Novel floating ZnO/alumina foam photocatalyst: a realistic approach for sustainable dye degradation in wastewater

Abstract

In the present investigation, a highly porous pure α-Al₂O₃ foam (92% porosity) was fabricated using a thermo-foaming method. Later, zinc oxide (ZnO) nanorods were grown on the α-Al₂O₃ foam by a solvothermal method to produce ZnO nanorod-decorated alumina foam (ZnO–Al₂O₃). The morphologies and crystallographic orientations of pure α-Al₂O₃ and ZnO–Al₂O₃ foams were verified using Field Emission Scanning Electron Microscopy (FESEM) and X-ray diffraction (XRD), respectively. The photocatalytic degradation of methylene blue (MB) was evaluated using pristine Al₂O₃ foam and ZnO–Al₂O₃ foam under optimized conditions. ZnO–Al₂O₃ foam exhibited a degradation efficiency of approximately 57%, which was significantly higher than the ∼27% achieved by the pure Al₂O₃ foam. The synergistic interaction between ZnO's photocatalytic activity and the structural support provided by the alumina scaffold plays an important role in the degradation of the dye. The ZnO–Al₂O₃ foam utilizes both adsorption and photocatalytic degradation, whereas the Al₂O₃ foam removes dyes only through physical adsorption. The ZnO–Al₂O₃ foam provides an effective photocatalyst and a real-life solution for mitigating water pollution.

Floatable ZnO–Al₂O₃ foam for degradation of methylene blue. The image was enhanced using Microsoft Copilot AI tool.

1. Introduction

Water pollution is one of the most serious environmental challenges as it affects human health. Every year, millions of people die due to the consumption of contaminated water. In India, industries such as textiles, cosmetics, paints, plastics, pharmaceuticals, paper, and petrochemicals discharge approximately 300–400 million tonnes of pollutants into water bodies annually, significantly contributing to the pollution of rivers and groundwater.^1–3

Among all these pollutants, synthetic dyes stand out due to their high toxicity and poor degradability. These organic dyes are widely used in textile manufacturing, printing, and the food industry.^4,5 Methylene blue (MB), a phenothiazine derivative, is commonly used in the textile industry and was selected as a representative example. It is highly toxic and a known carcinogen.⁶

Conventional treatment methods, such as adsorption and ozonation, have been widely used to remove hazardous dyes. However, due to their inherent limitations, these pollutants often remain in the treated wastewater.⁷ Because of this, there is an urgent need to develop effective and practical methods to treat dye-contaminated wastewater before it enters our natural water bodies. Several techniques have been developed to address this issue, including physical, biological, chemical, electrochemical, and photocatalytic methods. Among these, photocatalysis has emerged as one of the most promising and effective approaches for removing organic pollutants from wastewater.

In recent years, ZnO has gained significant attention as a semiconductor photocatalyst, primarily owing to its suitable band gap, which enables efficient light absorption and subsequent photocatalytic activity.⁸ In addition to its photocatalytic applications, ZnO is also valued for its use in transparent thin-film transistors, transducers, light-harvesting devices, and various other optical and electronic devices.⁸ Researchers have extensively evaluated the degradation efficiency of pure ZnO and composites of ZnO for the removal of various organic pollutants under different experimental conditions, including varying temperature, pH, and irradiation time.

For a semiconductor to function as an effective photocatalyst, it should possess several key attributes, such as a suitable band gap and appropriate energy band positions,⁹ high photostability, a low electron–hole recombination rate, low cost, and non-toxicity. Over the years, a wide range of semiconductors, including TiO₂, ZnO, Fe₂O₃, ZnS, SnO₂, WO₃, CdS, CdSe, CuO, Nb₂O₅, SrTiO₃, and Sn₃O₄ (ref. 10–18), have been investigated for photocatalytic applications. Several comprehensive reviews have also been published on their use in environmental remediation.^19–21 Photocatalytic reactions rely heavily on the generation and separation of electron–hole (e⁻–h⁺) pairs, which are initiated by radiation in the presence of catalysts and play a crucial role in reduction and oxidation reactions.²²

