Introduction

Water resources are essential for sustaining life, ecosystems, and socio-economic development1. Rapid industrialization and population growth have exacerbated water pollution, introducing complex contaminants such as pharmaceuticals, pesticides, and synthetic dyes into aquatic systems2,3. Synthetic dyes such as azo compounds are widely used in the textile and dyeing industries but pose serious environmental concerns due to their complex molecular structures, low biodegradability, and potential toxicity to aquatic life. These compounds are difficult to remove using conventional treatments, highlighting the need for an efficient photocatalytic degradation approach4,5,6,7. Conventional water treatment methods, including filtration and chlorination, are often inadequate for removing these persistent pollutants, necessitating the development of advanced and efficient purification strategies8. Advanced oxidation processes (AOPs) have gained prominence due to their ability to degrade recalcitrant organic pollutants through the generation of highly reactive species, such as hydroxyl radicals9. Photocatalysis, a key AOP, utilizes light-activated catalysts to achieve effective pollutant degradation10. However, challenges such as inefficient light utilization, catalyst instability, and limited scalability hinder its practical implementation. Photoreactors play a vital role in addressing these limitations by optimizing light distribution and maximizing catalyst-pollutant interaction9,11.

The stabilization of photocatalysts is critical to improve their durability and performance in AOPs12. Traditional stabilization methods, including suspension or immobilization on rigid supports, often face issues such as catalyst detachment, light penetration inefficiencies, and deactivation over time13,14,15. In contrast, silicone adhesive-based stabilization offers a simpler and more effective alternative, ensuring strong adhesion, enhanced mechanical stability, and resistance to harsh environmental conditions while maintaining efficient UV light penetration16,17,18. Our research team has previously contributed on to the development of efficient photocatalytic systems and reactor designs aimed at pollutant removal from aqueous environments. In our earlier studies, we successfully demonstrated the degradation of various organic contaminants using innovative reactor configurations and modified photocatalysts19,20,21. Recent studies on nanocomposite photocatalysts have demonstrated significant advances in pollutant degradation and antibacterial performance. Hybrid structures such as TiO2/ZnO/rGO, carbon-supported TiO2/ZnO, and Ag–TiO2 combined with micro-nanobubbles have shown enhanced performance due to synergistic effects, improved charge separation, and higher surface area. These systems exhibit excellent photocatalytic activity under UV and visible light, making them promising candidates for advanced wastewater treatment22,23,24,25,26,27. This study explores a hybrid TiO2-clay photocatalyst system, leveraging the natural adsorptive properties of clay and the high reactivity of titanium dioxide under UV light28. Clay acts as a supportive matrix, preventing TiO2 aggregation and providing a cost-effective solution for large-scale applications29. The combination of adsorption and photocatalysis enhances pollutant removal efficiency, making this system ideal for addressing complex water contaminants30,31. To further optimize photocatalytic degradation, a novel rotating photoreactor was designed to improve UV light penetration by creating a thin water layer over the photocatalyst surface. Factors such as pollutant concentration, light source positioning, and rotation speed were systematically investigated. The system’s performance was evaluated using BR 46 as a model pollutant, with both experimental and DFT-based theoretical approaches applied to elucidate degradation mechanisms32,33,34. The results demonstrate a highly complex degradation pathway for BR 46, driven by hydroxyl radicals. The proposed mechanism, supported by DFT calculations, highlights the critical role of reactive species in breaking down dye molecules into smaller, less harmful fragments. This innovative system achieved 92% TOC removal after 90 min of UV irradiation, showcasing its potential as a high-performance solution for industrial wastewater treatment35,36,37. In contrast to conventional systems, this study introduces a rotary photoreactor combined with a flexible TiO2–clay composite bed that enhances both UV light penetration and mass transfer. The novelty lies in the effective immobilization method using silicone adhesive, the integration of a thin water film over a rotating photocatalytic surface, and the use of DFT simulations to elucidate the degradation mechanism of BR46. Unlike previous studies that often relied solely on either theoretical or experimental approaches, this work presents a synergistic combination of both, yielding deeper insights into photocatalytic behavior and dye degradation pathways. Despite the relatively narrow spectral range of UV light in solar radiation (~ 4%), it was selected in this study due to its high quantum efficiency and strong photocatalytic activation of TiO2. UV sources provide consistent irradiation with sufficient energy to excite electrons from the valence band to the conduction band of TiO2, leading to the generation of reactive oxygen species (ROS)38. This makes UV-based systems highly effective for initial performance assessment of photocatalysts, especially under controlled laboratory conditions.

The photocatalytic mechanism begins with the absorption of UV light by TiO2, resulting in the formation of electron-hole (e–/h+) pairs. These charge carriers participate in redox reactions: photogenerated holes oxidize water molecules or hydroxide ions to produce hydroxyl radicals (OH·), while electrons reduce the molecular oxygen to generate superoxide radicals (O2–·). These reactive species are responsible for attacking and degrading the dye molecules into smaller, less toxic compounds. This fundamental mechanism underpins the observed degradation of BR 46 in our system and aligns with both experimental observations and DFT-predicted pathways39,40,41.

Experimental details

Materials

Titanium dioxide TiO2-P25 was procured from Degussa (Germany). Industrial clay was utilized to produce clay powder. Silicone adhesive for bed assembly was acquired from Razi (Iran). The model organic pollutant, Basic Red 46 (BR46, C18H21BrN6), was acquired from Alvan Sabet (Iran). All chemicals are of analytical grade, indicating a purity of ≥ 99.0%.

