the utilization of black liquor generated in the cellulose production in the synthesis of carbon based materials

Inspired by previous research on rice-straw-generated cellulose and the utilization of black liquor, this work focused on the extreme exploitation of rice straw for the production of var

LITERATURE REVIEW

Rice straw and its agricultural advantages

Rice straw serves multiple agricultural purposes, encompassing applications such as mulching, green manure, and livestock bedding Studies in Vietnam show that rice straw mulch can reduce soil evaporation by up to 20% [1] and inhibit weed growth by about 70% Incorporating rice straw into the soil as green manure increases soil organic carbon content, enhances nutrient availability, and boosts crop yields Adding

5 to 8 tons of rice straw per hectare as green manure can increase rice yields by 10 to 15% Rice straw is also used as supplementary livestock feed, especially when combined with other nutrient-rich sources, improving livestock performance, and reducing feed costs Dairy cattle fed with rice straw exhibit enhanced milk production and reduced feed expenses by up to 30% Additionally, rice straw serves as a bedding material, improving livestock comfort, moisture absorption, and waste management Therefore, the incorporation of this type of biomass into agricultural systems promotes nutrient cycling and carbon sequestration, facilitating the release of essential nutrients and mitigating greenhouse gas emissions

Chemical composition determines the nutritional quality of rice straw, which is important for livestock feed, anaerobic digestion, and soil amendment Rice straw has low nutritional value and research has been done to improve it Jenkins (1998) indicated that the typical components of plant biomass are moisture cellulose, hemicelluloses, lignin, lipids, proteins, simple sugars, starches, water, hydrocarbon, ash, and other compounds The concentrations of these compounds depend on the plant species, type of tissue, growth stage, and growing conditions Rice straw is considered a lignocellulosic biomass that contains 38% cellulose, 25% hemicellulose, and 12% lignin Compared to the biomass of other plants, such as softwood, rice straw is lower in cellulose and lignin and higher in hemicellulose content Table 1-1 shows the comparison of rice straw with other biomass sources

Table 1-1 Lignocellulose contents in 4 different types of biomass [2]

Rice straw Pineapple leaves Wheat straw Coco-peat

Table 1-2 Proximate composition and selected major elements of ash in rice straw, rice husk, and wheat straw [3]

Rice straw Rice husk Wheat straw Proximate analysis (%dry fuel)

The chemical composition of feedstock has a major influence on the efficiency of bioenergy generation Table 1-2, Lists the chemical properties (fuel properties) of three agricultural residues: rice straw, rice husk, and wheat straw to highlight the particulate differences in feedstock Rice straw has low-quality feedstock primarily determined by a high ash content (10 - 17%) as compared to wheat straw (around 3%) and high silica content in ash in rice (SO2 is 75%) straw and in wheat 55% On the contrary rice straw feedstock has relatively low total alkali content (Na2O and K2O comprise < 15% of total ash) whereas wheat straws have < 25 alkali content in ash Based on its slagging index, Rs 0.04, fouling index, Rf 0.24 Rice straw is not expected to have significant operating problems or different emissions compared with wheat straw and rice husk under similar operating conditions Rice husk which is also of poor feed quality, is caused mainly by high silica content, but its uniformity in size and easy procurement are advantages Thus the preferred use of this biomass for energy is related to both availability and quality

The calorific value is an essential parameter that shows the energy value of rice straw if to be used for bioenergy The energy efficiency of rice straw can be calculated by dividing its energy output by its calorific value, which may be expressed as the higher-heating value (HHV), wherein latent heat of the water is included, or lower- heating value (LHV) In terms of calorific value, rice straw has an HHV that ranges from 14.08 to 15.09 MJ kg −1 [3], as determined by different studies Fixed carbon refers to the carbon left after the volatiles are driven off Rice straw has a fixed carbon ranging from 11.10% to 16.75%, which is also comparable to other biomass The ultimate analysis reveals the elemental carbon, hydrogen, oxygen, nitrogen, and sulfur composition of rice straw In contrast to fossil fuels, rice straw biomass has a lower carbon content, but higher oxygen and hydrogen contents The ash content of rice straw, comprising noncombustible residues, ranges from 18.67% to 29.1% [3] The elevated silica content in rice straw poses challenges in processing machinery, such as conveyors and grinders, as well as in boilers Additionally, it diminishes the digestibility of rice straw when utilized as fodder Compared to wood and coal, rice straw exhibits a higher volatile matter and a lower fixed carbon content The substantial ash content in rice straw not only reduces its calorific value but also poses challenges in energy conversion processes The significant potassium and alkali content in the ash can contribute to corrosion and fouling issues in grates, as alkali metals are recognized catalysts for these phenomena

Rice straw has various agricultural uses, including mulching, green manure, and livestock bedding Its utilization provides benefits such as soil moisture conservation, weed suppression, improved soil fertility, and nutrient cycling Studies in Vietnam show that rice straw mulch can reduce soil evaporation by up to 20% [4] and inhibit weed growth by about 70% Incorporating rice straw into the soil as green manure increases soil organic carbon content, enhances nutrient availability, and boosts crop yields Adding 5 to 8 tons of rice straw per hectare as green manure can increase rice yields by 10 to 15% Rice straw is also used as supplementary livestock feed, especially when combined with other nutrient-rich sources, improving livestock performance and reducing feed costs Dairy cattle fed with rice straw exhibit enhanced milk production and reduced feed expenses by up to 30% Additionally, rice straw serves as a bedding material, improving livestock comfort, moisture absorption, and waste management Its incorporation into agricultural systems promotes nutrient cycling and carbon sequestration, facilitating the release of essential nutrients and mitigating greenhouse gas emissions

Rice straw has great potential as a feedstock for renewable energy production, specifically in biogas generation and bioethanol production Biogas yield from rice straw can range from 120 to 160 cubic meters per metric ton [5], offering a clean energy source and waste management benefits Rice straw can also be converted into bioethanol, with a yield of approximately 200 liters per metric ton [6], reducing greenhouse gas emissions compared to fossil fuels Moreover, rice straw can fuel biomass power plants, generating electricity at capacities ranging from 1 to 10 megawatts [7] This renewable energy utilization of rice straw reduces reliance on fossil fuels, mitigates climate change, improves air quality by avoiding open burning, and contributes to rural electrification However, challenges such as collection, transportation, and technological advancements need to be addressed Further research, infrastructure development, and supportive policies are essential to fully harness the potential of rice straw for sustainable energy production in Vietnam

Innovative applications of rice straw can transform it into value-added products, minimizing waste and creating economic opportunities This section explores specific data on the utilization of rice straw in the production of biochar, compost, and mushroom cultivation It highlights the environmental and economic benefits of these value-added products, such as carbon sequestration, soil improvement, and the creation of alternative revenue streams A very popular product that is derivative from rice straw is biochar which can be known as a carbon-rich material produced through the pyrolysis of biomass Research studies conducted in Vietnam demonstrate the potential of rice straw biochar in improving soil fertility and carbon sequestration Specific data reveals that biochar produced from rice straw can have a carbon content ranging from 30% to 40%, making it a valuable soil amendment [8] Furthermore, the addition of rice straw biochar to the soil can enhance water retention, nutrient availability, and overall soil health Moreover, lignin and silica can be extracted from rice straw, offering valuable components for various industries Lignin has applications in renewable chemicals, bioplastics, and biofuels, reducing reliance on non-renewable resources Silica, abundant in rice straw, is used in construction, electronics, and agriculture Its extraction from rice straw creates value-added products and reduces environmental impact Research and development efforts continue to optimize extraction techniques and explore new applications for these components such as dye pollution treatment by adsorption method, promoting their use in a range of industries.

Black liquor and lignin-silica composite from rice straw

1.2.1 Black liquor from rice straw

Black liquor, commonly known as a waste product of the paper pulping industry process, is a mixture containing mostly lignin, cellulose, hemicellulose, and many other organic and inorganic compounds recovered after the separation of cellulose from the structure of lignocellulose materials Throughout many years of development history, this waste stream has been treated simply by burning as fuels for the generation of steam and electricity in the paper industry [9] Realizing the urgent environmental problems, it was not until recent decades that many studies related to the use of black liquor as a raw material in the production of advanced biofuels and biochemicals have been implemented, especially in countries with strong policies that support the development of renewable energy and the bioeconomy In the European Union, the Renewable Energy Policy requires its member to increase the share of renewable energy in their transport sector to 10% by 2020 and up to 14% by 2030 [10] Accordingly, black liquor is considered a potential candidate that can meet these goals To date, several research projects and pilot plants have been conducted to test the feasibility of using black liquor for this purpose In the United States, black liquor is also considered as a potential raw material for the production of value-added products The Renewable Fuels Standards Program, established under the Energy Policy Act of 2005, requires a certain amount of renewable fuels to be blended into transportation fuels each year and black liquor is again a source of material that can be used to fulfill these requirements

However, up until this recent decade, black liquor was still only utilized in the paper industry, until the concept of biorefinery was being developed Black liquor recovered from the alkali pretreatment process of straw biomass contains significant amounts of silica and lignin (25.14% and 51.81% respectively) [11] Therein, lignin is a highly branched polymer with an amorphous structure and a molecular weight of

1000 to 20000 g/mol This polymer is made up of monolignol, also known as lignol, which is all derived from phenylpropane: coniferyl alcohol (4-hydroxy-3- methoxyphenylpropane) (G or guaiacyl), sinapyl alcohol (3,5-dimethoxy-4- hydroxyphenylpropane) (S or syringyl), and paracoumaryl alcohol (4- hydroxyphenylpropane) (H or 4-hydroxyphenyl) (Figure 1.1) The proportions of these monomer precursors vary by plant origin: hardwoods and grasses are rich in coniferyl and sinapyl units, and softwoods are rich in conifery units All lignins contain small amounts of incomplete or modified lignols, and other monomers (which are abundant in non-woody plants) [12] Natural lignin is usually colorless or pale yellow, but after treatment with acid or alkali, its color tends to change into brown or dark brown Owing to the structure of a chain of non-polar aromatic rings, lignin is almost insoluble in water or polar solvents and even in concentrated H2SO4 acids but becomes easy to be dissolved in hot alkalis solution due to the presence of phenolic hydroxyl groups In addition, this organic compound exhibits great solubility in organic solvents such as ethanol, tetrahydrofuran, or dimethyl sulfoxide The diverse number of functional groups including free phenolic hydroxyl, methoxy, benzylic hydroxyl, benzylic, or phenolic ethers with linear alcohols and carbonyl groups leads to compatibility with a wide range of industrial chemicals Lignin demonstrates similar properties and advantages to those of other renewable biopolymers including antioxidant, anti-fungal, and antibacterial activities, availability in large quantities as a by-product from industrial waste, and biodegradable [13]

Figure 1.1 Structure of lignin With these mentioned properties, lignin from lignocellulose structure can be applied directly without any further chemical modification and is able to be used as a component or filler in thermoplastics as a UV radiation stabilizer, antioxidant, flame retardant, and additive to promote product plasticity and flowability On the other hand, in some cases, this compound has been chemically modified, in which lignin undergoes the depolymerization process to form base compounds such as aromatics, alkanes, phenols, and derivatives – the input source for polymer synthesis or to produce chemicals, fuels, and high-value products [14]

During the black liquor treatment process, silica holds a significant proportion of the recovered solid, which is generated through the absorption of water-soluble silicic acid (H4SiO4) combined with polymerization reactions to form insoluble polysilicic acid, then precipitating as amorphous silica and depositing in the plant cell wall [15]

