The Development of a New Bi₁₂ZnO₂₀/AgI Heterosystem for the Degradation of Dye-Contaminated Water by Photocatalysis Under Solar Irradiation: Synthesis, Characterization and Kinetics

Abstract

This study explores the efficiency of heterogeneous photocatalysis in wastewater treatment, which is recognized for inducing significant rates of degradation and mineralization of various contaminants, including dyes. The study focuses on the development of an innovative composite via a combination of the sillenite type semiconductor Bi₁₂ZnO₂₀ and the halide-type semiconductor AgI. Both semiconductors were synthesized via co-precipitation, and their phases were identified using X-ray diffraction and characterized by scanning electron microscopy, Raman spectroscopy, Brunauer–Emmett–Teller analysis for specific surface area, UV–Visible diffuse reflectance spectroscopy, and the point of zero charge. The evaluation of the photocatalytic activity of the Bi₁₂ZnO₂₀/AgI heterosystem was carried out by monitoring the degradation process of Basic Blue 41 (BB41) under solar irradiation conditions. The results of this study revealed that the Bi₁₂ZnO₂₀/AgI heterosystem achieved the efficient degradation of BB41, with a removal rate of 98% after 150 min of treatment. The mineralization study showed that the TOC value decreased from 19.89 mg L⁻¹ to 6.87 mg L⁻¹, indicating that a significant portion of BB41 was mineralized. Via kinetic research, it was established that the degradation process followed a pseudo-first-order mechanism. Furthermore, recycling tests showed that the synthesized heterostructures maintained good structural stability and acceptable reusability over several cycles. These findings highlight the potential of heterogeneous photocatalysis as a promising approach to addressing environmental challenges associated with azo dyes.

1. Introduction

One of the major causes contributing to the scarcity of drinking water and freshwater is the degradation of the aquatic environment due to pollution. The deterioration of water quality is primarily caused by human activities, whether industrial, medical, agricultural, or domestic [1]. To date, industrial pollution ranks among the main factors significantly impacting the degradation of aquatic ecosystems [2]. The textile industry has become one of the leading contributors to the pollution of surface and groundwater reservoirs. The production and use of synthetic dyes for fabric dyeing have thus grown into massive industries. More than 10,000 pigments and dyes are used in the textile sector [3]. Each year, approximately 60,000 tons of dyes, nearly 80% of which are azo dyes, are released into the environment worldwide as waste [4]. Even at low concentrations (≤1 ppm), textile dyes can significantly harm aquatic ecosystems due to their high chemical oxygen demand (COD) and biological oxygen demand (BOD). These metrics indicate the potential for oxygen depletion in water bodies, which is critical for aquatic life. Most textile dyes are not only toxic but also highly resistant to biodegradation [5]. Their complex chemical structures prevent effective breakdown by microorganisms, leading to sustained high BOD levels in water. The elevated BOD levels resulting from the presence of these non-biodegradable dyes can lead to hypoxic conditions, which are detrimental to fish and other aquatic organisms. Azo dyes account for 50% of the dyes used in the textile industry. This type of dye is known for its persistence in the environment and the complexity of its degradation [6]. Currently, 2000 types of azo dyes are used worldwide, with 70,000 tons produced annually [7]. These dyes contain a double bond between two nitrogen atoms (-N=N-) [8]. Basic Blue 41 was selected as the model azo dye for this study, as it is among the most widely used dyes in the textile industry [9]. Furthermore, this dye is classified as a substance with a high level of toxicity [10].

In response to this challenge, research must focus on developing economically viable and competitive processes compared to conventional water treatment technologies. Heterogeneous photocatalysis emerges as a promising alternative for wastewater purification [11,12,13,14]. Within the framework of advanced oxidation processes (AOPs), the application of heterogeneous photocatalysis stands out as an optimal solution for the removal of organic pollutants, primarily due to its ability to efficiently harness natural and renewable solar energy [15]. This approach is based on the principle of photoexcitation of a semiconductor material, achieved through a light source with sufficient energy (hʋ ≥ Eg). Several high-performance semiconductor nanomaterials have been developed and utilized for this purpose. Among these photocatalysts, bismuth oxide-based semiconductors have been attracting increasing attention from researchers due to their numerous applications in photocatalysis, as well as their unique properties, such as chemical and thermal stability, strong visible light absorption, rapid interfacial charge separation, and low optical band gap energy [16,17].

