Green Development of Titanium Dioxide Using Astragalus boeticus for the Degradation of Cationic and Anionic Dyes in an Aqueous Environment

Abstract

Wastewater discharge from the textile industry poses significant health problems for humans. As a result, the effluent waters are often rich in dyes, whose low natural decomposition capacity makes their treatment complex, thus contributing to environmental degradation. It becomes imperative to implement effective solutions for treating these contaminated waters, with a primary goal: to make them fit for human consumption. The present study focuses on the development of green TiO₂ nanoparticles (TiO₂-NP) using titanium (IV) isopropoxide as a precursor, along with the extract of Astragalus boeticus (A.B). These green TiO₂ nanoparticles have been developed for use as highly efficient photocatalysts for the degradation of two types of dyes: Reactive Yellow 161 (RY161), an anionic dye, and Crystal Violet (CV), a cationic dye. The structural, microstructural, and optical properties of the synthesized material were characterized using XRD, FTIR, SEM, EDX, and UV-Vis methods. The results of these analyses revealed that the nanoparticles have a size of approximately 68 nm, possess an anatase structure, exhibit a spherical surface morphology, and have a band gap of 3.22 eV. The photocatalytic activity of the synthesized material demonstrated a 94.06% degradation of CV dye in a basic environment (pH = 10) within 30 min, with an initial CV concentration of 10 mg/L and a catalyst mass of 1 g/L. Additionally, it achieved a 100% degradation of RY161 dye in an acidic environment (pH = 4) within 90 min, with an initial RY161 concentration of 30 mg/L and a catalyst mass of 1 g/L. Furthermore, the recycling study indicated that the green TiO₂ NPs catalyst could be effectively reused for up to five cycles. These experimental findings suggest that the developed TiO₂ catalyst holds significant potential as an eco-friendly solution for remediating aqueous media polluted by both anionic and cationic dyes.

1. Introduction

In recent years, it has been observed that the Earth’s atmosphere has begun to deteriorate due to the increase in and rapid expansion of industrialization, urbanization, and population growth. These factors have led to the massive release of dangerous and undesirable substances, such as dyes, pesticides, heavy metals, and antibiotics [1,2]. The textile sector, which is a significant consumer of water, generates a high level of pollution in the aquatic environment, with discharges heavily contaminated by dyes. Among these dyes, Crystal Violet (CV), a cationic dye, and Reactive Yellow 161 (RY161), an anionic dye, exhibit persistent effects in aqueous media. CV was chosen for its widespread use in various industries, including the textile, paint, chemical manufacturing, and biotechnology sectors [3,4]. RY161, on the other hand, was selected for its commercial azo character, commonly used for dyeing cotton textiles [5].

In recent years, the treatment of contaminated waters has become a significant challenge due to the complexity of their chemical structure, particularly the presence of recalcitrant aromatic rings [6]. Conventional methods struggle to effectively degrade these pollutants. Therefore, urgent action is required to treat and ensure the safety of these contaminated waters before discharge.

Advanced oxidation processes (AOPs) have gained popularity for their ability to transform recalcitrant pollutants into biodegradable products, mineralize most pollutants into harmless compounds like CO₂ and H₂O, and consume less energy compared to methods like incineration. Over the past few decades, various techniques for treating organic contaminants in wastewater and air have been explored [7,8].

One promising approach is photodegradation using metal nanoparticle photocatalysts [9,10,11]. Nanoparticles (NPs) are tiny particles with dimensions between 1 and 100 nm, and metal oxide NPs exhibit remarkable chemical, physical, magnetic, electronic, and optical properties [10,12]. Several metal oxide nanoparticles have been synthesized, including ZnO [13,14], ZnAl₂O₄@ZnO [15], Ag₂O-Ag/ZnAl-oxide [16], Fe [17], ZnO/AC [18], CoMnCrO₄ [19], MgFeCrO₄ [20], and TiO₂ [10,12,21,22,23].

Among these, TiO₂ stands out as a semiconducting photocatalyst with a wide bandgap, making it an excellent absorber in the ultraviolet range [24,25,26,27]. It is highly efficient and possesses key properties for wastewater treatment and medicinal purposes, including high chemical and thermal stability, cost-effectiveness, non-toxicity, and biocompatibility [10,28,29].

