Fly Ash-Derived Mesoporous Silica–Alumina Aerogel via an Optimized Water-Acid Leaching Process for Effective Methylene Blue Removal

Abstract

Adsorption is a common method for solving the contamination of methylene blue (MB) in dyeing wastewater. Aerogel adsorbents with high porosity and specific surface areas have attracted increasing attention. However, the high costs of raw materials for aerogel preparation restrict their large-scale production and application. Fly ash (FA), a by-product of coal-fired power plants, is rich in silica and aluminum elements and has the potential to prepare aerogel adsorbents. This study proposed a modified recycling route for FA to synthesize silica–alumina composite aerogel with high specific surface area. FA was pretreated by three steps of alkali fusion, water leaching and acid leaching to obtain a solution rich in silicon and aluminum elements, with a total leaching efficiency of 96.92% and 91.36% for silicon and aluminum, respectively, under optimized alkaline fusion conditions of FA:NaOH mass ratio of 1:1.2, calcination time of 2 h, and calcination temperature of 550 °C. Silica–alumina aerogel with a specific surface area of 661.3 m²/g was then synthesized from the leaching solution through a sol–gel method, exhibiting well-developed mesopores and achieving an adsorption capacity of 52.22 mg/g for MB. The adsorption kinetics and isotherms of MB adsorption by FA-derived silica–alumina composite aerogel was investigated. FTIR characterization confirmed that the adsorption of MB by FA-derived aerogel was mainly physical adsorption. This study provides a new approach for the resource utilization of FA, and the high-specific-surface-area FA-derived aerogel holds potential as an alternative adsorbent for the removal of dyes in wastewater.

1. Introduction

Dyes are widely used in industries such as textiles, printing and dyeing, and papermaking [1]. However, the industrial application of dyes leads to the generation of a considerable amount of dye wastewater that requires appropriate treatment. Most dye molecules are aromatic compounds with complex and relatively stable structures, making them challenging to biodegrade effectively [2]. Furthermore, dye wastewater presents issues such as high concentration, deep color, high chemical oxygen demand, complex organic composition, elevated inorganic salt content, and high toxicity, thus increasing the difficulty in its treatment [3]. Particularly, methylene blue (MB), a typical azo dye, is an aromatic heterocyclic compound with a relatively stable structure due to its benzene ring composition [4], and, therefore, it tends to be persistent and potentially hazardous to the environment. The adsorption method has become a common approach for dye wastewater removal due to its simplicity, low cost, and effectiveness [5]. Although activated carbon, bentonite, and zeolite are common traditional adsorbents for dye adsorption [6], they may exhibit high cost, low adsorption efficiency, poor recycling performance and high energy consumption, which necessitates the development of novel adsorption materials.

Aerogel, as a novel nanomaterial with low solid density, high porosity, and large specific surface area, is emerging as an ideal choice for adsorbents [7]. Among them, inorganic silica-based aerogels have been extensively studied. However, the high costs of raw materials for silica-based aerogel preparation, such as tetraethyl orthosilicate and sodium silicate, restrict their large-scale production and application [8]. Under these circumstances, some researchers are turning their attention to silicon-rich solid wastes as inexpensive raw materials for aerogel preparation. Coal fly ash (FA), an industrial waste generated during the coal combustion process, has become one of the primary solid wastes in China as the installation of coal-fired power plants increases annually [9]. With a resource utilization ratio of only about 80% [10], the underutilized FA can lead to severe land occupation issues and environmental pollution. Since FA is rich in silicon and aluminum elements, making it a potential material to prepare aerogel to realize effective utilization [11,12,13,14].

The most used method to prepare aerogels from FA involves two pretreatment steps, alkali fusion and acid leaching, to obtain a leaching solution rich in silicon and aluminum elements [15]. Subsequently, the pH of the acid leaching solution was adjusted by adding alkali to initiate the sol–gel process to obtain the aerogel product. For example, Fan et al. prepared silica aerogels from FA by alkali fusion and acid leaching, which was further grafted with 3-(Aminopropyl) triethoxysilane (APTES) via a simple ambient pressure drying process, and verified as an efficient adsorbent for CO₂ capture [16]. Zhang et al. used FA acid leaching residue as the silicon source to prepare sodium silicate solution by alkali leaching, and then prepared wet gel by the sol–gel method. Hexamethyldisiloxane (HMDSO) was used to modify the surface of the wet gel to reduce the capillary tension during the atmospheric pressure drying process, and finally a silica aerogel material with hydrophobic properties was obtained [17]. Nevertheless, this alkali fusion, acid leaching and alkali leaching method faces challenges such as high consumption of acids and alkalis, as well as low leaching efficiency of silicon and aluminum elements. Hence, reducing the leaching costs and enhancing the leaching efficiency of silicon and aluminum from FA are crucial for achieving large-scale production of FA-derived aerogel.

