Ultrasound-Assisted Deposition and Supercritical Reduction of Graphene Oxide on θ-Al₂O₃ Microspheres for Selective Adsorption of Methylene Blue

Abstract

A composite based on θ-Al₂O₃ microspheres coated with graphene oxide (GO) and reduced graphene oxide (RGO) was prepared and evaluated as a sorbent for the removal of synthetic dyes from aqueous solutions. GO was synthesized by a modified Hummers’ method and deposited onto alumina microspheres via ultrasound-assisted treatment under various conditions, followed by supercritical reduction to obtain the Al₂O₃_RGO composite. The structure, morphology, and composition of the materials were characterized by Raman spectroscopy, SEM, TGA/DSC, FTIR, and XRD, revealing the formation of mono- and few-layer GO/RGO coatings on the substrate surface. Adsorption tests for cationic methylene blue (MB) dye and anionic methyl orange (MO) dye demonstrated that the alumina substrate was inactive, whereas GO- and RGO-coated microspheres exhibited high adsorption efficiency for MB and partial uptake of MO from water solutions. In mixed-dye solutions, both Al₂O₃_GO and Al₂O₃_RGO composites showed selectivity toward MB, and the RGO-based composite demonstrated enhanced MB adsorption at low concentrations. The results highlight GO/RGO-coated θ-Al₂O₃ microspheres as convenient and selective composite sorbents for water purification processes.

1. Introduction

Currently, intensive economic growth is closely linked to industrial expansion, which significantly increases the load on the environment. With the development of the textile industry, water resources are primarily suffering from the risk of contamination by various dyes [1], oils [2], pharmaceuticals [3], heavy metal ions [4], and other pollutants. The highest levels of contamination are generally observed in developing countries due to the limited capacity to implement a multi-stage wastewater treatment system at industrial facilities [5].

Synthetic organic dyes enter water media mainly through industrial effluents and landfills, posing a serious threat to the aquatic ecosystem. In addition, dyes have negative effects on human health, causing dermatitis, damage to internal organs, and potential reproductive toxicity [6,7]. Consequently, the development of simple, cost-effective wastewater treatment methods that do not require expensive equipment remains a highly relevant task for material science and environmental engineering.

Carbon-containing materials have long been widely applied as efficient sorbents [8]. Activated carbon is considered a standard due to its high specific surface area, which largely determines its adsorption performance [9]. However, activated carbon has some disadvantages, including limited selectivity and difficulties in regeneration and recycling [10].

The use of graphene-based materials (carbon nanotubes, graphene oxide, reduced graphene oxide, etc.) allows for enhancing the adsorption characteristics of next-generation sorbents [11,12]. According to the literature [13,14], graphene oxide (GO) is particularly attractive for adsorption processes, as it represents a planar graphene surface decorated with a large number of oxygen-containing functional groups. Due to its negatively charged surface, GO exhibits excellent adsorption efficiency towards cationic dyes, such as methylene blue (MB) [15,16]. At the same time, GO is highly hydrophilic in aqueous or alcoholic media, which complicates its separation from purified solution and requires additional stages such as centrifugation or filtration.

Immobilization of GO on a solid support can help to solve the problem of sorbent recovery, purification, and reuse [17]. The main methods for GO deposition onto support, for example, ceramic particles, include drop-casting [18], vacuum filtration [19,20], spray-coating [21], and dip-coating [22,23]. Vacuum filtration is a quite fast method and is convenient for preparing membranes based on GO and polymers [24]. However, it is not suitable for ceramic materials, which typically possess a limited number of surface functional groups, resulting in poor adhesion of GO layers to the substrate and delamination in water solution. This was shown in [25], where GO was deposited on flat ZrO₂-TiO₂ substrates and used for the anaerobic decolorization of azo dyes. Two additional drying steps at 80 °C for 24 h and at 100 °C for 72 h were required to improve adhesion between the ceramic support and GO.

θ-Al₂O₃ is particularly suitable as a substrate for the deposition of GO for several reasons: (i) it is a chemically and thermally stable material, which is critical for adsorption/desorption processes under various conditions; (ii) the spherical morphology ensures good hydraulic properties and facilitates easy separation of the composite from purified aqueous solutions; and (iii) high-purity alumina microspheres are an inexpensive and commercially available raw material suitable for the composite preparation [26,27].

