Upcycling Hazardous Fly Ash into High-Performance Lightweight Xerogels for Thermal Insulation, CO2 Adsorption, and Wastewater Purification

Upcycling Hazardous Fly Ash into High-Performance Lightweight Xerogels for Thermal Insulation, CO2 Adsorption, and Wastewater Purification | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Upcycling Hazardous Fly Ash into High-Performance Lightweight Xerogels for Thermal Insulation, CO 2 Adsorption, and Wastewater Purification Sunanda Roy, Sajal Nandi, Barnali Dasgupta Ghosh, Kheng Lim Goh This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8275181/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract Each year, millions of tons of fly ash (FA), a hazardous byproduct of coal-fired power plants, are disposed of in landfills, causing serious environmental pollution and substantial health risks. Consequently, effective FA management has become critically important. Despite its popularity in the construction sector, FA has rarely been used to create multifunctional porous materials capable of addressing heat insulation, CO 2 capture, and wastewater purification. This paper reports the successful development of highly durable, lightweight, flexible and multifunctional xerogels derived from FA. The FA was chemically modified and blended with cellulose nanofibers (CNFs), a binder, and a crosslinker to enhance structural stability and multifunctionalities. The resulting xerogel exhibited a compressive strength of 293.5 ± 9.6 kPa and a Young’s modulus of 264.1 ± 6.8 kPa at 70% strain, representing an increase of 86.6% in strength and 94% in modulus compared to pure CNF xerogel. Moreover, it exhibited excellent thermal conductivity of 32.1 mW/m.K, and a CO 2 adsorption capacity of 1.67 mmol/g, superior to many contemporary materials. Interestingly, the xerogel also performed extremely well in selectively removing both cationic and anionic dyes from water. These results highlight the effectiveness of our formulation and design approach in producing a robust multifunctional xerogel. With its simple fabrication process, lightweight structure, mechanical robustness and multifunctionality, the developed xerogel emerges as an attractive solution for wastewater management, CO 2 mitigation and building insulation. Xerogel thermal conductivity fly ash dye cellulose nanofibers porous Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Fly ash (FA) is one of the hazardous industrial wastes generated in coal-fired thermal power plants when coal is burned. It contains various toxic substances, including heavy metals and naturally radioactive materials such as arsenic, chromium, mercury, lead, thorium, and uranium [ 1 , 2 ]. As electricity demand rises due to a growing population, the FA production is also increasing. Currently, ~ 70% of India’s power supply comes from coal. These plants produce nearly 120–150 million tons of FA annually. Ironically, despite its known toxicity, a large portion of FA is dumped in open landfills, causing severe environmental pollution and threatening the entire ecosystems. This has raised widespread global concern and emphasised the urgent need for measures to combat pollution. To date, FA has been primarily used in construction applications such as cement, brick, tiles and roadworks [ 3 – 6 ]. Some recent studies have also explored its potential in agriculture, catalysts, insulation panels, wastewater treatment, coatings, and polymer composites [ 7 – 10 ]. However, these utilizations account for only 30–40% of the total FA volume. This situation demands more scientific, sustainable strategies that can reduce pollution also transform it into valuable resources for advanced technologies. This research aims to convert FA into novel, multifunctional, lightweight, and robust xerogels useful for sustainable development through applications for heat shielding and building insulation, CO 2 capture, and wastewater treatment via dye removal. The objective was to design one material capable of addressing multiple challenges. However, developing such a xerogel is challenging due to certain intrinsic limitations of FA, including its non-functional surfaces, variable particle size, poor dispersion within the polymer matrix and weak interfacial adhesion, all of which result in brittle and non-functional structures [ 2 , 11 ]. Recently, some studies have investigated the effects of FA on polymer composites; however, most of the resulting composites exhibited weaker mechanical properties than the pristine polymer due to the aforementioned challenges [ 11 – 14 ]. Moreover, to our knowledge, no report exists on such FA-polymer composite xerogel. Aerogels and xerogels are known for their exceptional properties, lightweight structure, ultra-low density, high porosity, large surface area, and superior thermal insulation, making them ideal for applications in aerospace, construction, advanced composites, and sustainable technologies. Aerogels and xerogels are porous solids with a three-dimensional network structure derived from gels, mainly differing in their drying methods. Aerogels are produced using supercritical drying to preserve their structure and achieve low density, whereas xerogels are formed through evaporative drying, which results in partial shrinkage and higher density; however, the process is more cost-effective and faster [ 15 ]. There are a few articles on FA-based aerogels, but those have only focused on extracting SiO 2 and Al 2 O 3 from FA and then using them to synthesise aerogels via the sol–gel method [ 16 – 18 ]. These resulting aerogels are often fragile and brittle. In contrast, our approach uses FA as a functional filler and mixes it with polymer to develop a flexible, robust composite xerogel. The xerogel was formulated to achieve high thermal insulation, effective CO 2 capture, and rapid wastewater purification. The xerogel contains polydopamine coated FA (P-FA) as a functional filler, cellulose nanofibers (CNFs) as the structural matrix, polyvinyl alcohol (PVA) as a binder, polyethylenimine (PEI) as both a surface modifier and intermediate bridging agent, and glutaraldehyde (GA) as a crosslinking agent. Given the complex surface chemistry of FA, polydopamine was selectively chosen for surface functionalization due to its fast and strong adhesion to a wide variety of substrates [ 19 , 20 ] and the ability to provide a uniform surface coating. This coating further facilitates the attachment of other functional groups such as amines and thiols, thus enhancing material functionality, interfacial adhesion and overall performance. Cellulose was selected as the main matrix due to its biodegradability, renewability, low cost, lightweight, and excellent mechanical properties [ 21 , 22 ]. Moreover, cellulose, PEI and polydopamine coated FA can form a strong intermolecular network and crosslinking within the system due to their diverse and rich functional surfaces. Owing to the unique formulation and fabrication strategy, the resulting xerogel exhibited ~ 86.6% and ~ 94% higher compressive strength and Young’s modulus than the pure CNF xerogel. Moreover, it showed excellent heat shielding, thermal conductivity of 32.1 mW/m.K, and a CO 2 adsorption capacity of 1.67 mmol/g, better than many contemporary materials. To the best of our knowledge, this is the first FA-CNF composite xerogel that addresses three critical and challenging issues: high thermal insulation, efficient CO 2 capture, and rapid, efficient wastewater purification. To understand the material’s properties and performance, an in-depth characterization was performed. Overall, this research shows that with strategic planning and processing, hazardous FA can be turned into a valuable resource for advanced, sustainable applications. 2. Experimental section 2.1. Materials Fly ash (FA) was collected from the Kolaghat thermal power plant in West Bengal, India. Cellulose nanofibers (CNFs) were prepared from hardwood pulp via TEMPO-mediated oxidation, following previously reported protocols [ 22 , 23 ]. Polyvinyl alcohol (PVA; Mw 13,000–23,000, 87–89% hydrolyzed), branched polyethylene-imine (PEI; average Mw ~ 800, Mn ~ 600), glutaraldehyde (GA), dopamine hydrochloride (≥ 98%), Trizma base (Mw 121.14), and all other solvents were purchased from Merck, India. 2.2. Fabrication of P-FA/CNF xerogel To prepare polydopamine-functionalized fly ash (P-FA), approximately 1 g of FA was dispersed in 10 mL of Trizma buffer (10 mmol, pH 8.68), followed by the addition of 100 mg dopamine hydrochloride. The suspension was stirred magnetically at 40°C for 8 hours. The resulting dark brown modified FA was collected by vacuum filtration and rinsed thoroughly with deionized water to remove unreacted components. The product was labelled as P-FA, where P indicates polydopamine coating. For the composite xerogel, ~ 60 g of CNF suspension (2.2 wt% on a dry basis) was homogenized with ~ 60 mg of P-FA at 10,000 rpm for 10 minutes. A 10 wt% PVA solution was separately prepared by dissolving 5 g of PVA in 45 mL of DI water at 80°C for 2 hours. Next, a composite mixture was prepared by mixing P-FA, PVA and PEI (dissolved in hot water). The ratio of P-FA: PVA: PEI was maintained at 4:95.50:0.50. The mixture was then homogenized for 10 min at 10,000 rpm followed by 10h magnetic stirring at 70°C. During this process, a small amount of GA (crosslinker) and HCl were added to initiate crosslinking. The final mixture was then poured into cylindrical plastic molds and frozen for 24h to facilitate solidification. Subsequently, the molds with frozen gels were slowly immersed in a 1L acetone bath. This step was repeated twice. The wet xerogel samples were then dried in an oven at 100°C for 1 hour to obtain the final porous lightweight dry xerogels, designated as P-FA/CNF xerogel. For comparison, a control xerogel was prepared using pure CNF, referred to as CNF xerogel. Although a pristine (unmodified) FA/CNF xerogel was also prepared following the same formulation and process, it was brittle and did not form a xerogel-like structure. Thus, the pure CNF xerogel was chosen as the reference sample for comparison. However, in some cases, the properties of unmodified FA/CNF were also discussed for additional clarity. 2.3. Dye adsorption test The dye adsorption tests were carried out on P-FA/CNF xerogel using aqueous solutions of cationic and anionic dyes. Methylene blue (MB) and Nigrosin are both dyes were used as the model cationic and anionic dye, respectively. Three independent tests were carried out with each dye sample to ensure reproducibility. During the experiment, approximately 120 mg of the dry xerogel (adsorbent) was placed in a glass beaker containing 50 mL of dye solution (50 mg/L) and stirred at constant speed at 35°C. After certain time intervals, fixed aliquots were withdrawn, diluted with an equal volume of DI water, and the residual dye concentration was determined using UV–vis spectroscopy. The characteristic absorption maxima for MB and Nigrosin were observed at 663 nm and 578 nm, respectively. 2.4. Characterization The surface chemical composition of the xerogels was identified using Fourier Transform Infrared (FTIR) spectroscopy (PerkinElmer Frontier) between 600–4000 cm − 1 with 16 scan rate. Thermal properties were evaluated using Shimadzu DTG-60 thermogravimetric analyser under nitrogen atmosphere from 100°C to 600°C at a heating rate of 10°C/min. X-ray photoelectron spectroscopy (XPS) was performed with a K-Alpha Thermo Scientific system equipped with monochromatic Al Kα radiation, and all spectra were calibrated against the C 1s peak at 284.8 eV. Surface morphology was observed using a field emission scanning electron microscope (FESEM, Hitachi S4200). Thermal insulation performance was assessed using the Transient Planar Source (TPS) method in accordance with ISO 22007-2:2015, employing a HotDisk® TPS 2500S thermal conductivity analyzer (Göteborg, Sweden). Measurements were carried out under ambient conditions with a heating power of 15 mW applied for 5s. Two identical samples were positioned symmetrically on either side of a Kapton sensor, with a 10-minute interval between successive tests. The reported values represent the average of four samples. The Micromeritics ASAP 2020 analyzer was used to measure the Brunauer-Emmett-Teller (BET) surface area, micropore volume, and pore size distribution via nitrogen adsorption–desorption isotherms over a relative pressure range (p/p⁰) of 0–1 bar. CO 2 adsorption isotherms were also measured using the same instrument. Before analysis, the xerogels were degassed at 100°C to remove moisture and residual impurities. Adsorption experiments were conducted with a circulating water bath at different temperatures, with 30 s equilibration per pressure point. The regeneration study was conducted via adsorption-desorption cycles for up to eight cycles. The dye adsorption performance was evaluated using a UV-Vis spectrophotometer (UV-2501PC, Shimadzu). Figure 1 schematically illustrates the synthesis of P-FA/CNF composite aerogel and plausible intermolecular interactions among its components. The bonding mechanism in this system is complex, involving multiple polar functional groups, including hydroxyl and amine groups. The main interactions arise from hydrogen bonding among the -OH and -NH 2 groups of P-FA, CNF, PVA, and PEI. Additionally, the covalent acetal linkages formed through glutaraldehyde (GA) assisted crosslinking between P-FA/CNF, P-FA/PVA, and PVA/CNF further strengthen the xerogel structure. Under mildly acidic conditions, the aldehyde groups (-CHO) of GA readily react with two adjacent hydroxyl groups of polyols (CNF, PVA, and P-FA), forming stable acetal and ether bonds [ 24 , 25 ]. Simultaneously, GA can form imine (C = N) linkages with the -NH 2 of PEI. These interactions collectively impart mechanical strength and structural integrity to the P-FA/PVA xerogel. Figure 2 a presents the FTIR spectra of raw fly ash (FA) and polydopamine-coated fly ash (P-FA). Although both samples exhibit similar overall spectral profiles, notable differences are observed around 3300 cm⁻¹ and 2800–2700 cm⁻¹. The broad absorption band near 3448 cm⁻¹ in raw FA corresponds to O–H stretching vibrations associated with amorphous silicates or hydrated aluminosilicate species. In contrast, the corresponding band in P-FA indicates the presence of hydroxyl (–OH) and primary/secondary amine (–NH) groups originating from the polydopamine coating. In P-FA, distinct doublet peaks in the 2850–2930 cm⁻¹ range, attributed to the asymmetric and symmetric stretching of CH 2 and CH 3 groups [ 20 ], confirm the successful deposition and functionalization of PDA. The common peaks in both samples, i.e., at 1634, 1092, and 793 cm⁻¹, correspond to O–H bending of adsorbed water molecules, asymmetric Si-O-Si stretching, and symmetric Si-O-Si/Si-O-Al stretching modes, respectively [ 2 ]. The XPS analysis from our previous study confirmed that P-FA contains C, O, and N content with 74.12% carbon, 19.98% oxygen, and 4.50% nitrogen [ 2 ]. The surface morphology of raw FA and P-FA was discussed next (Fig. 2 b) to understand the surface modification. SEM images revealed that the raw FA particles are smooth and spherical, whereas the P-FA particles exhibit a rougher texture with visible polydopamine particles. These morphological variations confirm the successful polydopamine coating onto FA. Additionally, the inherent variation in particle size was clearly observed in the SEM image. The paper will now focus on the properties of the xerogels. Figure 3 a compares the XPS survey spectra of pure CNF xerogel and P-FA/CNF xerogel. The XPS spectra of pure CNF xerogel showed only carbon and oxygen elemental peaks, suggesting no contamination or impurities in the neat CNF. In contrast, the P-FA/CNF xerogel shows a new peak at 399.8 eV, corresponding to N1s of nitrogen element. The appearance of the N1s peak confirms the presence of polydopamine and PEI molecules in the P-FA/CNF xerogel. The atomic