Okara derived hydrochar effects of activation on the solid fuel properties and adsorption behaviors of the cationic dye (brilliant green)

VIETNAM NATIONAL UNIVERSITY, HA NOI
VIETNAM JAPAN UNIVERSITY

NGUYEN THI THU HOA

OKARA-DERIVED HYDROCHAR:
EFFECTS OF ACTIVATION ON THE SOLID
FUEL PROPERTIES AND ADSORPTION
BEHAVIORS OF THE CATIONIC DYE
(BRILLIANT GREEN)

MASTER’S THESIS


VIETNAM NATIONAL UNIVERSITY, HA NOI
VIETNAM JAPAN UNIVERSITY

NGUYEN THI THU HOA

OKARA-DERIVED HYDROCHAR:
EFFECTS OF ACTIVATION ON THE SOLID
FUEL PROPERTIES AND ADSORPTION
BEHAVIORS OF THE CATIONIC DYE
(BRILLIANT GREEN)

MAJOR: ENVIRONMENTAL ENGINEERING
CODE: 8520320.01

RESEARCH SUPERVISORS:
Dr. NGUYEN THI AN HANG
Dr. NGUYEN HONG NAM

Hanoi, 2021


ACKNOWLEDGEMENTS
First of all, I would like to express my sincere thanks to my principal supervisor Dr.
Nguyen Thi An Hang, lecturer and director of the Environmental Engineering program
at Vietnam Japan University-Vietnam National University, Hanoi for supporting,
helping and teaching me in the process of studying and conducting my master's thesis.
She was always encouraging and ready to help me when I have difficulties, imparting
skills and knowledge to help me work in the most effective way. This helps me not only
improve my professional knowledge but also improve my life skills.
I would also like to thank Dr. Nguyen Hong Nam, lecturer at Vietnam Japan University
for his comments, support and creating conditions for me during the process of writing
my thesis as well as doing my internship at Viet Phap University.
I would also like to thank Ms. Nguyen Thi Xuyen, the project staff who supported me
in conducting experiments as well as analyzing environmental parameters in the master's
thesis.
I would like to acknowledge the VJU's JICA research fund (2021-2023, Principal
Investigator Dr. Nguyen Thi An Hang) for providing the financial support.
I would like to express my sincere thanks and gratitude to my family and friends for
facilitating my studies and encouraging me.
Hanoi, June 14th, 2021.
Nguyen Thi Thu Hoa


TABLE OF CONTENTS

ACKNOWLEDGEMENTS ........................................................................................
TABLE OF CONTENTS .............................................................................................

LIST OF TABLES .......................................................................................................i
LIST OF FIGURES ................................................................................................... ii
LIST OF ABBREVIATIONS ................................................................................. iii
CHAPTER 1: INTRODUCTION ............................................................................1
1.1. Research background............................................................................................1
1.2. Research objectives ..............................................................................................2

CHAPTER 2: LITERATURE REVIEW ...............................................................3
2.1. Dye pollution in the world and in Vietnam ..........................................................3
2.1.1. Environmental concerns of dyes in the world ...............................................3
2.1.2. Dye treatment technologies in the world .......................................................3
2.2. Potential and challenges of agrowaste ..................................................................5
2.2.1. Generation and disposal of agrowaste in Vietnam ........................................5
2.2.2. Sources, current use and disposal of okara....................................................7
2.3. Agrowaste thermal conversion technologies ........................................................9
2.3.1. Hydrothermal carbonization (HTC) ..............................................................9
2.3.2. Pyrolysis ......................................................................................................12
2.3.3. Incineration ..................................................................................................13
2.4. Application of agrowaste-derived hydrochars ...................................................13
2.4.1. Solid fuels ....................................................................................................13
2.4.2. Environmental materials ..............................................................................13
2.4.3. Soil reclamation ...........................................................................................14
2.4.4. Carbon sequestration ...................................................................................14

CHAPTER 3: MATERIALS AND METHODS.................................................15
3.1. Materials .............................................................................................................15
3.1.1. BG dye .........................................................................................................15
3.1.2. Okara ...........................................................................................................15

3.2. Experiment setup and equipment .......................................................................15
3.2.1. Hydrochar fabrication ..................................................................................15
3.2.2. Hydrochar modification...............................................................................15
3.2.3. Hydrochar characterization .........................................................................16
3.2.4. Fuel properties of raw and activated hydrochars.........................................16
3.2.5. BG dye adsorption by the selected activated hydrochar .............................16
3.3. Statical analysis ..................................................................................................19

CHAPTER 4: RESULTS AND DISCUSSION ...................................................20
4.1. Factors influencing the fabrication of okara-derived hydrochars ......................20
4.1.1. Effect of temperature ...................................................................................20
4.1.2. Effect of contact time ..................................................................................21
4.1.3. The okara/water ratio ...................................................................................22
4.2. Effect of modification on the fuel and adsorption properties of hydrochars .....23
4.3. Characterization of hydrochars...........................................................................25
4.4. The fuel properties of the selected activated hydrochar .....................................28
4.5. BG dye adsorption behaviors of the selected activated hydrochar ....................28
4.5.1. Influential factors.........................................................................................28
4.5.2. Adsorption isotherm ....................................................................................30
4.5.3. Adsorption kinetics ......................................................................................33