Dip-coating is a more versatile and convenient approach for forming a GO layer on the support of a complex shape. For example, in [23], dip-coating was applied for GO deposition onto a mullite foam matrix, and the resulting composite completely removed MB dye with a 5 ppm concentration from water in less than 30 min.

Removal of oxygen-containing functional groups from the GO surface results in the preparation of reduced graphene oxide (RGO) [28,29]. RGO, both in pristine form [30] and as a component of composites [31,32], can be used as a sorbent for organic compounds due to its layered structure, high defect density, and residual oxygen-containing groups. Therefore, the development of new approaches to GO/RGO-based composites is of great interest for enhancing water purification technologies.

In the present study, a composite material consisting of alumina microspheres coated with GO and RGO layers was prepared. The GO coating exhibits strong adhesion to the ceramic surface and does not peel off even after treatment in supercritical isopropanol used to reduce GO to RGO. The resulting composites were applied for the adsorption of methylene blue (MB) and methyl orange (MO) from water solution, as well as from their binary mixture in water.

2. Materials and Methods

2.1. Chemicals and Materials

Reagents, including potassium permanganate (KMnO₄, CAS No. 7722-64-7), concentrated sulfuric acid (96% H₂SO₄, CAS No. 7664-93-9), hydrogen peroxide (30% H₂O₂, CAS No. 7722-84-1), hydrochloric acid (37% HCl, CAS No. 7647-01-0), isopropanol (99.9% (CH₃)₂CHOH, CAS No. 67-63-0), and sodium hydroxide (NaOH, CAS No. 1310-73-2), were obtained from Reakhim (Moscow, Russia).

Commercial θ-Al₂O₃ (Al₂O₃, CAS No. 1344-28-1) from Mikrointek LLC (Moscow, Russia) with spherical particles (fraction of 20–80 μm) was used as the composite matrix.

Natural graphite (Kahatagaha Graphite, Sri Lanka, 100 mesh fraction) (C, CAS No. 7782-42-5) was used for GO preparation.

All reagents were used without further purification.

2.2. Synthesis of Graphene Oxide (GO)

GO was synthesized by a modified Hummers’ method, as described in [33]. Natural graphite (1 g) was placed in 60 mL of concentrated sulfuric acid and stirred at 350 rpm for 30 min at 25 °C. Then, 1 g of KMnO₄ was added in four equal portions at equal time intervals, and the oxidation was continued for 24 h. The reaction mixture was then poured into ice-cold distilled water, followed by the addition of 30 mL of H₂O₂. After sedimentation, the supernatant was decanted, and the solid product was repeatedly washed with water containing 10 mL of 1 M HCl. The suspension was centrifuged at 7500 rpm until the pH reached 7. The resulting GO was transferred into Petri dishes and dried in air to a constant weight.

2.3. Synthesis of Composites Al₂O₃_GO

GO was pre-dispersed in water to obtain a dispersion of 1 mg/mL. Then, 1 g of spherical θ-Al₂O₃ was added to the GO dispersion, and the mixture was sonicated (ultrasound (US) power 1.5 W/cm³) for 1 h using an ice bath to prevent overheating. To study the activation of the θ-Al₂O₃ microsphere surface, the process was carried out under different conditions (

Table 1. Preparation conditions of composites.

Sample	Composition	Synthesis Conditions
Al2O3	θ-Al2O3	Commercial θ-Al2O3 (20–80 μm)
GO	GO	GO synthesized by the Hummers’ method
Al2O3_GO1	Al2O3 and GO	US treatment, τ = 15 min.
Al2O3_GO2	Al2O3 and GO	US treatment, τ = 1 h.
Al2O3_GO3	Al2O3 and GO	US treatment, τ = 2 h.
Al2O3_GO4	Al2O3 and GO	US treatment, τ = 1 h. HCl solution (pH = 3)
Al2O3_GO5	Al2O3 and GO	US treatment, τ = 1 h. NaOH solution (pH = 10)
Al2O3_GO6	Al2O3 and GO	Mechanical mixing of GO and θ-Al2O3, τ = 24 h.
Al2O3_RGO	Al2O3 and RGO	Treatment Al2O3_GO2 in supercritical isopropanol.