While chemical synthesis of nanoparticles can be toxic and environmentally harmful, green synthesis methods avoid the use of harmful and toxic agents [10]. Plant extracts offer a convenient and environmentally friendly approach to biosynthesis. They prevent nanoparticle agglomeration, enhance stability, and are cost-effective. Plant substances act as reducing and capping agents, making this method natural and safe [10,30,31].

Previous research has successfully prepared TiO₂ nanoparticles using various plant extracts, such as Jatropha curcas L., Citrus Limetta, Cinnamon, Syzygium Cumini, Citrus Limon juice, and costus speciosus. These nanoparticles have found applications in areas like antioxidant and antimicrobial studies [18,32,33,34,35].

Based on the foregoing, this study focuses on the green synthesis of TiO₂ NPs using titanium isopropoxide (TTIP) as a titanium source and Astragalus boeticus leaf extract as a reducing and protecting agent. The choice of this plant is justified by its numerous environmental and natural qualities, as well as the fact that it has never been used in wastewater treatment. Furthermore, it is worth noting that this plant is environmentally friendly, as evidenced by its historical use in various fields, including medicine [36,37], and its seeds have been used as a coffee substitute [38]. This plant offers the advantage of providing a safe and environmentally friendly extract for the reduction in and coating of nanoparticles, a crucial step in the preparation of nanoscale materials. Quantitative tests have shown that extracts from this plant are rich in natural antioxidants, such as polyphenols and flavonoids, which possess well-known antioxidant properties. These compounds play a crucial role in protecting materials against damage caused by free radicals [39] generated during the photodegradation process. The synergy between environmental benefits and the antioxidant properties of this plant makes it an ideal choice for our research.

The synthesized material was characterized and used as a catalyst for the photocatalytic degradation of both cationic (CV) and anionic (RY161) textile dyes in an aqueous medium under UV irradiation. These two dyes were selected due to their persistent effects. The effect of various parameters, such as the catalytic dose, solution pH, and initial dye concentration, was studied. Detailed information on the reuse of the TiO₂ nanoparticles prepared after the photodegradation process was also provided.

2. Materials and Methods

2.1. Materials

The leaves of Astragalus boeticus (A.B) were meticulously gathered from the local fields situated in the Ain Aouda region of Rabat, Morocco. These leaves served as a vital source of natural materials for the subsequent experiments and studies. For the chemical components and substances involved in the experiments, Titanium (IV) isopropylate, also known as titanium (IV) isopropoxide (C12H28O4Ti), was procured from Sigma-Aldrich with a stated purity level of 97%. This compound played a central role in the synthesis or processes being conducted. Sodium hydroxide (NaOH) and hydrochloric acid (HCl, 37%) were obtained from Panreac Appli-chem. The Reactive Yellow dye utilized in the experiments was sourced from Dystar Textilfarben, GmbH, and Co. Deutschland KG D-6007 Frankfurt. Additionally, Crystal Violet dye was purchased from Sigma-Aldrich. To prepare the necessary solutions and reagents for the experiments, double-distilled water was used. This ensured the purity of the solvent, reducing the likelihood of any impurities affecting the results. It is worth noting that all the reagents were used as obtained without undergoing any additional purification processes.

2.2. Preparation of Astragalus boeticus Extract

The fresh leaves underwent a thorough washing process, first with tap water to remove surface impurities, followed by rinsing with double-distilled water to ensure cleanliness. Subsequently, they were left to air dry at room temperature for a period ranging from 4 to 6 days until they reached an appropriate level of dryness. Once dried, the leaves were finely ground into a powder by crushing them in a mortar using a porcelain pestle. The preparation of the leaf extract then involved taking 30 g of the obtained powder and mixing it with 150 mL of distilled water. This mixture was then heated to 80 °C for a duration of 80 min. The heat facilitated the extraction of the desired compounds from the leaves into the water, creating a solution of the leaf extract. Following the heating process, the obtained extract was carefully filtered through Whatman filter paper with a pore size of 0.45 µm.