CHAPTER 5: CONCLUSION AND RECOMMENDATION ........................35
5.1. Conclusions ........................................................................................................35
5.2. Recommendations ..............................................................................................35

REFERENCES ..........................................................................................................36
APPENDIX .................................................................................................................41


LIST OF TABLES

Table 4.1. Effects of activation methods on the solid fuel properties of okara-derived
hydrochar .......................................................................................................................24
Table 4.2. Comparing the HHV of AH2 with other materials......................................25
Table 4.3. Langmuir and Freundlich isotherm parameters for BG adsorption on AH2
.......................................................................................................................................31
Table 4.4. Comparing qmax of AH2 with those of other hydrochars .............................32
Table 4.5. Pseudo-first-order and pseudo-second-order kinetic parameters for BG
adsorption onto AH2 .....................................................................................................33

i


LIST OF FIGURES
Figure 4.1. Effect of the temperature on okara-derived hydrochar fabrication ............20
Figure 4.2. Effect of the contact time on okara-derived hydrochar fabrication ...........21
Figure 4.3. Effect of the okara: water ratio on okara-derived fabrication ....................22
Figure 4.4. Effects of activation methods on BG adsorption .......................................23
Figure 4.5. SEM results of RH .....................................................................................26
Figure 4.6. SEM results of AH1 ...................................................................................27
Figure 4.7. SEM results of AH2 ...................................................................................27
Figure 4.8. FTIR result .................................................................................................28
Figure 4.9. Point of zero charge (pHpzc) for the selected hydrochar (AH2) ...............29
Figure 4.10. Effect of pH on BG adsorption by AH2 ..................................................29
Figure 4.11. Effect of AH2 dose on BG adsorption .....................................................30
Figure 4.12. Langmuir adsorption isotherm curve for BG adsorption on AH2 ...........31
Figure 4.13. BG adsorption isotherms on AH2 ............................................................31
Figure 4.14. Freundlich adsorption isotherm curve for BG adsorption on AH2 ..........32
Figure 4.15. Pseudo-first-order and Pseudo-second-order kinetic curves for BG
adsorption on AH2 ........................................................................................................34

ii


LIST OF ABBREVIATIONS
BG
HTC
AOP
RH
SEM
BET
FTIR
HHV
pHzpc
COD

Brilliant Green
Hydrothermal Carbonization
Advanced Oxidation Process
Raw Hydrochar
Scanning electron microscopy
Brunauer-Emmett-Teller
Fourier transform infared spectroscopy
High Heating Value
pH zero point charge
Chemical oxygen demand

iii


CHAPTER 1: INTRODUCTION

1.1. Research background
Currently, the increased demand for the use of synthetic dyes is gaining popularity. The
textile industries use a lot of water, energy as well as emit a large amount of wastewater
and many harmful chemicals (Ito et al., 2016). Therefore, the use of dyes has made dye
pollution becoming worsen. The brilliant green (BG) dye is widely used in many
industries such as textiles, plastics and paper printing. The BG dye can cause several
health risks which include eye burns, skin irritation, coughing and shortness of breath,
nausea, vomiting and diarrhea. Thus, the treatment of dyestuff wastewaters is necessary
(Chequer et al., 2013; Kismir and Aroguz., 2011). There are many treatment techniques
being used to eliminate dye compounds from water such as biodegradation, coagulation,
reverse osmosis and adsorption. Among them, adsorption is considered as the most
effective method due to its easy operation and high efficiency. However, the large-scale
use of activated carbon is limited as the result of its high prices (Mansoout et al., 2020).
Hydrothermal carbonization (HTC) is a promising heat treatment method for converting
raw materials into value-added products. The two main products of the HTC process are
hydrochar and bio-oil, of which hydrochar accounts for 40-70% by volume. HTC is
usually performed at relatively low temperatures (180-350oC) compared to other heat
treatment technologies. Since it does not require drying in advance, which is an energyintensive consumption process, HTC is economical. Compared with the conventional
pyrolysis method, the HTC method has outstanding advantages, such as low energy
consumption, high yield and minimal emissions. This unique features of HTC have
attracted the attention of researchers studying hydrochar as an alternative to fossil fuels
used in various processes (Cao et al., 2007). In recent years, the production of activated
carbon from agro-waste is being widely studied. The use of agrowastes as raw materials
for the production of activated carbon will be highly economical because they are
abundant, cheap, renewable and sustainable precursor. Among agrowastes okara is
known as a very potential material. Annually, large quantities of okara are produced
causing environmental pollution due to its quick decomposition. A lot of research has
been done to recycle of okara as the additive in snacks. Howerver, okara’s usage as a
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human food is limited by its high fiber content. In constrast, as okara contains crude
fiber (e.g cellulose, hemicellulose and lignin), it can be useful for dye removal through
different mechanisms.
Based on the principle of waste control by waste, the present work aims at developing a
novel adsorbent for removing BG dye from wastewater. The use of okara as a low-cost
adsorbent will add the economic value to reduce waste disposal costs, and remedy dye
pollution. Besides the agro-waste disposal discussed above, energy is another important
issue. The global demand for energy has increased and fossil fuel reserves are depleting
at an alarming rate (Change., 2006). Global prediction of the increased fuel use has
added to the uncertainty about the balance of fossil fuel supply and demand (Dincer.,
2006). Hence, it is urgent to produce energy from cheaper and alternative renewable
sources. Meanwhile, HTC is known to be capable of producing high energy products
from agricultural by-products. As a results, the combustion behavior of agrowastederived hydrochar needs to be further investigated in the future.
1.2. Research objectives
The overall goal of this study is to (i) prepare hydrochar from okara using HTC
technology for the removal of BG dye from aqueous solutions and ii) investigate the
fuel properties of the fabricated hydrochar. The specific objectives of the study are listed
as below:
• Optimization of the okara-derived hydrochar fabrication process.
• Selection of the most effective activation method to enhance the BG dye adsorption of
the fabricated okara.
• Evaluation of the physicochemical and fuel properties of the pristine hydrochar and
activated hydrochar.
• Investigation of the adsorptive removal of BG dye from aqueous solutions using the
fabricated hydrochar.

