Synthesis and application of aerogel composites from agricultural by products in wastewater treatment

MISSIONS AND CONTENTS:  Synthesis and investigation of physical-chemical properties of cellulose aerogel composites from pineapple leaf fibers and cotton waste fibers  Evaluation of a

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

HO CHI MINH CITY, January 2023

VU VAN PHU

SYNTHESIS AND APPLICATION OF AEROGEL

COMPOSITES FROM AGRICULTURAL BY-PRODUCTS IN

WASTEWATER TREATMENT

Major: Chemical Engineering

Code: 8520301

MASTER THESIS

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THIS RESEARCH WAS CONDUCTED AT

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY – VNU-HCM

Supervisor: Assoc Prof Dr Le Thi Kim Phung

Reviewer 1: Assoc Prof Dr Nguyen Truong Son

Reviewer 2: Dr Tran Phuoc Nhat Uyen

The master thesis was defended at Ho Chi Minh city University of Technology, VNU-HCM, January 6, 2023

The members of the Assessment Committee including:

Confirmation of the Assessment Committee Chairman and the Head of

Faculty after the thesis has been corrected (if any)

Assessment Committee Chairman Dean of Chemical Engineering Faculty

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Vietnam National University – HCM City SOCIALIST REPUBLIC OF VIETNAM HCMC University of Technology Independence - Freedom – Happiness

MASTER THESIS MISSIONS

Full Name: VU VAN PHU Student’s ID: 2070481

Date of Birth: 25/10/1998 Place of Birth: Binh Phuoc Major: Chemical Engineering Code: 8520301

I THESIS TITLE:

SYNTHESIS AND APPLICATION OF AEROGEL COMPOSITES FROM

AGRICULTURAL BY-PRODUCTS IN WASTEWATER TREATMENT

TỔNG HỢP VÀ ỨNG DỤNG TỔNG HỢP AIRGEL TỪ PHỤ PHẨM

NÔNG NGHIỆP TRONG XỬ LÝ NƯỚC THẢI

II MISSIONS AND CONTENTS:

 Synthesis and investigation of physical-chemical properties of cellulose aerogel composites from pineapple leaf fibers and cotton waste fibers

 Evaluation of adsorption performance for dye and oil of cellulose aerogel composites in wastewater treatment

 Synthesis and investigation of physical-chemical properties of silica-based aerogel composites from rice husk ash and shrimp-based chitosan

 Evaluation of adsorption performance for the dye of chitosan-silica aerogel composites

in wastewater treatment

III DATE OF ASSIGNMENT: 01/2022

IV DATE OF COMPLETION: 12/2022

V SUPERVISOR: Assoc Prof Dr Le Thi Kim Phung

Ho Chi Minh City, December 28, 2022.

DEAN OF CHEMICAL ENGINEERING FACULTY

I am most grateful to the collaborators for giving me many memories at RPTC There is no way to express how much it meant to me to have been a member of RPTC

Last but not the least, I would like to express my gratitude to my family for their unfailing emotional support, unconditional trust, timely encouragement, and endless patience

I acknowledge the support of time and facilities from Ho Chi Minh City University

of Technology (HCMUT), VNU-HCM for this study

Ho Chi Minh City, December 2022

Vu Van Phu

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ABSTRACT

In a world where demands for freshwater are ever-growing, wastewater remediation becomes a global concern However, the development of innovative processes for wastewater treatment is still a major obstacle Therefore, the utilization of agricultural byproducts as a feedstock for successfully making adsorbents not only meaningful turns waste into high-value materials but also lowers production costs Concerning their fast removal rate and environmental compatibility, cellulose aerogel composites and silica-based aerogel composites are recently considered potential contributors to water remediation In this study, cellulose aerogel composites are fabricated using the sol-gel method from pineapple leaf fibers and cotton waste fibers in the alkali-urea solution followed by freeze-drying Meanwhile gelation of silica extracted from rice husk ash and shrimp-based chitosan catalyzed by hydrochloric acid (HCl), followed

by aging, solvent exchange, surface modification, and cost-effective ambient drying results in monolithic chitosan-silica aerogel composites without fragmented The obtained chitosan-silica aerogel composites show higher equilibrium methylene blue (MB) uptake (53.81 mg/g) than those of chitosan-free silica aerogel (47.52 mg/g) at the MB concentration of 125 mg/L Although the aerogel composites showed both lower porosity (84.67 – 90.54%) and surface area (21.40 – 81.49 m2/g) compared with chitosan-free silica aerogel (94.84% and 457 m2/g), the presence of amine groups enhanced the adsorption efficiency of the aerogel composites The synthesized cellulose aerogel composites are directly applied to adsorb cationic methylene blue

aerogel composites also show their ability to deal with oil pollution with a maximum oil adsorption capacity of 15.8 g.g-1 within only 20 sec Overall, our developed natural feedstock-derived aerogel composites demonstrated their great adsorption perfromance based on their ability to eliminate methylene blue, making them a potential material in dye-contaminated water treatment

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TÓM TẮT

Ngày nay khi nhu cầu về việc sử dụng nước sạch ngày càng tăng thì việc xử lý nước thải trở thành mối quan tâm mang tính toàn cầu Tuy nhiên, việc phát triển các quy trình để xử lý nước thải vẫn là một trở ngại lớn Vì thế, việc tận dụng phụ phẩm nông nghiệp làm nguyên liệu để chế tạo thành công vật liệu hấp phụ không chỉ có ý nghĩa biến phế thải thành vật liệu có giá trị sử dụng cao mà còn giảm chi phí sản xuất Vật liệu cellulose aerogel composite và vật liệu aerogel composite có nguồn gốc từ silica gần đây được coi là những ứng cử viên tiềm năng cho việc xử lý nước Trong nghiên cứu này, vật liệu cellulose aerogel composite được chế tạo bằng phương pháp sol-gel