The present study focuses on the sillenite family, characterized by the general formula Bi₁₂MO₂₀ [18], as a promising new series of bismuth oxide-based photocatalysts, where M (Zn, Ti, Co, Si, Ge,…) is a tetravalent ion. The crystalline structure of sillenite can be described as an interconnection between MO₄ tetrahedra and BiO₈polyhedra through oxygen atoms [19], as shown in Figure 1. The high recombination rate of electron-hole pairs in semiconductors is a significant limitation to their efficiency as photocatalysts, as well as their practical applications in pollution remediation [20]. Consequently, research is focused on overcoming this challenge by exploring innovative strategies. This research aims to develop a new photocatalyst with a high photocatalytic performance for the elimination of an organic pollutant (textile dye) in an aqueous solution, with the perspective of providing an economically and ecologically viable solution. One crucial approach involves combining these photocatalysts with other semiconductor nanomaterials, thereby forming heterosystem photocatalysts. Our choice focused on combining a zinc oxide sillenie (Bi₁₂ZnO₂₀) with silver iodide (AgI), which efficiently absorbs visible light [21]. This selection aims to maximize the advantages of each component to achieve more effective and environmentally friendly pollution remediation. The synthesized Bi₁₂ZnO₂₀/AgI heterosystem was used for the photodegradation of a textile dye (Basic Blue 41, BB41). Various characterization techniques were employed to determine the structural, morphological, and optical behavior of the nanomaterials and the developed heterosystem, notably TGA-DSC, XRD, SEM, Raman, optical band gap, and pHpzc. Parametric analysis was performed to study pollutant elimination in detail, with the aim of optimizing the operating conditions to ensure efficient removal. To our knowledge, this is the first study to report both the synthesis of this specific heterostructure and its successful application for BB41 removal under natural sunlight conditions, offering a sustainable and cost-effective alternative to existing photocatalytic systems.

2. Materials and Methods

2.1. Materials

All necessary precautions were taken before the start of our study to prevent any contamination of the raw materials and reagents used. The chemicals used are as follows: bismuth nitrate pentahydrate Bi(NO₃)₃·5H₂O (98.5%, Chem-Lab Algiers, Algeria), zinc acetate dihydrate (CH₃COO)₂Zn·2H₂O (99.5%, Chem-Lab), silver nitrate AgNO₃ (99.8%, Biochem Chemo-pharma Algiers, Algeria), potassium iodide KI (99%, Chem-Lab), and nitric acid HNO₃ (65%, Chem-Lab). These compounds were used for the synthesis of semiconductors as well as the Bi₁₂ZnO₂₀/AgI heterosystem. Additionally, HCl (37%, Biochem Chemo-pharma) and NaOH (98%, Biochem Chemo-pharma) were also used to adjust the pH of the different solutions. Double-distilled water was used for solution preparation, and Basic Blue 41 C₂₀H₂₆N₄O₆S₂ was supplied by the textile industry Fital.

2.2. Synthesis of Semiconductors and Preparation of the Heterosystem

The sillenite Bi₁₂ZnO₂₀ was synthesized via the co-precipitation method [22] using stoichiometric amounts of Bi(NO₃)₃·5H₂O and (CH₃COO)₂Zn·2H₂O. A precise quantity of each reagent was measured and then separately dissolved in distilled water with the addition of 5% nitric acid (HNO₃), followed by stirring. The two homogeneous solutions were then mixed under continuous stirring, and NaOH was added dropwise until a pH of 12 was reached. The resulting suspension was left to settle for 24 h in a closed environment to allow complete precipitation. The precipitate was regularly washed with distilled water until a pH of 7 was reached; then, it was filtered and dried in an oven at 100 °C for 24 h. The dried solid was ground using an agate mortar. Finally, the obtained powder was subjected to calcination at 850 °C for 4 h (this temperature was selected based on the results of the TG-DSC analysis) to obtain the desired zinc oxide sillenite phase. Figure 2 illustrates the key steps of the sillenite formation process.

Silver iodide was synthesized by separately dissolving 16.98 g of AgNO₃ and 16.6 g of KI in 100 mL of distilled water under constant stirring for 20 to 30 min. The KI solution was then gradually added to the AgNO₃ solution. The resulting light green precipitate was filtered, dried at 80 °C for 48 h, and then ground in an agate mortar.

The Bi₁₂ZnO₂₀/AgI heterosystem was obtained through a simple physical mixing process. The synthesized zinc oxide sillenite was combined with AgI in a ratio of 75% Bi₁₂ZnO₂₀ and 25% AgI. The two powders were carefully homogenized and then ground in an agate mortar until a fine and homogeneous powder was obtained.

