The procedure starts with the creation of a colloidal solution, often known as a sol. A solution of reactants and solvents contains solid nanoparticles or initiator materials.
Sol–gel products may be manufactured from a variety of substances, including oxides (such as silicon dioxide and oxide minerals), natural compounds (such as large molecules like plant-derived materials), and carbon-based substances (such as 2D carbon allotropes and carbon nanopipes)
[60][62]. In this study, pineapple-fiber (PF) aerogels were successfully created by pretreating PFs with naturally decomposable polyvinyl alcohol (PVA). The PVA solution preparation was combined with PFs and freeze-dried. According to the findings, the PFs have high porosities (~99%), ultralow densities, and microporous formations, as shown by field-emission scanning electron microscopy, Brunauer–Emmett–Teller isotherm, and X-ray diffraction analysis. The exceptionally low thermal conductivity of the PF aerogel demonstrated its applicability for thermal barrier uses. A thermal coat wrapped over a water bottle with a PF aerogel filling can unquestionably keep the water temperature near 0 °C (just above the freezing temperature) for up to 6 h (initial temperature: −3 °C) and above 40 °C for up to 2.5 h (initial temperature: 90 °C). The thermal coat has a potential thermal barrier that is nearly three times that of a product that is currently on the market
[63]. In another study, the goal was to develop a thermal coat for army canteens based on a paper waste cellulose aerogel to increase the life of ice slurry for dynamic army troops in exercises or operations. However, because of the minimal stretching capacity and the ease with which the bio-based aerogel structure can be damaged, the bio-based aerogel must be sandwiched between two protective layers to make the thermal coat more durable. The paper waste was combined with deionized water and crosslinked with
Kymene chemicals (crosslinkers based on polyamide-epichlorohydrin resin) before being frozen overnight. After freezing, the gel was dried using the lyophilization drying technique at −91 °C to create cellulose aerogels, followed by the crosslinking process in the dryer for 3 h at 120 °C. Following all measurements, the results showed that the heat barrier function of the developed thermal coats was significantly better than that of marketed thermal flasks and similar to that of vacuum flasks for the same duration of 4 h and the same surrounding temperatures
[64]. Cellulose aerogels were made from dissolvable cellulose filaments in melts of calcium thiocyanate salt hydrate in this study, followed by regenerating in ethanol and drying under supercritical CO
2. It is possible to create uniform-structured bio-based aerogels with minimal bulk mass. The microstructure of bio-based aerogels exhibited a continuous 3D network with a large specific surface ratio coupled with a significantly sponge-like structure (up to 98%). This research enabled the examination of increased cellulose amounts of up to 6 wt%. Bio-based aerogels displayed remarkable physical strength and heat transfer efficiency for textile applications at atmospheric pressure. Moreover, the Young’s modulus of cellulose aerogels showed that it can be reached at 13.5 MPa, while the Poisson ratio was near zero
[65]. Yangyang exploited discarded cotton textiles to enhance the anti-flaming capabilities of cellulose aerogels by producing magnesium hydroxide nanoparticles in situ in cellulose gel nanostructures. In addition, three-dimensional nanoporous cellulose gels were produced by disintegrating and coagulating cellulose in an aqueous NaOH/urea solution, and these were employed as patterns for the unclustered production of magnesium hydroxide nanoparticles. According to the findings, the produced mixture–matrix aerogels have extremely porous architectures and exceptional thermal isolation characteristics with minimal heat transfer. In addition, effective flame-retardant and mechanical characteristics were obtained
[66].
3. Multifunctional Application of Cellulose-Based Aerogels on Textile Structures
Due to the robust chemical reactivity of cellulose, the wide range of diverse derivatives with various functions, the adaptable construction process, and the multiple methods of modification, bio-based aerogels exhibit multifunctionality. There exist three primary methods for modifying cellulose aerogels
[11]:
- -
-
Other components can be added to the cellulose solution/suspension
[11]. For example, the reaction of CNF with N-methylol-dimethyphospylpropionamide (MDPA) and further crosslinking by 1,2,3,4-butane tracarboxylic acid (BTCA) yields a flame retardant with good flexibility and self-extinguishment
[106].
