Artificial Visible Light-Driven Photodegradation of Orange G Dye Using Cu-Ti-Oxide (Cu₃TiO₅) Deposited Bentonite Nanocomposites

Abstract

Bentonite-supported TiO₂ (Montmorillonite (MMT)-TiO₂) and Cu₃TiO₅ oxides (MMT-Cu₃TiO₅) nanomaterials were synthesized via a facile and sustainable sol–gel synthesis approach. The XRD results indicate the presence of mixed phases, namely, TiO₂ anatase and a new semiconductor, Cu₃TiO₅, in the material. The specific surface area (S_BET) exhibits a notable increase with the incorporation of TiO₂ and Cu₃TiO₅, rising from 85 m²/g for pure montmorillonite to 245 m²/g for MMT-TiO₂ and 279 m²/g for MMT-Cu₃TiO₅. The lower gap energy of MMT-Cu₃TiO₅ (2.15 eV) in comparison to MMT-TiO₂ (2.7 eV) indicates that MMT-Cu₃TiO₅ is capable of more efficient absorption of visible light with longer wavelengths. The immobilization of TiO₂ and Cu₃TiO₅ on bentonite not only enhances the textural properties of the samples but also augments their visible light absorption capabilities, rendering them potentially more efficacious for adsorption and photocatalytic applications. The photocatalytic efficacy of both MMT-TiO₂ and MMT-Cu₃TiO₅ was evaluated through the monitoring of the degradation of Orange G, an anionic azo dye. The MMT-Cu₃TiO₅ photocatalyst was observed to induce complete degradation (100%) of the Orange G dye in 120 min when tested in an optimized reaction medium with a pH of 3 and a catalyst concentration of 2 g/L. MMT-Cu₃TiO₅ was demonstrated to be an exceptionally effective catalyst for the degradation of Orange G. Following the synthesis of the catalyst, it can be simply washed with the same recovered solution and reused multiple times for the photocatalytic process without the need for any chemical additives.

1. Introduction

The environmental impact of dyes is considerable, largely due to their extensive use and improper disposal by industries such as textiles, paper, leather, and food production [1]. These persistent pollutants have been demonstrated to disrupt aquatic ecosystems by reducing sunlight penetration and harming organisms [2,3]. Moreover, a number of dyes, particularly azo dyes, have been identified as toxic and carcinogenic [4,5,6]. It is imperative that stricter regulations and sustainable practices be implemented in order to adequately address this issue. Fortunately, a number of techniques exist for the removal of dyes, including biodegradation, coagulation/flocculation, and advanced oxidation techniques [7,8,9]. Among these, adsorption and photocatalysis are particularly promising due to their speed, durability, and environmental friendliness [10]. This research presents a notable advancement: bifunctional adsorbent photocatalysts, which combine the adsorption of dyes onto a material’s surface with their subsequent degradation via photocatalysis under light irradiation. The evidence supporting this claim is evident in the materials’ capacity to integrate these two processes seamlessly, as demonstrated by their high adsorption capacity and efficient photocatalytic degradation rates [11]. The optimization of these materials and reduction in production costs have the potential to revolutionize wastewater treatment.

Bentonite, a natural clay, is emerging as a powerful tool for dye removal due to its exceptional adsorption capacity, as evidenced by research in references [12,13]. This abundant and inexpensive material, particularly in its fine particle form, offers a large surface area ideal for the capture of pollutants [14]. It is noteworthy that the versatility of bentonite extends beyond its inherent attraction [15,16]. It can be modified to target a wider range of dyes, including those with a negative charge. These modifications address the initial repulsion between bentonite and certain dyes, thereby enhancing its effectiveness. Scientists are developing functionalized bentonite-based semiconductor photocatalysts, which represent a highly promising approach for wastewater treatment. By modifying the clay’s surface, researchers have enhanced the material’s conductivity, which is vital for photocatalysis, and its surface area, which allows for improved capture of pollutants. The functionalization of the clay provides additional space for the trapping of contaminants, thereby rendering these materials highly promising for the treatment of wastewater [17,18].

Titanium dioxide (TiO₂) has become a significant component in innovative wastewater dye treatment technologies, as evidenced by references [19,20,21,22]. As a semiconductor, TiO₂ plays a pivotal role in these transformative technologies. Its exceptional photocatalytic activity, stability under light and chemical conditions, and availability contribute to its popularity as a choice material. Nevertheless, the extensive implementation of this technology is constrained by its restricted light absorption capacity within the ultraviolet (UV) spectrum and the difficulty in separating and recovering TiO₂ particles following the treatment process [23,24]. These limitations are being addressed by scientists with innovative solutions. One promising approach involves the coupling of TiO₂’s unique structure with visible light sensitizers, which significantly enhances its ability to capture light [25]. Furthermore, the development of more efficient and cost-effective methods for the recovery of particles is crucial for the economic viability of this process.

Copper oxide (CuO_x) is emerging as a particularly promising visible-light sensitizer for coupling with TiO₂. It is noteworthy that when these two materials are brought into close proximity, they can interact under specific conditions to form a new material: copper titanium oxide (Cu₃TiO₅). This intriguing phenomenon is one of the reasons this combination is being investigated for its effectiveness in the degradation of dyes during wastewater treatment [26,27]. Cu₃TiO₅ itself is non-toxic and does not present any environmental or health hazards [28]. Moreover, the production process does not entail the use of harmful chemicals, thereby reducing the environmental impact. The narrow bandgap energy of Cu₃TiO₅ is a significant advantage, as it enables activation by visible light, in contrast to the widely used photocatalyst titanium dioxide (TiO₂) [29]. Moreover, copper oxide displays intrinsic antimicrobial characteristics, thus offering an additional purification stage during wastewater treatment [30].

The strategic combination of TiO₂ and CuO_x nanoparticles immobilized on bentonite clay generates a powerful photocatalytic system, wherein the synergistic effect between TiO₂ and CuO_x markedly enhances the system’s efficiency in dye degradation. This system also paves the way for the removal of a wider range of pollutants. In the presence of light, titanium dioxide (TiO₂) generates electron–hole pairs, while the oxidized copper (CuO_x) acts as an electron acceptor, promoting charge separation and inhibiting recombination. This enhanced process results in an increased production of reactive oxygen species (ROS), which are the primary agents responsible for dye degradation [31,32]. Bentonite not only provides a substantial surface area for effective dye adsorption but also extends the light absorption range of the composite to include visible light [33]. This process maximizes the number of active sites and prevents nanoparticle agglomeration, thereby ensuring long-term stability and consistent photocatalytic performance. It is of particular significance that the interaction between TiO₂ and CuO_x on the bentonite surface facilitates the formation of Cu₃TiO₅, a distinctive semiconductor material within the composite. The formation of Cu₃TiO₅ plays a pivotal role in the enhancement of photocatalytic activity, facilitating charge transfer and augmenting the production of ROS. Moreover, Cu₃TiO₅ displays the biocidal characteristics of CuO, thus broadening the scope of this system’s capacity to address a more diverse array of pollutants. The composite is capable of effectively degrading not only organic dyes but also microbial contaminants, rendering it a promising candidate for wastewater treatment applications [34]. In conclusion, the synergistic system formed by Cu₃TiO₅ and bentonite provides a robust and versatile approach to dye degradation and the removal of a diverse range of pollutants. The system effectively combines pollutant adsorption with catalysis through the in situ formation of Cu₃TiO₅ on bentonite.

