Photocatalytic Degradation of Organic Dyes by Magnetite Nanoparticles Prepared by Co-Precipitation

Iron oxide nanoparticles were synthesized by co-precipitation using three different iron salt stoichiometric mole ratios. Powder X-ray diffraction patterns revealed the inverse cubic spinel structure of magnetite iron oxide. Transmission electron microscopic images showed Fe3O4 nanoparticles with different shapes and average particle sizes of 5.48 nm for Fe3O4-1:2, 6.02 nm for Fe3O4-1.5:2, and 6.98 nm for Fe3O4-2:3 with an energy bandgap of 3.27 to 3.53 eV. The as-prepared Fe3O4 nanoparticles were used as photocatalysts to degrade brilliant green (BG), rhodamine B (RhB), indigo carmine (IC), and methyl red (MR) under visible light irradiation. The photocatalytic degradation efficiency of 80.4% was obtained from Fe3O4-1:2 for brilliant green, 61.5% from Fe3O4-1.5:2 for rhodamine B, and 77.9% and 73.9% from Fe3O4-2:3 for both indigo carmine and methyl red. This indicates that Fe3O4-2:3 is more efficient in the degradation of more than one dye. This study shows that brilliant green degrades most effectively at pH 9, rhodamine B degrades best at pH 6.5, and indigo carmine and methyl red degrade most efficiently at pH 3. Recyclability experiments showed that the Fe3O4 photocatalysts can be recycled four times and are photostable.


Introduction
Azo dyes are the most prevalent and biggest group of artificial dyes that are extensively used in the textiles, beauty products, pharmaceutical, and food industries [1,2].The use of azo dyes has seen a dramatic increase in the textile industry over the past decades for coloring cotton and wool because of their vibrant colors, affordability, and excellent durability upon exposure to light and washing [3].Most of these dyes have mutagenic, cancer-causing, genotoxic, and teratogenic properties [4].Furthermore, their presence in water bodies inhibits solar radiation from penetrating, lowering oxygen quantity, interfering with photosynthesis, and damaging terraqueous ecosystems [5,6].As a result, researchers have long been involved in developing an efficient, cost-effective, and dependable method for eliminating dyes from wastewater [7,8].
Photodegradation is commonly used to remove contaminants from the environment [9].It involves subjecting pollutants to light, which causes their photochemical breakdown into simpler, non-toxic compounds.This method is frequently preferred due to its economic viability, sustainability, absence of secondary waste, and broad-spectrum degradation of different organic dyes into CO 2 and H 2 O [10,11].The practicality of the procedure can be improved by using a photocatalyst, which is a semiconducting material that is excited by absorbing light photons.Iron oxide is one of the materials that show potential for photocatalytic applications because of its cost-effective synthesis, magnetic properties, and n-type semiconducting effects [12].
Previous studies have shown the properties of superparamagnetic iron oxide nanoparticles synthesized by co-precipitation could be influenced by variations of the preparative parameters [3,13,14].Alibeigi and Vaezi investigated the effect of Fe 2+ /Fe 3+ molar ratios and observed that increasing the iron salt ratio increases crystallinity and specific saturation magnetization [15].A study by Dehghanpour also demonstrated that adjusting the mole ratio of Fe 2+ to Fe 3+ ions, pH, and capping agents may influence saturation magnetization and the size of the nanoparticles [16].Another study reported that changing the stoichiometric ratio of 1:2 for Fe 2+ and Fe 3+ salts decreased electrical conductivity and led to the phase change of the nanoparticles from Fe 3 O 4 to γ-Fe 2 O 3 [17].Iida et al. [18] synthesized Fe 3 O 4 nanoparticles by hydrolysis in an aqueous solution comprising ferrous and ferric salts in varying ratios, with 1,6-hexane-diamine as the base.An increase in the ratio of Fe 2+ to Fe 3+ ions resulted in an increase in the size of Fe 3 O 4 nanoparticles from 9 to approximately 37 nm. Lee et al. [19] studied the antioxidant properties and physiological effects of maghemite nanoparticles (γ-Fe 2 O 3 ) prepared by varying Fe 2+ /Fe 3+ molar ratios using ferrous/ferric salt co-precipitation.Thus, studies have shown that the physical and chemical changes that occur in iron oxide nanoparticles when the Fe 2+ /Fe 3+ molar ratio varies throughout the co-precipitation process.However, no work has studied the photocatalytic potential of Fe 3 O 4 with respect to different Fe 2+ to Fe 3+ molar ratios.
The surface properties, crystalline structure, and morphology of the nanoparticles, which are all controlled by the synthetic technique, could influence the photocatalytic degradation efficiency of magnetite nanoparticles [20].Studies have shown that a change in dye pH, light source, and surface charge influences the photocatalytic activity of Fe 3 O 4 nanoparticles against organic dyes such as rhodamine B [21], methyl red [22], brilliant green [23], and indigo carmine [24].In the current study, we evaluate the influence of the Fe 2+ /Fe 3+ salt ratios on the structural and morphological optical and photocatalytic properties of Fe 3 O 4 nanoparticles synthesized via the co-precipitation method.The potential of the as-synthesized iron oxide nanoparticles as photocatalysts for the photodegradation of brilliant green, rhodamine B, indigo carmine, and methyl red under visible light was studied.Additionally, the photostability of the nanoparticles, the effect of pH, and scavengers on the photodegradation of the organic dyes by the iron oxide nanoparticles were evaluated.