In terms of properties, the solubility of silica is a temperature-dependent ability; as shown in Figure 1.2, this value peaks at 340 °C [16] In particular, silica can react with alkalis and base oxides at high temperatures to create a solution of silicate salt [17]

Figure 1.2 Solubility of silica depending on temperature

The thermal stability of silica is significantly greater than cellulose, hemicellulose, and lignin, with a melting point of 1713 °C Furthermore, amorphous silica obtained from biomass has a large specific surface area, which can be up to 580 m 2 /g [18] Therefore, silica is one of the most important components and can be found in various fields such as biological materials production, environmental treatment, construction, synthesis of composite materials, and the production of medical devices [19] The main reason for the wide range of applications of silica could be deduced by the unique properties of this material relating to its chemical, physical, and thermal stability, its compatibility with numerous materials, and its availability (existing in agricultural by-products) with reasonable prices

1.2.2 Lignin-silica composite from black liquor

In recent years, a new approach towards biorefinery of agricultural by-products has been introduced as a ‘lignin-first approach’ with the concept of reducing the depolymerization of lignin as the key factor in consideration [20] This approach may provide both an enhanced property of recovered lignin and provide a better framework for holocellulose production

With the applicability of lignin/silica composite as mentioned above, many efforts have been conducted to find out the methods to extract lignin and silica from black liquor and combine it to become a valuable composite In which, some methods have successfully taken advantage to prepare lignin/silica hybrids such as solvent blending or in-situ polymerization methods [21]–[24] In the solvent blending method, lignin is dissolved in a solvent, and silica particles are dispersed in the solution After removing the solvent, the lignin/silica composite is obtained When it comes to in-situ polymerization, monomers are polymerized in the presence of lignin and silica, creating a composite with a chemically bonded polymer matrix These methods enable the synthesis of lignin/silica composites with tailored properties and diverse applications Overall, these mentioned methods require a complex process and specific chemicals to conduct the operation for preparing lignin/silica composite Hence, it is necessary to find novel methods that could overcome all these aforementioned drawbacks In the journey of seeking alternative processes, the sol- gel method can satisfy all requirements to assure the evaluative criteria of product quality and environmental safety During this process, the composite is co- precipitated with sulfuric acid H2SO4 solution for simple production capable of industrial applications Specifically, in a previous publication by our research group [25], lignin-silica materials were synthesized through an acidification step to adjust the pH of the black liquor after the alkali treatment of rice straw from a value of 12 to values of 9 and 3 By combining other analysis methods, including XRD, SEM, and FTIR, the material was proven to consist of silica particles deposited on lignin chains, as well as chemical bonds between these two substances

Many studies have shown evidence of a chemical connection between lignin and silicon in plants, including FTIR adsorption band of 940 cm −1 indicating a connection between Si-O and lignin (Figure 1.3a) and Raman scan of a strong band of 973 and

802 cm −1 , suggesting a Si-O-Si and Si-C connection [26] In another study, silicon connection with organic compounds was proposed to include 3 possible linkages: phosphoryl, carboxyl, and hydroxyl connection [27] Biological studies propose the idea of the hydroxyl group in lignin, tannin, and polysaccharides of plants crosslinking with monosilicic acid [28], supporting the previous hypothesis Until recently, these assumptions of the exact connection and mechanism of formation between lignin and silica bonds remained unclear until one research precipitates lignin and silica simultaneously and evaluates any chemical bond changes in NMR

29Si SP/MAS NMR spectra of silica – lignin precipitated compounds display the formation of hydrogen bonds (Figure 1.3b) – which is believed to be the main connection between silanol groups The formation of hydrogen bonds is caused by the substitution mechanism between lignin and silica, which is suggested by the differences in the distribution of hydroxyl groups before and after on silica surfaces

Figure 1.3 Lignin-silica linkage in plant Besides, other studies have indicated that lignin-silica samples synthesized through this method may exhibit variations in the silica and lignin content (with silica content being four times higher than the lignin content [29] and silica particle diameter of over 1000 nm [30] This can be explained by the complex branching structure of lignin, which may differ depending on the specific biomass source and the influence of the surrounding environment Similarly, silica particles in plants can vary from 5.69% to 9.95% in different growing seasons, and so on Based on these findings, in-depth studies are needed to ensure the stability and essential properties of lignin-silica structures for each specific feedstock source, taking into account factors such as cultivation seasons, soil nutrient levels, initial biomass composition, and more

1.2.3 Carbon materials derived from lignin

As mentioned above, lignin is a complex polymer containing diverse functional groups such as hydroxyl, methoxy, and phenolic groups This diversity imparts chemical and mechanical properties to lignin when it is transformed into carbon materials The complicated structure of lignin allows for the creation of carbon materials with diverse and tunable properties For instance, through thermal or chemical treatments, this organic compound can be converted into different forms of carbon, such as activated carbon, carbon powder, or carbon fibers The lignin network also affects the physical properties of carbon materials, including hardness, strength, flexibility, and electrical conductivity The versatility and tunability of carbon materials from lignin open up numerous potential applications For instance, carbon materials derived from lignin is able to be used in the production of lightweight, flexible, and sustainable construction materials They can also be applied in the electronics industry and renewable energy applications such as lithium-ion batteries However, the complex structure of lignin also requires more sophisticated production and processing methods to obtain high-quality carbon materials Research and development of advanced technologies to take full advantage of lignin are important challenges for the development of carbon materials from renewable and sustainable sources

Due to the high carbon content and abundant aromatic compounds, lignin is considered a suitable feedstock for carbon material production through various large- scale thermochemical processes The fundamental principle of these processes involves depolymerizing lignin molecules and transforming them into biochar materials possessing aromatic hydrocarbon structures, with the assistance of catalysts, high temperatures, or high pressures Recent studies have shown that there are common carbonization methods for lignin-derived materials, such as pyrolysis, hydrothermal carbonization, and plasma-assisted pyrolysis

Composite material obtained from the pyrolysis route

Introduction of micro-fibrillated cellulose (MFC)

MFC, also called cellulose microfibril, microfibrillar cellulose, has been reviewed quite recently, particularly in terms of nanocomposite applications [45] Microcellulose fibers have an area network of bundled microfibrils, which form slender and seemingly infinite rods When these microfibrils are dissolved in strong acids, they form cellulose microcrystals, which are short crystalline rods with a diameter of 9–16 μm [46] MFC is manufactured by fibrillation of cellulose fibers into nanoscale components and requires a lot of mechanical treatment Chemical treatments may be applied prior to mechanical fibrillation, depending on the raw material and the degree of processing These chemical methods generate purified cellulose, such as bleached cellulose pulp, which can then be treated further There are alternative ways with lower energy needs, such as enzymatic pre-treatment followed by mechanical treatments to isolate cellulose microfibrils

Pretreatment of cellulose fibers is a technique for lowering the energy consumption of mechanical nanofibrillation procedures while increasing the degree of nanofibrillation The pretreatment process has become a crucial step because energy expenditure is a large drawback for producing nanofibers [47]

Enzymatic pretreatments enable the manufacture of MFC using much less energy

In nature, cellulose is not degraded by a single enzyme, but a set of cellulases is included These can be categorized as A- and B- type cellulases, also known as cellobiohydrolases, which may target highly crystalline cellulose; C- and D-type cellulases or endoglucanases which generally require some structural to degrade cellulose Svagan et al and López-Rubio et al both used a combination of mechanical and enzymatic treatments [48] The pulp was treated in four different processes to delaminate the cell walls: a refining step using an Escherwyss refiner to increase the accessibility of the cell wall to the subsequent enzyme treatment, an enzymatic treatment step with monocomponent endoglucanase, a second refining stage, and finally a step where the pulp slurry was passed through a high-pressure microfluidizer

Alkaline–acid pretreatment is the typical approach method for lignin, hemicellulose, and pectin solubilization removal before mechanical isolation of nanocellulose The steps in this process were as follows: Firstly, the fibers were soaked in an alkaline solution This raises the surface area of cellulosic fibers and facilitates the hydrolysis before immersing them in an acid solution; this process solubilizes the hemicelluloses Finally, a greater concentration of alkaline solution is used The lignin structure is disrupted, and the carbohydrate-lignin connections are broken [49]

MFC produced by mechanical treatments was first reported by Herrick et al and Turbak et al in 1983 Their methods were based on passing dilute cellulosic wood pulp-water solutions through a mechanical homogenizer with a large pressure drop to aid microfibrillation MFC is currently a commercial product that can be purchased from various companies and other organizations (e.g Rettenmaier, Germany; Daicel, Japan or Innventia AB, Sweden) [50] Mechanical treatment, which includes high- pressure homogenization, microfluidization, grinding, cryocrushing, high-intensity ultrasonication, and ball milling, is currently used to create MFC, which is based on refining and high-pressure homogenizing process steps The fibers undergo irreversible modifications as a result of the mechanical treatment, boosting their bonding potential by altering their shape and size [51] Mechanical refining procedures, on the other hand, either damage the microfibril structure by lowering molar mass and degree of crystallinity or fail to disintegrate the pulp fiber enough Due to the fact that the refining process causes exterior fibrillation of fibers by progressively peeling off the external cell wall layers, it is done before homogenization Internal fibrillation loosens the fiber wall, allowing the pulp fibers to be homogenized thereafter [52]

MFC is characterized by its high porosity, great absorbency, exceedingly low thermal conductivity, high strength-to-weight ratio, and large surface-to-volume surface As a result, considerable research has been conducted to develop porous materials, such as aerogels, from this natural, renewable, and biodegradable cellulose resource Also, various active substances can be immobilized in cellulosic substrates Those characteristics of MFC are a consequence of its microstructure [53]

The cross-linking method employed during the gelation process has a significant impact on the final structure and performance of porous materials Physical crosslinking is often based on weaker forces like hydrogen bonding and electrostatic interaction Chemical crosslinking, on the other hand, frequently results in stronger interactions and networks, such as covalent bonding and polymerization The most prevalent gelation technique is hydrogen bonding between nanocellulose or between nanocellulose and additional additives Typically, electrostatic interactions usually occur between the negatively-charged nanocellulose and positively charged crosslinking agents Divalent cations are commonly utilized to electrostatically generate ionic crosslinking between anionic nanocellulose Covalent crosslinking is created by the action of covalent bonds to produce crosslinking between nanocellulose and reactive coupling agents Both covalent and polymerization crosslinking produce gelation by forming new covalent bonds, considerably improving network mechanical strength [54]