This work presents a novel approach to the synthesis of an active Cu₃TiO₅/bentonite nanocomposite. The results demonstrate the adsorption and photocatalytic degradation of Orange G, a representative azo dye commonly used commercially.

2. Results and Discussion

2.1. Chemical Analysis and Cationic Exchange Capacity

Table 1. Chemical composition of MMT, MMT-TiO₂, and MMT-Cu₃TiO₅ composites (in mass%) from XRF analysis.

Sample	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	K2O	TiO2	CuO	CEC (meq/100 g)
MMT	63.07	14.37	6.91	3.5	6.68	2.78	0.31	0.68		213
MMT-TiO2	34.59	7.94	3.61		3.52			48.79		300
MMT-Cu3TiO5	32.66	7.23	3.46		2.52			52.14	0.02	295

presents a comprehensive analysis of the chemical composition of MMT, MMT-TiO₂, and MMT-Cu₃TiO₅. As the primary clay mineral, MMT is primarily composed of silica (SiO₂) and alumina (Al₂O₃), comprising approximately 77.44% of its total weight. This suggests that it should be classified as an aluminosilicate material. These oxides are present within the structure of montmorillonite, which is the most prevalent clay mineral in bentonite. As reported by Ghrair et al. [35], montmorillonite is the dominant mineral in bentonite, a type of clay rock. The high alumina (Al₂O₃) content suggests the possibility of substitution within the tetrahedral or octahedral sheets of the montmorillonite structure. Conversely, elevated levels of magnesium oxide (MgO), calcium oxide (CaO), potassium oxide (K₂O), and sodium oxide (Na₂O) indicate their presence as readily exchangeable cations within bentonite. Furthermore, the SiO₂/Al₂O₃ ratio, which is 4.39 for bentonite, offers valuable insights. This ratio falls within the 2–5 range, which is characteristic of smectite clays, thus further suggesting a high cation exchange capacity (CEC). The XRF results indicate that the bentonite composition contains insignificant quantities of TiO₂ and CuO. In order to differentiate between initial bentonite and modified bentonite (MMT-TiO₂ and MMT-Cu₃TiO₅) composites, it is necessary to identify the elemental composition present in each. This is presented in Table 1.

The observed disappearance of Ca²⁺, Na⁺, and K⁺ in the treated samples, despite the absence of direct cation exchange in the preparation procedure, can be explained by several interconnected factors. The highly acidic environment generated during TiCl₄ hydrolysis likely promotes partial cation exchange, where protons (H⁺) replace exchangeable cations in the clay’s interlayer spaces, leading to their removal during washing. Furthermore, the occurrence of minor structural changes in the clay, such as edge dissolution due to the presence of acidic conditions, can render certain cations more accessible and susceptible to displacement. In addition, multivalent cations, such as Ca²⁺, can react with hydrolyzed titanium species or chloride ions to form insoluble precipitates that are subsequently removed during the washing process. The deposition of TiO₂ or CuO nanoparticles has been observed to further alter the clay structure by obstructing ionic exchange sites, thereby reducing cation retention. The combined effects of these processes, along with the slight leaching observed in the reduction in major oxides (SiO₂, Al₂O₃, Fe₂O₃), collectively explain the disappearance of these cations in the treated samples. During the synthesis process, titanium species are primarily deposited onto the surface or interlayer spaces of bentonite rather than integrating into its crystalline framework. This deposition leads to a composite material with titanium species distributed across the clay, influenced by the synthesis conditions and interactions between titanium precursors and the clay matrix. This interaction, in conjunction with the heterogeneous and porous nature of bentonite, may account for the observed TiO₂ content of approximately 48.79% and 52.14% in MMT-TiO₂ and MMT-Cu₃TiO₅, respectively, despite the initial 2:1 Ti/clay ratio. The results demonstrate the successful integration of TiO₂ nanoparticles into the bentonite matrix.

Additionally, the presence of CuO is observed exclusively in MMT-Cu₃TiO₅, with a content of 0.02%. This confirms the targeted introduction of CuO alongside TiO₂ during the modification process, resulting in a change in the chemical formula for MMT-TiO₂: (Si_2.28Al_0.61Fe_0.17Mg_0.34Ti_2.42)O₁₀(OH)₂ and for MMT-Cu₃TiO₅: (Si_2.17Al_0.56 Fe_0.17Mg_0.25Ti_2.61Cu_0.001)O₁₀(OH)₂. These observations underscore the substantial influence of TiO₂ and TiO₂-CuO modification on the original bentonite composition. Such modifications have the potential to induce novel functional properties, including photocatalytic activity and antibacterial behavior.

2.2. Powder X-Ray Diffraction (PXRD) Analysis

The structural characteristics of MMT, MMT-TiO₂, and MMT-Cu₃TiO₅ were investigated using X-ray diffraction (XRD), as illustrated in Figure 1. The diffraction patterns for all three samples exhibited the characteristic features of layered structures. The diffractograms of the three catalyst materials exhibited distinct peaks corresponding to hkl reflections of type (100), (105), (210), and (300), located at 2θ angles of 19.89°, 35.02°, 54.23°, and 62.02°, respectively. A comparison of these data with the JCPDS Card N0 00-029-1498 crystallographic card allows for the standard structure of Na-montmorillonite to be identified [36,37]. The most prominent diffraction peak for bentonite was observed at a 2θ value of 6.49°, which is characteristic of the 001 plane of montmorillonite. The corresponding interlayer spacing, d₀₀₁, was determined to be approximately 13.60 Å. The presence of quartz and cristobalite as the main impurities is confirmed by the observation of characteristic reflections at 2θ = 26.56°, 29.19°, and 73.18°, which align with the standard JCPDS cards for cristobalite (No. 00-039-1425) and quartz (No. 00-033-1161) [38,39]. A comparison of the DRX analysis of unmodified bentonite with that of MMT-TiO₂ and MMT-Cu₃TiO₅ photocatalysts reveals the presence of higher background peaks, which can be attributed to the semi-crystalline nature of these materials.

However, the d₀₀₁ value remains largely unaltered following impregnation with TiO₂ or TiO₂-CuO heterojunction oxides, exhibiting 2θ values of 6.61° and 6.01°, respectively. The introduction of Ti⁴ and Cu²⁺ ions, which have larger ionic radii (R (Ti⁴⁺) = 0.61 Å and R (Cu²⁺) = 0.73 Å) compared to Al³⁺(octahedral layer) and Si⁴⁺(tetrahedral layer) ions in montmorillonite, does not directly result in the substitution of these ions. Rather, it leads to the formation of the TiO₂ and Cu₃TiO₅ phases. The interaction of these cations with the clay structure, particularly in the interlayer spaces and on the surface, may influence the overall structure and ordering of the montmorillonite. This interaction may result in a reduction in the intensity of diffraction peaks, indicating that the layered structure may undergo some degree of distortion or disorder along the C axis during the synthesis process via the sol–gel method. The MMT-Cu₃TiO₅ composite displays the most substantial structural impact, likely due to the synergistic interaction between the materials. Additional reflections observed in the X-ray diffraction (XRD) patterns indicate the crystallization of the Cu₃TiO₅ phase. Despite the low copper content (0.02%), the detection of Cu₃TiO₅ is feasible, as the formation of this phase is not solely dependent on the absolute concentration of copper.

