Toxic Congo Red Dye Photodegradation Employing Green Synthesis of Zinc Oxide Nanoparticles Using Gum Arabic

Abstract

A green synthesis method for producing zinc oxide nanoparticles (ZnO NPs) was presented using natural Gum Arabic (GA) as a natural stabilizing agent. For the first time, the as-synthesized ZnO NPs were employed to photodegrade the toxic Congo Red (CR) dye in an aqueous solution. The structural and morphological characterizations confirmed the successful synthesis of ZnO NPs. The ZnO NPs possessed an average crystallite size of 42.7 nm. In addition, it was found that a concentration of 20 mg L⁻¹ of CR dye yielded the most favorable photodegradation results, and 4 mg mL⁻¹ of the photocatalyst was the optimal amount. The results showed a maximum degradation percentage of 99.5% at pH 8 after 30 min of irradiation. This indicates that the as-synthesized ZnO NPs have remarkable photocatalytic properties. Moreover, the study demonstrated the suitability of the pseudo-first-order kinetic model for representing the photodegradation process through kinetic studies of the photocatalyst process of CR dye by ZnO NPs using the Langmuir-Hinshelwood (L-H) model.

1. Introduction

Industrial developments in several sectors have significantly impacted environmental safety. One such sector is the production of dyes, which contaminate the aquatic environment when released into various water sources [1,2]. They are widely used in textiles, food, medicine, cosmetics, household products, and agricultural items [3,4,5]. Even in low quantities, they can cause severe diseases due to their hazardous properties, complex structure, and challenges associated with their decomposition [6,7]. Therefore, purifying the dye wastewater is a necessity and an urgent priority [8].

The complexity and resistance of dye molecules to standard treatment technologies have posed a significant challenge to their removal from wastewater [9]. Adsorption, coagulation, biodegradation, photocatalysis, and membrane filtering are among the techniques considered [10]. However, most of them often have limitations [11,12,13,14,15]. They can be costly due to the requirement for specialized equipment, energy consumption, or expensive materials [16]. Additionally, some processes might have slow kinetics, resulting in longer treatment times or requiring higher energy inputs to speed up the reactions [17]. Therefore, researchers are persistently exploring strategies to overcome these limitations. Currently, novel technologies and adaptations for several techniques are being developed to boost efficacy, minimize expenses, accelerate response times, and address challenges. Given the significant impact of dye wastewater on water quality, ongoing research is dedicated to developing more efficient, cost-effective, and long-term treatment solutions to address the shortcomings of previous procedures.

The photocatalytic degradation of dyes via semiconductor photocatalyst materials has emerged as an efficient method for removing dyes from wastewater [18]. It can degrade a wide range of organic and inorganic pollutants while avoiding the generation of undesirable byproducts [19,20,21]. Among semiconductor materials, zinc oxide (ZnO) is a superior choice for degrading organic dyes. It possesses a suitable band gap, rapid photocatalytic activity, low toxicity, easy preparation, and high efficiency [22,23]. It has a large exciton binding energy (60 meV) [24] like TiO₂, suggesting a more stable and longer lifetime as a catalyst. Furthermore, ZnO has many active sites, making it a highly active photocatalyst. Therefore, it is ideal for removing toxic dyes from aqueous solutions. However, the synthesis methods used to create these nanoparticles should prioritize economic feasibility and sustainability. The size of these nanoparticles is also crucial, as it can be influenced by various factors such as temperature, stabilizers, and catalysts. These factors can impact the size of the nanoparticles, potentially leading to aggregation and the formation of larger particles due to thermodynamic instability [25,26,27]. The fast-paced development of nanomaterial production using different strategies has significantly increased the use of ZnO NPs in various applications. However, this progress has also brought several environmental issues to the forefront. As a result, there is a pressing need to find alternative synthesis strategies that can help mitigate these environmental problems. One promising approach is the green synthesis of ZnO using natural resources, such as plant extracts, microbes, and algae, as bio-stabilizer agents. By developing and implementing such green synthesis strategies, we can effectively address the environmental contamination caused by the high consumption of toxic chemicals and the discharge of hazardous waste in synthesizing ZnO NPs.

