PREFACE
Study background
Environmental pollution, particularly water pollution, is currently a major global issue imposing a serious threat to both human and ecosystem health According to statistics, water pollution is responsible for roughly 14.000 fatalities each day [1] Amongst various pollution causes, organic pollutants comprising soluble dyes and insoluble oils are the main contaminants in wastewater [1,2] The amount of these pollutants has been significantly increasing due to an increase in industrial activities and oil spill incidents [3] Oily wastewater devastates both aquatic and terrestrial lifeforms in marine areas and has even detrimental economic impacts on tourism and fishing Interaction with spilled oils has not only killed innumerable aquatic organisms (such as sea birds, mammals, algae, and so on) but the sand on seashore beaches has also been badly polluted [4,5] Additionally, the presence of dye in effluents is a major environmental concern due to its high concentration in wastewater and low biodegradability in aquatic ecosystems [6,7] The toxic effects of dye- contaminated water relate to the risk of dyes degrading into carcinogenic amine substances whilst the dark hue of certain dyes interferes with the photosynthetic activity of marine plants [8]
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.
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
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.
LITERATURE REVIEW
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/m 3 ), high porosity (84.0 – 99.9%), and large surface area (1000 – 2000 m 2 /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, polystyrene, ) [13,15], natural substances (cellulose, hemicellulose, 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]
Architectural and appliance insulation, portable coolers, transport vehicles, pipes, skylights
Catalysts, sorbents, sensors, fuel storage, ion exchange
Targets for ICF, X-ray lasers
Lightweight optics, light guides, special effect optics
lowest sound speed impedance matches for transducers, range finders, speakers Mechanical
lightweight energy absorber, hypervelocity particle trap
spacers for vacuum 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 can reach about 0.0191 W/m.K at room temperature and 0.0489 W/m.K at 500 o C [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 have a low density of 0.143 – 0.177 g/cm 3 and Young's modulus increased from 42.7 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 o C [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 (CVD) using chemicals such as trimethylchlorosilane (TMCS), 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.
Cellulose aerogel composite
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 o C 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 leaf fiber has good compressive strength (362 – 748 MN/m 2 ), and elongation before 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]
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
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
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 m 2 /g, porosity up to 99.9%, and density of about 0.0005 – 0.35 g/cm 3 [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/cm 3 ), good mechanical strength with tension from 413 - 627 GN/m 2 , compression modulus from 34.50 – 82.52 GN/m 2 [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/cm 3 ) 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].
Silica-based aerogel composite
Silica is another name for the chemical compound silicon dioxide Each unit of silica includes one atom of silicon and two atoms of oxygen The dioxide of silicon SiO2 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 replacing H in Si-OH with -CH3 using a mixed solution of Methyltrimethoxy silane (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/cm 3 , low thermal conductivity at approximately 0.013 W/m.K, and very high surface area (up to 1000 m 2 /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
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
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 m 2 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
CS/silica Acid red 88 7.0 25 Langmuir [6]
CS/silica/ZnO MB 7.0 293 Langmuir [73]
CS/silica/PVA Direct Red
CS/silica Congo Red - 150 Langmuir [75]
Nevertheless, these chitosan-based materials, and especially the emerging chitosan- silica-based composites, have a promising future as adsorbent materials.
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]
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 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 (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
Freeze drying is the simplest drying method to remove water from hydrogels Sublimation is the process by which a substance changes from a solid to a vapor without passing through the liquid phase on that substance's phase diagram (Figure 2.12) [78]
Figure 2.12 A Typical Phase Diagram for a Substance That Exhibits Three Phases—
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 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 (DMF) [81], SO2/amin [82], Me2SO/araformaldehyde [83], 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]
When dispersed in NaOH/Urea/H2O solution, at low-temperature NaOH forms a 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 studied NaOH/H2O system, water molecules can only penetrate inside the cellulose 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
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 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 m 2 g -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]
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 mechanical state is often very dependent on the number of bonds in the network [95]
Two reactions used to describe the sol-gel process are hydrolysis (Eq.1) and condensation (Eq.2 and Eq.3)
Si-(OR)4 + H2O → HO-Si-(OR)3 + ROH, R=alkyl group (Eq.1)
(RO)3-Si-OH + HO-Si-(OR)3 → (RO)3-Si-O-Si-(OR)3 + H2O (Eq.2) (RO)3-Si-OH + RO-Si-(OR)3 → (RO)3-Si-O-Si-(OR)3 + ROH (Eq.3)
Application of aerogel in wastewater treatment
The attractive property of aerogel is that the adsorbents can be facilely separated from the sample solution Some treatment methods like biological treatment, chemical oxidation, ozone treatment, ion exchange, photocatalysis, and adsorption have been widely investigated in the process of removing dyes from an aqueous solution Compared with other methods, adsorption is the most used method due to its simplicity, rapidity, and high efficiency in the removal of dyes from aqueous solutions Aerogels as a new type of nanometer material, a low-density solid, low cost, high porosity, and large specific surface area, is becoming the ideal choice of adsorbent In general, the adsorption capacity of the adsorbent mainly depends on its surface properties including surface areas and surface functional group
Magdalena Blachnio et al [71] used Chitosan–Silica Hybrid Composites for the Removal of Sulfonated Azo Dyes from Aqueous Solutions H.Han et al [59] showed that hydrophobic/hydrophilic silica aerogel can be used to remove cationic dyes from an aqueous solution Niyaz Mohammad Mahmoodi et al [104] proved that Amine- functionalized silica nanoparticles can remove anionic dyes Tetyana M Budnyak et al [105] synthesized successfully chitosan-silica nanocomposite and then applied it to the adsorption of microquantities of V(V), Mo(VI), and Cr(VI) oxoanions from the industrial wastewater M.Jabli [106] evaluated the adsorption efficiency of cationic and anionic dyes of the functional silica-immobilized chitosan biopolymer
The effluents from textile, leather, food processing, dyeing, cosmetics, paper, and dye manufacturing industries are important sources of MB pollution [107] It is difficult to remove the dyes from the effluent because dyes are not easily degradable and are generally not removed from wastewater by conventional wastewater systems Therefore, color removal was extensively studied with physical-chemical methods such as coagulation, ultra-filtration, electrochemical adsorption, and photo-oxidation [108]
Methylene Blue is a frequently-used cationic dye, with high solubility, stability, and persistence in water, environmental problems arising from this aroused people's attention In particular, Methylene blue is a colorful organic chloride salt compound
It is known as Methylthioninium chloride or Swiss blue
Figure 2.19 Chemical structure of MB
The chemical structure MB solution is shown in Figure 2.19, represented by the formula C16H18ClN3S, and is typified as a cationic dye and it’s a molecular weight of 319.85 g/mol At room temperature, it appears as a solid, odorless, dark green powder But MB solutions is deep blue MB’s melting point is from 100 to 110 ⁰C (with decomposition) About the solubility of MB, it is soluble in water, ethanol glacial acetic acid, glycerol, and chloroform; slightly soluble in pyridine; but that compound is insoluble in ethyl ether, xylene, and oleic acid Additionally, MB gets decomposed while heating and emits toxic fumes of nitrogen oxides, sulfur dioxides, and chlorides, that makes a bad effect on the natural environment It was chosen in this study because of its known strong adsorption onto solids The dye is not regarded as acutely toxic, but it can have various harmful effects.
