped CNCs with
the dimension of 10-15 nm in width and several hundred nanometers in length. The crystallinity index increased
considerably from the raw material to CNCs due to the removal of lignin and amorphous regions in the structure.

3.5 Cellulose nanofibrils (CNFs)

Despite the similarities in chemical composition when compared with CNC, CNF contains both crystal and
amorphous regions in the structure (Figure 2) (Abitbol et al., 2016). To modify the size of the product, mechanical
methods were usually used without the requirement of chemicals. This method is easily influenced by a lot of
processing factors such as temperature and operating pressure. This process is usually incorporated with other
treatment methods to reach a higher extraction yield (Tu and Hallett, 2019).
For example, Dilamian et al. (2019) employed treatments including high-shear homogenization and high-
intensity ultrasonication to acquire CNFs. The obtained product has the diameters in the range of 6–20 nm and
the crystallinity index increased to 65 % when compared to raw material. In another research, Zhao et al. (2019)
investigated the effects of isolation methods on the properties of nanocellulose from rice straw. After being

ground at high pressure for 5 times, the CNFs and CNCs from rice straw had the average diameter of 13.3 and
11.4 nm. Also, the results revealed that the CNFs possessed the crystallinity index of 54.4 % and exhibited
higher thermal stability when compared with CNCs.

3.6 Bacterial nanocellulose (BNC)

Bacterial nanocellulose is another type of nanocellulose. CNCs and CNFs are extracted from lignocellulose
through top-down process. Meanwhile, bacterial nanocellulose is created from the assembly of low-weight sugar
molecules known as bottom-up process. BNC has the same chemical composition as two mentioned kinds of
nanocellulose, even with a larger surface area per unit. Also, BNC is a hydrophilic material with high purity and
contains a substantial amount of absorbed water. It is composed of twisting ribbons which have the diameter in
the range of 20-100 nm, and the length of several μm (Kargarzadeh et al., 2017). Though the extraction of BCN
does not require former pretreatment to remove lignin or silica from rice straw, the production of BCN has its
own drawbacks due to the high cost (Gan et al., 2020). Until now, there is still a lack of studies about the bacterial
nanocellulose synthesis from rice straw.

4. Conclusions
With recent achievements in nanomaterials field, this review summarized common methods for the pretreatment
step, which is a crucial stage in the preparation of purified cellulose for further conversion. Among various types
of biomass, rice straw is considered to be a potential candidate for the production of nanocellulose. It is
noteworthy that by varying the conditions in the pretreatment step, synthesized materials exhibited differences
in terms of particle sizes, crystallinity, and morphology despite having similar chemical compositions. It was also
found the chemical methods, especially alkaline pretreatment have emerged as the most effective approach for
the isolation of cellulose from lignocellulosic network. In reality, although the combination of various methods in
the production process leads to higher efficiency than the employment of a single one, the production of both
CNCs and CNFs comprise multistep and require huge consumption of chemicals and energy. Therefore, in the
future, single-step processes carried out under mild conditions need more widely investigation. Furthermore,
this work exhibits a solution for utilizing agro-waste in the production of bio-based materials and its great
potential in industrial applications.

Acknowledgments

This research is funded by Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number 562-
2022-20-04. We acknowledge the support of time and facilities from Ho Chi Minh City University of Technology
(HCMUT), VNU-HCM for this study.

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Review of The Role of Pretreatment Step in Nanocellulose Production from Rice Straw