Preparation and immobilization of TiO2-clay nanocomposite

To synthesize the TiO2-clay nanocomposite for photocatalytic immobilization, 0.3 g of clay and 0.7 g of titanium dioxide were meticulously combined in a beaker. A volume of 5–10 mL of distilled water was added to the mixture, and the solution was agitated continuously with a magnetic stirrer for 4 h at ambient temperature. Following the stirring process, the resulting mixture was placed in an oven and heated at 60 °C for 6 h. The dried product was then ground into a fine powder using a mortar and pestle42. For immobilization, flexible plastic (talc) substrates, measuring 17 cm × 35 cm, were prepared as the primary surface for the reactor’s bed. A thin layer of silicone adhesive was applied to the surface of the substrates. The powdered TiO2-clay composite was uniformly applied to the adhesive-coated substrate utilizing a sieve. The coated substrates were allowed to dry at ambient temperature for 24 h. A diagram of the manufacturing process of the TiO2–clay photocatalytic immobilized bed is shown in Fig. 1a.

Fig. 1
figure 1

(a) Diagram of the manufacturing process of the TiO2-clay photocatalytic immobilized bed, (b) Schematic of the rotary photoreactor with immobilized photocatalyst, (c) The light source Position inside the photoreactor.

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Design and manufacturing of the rotary photoreactor

A rotary photoreactor was designed and constructed for the implementation of the present study. The device comprises a water tank with a capacity of approximately 500 mL, an electric motor to drive the rotation of the cylinder, a PVC cylinder with a length of 17 cm and a diameter of 11 cm, a quartz cylindrical tube serving as a lamp protector, and an ultraviolet (UV) light source positioned within the quartz tube. The quartz tube was chosen for its ability to transmit UV light. The radiation source utilized in this system is an 8-watt UV-C lamp. A schematic representation of the photoreactor is provided in Fig. 1b. Key features of this photoreactor include the adjustable rotational speed of the cylinder, the placement of the photocatalyst-coated sheet inside the rotating cylinder, and the ability to modify the positioning of the UV lamp within the reactor. During operation, the contaminated solution comes into contact with the rotating cylinder and the immobilized photocatalyst. In this study, the parameters of engine speed, lamp positioning, and pollutant concentration were systematically investigated using the rotary photoreactor.

Characterization

Characterization of the synthesized photocatalysts was performed using various analytical techniques. The crystallinity and phase structure were analyzed by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation (λ = 1.5418 Å). The microscopic morphology, chemical composition, and elemental distribution were examined using field emission scanning electron microscopy (FE-SEM KYKY, model EM8000F, China). The UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained using a UV–vis spectrometer (Specord 210 Plus, Analytikjena, Germany), and UV–vis absorbance spectra were recorded using a UV–vis spectrophotometer (Shimadzu UV-DR 2800).

A Belsorp Mini II analyzer (Microtrac Bel Corp, Japan) was employed to determine the Brunauer-Emmett-Teller (BET) surface area through N2 adsorption-desorption isotherm measurements for the synthesized photocatalysts.

The effectiveness of dye removal was evaluated using a TOC analyzer (Shimadzu TOC-5000). The hydrophilicity of the modified bed surfaces was assessed through contact angle measurements (JC2000D2M Contact Angle Meter), with each sample being measured in triplicate to obtain average values.

In order to evaluate the degradation byproducts of the dye, gas chromatography-mass spectrometry (GC-MS, Agilent 7890B GC and Agilent 5977 A MS, Agilent Technologies, USA) was employed. Helium with a purity of 99.999% was used as the carrier gas. The injector temperature was set to 280 °C with an injection volume of 1 µL. The column, with dimensions of 60 m × 0.25 mm × 0.25 μm, was programmed with an initial temperature of 38 °C, held for 7 min, followed by a temperature increase to 290 °C at a heating rate of 13 °C min-1, and maintained for 10 min. The procedure was carried out as follows: a BR46 solution (20 ppm, 500 mL) was exposed to UV radiation for 90 min. After the treatment, residual BR46 was extracted three times using 12 mL of dichloromethane. The final extracts were analyzed with a GC-MS instrument, respectively.

Analysis of the organic dyes

The photocatalytic activity of the TiO2-clay nanocomposite was evaluated through the degradation of BR46 dye under UV irradiation. Before photocatalytic testing, the adsorption/desorption equilibrium of the BR46 molecules on the surface of the immobilized photocatalyst was established by stirring the dye solution in the dark for 3 h. Following equilibrium, the dye solution was continuously stirred under UV light exposure. Photocatalytic experiments were performed using an 8-W UV lamp as the radiation source, with all reactions conducted at room temperature.

The absorption spectrum of the BR46 dye was recorded using a UV–Vis spectrophotometer over the wavelength range of 200–800 nm to determine the maximum absorption wavelength (λmax), which was identified as 532 nm for BR46. The dye removal efficiency was calculated using the following equation:

$${\text{Removal }}\left( \% \right){\text{ }}={\text{ }}\left( {{\text{1}}\, - \,{\text{C}}/{{\text{C}}_0}} \right){\text{ }} \times {\text{ 1}}00$$

where C0 and C represent the initial and final concentrations of the dye, respectively.

Kinetic modeling of BR46 photodegradation

After a 30-min dark-adsorption period, the initial concentration C0 was defined at t = 0 (start of irradiation). Apparent pseudo-first-order (PFO) kinetics were evaluated by plotting ln(C0/Ct) versus time (t), where the slope of the linear fit gives the apparent rate constant k (min–1). When UV–Vis absorbance was used as a proxy for concentration, Beer–Lambert proportionality (A ∝ C) was assumed at the dye’s absorption maximum (λmax = 532 nm) and ln(A0/At) was plotted equivalently. Linear least-squares regressions yielded k, the coefficient of determination (R2), and 95% confidence intervals. Where appropriate, trends were interpreted within the Langmuir–Hinshelwood (L–H) framework, noting that at low concentrations L–H kinetics reduce to PFO behavior.