Several synthesis methods use stabilizer materials to prevent aggregation by minimizing surface energy [28]. Generally, ZnO material is not stable. Due to water’s high polarity, ZnO can agglomerate, leading to deposition. Several methods have been employed to improve its stability, including direct and homogeneous precipitations, hydrothermal/solvothermal methods, the sonochemical method, reverse micelles, thermal decomposition, and the sol-gel method. Unfortunately, most previous techniques utilized toxic chemicals as reducing and/or capping agents, generating environmentally risky by-products. Thus, this issue can be addressed by using green synthesis methods to achieve more sustainable processes, focusing on reducing or preventing the use of toxic and risky substances. In this pursuit, researchers have used several stabilizer materials, such as natural polymers [29,30]. Gum Arabic (GA), a naturally derived polymer, is utilized in several applications. It is produced from plants and has been extensively documented in many applications due to its remarkable characteristics, including its increased viscosity and stability, lack of toxicity, environmentally friendly nature, easy accessibility, cost-effectiveness, and surface activity [31,32]. It can serve the twin purposes of stabilizing and reducing metal ion agents during nanoparticle production. Depending on the pH of the solution, the polymer either forms a stiff barrier or a charge patch when it binds to the particle’s surface [33,34]. Several research studies have been published on synthesizing ZnO NPs utilizing GA derived from various plants, including cashew gum, frankincense gum, and acacia Arabic. The studies have shown impressive efficacy in preparing ZnO NPs and remarkable practical application results in the degradation of several dyes, such as direct blue 129 (DB129) and Malachite Green [35,36,37,38]. However, there are no reports in the literature about the green synthesis of ZnO NPs using GA for toxic Congo Red dye photodegradation in an aqueous solution.

Congo Red (CR) is an anionic diazo dye with the chemical formula C₃₂H₂₂N₆Na₂O₆S₂ [39,40]. Several studies have evidenced that its presence in the human body might have harmful effects, resulting in various illnesses that, if severe, could potentially be fatal [37], as displayed in

Table 1. Harmful and toxic effects of Congo Red and the affected targets.

Harmful Effects	Targets
Undesirable mutagenicity	Living organisms
Carcinogenic textile dye	Humans and animals
Phytotoxicity	Plants
Allergic dermatitis, skin, eye, and gastrointestinal irritation	Humans
Difficulty in breathing, chest pain, and severe headache	Humans

[41,42,43]. Possible consequences include mutagenicity, carcinogenicity, cytotoxicity, neurotoxicity, chemotoxicity, and genotoxicity. The eyes, skin, lungs, and reproductive systems are all impacted. The structural stability of the CR dye makes it non-biodegradable. Consequently, it is classified as an organic contaminant, and its removal from wastewater is necessary.

In this work, we fabricated ZnO NPs using GA material as a green synthesis method. The GA material played a crucial role as a stabilizing and reducing agent. Notably, the as-prepared ZnO NP catalyst is used for the first time to effectively photodegrade toxic CR dye in an aqueous solution. Our photodegradation studies, considering parameters such as pH level, contact time, dye concentration, and catalyst dosage, demonstrated an impressively high level of photocatalytic activity against the CR dye. Importantly, we achieved a remarkable degradation percentage of 99.5% when the concentration of the CR dye was 20 mg L⁻¹, underscoring the potential of ZnO NPs as a catalyst.

2. Materials and Methods

2.1. Materials

Zinc (II) sulfate hexahydrate (ZnSO₄.6H₂O, >98%, Sigma-Aldrich, St. Louis, MO, USA), Congo Red dye (CR, Thermo Fisher Scientific, Waltham, MA, USA), sodium hydroxide pellets (NaOH, 98.0%, Loba Chemie, Mumbai, India), hydrochloric acid (HCl, 36.5%, Fluka Chemika, Seelze, Germany), and ethanol (C₂H₅OH, 99.9%, Techno Pharmchem, Rohtak, India) were used without further purification. Solutions were prepared by dissolving the appropriate amount of the dye in double-distilled water before each experiment.