2.3. Characterization Techniques

To deepen our understanding of the physicochemical properties as well as the structural, morphological, and optical behavior of the synthesized nanomaterials, various characterization techniques were employed. The temperature required for the formation of the sillenite zinc oxide phase was determined using thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC); this was conducted with a Setaram KEP TECHNOLOGIES analyzer. The identification of the crystalline phases of the nanomaterials was carried out by X-ray diffraction (XRD) using a PANalytical Empyrean diffractometer equipped with monochromatic copper radiation Cu Kα (λ = 1.540598 Å). We scanned within the 2θ range (5–90°). Surface morphology analysis was performed using scanning electron microscopy (SEM) with a JEOL JCM 6000 microscope. The synthesized samples were analyzed by Raman spectroscopy using a RENISHAW Centrus 0LL824 microscope system. The measurements were performed with a 532 nm excitation laser, scanning the spectral range between 5 and 700 cm⁻¹. Surface area measurements were conducted via nitrogen physisorption analysis employing the BET (Brunauer–Emmett–Teller) method. The experiments were performed at a set liquid nitrogen temperature (77.35 K) using N₂ gas with a molecular cross-sectional area of 16.2 Ų and a liquid density of 0.808 g/cm³. To enhance the electrical conductivity of the samples and optimize image clarity, a gold coating was applied via metallization for 2 min. The optical band gap energy (E_g) was determined using UV–Visible diffuse reflectance spectroscopy with a SPECORD 210 PLUS spectrophotometer. This instrument operates within a wavelength range of 250 to 1100 nm at room temperature, with a scanning speed of 10 nm s⁻¹, and the point of zero charge (pH_PZC) of the heterosystem was determined following the method reported by Ferro Garcia [23].

2.4. Study of the Photodegradation of Basic Blue 41

To assess the ability of the synthesized heterosystem to adsorb BB41, a kinetic study was conducted to determine the adsorption equilibrium, thereby facilitating subsequent photocatalytic process. Adsorption was performed by mixing 0.2 g of the photocatalyst at a dosage of 1 g L⁻¹ in 200 mL of the polluted solution with an initial concentration of 10 mg L⁻¹. The solution was continuously stirred in the dark at room temperature for 4 h using a magnetic stirrer at an average speed of 350 rpm to establish adsorption–desorption equilibrium while preventing any unwanted photocatalytic activity by the semiconductors.

All photocatalytic experiments were conducted in an open batch system during peak sunlight hours (10:00 AM to 2:00 PM) on clear days in June and July to ensure consistent solar irradiation intensity. The reactors, which had previously undergone the adsorption process, were exposed to solar irradiation; the average light source intensity was approximately 910 W·m⁻². The solution was continuously stirred to maintain mixture homogeneity. At regular time intervals, 5 mL aliquots were collected, protected from light using opaque packaging, and then centrifuged at 4500 rpm for 15 min before being analyzed by UV–Visible spectrophotometry at a wavelength of 610 nm (Specord 200 Plus). Photolysis experiments were also performed to assess the influence of direct solar irradiation on pollutant degradation in the absence of the photocatalyst. All photocatalytic experiments were conducted in 250 mL cylindrical glass reactors (68 mm diameter × 90 mm height) under standardized conditions.

The photocatalytic degradation efficiency was evaluated using Equation (1):

$$R(\%)={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(1)

where C₀ is the initial concentration of BB41 after adsorption (mg L⁻¹) and C_t is the concentration at time t (mg L⁻¹).

3. Results and Discussion

3.1. Physicochemical Characterization of Nanomaterials

3.1.1. Evaluation of Thermal Properties

The thermal decomposition of the Bi₁₂ZnO₂₀ mass occurs primarily in three stages during heating in the temperature range of 27 to 1000 °C (Figure 3). The first weight loss (3.75%) is mainly attributed to evaporation, induced by the breaking of molecular water bonds, as well as the elimination of volatile compounds present in the precursors (CH₃COOC)₂Zn·2H₂O and Bi(NO₃)₃·5H₂O. A second weight loss of 1.5% is observed between 250 and 350 °C, due to the decomposition of organic matter, including the acetate CH₃COO, followed by a mass loss of 1.1%, which is attributed to the complete desorption of water. Consequently, it can be concluded that the material’s mass decreases as the temperature increases during thermal analysis [24,25]. The DSC curve reveals a significant endothermic peak at 717 °C, attributed to the decomposition of water molecules, followed by an exothermic peak at 850 °C, associated with the formation of the crystalline structure of Bi₁₂ZnO₂₀. The calcination temperature determined in this study is close to the value reported in the literature [26].