Herein, the present study proposes a modified leaching method for extracting silicon and aluminum elements from FA to prepare silica–alumina composite aerogel. Initially, the alkaline fusion conditions of FA were optimized to maximize the activation of silicon and aluminum elements. A solution rich in silicon and aluminum elements was then obtained through a two-step process of water leaching and acid leaching. Subsequently, the water leaching solution with high alkalinity was used to adjust the pH of the acid leaching solution to initiate the sol–gel process to produce FA-derived silica–alumina composite aerogel materials. The feasibility of utilizing the as-prepared aerogel materials as low-cost adsorbents for removing MB was assessed, and the adsorption mechanism was analyzed.

2. Experiment

2.1. Raw Materials

The raw material of FA was obtained from a Zhejiang Zheneng Technology & Environment Group Co., Ltd., Zhejiang province, Lanxi, China. The chemical compositions of the FA given in Table S1 indicate that its main components are SiO₂ and Al₂O₃, with proportions of 55.13 wt% and 33.22 wt%, respectively. The X-ray diffraction (XRD) pattern of the FA shown in Figure S1a indicates that its crystalline phases mainly include mullite (3Al₂O₃·2SiO₂) and quartz (SiO₂), and amorphous substances. SEM image of FA shown in Figure S1b suggests that the FA contains glass beads of various sizes, as well as spongy vitreous bodies with irregular shapes.

Anhydrous ethanol (EtOH, C₂H₅OH, AR ≥ 99.5%), MB (C₁₆H₁₈N₃SCl₃·3H₂O, AR ≥ 98.0%), sodium hydroxide (NaOH, AR ≥ 96.0%), hydrochloric acid (HCl, AR ≥ 99.5%), and ammonia water (NH₃·H₂O, ≥28% in H₂O) were obtained from Sinopharm Co., Ltd. (Beijing, China) All chemicals were used without any further purification. Deionized (DI) water was made from laboratory.

2.2. Preparation of FA-Derived Silica–Alumina Composite Aerogel

The synthesis method of FA-derived aerogel is provided in Figure 1. FA and NaOH were mixed at a certain mass ratio (2:1, 1.25:1, 1:1, 1:1.2, and 1:1.5), and then ground together. Then, the mixture was placed into the muffle furnace and calcined at a pre-set temperature (350, 450, 550, 650 °C) for a certain time (1, 2, 3, 4 h). The alkali-fused ash samples obtained at different calcination conditions were then treated by water leaching and acid leaching under pre-optimized conditions. Specifically, the alkali-fused ash was ground using an agate mortar, and then ultrapure water was added with a liquid/solid (L/S) ratio of 25 mL/g. The mixture was stirred at 500 rpm for 1 h, then centrifuged to obtain a water leaching residue and a water leaching solution containing silicon and aluminum components. Then, the water leaching residue was mixed with HCl (5 mol/L) at a L/S ratio of 10 mL/g and stirred at 500 rpm for 1 h to obtain an acid leaching solution containing abundant silicon and aluminum elements. After that, the water and acid leaching solutions were mixed to obtain a silicon and aluminum-rich solution. This solution pH was adjusted by NH₃·H₂O under stirring conditions, and aged at 25 °C for certain days to obtain a wet gel. The wet gel was then washed with water, solvent exchanged with ethanol, and dried under atmospheric pressure to obtain the final silica–alumina composite aerogel.

The leaching efficiency of silicon and aluminum elements during the water and acid leaching processes are shown in Equations (1)–(4).

η_Si1 = C_Si1 V₁/m₁w_Si × 100%,

(1)

η_Al1 = C_Al1V₁/m₁w_Al × 100%,

(2)

η_Si2 = C_Si2 V₂/m₁w_Si × 100%,

(3)

η_Al2 = C_Al2V₂/m₁w_Al × 100%,

(4)

where η_Si1, η_Al1 and η_Si2, η_Al2 represent the leaching efficiency (%) of silicon and aluminum elements during the water and acid leaching processes, respectively, m₁ is the mass of the FA (g), w_Si and w_Al are the content of silicon and aluminum elements in the FA (wt%), V₁ and V₂ are the volume of the water and acid leaching solutions (L), C_Si1 and C_Si2 are the concentration of silicon element in the water and acid leaching solutions (mg/L), and C_Al1 and C_Al2 are the concentration of aluminum element in the water and acid leaching solutions (mg/L).