2.3. Synthesis of TiO₂ NPs by the Extract of Astragalus boeticus

TiO₂ nanoparticles were synthesized through the hydrolysis of titanium isopropoxide (TTIP), which is the primary method for producing TiO₂ NPs [40].

In this process, 75 mL of Astragalus boeticus (A.B) extract was slowly added drop by drop to a solution containing 75 mL of TTIP. The resulting mixture was stirred for 9 h at room temperature. After the stirring period, the mixture was subjected to centrifugation at 8000 rpm for 8 min to separate the two phases. The resulting nanoparticles (NPs) were then dried at 110 °C for 24 h and subsequently subjected to calcination at 570 °C in a muffle furnace (Nabertherm GmbH) for 3 h (see Figure 1).

2.4. Characterization Techniques

The TiO₂ NPs underwent comprehensive characterization, including structural, morphological, and optical assessments. The crystalline phase of the synthesized titanium nanoparticles (TiO₂ NPs) was determined using X-ray powder diffraction (XRD) with a Shimadzu XDR-6100 instrument, employing Cu K radiation (λ = 1.5406 Å). To identify the functional groups present in TiO₂, Fourier-transform infrared spectroscopy (FTIR) analysis was performed in the range of 4000–400 cm⁻¹ using a Bruker Alpha Platinum-ATR spectrometer with a KBr disk. Scanning electron microscopy and energy-dispersive analysis (EDX) were utilized to visualize the morphology of TiO₂ NPs, employing a Quanta 200 instrument. UV-visible transmission spectra were acquired using a Jasco V-673 spectrophotometer.

Photocatalytic experiments with the nanoparticles involved the assessment of two dyes: one anionic, Reactive Yellow 161, and the other cationic, Crystal Violet. These experiments were conducted using a UV-5800 spectrophotometer from Metash.

2.5. Photodegradation Test of RY161 and CV Dyes by Elaborated Green TiO₂ NPs

The photolysis and adsorption phenomena were assessed by monitoring the degradation efficiency of the organic compounds Reactive Yellow (RY61) and Crystal Violet (CV) in aqueous solutions using green-synthesized NPs.

The photocatalytic process for both dyes was conducted in a 500 mL photochemical reactor under visible light irradiation. Prior to the photodegradation, various amounts of TiO₂ NPs were brought into contact with separately prepared 100 mL solutions of each dye. The pH of the solutions was adjusted to 4 and then to 10 using 0.1M HCl and NaOH solutions. The solutions were stirred magnetically in the dark for 30 min. This adsorption step was essential to immobilize the pollutants on the support surface.

Subsequently, the solution of RY161 with the semiconductor was exposed to visible light (125 W) and stirred for 3 h. Samples of 5 mL each were collected at 15 min intervals during the first hour and at 30 min intervals for the remaining two hours. The solutions were then centrifuged using a SIGMA laborzentrifugen 2–15. The concentration changes in both dyes during the photocatalytic process were monitored using a UV-Visible spectrophotometer, with maximum wavelengths of 435 nm and 580 nm for the RY161 and CV dyes, respectively.

Ultraviolet light was emitted by a Radium HRL 125W/230/E27 mercury vapor lamp with a wavelength between 254 and 380 nm.

The yield of photocatalytic degradation was calculated from Equation (1):

$$R\%=\frac{C_0-C_t}{C_0}\cdot 100$$

(1)

where C₀ represents the initial concentration of the aqueous solutions of RY161 and CV, and C_t is the concentration of the solutions (RY161 and CV) during irradiation [41].

The photocatalytic degradation kinetics of cationic and anionic dye was evaluated by the Langmuir–Hinshelwood kinetics equation [42], as presented below (Equation (2)):

$$Ln \left(\frac{C_0}{C_t}\right)=k_{app }t+Constant$$

(2)

K_app is the apparent first-order constant (min⁻¹).