percentages of C, O and N are presented in the insets of the respective figures. Compared to the pure CNF xerogel (0.43), the P-FA/CNF xerogel shows a lower O/C ratio (0.22). This indicates that P-FA/CNF xerogel is composed of other organic molecules, such as PVA, PEI and GA. This reduction in the O/C ratio is attributed to the utilization of free hydroxyl groups of CNFs and PVA during the formation of covalent acetal linkages with GA. Figure 3 b presents the FTIR spectra of the pure CNF xerogel and P-FA/CNF xerogel, where several characteristic changes are clearly observed. The pure CNF shows a big and intense O-H stretching band at 3293 cm − 1 due to its abundant hydroxyl groups. Other characteristic peaks appear at 1160 cm − 1 , 1100 − 1000 cm − 1 , and 893 cm − 1 , corresponding to -COO⁻ stretching, C-O-C pyranose ring vibrations, and β-glycosidic linkages, respectively [ 21 , 26 ]. A band at 1600 cm − 1 indicates carboxylate groups (-COO) from TEMPO oxidation [ 22 ]. In contrast, the P-FA/CNF xerogel shows a reduced and relatively flat O–H band (3200–3500 cm − 1 ) band, indicating the consumption of -OH groups and the successful formation of extensive crosslinking facilitated by GA. In this system, the crosslinking occurs at various sites of the materials, such as P-FA/CNF, CNF/CNF, P-FA/PVA, CNF/PEI, and PVA/PEI interfaces. The incorporation of PEI provides additional crosslinking sites (via imine bonds, detected at 1658 cm − 1 ) and hydrogen bonding between the filler and the matrix [ 19 ], thereby enhancing the mechanical strength of the xerogel. Another prominent change was observed in the intensity of the C-H stretching of the methylene group between 2900 − 2800 cm⁻¹. This suggests the presence of PEI, GA and PVA molecules in the system [ 19 , 25 ]. The acetal groups represented by C-O-C stretching bands often overlap with the parent C–O signals of CNF and PVA. However, in P-FA/CNF xerogel, the distinct spectral changes in the 1450–1000 cm⁻¹ region clearly indicate the formation of covalent acetal linkages along with other bonding interactions [ 25 ]. Next, the thermal stability of the pristine FA, P-FA and xerogels was investigated to gain insights into their thermal behaviour and potential application boundaries. Figure 3 c shows the TGA curves of the pure CNF xerogel and P-FA/CNF xerogel, while the inset shows the thermal stability profiles of the pristine FA and P-FA samples. All experiments were performed under nitrogen from 25°C to 600°C. The TGA curve of pristine FA shows high thermal stability due to its inherent thermal insulation property [ 27 ]. The P-FA also displayed a high thermal stability, though slightly reduced by the polydomaine surface coating, which was about 7.6% in mass. The thermograms of xerogels were discussed next. Among the three xerogels (CNF xerogel, pristine FA/CNF xerogel and P-FA/xerogel), the pristine FA/CNF xerogel exhibits the lowest thermal degradation stability (204.6°C at 10% mass loss). Its faster degradation relative to the pure CNF xerogel (215.3°C at 10% mass loss) results from poor interfacial adhesion and incompatibility between pristine FA and CNF matrix. In contrast, the P-FA/CNF xerogel shows a remarkably higher (the highest) thermal decomposition temperature of 239.7°C at 10% mass loss. The nearly 35°C increase in the thermal decomposition temperature compared to the FA/CNF xerogel is attributed to strong interfacial bonding, effective crosslinking and overall robustness of the materials formulation. Notably, the minor mass loss below 210°C for all xerogels corresponds to the evaporation of bound water molecules in CNF and PVA [ 21 , 28 ]. The next major decomposition step between 210–400°C is mainly associated with the depolymerization and thermal decomposition of the CNF, PVA, and other organic components. The higher char residue of the P-FA/CNF xerogel further indicates its improved thermal stability. Figure 4 presents the morphologies of the three xerogels. The pristine CNF xerogel displays a randomly oriented 3D porous network structure with loose, thin cell walls (Fig. 4 a). This structure forms because CNFs are uniformly dispersed in the hydrogels and pack tightly through hydrogen bonding. As mentioned before, the xerogels were fabricated by freezing the precursor hydrogels in a freezer, followed by a solvent exchange. During freezing, ice crystals grow within the material, compressing the nanofibers together and facilitating physical entanglement. During solvent exchange and drying, capillary forces pull the nanofibers tightly together, creating a compact and sheet-like porous network. In contrast, both the FA/CNF and P-FA/CNF xerogels have very different architectures. The cell walls appeared denser, thicker and lower porosity (Fig. 4 b and 4 c). This occurs because the FA particles increase the solid concentration and restrict complete fiber exfoliation. Compared with the FA/CNF xerogel, the P-FA/CNF xerogel shows more robust cell walls with more evenly distributed FA particles owing to extensive crosslinking between CNF and P-FA particles. Figure 4 d further confirms the homogeneous dispersion and embedding of P-FA particles within the matrix. The uniform distribution and strong adhesion resulted in a more robust xerogel structure. Table 1 Physical properties of CNF xerogel and P-FA/CNF xerogel. Sample SBET (m2/g) Porosity (%) Pore size (nm) Compression strength at 70% strain (kPa) Compressive modulus (kPa) Thermal conductivity (mW/m.K) CO2 adsorption (mmol/g) CNF xerogel 69.0 97.8 16.1 157.2 (± 8.3) 136.1 (± 7) 36.3 (± 3.0) 0.52 Raw FA/ CNF xerogel 49.8 89.7 29.1 174.6 (± 9.1) 159.4 (± 7.4) 37.1 mW/m.K 0.42 P-FA/ CNF xerogel 63.8 96.3 17.4 293.5 (± 9.6) 264.1 (± 6.8) 32.1 (± 3.3) 1.67 Since the specific surface area, pore size and pore size distribution play a crucial role in evaluating the adsorption efficiency and thermal insulation properties of a xerogel, they are discussed next. Nitrogen adsorption was used to determine the specific surface area (SSA) and porosity characteristics of the CNF xerogel and P-FA/CNF xerogel. According to the IUPAC classification [ 29 ], if both adsorption and desorption isotherms of a porous material follow a type IV isotherm, the material belongs to mesoporous types and exhibits strong adsorbate-adsorbent interactions. All our xerogels belong to mesoporous materials. Table 1 presents the physical properties of all three xerogels (pure CNF xerogel, FA/CNF xerogel and P-FA/CNF xerogel). The pure CNF xerogel exhibits a higher specific surface area (69 m 2 /g) and porosity (97.8%), indicating a highly porous and lightweight structure. In contrast, the P-FA/CNF xerogel shows a slightly reduced surface area of 63.8 m 2 /g and porosity (96.3%), due to its relatively denser and compact architecture. Nonetheless, the xerogel remains very lightweight, highly porous and fluffy in nature. Figure 5 compares the sorption isotherms of the CNF xerogel and P-FA/CNF xerogels. Figure 6 . (a) Compressive stress vs strain curves of CNF xerogel and P-FA/CNF xerogel. The inset shows the Young’s modulus of the same samples. (b) Comparison of the thermal conductivity of our xerogels with some existing thermal insulation materials. (c) Digital images show the infrared thermometer readings of the P-FA/CNF xerogel surface temperature under heating. As the pristine FA/CNF xerogel shows inferior properties and is structurally weak, the discussion hereafter will focus only on the CNF xerogel and P-FA/CNF xerogel. To evaluate the structural stability of the P-FA/CNF xerogel, compressive tests were performed. Interestingly, the P-FA/CNF xerogel shows a remarkable improvement in mechanical strength (Fig. 6a), reaching a compressive strength of 293.5 ± 9.6 kPa and a Young’s modulus of 264.1 ± 6.8 at 70% strain, compared to 157.2 kPa and 136.1 kPa for the pure CNF xerogel, an enhancement of 86.6% in strength and 94% in modulus. This large improvement is attributed to strong intermolecular interactions between filler and matrix and the structural reinforcement provided by the P-FA. For curiosity, when a xerogel was prepared with the same amount of pristine (unmodified) FA following the same formulation, the strength and modulus were increased by only 9.3% and 11.2%, respectively, compared to the neat CNF xerogel. This small increment is due to the inherent reinforcing property of FA. This also highlights the weak interfacial bonding and incompatibility between pristine FA and the CNF matrix. It is worth noting that structural stability is crucial for the practical use of xerogels since a well-defined porous structure can provide greater thermal insulation, higher absorptivity, and mechanical robustness. In view of this, the P-FA/CNF xerogel can be a promising porous material for various applications such as gas adsorption, water treatment and thermal insulation. Next, the thermal conductivity was measured as it defines the material’s ability to conduct heat. As can be seen from Table 1 , the P-FA/CNF xerogel exhibited ~ 11.6 and 13.5% improvement in thermal conductivity (32.1 mW/m.K) compared to the pure CNF xerogel (36.3 mW/m.K) and FA/CNF xerogel, respectively. This enhancement confirms the presence of uniformly dispersed P-FA particles within the matrix, enabling the xerogel to perform as a good heat-insulating material with low thermal conductivity. Notably, the thermal conductivity of P-FA/CNF xerogel is much lower than that of the aerogel (47 mW/m.K) reported by Nguyen Do [ 17 ], composed of 5 wt.% FA/PVA/carboxy-methyl cellulose (CMC) and several commercial insulation materials, including mineral wool (30–40 mW/m.K), fiberglass (33–44 mW/m.K), and expanded polystyrene (EPS, 30–40 mW/m.K) [ 30 ] (Fig. 6b). To further confirm the thermal insulation properties, a small piece of P-FA/CNF xerogel was placed on a preheated hot plate at 80°C for 90 s and its surface temperature was measured using an infrared (IR) thermometer. This experiment provides a clear indication of the xerogel’s heat shielding performance. For visual understanding, a video was provided in the supplementary document. Surprisingly, the top surface of the xerogel was at a considerably lower temperature (~ 16.8°C) than that of the reference hot plate (65.4°C-82.2°C, Fig. 6c), demonstrating its excellent heat resistance ability. It is important to note that only 4 wt.% of P-FA was incorporated into the P-FA/CNF xerogel, and the addition of more P-FA would further lower its thermal conductivity. After thermal insulation analysis, the xerogel was evaluated for CO 2 adsorption performance. Figure 7 a compares the CO 2 adsorption capacity of the CNF xerogel and P-FA/CNF xerogel. Compared to CNF xerogel (0.52 mmol/g), the P-FA/CNF xerogel showed about 3.2 times higher CO 2 uptake (1.67 mmol/g at 28°C). This significant improvement can be explained due to the synergistic effects of the xerogel’s porous structure and its amine-rich surfaces. The amine groups act as active chemisorption sites for CO 2 [ 31 , 32 ]. Generally, in amine-functionalized porous adsorbents, CO 2 adsorption proceeds via carbamate formation through a zwitterionic mechanism, as illustrated in Fig. 7 b [ 31 – 33 ]. Therefore, the enhanced CO 2 adsorption capacity of P-FA/CNF xerogel stems from the cooperative contribution of polydopamine-derived pyrrolic nitrogen and free amine groups in PEI. Notably, CO 2 capture is influenced by various factors, including the nature of the adsorbent, pore architecture, pore volume, surface area, type and density of functional groups, their spatial distribution, and the accessibility of reactive sites [ 34 ]. Although the pristine CNF xerogel shows some CO 2 uptake, this is mainly driven by its highly porous structure and large surface area, enabling effective CO 2 diffusion and adsorption. Figure 7 b presents temperature-dependent CO 2 adsorption of the xerogel at a constant pressure of 1 bar. As the temperature increased, the CO 2 adsorption capacity decreased, indicating a typical exothermic adsorption behaviour [ 35 – 38 ]. This result helps in understanding the suitability of the xerogel for practical applications, especially under conditions related to industrial CO 2 capture. As FA-based aerogels or xerogels have not been widely explored for CO 2 adsorption, comparing them directly with high surface area activated carbons or inorganic adsorbents may not be relevant. Nevertheless, the P-FA/CNF xerogel exhibits a competitive CO 2 adsorption capacity compared to other porous adsorbents [ 39 – 45 ]. Figure 7 c illustrates the cyclic stability or regeneration performance of the P-FA/CNF xerogel over six adsorption-desorption cycles. Regeneration was carried out at 90°C under a pure N 2 atmosphere. As observed, the P-FA/CNF xerogel maintains nearly the same CO 2 adsorption capacity as the original sample. After six consecutive cycles, a minor loss of about 2–2.5% in adsorption capacity was observed, most likely due to partial pore blockage or pore collapse, and the breakdown of carbamate species, which facilitates the release of adsorbed CO 2 [ 31 ]. Nonetheless, the efficiency remained over 97%. These findings suggest that this xerogel can be readily integrated into modern building materials, where both thermal insulation and CO 2 mitigation are critically important. The adsorption performance of the P-FA/CNF xerogel for water purification was evaluated using methylene blue (MB) and nigrosin as model dye contaminants, owing to their widespread industrial use and hazardous nature. Approximately 120 mg of xerogel was immersed in the dye solution and gently stirred until the absorption was complete. UV-Vis spectroscopy was used to assess the adsorption behaviour. It can be seen from Figs. 8 a and 8 b that the absorbance of both dyes decreases steadily with time, confirming efficient dye removal. Interestingly, the xerogel adsorbed over 98% of nigrosin dye from the aqueous mixture within just 2 min, whereas MB required 35 min to reach 96.7% adsorption. These rates are faster than many previously reported adsorbents [ 46 – 53 ]. To illustrate the colour fading more clearly, the digital images of the dye solutions at different time intervals were presented in Figs. 8 c and 8 d. Next, a mixed dye solution (MB + nigrosin) was prepared to evaluate the xerogel’s performance, as actual wastewater often contains multiple dyes. Interestingly, the P-FA/CNF xerogel demonstrated efficient and fast removal of both dyes. Figures 8 e and 8 f show the time-dependent UV-Vis spectra and corresponding digital images of the dye colour, respectively. After mixing the two dyes, the spectral profiles changed; however, the characteristic MB peaks remained nearly the same, shifted only by 2–3 nm, due to dye-dye interactions. The MB peaks at 664 and 611 nm disappeared completely within 30 minutes. Although nigrosin didn’t show a distinct peak, its adsorption occurred rapidly. The excellent adsorption performance of the P-FA/CNF xerogel is attributed to π–π interactions, hydrogen bonding, and electrostatic attractions between its surface functional groups and the dye molecules (Fig. 8 g). To the best of our knowledge, this is the first study that presented a robust fly ash-based xerogel with three compelling functionalities. Collectively, these results highlight the exceptional dye adsorption capacity of the P-FA/CNF xerogel. Furthermore, the findings confirm the potential of FA to be upcycled into a robust, multifunctional xerogel, thereby reducing its environmental impact while enabling new pathways for sustainable utilization. 