Crosslinking via physical hydrogen bonding before freezing is widely adopted to achieve gelation [55] Physical gelation of nanocellulose-based lightweight porous materials tends to show limited structural stability Chemical gelation by covalent crosslinking, on the other hand, is more advantageous for improving mechanical performance To achieve covalent gelation of nanocellulose, various crosslinking agents, such as maleic acid (MA), butane tetracarboxylic acid (BTCA) [56], polyamide epichlorohydrin (PAE) [57], γ-glycidopropyltrimethoxysilane (GPTMS) [58], methyltrimethoxysilane (MTMS) [59], dialdehyde starch [60], and 3- isocyanatopropyltriethoxysilane, etc Zhang et al [61] developed an NFC/MFC aerogel with excellent wet strength by utilizing a PAE cross-linker that generated both self-crosslinking and external crosslinking with cellulose The crosslinked aerogels with hierarchical pore structures absorbed a lot of water and recovered their shape in both air and water quickly Guo et al [62] prepared N-methylol dimethylphosphonopropionamide and BTCA as cross-linkers to create a flame- retardant NFC aerogel The composite aerogel’s microstructure was comparable to that of the pure NFC aerogel, but the interlayer gap increased, and a more fibrous structure emerged at the interlayers The composite aerogels exhibited exceptional ductility due to the development of covalent ester linkages between the hydroxyls of NFC and the carboxyls of BTCA Cheng et al covalently crosslinked a durable and flexible NFC cryogel using GPTMS and PEI The highest shape recovery rate of NFC cryogel under 50% compression strain was as high as 93% of its original thickness due to the interaction of the polysiloxane layer and the high degree of cross-linking Jiang et al (2020) [63] combined CNF with MTMS and fumed silica with a simple method to create a porous and hydrophobic CNF aerogel with extremely low thermal conductivity (0.027 W.m -1 K), MTMS in the research functions as a crosslinker as well as a hydrophobic modifier

Mainly, most freeze-dried nanocellulose aerogels are formed by immersing the precursor dispersion/gel directly into liquid nitrogen (-196 o C) or refrigeration (-18 oC) After sublimation, The ultralow liquid nitrogen leads the water solvent to form ice crystals quickly, leaving dense and small pores in the aerogel after sublimation Especially, Because of the slower freezing rate, it takes longer to produce bigger pores when employing a cold source with a higher temperature However, this traditional freezing method also presents limitations, mainly that the microporous structure of the aerogel is usually isotropic The chaotic structure prohibits the aerogel from obtaining a directional mass transfer, heat transfer, electrical conductivity, and other functions

The drying procedure is crucial for regulating and controlling pore structure The common method for preparing nanocellulose-based aerogel is air-drying However, it does has some serious issues, such as adhesion produced by solvent surface tension and collapse of the internal network structure caused by capillary action of the solid matrix In order to reduce these problems, freeze-drying can be used to cross the solid- gas boundary, which is known as sublimation Sublimation is the process by which water alters its form from solid to vapor without passing the liquid phase at the Eutetti point on the diagram of water (Figure 1.5) The second method for avoiding the liquid-gas interface is to raise the temperature and pressure over the critical point, a process known as supercritical drying Both freeze-drying and supercritical-drying can efficiently solve the aforementioned issues while retaining the porosity structure [64]

Figure 1.5 Water phase diagram Additionally, the advantages and disadvantages of some drying methods are listed in Table 1-3 below:

Table 1-3 Comparison of some drying methods

` Criteria Requirement Limitation Cost/risk

Often requires material structure to be hydrophobic

Only suitable for aerogel with

Low cost, easy to do room pressure density >0.1 g.cm -3

The solvent must be compatible with CO2 under the drying conditions of the process The Hydrogels must exchange the solvent

The kinetic energy of the drying process depends on the exchange of

CO2/solvent in/out to form the structure

This type of drying is suitable for gels

Suffer significant energy loss due to CO2 compression process, risk of explosion due to low- temperature combined with

It is crucial to limit shrinkage and ensure the freezing point of the material

It is difficult to reach the goal of drying to a density of 0.03 g.cm -3

High energy consumption due to the demand of maintaining a vacuum state

For instance, in Vietnam, the agricultural industry is one of the key contributors to developing the country Along with a high population and birth rate, the increasing demand for food production results in massive waste generation Those residues contribute to many environmental problems because they are disposed of by burning, left to decompose, or are landfilled directly Because of their numerous supplies, biodegradability, and largely pollution-free agricultural waste, the creation of high- performance materials from agricultural waste has piqued the interest of researchers and government officials

Figure 1.6 FE-SEM images of the developed PF aerogels with different PF concentrations of (a) 0.5 wt.%, (b) 1.0 wt.%, (c) 2.0 wt.%, (d) water contact angles of the sample Research group of Assoc Le Thi Kim Phung, Refinery and Petrochemical Technology Research Center (RPTC) has conducted many projects in this field For example, in 2020, N H Do et al fabricated pineapple aerogels from agricultural waste to create a heat and sound insulation material [65] Firstly, pineapple leaf fibers were blended to a diameter below 20 àm, followed by dispersing in 0.2 wt% PVA solution The mixture is sonicated and crosslinked at 80 ℃ for 2 hours for the formation of cross-linking between PVA and pineapple leaf fibers before freezing in a refrigerator Freeze-drying was carried out to obtain the aerogel sample Obtain aerogel was modified from hydrophilic to hydrophobic by MTMS for adsorption applications Aerogels at different pineapple fibers (0.5 wt%, 1.0 wt%, and 2.0 wt%) were evaluated FESEM image (Figure 1.6) showed the success in three-dimensional structure via hydrogen bonding in which the pore size ranges from 1.38 to 2.21 nm Obtained aerogels had very low densities (12.72 to 32.63 mg.cm -3 ) and high porosities (96.98 to 98.85%) (Figure 1.7) Furthermore, after coating MTMS, aerogels exhibited the hydrophobic property, where the contact angle was 146.1 o Therefore, the pineapple leaf fibers aerogels displayed an excellent oil adsorption capacity of 26.6 to 37.9 g.g -1

Figure 1.7 Densities and porosities of pineapple leaf fibers aerogel with various fiber concentrations

On the other hand, they investigated the thermal insulation abilities of the as- synthesized aerogel, and their K-values are measured via the C-Therm TCi Thermal Conductivity Analyzer System The thermal conductivities for all the developed aerogels are extremely low (0.030 – 0.034) W.m -1 K -1 , making them very good thermal insulators Because of their highly porous structures, this aerogel is mainly composed of air Therefore, the aerogel’s low thermal conductivity might be attributed to its high porosity since air is a good heat insulator (0.026 W.m -1 K -1 ) (Table 1-4) at ambient temperature and pressure Moreover, the thermal conductivity of PF aerogel increases along with increasing fiber concentration This observation is due to a decrease in pore volume, meaning less volume of air captured inside Also, the thermal path through aerogel consists of air and fiber paths; an increase in fiber concentration increases the fiber pathways, with a consequent increase in the overall thermal conductivity The low thermal conductivity, the eco-friendliness of cellulose aerogel, and the cost-effectiveness of pineapple waste make PF aerogel a promising candidate for heat insulation

Table 1-4 Advanced multi-properties of the cellulose aerogels

Lignin and cellulose-based electrodes

1.4.1 Working mechanism of lithium-ion batteries

During the charging process, the cathode LiMO2, composed of M and oxygen, forms a strong bond, borrowing electrons from Li This occurs because MO2 has a high affinity for electrons, and Li readily provides them When the battery is connected to the charger, Li + ions are released from the crystal structure into the electrolyte solution Simultaneously, electrons are freed from the cathode and flow towards the anode However, MO2 becomes more electronegative, preventing the simultaneous release of ions and electrons during charging As each electron is removed from the cathode, the cathode becomes more electronegative, requiring higher voltage to separate further electrons due to the stronger hold The Li + ions naturally move towards the anode through diffusion

During the discharge, the Li + is stored at a ratio of 1 Li + per 6 carbon atoms, which is the most thermodynamically stable The battery is now fully charged Li now has electrons that can be readily removed but there is nowhere for those to go due to the non-conducting electron of the electrolyte When the electrical pathway opens between the anode and cathode, all electrons sense the energy imbalance The anode has an abundance of electrons whereas the cathode is seeking electrons Therefore, the Li releases electrons to carbon material, which travels to the current collector and then the wire The MO2 structure accepts the electron combined with the Li + and then returns to its original LiMO2 state The detailed reaction of the whole process is displayed as follows:

LiMO Li  MO e  Li  MO 2 e  LiMO 2

C Li  e  LiC LiC 6 C 6 Li  e  Cell reaction LiMO 2 C 6 MO 2 LiC 6 MO 2 LiC 6 LiMO 2 C 6 1.4.2 Lignin-based electrodes

Carbon materials with high surface area, porous structures with certain electrical conductivity, high mechanical strength, and good charge-carrying capacity are increasingly being studied in energy storage applications, especially in batteries, fuel cells, and supercapacitors These supercapacitors, fuel cells, solar cells, and other electrical energy storage devices typically require easily tunable pores, thin pore walls, large surface areas, and nanoscale structures to create favorable conditions and regulate the adsorption of charge carriers such as ions and electrons Therefore, carbon materials from biomass are also of interest

Upon pyrolysis, lignin undergoes carbonization under a solid state, meaning that no liquid is formed thanks to the oxygen-rich structure of the lignin molecules Therefore, in contrast to carbon materials from petroleum precursors, carbon materials from lignin are harder, have an amorphous structure rich in sp 3 hybridization in their C–C bonds, and are difficult to be graphitized at elevated temperatures [66] With its high mechanical strength and large specific surface area, lignin-based carbon materials have attracted much interest in energy storage applications Lignin was recovered from the black liquor of rice husk to convert into porous carbon material to be applied in the anodic electrode of sodium and potassium batteries Activated carbon was synthesized from lignin precursors under different activation conditions, namely KOH, H3PO4, and steam, exhibiting suitable properties as high surface area (1152, 1961, and 1649 m 2 /g, respectively), retaining the lignin- derived functional groups and a dense system of micropores These properties increase the area of interaction between the electrolyte and the positive electrode, elongating its life cycle and enhancing the maximum discharge capacity to 7.2 mAh/cm 2 for KOH-activated lignin-derived carbon material [67]

In addition, thanks to the high lignin and silica content with the high potential of Si-based compounds in energy storage, new methods for the synthesis of composite materials from the black liquor of rice straw are being studied for applications in energy storage Silica has been used as multiple components of a fuel cell – a device that converts the chemical energy of common hydrogen sources such as gaseous hydrogen, formic acid, and ethanol into electrical energy The hygroscopic properties of silica can be maintained at high temperatures and low humidity conditions, providing the basis for the utilization as a proton exchange membrane with high performance at critical operating parameters that are required for better electrical oxidation properties [68] In solar cells, SiO2 with a stable high surface area is combined with TiO2 into a semiconductor material in dye-sensitive solar cells (DSSC) The higher surface area of TiO2/SiO2 nanocomposites exhibits higher photoelectric enhancement and higher field-dependent photoconductivity compared to individual TiO2 nanoparticles [69]

Similarly, LIB with silica anodes often undergo a drastic change in volume during the charge and discharge of lithium ions The hard structure of silica that undergoes this intense expansion and contraction can easily break, resulting in a decrease in the electrical capacity over time One solution to this phenomenon is to introduce carbon materials with porous structures [70] into SiO2 or Si electrodes to form complexes in the form of SiO2/C [71] or Si/C [72] to exhibit high and stable long-cycle charging and discharging performance On this basis, the direct carbonization of lignin-silica substrates to obtain SiO2/C and Si/C composite materials offers a lot of potential A study has successfully synthesized Si/C materials from rice husks with advanced technologies of ball milling, thermal magnetic reduction, and activity addition with excellent electrochemical properties such as high specific capacity (572 mAh/g at 1A/g) and high stability even after 1000 cycles

Figure 1.8 provides a comprehensive comparison of the electrochemical behaviors of various anode materials derived from lignin and those without lignin The "Lignin/Cellulose" electrode, prepared from a mixture of KL and CA, exhibited a carbon network with specific characteristics: an interplanar spacing of 0.384 nm,

