The Photocatalytic Performance of Nd2O3 Doped CuO Nanoparticles with Enhanced Methylene Blue Degradation: Synthesis, Characterization and Comparative Study

The growth of the textile industry results in a massive accumulation of dyes on water. This enormous rise in pigments is the primary source of water pollution, affecting the aquatic lives and our ecosystem balance. This study aims to notify the fabrication of neodymium incorporated copper oxide (Nd2O3 doped CuO) nanoparticles by combustion method for effective degradation of dye, methylene blue (MB). X-ray diffraction (XRD), Field emission Scanning electron microscopy (FESEM), Zeta potential have been applied for characterization. Photocatalyst validity has been evaluated for methylene blue degradation (MB). Test conditions such as time of contact, H2O2, pH, and photo-Fenton have been modified to identify optimal degradation conditions. Noticeably, 7.5% Nd2O3 doped CuO nanoparticle demonstrated the highest photocatalytic efficiency, up to 90.8% in 80 min, with a 0.0227 min−1 degradation rate. However, the photocatalytic efficiency at pH 10 becomes 99% with a rate constant of 0.082 min−1. Cyclic experiments showed the Nd2O3 doped CuO nanoparticle’s stability over repeated use. Scavenge hydroxyl radical species responsible for degradation using 7.5% Nd2O3 doped CuO nanoparticles have been investigated under visible irradiation.


Introduction
Over the last decades, because of rapid industrialization, much toxic w released into water bodies. Dyes are involved in various industries, including tic, skincare products, food processing, medicines, etc. [1]. The dye can nega the aquatic environment due to reduced solar permeation. Furthermore, cert cause severe illnesses such as mutation and cancer [2]. Significant consequ lack of potable water, domestic use, agriculture, farming, etc. [3]. Methylene a cationic thiazine dye, which forms a blue water solution (see Scheme 1). various applications, including colored paper, the temporary coloring of cotto paper stock dyeing. MB leads to different toxic effects, including eye burnin tion, and gastrointestinal tract. So, it is essential to remediate the dyestuffs w save water and the environment [4]. Consequently, water pollution must be eliminated [5]. Physical techni adsorption and chemical techniques are employed to remove the textile dye vanced oxidation process includes heterogeneous photocatalysis, low-cost a ble technology for water purification that takes place on photocatalyst pa presence of photons, causing them to be mineralized [7].
Reusable photocatalysts are used to degrade organic contaminants in ph As particle size is reduced, the photocatalyst's surface area increases. As a res ber of activating sites has increased, resulting in a higher reactivity. After th of light, the electron has been excited, generating a hole, O2 .− and OH . has due to the reaction between e − with O2 and h + with OH − or H2O [8]. Many rese examined the degradation of various reactive dyes with visible light-ass (H2O2/Fe 2+ or Fe 3+ ) and photo-Fenton (UV-Vis/H2O2/Fe 2+ or Fe 3+ ) reactions mechanism and kinetics of the response have been widely explored [9]. Fent the mixture of H2O2 and ferrous iron, produces hydroxyl radicals with the ions, which initiate and catalyze H2O2 decomposition.
Metal oxide nanoparticles (NPs) such as ZnO, CoO, TiO2, SnO2, and C one strategy to lower aqueous organic dyes. In particular, CuO NPs have si tential because of their low cost, low toxicity, easy availability, optical, cataly microbial characteristics. In addition, copper oxide nanoparticles are notewo of their surface area and increased adsorbed oxygen capacity, leading its b ranged from 1.35 to 3.5 eV [10,11]. The strong ability of CuO-NPs for mole absorption has a significant impact on the electron-hole pair rearrangemen moval of the photogenerated electron [12]. Nd2O3 is rare earth oxide that Consequently, water pollution must be eliminated [5]. Physical techniques such as adsorption and chemical techniques are employed to remove the textile dyes [6]. The advanced oxidation process includes heterogeneous photocatalysis, low-cost and sustainable technology for water purification that takes place on photocatalyst particles in the presence of photons, causing them to be mineralized [7].
Reusable photocatalysts are used to degrade organic contaminants in photocatalysis. As particle size is reduced, the photocatalyst's surface area increases. As a result, the number of activating sites has increased, resulting in a higher reactivity. After the absorption of light, the electron has been excited, generating a hole, O − 2 and OH . has been formed due to the reaction between e − with O 2 and h + with OH − or H 2 O [8]. Many researchers have examined the degradation of various reactive dyes with visible light-assisted Fenton (H 2 O 2 /Fe 2+ or Fe 3+ ) and photo-Fenton (UV-Vis/H 2 O 2 /Fe 2+ or Fe 3+ ) reactions [4]. Both the mechanism and kinetics of the response have been widely explored [9]. Fenton's reagent, the mixture of H 2 O 2 and ferrous iron, produces hydroxyl radicals with the ferrous (Fe 2+ ) ions, which initiate and catalyze H 2 O 2 decomposition.
Metal oxide nanoparticles (NPs) such as ZnO, CoO, TiO 2 , SnO 2 , and CuO represent one strategy to lower aqueous organic dyes. In particular, CuO NPs have significant potential because of their low cost, low toxicity, easy availability, optical, catalytic, and antimicrobial characteristics. In addition, copper oxide nanoparticles are noteworthy because of their surface area and increased adsorbed oxygen capacity, leading its bandgap to be ranged from 1.35 to 3.5 eV [10,11]. The strong ability of CuO-NPs for molecular oxygen absorption has a significant impact on the electron-hole pair rearrangement and the removal of the photogenerated electron [12]. Nd 2 O 3 is rare earth oxide that improves the efficiency of CuO photocatalysis for MB photodegradation in the presence of light with a lower cost [13]. The preparation and characterization of Nd 2 O 3 NPs have drawn much attention to be studied because of their nano-size structure and surface.
In this work, we synthesized CuO and Nd 2 O 3 doped CuO photocatalysts with different concentrations of neodymium oxide (Nd 2 O 3 ) and characterized by various experi-examine the photocatalytic activity of the samples. Experiments were performed to enhance the conditions, such as the percentage of Nd2O3 incorporating CuO, H2O2, pH, and Photo-Fenton.