2.3. Adsorption Test

The FA-derived silica–alumina composite aerogel was passed through a 100-mesh sieve before the adsorption test. The aerogel sample was added to a 50 mL MB solution at different solid/liquid ratios. The pH of the MB solution was adjusted to different pH values between 2.0 and 11.0 using HCl (0.5 mol/L) or NaOH (0.5 mol/L). The mixture was placed in a constant temperature oscillator at a speed of 250 rpm for certain time. Samples were taken and filtered through a 0.22 µm filter membrane. The adsorption capacity of FA-derived aerogel for MB was calculated through the following Equation (5).

$$q_e = {\displaystyle \frac{\left(C_0 - C_e\right)V}{m}}$$

(5)

where q_e (mg/g) is the adsorption capacity, C₀ and C_e (mg/L) are the initial and equilibrium concentrations of MB, V (L) and m (g) are the solution volume and the weight of aerogel, respectively.

2.4. Characterization Methods

The chemical compositions of FA were determined by X-ray fluorescence (XRF, PANalytical Axios^max, Almelo, The Netherlands) using melting method with testing voltage of 60 kV and current of 50 mA. Mineral phases of samples were investigated by X-ray diffraction (XRD, PANalytical X’pert PRO, Almelo, The Netherlands) using Cu Kα radiation, with a 2-theta scanning range of 10–80°, a scanning voltage of 40 kV, a current of 40 mA and a sweep speed of 10°/min. The specific surface area and pore characteristics of samples were determined by N₂ physisorption isotherms (BET, JW-BK100, Beijing, China) at 77 K. The microscopic morphology of samples was characterized by scanning electron microscopy (SEM, GeminiSEM 300, Baden-Württemberg, Germany) after coating the samples with a thin layer of gold. Fourier Transform Infrared Spectrometer (FTIR, Thermo Scientific Nicolet iS20, Waltham, MA, USA) was used to analyze the samples’ surface functional group within a wavelength range of 400–2000 cm⁻¹. Ion concentration was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Perking Elmer Optima 8300, Shelton, CT, USA). The equilibrium concentration of MB was determined by the UV–vis spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan). The measurements were made at the wavelength λ = 665 nm which corresponds to maximum absorbance.

3. Results and Discussion

3.1. Optimization of the Alkali Fusion Conditions of FA

The effects of FA:NaOH mass ratio, alkali fusion time, and alkali fusion temperature on the mineral phases of alkali-fused ash, as well as the further silicon and aluminum leaching efficiency under water leaching and acid leaching were investigated, as illustrated in Figure 2, and compared with that under direct acid leaching (Figure S2).

After calcination at 350 °C for 1 h, the peak intensities of the water-soluble phases Na₄SiO₄ and Na₂SiO₃ in the product gradually increased as the FA:NaOH mass ratio decreased from 2:1 to 1:1.5, while the peak intensity of the water-insoluble phase NaAlSiO₄ first decreased and then increased (Figure 2a). The corresponding silicon and aluminum leaching efficiencies are shown in Figure 2b. When the FA:NaOH mass ratio was 1:1.2, favorable leaching efficiency was achieved through both the direct acid leaching method and two-step water leaching-acid leaching method. The direct acid leaching efficiency of silicon and aluminum were 64.78% and 72.38% (Figure S2a), respectively, which increased to 74.48% and 80.93%, respectively, using the two-step water leaching-acid leaching method. Therefore, the subsequent FA:NaOH mass ratio was chosen to be 1:1.2.

There was no significant change in the phases of the alkali-fused ash obtained under different calcination time conditions (Figure 2c). Extending the calcination time from 1 to 2 h partially improved the silicon and aluminum leaching efficiency for both direct acid leaching and water leaching-acid leaching methods (Figure 2d and Figure S2b). When the calcination time exceeded 2 h, the leaching efficiency of silicon and aluminum remained relatively constant. Therefore, a calcination time of 2 h was chosen for subsequent processes based on cost considerations.