MATERIALS AND METHODS
Cellulose aerogel composite
PFs with a cellulose content of 75.2% were purchased from Conifer Handmades Company, India Tensile strength and Young’s modulus of PFs are respectively 29.8 and 981 MPa according to the supplier CFs gathered from local textile companies after carding have a high cellulose content (85%), a length of 21.78 mm, a maximum ash level of 1.8%, insoluble matter content of 4.0 – 6.0%, the moisture of 8.0%, and specific strength of 23 g.tex −1 Commercial 5w30 motor oil (ExxonMobil Corporation, USA) was bought from the local market Sodium hydroxide (NaOH, 96.0%), ethanol (C2H5OH, 99.5%), urea (H2NCONH2, 99.0%), methylene blue (MB) trihydrate (C16H18ClN3S.3H2O, 98.0%) were purchased from Xilong Scientific Co., Ltd, China Methyltrimethoxysilane (MTMS, 97%) from Alfa Aesar All solutions were prepared in distilled water
3.1.2 Synthesis of PF/CF aerogel composites
NaOH/Urea/H2O solution was firstly prepared in a mass ratio of 7:12:81 at room temperature PFs were milled to a fine powder (ca 100 àm) before being mixed with CFs in a different solid mass proportion of 1:1, 2:1, and 4:1, respectively Each mixture of PFs and CFs was then dispersed into the NaOH/Urea/H2O solution, refrigerated to 0°C for 15 minutes, and magnetically stirred to achieve a homogeneous dispersion containing the total fiber content of 4 wt% The system was frozen for 24 hours and then defrosted to create strong hydrogen bonding between the fibers The suspension was poured into a cylinder mold and incubated with 60°C ethanol to promote gelation before being washed with DW to remove excessive NaOH and urea Finally, the whole mixture was frozen at -50°C for 4 hours and freeze-dried for roughly 50 hours to yield PF/CF aerogel composites The hydrophobic aerogel composites were obtained via a simple chemical vapor deposition with MTMS as described in previous work [80]
3.1.3 Study on oil removal of hydrophobic PF/CF aerogel composites
In this study, 5w30 motor oil is used for evaluating oil removal of the aerogel composites The initial mass mo of the sample is firstly weighed by a 4-digit analytical balance The sample is then placed in the beaker containing 5w30 motor oil for more than 1 min The material sample is submerged in a beaker of 5w30 motor oil for 1 hour to guarantee that the most oil could penetrate the pore structure of the sample to evaluate the maximum adsorption Finally, the sample is placed on a sieve, allowed to drain for 2 minutes, and then the adsorbed mass is reweighed The experiment is repeated three times to determine the average value, which is the maximum oil adsorption capacity of the aerogel composite
Oil holding capacity is measured at different times 5, 10, 15, 20, 30, 40, and 50 (seconds) to identify the most suitable adsorption kinetic model for the aerogel composite
The adsorption capacity of each sample at the time t is calculated using the following
Where m t (g) is the composite weight at the time t, and mo (g) is the composite weight at the initial time
In the adsorption mechanism, kinetic prediction of the adsorption capacity and adsorption time play an important role Kinetic data are analyzed using pseudo-first order and pseudo-second order models to characterize the dependency of the oil adsorption on time
The pseudo-first order model can be expressed as Eq (2):
Where q e and q t (g.g -1 ) are the amounts of 5w30 oil (g.g -1 ) adsorbed on adsorbent at equilibrium and various time t (sec), respectively k 1 (sec -1 ) is the kinetic rate constant of the pseudo-first-order model of adsorption
The pseudo-second-order model is another kinetic model that includes several types of adsorptions such as external film diffusion, adsorption, and internal particle diffusion equation can be given as Eq (3):
Where q e and q t (g.g -1 ) are the amounts of 5w30 oil (g.g -1 ) adsorbed on adsorbent at equilibrium and various time t (sec) k 2 (g g -1 sec -1 ) represents the rate constant of the pseudo-second order model of adsorption
3.1.4 Study on MB removal of PF/CF aerogel composites
To process the experiment, 0.1 g of solid MB is dissolved in distilled water in a 1000 mL cylinder to create a 1000 ppm MB standard sample solution The standard sample is diluted into MB solutions with concentrations of 25, 50, 75, 100, and 125 (ppm) for kinetic and isothermal adsorption studies, respectively
PF/CF aerogel composites with different proportions of fibers (PC41, PC21, PC11) are weighed to 0.05 g, placed in 25 mL of MB color solution at room temperature, and shaken well under time conditions After adsorption, the sample is removed, and UV−VIS analyzes the MB color solution adsorption at 640 nm; the result obtained is the solution’s absorbance Each composition is performed three times to determine the average value and standard deviation
The adsorption capacity at the time t (q t , mg.g -1 ) is calculated by Eq (4):
Where C o , C t (ppm) are the concentrations of MB in the initial solution and at the time t, respectively; V (L) is the volume of solution, and m (g) is the initial aerogel composite mass
To conduct a kinetic adsorption experiment, the investigation time for the experiment is 15, 20, 25, 30, 45, 60, and 75 (min) with MB color solution with an initial concentration of 50 ppm The adsorption parameters of the research experiment according to the first and second−order apparent kinetic models of adsorption are the same as those of the oil adsorption experiment Experiment for isothermal studies with MB color solution starting concentrations of 25, 50, 75, 100, and 125 (ppm)
Finally, the removal efficiency of MB (H%) refers to the total amount of adsorbed concerning the total initial concentration of adsorbent is calculated according to Eq (5):
Where C o and C e refer to the initial and equilibrium concentration of MB (ppm), respectively
The kinetics study of MB adsorption is done by using the pseudo-first-order model and pseudo-second-order model followed by Eq (2) and Eq (3), as mentioned above
The Langmuir isotherm model is suitable for the monolayer adsorption occurring on a homogeneous surface without subsequent interaction among the adsorbed species The model can be expressed as Eq (6) [109]:
Where K L (L.mg −1 ) is the Langmuir constant q e , q m (mg.g −1 ) is the equilibrium adsorption and the maximum adsorption capacity of adsorbate, respectively C e (ppm) is the equilibrium concentration of the adsorbate
The constant K L characterizes the interaction force between the adsorbent and the adsorbate at a certain temperature, or in other words, K L describes the adsorption center’s selectivity Construct a graph C e /q e based on Ce to determine the adsorption constants K L and R 2 as well as the maximum adsorption q m From the K L value, it is possible to assess whether the adsorption process is suitable or not in the investigated concentration range of the adsorbent through the equilibrium parameter R L determined as Eq (7) [110]:
Where R L is the equilibrium parameter, K L is the Langmuir constant, and C o (ppm) is the initial concentration of the adsorbate
The Freundlich isotherm model can be described as Eq (8) [111]: e e F lnq = lnK +lnC n (8)
Where K F is the Freundlich adsorption constant, and n is the empirical constant, indicating the quantity that evaluates the degree of heterogeneity of the adsorbent surface and describing the appropriateness of adsorbent molecule distribution on the surface of the adsorbent A value of n higher than 1 implies that molecules are adsorbing well onto the adsorbent surface The higher the value n, the higher the adsorption strength Construct a graph of lnq e dependent on lnC e , thereby determining
R 2 , the Freundlich constant K F, and the empirical constant n.
Silica-based aerogel composite
Rice husk ash (RHA) having a silica content of 85.7 wt% was collected from the local incinerator in Vietnam Chitosan powder (degree of deacetylation 75.0%, viscosity
150 – 500 cP) was purchased from Vietnam Food Joint Stock Company All reagents purchased from commercial suppliers include sodium hydroxide (NaOH), sulfuric acid (H2SO4), hydrochloric acid (HCl), ethanol, n-hexane, methyltrimethoxysilane (MTMS), acetic acid (CH3COOH), and methylene blue trihydrate
Figure 3.1 The procedure of synthesis of silica aerogel
The amorphous silica was extracted by mixing RHA with NaOH solution (RHA and NaOH ratio of 5:3 w/w) at 90°C for 2 h under stirring After cooling to room temperature, the residue was removed by vacuum filtration to obtain sodium silicate (Na2SiO3) The silicate solution was then poured into the mold, followed by the addition of HCl at a low feeding rate of 2.0 mL/min The mixture was shaken gently to produce a homogenous silica gel After feeding HCl, the gel was aged for 24 h at room temperature to strengthen its structure Before drying, the solvent in the silica gel should be replaced with a non-polar solvent to reduce the capillary stress The aged gel was then immersed in ethanol for 24 h to exchange water with ethanol The gel after the 1st solvent exchange was subsequently exchanged with n-hexane by the same procedure To enhance the hydrophobicity of the gel, a surface modification was performed by soaking the gel in MTMS/n-hexane solution (a volumetric ratio of 1:10 mL/mL) at room temperature for the next 24 h Finally, the hydrophobic silica aerogel was produced from the wet gel by drying it under ambient pressure at different temperatures of 60, 80, 120, and 150°C for 3, 3, 2, and 2 h, respectively
3.2.3 Synthesis of chitosan-silica aerogel composites
Figure 3.2 The procedure of chitosan-silica aerogel composite
Chitosan-silica aerogel composites were synthesized similarly to silica aerogels but with several additional steps Chitosan was dissolved in 6% CH3COOH at 50 °C, stirred at 500 rpm for 3 h, and then filtered to get a viscous, pale yellow chitosan solution The chitosan solution was mixed with the sodium silicate for 5 min, followed by the addition of HCl to produce a chitosan-silica gel The heat was then applied at 60 °C for 5 min to promote the hydrolysis and condensation of silica After aging for 24 h at room temperature, the chitosan-silica gel was solvent exchanged with ethanol, n-hexane, and surface modified with n-hexane/MTMS before drying at 80°C to fabricate silica-chitosan aerogel composite
3.2.4 Study on MB cationic dye removal of silica-based porous material
Adsorption experiments were carried out by adding 5 mg of sample into 25 mL MB solution at room temperature After a fixed adsorption time, the sample was taken and the remaining amount of MB in the solution was determined The adsorption capacity, q t (mg/g), and removal efficiency, H (%) of the adsorbent were calculated based on Eqs (9) and (10), respectively The effects of three parameters pH (3 − 9), time (0 − 150 min), and initial MB concentration (0 − 125 mg/L) on the adsorption capacity of each sample were studied
Where C o and C e (mg/L) are the initial and final MB concentrations V (L) is the volume of MB solution (25 mL), and m (g) is the weight of the adsorbent before adsorption
Each sample was exposed to the same MB aqueous solution having fixed conditions of initial MB concentration 50 mg/L, pH 7 The MB adsorption capacity of the investigated sample at the time of 15, 30, 60, 90, 120, and 150 min was determined and used as experimental data for studying a suitable adsorption kinetics model The applied linearized kinetic models, namely the pseudo-first-order and pseudo-second- order models, are expressed based on the following Eqs (11) and (12), respectively
Where q e and q t (mg/g) are the adsorption capacities of adsorbent at equilibrium and at the time t; k 1 and k 2 represent the kinetic rate constants for pseudo-first-order and pseudo-second-order models
The Langmuir and Freundlich isotherm models are chosen to study the maximum adsorption capacity of silica aerogels and chitosan-silica aerogel composites over the initial MB concentration The two isotherm models are expressed in Eqs (13) and (14), where the equilibrium constants k L , k F are fitted for Langmuir and Freundlich models; q max (mg/g) is the maximum adsorption capacity of adsorbent for MB max max e e 1 e L
Ultraviolet-visible (UV−VIS) spectrometer Shimadzu UV−1800 is utilized to analyze the absorption of light intensity of incoming beam through a sample solution placed in a cuvette The wavelength of the incoming beam is 640 nm corresponding to the color wavelength of MB.