The particular synthesis conditions, such as the sol–gel method, may facilitate the coordination of copper ions with titanium species, even at low concentrations. XRD analysis, indexed with the ASTM reference card (JCPDS: 00-018-0461), revealed the presence of peaks corresponding to the Cu₃TiO₅ phase at 2θ = 35.02°, 37.60°, 59.76°, and 61.54°, which can be attributed to the (310), (311), (511), and (204) planes, respectively. The presence of these peaks thus corroborates the identification of the Cu₃TiO₅ phase despite the relatively low concentration of copper. The high crystallinity of Cu₃TiO₅ and the sensitivity of XRD provide further evidence that this phase can be detected at trace concentrations. Furthermore, the TiO₂ present in the MMT-TiO₂ structure is predominantly in the anatase phase, as indicated by peaks at 2θ = 25.3° (101), 36.9° (103), 37.73° (004), 48.8° (200), and 53.8° (105), which align with the standard (JCPDS: 01-083-5916). Additionally, the rutile phase of TiO₂ was observed at 2θ = 27.50° (110), 36.04° (101), and 54.23° (211), though with lower intensity. The dominance of the anatase phase can be attributed to the confinement of TiO₂ nanoparticles within the silica-based matrix, which restricts their growth and results in smaller particle sizes, thereby enhancing their photocatalytic performance [40].

The stabilization of the anatase phase is attributed to the formation of Si–O–Ti bonds and the high dispersion of TiO₂ particles across the silica-rich clay matrix, which effectively prevents the transformation of anatase to the rutile phase [18].

The X-ray diffraction analysis of Cu-doped TiO₂-modified bentonite, as illustrated in Figure 2, indicates the existence of mixed phases. The presence of the anatase phase of TiO₂ is confirmed by the observation of characteristic peaks at 25.31° and 48.07°, which correspond to the (101) and (200) planes, respectively. The relative intensity of these peaks indicates that the anatase tetragonal structure is the predominant phase.

New peaks emerge at 2θ = 35.02°, 37.60°, 59.76°, and 61.54°, which are attributed to the (310), (311), (511), and (204) planes, respectively. These peaks corroborate the formation of the tetragonal structure of the Cu₃TiO₅ semiconductor in accordance with the JCPDS file 00-018-0461.

As previously stated, a reduction in crystallinity can result in an elevated number of defects in the material relative to a perfectly crystalline material. The average crystallite size of the phases can be calculated from the XRD data using the following formula [41,42] D = kλ/βcosθ, where k is a constant approximately equal to 0.9; λ is the wavelength of the XRD (λ = 1.5406Å); β represents the full width at half maximum (FWHM) of the reflection in radians, and θ is the angle between the incident and diffracted beams.

The average crystallite sizes of the MMT, MMT-TiO₂, and MMT-Cu₃TiO₅ nanocomposites are approximately 5.53 µm, 0.52 µm, and 0.14 µm, respectively. The crystallite size of MMT-TiO₂ is markedly smaller than that of pure MMT, indicating a substantial reduction resulting from the interaction between TiO₂ particles and the bentonite surface, which impedes crystallite growth. This reduction is further amplified in MMT-Cu₃TiO₅, potentially due to the formation of a distinct crystal structure with Cu₃TiO₅, which involves more robust chemical interactions and more constrained crystal growth. Such alterations in crystallite size may impact the physicochemical properties of the materials.

2.3. N₂ Adsorption/Desorption

To correlate the textural characteristics of the MMT-TiO₂ and MMT-Cu₃TiO₅ catalysts with their catalytic performance, a BET N₂ adsorption–desorption isotherm was utilized (Figure 2), and the corresponding textural parameters are presented in

Table 2. Textural parameters determined by nitrogen adsorption for bentonite, MMT-TiO₂, and MMT-Cu₃TiO₅.

Samples	SBET (m2/g)	Vm (cm3/g)	Sext (m2/g)	Slangmuir (m2/g)	VµP Microporous (BJH)	APD (Å) Horvath–Kawazoe	Total Pore Volume (cm3/g)
Bentonite (MMT)	85	0.08	53	216	0.01	5.84	0.03
MMT-TiO2	245	0.15	162	438	0.03	6.76	0.10
MMT-Cu3TiO5	279	0.20	215	568	0.02	6.74	0.11

. In accordance with the classification system established by the International Union of Pure and Applied Chemistry (IUPAC), all three isotherms exhibit a pronounced H3 hysteresis loop, a characteristic feature of porous materials with a mesoporous structure. This aligns with the Type IV isotherm classification, frequently associated with multilayer adsorption followed by capillary condensation [43,44].

The modified MMT-TiO₂ and MMT-Cu₃TiO₅ samples display a wider hysteresis loop, indicative of a broader pore size distribution and more pronounced mesoporous porosity. The hysteresis loop is indicative of plate particle aggregates or slit pores [45]. At low pressures (P/P₀ < 0.2), rapid initial adsorption occurs due to adsorption on the external surface and in the micropores. At higher relative pressures (P/P₀ > 0.4), the adsorption in the mesopores becomes more significant, as indicated by the continuous increase in the adsorbed volume. The hysteresis loop initiates at P/P₀ = 0.45 and culminates near P/P₀, indicative of the presence of moderate mesoporous pores.

A comparative analysis of the textural parameters of the samples presented in Table 2 reveals notable disparities between the pure montmorillonite, MMT-TiO₂, and MMT-Cu₃TiO₅. The nitrogen adsorption results indicate a notable enhancement in the specific surface area (S_BET), total pore volume (V_tot), and micropore volume (V_micro) of the modified materials (MMT-TiO₂ and MMT-Cu₃TiO₅) in comparison to pure montmorillonite. The S_BET increases significantly with the addition of TiO₂ and Cu₃TiO₅, rising from 85 m²/g for pure MMT to 245 m²/g for MMT-TiO₂ and 279 m²/g for MMT-Cu₃TiO₅. The mean pore diameter (D) exhibited a notable increase from 5.84 Å for the MMT sample to 6.76 Å for the MMT-TiO₂ sample and 6.74 Å for the MMT-Cu₃TiO₅ sample. This suggests that the modified materials possess a more developed pore structure. This phenomenon has been previously documented in the literature for catalysts obtained via impregnation of active phases [46]. This increase can be attributed to the formation of novel adsorption sites and the expansion of the pore size distribution, particularly within the mesoporous range. The modified materials exhibit an elevated average pore diameter and a more extensive pore distribution, indicating an enhancement in their textural characteristics. The micropore volume represents 19% and 11% of the total pore volume, respectively, while the external surface area contributes 67% and 78% to the total specific surface area of the MMT-TiO₂ and MMT-Cu₃TiO₅ samples. The mesoporosity, which can be described as a “house of cards” type porous organization, indicates that the external surface is the primary site of interaction with the adsorbed molecules.

These findings suggest that the incorporation of TiO₂ and Cu₃TiO₅ into the montmorillonite structure results in the formation of more porous materials, which exhibit enhanced potential for applications in adsorption and catalysis.

2.4. Morphological Characterization by SEM

Figure 3 depicts SEM images of MMT, MMT-TiO₂, and MMT-Cu₃TiO₅. Bentonite particles manifest as irregular aggregates with considerable variation in size, as illustrated in Figure 3a. The surface is observed to be rough and porous, a quality also exhibited by clays such as bentonite [47]. Micrometer-sized filler clusters form agglomerates, as can be observed in Figure 3b,c. It is evident that MMT-TiO₂ also exhibits irregularly shaped particles but with a notable difference from those observed in pure bentonite. The particles appear more compact and have more defined structures, which is likely due to the presence of titanium dioxide. The surface displays a reduction in cracking and an increase in uniformity when compared to bentonite alone [48]. This phenomenon may be attributed to the adsorption or coverage of TiO₂ particles on the bentonite surface. TiO₂ particles have the potential to act as a protective layer, modifying the bentonite surface.