3. Results and Discussion Figure 1 schematically illustrates the synthesis of P-FA/CNF composite aerogel and plausible intermolecular interactions among its components. The bonding mechanism in this system is complex, involving multiple polar functional groups, including hydroxyl and amine groups. The main interactions arise from hydrogen bonding among the -OH and -NH 2 groups of P-FA, CNF, PVA, and PEI. Additionally, the covalent acetal linkages formed through glutaraldehyde (GA) assisted crosslinking between P-FA/CNF, P-FA/PVA, and PVA/CNF further strengthen the xerogel structure. Under mildly acidic conditions, the aldehyde groups (-CHO) of GA readily react with two adjacent hydroxyl groups of polyols (CNF, PVA, and P-FA), forming stable acetal and ether bonds [24,25]. Simultaneously, GA can form imine (C=N) linkages with the -NH 2 of PEI. These interactions collectively impart mechanical strength and structural integrity to the P-FA/PVA xerogel. Figure 2a presents the FTIR spectra of raw fly ash (FA) and polydopamine-coated fly ash (P-FA). Although both samples exhibit similar overall spectral profiles, notable differences are observed around 3300 cm⁻¹ and 2800–2700 cm⁻¹. The broad absorption band near 3448 cm⁻¹ in raw FA corresponds to O–H stretching vibrations associated with amorphous silicates or hydrated aluminosilicate species. In contrast, the corresponding band in P-FA indicates the presence of hydroxyl (–OH) and primary/secondary amine (–NH) groups originating from the polydopamine coating. In P-FA, distinct doublet peaks in the 2850–2930 cm⁻¹ range, attributed to the asymmetric and symmetric stretching of CH 2 and CH 3 groups [20], confirm the successful deposition and functionalization of PDA. The common peaks in both samples, i.e., at 1634, 1092, and 793 cm⁻¹, correspond to O–H bending of adsorbed water molecules, asymmetric Si-O-Si stretching, and symmetric Si-O-Si/Si-O-Al stretching modes, respectively [2]. The XPS analysis from our previous study confirmed that P-FA contains C, O, and N content with 74.12% carbon, 19.98% oxygen, and 4.50% nitrogen [2]. The surface morphology of raw FA and P-FA was discussed next (Figure 2b) to understand the surface modification. SEM images revealed that the raw FA particles are smooth and spherical, whereas the P-FA particles exhibit a rougher texture with visible polydopamine particles. These morphological variations confirm the successful polydopamine coating onto FA. Additionally, the inherent variation in particle size was clearly observed in the SEM image. The paper will now focus on the properties of the xerogels. Figure 3a compares the XPS survey spectra of pure CNF xerogel and P-FA/CNF xerogel. The XPS spectra of pure CNF xerogel showed only carbon and oxygen elemental peaks, suggesting no contamination or impurities in the neat CNF. In contrast, the P-FA/CNF xerogel shows a new peak at 399.8 eV, corresponding to N1s of nitrogen element. The appearance of the N1s peak confirms the presence of polydopamine and PEI molecules in the P-FA/CNF xerogel. The atomic percentages of C, O and N are presented in the insets of the respective figures. Compared to the pure CNF xerogel (0.43), the P-FA/CNF xerogel shows a lower O/C ratio (0.22). This indicates that P-FA/CNF xerogel is composed of other organic molecules, such as PVA, PEI and GA. This reduction in the O/C ratio is attributed to the utilization of free hydroxyl groups of CNFs and PVA during the formation of covalent acetal linkages with GA. Figure 3b presents the FTIR spectra of the pure CNF xerogel and P-FA/CNF xerogel, where several characteristic changes are clearly observed. The pure CNF shows a big and intense O-H stretching band at 3293 cm -1 due to its abundant hydroxyl groups. Other characteristic peaks appear at 1160 cm -1 , 1100-1000 cm -1 , and 893 cm -1 , corresponding to -COO⁻ stretching, C-O-C pyranose ring vibrations, and β-glycosidic linkages, respectively [21,26]. A band at 1600 cm -1 indicates carboxylate groups (-COO) from TEMPO oxidation [22]. In contrast, the P-FA/CNF xerogel shows a reduced and relatively flat O–H band (3200–3500 cm -1 ) band, indicating the consumption of -OH groups and the successful formation of extensive crosslinking facilitated by GA. In this system, the crosslinking occurs at various sites of the materials, such as P-FA/CNF, CNF/CNF, P-FA/PVA, CNF/PEI, and PVA/PEI interfaces. The incorporation of PEI provides additional crosslinking sites (via imine bonds, detected at 1658 cm -1 ) and hydrogen bonding between the filler and the matrix [19], thereby enhancing the mechanical strength of the xerogel. Another prominent change was observed in the intensity of the C-H stretching of the methylene group between 2900-2800 cm⁻¹. This suggests the presence of PEI, GA and PVA molecules in the system [19,25]. The acetal groups represented by C-O-C stretching bands often overlap with the parent C–O signals of CNF and PVA. However, in P-FA/CNF xerogel, the distinct spectral changes in the 1450–1000 cm⁻¹ region clearly indicate the formation of covalent acetal linkages along with other bonding interactions [25]. Next, the thermal stability of the pristine FA, P-FA and xerogels was investigated to gain insights into their thermal behaviour and potential application boundaries. Figure 3c shows the TGA curves of the pure CNF xerogel and P-FA/CNF xerogel, while the inset shows the thermal stability profiles of the pristine FA and P-FA samples. All experiments were performed under nitrogen from 25°C to 600°C. The TGA curve of pristine FA shows high thermal stability due to its inherent thermal insulation property [27]. The P-FA also displayed a high thermal stability, though slightly reduced by the polydomaine surface coating, which was about 7.6% in mass. The thermograms of xerogels were discussed next. Among the three xerogels (CNF xerogel, pristine FA/CNF xerogel and P-FA/xerogel), the pristine FA/CNF xerogel exhibits the lowest thermal degradation stability (204.6°C at 10% mass loss). Its faster degradation relative to the pure CNF xerogel (215.3°C at 10% mass loss) results from poor interfacial adhesion and incompatibility between pristine FA and CNF matrix. In contrast, the P-FA/CNF xerogel shows a remarkably higher (the highest) thermal decomposition temperature of 239.7°C at 10% mass loss. The nearly 35°C increase in the thermal decomposition temperature compared to the FA/CNF xerogel is attributed to strong interfacial bonding, effective crosslinking and overall robustness of the materials formulation. Notably, the minor mass loss below 210°C for all xerogels corresponds to the evaporation of bound water molecules in CNF and PVA [21,28]. The next major decomposition step between 210-400°C is mainly associated with the depolymerization and thermal decomposition of the CNF, PVA, and other organic components. The higher char residue of the P-FA/CNF xerogel further indicates its improved thermal stability. Figure 4 presents the morphologies of the three xerogels. The pristine CNF xerogel displays a randomly oriented 3D porous network structure with loose, thin cell walls (Figure 4a). This structure forms because CNFs are uniformly dispersed in the hydrogels and pack tightly through hydrogen bonding. As mentioned before, the xerogels were fabricated by freezing the precursor hydrogels in a freezer, followed by a solvent exchange. During freezing, ice crystals grow within the material, compressing the nanofibers together and facilitating physical entanglement. During solvent exchange and drying, capillary forces pull the nanofibers tightly together, creating a compact and sheet-like porous network. In contrast, both the FA/CNF and P-FA/CNF xerogels have very different architectures. The cell walls appeared denser, thicker and lower porosity (Figure 4b and 4c). This occurs because the FA particles increase the solid concentration and restrict complete fiber exfoliation. Compared with the FA/CNF xerogel, the P-FA/CNF xerogel shows more robust cell walls with more evenly distributed FA particles owing to extensive crosslinking between CNF and P-FA particles. Figure 4d further confirms the homogeneous dispersion and embedding of P-FA particles within the matrix. The uniform distribution and strong adhesion resulted in a more robust xerogel structure. Table 1. Physical properties of CNF xerogel and P-FA/CNF xerogel. Sample SBET (m2/g) Porosity (%) Pore size (nm) Compression strength at 70% strain (kPa) Compressive modulus (kPa) Thermal conductivity (mW/m.K) CO2 adsorption (mmol/g) CNF xerogel 69.0 97.8 16.1 157.2 (±8.3) 136.1 (±7) 36.3 (±3.0) 0.52 Raw FA/ CNF xerogel 49.8 89.7 29.1 174.6 (±9.1) 159.4 (±7.4) 37.1 mW/m.K 0.42 P-FA/ CNF xerogel 63.8 96.3 17.4 293.5 (±9.6) 264.1 (±6.8) 32.1 (±3.3) 1.67 Since the specific surface area, pore size and pore size distribution play a crucial role in evaluating the adsorption efficiency and thermal insulation properties of a xerogel, they are discussed next. Nitrogen adsorption was used to determine the specific surface area (SSA) and porosity characteristics of the CNF xerogel and P-FA/CNF xerogel. According to the IUPAC classification [29], if both adsorption and desorption isotherms of a porous material follow a type IV isotherm, the material belongs to mesoporous types and exhibits strong adsorbate-adsorbent interactions. All our xerogels belong to mesoporous materials. Table 1 presents the physical properties of all three xerogels (pure CNF xerogel, FA/CNF xerogel and P-FA/CNF xerogel). The pure CNF xerogel exhibits a higher specific surface area (69 m 2 /g) and porosity (97.8%), indicating a highly porous and lightweight structure. In contrast, the P-FA/CNF xerogel shows a slightly reduced surface area of 63.8 m 2 /g and porosity (96.3%), due to its relatively denser and compact architecture. Nonetheless, the xerogel remains very lightweight, highly porous and fluffy in nature. Figure 5 compares the sorption isotherms of the CNF xerogel and P-FA/CNF xerogels. As the pristine FA/CNF xerogel shows inferior properties and is structurally weak, the discussion hereafter will focus only on the CNF xerogel and P-FA/CNF xerogel. To evaluate the structural stability of the P-FA/CNF xerogel, compressive tests were performed. Interestingly, the P-FA/CNF xerogel shows a remarkable improvement in mechanical strength (Figure 6a), reaching a compressive strength of 293.5±9.6 kPa and a Young’s modulus of 264.1±6.8 at 70% strain, compared to 157.2 kPa and 136.1 kPa for the pure CNF xerogel, an enhancement of 86.6% in strength and 94% in modulus. This large improvement is attributed to strong intermolecular interactions between filler and matrix and the structural reinforcement provided by the P-FA. For curiosity, when a xerogel was prepared with the same amount of pristine (unmodified) FA following the same formulation, the strength and modulus were increased by only 9.3% and 11.2%, respectively, compared to the neat CNF xerogel. This small increment is due to the inherent reinforcing property of FA. This also highlights the weak interfacial bonding and incompatibility between pristine FA and the CNF matrix. It is worth noting that structural stability is crucial for the practical use of xerogels since a well-defined porous structure can provide greater thermal insulation, higher absorptivity, and mechanical robustness. In view of this, the P-FA/CNF xerogel can be a promising porous material for various applications such as gas adsorption, water treatment and thermal insulation. Next, the thermal conductivity was measured as it defines the material’s ability to conduct heat. As can be seen from Table 1, the P-FA/CNF xerogel exhibited ~11.6 and 13.5 % improvement in thermal conductivity (32.1 mW/m.K) compared to the pure CNF xerogel (36.3 mW/m.K) and FA/CNF xerogel, respectively. This enhancement confirms the presence of uniformly dispersed P-FA particles within the matrix, enabling the xerogel to perform as a good heat-insulating material with low thermal conductivity. Notably, the thermal conductivity of P-FA/CNF xerogel is much lower than that of the aerogel (47 mW/m.K) reported by Nguyen Do [17], composed of 5 wt.% FA/PVA/carboxy-methyl cellulose (CMC) and several commercial insulation materials, including mineral wool (30–40 mW/m.K), fiberglass (33-44 mW/m.K), and expanded polystyrene (EPS, 30–40 mW/m.K) [30] (Figure 6b). To further confirm the thermal insulation properties, a small piece of P-FA/CNF xerogel was placed on a preheated hot plate at 80°C for 90 s and its surface temperature was measured using an infrared (IR) thermometer. This experiment provides a clear indication of the xerogel’s heat shielding performance. For visual understanding, a video was provided in the supplementary document. Surprisingly, the top surface of the xerogel was at a considerably lower temperature (~16.8 °C) than that of the reference hot plate (65.4°C-82.2°C, Figure 6c), demonstrating its excellent heat resistance ability. It is important to note that only 4 wt.% of P-FA was incorporated into the P-FA/CNF xerogel, and the addition of more P-FA would further lower its thermal conductivity. After thermal insulation analysis, the xerogel was evaluated for CO 2 adsorption performance. Figure 7a compares the CO 2 adsorption capacity of the CNF xerogel and P-FA/CNF xerogel. Compared to CNF xerogel (0.52 mmol/g), the P-FA/CNF xerogel showed about 3.2 times higher CO 2 uptake (1.67 mmol/g at 28 °C). This significant improvement can be explained due to the synergistic effects of the xerogel’s porous structure and its amine-rich surfaces. The amine groups act as active chemisorption sites for CO 2 [31,32]. Generally, in amine-functionalized porous adsorbents, CO 2 adsorption proceeds via carbamate formation through a zwitterionic mechanism, as illustrated in Figure 7b [31–33]. Therefore, the enhanced CO 2 adsorption capacity of P-FA/CNF xerogel stems from the cooperative contribution of polydopamine-derived pyrrolic nitrogen and free amine groups in PEI. Notably, CO 2 capture is influenced by various factors, including the nature of the adsorbent, pore architecture, pore volume, surface area, type and density of functional groups, their spatial distribution, and the accessibility of reactive sites [34]. Although the pristine CNF xerogel shows some CO 2 uptake, this is mainly driven by its highly porous structure and large surface area, enabling effective CO 2 diffusion and adsorption. Figure 7b presents temperature-dependent CO 2 adsorption of the xerogel at a constant pressure of 1 bar. As the temperature increased, the CO 2 adsorption capacity decreased, indicating a typical exothermic adsorption behaviour [35–38]. This result helps in understanding the suitability of the xerogel for practical applications, especially under conditions related to industrial CO 2 capture. As FA-based aerogels or xerogels have not been widely explored for CO 2 adsorption, comparing them directly with high surface area activated carbons or inorganic adsorbents may not be relevant. Nevertheless, the P-FA/CNF xerogel exhibits a competitive CO 2 adsorption capacity compared to other porous adsorbents [39–45]. Figure 7c illustrates the cyclic stability or regeneration performance of the P-FA/CNF xerogel over six adsorption-desorption cycles. Regeneration was carried out at 90 °C under a pure N 2 atmosphere. As observed, the P-FA/CNF xerogel maintains nearly the same CO 2 adsorption capacity as the original sample. After six consecutive cycles, a minor loss of about 2–2.5% in adsorption capacity was observed, most likely due to partial pore blockage or pore collapse, and the breakdown of carbamate species, which facilitates the release of adsorbed CO 2 [31]. Nonetheless, the efficiency remained over 97%. These findings suggest that this xerogel can be readily integrated into modern building materials, where both thermal insulation and CO 2 mitigation are critically important. The adsorption performance of the P-FA/CNF xerogel for water purification was evaluated using methylene blue (MB) and nigrosin as model dye contaminants, owing to their widespread industrial use and hazardous nature. Approximately 120 mg of xerogel was immersed in the dye solution and gently stirred until the absorption was complete. UV-Vis spectroscopy was used to assess the adsorption behaviour. It can be seen from Figures 8a and 8b that the absorbance of both dyes decreases steadily with time, confirming efficient dye removal. Interestingly, the xerogel adsorbed over 98% of nigrosin dye from the aqueous mixture within just 2 min, whereas MB required 35 min to reach 96.7% adsorption. These rates are faster than many previously reported adsorbents [46–53]. To illustrate the colour fading more clearly, the digital images of the dye solutions at different time intervals were presented in Figures 8c and 8d. Next, a mixed dye solution (MB+nigrosin) was prepared to evaluate the xerogel’s performance, as actual wastewater often contains multiple dyes. Interestingly, the P-FA/CNF xerogel demonstrated efficient and fast removal of both dyes. Figures 8e and 8f show the time-dependent UV-Vis spectra and corresponding digital images of the dye colour, respectively. After mixing the two dyes, the spectral profiles changed; however, the characteristic MB peaks remained nearly the same, shifted only by 2–3 nm, due to dye-dye interactions. The MB peaks at 664 and 611 nm disappeared completely within 30 minutes. Although nigrosin didn’t show a distinct peak, its adsorption occurred rapidly. The excellent adsorption performance of the P-FA/CNF xerogel is attributed to π–π interactions, hydrogen bonding, and electrostatic attractions between its surface functional groups and the dye molecules (Figure 8g). To the best of our knowledge, this is the first study that presented a robust fly ash-based xerogel with three compelling functionalities. Collectively, these results highlight the exceptional dye adsorption capacity of the P-FA/CNF xerogel. Furthermore, the findings confirm the potential of FA to be upcycled into a robust, multifunctional xerogel, thereby reducing its environmental impact while enabling new pathways for sustainable utilization. 