3.1.2. Identification of the Crystalline Phase

The confirmation of the crystalline phases of the synthesized nanomaterials and the formed heterosystem was carried out using X-ray diffraction (XRD), followed by XRD data processing with HighScore. The results of this analysis are presented in Figure 4.

The observed diffraction peaks (Figure 4b) correspond to the standard pattern of a zinc oxide sillenite, indexed in terms of cubic symmetry with space group I23 (PDF 01-078-1325) [27]. The presence of three intense diffraction peaks at 27.619°, 30.316°, and 32.822°, respectively, associated with the crystallographic planes (310), (222), and (321), confirms the well-established crystallization of the sillenite and the formation of a pure phase. The average crystallite size of Bi₁₂ZnO₂₀ was estimated using Scherrer’s formula [28], yielding a size of 53 nm.

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

(2)

The X-ray density was also evaluated (Equation (3)). The results of these parameters are listed in

Table 1. Structural parameters of Bi₁₂ZnO₂₀ sillenite.

Peak Position (°2th)	FWHM (°2th)	Crystal Size (nm)	Average Crystal Size (nm)	Density (g cm−3)
24.676	0.133	61.157	53	9.03
27.645	0.138	59.431
30.344	0.141	58.559
32.842	0.148	55.776
52.272	0.194	45.729
53.907	0.194	46.011
55.509	0.203	44.142

.

$$\rho ={\displaystyle \frac{ZM}{N_{A}V}}$$

(3)

where k is a dimensional form factor (=0.9), β is the wavelength of the X-ray (radian), and θ is the Braggs angle. Z is the number of formula units in a single unit cell (Z = 2), M is the molecular weight of Bi₁₂ZnO₂₀ (2893.12 g mol⁻¹), N_A is Avogadro’s number (6.02 × 10²³ mol⁻¹) and V is the unit cell volume (1062.77 × 10⁶ pm³).

The specific surface area was calculated using the crystallite diameter and the X-ray density of BZO (Equation (4)) [29].

$$S={\displaystyle \frac{6 \times {10}^3}{D \times \rho }}$$

(4)

A previous study [25] on the same photocatalyst, synthesized using a different sol–gel method, reported a specific surface area of 11.22 m² g⁻¹, which is lower than the value obtained in our research (12.54 m² g⁻¹). This discrepancy highlights the efficiency of our synthesis method, co-precipitation, while the observed difference is primarily attributed to variations in crystallite size.

The diffraction data of the AgI sample (Figure 4a) reveal the presence of peaks at 2θ values of 22.41°, 23.63°, 39.28°, and 46.28°, corresponding to the crystallographic planes (010), (002), (110), and (112), respectively [30]. All these peaks are indexed to a hexagonal unit cell, in accordance with the JCPDS card No. 98-007-9678. Furthermore, the diffractogram of Bi₁₂ZnO₂₀/AgI (Figure 4c) confirms the successful formation of the heterosystem, highlighting the presence of all the crystallization planes of both semiconductors, Bi₁₂ZnO₂₀ and AgI, without any shift or deformation.

3.1.3. Surface Analysis

The surface morphology of the synthesized nanomaterials, Bi₁₂ZnO₂₀ and AgI, and the formed Bi₁₂ZnO₂₀/AgI heterosystem was examined using SEM. The results of the imaging are presented in Figure 5. The morphology of Bi₁₂ZnO₂₀ sillenite is characterized by a regular and homogeneous arrangement of small particles (aggregates), taking the form of spheres and nanotubes, as shown in the SEM images (Figure 5a). The average size of these particles is estimated at 1.6 μm. Pure AgI nanoparticles exhibit both hexagonal and spherical shapes (Figure 5b). This observation was made at magnifications of 20 μm and 10 μm, revealing an average particle size of 5 μm. The presence of AgI particles and their homogeneous distribution on the surface of the sillenite confirms the successful formation of the Bi₁₂ZnO₂₀/AgI heterosystem (Figure 5c).

3.1.4. Raman Spectroscopy

The Raman spectrum analysis of Bi₁₂ZnO₂₀ (Figure 6) reveals 10 distinct spectral bands spanning a broad range of 5 to 700 cm⁻¹. These bands show excellent correspondence with the characteristic vibrational modes of sillénite [18,31], thereby confirming its crystalline phase. A detailed comparison between the Raman peaks observed in our study and those reported in the literature is presented in

Table 2. Comparison of Raman peaks of Bi₁₂ZnO₂₀ sillenite with those reported in literature.

