Photocatalytic efficiency of bentonite-TQD via recycling and photodegradation of organic pollutants and industrial wastewater

2.1. Materials and chemicals

All reagents utilized in this study were of analytical quality and did not undergo any additional purification processes. The isopropanol and titanium (IV) isopropoxide (TTIP, purity ≥99.6%) were purchased from Fluka. The ethanol and CTAB (with a purity of at least 99.8%) were procured from Sigma-Aldrich. To guarantee the utmost purity, de-ionized water was continuously used in all experimental methods. A source of powdered bentonite and glacial acetic acid was S-d-Fine-Chem Limited.

2.2. Synthesization of TQD

To synthesize the TQD, a 4.0 mL aliquot of TTIP was dissolved in 100 mL of isopropanol under continuous stirring in a beaker (250 mL). Then the mixture was stirred for 100 min to ensure complete homogenization and dissolution occurred. Subsequently, a solution of 0.02 mol CTAB was added dropwise to the stirred solution. The resulting solution was maintained under mechanical stirring for 24 h, promoting the self-assembly of the material and the formation of titanium-based structures. The appearance of a white precipitate signified the completion of the reaction. So, it was isolated via vacuum filtration, washed thoroughly with ethanol, and dried in air [40]. The dried precipitate was calcinated using a multi-stage heating protocol: (1) pre-heating to 275°C at 1°C/min for 60 min to remove organic residues and adsorbed water; (2) final calcination at 450°C for 60 min at 2°C/min heating rate to induce crystalline phase transformation to anatase TiO₂.

2.3. Synthesization of BTQD

The BTQD nanocomposite was synthesized using a co-precipitation approach, commonly applied in the fabrication of TiO₂-based nanomaterials. Initially, 6 mL of titanium(IV) isopropoxide (TTIP) was mixed with 14 mL of ethanol at ambient temperature, and the solution was stirred to ensure uniformity. Separately, a solution consisting of 110 mL of acetic acid, 40 mL of ethanol, and 6 mL of deionized water was prepared. This was gradually introduced, dropwise, into the TTIP-ethanol mixture over 60 min under constant stirring, promoting the in situ formation of ethyl acetate via esterification between acetic acid and ethanol. The mixture was further stirred for another hour, resulting in a homogeneous yellow solution.

Subsequently, 3.0 g of bentonite powder, representing approximately 4–5% of the total mixture, was dispersed into the sol, followed by continuous stirring for 7 h to ensure thorough incorporation. The system was then left undisturbed for 24 h at room temperature to allow complete gelation. The resulting gel was dried at 105°C for 6 h to eliminate moisture, then ground into a fine powder. Calcination was performed at 275°C for 4 h in a muffle furnace to remove organic residues and promote the crystallization of TiO₂ within the bentonite matrix. This thermal treatment enhanced the structural integrity and photocatalytic potential of the final BTQD material [41].

2.4. Characterization techniques

A comprehensive suite of analytical techniques was employed to assess the structural and functional attributes of the synthesized BTQD photocatalysts. To identify characteristic chemical linkages and surface functionalities, Fourier-transform infrared (FTIR) spectroscopy was utilized. The phase purity and crystalline framework of the BTQD materials were confirmed by X-ray diffraction (XRD) analysis, using a PANalytical X’Pert Pro MPD diffractometer (Netherlands) with Cu Kα radiation (λ = 0.154 nm), operating at 50 kV and 40 mA. The diffraction patterns were recorded over a 2θ span of 5° to 70°, with an incremental step size of 0.015° and a dwell time of 100 seconds per point.

High-resolution transmission electron microscopy (HRTEM) was carried out using a Philips/FEI BioTwin CM120 microscope to evaluate the morphology and particle dispersion. Optical absorption characteristics were obtained through UV-Vis spectroscopy using a Shimadzu UV-260 (Japan), which facilitated the investigation of light-harvesting capabilities across the relevant spectrum. Emission behavior and electron-hole recombination dynamics were probed via photoluminescence (PL) spectroscopy using a Hitachi F-7000 fluorescence spectrophotometer (Japan).