từ sợi lá dứa và sợi cotton thải trong dung dịch kiềm-urê sau đó được sấy thăng hoa Trong khi đó, quá trình gel hóa silica chiết xuất từ tro trấu và chitosan từ tôm được xúc tác bởi axit clohydric (HCl), sau đó là quá trình lão hóa, trao đổi dung môi, biến đổi bề mặt và làm khô môi trường xung quanh hiệu quả về chi phí tạo ra vật liệu tổng hợp aerogel chitosan-silica nguyên khối mà không bị phân mảnh Chitosan-silica aerogel composite thu được cho thấy lượng hấp phụ methylene blue (MB) ở trạng thái cân bằng (53,81 mg/g) cao hơn so với silica aerogel không chứa chitosan (47,52 mg/g) ở nồng độ MB là 125 mg/L Mặc dù chitosan-silica aerogel composite cho thấy

cả độ xốp (84,67 – 90,54%) và diện tích bề mặt (21,40 – 81,49 m2/g) thấp hơn so với

nhóm amin đã tăng cường khả năng hấp phụ hiệu quả của vật liệu aeogel composite Vật liệu cellulose arogel composite được ứng dụng trực tiếp để hấp phụ methylene blue và cho thấy đương lượng hấp phụ tối đa là 34,01 g.g-1 Vật liệu cellulose arogel composite phủ MTMS cũng cho thấy khả năng xử lý nước nhiễm dầu với khả năng hấp phụ dầu tối đa là 15,8 g.g-1 chỉ trong vòng 20 giây Nhìn chung, vật liệu aerogel composite có nguồn gốc từ nguyên liệu tự nhiên của chúng tôi đã chứng minh khả năng hấp phụ tốt dựa trên khả năng loại bỏ màu methylene blue, khiến chúng trở thành vật liệu tiềm năng trong xử lý nước bị nhiễm thuốc nhuộm

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DECLARATION

I hereby declare that this thesis is my original work and it has been written by me in its entirety I have duty acknowledged all the sources of information that have been used in the thesis

This thesis has also not been submitted for any degree in any university previously

Master student

Vu Van Phu

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CONTENT

LIST OF ABBREVIATIONS ix

LIST OF FIGURES x

LIST OF TABLES xii

CHAPTER 1 PREFACE 1

1.1 Study background 1

1.2 Research aims and Objectives 2

1.3 Outline of thesis 3

CHAPTER 2 LITERATURE REVIEW 4

2.1 Aerogel and aerogel composite 4

2.2 Cellulose aerogel composite 9

2.2.1 Pineapple leaf fiber 9

2.2.2 Cotton waste fiber 11

2.2.3 Cellulose aerogel 13

2.3 Silica-based aerogel composite 14

2.3.1 Silica aerogel 14

2.3.2 Chitosan 16

2.3.3 Chitosan-silica aerogel composite 19

2.4 Synthesis method of aerogel 21

2.4.1 Synthesis of cellulose aerogel composite 23

2.4.2 Synthesis of silica-based aerogel composite 25

2.5 Application of aerogel in wastewater treatment 31

CHAPTER 3 MATERIALS AND METHODS 33

3.1 Cellulose aerogel composite 33

3.1.1 Chemicals and materials 33

3.1.2 Synthesis of PF/CF aerogel composites 33

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3.1.3 Study on oil removal of hydrophobic PF/CF aerogel composites 34

3.1.4 Study on MB removal of PF/CF aerogel composites 35

3.2 Silica-based aerogel composite 37

3.2.1 Chemicals and materials 37

3.2.2 Synthesis of silica aerogels 38

3.2.3 Synthesis of chitosan-silica aerogel composites 39

3.2.4 Study on MB cationic dye removal of silica-based porous material 40

3.3 Characterization 41

3.3.1 Density and porosity 41

3.3.2 Surface morphology analysis 42

3.3.3 Specific surface area and pore analysis 43

3.3.4 Mechanical strength analysis 45

3.3.5 Infrared spectrum analysis 45

3.3.6 Thermogravimetric analysis 46

3.3.7 Thermal conductivity analysis 47

CHAPTER 4 RESULTS AND DISCUSSIONS 48

4.1 Cellulose aerogel composites 48

4.1.1 Characterization of PF/CF aerogel composites 48

4.1.2 Dye removal of PF/CF aerogel composites 53

4.1.3 Oil adsorption of the MTMS-coated PF/CF aerogel composites 57

4.2 Silica-based aerogel composite 58

4.2.1 Effect of acid concentration on gelation of silica aerogels from RHA 58

4.2.2 Characterization of silica aerogels and chitosan/silica aerogel composites 59

4.2.3 Dye adsorption of chitosan-silica aerogel composite 62

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CHAPTER 5 CONCLUSION AND FUTURE WORK 67

5.1 Conclusions 67

5.2 Future work recommendations 68

LIST OF PUBLICATIONS 69

REFERENCES 70

SHORT CURRICULUM VITAE 79

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LIST OF ABBREVIATIONS

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LIST OF FIGURES

Figure 1.1 Graphical abstract of this study 3

Figure 2.1 Several forms of aerogel have been produced commercially, such as (a) Clear glass from silica aerogel; (b) Polyurethane aerogel insulation sheet; (c) Silica aerogel towels; (d) Silica aerogel powder 5

Figure 2.2 Raw pineapple fiber production process: (a) Pineapple field; (b) Pineapple tree; (c) Stripping of fibers from pineapple leaves; (d) Pineapple fiber 10

Figure 2.3 (a) Putting the pineapple leaves into the stripping machine, (b) harvesting the bundles from the stripping machine and (c) the pineapple leaves after drying 11

Figure 2.4 Cotton waste fiber 12

Figure 2.5 Cellulose aerogel from bagasse (a), wastepaper (b), and coir (c) 13

Figure 2.6 (a) PF aerogel; (b) CF aerogel 14

Figure 2.7 Extraction silica aerogel from rice husk 15

Figure 2.8 Silica Aerogel with blue smoke color 16

Figure 2.9 Process of obtaining chitosan by the deacetylation alkaline treatment of chitin from shrimp shell wastes 17

Figure 2.10 Chemical structure of Chitin and Chitosan 18

Figure 2.11 Schematic for the chemical structure of silica-chitosan hybrid scaffolds 20

Figure 2.12 A Typical Phase Diagram for a Substance That Exhibits Three Phases—Solid, Liquid, and Gas 22

Figure 2.13 General procedure for the synthesis of cellulose aerogel 24

Figure 2.14 Cellulose fibers in NaOH/Urea/H2O solution The fiber part is not swollen (A), the part is swollen like a bubble (B) 25