2

CHAPTER 2: LITERATURE REVIEW
2.1. Dye pollution in the world and in Vietnam
2.1.1. Environmental concerns of dyes in the world
Dyes are widely used in textile dyeing, paper printing, color photography,
pharmaceuticals, food, cosmetics and leather industries (Ali., 2010). It is estimated that
more than 10,000 different dyes and pigments are used in industry and more than 7x107
tons of synthetic dyes are produced annually worldwide (Feng et al., 2017). Pulp and
paper mills, textiles, dyes, and tanneries are some of the industries that emit dark colored
wastewater. The textile industry is one of the largest sources of wastewater pollutants
because the large amount of water is used in the dyeing process. About 20% of industrial
water pollution is caused by textile production. Textile dyeing is the second largest cause
of water pollution globally. The fashion industry alone used 93 million tons of chemicals
in textile production each year (Niinimäki et al., 2020). It is estimated that 280,000 tons
of waste textile dyes are lost to wastewater each year during dyeing and finishing, due
to inefficient dyeing processes. The direct use of untreated dyeing wastewater in
agriculture has serious impacts on the environment and human health. The discharge of
untreated dyeing wastewater, without any treatment, into water bodies causes serious
environmental and health hazards. The thin layer of waste dye, which forms on the
surface of the receiving water bodies also reduces the amount of dissolved oxygen in
the water and the penetration of light into the receiving waters bodies thus adversely
influencing the photosynthetic activity of aquatic flora. Many dyes are visible in water
at concentrations as low as 1 mg/L. Therefore, in addition to affecting the health of
plants and animals, synthetic dyes are also undesirable in water bodies from an aesthetic
point of view (Ali., 2010).
2.1.2. Dye treatment technologies in the world
Current dye removal methods are classified into three main categories: physical
methods, oxidizing methods and biological methods.
2.1.2.1. Physicochemical
Physical dye removal methods include coagulation & flocculation, adsorption and
3

filtration. Coagulation is used to remove dispersed dyes in wastewater by changing the
characteristics of suspended particles causing them to agglomerate and form particles.
However, the disadvantages of this method are the use of a large amount of chemicals
and an increase in the volume of the sludge (Nguyen and Juang., 2013). Another
physical method is filtration, where techniques such as reverse osmosis and
ultrafiltration are used to remove dyes in wastewater. This method enables the recovery
and reuse of dyes for commercial purposes. The drawbacks of this method include the
high cost of the membranes and their maintenance requirement. The adsorption is known
as an effective and inexpensive wastewater treatment method. Dye removal is based on
physical and chemical properties as well as the type of the selected material. Activated
carbon is an effective adsorbent for many dyes. However, high cost and difficult
regeneration are major limitations of the technique. Therefore, there is a growing trend
to replace coal-based adsorbents with bio-based adsorbents.
2.1.2.2. Oxidation
Oxidation is a chemical method used to break down dyes. The oxidation techniques can
be classified as advanced oxidation processes and chemical oxidation. These processes
are capable of degrading primary toxicants and dye by-products partially or completely.
Advanced oxidation processes (AOP) are processes in which hydroxyl radicals, strong
oxidizing compounds, are generated inside. These oxidants are more reactive than
conventional oxidants. The chemical oxidation uses oxidants such as O3 and H2O2.
Ozone and H2O2 form strongly non-selective hydroxyl radicals at high pH values. These
high-oxidation potential-induced radicles can effectively disrupt the conjugated double
bonds of the dye pigment cells as well as other functional groups such as the complex
aromatic rings of the dye. Thereafter, the formation of smaller non-pigmented molecules
reduces the color of the wastewater. The advantage of ozonation is that it does not
increase the volume of wastewater and create sludge because ozone is used in the
gaseous state. However, the downside of using ozone is that it isn’t cost friendly. In
addition to the use of ozone, dye can also be degraded by combining UV light and H2O2.