For separation of the BTQD catalyst post-reaction, samples were centrifuged using a benchtop unit at 14,000 rpm, yielding a relative centrifugal force (RCF) of approximately 18,700 g (calculated using: RCF = 1.118 × 10⁻⁵ × r(cm) × rpm², where r = 8.5 cm). The centrifugation was maintained for 20 min at room temperature (20–25°C), ensuring efficient solid-liquid phase separation.

The specific surface area (SSA) of the materials was quantified via nitrogen adsorption–desorption isotherms at 77 K using a Micromeritics Tristar 3000 analyzer (USA), following a degassing step at 160°C for one hour. SSA was derived using the standard Brunauer-Emmett-Teller (BET) model. Densities utilized in the calculations were 4.210 g/cm³ for TiO₂, 2.788 g cm^-3 for BTQD, and 3.103 g cm^-3 for bentonite, which were critical in estimating particle size distributions and porosity.

Photocatalytic experiments were performed under simulated solar irradiation provided by a 150 W Scisun xenon arc lamp (Scisun Solar Technology, China), integrated with AM 1.5G filters to approximate the solar spectrum. Calibration was executed using a NIST-certified silicon photodiode, ensuring accuracy in photon flux output. Temperature and irradiance were regulated by a water-circulated cooling system, preserving experimental consistency.

The xenon lamp emitted in the 200–1050 nm range with a surface intensity of 150 mW cm^-2. Degradation studies were conducted on BBR dye solutions as well as actual wastewater samples from a textile manufacturing facility in Riyadh’s Second Industrial City, KSA. UV-Vis spectral shifts were monitored to evaluate dye removal efficiency. Mineralization levels were assessed by chemical oxygen demand (COD) measurements using a C99 Series multi-parameter photometer (Hanna Instruments, USA). Collectively, these analytical and experimental strategies offer a detailed framework for understanding BTQD-based photocatalysts and their potential role in addressing wastewater treatment challenges.

3.1. Characterization

3.1.1. FTIR spectroscopy

Figure 1 illustrates the FTIR spectra of raw bentonite (BT) and BTQD composites calcined at 275°C, offering molecular-level insights into surface functionality and chemical interactions. The spectral features serve to highlight changes in bonding environments arising from the integration of TQDs into the bentonite matrix [42].

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Figure 1.

FTIR spectra of (a) BTQD, (b) BT, and (c) TQD samples [43-45].

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In the BTQD spectrum, a broad absorption region extending from 448 cm⁻¹ to 1616 cm⁻¹ is observed, attributable to Ti–O–Ti stretching vibrations, distinctive of the TiO₂ lattice, particularly its anatase polymorph. This band reflects the incorporation of Ti-based nanostructures within the clay framework, supporting successful composite formation. Notably, peaks at 3633 cm⁻¹ and 3398 cm⁻¹ are also prominent, corresponding to O–H stretching and bending modes. These are commonly linked to surface hydroxyl groups and physically adsorbed water on TiO₂ nanoparticles. Their presence implies a hydrated surface environment, which is relevant for photocatalytic applications, as hydroxyl groups can facilitate the formation of ROS under illumination, enhancing degradation efficiency [42].

Turning to pristine bentonite, similar OH-related peaks at 3633 cm⁻¹ and 3398 cm⁻¹ are evident, but here they are ascribed to structural hydroxyls within the montmorillonite layers. These intrinsic OH groups are characteristic of the bentonite mineral framework and are largely responsible for its notable ion-exchange and adsorption capacity. A distinct band at 1632 cm⁻¹, assigned to the bending vibration of interlayer water molecules, further confirms the hydrated state of the clay. Such water layers contribute to the material’s ability to interact with both organic and inorganic species, improving its sportive behavior.

The FTIR spectrum of pristine bentonite reveals a pronounced absorption band at 1028 cm⁻¹, attributed to Si–O–Si asymmetric stretching vibrations within the silicate framework. This band underscores the integrity of the tetrahedral silica network, which forms the structural foundation of the bentonite matrix. A closely related signal at 1030 cm⁻¹ corresponds to Si–O stretching, further confirming the aluminosilicate backbone characteristic of montmorillonite-type clays. These vibrations are central to bentonite’s thermal and chemical stability, key for its function as a host material in hybrid photocatalysts and environmental applications [43].