Figure 2.15 Sol-gel and gel-sol process 26

Figure 2.16 Crosslinking of polymer chains 27

Figure 2.17 Schematic representation of the formation of SiO2–chitosan–MTMS (SCM) 29

Figure 2.18 Phase diagram and paths of drying technique: freeze-drying (left), conventional drying (middle), and supercritical drying (right) 31

Figure 2.19 Chemical structure of MB 32

Figure 3.1 The procedure of synthesis of silica aerogel 38

Figure 3.2 The procedure of chitosan-silica aerogel composite 39

Figure 3.3 FE-SEM Hitachi S-4800 43

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Figure 3.4 Nova 2200E (Quantachrome) 43

Figure 3.5 IUPAC adsorption-desorption isotherms 44

Figure 3.6 Mechanical meter Zwick Roell Z010 45

Figure 3.7 Fourier transform infrared spectrometer (MIR/NIR Frontier) 46

Figure 3.8 Thermogravimetric Analyzer 46

Figure 3.9 Thermal conductivity meter HFM-100 47

Figure 4.1 SEM images of aerogel composites with PF/CF ratios (a) 4:1; (b) 2:1; (c) 1:1 48

Figure 4.2 FTIR spectra of PF/CF aerogel composites 49

Figure 4.3 Stress-strain (a) and TGA curves (b) of PF/CF aerogel composites with different fiber ratios 51

Figure 4.4 Thermal conductivity of PF/CF aerogel composites with different fiber ratios 53

Figure 4.5 (a) MB adsorption kinetics of PF/CF aerogel composite with different ratios (b) Effect of contact time on MB absorption by PF/CF aerogel composite 54

Figure 4.6 (a) Adsorption isothermal of PF/CF aerogel composite (b) Effect of initial concentration of MB on absorption by PF/CF aerogel composite 55

Figure 4.7 Isotherms of PF/CF aerogel composite (a) Langmuir; (b) Freundlich 56

Figure 4.8 (a) PF/CF aerogel composite coated MTMS; evaluating 5w30 oil adsorption of hydrophobic PF/CF aerogel composites (b) before, (c) after 2 minutes 57

Figure 4.9 (a) 5w30 oil adsorption capacity of PF/CF aerogel composite and (b) Oil adsorption kinetics of PF/CF aerogel composite with different ratios 58

Figure 4.10 Morphology of silica gels when using various acid concentrations 59

Figure 4.11 FE-SEM images of: (a) silica aerogel, (b) chitosan-silica aerogel composite 61 Figure 4.12 FTIR spectra of silica aerogel and chitosan-silica aerogel composite 62

Figure 4.13 Effect of contact time on MB adsorption 63

Figure 4.14 Effect of initial MB concentration on adsorption of (a) CS00, (b) CS100 65

Figure 4.15 Effect of pH on MB uptake of chitosan-silica aerogel composite CS100 66

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LIST OF TABLES

Table 2.1 Aerogel application 5

Table 2.2 Composition of cotton waste fiber 12

Table 2.3 Chitosan precursor properties from Vietnam Food Joint Stock company 19

Table 2.4 Adsorption performance of chitosan composites in wastewater treatment 20

Table 4.1 Summary of density, porosity, and compressive strength of PF/CF aerogel composites 50

Table 4.2 Kinetics parameters for the MB adsorption of PF/CF aerogel composite 54

Table 4.3 Isotherm parameters for the MB dyes adsorption of PF/CF aerogel composite 56 Table 4.4 MB dyes adsorption of various adsorbents 56

Table 4.5 Kinetics parameters for the oil adsorption of PF/CF aerogel composite 58

Table 4.6 Physical properties of silica aerogel and chitosan-silica aerogel composites 60

Table 4.7 Parameters of two kinetic models 64

Table 4.8 The Langmuir and Freundlich isotherm models 65

One of the most common dyes is methylene blue (MB), which is utilized extensively

in a variety of industries including textile, pharmaceutical, printing, food, paint, and cosmetics After being used in production, a sizable portion of the leftover MB is discharged into sewage, endangering both the environment and human health Under normal circumstances, MB molecules are hard to degrade due to their high stability and toxicity [9] Various methods have been studied in the attempt to the treatment

of contaminated wastewater such as microbiological decomposition, photocatalytic degradation, filtration, oxidation, and adsorption [9,10] Amongst them, adsorption

is the common technique for wastewater purification to remove organic molecules at the industrial scale because of its low cost and ease of usage [10] Aerogels are highly porous materials with distinct properties, such as ultralight weight, small pore size (~ 1-50 nm), high surface area, and strong adsorption Owing intriguing properties making them versatile absorbents for various potential applications including wastewater treatment [11] Although aerogels can be fabricated from many different precursors, most of the aerogels are mainly composed of inorganics and/or petroleum-based organics With quite complicated preparation processes, neither the raw materials nor the prepared aerogels are degradable, and thus further treatment procedures are required for the protection of the environment and economic benefits

On the contrary, bio-based aerogels are widely available and environmentally friendly [12] Cellulose and silica are two outstanding biomasses and have received much attention from researchers to utilize for fabricating aerogel purposes from natural sources

Many studies show that silica aerogel and cellulose aerogel are two potential materials as pollutant adsorbents In addition, cellulose and silica are the most potential renewable biomass resources that exist in large quantities in agricultural wastes such as pineapple, cotton, rice husk ash, etc However, most of the current treatment methods are incineration, disposal, or without reasonable treatment methods, causing waste and environmental pollution Therefore, the study of taking advantage of these two biomass sources to synthesize aerogel not only creates a potential material with high applicability but also proposes a useful solution for the effective treatment of by-products This indirectly reduces the negative impact on the environment

1.2 Research aims and Objectives

The main objectives of this research were studied to utilize agricultural by-products

to produce cellulose aerogel composite and silica aerogel composite for adsorption

applications in wastewater treatment as shown in Figure 1.1 The specific objectives

were:

- Synthesis of chitosan-silica aerogel composite from silica extracted from rice husk ash and shrimp-based chitosan

- Synthesis of cellulose aerogel composite from pineapple leaf fibers and cotton waste fibers