2.2. Collection and Purification of Gum Arabic (GA)

The GA sample was collected from the Al-Mahawil district in the local area (Babylon Governorate, Iraq) based on its availability and effectiveness. After that, it was dried appropriately under sunlight and ground to a fine powder using a mortar and pestle. The GA solution (1% w/v) was prepared and purified using distilled water and hydrated for 24 h at 3 °C. The solution was then mixed with isopropanol at a 1:2 ratio and held at 3 °C for 6 h. After that, the solution was centrifugated, and the precipitate was retained. Finally, the precipitate was dried under a vacuum for 48 h.

2.3. Synthesis of ZnO NPs

A solution of GA (40 mL, 1.0% (w/v)) was added to 50 mL of 0.1 M ZnSO₄ 6H₂O. The mixture was subjected to stirring for 30 min. The pH of the mixture was adjusted to 10 by slowly adding a 1 M NaOH solution. Subsequently, it was agitated for 180 min at room temperature. The colloidal solution acquired underwent a 24 h aging process at room temperature and was collected by centrifugation. The resultant product was washed with C₂H₅OH and distilled water, ensuring no impurities remained, and then dried at 45 °C in an electrical oven. The dried sample underwent calcination for 1 h at 400 °C using a muffle furnace [44]. Figure 1 shows the synthesis method for the ZnO NPs using GA material.

2.4. Material Characterization

The X-ray diffractometer (XRD) used in this study was the PW-1730 Philips model (Almelo, Netherlands), equipped with filtered Cu-Kα radiation with a wavelength of 0.15406 nm. The Bruker Vertex 70 Fourier transform infrared (FT-IR) system, Shimadzu 1800, Japan, was used to analyze the structure of GA and ZnO NPs at a resolution of 4 cm⁻¹ throughout the 400–4000 cm⁻¹ spectral region. The surface morphology of the GA material and ZnO NPs was thoroughly investigated using scanning electron microscopy (SEM, Tescan Vega, Czech). The absorbances for each sample were measured using a Perkin-Elmer UV-visible spectrophotometer with a wavelength range of 190 to 1100 nm. A quartz cell with a path length of 10 mm was used for the measurements, ensuring accurate and reliable data.

2.5. Photocatalytic and Kinetic Study

To evaluate the ZnO NPs sample’s ability to degrade CR dye, 20 mL of a 100 mg L⁻¹ dye solution was added to a 250 mL beaker, which consists of two vessels (inner and outer) with water passing between them to control the temperature during the process of irradiating the solution, with 20 mg of ZnO NP powder. The mixture was then stirred for about 30 min in a dark environment, a crucial step to achieving equilibrium between adsorption and desorption, ensuring the accuracy of the results. A 60-watt lamp was used as a UV-VIS light source, positioned 10 cm from the solution’s base. At regular intervals of 15 min, a sample of the solution containing CR dye was extracted, and the absorbance was measured at a specific wavelength of 495 nm. The percentage of dye removal and the rate of removal constant were determined using Equations (1) and (2), respectively [45].

$$\mathrm{\%}\mathrm{D}\mathrm{y}\mathrm{e} \mathrm{d}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=(C_o-C_t)/\left(C_o\right)\times 100$$

(1)

$$kt=\ln (C_o)/C_t$$

(2)

where C_o is the initial dye concentration, C_t is the dye concentration after photodegradation, k is the reaction rate constant, and t is the time of reaction. All the experimental procedures were replicated three times under the examined parameters, including irradiation time, catalyst dose, pH level, and dye concentration.