Synthesis of Nd2O3 Doped CuO Nanostructured
The Nd2O3 doped CuO nanoparticles were prepared by the combustion method. First, 5 g of Copper nitrate (Sigma-Aldrich company, Burlington, MA, USA) has been taken, neodymium nitrate(Sigma-Aldrich company, Burlington, MA, USA) with different weights (0.11, 0.28, 0.58, 0.89, 1.21 g), for (1, 2.5, 5, 7.5, 10%) then 10 g starch and 30 mL distilled water in 6 crucibles and mixed well. Then the crucibles were dried in an oven at 120 °C for 12 h and calcined at 550 °C for 2 h to obtain the proper catalyst, as illustrated in Scheme 2. Scheme 2. Schematic diagram of the synthesis of Nd2O3 doped CuO nanocomposites.

Characterization Techniques and Devices
The structure of The Nd2O3 doped CuO NPs was obtained by a PAN analytical X'Pert PRO, (Philips, Eindhoven, Netherlands) Philips X-ray diffractometer with monochromatic CuKα operating radiation at 40 kV and 40 mA.
Field emission scanning electron microscopy (FESEM) of The Nd2O3 doped CuO nanoparticles have been examined by HR-SEM, QUANTA FEG 250, USA (FEI, Hillsboro, OR, USA), to identify their surface morphology.
A Nano Series, Zeta sizer(Malvern Panalytical comapny, MALVERN, UK), has been used to determine the surface potentials. The samples have been suspended at 25 °C into the water to determine the average zeta potential (ζav, mV). In addition, dynamic light scattering (DLS) (Malvern Panalytical comapny, MALVERN, UK), has been used to measure conductivity. The XRD, FE-SEM, and DLS devices are located in the Egyptian Petroleum Research Institute, Naser City, Egypt.

Photocatalytic Performance of Nd2O3 Doped CuO Nanopowders
The photocatalytic validity of all prepared samples has been assessed through a wood photoreactor. The photoreactor at NLEBA, Ain Shams University (ASU), was developed by I.S. Yahia and his team [14] (see Figure 1). It stands 100 cm tall and 65 cm broad from the outside. Each of the white and blue bulbs has an output of 18 watts. A 15-point, Scheme 2. Schematic diagram of the synthesis of Nd 2 O 3 doped CuO nanocomposites.

Characterization Techniques and Devices
The structure of The Nd 2 O 3 doped CuO NPs was obtained by a PAN analytical X'Pert PRO, (Philips, Eindhoven, Netherlands) Philips X-ray diffractometer with monochromatic CuK α operating radiation at 40 kV and 40 mA.
Field emission scanning electron microscopy (FESEM) of The Nd 2 O 3 doped CuO nanoparticles have been examined by HR-SEM, QUANTA FEG 250, USA (FEI, Hillsboro, OR, USA), to identify their surface morphology.
A Nano Series, Zeta sizer(Malvern Panalytical comapny, MALVERN, UK), has been used to determine the surface potentials. The samples have been suspended at 25 • C into the water to determine the average zeta potential (ζ av , mV). In addition, dynamic light scattering (DLS) (Malvern Panalytical comapny, MALVERN, UK), has been used to measure conductivity. The XRD, FE-SEM, and DLS devices are located in the Egyptian Petroleum Research Institute, Naser City, Egypt.