- Utilization of the fabricated materials for adsorption application

- Comparison and evaluation of the efficiency of wastewater treatment

Figure 1.1 Graphical abstract of this study

1.3 Outline of thesis

This work is split into two sections: a theoretical section (which includes a literature review) and an experimental section In chapter 2 Literature Review, the theoretical portion is discussed This section contains a literature analysis of the raw material as well as relevant chemical techniques for understanding the definitions and approaches in this study The experimental part of the thesis is described in chapter

3 Materials and Methods, and discusses the planning of experiments The results from the experiments, and finally, conclusions and future work are presented in chapter 4 Results and Discussions, and chapter 5 Conclusion and Future Work

CHAPTER 2

LITERATURE REVIEW

2.1 Aerogel and aerogel composite

Aerogel is a material with low density, high porous structure, and large specific surface area synthesized from the traditional sol-gel method combined with modern drying methods Aerogels were first synthesized from silica (silica aerogel) in 1931

by S Kistler by replacing the liquid in the agar mass with air without causing structural collapse According to Smirnova and Gurikov, an aerogel is defined as a colloidal system or a macromolecular compound consisting of loose masses bound together by particles or fibers that are distributed throughout the volume by air, so it exhibits very light properties and has a large specific surface area With the properties

of solid material with very small density (0.0030 – 0.15 kg/m3), high porosity (84.0 – 99.9%), and large surface area (1000 – 2000 m2/g), can be hydrophilic or hydrophobic structure, has thermal insulation properties (0.005 W/m.K), good sound resistance, can become transparent (~1.01 - about the same transparency as glass ) [13] With the current development of science and technology, aerogels can be synthesized from carbon (carbon, carbon nanotube, graphene, graphene oxide, ) [14], inorganic (SiO2, TiO2, SnO2, ), organic (PVC, polyurethane, polyimide,

polysaccharides, chitosan, proteins, ) [14,16] and can be modified surface suitable

for different purposes or applications as shown in Figure 2.1

Figure 2.1 Several forms of aerogel have been produced commercially, such as (a) Clear

glass from silica aerogel; (b) Polyurethane aerogel insulation sheet; (c) Silica aerogel

towels; (d) Silica aerogel powder

Aerogel is widely used in many fields of science, technology, and life such as insulators for construction, reactors, pipes, insulators for goods during transportation, clothing, and special protective shoes Aerogel is also used in the aerospace industry

as an adsorbent Because their features have enabled aerogel in becoming a promising material in numerous fields of application including building insulation, optics, energy storage, filtration/separation, space travel (as stardust collectors), etc [17]

Table 2.1 Aerogel application

probes

Mechanical

hypervelocity particle trap

Electrical

electrodes, vacuum display spacers, capacitors

Table 2.1 shows that the aerogel market is seen as having a potential development, as having high growth potential, as lab-scale process breakthroughs described in the scientific literature, show the potential of being translated into industrial-scale processes

The aerogel market according to IDTechEx statistics has reached 638 million USD

in 2020 and is aiming for 1,045 million USD in 2025, with a compound annual growth rate (CAGR) of 10.4% Aerogel is forecasted to grow strongly in the field of construction, but the strongest position is still in the oil and gas sector

Over a century of research and development, silica aerogel is the most widely used material Meanwhile, aerogels made from cellulose not only have more outstanding features but also have abundant, abundant, and environmentally friendly raw materials Therefore, nearly a decade ago, cellulose aerogel began to be studied more widely

Although aerogel has many outstanding properties, with its fragile structure and low compressive strength, it is easy to fracture, which limits the material's practical application [18] Although aerogel particles have good insulation properties, when used in construction, heat can be lost through the pores [19] In addition, the hygroscopicity of the aerogel due to its good polarity also affects the quality of the material under environmental conditions Typically, the insulation properties of silica aerogel will degrade due to moisture absorption or condensation on the surface in high-humidity environments [20] To improve these disadvantages, aerogel composite materials are researched and developed by scientists Aerogel composites are materials that are combined from two or more different materials to create a new material that has superior properties compared to the original materials Therefore, combining aerogels with other materials to form aerogel composites will overcome these limitations, helping to bring this potential material closer to reality Liu et al have synthesized a silica aerogel material with a thermal conductivity of 0.0486 W/m.K [21] However, when silica aerogel is combined with some types of reinforcing fibers, the aerogel composite has a lower thermal conductivity than the original aerogel Specifically, the thermal conductivity of aerogel composite from silica aerogel reinforced by aramid fiber and mineral fiber is 0.0232 W/m.K and 0.025 W/m.K, respectively [22] Most recently, the study of Shang et al used anti-infrared radiation fibers as reinforcement for composite insulation materials from silica aerogel The results show that the thermal conductivity of silica aerogel composite

[19] In addition, commonly used industrial chemicals such as polyvinyl alcohol (PVA) are also used as binders for silica aerogel composite with thermal conductivity

of 0.01892 W/m.K, lower than polyethylene insulation sheet (0.02525 W/m.K) [23] Another study in 2019 used fiberglass as a skeleton for support and shaping, reducing the shrinkage of polyimide aerogels during synthesis, and enhancing the material's mechanical strength and thermal stability The results show that these composites

to 113.5 MPa In addition, the thermal conductivity of the aerogel composite materials is very low from 0.023 to 0.029 W/m.K at room temperature and from 0.057

to 0.082 W/m.K at 500 oC [24] Thus, the addition of other materials to the aerogel to form an aerogel composite has significantly superior performance compared to the original aerogel material

With recent improvements in the synthesis of different types of composite aerogels, the potential application of this material has been and is being studied extensively One of their most common uses is as insulation in the construction industry The thermal conductivity of composite aerogel materials can be divided into three groups: heat transfer through the crystal lattice, through the gas phase, and radiation through

or within pores [25] According to the Knudsen effect, when the pore size in a porous material is close to the mean gas path (70 nm), the thermal conductivity of the material will be reduced because the pores will restrict the movement of the gas and inhibits convection The thermal conductivity of aerogel is usually less than 0.045 W/m.K at room temperature, equivalent to some insulation materials such as mineral wool (0.03 – 0.05 W/m.K), glass fiber (0.04 W/m.K) as well as commercial products such as polyurethane foam (0.026 W/m.K) and polypropylene foam (0.030 W/m.K) [26] Another popular application of composite aerogel is to treat oil spills and oil/water separation due to its good adsorption capacity and high porosity Oil spills occur frequently and the discharge of oil-containing wastewater streams during extraction by industry can cause significant economic and ecological damage Traditional adsorbents, including polypropylene, zeolite, and activated carbon, are commonly used to deal with these problems However, they have the disadvantages