C0 = initial concentration after dark adsorption (mg L–1); Ct = instantaneous concentration (mg L–1); t = time (min); k = apparent PFO rate constant (min–1); A0, At = UV–Vis absorbance at λmax; R2 = coefficient of determination. Unless otherwise stated, concentrations are reported in mg L–1 (≈ ppm for dilute aqueous solutions).

Results and discussion

Characterization of the TiO2-Clay photocatalyst

X-ray diffraction

The chemical composition and crystalline structure of the clay, TiO2, and TiO2–clay nanocomposite were analyzed using XRD, as shown in Fig. 2a. The XRD pattern of the clay shows distinct peaks attributed primarily to pyrophyllite (Al2(Si4O10)(OH)2), labeled as “Py,” with additional reflections corresponding to quartz impurities. These observations are consistent with previously reported data in the literature43,44.

Fig. 2
figure 2

(a) XRD of the clay, TiO2, and TiO2-clay, (b) The adsorption-desorption isotherms of the prepared samples, (c) BJH pore size distribution curves of the prepared samples, (d) FTIR spectra of pure TiO2 and TiO2–clay composite in the range of 4000–500 cm–1. (e) UV − vis absorption spectra, (f) plots of (αhv)1/2 vs. hv.

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The pure TiO2 sample displays characteristic diffraction peaks at 2θ values of approximately 25.5°, 27°, 37.8°, 48.2°, 54°, 55.1°, and 62.7°, which can be indexed to the (101), (110), (004), (200), (105), (211), and (204) planes of anatase and rutile phases. These peaks match the standard JCPDS cards for anatase (No. 21-1272) and rutile (No. 76–0319), confirming the biphasic crystalline nature of the synthesized TiO245,46,47.

In the XRD pattern of the TiO2–clay composite, peaks corresponding to both clay and TiO2 are still evident but with reduced intensity, likely due to the dilution of each phase in the hybrid material. Importantly, no significant peak shifts were observed, suggesting the preservation of the crystalline integrity of both the TiO2 and clay components after composite formation. However, the broadening of some peaks, particularly around the anatase (101) plane, may indicate nanoscale crystallinity or partial dispersion effects due to the clay matrix.

To further quantify the crystalline structure, the average crystallite size of pure TiO2 and the TiO2–clay composite was calculated using the Scherrer equation:

$$\frac{{K\lambda }}{{\beta cos\theta }}=D$$

where D is the crystallite size, K is the shape factor (taken as 0.9), λ is the X-ray wavelength (1.5406 Å for Cu Kα radiation), β is the full width at half maximum (FWHM) of the most intense diffraction peak in radians, and θ is the Bragg angle.

Using this method, the average crystallite size of pure TiO2 was found to be approximately 22.8 nm, while that of the TiO2–clay composite was slightly smaller, at 20.9 nm. This reduction in size implies that the presence of the clay matrix may act as a physical barrier, limiting TiO2 grain growth. Such confinement is beneficial, as it can enhance photocatalytic performance by increasing surface area and improving nanoparticle dispersion.

In addition, the intensity ratio between the anatase (101) peak at ~ 25.3° and the rutile (110) peak at ~ 27.4° was calculated to be around 4.5, confirming anatase as the dominant crystalline phase. The presence of a small fraction of rutile alongside anatase is known to facilitate interfacial charge separation, thereby enhancing photocatalytic efficiency. This favorable phase distribution in both the pure and composite samples supports the expected improvement in photocatalytic activity48,49.

FTIR analysis

To further investigate the possible chemical interactions between TiO2 and the clay matrix in the synthesized composite, Fourier-transform infrared (FTIR) spectroscopy was performed. Figure 2d shows the FTIR spectra of pure TiO2 and the TiO2–clay composite in the range of 4000–500 cm–1.

In the pure TiO2 spectrum, characteristic bands corresponding to Ti–O–Ti stretching vibrations are observed below 800 cm–1. In contrast, the composite sample exhibits additional absorption bands, particularly a broad band around ~ 1040 cm–1, attributed to Si–O stretching vibrations from the clay structure. The slight shift and broadening of this band, along with changes in the 920–950 cm–1 region, suggest the possible formation of Ti–O–Si interfacial bonds between TiO2 and the clay matrix.

Additionally, a broad band near 3400 cm–1 and a weak band around 1630 cm–1 are observed in both spectra, which correspond to the stretching and bending vibrations of –OH groups from adsorbed surface water—commonly present in clay-based materials50,51.

Overall, the FTIR data complement the XRD results and provide spectroscopic evidence for chemical interactions between TiO2 and the clay matrix, indicating successful structural integration without compromising the crystallinity of the components.

FE-SEM analysis

Scanning electron microscopy (FE-SEM) was employed to examine the morphology of the immobilized photocatalyst bed surface (Fig. 3). The SEM images of the substrate without TiO2-clay particles (Fig. 3a) display a smooth and uniform surface. However, after the immobilization of TiO2-clay particles onto the substrate using silicone adhesive, the nanoparticles were observed to be evenly distributed across the substrate’s surface (Fig. 3b-d). The SEM analysis of the TiO2-clay composite revealed that the particle size ranged between 20 and 30 nm.

Fig. 3
figure 3

FE-SEM images of (a) substrate without TiO2-clay and coating of TiO2-clay on the surface of the substrate. (b, c, d), Water contact angle of the substrates containing (h) 50% clay and 50% titanium dioxide; (g) 30% clay and 70% titanium dioxide; (f) 10% clay and 90% titanium dioxide; (e) pure titanium dioxide substrates; and (i) water contact angle. EDX analysis of the TiO2–clay nanocomposite: (j) combined elemental mapping showing distribution of O (cyan), Si (green), and Ti (red); (k–m) individual elemental maps of Ti, O, and Si, respectively; (n) EDX spectrum confirming the elemental composition of the composite.