Photocatalytic Performance of Nd 2 O 3 Doped CuO Nanopowders
The photocatalytic validity of all prepared samples has been assessed through a wood photoreactor. The photoreactor at NLEBA, Ain Shams University (ASU), was developed by I.S. Yahia and his team [14] (see Figure 1). It stands 100 cm tall and 65 cm broad from the outside. Each of the white and blue bulbs has an output of 18 watts. A 15-point, 500 rpm multi-position magnetic stirrer was used to stir the mixture of Nd 2 O 3 doped CuO nanopowders and MB solution (Sigma-Aldrich company) at room temperature, allowing us to evaluate many samples at once. 500 rpm multi-position magnetic stirrer was used to stir the mixture of Nd2O3 doped CuO nanopowders and MB solution (Sigma-Aldrich company) at room temperature, allowing us to evaluate many samples at once. A 100 mg of Nd2O3 doped CuO NPS was inserted in 50 mL of MB solution. Agitation in darkness results in adsorption-desorption equilibrium. The last step included turning on the white light to shine the prepared samples. The deteriorated solution was taken and centrifuged at ten-minute intervals to remove any nanopowders. To finish, the (LISCO-GmbH) UV-Visible spectrophotometer was used to evaluate the deteriorated solution alongside MB's stock solution. All operating parameters stay stable during photocatalytic processing.
The degradation of MB dye using white light assisted photo-Fenton reaction was investigated in the presence and absence of 7.5% Nd2O3 doped CuO catalyst. The photo-Fenton degradation rate was 0.00813 min −1 at 0.1 g of Fe +2 -ions in the presence of 7.5% Nd2O3 doped CuO, while the rate was 0.00276 min −1 at 0.05 g of Fe +2 -ions in the absence of 7.5% Nd2O3 doped CuO. This result means that the presence of catalyst enhanced the photo-Fenton degradation rate.

XRD of the Prepared Nanostructured Nd2O3 Doped CuO
XRD is the most successful approach for recognizing structural information of Nd2O3 doped CuO nanoparticles, and it investigated the crystallinity of nanomaterials. The XRD patterns for pure CuO and Nd2O3 doped CuO nanostructured are shown in Figure 2 [15,16]. The values of diffraction peaks of the as-prepared NPs were tabulated in Table 1. A 100 mg of Nd 2 O 3 doped CuO NPS was inserted in 50 mL of MB solution. Agitation in darkness results in adsorption-desorption equilibrium. The last step included turning on the white light to shine the prepared samples. The deteriorated solution was taken and centrifuged at ten-minute intervals to remove any nanopowders. To finish, the (LISCO-GmbH) UV-Visible spectrophotometer was used to evaluate the deteriorated solution alongside MB's stock solution. All operating parameters stay stable during photocatalytic processing.
The degradation of MB dye using white light assisted photo-Fenton reaction was investigated in the presence and absence of 7.5% Nd 2 O 3 doped CuO catalyst. The photo-Fenton degradation rate was 0.00813 min −1 at 0.1 g of Fe +2 -ions in the presence of 7.5% Nd 2 O 3 doped CuO, while the rate was 0.00276 min −1 at 0.05 g of Fe +2 -ions in the absence of 7.5% Nd 2 O 3 doped CuO. This result means that the presence of catalyst enhanced the photo-Fenton degradation rate.  [15,16]. The values of diffraction peaks of the as-prepared NPs were tabulated in Table 1.  The intensities and positions of the diffraction peaks for the CuO NPs agree with the reported data (JCPDS 048-1548). Indexes the peaks of the studied samples, and no peaks of impurities are found in the XRD pattern. It could be observed that the Nd 2 O 3 doped CuO NPs had the same XRD design as the CuO, indicating the crystalline structure of the photocatalyst. The excellent dispersion of Nd 2 O 3 via CuO, which acts as the host crystal, has also been confirmed. A small secondary phase appeared due to incorporating Nd 2 O 3 with different percentages (1, 2.5, 5, 7.5, and 10%) into the CuO matrix. Two crystalline peaks appeared at 2-theta: 23 • , and 32 • with hkl = [110] and [011]. They correspond to the peaks of Nd 2 O 3 as hexagonal phase (JCPDS card No. 01-074-2139). These two peaks strongly appeared at 10% Nd 2 O 3 doped CuO nanoparticles [13,17]. The equation of Scherrer was used to compute crystallite size [18].