Morphological Studies of the Iron Oxide Nanoparticles
Figure 1 displays the diffractograms of the iron oxide nanoparticles prepared using different iron salt ratios.The 2θ peaks observed at 18.23  220), (311), (400), (511), (440), (442), and (533) reflections of inverse cubic spinel structure of magnetite (JCPDS no: 19-0629) [25,26].The crystal growth direction inclination is toward the (311) plane.The diffraction patterns suggest that the iron salt ratios used have no apparent impact on the crystalline phases of the as-synthesized iron oxide nanoparticles.
The average crystallite sizes of the magnetite nanoparticles were calculated using the Debye-Scherrer equation [27]: where D, K, λ, β, and θ stand for crystallite diameter (nm), Scherrer constant, X-ray wavelength, full width at half-maximum (FWHM), and Bragg angle, respectively.The FWHM and θ were calculated by fitting all diffraction peaks to a Gaussian function.The calculated crystallite sizes were 4.45 nm, 5.89 nm, and 6.50 nm for Fe 3 O 4 -1:2, Fe 3 O 4 -1.5:2, and Fe 3 O 4 -2:3, respectively.The lattice parameter is another significant piece of information that may be retrieved from XRD patterns.The lattice parameter of magnetite nanoparticles was derived using Bragg's rule and the following equation [28].
Å, and 8.3574 Å for Fe3O4-1:2, Fe3O4-1.5:2, and Fe3O4-2:3, respectively.The results show that the calculated lattice constants of the as-prepared magnetite nanoparticles slightly vary from one another, and they are less than that of standard magnetite.This discrepancy could be attributed to the effect of various crystal size variations on the crystal lattice [29].TEM images in Figure 2 revealed that Fe3O4-1:2 nanoparticles are a combination of square-like, oval, rod, and rectangular shapes with a mean size of 5.48 nm, while Fe3O4-1.5:2nanoparticles are agglomerated square-like, rectangular, and oval-shaped with a mean size of 6.02 nm.Fe3O4-2:3 nanoparticles are also agglomerated and quasi-spherical, with an average size of 6.98 nm.There is no drastic change in average particle size with an increase in the stoichiometric ratios of the iron salts, as reported in other studies [16,18].However, there is apparent agglomeration for Fe3O4-1.5:2 and Fe3O4-2:3 nanoparticles and larger particle size distributions compared to Fe3O4-1:2 (Figure S1).
FTIR was used to study the interactions between octylamine and the as-synthesized Fe3O4 nanoparticles (Figure S2).The weak ν(N-H)asy and ν(N-H)sym vibrations at 3376 and 3291 cm −1 are assigned to the amine group of the capping agent [30].The C-H characteristic peaks of the octylamine alkyl chain are at 2919 and 2851 cm −1 [31].The C-N bending vibrations appear at 1460 cm −1 .The capping agent vibrational bands observed also appear in the spectra of the iron oxide nanoparticles but with low intensity, which suggests the presence of an octylamine capping agent on the surface of iron oxide nanoparticles.However, the N-H bands were not observed in the Fe3O4 nanoparticles spectra, signifying that the capping of the nanoparticles took place through the -NH2 group of octylamine [32].Furthermore, the Fe-O peak at 550 cm −1 confirms the synthesis of iron oxide nanoparticles [33].The results show that the calculated lattice constants of the as-prepared magnetite nanoparticles slightly vary from one another, and they are less than that of standard magnetite.This discrepancy could be attributed to the effect of various crystal size variations on the crystal lattice [29].TEM images in Figure 2 revealed that Fe 3 O 4 -1:2 nanoparticles are a combination of square-like, oval, rod, and rectangular shapes with a mean size of 5.48 nm, while Fe 3 O 4 -1.5:2nanoparticles are agglomerated square-like, rectangular, and oval-shaped with a mean size of 6.02 nm.Fe 3 O 4 -2:3 nanoparticles are also agglomerated and quasi-spherical, with an average size of 6.98 nm.There is no drastic change in average particle size with an increase in the stoichiometric ratios of the iron salts, as reported in other studies [16,18].However, there is apparent agglomeration for Fe 3 O 4 -1.5:2 and Fe 3 O 4 -2:3 nanoparticles and larger particle size distributions compared to Fe 3 O 4 -1:2 (Figure S1).
FTIR was used to study the interactions between octylamine and the as-synthesized Fe 3 O 4 nanoparticles (Figure S2).The weak ν(N-H) asy and ν(N-H) sym vibrations at 3376 and 3291 cm −1 are assigned to the amine group of the capping agent [30].The C-H characteristic peaks of the octylamine alkyl chain are at 2919 and 2851 cm −1 [31].The C-N bending vibrations appear at 1460 cm −1 .The capping agent vibrational bands observed also appear in the spectra of the iron oxide nanoparticles but with low intensity, which suggests the presence of an octylamine capping agent on the surface of iron oxide nanoparticles.However, the N-H bands were not observed in the Fe 3 O 4 nanoparticles spectra, signifying that the capping of the nanoparticles took place through the -NH 2 group of octylamine [32].Furthermore, the Fe-O peak at 550 cm −1 confirms the synthesis of iron oxide nanoparticles [33].