With the increase in calcination temperature, the mass transfer rate between solid phases was accelerated, leading to a greater conversion of silicon and aluminum elements in FA to water-soluble amorphous substances. The intensity of the water-insoluble phase NaAlSiO₄ peak exhibited a decreasing and then increasing trend (Figure 2e), resulting in a trend of initially increasing and then decreasing silicon and aluminum leaching efficiency (Figure 2f). At a calcination temperature of 550 °C, the silicon and aluminum leaching efficiency were 81.46% and 85.84% through direct acid leaching (Figure S2c), surpassing the results reported by Shen et al. [14]. In addition, the silicon and aluminum leaching efficiency further increased to 96.92% and 91.36%, respectively, by employing the two-step water leaching-acid leaching method. Therefore, compared with the currently reported method for leaching silicon and aluminum elements from FA by direct acid leaching after alkali fusion, the inclusion of a water leaching step before acid leaching can effectively enhance the silicon and aluminum leaching efficiency. The water leaching process enabled the leaching of some silicon and a small amount of aluminum, thus avoiding the issue of high silicon component concentration during direct acid leaching, which could lead to decreased silicon and aluminum leaching efficiency. Additionally, water leaching of the alkali-fused ash facilitated the dissolution of certain alkaline substances, reducing the amount of HCl added during the later acid leaching stage and the quantity of alkali used in pH adjustment, which further reduced the production cost of fly ash-based aerogels.

3.2. Phase Transformation During Alkali Activation of Fly Ash

During the alkali fusion of FA, initially, mullite and quartz were converted into sodium aluminosilicate (NaAlSiO₄), sodium silicate (Na₂SiO₃), and sodium metasilicate (Na₄SiO₄) (Equations (6)–(8)) [11,18]. The alkali-fused ash was first subjected to water leaching, allowing partial dissolution of sodium silicate in the solution (Equations (9) and (10)). This water leaching step prevents excessive hydrolysis and condensation of silica during direct acid leaching, thereby avoiding the formation of silica sols and the associated increase in solution viscosity, which would result in a reduction in the leaching efficiency of silicon and aluminum. At the same time, the water leaching step can dissolve some alkaline substances, reducing the amount of HCl added in the subsequent acid leaching step (Equations (11)–(13)).

3Al₂O₃·2SiO₂ + 4SiO₂ + 6NaOH → 6NaAlSiO₄ + 3H₂O

(6)

SiO₂ + 2NaOH → Na₂SiO₃ + H₂O

(7)

SiO₂ + 4NaOH → Na₄SiO₄ + 2H₂O

(8)

Na₂SiO₃ + H₂O → 2NaOH + H₂SiO₃

(9)

Na₄SiO₄ + 3H₂O → 4NaOH + H₂SiO₃

(10)

NaAlSiO₄ + 4HCl → NaCl + AlCl₃ + H₂SiO₃ + H₂O

(11)

Na₂SiO₃ + 2HCl → H₂SiO₃ + 2NaCl

(12)

nH₂SiO₃ + mH₂O → nSiO₂ + (n + m)H₂O

(13)

The alkali-fused ash samples obtained under different calcination conditions were characterized by FTIR to further investigate the mechanism of alkali activation of FA. As shown in Figure 3a, in the original raw FA, the absorption peak at 469 cm⁻¹ corresponds to the bending vibration of O-Si-O in the silicate tetrahedron, the peak at 562 cm⁻¹ corresponds to the absorption peak of aluminum octahedron in mullite, the peak at 847 cm⁻¹ is related to the quartz mineral in FA, and the absorption peak at 1091 cm⁻¹ corresponds to the asymmetric stretching vibration of T-O (T = Si, Al) bond [19,20]. After calcination with NaOH, the absorption peaks at 562 cm⁻¹ and 847 cm⁻¹ in FA disappeared, indicating the destruction of the aluminum octahedron in mullite and the silicate tetrahedron structure in the quartz phase of FA. When the FA:NaOH mass ratio was 2:1, the newly appeared absorption peaks near 740 cm⁻¹ and 682 cm⁻¹ are related to the quaternary ring structure [21], indicating that the newly generated AlO₄ and SiO₄ tetrahedra can further combine to form quaternary ring structures, thereby forming aluminum silicates that are easily soluble in acid. The absorption peak around 880 cm⁻¹ in the alkali-fused ash corresponds to the C-O bond, which is because NaOH in the alkali fusion process would absorb CO₂ in the air, introducing CO₃²⁻ into the calcination product, and Na₂CO₃ phase was also detected by XRD [22]. When the FA:NaOH mass ratio was reduced from 2:1 to 1:1.2, the wavenumber of the absorption peak related to T-O (T = Si, Al) bond decreases from 990 cm⁻¹ to 967 cm⁻¹, which then increased to 970 cm⁻¹ when the FA:NaOH mass ratio was further reduced to 1:1.5. Since the Al-O bond is longer and weaker than the Si-O bond, as the proportion of aluminum octahedra in aluminosilicates increases, their stability decreases, resulting in a decrease in the wavenumber of the absorption peak corresponding to the asymmetric peak of T-O bond (T = Si, Al) [19,23]. When the FA:NaOH mass ratio was 1:1.2, the wavenumber of the absorption peak corresponding to the T-O bond is the smallest, indicating that the stability of aluminosilicates was the worst at this time, and the corresponding leaching efficiency of silicon and aluminum in FA was the highest. It is consistent with the optimal silicon and aluminum leaching conditions obtained in Figure 2b, where the mass ratio of FA:NaOH was 1:1.2.