XRD of the Prepared Nanostructured Nd 2 O 3 Doped CuO
In this case, D represents the crystallite size and X-ray wavelength (λ); FWHM represents the full width at half maximum (FWHM). The lattice strain (ε) was determined using the following method [18]: Dislocation density (δ) has been achieved by [18,19]: Moreover, where the diffraction peak angle is θ. The calculated mean values of the grain size (D) are tabulated in Table 2. The first phase's average crystallite domain size ranges from 22.0 to 30.1 nm, while the second phase ranges from 27.1 to 36.7 nm. The value of (D) for CuO is found in the range of 20-30 nm. It was found that (D) of Nd 2 O 3 sample with Scherrer formula is about 35 nm. The ε and δ were calculated and tabulated in Table 2. It was found that Nd 2 O 3 and CuO atoms bond with O − atoms by increasing Nd 2 O 3 content. Lattice strain (ε) arises from the slight variation in the atomic radii of Nd 2+ -and Cu 2+ -beside Nd 2 O 3 content, where Cu 2+ is replaced with Nd 2+ in the host lattice. The decrease in lattice strain (1.2 × 10 −3 ) at 7.5% Nd 2 O 3 doped CuO nanoparticle (Table 2) because of atomic radii and dopant concentration reduces local distortion of the crystal lattice. The crystallite size (30.1 nm) of the 7.5% of Nd-CuO sample was increased because of a decrease in δ (1.4 × 10 −3 nm 2 ) and intrinsic micro-strain [20].

FESEM Analysis of Nd 2 O 3 Doped CuO Nanostructured
The efficiency and activity of photocatalysts depend on the morphological parameters [21]. Therefore, the morphology characterization of the CuO nanoparticles was investigated by FESEM, as shown in Figure 3A. The nanoparticles have a uniform size, spherical shape, and appropriate separation [22,23]. Figure 3B-F exhibit the FESEM images of Nd 2 O 3 doped CuO NPs at various concentration of Nd 2 O 3 . It is evident that the particles had quasi-spherical shapes and aggregated homogeneously. Empty spaces between the particles are pores arising from the inter-aggregation of particles [15]. From the FESEM result, the average crystalline size of the prepared nanocomposite materials was found in the range of 17-39 nm. As shown in the Table 3, 7.5%Nd 2 O 3 doped CuO has the smallest value of particle size (17 nm), which means that increasing surface area of catalyst thereby, the improvement of photodegradation efficiency of catalyst [15].   EDX image depicts the peaks of CuO and Nd 2 O 3 beside atomic percentage of elements (see inset of Figure 3G) contained in 7.5% Nd 2 O 3 doped CuO nanoparticle, where it verified the production of Nd 2 O 3 doped CuO nanoparticle by homogeneous chemical structure.  Figure 4b depicts the conductivity enhancement by increasing Nd 2 O 3 content, which was 0.025 mS/cm at 7.5% Nd 2 O 3 doped CuO nanoparticle. Enhancement of conductivity can be attributed to an increase of ζ av -potential value where Photocatalytic activity increases up to 7.5 percent with the increasing mobility of holes causing a photocatalytic activity enhancement [21].

Effect of Nd +3 Doping
The as-prepared nanomaterials (100 mg) have been subjected to visible (80 min irradiation) photodegradation of MB dye. Figure 5 illustrates the MB absorption spectra in the presence of visible illumination with pure CuO and Nd2O3 doped CuO nanoparticles. The λmax of MB is 664 nm. It is obvious from Figure 5 that the intensity of all samples decreases with increasing time, which means that the dye's photodegradation rate increased with time. The results revealed that the pure CuO catalyst achieved 87% degradation of MB, while 7.5%Nd2O3 doped CuO catalyst achieved the highest photocatalytic efficiency, up to 90.8% within 80 min under visible light irradiation. The rate of MB degradation k (min −1 ) was achieved by [24]:  The as-prepared nanomaterials (100 mg) have been subjected to visible (80 min irradiation) photodegradation of MB dye. Figure 5 illustrates the MB absorption spectra in the presence of visible illumination with pure CuO and Nd 2 O 3 doped CuO nanoparticles. The λ max of MB is 664 nm. It is obvious from Figure 5 that the intensity of all samples decreases with increasing time, which means that the dye's photodegradation rate increased with time. The results revealed that the pure CuO catalyst achieved 87% degradation of MB, while 7.5% Nd 2 O 3 doped CuO catalyst achieved the highest photocatalytic efficiency, up to 90.8% within 80 min under visible light irradiation. The rate of MB degradation k (min −1 ) was achieved by [24]: where absorption at 0 min illumination is A o (mg.L −1 ), and absorption at irradiation time t (min), is A. The k values for CuO and Nd 2 O 3 doped CuO nanoparticles are depicted in Figure 6. The higher the doping level, the higher the rate of deterioration up to 7.5% Nd 2 O 3 doped CuO nanoparticle, while at 10% Nd 2 O 3 doped CuO nanoparticle, the degradation begins to decrease, which means that 7.5% Nd 2 O 3 doped CuO nanoparticle has the highest degradation rate constant of 0.02274 min −1 (as seen in Table 4).  The synthesis of HO − is based on H2O2, one of the cleanest and optimal oxidants for the dye's degradation and organic pollutants in wastewater [24]. In this investigation, five different concentrations were utilized of H2O2 (1, 2, 3, 4 and 5 mL) in the presence of 0.1 g of 7.5% Nd2O3 doped CuO nanoparticle for visible photodegradation of each one of MB dye. Figure 7 depicts the effect of H2O2 concentration on photodegradation. As can be seen, the photodegradation rate increases with increasing H2O2 concentration until it reaches its maximum at 5 mL (as shown in Table 4). According to Equation (5), the greater reaction rates following the addition of H2O2 are due to an increase in the concentration of hydroxyl radical [25]:  The synthesis of HO − is based on H 2 O 2 , one of the cleanest and optimal oxidants for the dye's degradation and organic pollutants in wastewater [24]. In this investigation, five different concentrations were utilized of H 2 O 2 (1, 2, 3, 4 and 5 mL) in the presence of 0.1 g of 7.5% Nd 2 O 3 doped CuO nanoparticle for visible photodegradation of each one of MB dye. Figure 7 depicts the effect of H 2 O 2 concentration on photodegradation. As can be seen, the photodegradation rate increases with increasing H 2 O 2 concentration until it reaches its maximum at 5 mL (as shown in Table 4). According to Equation (5), the greater reaction rates following the addition of H 2 O 2 are due to an increase in the concentration of hydroxyl radical [25]: As a result, adding hydrogen peroxide at the required quantities can speed up the photodegradation of MB dye.