- -
-
Coating or adding additional substances to the aerogel structure
[11], such as the polyacrylonitrile–silica aerogel coating over viscose nonwoven fabric for protection and comfort
[107]. Another area of study is the application of molecular layer-by-layer (m-LBL) technology. This technique enables the deposition of ultrathin layers onto a surface through sequential covalent processes. As a consequence, a precise molecular-scale coating is generated, mostly by surface oligomerization, which is not possible with bulk synthesis techniques
[108][109][110].
- -
-
Surface modification of cellulose aerogels may be attained using a number of methods
[11], including dip-coating with PDMS (poly(dimethyl siloxane))
[111].
- -
-
Cellulose aerogels are lightweight 3D porous materials. They are currently employed mostly in insulation, flame retardants
[66][112][113], and biological applications
[4][11]. Additionally, they find applications in carbon aerogel production, as well as the transportation of metal nanoparticles and metal oxides
[11].
3.1. Thermal Insulation Materials
Materials are classified according to their thermal conductivity as thermal conductors (λeff ≥ 0.1 W/(mK), insulators (0.1 W/(mK) > λeff > 0.025 W/(mK), and superinsulators (λeff ≤ 0.025 W(mK). It is known that the thermal conductivity of dry air is around 0.025 W/(mK), which is generally slightly dependent on temperature and moisture content.
Due to their thermal conductivity levels spanning from tens to hundreds of W/(mK), metals are good thermal conductors. Expanded polystyrene, extruded polystyrene, glass wool, mineral wool, and wood exhibit thermal conductivities within the range of 0.1 to 0.026 W/(mK), making them effective insulators against heat transfer
[114][115]. Silica aerogels, vacuum insulation panels, and vacuum glasses are regarded as superinsulating materials because their thermal conductivity is below 0.025 W/(mK)
[116].
The thermal conduction of aerogels can be classified as solid-state, gas-phase, open-pore, or radiation thermal conduction. Once the pore size of a porous material approaches the average free path of the gas (which is approximately 70 nm when vented), the thermal conductivity of the substance decreases. This is attributed to the fact that the pores impede gas flow and restrict convection, thereby hindering heat transfer. The thermal conductivity of mesoporous cellulose aerogels primarily depends on two factors: solid-state thermal conduction and gas-phase thermal conduction. These factors, in turn, are closely associated with the aerogel’s density (determined by the initial cellulose concentration), the pore size distribution, and the surface structures of the aerogel material
[11].
Regenerated cellulose aerogels possess a porous structure with a relatively higher fraction of large pores compared to other cellulose aerogels. This increased presence of large holes within the aerogel structure enhances heat conductivity as it facilitates improved gas transport
[11].
Antlauf et al. conducted a study in which cellulose fibers (CFs) and cellulose nanofibers (CNFs) were produced from commercially available birch pulp. The production process involved varying pressure and temperature parameters as experimental variables. For temperatures ranging from 80 to 380 °C, their results exhibited very little fluctuation in thermal conductivity with density (ρ
sample = 1340–1560 kg/m
−3). Furthermore, temperature dependency is independent of fiber size, density, and porosity.
Figure 3 depicts their studies on thermal conductivity
[117].
Figure 3. Thermal conductivity against pressure.
3.2. Flame Retardancy
Aerogels with a lightweight composition derived from bio-based materials draw the attention of academics because of their exclusive properties, some examples of which include being environmentally conscious, sustainable, and possessing amazing thermal insulation effectiveness
[118][119]. The fire-resistant clothing used for firefighting is a form of specific thermal protection clothing used by firemen during firefighting operations
[120][121]. As a result, advanced flame-resistant and thermally insulating materials with exceptional performance are vital in thermal protective garments to safeguard firefighters. Para-aramid polymer is now used mostly in thermal protective gear as a material that provides flame retardancy due to thermal insulation of porous fabrics created from it
[122][123]. Using the wet spinning procedure, Liu et al.
[122] revealed that a lab-scale nanofibril Kevlar aerogel exhibited strong flame-suppressing properties, characterized by a comparatively slow combustion rate (0.013 cm/s) and the ability to extinguish itself.