13.26% oxygen content, and a specific surface area of 540.95 m 2 /g This nanocarbon network structure demonstrated a specific capacity of 290 mAhg 1 with a coulombic efficiency of 52% in the first cycle and an improved capacity of 340 mAh/g with a coulombic efficiency of 99% after 200 cycles at a current density of 50 mA/g In contrast, the "Cellulose" electrode, derived solely from CA, displayed an initial capacity of around 30 mAh/g lower and a coulombic efficiency of 46% after 200 cycles The carbon network formed from KL/CA nanofibers retained parts of the fibrous structure, facilitating both electron and ion transport [73]

Figure 1.8 Electrochemical performance of lignin-based anode and non-lignin- based anode: (a) cycle performance of the lignin-derived carbon anodes and lignin- free anodes, and charge-discharge pro f iles of (b) carbonized cellulose and lignin/cellulose (E-KL/CA-C, E-CA-C) at 50 mA g 1, (c) Si and lignin/Si(Si with lignin-derived carbon composite) at 0.1 A/g, (d) carbonized lignin with/without KOH treatment at 200 mA g 1, and (e) carbonized lignin with/without nitrogen doping 25 mA/g

Anodes with a well-defined pore structure play a pivotal role in Li-ion transport and offer an expanded surface area for swift reaction kinetics The "Lignin/KOH" sample, resulting from the blending of alkali lignin and KOH, demonstrated a capacity of 470 mAh/g over 400 cycles at a current density of 200 mA/g Conversely, the "Lignin without KOH" sample, derived from lignin without chemical activation, achieved a capacity of 180 mAh/g after 400 cycles

Lignin-derived carbon proves versatile for hosting various redox-active materials, with silicon (Si) being a particularly promising anode solution for future LIBs due to its excellent theoretical capacity and cost-effectiveness To address challenges like low electrical conductivity and volume changes during the lithiation– delithiation process, the "Si/Lignin" sample, formed through a coprecipitation process of Si/lignin composite and subsequent annealing, displayed an initial charge capacity of 1016.8 mAhg 1 It maintained high-capacity retentions of 74.5% and 57.5% after 100 and 200 cycles, respectively, at 0.2 A/g [74]

Enhancing the electrochemical performance of carbons involves incorporating surface functionalities such as phosphorus (P) and nitrogen (N) The "n-doped Lignin" sample, obtained through carbonization with a 3-aminophenol (nitrogen source), displayed characteristics like a small graphitic region, significant micro-, meso-, and macro-pores, and an enlarged interlayer space In comparison, the

"undoped Lignin" sample, without nitrogen doping, exhibited a lower specific area and pore volume N-doped carbons demonstrated a high reversible specific capacity of 374 mAh/g at 25 mA/g and excellent capacitance retention of 90% after 300 cycles at 100 mA/g, showcasing stable sodium-ion intercalation/adsorption [75], [76]

A versatile array of electrode materials, stemming from a cellulose like paper sheets and textile fibers, finds practical utility in Li-ion batteries Integration of current collectors with paper sheets, achieved through coating or printing techniques, results in the formation of electrode assemblies Pioneering the field, Hu et al introduced the Meyer rod coating technique in 2009 to craft highly conductive cellulose papers, exploring their efficacy in energy storage materials [77] By enveloping commercial Xerox papers with carbon nanotubes (CNTs) and silver nanowires, they achieved a sheet resistance of 1 Ω per square Examined in a supercapacitor, the resulting sample exhibited outstanding specific capacitance, durability, and a cycling life exceeding 40,000 cycles With a specific power of 200 KW/Kg and specific energy ranging from 30 to 47 Wh/kg, the study underscored the high conductivity, porosity, and mechanical and chemical stability of the derived cellulosic films, contributing to the advancement of high-performance lithium-ion batteries

In a novel lamination approach, all LIB components were consolidated onto a single sheet of cellulose paper coated with polyvinylidene fluoride (PVDF) This technique significantly enhanced the mechanical integrity of electrode materials compared to direct encapsulation on cellulose paper, effectively preventing leakage through the paper separator The resulting paper battery, with a mere 300 μm thickness, displayed exceptional flexibility and an energy density of 108 mWh/g [78] Leveraging the innate flexibility and mechanical strength of cellulose proved advantageous in anode preparation Flexible anodes for Li-ion batteries, based on cellulose/graphite paper, exhibited Young moduli spanning from 60 to 450 MPa, showcasing commendable tensile characteristics [78] Additionally, recording discharge capacities of 350 mA h/g, the study demonstrated comparable cycling performance to conventional graphitic anodes

Application of lignin in food preservation

1.5.1 Superior properties of lignin in food packaging film

There are generally three types of agents used for UV blocking, antioxidant, and antimicrobial purposes: organic, inorganic, and synthesized substances Therein, the employed synthetic material is typically known as metal-organic framework [84], zinc oxide–treated halloysite nanotubes, multi-walled carbon nanotubes and titanium dioxide [85], or polymer latex coatings which are usually fabricated via a complicated process For instance, Khan carried out a similar investigation in which they synthesized carbon dots derived from green tea, specifically nitrogen and phosphorus-doped green tea carbon dots (NP-CDs) These NP-CDs were integrated into multifunctional nanocomposite films based on chitosan and starch The addition of NP-CDs significantly improved the UV-blocking capabilities, achieving 93.1% for UV-A and 99.7% for UV-B radiation Subsequently, these fabricated films were assessed for their meat packaging applications, demonstrating that the UV-blocking films effectively extended the shelf life of the meat [86] Meanwhile, inorganic materials provide effective solutions for UV-blocking, incorporating compounds such as silver oxide, titanium dioxide, zinc oxide, and copper oxide These elements are traditionally integrated into skincare products, safeguarding the skin from UV radiation damage Ongoing research explores inorganic nanoparticles due to their high surface area-to-volume ratio, enhancing their efficiency in absorbing ultraviolet radiation [87] Notably, zinc oxide, widely used in skincare items like sunscreen, is valued for its photocatalytic, antibacterial, and optical properties, making it a versatile choice in sun protection products [88] Muhammad et al fabricated an MnS2-SiO2 composite to work as a photocatalyst, antioxidant substance, and antimicrobial agent The result shows that, at the DPPH concentration of 200 ppm, the obtained material performed nearly 80% scavenging activity with greater antifungal attributes vs T veride, and A flavus as compared to other catalysts like SiO2 [89] In another circumstance, Channa et al achieved a substantial 40% enhancement in UV absorption by developing nanocomposite films with polyvinyl alcohol and zinc oxide The fabricated UV-blocking films were employed to package apples and examine the oxidation rate The findings demonstrated a notable extension of the time it took for oxygen levels to rise, with a delay of 10 hours compared to the unpackaged apples [90] The potential migration of these nanoscale materials from food packaging to food items raises concerns not only about human health but also regarding the cost of the final Moreover, these substances exhibit limitations concerning renewability and biodegradability, which can have adverse environmental implications The tendency of these nanomaterials to aggregate within the polymer matrix diminishes their UV- blocking efficiency and increases opacity Hence, it is imperative to explore alternative solutions using environmentally friendly materials on a broader scale

At present, biopolymers are particularly useful in a variety of food-packaging applications due to their availability, eco-friendly connotation, and distinct properties [91] Chitosan is a linear amino polysaccharide containing d-glucosamine and N- acetyl-d-glucosamine units that is derived from chitin following deacetylation Chitosan has been used in a variety of industries, including medicine, agriculture, food, textiles, the environment, and bioengineering, due to its antibacterial activity, nontoxicity, biocompatibility, biodegradability, and chelating capabilities… [92] Besides, chitosan can be produced from many inexpensive sources like crustacean waste, fungi, and insects Therefore, many studies have developed techniques to produce packaging film from chitosan such as antimicrobial film, reducing weight loss packaging, and antibacterial film [93] However, according to the report of Nair [94], incorporating macromolecules into chitosan-based films is a viable strategy to improve their performance Lignin is a natural aromatic polymer made up of three precursors linked by C–O and C–C bonds: guaiacyl alcohol (G), syringyl alcohol (S), and p-coumaryl alcohol (H)) Due to its structure with phenolic hydroxyl groups, and aliphatic hydroxyl groups, lignin was considered a source of antioxidants and a natural UV protector Also, lignin has an aromatic and strongly cross-linked structure, and its functional groups make it particularly reactive As a result, lignin may interact with a wide range of polymers, altering their water solubility, fire resistance, and mechanical characteristics[95] Specifically, lignin has phenolic units, ketones, and chromophore functional groups therefore lignin can absorb a broad spectrum of UV light in a range of 250 - 400 nm Lignin is used as a natural chemical as a UV blocking agent for products such as sunscreen cream, transparent film, paints, varnishes, and microorganism protection Lignin from pulping process waste solution contains UV chromophore functional groups such as quinones and methoxy substituted phenoxy groups, that can be conjugated with double bonds or carbonyl functional groups Lignin has a brown/black color due to the absorption of visible light of the unsaturated functional groups The chromophores in the lignin structure have different UV absorption spectra as shown in Figure 1.10

The electrical conjugation between the vinyl group para and phenolic OH, which provides UV chromophores at the coupling sites in lignin, is lost during the polymerization of monolignols to produce lignin The capacity to absorb UV light is given to lignin by these chromophores, Figure 1.11 shows the strong ability to absorb

UV light of the material with different lignin concentrations In order to provide UV protection without burdening the environment, lignin derivatives obtained from plants are anticipated However, the application of common lignin derivatives as efficient UV absorbers is challenging without the deep coloring of mixed resins/polymers due to the deep brown or black coloring of lignin derivatives derived by a thermochemical extraction procedure

Figure 1.10 UV absorption spectra of different chromophores in lignin

Figure 1.11 Absorbance at wavelengths 200−800 nm with different lignin concentration %w/w

The antibacterial properties of lignin are a result of the phenolic units and oxygen- containing functional groups like carboxyl, ether, and carbonyl (aldehydes and ketones) Lignin has antibacterial properties against both gram-positive and gram- negative bacteria under normal and low-temperature settings, according to reports

The phenolic component in the side chain structure of lignin is believed to be the source of the substance's anti-microorganism effects, according to certain research [96] The double bond in α and β positions of the side chain and methyl group in the γ position gives the phenolic of lignin strong antibacterial properties Lignin is frequently coupled with other antimicrobial components to increase its effectiveness

In some cases, Chitosan is an excellent biopolymer that can incorporate with lignin through the polysaccharides' hydroxyl and ether groups leading to the formation of a number of ionic interactions Chitosan has antibacterial capabilities due to the amine functional group and is also biodegradable The combination of lignin and chitosan is useful for applications such as preservation biofilm that assist in addressing the environmental problem brought on by non-degrading plastic

1.5.2 Chitosan-lignin composite in food packaging film

Chitosan is a linear polysaccharide, derived from chitin through a deacetylation process in a strong alkaline solution It is an abundant biopolymer available in the exoskeleton of crustacea, cuticles of insects, and algae, and in the cell wall of fungi Chitosan is mostly produced from deacetylated chitin from crustacean shells since it is cheap and rich in seafood (crustacean) waste Nowadays, vegans are demanding more chitosan that is produced from fungi This chitosan type has countably better properties than the original manufactured chitosan, it has lower viscosity and higher deacetylation degree Chitosan consumption is increasing continuously due to the development of modern pharmaceutical and biomedical, cosmetics, food, and beverage markets, and water treatment - which chitosan is a significant material with high application due to its beneficial biological properties which will be discussed later The global market size of chitosan was 6.8 billion USD and is expected to expand at a revenue-based CAGR of 24.7% between 2020 and 2027 [97]