The MMT-Cu₃TiO₅ particles (Figure 3c) are not discrete entities; rather, they form irregular aggregates of micrometric dimensions. The surface of the aggregates is characterized by a high degree of roughness and the presence of numerous pores. This phenomenon can be attributed to the lamellar structure of the bentonite and the incorporation of the semiconductor Cu₃TiO₅. It is challenging to discern individual Cu₃TiO₅ particles in this image. This indicates the presence of either a highly dispersed TiO₂ phase within the bentonite matrix or a strong interaction between the two phases, which obscures the visibility of the TiO₂ particles. The particles and aggregates appear to be micrometric in size, which is consistent with the nature of the bentonite. Nevertheless, a certain particle size distribution is evident, with the presence of larger and smaller aggregates.

2.5. FTIR Characterization

FTIR spectroscopy provides valuable insight into the molecular structure and intermolecular interactions of materials. In the present case, a comparative analysis of the spectra of MMT, MMT-TiO₂, and MMT-Cu₃TiO₅ reveals the modifications induced by the incorporation of TiO₂ and Cu₃TiO₅ into the bentonite structure. Figure 4 depicts the FTIR spectra of MMT, MMT-TiO₂, and MMT-Cu₃TiO₅, which collectively demonstrate that all bentonite types exhibit absorption peaks that are characteristic of montmorillonite. The spectra display the characteristic bands of MMT at 3613, 3400, and 3229 cm⁻¹. These broad, intense bands are attributed to the valence vibrations of the O–H bonds of adsorbed water molecules and the structural hydroxyl groups of MMT [49,50].

The peak at 1631 cm⁻¹ is indicative of the angular deformation of adsorbed water molecules. The bands at 1106, 1003, 973, 916, 840, and 786 cm⁻¹ are indicative of the valence and strain vibrations of Si–O and Al–O bonds within the tetrahedra and octahedra of the MMT structure, as previously documented in references [51,52,53]. In comparison to MMT, the spectra of MMT-TiO₂ and MMT-Cu₃TiO₅ exhibit notable alterations. The O–H bands exhibit a shift and broadening. This indicates the presence of an interaction between water molecules and the TiO₂ and Cu₃TiO₅ species introduced into the montmorillonite structure. The appearance of new bands in the low wavenumber regions (below 1000 cm⁻¹) can be attributed to vibrations that are characteristic of Ti–O and Cu–O bonds. This evidence corroborates the incorporation of TiO₂ and Cu₃TiO₅ into the bentonite matrix. Furthermore, alterations in the relative intensity of the Si–O and Al–O bands suggest a disruption in the bentonite structure following the introduction of the metal species. These findings indicate that the integration of TiO₂ and Cu₃TiO₅ into the bentonite matrix alters its local environment by forming new bonds and disrupting existing ones. These modifications have the potential to significantly impact the physico-chemical properties of bentonite, including its CEC. As previously observed, there has been a notable enhancement in the cation exchange capacity and external specific surface area of bentonite. The external surface area of MMT-TiO₂ exhibited a 300% increase, while that of MMT-Cu₃TiO₅ demonstrated a 403% increase in comparison to the initial external surface area of MMT. This increase had a significant impact on the catalytic properties.

2.6. Thermogravimetric Analysis

The TGA thermograms of the materials illustrated in Figure S1 demonstrate distinct thermal behaviors. For MMT, a 1% mass loss is observed between 30 and 150 °C, which is attributed to the desorption of physically adsorbed water. A further loss of 5.72% is observed between 200 and 500 °C, extending up to 700 °C, corresponding to the dehydroxylation of structural hydroxyl groups. The profiles of MMT-TiO₂ and MMT-Cu₃TiO₅ are similar but show slight discrepancies. A mass loss of 3% is observed between 30 and 200 °C, which is attributed to the desorption of water and the elimination of hydroxyl groups.

A 7% loss is observed between 200 and 800 °C, which is attributed to phase transformations and dehydroxylation. Although these processes result in mass loss, they do not indicate a loss of thermal stability. The curve for MMT-Cu₃TiO₅ is slightly below that for MMT-TiO₂, indicating a stronger interaction between Cu₃TiO₅ and MMT. This interaction enhances thermal stability by preserving structural integrity during phase transformations and dehydroxylation.

In conclusion, the materials MMT-TiO₂ and MMT-Cu₃TiO₅ exhibit improved thermal stability compared to pure MMT, with MMT-Cu₃TiO₅ demonstrating the most favorable thermal stability, likely due to optimal interactions between Cu₃TiO₅ and the MMT matrix. The observed thermal stability beyond 500 °C for all samples indicates their potential for high-temperature applications. Additionally, the combined properties of montmorillonite, TiO₂, and Cu₃TiO₅ may offer promising prospects for catalytic or sorption applications.

2.7. Zeta Potential

In the Orange G (OG) solution (Figure 5a), the zeta potential of the bentonite decreases with increasing pH, indicating that the bentonite surface becomes increasingly negative. This phenomenon may be attributed to the deprotonation of the adsorption sites on the bentonite. In comparison, the zeta potential values of the bentonite in distilled water (Figure 5b) exhibit minimal variation, indicating that the adsorption sites remain predominantly protonated. The lack of interaction between bentonite and OG dye molecules, as evidenced by the decrease in the zeta potential of MMT-TiO₂ and MMT-Cu₃TiO₅ with increasing pH, also suggests that the surface of MMT-TiO₂ and MMT-Cu₃TiO₅ becomes increasingly negative. However, this reduction is less pronounced compared to that observed in bentonite. This phenomenon may be attributed to the deprotonation of adsorption sites. The acid activation of bentonite and the presence of TiO₂ may also result in the creation of new adsorption sites that exhibit differing reactivity with the dye compared to pure bentonite.

This is evidenced by the observed change in zeta potential with the presence of MMT-TiO₂ and MMT-Cu₃TiO₅ on OG, with the peak change occurring at pH = 3, followed by a gradual decrease as the pH value increases and then decreases. The zeta potentials of MMT-Cu₃TiO₅ are less negative than those of pure bentonite and MMT-TiO₂ at low pHs, indicating the potential for interaction between the copper in the composite and the dye, which could affect the surface charge. Zeta potential measurements demonstrate that the interactions between Orange G and the different materials are complex and are strongly influenced by pH. The addition of TiO₂ and Cu to the composites appears to modulate the effect of pH on the surface charge, which could have a significant impact on the adsorption capacity of the dye.

2.8. UV Spectroscopy of Photocatalysts

As illustrated in Figure S2, the UV–visible spectrum provides the evaluated values of the band gap energy (Eg) for the prepared materials. These values were determined by plotting (αhν)^0.5 as a function of photon energy (hν), where α represents the absorption coefficient; h is the Planck constant (4.14 × 10⁻¹⁵ eV·s), and ν is the photon frequency (Hz). The resulting UV–visible absorbance spectrum was obtained by measuring the absorbance (A) as a function of wavelength. The absorption coefficient (α) can be calculated from a spectrum [54] using the following equation:

α = 2.303A/t

(1)

where t is film thickness; α varies with the bandgap length of the semiconductor (E_g) and with the energy of the absorbed photon (hν) according to the Tauc equation:

αhν ∝ (hν − E_g)n

(2)

The experimental band gap of sample E can be obtained by identifying the point of intersection between the tangent to the linear portion of the curve and the abscissa axis (αhν)^1/n = 0. The value of n is contingent upon the type of electronic transition involved (e.g., n = 1/2 for a direct allowed transition).