It is vital to identify environmentally acceptable thermal insulation materials designed for firefighting apparel
[124]. Researchers have recently expressed interest in flame-retardant aerogels made from low-cost biomaterials because of their sustainable nature, eco-friendliness, affordability, lightweight properties, and strong thermal insulation properties
[125][126][127]. Due to their high porosity, low thermal conductivity, lightweight structure, and excellent thermal insulation properties, aerogels find extensive utilization in various applications, such as fire resistance and thermal insulation
[128][129]. Polymers derived from natural polysaccharides are common renewable biomass resources that are more biodegradable and environmentally friendly compared to fossil-based products
[130]. In consequence, several efforts have been undertaken to create aerogels based on polysaccharides that exhibit remarkable low density, porosity, non-toxicity, biodegradability, and bio-sustainability
[124]. Among the notable examples are magnesium hydroxide nanoparticles (MH NPs) in waste cotton fabric-based cellulose gel nanostructures
[66] obtained by the freeze-drying method, which have demonstrated that the addition of magnesium hydroxide to the gel structure effectively enhances the flame-retardant properties of the aerogel in foam form. According to an experiment conducted by N. Le Thanh, adding NaHCO
3 to the material showed a decrease in the combustibility of the material and the burning rate.
Figure 4 shows this combustion example accordingly. As the figure and the experiments show, while the pure cellulose aerogel burned quickly (at an average speed of 3.45 mm/s), by increasing the NaHCO
3 concentration in the material by between 1–2 and 3%, the combustibility of the material fell and its rate of burning also decreased
[131].
Figure 4. Images of a paper cellulose aerogel after a 10 s burn; the following samples were observed: (a) pure cellulose aerogel, (b) cellulose aerogel with 1% NaHCO3, (c) cellulose aerogel with 2% NaHCO3, and (d) cellulose aerogel with 3% NaHCO3.
3.3. Medical Applications
As the most common polymer on the planet, cellulose is mostly obtained from plants and microbiological sources
[132]. Nevertheless, due to its unique properties, such as decomposability, compatibility with living systems, and low cytotoxicity, it is one of the most commonly used polymers for manufacturing aerogels
[133]. Bio-based aerogels are widely employed in medical treatments such as biological detection, drug release systems, regenerative scaffolds, and anti-infective wound wrap materials
[134]. Several studies have previously been published on the sequential evolution of aerogels’ formation and the therapeutic uses of nanofibrillated cellulose aerogels
[17]. Nevertheless, little research has been conducted on the utilization of bio-based aerogels for bactericidal administration and wound treatment in textile applications. Many studies have explored strategies for wound healing, including the use of a composite aerogel with collagen and cellulose
[135]. Collagen, which is valued for its adhesive, biodegradable, and biocompatible properties, proves suitable for wound dressings. Oxygen permeability and moisture management are crucial for normal cellular function in wound healing
[136]. Cellulose-based nanoparticles, particularly nanocellulose polymers, leverage the effective surface area of filamentous biomaterials for cellular absorption. Combined with an antimicrobial substance
[137], CNF aerogels emerge as a potential wound dressing solution.
3.4. Water Treatment Containing Textile Dyes
Water pollution, a global concern affecting both water supplies and public health
[138], involves contaminants like heavy metals
[139], petroleum products, dyes, and various chemical compounds
[140]. Ongoing efforts seek optimized methods for removing pollution sources, focusing on sustainable and environmentally friendly materials for water purification
[138]. The water treatment method chosen depends on the water composition, quality criteria, and intended usage
[141]. Iron removal is crucial for technical purposes to prevent water from becoming unsuitable due to high iron levels and discoloration. Dye release, a minor contributor to water pollution, is visible and undesirable even at low concentrations
[140]. Annual dye production, surpassing 700 thousand tons, includes synthetic types posing risks to aquatic organisms and humans
[142][143]. In textile manufacturing, dyes fall into categories such as anionic, cationic, or non-ionic
[140]. Effluents from the dyeing process exhibit increased levels of color, suspended solids, biochemical oxygen demand, chemical oxygen demand, temperature, metals, and salts
[144]. Continuous monitoring and comparison of these parameters with established concentrations are essential in treatment procedures before releasing effluents into water bodies. The assessment of treatment effectiveness also considers additional parameters such as total organic carbon, nitrate–nitrogen, ammonia–nitrogen, and orthophosphate–phosphorus
[145]. Capturing dyes in fabrics during dyeing poses a challenge due to their pronounced water solubility, resulting in the generation of considerable wastewater with substantial quantities of these organic compounds
[144]. The composition of wastewater in the textile industry exhibits global variations influenced by factors such as the manufacturing process, fabric type, factory equipment, applied chemicals, fabric weight, season, and fashion trends
[145].