Chitosan structure composed of varying amounts of (β1→4) linked residues of deacetylated unit 2-Amino-2-deoxy-glucose (D-glucosamine, GlcN) and acetylated unit 2-amino-2-deoxy-D-glucose (N-acetyl-glucosamine, GlcNAc) Deacetylation of amide groups (R-NH-CO-CH3) induces the formation of ammonium groups (R-

NH3+; pKa ≈ 6.5) which provides chitosan good film forming properties and improved water solubility

Chitosan and lignin biopolymer composite films were examined for their mechanical, barrier, surface, and antioxidant characteristics in conjunction with a thorough microscopic examination of both their exterior and interior structures Lignin as an antioxidant provides the film with a radical scavenging activity, essentially governed by a surface activity mechanism, therefore, lignin is a promising compound for a good added-value due to radical scavenging in a chitosan matrix Recent works already focused on lignin use as an added-value compound for thermoplastic or packaging film by developing sustainable composites using chitosan matrix As a cheap phenolic biopolymer, lignin offers attractive potential as a filler and additive, especially with respect to the modification of biodegradable polymers such as chitosan

In the matrix, chitosan connects with other chitosan particles through ionic crosslink bonding However, chitosan creates linkages with lignin by hydrogen bonding between the hydroxyl, amine, carboxyl, and carbonyl groups, this leads to the reduction of stronger ionic bonding (Figure 1.12) Therefore, the mechanical durability of the composite - which strongly depends on the structure bonding - varies on the concentration between chitosan and lignin

Many studies showed that the combination of chitosan with other materials can improve the properties of chitosan films for food packaging, but the choice of material and the specific properties of the resulting composite film will depend on the specific application and the desired characteristics of the film For example, chitosan can be combined with other biopolymers such as gelatin, alginate, or cellulose to improve the mechanical strength and barrier properties of the film [98] These composite films may also have improved antimicrobial properties and may be more effective at extending the shelf life of fresh foods Much research also noticed that chitosan can be combined with synthetic polymers such as polyethylene or polypropylene to improve the strength and barrier properties of the film These composite films may also be more resistant to puncture and tearing, making them more suitable for use in food packaging

Figure 1.12 Chitosan and lignin cross-linking structure Lignin-based films can be made by extracting lignin from plant material and then dissolving it in a solvent to form a solution The solution can then be cast or extruded into a film, which can be dried and used as a packaging material However, lignin films can be difficult to process and may have low mechanical strength and poor flexibility, which may limit their use in certain food packaging applications In addition, lignin films may be prone to swelling and shrinking, which can affect their performance as a packaging material

While conventional chitosan is recognized as an antioxidant agent, chitosan with a broad range of molecular weight often exhibits less stability and effectiveness compared to nanochitosan Therefore, in some cases, the antioxidant activity of the film engineered in this work primarily relies on the performance of lignin Figure 1.13 illustrates the plausible pathways detailing the interaction between lignin and DPPH during the antioxidant stage Initially, lignin donates hydrogen from Phe-OH to capture DPPH molecules Then, the OMe group effectively stabilizes the Phe-OH radical, indicating the positive impact of the capacity of OMe on the great antioxidant activity of lignin Moreover, there is the possibility of DPPH free radical molecules finding stability by forming electron pairs through binding with aryl radicals [99]

Figure 1.13 The mechanism of DPPH scavenging process of lignin

Motivations and objectives

As mentioned above, MFC, lignin, and silica derived from rice straw possess notable properties such as great electrical conductivity, antibacterial capability, UV resistance, and ease of application in composite synthesis However, research about the valorization of MFC aerogel or lignin-silica for energy-related applications is facing certain drawbacks when multiple modification steps are required Additionally, in the field of nanocomposites, where a fused structure between an organic polymer and inorganic particles is fundamental, the interest in the outstanding performance of such materials compared to their pristine forms has been growing rapidly in recent years The refinement of these characteristics relates to an even dispersion of inorganic nanoparticles or layered clay particles within the polymer matrix Nevertheless, challenges regarding the agglomeration of nanoparticles and the potential phase separation during physical blends, which can further result in the compromised mechanical and optical properties of the final products, have arisen To address these challenges, the in-situ formation of the polymer matrix and inorganic networks becomes crucial The sol–gel chemistry facilitates the precipitation of nanoscale silica particles within a polymer host, ensuring their uniform dispersion and consequently enhancing the overall properties of the resulting product, which is one of the potential solutions A noteworthy inorganic material produced through the sol–gel method is SiO2 networks, utilizing Si(OR)4-type precursors Therefore, alongside the MFC production process following the procedure of Pham et al [100], this thesis aims to synthesize lignin-silica hybrids using the sol-gel method The chemical structure and electrochemical properties of the C-MFCA and CLS systems will be investigated

Additionally, with the goal of utilizing by-products from the rice straw processing, the thesis also focuses on the recovery of lignin from the waste stream after LS9 production and its application in the synthesis of lignin/chitosan composites The obtained films will be analyzed for their structure and the evaluation of their antibacterial, antifungal, antioxidant, and UV-blocking properties.

EXPERIMENTAL SECTION

Research contents

- Synthesis and characterization of carbonized lignin-silica hybrids (CLS)

- Investigation into the effect of pH value (3 and 9) on the electrochemical behavior of CLS samples as an anode of lithium-ion batteries

- Synthesis characterization of carbonized MFC aerogel (C-MFCA)

- Investigation of the electrochemical behavior of CLS samples as an anode of lithium-ion batteries

- Fabrication and characterization of lignin/chitosan film with different lignin content (2 – 10%)

- Evaluation of the UV blocking, antioxidant performance, and antibacterial activity of lignin/chitosan film.

Research methodology

- The crystal structure of materials was analyzed by powder X-ray diffraction (PXRD)

- Information about morphology and particle sizes of CLS and lignin/chitosan film sample analogues was collected by scanning electron microscopy (SEM)

- Insights about material porosity were acquired by nitrogen isotherm adsorption

- Composition and elemental distribution of CLS samples was determined using Energy Dispersive X-ray (EDX)

- Electrochemical properties of CLS were characterized by employing Cyclic voltammetry (CV) and Galvanostatic Charge-Discharge (GCD) analysis

- The thermal degradation of materials was determined using Thermogravimetric analysis

- DPPH scavenging was measured by employing UV-Vis spectroscopy

- The water solubility of the film was calculated by drying 1.5 × 3 cm 2 rectangles pieces of the film at 106 °C for 3h and then weighted, then the pieces were immersed in 50 mL of distilled water with stirring for 24h at room temperature The undissolved films are dried at 106 °C for 24h and then weighed The water solubility is calculated by the equation: (%) o  1 o

WS W Where Wo is the weight before water immersion (g), and W1 is the weight of the undissolved film portion after drying (g)

- Water permeability of the sample films was conducted by the wet-cup water method The 90 mm in diameter films are sealed on a plastic cup containing 40 ml of distilled water The area of the open-mouth circle has a diameter of 50 mm The water is weighed every hour for 8 hours in a determined temperature and relative humidityThe water vapor transmission is calculated by the following equation:

WVT A t Where G is the water weight change (from the line) (g), t is the time (h), G/t is the slope of the line (g/h), and A is the test area (cup mouth area) (m 2 ) The water vapor permeability is calculated by the following equation:

P S R R Where t is the thickness of the sample (mm), 𝛥P is the vapor pressure difference (kPa), S is the saturation vapor pressure at test temperature (kPa), R1 is the relative humidity in the dish, R2 is the relative humidity at the vapor sink, and WVP is the water vapor permeability of sample (g.mm/m 2 h.kPa).

Materials and Instrumentations

All essential reagents and starting materials in this study were commercially obtained and used as received without any further purification procedures These chemicals are listed in Table 2-1

Table 2-1 List of chemicals purchased and used in the study

Acetic acid, 99.5% CH3COOH, Xilong

Rice straw - Loc Troi Company

Powder X-ray diffraction (PXRD) analyses were performed on the Bruker Advance D8 Diffractometer (Bruker AXS, Germany), which was equipped with monochromatized Cu-Kα radiation (λ = 1.5418 Å), operated at 40 kV and 40 mA, room temperature The data were collected in the 2θ range of 2-30° with a scan rate of 0.6°.min-1 and a step size of 0.01°

Nitrogen physisorption measurements were conducted on the Micromeritics ASAP 2020 apparatus Samples were pretreated by heating under vacuum at 150 ℃ for 5 h in advance of isotherm adsorption at 77 K using high-purity nitrogen gas The equivalent specific surface area was calculated by applying the Brunauer-Emmett-Teller (BET) model in a relative pressure range of 24 0.05-0.30 p/p0 while the pore size distribution was evaluated using the density functional theory (DFT) method The EDX technique is performed with a Scanning Electron Microscope where a beam of high-energy electrons is emitted and excites the atoms on the sample surface

From there, x-rays with specific wavelengths that are characteristic of the atomic structure of the elements are emitted and detected by an energy-dispersive detector

In this study, SEM-EDX analyses were performed at the Nanotechnology Laboratory of Saigon Hi-tech Park using an S-4800 device at 10000V

Thermogravimetric analysis (TGA) was performed to navigate the thermal stabilization of the materials and was conducted on a LINSEIS DSC PT 1600 in air atmosphere at a heating rate of 5 °C/min from room temperature to 800 °C The mechanical strength was performed on an X350 Testometric machine with a pulling rate of 5mm/min and a pretension of 1 N

The charge–discharge tests were performed on a CR2032 two-electrode coin-type cell system The anodic electrode was prepared by mixing the dried carbonized lignin-silica powder with Super P conductive carbon black and polyvinylidene fluoride (PVDF) 10% at a ratio of (70:15:15), using 1-methyl-2- pyrrolidinone (NMP) as the solvent The attained slurry is then spread onto thin copper foil and dried at 110 °C under vacuum to evaporate the NMP solution The coin cell was assembled in an Argon-filled glove box with the anodic electrode being the aforementioned carbonized lignin-silica/C-MFCA electrode, a lithium electrode 15 mm in diameter as the counter electrode, and using Celgard 2400 polypropylene as the separator LiPF6 1 M is the electrolyte in a 1:1 volumetric ratio cosolvent of ethylene carbonate (EC) and diethyl carbonate (DEC) Electrochemical measurements were performed using a battery cycler system with a voltage range of 0.01–3.00 V and a constant current density of 100 mA g-1 (Arbin Instruments, BT2143) In this paper, the charge and discharge processes are defined as the respective lithiation and de-lithiation of the carbonized lignin-silica hybrids/C- MFCA The specific capacity is calculated based on the mass of the active material

A modified dilution method is applied to assess the antibacterial and antifungal properties of the attained film samples Escherichia coli (E coli - Gram-negative bacteria) and Candida albican (C albican – fungi) were first activated in Tryptic Soy Agar and Sabouraud Dextrose Agar respectively, and adjusted to reach 1.5 × 10 8 CFU/mL (McFarland 0.5) for bacteria and 10 7 CFU/mL for fungi The microbials are then added into 25mL of NaCl 0.9% so that their density reaches 10 6 CFU/mL Film samples of the same area (55 cm 2 ) were prepared by sterilizing using a 30W UV light for 5 minutes Afterwards, the films are added to the prepared microbial solutions and incubated for 24 hours Microbial counts were performed by diluting the microbial in folds of 10 and spreading them on Mueller Hilton Agar where, for C albican, 2% glucose is introduced After 24 hours, the number of microbial colonies was counted and calculated for inhibition capacity determination The film samples were blank and LN2 and were compared with respect to the control with no films added

The antioxidant property of the samples was determined using a 2,2-diphenyl-1- picrylhydrazyl (DPPH) radical scavenging activity assay The samples were cut into small pieces first and 100 mg of samples were submerged in 4ml 0.1 mM DPPH in 80% methanol solution and incubated in a dark area for 4 hours The absorbance of the solutions at the wavelength of 517 nm was determined by the Shimadzu 2550 UV–vis spectrometer The DPPH radical scavenging activity is calculated by the following equation:  o  t 100% o

A Where A o is the absorbance of the control sample (DPPH in 80% methanol solution), and At is the absorbance of the solution with incorporation of the film samples.