2.6. Reusability Test

The reusability test involved the recovery and regeneration of the green TiO₂ NPs’ catalyst following the photocatalytic degradation experiment. After the completion of the photocatalytic reaction, the synthesized photocatalyst was subjected to a thorough washing process using double-distilled water for a duration of 1 h. This washing step was essential to remove any residual contaminants, reaction by-products, or adsorbed species from the surface of the catalyst, ensuring its cleanliness and readiness for subsequent use. Once the washing process was completed, the TiO₂ NPs catalyst was carefully collected and prepared for reuse. To do so, it was subjected to an oven-drying procedure, allowing it to dry thoroughly overnight at a temperature of 100 °C. This drying step aimed to remove any remaining moisture and ensure that the catalyst was in a completely dry state, ready to be employed in further photocatalytic degradation experiments.

3. Results and Discussion

3.1. Sample Characterization

3.1.1. XRD Analysis

The XRD spectrum of synthesized TiO₂ NPs is shown in Figure 2. Notable peaks in a 2θ range of 10° < 2θ < 70° were observed at 25.36°, 36.79°, ° 37.86°, 38.22°, 48.08°, 53.93°, 54.97°, and 62.67°, corresponding to the following values of Miller indices (hkl): (101), (103), (004), (112), (200), (105), (211), and (213), respectively [43]. All diffraction peaks are well indexed to the pure anatase phase according to the standard JCPDS map No. 99-101-0679. This has been validated by similar results from several published works [7,44].

The Debye–Scherrer relationship is used to calculate the average grain size of NPs [10,45]. The measured grain size of the synthesized TiO₂ nanoparticles was about 68 nm.

$$D=\frac{k\cdot \lambda }{\beta cos cos \theta }$$

(3)

where

D: Crystal size of the NPs;

λ: Wavelength of the X-ray source;

K: Constant crystal form factor;

Θ: Bragg diffraction angle;

β: Total angular width at half maximum (FWHM) of the XRD peaks recorded at diffraction angle 2θ.

3.1.2. FTIR Analysis

The FTIR spectra of TiO₂ nanoparticles are depicted in Figure 3. The band observed around 500 cm⁻¹ to 600 cm⁻¹ can be attributed to the stretching vibration of Ti-O and Ti-O-Ti bonds within the TiO₂ structure [46]. This band is indicative of the presence of chemical bonds within the material and is crucial for characterizing the TiO₂ structure. Another significant peak is observed at 1645 cm⁻¹, corresponding to the characteristic bending vibration of the—OH group. The presence of this hydroxyl group is important as it suggests an interaction with water or other compounds containing hydroxyl groups. This can have significant implications in various applications of TiO₂, including its reactivity with water or its ability to adsorb compounds containing hydroxyl groups. Lastly, a broad band in the range of 3650–3100 cm⁻¹ is seen, corresponding to the intermolecular interaction between the TiO₂ surface and the hydroxyl group of water molecules [47]. This interaction is crucial in many TiO₂ applications, particularly in heterogeneous catalysis and photocatalysis, where water is often present as a reactant or reagent. Analyzing this band can provide insights into the nature of the interaction between TiO₂ and water, which is essential for understanding the reaction mechanisms involved in these processes.

3.1.3. SEM with EDX Analysis

The structure, morphology, and apparent particle size of the synthesized nanoparticles were analyzed by scanning electron microscopy (SEM). The SEM image of TiO₂ NPs is presented in Figure 4a, which shows that the particles are uniformly distributed and spherical in shape.

The EDX spectrum of the NPs is presented in Figure 4. The inset in Figure 4b shows the weight and atomic percentages of Ti (63.55 wt% and 36.32 atomic %) and O (32.41 wt% and 54.59 atomic %). The presence of the titanium and oxygen peaks alone confirms the formation of pure anatase TiO₂. The carbon (C) peak is due to the carbon support used during this analysis. No other impurity peaks were observed.

3.1.4. UV-Vis Analysis

The optical properties are essential for the photocatalytic study because they allow one to describe the number of photons absorbed during the photocatalytic treatment [12]. The absorption spectrum and Tauc graph of prepared samples that were analyzed by UV-vis spectroscopy are shown in Figure 5. The UV-vis absorption pattern shows that the highest absorption is in the range of 250–400 nm, as shown in Figure 5a. The band gap of the synthesized nanoparticles was calculated by the Tauc Equation (4) [48].

$${\left(\alpha h\nu \right)}^{2/n}=A \left(h\nu -Eg\right)$$

(4)

where:

α: Absorption coefficient,

A: Constant proportionality,

hν: Energy of the incident photon,

Eg: Energy of the band gap.