Despite the wide range of strategies developed to improve photocatalytic performance, including band gap engineering, construction of semiconductor composites, metal and non-metal doping, morphological control at the nanoscale, and deliberate introduction of lattice defects, efficient separation and utilization of photogenerated charge carriers still remain two of the most persistent and formidable challenges in the field. Among various semiconductor photocatalysts, ZnO is highly attractive due to its low cost, non-toxicity, excellent chemical stability, and superior redox capabilities. Crucially, ZnO possesses a low-lying conduction band bottom and a deep valence band top, providing strong thermodynamic driving forces for redox reactions. The deep valence band edge facilitates the direct oxidation of water/hydroxyl ions into highly reactive hydroxyl radicals (˙OH), while the conduction band edge enables the efficient reduction of dissolved oxygen to superoxide radicals (˙O₂⁻). These favorable band positions make ZnO exceptionally potent for generating the free radicals necessary for pollutant degradation. However, its wide band gap (∼3.2 eV) limits photoexcitation to the ultraviolet region, which accounts for only ∼4–5% of the solar spectrum. Additionally, pristine ZnO nanoparticles suffer from rapid electron–hole recombination and photocorrosion under prolonged light irradiation, resulting in low quantum efficiency. Furthermore, when ZnO is used in slurry form, it poses serious practical challenges, including difficulty in recovery and separation from treated water, potential aggregation, and secondary pollution risks, which motivate the development of floating foam architectures.^23–25

To overcome these limitations, immobilizing nanostructured ZnO onto a highly porous, inert support such as alumina (Al₂O₃) foam offers a promising strategy. Alumina foam provides a three-dimensional, interconnected macroporous structure with high surface area, excellent mechanical strength, superior thermal and chemical stability, and good adsorption capacity, thereby enhancing pollutant–catalyst contact and facilitating efficient light penetration and mass transfer. The floatable ZnO-decorated alumina foam offers several key advantages in the photocatalytic degradation of methylene blue dye, a common organic pollutant in wastewater. Its buoyant design allows the material to remain at the water surface, maximizing exposure to natural sunlight and facilitating efficient light harvesting while enhancing oxygen accessibility at the air–water interface, which promotes the generation of reactive oxygen species for faster MB breakdown. This combination makes the material particularly suitable for scalable, eco-friendly water treatment applications under ambient conditions.

In this research, we report for the first time the development of a floatable ZnO nanorod decorated alumina foam as a noble photocatalyst for wastewater treatment. The work focuses on synthesizing highly porous alumina foam via a thermo-foaming method and subsequently growing ZnO nanorods via the seed method on the Al₂O₃ surface, followed by a solvothermal process to form ZnO–Al₂O₃ foam. The buoyant nature of the foam allows it to float on the water surface, improving light absorption and oxygen availability and thereby enhancing photocatalytic activity. The photocatalytic performance was evaluated by studying the degradation of methylene blue under UV irradiation. We also investigated the reaction kinetics, reusability, and long-term stability of the photocatalyst. This unique floatable ZnO–Al₂O₃ foam offers a practical, cost-effective, eco-friendly, and scalable solution for the efficient treatment of dye contaminated wastewater.

2. Experimental section

2.1. Materials used

α-Al₂O₃ powder of >99.5% purity, with an average particle size of 0.1 µm, was obtained from Rohini Industries, Pune, India. Zinc acetate dihydrate, zinc nitrate hexahydrate, hexamethylenetetramine (HMTA), sucrose, and ethyl alcohol were purchased from Merck Chemicals, India and used without any purification.

2.2. Synthesis of alumina foam

The precursor mixture was prepared by mixing α-Al₂O₃ powder with sucrose at a fixed weight ratio of 1.2 : 1 (w/w) in acetone as the dispersing medium. The mixing was carried out in a ball mill for 5 hours using zirconium balls for grinding, resulting in a uniformly dispersed suspension. After milling, the resulting slurry containing α-Al₂O₃ and sucrose was transferred into a two litre glass tray. The glass tray was then placed in a hot-air oven and heated to 185 °C to melt the sucrose. Once the slurry had melted, it was manually stirred with a glass rod to evenly distribute the α-Al₂O₃ particles throughout the molten sucrose. The stirring step was particularly important for achieving a homogeneous mixture and promoting a uniform pore structure in the final product. The Al₂O₃ dispersed molten sucrose was subsequently heated to 180 °C, enabling foaming and setting of the mixture. These green foam samples were sintered at 1650 °C to achieve full density. The complete fabrication of alumina foam is discussed in our previous work and shown in Fig. 1.²⁶

Fig. 1. Schematic of the fabrication of Al₂O₃ foam and ZnO nanorods on Al₂O₃ foam.