The advantage of this method is that it does not create sludge and reduces odors. UV is
used to trigger the decomposition of H2O2 into hydroxyl radicals. The hydroxyl radicals
cause chemical oxidation of the dyes. (Navin et al., 2018)

4


2.1.2.3. Biological methods
Biological methods used to remove dyes are based on the adaptability of the
microorganism and the enzymes secreted directly from the microorganism or the free
enzymes. The ratio between the organic load on the dye and the microbial load, its
temperature and the oxygen concentration in the system will affect the removal
efficiency. Biological methods can be classified into aerobic, anaerobic techniques. The
advantages of the biological method are that it is environmental friendly, economically
beneficial, producing less sludge, producing non-hazardous metabolites or fully
mineralization. (Navin et al., 2018)
2.2. Potential and challenges of agrowaste
2.2.1. Generation and disposal of agrowaste in Vietnam
2.2.1.1. Generation of agrowaste in Vietnam
Agricultural waste is waste generated during agricultural activities. The origin of
agricultural waste is from the processing of agricultural crops, food, fruits and food.
Agricultural wastes are mainly rice husks, sawdust, bagasse, soybean residue and so on.
This is a huge source of raw materials that always exists and is increasing. According to
statistics of the Ministry of Agriculture and Rural Development, in 2010 the whole
country had about 7.47 million ha of rice cultivation area, 1.1 million ha of maize, 498
thousand ha of cassava, 173,000 ha of soybeans, 210 thousand ha of peanuts and 269
thousand ha of sugarcane. These crops all leave a huge source of agricultural byproducts after harvest. Thus, with the cultivated area in 2010, the estimated amount of
by-products from cultivation by the Institute of Agricultural Environment shows that
our country has about 61.43 million tons of by-products (Tin., 2017). With such a large
amount of by-products, if not handled properly, there will be effects on the surrounding

environment due to decomposition, misuse or burning. In fact, organic sources from
crop waste can be reused and treated to become a valuable organic source that ensures
environmental friendliness and brings economic efficiency.
2.2.1.2. Disposal of agrowaste in Vietnam
Due to its high nutritional value, the ability to provide large calories and fiber content,
if appropriate technologies are applied, agricultural by-products will become valuable
5


products. However, at present, only about 10% of agricultural by-products are used as
fuel such as cooking in households, brick kilns, 5% are industrial fuels such as rice husks
and bagasse to produce heat in boilers, drying system, 3% for animal feed, flavoring,
fertilizer for the soil and more than 80% of agricultural by-products that have not been
used, which are directly discharged into the environment, causing food to be dumped
into canals, ditches and rivers, causing serious damage to the environment and
obstructing the flow (Tin., 2017). Therefore, currently many technologies are being
applied in Vietnam to prevent pollution such as:
Compost
Composting is a widely used solution in countries with good classification systems,
based on the natural aerobic decomposition of microorganisms that turn waste into jams
and nutrients for plants. The advantage of the method is that it turns non-valued waste
into organic fertilizer that increases the amount of humus in the soil, improves the
physicochemical properties of the soil, so it is good for plants and the price is reasonable.
The disadvantage of this method is that the composting process depends on climate,
weather, the decomposition process takes place in a complex, multi-stage process and
can create odors and unsightly.
Production of biochar
This method uses high temperatures to produce biochar from agricultural by-products.
During the biochar production process, the temperature and the type of material used
will affect the biochar yield and product properties. When the pyrolysis temperature

increases, the proportion of coal and concentrated liquid decreases, for example, when
pyrolysis is at 280oC, the coal yield is about 30-50% and gradually decreases to 20-30%
when the temperature is increased to 850oC. The current fuel sources for biochar
production are shrubs, trash trees, waste wood in processing zones, crop by-products in
cultivation, agricultural product processing, animal waste in livestock and organic
waste. The disadvantage of this method is that the biochar product produced is not
homogeneous.
Serving and processing animal feed
In order to reduce environmental pollution in livestock production and create a source
6


of clean food, the use of waste products such as: Rice, corn, soybeans, corn residues,
etc. to be processed as animal feed will help save costs and get high profits. Currently,
agricultural by-products are gradually being used in livestock such as:
Rice straw is gradually being used in cattle rearing for plowing and breeding. It is also
a very good source of fiber to combine with puree in dairy farming and fattening of beef
cattle. Because rice straw is rich in potassium but lacks calcium absorption, so when
cattle are fed with rice straw, it is necessary to add an easily digestible source of calcium.
Currently, rice straw is composted with 4-5% urea to increase digestibility and energy
value increases from 4.74 MJ to 5.49 MJ/kg dry matter (Cai., 2002). In addition to rice
straw, some other types are used in livestock such as bagasse, rice bran, wine residue
and okara. However, only a small amount of waste is reused, due to weak agricultural
waste management and planning systems and therefore environmental pollution in rural
areas is still alarming in many places.
2.2.2. Sources, current use and disposal of okara
2.2.2.1. Sources
Okara is a by-product created during the production of tofu or soy milk. About 1.2 kg
of fresh okara is produced from 1 kg of dry soybeans to be processed into tofu. A large
number of okara are produced worldwide. In Japan about 800,000 tons, in Korea about