Characterization
The bulk density of each aerogel composite is determined via its weight and volume Its porosity (φ) is calculated based on its bulk density (ρ a ) and the average density of components (ρ b ) as indicated by Eq (15) [42], [112]:
(15) where b is determined by using Eq (16) or Eq (17) [112]:
(17) where ρ PF = 1.07 g.cm -3 and ρ CF = 1.56 g.cm -3 are the density of PF and CF, respectively C PF and C CF are the mass concentration of PF and CF, respectively Additionally, ρ CS = 1.01 g.cm -3 and ρ S = 2.33 g.cm -3 are the density of chitosan and silica, respectively The mass concentration of chitosan and silica, respectively, are
Scanning electron microscopy is a useful tool for observing the surface morphology of samples The surface morphology of aerogels and aerogel composites was analyzed by using Field-Emission Scanning Electron Microscope (FE-SEM S4800) of Hitachi, Japan (Figure 3.3) All samples were coated with a thin Pt for 30 seconds layer before measurement to enhance the secondary electron signal, the current used in the analysis has a voltage of 10 kV The principle of operation of this device is to use a narrow, low-energy electron beam (electron beam) to scan the surface of the sample The electrons interact with the atoms in the sample, producing secondary electrons that contain information about the surface topography and composition of the sample These secondary electrons are detected by the probe The position information of the electron beam is combined with the strength of the signals received by the probe to produce an image of the sample surface
Figure 3.3 FE-SEM Hitachi S-4800 3.3.3 Specific surface area and pore analysis
The specific surface and pore analysis method were performed on the Nova 2200E instrument of Quantachrome Instruments, USA (Figure 3.4)
Use the N2 adsorption-desorption isotherm and the Brunauer-Emmett-Teller (BET) equation to determine the specific surface area of the material The amount of gas (vapor) adsorbed (V) expressed as a volume is a quantity that characterizes the number of molecules adsorbed, depending on the equilibrium pressure (P), temperature (T), nature of gases, and solid materials V is a covariate function with equilibrium pressure When the pressure is increased to the saturation pressure (Po) of the adsorbent at a given temperature, the relationship between V and P is called
"adsorption isotherm" After the saturation pressure (Po) has been reached, the adsorbed gas volume values are measured at decreasing relative pressures (P/Po) and a “desorption isotherm” is obtained The adsorption/desorption isotherms of solids are proposed by the International Union of Pure and Applied Chemistry (IUPAC) as shown in Figure 3.5
Figure 3.5 IUPAC adsorption-desorption isotherms
Type I: material with micropore size
Class II: material without large capillaries
Class III: uncommon, weak adsorbent-desorption interactions
Class IV: medium capillary material
Type V: uncommon, materials with medium capillary and weak interactions
Type VI: stepwise multilayer adsorption on non-capillary surfaces
On the adsorption-desorption isotherm on type IV materials, hysteresis rings appear, indicating capillary condensation Then, use the Barrett-Joyner-Halenda (BJH) method to determine the average capillary volume and size
The mechanical properties of the cellulose aerogel composite were determined using the Zwick Roell Z010 instrument (Figure 3.6) at the Vietnam - Russia Tropical Center, HCMC The material compression test was performed with a heavy load speed of 1 mm/min using a 100 N weight A stress-strain curve was plotted based on the results obtained The line that best fits the R 2 value remaining at greater than 0.95 is drawn at the top of the curve, where it is still relatively linear (about 10%) Young's modulus of the material is determined from the slope of the line just drawn
Figure 3.6 Mechanical meter Zwick Roell Z010 3.3.5 Infrared spectrum analysis
The Fourier transform infrared (FT-IR) spectroscopy is used to determine functional groups present in the chemical structure of the aerogel composites The equipment used is a Fourier transform infrared spectrometer MIR/ NIR Frontier of PerkinElmer, USA (Figure 3.7), wave number in the range 550 – 4000 cm -1 , division 4 cm -1 , and each sample was scanned 64 times Before analysis, the sample was dried at 50 °C to constant mass The position and intensity of the peaks represent the functional groups present in the analyzed sample, thereby determining the change in the composition of the sample after each processing step The principle of operation of this analyzer is to shine a beam of light of different frequencies at the same time on the sample and measure how many frequencies are absorbed by the sample Then repeat with other beams of different frequencies to obtain the next data points This process repeats many times in a short period Finally, the computer converts this data into a universal form through a popular algorithm called the "Fourier transform"
Figure 3.7 Fourier transform infrared spectrometer (MIR/NIR Frontier)
Thermogravimetric analysis (TGA) is used to determine the properties of a substance through the change in mass with temperature The analysis is performed by gradually increasing the temperature from room temperature to the target temperature and simultaneously measuring the mass of the sample at each temperature point For aerogel composite samples in this study, the LabSys Evo TG/DSC 1600 instrument from Setaram (France) (Figure 3.8) was used
The thermal conductivity, also known as the K-value, of the composite aerogel materials was determined by thermal flow measurement with the HFM-100 device (Figure 3.9) The HFM-100 instrument is designed and engineered for the highest accuracy, repeatability, and widest temperature range The thermal conductivity of the composite aerogel sample was measured according to ASTM C518 Composite aerogel samples were synthesized with a rectangular shape measuring 14 x 7 cm Samples were measured at a room temperature of 25 °C
Figure 3.9 Thermal conductivity meter HFM-100
RESULTS AND DISCUSSIONS
Cellulose aerogel composites
4.1.1 Characterization of PF/CF aerogel composites a) Morphology
Lyophilization is the preferred technique in the synthesis of aerogel composites from PFs and CFs to avoid structural damage As the result, the porous structure of developed aerogel composites with high porosity is observed through SEM images in Figure 4.1 The pores inside the structure of aerogel composites created by PFs (straight, sizes 40 − 60 àm) and CFs (twisted, sizes 10 −20 àm) were primarily macropores, ranging from 30 to 90 àm Furthermore, the increase in CF concentration causes a more packed structure, resulting in a remarkable decrease in the diameter of the pores [53] The results are consistent with previous studies on PF/CF aerogel composites [42] At the same time, the arrangement and linkage between the fibers are disordered as they are cast on the mold randomly before adding the NaOH/Urea/H2O solution, which makes the cellulose chains swell and dissolve into solution, then gelation caused by physical cross-linking of cellulose chains to form the structure of the material [87]
Figure 4.1 SEM images of aerogel composites with PF/CF ratios (a) 4:1; (b) 2:1; (c) 1:1 b) FTIR spectroscopy
As can be seen from Figure 4.2, the appearance of the peak in the region 3450 – 3600 cm -1 proves the presence of fluctuating linkages between O-H groups that appear mostly in cellulose [113] The peak at 2894 cm -1 characterizes the elastic fluctuations of the C-H group in cellulose The peaks located at wave number 1650 cm -1 are the fluctuations of the two functional groups C-N [114], respectively, indicating the presence of urea in the sample [113] The C=C functional group appeared through the peak at 1505 cm -1 due to the oscillation of the aromatic ring inside the lignin, showing that a small part of lignin still exists inside the material The peak at 1338 –
1464 cm -1 is due to asymmetrical C-OH group bending and 1020 cm -1 is due to asymmetrical vibration of the C-O-C group [115]
Figure 4.2 FTIR spectra of PF/CF aerogel composites c) Physical properties
The physical properties of the aerogel composite samples in terms of density and porosity were measured and are shown in Table 4.1 The results reveal that the obtained aerogel composites possess a low density in the range of 0.053 − 0.069 g.cm −3 and high porosity of 94.7 − 95.2%, indicating their highly porous structure after synthesis Increasing the CF content whilst remaining the total fiber content unchanged (4%) results in an increase in the density of the aerogel composite from
0.053 to 0.069 g.cm −3 This can be explained by the fact that the density of CF (1.56 g.cm −3 ) is greater than that of PF (1.07 g.cm −3 ) [42] Compared to the previous work, our PF/CF aerogel composite have a higher density and lower porosity (density of 0.019 – 0.046 g.cm −3 and porosity of 96.1 – 98.4%) [42] s
Table 4.1 Summary of density, porosity, and compressive strength of PF/CF aerogel composites
Sample PF:CF (g/g) Density (g.cm −3 ) Porosity (%) Compressive modulus (kPa)
The correlation between compressive stress and corresponding strain of the PF/CF aerogel composite with different mixing ratios of the two fiber compositions is shown in Figure 4.3a, and Young’s modulus parameter is tabulated in Table 4.1 From the results, it can be seen that the synthesized aerogel composites exhibit a high compressive modulus of 90.97 kPa (PC41), 145.28 kPa (PC21), and 203.72 kPa (PC11) These parameters are approximately 5−9 times as much as those of PF/CF aerogel composite produced via the crosslinking method with PVA (11.33–44.63 kPa) [42] and are much higher than those of PF-only aerogel (1.64–5.34 kPa) [115] The higher compressive strength of the material can be explained by the fact that when the fibers are dispersed with NaOH/Urea/H2O aqueous solution and then gelation with ethanol, the cellulose fibers are linked together by intramolecular and intermolecular hydrogen bonds which make the structure become tight aggregates than when using PVA as a cross-linking [87] The stress-strain curves of aerogel composites show a linear line at strains less than 10%; this linear elastic region shows the ability to fully recover to the original state before deformation A smooth plastic- yielding region witnessed at the compression strain of 10−60% demonstrates the collapse of the porous structure Finally, a densification area is observed for compression strains greater than 60%, where stress is dramatically increased As in