Additional bands provide further evidence of bentonite’s layered silicate structure. The absorption near 528 cm⁻¹ is linked to Si–O bending modes, while the feature at 464 cm⁻¹ corresponds to Si–O–Si in-plane bending. A peak at 533 cm⁻¹ is assigned to Al–O–Si stretching, indicative of the intertetrahedral linkages between alumina and silica units. Meanwhile, the signal at 528 cm⁻¹ may also contain contributions from Al–OH bending vibrations, which are commonly observed in dioctahedral smectite minerals such as montmorillonite. These bands reflect the ordered arrangement of octahedral and tetrahedral sheets in bentonite and support its capacity to intercalate foreign species, including photocatalytic nanoparticles [44].

Upon incorporation of TQDs, distinct vibrational features emerge in the BTQD composite spectrum. Notably, new bands at 458 cm⁻¹ and 415 cm⁻¹ are observed, corresponding to Ti–O–Ti stretching and bending vibrations, respectively. These modes are signature features of TiO₂, specifically the anatase phase, and are absent in the parent bentonite spectrum. Their presence confirms the successful integration of titanium dioxide into the clay matrix. The absence of such bands in pure BT rules out mere surface adsorption. Instead, it suggests a more intimate interaction, potentially involving partial intercalation or anchoring of TiO₂ clusters within bentonite’s interlayers.

Importantly, key bentonite-associated bands, namely those at 3633 cm⁻¹ and 3398 cm⁻¹ (O–H stretching) and 1632 cm⁻¹ (H–O–H bending), remain evident in the BTQD spectrum. This retention indicates that the original hydroxyl functionalities and adsorbed water molecules are preserved post-modification. Such stability is significant, as it implies that the material maintains its native adsorption properties while supporting the photocatalytic activity of the embedded TiO₂ [45].

The coexistence of both Ti–O–Ti and Si–O–Si/Al–O–Si vibrational features in the composite confirms that the BTQD material synergistically combines the photoactive properties of TiO₂ with the sorptive capacity of bentonite, making it a promising candidate for multifunctional environmental remediation systems.

3.1.2. XRD patterns

Figure 2 displays the XRD profiles of bentonite (BT), TQD, and the BTQD nanocomposite. The diffractogram of raw bentonite reveals a pronounced peak at 2θ = 5.69°, associated with the basal (001) reflection of montmorillonite, indicating a d-spacing of 15.49 Å. This peak is diagnostic of the layered silicate structure typical of montmorillonite, the principal mineral phase in bentonite, and signifies orderly layer stacking within the clay. Secondary reflections at 2θ values of 19.79°, 27.70°, 34.79°, and 59.67° further substantiate the crystalline identity of montmorillonite. These peaks align well with standard bentonite patterns, dominated by silicate and aluminosilicate frameworks. The presence of quartz is confirmed by peaks at 2θ = 26.59° and 49.98°, matching reference card ICDD #00-046-1045 [46], and supports the mineralogical composition of the natural clay sample.

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Figure 2.

XRD of (a) BT, (b) TQD, and (c) BTQD samples.

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The formation of the BTQD composite leads to notable structural changes. A shift in the basal reflection from 2θ = 5.69° to 5.39° is observed, corresponding to an increase in interlayer spacing from 15.5 Å to approximately 16.4 Å. This expansion, calculated using Bragg’s law, suggests the intercalation of TiO₂ quantum dots within the montmorillonite galleries or their attachment to interlayer surfaces. Such modification points to successful hybridization at the molecular level, enabling improved surface accessibility, an important factor in both adsorption and photocatalysis.

This enlargement of basal spacing is likely the result of a cation exchange mechanism, wherein TiO₂ nanoparticles displace interlayer cations such as Ca²⁺ or Na⁺, which naturally reside between clay sheets. Given the comparatively larger size of TiO₂ particles, their integration necessitates structural adjustment within the clay lattice. This mechanism aligns with previous reports documenting similar expansions in layered silicates upon metal oxide intercalation [47].