3. Results and Discussion

The possible reaction mechanism of the as-synthesized ZnO NPs can be summarized via Equations (3)–(6) [46]. When the Zn ions (Zn²⁺) are formed in water, they are stabilized using the GA solution. When added, NaOH can react with the Zn²⁺ ions to form Zn(OH)₂. Finally, the ZnO NPs were obtained when ${\left[\mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_2{\left(\mathrm{G}\mathrm{u}\mathrm{m} \mathrm{A}\mathrm{r}\mathrm{a}\mathrm{b}\mathrm{i}\mathrm{c}\right)}_{\mathrm{n}}\right]}_{\left(\mathrm{s}\right)}$ calcined at 400 °C for 1 h.

$$ZnS{O_4}_{\left(aq\right)}\to Z{n}_{ \left(aq\right)}^{2+}+S{O_4}_{ \left(aq\right)}^{2-}$$

(3)

$$Z{n}_{ \left(aq\right)}^{2+}+Gum Arabic \left(aq\right)\to {\left[Zn{\left(Gum Arabic\right)}_n\right]}_{ \left(aq\right)}^{2+}$$

(4)

$${\left[Zn{\left(Gum Arabic\right)}_n\right]}_{ \left(aq\right)}^{2+}+2O{H}_{ \left(aq\right)}^{-}\to {\left[Zn{\left(OH\right)}_2{\left(Gum Arabic\right)}_n\right]}_{\left(s\right)}$$

(5)

$${\left[Zn{\left(OH\right)}_2{\left(Gum Arabic\right)}_n\right]}_{\left(s\right)} \stackrel{∆}{\to } ZnO NPs$$

(6)

3.1. Structural Characterization of the ZnO NPs

The X-ray diffraction patterns of the GA sample and ZnO NPs are represented in Figure 2a. The XRD pattern of the GA sample demonstrated that the powder lacked a distinct diffraction pattern, indicating its amorphous nature with a broad peak at 2θ ~19.5°. This is a significant observation in our research. In contrast, the diffraction peaks of ZnO NPs are located at 31.58°, 34.23°, 36.07°, 47.35°, 56.41°, 62.67°, 66.19°, 67.76°, 68.91°, 72.62°, and 76.86°, which are indexed to ZnO (JCPDS card number: 36-1451, hexagonal phase) [47]. Furthermore, the crystallite size (D, nm) of the ZnO NPs was estimated using the Debye–Scherrer Equation (7) [48].

$$D={\displaystyle \frac{0.94\lambda }{\beta cos\theta }}$$

(7)

where 0.94 is Scherrer’s constant, λ is the X-ray wavelength, β is the full width at half maximum height (FWHM), and θ is the diffraction angle. The average crystallite size of the as-prepared ZnO NPs was 42.74 nm. The small crystallite size of ZnO NPs can speed up the photodegradation of CR dye by adding more active sites for adsorption.

FTIR analysis was used to identify the functional groups in the GA and ZnO NPs, as shown in Figure 2b. The band’s presence at 3415.9 cm⁻¹ in the GA spectrum can be attributed to the O–H stretching vibration, a typical feature of the glycosidic ring [49]. The amino group’s distinctive absorption band within the range of 3300–3500 cm⁻¹, though partially obscured by the broader absorption band of the O–H group, is a reliable finding. The C–H groups and specific sugar components in GA, such as galactose, arabinose, and rhamnose, are identifiable at 2926 cm⁻¹ [50]. The spectrum peak at 1632.1 cm⁻¹ is attributed to the vibrational stretching of the C=O bond. The typical band of C=C (stretching) was observed at around 1608.6 cm⁻¹. The glucuronic acids exhibit distinct vibrational modes, including a band at 1426.6 cm⁻¹ attributed to C=O symmetric stretching and another band at 1373.3 cm⁻¹ associated with –OH bending. The peaks within 1300 to 900 cm⁻¹ are attributed to the –C–O–C– and C–O stretching modes [51,52,53,54]. The presence of the bands at 416.9 and 617.2 cm⁻¹ was ascribed to the stretching of the Zn–O bond [53,54]. FE-SEM analysis shows that the exterior surface of the GA (Figure 3a) appears to be an irregular and rocky-like structure with many dents. Figure 3b represents the FE-SEM image of the ZnO NPs. It demonstrates the configuration of the nanoparticle.