of poor reusability, non-selective oil adsorption, and lack of biodegradability Therefore, with a porous structure, large specific surface area, and low matter density, composite aerogel shows good adsorption capacity for water, oil, and organic solvent The adsorption capacity of composite aerogel is higher than that of traditional adsorbents and twice that of commercial PP products [27] By increasing the surface roughness of the aerogel or by adding substances with low surface energy, the hydrophobicity and lipophilicity of composite aerogels can be improved, thereby greatly enhancing the oil-selective adsorption capacity Methods to increase the hydrophobicity of commonly used materials include chemical vapor deposition

methyltrimethoxysilane (MTMS), methyltrichlorosilane (MTCS), and octadecytrimethoxysilane (OTMS); molecular layer condensation; cold plasma treatment, hydrophobic transformation with isocyanate [28]; surface fluorination [29]

or esterification [30] Once modified, the wettability angle of the aerogel is usually greater than 135o and the adsorption capacity for oils and other organic solvents is typically in the range of 10 – 400 g/g In addition, the material's ability to separate oil from water can also be achieved by creating a hydrophilic rough surface Peng et al synthesized a super hydrophilic aerogel by mixing cellulose and chitosan together After immersing the aerogel in water, the rough surface of the aerogel forms a thin film, and thus it possesses superhydrophilic ability under water This material shows excellent separation of oil-water mixtures through a filter that uses an aerogel as the membrane [31] However, the ability to reuse this material is limited Besides, the applications of aerogels and composite aerogels are increasingly being expanded in many other fields such as catalysis, drug delivery systems, chemical engineering, and the environmental industry Due to its wide range of applications, composite aerogels are increasingly becoming high-performance materials and potential candidates in the twenty-first century

2.2 Cellulose aerogel composite

2.2.1 Pineapple leaf fiber

Agriculture is one of the key and indispensable economic sectors in Vietnam With a GDP growth rate of 3.76% in 2018, Vietnam's export value reached 40.02 billion USD – the highest export figure as of 2018 in which agricultural products are estimated to reach 40.02 billion USD calculate 19.51 billion USD In addition to the growth in the crop, the industry is the emission to the environment, specifically, the total annual production of biomass can reach from 8 to 11 million tons These by-products account for 30-40% of total agricultural production, which is particularly harmful to the environment as they are usually only disposed of by burning or being decomposed and buried These methods produce air pollutant gases such as COx, H2S,

NOx, and aromatic polycyclic hydrocarbons that directly affect the climate, living environment, and human health [32] Therefore, at present, the issue of sustainable development of the industry of renewable materials from agricultural by-products is

being focused on The research is conducted towards the reuse of these by-products

to create materials with high application and use value

Pineapple is a popular crop in the world, accounting for 20% of the total production

of tropical fruit trees, with delicious properties and good for human health In Vietnam, according to the Food and Agriculture Organization of the United Nations (FAO), pineapple production in 2018 was 654,800 tons By-products including unused peel, leafs, seeds, and flesh make up 50% of the total weight of harvested pineapple [33] It is estimated that 1 hectare of pineapple destroyed to replant after two harvests will leave about 50 tons of by-products

In the past few decades, pineapple fiber has been used as a reinforcing agent for polymer composites, low-density polyethylene (LDPE) composites, high biodegradability, and thermal and sound insulation applications, [34] This is possible thanks to the length and excellent properties of the finished pineapple leaf fiber after going through the processes from harvesting the fresh leafs to creating

fibers (Figure 2.2)

Figure 2.2 Raw pineapple fiber production process: (a) Pineapple field; (b) Pineapple tree;

(c) Stripping of fibers from pineapple leaves; (d) Pineapple fiber

The process of creating fiber from pineapple leaves is done by a spinning machine as

illustrated in Figure 2.3 The device has an internal blade unit that scrapes the leaf

sheath, leaving only bundles of fibers between 40 cm and 1 m in length The fiber bundles obtained from the machine will be dried at 40 oC for 48 hours [35]

Figure 2.3 (a) Putting the pineapple leaves into the stripping machine, (b) harvesting the

bundles from the stripping machine and (c) the pineapple leaves after drying

Studies show that the chemical composition of pineapple leaves includes main substances such as cellulose (66.2%), hemicellulose (19.5%), and lignin (4.2%) [36] The quality of the fiber depends on the cellulose content in the composition, which means that the more cellulose, the stronger the fiber On the other hand, pineapple leaves have lower ash content than other leaves, specifically pineapple leaves are 4.5% lower than palm leaves (9.0%) [37] The lignin content in pineapple leaves is only about 4% lower than that of the banana stem (18.6%), oil palm (20.5%), and coconut fiber (32.8%) [38] Because it is mainly composed of cellulose, pineapple

breaking is 2.0 – 2.8% Thanks to this mechanical property, pineapple leaf fiber is used as a fabric in the textile industry, or as reinforcement in polymer composites such as biodegradable plastics and natural rubber [40–42]

2.2.2 Cotton waste fiber

Besides agricultural problems, industrial by-products are also considered urgent to be solved in the current context, especially in the textile industry After the weaving

process, the excess cotton fiber (Figure 2.4) is discarded and considered a by-product

of little or no use

Figure 2.4 Cotton waste fiber

In 2010, in the United States, the textile industry emitted 13.1 million tons of fiber, but only 15% was reused, the rest was discharged into the environment [42] In Vietnam, the textile industry has developed strongly in recent years, the demand for importing cotton for cotton fabric production ranks third in the world with an output

of 1.5 million tons per year Cotton waste fiber is a by-product with a very high reuse value if collected properly Like pineapple leaf fiber, cotton waste fiber also contains

cellulose as the main ingredient that as shown in Table 2.2 Cellulose is a raw

sustainable material for making environmentally friendly and high-value materials - cellulose aerogel