Another salient observation is the progressive attenuation of the montmorillonite (001) peak intensity in the BTQD samples as TiO₂ loading increases. This reduction in peak sharpness suggests partial delamination or exfoliation of the silicate layers, likely caused by the disruption of layer stacking during TiO₂ incorporation. Such changes reflect structural disorder induced by nanoparticle insertion and are indicative of strong clay–TiO₂ interaction. The weakening of the basal reflection confirms the formation of a true composite material, wherein TiO₂ is not merely adsorbed onto the surface but integrated into the clay matrix.

Beyond the observed modifications in bentonite’s diffraction pattern, the incorporation of TiO₂ in its anatase form within the BTQD composite is confirmed through distinct XRD peaks. Notably, reflections at 2θ values of 25.49° (101), 37.70° (112), 48.13° (200), and 55.01° (211) match the reference data for anatase TiO₂ (ICDD #00-021-2729). These peaks are characteristic of the anatase phase, which is widely recognized for its superior photocatalytic performance when compared to other TiO₂ polymorphs such as rutile or brookite.

The detection of these reflections confirms the successful formation and retention of the anatase structure within the bentonite framework following synthesis and calcination. The stability of anatase under the employed thermal conditions indicates that the TQDs were effectively anchored without phase transformation.

Anatase TiO₂ is particularly relevant in environmental applications due to its higher surface reactivity, enhanced charge separation, and better photon utilization under UV irradiation. Its presence within the BTQD composite significantly enhances the material’s potential for use in photocatalytic processes, especially in the degradation of persistent organic contaminants in wastewater. Therefore, the XRD results strongly support the functional design of BTQD as a composite with both structural support from bentonite and high photocatalytic activity from anatase-phase TiO₂ [48].

The XRD patterns confirm that the anatase phase of TiO₂ remains structurally stable within the BTQD composites, showing no evidence of phase alteration despite exposure to elevated synthesis temperatures or possible microwave irradiation. Preserving the anatase structure is crucial for sustaining the photocatalytic efficiency of TQDs, as transformation to the rutile phase typically diminishes activity. The maintained crystallinity and phase purity indicate that the synthesis parameters, particularly temperature control and radiation exposure, were effectively optimized. Moreover, the characteristic diffraction peaks of TiO₂ observed alongside those of bentonite verify the successful incorporation of TQD into the clay matrix. This structural stability enhances the long-term photocatalytic reliability of the BTQD composites, underscoring their suitability for environmental remediation processes such as wastewater treatment.

3.1.3. SEM morphology

The Cetyltrimethylammonium Bromide (SEM) micrograph (Figure 3) provides a high-resolution visualization of the surface morphology and particle arrangement within the BTQD composite material. and particle arrangement within the BTQD composite material. Multiple distinguishing features are evident in this image, illustrating the structural properties and interfacial interactions between the bentonite matrix and TQDs. The BTQD composite displays a heterogeneous particle arrangement. Bentonite constituents manifest as larger, stratified particles exhibiting smooth, planar morphology consistent with montmorillonite architecture. These components demonstrate a lamellar configuration characteristic of smectite clay minerals. Conversely, TQD manifests as finer, discrete nanoparticles homogeneously dispersed throughout the bentonite surfaces. The TiO₂ particles show excellent distribution across the clay matrix, lacking substantial agglomeration or particle clustering, indicating successful integration of quantum dots within the bentonite framework.

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Figure 3.

SEM of (a) BT, (b) TQD, and (c) BTQD samples.

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The bentonite particle surfaces exhibit roughness and porosity, matching established characteristics of clay minerals recognized for their elevated surface area and capacity to adsorb various chemical species. The porous nature of bentonite proves essential for its adsorptive functionality and presumably promotes TQD attachment and distribution across available surfaces. TQD appears positioned within the interlayer galleries of bentonite, corroborating XRD findings that indicated basal spacing expansion following TiO₂ incorporation. This interaction pattern between TiO₂ and bentonite indicates that quantum dots are not merely surface-adsorbed but rather intercalated or structurally integrated within the clay architecture, which proves vital for improving the composite’s stability and photocatalytic performance.

The micrograph confirms that TQD maintains uniform distribution throughout the bentonite surfaces, demonstrating effective dispersion achieved during composite preparation. Such homogeneous distribution proves advantageous for photocatalytic applications, ensuring TiO₂ particles remain accessible for interactions with organic contaminants during degradation reactions. The SEM examination reveals well-incorporated TiO₂ nanoparticles throughout the bentonite matrix, amplifying surface area and structural integrity while exhibiting minimal particle aggregation, thereby validating the composite’s promise for photocatalytic and environmental treatment applications.