3.2. Catalytic Activity of ZnO NPs

The photocatalytic degradation of CR dye using ZnO NPs was detected at different contact times at λ_max of 495 nm. The results of a typical run are represented in Figure 4a. The concentration of the CR dye decreased with increasing irradiation time, indicating that the CR dye was degraded photocatalytically. The control experiments were achieved and indicated that the dye is degraded only with the existence of photocatalysts upon irradiation.

3.3. Effect of Irradiation Time

UV/Vis spectrophotometric analysis was used to study the effect of irradiation time on CR degradation. The photocatalytic degradation of the CR dye was carried out by adding 20.0 mg of ZnO NPs to the solution and irradiating it at different times. The absorbance was then measured to find the CR dye’s percentage of photodegradation (%D.). Figure 4b illustrates the impact of varying irradiation times on the degradation process of the CR dye. The results demonstrated a positive correlation between irradiation time and the degradation percentage. Specifically, the degradation percentage exhibited an upward trend, reaching a peak of 97.8% after 90 min of irradiation. Conversely, the lowest degradation percentage of 80.4% was seen after 15 min of irradiation.

3.4. Effect of Variation of Catalyst Dose on Photodegradation of CR Dye

The photocatalytic degradation process was carried out with various amounts of catalyst to determine how much of the mass of the photocatalyst affected its photocatalytic activity. Different mass catalysts (0.02, 0.04, 0.06, 0.08, and 0.1 g) were used to study what happens when the mass of ZnO NPs changes during the photodegradation process of the CR dye. As shown in Figure 5a, the percentage of photodegradation increases as the catalyst is raised from 0.02 to 0.08 g. In addition, Figure 5b,c show the rate of photodegradation constant (K), which increased with increasing doses of catalyst from 0.02 to 0.08 g. This increase in catalyst mass leads to a corresponding rise in active sites, which could explain the relationship between the dye percentage and the catalyst’s weight [55]. When the weight of the substance exceeds 100 mg, the percentage of destroyed dye decreases. The solution’s light catalyst particles may be to blame for this drop’s light blocking and scattering [56]. The CR degradation efficiency on ZnO NPs is compared with other related materials, as listed in

Table 2. The photodegradation percentage comparison of CR dye using as-synthesized ZnO NPs with other related materials.

ZnO Photocatalyst	Congo Red Concentration (mg L−1)	Degradation Efficiency (%)	Time (min)	Ref.
0.3 g L−1 of ZnO-PVP-St	20	65	---	[57]
SBT-ZnO/NF	15	99	80	[58]
150 mg of ZnO	100	93	150	[59]
ZnO	20	<90	1 h	[60]
0.17 g/100 mL of ZnO	5	96	---	[61]
4 mg mL−1 of ZnO NPs	20	99.5	30	This work

.

3.5. Effect of pH of Solution

The pH level is an essential factor in the photocatalytic degradation process [62]. It can be significantly affected by the size of particles, the charge on their surfaces, and where the boundaries of the valence and conduction bands are located [63]. The tested pH range was 5–9, as shown in Figure 6a, and the effect of pH on the rate constant (k) is listed in Figure 6b,c. The dye’s percentage of photodegradation led to an increase in free hydroxyl radical production, which raised the pH to 8. On the other hand, the dye’s photodegradation rate declined above a pH of 8, which may be due to the catalyst’s propensity for corrosion [64]. The rationale for this phenomenon is derived from the photocatalyst’s point of zero charge (PZC). Therefore, the surface charge was studied by assessing the PZC. The results revealed that the pH_PZC is 8.4 (Figure S1), where above this pH value, the catalyst system will be negatively charged, and the result was confirmed from the results of different pH experiments. As shown in the effect of the pH findings, the photodegradation activity was reduced dramatically by increasing the pH value above pH 8, and this was attributed to the repulsion between the negatively charged catalyst surface at this point with the anionic CR dye, which is negative in nature [54]. Thus, the findings highlighted the crucial role of adsorption. Hydroxyl radicals are the main oxidizing species that break down materials through photocatalysis. The positively charged surface of ZnO NPs and the negatively charged CR dye interact electrostatically, which produces more hydroxyl radicals [65].