4. Conclusion This study reported a novel strategy for upcycling fly ash into a multifunctional, robust xerogel, suitable for sustainable environmental applications. To enhance the mechanical strength and functionality of the xerogel, the FA was chemically functionalized with polydopamine and then mixed with cellulose nanofibers, polyvinyl alcohol, polyethyleneimine, and glutaraldehyde. The resulting xerogel (P-FA/CNF xerogel) exhibited outstanding heat shielding, thermal insulation, CO 2 capture, and dye removing capabilities, never reported before. The P-FA/CNF xerogel showed a compressive strength of 293.5 ± 9.6 kPa and a Young’s modulus of 264.14 ± 7.2 kPa at 70% strain, which are 86.6% and 94% higher in strength and modulus, respectively, compared to pure CNF xerogel. It also offered excellent thermal conductivity (33.4 mW/m.K), outperforming commercial insulators such as expanded polystyrene (EPS), mineral wool, and fiberglass. Additionally, the xerogel exhibited a high CO 2 adsorption capacity of 1.67 mmol/g, which is 3.2 times higher than that of the pure CNF xerogel. The xerogel also showed repeatable performances over six adsorption-desorption cycles. Not only that, the as-synthesised xerogel can quickly remove hazardous water-soluble industrial dyes, such as methylene blue and nigrosine from aqueous mixtures, highlighting its high potential in wastewater purification technologies. These results demonstrate that hazardous fly ash can be effectively transformed into high-value materials for multiple challenging applications when valorised scientifically. We hope that our simple, cost-effective and novel proof-of-concept approach for converting fly ash into multifunctional xerogel will serve as a valuable reference for researchers working in waste management, water purification, thermal heat management, CO 2 reduction and environmental sustainability. Declarations NOTES The authors declare no competing. Author Contribution Sunanda Roy: Conceptualization, Methodology, Investigation, Data Analysis, Writing- Original draft preparation; Sajal Nandi: Methodology, Investigation, Data Analysis, Software. Barnali Dasgupta Ghosh: Methodology, Investigation, Resources, Funding, Writing- Reviewing and Editing; Kheng Lim Goh: Investigation, Resources, Reviewing and Editing. ACKNOWLEDGEMENT This work is supported by the Anusandhan National Research Foundation (ANRF), formerly known as the Science and Engineering Research Board (SERB), Government of India ( CRG/2022/001610). References Bhatt, A. et al. (2019). Physical, chemical, and geotechnical properties of coal fly ash: A global review. Case Studies in Construction Materials . https://doi.org/https://doi.org/10.1016/j.cscm.2019.e00263 . Roy, S. et al. (2024). Facile and sustainable upcycling of fly ash into multifunctional durable superhydrophobic coatings. 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Advanced Composites and Hybrid Materials . https://doi.org/10.1007/s42114-020-00140-w . Martis, L.J. et al. (2022). Preparation, characterization, and methylene blue dye adsorption study of silk fibroin–graphene oxide nanocomposites. Environ. Sci.: Adv. https://doi.org/10.1039/D1VA00047K . Chen, Y. et al. (2020). Effective photocatalytic degradation and physical adsorption of methylene blue using cellulose/GO/TiO2 hydrogels. RSC Adv. https://doi.org/10.1039/D0RA04509H . Shenoy, M.R. et al. (2020). The effect of morphology-dependent surface charges of iron oxide on the visible light photocatalytic degradation of methylene blue dye. Journal of Materials Science: Materials in Electronics . https://doi.org/10.1007/s10854-020-04325-3 . Yuan, H. et al. (2019). Ultra-high adsorption of cationic methylene blue on two dimensional titanate nanosheets. RSC Adv. https://doi.org/10.1039/C8RA10172H . Li, D. et al. (2020). Nitrogen-doped carbon enhanced mesoporous TiO2 in photocatalytic remediation of organic pollutants. Research on Chemical Intermediates . https://doi.org/10.1007/s11164-018-3531-9 . Baig, N. et al. (2021). Selective removal of toxic organic dyes using Trӧger base-containing sulfone copolymers made from a metal-free thiol-yne click reaction followed by oxidation. RSC Adv. https://doi.org/10.1039/D1RA03783H . Additional Declarations No competing interests reported. Supplementary Files Movie1.mp4 Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 19 Feb, 2026 Editor assigned by journal 19 Feb, 2026 Submission checks completed at journal 04 Dec, 2025 First submitted to journal 03 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8275181","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":594149654,"identity":"67c73aee-5cd2-45d9-89a3-a45f44849aa6","order_by":0,"name":"Sunanda Roy","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3ElEQVRIiWNgGAWjYFADZsbGB0CKh49YDRIM7M2HDUBa2IjXwnMsTQLEIqiFv7332YOPe+zq+CVyzCq/5tjJsDEwP3x0A5/pZ46bG854liwhOSPH7LbstmSgw9iMjXPwaDGQSGOT5jnALGFwA6hFchszUAsPmzQRWuol7IFaiiW31ROt5bCEAdD7jB+3HSasReLMMXbDGQeOS8443nxYmnHbcR42ZgJ+4W9vY3vw4UA1P38zY+PHn9uq7fnZmx8+xqeFATkimHnAJH7lqFoYfxBWPQpGwSgYBSMQAAAH4EC08WC4EgAAAABJRU5ErkJggg==","orcid":"","institution":"Alliance University","correspondingAuthor":true,"prefix":"","firstName":"Sunanda","middleName":"","lastName":"Roy","suffix":""},{"id":594149655,"identity":"32097e89-38b8-420b-80b1-6c9aef08940e","order_by":1,"name":"Sajal Nandi","email":"","orcid":"","institution":"BIT Mesra: Birla Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Sajal","middleName":"","lastName":"Nandi","suffix":""},{"id":594149657,"identity":"74edf0ea-b994-4653-a2a0-f64c27b3d63f","order_by":2,"name":"Barnali Dasgupta Ghosh","email":"","orcid":"","institution":"BIT Mesra: Birla Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Barnali","middleName":"Dasgupta","lastName":"Ghosh","suffix":""},{"id":594149658,"identity":"0d4c3c8b-95cb-4283-97d9-d9c36c0afb96","order_by":3,"name":"Kheng Lim Goh","email":"","orcid":"","institution":"Newcastle University Singapore","correspondingAuthor":false,"prefix":"","firstName":"Kheng","middleName":"Lim","lastName":"Goh","suffix":""}],"badges":[],"createdAt":"2025-12-04 04:08:06","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8275181/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8275181/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104405162,"identity":"12c31f89-54cf-4135-b6db-613eebbaef19","added_by":"auto","created_at":"2026-03-11 12:21:58","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":239296,"visible":true,"origin":"","legend":"\u003cp\u003ePreparation of P-FA/CNF composite xerogels and their intermolecular interactions.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/9d956ebd35b7557ffd568cb0.png"},{"id":104252375,"identity":"644a5bc6-cef9-481e-8b42-50b2ae67679b","added_by":"auto","created_at":"2026-03-09 16:18:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":308571,"visible":true,"origin":"","legend":"\u003cp\u003e(a) FTIR and (b) SEM morphology for raw FA and P-FA. The P\u003cem\u003e-\u003c/em\u003eFA showed the coating and rough surfaces.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/bc7af5208272adf1a7178497.png"},{"id":104405184,"identity":"a3484970-79b9-4542-979a-00bc8cf365f6","added_by":"auto","created_at":"2026-03-11 12:22:02","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":111398,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XPS, (b) FTIR spectra, and (c) TGA profiles for CNF xerogel, FA/CNF xerogel and P-FA/CNF xerogel. The inset compares the TGA curves of FA and P-FA.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/be0b40ea060ee34488505185.png"},{"id":104252432,"identity":"40759a9c-1208-41bb-bd58-a4c061f6e971","added_by":"auto","created_at":"2026-03-09 16:18:21","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":702050,"visible":true,"origin":"","legend":"\u003cp\u003eSurface morphologies of (a) CNF xerogel, (b) FA (pristine)/CNF xerogel and (c) P-FA/CNF xerogel and (d) high magnification image of P-FA/CNF xerogel. The arrows show the dispersion of P-FA particles in the polymer matrix. Images of all xerogels are shown in the inset.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/8234f374338f2b494f0ae34f.png"},{"id":104252430,"identity":"8f7aace7-985c-4e95-a5ef-b2eed274954b","added_by":"auto","created_at":"2026-03-09 16:18:20","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":72504,"visible":true,"origin":"","legend":"\u003cp\u003eNitrogen adsorption-desorption isotherms of CNF xerogel and P-FA/CNF xerogel. Porosity ranges between 96.3–97.8%\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/3a75458e9f98c76a7d6bddb3.png"},{"id":104252399,"identity":"05624f21-d79e-44b9-a6fd-7f3e38d4d3cc","added_by":"auto","created_at":"2026-03-09 16:18:17","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":359071,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Compressive stress vs strain curves of CNF xerogel and P-FA/CNF xerogel. The inset shows the Young’s modulus of the same samples. (b) Comparison of the thermal conductivity of our xerogels with some existing thermal insulation materials. (c) Digital images show the infrared thermometer readings of the P-FA/CNF xerogel surface temperature under heating.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/add76efb7c0570ed61ed76eb.png"},{"id":104252369,"identity":"402d60dd-d9da-4b1f-b450-5b4e84cd2425","added_by":"auto","created_at":"2026-03-09 16:18:13","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":130338,"visible":true,"origin":"","legend":"\u003cp\u003e(a) CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherms of P-FA/CNF xerogel at a temperature of 28°C and pressure (0°C and 1bar). (b) CO\u003csub\u003e2\u003c/sub\u003e adsorption kinetics of the xerogel at different temperatures. (c) The CO\u003csub\u003e2\u003c/sub\u003e adsorption mechanism of xerogel. (d) The bar diagram shows the cyclic CO\u003csub\u003e2\u003c/sub\u003e adsorption/desorption.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/49a6794d6e3caaa3f72bda29.png"},{"id":104252374,"identity":"8c8ad069-da05-4abb-8074-cf44676f3447","added_by":"auto","created_at":"2026-03-09 16:18:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":174441,"visible":true,"origin":"","legend":"\u003cp\u003eThe UV-vis absorption spectra of (a) MB and (b) Nigrosin dyes. Digital images of the colour changes of (c) MB dye and (d) Nigrosin dye. (e) The UV-vis absorption spectra of the MB-Nigrosin dye blend. (f) Digital images showing the colour fading gradually. (g) Mechanism for the adsorption of MB and Nigrosin dyes onto the P-FA/CNF xerogel adsorbent.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/c31193c49cd860150edad0c3.png"},{"id":104409109,"identity":"3f82f097-d9f1-477b-a11a-b873ca520803","added_by":"auto","created_at":"2026-03-11 12:44:08","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2759622,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/d4041462-9ac4-4179-a486-59b52cf4afeb.pdf"},{"id":104252376,"identity":"87bfe462-3125-40a7-a1eb-4f7e60a3b76d","added_by":"auto","created_at":"2026-03-09 16:18:16","extension":"mp4","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":3719912,"visible":true,"origin":"","legend":"","description":"","filename":"Movie1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-8275181/v1/1fec5fe9ee7cb443259e19eb.mp4"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eUpcycling Hazardous Fly Ash into High-Performance Lightweight Xerogels for Thermal Insulation, CO\u003csub\u003e2\u003c/sub\u003e Adsorption, and Wastewater Purification\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFly ash (FA) is one of the hazardous industrial wastes generated in coal-fired thermal power plants when coal is burned. It contains various toxic substances, including heavy metals and naturally radioactive materials such as arsenic, chromium, mercury, lead, thorium, and uranium [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. As electricity demand rises due to a growing population, the FA production is also increasing. Currently, ~\u0026thinsp;70% of India\u0026rsquo;s power supply comes from coal. These plants produce nearly 120\u0026ndash;150\u0026nbsp;million tons of FA annually. Ironically, despite its known toxicity, a large portion of FA is dumped in open landfills, causing severe environmental pollution and threatening the entire ecosystems. This has raised widespread global concern and emphasised the urgent need for measures to combat pollution. To date, FA has been primarily used in construction applications such as cement, brick, tiles and roadworks [\u003cspan additionalcitationids=\"CR4 CR5\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Some recent studies have also explored its potential in agriculture, catalysts, insulation panels, wastewater treatment, coatings, and polymer composites [\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. However, these utilizations account for only 30\u0026ndash;40% of the total FA volume. This situation demands more scientific, sustainable strategies that can reduce pollution also transform it into valuable resources for advanced technologies.\u003c/p\u003e\u003cp\u003eThis research aims to convert FA into novel, multifunctional, lightweight, and robust xerogels useful for sustainable development through applications for heat shielding and building insulation, CO\u003csub\u003e2\u003c/sub\u003e capture, and wastewater treatment via dye removal. The objective was to design one material capable of addressing multiple challenges. However, developing such a xerogel is challenging due to certain intrinsic limitations of FA, including its non-functional surfaces, variable particle size, poor dispersion within the polymer matrix and weak interfacial adhesion, all of which result in brittle and non-functional structures [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Recently, some studies have investigated the effects of FA on polymer composites; however, most of the resulting composites exhibited weaker mechanical properties than the pristine polymer due to the aforementioned challenges [\u003cspan additionalcitationids=\"CR12 CR13\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Moreover, to our knowledge, no report exists on such FA-polymer composite xerogel.