). The resulting Al₂O₃_GO composites were washed repeatedly with distilled water (at least six times) until non-precipitated GO was completely removed. The general scheme of the synthesis procedure is shown in Figure 1.

2.4. Synthesis of Composite Al₂O₃_RGO

The Al₂O₃_RGO sample was prepared using supercritical isopropanol treatment (SCI) of the Al₂O₃_GO2 sample [34]. First, 100 mg of Al₂O₃_GO2 was placed in a quartz tube containing 5.7 mL of isopropyl alcohol. Then, the tube was loaded into a steel autoclave. The autoclave was heated in an oven to 280 °C and maintained at this temperature for 24 h. After cooling to room temperature, the autoclave was opened, and the product was collected. The resulting composite Al₂O₃_GO2 was washed three times with isopropanol and acetone and then dried in an oven at 120 °C to constant weight.

2.5. Characterization Techniques

FTIR spectra were recorded on a Bruker Alpha spectrometer (Karlsruhe, Germany) with a Platinum ATR accessory (the 400–4000 cm⁻¹ range, 4 cm⁻¹ resolution). X-ray powder diffraction (XRD) studies were performed on a Bruker D8 Advance diffractometer with CuKα1 radiation (λ = 0.154056 nm) in parallel-beam geometry. The scanning electron microscopy (SEM) images were obtained on a Carl Zeiss Supra 40 microscope (Zeiss, Oberkochen,Germany) at 10 kV (secondary electrons, 30 μm aperture). Raman spectra were registered on a Raman microscopeConfotec NR500 (SOL Instruments, Minsk, Belarus) using a green laser with a wavelength of 532 nm. UV–visible spectra were recorded on an Agilent Cary 60 spectrophotometer (Agilent, Tokyo, Japan) in a standard quartz 1 cm cuvette. Thermal analysis (TA) of samples was carried out using a synchronous thermal analyzer STA 449 F3 Jupiter combined with a QMS 403C Aëolos quadrupole mass spectrometer (NETZSCH, Selb, Germany). The samples were heated at a rate of 10 °C/min to a temperature of 800 °C in a dynamic argon flow (flow rate 50 mL/min). The change in sample mass was recorded with a resolution of 2 μg.

2.6. Adsorption Experiments

To determine the adsorption efficiency, MB and MO solutions with initial dye concentrations of 2 and 5 ppm were used. The volume of all solutions was 100 mL. For each experiment, 0.01 g of composite was added, and the aliquots for UV–vis analysis were taken over 24 h. Calibration curves were constructed for each dye type to determine the residual concentration of dye in solution.

The selective sorption performance of the GO- and RGO-containing composites toward the mixture of MB and MO was then evaluated. Sorbent samples (10 mg) were added to 100 mL of aqueous solutions containing 2 and 5 ppm organic dyes and stirred for 24 h.

To evaluate the reusability of the sorbents, cyclic experiments on the adsorption of MB with an initial concentration of 2 ppm were carried out. For each adsorption cycle, the solution volume was 100 mL, the sorbent mass was 0.01 g, and the duration of each cycle was 24 h. After each cycle, the sorbent was washed with isopropanol, dried, and reused in the subsequent experiment. A total of three cycles was performed.

Additional sorption experiments for MB removal (2 ppm) were performed under conditions simulating real ones: in a 0.01 M NaCl solution, in tap water (pH = 7.52), and in an acidic solution (pH = 2) adjusted with concentrated sulfuric acid.

The equilibrium adsorption capacity (q_E) was calculated using the following Equation (1):

$$q_E={\displaystyle \frac{(c_0-c_E)\times V}{m}}$$

(1)

where c₀ and c_E are the initial and equilibrium dye concentrations, V is the solution volume, and m is the mass of the sorbent.