Figure 4.3a, the more CF content increases, the greater the mechanical strength under the same pressure point This proves that CF plays a role in the reinforcement of the hollow structure of the PF/CF aerogel composite and increasing CF content makes the fabricated composites stiffer
Figure 4.3 Stress-strain (a) and TGA curves (b) of PF/CF aerogel composites with different fiber ratios
The graph of TGA analysis of PF/CF aerogel composite is shown in Figure 4.3b From the graph, it can be seen that changing the mixing PF/CF ratios has no significant effect on the mass loss of aerogel composites by thermal decomposition The PF/CF aerogel composites exhibit a typical thermogram with mass degradation in three phases by the temperature as follows: (i) 60 – 100 °C, (ii) 270 – 350 °C, and (iii) 360 – 460 °C In the first step, the sample weight decreased from 8 to 10%, attributed to the evaporation of moisture The reason for this phenomenon is that the PF/CF aerogel composites after drying still have a small amount of moisture inside and contain many hydroxyl groups on cellulose chains of two types of constituent fibers that absorb water in the air At the next stage from 270 to 350 °C, a drastic decline of about 67–70% is witnessed in the mass of all composite samples caused by the oxidative decomposition of organic compounds including cellulose, hemicellulose, and lignin [115] Eventually, the mass degradation of aerogel composites occurs rapidly from 360 °C until the material is completely decomposed at 460 °C, and the rest is ash As a result, the highest stable temperature range for PF/CF aerogel composites is around 230 − 280 °C, compared with PF/CF aerogel composites synthesized by Do et al., there is no significant difference in stable temperature range as they come from the same material d) Thermal conductivity
The thermal conductivity of PF/CF aerogel composites at the ambient condition ranges from 0.039 to 0.045 W/m.K as shown in Figure 4.4, making them very good thermal insulators Because of their highly porous structures, these aerogel composites are mainly composed of air Therefore, the aerogels’ low thermal conductivity might be attributed to their high porosity, since air is a good heat insulator (0.026 W/mK) at ambient temperature and pressure [115] The observation showed that the thermal conductivity of the PF/CF aerogel composite increased along with decreasing in porosity This can be explained by the fact that the decrease in the porosity of aerogel composite will results in a reduction in the pore volume of air within, thus an increase in thermal conductivity Comparing the thermal conductivity of PF/CF aerogel composite with composite aerogel with the same raw materials from pineapple leaf fiber and cotton waste fiber of previous research (0.041 – 0.043 W/m.K) gave similar results [42] In addition, the thermal conductivity of PF/CF aerogel composite is also comparable to that of good insulation materials such as wool (0.03 – 0.04) W/mK, recycled cellulose aerogels (0.032 W/mK), and Aspen aerogels products (0.021 W/mK) [115], [116]
Figure 4.4 Thermal conductivity of PF/CF aerogel composites with different fiber ratios 4.1.2 Dye removal of PF/CF aerogel composites a) Adsorption kinetic
It can be seen from Figure 4.5 that all three materials with different fiber ratios have a fast adsorption time in the first 20 min due to the diffusion process to the large surface without obstruction, as well as the number of empty adsorption centers At the same time, it can be observed that the lower the porosity of the material, the lower the adsorption capacity due to the decrease of empty adsorption centers in the material Adsorption equilibrium was reached about 30 min from when the material was placed in the MB solution The explanation for the short-time adsorption is because (1) the material structure is high porosity, (2) there is a hydrogen interaction between the hydroxide functional group (-OH) of cellulose and nitrogen atoms on
MB [117], (3) capillary force bring MB from the surface to the interior of the material
It can be concluded that the adsorption undergoes rapid adsorption stages on the surface of the material, so the adsorption increases the first time very quickly The diffusion process into the porous structure inside the material and this process continue until equilibrium is reached
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
Both common model pseudo-first-order and pseudo-second-order are also studied The R 2 values for both models are calculated and shown in Table 4.2 Kinetics parameters for the MB adsorption of PF/CF aerogel composite All PF/CF aerogel composites fit better into the pseudo-second-order model than the pseudo-first-order model This demonstrates that both adsorbent and adsorbate molecules influence adsorption [8]
Table 4.2 Kinetics parameters for the MB adsorption of PF/CF aerogel composite
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
Figure 4.6a shows that the adsorption capacity increases with increasing initial MB concentration With a fixed amount of material, there is a constant number of adsorption centers Therefore, when the concentration of MB is increased to a certain level, a saturated adsorption state will be reached which is followed by a drop in removal efficiency on MB absorption Two models isotherms of PF/CF aerogel composite are investigated (Figure 4.7), and the adsorption data according to Langmuir and Freundlich adsorption isotherm models (Table 4.3) gives results that fitted Langmuir’s model with higher R 2 , which indicates that the adsorption process is monolayer on the surface of the material [111] The RL values are all in the range between 0 and 1, representing the suitable initial concentrations chosen to investigate the Langmuir isotherm adsorption equation [118] Through Langmuir’s adsorption equation, the maximum adsorption capacity qm of PF/CF aerogel composite materials was determined as PC41 (34.01 mg.g −1 ), PC21 (33.22 mg.g −1 ), and PC11 (23.20 mg.g −1 )
Figure 4.7 Isotherms of PF/CF aerogel composite (a) Langmuir; (b) Freundlich
Table 4.3 Isotherm parameters for the MB dyes adsorption of PF/CF aerogel composite
Table 4.4 shows the comparison of MB adsorption of materials from various studies Even though the material porosity is large, the specific surface area is low due to the gelation method, hence the MB adsorption of PF/CF aerogel composite is still rather poor, lower than that of most materials from other cellulose sources However, the short adsorption period of PF/CF aerogel composite makes it a viable option for fast
Table 4.4 MB dyes adsorption of various adsorbents Adsorbents Adsorption times q m (mg/g) References
PF/CF aerogel composite 20 min 23.20−34.01 This study
4.1.3 Oil adsorption of the MTMS-coated PF/CF aerogel composites
The PF/CF aerogel composites after MTMS coating are hydrophobic because the -
OH groups of cellulose are displaced by -O-Si-(CH3)3 groups from MTMS, indicating by not absorbing water droplets on the surface as shown in Figure 4.8a A piece of this hydrophobic material was placed in a petri dish already containing 5w30 oil (Figure 4.8b) to evaluate the oil adsorption of the produced aerogel composite The results showed that the aerogel composite sample had almost completely absorbed all of the oil on the petri dish after only 2 minutes (Figure 4.8c)
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
The maximum oil adsorption of the material is 15.8 g.g −1 with sample PC41, and the lowest adsorption is 11.3 g.g −1 with sample PC11 (Figure 4.9a) This shows that the higher the porosity, the better the adsorption capacity The aerogel composites have higher oil adsorption when compared with coir-based aerogels prepared by the same alkali-urea method (10 g.g −1 ) [114]
Silica-based aerogel composite
4.2.1 Effect of acid concentration on gelation of silica aerogels from RHA
The gelation of silica sol is catalyzed by strong acid to perform hydrolysis and condensation of sodium silicate extracted from RHA The result of this process is silica gel with a network of stable polymer cross-linked by ≡Si-O-Si≡ bonds [94] One of the factors affecting silica gel formation is the concentration of acid The concentration of HCl used must be suitable so that a homogenous silica gel can be fabricated A series of HCl concentrations including 0.50, 0.75, 1.00, 1.50, and 2.00 is investigated in the preparation of silica gel The morphology of silica gels with different HCl concentrations is observed after aging and the solvent exchange (Figure 4.10)
The results show that the silica gel obtained by using 1.5 M HCl has a pH of 7−8 after aging and is hard and opalescent The gel structure is still stable after the solvent exchange When using higher HCl concentration, gelation occurs quickly but the gel formation is difficult to control because of the unstable structure The resulting gel is blocky and rigid after aging and broken into smaller fragments after solvent exchange At lower HCl concentrations, the silica gels are soft after aging and disintegrate in the next stage The HCl solution with a concentration of 1.5 M is chosen for the synthesis of silica aerogels and chitosan-silica aerogel composites in this study
Figure 4.10 Morphology of silica gels when using various acid concentrations
4.2.2 Characterization of silica aerogels and chitosan/silica aerogel composites a) Physical properties
The physical properties of silica aerogels and chitosan/silica aerogel composites in terms of density, porosity, surface area, and pore volume are tabulated in Table 4.6
The addition of chitosan in the structure of silica aerogels causes an increase in the density of aerogel composites, while a decrease in their porosity, pore volumes, and surface area is witnessed One probable explanation is that the presence of chitosan makes the aerogels to be more compact and reduces the pore size as well as the total surface area Despite the high porosity and surface area seen in the chitosan-free sample, the aerogel composites are expected to achieve higher adsorption capacity and stability than silica aerogels because of the improvement in affinity for dyes by chitosan
Table 4.6 Physical properties of silica aerogel and chitosan-silica aerogel composites