H O +• OH ⟶ HO • + H O,
As a result, adding hydrogen peroxide at the required quantities can speed up the photodegradation of MB dye.

Effect of pH on 7.5% Nd2O3 Doped CuO Nanoparticle
Due to the high pH dependency of certain aspects, such as semiconductor surface charge state and dissociation of chemicals in the solution, the pH of the solution impacts photodegradation processes. Therefore, photodegradation was studied in ideal circumstances with a pH range of 2 to 10 to evaluate the impact of pH on the photodegradation process. Figure 8a,b represents kinetic study and the rate constant (k) of photodegradation of MB at different pH values in the presence of 7.5% Nd2O3 doped CuO nanoparticle as a catalyst.

Effect of pH on 7.5% Nd 2 O 3 Doped CuO Nanoparticle
Due to the high pH dependency of certain aspects, such as semiconductor surface charge state and dissociation of chemicals in the solution, the pH of the solution impacts photodegradation processes. Therefore, photodegradation was studied in ideal circumstances with a pH range of 2 to 10 to evaluate the impact of pH on the photodegradation process. Figure 8a,b represents kinetic study and the rate constant (k) of photodegradation of MB at different pH values in the presence of 7.5% Nd 2 O 3 doped CuO nanoparticle as a catalyst.
The result exhibited a maximum efficiency (99%) at pH 10 after 50 min from the start of irradiation (as seen in Table 4). Several techniques were utilized to calculate the pH of the CuO NPs' point of zero charges (pH pzc ), which was between 8.5 and 9.5 [26,27]. The charge on the photocatalyst surface was positive and negative, below and above pH pzc , respectively. When the pH is less than pH pzc , repulsion between the positively charged 7.5% Nd 2 O 3 doped CuO catalyst and cationic dye reduces photodegradation efficiency [27]. Scheme 3 depicts the impact of CuO NPs' pH pzc on cationic dye degradation. Moondeep et al. [26] At greater pH, the CuO NP surface is charged negatively, which improves its interaction with cationic dye (VB). Meanwhile, the CuO NP surface is charged positively at lower pH, enhancing its interaction with anionic dye.
As a result, adding hydrogen peroxide at the required quantities can speed up the photodegradation of MB dye.

Effect of pH on 7.5% Nd2O3 Doped CuO Nanoparticle
Due to the high pH dependency of certain aspects, such as semiconductor surface charge state and dissociation of chemicals in the solution, the pH of the solution impacts photodegradation processes. Therefore, photodegradation was studied in ideal circumstances with a pH range of 2 to 10 to evaluate the impact of pH on the photodegradation process. Figure 8a,b represents kinetic study and the rate constant (k) of photodegradation of MB at different pH values in the presence of 7.5% Nd2O3 doped CuO nanoparticle as a catalyst. The result exhibited a maximum efficiency (99%) at pH 10 after 50 min from the start of irradiation (as seen in Table 4). Several techniques were utilized to calculate the pH of the CuO NPs' point of zero charges (pHpzc), which was between 8.5 and 9.5 [26,27]. The charge on the photocatalyst surface was positive and negative, below and above pHpzc, respectively. When the pH is less than pHpzc, repulsion between the positively charged 7.5% Nd2O3 doped CuO catalyst and cationic dye reduces photodegradation efficiency [27]. Scheme 3 depicts the impact of CuO NPs' pHpzc on cationic dye degradation. Moondeep et al. [26] At greater pH, the CuO NP surface is charged negatively, which improves its interaction with cationic dye (VB). Meanwhile, the CuO NP surface is charged positively at lower pH, enhancing its interaction with anionic dye.