The effect of different calcination time (Figure 3b) shows that the wavenumber of the absorption peak at 1091 cm⁻¹ in FA decreases first to 987 cm⁻¹ and then remains unchanged with the prolonging of the calcination time, indicating that the activity of aluminosilicates increases first and then remains unchanged with the prolonging of calcination time, which is consistent with the trend of effect of alkali fusion time on the leaching of silicon and aluminum elements in Figure 2d.

When the calcination temperature increased (Figure 3c), the wavenumber of the absorption peak at 1091 cm⁻¹ in FA first decreases to 975 cm⁻¹ and then remains unchanged, indicating that the activity of aluminosilicates increases first and then remains unchanged with the increase in calcination temperature, which is consistent with the trend of the effect of calcination temperature on the leaching of silicon and aluminum elements in Figure 2f. At different calcination temperatures, new absorption peaks appear at 738 cm⁻¹ and 684 cm⁻¹, indicating the formation of aluminum silicates such as nepheline with the quaternary ring as the basic unit, which is consistent with the XRD analysis in Figure 2e.

In conclusion, during the process of calcination, the alumina octahedral structure in FA was destroyed and transformed into alumina tetrahedral structure. The alumina tetrahedra and silica tetrahedra recombined to form a quaternary ring structure. Finally, the quaternary ring structure served as the basic unit to form silicoaluminate minerals such as nepheline, which could efficiently extract silicon–aluminum elements through simple processes of water leaching and acid leaching. After calcination at 550 °C for 2 h with a FA:NaOH mass ratio of 1:1.2, the silicon and aluminum leaching efficiency of FA could reach 96.92% and 91.36%, respectively, through a two-step process of water leaching and acid leaching.

3.3. Optimization of the Sol–Gel Reaction Conditions of FA-Derived Aerogel

The FA-derived aerogel was prepared by sol–gel method using the mixed water leaching and acid leaching solution from alkali-fused ash obtained under optimized alkali fusion conditions following the procedure in Figure 1. Effects of initial pH and aging time of sol–gel reaction on the N₂ adsorption–desorption curve and pore distribution of prepared FA-derived aerogel are shown in Figure 4, and the corresponding characteristic parameters of pore structure are provided in Tables S2 and S3.

As can be seen from Figure 4a,b, the N₂ adsorption isotherms of the aerogels prepared at pH = 1.5 and pH = 2.0 are typical type IV adsorption curves, which indicate the presence of certain mesopores. The N₂ adsorption isotherms of the aerogels prepared under the remaining pH conditions are a combination of typical type I and type IV adsorption curves, and there is a certain increase in the amount of N₂ adsorbed at low pressure (P/P₀ < 0.2), indicating that a small amount of micropores existed in the samples. There is a large hysteresis loop in the adsorption isotherm in the medium- and high-pressure section (0.4 < P/P₀ < 1.0), which belongs to the H₄-type hysteresis loop, indicating the presence of a large number of mesopores in the sample [24]. The specific surface area and total pore volume of the aerogel prepared at pH = 1.5 and pH = 2.0 are small, which are less than 30 m²/g and 0.5 cm³/g, respectively, and the average pore diameter of the aerogel is larger than 5 nm (Table S2). It indicates that at these pH conditions the pore structure of the aerogel is not well developed. The specific surface area of the prepared aerogel is significantly improved when the pH increased from 2.5 to 2.9, and the pore size of the aerogel contains micropores, mesopores, and macropores. At pH = 2.9, the specific surface area of the aerogel is the largest, 556.0 m²/g, and the total pore volume reaches 0.538 cm³/g. This could be attributed to the increased pH is closer to the equipotential point of the wet silica gel (pH ≈ 2–3), which make the formed spatial structure more compact and less prone to shrinkage during drying [25]. Further increase in pH from 2.9 to 3.1 results in a certain degree of decline in the specific surface area and total pore volume of the aerogel, which is due to the fact that the excessive addition of ammonia will make the structure formed after gelation looser, and thus the pore wall structure of aerogel collapses and shrinks to some extent when drying [26]. Therefore, initial pH of 2.9 was chosen as the pH condition for the subsequent preparation of aerogels.