Various methods, including filtration, oxidation, and microbial approaches, are employed to eliminate dyes from water. However, these methods are associated with high costs, low efficiency, and operational challenges. Despite dyes’ resistance to degradation, certain bacteria, such as
Pseudomonas sp. and
Sphingomonas sp., have demonstrated effectiveness in decolorizing and mineralizing them
[146]. The extensively used adsorption methods, employing traditional adsorbents like clay, bentonite, zeolite, or charcoal
[10][11][12][13][147][148][149], are preferred for their cost-effectiveness, high efficiency, and simplicity. Nonetheless, traditional adsorbents face limitations such as a short effective life, regeneration difficulties, and high consumption. Color removal from wastewater involves a range of physical, chemical, and biological treatment methods
[150], with adsorption widely acknowledged and employed
[151].
Dyes in wastewater pose a significant threat to ecosystems and living organisms due to their persistence, biotoxicity, and bioaccumulation
[152][153]. Consequently, there is a growing interest in developing efficient treatment strategies for dye effluents to address multipollutant removal
[154]. Among the available technologies, adsorption is considered to be the most competitive, offering easy operation, relatively low costs, and non-toxic byproducts
[155][156]. Cellulose-based aerogels are recognized as potential candidates for wastewater treatment due to cellulose’s renewable and biodegradable nature, easily functionalized properties, unique 3D network structure, and high surface area
[150][157][158][159]. However, challenges remain, including avoiding toxic crosslinking agents and achieving a balance between high adsorption capacity and selectivity
[150][160]. A green and effective strategy is essential to develop cellulose-based adsorption aerogel materials for dye wastewater purification
[161]. Produced via the transesterification of cellulose and acetoacetate reagents, cellulose acetoacetate (CAA) is a water-soluble derivative that offers active reaction sites, which enable the creation of functional materials in aqueous environments
[162][163][164]. In a noteworthy investigation, Liu and colleagues presented a self-repairing polysaccharide hydrogel formed by combining CAA and chitosan in a solution
[165]. This hydrogel, derived from cellulose and employing enamine bonds, exhibited reversible sol–gel transitions that were responsive to pH variations. This highlights the suitability of polymers with amino groups for creating CAA-based gels. Unlike conventional chemical crosslinking agents, the eco-friendly approach of constructing 3D network structures with dynamic enamine bonds can be swiftly achieved at room temperature. The interlocking 3D network structure is also capable of being disassembled for further applications. Additionally, the use of electrostatic attraction, recognized as a simple and effective force, is emphasized for selectively capturing dyes, especially considering that the majority of commercial dyes exist in ion form
[166]. In another study, a cationic cellulose aerogel (Q-CNF), derived from cellulose nanofibrils with trimethylammonium chloride groups, was prepared through freeze-drying and aliphatic triisocyanate crosslinking. The rigid porous aerogel demonstrated efficient adsorption of anionic dyes, with capacities of 250, 520, and 600 μmol g
−1 (approximately 160, 230, and 560 mg g
−1, respectively) for red, blue, and orange dyes, respectively. Electrostatic interactions between CNF surface positive sites and dye sulfonate groups were identified as the main contributors. The adsorption capacity was correlated with the specific surface area and cationic content of the aerogel. Regeneration with KCl in an ethanol–water mixture allowed for multiple adsorption–desorption cycles without significant capacity loss. Q-CNF aerogels exhibit promise as renewable, reusable adsorbents for treating dye-loaded water
[167].