The sucrose template (used for pore formation) undergoes complete combustion during the high-temperature sintering at 1650 °C in ambient air. Sucrose decomposition occurs in several stages: (i) caramelization and dehydration (150–300 °C), (ii) carbonization (300–500 °C), and (iii) complete oxidation of carbon to CO₂ (500–800 °C). The final sintering at 1650 °C under atmospheric conditions ensures that any residual carbon is fully oxidized and removed as CO₂.

2.3. Synthesis of ZnO nanorods on the α-Al₂O₃ foam surface

Firstly, zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) was dissolved in ethanol to prepare an 8 mM solution. Then, α-Al₂O₃ foam was immersed in this solution for 5 minutes to coat it, and the foam was annealed at 300 °C for 2 hours in ambient air to grow a ZnO seed layer on the alumina foam surface. In another beaker, 50 mL of 20 mM zinc nitrate hexahydrate solution was prepared in DI water, and 50 mL of 20 mM HMTA (C₆H₁₂N₄) solution was added under continuous stirring for 10 minutes to obtain a homogeneous solution. After that, the solution was placed on a hot plate. The α-Al₂O₃ foam was then immersed in the solution and continuously heated at 150 °C for 4 hours to grow nanorods on the foam. During the ZnO nanorod growth using C₆H₁₂N₄ (HMTA) at 150 °C, HMTA serves as a structure-directing agent and does not incorporate carbon into the ZnO lattice. At the growth temperature (150 °C), HMTA gradually decomposes, forming formaldehyde and ammonia, which are volatile and escape from the system. After the reaction, the samples were removed from the solution and dried at 100 °C, as shown in Fig. 1.²⁷

2.4. Photocatalytic testing

For the photocatalytic testing of the synthesized foam, a photochemical reactor (City Instruments, India) equipped with a xenon lamp (power: 250 W, wavelength: 300–780 nm, intensity: 2000 Lux) was used. Methylene blue (MB) solution was prepared by dissolving the required quantity of dye in deionized water of Milli-Q quality. The Al₂O₃ foam was immersed in 100 mL of an aqueous MB solution at a concentration of 10 mg L⁻¹. Prior to irradiation, the systems were magnetically stirred in the dark (“dark adsorption” experiment) for 30 min to ensure MB equilibrium on the foam surface. During the experiment, the MB aqueous solution was continuously stirred with a magnetic stirrer, and the distance between the light-emitting diode (LED) lamp with a radiation peak at 365 nm and the MB aqueous solution was 20 cm. The photocatalytic activity of Al₂O₃ foam was compared with that of ZnO–Al₂O₃ foam. The suspended foam used for the experiment was removed after the reaction, and the MB concentration in the solution was analysed using a UV-vis spectrophotometer. All experiments were done in triplicate, with errors below 3%.

2.5. Characterization of materials

Phase characterization of the fabricated Al₂O₃ foam was performed using an X-ray diffractometer in θ–2θ mode (D8 Advance Bruker diffractometer with a Cu Kα source, λ = 1.54 Å) at a specified scan rate of 2° per minute. The microstructural features, including Al₂O₃ foam and ZnO–Al₂O₃ foam pore morphology, size distribution, and strut thickness, were examined using a field emission scanning electron microscope (FE-SEM, JSM-7610FPlus, JEOL, Japan) coupled with an Energy Dispersive Spectroscopy (EDS) attachment. The sintered density of the foam was determined by measuring its weight and volume after sintering, and the porosity was estimated by comparing the measured foam density to the material's theoretical density. Raman spectroscopy measurements of the samples were performed using a Renishaw microscope with 514 nm He–Ne laser excitation.