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To prevent particle aggregation, the photocatalyst particles were gradually applied onto the substrate. Importantly, the use of silicone adhesive for immobilization did not compromise the photocatalytic activity of the TiO2-clay particles. The TiO2 nanoparticles appeared to be uniformly dispersed within the organoclay matrix, which exhibited a spongy morphology without any noticeable stacking of the clay layers. This method effectively facilitates the formation of a disordered, porous structure with TiO2 nanoparticles homogeneously distributed between clay particles, demonstrating the simplicity and efficacy of the immobilization process in producing stable, high-performance photocatalytic materials17,52.

Elemental mapping and composition analysis by EDX

To verify the successful integration and spatial distribution of TiO2 nanoparticles within the clay matrix, energy-dispersive X-ray spectroscopy (EDX) mapping and spectrum analysis were conducted. As shown in Fig. 3(j), the elemental map of the composite reveals a uniform and homogeneous dispersion of titanium (Ti, red), silicon (Si, green), and oxygen (O, cyan), suggesting effective interfacial contact between TiO2 and the clay support.

The individual elemental maps for Ti, O, and Si—presented in Fig. 3(k–m)—further confirm the spatial overlap and indicate strong dispersion without phase segregation. Moreover, the corresponding EDX spectrum in Fig. 3(n) displays prominent peaks corresponding to Ti, Si, and O, affirming the presence of these key elements in the composite material.

These results collectively support the successful formation of the TiO2–clay nanocomposite and provide strong evidence for the uniform distribution of TiO2 over the clay matrix, which is essential for achieving enhanced photocatalytic performance through increased surface contact and reactive site availability51.

UV–vis diffuse reflection spectra

UV–vis diffuse reflectance spectroscopy (DRS) was conducted to analyze the photoabsorption properties of TiO2, clay, and TiO2-clay nanocomposites. As shown in Fig. 2e, the TiO2-clay photocatalyst exhibited enhanced photoabsorption compared to pure TiO2, with a noticeable shift in the absorption edge toward longer wavelengths in the visible light range (400–700 nm). Despite this shift, the TiO2-clay photocatalyst remains inactive in the visible light region, and the improved photoabsorption does not significantly impact its photocatalytic activity.

The band gap energy (Eg) of the materials was calculated using the Kubelka-Munk transformation as.

$${(ahv)^{{\text{1}}/{\text{n}}}}={\text{A}}\left( {hv - {{\text{E}}_{\text{g}}}} \right)$$

where α is the absorption coefficient, hv is the photon energy, and A is a constant. The value of n depends on the type of electronic transition (with n = 1/2 for direct transitions and n = 2 for indirect transitions). Tauc plots were generated from the DRS spectra by plotting (αhv) 1/n against energy, and the linear regions of the curves were extrapolated to the X-axis to determine the band gap energies (Fig. 2f). The results indicated that the indirect band gap (Eg) values for pure TiO2 and TiO2-clay were 3.1 eV and 3.04 eV, respectively. The slight reduction in band gap energy following the addition of clay suggests minimal changes in the electronic structure of TiO253,54.

BET surface area and pore structure analysis

Nitrogen adsorption–desorption isotherms of TiO2 and TiO2–clay composites are shown in Fig. 2b. Both samples exhibit type IV isotherms with H2 hysteresis loops, according to IUPAC classification, which are characteristic of mesoporous materials with slit-like pores. The observed hysteresis at relative pressure P/P0 < 0.98 confirms the presence of capillary condensation in mesopores51,53.

The incorporation of clay led to a marked enhancement in the specific surface area from 52.12 to 65.35 m2/g (Table 1), along with increases in both total pore volume (from 0.126 to 0.147 cm2/g) and mean pore diameter (from 26.7 to 33.9 nm). This increase can be attributed to the better dispersion of TiO2 nanoparticles over the high-surface-area clay matrix, which prevents particle aggregation and introduces additional mesoporous channels. These structural features provide a larger number of active sites and facilitate reactant diffusion to photocatalytically active centers.

Table 1 Surface features obtained from the BET analysis for TiO2 and TiO2-clay samples.
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Furthermore, Fig. 2c displays the BJH pore size distribution curve for the TiO2–clay composite, showing a narrow mesopore size distribution predominantly between 10 and 30 nm. This size range is optimal for the diffusion of BR46 dye molecules, enabling effective adsorption and enhancing photocatalytic efficiency.

Although BET analysis was performed only for the optimal composition (30% clay, 70% TiO2), the influence of clay content on photocatalytic activity was investigated experimentally (see Sect. 3.2.4, Fig. 4d). Results showed that surface adsorption increased with higher clay content, but excessive adsorption at high clay ratios (> 30%) led to blockage of active sites and a decline in photocatalytic degradation due to reduced formation of reactive radicals such as OH· and O2–.

Fig. 4
figure 4

The effects of the operational variables are: (a) the initial concentration of BR 46; (b) the effect of light source Position; (c) the effect of rotating speed; and (d) the effect of clay dose.

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Together, the enhanced surface area, favorable pore size distribution, and optimized clay ratio contribute synergistically to the superior photocatalytic performance of the TiO2–clay composite55,56.