Optical Studies of the Magnetite Nanoparticles
Figure 3a displays the UV-Vis spectra of the as-prepared iron oxide nanoparticles, which exhibit sharp absorption peaks at 212.5 nm, 220.4,and 217.6 nm for Fe3O4-1:2, Fe3O4-1.5:2, and Fe3O4-2:3, respectively.To further investigate the optical properties of the magnetite nanoparticles, the Tauc plot was used to calculate the energy gap [34].The estimated energy gaps were 3.53 eV for Fe3O4-1:2, 3.44 eV for Fe3O4-1.5:2, and 3.27 eV for Fe3O4-2:3   3b).The bandgap is observed to decrease with an increase in iron salt ratios, with Fe 3 O 4 -2:3 nanoparticles having a larger average particle size and the smallest bandgap.The energy gap of the as-synthesized magnetite nanoparticles is greater than that of bulk magnetite (0.20 eV) due to the quantum confinement effect [35].The optical bandgap of Fe 3 O 4 nanoparticles has been reported to range from 3.23 to 3.9 eV [36,37].Moreover, Shamaila et al. synthesized Fe 3 O 4 nanoparticles with an energy bandgap of 2.5 eV and titanium-coated Fe 3 O 4 nanoparticles with a bandgap of 3.22 eV [38].The bare Fe 3 O 4 demonstrated poor photocatalytic activity, whereas the titanium-coated Fe 3 O 4 exhibited improved performance under visible light.The optical bandgaps of the magnetite nanoparticles in this study are comparable to previously reported bandgaps [36][37][38], and the nanoparticles have the potential to be used as photocatalysts for dye degradation under visible light.