3. Results and discussion

The X-ray diffraction (XRD) analysis of the α-Al₂O₃ foam and the ZnO–Al₂O₃ foam is presented in Fig. 2a. The diffraction pattern confirmed the presence of a single α-Al₂O₃ phase in the alumina foam, with all peaks matching the standard pattern (JCPDS # 46-1212). In the case of ZnO–Al₂O₃, diffraction peaks matching the hexagonal wurtzite structure of ZnO were present and indexed according to the JCPDS standard (#01-070-8070). The provided Raman spectrum compares the α-Al₂O₃ foam with ZnO nanorods grown on α-Al₂O₃, as shown in Fig. 2b. α-Al₂O₃ shows its characteristic vibrational bands, with the most prominent peak near ∼418 cm⁻¹ (A_1g mode) and smaller peaks in the lower wavenumber region (E_g mode), typical of sapphire/alumina's corundum structure. After ZnO nanorods are deposited, the intensity of Raman peaks changes, and additional ZnO-related modes become prominent. The peaks labelled E_g (∼330–340 cm⁻¹) and E₂ (∼437 cm⁻¹) correspond to the wurtzite ZnO lattice vibrations. The A_1g peak near ∼418 cm⁻¹ is prominent because both ZnO and α-Al₂O₃ contribute to it, indicating that substrate and nanorod signals overlap. The E₂ mode is particularly important because it indicates high crystallinity and low structural defect in ZnO. Overall, the spectrum confirms that ZnO nanorods have been successfully grown on the alumina foam, as evidenced by the appearance of ZnO specific vibrational modes superimposed on the α-Al₂O₃ substrate peaks. The intense E₂ mode in the ZnO–Al₂O₃ foam reflects the contribution from crystalline ZnO nanorods.²⁸

Fig. 2. (a) XRD patterns and (b) Raman spectra of α-Al₂O₃ foam and ZnO–Al₂O₃ foam.

The porosity (P) of the Al₂O₃ foam was determined using the relative density method, based on the bulk and theoretical densities of dense alumina. The porosity was calculated using the following equations and was found to be ∼92% (Table 1).ρ_b = bulk density, ρ_t = theoretical density of dense alumina (3.98 g cm⁻³).

Table 1. Density measurement of Al₂O₃ foam.

Sample	Mass (g)	Volume (cm3)	Bulk density (g cm−3)	Relative density	Porosity (%)
1	2.560	8.00	0.320	0.0804	91.96
2	2.552	8.00	0.319	0.0802	91.98
3	2.552	8.00	0.319	0.0802	91.98

The floatability of our material originates entirely from the low bulk density and highly porous architecture of the Al₂O₃ substrate, which keeps the composite buoyant even when fully saturated with water. Because the ZnO–Al₂O₃ foam is hydrophilic and wettable, the aqueous Methylene Blue (MB) solution easily penetrates the interconnected porous network via capillary action. This ensures direct contact between the MB molecules and the ZnO catalytic sites.

The surface morphology and interfacial characteristics of the ZnO–Al₂O₃ nanorods were examined using field emission scanning electron microscopy (FE-SEM), as illustrated in Fig. 3 at various magnifications. Fig. 3a shows the Al₂O₃ foam with uniformly distributed pores, and Fig. 3b–d shows the uniform distribution of ZnO nanorods and the distinct hexagonal cross-sections with a size of approximately 200 nm when viewed from the top, which is indicative of their crystalline wurtzite structure. The EDX spectrum shown in Fig. 4a and b shows the elemental composition of the ZnO–Al₂O₃ foam before and after photocatalytic activity. Fig. 4a confirms the presence of only zinc (Zn) and oxygen (O), and no additional elemental peaks were detected, indicating the high purity of the ZnO nanorods and no detectable impurities or contamination in their composition. Fig. 4b shows the presence of zinc (Zn) and oxygen (O) peaks, along with the MB elements (carbon (C), nitrogen (N), and sulfur (S)) on the photocatalyst surface after photocatalytic degradation of methylene blue (MB).