\u003c/p\u003e\u003cp\u003eAerogels and xerogels are known for their exceptional properties, lightweight structure, ultra-low density, high porosity, large surface area, and superior thermal insulation, making them ideal for applications in aerospace, construction, advanced composites, and sustainable technologies. Aerogels and xerogels are porous solids with a three-dimensional network structure derived from gels, mainly differing in their drying methods. Aerogels are produced using supercritical drying to preserve their structure and achieve low density, whereas xerogels are formed through evaporative drying, which results in partial shrinkage and higher density; however, the process is more cost-effective and faster [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. There are a few articles on FA-based aerogels, but those have only focused on extracting SiO\u003csub\u003e2\u003c/sub\u003e and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e from FA and then using them to synthesise aerogels via the sol\u0026ndash;gel method [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. These resulting aerogels are often fragile and brittle.\u003c/p\u003e\u003cp\u003eIn contrast, our approach uses FA as a functional filler and mixes it with polymer to develop a flexible, robust composite xerogel. The xerogel was formulated to achieve high thermal insulation, effective CO\u003csub\u003e2\u003c/sub\u003e capture, and rapid wastewater purification. The xerogel contains polydopamine coated FA (P-FA) as a functional filler, cellulose nanofibers (CNFs) as the structural matrix, polyvinyl alcohol (PVA) as a binder, polyethylenimine (PEI) as both a surface modifier and intermediate bridging agent, and glutaraldehyde (GA) as a crosslinking agent. Given the complex surface chemistry of FA, polydopamine was selectively chosen for surface functionalization due to its fast and strong adhesion to a wide variety of substrates [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] and the ability to provide a uniform surface coating. This coating further facilitates the attachment of other functional groups such as amines and thiols, thus enhancing material functionality, interfacial adhesion and overall performance. Cellulose was selected as the main matrix due to its biodegradability, renewability, low cost, lightweight, and excellent mechanical properties [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Moreover, cellulose, PEI and polydopamine coated FA can form a strong intermolecular network and crosslinking within the system due to their diverse and rich functional surfaces. Owing to the unique formulation and fabrication strategy, the resulting xerogel exhibited\u0026thinsp;~\u0026thinsp;86.6% and ~\u0026thinsp;94% higher compressive strength and Young\u0026rsquo;s modulus than the pure CNF xerogel. Moreover, it showed excellent heat shielding, thermal conductivity of 32.1 mW/m.K, and a CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of 1.67 mmol/g, better than many contemporary materials. To the best of our knowledge, this is the first FA-CNF composite xerogel that addresses three critical and challenging issues: high thermal insulation, efficient CO\u003csub\u003e2\u003c/sub\u003e capture, and rapid, efficient wastewater purification. To understand the material\u0026rsquo;s properties and performance, an in-depth characterization was performed. Overall, this research shows that with strategic planning and processing, hazardous FA can be turned into a valuable resource for advanced, sustainable applications.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials\u003c/h2\u003e\u003cp\u003eFly ash (FA) was collected from the Kolaghat thermal power plant in West Bengal, India. Cellulose nanofibers (CNFs) were prepared from hardwood pulp via TEMPO-mediated oxidation, following previously reported protocols [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Polyvinyl alcohol (PVA; Mw 13,000\u0026ndash;23,000, 87\u0026ndash;89% hydrolyzed), branched polyethylene-imine (PEI; average Mw\u0026thinsp;~\u0026thinsp;800, Mn\u0026thinsp;~\u0026thinsp;600), glutaraldehyde (GA), dopamine hydrochloride (\u0026ge;\u0026thinsp;98%), Trizma base (Mw 121.14), and all other solvents were purchased from Merck, India.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Fabrication of P-FA/CNF xerogel\u003c/h2\u003e\u003cp\u003eTo prepare polydopamine-functionalized fly ash (P-FA), approximately 1 g of FA was dispersed in 10 mL of Trizma buffer (10 mmol, pH 8.68), followed by the addition of 100 mg dopamine hydrochloride. The suspension was stirred magnetically at 40\u0026deg;C for 8 hours. The resulting dark brown modified FA was collected by vacuum filtration and rinsed thoroughly with deionized water to remove unreacted components. The product was labelled as P-FA, where P indicates polydopamine coating.\u003c/p\u003e\u003cp\u003eFor the composite xerogel, ~\u0026thinsp;60 g of CNF suspension (2.2 wt% on a dry basis) was homogenized with ~\u0026thinsp;60 mg of P-FA at 10,000 rpm for 10 minutes. A 10 wt% PVA solution was separately prepared by dissolving 5 g of PVA in 45 mL of DI water at 80\u0026deg;C for 2 hours. Next, a composite mixture was prepared by mixing P-FA, PVA and PEI (dissolved in hot water). The ratio of P-FA: PVA: PEI was maintained at 4:95.50:0.50. The mixture was then homogenized for 10 min at 10,000 rpm followed by 10h magnetic stirring at 70\u0026deg;C. During this process, a small amount of GA (crosslinker) and HCl were added to initiate crosslinking. The final mixture was then poured into cylindrical plastic molds and frozen for 24h to facilitate solidification. Subsequently, the molds with frozen gels were slowly immersed in a 1L acetone bath. This step was repeated twice. The wet xerogel samples were then dried in an oven at 100\u0026deg;C for 1 hour to obtain the final porous lightweight dry xerogels, designated as P-FA/CNF xerogel. For comparison, a control xerogel was prepared using pure CNF, referred to as CNF xerogel. Although a pristine (unmodified) FA/CNF xerogel was also prepared following the same formulation and process, it was brittle and did not form a xerogel-like structure. Thus, the pure CNF xerogel was chosen as the reference sample for comparison. However, in some cases, the properties of unmodified FA/CNF were also discussed for additional clarity.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Dye adsorption test\u003c/h2\u003e\u003cp\u003eThe dye adsorption tests were carried out on P-FA/CNF xerogel using aqueous solutions of cationic and anionic dyes. Methylene blue (MB) and Nigrosin are both dyes were used as the model cationic and anionic dye, respectively. Three independent tests were carried out with each dye sample to ensure reproducibility. During the experiment, approximately 120 mg of the dry xerogel (adsorbent) was placed in a glass beaker containing 50 mL of dye solution (50 mg/L) and stirred at constant speed at 35\u0026deg;C. After certain time intervals, fixed aliquots were withdrawn, diluted with an equal volume of DI water, and the residual dye concentration was determined using UV\u0026ndash;vis spectroscopy. The characteristic absorption maxima for MB and Nigrosin were observed at 663 nm and 578 nm, respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Characterization\u003c/h2\u003e\u003cp\u003eThe surface chemical composition of the xerogels was identified using Fourier Transform Infrared (FTIR) spectroscopy (PerkinElmer Frontier) between 600\u0026ndash;4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 16 scan rate. Thermal properties were evaluated using Shimadzu DTG-60 thermogravimetric analyser under nitrogen atmosphere from 100\u0026deg;C to 600\u0026deg;C at a heating rate of 10\u0026deg;C/min. X-ray photoelectron spectroscopy (XPS) was performed with a K-Alpha Thermo Scientific system equipped with monochromatic Al Kα radiation, and all spectra were calibrated against the C 1s peak at 284.8 eV. Surface morphology was observed using a field emission scanning electron microscope (FESEM, Hitachi S4200). Thermal insulation performance was assessed using the Transient Planar Source (TPS) method in accordance with ISO 22007-2:2015, employing a HotDisk\u0026reg; TPS 2500S thermal conductivity analyzer (G\u0026ouml;teborg, Sweden). Measurements were carried out under ambient conditions with a heating power of 15 mW applied for 5s. Two identical samples were positioned symmetrically on either side of a Kapton sensor, with a 10-minute interval between successive tests. The reported values represent the average of four samples. The Micromeritics ASAP 2020 analyzer was used to measure the Brunauer-Emmett-Teller (BET) surface area, micropore volume, and pore size distribution via nitrogen adsorption\u0026ndash;desorption isotherms over a relative pressure range (p/p⁰) of 0\u0026ndash;1 bar. CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherms were also measured using the same instrument. Before analysis, the xerogels were degassed at 100\u0026deg;C to remove moisture and residual impurities. Adsorption experiments were conducted with a circulating water bath at different temperatures, with 30 s equilibration per pressure point. The regeneration study was conducted via adsorption-desorption cycles for up to eight cycles. The dye adsorption performance was evaluated using a UV-Vis spectrophotometer (UV-2501PC, Shimadzu).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e schematically illustrates the synthesis of P-FA/CNF composite aerogel and plausible intermolecular interactions among its components. The bonding mechanism in this system is complex, involving multiple polar functional groups, including hydroxyl and amine groups. The main interactions arise from hydrogen bonding among the -OH and -NH\u003csub\u003e2\u003c/sub\u003e groups of P-FA, CNF, PVA, and PEI. Additionally, the covalent acetal linkages formed through glutaraldehyde (GA) assisted crosslinking between P-FA/CNF, P-FA/PVA, and PVA/CNF further strengthen the xerogel structure. Under mildly acidic conditions, the aldehyde groups (-CHO) of GA readily react with two adjacent hydroxyl groups of polyols (CNF, PVA, and P-FA), forming stable acetal and ether bonds [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Simultaneously, GA can form imine (C\u0026thinsp;=\u0026thinsp;N) linkages with the -NH\u003csub\u003e2\u003c/sub\u003e of PEI. These interactions collectively impart mechanical strength and structural integrity to the P-FA/PVA xerogel.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea presents the FTIR spectra of raw fly ash (FA) and polydopamine-coated fly ash (P-FA). Although both samples exhibit similar overall spectral profiles, notable differences are observed around 3300 cm⁻\u0026sup1; and 2800\u0026ndash;2700 cm⁻\u0026sup1;. The broad absorption band near 3448 cm⁻\u0026sup1; in raw FA corresponds to O\u0026ndash;H stretching vibrations associated with amorphous silicates or hydrated aluminosilicate species. In contrast, the corresponding band in P-FA indicates the presence of hydroxyl (\u0026ndash;OH) and primary/secondary amine (\u0026ndash;NH) groups originating from the polydopamine coating. In P-FA, distinct doublet peaks in the 2850\u0026ndash;2930 cm⁻\u0026sup1; range, attributed to the asymmetric and symmetric stretching of CH\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e3\u003c/sub\u003e groups [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], confirm the successful deposition and functionalization of PDA. The common peaks in both samples, i.e., at 1634, 1092, and 793 cm⁻\u0026sup1;, correspond to O\u0026ndash;H bending of adsorbed water molecules, asymmetric Si-O-Si stretching, and symmetric Si-O-Si/Si-O-Al stretching modes, respectively [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The XPS analysis from our previous study confirmed that P-FA contains C, O, and N content with 74.12% carbon, 19.98% oxygen, and 4.50% nitrogen [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe surface morphology of raw FA and P-FA was discussed next (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) to understand the surface modification. SEM images revealed that the raw FA particles are smooth and spherical, whereas the P-FA particles exhibit a rougher texture with visible polydopamine particles. These morphological variations confirm the successful polydopamine coating onto FA. Additionally, the inherent variation in particle size was clearly observed in the SEM image.\u003c/p\u003e\u003cp\u003eThe paper will now focus on the properties of the xerogels. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea compares the XPS survey spectra of pure CNF xerogel and P-FA/CNF xerogel. The XPS spectra of pure CNF xerogel showed only carbon and oxygen elemental peaks, suggesting no contamination or impurities in the neat CNF. In contrast, the P-FA/CNF xerogel shows a new peak at 399.8 eV, corresponding to N1s of nitrogen element. The appearance of the N1s peak confirms the presence of polydopamine and PEI molecules in the P-FA/CNF xerogel. The atomic percentages of C, O and N are presented in the insets of the respective figures. Compared to the pure CNF xerogel (0.43), the P-FA/CNF xerogel shows a lower O/C ratio (0.22). This indicates that P-FA/CNF xerogel is composed of other organic molecules, such as PVA, PEI and GA. This reduction in the O/C ratio is attributed to the utilization of free hydroxyl groups of CNFs and PVA during the formation of covalent acetal linkages with GA.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb presents the FTIR spectra of the pure CNF xerogel and P-FA/CNF xerogel, where several characteristic changes are clearly observed. The pure CNF shows a big and intense O-H stretching band at 3293 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e due to its abundant hydroxyl groups. Other characteristic peaks appear at 1160 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1100\u0026thinsp;\u0026minus;\u0026thinsp;1000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and 893 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to -COO⁻ stretching, C-O-C pyranose ring vibrations, and β-glycosidic linkages, respectively [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. A band at 1600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates carboxylate groups (-COO) from TEMPO oxidation [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. In contrast, the P-FA/CNF xerogel shows a reduced and relatively flat O\u0026ndash;H band (3200\u0026ndash;3500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) band, indicating the consumption of -OH groups and the successful formation of extensive crosslinking facilitated by GA. In this system, the crosslinking occurs at various sites of the materials, such as P-FA/CNF, CNF/CNF, P-FA/PVA, CNF/PEI, and PVA/PEI interfaces. The incorporation of PEI provides additional crosslinking sites (via imine bonds, detected at 1658 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and hydrogen bonding between the filler and the matrix [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], thereby enhancing the mechanical strength of the xerogel. Another prominent change was observed in the intensity of the C-H stretching of the methylene group between 2900\u0026thinsp;\u0026minus;\u0026thinsp;2800 cm⁻\u0026sup1;. This suggests the presence of PEI, GA and PVA molecules in the system [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The acetal groups represented by C-O-C stretching bands often overlap with the parent C\u0026ndash;O signals of CNF and PVA. However, in P-FA/CNF xerogel, the distinct spectral changes in the 1450\u0026ndash;1000 cm⁻\u0026sup1; region clearly indicate the formation of covalent acetal linkages along with other bonding interactions [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eNext, the thermal stability of the pristine FA, P-FA and xerogels was investigated to gain insights into their thermal behaviour and potential application boundaries. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec shows the TGA curves of the pure CNF xerogel and P-FA/CNF xerogel, while the inset shows the thermal stability profiles of the pristine FA and P-FA samples. All experiments were performed under nitrogen from 25\u0026deg;C to 600\u0026deg;C. The TGA curve of pristine FA shows high thermal stability due to its inherent thermal insulation property [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The P-FA also displayed a high thermal stability, though slightly reduced by the polydomaine surface coating, which was about 7.6% in mass.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe thermograms of xerogels were discussed next. Among the three xerogels (CNF xerogel, pristine FA/CNF xerogel and P-FA/xerogel), the pristine FA/CNF xerogel exhibits the lowest thermal degradation stability (204.6\u0026deg;C at 10% mass loss). Its faster degradation relative to the pure CNF xerogel (215.3\u0026deg;C at 10% mass loss) results from poor interfacial adhesion and incompatibility between pristine FA and CNF matrix. In contrast, the P-FA/CNF xerogel shows a remarkably higher (the highest) thermal decomposition temperature of 239.7\u0026deg;C at 10% mass loss. The nearly 35\u0026deg;C increase in the thermal decomposition temperature compared to the FA/CNF xerogel is attributed to strong interfacial bonding, effective crosslinking and overall robustness of the materials formulation. Notably, the minor mass loss below 210\u0026deg;C for all xerogels corresponds to the evaporation of bound water molecules in CNF and PVA [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The next major decomposition step between 210\u0026ndash;400\u0026deg;C is mainly associated with the depolymerization and thermal decomposition of the CNF, PVA, and other organic components. The higher char residue of the P-FA/CNF xerogel further indicates its improved thermal stability.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents the morphologies of the three xerogels. The pristine CNF xerogel displays a randomly oriented 3D porous network structure with loose, thin cell walls (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). This structure forms because CNFs are uniformly dispersed in the hydrogels and pack tightly through hydrogen bonding. As mentioned before, the xerogels were fabricated by freezing the precursor hydrogels in a freezer, followed by a solvent exchange. During freezing, ice crystals grow within the material, compressing the nanofibers together and facilitating physical entanglement. During solvent exchange and drying, capillary forces pull the nanofibers tightly together, creating a compact and sheet-like porous network. In contrast, both the FA/CNF and P-FA/CNF xerogels have very different architectures. The cell walls appeared denser, thicker and lower porosity (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). This occurs because the FA particles increase the solid concentration and restrict complete fiber exfoliation. Compared with the FA/CNF xerogel, the P-FA/CNF xerogel shows more robust cell walls with more evenly distributed FA particles owing to extensive crosslinking between CNF and P-FA particles. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed further confirms the homogeneous dispersion and embedding of P-FA particles within the matrix. The uniform distribution and strong adhesion resulted in a more robust xerogel structure.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePhysical properties of CNF xerogel and P-FA/CNF xerogel.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSample\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSBET (m2/g)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePorosity (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ePore size\u003c/p\u003e\u003cp\u003e(nm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCompression strength at 70% strain (kPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCompressive modulus\u003c/p\u003e\u003cp\u003e(kPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eThermal conductivity (mW/m.K)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eCO2 adsorption\u003c/p\u003e\u003cp\u003e(mmol/g)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCNF xerogel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e69.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e97.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e16.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e157.2 (\u0026plusmn;\u0026thinsp;8.3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e136.1 (\u0026plusmn;\u0026thinsp;7)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e36.3 (\u0026plusmn;\u0026thinsp;3.0)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.52\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eRaw FA/\u003c/p\u003e\u003cp\u003eCNF xerogel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e49.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e89.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e29.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e174.6 (\u0026plusmn;\u0026thinsp;9.1)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e159.4 (\u0026plusmn;\u0026thinsp;7.4)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e37.1 mW/m.K\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.42\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eP-FA/\u003c/p\u003e\u003cp\u003eCNF xerogel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e63.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e96.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e17.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e293.5 (\u0026plusmn;\u0026thinsp;9.6)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e264.1 (\u0026plusmn;\u0026thinsp;6.8)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e32.1 (\u0026plusmn;\u0026thinsp;3.3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e1.67\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSince the specific surface area, pore size and pore size distribution play a crucial role in evaluating the adsorption efficiency and thermal insulation properties of a xerogel, they are discussed next. Nitrogen adsorption was used to determine the specific surface area (SSA) and porosity characteristics of the CNF xerogel and P-FA/CNF xerogel. According to the IUPAC classification [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], if both adsorption and desorption isotherms of a porous material follow a type IV isotherm, the material belongs to mesoporous types and exhibits strong adsorbate-adsorbent interactions. All our xerogels belong to mesoporous materials. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the physical properties of all three xerogels (pure CNF xerogel, FA/CNF xerogel and P-FA/CNF xerogel). The pure CNF xerogel exhibits a higher specific surface area (69 m\u003csup\u003e2\u003c/sup\u003e/g) and porosity (97.8%), indicating a highly porous and lightweight structure. In contrast, the P-FA/CNF xerogel shows a slightly reduced surface area of 63.8 m\u003csup\u003e2\u003c/sup\u003e/g and porosity (96.3%), due to its relatively denser and compact architecture. Nonetheless, the xerogel remains very lightweight, highly porous and fluffy in nature. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e compares the sorption isotherms of the CNF xerogel and P-FA/CNF xerogels.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eFigure 6\u003c/b\u003e. (a) Compressive stress vs strain curves of CNF xerogel and P-FA/CNF xerogel. The inset shows the Young\u0026rsquo;s modulus of the same samples. (b) Comparison of the thermal conductivity of our xerogels with some existing thermal insulation materials. (c) Digital images show the infrared thermometer readings of the P-FA/CNF xerogel surface temperature under heating.\u003c/p\u003e\u003cp\u003eAs the pristine FA/CNF xerogel shows inferior properties and is structurally weak, the discussion hereafter will focus only on the CNF xerogel and P-FA/CNF xerogel. To evaluate the structural stability of the P-FA/CNF xerogel, compressive tests were performed. Interestingly, the P-FA/CNF xerogel shows a remarkable improvement in mechanical strength (Fig.\u0026nbsp;6a), reaching a compressive strength of 293.5\u0026thinsp;\u0026plusmn;\u0026thinsp;9.6 kPa and a Young\u0026rsquo;s modulus of 264.1\u0026thinsp;\u0026plusmn;\u0026thinsp;6.8 at 70% strain, compared to 157.2 kPa and 136.1 kPa for the pure CNF xerogel, an enhancement of 86.6% in strength and 94% in modulus. This large improvement is attributed to strong intermolecular interactions between filler and matrix and the structural reinforcement provided by the P-FA. For curiosity, when a xerogel was prepared with the same amount of pristine (unmodified) FA following the same formulation, the strength and modulus were increased by only 9.3% and 11.2%, respectively, compared to the neat CNF xerogel. This small increment is due to the inherent reinforcing property of FA. This also highlights the weak interfacial bonding and incompatibility between pristine FA and the CNF matrix. It is worth noting that structural stability is crucial for the practical use of xerogels since a well-defined porous structure can provide greater thermal insulation, higher absorptivity, and mechanical robustness. In view of this, the P-FA/CNF xerogel can be a promising porous material for various applications such as gas adsorption, water treatment and thermal insulation.\u003c/p\u003e\u003cp\u003eNext, the thermal conductivity was measured as it defines the material\u0026rsquo;s ability to conduct heat. As can be seen from Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the P-FA/CNF xerogel exhibited\u0026thinsp;~\u0026thinsp;11.6 and 13.5% improvement in thermal conductivity (32.1 mW/m.K) compared to the pure CNF xerogel (36.3 mW/m.K) and FA/CNF xerogel, respectively. This enhancement confirms the presence of uniformly dispersed P-FA particles within the matrix, enabling the xerogel to perform as a good heat-insulating material with low thermal conductivity. Notably, the thermal conductivity of P-FA/CNF xerogel is much lower than that of the aerogel (47 mW/m.K) reported by Nguyen Do [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], composed of 5 wt.% FA/PVA/carboxy-methyl cellulose (CMC) and several commercial insulation materials, including mineral wool (30\u0026ndash;40 mW/m.K), fiberglass (33\u0026ndash;44 mW/m.K), and expanded polystyrene (EPS, 30\u0026ndash;40 mW/m.K) [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] (Fig.\u0026nbsp;6b). To further confirm the thermal insulation properties, a small piece of P-FA/CNF xerogel was placed on a preheated hot plate at 80\u0026deg;C for 90 s and its surface temperature was measured using an infrared (IR) thermometer. This experiment provides a clear indication of the xerogel\u0026rsquo;s heat shielding performance. For visual understanding, a video was provided in the supplementary document. Surprisingly, the top surface of the xerogel was at a considerably lower temperature (~\u0026thinsp;16.8\u0026deg;C) than that of the reference hot plate (65.4\u0026deg;C-82.2\u0026deg;C, Fig.\u0026nbsp;6c), demonstrating its excellent heat resistance ability. It is important to note that only 4 wt.% of P-FA was incorporated into the P-FA/CNF xerogel, and the addition of more P-FA would further lower its thermal conductivity.\u003c/p\u003e\u003cp\u003eAfter thermal insulation analysis, the xerogel was evaluated for CO\u003csub\u003e2\u003c/sub\u003e adsorption performance. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ea compares the CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of the CNF xerogel and P-FA/CNF xerogel. Compared to CNF xerogel (0.52 mmol/g), the P-FA/CNF xerogel showed about 3.2 times higher CO\u003csub\u003e2\u003c/sub\u003e uptake (1.67 mmol/g at 28\u0026deg;C). This significant improvement can be explained due to the synergistic effects of the xerogel\u0026rsquo;s porous structure and its amine-rich surfaces. The amine groups act as active chemisorption sites for CO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Generally, in amine-functionalized porous adsorbents, CO\u003csub\u003e2\u003c/sub\u003e adsorption proceeds via carbamate formation through a zwitterionic mechanism, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003eb [\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Therefore, the enhanced CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of P-FA/CNF xerogel stems from the cooperative contribution of polydopamine-derived pyrrolic nitrogen and free amine groups in PEI. Notably, CO\u003csub\u003e2\u003c/sub\u003e capture is influenced by various factors, including the nature of the adsorbent, pore architecture, pore volume, surface area, type and density of functional groups, their spatial distribution, and the accessibility of reactive sites [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Although the pristine CNF xerogel shows some CO\u003csub\u003e2\u003c/sub\u003e uptake, this is mainly driven by its highly porous structure and large surface area, enabling effective CO\u003csub\u003e2\u003c/sub\u003e diffusion and adsorption.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003eb presents temperature-dependent CO\u003csub\u003e2\u003c/sub\u003e adsorption of the xerogel at a constant pressure of 1 bar. As the temperature increased, the CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity decreased, indicating a typical exothermic adsorption behaviour [\u003cspan additionalcitationids=\"CR36 CR37\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. This result helps in understanding the suitability of the xerogel for practical applications, especially under conditions related to industrial CO\u003csub\u003e2\u003c/sub\u003e capture. As FA-based aerogels or xerogels have not been widely explored for CO\u003csub\u003e2\u003c/sub\u003e adsorption, comparing them directly with high surface area activated carbons or inorganic adsorbents may not be relevant. Nevertheless, the P-FA/CNF xerogel exhibits a competitive CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity compared to other porous adsorbents [\u003cspan additionalcitationids=\"CR40 CR41 CR42 CR43 CR44\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ec illustrates the cyclic stability or regeneration performance of the P-FA/CNF xerogel over six adsorption-desorption cycles. Regeneration was carried out at 90\u0026deg;C under a pure N\u003csub\u003e2\u003c/sub\u003e atmosphere. As observed, the P-FA/CNF xerogel maintains nearly the same CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity as the original sample. After six consecutive cycles, a minor loss of about 2\u0026ndash;2.5% in adsorption capacity was observed, most likely due to partial pore blockage or pore collapse, and the breakdown of carbamate species, which facilitates the release of adsorbed CO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Nonetheless, the efficiency remained over 97%. These findings suggest that this xerogel can be readily integrated into modern building materials, where both thermal insulation and CO\u003csub\u003e2\u003c/sub\u003e mitigation are critically important.\u003c/p\u003e\u003cp\u003eThe adsorption performance of the P-FA/CNF xerogel for water purification was evaluated using methylene blue (MB) and nigrosin as model dye contaminants, owing to their widespread industrial use and hazardous nature. Approximately 120 mg of xerogel was immersed in the dye solution and gently stirred until the absorption was complete. UV-Vis spectroscopy was used to assess the adsorption behaviour. It can be seen from Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003ea and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003eb that the absorbance of both dyes decreases steadily with time, confirming efficient dye removal. Interestingly, the xerogel adsorbed over 98% of nigrosin dye from the aqueous mixture within just 2 min, whereas MB required 35 min to reach 96.7% adsorption. These rates are faster than many previously reported adsorbents [\u003cspan additionalcitationids=\"CR47 CR48 CR49 CR50 CR51 CR52\" citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. To illustrate the colour fading more clearly, the digital images of the dye solutions at different time intervals were presented in Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003ec and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003ed.