310,000 tons and China about 2,800,000 tons of okara are produced from the tofu
industry each year.
Dried okara contains about 50% fiber, 25% protein and 10% lipid. Other soy
components that may also be present in okra include isoflavones (genistein and
daidzein), lignans, phytosterols, coumestans, saponins, and phytates. These compounds
have various physiological and therapeutic functions, including antioxidant activity,
prevention of cardiovascular diseases, and effective chemopreventive agents against
several types of cancer. Therefore, okara has good nutritional value, which can be reused
in many aspects or recycled by recovering the nutritional components.
2.2.2.2. Current use and disposal of okara
Application in food
Okara has been used as a food for many years in China and Japan. It can be used in wet,
7


dry or paste form in food products ranging from meat to baked goods. Since okara
contains valuable ingredients including fiber and protein, it is relatively easy to add to a
product to help meet nutritional requirements. Okara has oil and moisture binding
properties making it an ideal low cost ingredient to help increase yield in meat products.
Okara also has a positive effect on shelf life in chocolate chip cookies at an optimal level
of 5% and prevents synthesis in cheese ravioli during freezing and thawing, the taste of
okara gives This allows it to be used to a relatively high degree without negatively
affecting the flavor or texture of meat and bread products (Li et al., 2012). Most largescale commercial soy milk or beverage manufacturers do not use or sell okara for food
purposes, only a few companies use okara in food products by freezing and stored
immediately after production. Therefore, the total amount of okara utilized and utilized
is relatively small compared to the total amount produced (Li et al., 2012).
Animal feed
Most of okara is being used as animal feed because Okara contains high content of
protein and non-fibrous carbohydrates and hence it can provide abundant and excellent
nutrients. Moreover, okara is much cheaper than soybean meal. To prepare animal feed,

okara must be completely dried and pelletized so that it can be easily transferred and
recycled into animal feed (Li et al., 2012).
Fermentation
Okara is rich in carbohydrates, proteins and other nutrients, making it a useful substrate
for microbial fermentation.
Fermented okara can be used as human food. Fermentation can increase the content of
soluble fiber, protein, amino acids, isoflavones and can break down phytic acid, a
substance that is not good for nutrition. Therefore, fermentation of okara will lead to
improved nutrition and processing properties. Lu et.al 2007 reported optimal conditions
for solid state fermentation of okara by Mucor as 75% initial moisture content and 1%
NaCl at 28 °C for 2 days. After fermentation, the protein enzyme activity in okara was
600 U/g, the crude protein content increased from 19.76% to 22.96%, and the amino
acid nitrogen content increased from 0.26% to 1.45% (Lu et al., 2007). In addition, okara
can be used as a fermentation medium for natto production and substrate for
8


fermentation such as in the production of iturin A, alcohol and citric acid.
2.3. Agrowaste thermal conversion technologies
2.3.1. Hydrothermal carbonization (HTC)
2.3.1.1. Definition
The process of converting organic biomass into a carbon-rich solid product through a
thermochemical process is known as hydrothermal carbonization (HTC). In this process,
the feed material is submerged in water between a temperature of 180 - 350°C (Zhao et
al., 2014) and a pressure of about 2 – 6 Mpa for 5 - 240 min (Libra et al., 2011). This
process produces three main products: solid (hydrochar), liquid and a small amount of
gas (mainly CO2). Coal is the main product obtained from the HTC process called
hydrochar with 40-70% mass yield (Saqib et al., 2019). Usually the reaction pressure is
not controlled in the process and is automatic with the saturated vapor pressure of the
water corresponding to the reaction temperature. At high temperatures, water with a high

ionization constant can facilitate hydrolysis and cleavage of ligniccellulosic biomass,
the water responsible for the hydrolysis of organic compounds, which can be catalyzed,
further reacted by acids or bases. In addition, increasing the temperature improves
hydrochar's fuel properties such as fuel ratio and calorific value. Dehydration and
decarboxylation reactions will produce a large amount of aromatisation with a
significant amount of oxygen-containing groups on the surface of the hydrochar. The
presence of an oxygen-containing functional group on the hydrochar surface clarifies its
compatibility with water and thus soil water holding capacity can be increased by its use
as a soil improver. In addition, wet biomass can be used directly as input material for
the HTC process and thus can save energy (Yoganandham et al., 2020).
2.3.1.2. Advantages and disadvantages
The advantage of the HTC method is that it is performed at relatively low temperatures
compared to incineration and pyrolysis. Furthermore, the feedstock does not require predrying prior to heat treatment and is carried out in a solvent environment thus resulting
in greater coal yield with more organic compounds dissolved in water. The gas products
of the HTC process, especially CO2, are smaller than those of other conversions due to
the limited oxygen exposure in the reactor. In addition, the chemical structure of HTC
manufactured coal has many similarities with natural coal (Medick et al., 2017). Because
9