Bi12ZnO20 Raman Peaks Observed in This Study	Raman Peaks of Zinc sillenite Bi12.66Zn0.33O19.33 (Ref. [31])	Raman Peaks of Sil-Lenite Bi12ZnO20 ([26])	Raman Peaks of BZnO Sillenite ([18])
	52	-	56
80	78	80	83
-	91	92	-
126	122	123	127
136	136	137	141
162	162	162	166
-	208	-	209
-	-	181	-
-	-	207	-
254	250	251	257
307	310	305	311
380	380	372	377
-	446	444	434
527	527	527	526
630	623	666	622

.

The intense peaks observed below 200 cm⁻¹ can be attributed to Bi-O bond vibrations, while the broad peaks between 200 and 600 cm⁻¹ correspond to oxygen atom vibrations and breathing modes. The weak Raman mode detected at 630 cm⁻¹ is associated with vibrational stretching within ZnO₄ tetrahedra [26]. The Raman analysis of AgI nanomaterials revealed two distinct vibrational modes at 83 and 103 cm⁻¹. These modes correspond to the symmetric stretching vibrations of silver iodide and are characteristic of the β-AgI polymorph, specifically its E₂ and A₁ transitions [32]. Remarkably, the Raman spectrum of the BZO/AgI heterosystem shows that the characteristic vibrational modes of both sillénite and AgI remain intact, exhibiting no peak shifts or distortions. This observation strongly suggests that both nanomaterials maintain their original crystalline structures despite being combined in the heterosystem.

3.1.5. BET Analysis

The nanomaterials’ specific surface area was determined through BET adsorption isotherm analysis, as graphically represented in Figure 7.

The BET surface area measured for AgI was remarkably low (0.116 m² g⁻¹). This primarily results from particle agglomeration, that reduces the available gas adsorption surface. Furthermore, semiconductors typically exhibit intrinsically low specific surface areas due to their crystalline structure and atomic arrangement. Interestingly, the sillénite Bi₁₂ZnO₂₀ showed a significant surface area increase from 0.757 m² g⁻¹ to 1.076 m² g⁻¹ upon forming the Bi₁₂ZnO₂₀/AgI heterosystem. This enhancement led to improved pollutant degradation efficiency when sillénite was incorporated into the heterostructure. Notably, the SBET value of zinc sillénite falls within the typical range reported for sillénites (~0.8 m² g⁻¹), confirming the consistency of our measurements with the established literature values [33].

3.1.6. Optical Properties Analysis

The analysis of the direct transition (αhʋ)² and indirect transition (αhʋ)^1/2 curves as a function of photon energy hʋ allowed us to determine the band gap energy E_g of the nanomaterials (Equation (5)) [31].

$${\left(\alpha hʋ\right)}^n=K\left(hʋ-E_g\right)$$

(5)

where α is the absorption coefficient, hʋ is the energy of the incident photon (hʋ = 1240/λ), K is the energy constant, and n is the electronic transition. The following values were obtained (Figure 8): 2.08 eV for AgI, 2.78 eV for Bi₁₂ZnO₂₀, and 1.58 eV for the Bi₁₂ZnO₂₀/AgI heterosystem. These results demonstrate that all photocatalysts exhibit strong absorption in the visible region, making them effective when exposed to visible light irradiation. Therefore, solar energy was used as an irradiation source for the application of these photocatalysts in heterogeneous photocatalysis. A significant reduction in the optical band gap was observed following the formation of Bi₁₂ZnO₂₀/AgI. This can be explained by the intimate contact between the two semiconductors, which facilitates interfacial charge transfer during photocatalytic excitation. The optical band gap values of AgI and Bi₁₂ZnO₂₀ are consistent with those reported in the literature [32,33].

3.1.7. Determination of the Point of Zero Charge

The isoelectric point (pH_PZC) of Bi₁₂ZnO₂₀/AgI was determined by plotting the evolution of the final pH as a function of the initial pH, as illustrated in Figure 9. The final pH of the recovered NaCl solution gradually increases with the rise in initial pH, reaching an isoelectric point at pH~8.45. This indicates that the Bi₁₂ZnO₂₀/AgI heterosystem exhibits a basic character, demonstrating that when pH values are lower than pH pH_PZC, the surface of the heterosystem acquires a positive charge, whereas when pH values are higher than pH pH_PZC, the surface carries a negative charge. These results provide valuable insights into the attraction and repulsion between the heterosystem’s surface and the molecules of the targeted pollutant.