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eNext, a mixed dye solution (MB\u0026thinsp;+\u0026thinsp;nigrosin) was prepared to evaluate the xerogel\u0026rsquo;s performance, as actual wastewater often contains multiple dyes. Interestingly, the P-FA/CNF xerogel demonstrated efficient and fast removal of both dyes. Figures\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003ee and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003ef show the time-dependent UV-Vis spectra and corresponding digital images of the dye colour, respectively. After mixing the two dyes, the spectral profiles changed; however, the characteristic MB peaks remained nearly the same, shifted only by 2\u0026ndash;3 nm, due to dye-dye interactions. The MB peaks at 664 and 611 nm disappeared completely within 30 minutes. Although nigrosin didn\u0026rsquo;t show a distinct peak, its adsorption occurred rapidly. The excellent adsorption performance of the P-FA/CNF xerogel is attributed to π\u0026ndash;π interactions, hydrogen bonding, and electrostatic attractions between its surface functional groups and the dye molecules (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003eg). To the best of our knowledge, this is the first study that presented a robust fly ash-based xerogel with three compelling functionalities.\u003c/p\u003e\u003cp\u003eCollectively, these results highlight the exceptional dye adsorption capacity of the P-FA/CNF xerogel. Furthermore, the findings confirm the potential of FA to be upcycled into a robust, multifunctional xerogel, thereby reducing its environmental impact while enabling new pathways for sustainable utilization.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eFigure 1 schematically illustrates the synthesis of P-FA/CNF composite aerogel and plausible intermolecular interactions among its components. The bonding mechanism in this system is complex, involving multiple polar functional groups, including hydroxyl and amine groups. The main interactions arise from hydrogen bonding among the -OH and -NH\u003csub\u003e2\u003c/sub\u003e groups of P-FA, CNF, PVA, and PEI. Additionally, the covalent acetal linkages formed through glutaraldehyde (GA) assisted crosslinking between P-FA/CNF, P-FA/PVA, and PVA/CNF further strengthen the xerogel structure. Under mildly acidic conditions, the aldehyde groups (-CHO) of GA readily react with two adjacent hydroxyl groups of polyols (CNF, PVA, and P-FA), forming stable acetal and ether bonds [24,25]. Simultaneously, GA can form imine (C=N) linkages with the -NH\u003csub\u003e2\u003c/sub\u003e of PEI. These interactions collectively impart mechanical strength and structural integrity to the P-FA/PVA xerogel.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 2a presents the FTIR spectra of raw fly ash (FA) and polydopamine-coated fly ash (P-FA). Although both samples exhibit similar overall spectral profiles, notable differences are observed around 3300 cm⁻\u0026sup1; and 2800\u0026ndash;2700 cm⁻\u0026sup1;. The broad absorption band near 3448 cm⁻\u0026sup1; in raw FA corresponds to O\u0026ndash;H stretching vibrations associated with amorphous silicates or hydrated aluminosilicate species. In contrast, the corresponding band in P-FA indicates the presence of hydroxyl (\u0026ndash;OH) and primary/secondary amine (\u0026ndash;NH) groups originating from the polydopamine coating. In P-FA, distinct doublet peaks in the 2850\u0026ndash;2930 cm⁻\u0026sup1; range, attributed to the asymmetric and symmetric stretching of CH\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e3\u003c/sub\u003e groups [20], confirm the successful deposition and functionalization of PDA. The common peaks in both samples, i.e., at 1634, 1092, and 793 cm⁻\u0026sup1;, correspond to O\u0026ndash;H bending of adsorbed water molecules, asymmetric Si-O-Si stretching, and symmetric Si-O-Si/Si-O-Al stretching modes, respectively [2]. The XPS analysis from our previous study confirmed that P-FA contains C, O, and N content with 74.12% carbon, 19.98% oxygen, and 4.50% nitrogen [2].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe surface morphology of raw FA and P-FA was discussed next (Figure 2b) to understand the surface modification. SEM images revealed that the raw FA particles are smooth and spherical, whereas the P-FA particles exhibit a rougher texture with visible polydopamine particles. These morphological variations confirm the successful polydopamine coating onto FA. Additionally, the inherent variation in particle size was clearly observed in the SEM image. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe paper will now focus on the properties of the xerogels. Figure 3a compares the XPS survey spectra of pure CNF xerogel and P-FA/CNF xerogel. The XPS spectra of pure CNF xerogel showed only carbon and oxygen elemental peaks, suggesting no contamination or impurities in the neat CNF. In contrast, the P-FA/CNF xerogel shows a new peak at 399.8 eV, corresponding to N1s of nitrogen element. The appearance of the N1s peak confirms the presence of polydopamine and PEI molecules in the P-FA/CNF xerogel. The atomic percentages of C, O and N are presented in the insets of the respective figures. Compared to the pure CNF xerogel (0.43), the P-FA/CNF xerogel shows a lower O/C ratio (0.22). This indicates that P-FA/CNF xerogel is composed of other organic molecules, such as PVA, PEI and GA. This reduction in the O/C ratio is attributed to the utilization of free hydroxyl groups of CNFs and PVA during the formation of covalent acetal linkages with GA.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 3b presents the FTIR spectra of the pure CNF xerogel and P-FA/CNF xerogel, where several characteristic changes are clearly observed. The pure CNF shows a big and intense O-H stretching band at 3293 cm\u003csup\u003e-1\u003c/sup\u003e due to its abundant hydroxyl groups. Other characteristic peaks appear at 1160 cm\u003csup\u003e-1\u003c/sup\u003e, 1100-1000 cm\u003csup\u003e-1\u003c/sup\u003e, and 893 cm\u003csup\u003e-1\u003c/sup\u003e, corresponding to -COO⁻ stretching, C-O-C pyranose ring vibrations, and \u0026beta;-glycosidic linkages, respectively [21,26]. A band at 1600 cm\u003csup\u003e-1\u003c/sup\u003e indicates carboxylate groups (-COO) from TEMPO oxidation [22]. In contrast, the P-FA/CNF xerogel shows a reduced and relatively flat O\u0026ndash;H band (3200\u0026ndash;3500 cm\u003csup\u003e-1\u003c/sup\u003e) band, indicating the consumption of -OH groups and the successful formation of extensive crosslinking facilitated by GA. In this system, the crosslinking occurs at various sites of the materials, such as P-FA/CNF, CNF/CNF, P-FA/PVA, CNF/PEI, and PVA/PEI interfaces. The incorporation of PEI provides additional crosslinking sites (via imine bonds, detected at 1658 cm\u003csup\u003e-1\u003c/sup\u003e) and hydrogen bonding between the filler and the matrix [19], thereby enhancing the mechanical strength of the xerogel. Another prominent change was observed in the intensity of the C-H stretching of the methylene group between 2900-2800 cm⁻\u0026sup1;. This suggests the presence of PEI, GA and PVA molecules in the system [19,25]. The acetal groups represented by C-O-C stretching bands often overlap with the parent C\u0026ndash;O signals of CNF and PVA. However, in P-FA/CNF xerogel, the distinct spectral changes in the 1450\u0026ndash;1000 cm⁻\u0026sup1; region clearly indicate the formation of covalent acetal linkages along with other bonding interactions [25].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNext, the thermal stability of the pristine FA, P-FA and xerogels was investigated to gain insights into their thermal behaviour and potential application boundaries. Figure 3c shows the TGA curves of the pure CNF xerogel and P-FA/CNF xerogel, while the inset shows the thermal stability profiles of the pristine FA and P-FA samples. All experiments were performed under nitrogen from 25\u0026deg;C to 600\u0026deg;C. The TGA curve of pristine FA shows high thermal stability due to its inherent thermal insulation property [27]. The P-FA also displayed a high thermal stability, though slightly reduced by the polydomaine surface coating, which was about 7.6% in mass.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe thermograms of xerogels were discussed next. Among the three xerogels (CNF xerogel, pristine FA/CNF xerogel and P-FA/xerogel), the pristine FA/CNF xerogel exhibits the lowest thermal degradation stability (204.6\u0026deg;C at 10% mass loss). \u0026nbsp;Its faster degradation relative to the pure CNF xerogel (215.3\u0026deg;C at 10% mass loss) results from poor interfacial adhesion and incompatibility between pristine FA and CNF matrix. In contrast, the P-FA/CNF xerogel shows a remarkably higher (the highest) thermal decomposition temperature of 239.7\u0026deg;C at 10% mass loss. The nearly 35\u0026deg;C increase in the thermal decomposition temperature compared to the FA/CNF xerogel is attributed to strong interfacial bonding, effective crosslinking and overall robustness of the materials formulation. Notably, the minor mass loss below 210\u0026deg;C for all xerogels corresponds to the evaporation of bound water molecules in CNF and PVA \u0026nbsp;[21,28]. The next major decomposition step between 210-400\u0026deg;C is mainly associated with the depolymerization and thermal decomposition of the CNF, PVA, and other organic components. The higher char residue of the P-FA/CNF xerogel further indicates its improved thermal stability.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 4 presents the morphologies of the three xerogels. The pristine CNF xerogel displays a randomly oriented 3D porous network structure with loose, thin cell walls (Figure 4a). This structure forms because CNFs are uniformly dispersed in the hydrogels and pack tightly through hydrogen bonding. As mentioned before, the xerogels were fabricated by freezing the precursor hydrogels in a freezer, followed by a solvent exchange. During freezing, ice crystals grow within the material, compressing the nanofibers together and facilitating physical entanglement. During solvent exchange and drying, capillary forces pull the nanofibers tightly together, creating a compact and sheet-like porous network. \u0026nbsp;In contrast, both the FA/CNF and P-FA/CNF xerogels have very different architectures. The cell walls appeared denser, thicker and lower porosity (Figure 4b and 4c). This occurs because the FA particles increase the solid concentration and restrict complete fiber exfoliation. Compared with the FA/CNF xerogel, the P-FA/CNF xerogel shows more robust cell walls with more evenly distributed FA particles owing to extensive crosslinking between CNF and P-FA particles. Figure 4d further confirms the homogeneous dispersion and embedding of P-FA particles within the matrix. The uniform distribution and strong adhesion resulted in a more robust xerogel structure.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1.\u0026nbsp;\u003c/strong\u003ePhysical properties of CNF xerogel and P-FA/CNF xerogel.\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 10.8153%;\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.98336%;\"\u003e\n \u003cp\u003eSBET (m2/g)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 11.3145%;\"\u003e\n \u003cp\u003ePorosity (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.15141%;\"\u003e\n \u003cp\u003ePore size\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(nm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.807%;\"\u003e\n \u003cp\u003eCompression strength at 70% strain (kPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.6406%;\"\u003e\n \u003cp\u003eCompressive modulus\u003c/p\u003e\n \u003cp\u003e(kPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 14.6423%;\"\u003e\n \u003cp\u003eThermal conductivity (mW/m.K)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.6456%;\"\u003e\n \u003cp\u003eCO2 adsorption\u003c/p\u003e\n \u003cp\u003e(mmol/g)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 10.8153%;\"\u003e\n \u003cp\u003eCNF xerogel\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.98336%;\"\u003e\n \u003cp\u003e69.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 11.3145%;\"\u003e\n \u003cp\u003e97.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.15141%;\"\u003e\n \u003cp\u003e16.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.807%;\"\u003e\n \u003cp\u003e157.2 (\u0026plusmn;8.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.6406%;\"\u003e\n \u003cp\u003e136.1 (\u0026plusmn;7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 14.6423%;\"\u003e\n \u003cp\u003e36.3 (\u0026plusmn;3.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.6456%;\"\u003e\n \u003cp\u003e0.52\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 10.8153%;\"\u003e\n \u003cp\u003eRaw FA/\u003c/p\u003e\n \u003cp\u003eCNF xerogel\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.98336%;\"\u003e\n \u003cp\u003e49.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 11.3145%;\"\u003e\n \u003cp\u003e89.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.15141%;\"\u003e\n \u003cp\u003e29.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.807%;\"\u003e\n \u003cp\u003e174.6 (\u0026plusmn;9.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.6406%;\"\u003e\n \u003cp\u003e159.4 (\u0026plusmn;7.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 14.6423%;\"\u003e\n \u003cp\u003e37.1 mW/m.K\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.6456%;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e0.42\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 10.8153%;\"\u003e\n \u003cp\u003eP-FA/\u003c/p\u003e\n \u003cp\u003eCNF xerogel\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.98336%;\"\u003e\n \u003cp\u003e63.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 11.3145%;\"\u003e\n \u003cp\u003e96.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 9.15141%;\"\u003e\n \u003cp\u003e17.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.807%;\"\u003e\n \u003cp\u003e293.5 (\u0026plusmn;9.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 15.6406%;\"\u003e\n \u003cp\u003e264.1 (\u0026plusmn;6.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 14.6423%;\"\u003e\n \u003cp\u003e32.1 (\u0026plusmn;3.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.6456%;\"\u003e\n \u003cp\u003e1.67\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSince the specific surface area, pore size and pore size distribution play a crucial role in evaluating the adsorption efficiency and thermal insulation properties of a xerogel, they are discussed next. Nitrogen adsorption was used to determine the specific surface area (SSA) and porosity characteristics of the CNF xerogel and P-FA/CNF xerogel. According to the IUPAC classification [29], if both adsorption and desorption isotherms of a porous material follow a type IV isotherm, the material belongs to mesoporous types and exhibits strong adsorbate-adsorbent interactions. All our xerogels belong to mesoporous materials. Table 1 presents the physical properties of all three xerogels (pure CNF xerogel, FA/CNF xerogel and P-FA/CNF xerogel). The pure CNF xerogel exhibits a higher specific surface area (69 m\u003csup\u003e2\u003c/sup\u003e/g) and porosity (97.8%), indicating a highly porous and lightweight structure. In contrast, the P-FA/CNF xerogel shows a slightly reduced surface area of 63.8 m\u003csup\u003e2\u003c/sup\u003e/g and porosity (96.3%), due to its relatively denser and compact architecture. Nonetheless, the xerogel remains very lightweight, highly porous and fluffy in nature. Figure 5 compares the sorption isotherms of the CNF xerogel and P-FA/CNF xerogels.