of this unique property of hydrochar, it is a potential material to replace fossil fuels in
the future.
The disadvantage of this process is the lack of reaction kinetic data including reaction
curve and mass transfer, which is an important parameter for process optimization and
reaction kinetics design in HTC process. In addition, liquid and solid separation adds to
process costs and reduces product yield.
2.3.1.3. Influential factors/ Process parameters
Influence of solvent
In the HTC process, water acts as an alternative reaction solvent for toxic solvents and
corrosive chemicals. Water acts as a non-polar solvent, increasing the solubility of

organic compounds including biomass. Water contains a high degree of ionization at
high temperature and pressure, which leads to dissociation into hydroxide ions (OH-)
and acidic hydronium ions (H3O+) which exhibit acidity and basicity (Marcus., 1999).
Subcritical water initiates the reaction mechanism by hydrolysis, which lowers the
activation energy levels of cellulose and hemicellulose leading to a rapid decrease in
decomposition and the formation of water-soluble products (Bobleter., 1994).
Influence of temperature
In the HTC process, temperature is an important factor as it affects the hydrochar's
characteristics. It is also a major determinant of the water properties that cause ionic
reactions to occur in the subcritical region. An increase in temperature will allow water
for easier penetration into the porous medium and thus further degradation of the
biomass. When the temperature reaches a certain point, it will affect the hydrolysis
reaction of the biomass and the higher temperature leads to dehydration,
decarboxylation, and condensation simultaneously. Variation due to temperature
difference can also be demonstrated by the elemental composition of the hydrochar
products. When the temperature increases from 230 to 250oC, the ratio of O/C and H/C
atoms decreases and at the same time the degree of aromaticization is high (Bobleter.,
1994). The energy content and thermal stability of hydrochar were also significantly
improved with increasing reaction temperature.
Influence of residence time
10


The reaction time has an effect on the yield of hydrochar. Longer residence time and
higher temperature will reduce hydrochar yield and conversely short residence time will
increase hydrochar yield. For lignocellulose materials, the formation of hydrochar is
dependent on residence time because soluble monomers require extensive
polymerization. As the retention time is reduced, less condensation products (high O/C
and H/C atom ratio) are obtained due to lower degree of hydrolysis and polymerization.
The residence time is also a factor to improve the structural and morphological

properties of hydrochar because increasing the residence time will release more volatiles
and carbonization will take place more (Newalkar et al., 2014). Two types of char are
formed: the first is the solid remainder of the biomass called primary char or nonliquefied remainder, the second is called polymerised char or secondary char. The
increased residence time leads mainly to the formation of secondary chars (Knezevic et
al., 2009). The formation of secondary char due to condensation and depolymerisation
reduces bio-oil conversion and yield at long residence times.
2.3.1.4. Mechanisms
The complex reaction which takes place during HTC is endothermic in nature which is
a combination of dehydration and decarboxylation reaction (Berge et al., 2011). The
main reactions in the HTC process are hydrolysis, dehydration, decarboxylation,
condensation, polymerisation and aromatisation.
In hydrolysis, water reacts with cellulose or hemicellulose and breaks ester and ether
bonds to give a variety of products. Hydronium ions (H3O+) are formed from the
cleavage of water molecules during the heating of biomass in water, facilitating
hydrolysis reactions. As the temperature increases, the biomolecules (chains of cellulose
and hemicellulose) initially hydrolyze to intermediate components such as oligomers
and glucose. As the reaction time increases. Oligomer and glucose will break down into
organic acids such as acetic acid, lactic acid and levulinic acid. This explains the
phenomenon that as the temperature increases, the pH of the solution decreases. The
formed product is further hydrolyzed to form the fuane, 5-HMF. Hemicellulose
hydrolysis starts at temperatures above 180°C while cellulose hydrolysis starts above
230°C. The physical and chemical process is primarily responsible for dehydration and
is the primary process in which oxygen removal takes place. The removal of the
11


hydroxyl group is a chemical dehydration process thus it reduces the H/C and O/C ratios.
In addition, the biomass was significantly carbonized leading to a significant reduction
in the O/C ratio. At temperatures above 230oC, the decomposition of the carboxyl group
takes place.

During polymerization, the intermediate monomers formed during hydrolysis are
polymerized to form a polymer chain. Intermediates such as 5-HMF and aldehydes are
unstable and polymerize by aldol-condensation and intermolecular dehydration. The
linear structure of cellulose during hydrolysis is also crosslinked to form a crosslinking
polymer similar to the polymerization of lignin. The lignin fragments were polymerized
in a few minutes at 300oC.
Lignin is naturally composed of many stable aromatic rings. As the temperature and
residence time in the reactor increased, the percentage of lignin increased compared to
the roughage. The linear carbohydrates chain of hemicellulose and cellulose is easily
aromaticized to form lignin.
Other minor mechanisms that can occur under the HTC process include demethylation,
pyrolytic reactions, fischer–tropsch reactions, transformation reactions and secondary
char formation.
2.3.2. Pyrolysis
Pyrolysis is the thermochemical conversion of organic biomass or raw materials in
which the raw material is calcined at high temperature (300 to 650oC) in the absence of
oxygen. This process produces three main products namely biochar, bio-oil and gas such
as CO2, CO, H2 and CH4. The pyrolysis process is divided into slow, intermediate and
fast stages depending on the reaction temperature, retention time and heating rate (Laird
et al., 2009). Slow pyrolysis is the main pyrolysis process for biochar production due to
its higher solid yield (25-35%) (Mohan et al., 2006). Slow pyrolysis will be heated in
the range of 300-650oC with low heating rate and long residual time (Onay and Kockar.,
2003). Reaction temperature, time, initial humidity, heating rate and pressure are the
main important parameters affecting the physicochemical properties and yield of
biochar. A low reaction temperature and a slow heating rate will yield a high solid yield,
and conversely, a high reaction temperature and heating rate will result in a low solids
12