Optical Studies of the Magnetite Nanoparticles
Int. J. Mol.Sci.2024, 25, x FOR PEER REVIEW 5 of 19 (Figure 3b).The bandgap is observed to decrease with an increase in iron salt ratios, with Fe3O4-2:3 nanoparticles having a larger average particle size and the smallest bandgap.
The energy gap of the as-synthesized magnetite nanoparticles is greater than that of bulk magnetite (0.20 eV) due to the quantum confinement effect [35].The optical bandgap of Fe3O4 nanoparticles has been reported to range from 3.23 to 3.9 eV [36,37].Moreover, Shamaila et al. synthesized Fe3O4 nanoparticles with an energy bandgap of 2.5 eV and titanium-coated Fe3O4 nanoparticles with a bandgap of 3.22 eV [38].The bare Fe3O4 demonstrated poor photocatalytic activity, whereas the titanium-coated Fe3O4 exhibited improved performance under visible light.The optical bandgaps of the magnetite nanoparticles in this study are comparable to previously reported bandgaps [36][37][38], and the nanoparticles have the potential to be used as photocatalysts for dye degradation under visible light.

Surface Properties
The point of zero charge (PZC) of the magnetite nanoparticles was determined, as shown in Figure 4.The pHpzc values of Fe3O4-1:2, Fe3O4-1.5:2, and Fe3O4-2:3 were found to be 6.22,5.56, and 4.55, respectively.The pHpzc values obtained in this study are comparable to some pHpzc values for magnetite found in the literature [39,40].At the PZC, the catalyst surface consists of an equal proportion of positive and negative functional groups, yielding a zero net charge.The surface accumulates a negative charge above the PZC and a positive charge below the PZC.This is due to the presence of FeOH groups on the surface of magnetite iron oxide nanoparticles in aqueous solution [41].These hydroxyl groups can protonate or deprotonate to produce FeOH2 + or FeO − , which changes in response to pH values below or above the pHPZC.

Surface Properties
The point of zero charge (PZC) of the magnetite nanoparticles was determined, as shown in Figure 4.The pH pzc values of Fe 3 O 4 -1:2, Fe 3 O 4 -1.5:2, and Fe 3 O 4 -2:3 were found to be 6.22,5.56, and 4.55, respectively.The pH pzc values obtained in this study are comparable to some pH pzc values for magnetite found in the literature [39,40].At the PZC, the catalyst surface consists of an equal proportion of positive and negative functional groups, yielding a zero net charge.The surface accumulates a negative charge above the PZC and a positive charge below the PZC.This is due to the presence of FeOH groups on the surface of magnetite iron oxide nanoparticles in aqueous solution [41].These hydroxyl groups can protonate or deprotonate to produce FeOH 2 + or FeO − , which changes in response to pH values below or above the pH PZC .

Adsorption Studies of Magnetite Nanoparticles
The adsorption capacity of Fe3O4 nanoparticles over BG, RhB, IC, and MR dyes was evaluated.The impact of contact time on dye adsorption on Fe3O4 nanoparticles was investigated to determine the equilibrium interval (Figure 5).Significant adsorption efficiency was noted during the first 45 min, indicating the availability of accessible sites [42,43].Maximum adsorption capacity was attained at 60 min; beyond this time, adsorption remained constant.Therefore, a 60 min equilibration time was decided upon for the

Adsorption Studies of Magnetite Nanoparticles
The adsorption capacity of Fe 3 O 4 nanoparticles over BG, RhB, IC, and MR dyes was evaluated.The impact of contact time on dye adsorption on Fe 3 O 4 nanoparticles was investigated to determine the equilibrium interval (Figure 5).Significant adsorption efficiency was noted during the first 45 min, indicating the availability of accessible sites [42,43].Maximum adsorption capacity was attained at 60 min; beyond this time, adsorption remained constant.Therefore, a 60 min equilibration time was decided upon for the adsorption period prior to visible light exposure for the photocatalytic studies.

Adsorption Studies of Magnetite Nanoparticles
The adsorption capacity of Fe3O4 nanoparticles over BG, RhB, IC, and MR dyes was evaluated.The impact of contact time on dye adsorption on Fe3O4 nanoparticles was investigated to determine the equilibrium interval (Figure 5).Significant adsorption efficiency was noted during the first 45 min, indicating the availability of accessible sites [42,43].Maximum adsorption capacity was attained at 60 min; beyond this time, adsorption remained constant.Therefore, a 60 min equilibration time was decided upon for the adsorption period prior to visible light exposure for the photocatalytic studies.