3. Results and Discussion

3.1. Study of the Initial GO and θ-Al₂O₃

The GO synthesized by the modified Hummers’ method, according to CHNS analysis, has the following elemental composition: 46 wt.% C, 2 wt.% H, 2 wt.% S, and 50 wt.% O. The atomic composition is identical to the literature data [14] and indicates the presence of a large number of oxygen-containing functional groups on the GO surface. This is confirmed by FTIR spectroscopy data (Figure 2a). In the FTIR spectrum of GO, a broad intense band appears in the 3000–3450 cm⁻¹ region, corresponding to the stretching vibrations of surface OH groups and adsorbed water molecules. The distinct and intense bands at 1720 cm⁻¹ and 1620 cm⁻¹ are assigned to the stretching vibrations of C=O groups and the bending vibrations of water molecules (δH-OH), respectively. Bands at 1368 cm⁻¹ and 1281 cm⁻¹ correspond to the deformation vibrations of OH groups in tertiary alcohols, whereas the bands at 1040 cm⁻¹ and 987 cm⁻¹ correspond to stretching vibrations of hydroxyl (OH-) and epoxy (C-O-C) groups. In addition, in the FTIR spectrum of GO synthesized by the Hummers’ method, characteristic bands at 1221 cm⁻¹ and 1410 cm⁻¹ are attributed to sulfate group vibrations [35].

The XRD pattern of the synthesized GO sample (Figure 2c) presents a single diffraction peak at 2θ = 11°, corresponding to the [002] plane; the graphite peak at 2θ = 26.5° is absent, indicating complete oxidation [36]. SEM images of GO powder deposited on a SiO₂ substrate are presented in Figure 3. According to the data analysis, GO exhibits a layered structure consisting of mono- and few-layer sheets with lateral dimensions ranging from tens or hundreds of nanometers to several micrometers, without visible defects. When compacted, GO layers form folds and wrinkles, which are almost absent on the smooth surface.

For the synthesis of Al₂O₃_GO composites, commercial Al₂O₃ with a particle fraction of 20–80 μm consisting of spherical particles was used; SEM images are shown in Figure 3. In the FTIR spectrum of Al₂O₃, characteristic absorption bands appear in the 400–1000 cm⁻¹ region, with distinct maxima near 441, 562, 636, 763, and 820 cm⁻¹, corresponding to Al-O bond vibrations. A weak and broadened band in the 3000–3600 cm⁻¹ region corresponds to the stretching vibrations of OH groups from surface-bound water molecules (Figure 2b). The XRD pattern of the Al₂O₃ sample corresponds to the JCPDS 35-0121 card and indicates that the material is single-phase θ-Al₂O₃ [37].

3.2. Composite Preparation and Study

The dip-coating method used for composite preparation involves immersing a ceramic substrate into an aqueous suspension, followed by withdrawal, drying, and repeating the process until a layer of the desired thickness is obtained [38]. In our study, the main objective was to understand whether, and under what conditions, the adhesion of a monolayer of GO to the surface of alumina microspheres can occur. To preliminarily activate the alumina surface, the suspension was treated with high-power ultrasound. This technique leads to cavitation [39], i.e., the formation of air bubbles in the liquid which, upon collapsing near the Al₂O₃ surface, cause surface activation. Under US treatment, GO sheets are also partially broken, generating new free bonds. To lower the free energy of the system, the deposition of GO onto the Al₂O₃ surface becomes thermodynamically favorable, resulting in composite formation. An alternative means of activating the matrix surface during GO coating deposition involves treatment with acids or alkalis.

The optimal conditions for preparing composites were determined by varying the duration of US treatment and the pH of the medium. For comparison, dry mixing of GO powder and alumina microspheres was also performed, and the obtained Al₂O₃_GO composite was treated in SCI in order to transform GO to RGO. All prepared composite samples are listed in Table 1.

Raman spectroscopy (Figure 4) and SEM (Figure 5) were employed to characterize the obtained composites. The use of XRD as well as FTIR spectroscopy is not suitable due to these methods being quantitatively insensitive to the low mass content of GO on the alumina support surface. The Raman spectra of the initial Al₂O₃ and GO are shown in Figure 4a. For commercial θ-Al₂O₃ under excitation with a 532 nm laser, characteristic modes are observed below 900 cm⁻¹, which are associated with A-O bonds. At the same time, Raman spectroscopy is the most suitable technique for investigating the structure and defect level of graphene-based materials. GO exhibits two intense bands in the Raman spectrum at approximately ~1350 cm⁻¹ and ~1580 cm⁻¹, referred to as the D and G bands, respectively [40]. The D band reflects the degree of disorder in the crystal structure and is assigned to fully symmetric vibrations of carbon atoms. The G band is observed in the Raman spectra of all carbon structures containing sp² bonds and arises from tangential in-plane stretching vibrations of carbon atoms [41].