3.2. Evaluation of the Adsorption Effect on the Degradation of Basic Blue 41

Adsorption shows a removal rate of 30%, which is relatively low in terms of adsorption efficiency (Figure 10). These results are consistent with those of previous studies [11]. It can also be observed that after approximately 120 min, the adsorption equilibrium is reached. For all subsequent photocatalytic experiments, it is essential to first conduct a 120 min adsorption step to enhance the efficiency of the photocatalysis process.

3.3. Study of the Photodegradation of Basic Blue 41

A parametric study was conducted to evaluate the effect of mass ratio, pH, photocatalyst dosage, and initial pollutant concentration on the photocatalytic degradation of BB41. The objective was to optimize the best operating conditions in order to maximize photocatalytic performance.

3.3.1. Influence of the Mass Ratio of the Bi₁₂ZnO₂₀/AgI Heterosystem

The histogram in Figure 11 shows that 69% and 49% of BB41 was degraded using Bi₁₂ZnO₂₀ and AgI, respectively. The addition of AgI to the Bi₁₂ZnO₂₀ surface led to a significant improvement in degradation efficiency, reaching an optimal value of 80% for a mixture of 75% BZO and 25% AgI. The efficiency of the heterosystem increases with the Bi₁₂ZnO₂₀ concentration, clearly highlighting the significant advantages of the Bi₁₂ZnO₂₀/AgI heterosystem for treating dye-contaminated water. This enhancement is attributed to the interfacial charge transfer phenomenon between AgI and Bi₁₂ZnO₂₀. This transfer facilitates the migration of photogenerated charge carriers until the Fermi levels of both semiconductors align, creating an electric field between their surfaces, which prevents recombination. As a result, the degradation efficiency of BB41 under visible light is significantly improved. These findings suggest that AgI could be a promising choice for photosensitizing Bi₁₂ZnO₂₀ during junction formation.

3.3.2. Influence of pH on the Photodegradation of Basic Blue 41

The effect of pH was studied by varying the pH range from an acidic medium (pH 5.3) to a basic medium (pH 10) in a suspension containing 10 mg L⁻¹ of BB41 and 1 g L⁻¹ of Bi₁₂ZnO₂₀/AgI, as shown in Figure 12. The results reveal a low dye removal efficiency at pH~7 and isoelectric point~5.3. These are characterized by a slower degradation rate. This behavior can be attributed to the weak adsorption of dye molecules on the Bi₁₂ZnO₂₀/AgI surface due to the similarity in surface charge. Since Bi₁₂ZnO₂₀/AgI is positively charged at pH ˂ 8.45, it experiences electrostatic repulsion with the cationic dye molecules, thereby reducing the degradation efficiency. The maximum level of BB41 degradation was observed at pH~10, achieving a 99% removal efficiency. However, a slight decrease in decolorization efficiency was noted at pH~8, with a yield of 97% after 150 min of treatment. This is due to the electrostatic attraction between the negatively charged Bi₁₂ZnO₂₀/AgI surface and the positively charged cationic BB41 molecules, which enhances dye adsorption onto the heterosystem surface. To maintain ecological balance and the health of aquatic ecosystems [34], pH~8 was selected as the optimal value. These results are consistent with the literature findings, confirming that BB41 photodegradation under UV and visible light irradiation is favored in a basic medium, particularly at pH values ranging from 8 to 10 [35,36].

3.3.3. Influence of Bi₁₂ZnO₂₀/AgI Dose on Photodegradation of Basic Blue 41

According to the results illustrating the variation in BB41 decolorization rate as a function of irradiation time (Figure 13), it is observed that the decolorization efficiency increases from 66% to 98% with the increase in photocatalyst dosage, reaching a maximum value at 0.75 g L⁻¹. This improvement is further enhanced by the small average crystallite size (~53 nm), which promotes both an increase in reactive sites and a larger photon absorption surface, thereby enhancing the efficiency of the photocatalytic treatment process. However, beyond this optimal dosage, the degradation decreases by 8% due to the saturation of the solution with the Bi₁₂ZnO₂₀/AgI heterosystem. These observations are consistent with the work of Y. Benrighi and colleagues [35], who reported that increasing the CoCr₂O₄ dosage from 0.125 to 0.5 g L⁻¹ enhanced BB41 degradation from 63% to 99% under visible light irradiation. They also observed that this maximum decolorization rate decreased to 86% when the photocatalyst dosage exceeded 1 g L⁻¹.