With the increase in aging time, the specific surface area and total pore volume of the aerogel show an increasing trend, and the specific surface area of the aerogel increases from 556.0 m²/g to 661.3 m²/g, the total pore volume from 0.538 cm³/g to 0.804 cm³/g, and the average pore size from 3.873 nm to 4.863 nm when the aging time is increased from 1 d to 3 d. It indicates that prolonging the aging time is conducive to improving the formation of aerogel pore structure, which is consistent with literature [27]. When the aging time is longer than 3 d, the specific surface area and total pore volume of the aerogel are basically unchanged, indicating that the gel reaction is basically completed at this time. Therefore, 3 d of aging was chosen as the reaction time for preparation of FA-derived aerogel.

FA-derived aerogel with a specific surface area of 661.3 m²/g, an average pore size of 4.86 nm, and a pore volume of 0.804 cm³/g prepared under optimized sol–gel reaction conditions with an initial pH of 2.9 and aging time of 3 d was characterized. The XRD pattern shown in Figure 5a reveals a broad peak between 20 and 30°, indicating the prepared aerogel is amorphous, which is consistent with the literature [14]. In the FTIR spectrum depicted in Figure 5b, the peak at 1645 cm⁻¹ corresponds to the H-O-H bond, indicating the presence of bound water within the aerogel [28]. At 1390 cm⁻¹, the Al-O-H bond is identified. The absorption peaks at 1050 cm⁻¹ and 465 cm⁻¹ are attributed to the stretching vibrations of the Si-O-Si bond. Additionally, peaks at 800 cm⁻¹ and 557 cm⁻¹ correspond to the Al-O and Si-O-Al bonds [29]. The formation of these bonds indicates the successful preparation of silicon–aluminum composite aerogel materials. The SEM image (Figure 5c) depicts a rough surface morphology of the aerogel, showing loosely connected nanoscale particles forming pores of various sizes. These pores are discontinuous and randomly dispersed, with diameters mainly falling within the 2–5 nm range, consistent with the BET results. The presence of clustered structures suggests that during normal pressure drying, significant capillary forces have disturbed the fundamental network structure, leading to nanoparticle aggregation [30]. Element contents in the FA-derived aerogel were measured by ICP-OES after acid digestion, and the results (Figure 5d) show that the prepared aerogel is mainly composed of silicon and aluminum elements, and the sum of the mass of these two elements accounts for 95.5% of the total, in addition to a small amount of Ca, Fe, Na impurities.

3.4. Adsorption Performance of the FA-Derived Silica–Alumina Composite Aerogel

3.4.1. Effect of Adsorption Parameters

The effect of different initial MB concentrations (1, 5, 10, 20, 50, 100, 200 mg/L) on the adsorption performance of FA-derived silica–alumina composite aerogel was studied under conditions of solid/liquid ratio of 1 g/L, adsorption time of 24 h, adsorption temperature of 298 K and solution pH of 7.0 (Figure 6a). The adsorption capacity of the FA-derived aerogel for MB continuously increases with the increase in initial concentration of MB, but the adsorption efficiency shows a decreasing trend. This is because a higher initial concentration of MB solution provides more adsorbate that can be adsorbed, and the greater the concentration difference between the high-concentration MB solution and the adsorbent surface, the greater the driving force for adsorption mass transfer and the easier adsorption occurs [31]. However, the adsorption capacity per unit mass of adsorbent is limited. As the concentration of MB increases, the adsorption capacity increases but at a decreasing rate, and the adsorption efficiency shows a decreasing trend because of the lack of available active sites under high concentration condition of MB.

Figure 6b presents the effect of contact time for MB adsorption by FA-derived aerogel under conditions of MB concentration of 100 mg/L, solid/liquid ratio of 1 g/L, adsorption temperature of 298 K and solution pH of 7.0. The adsorption capacity and adsorption efficiency of MB both increase with increasing adsorption time. Within 30 min, the adsorption rate of FA-derived aerogel is fast, and the adsorption capacity reaches 24.48 mg/g, which is 46.88% of the equilibrium adsorption capacity. This is because at the beginning of adsorption, the concentration of MB in the solution is high, resulting in a large concentration difference and a greater chance of contact between MB solution and FA-derived aerogel. Moreover, there are multiple adsorption sites available initially, leading to a fast adsorption rate. The rate of adsorption decreases with time and gradually reaches a plateau. This is due to the gradual decreases in the concentration driving force with time. Additionally, the number of adsorption sites on the adsorbent decreases, resulting in a slower increase in the adsorption capacity [32]. At 24 h, the adsorption reaction reaches equilibrium, and the adsorption capacity of the FA-derived aerogel is 52.22 mg/g.