Fig. 3. FE-SEM images of (a) Al₂O₃ foam and (b) ZnO–Al₂O₃ foam; (c) high-magnification image of ZnO–Al₂O₃ foam and (d) dried ZnO–Al₂O₃ foam after the photocatalytic activity cycle.

Fig. 4. The EDX analysis of (a) ZnO–Al₂O₃ foam before photocatalytic activity and (b) ZnO–Al₂O₃ foam after photocatalytic activity.

The photocatalytic degradation of methylene blue (MB) was investigated using Al₂O₃ foam and ZnO–Al₂O₃ foam under optimized conditions, employing a photocatalyst dosage of 20 mg and an initial dye concentration of 10 ppm, prepared in distilled water. The reaction mixture was irradiated with a suitable light source, and aliquots were withdrawn at 20 min intervals and analyzed by UV-vis spectrophotometry. The UV-vis absorption spectra (Fig. 5a and b) show a progressive decrease in the characteristic MB absorption peak, indicating decolourization due to the breakdown of chromophoric groups, as further supported by Fig. 6a and b. The degradation kinetics (Fig. 6c) is expressed as the ratio of concentration at time t to the initial concentration (C_t/C₀) versus time. The degradation profile clearly demonstrates that the ZnO–Al₂O₃ foam exhibits significantly superior photocatalytic activity compared to the bare α-Al₂O₃ foam. Over 120 minutes, the α-Al₂O₃ foam alone shows only a modest reduction in methylene blue concentration, with C_t/C₀ decreasing from 1.00 to 0.74. In contrast, the ZnO–Al₂O₃ composite foam achieves a substantially higher degradation efficiency, reducing the relative concentration to 0.44 within the same period. The enhanced performance of the ZnO-decorated foam is evident at every measured time point; for example, after 90 minutes, C_t/C₀ values are 0.77 for α-Al₂O₃versus 0.55 for ZnO–Al₂O₃. This marked difference indicates that while α-Al₂O₃ shows only minor adsorption or photolysis, the incorporation of ZnO nanoparticles provides active photocatalytic sites that generate reactive oxygen species under light irradiation, leading to accelerated methylene blue breakdown. Overall, the plot shows that ZnO functionalization dramatically enhances degradation kinetics, making ZnO–Al₂O₃ a far more efficient photocatalyst for dye removal applications.

Fig. 5. UV-vis analysis of MB degradation: (a) photocatalytic degradation of MB using Al₂O₃ foam and (b) photocatalytic degradation of ZnO–Al₂O₃ foam.

Fig. 6. Photocatalytic degradation kinetics curves of C_t/C₀versus irradiation time curves of MB: (a) MB solution at t = 0 min, (b) MB solution at t = 120 min, (c) MB degradation with time, (d) pseudo-first-order plot [−ln(C_t/C₀)] vs. time.

This improvement is attributed to the synergistic interaction between ZnO nanorods and the porous Al₂O₃ scaffold, in which ZnO provides active photocatalytic sites, while the alumina foam offers high surface area, structural stability, and improved mass transport.

The methylene blue degradation follows pseudo-first-order kinetics as shown in Fig. 6d. The integrated rate law for a pseudo-first-order reaction can be written as:where C_t = concentration at time t, C₀ = initial concentration, and k_app = apparent pseudo-first-order rate constant (min⁻¹).

The degradation of methylene blue by both the bare α-Al₂O₃ foam and the ZnO–Al₂O₃ foam follows pseudo-first-order kinetics, as confirmed by the linear regression of versus irradiation time. The linear regression of versus time yielded excellent correlation coefficients for both materials, with R² = 0.9892 for α-Al₂O₃ and R² = 0.9935 for ZnO–Al₂O₃, confirming that the degradation kinetics follow the pseudo-first-order model. The apparent rate constant (k_app) for bare α-Al₂O₃ was determined to be 0.0026 min⁻¹, indicating a very slow reduction in dye concentration, which is primarily attributed to weak physical adsorption rather than genuine photocatalytic activity. In contrast, the ZnO–Al₂O₃ composite exhibited a significantly higher rate constant of 0.0066 min⁻¹, which is approximately 2.5 times greater than that of bare α-Al₂O₃. This substantial increase in the rate constant clearly demonstrates that incorporating ZnO nanoparticles provides highly efficient photocatalytic active sites, leading to accelerated charge-carrier separation and enhanced generation of reactive oxygen species under light irradiation. It can be concluded that both materials follow pseudo-first-order kinetics; the markedly higher rate constant for ZnO–Al₂O₃ foam confirms its superior photocatalytic performance for methylene blue degradation.