3.3.4. Influence of the Initial Concentration of Basic Blue 41

Under visible light irradiation, the concentration of BB41 appears to have a negative effect on the photocatalytic degradation process occurring the surface of the Bi₁₂ZnO₂₀ /AgI heterosystem. A higher initial concentration leads to a decrease in degradation efficiency. The decolorization rate increases from 20% to 98% as the dye concentration decreases from 50 to 10 mg L⁻¹ (Figure 14a). At an initial concentration of 10 mg L⁻¹, the degradation process is remarkably fast, reaching a maximum efficiency of 98% in just 150 min of treatment. These results can be explained as follows: At high concentrations, the increased presence of dye creates an optical shielding effect, making the medium opaquer. This opacity prevents incident photons from reaching the surface of Bi₁₂ZnO₂₀/AgI, thereby reducing the generation of radicals responsible for pollutant decomposition. Conversely, at lower concentrations, incident photons can more easily reach the surface of the heterosystem particles. This enhances the interaction between pollutant molecules and the reactive species generated during photocatalytic activation. Many researchers have also observed a similar trend in the degradation of BB41 [37,38].

Using the Bi₁₂ZnO₂₀/AgI heterosystem, the evolution of the UV-Vis absorption spectrum during the photodegradation of Basic Blue 41 as a function of irradiation time is presented in Figure 14b. A decrease in absorbance in the visible region of the absorption spectrum is observed over time due to the fading of the suspension color. This phenomenon is attributed to the breakdown of the azo group (-N=N-), which is a characteristic feature of the BB41 molecule [39].

3.4. Photocatalytic Degradation Kinetics

The kinetic study of the photodegradation reaction of BB41 as a function of the initial concentration was conducted by varying it from 10 to 50 ppm, while keeping the optimal conditions of pH (~8) and photocatalyst dose (0.75 g L⁻¹) constant. Two kinetic models were used [22]: the pseudo-first-order model (Equation (6)) and the pseudo-second-order model (Equation (7)).

$$\ln {\displaystyle \frac{C_t}{C_0}}={-k}_{1}t$$

(6)

$${\displaystyle \frac{1}{C_t}}={\displaystyle \frac{1}{C_0}}+k_{2}t$$

(7)

where k₁ is the apparent first-order kinetic constant of degradation (min ⁻¹), k₂ is the apparent second-order kinetic constant of degradation (L mg⁻¹ min⁻¹), C₀ is the initial concentration of BB41 after adsorption (mg L⁻¹), C_t is the concentration at time t (mg L⁻¹), and t is irradiation time (min).

The graphical representation of these two models allows us to determine the kinetic order of the photocatalytic degradation reaction of BB41. The experimental results are presented in Figure 15 and the kinetic parameters of both models are summarized in

Table 3. Kinetic parameters of BB41 photodegradation using Bi₁₂ZnO₂₀/AgI heterosystem.

C0 (mg L−1)	Pseudo-First Order		Pseudo-Second Order
	R2	k1(min−1)	R2	k2 (L mg−1 min−1)
10	0.8408	0.0123	0.8788	0.0218
20	0.9671	0.0027	0.9443	0.0002
30	0.9733	0.0032	0.9532	0.0002
40	0.9554	0.0025	0.9458	0.00009
50	0.9647	0.0006	0.8765	0.00002

.

The kinetic curves, representing ln(C₀/C) as a function of irradiation time for different initial concentrations, exhibit good linearity, indicating that the photodegradation kinetics are described well by the pseudo-first-order model, with correlation coefficients very close to 1 (R²~0.94). These results are consistent with previous studies [39]. The initial rate r₀ (Equation (8)) and the half-reaction time t_1/2 (Equation (9)) can be calculated using the apparent rate constant k₁.

$$r_0=k_{app} C_0$$

(8)

$$t_{1/2}={\displaystyle \frac{ln2}{k_1}}$$

(9)

The results in

Table 4. Pseudo-first-order kinetic parameters of the BB41 photodegradation reaction.

C0 (mg L−1)	R2	kapp (min−1)	t1/2 (min)	r0 (mg L−1 min−1)
10	0.8408	0.0123	56.353	0.123
20	0.9671	0.0027	256.721	0.054
30	0.9733	0.0032	216.608	0.096
40	0.9554	0.0025	277.259	0.1
50	0.9647	0.0006	1155.245	0.03

show that the apparent rate constant, kapp, decreases with the increase in initial dye concentration C₀ due to the screening effect. This leads to a reduction in the solution’s permeability to visible radiation, preventing incident photons from reaching the surface of Bi₁₂ZnO₂₀/AgI and thereby reducing the generation of radicals responsible for dye decomposition. However, the half-reaction time (t_1/2) increases as the initial concentration C₀ increases.