3.6. Effect of Dye Concentration

The present investigation examined the impact of varying concentrations of CR dye on the photocatalytic reaction. Different concentrations (20, 40, 60, 80, 100, and 120 mg L⁻¹) were considered. The results are presented in Figure 7a,b. In addition, the effect of the dye concentration on the rate constant (K) is listed in Figure 7c,d. The degradation percentage and rate photodegradation constant are increased at lower dye concentrations, whereas they demonstrated a reduction at higher concentrations. The efficacy of the photocatalytic process is contingent upon the extent to which light can permeate the dye solution and then interact with the ZnO NPs, thereby enhancing the reduction of absorbed oxygen. At lower dye concentrations, the incident light can saturate the dye solution and absorb it from the photocatalyst. At elevated dye concentrations, a significant quantity of dye molecules will impede the passage of light photons to the photocatalyst. The rate of photoreaction is decreased by this phenomenon [66,67]. Conversely, when present in low quantities, it is shown that not all active sites are filled by dye molecules. Various free radicals contribute to the formation of distinct free radicals, which are responsible for the photodegradation of the dye. Consequently, the photoreaction rate significantly increases when the dye is in low quantities [68,69].

3.7. Photocatalytic Kinetic Study

Langmuir-Hinshelwood (L-H) kinetics are a common way to characterize the reaction between oxygen-containing molecules and surface-reducible reactants in photocatalysis. This is the most common kinetic model for photodegradation and heterogeneous catalytic processes. Based on certain assumptions of the Langmuir adsorption isotherm, it is utilized to determine the rate of a heterogeneous catalytic process. In addition, it is crucial to consider that the L-H model is affected by several elements, such as the characteristics of the catalyst, the light sources used, the parameters of mass transfer, and the operating circumstances. It can be expressed using Equation (8) as follows:

$$-{\displaystyle \frac{dC}{dt}}={\displaystyle \frac{k\mathrm{K}\mathrm{C}}{1+KC}}$$

(8)

where C is the dye concentration (mg L⁻¹), k is the rate constant (mg L⁻¹ min⁻¹), and K is the equilibrium adsorption constant (L mg⁻¹).

The photocatalyst concentration remains unchanged during the photocatalytic reaction. Therefore, the reaction rate depends on the dye concentration. Accordingly, the pseudo-term can be used to determine the reaction rate. The surface of the photocatalyst is saturated if the dye concentration is very high; therefore, the reaction rate constant does not depend on the dye concentration (a zero-order expression). As shown in Equation (9):

$$-{\displaystyle \frac{dC}{dt}}=\mathrm{K}$$

(9)

In contrast, when the dye concentration is low, the rate of photodegradation depends on the change in the dye concentration, meaning the rate of photodegradation is proportional to the dye concentration, as expressed in Equation (10):

$$-{\displaystyle \frac{dC}{dt}}={\mathrm{K}}_{1}C$$

(10)

Figure 8 shows the plot of (C_o-C_t), (ln C), and (1/C) versus time of irradiation (in min) of zero, first, and second-order reactions, respectively, based on the integrated equation for the CR dye at the optimum pH values of the solution and dose of the photocatalyst. In addition,

Table 3. Rate constant and regression coefficient values for CR dye photodegradation obtained by plotting relationships of zero-, first-, and second-order reaction at optimum values.

