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

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

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 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

FTIR Fourier transform infared spectroscopy

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 energy-intensive 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

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 agrowaste-derived 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

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 7x107tons 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

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)

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 by-products 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

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

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,

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 large-scale 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)

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

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 pre-drying 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

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

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 non-liquefied 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

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

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

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

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

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)

• 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