The gap energy for MMT-TiO₂ and MMT-Cu₃TiO₅ was determined to be 2.7 eV and 2.15 eV, respectively, corresponding to the absorbance wavelengths of 459.2 nm and 568.8 nm.

2.9. Adsorption Kinetics

In order to investigate the photocatalytic degradation of OG, it was first necessary to determine the adsorption kinetics of the dye onto MMT-TiO₂ and MMT-Cu₃TiO₅ catalysts. The adsorption process, which precedes photodegradation, is of critical importance in establishing the efficiency of the catalyst. Batch adsorption experiments were conducted at 25 °C and pH 7, with an initial OG concentration of 40 mg/L and an adsorbent concentration of 2 g/L. The adsorption kinetics were monitored in the absence of light for a period of 300 min to ascertain the time required to reach equilibrium. The results depicted in Figure 6 provide valuable insights into the adsorptive properties of these catalysts and their potential for effective OG removal. The graph illustrates the temporal evolution of the quantity of OG dye adsorbed (in mg/g) for the two adsorbent materials as a function of time (in minutes). The materials under consideration are MMT-Cu₃TiO₅ and MMT-TiO₂. The quantity of dye adsorbed exhibited a rapid increase within the initial 15 min of contact. This indicates that the most accessible adsorption sites are rapidly occupied by dye molecules. This rapid adsorption can be attributed to physisorption, a process whereby dye molecules are attracted to the material’s adsorption sites by weak forces, such as van der Waals forces. Subsequently, the quantity of dye adsorbed reaches a plateau, indicating that equilibrium has been reached with regard to the adsorption process. The number of available adsorption sites is reached, and the amount of adsorbed dye no longer varies significantly. The equilibrium uptake of dye on MMT-TiO₂ is reached in 30 min with 3.60 mg/g (37%) of dye removal, while on MMT-Cu₃TiO₅, equilibrium is reached in 120 min with 9.11 mg/g (41.50%) of dye removal. The MMT-Cu₃TiO₅ material exhibits a greater adsorption capacity than the MMT-TiO₂ material, attaining a higher maximum adsorption amount.

The findings indicate that the integration of copper into the MMT-Cu₃TiO₅ structure markedly enhances its adsorption capacity. This can be attributed to the increase in specific surface area and total pore volume (279 m²/g and 0.11 cm³/g versus 245 m²/g and 0.10 cm³/g for MMT-TiO₂), which results in the creation of additional active sites for dye adsorption. Furthermore, the presence of copper introduces new specific adsorption sites and promotes coordination interactions between metal ions and dye molecules. While MMT-TiO₂ exhibits promising adsorption capacity, the nature of its active sites, predominantly associated with titanium dioxide, appears less conducive to interactions with the dye under investigation. The surface coverage rate, which reflects the efficiency of an adsorbent, was evaluated for the materials MMT-TiO₂ and MMT-Cu₃TiO₅. In accordance with the principle of proportionality between the surface area occupied by a molecule and its molecular surface area [55,56,57], it was estimated that OG used as a model dye occupies a surface area of 169.39 Å. The results demonstrate that MMT-Cu₃TiO₅ achieves a coverage rate of 6.87%, which is significantly higher than that of MMT-TiO₂ (3.23%). Moreover, the enhanced pore structure and pore size, as demonstrated by BET analysis, could also facilitate the adsorption process. Therefore, it can be postulated that the interactions between the dye and the MMT-Cu₃TiO₅ surface may be more robust, thereby enhancing the adsorption process.

In order to investigate the adsorption mechanism, the nonlinear forms of the pseudo-first-order [58] and the pseudo-second-order [59] kinetic rate equations are provided as Equations (3) and (4), respectively.

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}\left(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}}\right)$$

(3)

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}{1+{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}\mathrm{t}}}\mathrm{t}$$

(4)

where q_t and q_e represent the amount of dyes adsorbed at equilibrium (mg/g) and at time t (min), and k₁ (min⁻¹) and k₂ (g/mg min) are the pseudo-first-order and pseudo-second-order rate constants, respectively. The rate constants k₁, k₂, and q_e(cal) were obtained from plots of Equations (3) and (4), as well as from q_e(exp) (Figure 6). The resulting values are gathered in

Table 3. Kinetic for the adsorption of Orange G dye onto the catalysts MMT-TiO₂ and MMT-Cu₃TiO₅. PFO and PSO correspond to pseudo-first-order and pseudo-second-order models, respectively.

Adsorbents	qe (exp) mg/g	PFO				PSO
		k1 min−1	qe1 cal mg/g	R2	χ2	k2 g/mg min	qe2 cal mg/g	R2	χ2
MMT-TiO2	3.60	0.180	3.61	0.997	0.003	0.206	3.66	0.999	0.0009
MMT-Cu3TiO5	9.11	0.099	8.79	0.968	0.303	0.021	9.14	0.939	0.075

, along with the correlation coefficients R² and the chi-square Test (χ²).

The pseudo-first-order and pseudo-second-order kinetic models demonstrated satisfactory fits to the experimental data. However, the rate constants (k₁ = 0.180 min⁻¹ and k₂ = 0.206 g/mg.min) for MMT-TiO₂ are significantly higher than those for MMT-Cu₃TiO₅. The rate constants (k₁ = 0.099 min⁻¹ and k₂ = 0.021 g/mg. min) indicate that the adsorption kinetics of OG dye on MMT-Cu₃TiO₅ follows a relatively slow process. With regard to the correlation coefficient, the value of 0.999 for MMT-TiO₂ is slightly higher than that of 0.997, indicating that the pseudo-second-order model provides a superior description of the adsorption kinetics for this material. In contrast, for MMT-Cu₃TiO₅, the value of 0.939 is less than 0.968, indicating that the first-order model may be more applicable to this material, although the fit is slightly inferior to that of MMT-TiO₂. The chi-square test (χ2) indicates a discrepancy between the models and the experimental data, with low values for the pseudo-second-order model. Nevertheless, the pseudo-second-order model demonstrates an excellent correlation with the experimental data, suggesting that this model more accurately describes the adsorption kinetics of the OG dye for both materials. These results indicate that the adsorption of the dye likely follows a more intricate mechanism, consistent with the assumptions of the pseudo-second-order model. This suggests that specific interactions, such as multilayer formation or chemical reactions at the active sites, play a pivotal role in the adsorption process. This finding reflects the heterogeneous nature of adsorption on MMT-TiO₂ and MMT-Cu₃TiO₅ materials, with the pseudo-second-order model offering the most accurate description of the observed process.