3.5. Analysis of Dye Mineralization

To evaluate the mineralization rate of organic matter in an aqueous solution, the temporal variation in total organic carbon was monitored. This study was conducted under the optimal conditions as determined by a parametric study of BB41 photodegradation. The results of this study are illustrated in Figure 16. A significant mineralization rate of 65.5% was observed, while the degradation rate determined by UV-Vis analysis reached 98%. After 360 min of treatment, the TOC value decreased from 19.89 mg L⁻¹ to 6.87 mg L⁻¹, indicating that a large portion of BB41 was mineralized. To achieve complete mineralization, the photocatalytic treatment process requires a sufficiently long duration to ensure the decomposition of BB41 organic molecules and the intermediate products formed.

3.6. Study of the Regeneration of the Bi₁₂ZnO₂₀/AgI Heterosystem

We conducted series of recycling tests using the Bi₁₂ZnO₂₀/AgI heterosystem under optimal reaction conditions for the photocatalytic degradation of BB41. After each experiment, the photocatalyst was separated from the aqueous suspension by filtration, cleaned with ethanol, then washed with distilled water to remove the adsorbed dye molecules [40], and finally dried at 80 °C for 24 h. After five recycling cycles (Figure 17), a decrease in degradation efficiency was observed, leading to a reduction in yield from 96% to 46% after 4 h of solar light irradiation. This indicates that the heterosystem is relatively stable and can retain 50% of its photocatalytic properties.

After the recycling experiments, the recovered Bi₁₂ZnO₂₀/AgI nanoparticles were characterized using X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Figure 18 illustrates the main results obtained. The XRD spectra of the Bi₁₂ZnO₂₀/AgI heterosystem before and after recycling (Figure 18a) exhibit peaks at similar 2θ positions, indicating the preservation of the main crystalline structure. SEM analysis reveals that the overall morphology of the nanoparticles remains remarkably unchanged (Figure 18b). However, a more intimate attachment between AgI and Bi₁₂ZnO₂₀ has been observed, which may contribute to enhancing the semiconductor performance of the heterosystem. The retention of all constituent elements of Bi₁₂ZnO₂₀/AgI after the recycling process (Figure 18c) confirms the purity, stability, and durability of the synthesized photocatalyst.

3.7. Comparison of BB41 Photodegradation with Previous Studies

It would be relevant at this stage to compare the photocatalytic performance of our heterostructure with that reported in previous studies on the photodegradation of Basic Blue 41. The main results are summarized in

Table 5. Comparison of BB41 photodegradation by different photocatalysts.

Photocatalyst	Light Source	[BB41]	pH	Dose	Degradation Rate (%)	Reference
		(mg L−1)		(g L−1)
BZO/AgI	Solar irradiation	10	8	0.75	98	This study
Cu2ZnSnS4	4 fluorescentlamps (4 W)	10	free	-	97.5	[41]
SrTiO3/Ag3PO4	Irradiation visible	20	free	5	99	[42]
C/ZnO/Zn	Lumière UV	12.5	free	0.1	96	[10]
TiO2- biochar based on lignin	Lumière UV	40	6.1	0.75	96.72	[43]
	(8 W)
TiO2/CaAlg	Solar irradiation	30	7	1	96	[11]
CoCr2O4	Tungsten lamp (200 W)	10	5	0.5	99	[35]

. These data confirm that the synthesized BZO/AgI heterostructure could represent a competitive solution for the treatment of textile wastewater under solar irradiation.

4. Conclusions

In this work, a Bi₁₂ZO₂₀/AgI heterosystem was prepared by combining sillenite-type zinc oxide and silver iodide. This heterosystem was used for the photodegradation of the textile dye Basic Blue 41. Various characterization techniques demonstrated that the prepared heterosystem exhibits significantly superior semiconductor properties compared to Bi₁₂ZnO₂₀ and AgI individually. the efficient degradation of BB41 with a removal rate of 98% was observed after 150 min of treatment under the following operating conditions: pH~8, dose = 0.75 g L⁻¹, [BB41] = 10 mg L⁻¹. Experimental results revealed that the photodegradation kinetics are perfectly described by the pseudo-first-order model. The recycling test demonstrated that the synthesized heterosystem retained its crystalline structure and morphology after use, while showing a moderate decrease in reuse capacity over successive cycles. In light of this limitation, future work will focus on the development and evaluation of alternative heterosystems with different compositions to enhance long-term photocatalytic activity and reusability. In summary, this Bi₁₂ZnO₂₀/AgI sillenite-based heterosystem has demonstrated remarkable photocatalytic efficiency, achieving the near-complete degradation of Basic Blue 41 dye. This exceptional performance positions the material as a highly promising solution for environmental remediation applications.