3.5. CO2 Capture
Carbon capture and storage (CCS) is a key component in the global effort to reduce carbon dioxide (CO
2) emissions by preventing the release of CO
2 into the atmosphere. Choosing the right materials is crucial to building dependable and secure infrastructure for CCS technology. Natural cellulose materials are a great option for CCS applications because of their impressive mechanical and physical qualities and eco-friendliness. As CO
2 adsorbents and catalyst carriers for CO
2 conversion, cellulose-based materials are useful in the field of carbon capture, utilization, and sequestration technologies
[168]. Cellulose is a flexible material that may be used as a matrix or filler to produce goods such as films, paper bases, aerogels, and hydrogels with adsorption capabilities. It is known for its renewability and degradability
[169][170][171][172]. Creating porous carbon with adsorption properties also requires it. Activation techniques employing physical or chemical activators can be used to modify carbon-based materials. Carbon capture technology appears to benefit from the use of cellulose and its derivatives, such as cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs), among other sophisticated materials
[170][173][174]. Rich and affordable, cellulose aerogels have the potential to revolutionize existing carbon capture techniques, especially when it comes to the manufacturing of commercial nanocellulose
[175][176][177]. Notwithstanding their smaller surface area, chemically altered cellulose and nanocellulose aerogels demonstrate adequate CO
2 chemisorption, highlighting their potential for use in carbon capture applications
[178][179]. By combining cellulose aerogels with large-pore hierarchical porous metal–organic frameworks (HP-MOFs), employing monocarboxylic acid (MA) as a modifier, and growing HP-UIO-66-NH
2 on the cellulose aerogels in situ, Yu et al. created hybrid aerogels. The chain length of MA was changed to modify the pore size of the HP-MOFs. The study revealed that the CO
2 adsorption capacity followed a trend of initially increasing and then decreasing with the increase in the MOFs’ pore size. Concurrently, the adsorption selectivity for CO
2 consistently grew. Notably, among all of the samples, MC-HUN-4, distinguished by a moderate pore size, exhibited the highest CO
2 adsorption capacity (1.90 mmol/g at 298 K and 1 bar) and superior adsorption selectivity (13.02 and 2.40 for CO
2/N
2 and CO
2/CH
4)
[180]. Zhou et al. applied a sol–gel method to create composite aerogels (CSA) by using silica from skimmed cotton and cellulose whiskers. Employing tetraethyl orthosilicates (TEOs) and an alkaline silica solution as precursors, the CSA-TEPA 70% aerogel, with a 70% TEPA (tetraethylenepentamine) loading, demonstrated impressive adsorption, achieving a maximum capacity of 2.25 mmol/g. These adsorbents hold promise for CO
2 capture. Additionally, the heightened research interest in metal–organic frameworks (MOFs) is attributed to their high surface area, CO
2 affinity, structural diversity, tunable microporosity, and adaptable structure
[181]. Using bacterial cellulose (BC) as a substrate, Ma et al. created composite aerogels containing amino-functionalized ZIF-8 (zeolitic imidazolate frameworks) (ZIF-8-NH
2). Zinc ions and hydroxyl groups chelated to produce composites with strong interfacial attraction and compatibility when ZIF crystals were evenly encased around cellulose fibers. The resultant aerogel showed a noteworthy CO
2 adsorption capability of 1.63 mmol/g. Zinc ions combined with the hydroxyl and oxygen groups in cellulose to create complexes. ZIF-8-NH
2 crystals were created as linkers by adding 2-methylimidazole and 2-aminobenzimidazole by wrapping BC chains without the need for binders. By varying the amount of organic linker, the ZIF-8 amino group loading was optimized
[182].
Mesoporous cellulose aerogels, derived from old corrugated containers (OCCs) through freeze-drying, exhibited efficient CO
2 capture. The aerogel synthesis induced a transition in cellulose crystals from form I to form II while preserving their chemical structures. These aerogels featured excellent thermal stability, comprising highly porous networks with fibrils below 50 nm wide. Their mesopore volumes ranged from 0.73 to 1.53 cm
3 g
−1, and their specific surface areas varied from 132.72 to 245.19 m
2 g
−1. Furthermore, the research demonstrated exceptional CO
2 adsorption capacities within the range of 1.96–11.78 mmol g
−1. In comparison to other sorbents, the CA-2 material (2% weight cellulose) exhibited superior CO
2 adsorption capacity at room temperature
[183].