Photocatalytic Studies of Magnetite Nanoparticles
The photocatalytic degradation efficiencies of the Fe 3 O 4 nanoparticles were evaluated using BG, RhB, IC, and MR under visible light (Figure 6). 3 nanoparticles can be ascribed to their small particle size, leading to a greater surface area that facilitates more interactions with dye molecules.It has been demonstrated that size is a critical factor influencing the activity of photocatalysts.The catalyst's size and shape affect its surface structure, which subsequently influences its interaction with dye molecules [45,46].
The comparisons between the current work and previous studies on the degradation of BG, IC, MR, and RhB dyes are presented in Table 1.In most studies, doped or supported nanoparticles are used under UV light or high-wattage visible light.However, in this study, high degradation efficiency was achieved using a light source with lower wattage and unsupported magnetite nanoparticles.The high degradation efficiency of the magnetite nanoparticles may be attributed to the presence of both Fe 2+ and Fe 3+ ions that can facilitate the generation of •OH radicals, improving the photocatalytic reaction [47].The comparisons between the current work and previous studies on the degradation of BG, IC, MR, and RhB dyes are presented in Table 1.In most studies, doped or supported nanoparticles are used under UV light or high-wattage visible light.However, in this study, high degradation efficiency was achieved using a light source with lower wattage and unsupported magnetite nanoparticles.The high degradation efficiency of the magnetite nanoparticles may be attributed to the presence of both Fe 2+ and Fe 3+ ions that can facilitate the generation of •OH radicals, improving the photocatalytic reaction [47].The photodegradation rate constants of organic dyes by the as-prepared Fe 3 O 4 nanoparticles were further studied using the Langmuir-Hinshelwood pseudo-first-order kinetic model [59,60].The reaction rate constants were calculated from the gradient of the ln(C t /C 0 ) versus time plot (Figure S3) and presented in Table 2.The photodegradation rate constants were calculated to be in the range of 0.0061-0.0091min −1 for BG, 0.0024-0.0053min −1 for RhB, 0.0053-0.0084min −1 for IC, and 0.0021-0.0074min −1 for MR.In accordance with the degradation efficiency curves, the Fe 3 O 4 -2:3 degradation rate constant was higher for anionic dyes but lower for cationic dyes.The correlation coefficient (R 2 ) values of <0.97 indicate that the photocatalytic decomposition of all dyes fitted the pseudo-first-order model [61].The photocatalytic degradation reaction is regulated by a movement of electron-hole pairs photogenerated on the surface of the catalyst.Therefore, it is crucial to establish the effect of the reactive species in the photodegradation reaction [62,63].Hence, methanol alcohol, acrylamide, and ammonium oxalate were introduced as scavengers to quench •OH, •O 2 − , and h + in the photocatalytic reaction, respectively.Figure 7 illustrates the percentage degradation of the organic dyes when different scavengers are added to the reaction system.The bar graphs show that the photodegradation efficiency of Fe 3 O 4 nanoparticles was significantly reduced upon the introduction of methanol (MeOH), which suggests that •OH plays a major part in the degradation of BG, RhB, and MR.The photodegradation efficiency of BG decreased to 32.6-53.7%and 59.4-70.2%after adding AC and AO, respectively.This indicates that •OH and •O 2 − are the major active species in the degradation of BG, with h + serving as a secondary species.This trend is consistent with the literature [64,65].Adding MeOH and AO inhibited the photodegradation of rhodamine B drastically, indicating that h + and •OH are the major active species, while •O 2 − plays a limited part in the photodegradation process [66,67].For indigo carmine, the order of reactive oxidative species responsible for photocatalytic degradation by the Fe 3 O 4 was different for all nanoparticles.The observed trend for Fe 3 O 4 -1:2 was •OH > h + > •O 2 − , which is the same trend observed for all nanoparticles against rhodamine B. Fe 3 O 4 -1.5:2followed the order [68][69][70][71].The addition of MeOH in methyl red reaction media inhibited degradation efficiency from 30.6 to 8.8% (for Fe 3 O 4 -1:2), 73.9 to 4.8% (for Fe 3 O 4 -2:3), and 47.4 to 6.1% (for Fe 3 O 4 -1.5:2).The significant reduction in efficiency suggests that the hydroxyl radicals are major reactive species during the photodegradation.Similarly, the addition of AC reduced the performance of Fe 3 O 4 -1:2 (9.6-30.6%),Fe 3 O 4 -1.5:2(2.9-47.4%),and Fe 3 O 4 -2:3 (12.8-73.9%),suggesting that •O 2 − also has a major role in the photocatalytic degradation of methyl red.The results show that Fe 3 O 4 -1:2 and Fe 3 O 4 -2:3 follow a reactive species sequence of [72,73].