Figure 5. (a) UV-Vis absorbance spectrum and (b) Tauc’s plot (inset) of TiO₂ nanoparticles.

Figure 5. (a) UV-Vis absorbance spectrum and (b) Tauc’s plot (inset) of TiO₂ nanoparticles.

The value of n is related to the kind of optical transition of the semiconductor (for the direct transition n = 1 and for the indirect transition n = 4) [10,49]. The calculated direct band gap of the synthesized TiO₂ NPs is 3.22 eV, as shown in Figure 4b, which is also consistent with the literature [12,50].

3.2. Photocatalytic Activity of TiO₂ NPs for Degradation of Dyes

In the context of this research theme, the study of the degradation of RY161 and CV and the impact of specific key parameters on the photocatalytic activity of TiO₂ nanoparticles (TiO₂ NPs) in dye degradation are explored. The mechanism of heterogeneous photocatalysis begins with the careful selection of an appropriate semiconductor, possessing the required electronic structure characterized by an empty conduction band and a filled valence band. This configuration facilitates the light-induced redox process, which plays a pivotal role in dye degradation [51]. The investigation starts by analyzing the influence of dye concentration in the solution and conducting a kinetic study to clarify how this variable affects the degradation process. Simultaneously, exploration extends to the effect of the mass of TiO₂ NPs on the photocatalytic reaction, involving a kinetic study to evaluate how the quantity of photocatalyst impacts degradation efficiency. Additionally, examination delves into the impact of solution pH on the degradation process, encompassing another kinetic study to gain a deeper understanding of how variations in pH can modulate the photocatalytic activity of TiO₂ NPs. Finally, the proposed mechanism is comprehensively addressed to explain how TiO₂ NPs act as catalysts in dye degradation. This thorough investigation contributes to a profound understanding of the effectiveness of this dye degradation technology and the optimization of its operational parameters for a more efficient and environmentally friendly application.

3.2.1. Effect of Photolysis and Adsorption

A preliminary study of the degradation efficiency of two dyes, RY161 (30 mg/L) and CV (10 mg/L), was conducted. The study was divided into two parts: the first part involved direct photolysis of the two dyes (without the addition of a catalyst) under UV irradiation, and the second part focused on the adsorption process with the addition of 1 g/L of TiO₂ NPs based on A.B extract, both in the absence and presence of UV irradiation (referred to as adsorption and photocatalysis, respectively). The initial pH of each solution was maintained without adjustment (pH RY161 = 6.1 and pH CV = 5.9).

The results, presented in Figure 6, indicate that after 3 h of direct exposure of the two dyes to UV radiation without the addition of catalytic particles, the degradation rate remained negligible. This highlights the crucial role of the catalyst in the reaction medium (Figure 6a,b). However, an adsorption removal rate of 26.4% for RY161 and 20.65% for CV was achieved after 3 h using the synthesized catalyst. Furthermore, significantly higher removal efficiency, reaching 99.2% for RY161 and 96.34% for CV, was obtained after 3 h of contact with the synthesized TiO₂ NPs under UV irradiation.

Comparing the experimental results, it is evident that the degradation efficiency of the two target dyes is greatly improved when in contact with TiO₂ NPs under UV irradiation (photocatalysis) compared to adsorption alone. Direct photolysis, on the other hand, did not exhibit any advantage in the degradation process of the two pollutants.

These findings confirm that TiO₂ NPs based on Astragalus boeticus extract, in combination with UV irradiation, exhibit superior photocatalytic properties for RY161 and CV molecules compared to TiO₂ NPs without UV.

Based on these results, the decision was made to further investigate the photocatalytic efficiency of TiO₂ NPs using UV radiation and to identify the key factors that positively or negatively impact their efficiency.

3.2.2. Effect of TiO₂ NPs Mass and Kinetic Study

It is well known that the rate of photocatalytic degradation is closely related to the appropriate catalyst dose [52].