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAs the pristine FA/CNF xerogel shows inferior properties and is structurally weak, the discussion hereafter will focus only on the CNF xerogel and P-FA/CNF xerogel. To evaluate the structural stability of the P-FA/CNF xerogel, compressive tests were performed. Interestingly, the P-FA/CNF xerogel shows a remarkable improvement in mechanical strength (Figure 6a), reaching a compressive strength of 293.5\u0026plusmn;9.6 kPa and a Young\u0026rsquo;s modulus of 264.1\u0026plusmn;6.8 at 70% strain, compared to 157.2 kPa and 136.1 kPa for the pure CNF xerogel, an enhancement of 86.6% in strength and 94% in modulus. This large improvement is attributed to strong intermolecular interactions between filler and matrix and the structural reinforcement provided by the P-FA. For curiosity, when a xerogel was prepared with the same amount of pristine (unmodified) FA following the same formulation, the strength and modulus were increased by only 9.3% and 11.2%, respectively, compared to the neat CNF xerogel. This small increment is due to the inherent reinforcing property of FA. This also highlights the weak interfacial bonding and incompatibility between pristine FA and the CNF matrix. It is worth noting that structural stability is crucial for the practical use of xerogels since a well-defined porous structure can provide greater thermal insulation, higher absorptivity, and mechanical robustness. In view of this, the P-FA/CNF xerogel can be a promising porous material for various applications such as gas adsorption, water treatment and thermal insulation.\u003c/p\u003e\n\u003cp\u003eNext, the thermal conductivity was measured as it defines the material\u0026rsquo;s ability to conduct heat. As can be seen from Table 1, the P-FA/CNF xerogel exhibited ~11.6 and 13.5 % improvement in thermal conductivity (32.1 mW/m.K) compared to the pure CNF xerogel (36.3 mW/m.K) and FA/CNF xerogel, respectively. This enhancement confirms the presence of uniformly dispersed P-FA particles within the matrix, enabling the xerogel to perform as a good heat-insulating material with low thermal conductivity. Notably, the thermal conductivity of P-FA/CNF xerogel is much lower than that of the aerogel (47 mW/m.K) reported by Nguyen Do [17], composed of 5 wt.% FA/PVA/carboxy-methyl cellulose (CMC) and several commercial insulation materials, including mineral wool (30\u0026ndash;40 mW/m.K), fiberglass (33-44 mW/m.K), and expanded polystyrene (EPS, 30\u0026ndash;40 mW/m.K) [30] (Figure 6b). To further confirm the thermal insulation properties, a small piece of P-FA/CNF xerogel was placed on a preheated hot plate at 80\u0026deg;C for 90 s and its surface temperature was measured using an infrared (IR) thermometer. This experiment provides a clear indication of the xerogel\u0026rsquo;s heat shielding performance. For visual understanding, a video was provided in the supplementary document. Surprisingly, the top surface of the xerogel was at a considerably lower temperature (~16.8 \u0026deg;C) than that of the reference hot plate (65.4\u0026deg;C-82.2\u0026deg;C, Figure 6c), demonstrating its excellent heat resistance ability. It is important to note that only 4 wt.% of P-FA was incorporated into the P-FA/CNF xerogel, and the addition of more P-FA would further lower its thermal conductivity.\u003c/p\u003e\n\u003cp\u003eAfter thermal insulation analysis, the xerogel was evaluated for CO\u003csub\u003e2\u003c/sub\u003e adsorption performance. Figure 7a compares the CO\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eadsorption capacity of the CNF xerogel and P-FA/CNF xerogel. Compared to CNF xerogel (0.52 mmol/g), the P-FA/CNF xerogel showed about 3.2 times higher CO\u003csub\u003e2\u003c/sub\u003e uptake (1.67 mmol/g at 28 \u0026deg;C). This significant improvement can be explained due to the synergistic effects of the xerogel\u0026rsquo;s porous structure and its amine-rich surfaces. The amine groups act as active chemisorption sites for CO\u003csub\u003e2\u003c/sub\u003e [31,32]. Generally, in amine-functionalized porous adsorbents, CO\u003csub\u003e2\u003c/sub\u003e adsorption proceeds via carbamate formation through a zwitterionic mechanism, as illustrated in Figure 7b [31\u0026ndash;33]. Therefore, the enhanced CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of P-FA/CNF xerogel stems from the cooperative contribution of polydopamine-derived pyrrolic nitrogen and free amine groups in PEI. Notably, CO\u003csub\u003e2\u003c/sub\u003e capture is influenced by various factors, including the nature of the adsorbent, pore architecture, pore volume, surface area, type and density of functional groups, their spatial distribution, and the accessibility of reactive sites [34]. Although the pristine CNF xerogel shows some CO\u003csub\u003e2\u003c/sub\u003e uptake, this is mainly driven by its highly porous structure and large surface area, enabling effective CO\u003csub\u003e2\u003c/sub\u003e diffusion and adsorption.\u003c/p\u003e\n\u003cp\u003eFigure 7b presents temperature-dependent CO\u003csub\u003e2\u003c/sub\u003e adsorption of the xerogel at a constant pressure of 1 bar. As the temperature increased, the CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity decreased, indicating a typical exothermic adsorption behaviour [35\u0026ndash;38]. This result helps in understanding the suitability of the xerogel for practical applications, especially under conditions related to industrial CO\u003csub\u003e2\u003c/sub\u003e capture. As FA-based aerogels or xerogels have not been widely explored for CO\u003csub\u003e2\u003c/sub\u003e adsorption, comparing them directly with high surface area activated carbons or inorganic adsorbents may not be relevant. Nevertheless, the P-FA/CNF xerogel exhibits a competitive CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity compared to other porous adsorbents [39\u0026ndash;45].\u003c/p\u003e\n\u003cp\u003eFigure 7c illustrates the cyclic stability or regeneration performance of the P-FA/CNF xerogel over six adsorption-desorption cycles. Regeneration was carried out at 90 \u0026deg;C under a pure N\u003csub\u003e2\u003c/sub\u003e atmosphere. As observed, the P-FA/CNF xerogel maintains nearly the same CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity as the original sample. After six consecutive cycles, a minor loss of about 2\u0026ndash;2.5% in adsorption capacity was observed, most likely due to partial pore blockage or pore collapse, and the breakdown of carbamate species, which facilitates the release of adsorbed CO\u003csub\u003e2\u0026nbsp;\u003c/sub\u003e[31]. Nonetheless, the efficiency remained over 97%. These findings suggest that this xerogel can be readily integrated into modern building materials, where both thermal insulation and CO\u003csub\u003e2\u003c/sub\u003e mitigation are critically important.\u003c/p\u003e\n\u003cp\u003eThe adsorption performance of the P-FA/CNF xerogel for water purification was evaluated using methylene blue (MB) and nigrosin as model dye contaminants, owing to their widespread industrial use and hazardous nature. Approximately 120 mg of xerogel was immersed in the dye solution and gently stirred until the absorption was complete. UV-Vis spectroscopy was used to assess the adsorption behaviour. It can be seen from Figures 8a and 8b that the absorbance of both dyes decreases steadily with time, confirming efficient dye removal. Interestingly, the xerogel adsorbed over 98% of nigrosin dye from the aqueous mixture within just 2 min, whereas MB required 35 min to reach 96.7% adsorption. These rates are faster than many previously reported adsorbents \u0026nbsp;[46\u0026ndash;53]. To illustrate the colour fading more clearly, the digital images of the dye solutions at different time intervals were presented in Figures 8c and 8d.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNext, a mixed dye solution (MB+nigrosin) was prepared to evaluate the xerogel\u0026rsquo;s performance, as actual wastewater often contains multiple dyes. Interestingly, the P-FA/CNF xerogel demonstrated efficient and fast removal of both dyes. Figures 8e and 8f show the time-dependent UV-Vis spectra and corresponding digital images of the dye colour, respectively. After mixing the two dyes, the spectral profiles changed; however, the characteristic MB peaks remained nearly the same, shifted only by 2\u0026ndash;3 nm, due to dye-dye interactions. The MB peaks at 664 and 611 nm disappeared completely within 30 minutes. Although nigrosin didn\u0026rsquo;t show a distinct peak, its adsorption occurred rapidly. The excellent adsorption performance of the P-FA/CNF xerogel is attributed to \u0026pi;\u0026ndash;\u0026pi; interactions, hydrogen bonding, and electrostatic attractions between its surface functional groups and the dye molecules (Figure 8g). To the best of our knowledge, this is the first study that presented a robust fly ash-based xerogel with three compelling functionalities.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eCollectively, these results highlight the exceptional dye adsorption capacity of the P-FA/CNF xerogel. Furthermore, the findings confirm the potential of FA to be upcycled into a robust, multifunctional xerogel, thereby reducing its environmental impact while enabling new pathways for sustainable utilization.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study reported a novel strategy for upcycling fly ash into a multifunctional, robust xerogel, suitable for sustainable environmental applications. To enhance the mechanical strength and functionality of the xerogel, the FA was chemically functionalized with polydopamine and then mixed with cellulose nanofibers, polyvinyl alcohol, polyethyleneimine, and glutaraldehyde. The resulting xerogel (P-FA/CNF xerogel) exhibited outstanding heat shielding, thermal insulation, CO\u003csub\u003e2\u003c/sub\u003e capture, and dye removing capabilities, never reported before. The P-FA/CNF xerogel showed a compressive strength of 293.5\u0026thinsp;\u0026plusmn;\u0026thinsp;9.6 kPa and a Young\u0026rsquo;s modulus of 264.14\u0026thinsp;\u0026plusmn;\u0026thinsp;7.2 kPa at 70% strain, which are 86.6% and 94% higher in strength and modulus, respectively, compared to pure CNF xerogel. It also offered excellent thermal conductivity (33.4 mW/m.K), outperforming commercial insulators such as expanded polystyrene (EPS), mineral wool, and fiberglass. Additionally, the xerogel exhibited a high CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of 1.67 mmol/g, which is 3.2 times higher than that of the pure CNF xerogel. The xerogel also showed repeatable performances over six adsorption-desorption cycles. Not only that, the as-synthesised xerogel can quickly remove hazardous water-soluble industrial dyes, such as methylene blue and nigrosine from aqueous mixtures, highlighting its high potential in wastewater purification technologies. These results demonstrate that hazardous fly ash can be effectively transformed into high-value materials for multiple challenging applications when valorised scientifically. We hope that our simple, cost-effective and novel proof-of-concept approach for converting fly ash into multifunctional xerogel will serve as a valuable reference for researchers working in waste management, water purification, thermal heat management, CO\u003csub\u003e2\u003c/sub\u003e reduction and environmental sustainability.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eNOTES\u003c/h2\u003e\n\u003cp\u003eThe authors declare no competing.\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eSunanda Roy: Conceptualization, Methodology, Investigation, Data Analysis, Writing- Original draft preparation; Sajal Nandi: Methodology, Investigation, Data Analysis, Software. Barnali Dasgupta Ghosh: Methodology, Investigation, Resources, Funding, Writing- Reviewing and Editing; Kheng Lim Goh: Investigation, Resources, Reviewing and Editing.\u003c/p\u003e\u003ch2\u003eACKNOWLEDGEMENT\u003c/h2\u003e\u003cp\u003eThis work is supported by the Anusandhan National Research Foundation (ANRF), formerly known as the Science and Engineering Research Board (SERB), Government of India ( CRG/2022/001610).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBhatt, A. \u003cem\u003eet al.\u003c/em\u003e (2019). 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Selective removal of toxic organic dyes using Trӧger base-containing sulfone copolymers made from a metal-free thiol-yne click reaction followed by oxidation. \u003cem\u003eRSC Adv.\u003c/em\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/D1RA03783H\u003c/span\u003e\u003cspan address=\"10.1039/D1RA03783H\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Xerogel, thermal conductivity, fly ash, dye, cellulose nanofibers, porous","lastPublishedDoi":"10.21203/rs.3.rs-8275181/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8275181/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEach year, millions of tons of fly ash (FA), a hazardous byproduct of coal-fired power plants, are disposed of in landfills, causing serious environmental pollution and substantial health risks. Consequently, effective FA management has become critically important. Despite its popularity in the construction sector, FA has rarely been used to create multifunctional porous materials capable of addressing heat insulation, CO\u003csub\u003e2\u003c/sub\u003e capture, and wastewater purification. This paper reports the successful development of highly durable, lightweight, flexible and multifunctional xerogels derived from FA. The FA was chemically modified and blended with cellulose nanofibers (CNFs), a binder, and a crosslinker to enhance structural stability and multifunctionalities. The resulting xerogel exhibited a compressive strength of 293.5\u0026thinsp;\u0026plusmn;\u0026thinsp;9.6 kPa and a Young\u0026rsquo;s modulus of 264.1\u0026thinsp;\u0026plusmn;\u0026thinsp;6.8 kPa at 70% strain, representing an increase of 86.6% in strength and 94% in modulus compared to pure CNF xerogel. Moreover, it exhibited excellent thermal conductivity of 32.1 mW/m.K, and a CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of 1.67 mmol/g, superior to many contemporary materials. Interestingly, the xerogel also performed extremely well in selectively removing both cationic and anionic dyes from water. These results highlight the effectiveness of our formulation and design approach in producing a robust multifunctional xerogel. With its simple fabrication process, lightweight structure, mechanical robustness and multifunctionality, the developed xerogel emerges as an attractive solution for wastewater management, CO\u003csub\u003e2\u003c/sub\u003e mitigation and building insulation.\u003c/p\u003e","manuscriptTitle":"Upcycling Hazardous Fly Ash into High-Performance Lightweight Xerogels for Thermal Insulation, CO2 Adsorption, and Wastewater Purification","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-09 16:18:06","doi":"10.21203/rs.3.rs-8275181/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-19T23:34:51+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-19T23:30:03+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-05T04:09:29+00:00","index":"","fulltext":""},{"type":"submitted","content":"Cellulose","date":"2025-12-04T03:51:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"880ff582-0f55-4bfa-af36-b14f1bb42e65","owner":[],"postedDate":"March 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-14T03:09:29+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-09 16:18:06","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8275181","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8275181","identity":"rs-8275181","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}