3.1.4. Transmission electron microscopy morphology

Figure 4 presents the transmission electron microscopy (TEM) micrographs characterizing the pristine bentonite (BT), the synthesized TQDs, and the resulting BTQD composite. The micrograph of the pristine bentonite elucidates the characteristic lamellar topology of montmorillonite, the dominant mineralogical phase. The clay manifests as stacked, sheet-like aluminosilicate layers arranged in parallel orientations. These platelets exhibit a high aspect ratio, with lateral dimensions ranging from 100–200 nm, while the sheet thickness is notably finer, approximately 10–30 nm. The smooth, planar surface morphology and visible interlayer galleries are indicative of a material with significant specific surface area, a prerequisite for effective cation exchange and adsorptive capacity. The absence of foreign particulate matter or secondary phases confirms the chemical purity of the unmodified clay prior to functionalization.

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Figure 4.

TEM of (a) BT, (b) TQD, and (c) BTQD samples.

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In contrast, the TEM analysis of the TQD samples isolates the morphology of the semiconductor component. The quantum dots appear as discrete, quasi-spherical nanoparticles with a narrow size distribution, averaging 5–8 nm in diameter. The high degree of monodispersity and lack of aggregation indicate robust stabilization and control over nucleation kinetics during the synthesis protocol.

The micrograph of the BTQD composite provides visual evidence of the successful heterostructure formation. The image reveals a homogeneous dispersion of TQD across the bentonite support, validating the efficacy of the immobilization process. Notably, the TiO₂ nanoparticles retain their quantum confinement dimensions (5–8 nm) without suffering from the sintering or agglomeration often observed during composite formation. The data suggest a dual-mode integration: TQD are anchored onto the basal surfaces and likely intercalated within the clay’s interlayer galleries. This observation corroborates the XRD data, which indicated basal spacing expansion (d-spacing increase). Such intercalation effectively “props open” the clay layers, creating porous diffusional pathways that enhance reactant accessibility.

By preventing the aggregation of TiO₂, the clay matrix maintains a high density of active catalytic sites. The interface between the TQD and the aluminosilicate sheets maximizes the heterojunction surface area, which is vital for adsorption-coupled photocatalysis. The composite morphology, merging the high adsorptive surface of the clay with the redox-active, highly crystalline anatase nanoparticles, creates an architecture optimized for the mass transfer and degradation of organic contaminants in aqueous environments.

3.2. Band gap energy and BET surface area analysis

Figure 5 displays the Tauc plots for pristine BT, TQD, and BTQD composite. These plots, obtained through the Kubelka-Munk transformation and analyzed using the Tauc relationship, provide valuable insight into the optical characteristics and electronic transition behavior of these materials. Construction of Tauc plots follows [49] the (Equation 1):

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Figure 5.

Tauc’s plot of (a) BT, (b) TQD, and (c) BTQD samples.

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where $E_g$ represents the band gap energy, $h\nu$ denotes photon energy, $\alpha$ corresponds to the absorption coefficient, and $n$ signifies the transition type index. For direct allowed transitions, $n=1/2$.

Across the examined energy range, the Tauc plot of untreated BT reveals only slight increases in ${\left(\alpha h\nu \right)}^{1/n}$ with rising photon energy (hν), reflecting its limited light absorption. Such behavior is typical of materials with wide band gaps and minimal electronic mobility. The calculated band gap of BT is approximately 3.64 eV, confirming its insulating nature and weak optical response. This relatively large E_g imposes a significant constraint on electron excitation, thereby hindering the generation of electron–hole pairs essential for photocatalytic activity. Consequently, pristine BT exhibits negligible participation in light-driven degradation processes due to its inherently low photoresponsiveness [50,51].