2.10. Study of the pH Effect on the Adsorption and Photodegradation of Orange G onto MMT-TiO₂ and MMT-Cu₃TiO₅

The adsorption process is significantly influenced by pH, as it affects the surface charge of the adsorbents (MMT-TiO₂ and MMT-Cu₃TiO₅) and the ionic form of the anionic dye OG (Figure S3). The adsorption of the anionic dye OG on MMT is found to be negligible under all pH conditions, indicating that pure montmorillonite is not an effective adsorbent for the dye at any pH. The highest adsorption is observed at pH 3, reaching approximately 52.48% on MMT-Cu₃TiO₅ and 31.01% on MMT-TiO₂, demonstrating excellent efficiency under acidic conditions. The zeta potential was found to be approximately +2.81 mV for MMT-Cu₃TiO₅ and +1.05 mV for MMT-TiO₂, indicating a significant positive charge. The anionic form of OG was the predominant species at pH values greater than 1, resulting from the deprotonation of its two SO₃H groups (pK_a = 1) [60]. This finding is consistent with the high adsorption of the dye at this pH, which suggests a favorable electrostatic interaction between the anionic dye and the positively charged material. At higher pH values, above pH = 7, the zeta potential becomes slightly negative (approximately −0.49 mV and −11.6 mV for MMT-Cu₃TiO₅ and MMT-TiO₂, respectively), which may account for the slight decrease in adsorption observed. The efficiency of the MMT-Cu₃TiO₅ material declines, though it remains relatively stable at approximately 35% between pH 4 and 7 and between 24.28% and 19.43% between pH 4 and 9 for MMT-TiO₂. The decline in efficiency with rising pH levels suggests that the reaction mechanism is more complex than a straightforward electrostatic interaction between oppositely charged species. Nevertheless, the rate of adsorption declines precipitously at pH values exceeding 9. At elevated pH levels, the repulsion between the negatively charged surfaces of the materials and the anionic dye (OG) increases, resulting in a reduction in adsorption efficiency. This is due to a reduction in favorable electrostatic interactions, which results in a decline in the efficacy of the adsorption process.

The adsorption process of the anionic azo dye OG on the nanocomposites MMT-TiO₂ and MMT-Cu₃TiO₅ appears to be primarily regulated by physisorption forces. At acidic and slightly acidic pH levels, electrostatic interactions are established between the sulfonate groups (SO³⁻) of the anionic dye and the positive surface charges of the nanocomposites. Moreover, polar forces generated by an electric field within the micropores of the nanocomposites, as well as potential hydrogen bonds with the silanol (-SiOH) and aluminol (-AlOH) groups present on some nanocomposites, may also play a pivotal role in the adsorption process [61]. Therefore, the capacity of bentonite to adsorb anionic azo dye molecules can be markedly augmented through the immobilization of Cu₃TiO₅ nanoparticles on its surface. The efficiency of OG degradation using MMT-Cu₃TiO₅ at pH 3 was 99% after 120 min.

2.11. Photolytic Behavior of Orange G Dye

Figure 7 illustrates the photolysis kinetics of the degradation of OG in an aqueous solution with an initial concentration of 70 mg/L at pH 3 under LC8 lamp irradiation. As the duration of irradiation increases, the degradation of OG initially rises, reaching a maximum of 1% at 30 min. Further exposure beyond this point has a negligible impact. This indicates that the optimal concentration of hydroxyl radicals is reached within 30 min, thereby limiting further degradation of the dye’s functional groups [62]. In the absence of a catalyst, the degradation of OG remains low, irrespective of the duration of irradiation.

The results presented in Figure 7 illustrate the substantial influence of the MMT-TiO₂ and MMT-Cu₃TiO₅ catalysts on the photocatalytic degradation of OG. The MMT-TiO₂ catalyst markedly enhances the degradation process compared to simple photolysis, although complete degradation is not attained within a 300-minute period, with a rate of 96%. The addition of montmorillonite serves to stabilize the TiO₂, thereby optimizing its photocatalytic performance. Nevertheless, the incomplete degradation may be attributed to saturation of the active sites or diffusion limitations. The MMT-Cu₃TiO₅ catalyst demonstrated significantly enhanced performance. Its structure, with a band gap of 2.15 eV in comparison to 2.7 eV for MMT-TiO₂, allows for greater absorption of visible light. The immobilization of Cu₃TiO₅ to the montmorillonite system also appears to facilitate the generation of reactive radicals (OH•) and enhance selectivity in the attack on dye molecules, resulting in nearly complete degradation (100%) of the dye within 150 min. This distinctive MMT-Cu₃TiO₅ combination may be attributed to the enhanced separation of electron–hole pairs, which reduces recombination and enhances the efficiency of the photocatalytic reaction.

In order to ascertain the order of the OG degradation reaction, the experimental data were fitted to nonlinear kinetic models of pseudo-first-order (PFO) in accordance with Equation (5) and pseudo-second-order (PSO) in accordance with Equation (6). The best fit was identified through evaluation of the coefficient of determination (R²) and the chi-square test (χ2), which revealed that the predominant reaction order was as follows:

$${\displaystyle \frac{\mathrm{C}}{{\mathrm{C}}_0}}=e^{-{\mathrm{k}}_1\mathrm{t}}$$

(5)

$${\displaystyle \frac{\mathrm{C}}{{\mathrm{C}}_0}}={\displaystyle \frac{1}{1+{\mathrm{k}}_{2 }{\mathrm{C}}_0\mathrm{t}}}$$

(6)

where C₀ represents the initial dye concentration (in mg/L); C denotes the concentration at time t (in mg/L); k₁ signifies the rate constant of pseudo-first-order degradation (in units of minutes/L), and k₂ denotes the rate constant of pseudo-second-order degradation (in units of L mg⁻¹ min⁻¹).

The experimental results were analyzed using pseudo-first- and pseudo-second-order kinetic models. Figure 7 presents a comparison of the two models, thereby enabling the identification of the model that most accurately describes the kinetics of the studied reaction. The regression coefficients and the chi-square (χ²) test (

Table 4. Kinetics constants for pseudo-first- and pseudo-second-order models for adsorption of Orange G dye on MMT-TiO₂ and MMT-Cu₃TiO₅.

Sample	Pseudo—First—Order			Pseudo—Second—Order
	k1 (1/min)	R2	χ2	k2 (L mg−1 min−1)	R2	χ2
MMT-TiO2	6.76 10−3	0.874	0.0187	1.61 10−4	0.743	0.0384
MMT-Cu3TiO5	12.3 10−3	0.827	0.034	2.80 10−4	0.708	0.0577

) demonstrated that the pseudo-first-order model provided the most accurate representation of the experimental data. This observation is consistent with prior research on the photodegradation kinetics of OG dye on diverse catalyst surfaces [63].

The photodegradation of OG dye using the MMT-Cu₃TiO₅ catalyst was found to be markedly more efficient than that achieved with the MMT-TiO₂ material. This discrepancy in performance can be attributed to the enhanced specific surface area and positive surface charge of MMT-Cu₃TiO₅, which facilitate superior photocatalytic activity. The MMT-Cu₃TiO₅ nanocomposite was tested at varying catalyst concentrations (2, 2.66, 3.33, 4, and 4.66 g/L) to ascertain the optimal concentration for the photodegradation of OG. The results are presented in Figure 8. In contrast with conventional methodologies, the reaction medium was not maintained in a dark environment prior to irradiation; instead, it was directly exposed to the UV–Visible lamp. Time point t₀ = 0 corresponds to the initiation of the solution’s exposure to the light, thereby marking the commencement of the photocatalysis experiment. However, the photolysis of OG dye in the absence of any photocatalyst resulted in minimal dye degradation, indicating that the degradation process was exclusively photocatalytic, as OG was not susceptible to UV radiation in the absence of the photocatalyst. This result is consistent with those reported by Derya et al. [64], wherein the photolysis of OG resulted in minimal dye degradation. The results demonstrate a rapid reduction in the concentration of OG, reaching 100% after only 180 min of irradiation for catalyst concentrations of 2.66, 3.33, 4, and 4.66 g/L of MMT-Cu₃TiO₅. Furthermore, additional experiments were conducted at a catalyst concentration of 2 g/L of MMT-Cu₃TiO₅, as higher concentrations resulted in an excessively rapid degradation process, yielding a limited number of exploitable data points. In the study of the rate at which a reaction occurs on the surface of a catalyst (heterogeneous catalysis), the Langmuir–Hinshelwood model is a commonly employed model. The model posits that reactant molecules initially adsorb onto the catalyst surface before undergoing a reaction with one another. The interaction on the catalyst surface directly influences the reaction rate, which can be quantified using Equation (7).