The obtained results (Figure 4a) indicate the presence of a GO coating on all Al₂O₃_GO composite samples, regardless of the preparation conditions. In addition, characteristic θ-Al₂O₃ modes are observed for each sample, which implies the coating is formed by a thin GO layer. In the Al₂O₃_GO6 sample prepared by mechanical mixing of Al₂O₃ and GO powders, the modes associated with θ-Al₂O₃ exhibit very low intensity, which is related to the thickness of the GO platelets adhered to the substrate surface. In the Al₂O₃_RGO sample, a clear change in the shape of the G band, typical of RGO, is observed [42], along with more intense θ-Al₂O₃ modes, indicating the formation of a graphene monolayer on the alumina surface. The evolution of the D and G bands’ intensity (I_D/I_G) is presented in Figure 4b. Although the I_D/I_G is commonly used to assess the defect density of graphene-based materials, in the present study, it correlates well with the increasing degree of coverage of the alumina matrix by GO or RGO layers.

SEM images of the composites are shown in Figure 5 and Figure S1. The surface of the pure alumina exhibits some irregularities and roughness but is generally rather smooth (Figure 5a). The Al₂O₃_GO1 sample was prepared after 15 min of US treatment. GO layers only negligibly cover the surface of Al₂O₃ microspheres (Figure 5b). At the same time, it can be seen that the resulting coating consists of monolayer GO sheets, which may also adhere to each other (Figure S1a,b). Even short US treatment leads to pronounced fragmentation of GO sheets, with their lateral size in the range of 10–500 nm. Increasing the US treatment time to 1 h (sample Al₂O₃_GO2) results in the formation of a sufficiently uniform GO layer on the surface of alumina microspheres (Figure 5c). This explains the decrease in the GO sheet lateral size. Further increase in the US exposure (sample Al₂O₃_GO3) does not enhance this positive effect. The degree of substrate coverage by GO sheets increases; however, microcracks appear on the Al₂O₃ surface (Figure 5d).

During deposition in an acidic medium at pH = 3, an intensive coverage of the Al₂O₃ with GO sheets is also observed (sample Al₂O₃_GO4) (Figure 5e). However, complete and uniform coverage of the substrate still is not achieved. The opposite situation is observed in an alkaline medium at pH = 10. In Figure 5f, the surface of the Al₂O₃_GO5 is non-uniformly covered by both isolated GO monolayers and GO sheet agglomerates. This behavior is associated with the fact that in an alkaline medium, GO is partially reduced and degrades into epoxy and hydroxyl groups [43]. As a result, the partially reduced GO domains can adhere to each other, leading to the formation of agglomerates observed in the SEM images.

The Al₂O₃_GO6 sample prepared by mechanical mixing of the initial components consists of Al₂O₃ microspheres coated with multilayer GO platelets. Interestingly, even without US treatment, the lateral size of the platelets does not exceed 400 nm (Figure 5g). The interaction between the alumina microsphere surface and powdered GO occurs through electrostatic forces. However, this approach does not allow the formation of a GO monolayer coating on the Al₂O₃ surface.

According to the Raman spectroscopy and SEM data, the optimal method for composite fabrication is ultrasound treatment of GO and Al₂O₃ suspended together in a neutral medium. Based on this conclusion, the Al₂O₃_GO2 composite sample was reduced in supercritical isopropanol in order to remove oxygen-containing functional groups from the GO plane, yielding the Al₂O₃_RGO composite [44]. The resulting Al₂O₃_RGO microspheres exhibit a dark gray color, in contrast to the light brown color of the Al₂O₃_GO composites, and SEM images (Figure 5h and Figure S1c,d) show that thin graphene layers uniformly cover the surface of the substrate.

The GO content on the Al₂O₃ surface was determined by TGA-DSC analysis. The thermogravimetric curves (Figure 6a) show a mass loss in the temperature range of 50–400 °C for all samples, which corresponds to the removal of oxygen-containing functional groups and the decomposition of GO. At higher temperatures, a plateau is observed for Al₂O₃_GO1-5, whereas for the Al₂O₃_GO6 sample prepared by mechanical mixing of the components and for the Al₂O₃_RGO composite, the mass continues to decrease slightly. The Al₂O₃_RGO sample contains RGO on the microsphere surface, which is more thermally stable than GO [45]. In the Al₂O₃_GO6 sample, multilayered GO platelets are present on the alumina microsphere surface, and upon heating, a slow reduction process of these platelets is likely to occur.