The UV-vis absorption spectra of pristine Al₂O₃ and ZnO–Al₂O₃ composite were recorded in an aqueous suspension (0.5 mg mL⁻¹) using a quartz cuvette against a water blank. The spectra were used to determine the absorption onset and estimate the band gap energy.²⁹

For indirect band gap semiconductors, the optical band gap was estimated by extrapolating the linear region of the Tauc plot of (αhν)²versus photon energy hν, where α is the measured absorption coefficient of the material. As shown in Fig. 7, pristine Al₂O₃ shows an absorption peak at around 260 nm, confirming its wide band gap (>5.0 eV). In contrast, the ZnO–Al₂O₃ composite displays two distinct peaks at approximately 270 nm and 370 nm. The corresponding Tauc plot yields a band gap of ∼3.68 eV, which is characteristic of the ZnO composite's semiconductor phase. It is important to note that the Tauc plot derived from a liquid suspension provides an approximation of the band gap due to scattering effects from the dispersed particles. Nevertheless, the clear shift in absorption onset and the significant reduction in band gap compared to pristine Al₂O₃ qualitatively explain the enhanced photocatalytic activity: the narrower band gap of the composite enables efficient generation of electron–hole pairs under UV-A/visible irradiation, which are the primary drivers of the photodegradation reactions observed in this study.

Fig. 7. UV-vis analysis of (a) pristine Al₂O₃ foam and (b) ZnO–Al₂O₃ foam (Tauc's plot in the inset).

The reusability and stability of the ZnO–Al₂O₃ foam were studied over multiple photocatalytic cycles for the degradation of MB, as shown in Fig. 8. In the first cycle, the composite foam exhibits a degradation efficiency of 57%, and notably maintains nearly consistent performance in subsequent cycles, with efficiencies of 57%, 56.8%, and 56.6% in the second, third, and fourth cycles, respectively. The minimal decline in degradation efficiency demonstrates excellent structural stability, strong adhesion of ZnO nanorods to the alumina scaffold, and resistance to catalyst leaching. The negligible loss of activity across multiple cycles demonstrates the photocatalyst's stability and reusability, highlighting its potential for practical, sustainable wastewater treatment applications.

Fig. 8. Degradation efficiency (%) vs. number of photocatalytic cycles for ZnO–Al₂O₃ foam.

The XRD pattern of the ZnO–Al₂O₃ foam after four photocatalytic cycles is shown in Fig. 9a. The XRD pattern shows no significant differences, with all characteristic peaks of ZnO (hexagonal wurtzite structure) and α-Al₂O₃ (corundum structure) well preserved after recycling. No new peaks corresponding to ZnO dissolution, phase transformation, or the formation of secondary phases were detected. This confirms the excellent structural stability of the composite.

Fig. 9. (a) XRD pattern and (b) Raman spectrum of ZnO–Al₂O₃ foam after the photocatalytic reaction.

The Raman spectrum of the ZnO–Al₂O₃ foam after recycling (Fig. 9b) shows that the characteristic ZnO E₂ mode (∼437 cm⁻¹) and the α-Al₂O₃ A_1g mode (∼418 cm⁻¹) are fully retained after four cycles. No additional peaks or significant shifts were observed, indicating that the chemical bonding and crystal structure remain intact. These analyses, along with the FE-SEM images of the ZnO–Al₂O₃ foam after MB adsorption (Fig. 3d), reveal that the ZnO nanorods maintain their original morphology and distribution on the alumina foam surface. No significant agglomeration, detachment, or morphological degradation was observed.