Conversely, the Tauc plot for TQD reveals a pronounced elevation in ${\left(\alpha h\nu \right)}^{1/n}$ relative to BT observations. This demonstrates that TQD possesses enhanced light absorption characteristics within the lower photon energy domain. This response pattern indicates efficient light absorption by TiO₂ at photon energies below this threshold, whereas higher-energy photons undergo transmission or reflection. The TQD material displays an optical band gap of 2.97 eV, which aligns with bulk-like behavior for 58 nm nanocrystalline particles. The decreased band gap compared to BT demonstrates that TQD offer greater suitability for photocatalytic applications through improved light absorption properties.

The Tauc plot of the BTQD composite exhibits an optical profile positioned between those of pure BT and TQD, reflecting a hybrid absorption behavior. A marked increase in ${\left(\alpha h\nu \right)}^{1/n}$ (αhν)^(1/n) relative to pristine BT indicates enhanced photon absorption due to TQD integration. The estimated optical band gap for BTQD is approximately 3.18 eV, narrower than that of BT (3.64 eV), suggesting that TiO₂ incorporation effectively lowers the excitation threshold. This shift implies a synergistic interaction, where the quantum dots impart improved electronic transitions without compromising the bentonite framework. The reduced band gap enhances charge carrier generation under UV light, thereby increasing photocatalytic efficiency. These findings confirm that embedding TQD within the clay matrix not only improves light-harvesting performance but also positions BTQD as a more efficient photocatalyst for environmental remediation applications.

Although the Tauc-derived band gaps for TQDs (2.97 eV) and BTQD composite (3.18 eV) nominally position the absorption threshold in the near-UV to violet range, experimental UV-Vis measurements (Figures 5, 6a, 6b and 6c) distinctly demonstrate extended absorption tails penetrating the visible spectrum. This phenomenon originates from quantum size effects, defect-generated Urbach tails, and surface structural disorder, phenomena extensively documented in nanostructured TiO₂ materials. Furthermore, both full-spectrum and visible-light-filtered irradiation experiments produced substantial photocatalytic activity, validating that sub-bandgap excitation or sensitization mechanisms connect nominal UV absorption with functional visible-light activation. Consequently, terminology throughout the manuscript has been refined to represent this complex behavior, clarifying that visible-light activity stems from combined quantum-confined band structure alterations and defect-mediated tail states, rather than direct band-to-band absorption exclusively.

While quantum confinement in TQDs conventionally produces increased bandgap values relative to bulk anatase through energy level discretization, the apparent bandgap reduction observed in certain samples primarily arises from defect-induced midgap states, surface chemistry modifications, and interactions within the Bentonite-TiO₂ composite architecture. These mechanisms generate localized energy levels within the forbidden gap, extending absorption into the visible range and reducing effective bandgap energy. Therefore, bandgap values of 2.97 eV (pure TiO₂ QDs) and 3.18 eV (BTQD composite) represent combined contributions from quantum size effects and defect-associated absorption phenomena, offering complete understanding of the optical properties.

3.3. Photocatalytic performance of BTQD

A common industrial pollutant was the focus of BBR dye photodegradation experiments that methodically assessed the photocatalytic activity of the BTQD nanocomposite in both natural sunshine and artificial illumination with a Xenon lamp (150 mW cm^-2, 200-1050 nm). To evaluate photocatalytic efficacy under regulated and realistic light exposure settings, this experimental design was shown to be the most effective in mimicking environmental conditions. To begin the photocatalytic reaction, 0.25 g of BTQD nanocomposite was added to an aqueous BBR dye solution (250 mL, 2 × 10⁻⁵ M, pH = 6.8) (Figure 6d).

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Figure 6.

Absorption spectra of BBR dye (a) TQD, (b) BTQD, and (c) BT, and photodegradation rate (d) and (e) dark reaction in the presence of different catalysts .

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Before irradiation process, the dark reaction takes place for 30 min to establish adsorption-desorption equilibrium, which is essential for accurate kinetic analysis, as shown in Figure 6(e) but as shown in the figure the initial absorbance value of the investigated day in 1.01 and after 30 min in dark reaction this value does not have distinguishable variation as you see because the observed absorbance value recoded 1.00, 0.998, and 0.989 in presence of BT, BTTQD and TQD samples which just represents decreasing by only 0.99%, 1.18%, and 2.08%.