According to the DSC data (Figure 6b), heating up to 400–500 °C is accompanied by exothermic effects associated with the removal of surface oxygen-containing functional groups. Endothermic effects, which are predominantly observed after heating to ≥500 °C, are related to the gradual decomposition of GO and RGO. The differences observed for the Al₂O₃_GO1 sample are due to the lower number of GO layers present on the alumina surface and their rapid decomposition upon heating.

The largest mass loss (1.59 wt.%) is observed for the composite obtained in an acidic medium. This may also be partly attributed to intercalated chlorine ions, which are removed during heating. A slightly lower mass (1.42 wt.%) is demonstrated by the sample prepared by mechanical mixing of GO with alumina; however, in this composite, a uniform GO monolayer is not formed, and GO is present on the microsphere surface in the form of multilayer platelets. The duration of US treatment indeed affects the GO content on the alumina microsphere surface, but the maximum mass loss recorded for composites obtained in a neutral medium does not exceed 1 wt.%. The minimum mass loss is recorded for the Al₂O₃_GO5 composite sample prepared in alkaline medium, which correlates with the SEM and Raman spectroscopy data. For the Al₂O₃_RGO sample, an increase in the mass loss is observed compared with the initial Al₂O₃_GO2 sample, which is associated with the formation of more thermally stable RGO.

The conducted studies have shown that US treatment of an alumina and GO suspension principally leads to the formation of a thin GO layer on the surface of the substrate. The GO content in the composite ranges from 0.6 to 1.6 wt.%. An increase in the sonication time promotes an increase in the GO coating thickness; however, it also results in severe fragmentation of GO sheets and the formation of microcracks in the Al₂O₃. An increase in the pH of the medium leads to partial GO reduction, and the coating is of poor quality. Therefore, the optimal conditions for composite preparation are US treatment for 1 h in a neutral or acidic medium.

3.3. Adsorption Experiments

For the sorption of organic dye experiments, the Al₂O₃_GO2 and Al₂O₃_RGO composites were selected as adsorbents. The tests were carried out using a dilute aqueous solution of the cationic dye (MB) and anionic dye (MO), as well as their mixture. These dyes are commonly used for dyeing denim and cotton fabrics and also find applications in analytical chemistry and pharmaceuticals; their uncontrolled release into industrial wastewater can have a detrimental impact on aquatic ecosystems [46]. The use of GO as an efficient sorbent for synthetic dyes has already been demonstrated in several studies [14,16], while its deposition on an inert alumina support can provide a more convenient way to introduce and remove the sorbent in a water treatment system. Figure S2 (in the Supplementary File) shows images of GO, Al₂O₃_GO, and Al₂O₃_RGO sorbents dispersed in water. It is obvious that, despite the high adsorption capacity of GO, there are certain limitations associated with its use in pristine powdered form for water purification. In contrast, the Al₂O₃_GO and Al₂O₃_RGO composites immediately settle to the bottom after the sorption processes, which enables straightforward separation of the purified solution without additional steps such as centrifugation or filtration.

To plot calibration curves (Figure 7), a series of diluted MB and MO solutions with different concentrations was prepared. UV–visible spectroscopy was employed, since MB and MO exhibit a distinct adsorption maximum at λ = 665 nm and 464 nm, respectively. The calibration curves were used to determine the dye concentration in solution during sorption tests of the Al₂O₃_GO and Al₂O₃_RGO composites.