4. Plausible photocatalytic mechanism

Based on the well-established photocatalytic principles for ZnO semiconductors and the observed enhancement in MB degradation, a plausible photocatalytic mechanism for the ZnO–Al₂O₃ foam is proposed. Under UV irradiation (λ < 387 nm), ZnO absorbs photons and generates electron–hole pairs as follows:³⁰

ZnO + hν → e⁻(CB) + h⁺(VB) 1

The photogenerated holes react with water and hydroxyl ions to produce hydroxyl radicals, while electrons reduce dissolved oxygen to form superoxide radicals:²²

h⁺ + H₂O → ˙OH + H⁺ 2

h⁺ + OH⁻ → ˙OH 3

e⁻ + O₂ → O₂˙⁻ 4

The superoxide radicals further undergo protonation and disproportionation reactions to form hydrogen peroxide:

O₂˙⁻ + H⁺ → HO₂˙ 5

2HO₂˙ → H₂O₂ + O₂ 6

Hydrogen peroxide subsequently generates additional hydroxyl radicals through electron interaction or photolysis:²³

H₂O₂ + e⁻ → ˙OH + OH⁻ 7

H₂O₂ + hν → 2˙OH 8

These reactive oxygen species (ROS) attack the MB molecule, initially cleaving the chromophoric C–S⁺ C bond, followed by ring opening, formation of intermediate species such as Azure B, and eventual mineralization into CO₂, H₂O, SO₄²⁻, NO₃⁻, and Cl⁻, as represented by:

C₁₆H₁₈ClN₃S + 25.5O₂ → 16CO₂ + 3HNO₃ + H₂SO₄ + HCl + 8H₂O 9

This proposed mechanism is consistent with the literature on ZnO-mediated photocatalysis.²⁷ However, we acknowledge that definitive confirmation of the involvement and relative contributions of ˙OH, O₂˙⁻, and h⁺ in the degradation process requires systematic radical quenching experiments (using scavengers such as t-butyl alcohol for ˙OH, p-BZQ for O₂˙⁻, and Na₂-EDTA for h⁺) and direct detection of radical species by electron spin resonance (EPR) spectroscopy.^31,32 These mechanistic studies are planned as part of our ongoing investigation and will be reported in a subsequent communication.

It is important to emphasize that the primary contribution of the present work lies in demonstrating the practical feasibility of the floatable ZnO–Al₂O₃ foam as a recoverable, reusable, and scalable photocatalyst for wastewater treatment, rather than providing a comprehensive mechanistic investigation.

The enhanced photocatalytic activity of ZnO–Al₂O₃ foam can be attributed to the following synergistic factors:

(i) The porous alumina scaffold prevents ZnO agglomeration and provides a high surface area for dye adsorption;

(ii) The foam's three-dimensional interconnected pore network facilitates efficient mass transport and light penetration;

(iii) The floatable design maximizes light harvesting at the air–water interface;

(iv) The air–water interface enhances oxygen availability, promoting ROS generation;

(v) The strong adhesion of ZnO nanorods to the alumina scaffold ensures long-term stability and reusability;

(vi) The easy recovery of the foam eliminates the need for centrifugation or filtration, minimizing secondary pollution.

5. Conclusions

A ZnO–Al₂O₃ composite foam was successfully synthesized, and the study showed that the floatable foam degrades methylene blue effectively under UV light. ZnO–Al₂O₃ foam degraded about 57% of the dye in just 2 hours, which was nearly double that of what the pristine alumina foam could do (∼27%). The ZnO nanorods were responsible for photocatalysis, while the porous alumina scaffold provided a sturdy and high surface area support. The floatable foam remains at the air–water interface, absorbing light and allowing oxygen in, which is responsible for the formation of reactive species that drive dye degradation. The foam remained stable after several uses, with a negligible drop in activity. This kind of durability matters for real-world use of such materials. This process does not require any centrifugation or filtration to recover the photocatalyst, as the foam can be easily separated from water. The ZnO–Al₂O₃ foam provided a simple, solid, and environmentally friendly option for cleaning up wastewater containing organic dyes.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors declare that all the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format, they are available from the corresponding author upon reasonable request.