Morphologies of silica aerogel and chitosan/silica aerogel composite are analyzed by SEM images shown in Figure 4.11 The porous structure of silica aerogel (Figure 4.11a) is primarily made up of spherical solid clusters and small aerogel particles The pore size distribution is uneven with voids of different sizes between the particles and aggregates Regarding the chitosan-silica aerogel composite (Figure 4.11b), its surface texture becomes rougher with larger particles of silica aerogel and smaller pores between them However, the distribution of aerogel particles is more uniform with their binding to chitosan into larger aggregates Although chitosan plays a key role to shape the silica aerogels into a monolith, it causes significant shrinkage of the aerogel composites after ambient drying
Figure 4.11 FE-SEM images of: (a) silica aerogel, (b) chitosan-silica aerogel composite c) FTIR spectroscopy
Structural characteristics of silica aerogel and chitosan-silica aerogel composite are characterized using FTIR spectra methodology (Figure 4.12) For pure SiO2 aerogel, the strong and weak peaks near 453 cm -1 and 1043 cm -1 describe the Si-O-Si bonding The peaks at 1275 and 2974 cm -1 correspond to the IR absorption by Si-CH3 groups, which are attributed to the MTMS-coated layer [125]
The wideband at 3590 – 3650 cm -1 belongs to the stretching vibrations of O-H bonds in water molecules, bonded by hydrogen bonds The peaks at 1567 cm -1 correspond to the deformation vibrations of water molecules The bands at 3666 cm -1 correspond to the stretching vibrations O-H of hydroxyl groups bound with a silicon atom, and silanol Si-OH groups at 3666 cm -1 indicate the polarity of silica aerogel These hydroxyl peaks are less stretched for the silica aerogel beads, indicating better hydrophobicity
Chitosan-silica hybrid was also reported to have Si-OH and Si-O-Si with bands in the regions of 3590 – 3650 cm -1 and 800 – 928 cm -1 , respectively FTIR spectra showed an intensive broad absorption band located at 1099 cm −1 showing an intensity increase due to the absorption from the Si-O-Si bonds This is due to the overlapping of Si-O-Si, Si-O-C, and C-O stretching peaks IR band in 3700–3200 cm -1 is assigned to the overlapping of -OH stretch and amine band The absorption peak centered at about 3418 cm −1 is assigned to free or adsorbed water and is accompanied by the peak observed at 1863 cm −1 , a characteristic peak for the Si-OH bond, accounting for the hydrophilic behavior of the silica aerogels A peak of 1275 cm -1 to the asymmetric CO-NH2 stretching vibrations
Besides covalent bonding, most likely the chitosan is infused into the silica matrix through Van der Waals interaction between amino (-NH2) groups and silanol groups (Si-OH), and hydrogen bonding between acetamide (-CO-NH2) groups and silanol (Si-OH) groups Silica interaction with chitosan can also be seen from the decrease of the stretching intensity of the -N-H bond in the 3270 – 3290 cm -1 zone that overlaps with the –OH bond’s absorption zone, indicating that the -N-H bond of the added chitosan interacted with the silica
Figure 4.12 FTIR spectra of silica aerogel and chitosan-silica aerogel composite
4.2.3 Dye adsorption of chitosan-silica aerogel composite a) Effect of contact time on adsorption capacity of silica-based adsorbents
The effect of contact time on the MB adsorption capacity of the chitosan-silica aerogel composites with various chitosan content is presented graphically in Figure 4.13 The burst MB adsorption in the first 15 min is observed in all samples, followed by sustained adsorption to reach an equilibrium state after 120 min This phenomenon is explained by the immediate availability of adsorption sites in aerogels and aerogel
-Si-O-H928-800Si-O-Si composites during the first stage [126] After a while, these accessible sites on the surface of the adsorbent gradually react with MB until saturation, resulting in the equilibrium adsorption capacity of the aerogel composite after 120-150 min Consequently, the equilibrium time is 120 min in our experiment for the adsorption of MB onto the adsorbent Moreover, the addition of chitosan helps to boost the adsorption capacity of chitosan-silica aerogel composites, in particular, the equilibrium adsorption capacity of silica-based aerogel composite CS100 is roughly
5 mg/g higher than that of chitosan-free silica aerogel CS00
Figure 4.13 Effect of contact time on MB adsorption
Based on the experimental data, the adsorption kinetics model which is suitable to describe the MB adsorption mechanism of silica aerogels and chitosan-silica aerogel composites over time is determined with calculated parameters presented in Table 4.7 As can be seen, all samples are well fit to pseudo-second-order kinetics because of the high R-squared value close to 1 It is demonstrated that the MB adsorption process of silica aerogel and chitosan/silica aerogel composites takes place by the mutual affinity between adsorbent and adsorbate [8]
Table 4.7 Parameters of two kinetic models
Sample Pseudo-first order Pseudo-second order k1 R² Calculated qe (mg/g)
CS100 0.018 0.755 22.472 22.276 0.011 0.998 b) Effect of initial MB concentration on adsorption capacity of silica-based adsorbents
Figure 4.14 depicts the influence of varied initial MB dye concentrations on the adsorption of chitosan-free silica aerogel (CS00) and chitosan/silica aerogel composite (CS100) It is obvious that increasing the dye concentration from 12.5 to
50 mg/L causes a rise in the removal efficiency for both samples CS100 and CS00 The support of chitosan in improving the MB adsorption capacity is once again demonstrated by a difference in removal efficiency of up to 12% between the two samples at the same MB initial concentration of 50 mg/L At a higher MB concentration of 100 mg/L, the MB removal efficiency of aerogels and aerogel composites decreases and tends to remain unchanged when the MB concentration is
125 mg/L The leveling off for the adsorption process with dye concentrations higher than 100 mg/L is attributed to the saturation of active adsorption sites within samples
In addition, excessive concentrations of dye molecules not only rapidly saturate the surface adsorption sites but also slow down the adsorption process [126] Therefore, an appropriate MB concentration of 50 mg/L provides high removal efficiency Moreover, at the MB concentration of 125 mg/L chitosan-silica aerogel composite CS100 processes a higher adsorption capacity (53.81 mg/L) and removal efficiency (88.3%) than that of silica aerogel CS00 (47.52 mg/L and 78.3%) This further reinforces the abovementioned statement that adding chitosan enhances the adsorption capacity of the synthesized composites by 13.24%
Figure 4.14 Effect of initial MB concentration on adsorption of (a) CS00, (b) CS100
Two common isothermal equilibrium models, namely Langmuir and Freundlich, are used to model the equilibrium adsorption capacity of silica aerogels and chitosan- silica aerogel composites following the change in the initial MB concentration The analyzed parameters of the MB adsorption isotherms onto CS00 and CS100 samples are listed in Table 4.8 According to the results, the adsorption equilibrium data of the two samples can be presented by the Freundlich model in all solutions because of the higher linear regression coefficient, as opposed to the Langmuir model This adsorption behavior suggests that adsorption of MB by silica aerogels and chitosan- silica aerogel composites occurs heterogeneously due to the multi-layer MB adsorption on the adsorbent and the π-π bond among the MB molecules [8]
Table 4.8 The Langmuir and Freundlich isotherm models
Langmuir Freundlich kL (L/mg) R 2 n (g/L) kF (mg/g) R 2
CS100 0.0004 0.012 1.004 4.185 0.938 c) Effect of pH on adsorption capacity of chitosan-silica aerogel composite
The influence of pH on MB uptake of silica-based adsorbents is shown in Figure 4.15 As the pH value increases from 3 to 5, a significant enhancement in MB
CONCLUSION AND FUTURE WORK
Conclusions
In this study, aerogel composites based on agricultural by-products are an opportunity to minimize waste production In addition, the study and application of the adsorption capacity of these materials also bring meaning to the problem of solving environmental pollution, specifically here, wastewater treatment The following findings were obtained as a result of the study:
PF/CF aerogel composites were successfully synthesized from the sol-gel process with the NaOH/Urea/H2O solvent system with a mass ratio of 7/12/81 The fiber mixing ratio of 4/1 gives the highest porosity (95.2%) and the lowest density (0.053 kg/m3) The insulation of this fiber mix ratio is the best (0.039 W/m.K) The PF/CF fiber mix ratio is 1/1 for the highest specific surface area (19.37 m 2 /g) with pore sizes ranging from micro to macroporous At the same time, this fiber mixing ratio also gives better muscle strength (203.72 kPa) than the other two yarn mixing ratios, 2/1 (145.28 kPa) and 4/1 (90.97 kPa) The highest measured oil adsorption (15.8 g/g) at the PF/CF fiber blend ratio was 4/1 In addition, MB color adsorption is also the highest at this ratio (34.01 mg/g)
Chitosan-silica aerogel composites were fabricated by in situ formations of an inorganic network in the presence of a preformed organic polymer Low-density silica aerogel was synthesized from rice husk ash via the sodium silicate route Both silica aerogel and chitosan/silica aerogel composite were prepared by sol-gel method and dried at ambient pressure after surface modification with Methyltrimethoxy Silane (MTMS) solution
In the process of synthesizing silica aerogel, the optimal conditions for good gels are neutral pH of about 7 – 8 and neutralized HCl acid concentration of 1.5 M
Both silica aerogel and chitosan/silica aerogel composite samples proved to be effective adsorbents for the removal of MB from an aqueous solution The percentage of adsorption was maximal at a pH value of 5.0 and decreased in less acidic MB solutions The adsorption kinetics was well described by the pseudo-second-order model equation An adsorption isotherm was fitted by the Freundlich model, with the maximum adsorption capacity of MB on composite material calculated at 52.5 mg/g for composite and 47.5 mg/g for silica aerogel The candidate materials exhibited a high maximum removal capacity and therefore, these hybrid materials behave as good candidates for MB removal in water purification.