Surface wettability

The effect of clay particles on the surface hydrophilicity of the fabricated photocatalytic beds was assessed using contact angle measurements and the captive bubble technique, with the results presented in Fig. 3. The addition of clay to the TiO2 composition significantly enhanced the hydrophilic properties of the photocatalytic surfaces compared to pure TiO2. In substrates containing different ratios of clay to TiO2—50% clay and 50% TiO2 (Fig. 3h), 30% clay and 70% TiO2 (Fig. 3g), 10% clay and 90% TiO2 (Fig. 3f), and pure TiO2 (Fig. 3e)—the contact angles of water on the surface were measured at 45.40°, 54.50°, 59.28°, and 87.72°, respectively, as shown in Fig. 3i. The incorporation of clay into the TiO2 matrix increased surface hydrophilicity, resulting in the formation of a thin water layer, which enhanced pollutant adsorption onto the substrate surface. This improved hydrophilicity contributed to enhanced removal efficiency in the rotating photoreactor. However, increasing the clay content beyond an optimal level led to excessive absorption on the surface, which negatively impacted removal efficiency. The hydrophilization of the photocatalyst surface promotes mass transfer between the liquid phase and the photocatalyst, thereby improving the overall photocatalytic efficiency. This enhancement in surface wettability and roughness, induced by the presence of clay particles, likely explains the observed increase in photocatalytic performance52,57.

Determination of point of zero charge (PZC)

The point of zero charge (PZC) of the TiO2–clay nanocomposite was determined using the pH drift method. A series of 50 mL NaCl aqueous solutions (0.01 M) was adjusted to different initial pH values ranging from 3 to 9 using either 0.1 M HCl or NaOH. Then, 0.05 g of the TiO2–clay catalyst was added to each solution and stirred at room temperature for 24 h. After equilibration, the final pH of each sample was measured. The PZC was identified as the point where the difference between the initial and final pH values (ΔpH = pH_final – pH_initial) was zero. The resulting data were plotted as ΔpH versus initial pH, and the intersection point indicated the PZC value of the composite58.

The point of zero charge (PZC) of the TiO2–clay nanocomposite was determined to be approximately 5.8, as shown in Fig. 5a. This value indicates that the surface of the photocatalyst becomes positively charged at pH values below 5.8 and negatively charged above this threshold. Since the photocatalytic experiments were conducted at near-neutral pH (~ 7), the surface of the catalyst was negatively charged, which promoted the electrostatic adsorption of the cationic BR46 dye. This behavior enhanced the contact between dye molecules and active sites, contributing to the overall photocatalytic efficiency. The observed PZC value, being slightly lower than that of pure TiO2, reflects the partial influence of the clay component, which typically possesses lower surface charge neutrality points. This synergistic surface interaction underlines the role of composite formation in tuning the surface properties of the material59,60.

Fig. 5
figure 5

(a) Determination of the point of zero charge (PZC) of TiO2–clay nanocomposite using the pH drift method, (b) Evaluation of active species involved in BR46 degradation using radical scavenger experiments, Pseudo-first-order kinetic plots for BR46 photodegradation under variations of (c) initial concentration, (d) lamp position, (e) rotational speed, and (f) clay content.

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Operating parameters’ impact on a rotary photoreactor’s photocatalytic degradation

Impact of the initial dye concentration

The effect of initial dye concentration on the photocatalytic degradation of BR46 was investigated using different concentrations (20, 30, and 50 mg/L), as shown in Fig. 4a. The results indicate that the highest degradation efficiency was achieved at the lowest concentration (20 mg/L), with the efficiency decreasing as the dye concentration increased. This suggests that lower initial concentrations of the dye enhance the removal efficiency. The decrease in degradation efficiency at higher concentrations can be attributed to the saturation of active sites on the photocatalyst surface by the dye molecules. As the dye concentration increases, a larger number of dye molecules are adsorbed onto the photocatalyst, occupying active sites and limiting the adsorption of essential species such as O2 and OH–, which are required for the generation of reactive radicals. Moreover, at higher dye concentrations, photon absorption by the photocatalyst is reduced due to the dye molecules obstructing the light before it reaches the catalyst surface, further inhibiting the photocatalytic process. This combination of factors results in diminished photocatalytic performance at higher initial dye concentrations61,62.

Effect of light source positioning

Figure 4b illustrates the effect of light source positioning on the removal efficiency of BR46 after 90 min of photocatalytic treatment. The positioning of the UV lamp inside the photoreactor is also depicted in Fig. 1c. The optimal location for the light source was determined to be on the left side of the rotating cylinder, at a distance of approximately 1 cm from the substrate.

As the cylinder rotates clockwise, it carries a thin layer of the dye solution along its surface, and positioning the light source on the left side ensures maximum exposure to UV radiation. This configuration provides the highest intensity of radiation to the thin film of the pollutant solution, thereby enhancing the photocatalytic degradation process. The close proximity of the light source to the substrate also ensures more effective photon absorption by the photocatalyst, further contributing to improved dye removal efficiency63,64.

Effect of rotating speed

The rotational speed of the disk plays a critical role in mass transfer and affects the thickness of the aqueous film formed on the cylinder surface. Figure 4c illustrates the impact of rotation speed on the removal efficiency of BR46 after 90 min of treatment. An increase in the cylinder’s rotation speed correlates with an improvement in pollutant degradation, up to an optimal speed of 5.5 rpm. This enhancement is attributed to the improved mass transfer of both the pollutant and oxygen across the cylinder’s surface at higher speeds. However, beyond 5.5 rpm, the removal efficiency decreases, likely due to the reduced contact time between the pollutant and the photocatalyst surface, as the increased speed causes the pollutant solution to pass more rapidly over the substrate. Additionally, higher rotation speeds lead to increased energy consumption by the motor, with the maximum power usage observed at speeds of 10.5 rpm or higher. Based on the balance between pollutant removal efficiency and energy consumption, 5.5 rpm was identified as the optimal rotation speed for the photoreactor in subsequent experiments65.

Effect of clay dose

Figure 4d demonstrates the reduction in BR46 concentration after 90 min of UV exposure for four different substrate compositions: 50% clay and 50% TiO2, 30% clay and 70% TiO2, 10% clay and 90% TiO2, and pure TiO2. The results indicate that the substrate containing 30% clay and 70% TiO2 exhibited the greatest reduction in dye concentration, outperforming the other compositions.