Effect of Fenton and Photo-Fenton on MB Photodegradation
Fenton and Photo-Fenton processes have been demonstrated to be the most promising strategies for wastewater treatment. These techniques contribute to improving the environment through a reduction in the pollution resulting from complicated dye molecules and other organic substances as a green chemical and ecological method [9]. The current study investigates photochemical degradation of MB dye by photo-Fenton reaction in the presence and absence of 7.5% Nd2O3 doped CuO catalyst. Figure 9a shows the effect of ferrous sulfate concentration [0.05 to 0.2 g Fe +2 -ions] on the rate of photochemical The result exhibited a maximum efficiency (99%) at pH 10 after 50 min from the star of irradiation (as seen in Table 4). Several techniques were utilized to calculate the pH o the CuO NPs' point of zero charges (pHpzc), which was between 8.5 and 9.5 [26,27]. Th charge on the photocatalyst surface was positive and negative, below and above pHpz respectively. When the pH is less than pHpzc, repulsion between the positively charged 7.5% Nd2O3 doped CuO catalyst and cationic dye reduces photodegradation efficienc [27]. Scheme 3 depicts the impact of CuO NPs' pHpzc on cationic dye degradation. Moon deep et al. [26] At greater pH, the CuO NP surface is charged negatively, which improve its interaction with cationic dye (VB). Meanwhile, the CuO NP surface is charged posi tively at lower pH, enhancing its interaction with anionic dye.

Effect of Fenton and Photo-Fenton on MB Photodegradation
Fenton and Photo-Fenton processes have been demonstrated to be the most promis ing strategies for wastewater treatment. These techniques contribute to improving the en vironment through a reduction in the pollution resulting from complicated dye molecule and other organic substances as a green chemical and ecological method [9]. The curren study investigates photochemical degradation of MB dye by photo-Fenton reaction in th presence and absence of 7.5% Nd2O3 doped CuO catalyst. Figure 9a shows the effect o ferrous sulfate concentration [0.05 to 0.2 g Fe +2 -ions] on the rate of photochemica

Effect of Fenton and Photo-Fenton on MB Photodegradation
Fenton and Photo-Fenton processes have been demonstrated to be the most promising strategies for wastewater treatment. These techniques contribute to improving the environment through a reduction in the pollution resulting from complicated dye molecules and other organic substances as a green chemical and ecological method [9]. The current study investigates photochemical degradation of MB dye by photo-Fenton reaction in the presence and absence of 7.5% Nd 2 O 3 doped CuO catalyst. Figure 9a shows the effect of ferrous sulfate concentration [0.05 to 0.2 g Fe +2 -ions] on the rate of photochemical degradation in the presence of 7.5% Nd 2 O 3 doped CuO catalyst, maintaining all other parameters constant. The findings displayed in Figure 9a enhance the photo-Fenton degradation rate by increasing the content of Fe +2 -ions up to 0.1 g by a continuous rate of 0.00813 min −1 . Increasing the number of Fe +2 -ions up to this specific Fe +2 concentration improves • OH radical production and increases the photochemical degradation rate. In addition, the reaction rate was reported to be lowered with increasing Fe +2 -ions concentration. Figure 9b shows the effect of Ferrous sulfate concentration [0.05 to 0.2 g Fe +2 -ions] on the rate of photochemical degradation in the absence of 7.5% Nd 2 O 3 doped CuO catalyst.
H O → HO . + H , Fe + OH . → Fe + OH , Fe + HO . → Fe + O + H OH . + OH . → H O , MB Dye + OH . →Degraded Product (11)  The result showed that the optimum rate constant was 0.00276 min −1 at 0.05 g of Fe +2 ions concentration without a catalyst. The constant rate in the presence of a catalyst is higher than the absence of them. The rate constant (k) values and degradation (%) at different Nd 2 O 3 concentrations, H 2 O 2, and pH were calculated and tabulated in Table 4.
The reaction in Fenton was initially identified by H.J. Fenton [28] and is classified as an improved oxidative potential for H 2 O 2 in acidic media when iron (Fe) is a catalyst. The reactions included in Fenton processes are [29]: 14 of 20 MB Dye + OH . → Degraded Product (11)