$${ \mathrm{V}}_0={\displaystyle \frac{\mathrm{k}{\mathrm{K}}_{\mathrm{L}}\mathrm{C}}{1+\mathrm{k}{\mathrm{K}}_{\mathrm{L}}\mathrm{C}}}$$

(7)

In this equation, k represents the reaction rate constant; K_L is the adsorption equilibrium constant that characterizes the affinity between the reactant and the catalyst surface, and C denotes the instantaneous concentration of the reactant in the solution. Daneshvar and colleagues [65] proposed an adaptation of the Langmuir–Hinshelwood model for the specific purpose of investigating the impact of the quantity of TiO₂ employed as a catalyst Equation (8).

$${ \mathrm{V}}_0=\mathrm{k}\mathrm{K}\left[{\mathrm{T}\mathrm{i}\mathrm{O}}_2\right]={\mathrm{k}}_{\mathrm{a}\mathrm{p}\mathrm{p}}\left[{\mathrm{T}\mathrm{i}\mathrm{O}}_2\right]$$

(8)

In this context, k_app represents the apparent first-order rate constant. The experimental data are presented in Figure S4 for purposes of illustration.

Figure S4 illustrates that an increase in the concentration of MMT-Cu₃TiO₅ is accompanied by an enhancement in the initial reaction rate. This evidence substantiates the hypothesis that MMT-Cu₃TiO₅ plays a catalytic role in the degradation of the OG dye. The relationship between V₀ and catalyst concentration appears to be approximately linear over the range of catalyst concentrations studied. This figure demonstrates that Equation (8) is verified, thereby validating the Langmuir–Hinshelwood model within the specified range of catalyst concentration. This study allows us to conclude that the photodegradation of OG dye in the presence of MMT-Cu₃TiO₅ can occur via two simultaneous processes: adsorption and photocatalysis, with a rate constant of 0.1646 min⁻¹.

2.12. Regeneration of MMT-Cu₃TiO₅ Catalyst After Orange G Adsorption and Photodegradation

The potential for repurposing the MMT-Cu₃TiO₅ photocatalyst is of significant economic and practical value, necessitating comprehensive life cycle assessment and the identification of optimal treatment methodologies.

The potential for reusing MMT-Cu₃TiO₅ was investigated for up to four cycles of OG degradation under the following experimental conditions: an OG concentration of 70 mg/L and a catalyst concentration of 2 g/L, comprising MMT-TiO₂ and MMT-Cu₃TiO₅. The adsorption process was monitored for a duration of 30 min, and the photocatalytic process was monitored for a duration of 120 min at a pH of 3 and a temperature of 25 °C. Subsequent to the completion of each process, the catalyst was extracted from the reaction mixture via centrifugation and subjected to a wash with the same solution that was recovered following the catalyst synthesis process, thereby maintaining the same acidity. Subsequently, the material was washed with distilled water, recovered by centrifugation, and dried at 80 °C. The catalyst’s weight was measured both before and after the regeneration process, and no appreciable loss of the catalyst was observed during this procedure.

As illustrated in Figure 9, the adsorption capacity of the catalyst exhibited a decline over time. However, the catalyst demonstrated the capacity to sustain the degradation of the dye throughout the same period, which substantiates its ability to maintain the production rate of and the same dye degradation rates. The findings demonstrate the capacity of the MMT- Cu₃TiO₅ catalyst to sustain its photocatalytic efficacy following four successive degradation cycles, exhibiting a modest decline in the extent of OG degradation.

Initially, the capacity of the catalyst to purify itself from adsorbed materials was observed, as evidenced by the DRX and FTIR results (Figure 10 and Figure 11, respectively). The absence of new materials after use indicates that the catalyst utilized the photocatalytic properties of the dye to achieve complete degradation.

3. Materials and Methods

3.1. Catalysts Preparation

In this study, the support material was untreated bentonite from Sigma-Aldrich, an affiliate of Merck KGaA, Darmstadt, Germany. The bentonite utilized is predominantly composed of montmorillonite (MMT). Moreover, the composition is validated by mineralogical and chemical analyses. The particle size is ≤25 µm, and the bentonite in question exhibits a high cation exchange capacity (CEC) of 213 meq/100 g. The general structural formula is (Si_3.89Al_0.10)^IV(Al_0.93Fe_0.32Mg_0.61Ti_0.02)^VIO₁₀Na_0.33Ca_0.23K_0.02(OH)₂,nH₂O. The basic chemical composition is presented in Table 1.

The sol–gel method was employed to synthesize two catalysts, designated as MMT-TiO₂ and MMT-Cu₃TiO₅ (Figure 12). The synthesis of MMT-TiO₂ was initiated with the preparation of a solution of TiCl₄. To this end, 2.8 mL of TiCl₄ (99.9%, Fluka/Honeywell, Charlotte, NC, USA) was mixed with ethanol (99.8%, Fluka/Honeywell) and distilled water (2 mL, at 5 °C). This solution was then added, under continuous stirring, to a suspension of montmorillonite clay at a concentration of 1% (w/w), maintaining a Ti/clay mass ratio of 2:1. This ratio was selected to ensure adequate titanium loading for effective immobilization of TiO₂ on the clay surface. After a 24-h aging period, the resulting material was subjected to a thorough washing process to remove any residual unreacted precursors. It is important to note that, rather than achieving a completely homogeneous coverage of the clay surface, the final material consists of a composite of TiO₂ and montmorillonite. The titanium species involved include both TiO₂ and potential residual titanium compounds (such as titanium hydroxides), which are not uniformly distributed on the clay surface. The composite was then dried at 80 °C for 24 h and sieved in order to obtain the MMT-TiO₂ catalyst. In the case of MMT-Cu₃TiO₅, copper was introduced as a modifying agent in a controlled amount. A specific quantity of CuCl₂ was dissolved in 2 mL of distilled water and subsequently added to the TiO₂ precursor solution. The quantity of CuCl₂ was calculated in order to achieve a Cu/Ti mass ratio of 0.02/2. This low ratio was deliberately selected to optimize the synergistic interaction between Cu and TiO₂; excessive Cu loading, which could lead to particle agglomeration or interference with photocatalytic activity, was, thus, avoided. Subsequently, the Cu-modified TiO₂ solution was immobilized onto montmorillonite in accordance with the same procedure employed for MMT-TiO₂ synthesis. Following immobilization, the MMT-Cu₃TiO₅ catalyst was subjected to the same washing, drying (80 °C for 24 h), and sieving steps as MMT-TiO₂ synthesis. Although the low Cu/Ti ratio may result in the limited detectability of Cu-containing phases by XRD, the sol–gel synthesis ensures the homogeneous distribution of Cu species. Furthermore, the incorporation of Cu enhances the visible light absorption and photocatalytic performance of the final material.

3.2. Characterization of Catalysts

A variety of physicochemical techniques were utilized to characterize the catalyst materials. The chemical composition was determined using an Agilent Technologies Inductively Coupled Plasma Emission Spectrometer (ICP-AES 5110, dual view, Santa Clara, CA, USA). The structure and crystallinity were analyzed by PXRD using a Siemens D-5000 diffractometer with CuKα radiation (1.5406 Å). Patterns were recorded over the 5–80° 2θ range in steps of 0.04° with a counting time per step of 8 s. The infrared absorption (IR) spectra were determined between 4000 and 400 cm⁻¹ using a PHILIPS PU 9714 spectrophotometer.