As the clay content increased, surface adsorption of the dye also increased. This rise in adsorption leads to the saturation of active sites on the photocatalyst surface, with dye molecules occupying a large number of these sites. Consequently, the photocatalyst’s ability to adsorb essential species such as O₂ and OH⁻ is reduced, limiting the generation of reactive radicals. This reduction in radical renewal ultimately results in a lower degradation rate. Thus, while increasing clay content enhances adsorption, it also diminishes the photocatalytic efficiency beyond an optimal concentration28,29.

Kinetics and mechanistic implications

We fitted the BR46 degradation profiles to a pseudo-first-order model (ln(C0/Ct) = k·t). Figure 5c–f presents the kinetic plots under four parameter sets together with the corresponding linear fits, k values, and R2.

Figure 5c — Initial concentration (20, 30, 50 mg L–1): The apparent rate constant decreased with increasing C0, consistent with light attenuation and active-site saturation (20 mg L–1: k = 0.0158 min⁻¹, R2 = 0.9799; 50 mg L–1: k = 0.0069 min–1, R2 = 0.9844).

Figure 5d — Lamp position (left/center/right/above-center): The “left” configuration delivered the highest rate (k = 0.0158 min–1; R2 = 0.9799), highlighting the importance of uniform irradiation across the active area.

Figure 5e — Rotational speed (3.5, 5.5, 8.5, 10.5 rpm): Mixing enhanced kinetics up to 5.5 rpm (k = 0.0138 min–1; R2 = 0.9916); decline at 10.5 rpm likely reflects excessive turbulence and splashing, which reduce effective irradiation time.

Figure 5f — Clay content (0, 10, 30, 50% w/w in the composite): 30% clay maximized the rate (k = 0.0158 min–1; R2 = 0.9799) via adsorption/charge-separation synergy; at 50% clay, the rate dropped (k = 0.0066 min–1), likely due to light scattering and diminished penetration.

Overall, PFO provided excellent fits across all conditions (R2 > 0.97), confirming kinetic consistency and enabling quantitative comparison with related TiO2 systems. The observed trends (lower k at higher C0, sensitivity to irradiation geometry and hydrodynamics, and an optimum clay fraction) align with a surface-reaction-controlled mechanism in which radical generation and mass transfer jointly govern the overall rate66,67.

Computational details

To introduce valid mechanisms theoretically, the use of a suitable DFT method for simulating the reactions of target molecules is an important task. Therefore, an effective literature survey led us to select the best method to design target mechanisms. To this end, the following strategy is employed. Our experience, accompanied by literature information, indicates that the meta hybrid M06-2X68 method is the best among all to simulate reactions involving H abstraction, addition/elimination, and SN2 mechanisms. Thus, all involved species in the designed reactions are optimized by the M06-2X method in conjunction with the Pople type 6-31G(d, p) basis set. After that, the same level (M06-2X/6-31G(d, p) level) is utilized for the harmonic vibrational frequency calculations of the obtained stationary points. The final results show that all minima structures have frequencies only, and the optimized structures for all saddle points have just one imaginary frequency in addition to real frequencies. It is worth noting that the simulated reactions occur in the liquid phase. Therefore, a valid chemistry model to design the liquid phase is required. Nowadays, chemists use implicit solvation models when employing quantum chemical methods to describe their study systems. These models employ implicit solute-solvent interactions instead of explicit solvent molecules around solute molecules. Here, we use the Solvation Model based on Density (SMD) to include the solvent effect. The SMD is a continuum solvation model that is applied for all charged and uncharged solute molecules due to a large amount of data, including 2821 solvation-free energies for parametrizing this model69. Therefore, this model is the best among all to simulate the reactions of BR 46 in water solvent. Also, the used solvent is water, as stated in the experiment section. The Gaussian 09 (G09) software package70 was used to perform DFT calculations.

To further establish the reliability of the chosen methodology, it is important to highlight that the M06-2X functional has undergone extensive benchmarking and offers accurate descriptions of thermochemical kinetics and noncovalent interactions, which are crucial for modeling photocatalytic degradation pathways71,72,73. In this study, all intermediates and transition states were fully optimized, with their relative energies calculated at the SMD-M06-2X/6-31G(d, p) level. These calculated energies were then directly compared with the experimentally identified intermediates from GC–MS analysis, revealing consistent trends that validate the proposed stepwise degradation mechanism. Consequently, the integration of DFT calculations with experimental findings ensures that the theoretical results are not only accurate but also reliable, effectively supporting the proposed photocatalytic degradation pathway.

The OH addition to different sites of BR 46

First of all, it should be pointed out that there are a large number of reactions for molecules with several reacting centers, like BR 46, under conditions including OH radicals and UV light. Therefore, our effort is to present a valid and simple pattern for BR 46 degradation with the support of DFT computations. Therefore, we follow just some probable paths energetically and skip others due to encountering a large number of reactions that are impossible to consider here. The Energy barriers of the saddle points involved in the OH addition to different sites of BR 46 are listed in Table 2. Figure 6 shows the structures of the OH addition to different sites of BR 46. Schematic representation of hydroxyl radical addition to different sites of BR 46 and decomposition to I and K species form the B adduct is displayed in Fig. 7.

Table 2 Energy barriers (\(\Delta {E^ \ne }\)) in Kcal mol− 1 for the transition States involved in the OH addition to different sites of BR 46.
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Fig. 6
figure 6figure 6

The structures of the OH addition to different sites of BR 46.

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Fig. 7
figure 7

Schematic representation of hydroxyl radical addition to different sites of BR 46 and decomposition to I and K species form the B adduct.