Future work recommendations
Cellulose-based fibers have the ability to insulate heat and sound due to their low thermal conductivity and high sound insulation coefficient Therefore, it is advisable to study other applications of PF-CF aerogel composites
In addition, many studies show that chitosan has an antibacterial ability, so there is potential for the application of materials made from chitosan to antibacterial Therefore, it is necessary to study other applications of chitosan-silica aerogel composites
LIST OF PUBLICATIONS International journal
1 P V Vu, T M Le, V T Tran, and P K Le, “Effects of Process Parameters on Conversion of Rice Straw-Lignin into Bio-Oil by Hydrothermal Liquefaction,” Chem
2 V T Tran, T M Le, P V Vu, H M Nguyen, Y H P Duong, and P K Le,
“Depolymerization of Rice Straw Lignin into Value-Added Chemicals in Sub- Supercritical Ethanol,” Sci World J., vol 2022, 2022
3 P V Vu, T D Doan, G C Tu, N H N Do, K A Le, and P K Le, “A novel application of cellulose aerogel composites from pineapple leaf fibers and cotton waste: Removal of dyes and oil in wastewater,” J Porous Mater., vol 29(4), pp 1–11, 2022
[1] B Li, L Wu, L Li, S Seeger, J Zhang, and A Wang, “Superwetting double-layer polyester materials for effective removal of both insoluble oils and soluble dyes in water,” ACS Appl Mater Interfaces, vol 6, no 14, pp 11581–11588, 2014
[2] J Xiao, J Zhang, W Lv, Y Song, and Q Zheng, “Multifunctional graphene/poly(vinyl alcohol) aerogels: In situ hydrothermal preparation and applications in broad-spectrum adsorption for dyes and oils,” Carbon N Y., vol 123, pp 354–363, 2017
[3] J Wang et al., “Recyclable textiles functionalized with reduced graphene
Oxide@ZnO for removal of oil spills and dye pollutants,” Australian Journal of Chemistry, vol 67, no 1 pp 71–77, 2014
[4] Q B Thai et al., “Advanced aerogels from waste tire fibers for oil spill-cleaning applications,” J Environ Chem Eng., vol 8, no 4, p 104016, 2020
[5] T Zhang et al., “Recent progress and future prospects of oil-absorbing materials,” Chinese J Chem Eng., vol 27, no 6, pp 1282–1295, 2019
[6] R D C Soltani, A R Khataee, M Safari, and S W Joo, “Preparation of bio- silica/chitosan nanocomposite for adsorption of a textile dye in aqueous solutions,”
Int Biodeterior Biodegradation, vol 85, pp 383–391, 2013
[7] Y Bulut and H Karaer, “Adsorption of Methylene Blue from Aqueous Solution by Crosslinked Chitosan/Bentonite Composite,” J Dispers Sci Technol., vol 36, no 1, pp 61–67, 2015
[8] S M El-Kousy, H G El-Shorbagy, and M A A El-Ghaffar,
“Chitosan/montmorillonite composites for fast removal of methylene blue from aqueous solutions,” Mater Chem Phys., vol 254, 2020
[9] S Bin Kang, Z Wang, and S W Won, “Adsorption characteristics of methylene blue on PAA-PSBF adsorbent,” Chem Eng Trans., vol 78, pp 205–210, 2020
[10] B De Caprariis, P De Filippis, E Petrucci, and M Scarsella, “Activated biochars used as adsorbents for dyes removal,” Chem Eng Trans., vol 65, pp 103–108, 2018
[11] M Fauziyah, W Widiyastuti, R Balgis, and H Setyawan, “Production of cellulose aerogels from coir fibers via an alkali–urea method for sorption applications,”
[12] W.-J Yang et al., “Recent progress in bio-based aerogel absorbents for oil/water separation,” Cellulose, vol 26, no 11, pp 6449–6476, 2019
[13] I Smirnova and P Gurikov, “Aerogel production: Current status, research directions, and future opportunities,” J Supercrit Fluids, vol 134, pp 228–233, 2018
[14] A E Aliev et al., “Giant-stroke, superelastic carbon nanotube aerogel muscles,” Science (80- )., vol 323, no 5921, pp 1575–1578, 2009
[15] I Smirnova and P Gurikov, “Aerogels in chemical engineering: Strategies toward tailor-made aerogels,” Annu Rev Chem Biomol Eng., vol 8, no March, pp 307–
[16] O Masson, V Rieux, R Guinebretière, and A Dauger, “Size and shape characterization of TiO2 aerogel nanocrystals,” Nanostructured Mater., vol 7, no 7, pp 725–731, 1996
[17] J L Gurav, I K Jung, H H Park, E S Kang, and D Y Nadargi, “Silica aerogel: Synthesis and applications,” J Nanomater., vol 2010, 2010
[18] H Ma, X Zheng, X Luo, Y Yi, and F Yang, “Simulation and analysis of mechanical properties of silica aerogels: from rationalization to prediction,” Materials (Basel)., vol 11, no 2, p 214, 2018
[19] L Shang, Y Lyu, and W Han, “Microstructure and thermal insulation property of silica composite aerogel,” Materials (Basel)., vol 12, no 6, p 993, 2019
[20] A Hoseini and M Bahrami, “Effects of humidity on thermal performance of aerogel insulation blankets,” J Build Eng., vol 13, pp 107–115, 2017
[21] Z Liu, Y Ding, F Wang, and Z Deng, “Thermal insulation material based on SiO2 aerogel,” Constr Build Mater., vol 122, pp 548–555, 2016
[22] Z Li, L Gong, X Cheng, S He, C Li, and H Zhang, “Flexible silica aerogel composites strengthened with aramid fibers and their thermal behavior,” Mater Des., vol 99, pp 349–355, 2016
[23] K.-J Lee, Y.-J Choe, Y H Kim, J K Lee, and H.-J Hwang, “Fabrication of silica aerogel composite blankets from an aqueous silica aerogel slurry,” Ceram Int., vol
[24] Z Zhu et al., “Fiber reinforced polyimide aerogel composites with high mechanical strength for high temperature insulation,” Macromol Mater Eng., vol 304, no 5, p
[25] C A García-González, M Alnaief, and I Smirnova, “Polysaccharide-based aerogels
- Promising biodegradable carriers for drug delivery systems,” Carbohydr Polym., vol 86, no 4, pp 1425–1438, 2011
[26] D Illera, J Mesa, H Gomez, and H Maury, “Cellulose aerogels for thermal insulation in buildings: trends and challenges,” Coatings, vol 8, no 10, p 345, 2018
[27] O Karatum, S A Steiner III, J S Griffin, W Shi, and D L Plata, “Flexible, mechanically durable aerogel composites for oil capture and recovery,” ACS Appl
Mater Interfaces, vol 8, no 1, pp 215–224, 2016
[28] F Jiang and Y.-L Hsieh, “Cellulose nanofibril aerogels: synergistic improvement of hydrophobicity, strength, and thermal stability via cross-linking with diisocyanate,”
ACS Appl Mater Interfaces, vol 9, no 3, pp 2825–2834, 2017
[29] H Jin et al., “Superhydrophobic and superoleophobic nanocellulose aerogel membranes as bioinspired cargo carriers on water and oil,” Langmuir, vol 27, no 5, pp 1930–1934, 2011
[30] M Fumagalli, D Ouhab, S M Boisseau, and L Heux, “Versatile gas-phase reactions for surface to bulk esterification of cellulose microfibrils aerogels,”
[31] H Peng et al., “A facile approach for preparation of underwater superoleophobicity cellulose/chitosan composite aerogel for oil/water separation,” Appl Phys A, vol
[32] M Asim et al., “A review on pineapple leaves fibre and its composites,” Int J Polym Sci., vol 2015, 2015
[33] A Roda and M Lambri, “Food uses of pineapple waste and by‐products: a review,”
Int J Food Sci Technol., vol 54, no 4, pp 1009–1017, 2019
[34] H P S Abdul Khalil et al., “A review on plant cellulose nanofibre-based aerogels for biomedical applications,” Polymers (Basel)., vol 12, no 8, 2020
[35] A R S Neto et al., “Comparative study of 12 pineapple leaf fiber varieties for use as mechanical reinforcement in polymer composites,” Ind Crops Prod., vol 64, pp 68–
[36] D Zawawi, Z Mohd, S Angzzas, and M A Ashuvila, “Analysis of the chemical compositions and fiber morphology of pineapple (Ananas comosus) leaves in Malaysia.,” J Appl Sci., vol 14, no 12, pp 1355–1358, 2014
[37] R Khiari, M F Mhenni, M N Belgacem, and E Mauret, “Chemical composition and pulping of date palm rachis and Posidonia oceanica–A comparison with other wood and non-wood fibre sources,” Bioresour Technol., vol 101, no 2, pp 775–
[38] H P S A Khalil, M S Alwani, and A K M Omar, “Chemical composition, anatomy, lignin distribution, and cell wall structure of Malaysian plant waste fibers,”
[39] P S Mukherjee and K G Satyanarayana, “Structure and properties of some vegetable fibres,” J Mater Sci., vol 21, no 1, pp 51–56, 1986
[40] W Liu, M Misra, P Askeland, L T Drzal, and A K Mohanty, “‘Green’composites from soy based plastic and pineapple leaf fiber: fabrication and properties evaluation,”