Experimental section

2.4.1 The synthesis of lignin-silica hybrid and carbon material derived from lignin- silica hybrid

First, 1 kg ground rice straw was dissolved in 20-liter sodium hydroxide 1% at

90 °C for 2 hours in a continuous stirring tank Then, the cooked mixture was filtered by a colander to obtain liquor which mainly contains sodium lignin and sodium silicate, that called “black liquor” (BL) Subsequently, the BL was precipitated by slowly adding a sulfuric acid solution of 20%, simultaneously well-mixing was carried out to ensure the highest precipitation yield The pH value of BL was adjusted to 3 and 9 Then, the system was left aging for 24 hours for stabilizing The mixture was filtered in a vacuum filter to obtain precipitated lignin and silica, the final product described as lignin-silica hybrid samples (LS3, LS9) was washed with RO water several times to remove impurities and dried in an oven at 105 °C for 24h (Scheme 2-1)

Scheme 2-1 Synthesis process of lignin/silica composite from rice straw

The lignin-silica hybrids were pulverized prior to carbonization The materials were subsequently placed in a calcination ceramic boat and inserted into the horizontal tube furnace under nitrogen flow The following activation process was conducted at 700 °C with a heating rate of 10 °C/min The resulting solids were designated as carbonized lignin–silica materials, with the samples obtained from lignin silica, precipitated at pH 3 being CLS3 and pH 9 being CLS9 as displayed in Scheme 2-2

Scheme 2-2 The general synthesis of CLS samples 2.4.2 The fabrication of carbonized micro-fibrillated cellulose aerogel (C-MFCA)

The rice straw was grounded and then treated with NaOH 1wt.% at a liquid-to- solid ratio of 20:1 for 2 hours, at 80 ° C The obtained slurry was rinsed with reverse osmosis water (RO) until it acquired a neutral pH value Subsequently, the solid phase was treated with NaOH 6wt% at a liquid-to-ratio of 20:1 in 1.5h, at 80 °C Afterward, the system was then mixed with a mixture of 10 wt% H2O2 and 0.5 wt.% NaOH solutions at 60 °C for 45 min The ratio of the liquid mixture to powder was 20:1 (mL:g) The obtained powder was washed with RO water until pH-balanced before drying in an oven at 80 °C overnight

The obtained cellulose was hydrolyzed with 40 wt% H2SO4 at 45 °C for 1 hrs The hydrolysis step was terminated by adding water The suspension was settled down over time and then the upper layer of water separated from the suspension was removed before introducing RO water into the residue This process was conducted periodically until the suspension reached neutral pH After that, the entire mixture was homogenized for 2 hrs at 15000 rpm to finally achieve the MFC suspension

A mixture of an aqueous suspension of 50g cellulose fibers (1 wt%) and PAE (20-

80 wt% of dry cellulose fibers) was vigorously agitated for 15 min The suspension was then poured into a stainless-steel mold Next, the sample is frozen in liquid nitrogen to obtain the hardened form, followed by freeze-dried by Toption TPV-50F lyophilizer under vacuum conditions for 48 hrs The MFC aerogel was yielded via a heat-annealing process at 120 ℃ for 3 hours Finally, the carbon aerogel was obtained after the carbonization process at 700 °C for 2 hours as shown in Scheme 2-3

Scheme 2-3 Schematic for synthesizing MFC carbon aerogel

2.4.3 Preparation of lignin/chitosan film

The black liquor obtained after LS9 recovery was used in the retrieval of lignin using sulfuric acid for the acidification to a pH value of 3 The lignin was then filtered and washed several times with water to neutralize the pH and remove any salt or remaining acid, the lignin was then dried in the oven at 90 o C to remove moisture Scheme 2-4

Scheme 2-4 Scheme of preparing lignin from rice straw

The 2% (w/v) chitosan solution was prepared by dissolving chitosan in 2% (v/v) acetic acid solution at room temperature and vigorously stirred for 3 hours then filtered undissolved chitosan and residue with vacuum filtration Glycerol was added to the solution with the mass ratio of chitosan 1:10 as a plasticizer to improve flexibility Sodium hydroxide solution 0.1 N was used to dissolve different amounts of lignin to obtain different lignin solutions, with concentrations of 1, 2, 4, 6, and 8 g/L The mixture of 25 mL chitosan-glycerol solution and 15 mL of lignin solution was well mixed and degassed at room temperature then cast on a petri dish with a diameter of 90 mm The film was allowed to dry naturally at ambient room temperature for 24 hours to remove any remaining air bubbles and facilitate the formation of bonds between the chitosan and lignin materials, then oven-dried at 50 oC for 24 hours to remove excess water The film without lignin added was a blank sample (LN0) while others are defined as LN1, LN2, LN4, LN6, and LN8 corresponding with the lignin/chitosan film with the lignin concentration of 1, 2, 4,

Scheme 2-5 The general synthesis of lignin/chitosan film

PERFORMANCE OF CELLULOSE AND LIGNIN-BASED

Structural characteristics of carbon materials

3.1.1 Characterization of carbonized lignin-silica (CLS)

Figure 3.1 FT-IR spectra of LS samples

As shown in Figure 3.1, the FTIR analysis of lignin-silica composites reveals the structural characteristics of LS samples One prominent feature is the presence of hydroxyl groups, represented by a broad peak in the range of 3200-3500 cm -1 This peak indicates the abundance of hydroxyl groups contributed by both the lignin and silica components, providing valuable insights into the potential for hydrogen bonding and hydrophilicity of the composite Another important observation is the stretching vibrations of aliphatic C-H bonds, typically appearing in the region of 2800-3000 cm -1 These vibrations arise from the aliphatic chains in lignin as well as the silica component, signifying the presence of aliphatic hydrocarbons within the composite structure Additionally, the FTIR analysis allows the identification of carbonyl groups in the lignin-silica composite The stretching vibrations of C=O bonds associated with carbonyl functionalities are observed in the range of 1700-1750 cm -1 , providing insights into the presence and nature of carbonyl moieties in the composite Moreover, the analysis reveals the stretching vibrations of C=C bonds present in aromatic rings of lignin, appearing as characteristic peaks in the range of 1500-1600 cm -1 This aromatic fingerprint region offers valuable information about the aromatic nature and degree of condensation within the lignin-silica composite Furthermore, bands originating from the SiO2 point at the successful interconnection of lignin and silica in the hybrid material, namely symmetrical and asymmetrical stretching vibrations of the Si-O-Si group (at 1096 and 807 cm -1 , respectively), stretching vibrations of the Si-OH group (at 956 cm -1 ), and bending vibrations of Si-

O group (at 470 cm -1 ) The presence of bands originating from silica as well as lignin confirms that the proposed method of hybrid material synthesis was performed with satisfactory results Bonds that are formed between the precursors ensure their strong interconnection Based on these basics, FTIR analysis of lignin-silica composites reveals the presence of hydroxyl groups, aliphatic C-H stretching, carbonyl groups, and aromatic C=C stretching Additionally, it provides insights into the Si-O-Si stretching and Si-OH bending, contributing to a comprehensive understanding of the composition and bonding interactions within the composite

Figure 3.2 FT-IR spectra of CLS samples Owing to the restructuring of carbohydrates after the carbonization stage, CLS samples had some changes in the structural characteristics when compared to the corresponding precipitate It is obvious that the only peak for CLS3 was observed at around 1600 cm -1 , which can be consistent with the high conjugated C=O bonds [101] Whereas, with the higher silica content, the transmittance of CLS9 decrease along with the decline in the intensity of 1600 cm -1 peak (Figure 3.2)

Furthermore, the FTIR spectrum of CLS9 also indicates the appearance of Si-N-

Si with an intensive band at 900-800 cm -1 , which can be assigned to the generation of Si3N4 after the nitrification SiO2 in the presence of C, following the equation [102]:

4SiO (s) 8C(s) N (g)  SiC(s) Si N (s) 8CO(g)  3.1.1.2 Phase composition and morphology of CLS samples

As mentioned above, CLS samples have their signature structure which depends on the source of lignin, the SiO2 content in biomass, and even the employed method for the pyrolysis process Therefore, powder X-ray Diffraction (PXRD) is one of the most important and effective ways to characterize CLS materials, by comparing the obtained PXRD patterns to the simulated ones or previous reports

Figure 3.3 XRD patterns of CLS samples Figure 3.3 illustrates the X-ray diffraction (XRD) patterns that reveal the crystal structures of the samples following pyrolysis and carbonization Upon pyrolyzing at

700 °C under a nitrogen (N2) atmosphere, the resulting composite materials consisted of amorphous carbon (C), silicon dioxide (SiO2), and crystalline silicon carbide (SiC) and silicon nitride (Si3N4) Specifically, in the case of CLS3, the XRD pattern exhibited a broad peak spanning the range of 16° to 35°, centered around 24° This peak can be attributed to the overlapping diffraction peaks of amorphous carbon and amorphous silicon dioxide Another broad peak within the range of 38° to 50°, centered around 44°, was also identified as amorphous carbon

For CLS9, a complex crystal structure was formed, characterized by numerous signals that potentially indicate the presence of SiC, Si3N4, and Si The XRD pattern exhibited distinct crystalline phases of both -Si3N4 and -Si3N4 (identified as JCPDS no.41-0360), which aligns with the Si-N-Si bonding observed in the FTIR results Additionally, sharp peaks corresponding to multiple forms of Si, such as SiC and

Si, were observed These peaks are indicative of the reduction of silica (SiO2) under the nitrogen (N2) atmosphere during the pyrolysis process [103]

Figure 3.4 SEM/EDX results of CLS3 (a) and CLS9 (b)

As depicted in Figure 3.4, the EDX spectra confirm the expected composition of

C and Si CLS3 shows a high carbon composition of 78.96 wt%, while CLS9 exhibits a much higher silicon content of 27.92 wt%, approximately 20 times greater than CLS3 Additionally, signals of nitrogen (N) are present in CLS9, which aligns with the suggested presence of the Si3N4 phase

The SEM image of the CLS3 sample reveals an amorphous and resistant structure with varying particle sizes ranging from approximately 1 to 3 μm In contrast, for CLS9, smaller particles of SiC, SiO2, and Si3N4 are observed on the carbon surface, with sizes ranging from 80 to 150 nm

3.1.1.3 Specific area and porosity of LS and CLS samples

Porosity is also one of the most important features of porous materials in general, determined through nitrogen isotherm adsorption at 77 K under low pressure According to this result, the pore distribution was also provided To perform the measurements, about 0.1 g of activated samples were weighed and placed in the dried cell filled with a rod and capped with a frit The samples were then heated at 150 ℃ under vacuum for 4 h prior to physisorption