Table 2.2 Composition of cotton waste fiber

In 2020, the development of ecologically friendly materials such as natural materials

or recycled materials (from rubber, synthetic fiber, PE plastic, etc.) is under great interest due to the issues of environmental pollution, urbanism, and most notably, global warming One of the most promising alternatives among them is aerogel,

which is the topic of extensive research and development Due to the cost and technical requirements of large-scale production, aerogels have not yet appeared commonly on the market However, in the near future, this material promises to become more popular and appear widely in all areas of life

2.2.3 Cellulose aerogel

Cellulose is the most abundant biopolymer in the world and is an integral part of the structure of plant cell walls Cellulose is a linear polysaccharide composed of β-(1,4) D-glucose chains [26] Thanks to the intramolecular hydrogen bonding between the hydroxyl and oxygen groups of adjacent sugar molecules, it strengthens the bonds and thus forms a linear structure [43] Because of its natural origin, cellulose contains self-regeneration, biocompatibility, and biodegradability [26] Based on these special

properties, cellulose has been studied as an aerogel composite material (Figure 2.5)

with a specific surface area from 10 to 975 m2/g, porosity up to 99.9%, and density

of about 0.0005 – 0.35 g/cm3 [45–47] However, cellulose aerogel has greater mechanical compressive strength than silica aerogel (5.2 kPa – 16.67 MPa) and biodegradability [44] Similar to traditional silica aerogel, cellulose aerogel materials have many applications such as insulation in construction because of their poor thermal conductivity down to 0.018 W/m.K [48,49]; carrier in drug delivery [49]; oil spill treatment, oil/water emulsion separation and dye adsorption in wastewater [50]–[52]

Figure 2.5 Cellulose aerogel from bagasse (a), wastepaper (b), and coir (c)

Pineapple fiber is one of the potential raw materials to make cellulose aerogel because

it is an agricultural by-product that accounts for a very large amount in Vietnam Pineapple accounts for cellulose content from 70 – 82%, small density (1.07 – 1.53

g/cm3), good mechanical strength with tension from 413 - 627 GN/m2, compression modulus from 34.50 – 82.52 GN/m2 [32] Thereby, it is suitable for creating pineapple

fiber aerogel (PF aerogel) material as shown in Figure 2.6a Besides, cotton waste

fiber (85% cellulose, density from 1.50 – 1.54 g/cm3) discharged from the textile industry is also a renewable source of raw materials to make use of to produce cotton

waste fiber aerogel (CF aerogel) as shown in Figure 2.6b [53]

Figure 2.6 (a) PF aerogel; (b) CF aerogel

2.3 Silica-based aerogel composite

2.3.1 Silica aerogel

Silica is another name for the chemical compound silicon dioxide Each unit of silica

occurs in crystalline, amorphous, and impure forms (as in quartz, opal, and sand respectively) Silica has very good physical, mechanical and thermal stability and can

be easily functionalized due to its hydroxyl groups In particular, mesoporous silica

is a fascinating material, which first gained prominence in the 1990s with a regular mesostructure, uniform pore distribution and tunable pore sizes, and very high specific surface areas, combined with thermal and mechanical stability [54] It is attracting considerable interest as an adsorbent material [55] These materials can be formed by a simple sol-gel synthesis route comprising hydrolysis, condensation, and polycondensation reactions using various templates or surfactant molecules [56]

To pursue the massive application of silica aerogel, a low-cost and green synthesis pathway is the prior task Conventional production methods using organic silica sources, like tetraethoxysilane (TEOS), polyethoxydisiloxane (PEDS), or

methltrimethoxysilane (MTMS) are generally expensive and hard to be applied to extensive industrial production of silica aerogel [57] For massive industrial processes, abundant biomass wastes like rice husks, bamboo, and wheat husks were gradually considered promising silicon sources

Figure 2.7 Extraction silica aerogel from rice husk [58]

Especially, rice husk ash is rich in amorphous silica, containing 90% SiO2, and it can

easily react with sodium hydroxide solution to form water glass (Figure 2.7) After

the sol-gel process, supercritical drying is usually conducted to obtain dry products However, it is highly energy-consuming and unsafe, it is desirable to prepare silica aerogels at ambient pressure drying and minimize the capillary stress by solvent exchange and surface modification [58] For solvent exchange, anhydrous ethanol and n-hexane are chosen due to their low surface tension There is a great deal of -

OH groups on the surface of wet gel which will incur dehydration at the ambient pressure, causing damage to the structure of silica aerogel Surface modification by

(MTMS) and n-hexane is a practical method [57] to avoid hydration In summary, the present study proposes to synthesis silica aerogel by processing the rice husk ash in water glass preparation, ion exchange, sol-gel, solution exchange, surface modification, and ambient pressure drying

Silica aerogel has a translucent blue color (Figure 2.8), commonly referred to as blue

smoke Silica aerogel is a lightweight, highly porous solid material with specific gravity as low as 0.025 g/cm3, low thermal conductivity at approximately 0.013 W/m.K, and very high surface area (up to 1000 m2/g) Due to this, many studies have been done on the synthesis of silica aerogel and to apply it on catalytic supports, absorbent of pollutants, thermal insulation materials of buildings, and drug carrier materials Hekun Han et al [59] prepared hydrophobic silica aerogel and hydrophilic silica aerogel from TEOS, which exhibited high adsorption capacity for Methylene Blue and Rhodamine B, respectively

Figure 2.8 Silica Aerogel with blue smoke color

For a long time, silica aerogel, as well as inorganic and carbon aerogel are commercially available The main drawback of silica aerogels is their brittleness One

of the ways to improve aerogel brittleness is the use of fibers, alumina fibers, and alumina borosilicate fibers [60] Consequently, to solve this problem, an effective supporting skeleton is necessary

2.3.2 Chitosan

Chitosan is a natural linear polysaccharide derived from the partial deacetylation of

chitin (Figure 2.9) - a natural compound in the exoskeleton of crustaceans, such as

shrimps, and crabs The abundance of hydroxyl groups and highly reactive amino groups in chitosan with a strong tendency for intra and inter-molecular hydrogen bonding results in the formation of linear aggregates and rigid crystalline domains However, chitosan is usually less crystalline than chitin, consequently, more soluble

The chitosan structure has one primary amine and two free hydroxyl groups for each monomer [61]

Figure 2.9 Process of obtaining chitosan by the deacetylation alkaline treatment of chitin

from shrimp shell wastes [61]