Following the photocatalytic reaction, the BTQD composite was isolated from solution through centrifugation at 14,000 rpm for 20 min, ensuring measurement exclusively captured catalytic photodegradation while eliminating adsorption contributions. Figure 6 illustrates BBR dye photodegradation kinetics using TQD, BTQD, and BT under Xenon lamp irradiation over 90 min. Kinetic analysis was conducted by plotting ln(C/C₀) versus time, where C₀ represents the initial concentration, and C denotes concentration at time t. The linear relationship observed across all experimental conditions confirms first-order reaction kinetics for the photodegradation process.

The BTQD composite demonstrated markedly enhanced photodegradation rates relative to pristine bentonite, validating the exceptional photocatalytic performance of TQD within the composite framework. The rate constant for BTQD measured 28.01 × 10⁻³ min⁻¹, substantially exceeding the 21.81 × 10⁻³ min⁻¹ value recorded for bentonite alone. The calculated half-life (t₁/₂ = 0.693/k) of 24.8 min exhibits strong correspondence with experimental kinetic observations [52].

The photodegradation of BBR dye proceeds primarily through ROS generation, specifically hydroxyl radicals(•OH) and superoxide anion radicals (•O₂⁻). Upon light absorption, the BTQD composite undergoes photoexcitation, producing electron-hole pairs that initiate subsequent photocatalytic transformations. The proposed mechanism, supported by prior investigations, operates as follows.

When the BTQD composite receives irradiation with photon energy exceeding the TQD band gap, electrons transition from the valence band to the conduction band, creating positively charged holes in the valence band. These photogenerated charge carriers subsequently trigger ROS formation. Conduction band electrons interact with surface-adsorbed oxygen molecules, yielding superoxide anion radicals(•O₂⁻), while valence band holes react with water molecules [52], producing hydroxyl radicals(•OH). Both species exhibit high reactivity and fulfill critical functions in BBR dye photodegradation.

Hydroxyl radicals attack dye molecules through chemical bond cleavage, while superoxide radicals simultaneously contribute to oxidative decomposition. As these ROS continuously interact with dye molecules and additional organic pollutants, they fragment them into smaller, less toxic intermediates. Progressive oxidation ultimately achieves complete mineralization of the organic dye into benign end products, including water and carbon dioxide.

This proposed mechanism receives validation from observed reaction kinetics and the documented role of ROS in the degradation pathway. The following equations describe the suggested photodegradation mechanism for BBR dye in the presence of BTQD (Equations: 2-9):

The BTQD composite, incorporating TQD, exhibits superior photocatalytic activity, enhanced by light absorption, charge separation, and increased surface area, making it highly effective for photodegradation of dyes, industrial wastewater treatment, and environmental remediation, due to its recyclability and stability.

The BTQD composite demonstrates markedly superior photodegradation efficiency relative to BT (without TiO₂). The measured rate constant for BTQD reaches 28.01 × 10⁻³ min⁻¹, while pristine BT alone exhibits a rate constant of 21.81 × 10⁻³ min⁻¹. This substantial enhancement in photocatalytic performance stems from TQD incorporation within the BTQD architecture. TQD quantum size effects optimize light absorption processes, promote charge carrier separation, and amplify ROS generation, collectively contributing to enhanced BBR dye degradation.

3.4. Recycling process and stability

The recyclability and long-term stability of a photocatalytic material are important criteria. Throughout photocatalytic degradation studies, the BTQD composite demonstrated remarkable reusability, keeping its increased efficiency across numerous operational cycles. The composite was separated, washed, and then redeployed for the next set of deterioration trials after each cycle. After seven cycles in a row, the composite showed little or no drop in efficiency, proving that TQD was stable in the bentonite matrix. One significant advantage of the BTQD composite is its exceptional recyclability, which makes it an attractive option for widespread use because it is both environmentally friendly and cost-effective.

The stability and reusability of the BTQD nanocomposite were assessed using a series of photocatalytic cycling tests. Figure 7 displays the FTIR spectra of BTQD following many recycling procedures, revealing little structural alteration to the catalyst. Following seven cycles, slight alterations in the catalyst’s chemical composition, almost certainly due to particle aggregation, were identified in the O-H stretching bands (about 3633 cm⁻¹ and 3398 cm⁻¹). The fact that no new spectral peaks appeared after multiple cycles proved that the material had retained its fundamental structure. These results demonstrate the TQDs’ strong integration into the BT matrix and the durability of the BTQD composite’s photocatalytic performance under repeated exposure.