Table 3-1 Specific surface area and average pore diameter of LS and CLS samples

Figure 3.5 Isotherm adsorption of (a) LS3, (b) CLS3, (c) LS9, and (d) CLS9 The specific surface area of LS and CLS samples are displayed in Table 3-1 Owing to the low SiO2 content, the LS3 possessed the lowermost SBET of 6.7 m 2 /g among four samples, indicating the non-porous structure of this material It is noteworthy that the highest value was recorded as SBET of CLS9 (292.7 m 2 /g) which was twice as much as this number of uncarbonized ones from wheat straw and shows significant enhancement in comparison to LS9 Additionally, after a three-step process, Samy et al reported that the obtained amorphous silica only possessed a lower SBET (Table 3-2) than this value of CLS9 It indicates that although CLS samples were produced from waste liquid through an uncomplicated procedure, these exhibited superior properties not only in the high specific surface area of material but also in the ability to be employed in various applications due to the presence of both carbon structure and SiO2

Table 3-2 BET surface area of CLS constituents

3 Lignin-silica microparticles from wheat husk 117.4 [106]

The sorption curves of LS and CLS samples are shown in Figure 3.5 In the physisorption results of LS9 and CLS9, the sorption curve resembled type IV, which is a common signal for mesoporous materials [107] Although LS9 presented a noticeable rise in the N2 uptake in comparison with LS3 (more than 100 times differential), the nitrogen sorption curve of CLS3 was suitable with type III with better-adsorbed volume due to the effects of the restructuring after the carbonization process on the obtained material

3.1.2 Characterization of carbon micro-fibrillated cellulose aerogel (C-MFCA) 3.1.2.1 Chemical structure

Figure 3.6 FT-IR spectra of MFC, MFCA, and C-MFCA

The FTIR spectra of cellulose aerogel with PAE 60 wt% and PL following acid hydrolysis are shown in Figure 3.6 When comparing two samples, there is a significant difference between them, suggesting that there are chemical differences between them before and after aerogel synthesis due to PAE The peak at 1061 cm1 is caused by cellulose's C-O-C (pyranose ring) stretching and C-H rock vibrations

Both the MFC and MFCA’s FTIR spectra contain broad peak bands at wave number

3344 cm -1 , which correspond to O-H stretching vibrations The C-H and C-O bond vibrations in the cellulose polysaccharide ring correspond to a wavenumber of 1368 cm -1 The observed differences are 1630 cm -1 peak represents the amide group vibration of PAE and 1724 cm -1 represents the C=O bond from the ester function in the crosslinking network [108] Through undergoing the heating and drying process, the active azetidinium establishes covalent bonds with carboxyl groups on fibers Simultaneously, crosslinks form within and between PAE chains, and strong hydrogen bonds as reported in previous studies [109] Therefore, a network of crosslinked polymers is created, linking the surfaces of adjacent fibers (Figure 3.7) Besides, after the carbonization process, the obtained material exhibited distinct structural characteristics compared to its initial structure As can be seen, a slight peak around 1600 cm -1 in C-MFCA corresponds to the presence of highly conjugated C=O bonds, signifying the altered structure of the carbohydrate during the process

Figure 3.7 The bonding of PAE and MFC after the annealing stage

3.1.2.2 Specific area, porosity, and morphology of aerogels

Figure 3.8 provides details regarding the composition of MFCA and C-MFCA The majority of cluster PAE-MFC in MFCA had a diameter ranging from 10 μm to nearly 40 àm Following the carbonization process, the diameter was dramatically reduced to 3-6 μm C-MFCA possessed a porous structure with interconnected three- dimensional patterns and pore sizes ranging from 10-20 μm Cellulose microfibers are still bound together via thin PAE films through robust hydrogen bonding

Figure 3.8 SEM micrographs of CA (a,b) and CCA (c,d)

Regarding the density and porosity of MFCA with C-MFCA (Table 3-3), the bulk density of carbon aerogel decreases after carbonization, while the porosity rises also This is considered to be owing to the loss of functional groups to leave carbon on the 3-dimensional structure, causing the cellulose fibers to shrink and the pore size in the carbon aerogel to grow Furthermore, the nitrogen content in C-MFCA is significantly higher when compared to this value in MFCA Due to the restructuring and diminishment of some functional groups in the structure of MFCA (Oxygen- containing group), the N increased and accounted for 10.49 wt.% with better distribution in C-MFCA (Figure 3.9), which has a great potential for lithium-ion battery

Table 3-3 The comparison of the elemental composition, specific surface area, and porosity of MFCA and C-MFCA

Figure 3.9 The EDX-mapping result of carbon, oxygen, and nitrogen in CA (a,b, and c) and CCA (d,e, and f)

3.2 Behavior of CLS samples and carbon aerogel as anode in lithium-ion batteries

3.2.1.1 Galvanostatic charge-discharge ability of obtained anode

Figure 3.10 Galvanostatic Charge–Discharge profile for a CLS3, b CLS9 at a current density of 1C after 100 cycles

APPLICATION OF LIGNIN/CHITOSAN FILM FOR UV-

Properties of lignin/chitosan film

The thickness of the various samples, as indicated in Table 4-1, ranging from 90 to 100 àm, was found to be independent of the lignin concentration, implying that the addition of lignin in the composite did not affect the thickness of obtained films The data presented in Table 4-1 suggests that higher amounts of lignin in the sample correspond to lower levels of water solubility (from 25.05 to 21.26) and moisture content (from 1.53 to 0.38), which can be explained by the hydrophobic nature of lignin and the formation of cross-linkages between the lignin and chitosan molecules Specifically, the reaction of lignin with the amino and hydroxyl groups in chitosan led to the formation of hydrogen and electrostatic bonds, resulting in the reduction of the availability of hydrophilic groups on the surface of the films

Table 4-1: Thickness, water solubility, and moisture content of the film samples Sample Thickness (μm) Water solubility (%) Moisture content (%) Blank 97.51±33.41 35.05±0.04 1.53 ± 0.03

Figure 4.1 presents the SEM micrographs illustrating the morphology of lignin and the surface of chitosan and lignin/chitosan film Both chitosan and lignin/chitosan film showed smooth surfaces while the obtained lignin had a very rough surface and appeared with granules of various dimensions, as illustrated in Figure 4.1a Notably, the lignin/chitosan film is more homogenous than the only chitosan film, which may be caused by the interaction of functional groups of –NH2 in chitosan and –OH in lignin However, there are some agglomerates observed in lignin/chitosan film due to the precipitation of lignin in neutral and acid pH values

For the lignin/chitosan films, the surface morphology differs in all samples as shown in Figure 4.1c-g In general, the addition of lignin creates a rougher surface with the formation of irregularities and pores on the surface of the chitosan film The degree of roughness can depend on the concentration of lignin added, with higher concentrations leading to more pronounced roughness as shown in the SEM micrographs With the presence of lignin, the chitosan film matrix gaps should be filled with lignin particles, therefore reducing the cavity amount, this could benefit the materials in some aspects such as water resistance, oxygen permeability, and water vapor permeability On the lignin/chitosan film, it could be observed that there are some small pellets, which can be assumed as lignin precipitate due to the pH drop caused by the acetic acid of the chitosan solution With the precipitation of lignin, the connection between lignin and chitosan via functional groups of –NH2 in chitosan and –OH in lignin, which formed clusters of chitosan and lignin due to the differences in pH level of the two materials in the mixing between two solutions

Figure 4.1: SEM micrographs lignin (a), chitosan film (b), LN1 (c), LN2(d),

LN4(e), LN(f) and LN8 (g) 4.1.3 Chemical structure

The XRD patterns (Figure 4.2a) of lignin, biofilm with/without lignin shows a broad peak around 20°, suggesting all samples were amorphous The XRD patterns of blank film and LN2 film show similar results with other findings which have 2 broad peaks at around 20.5° and 28°, indicating the typical fingerprints of semi- crystalline chitosan The peaks around 20.5° and 28° are related to crystal-I and crystal-II in chitosan structure [128] however the high crystallinity characteristic of chitosan presented in the two peaks of the films is not as intense as shown in the XRD patterns This is due to the formation of chitosan acetate which decreases the degree of deacetylation, thus reducing the intensity of the two peaks There are different intensities at the characteristic diffraction peak between LN2 and LN0 Thus, the introduction of lignin into the chitosan film matrix induces changes in the crystallinity of the materials, which showed similarities to reported research [129] The pattern of lignin shows a sharp peak at 32°, which represents the presence of sodium sulfate in the material formed from the neutralization in the acidification step [130] The sodium sulfate salt is transformed due to the reaction of the remaining sulfuric acid from acidification and sodium hydroxide of lignin solution

Figure 4.2: X-ray diffraction patterns of lignin, blank film, and LN2 (a), and

FTIR spectra of lignin, blank sample, and LN2 sample (b)

The FTIR (Figure 4.2b) shows a good agreement with the XRD pattern of the materials The intense bands at 1605, 1516, and 1425 cm -1 correspond to aromatic skeletal vibrations of lignin A wide adsorption band located at 3420 cm -1 is assigned to aromatic and aliphatic O-H groups Peaks at 1217 cm -1 and 1119 cm -1 have been assigned to syringyl ring breathing with CO stretching and C-O stretching for secondary alcohols respectively [131] For the chitosan spectrum, a strong band in the region 3325 cm −1 to 3376 cm −1 corresponds to N-H and O-H stretching The absorption bands at around 2885 and 2926 cm −1 can be attributed to C-H asymmetric stretching and symmetric, respectively A band at 1455 cm −1 corresponds to the N-H bending of the primary amine The presence of residual N-acetyl groups was confirmed by the bands at around 1600 cm −1 (C=O stretching of amide I) [132] It can be observed that several noticeable changes occur in the spectrum of lignin/chitosan (20%) in comparison with the spectrum of pure chitosan The band of chitosan at 3325 cm -1 for hydroxyl groups is shifted to 3420 cm -1 , and the intensity is weakened; 1600 cm -1 for amide I is shifted to 1589 cm -1 ; 1080 cm -1 for C-O stretching is shifted to 1073 cm -1 All the clearly measurable changes in wave number indicate obvious interactions among the hydroxyl, carbonyl, and ether groups of the two components It can be attributed to the hydrogen bonding possibly formed between hydroxyl, carbonyl groups in chitosan and carbonyl, hydroxyl, and ether groups in lignin However, not much change of N-H stretching can be observed, which is located at 3376 cm -1 for both the blend and pure chitosan The same situation can be seen for the bands of amide II This result suggests that the hydrogen bonding is mainly formed between the hydroxyl and carbonyl groups in chitosan with lignin

Figure 4.3 TGA profiles of different lignin/chitosan film samples

The thermogravimetric analysis was used to determine how the addition of lignin affected the chitosan films, and the TGA curves are displayed in Figure 4.3 In general, each sample showed three distinct phases in the deterioration process The evaporation of water and acetic acid residue caused the initial stage of degradation to take place below 100 °C to 250 °C The loss of the low molecular weight molecules, or glycerol molecules, was attributed to temperatures between 125 °C and 150 °C at the commencement and 200 °C at the maximal degradation The second stage, which was noticed between 275 °C and 325 °C, was related to the breakdown of chitosan Additionally, lignin may degrade between 200 °C and 500 °C Overall, the findings showed that the films had higher thermal stability than the addition of lignin The fact that the inserted additives lessen the intramolecular interactions between the various elements of the film matrix may be the cause of this effect