Proposed Mechanism
The proposed mechanism for the photocatalytic degradation of organic dyes by the as-prepared Fe

Proposed Mechanism
The proposed mechanism for the photocatalytic degradation of organic dyes by the as-prepared Fe3O4 nanoparticles is illustrated in Scheme 1.When the mixture of the dye and iron oxide (Fe3O4) nanoparticles is subjected to visible light, electron-hole pairs are generated as electrons (e − ) transition from the valence band to the conduction band.The electrons (e − ) react with oxygen molecules to generate oxygen peroxide radicals (•O2 − ), while the positive charge holes (h + ) react with H2O to produce hydroxyl radicals (•OH).The oxygen peroxide radical undergoes protonation to form hydroperoxyl radicals (•O2H) and eventually forms H2O2, which also dissociates into hydroxyl radicals [55].The organic dye molecules are degraded by oxidation reactions with •O2 − and •OH radicals, resulting in CO2, H2O, and some mineralization products.Scheme 1. Mechanism of the photocatalytic degradation of MR, IC, RhB, and BG dyes.

Effect of pH
The surface charge of a photocatalyst and the charge of organic dyes are both strongly impacted by pH [74][75][76].Figure 8 illustrates the findings of a study conducted in neutral, acidic, and alkaline dye solutions to examine the impact of pH on degradation efficiency.The degradation percentage of the BG dye by the iron oxide nanoparticles was lower at a pH of 3 and higher at a pH of 9.The high degradation efficiency observed in the alkaline Scheme 1. Mechanism of the photocatalytic degradation of MR, IC, RhB, and BG dyes.

Effect of pH
The surface charge of a photocatalyst and the charge of organic dyes are both strongly impacted by pH [74][75][76].Figure 8 illustrates the findings of a study conducted in neutral, acidic, and alkaline dye solutions to examine the impact of pH on degradation efficiency.The degradation percentage of the BG dye by the iron oxide nanoparticles was lower at a pH of 3 and higher at a pH of 9.The high degradation efficiency observed in the alkaline solution can be attributed to the tertiary carbon of brilliant green, making the dye molecule a carbon-centered electrophile and the excess OH − ions available, making the degradation process more favorable [77].Rhodamine B degrades most efficiently at neutral pH.When the dye solution is adjusted to either alkaline or acidic, the degradation efficiency is drastically reduced.The dye is cationic (RhB + ) at pH 3, and high H + concentration reduces the catalyst's accessible sites for interactions with positively charged RhB dye molecules [78,79].The RhB + undergoes deprotonation at pH 9 and forms a zwitter ion, which results in the repelling of negatively charged dye molecules and the catalysts due to coulombic forces [80].A similar phenomenon was observed in other studies using different catalysts [78,81,82].For indigo carmine, the results show that an increase in the pH of the reaction media decreases the photocatalytic degradation efficiency.At a pH of 3, the highest degradation efficiency was 98.8% after 180 min for Fe 3 O 4 -2:3, while at a pH of 9, the lowest degradation of 11.5% for Fe 3 O 4 -1.5:2 was obtained.Indigo carmine is negatively charged over wide pH ranges, and its structure breaks into hydrocarbon anion fragments with negatively charged sulfonate groups and sodium ions [83,84].The drastic differences in the degradation efficiency observed over the 3-9 pH range could be due to the coulomb interactions between the surface of the catalysts and the IC dye molecules [85].Similar results were obtained in the study by Bhakpar et al. using ZnO nanoparticles [75].The degradation percentage was also found to be higher in an acidic medium than in neutral and alkaline pH for methyl red.Low degradation efficiency (11.7% for Fe 3 O 4 -1:2, 15.9% for Fe 3 O 4 -1.5:2, and 19.3% for Fe 3 O 4 -2:3) were observed at pH 9, while high degradation efficiency of 68.9%, 71.8% and 94.0% for Fe 3 O 4 -1:2, Fe 3 O 4 -1.5:2, and Fe 3 O 4 -2:3 were obtained at pH 3.This is mainly because pH has a considerable effect on the molecular structure of methyl red.In an acidic medium, the structure of methyl red transforms into quinoid form, which is unstable and easily degradable.In contrast, the dye molecules in basic media are in the benzenoid form, which is stable and difficult to degrade [86,87].The findings are consistent with those of previous studies [88,89].