The factors which attract scientists of chitosan are biocompatible and using of chitosan as a precursor to prepare the final aerogels are summarized as follows: (i) Chitosan which is the second most abundant renewable biopolymer after cellulose, can be extracted from diverse organisms;

(ii) The existence of these free amino (-NH2) groups and hydroxyl (-OH) groups active groups allows the adsorption of other pollutants, such as phenol, antibiotics, and pesticides

(iii) Chitosan is a kind of unique polysaccharide, with many favorable characteristics, including biocompatibility, biodegradability, nontoxicity, and chemical activity [62]

Figure 2.10 Chemical structure of Chitin and Chitosan

The main property of chitosan is summarised in Table 2.3 Because of these

outstanding properties, chitosan can be selected as a useful and effective polymer matrix in composite materials Moreover, its hydroxyl and amino groups are beneficial for the formation of hydrogen-bond interactions and homogeneous phases

in composite structures; these provide chitosan with great advantages as a skeleton material [63] In addition, some of the intrinsic properties of chitosan, such as its polycationic character in acid media, its ability to form hydrogen bonds, Van der Walls, and electrostatic interactions, make it an efficient adsorbent material [61] Besides that, chitosan biopolymer is a very much attractive material that is considered

an excellent adsorbent owing to its nitrogen and oxygen richness A significant number of publications confirmed the effectiveness of the functionalization of chitosan with silica particles to treat contaminated waters [64] Antonio et al, [65] described the synthesis of silica/chitosan for the adsorption of anionic dyes Zhao et al.,[66] prepared porous chitosan/silica hybrid microspheres and studied their performance for the removal of copper ions Recent developments, D Alves et al [86] indicated that chitosan is a good-based absorbent for the removal of pollutants from aqueous environments, his point of view is chitosan can combine with activated

carbon, graphene, silica, and other inorganic absorbent material Table 2.3

presented some typical physical properties of chitosan, which is supplied by the Vietnam Food Joint Stock company

Table 2.3 Chitosan precursor properties from Vietnam Food Joint Stock company

2.3.3 Chitosan-silica aerogel composite

Chitosan-silica aerogel composites have been formed using a variety of methods which can be grouped into two main approaches, comprising silica-supported chitosan, where the chitosan is coated or adsorbed onto the silica support, and secondly, a chitosan-silica hybrid that is fabricated using the sol-gel methodology Sol-gel synthesis is used commonly to form a chitosan-silica hybrid layer on silica bead/particle supports [67]

Several reports have focused on SiO2 as a bead, particle, nanoparticle, or powder, where the SiO2 particles are added to the chitosan solution phase to give a chitosan-coated particle [68] The SiO2 particles can also be functionalized with amine and carboxylic groups to give more efficient binding with the chitosan [69] Silica layers have also been added to previously formed chitosan-based beads to give organic-inorganic (chitosan-silica) layered structures, with greater stability [69] and sol-gel synthesis has been employed to immobilize chitosan onto silica particles [6]

For example, Xu et al [70] covalently linked chitosan with an epoxide-containing siloxane through the sol-gel process to give a hybrid chitosan layer on silica particles

Blachnio et al [71], concluded that the adsorbed chitosan had a higher adsorption capacity for dye molecules, although the chitosan-silica aerogel composite fabricated using the sol-gel synthesis had a high surface area of 600 m2 g−1

Figure 2.11 Schematic for the chemical structure of silica-chitosan hybrid scaffolds

The cross-linking method can be employed to decorate mesoporous silica with chitosan Cross-linking agents, such as glutaraldehyde, formaldehyde, and epoxides [72], can be used In general, the surface area, pore size, and volume of the mesoporous silica are reduced as higher amounts of chitosan are added and partially fill the pores However, these chitosan and mesoporous silica composites possess good surface areas with a high density of functional groups and with the potential to give magnetic separation

The CS/silica composites are only emerging as potential adsorbents and compared with the chitosan-carbon-based systems, there are much fewer reports focused on the removal of dyes and organic molecules with these adsorbents This may be due in part to the hydrophilic silanol groups, which easily form hydrogen bonds with water, thus limiting the adsorption process However, the mesoporous silica surfaces can be functionalized, and this provides the opportunity to design more hydrophobic surfaces that can be tailored to adsorb organic molecules [61] Indeed, there is clear

evidence in Table 2.4 that the CS/silica composites can be employed in the removal

of dyes

Table 2.4 Adsorption performance of chitosan composites in wastewater treatment

Capacity

Isotherm model

Ref

CS/silica/ZnO MB 7.0 293 Langmuir [73]

CS/silica/PVA Direct Red

80

Nevertheless, these based materials, and especially the emerging silica-based composites, have a promising future as adsorbent materials

chitosan-2.4 Synthesis method of aerogel

The preparation of aerogel mostly consists of two main critical steps: (1) synthesis of

a wet gel through a wet chemical synthesis approach, mainly the sol-gel technique,

to make a three-dimensional gel body, and (2) an appropriate drying approach to turn the obtained wet gel into a solid material with almost the same dimensions of the initial wet gels [76]

 Sol-gel process

According to IUPAC, a sol-gel process is defined as a structural network formed from

a solution that transforms into a colloidal dispersion (sol) and then forms a flocculation system or polymer network (gel) Sol-gel methods can include solvent dissolution, polymerization, coagulation, and cross-linking (chemical and physical) [15] Currently, the sol-gel method is a widely used technique to create bulk materials, thin films with nanostructured, high-fineness powders, or filaments with a polycrystalline or amorphous structure The advantage of this method is that it is inexpensive, can produce many materials and the materials that can be fabricated are very diverse (inorganic, organic, metal materials) Several studies reported that the structure and properties of hybrid materials prepared by the sol-gel process could be altered and controlled by different parameters, such as reaction pH, catalyst, type and content of alkoxide precursor, temperature, and reaction time Compared to other methods, the sol-gel method can control the properties of the gel, thus controlling the properties of the product But the downside is that the efficiency is not high, and the compounds that can bind to water molecules are not much [77]