Polymer (Guildf)., vol 46, no 8, pp 2710–2721, 2005
[41] S Debnath, “Pineapple leaf fibre—a sustainable luxury and industrial textiles,” in
Handbook of sustainable luxury textiles and fashion, Springer, 2016, pp 35–49
[42] N H N Do et al., “Recycling of Pineapple Leaf and Cotton Waste Fibers into
Heat‑insulating and Flexible Cellulose Aerogel Composites,” J Polym Environ., vol
[43] R J Moon, A Martini, J Nairn, J Simonsen, and J Youngblood, “Cellulose nanomaterials review: structure, properties and nanocomposites,” Chem Soc Rev., vol 40, no 7, pp 3941–3994, 2011
[44] L Y Long, Y X Weng, and Y Z Wang, “Cellulose aerogels: Synthesis, applications, and prospects,” Polymers (Basel)., vol 8, no 6, pp 1–28, 2018
[45] Q B Thai et al., “Cellulose-based aerogels from sugarcane bagasse for oil spill- cleaning and heat insulation applications,” Carbohydr Polym., vol 228, p 115365, Jan 2020
[46] M Fauziyah, W Widiyastuti, R Balgis, and H Setyawan, “Production of cellulose aerogels from coir fibers via an alkali–urea method for sorption applications,”
[47] B Seantier, D Bendahou, A Bendahou, Y Grohens, and H Kaddami, “Multi-scale cellulose based new bio-aerogel composites with thermal super-insulating and tunable mechanical properties,” Carbohydr Polym., vol 138, pp 335–348, 2016
[48] Y Kobayashi, T Saito, and A Isogai, “Aerogels with 3D ordered nanofiber skeletons of liquid‐crystalline nanocellulose derivatives as tough and transparent insulators,”
Angew Chemie, vol 126, no 39, pp 10562–10565, 2014
[49] Y Bin Choy, H Choi, and K Kim, “Uniform ethyl cellulose microspheres of controlled sizes and polymer viscosities and their drug‐release profiles,” J Appl Polym Sci., vol 112, no 2, pp 850–857, 2009
[50] B F Martins, P V O de Toledo, and D F S Petri, “Hydroxypropyl methylcellulose based aerogels: Synthesis, characterization and application as adsorbents for wastewater pollutants,” Carbohydr Polym., vol 155, pp 173–181, 2017
[51] M X Bao, S Xu, X Wang, and R Sun, “Porous cellulose aerogels with high mechanical performance and their absorption behaviors [J],” Bioresources, vol 11, no 1, pp 8–20, 2016
[52] M Kaya, “Super absorbent, light, and highly flame retardant cellulose‐based aerogel crosslinked with citric acid,” J Appl Polym Sci., vol 134, no 38, p 45315, 2017
[53] H Cheng, B Gu, M P Pennefather, T X Nguyen, N Phan-Thien, and H M Duong,
“Cotton aerogels and cotton-cellulose aerogels from environmental waste for oil spillage cleanup,” Mater Des., vol 130, pp 452–458, 2017
[54] A Walcarius and L Mercier, “Mesoporous organosilica adsorbents: Nanoengineered materials for removal of organic and inorganic pollutants,” J Mater Chem., vol 20, no 22, pp 4478–4511, 2010
[55] A Shahat, H M A Hassan, H M E Azzazy, E A El-Sharkawy, H M Abdou, and
M R Awual, “Novel hierarchical composite adsorbent for selective lead(II) ions capturing from wastewater samples,” Chem Eng J., vol 332, no Ii, pp 377–386,
[56] S M L Dos Santos, K A B Nogueira, M De Souza Gama, J D F Lima, I J Da Silva Júnior, and D C S De Azevedo, “Synthesis and characterization of ordered mesoporous silica (SBA-15 and SBA-16) for adsorption of biomolecules,”
Microporous Mesoporous Mater., vol 180, pp 284–292, 2013
[57] S S Prakash, C J Sankaran, A J Hurd, and S M Rao, “Silica aerogel films prepared at ambient pressure by using surface derivatization to induce reversible drying shrinkage,” Nature, vol 374, no 6521 pp 439–443, 1995
[58] Q Feng et al., “Synthesis of high specific surface area silica aerogel from rice husk ash via ambient pressure drying,” Colloids Surfaces A Physicochem Eng Asp., vol
[59] H Han, W Wei, Z Jiang, J Lu, J Zhu, and J Xie, “Removal of cationic dyes from aqueous solution by adsorption onto hydrophobic/hydrophilic silica aerogel,”
Colloids Surfaces A Physicochem Eng Asp., vol 509, pp 539–549, 2016
[60] Y Shunin, S Bellucci, A Gruodis, and T Lobanova-Shunina, Nanotechnology Application Challenges: Nanomanagement, Nanorisks and Consumer Behaviour
[61] D C da Silva Alves, B Healy, L A d A Pinto, T R S Cadaval, and C B Breslin,
“Recent developments in Chitosan-based adsorbents for the removal of pollutants from aqueous environments,” Molecules, vol 26, no 3 Multidisciplinary Digital
[62] S Wei, Y C Ching, and C H Chuah, “Synthesis of chitosan aerogels as promising carriers for drug delivery: A review,” Carbohydr Polym., vol 231, p 115744, 2020
[63] Q Ma, Y Liu, Z Dong, J Wang, and X Hou, “Hydrophobic and nanoporous chitosan-silica composite aerogels for oil absorption,” J Appl Polym Sci., vol 132, no 15, pp 1–11, 2015
[64] S Smitha, P Shajesh, P Mukundan, and K G K Warrier, “Sol-gel synthesis of biocompatible silica-chitosan hybrids and hydrophobic coatings,” J Mater Res., vol
[65] A R Cestari, E F S Vieira, A A Pinto, and E C N Lopes, “Multistep adsorption of anionic dyes on silica/chitosan hybrid: 1 Comparative kinetic data from liquid-and solid-phase models,” J Colloid Interface Sci., vol 292, no 2, pp 363–372, 2005
[66] H Zhao, J Xu, W Lan, T Wang, and G Luo, “Microfluidic production of porous chitosan/silica hybrid microspheres and its Cu(II) adsorption performance,” Chem Eng J., vol 229, pp 82–89, 2013
[67] K Ebisike, A E Okoronkwo, and K K Alaneme, “Adsorption of Cd (II) on chitosan–silica hybrid aerogel from aqueous solution,” Environ Technol Innov., vol
[68] J Wang, M Mao, S Atif, and Y Chen, “Adsorption behavior and mechanism of aqueous Cr(III) and Cr(III)-EDTA chelates on DTPA-chitosan modified Fe3O4@SiO2,” React Funct Polym., vol 156, no May, p 104720, 2020
[69] M Aden, R N Ubol, M Knorr, J Husson, and M Euvrard, “Efficent removal of nickel(II) salts from aqueous solution using carboxymethylchitosan-coated silica particles as adsorbent,” Carbohydr Polym., vol 173, pp 372–382, 2017
[70] X Xu, P Dong, Y Feng, F Li, and H Yu, “A simple strategy for preparation of spherical silica-supported porous chitosan matrix based on sol-gel reaction and simple treatment with ammonia solution,” Anal Methods, vol 2, no 5, pp 546–551, 2010
[71] M Blachnio, T M Budnyak, A Derylo-Marczewska, A W Marczewski, and V A Tertykh, “Chitosan-Silica Hybrid Composites for Removal of Sulfonated Azo Dyes from Aqueous Solutions,” Langmuir, vol 34, no 6, pp 2258–2273, 2018
[72] Y Li, Y Zhou, W Nie, L Song, and P Chen, “Highly efficient methylene blue dyes removal from aqueous systems by chitosan coated magnetic mesoporous silica nanoparticles,” J Porous Mater., vol 22, no 5, pp 1383–1392, 2015
[73] H Hassan, A Salama, A K El-ziaty, and M El-Sakhawy, “New chitosan/silica/zinc oxide nanocomposite as adsorbent for dye removal,” Int J Biol Macromol., vol 131, pp 520–526, 2019
[74] N M Mahmoodi, Z Mokhtari-Shourijeh, and J Abdi, “Preparation of mesoporous polyvinyl alcohol/chitosan/silica composite nanofiber and dye removal from wastewater,” Environ Prog Sustain Energy, vol 38, no s1, pp S100–S109, 2019
[75] J Wang et al., “Chitosan–silica composite aerogels: preparation, characterization and Congo red adsorption,” J Sol-Gel Sci Technol., vol 76, no 3, pp 501–509, 2015
[76] S Montes and H Maleki, “12 - Aerogels and their applications,” in Metal Oxides, S Thomas, A Tresa Sunny, and P B T.-C M O N Velayudhan, Eds Elsevier, 2020, pp 337–399
[77] H Liu, B Geng, Y Chen, and H Wang, “Review on the Aerogel-Type Oil Sorbents Derived from Nanocellulose,” ACS Sustain Chem Eng., vol 5, no 1, pp 49–66,
[78] L E Garcia-Amezquita, J Welti-Chanes, F T Vergara-Balderas, and D Bermúdez- Aguirre, “Freeze-drying: The Basic Process,” Encycl Food Heal., pp 104–109, 2015
[79] X Zhang, Y Yu, Z Jiang, and H Wang, “The effect of freezing speed and hydrogel concentration on the microstructure and compressive performance of bamboo-based cellulose aerogel,” J Wood Sci., vol 61, no 6, pp 595–601, 2015
[80] N H N Do et al., “Advanced fabrication and application of pineapple aerogels from agricultural waste,” Mater Technol., vol 35, no 11–12, pp 807–814, 2020