Comparison of the Photocatalytic Activity of Several Compounds Based on CuO
In the present work, 7.5% Nd 2 O 3 doped CuO nanocomposite exhibited the highest photocatalytic efficiency, up to 90.8% in 80 min, with a 0.0227 min −1 degradation rate. At pH 4, the degradation efficiency reaches its maximum value to be 99%. When comparing the data acquired with other research, the capacity to study, evaluate and contrast outcomes may be demonstrated. Table 5 indicates the comparison of photocatalytic degradation of MB and other dyes in the presence of Nd-doped metal oxide with different previous work samples. M. Arunpandian et al. [30] found that 0.3 wt% of Nd 2 O 3 -doped ZnO was the highest active, showing high photocatalytic activity for the degradation of MB dye. Arun Pandian M. et al. [16] It was shown that MB degradation was by Ag/Nd 2 O 3 -ZnO Nanocomposite within 30 min with a 98.12 percent efficiency under visible light. Dhanya et al. [31] Including Nd 3+ in the SnO 2 lattice decreases bandgap and inhibits the electron-hole recombination. Thus, visible light can excite electrons from the valence band to the conduction band. Samuel et al. [32] Nd-ZnO showed good photocatalytic activity. In comparison to ZnO, nanocomposite [Nd-ZnO-GO (0.3% of Nd)] demonstrated higher photocatalytic efficiency and could be regarded for promise as a photocatalyst in organic wastewater treatment. Samuel Osei-Bonsu Oppong et al. [33] The photodegradation efficiency of indigo carmine solutions exposed to simulated solar light for 3 h with Nd-TiO 2 -GO (0.6 percent Nd) nanocomposites was found to be 92 percent. Umair Alam et al. [34] showed that the Nd-doped ZnO is the best activity with 98% degradation efficiency compared used other rare earth metals. Sonali et al. [35] The optimum photocatalytic activity for MB degradation was ob-served for 3.0 mol. Percent Ce-CuO, which led to 98 percent degradation in 180 min. Xia Shao et al. [36] When the ratio of Nd to TiO 2 is 3-4 wt%, the highest activity is reached, and MB is degraded by UV-light in 160 min. Reda M. Mohamed et al. [37] For 2 h, the photocatalytic efficiency of mesoporous 3 percent Nd 2 O 3 /ZnO for TC degradation reached 100%. Moondeep et al. [26] under diverse energy sources, such as UV-visible irradiation and UV radiations, the degradation efficiency of CuO NPs on cationic (VB) and anionic (DR) dyes were measured.  3.4.6. Reusability and Stability In practical applications, the photostability of a photocatalyst is an essential feature. The synthesized Nd 2 O 3 doped CuO photocatalyst was exposed to five photocatalytic trial runs to determine its photostability by adding the reused photocatalyst to new MB solutions under the same experimental conditions. The photocatalyst was reused after centrifugation without regeneration. Figure 10 depicts the recycling of 7.5% Nd 2 O 3 doped CuO nanostructured. It was found that, in five successive experimental runs, photocatalyst activity reached 88% of MB degradation, which promotes the prepared samples for its photocatalytic performance in environmental treatment.

Influence of Radicals' Scavengers on Photocatalytic Activity
The photocatalytic reaction occurs in 3 stages: absorption of light, generation and separation of electron-hole, and oxidation-reduction response on the photocatalyst's surface. e − and h + are extremely recombined so, the trapping and transfer of surface charges are limited. Therefore, it is vital to eliminate the recombination of surface charge and promote charge transfer to the surface-active sites to boost the carriers' dynamics and photocatalytic activity. There have thus been significant attempts to increase the effectiveness of photocatalysts [21].
The scavenging of active characters responsible for photodegradation in the presence of 7.5% Nd2O3 doped CuO nanoparticle is examined as demonstrated in Figure 11a. The scavengers applied such as NaCl for h + , NaNO3 for e − , IPA for OH . and Ascorbic acid for O . . From the data shown in Figure 11b, the use of NaCl, NaNO3, IPA, and Ascorbic acid as scavengers is responsible for eliminating MB contaminants of around 56, 36.7, 19.4, and 98 percent. The most efficient agent in eliminating pollutants may be regarded as hydroxy