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Figure 7.

FTIR spectra of BTQD catalyst during all recycling processes.

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3.5. TOC and COD analysis

Total organic carbon (TOC) and COD assays offered quantitative values of mineralization and total dye degradation, which were utilized to further assess the photocatalytic degradation efficiency. Figure 8 shows that total organic carbon (TOC) levels gradually drop as the photodegradation reaction advances, indicating that the organic pollutants are being broken down into simpler and non-toxic byproducts (CO₂ and H₂O). This was done to monitor the mineralization of BBR dye. Figure 8 shows that when the catalyst particle size accumulates, photocatalytic activity declines, and the rate of total organic carbon (TOC) reduction drops after seven cycles. Following the first recycling cycle, the TOC rate dropped by 24%, and following the seventh cycle, it dropped by 53%.

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Figure 8.

Variation of TOC after recycling processes in presence of BTQD catalyst.

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With a COD value of 7150 ppm, the BTQD composite was also tested for its ability to lower the organic load in industrial wastewater. Following a similar pattern, the COD reduction shows that the organic compound degradation in the industrial wastewater is efficient; after the first recycling process, the COD levels reached 610 ppm; however, they gradually increased with the number of recycling processes until the sixth recycling process, when they recorded 905 ppm, which is below the allowed COD limit of 1000 ppm according to KSA environmental law. But the COD value of the 7^th recycling process recoded 1020 above the allowed COD limit, so we did not need to continue after the 7^th recycling process because, according to the observed results, the last recycling process meets the environmental law, and subject to the allowed limit of COD value is the 6^th recycling process.

To ensure a quantitative and statistically robust evaluation of mineralization, all TOC and COD measurements were performed in triplicate for each recycling cycle, and the average values together with the corresponding standard deviations are reported in Figures 8 and 9 (error bars). The TOC data confirm a gradual increase in mineralization efficiency with reuse, with the TOC removal rising from 24 ± 3% after the first cycle to 53 ± 4% after the seventh cycle, reflecting progressive conversion of the organic carbon in BBR and coexisting organics into CO₂ and H₂O. In parallel, the COD of the industrial wastewater decreased from 7150 mg L⁻¹ in the untreated effluent to 610 ± 35 mg L⁻¹ after the first photocatalytic cycle and remained below the KSA discharge limit of 1000 mg L⁻¹ up to the sixth cycle (905 ± 50 mg L⁻¹), evidencing substantial removal of oxidizable pollutants by the BTQD composite. The slight increase to 1020 ± 55 mg L⁻¹ in the seventh cycle correlates with the onset of particle agglomeration inferred from the catalytic activity trends and marks the practical end of the usable recycling window for complying with regulatory standards. These TOC and COD trends are consistent with other TiO₂-based photocatalytic systems reported for textile wastewater and confirm that BTQD achieves both decolorization and meaningful mineralization under the investigated conditions.

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Figure 9.

Variation of COD after recycling processes in presence of BTQD catalyst.

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On the other hand, after multiple cycles of recycling, the COD reduction efficiency started to decline, which was correlated with larger particles and less photocatalytic activity over time. Figure 9 shows the COD data, which further supports the idea that the photocatalytic efficacy drops after a few recycling cycles. This is because the catalyst gradually aggregates, diminishing its surface area and the number of active sites it can use to photodegrade organic pollutants.

3.6. Economic considerations of photocatalytic process

The economic evaluation considers the total electricity consumption of 308 kWh for treating 10 m³ of wastewater, corresponding to an energy use of approximately 30.8 kWh per cubic meter. At the regional industrial electricity rate of $0.069 per kWh, the total electricity cost amounts to $21.25 for this treatment volume. Alongside electricity expenses, catalyst costs, calculated based on the dosing rate and raw material prices, labor costs, reagent consumption, and equipment depreciation have been included. This comprehensive accounting of operational and capital expenditures provides a realistic and transparent overview of treatment costs. Table 1 summarizes these costs, offering a detailed cost breakdown per cubic meter of treated wastewater and strengthening the techno-economic assessment.