[81] J R Lin et al., “Studies on the regioselectivity of acetylation-bromination in pregnanetriol,” Acta Chim Sin., vol 64, no 12, pp 1265–1268, 2006
[82] A Isogai and R H Atalla, “Amorphous celluloses stable in aqueous media: Regeneration from SO2–amine solvent systems,” J Polym Sci Part A Polym Chem., vol 29, no 1, pp 113–119, 1991
[83] J F Masson and R S J Manley, “Cellulose/poly (4-vinylpyridine) blends,”
[84] S Striouk and B A Wolf, “Fractional dissolution of ‘solid’ unsubstituted cellulose,”
Macromol Chem Phys., vol 201, no 15, pp 1946–1949, 2000
[85] D Ruan, L Zhang, J Zhou, H Jin, and H Chen, “Structure and properties of novel fibers spun from cellulose in NaOH/thiourea aqueous solution,” Macromol Biosci., vol 4, no 12, pp 1105–1112, 2004
[86] J Shi, L Lu, W Guo, M Liu, and Y Cao, “On preparation, structure and performance of high porosity bulk cellulose aerogel,” Plast Rubber Compos., vol 44, no 1, pp 26–32, 2015
[87] S Zhang, W C Wang, F X Li, and J Y Yu, “Swelling and dissolution of cellulose in NaOH aqueous solvent systems,” Cellul Chem Technol., vol 47, no 9–10, pp 671–679, 2013
[88] L Druel, A Kenkel, V Baudron, S Buwalda, and T Budtova, “Cellulose Aerogel Microparticles via Emulsion-Coagulation Technique,” Biomacromolecules, vol 21, no 5, pp 1824–1831, 2020
[89] T Budtova and P Navard, “Cellulose in NaOH–water based solvents: a review,”
[90] A Zaman, F Huang, M Jiang, W Wei, and Z Zhou, “Preparation, Properties, and Applications of Natural Cellulosic Aerogels: A Review,” Energy Built Environ., vol
[91] R D Badley, W T Ford, F J McEnroe, and R A Assink, “Surface Modification of Colloidal Silica,” Langmuir, vol 6, no 4, pp 792–801, 1990
[92] A Van Blaaderen, J Van Geest, and A Vrij, “Monodisperse colloidal silica spheres from tetraalkoxysilanes: Particle formation and growth mechanism,” J Colloid Interface Sci., vol 154, no 2, pp 481–501, 1992
[93] C Daniel, C Dammer, and J M Guenet, “On the definition of thermoreversible gels: the case of syndiotactic polystyrene,” Polymer (Guildf)., vol 35, no 19, pp 4243–
[94] K Zheng and A R Boccaccini, “Sol-gel processing of bioactive glass nanoparticles:
A review,” Adv Colloid Interface Sci., vol 249, pp 363–373, 2017
[95] ͆losarczyk Agnieszka, S Wojciech, Z Piotr, and J Paulina, “Synthesis and characterization of carbon fiber/silica aerogel nanocomposites,” J Non Cryst Solids, vol 416, pp 1–3, 2015
[96] S Sankar et al., “Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by a chemical method,” Ceram Int., vol 42, no 4, pp 4875–
[97] W Wichaita, Y G Kim, P Tangboriboonrat, and H Thérien-Aubin, “Polymer- functionalized polymer nanoparticles and their behaviour in suspensions,” Polym Chem., vol 11, no 12, pp 2119–2128, 2020
[98] J Estella, J C Echeverría, M Laguna, and J J Garrido, “Effects of aging and drying conditions on the structural and textural properties of silica gels,” Microporous Mesoporous Mater., vol 102, no 1–3, pp 274–282, 2007
[99] S Affandi, H Setyawan, S Winardi, A Purwanto, and R Balgis, “A facile method for production of high-purity silica xerogels from bagasse ash,” Adv Powder Technol., vol 20, no 5, pp 468–472, 2009
[100] A P Rao, A V Rao, and G M Pajonk, “Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents,” Appl Surf Sci., vol 253, no 14, pp 6032–6040, 2007
[101] S D Bhagat, Y H Kim, K H Suh, Y S Ahn, J G Yeo, and J H Han,
“Superhydrophobic silica aerogel powders with simultaneous surface modification, solvent exchange and sodium ion removal from hydrogels,” Microporous Mesoporous Mater., vol 112, no 1–3, pp 504–509, 2008
[102] A S Dorcheh and M H Abbasi, “Silica aerogel; synthesis, properties and characterization,” J Mater Process Technol., vol 199, no 1–3, pp 10–26, 2008
[103] M D Haw, M Gillie, and W C K Poon, “Effects of phase behavior on the drying of colloidal suspensions,” Langmuir, vol 18, no 5, pp 1626–1633, 2002
[104] N M Mahmoodi, S Khorramfar, and F Najafi, “Amine-functionalized silica nanoparticle: Preparation, characterization and anionic dye removal ability,”
[105] T M Budnyak, I V Pylypchuk, V A Tertykh, E S Yanovska, and D Kolodynska,
“Synthesis and adsorption properties of chitosan-silica nanocomposite prepared by sol-gel method,” Nanoscale Res Lett., vol 10, no 1, pp 1–10, 2015
[106] K Chen, Q Feng, D Ma, and X Huang, “Hydroxyl modification of silica aerogel:
An effective adsorbent for cationic and anionic dyes,” Colloids Surfaces A Physicochem Eng Asp., vol 616, no February, p 126331, 2021
[107] A Bhatnagar and A K Jain, “A comparative adsorption study with different industrial wastes as adsorbents for the removal of cationic dyes from water,” J Colloid Interface Sci., vol 281, no 1, pp 49–55, 2005
[108] W Sumarni, R S Iswari, P Marwoto, and E F Rahayu, “Physical characteristics of chitosan-silica composite of rice husk ash,” IOP Conf Ser Mater Sci Eng., vol 107, no 1, 2016
[109] F Ren et al., “Facile preparation of 3D regenerated cellulose/graphene oxide composite aerogel with high-efficiency adsorption towards methylene blue,” J Colloid Interface Sci., vol 532, pp 58–67, 2018
[110] J H Beh, T H Lim, J H Lew, and J C Lai, “Cellulose nanofibril-based aerogel derived from sago pith waste and its application on methylene blue removal,” Int J
[111] M D LeVan and T Vermeulen, “Binary Langmuir and Freundlich isotherms for ideal adsorbed solutions,” J Phys Chem., vol 85, no 22, pp 3247–3250, 1981
[112] L Zhou, S Zhai, Y Chen, and Z Xu, “Anisotropic Cellulose Nanofibers/Polyvinyl Alcohol/Graphene Aerogels Fabricated by Directional Freeze-drying as Effective Oil Adsorbents,” Polymers , vol 11, no 4 2019
[113] N T Son, “Rice Straw Cellulose Aerogels,” Vietnam J Sci Technol., vol 56, no 2A, pp 118–125, 2018
[114] M Fauziyah, W Widiyastuti, and H Setyawan, “A hydrophobic cellulose aerogel from coir fibers waste for oil spill application,” IOP Conf Ser Mater Sci Eng., vol
[115] N H N Do et al., “Heat and sound insulation applications of pineapple aerogels from pineapple waste,” Mater Chem Phys., vol 242, p 122267, Feb 2020
[116] S T Nguyen, J Feng, S K Ng, J P W Wong, V B C Tan, and H M Duong,
“Advanced thermal insulation and absorption properties of recycled cellulose aerogels,” Colloids Surfaces A Physicochem Eng Asp., vol 445, pp 128–134, 2014
[117] T Ahmad et al., “The use of date palm as a potential adsorbent for wastewater treatment: A review,” Environ Sci Pollut Res., vol 19, no 5, pp 1464–1484, 2012
[118] T M Budnyak et al., “Methylene Blue dye sorption by hybrid materials from technical lignins,” J Environ Chem Eng., vol 6, no 4, pp 4997–5007, 2018
[119] Q Bai, Q Xiong, C Li, Y Shen, and H Uyama, “Hierarchical porous cellulose/activated carbon composite monolith for efficient adsorption of dyes,”
[120] V K Garg, R Gupta, A B Yadav, and R Kumar, “Dye removal from aqueous solution by adsorption on treated sawdust,” Bioresour Technol., vol 89, no 2, pp 121–124, 2003
[121] J Liu et al., “Facile fabrication of carboxymethyl cellulose sodium/graphene oxide hydrogel microparticles for water purification,” RSC Adv., vol 6, no 55, pp 50061–
[122] Y Li et al., “Comparative study of methylene blue dye adsorption onto activated carbon, graphene oxide, and carbon nanotubes,” Chem Eng Res Des., vol 91, no 2, pp 361–368, 2013
[123] X Chang, D Chen, and X Jiap, “Chitosan-based aerogels with high adsorption performance,” J Phys Chem B, vol 112, no 26, pp 7721–7725, 2008
[124] Z E Lim et al., “Functionalized pineapple aerogels for ethylene gas adsorption and nickel (II) ion removal applications,” J Environ Chem Eng., vol 8, no 6, p 104524,
[125] H El Rassy and A C Pierre, “NMR and IR spectroscopy of silica aerogels with different hydrophobic characteristics,” J Non Cryst Solids, vol 351, no 19–20, pp 1603–1610, 2005
[126] M Jabli, “Synthesis, characterization, and assessment of cationic and anionic dye adsorption performance of functionalized silica immobilized chitosan bio-polymer,”
Int J Biol Macromol., vol 153, pp 305–316, 2020.