yield and in addition it affects the surface area and heating value (HHV) and carbon

content (Karaosmanoğlu et al., 1999).
2.3.3. Incineration
Incineration is the technology of converting and burning waste products into heat and
energy. The generated heat can be used in various industries. Incineration reduces the
amount of solid waste that goes to landfills. However, the disadvantage of this method
is air pollution because the exhaust gas from the incinerator contains dioxins and heavy
metals (Psomzopoulos et al., 2009). Some studies have demonstrated that some heavy
metals such as Cd, Hg, Pb, Cu, Cr and Zn are generated from combustion and released
into the environment (Haiying et al., 2010). This method, although it reduces the amount
of waste going to landfill and can recover heat, produces a large amount of fly ash and
chemicals that are hazardous to the ecosystem, people and the environment.
2.4. Application of agrowaste-derived hydrochars
2.4.1. Solid fuels
The HTC process can transform biomass into activated carbon with high
physicochemical properties and can use a coal substitute for energy production. The
increase in the C/O ratio is due to the transformation of cellulose and hemicellulose from
the biomass during the HTC process thereby increasing the HHV value of the solid
product. The calorific value (HHV) of lignite coal ranges from 15.08 to 21.74 MJ/kg
(Liu et al., 2014). Meanwhile, hydrochar derived from pellets has a higher calorific value
than coal and is up to 21.74MJ/kg. In addition, the removal of hemicellulose from the
raw material can enhance the hydrophobicity of the hydrochar, thereby reducing the
hygroscopicity and facilitating the combustion of the hydrochar. The results show that
hydrochar is a material with properties similar to coal and is a potential material to
replace coal in the future.
2.4.2. Environmental materials
Hydrochar is used as a low-cost adsorbent to remove pollutants in water. The hydrochar
production conditions and the properties of the starting biomass will affect the
adsorption capacity of hydrochar. Primary hydrochar has a small surface area and pore
volume, which results in a reduced adsorption capacity. In addition, the presence of
13

oxygen-rich functional group on the hydrochar surface has made it a potential material
to remove positively charged pollutants and conversely, its ability to remove negatively
charged pollutants is reduced. Therefore, modification of hydrochar through activation
may provide desirable properties for contaminant treatment.
2.4.3. Soil reclamation
Hydrochar derived from plants usually have a low nutrient content and therefore it is
added to the soil to enhance the effect of the fertilizer by reducing the amount of fertilizer
lost through run-off surface. The nutrients will be absorbed into the pores in the surface
of the hydrochar, then the nutrients will be slowly released into the soil over time for
the plants to absorb (Yao et al., 2013); Fang et al., 2018). Some studies show that, when
adding hydrochar to soil, some physiological properties of the soil are improved such as
water holding capacity, stable agglomeration in water, pH, exchange of cations and
anions and extractable nutrients.
2.4.4. Carbon sequestration
When biomass is converted to solid coal and introduced into the soil, the process is
known as carbon sequestration. However, some studies suggest that hydrochar has a low
C sequestration potential because a large part of C remains in soluble form and can be
rapidly degraded (Malghani et al., 2013). Compared with biochar, hydrochar has a
higher ratio of H/C and O/C, so it is easier to decompose by microorganisms and it can
transform microorganisms before long-term C storage (Ramke and Blohse., 2009).
Because the duration of C sequestration may depend on interactions between dissolved
C or modified organisms and the mineral surface of the soil, soil properties and texture
also play an important role in the fate of hydrochar C amendments. Currently, the
majority of research indicates that hydrochar is not useful in carbon sequestration due
to its low stability in soil. Therefore, further studies are needed to optimize the
application of hydrochar in soil to absorb carbon with better stability.

14

CHAPTER 3: MATERIALS AND METHODS
3.1. Materials
3.1.1. BG dye
Brilliant green (molecular formula C27H34N2O4S, molecular weight = 482.65 g) was
used in the experiment as a dye pollutant. A standard stock solution of 1000 mg/l was
prepared by dissolving the required amount of BG dye in deionised water. The working
dye solutions with desired concentrations were prepared by diluting the standard
solution with deionised water. The pH values of working dye solutions were adjusted
with 0.1M HCl and 0.1M NaOH in the solutions.
3.1.2. Okara
Okara was collected from a household scale tofu production facility in Yen Hoa street,
Cau Giay district, Hanoi and then was washed with distilled water to remove the
impurities and then dried in an oven at 105oC until a constant mass was obtained. The
dried okara was then blended and sieved to 150 𝜇𝑚.
3.2. Experiment setup and equipment
3.2.1. Hydrochar fabrication
Hydrothermal carbonization of okara was performed with lab-scale Teflonlined
stainless steel autoclave reactors, which were placed in a furnace. HTC was heated at
different conditions regarding temperature 180, 220 and 260oC, reaction time 3, 6 and 9
hours and the ratio of solid to liquid was from 1, 3, 5, 7 g/30mL. At the end of treatment,
each autoclave was removed from the furnace and quickly cooled down using tap water.
Then, the solid product was separated from the liquid using vacuum filtration. The
separated solid product was allowed to dry at 105oC for 24 hours to obtain the raw
hydrochar (RH).
3.2.2. Hydrochar modification
The raw hydrochars (RH) 5 g were activated using different methods as follows:
• Method 1: Mixing with 200 ml of 1M NaOH solution (AH1)
15