In order to determine the effect of the added catalyst dose on the degradation of 30 mg/L of RY161 dye and 10 mg/L of CV, a variation of the catalyst mass from 0.5 to 2 g/L was tested while keeping the pH of the two solutions fixed (pH_S(RY161) = 6.1 and pH_S(CV) = 5.9).

The results presented in Figure 7a,c demonstrate that the degradation efficiency of both dyes increases with the augmentation of the catalyst’s mass. In fact, an increase in the quantity of synthesized TiO₂ NPs from 0.5 to 1 g/L significantly enhances the degradation rate. This increase can be attributed to the multiplication of active reaction sites on the TiO₂ NPs [53] and the heightened capture of photons at the catalyst’s surface. A substantial number of high-energy dynamic sites on the catalyst’s surface could overcome the resistance to the mass transfer of pollutants between the solid and aqueous phases, leading to an enhancement in degradation [54]. Above this value (1 g/L), the degradation rate decreases, which is likely due to an excess of catalyst, and the agglomeration of the grains between them, which masks a good part of the photosensitive surface [24].

This was confirmed by the determined value of the apparent rate constants. In fact, the Kapp values obtained for a dose of 2 g/L (0.00872 min⁻¹) were significantly lower than that obtained at 0.5 and 1 g/L (0.02187 min⁻¹, 0.02617 min⁻¹ respectively) for CV (Figure 7d). Meanwhile, for the RY161 dye, a Kapp value of 0.5 g/L (0.01146 min⁻¹) was lower than 1 g/L (0.04865 min⁻¹) and 2 g/L (0.01636 min⁻¹), which is quite normal because the amount chosen (0.5 g/L) is lower compared to the initial concentration (30 mg/L) Figure 7b.

3.2.3. Effect of Concentration and Kinetic Study

Generally, the concentration of pollutants is one of the key parameters affecting the rate of photocatalytic degradation as a high concentration of pollutants limits the penetration of photons on the catalyst surface [24].

The effect of the initial pollutant concentration on the photocatalytic activity of the two dyes was evaluated by varying the initial concentration from 30 to 60 mg/L for RY161 and from 5 to 15 mg/L for CV. The results are presented in Figure 8, which illustrates the degradation kinetics of the dyes as a function of radiation time in the presence of supported TiO₂ NPs (1 g/L). It is observed that the higher the initial concentration of the dyes, the longer the time required for their degradation.

Furthermore, increasing the concentration of RY161 from 30 to 60 mg/L leads to a decrease in the rate of the photocatalytic reaction from 0.04865 to 0.01292 min⁻¹ (Figure 8b). Similar behavior was observed for the cationic dye, with a decrease in the rate constant from 0.02563 to 0.02172 min⁻¹ for concentrations ranging from 5 to 15 mg/L, respectively (Figure 8d).

This decrease in efficiency at high concentrations can be attributed to the reduction in OH^• radicals generated on the surface of the catalyst as the active sites become occupied by dye ions. Additionally, at higher concentrations, a significant amount of UV radiation can be absorbed by the dye molecules, reducing the transmittance of light. This is unfavorable for the absorption of light energy by the catalysts [42].

3.2.4. Effect of Solution pH and Kinetic Study

pH is an important factor that influences the surface properties of solids, surface charge, and particle aggregate sizes in water [55,56].

Its impact on the photocatalytic activity is important to allow the evaluation of the efficiency of the technique in the case of a pollutant that is partially ionized or carrying charged functions, and it is necessary to determine the electrostatic interactions that occur between the adsorbent and the adsorbate. Indeed, depending on the point of zero charge (PZC) of the material, the surface charge of the latter depends on the pH. Thus, for TiO₂ where PZC = 6.5, the surface is positively and negatively charged as shown in Equations (5) and (6), respectively [57,58]:

$$\mathrm{pH}<6.5 {\mathrm{TiOH}}_2^{+}\to \mathrm{TiOH}+{\mathrm{H}}^{+}$$

(5)

$$\mathrm{pH}>6.5 \mathrm{TiOH}\to {\mathrm{TiO}}^{-}+{\mathrm{H}}^{+}$$

(6)

The photocatalytic degradation of RY161 (30 mg/L) and CV (10 mg/L) in the presence of synthesized TiO₂ NPs at different pH levels (4, initial pH of the solution, and 10) was investigated.