The volume of dye solutions used was 100 mL, with dye concentrations of 2 ppm and 5 ppm; the total sorbent (composite) mass was 10 mg, and the mass of GO in the sorbent was 70–100 μg. The corresponding UV–visible spectra are shown in Figure 8. As shown in Figure 8a,b, pure alumina microspheres do not exhibit any noticeable adsorption activity toward the selected dyes, as the recorded UV–visible spectra almost coincide with the initial ones. GO, due to the large number of oxygen-containing surface groups, is an excellent sorbent for MB, while pristine RGO is also capable of adsorbing the MB (Figure 8a) with a removal efficiency of approximately 72%. The GO and RGO, consistent with literature data [47,48], possess a negatively charged surface and a large number of oxygen-containing functional groups (residual functional groups in RGO). The high adsorption efficiency toward cationic MB is attributed to electrostatic interactions between the surface functional groups of GO/RGO and the positively charged dye molecules, as well as π-π stacking. These properties are retained upon deposition of GO and RGO onto the surface of Al₂O₃ microspheres. In the Al₂O₃_GO2 composite, the removal efficiency reaches ~94%, and for the Al₂O₃_RGO composite, it increases to ~85%, which can be explained by the participation of an extended graphene monolayer on the substrate in the adsorption process, whereas compacted RGO is quite defective and has a significantly smaller surface area. In highly diluted dye solutions, RGO exhibits a removal efficiency of MB that is higher than that of GO. The removal efficiency of the composites toward the MB (2 ppm) dye during three adsorption cycles is presented in Figure S3. It can be seen that after the third cycle, the performance of both Al₂O₃_GO and Al₂O₃_RGO remains above 60%. SEM images of composites after three adsorption cycles, presented in Figure S4, demonstrate the strong adhesion of the graphene-based materials to the alumina substrate, as the GO and RGO layers are clearly visible on the Al₂O₃ surface.

To evaluate the potential for practical application, three additional sorption experiments for MB removal were carried out under conditions simulating real systems: in a 0.01 M NaCl solution, in tap water (pH = 7.52), and in an acidic solution with pH = 2. The corresponding results are shown in Figure S5. Interestingly, in all series of experiments, the adsorption performance of GO and RGO is essentially identical. The composites adsorb MB dye, exhibiting a removal efficiency of ~50% in tap water and acidic media, whereas in a 0.01 M NaCl solution, the removal efficiency reaches about 70%.

In the literature, it has been reported that GO or RGO are not capable of adsorbing MO quantitatively [48], but this effect does not appear for a highly diluted solution. As demonstrated by the present experiments, both GO and RGO are able to remove MO to some extent from a solution with a concentration of 5 ppm (Figure 8b). The interaction with MO occurs via π-π stacking between the aromatic system of the dye molecules and the graphene domains with sp² hybridization in GO and RGO [49].

In adsorption experiments with mixed-dye solution (Figure 8c,d), both composites exhibit selective uptake of the cationic dye. A decrease in the absorption maxima of MB is observed in UV–visible spectra for solutions with dye concentrations of 2 ppm and 5 ppm, whereas the adsorption of the anionic MO is not detected, indicating that the electrostatic interaction of oxygen-containing functional groups of GO/RGO with cationic dye molecules is the dominant process. Interestingly, MB adsorption from the mixed solution at lower dye concentration (2 ppm) proceeds more efficiently on the Al₂O₃_RGO composite than on Al₂O₃_GO. This can be explained by the fact that the monolayer RGO on the alumina microsphere surface is more accessible sterically for interaction with MB molecules than the GO, which is less planar due to the presence of surface functional groups.

4. Conclusions

In the present study, composite samples consisting of alumina microspheres coated with a graphene oxide layer were prepared. The optimal conditions for composite fabrication were determined as 1 h of ultrasonic treatment in acidic or neutral media, which allows coating of the alumina surface of mono- and few-layer GO. In contrast, longer US treatment or the use of alkaline media leads to partial reduction of GO, severe fragmentation of GO sheets, and the appearance of microcracks on the alumina surface. The composites were characterized by Raman spectroscopy, TGA/DSC, and SEM. Thermal analysis showed that the GO content in the composites ranged from 0.6 to 1.6 wt.%. Treatment of the Al₂O₃_GO composite in supercritical isopropanol resulted in the reduction of GO to reduced graphene oxide on the microsphere surface, as confirmed by Raman spectroscopy and TGA.

The Al₂O₃_GO and Al₂O₃_RGO composites were evaluated in model adsorption experiments with methylene blue and methyl orange in aqueous solutions. Both composites exhibited high adsorption efficiency toward the cationic dye MB, partial adsorption of MO, and selective removal of MB from a binary MB/MO solution with dye concentrations of 2 ppm and 5 ppm. These findings demonstrate the potential of Al₂O₃ microspheres coated with GO/RGO as convenient and efficient sorbents for the selective removal of cationic dyes from industrial wastewater.