To investigate the effect of FA-derived aerogel dose on the MB adsorption, experiments were carried out with an initial MB concentration of 100 mg/L (pH = 7.0) and varying adsorbent dose at 298 K for 24 h, and the results are shown in Figure 6c. As the dose of FA-derived aerogel increases, the adsorption capacity decreases, leading to an increase in the adsorption efficiency of MB. This is because the number of adsorption sites increases with increasing dosage of the adsorbent, resulting in an increase in the adsorption capacity [33]. However, at this point, the adsorption capacity per unit mass of the adsorbent is limited, and adsorbent powers tend to agglomerate, which leads to no significant increase in effective adsorption sites.

The pH of MB solution plays a significant role in the whole adsorption process, which influences the chemistry properties for both MB molecules and adsorbents [34]. The effect of pH on the MB adsorption by FA-derived aerogel was performed at a pH range of 2.0–11.0, MB concentration of 100 mg/L, adsorbent dose of 1 g/L, temperature of 298 K, and equilibrium time of 24 h. As shown in Figure 6d, the adsorption amount of MB by FA-derived aerogel is positively correlated with the pH. It is possible that the H⁺ in the solution could compete with MB (cationic dye) for adsorption at a low pH value [35]. At the same time, the deprotonation of surface polar functional groups causes the aerogel to generate a certain amount of H⁺ on the surface, which results in electrostatic repulsion with MB [36]. This effect weakens as the pH increases, and the deprotonation of functional groups on the adsorbent surface favors the adsorption of MB.

3.4.2. Adsorption Mechanism Study

To investigate the adsorption processes and mechanism of MB onto FA-derived aerogel, different adsorption kinetics and isotherms models were introduced for analysis. The various models and their parameters used in this study are shown in Table S4. The fitting results of adsorption kinetics using the pseudo-first-order and pseudo-second-order models are shown in Figure S3. The pseudo-second-order kinetic model fits better than the pseudo-first-order kinetic model, with a correlation coefficient R² = 0.9965. The theoretical saturated adsorption amount q_e is 52.08 mg/g, which is very close to the experimental value q_e (52.22 mg/g).

Effect of different adsorption temperatures (15, 25, 35 °C) on the adsorption isotherms of MB by FA-derived aerogel was also studied. The initial concentrations of the MB solution were varied as 1, 5, 10, 20, 50, 100, and 200 mg/L. The solid/liquid ratio was maintained at 1 g/L, and the solution pH was set to 7.0. As shown in Figure S4, the adsorption capacity increased with temperature, indicating that the adsorption of MB by FA-derived aerogel is an endothermic reaction. The thermal motion, the solubility, and the chemical potential of dye molecules increase with the increase in temperature [23]. Moreover, when the temperature increases, it leads to the force the monomer ↔ dimer equilibrium towards more monomer species formation [37]. This results in an increased quantity of monomer MB molecules, as well as a decrease in adsorbent particle size and an increase in the number of adsorbable substances, which can also lead to an increase in adsorption capacity.

The adsorption isotherms at different temperatures were fitted using the Langmuir and Freundlich isotherm models. The results are presented in Figure S5 and

Table 1. Langmuir and Freundlich isotherm model parameters of adsorption of MB by FA-derived aerogel.

Temperature (°C)	Langmuir Model			Freundlich Model
	qm (mg/g)	KL (L/mg)	R2	n	KF (L/mg)	R2
15	51.21	0.10	0.9973	18.0	4.48	0.9543
25	55.42	0.17	0.9960	37.9	7.71	0.9833
35	65.35	0.33	0.9989	25.1	11.15	0.9620

. The Langmuir isotherm model is found to represent the adsorption isotherm data better than the Freundlich isotherm model due to higher regression coefficient. This indicates that the MB is adsorbed in monolayer on the surface of FA-derived aerogel, and the adsorption sites on the surface are uniformly distributed. The K_L value, ranging between 0 and 1, signifies a favorable adsorption process. The adsorption capacity of MB by FA-derived aerogel at room temperature is comparable to other reported aerogel adsorbents as listed in

Table 2. Partial reported study of MB adsorption by aerogel materials.