Figure 9 illustrates the removal of RY161 and CV as a function of irradiation time for various pH values. The results indicate that photodegradation is more significant at acidic pH (pH = 4) for RY161 and at basic pH (pH = 10) for CV.

This can be explained by the charge nature of these two pollutants. In an acidic medium, a substantial adsorption of the anionic dye RY161 onto TiO₂ nanoparticles is observed, likely due to the electrostatic attraction between the positive charge of the synthesized TiO₂ NPs and the negative charge of the dye [55]. However, the rate of photocatalytic degradation decreases with increasing pH, transitioning from 0.0507 to 0.009954 min⁻¹ (Figure 9b).

Conversely, the best adsorption and degradation were observed in a basic medium where TiO₂ carries a negative charge, favoring the adsorption of the cationic dye CV. Increasing the pH from 4 to 5.9 resulted in an increase in the rate of photocatalytic reactions from 0.00972 to 0.02015 min⁻¹, with the degradation process following the Langmuir–Hinshelwood (L-H) model (Figure 9d). However, when the CV solution was highly basic (pH = 10), 94.06% of the CV molecules were removed within the first 30 min, confirming the high degradation efficiency of the catalyst. Therefore, its degradation performance did not conform to the L-H model throughout the entire 180 min process.

3.2.5. Mechanism Proposed for the Photocatalytic Activity of TiO₂ NPs

Under the effect of visible light and after the excitation of the semiconductor, the electrons of the valence band (BV) pass towards the conduction band (BC) whose energy of the photons is higher or equal to the difference of the energy between these two bands of TiO₂ NPs, which involves the production of an electron-hole pair (e−,h⁺), as shown in Equation (7) (Figure 10) [59].

These charge carriers generate and perform redox reactions, and oxygen molecules are oxidized to give superoxide ions $({\mathrm{O}}_2^{.-})$ Equation (8); the latter can react with hydrogen peroxide to form elements like $({\mathrm{OH}}^{.}),({\mathrm{OH}}^{-}) \mathrm{and} ({\mathrm{O}}_2)$ Equation (9), while water molecules and hydroxyl anions are reduced to hydroxyl radicals (${\mathrm{OH}}^{.})$ according to Equation (10) and Euation (11), respectively. These free radicals are highly reactive and non-selective species, capable of oxidizing persistent organic pollutants to form intermediates that will themselves react to full mineralization, forming degradation products such as (CO₂) and (H₂O) Equation (12) [8,44].

The schematic mechanism of the photocatalytic activity is presented in Figure 10, and the corresponding reactions are also presented below.

$${\mathrm{TiO}}_2+h\vartheta \to {\mathrm{TiO}}_2 \left({\mathrm{h}}^{+},{\mathrm{e}}^{-}\right)$$

(7)

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{O}}_2^{.-}$$

(8)

$${\mathrm{O}}_2^{.-}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{OH}}^{.}+{\mathrm{OH}}^{-}+{\mathrm{O}}_2$$

(9)

$${\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)+{\mathrm{H}}_2\mathrm{O}\to { \mathrm{TiO}}_2+{\mathrm{H}}^{+}+{\mathrm{OH}}^{.}$$

(10)

$${\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)+{\mathrm{OH}}^{-}\to {\mathrm{TiO}}_2+{\mathrm{OH}}^{.}$$

(11)

$$\mathrm{Organic} \mathrm{Matter}+{\mathrm{OH}}^{.}\to \mathrm{intermediate} \mathrm{products}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$$

(12)

3.3. Reusability of the catalyst TiO₂ NPs

The reuse of TiO₂ NPs synthesized from Astragalus boeticus extract has been verified. Indeed, five recycling attempts operating under optimal operating conditions (pH_RY161 = 4, C_RY161 = 30 ppm, m = 1 g/L and pH_CV = 10, C_CV = 10 ppm, m = 1 g/L) were tested to prove the stability of the synthesized catalyst (Figure 11).

The obtained results show that after the fifth cycle, the photocatalytic efficiency decreased by only 7% and 9% for RY161 (Figure 10a) and CV (Figure 10b), respectively.