Recyclability of Iron Oxide Nanoparticles
The stability of the photocatalyst is a significant parameter for its potential practical application in industry [90].As a result, the recyclability of the Fe 3 O 4 photocatalysts was assessed in four runs.A degradation efficiency reduction of about 6.1-8.7%,2.4-7.2%,8.7-18.7%,and 1.7-4.5% was observed for BG, RhB, IC, and MR, respectively, as shown in Figure 9.This suggests that the as-synthesized iron oxide nanoparticles exhibit good photostability and can be recycled in dye photodegradation reactions (Figures S4-S7).The decrease in degradation percentage may be attributed to the adsorption of the intermediates produced by the degradation of organic dyes onto the active sites of the nanoparticle's active sites [91].
degradation percentage was also found to be higher in an acidic medium than in neutral and alkaline pH for methyl red.Low degradation efficiency (11.7% for Fe3O4-1:2, 15.9% for Fe3O4-1.5:2, and 19.3% for Fe3O4-2:3) were observed at pH 9, while high degradation efficiency of 68.9%, 71.8% and 94.0% for Fe3O4-1:2, Fe3O4-1.5:2, and Fe3O4-2:3 were obtained at pH 3.This is mainly because pH has a considerable effect on the molecular structure of methyl red.In an acidic medium, the structure of methyl red transforms into quinoid form, which is unstable and easily degradable.In contrast, the dye molecules in basic media are in the benzenoid form, which is stable and difficult to degrade [86,87].The findings are consistent with those of previous studies [88,89].

Recyclability of Iron Oxide Nanoparticles
The stability of the photocatalyst is a significant parameter for its potential practical application in industry [90].As a result, the recyclability of the Fe3O4 photocatalysts was assessed in four runs.A degradation efficiency reduction of about 6.1-8.7%,2.4-7.2%,8.7-18.7%,and 1.7-4.5% was observed for BG, RhB, IC, and MR, respectively, as shown in Figure 9.This suggests that the as-synthesized iron oxide nanoparticles exhibit good photostability and can be recycled in dye photodegradation reactions (Figures S4-S7).The decrease in degradation percentage may be attributed to the adsorption of the intermediates produced by the degradation of organic dyes onto the active sites of the nanoparticle's active sites [91].

Preparation of Iron Oxide Nanoparticles
Iron oxide nanoparticles were prepared using the co-precipitation approach [92].Two separate solutions of FeCl 2 •4H 2 O and Fe 2 (SO 4 ) 3 •H 2 O were made using 50 mL of water and co-precipitated with 15 mL 25% ammonia solution at 90 • C under nitrogen flow.After adding base to the solution, the reaction mixture was stirred for 30 min; thereafter, 20 mL of octylamine was introduced into the solution, and the resulting mixture was further stirred for 1 h.Three different Fe 2+ :Fe 3+ mole ratios of 1:2, 1.5:2, and 2:3 were used to prepare Fe 3 O 4 -1:2, Fe 3 O 4 -1.5:2, and Fe 3 O 4 -2:3, respectively.The nanoparticles were separated using a magnet, rinsed with distilled water and ethanol, and then oven-dried at 70 • C for 3 h.

Adsorption Studies
Adsorption studies were conducted by dispersing 5 mg of the as-prepared Fe 3 O 4 nanoparticles in 5 mL of 10 mg/L dye solution in seven test tubes.The dye-adsorbent mixture was agitated for 90 min with a magnetic stirrer bar.At 15 min intervals, sample aliquots were collected, and a magnet was used to separate the adsorbent from the solution.The dye adsorption was monitored using a UV-Visible spectrometer.The dye adsorption Qt (mg/g) was determined by Equation (3) [93].
C 0 represents the initial dye concentration (mg/L), C t represents the dye concentration (mg/L) at the time 't', V represents the dye volume (L), and W represents the adsorbent mass (g).