• Method 2: Mixing with 200 ml of 1M NaOH solution followed by heating at 700oC,
for 30 min, at the heating rate of 5oC/min in the supply of N2 (AH2)
• Method 3: Heating at 700oC, for 30 min, at the heating rate of 5oC/min in the supply
of N2 (AH3)
• Method 4: Heating at 700oC, for 30 min, at the heating rate of 5oC/min in the supply
of N2 followed by mixing with 200 ml of 1M NaOH solution (AH4).
Using filtration, the solid products were collected, then washed with 0.1M NaOH and
0.1M HNO3 to obtain pH value of 7-8. Then, the modifed hydrochars dried at 105oC for
24h.
3.2.3. Hydrochar characterization
The morphology features of the RH (raw hydrochar) and AH2 (NaOH modified
hydrochar with temperature) were identified by scanning electron microscopy (Tabletop
Microscopes TM4000 Plus-SEM), their specific surface areas were measured using the
Brunauer-Emmett-Teller (The NOVAtouch LX-BET) method and their chemical
functional groups were identified with Fourier transform infrared (JASCO Asia Portal FT/IR-4600-FTIR) spectroscopy.
3.2.4. Fuel properties of raw and activated hydrochars
High heating value (HHV) of RH, AH1, AH2, AH3 and AH4 were measured using Parr
6200 calorimeter.
3.2.5. BG dye adsorption by the selected activated hydrochar
To determine the adsorption isotherms, batch adsorption experiments were performed
in a set of 250 ml beakers containing 50 ml of BG with different initial concentrations
(5, 20, 40, 60, 80 and 100 mg/L), pH equal to 7 and dose 0.25 g/l. Subsequently, the
mixtures were shaken at the speed of 120 rpm for 4.5 hours to attain equilibrium. The
batch experiments was duplicated. The adsorption capacity of the BG dye by AH2 was
determined from the difference between the initial and final dye one in the aqueous
solution. The concentration of the BG dye was analyzed using a spectrophotometer
(UV/VIS spectrophotometer, S2150 UV, Unico) at the maximum wavelength (max) of
624 nm. The amount of dye retained per unit mass of the adsorbent and the percent

16


removal (%R) of dye were calculated using the following equations:
%𝑅 =

𝐶𝑖 −𝐶𝑒
𝐶𝑖

∗ 100 (Eq.1)

𝑞𝑒 = (𝐶𝑖 − 𝐶𝑒 ) ∗

𝑉
𝑚

(Eq.2)

Where qe is the adsorption capacity (mg/g), Ci is the initial concentration of the BG dye
in aqueous solution (mg/l), Ce is the concentration of the BG dye in aqueous at
equilibrium (mg/l), V is the volume of the volume of the solution (l) and m is the amount
of the adsorbent (g).
Point of zero charge (pHzpc)
To describe the properties of the adsorbent, it is common to use pHzpc, because at that
pH, the surface of the material has zero charge. When pH < pHzpc, the adsorption surface
has a positive charge and vice versa when pH > pHzpc, the surface of the material is
negatively charged. In this experiment, the determination of pHzpc was performed as
follows: 50ml of 0.1M KNO3 solution was put into a conical flask, the initial pH of the
solution was adjusted from 3 to 10 with 0.1M HCl and 0.1M NaOH. Then 0.0125 g of
adsorbent was added to the solution. The mixture was stirred for 24 h and then filtered

with filter paper. The pH values of the remaining solutions were measured. The value
of pHPZC can be determined from the pHi curve of the plot pHf - pHi versus pHi, where
pHi and pHf were the pH values of the initial and remaining solutions, respectively.
3.2.5.1. Influential factors
The effects of pH on the BG dye uptake by hydrochars were investigated in the pH range
of 3-10. The pH was adjusted using 0.1M NaOH and 0.1M HCl. The concentration of
BG was 20 mg/l while adsorbent dose was 0.25 g/l. The solution was shaken for 4.5h at
the speed of 120 rpm. Then mixture was filtered the BG concentration in the remaining
solution was determined using UV-VIS spectrophotometer at max = 624 nm.
3.2.5.2. Adsorption isotherm study
The adsorption isotherms provides information on of the concentration and quantity of
the pollutant adsorbed. Equilibrium adsorption isotherms are used to describe the
interaction behavior between solute and adsorbent. The equilibrium adsorption isotherm
17