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As shown in Fig. 7, the first step in the degradation of BR 46 is the addition of hydroxyl radicals to different reacting sites, leading to the formation of A, B, C, D, E, and F species. The saddle point TS2 has a smaller energy barrier (-2.53 kcal mol− 1) than TS1 (-1.57 kcal mol− 1) and TS3-TS6 (-1.98, 2.05, 7.35, and 4.05, respectively). This sentence does not mean that the production probability of other species is negligible, but the meaning of this sentence is that the likelihood of generating B species is more than that of others. Therefore, we focus on the reactions of B species only and skip others.

The transition state 2 (TS2) shows the addition of an OH radical to the N1 atom of BR 46. The result of this addition is B species. After this addition, other nitrogen gets a radical center. Thus, a collision of another OH with this center produces G species. Through these processes, the double bond of the N = N bond is transformed into a single sigma bond. Then, the addition of an OH radical to the carbon with the number 13 by TS7 (-8.37 kcal mol− 1), and the H compound is generated. Then, UV light breaks the N3-C13 bond in the H species. This process is probable because the cleavage of the N1-C1 bond leads to the formation of the benzene ring resonance. Thus, the I and J species are formed. The J species is a radical species, so a collision of one hydroxyl radical with the radical center (N atom) produces the K species (see Fig. 7).

The reactions of I species

The Energy barriers of the saddle points involved in the reactions of I species are collected in Table 3. Figure 8 shows a schematic representation of the I molecule reactions under OH radicals and UV light.

Table 3 Energy barriers (\(\Delta {E^ \ne }\)) in Kcal mol− 1 for the transition States involved in the reactions of I species.
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Fig. 8
figure 8

Schematic representation of the I molecule reactions under OH radicals and UV light.

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Figure 8 shows that the I species, by the addition of an OH radical, converts to I-1, and the I-1 intermediate breaks to I-1 A and I-1B under UV light. I-1B is a radical species with a radical center on the nitrogen atom. Thus, I-1 C is generated by the collision of an OH radical with the mentioned center. From I-1 C, the CH3N(OH)2 and CH3N(OH)CH2OH species are eliminated by TS17a (with an energy barrier of 36.93 kcal mol− 1) and TS17b (1.40 kcal mol− 1), respectively. The residual species, I-1C2 and I-1E, can undergo other reactions. I-1E species via TS18a (41.57 kcal mol− 1) yields CH2(OH)2 and I-1 F. The intermediate I-1 F is a radical compound, and by collision with an OH radical, it converts to the I-1C2 species. Hydroxyl radicals can be added to the benzene rings of I-1 A, I-1C2, and I-1G compounds. In the next step, the reactions can go back to the elimination of OH radicals or hydrogen atoms to recover the resonance of the benzene ring. By these processes, the hydrogens of the benzene ring can be replaced by the OH radicals (see Fig. 8 and Figure S1).

The reactions of K compound

Schematic representation of the K molecule decomposition under OH radicals and UV light is shown in Fig. 9.

Fig. 9
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Schematic representation of the K molecule decomposition under OH radicals and UV light.

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In Fig. 9, the decomposition paths of the K species under OH radicals and UV light are displayed. The Energy barriers of the saddle points involved in the reactions of the K compound are listed in Table 4. The formed K species (see Fig. 7) can undergo several reactions (TS29-TS53) and produce different compounds such as L-S and compounds with smaller mass such as R1-1 - R1-4, R1-b1 - R1-b4, R1-b2a - R1b-2b, R2-1, R2b-1 - R2b-b3, R2b-2a, R2b-b2, CH3N(OH)CHOHBr, CH3N(OH)C(OH)3, C(OH)3N(OH)CHOHBr, CH3N(OH)N(OH)2, C(OH)3N(OH)N(OH)2, and CH3N(OH)CH2OH, CH3N(OH)2, CH(OH)2Br, CH3OH, N(OH)2CHOHBr, C(OH)4, C(OH)3N(OH)2, N(OH)2CH2OH, CH2(OH)2, N(OH)3, and N2(OH)4. The calculated energy barriers for the mentioned reactions are in a range from − 7.72 to 57.94 kcal mol− 1 (see Table 4).

Table 4 Energy barriers (\(\Delta {E^ \ne }\)) in Kcal mol− 1 for the transition States involved in the reactions of the K compound.
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Figure 10 shows the transformation of the produced compounds by the previous processes to smaller species. The compound CH3N(OH)CHOHBr, CH3N(OH)C(OH)3, C(OH)3N(OH)CHOHBr, CH3N(OH)N(OH)2, C(OH)3N(OH)N(OH)2, and CH3N(OH)CH2OH is broken down to the compound with smaller mass, such as CH3N(OH)2, CH(OH)2Br, CH3OH, N(OH)2CHOHBr, C(OH)4, C(OH)3N(OH)2, N(OH)2CH2OH, CH2(OH)2, N(OH)3, and N2(OH)4 by TS54-TS70. The energy barriers of the saddle points, TS54-TS70, are collected in Table 5, which are in a range from 14.00 to 50.76 kcal mol-1.

Fig. 10
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Schematic representation of the decomposition of some species produced from previous reactions under OH radicals and UV light.

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Table 5 Energy barriers (\(\Delta {E^ \ne }\)) in Kcal mol− 1 for the transition States involved in the reactions of small species production.
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Characterization of BR46 degradation products by GC–MS

Gas chromatography-mass spectrometry (GC–MS) analysis was performed to identify the degradation byproducts of BR46. The results indicate that BR46 molecules were effectively degraded, leading to the fragmentation into smaller molecular structures. Specifically, the photodegradation process converted the toxic BR46 dye into non-toxic small molecules and low-carbon aliphatic compounds. Several degradation products were detected (see Table 6). Furthermore, these aliphatic compounds can undergo further oxidation to produce carbon dioxide (CO₂) and water (H₂O). Figure 11 displays the GC–MS spectra of the intermediate compounds present in the BR46 solution during the photocatalytic process74,75. Additional GC–MS chromatograms and the full list of identified degradation compounds are provided in the Supporting Information (Tables S1–S3 and Figures S3–S6).