Influence of Radicals' Scavengers on Photocatalytic Activity
The photocatalytic reaction occurs in 3 stages: absorption of light, generation and separation of electron-hole, and oxidation-reduction response on the photocatalyst's surface. e − and h + are extremely recombined so, the trapping and transfer of surface charges are limited. Therefore, it is vital to eliminate the recombination of surface charge and promote charge transfer to the surface-active sites to boost the carriers' dynamics and photocatalytic activity. There have thus been significant attempts to increase the effectiveness of photocatalysts [21].
The scavenging of active characters responsible for photodegradation in the presence of 7.5% Nd 2 O 3 doped CuO nanoparticle is examined as demonstrated in Figure 11a. The scavengers applied such as NaCl for h + , NaNO 3 for e − , IPA for OH . and Ascorbic acid for O −.
2 . From the data shown in Figure 11b, the use of NaCl, NaNO 3 , IPA, and Ascorbic acid as scavengers is responsible for eliminating MB contaminants of around 56, 36.7, 19.4, and 98 percent. The most efficient agent in eliminating pollutants may be regarded as hydroxy radicals. Sahar Zinatloo et al. [38] reported that in the presence of Nd 2 O 3 -SiO 2 , hydroxyl radicals were the most efficient agent for the degradation of methyl violet. Photocatalysis and the photo-Fenton procedure are responsible for MB degradation. During the reaction produced hydroxyl radicals that are represented in Scheme 4. After irradiation, an electron has been excited, and holes are generated to react with H2O and . Figure 11. Trapping experiments of active species for the MB photocatalytic reaction using 7.5% Nd 2 O 3 doped CuO under visible light irradiation using 200 mM scavengers, (a) kinetics, (b) Degradation efficiency.

Mechanism of Photocatalysis for Nd 2 O 3 Doped CuO Nanostructured
Photocatalysis and the photo-Fenton procedure are responsible for MB degradation. During the reaction produced hydroxyl radicals that are represented in Scheme 4. After irradiation, an electron has been excited, and holes are generated to react with H 2 O and dissolved O 2, producing OH . radicals that cause MB degradation [39,40]. The proposed degradation process for MB pollutants can be represented as: 2h OH . + MB contaminant → Degradation products, h + H O ⟶ OH . + H , 2h + 2H O ⟶ H O + 2H H O ⟶ 2OH . , H O ⟶ 2OH . , OH . + MB contaminant ⟶ Degradation products,

Conclusions
The combustion method was applied to synthesize pure CuO, and different concentrations of Nd2O3 incorporated CuO nanocomposites. The addition of neodymium to CuO improved its structural and morphological characteristics and improved the photocatalytic activities for the apparent photodegradation of MB dye. The crystallinity of the synthesized samples was shown through XRD analysis. The first phase's average crystallite domain size ranges from 22.081 to 30.109 nm, while the second phase ranges from 27.180 to 36.744 nm. In addition, ε and δ have been calculated. SEM investigation indicated that particles are aggregated homogenously with almost spherical forms. ζav-potentials dataenhanced via rising Nd2O3 content up to 7.5% Nd2O3 doped CuO nanoparticle reaching ~27.0 mV with the conductivity of 0.024 mS/cm. The as-prepared 7.5% Nd2O3 doped CuO nanoparticle exhibits the optimum photocatalytic efficiency comes 90.8% within 80 min with a degradation rate of 0.02274 min −1 for MB dye. In comparison, the photocatalytic

Conclusions
The combustion method was applied to synthesize pure CuO, and different concentrations of Nd 2 O 3 incorporated CuO nanocomposites. The addition of neodymium to CuO improved its structural and morphological characteristics and improved the photocatalytic activities for the apparent photodegradation of MB dye. The crystallinity of the synthesized samples was shown through XRD analysis. The first phase's average crystallite domain size ranges from 22.081 to 30.109 nm, while the second phase ranges from 27.180 to 36.744 nm. In addition, ε and δ have been calculated. SEM investigation indicated that particles are aggregated homogenously with almost spherical forms. ζ av -potentials data-enhanced via rising Nd 2 O 3 content up to 7.5% Nd 2 O 3 doped CuO nanoparticle reaching~27.0 mV with the conductivity of 0.024 mS/cm. The as-prepared 7.5% Nd 2 O 3 doped CuO nanoparticle exhibits the optimum photocatalytic efficiency comes 90.8% within 80 min with a degradation rate of 0.02274 min −1 for MB dye. In comparison, the photocatalytic efficiency was 87% within 80 min with the rate of 0.01683 min −1 for MB by CuO.
The degradation efficiency enhances at pH 10 to be 99% with a rate of degradation of 0.082 min −1 . The stability of the Nd 2 O 3 doped CuO nanoparticle has been examined through recycling. The results of trapping experiments were indications that the hydroxyl radicals are responsible for the elimination of pollutants. Finally, Nd 2 O 3 -doped CuO nanostructured was a promising material under visible light to treat vast quantities of wastewater.

Informed Consent Statement: Not applicable.
Data Availability Statement: The data supporting this study's findings are available from the corresponding author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.