Thermogravimetric analysis (TGA) was performed with a NETZSCH TG 209F1 Libra (Selb, Germany) with a mass resolution of 3 µg and using HT platinum plates. The analysis of the samples with an average weight of 8 mg was conducted under a nitrogen atmosphere, with a flow rate of 20 mL·min⁻¹. The samples were subjected to a heating ramp of 10 °C·min⁻¹ within the temperature range of 40 to 800 °C. The textural properties of the prepared samples were determined through the analysis of nitrogen adsorption/desorption isotherms, which were measured with a Micromeritics ASAP 2020 analyzer (Norcross, GA, USA). The specific surface area was determined by the Brunauer–Emmett–Teller (BET) multi-point analysis method from nitrogen adsorption at 77 K. The total pore volume was calculated from the amount of gas adsorbed at (P/P₀) = 0.99, and the micropore volume was calculated using the Barrett–Joyner–Halenda (BJH) model on the desorption branch.

Scanning electron microscopy (SEM) observations were conducted using a JEOL JSM-7800F apparatus (Tokyo, Japan), operating from 0.5 to 30 kV, which was equipped with an energy dispersive spectrometer (EDX) for chemical analysis. The powder material was dispersed on a support with a double-sided conductive carbon tape. To prevent electronic charging, a nanometer-thick chrome deposit was applied to the preparation.

Zeta-potential measurements were conducted at room temperature using a Zetasizer (Nano-ZS model, Malvern Instruments, Malvern, UK). To investigate the impact of pH on the surface charge of MMT, MMT-TiO₂, and MMT-Cu₃TiO₅, a series of aqueous solutions with varying pH values were prepared by adjusting the pH of the MMT/water mixtures with 0.1 N HCl or NaOH solutions. The material was dissolved in a solution of 2 g·L⁻¹ at a volume of 10 mL. Each zeta-potential value was determined in triplicate using three distinct samples.

The concentration of the dye in the solution was determined using a Cary 100 spectrophotometer (Agilent, Santa Clara, CA, USA), which was operated in absorbance mode. The bandgap energy of the photocatalyst material was calculated using the Tauc equation. In order to select an appropriate light source for photodegradation experiments, a Hamamatsu LC8 light source was chosen, as it emits light with energy greater than or equal to the bandgap energy of the materials, thereby generating the (e, hole) couple and allowing for a series of photodegradation processes to be performed.

3.3. Photoreactor and Analytical Methods

Irradiation experiments were conducted in a photocatalytic oxidation reactor connected to a thermostatic bath equipped with a water-cooling circuit to absorb infrared radiation, thus preventing the solution from heating (Figure S5). At the core of the cylindrical reactor is a 150-watt xenon lamp (Hamamatsu LC8) coupled to an optical fiber positioned perpendicular to the UV–visible light source. The lamp emits a broad-spectrum irradiation, as illustrated in Figure S6, which depicts a peak in the visible range. The optical fiber tip was maintained at a distance of 3 cm from the reactor, and the reaction mixture was continuously agitated using a magnetic stirrer.

A solution of Orange G dye at a concentration of 0.5 g/L was prepared using distilled water as the solvent. The requisite concentrations for the photocatalysis experiments were subsequently derived from this stock solution. In particular, the impact of solution acidity on the adsorption process was examined using a dye concentration of 70 mg/L in 30 mL of solution. The pH of the solution was maintained through the addition of 0.1 N NaOH or 0.1 N HCl. Subsequently, the impact of varying the catalyst quantity on the photocatalytic degradation of the dye was investigated, with the optimal pH being identified as 3. This was performed to identify the appropriate catalyst dosage that maximizes the photocatalytic degradation efficiency and minimizes the treatment time. The degradation of OG (Orange G) was monitored by measuring absorbance using a UV–vis spectrophotometer at a wavelength of 478 nm. The absorbance measurements were converted to concentrations using a standard calibration method (r = 0.9999).

In order to prevent photodegradation, which could otherwise occur during the course of an adsorption experiment, tests are conducted in the dark. This ensures that only adsorption occurs. Once equilibrium has been reached, the catalyst is separated from the solution, and the concentration of the dye is then measured using visible spectroscopy in order to determine the amount adsorbed. In photodegradation experiments, the lamp (LC8) is activated to initiate the photodegradation process. In this instance, the catalyst is subjected to illumination, and any diminution in the concentration of the dye is attributable to degradation rather than adsorption. The presence or absence of light serves as a clear distinction between the two processes. When the material is exposed to light (hν), an electron is excited from a valence band to a conduction band, thereby creating an electron–hole pair.

$$\mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5+h\nu \to \mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5\left(h^{+}+e^{-}\right)$$

A positive hole (h⁺) can react with water adsorbed on the catalyst surface to form hydroxyl radicals (OH^•), which are highly reactive oxidizing agents.

$$\mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5\left(h^{+}\right)+H_{2}O\to \mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5\left({OH}^{•}\right)+H^{+}$$

The electron (e⁻) is capable of reacting with molecular oxygen that has been adsorbed on the catalyst surface, resulting in the formation of superoxide anions (O₂^−•). These species are also powerful oxidizers.

$$\mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5\left(e^{-}\right)+O_2\to \mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5\left({O_2^{-}}^{•}\right)$$

Hydroxyl radicals (${OH}^{•}$) can directly attack dye molecules, resulting in the oxidation and degradation of these molecules into inorganic products.

$$\mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5\left({OH}^{•}\right)+dye\to degradation product+\mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5$$

Additionally, superoxide anions $\left({O_2^{-}}^{•}\right)$ have been demonstrated to facilitate the degradation of dyes, either directly or by the generation of other reactive species.

$$\mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5\left({O_2^{-}}^{•}\right)+dye\to degradation product+\mathrm{M}\mathrm{M}\mathrm{T}-{\mathrm{C}\mathrm{u}}_3{\mathrm{T}\mathrm{i}\mathrm{O}}_5$$

This type of mechanism is frequently employed to elucidate the phenomenon of heterogeneous photocatalysis, which has a multitude of applications in the remediation of water pollution.

4. Conclusions

This study successfully synthesized and characterized MMT-TiO₂ and MMT-Cu₃TiO₅ nanocomposites using a sustainable sol–gel method, thereby demonstrating notable enhancements in their structural and optical properties. The specific surface area exhibited a notable increase, with XRD analysis confirming the presence of TiO₂ anatase and Cu₃TiO₅. The optical analysis demonstrated that MMT-Cu₃TiO₅, exhibiting a lower bandgap energy of 2.15 eV, demonstrated enhanced absorption of longer wavelengths of visible light in comparison to MMT-TiO₂. The MMT-Cu₃TiO₅ catalyst demonstrated complete degradation of Orange G dye within 120 min under optimal conditions (pH 3, catalyst concentration 2 g/L), outperforming the MMT-TiO₂ catalyst. Furthermore, the MMT-Cu₃TiO₅ catalyst can be washed and reused multiple times without the addition of chemical agents, thus demonstrating its sustainability. In conclusion, the MMT-Cu₃TiO₅ nanocomposites display enhanced structural, optical, and photocatalytic properties, rendering them highly promising for wastewater treatment applications, particularly in the degradation of organic contaminants such as Orange G dye.