 Drying

Drying is the process of applying heat to evaporate moisture out of a solid or liquid material To reduce the volume of materials, increase the durability of materials,

preserving well for a long time, There is some popularity of drying methods: such

as ambient pressure drying (APD), supercritical drying (SCD), and freeze-drying

Supercritical drying

Supercritical drying (SCD) is a process by which the liquid in a substance is transformed into gas in the absence of surface tension and capillary stress, thus avoiding structural collapse in aerogels Organic solvents often have a critical point

at very high pressures and temperatures that are explosive, while CO2 has a suitable critical point (304 K, 7.4 MPa) safe and low cost Therefore, supercritical CO2 is most used as a solvent to transform gels into aerogels However, the supercritical drying method requires very expensive machinery and equipment and requires compressed air at high pressure

Figure 2.12 A Typical Phase Diagram for a Substance That Exhibits Three Phases—

Solid, Liquid, and Gas

Before drying, the hydrogel will be completely frozen The formation of pores inside the aerogel depends a lot on the freezing stage, time, or temperature, the structure and size of the pores also change According to some studies, cooling with liquid nitrogen helps liquid crystals to freeze faster, avoiding the phenomenon of agglomeration into large crystals This makes the pore distribution of the cellulose aerogel more orderly and has a smoother surface [79] A porous structure with many pores is one of the important factors for cellulose aerogel materials to be applied in many different fields

Ambient pressure drying

Ambient pressure drying is safer and less expensive than the supercritical drying process In this method, drying of the silica gel starts with the warming of the material Following the warming, evaporated solvent from the silica gel balances with the volume loss of the gel After partially flowing solvent through the pores, the solvent is removed by vapor diffusion to the material surface The material is termed silica aerogel at the end of the ambient pressure drying process The strong capillary force caused by liquid-gas surface tension and liquid-solid adhesive forces within the small pores of the gel, tend to destroy the delicate solid structure, leading to pore collapse and densification However, these problems can be overcome easily with the help of some surface modification

2.4.1 Synthesis of cellulose aerogel composite

The preferred raw materials for the synthesis of cellulose aerogel are perennial crops (wood), agricultural-industrial by-products such as rice husk, bagasse, pandan fiber, wood pulp, cotton, etc Depending on the properties of the material such as the length

of the cellulose chain, density, size, thermal properties, etc., different material properties will be produced [44] Cellulose aerogel is prepared in three main steps: raw material treatment (alkylation and bleaching to remove lignin in fibers), the sol-gel process including dispersing cellulose into solution (forming sol system), exchange solvent change or aging (gel formation), and finally drying (removal of

water) as shown in Figure 2.13 Which, two decisive stages are the sol-gel process

and drying

Figure 2.13 General procedure for the synthesis of cellulose aerogel

Finding the right solvent for dispersing cellulose has become a challenge in science [80] Many solvents have been proposed such as N2O4/N,N-dimethylformamide

LiCl/N,N-dimethylacet-amide (DMAc) [84], N-methyl-morpholine-N-oxide (NMMO) [85], However, these solvents are often volatile, toxic, and expensive, so research is still limited Zhang et al [54,82] found that NaOH/Urea or NaOH/Thiourea solutions can both disperse cellulose directly and rapidly Moreover, the ability to disperse cellulose in solution is long-lasting (with less sedimentation) [86]

hydrate layer with a lot of water molecules, this facilitates water molecules to enter

inside and causes cellulose molecules to swell like bubbles (Figure 2.14) This

swelling is the first step of the dispersion process Meanwhile, with the previously

fiber structure but have not been able to break the hydrogen bonds in this cellulose network [87] The addition of urea increases the “quality” of the solvent, allowing maximum swelling to break the bubbles, promoting the dissolution of the cellulose fibers, and preventing re-bonding [88] In addition, the concentration of NaOH and the temperature of the solution are also important factors affecting the swelling of cellulose It has been demonstrated that a solution with a NaOH concentration of 7 – 8% and a temperature range of -5 to 0 °C is appropriate for the efficient solubilization

of cellulose fibers [89] After creating the sol system, the system is gelled with ethanol

or annealed to remove the solvate shells generated by the NaOH/Urea/H2O system, promoting the gelation process between the cellulose fibers [87,92]

Figure 2.14 Cellulose fibers in NaOH/Urea/H2O solution The fiber part is not swollen

(A), the part is swollen like a bubble (B)

2.4.2 Synthesis of silica-based aerogel composite

Sol-gel

The process of forming gels, or gelation, from sol, is called sol-gel, which involves free-moving colloidal particles (sol) aggregating into large chains making the solution viscous enough to form a gel structure When pouring sol into the mold, due

to the conversion from sol to wet gel, wet gel takes the shape of the mold

Silica sol

Silica sol is SiO2 colloidal particles dispersed in solution It is usually prepared by a reaction between silicate solution and acid, at pH > 8 The silica sol particles are spherical, and discrete, with a diameter of about 4 – 60 nm Specific surface area is usually ranged from 50 – 70 m2g-1 The color of silica sol depends on the particle size and concentration of SiO2: if the particle size is large and the high concentration, silica sol is opaque the medium particle size is opalescent and it almost presents transparent with small size particles [91]

Silica gel

The colloidal sol particles combine and form a three-dimensional network as shown

in Figure 2.15 Silica gel is usually prepared by reacting silicate solution with acids

at pH ~ 4 Depending on the surface tension, the nature of the liquid, the capillary pressure, and the rate of moisture escape will affect the gel’s volume of shrinkage after drying If the gel approaches supercritical drying, the shrinkage will be negligible, yielding a special gel called an aerogel [92] When the sol is in gelation, the solution becomes viscous, the viscosity increases, and gradually hardens The sol particles bind together into a three-dimensional network, retaining the liquid in the capillaries

Figure 2.15 Sol-gel and gel-sol process

Gelation is induced by various factors including concentration and change in pH [93]

It is based on hydrolysis and condensation The sol-gel process consists of two phases [94]:

 In the first stage, the monomers are dispersed in solvents Sol is a suspension containing particles about 1 - 1000 nm in diameter dispersed in

an aqueous environment This process involves converting monomers into

a colloidal solution (sol) that serves as a precursor to an integrated lattice (or gel) of one of the discrete particles or lattice polymers

 The second stage is the gelation process This is the process by which a freely moving sol is converted into a 3D solid grid surrounded by a solvent environment [64] The parameters that mainly affect the sol-gel process are

pH, catalyst concentration, temperature, time, etc The starting point of gelation is usually determined by a sudden increase in viscosity The gel