Raw Material	Products	Specific Surface Area (m2/g)	Adsorption Conditions	Adsorption Capacity of MB (mg/g)	Ref.
Ethyl orthosilicate	Hydrophobic SiO2 aerogel	880.5	Initial MB concentration 250 mg/L, dose1 g/L, 24 h, 298 K, pH 7.0	59.78	[38]
Ethyl orthosilicate	Hydrophilic SiO2 aerogel	628.5	Initial MB concentration 250 mg/L, dose1 g/L, 24 h, 298 K, pH 7.0	41.08	[38]
Ethyl orthosilicate	SiO2 aerogel/hollow silica microsphere composites	213.0	Initial MB concentration: 40 mg/L, dose 2 g/L, 2 h, 303 K, pH 10.0	29.20	[39]
Cellulose	Cellulose aerogel	—	Initial MB concentration 20 mg/L, dose 4 g/L, 2 h, 298 K, pH 7.0	2.28	[40]
Bamboo cellulose fiber	TiO2 coated carbon aerogel	—	Initial MB concentration 20 mg/L, 48 h, 298 K	18.50	[41]
Fly ash	Silica–alumina composite aerogel	661.3	Initial MB concentration 100 mg/L, dose1 g/L, 24 h, 298 K, pH 7.0	52.22	This study

.

In order to determine whether the adsorption of FA-derived aerogel on MB is attributed to physical or chemical adsorption, the adsorption isotherm was further fitted using the D-R model, and the average adsorption energy (E) which serves as a crucial indicator for determining the adsorption type, was calculated based on calculation formula provided in Table S4. The calculated average adsorption energy was found to be 1.31 kJ/mol, which is lower than 8 kJ/mol, indicating that the adsorption of MB onto FA-derived aerogel is mainly of a physical adsorption [42,43].

The characterization of the functional groups before and after the adsorption of MB by FA-derived aerogel was performed to further verify the physical adsorption of MB by FA-derived aerogel (Figure 7a). It can be observed that there is minimal difference in the infrared spectra of FA-derived aerogel before and after the adsorption of MB, indicating that there is essentially no significant change in the functional groups of the aerogel during the entire adsorption process, thus suggesting that physical adsorption is the main mechanism involved.

3.4.3. Adsorption Cycle Test

The adsorption/desorption cycle performance of MB by FA-derived aerogel was examined (Figure 7b). The adsorption procedure was performed under conditions: MB concentration of 100 mg/L, solid/liquid ratio of 1 g/L, adsorption temperature of 298 K, solution pH of 7.0, and adsorption time of 24 h. After adsorption and filtration, the aerogel adsorbent was regenerated with a methanol solution, washed, dried, and then used for the next round of adsorption experiments. The adsorption capacity of FA-derived aerogel exhibited an indiscernible change after five adsorption–desorption cycles, suggesting its promising sustainability for the purification of MB-containing wastewater.

4. Conclusions and Future Perspectives

This study proposed a modified recycling route for fly ash to synthesize silica–alumina composite aerogel. The alkali fusion conditions of fly ash were first optimized, followed by a two-step leaching method involving water leaching and acid leaching to achieve leaching efficiency of silicon and aluminum elements in fly ash of 96.92% and 91.36%, respectively. The optimized alkaline fusion conditions were FA:NaOH mass ratio of 1:1.2, calcination time of 2 h and calcination temperature of 550 °C. Phase transformation study showed that during alkali fusion of FA, mullite, and quartz were converted into sodium aluminosilicate, sodium silicate, and sodium metasilicate. When the alkali-fused ash was subjected to water leaching, partial sodium silicates were dissolved into the solution. This water leaching step prevents excessive hydrolysis and condensation of silica during direct acid leaching, thereby avoiding the formation of silica sols and the associated increase in solution viscosity. At the same time, the water leaching step can dissolve some alkaline substances, reducing the amount of HCl added in the subsequent acid leaching step. The prepared fly ash-derived silica–alumina composite aerogel had a high specific surface area of 661.3 m²/g under optimized sol–gel reaction conditions with an initial pH of 2.9 and aging time of 3 d. MB adsorption experiments revealed that the adsorption of MB by the fly ash-derived aerogel was attributed to physical adsorption, with an adsorption capacity of 52.22 mg/g, surpassing some reported aerogel materials, indicating promising prospects. The adsorption capacity of FA-derived aerogel exhibited an indiscernible change after five adsorption–desorption cycles. Future work could be focused on further reducing the consumption of acid and base, optimizing the drying method, and investigating the performance of aerogel adsorbents for mixed dye wastewater.