Indeed, during the first two cycles for both target pollutants, the decomposition capacity remained stable at 100%, then from the third and fourth cycle, this photocatalytic oxidation rate decreased very slightly and reached 2% for RY161 and 4% for CV.

Moreover, a degradation capacity of 93% and 91% was obtained for RY161 and CV, respectively, after the fifth cycle. Therefore, TiO₂ NPs are stable, possess excellent reusability, and have the potential to be used multiple times without a considerable change in activity.

3.4. Comparative Study of the Photocatalytic Degradation of Organic Pollutants by Nanomaterials Synthesized Using a Green Way

Photocatalytic degradation of various organic pollutants using catalytic TiO₂ nanoparticles synthesized by the green route using plant extracts was examined by comparing the results obtained from each study (

Table 1. Comparative study of previous and current data on photocatalytic degradation of pollutants with green-synthesized TiO₂ nanomaterials.

Plant Used for the Synthesis of TiO2 NPs	Pollutant Conc.	Catalyst Conc.	Time (min) Percentage	Morphology	Size (nm)	Ref
Jatropha curcas	Tannery wastewater (Cr) 6.88 mg/L	5 g/5 L	5 h (76.48%)	Spherical	75 nm	[9]
Citrus limetta	RhB dye 10 mg/L	0.7 g/50 mL	80 min (90%)	Spherical	80–100 nm	[10]
Syzygium cumini	Lead (Pb) 8.621 mg/L	0.3 g/500 mL	17 h (82.53%)	Spherical	10 nm	[29]
Citrus limon juice	RG-19 6.7 Mm	0.03 g/100 mL	60 min (99.88%)	Spherical	10–21 nm	[32]
Piper betel	Vert Malachite 100 ppm	100 mg/50 mL	50 min (100%)	-	6.6 nm	[60]
Ocimum tenuiflorm			50 min (100%)		7.0 nm
Moringa oleifera			30 min (100%)		6.6 nm
Coriandrum sativum			50 min (100%)		6.8 nm
Astragalus boeticus	RY161 30 mg/L CV 10 mg/L	0.1 g/100 mL	90 min (100%) 30 min (94.06%)	Spherical	68 nm	This work

). The latter clearly shows that TiO₂ NPs synthesized from Astragalus boeticus extract are one of the most efficient heterogeneous catalysts, with a percentage of degradation of 100% and 94.06%, respectively, for Reactive Yellow 161 and Crystal Violet for the degradation of persistent pollutants due to the presence of substances that function as a reducing and capping agent.

4. Conclusions

The green synthesis of TiO₂ nanoparticles was carried out using an extract from the leaves of Astragalus boeticus, a plant widely available in recycling contexts, making it an even more environmentally friendly choice. This green synthesis method is cost-effective, non-toxic, natural, and environmentally friendly. The crystalline phase and morphology of TiO₂ NPs were characterized through XRD, FTIR, UV-Vis, and SEM/EDX analyses. The characterization of the synthesized TiO₂ NPs revealed that the nanoparticles have a size of approximately 68 nm, an anatase crystal structure with a spherical surface morphology, and a calculated band gap of 3.22 eV, which is in line with the literature. Photocatalytic activity demonstrated that the cationic pollutant rapidly degrades in a basic medium (pH = 10) with a yield of 94.06% within 30 min, while the anionic pollutant achieves a yield of 100% after 90 min in an acidic medium (pH = 4). A recyclability study revealed the effective reuse of the green TiO₂ NPs catalyst for up to five cycles. This excellent reusability and photocatalytic activity demonstrate that the synthesized photocatalyst is not only environmentally friendly but also holds potential applications in water purification.

While the green synthesis of TiO₂ nanoparticles using Astragalus boeticus leaf extract offers numerous advantages, it is essential to acknowledge certain limitations of this approach. Firstly, the long-term stability of TiO₂ NPs in complex environments needs to be further investigated. Additionally, the present study focused on the degradation of specific dyes, and the efficiency of this photocatalyst may vary depending on the pollutants present in the water. Further research is required to assess its applicability to other pollutants. Lastly, the scale of TiO₂ NP production should be considered for potential large-scale applications.