The results show that as the lignin concentration increases, the water vapor permeability of the film decreases as demonstrated in Figure 4.4 The water vapor permeability of the chitosan film and lignin/chitosan biofilm is higher than that of PVC plastic wrap film, with a permeability of 5.44´10 -4 and 5.53´10 -4 g.mm/m 2 h.kPa, respectively However, the water vapor permeability gradually decreases to 1.42´10 -

4 g.mm/m 2 h.kPa as the concentration of lignin increases in the composite This reduction in water vapor permeability is a desirable property for packaging materials, as it can help to prevent moisture loss or gain in food and other perishable items

Figure 4.4: Water vapor permeability of the sample films 4.1.6 Mechanical strength

Table 4-2 provides a detailed summary of the different mechanical properties of each sample The elongation at break initially increased with the incorporation of lignin, with the LN1 sample demonstrating the highest elongation at break among all samples However, after the LN2 sample, the trend shows a decreasing trend This observation can be attributed to the inherent characteristics of lignin, which is a natural polymer that can interact with chitosan and promote the formation of hydrogen bonds These bonds can lead to improved adhesion between the lignin and chitosan molecules, resulting in increased elongation at break However, as the lignin concentration increases beyond a certain point, which is a similar trend to the previous study – citation around 20% w/w which is equivalent to LN8 [133]

Table 4-2: Mechanical strength of film samples

UV blocking, antioxidant and antibacterial activity of lignin/chitosan film

Figure 4.6a shows the transmittance of the films from 200 to 700 nm (UV–Vis light) As expected, the incorporation of lignin into the chitosan films decreases their transmittance; and, interestingly, the spectra profile differs depending on the lignin concentration All films containing lignin showed a drastic decrease in transmittance around 350 nm, showing to be a great barrier against UVC and UVB radiation, probably due to the presence of phenolic compounds The LN4, LN6, and LN8 films exhibited very low transmittance in the visible region and a total blocking in the UV region

As the lignin concentration increased in the films, the films became less transparent, with a noticeable decrease in visual transmittance (Figure 4.6b) This is because lignin has a high color level that reduces the transparency of the material to visible light wavelengths [134] Interestingly, the LN1 and LN2 samples did not show a significant increase in opacity as the lignin concentration increased, despite the drop in visual transmittance However, the UV absorbance significantly increased in these samples, suggesting that the introduction of lignin into the film has a positive effect on UV blocking while still maintaining the clarity of the film

Figure 4.5: UV blocking activity of lignin/chitosan film The UV light-blocking mechanism of lignin/chitosan film is depicted in Figure 4.5 Specifically, as exposed to UV light, lignin undergoes electron transfer leading to the generation of the carbonyl group which facilitates extinguishing the UV light [135] Then, the photon energy from this source is transformed into heat energy or re- emitted at lower energy, making lignin/chitosan film effective in protecting against

UV radiation The complex structure of lignin, which includes aromatic rings and a variety of functional and chromophore groups, is interconnected through ether or single bonds, endowing it with excellent UV light absorption capabilities In comparison to films engineered with more intricate agents (CNF and nano zinc oxide) and polymer matrix (CMC and MFC), the lignin/chitosan in this work film examined in this study exhibited exceptional UV blocking efficiency of 100% (Table 4-3) Significantly, despite being fabricated via an uncomplicated route, the obtained film outperformed others incorporating cinnamaldehyde, tannic acid, or curcumin/CNF

Table 4-3 Comparison of UV blocking efficiency of different agents

UV blocking agent Polymer matrix Blocking efficiency (%) Reference

Tannic acid Micro-fibrillated cellulose and gelatin 98 [139]

Figure 4.6: UV transmittance of film samples with the wavelength in a range of

200-700 nm (a )and opacity of the film samples (b)

Figure 4.7: DPPH scavenging activity of different film samples

The antioxidant activity of the chitosan-lignin biofilm was evaluated using the DPPH radical scavenging assay As shown in Figure 4.7, while the blank film containing only chitosan had a moderate antioxidant activity with a value of 56.14%, the addition of lignin to the chitosan film significantly increased its DPPH radical scavenging activity to more than 91% This is deduced by the presence of the phenolic group in lignin which is considered as a crucial factor in determining the antioxidant action of lignin [140] As the concentration of lignin in the film increased, the radical scavenging activity increased up to a certain point, after which there was no significant change in this activity In comparison with previous research that utilized the lignin nanoparticle in fabricating biofilm, the lignin/chitosan film in this study exhibited the same DPPH radical scavenging activity to this value of lignin/chitosan nanoparticle despite using the low amount of lignin [141]

While conventional chitosan is recognized as an antioxidant agent, chitosan with a broad range of molecular weight often exhibits less stability and effectiveness compared to nanochitosan Therefore, the antioxidant activity of the film engineered in this work primarily relies on the performance of lignin Figure 4.8 illustrates the plausible pathways detailing the interaction between lignin and DPPH during the antioxidant stage Initially, lignin donates hydrogen from Phe-OH to capture DPPH molecules Then, the OMe group effectively stabilizes the Phe-OH radical, indicating the positive impact of the capacity of OMe on the great antioxidant activity of lignin Moreover, there is the possibility of DPPH free radical molecules finding stability by forming electron pairs through binding with aryl radicals [142]

Figure 4.8: The mechanism of DPPH scavenging process of lignin

As analyzed above, all lignin/chitosan films exhibited great potential in both chemical properties and antioxidant activity Among these, LN2 could be considered the most prospective candidate as it can meet the essential requirements of a biofilm by using a low amount of lignin Therefore, this sample was used to evaluate the antimicrobial ability of lignin/chitosan film

Table 4-4: Inhibition rate of LN0 and LN2 film for E Coli and C Abicans and

Microbials Inhibition rate MIC value (%v/v)

The results presented in Table 4-4 demonstrate the effectiveness of the lignin/chitosan film in inhibiting the growth of microorganisms The inhibition rates of E coli and C albicans were found to be 80.39% and 99.77% for LN2, respectively

On the other hand, the blank film with only chitosan experiences a lower inhibition rate of 15.17% for E coli and 36.26% for C albicans The presence of lignin in the film is believed to have contributed to its enhanced antimicrobial activity Lignin is known to possess inherent antimicrobial properties, as it is a complex polymer composed of various phenolic compounds that exhibit broad-spectrum antimicrobial activity against bacteria and fungi [143]

Figure 4.9: The antibacterial mechanism of lignin

To get a better understanding of the antibacterial activity of lignin/chitosan film, the plausible mechanism of this process was conducted (Figure 4.9) Researchers have indeed observed that polyphenols stimulate the production of hydrogen peroxide, creating oxidative stress within bacterial cells This antibacterial effect arises from polyphenols' ability to inhibit crucial enzymes and weaken bacterial membranes, thereby enhancing cell permeability Lignin has the ability to enter cells, where phenolic compounds like cinnamaldehyde, derived from lignin, lower intracellular pH and deplete ATP, consequently reducing bacterial metabolic activity Studies have demonstrated that even at subinhibitory concentrations, lignin diminishes bacterial metabolism Furthermore, owing to its hydrophobic nature, lignin can intermingle with bacterial envelope lipids, leading to disruptions in the cell membrane [144]

Table 4-5: Comparison of antifungal activity of different agents

Polymer matrix Concentration Inhibition rate (%) Reference Scoured jute fabrics with chitosan- metal complex

Cell-free supernatant- embedded bacterial nanocellulose film

Numerous studies have examined various antifungal materials used in active packaging films, including natural products, essential oils, preservatives, fungicides, nanoparticles, and chitosan as shown in Table 4-5 The obtained film in this work exhibited great antifungal properties when compared to other natural components like Methyl cellulose incorporated with Cinnamaldehyde essential oil which performed an inhibition rate of only 12% In another circumstance, the cell-free supernatant embedded bacterial nanocellulose film inhibited only nearly 60% after 24h for the testing process.

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

Overall, the thesis has effectively utilized rice straws and black liquor after the alkaline treatment process to generate valuable and highly applicable products in energy storage systems and food preservation Specifically, rice straw produced during agricultural production was utilized in the synthesis of C-MFCA by a cross- linking and carbonization method Following the pyrolysis phase, the obtained material showed distinct structural characteristics Results from FT-IR, XRD, and SEM-EDX analyses indicated the presence of a crystalline structure of cellulose, with a high nitrogen content of 10.49 wt.% In the context of battery electrode fabrication, the C-MFCA exhibited superior stability compared to graphite and metal-containing composites that had been done in numerous previous works In particular, the C- MFCA displayed an impressive initial lithiation-specific capacity of up to 348.6 mAh/g and maintained a stable reversible capacity of 257.5 mAh/g

In addition, the black liquor generated from rice straw alkali pretreatment was employed in the syntheses of lignin-silica hybrids using an acidic pH adjustment method The SEM–EDX analysis suggested a carbonized lignin-silica crystalline structure of SiC, SiO2, and Si3N4 with a high Si content of 27.92 wt% for samples adjusted to pH 9 Meanwhile, an amorphous structure of SiO2 and C was observed for samples at pH 3 Both composites have demonstrated improved stability in comparison to Si as the anodic electrode in lithium-ion batteries More specifically, a high initial lithiation-specific capacity of up to 1668 mAh/g and a stable reversible capacity of 328 mAh/g were observed for the carbonized lignin-silica sample Therefore, this study not only exhibited the tremendous potential of these carbon- based materials in electrochemistry applications but also proposed a new solution for the environmental remediation problem With the current increasing demands in energy, this research also demonstrated an approach to the valorization of black liquor in the fields of electrical storage devices such as lithium-ion batteries

To take all the advantages of rice straw and significantly lessen the amount of waste stream into the environment, this thesis also employed the black liquor after the recovery of LS9 in the generation of the lignin/chitosan film employing an eco- friendly and energy-saving approach, reducing the need for nanoscale alterations by utilizing a solution of lignin and chitosan instead of their respective suspension It was found that the obtained film exhibited a good performance in blocking UV radiation waves, which prevents food degradation and oxidation, the lowest concentration of lignin (1 g/L of NaOH 0.1 N) completely absorbed the UV transmittance but maintained clear transparency compared to LN0 sample Moreover, the presence of lignin in chitosan fills the gap of the polymer matrix, thus decreasing the water vapor permeability of the films, dropping from 5.44×10 –4 g mm/m 2 h.kPa in blank film to 1.42×10 –4 g mm/m 2 h.kPa in LN8 (reduced 73.9% from initial value), which is crucial for water vapor resistance ability for food preservation When compared to the chitosan film only, the addition of lignin in the lignin/chitosan film sample further boosted the DPPH radical scavenging activity from 56.14 to 91.31% Moreover, LN2 film could be considered as a prospective candidate for preventing bacterial and fungal growth due to its great inhibition rate for Coli and C Abicans of 80.39 and 99.77%, respectively Furthermore, the LN2 sample exhibited satisfactory

UV absorption performance with minimal opacity and viability of lignin/chitosan films for usage in food packaging applications.

Recommendations

It is essential to examine the battery's performance under diverse charging conditions Despite demonstrating promising attributes, the specific capacity of the battery remained unstable due to the disruption of active sites Research on utilizing lignin as a binder needs further exploration to maintain a consistently high specific capacity

Regarding the food packaging application, the practical storage effectiveness of the lignin/chitosan film on various types of fruit or meat should be tested Studies relating directly using the black liquor for the mixture before casting, and skipping the lignin recovery step should be conducted

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