Table 6 Degradation byproducts of BR46 identified by GC-MS in the experimental section of the Study.
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Fig. 11
figure 11

Mass spectrum of degradation products of BR46 measured by GC-MS.

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Removal of TOC

The removal of TOC from BR46 was evaluated under optimized conditions to assess the overall oxidative degradation efficiency of the photocatalytic system. The TOC analysis was employed as it provides a comprehensive measure of the total organic compounds present in the solution, indicating the extent of mineralization and degradation of organic pollutants. As illustrated in Fig. 12a, after 90 min of UV irradiation, the TOC removal efficiency reached 92%. Notably, extending the irradiation time to 180 min or longer resulted in minimal changes in TOC removal, suggesting that the system achieved saturation in terms of degradation efficiency. These findings align with previous studies, which demonstrated similar trends in the removal of organic compounds from aqueous solutions during photocatalytic processes. The consistent TOC reduction indicates that the TiO2-clay photocatalyst effectively facilitates the transformation of organic pollutants into less harmful or non-toxic end products76.

Fig. 12
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(a) TOC chromatograms with treatment time, (b) removal (%) of BR 46 in recycling runs.

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Recycling and stability assessment of synthesized photocatalyst

Monitoring recyclability, along with determining the stability and activity of synthesized photocatalysts under UV irradiation, is essential for evaluating their practical applications. To evaluate the photocatalytic performance of the TiO2-clay composite, recycling experiments were conducted under consistent experimental conditions (see Fig. 12b). The TiO2-clay sample showed stable degradation efficiency across six consecutive runs, indicating its high photostability and sustained photocatalytic activity. Furthermore, the stability of the TiO2-clay composite was confirmed by X-ray diffraction (XRD) analysis. The XRD patterns obtained after 600 min of UV exposure confirmed that the composite retained its crystalline structure and stability throughout the treatment, highlighting its durability under prolonged irradiation. These results highlight the robustness of the TiO2-clay photocatalyst, positioning it as a strong candidate for continuous photocatalytic applications in wastewater treatment.

Radical scavenger tests

To elucidate the dominant reactive species involved in the photocatalytic degradation of BR 46, a series of radical trapping experiments was performed using specific scavengers: isopropanol (IPA) for hydroxyl radicals (OH·), benzoquinone (BQ) for superoxide radicals (O₂⁻·), and ethylenediaminetetraacetic acid (EDTA) for photogenerated holes (h+). As shown in Fig. 5b, the degradation efficiency significantly decreased in the presence of IPA, suggesting that OH· radicals play a primary role in the degradation mechanism. The slight suppression observed with BQ and EDTA indicates a secondary contribution from O₂⁻· and h+ species, respectively. These findings confirm that hydroxyl radicals are the main oxidative species responsible for BR46 decomposition in the TiO2–clay photocatalytic system under UV irradiation77,78,79.

Conclusion

This study presents a comprehensive investigation of a novel rotary photoreactor integrated with a TiO2–clay nanocomposite immobilized via silicone adhesive, offering both mechanistic insights and technological advancements for wastewater treatment. Structural characterization by XRD and FTIR confirmed the successful integration of TiO2 with the clay matrix without compromising crystallinity, while EDX mapping revealed homogeneous dispersion of nanoparticles across the support. BET and contact angle measurements demonstrated that clay addition increased surface area, porosity, and hydrophilicity, thereby enhancing adsorption and facilitating pollutant–catalyst interactions. The PZC analysis further established that at near-neutral pH (~ 7), the negatively charged composite surface promotes electrostatic attraction of the cationic BR46 dye, strengthening the adsorption–degradation synergy.

Photocatalytic performance testing revealed that the rotary design and optimized clay fraction significantly improved BR46 removal efficiency. Operational parameters, including initial dye concentration, lamp position, and rotational speed, were shown to critically influence degradation rates, with optimum performance achieved at 20 mg L–1 dye concentration, left-side lamp positioning, and 5.5 rpm rotation. Kinetic analysis confirmed pseudo-first-order behavior (R2 > 0.97) across all conditions, with K values highlighting the balance between adsorption, light penetration, and radical generation. Radical scavenger experiments identified hydroxyl radicals (OH·) as the dominant reactive species, supported by DFT simulations, which elucidated the stepwise dye degradation mechanism. Theoretical modeling demonstrated agreement with experimental findings, reinforcing the central role of OH· radicals in azo bond cleavage and subsequent mineralization pathways.

GC–MS analysis of reaction intermediates revealed a complex multistep degradation route involving aromatic cleavage, dealkylation, and progressive oxidation, consistent with DFT-predicted pathways. High mineralization efficiency was confirmed by TOC removal (92% within 90 min), while recyclability and structural stability across six cycles validated the robustness of the TiO2–clay composite under prolonged operation.

Collectively, these findings establish that the TiO2–clay nanocomposite, in combination with the innovative rotary photoreactor, provides a highly effective and durable system for azo dye degradation. The integration of adsorption, optimized light utilization, and radical-driven photocatalysis delivers superior performance compared to conventional fixed-bed or slurry systems. Beyond demonstrating high removal efficiencies, this study advances the mechanistic understanding of dye degradation through the combined application of scavenger testing, kinetic modeling, and DFT analysis. The insights gained here not only deepen the scientific basis of photocatalytic processes but also offer practical guidance for scaling up advanced oxidation systems in industrial wastewater treatment.