Kinetic Model	Regression Coefficient (R2)	Rate Constant
Zero-Order Reaction	0.723	0.9753 K M min−1
1st-Order Reaction	0.998	0.067 K (min−1)
2nd-Order Reaction	0.627	0.0344 K M−1 min−1

shows the values of the rate constant and regression coefficient for zero, first, and second-order reactions. Based on the findings, the reaction obeyed pseudo-first-order kinetics because it produced a straight line with a high regression coefficient (R² = 0.998).

3.8. Photocatalytic Performance

To evaluate the recyclability and stability of the as-synthesized ZnO NP photocatalyst, tests were conducted by executing recycling reactions five times for the photodegradation of CR dye under a UV-VIS light source. As shown in Figure 9, no noticeable loss of the photocatalytic activity of the ZnO NPs was observed for the CR dye degradation reaction after repeated cycles. This indicates that ZnO has good recyclability and stability and could be a potent material for practical photocatalysis applications.

3.9. Photocatalytic Mechanism

The UV-VIS diffuse reflectance spectrum (DRS) (Figure S2a,b) was conducted to estimate the band gap (E_g) value of the ZnO NPs. It was calculated from Equation (11) to be 2.9 eV.

Eg = 1240.2/λ(nm)

(11)

In addition, from the high-resolution XPS spectra in Figure S2c,d, the valence band (VB) was calculated to be 2.42 eV, indicating enough oxidation potential to produce •OH from OH⁻. The conduction band (CB) was also calculated from this relation, E_CB = E_g − E_VB, to be −0.48 eV, more than the reduction potential of O₂/•O2⁻ (−0.33 eV) [70]. Therefore, we can conclude that O₂ and •OH play the main roles in the photodegradation of CR dye.

Figure 10 represents the CR dye photocatalytic degradation mechanism using a ZnO NP catalyst. Based on several studies, violet light can transfer electrons from the VB to the CB [71,72]. This creates holes (h⁺) and electrons (e⁻) on the surface of the Zn NP catalyst, which is a vital part of breaking down the CR dye. The diazo groups in the CR dye are cleaved due to the generation of •OH radicals on the surface of the photocatalyst, which arises via the oxidation of the H₂O molecule [73,74]. Furthermore, a photoinduced electron transforms the adsorbed O₂ molecule, forming O•²⁻ radicals via the reduction process. The O•²⁻ radical then reacts with the H₂O molecule, forming an H₂O₂ molecule. This molecule is further reduced by electrons, leading to the production of •OH radicals. Following a sequence of oxidation reactions, the CR dye converts into carbon dioxide (CO₂), H₂O, and non-toxic molecules.

4. Conclusions

In summary, the green synthesis of the zinc oxide nanoparticle (ZnO NP) catalyst was successfully demonstrated using Gum Arabic material as a natural stabilizing agent by calcination. Gum Arabic was used as an alternative to potentially harmful and toxic materials. It was employed for the photodegradation of Congo Red (CR) dye in an aqueous solution for the first time. The structural and morphological characterizations revealed that the green synthesis of the ZnO NPs via Gum Arabic improved its properties, which further helped enhance the dye adsorption performance. It possessed a small crystallite size of 42.7 nm that could speed up the photodegradation of CR dye by adding more active sites for adsorption. Furthermore, the findings indicated that the degree of CR dye degradation was affected by dye concentration, photocatalyst dose, and pH value. The optimal parameters were shown to be CR dye concentration = 20 mg L⁻¹, amount of photocatalyst = 4 mg mL⁻¹, and pH = 8 for a degradation percentage of 99.5% during a 30 min irradiation time. In addition, the CR dye photokinetics of the ZnO-NPs were examined systematically using various kinetic models. The results suggested that the pseudo-first-order model was deemed the most fitting, identified by maximum regression coefficient values. The as-synthesized ZnO NP catalyst exhibited excellent catalytic activity, and it is recommended as an efficient photocatalyst to remove CR dye from wastewater.