Photocatalysis Experiment
The photocatalytic potential of the iron oxide nanoparticles was studied by evaluating the degradation of brilliant green (BG), rhodamine B (RhB), indigo carmine (IC), and methyl red.A total of 5 mg of the as-prepared Fe 3 O 4 nanoparticles was dispersed in 5 mL of 10 mg/L dye solution in seven test tubes.The heterogeneous solution was sonicated for 30 min.and then magnetically agitated in the dark for 60 min to reach adsorptiondesorption equilibrium.The dye-photocatalyst mixture was then subjected to an OSRAM VIALOX 70 W mercury lamp (OSRAM, Munich, Germany) with continuous agitation for 180 min at room temperature.At 30 min intervals, 5 mL aliquots were collected, magnet-separated, and the dye was analyzed with a UV-Vis spectrophotometer.The degradation efficiency was determined using Equation (4) [94]: D = (C 0 − C t )/C 0 × 100 (4) D denotes degradation efficiency, and C 0 and C t denote the dye concentrations at t 0 and time t, respectively.
The catalysts were recycled four times to assess their photostability and reusability.Following each run, the catalyst was magnetically removed from the dye solution, rinsed with distilled water and ethanol, and oven-dried at 70 • C for 3 h.

Effect of pH
The effect of pH on the photodegradation of BG, RhB, IC, and MR by Fe 3 O 4 was investigated at pH 3, pH 6.5, and pH 9. 0.1 M aqueous hydrochloric acid and sodium hydroxide solutions were used to adjust the pH of the dye solutions.A 0.01 M NaCl solution was used for the point of zero charge (pH zpc ) test for magnetite nanoparticles, which was conducted between the pH values of 2 and 10.Fe 3 O 4 nanoparticles (0.03 g) were dispersed in 30 mL of NaCl solution and shaken for 24 h at the ambient temperature.The pH of the supernatant was determined.The change in the solution pH was calculated and plotted over the initial solution pH to determine the pH zpc .

Effect of Scavengers
The quenching experiment was used to examine the reactive species responsible for the photocatalytic degradation of the BG, RhB, IC, and MR dyes by the magnetite nanoparticles.Acrylamide (AC), ammonium oxalate (AO), and methanol (MeOH) were used as •O 2 − , h + , and •OH quenchers.Scavengers at a concentration of 10 mM were added to the dye solutions, and the reaction was carried out using the same reaction parameters as the photocatalysis experiment.

Conclusions
Magnetite iron oxide nanoparticles were synthesized by co-precipitating using three different iron (II) and iron (III) salt mole ratios.The p-XRD patterns correspond to the magnetite crystalline phase of iron oxide.The as-synthesized iron oxide nanoparticles consist of nanoparticles with different mixed shapes with average sizes of 5.48 nm, 6.02 nm, and 6.98 nm for Fe − , and h + reactive species; and pH influence the photodegradation efficiency of BG, RhB, IC, and MR by the as-prepared Fe 3 O 4 nanoparticles.The as-prepared Fe 3 O 4 nanoparticles are recyclable and are photostable for the degradation of the organic dyes.Photocatalytic degradation efficiencies of over 70% were obtained against some of the dyes, and their general photostability indicates that the synthetic magnetite iron oxide nanoparticles could be fine-tuned for practical applications.

Figure 1 .
Figure 1.XRD patterns of Fe3O4 nanoparticles were prepared using different iron salt stochiometric ratios.

Figure 1 .
Figure 1.XRD patterns of Fe 3 O 4 nanoparticles were prepared using different iron salt stochiometric ratios.

Figure
Figure3adisplays the UV-Vis spectra of the as-prepared iron oxide nanoparticles, which exhibit sharp absorption peaks at 212.5 nm, 220.4,and 217.6 nm for Fe 3 O 4 -1:2, Fe 3 O 4 -1.5:2, and Fe 3 O 4 -2:3, respectively.To further investigate the optical properties of the magnetite nanoparticles, the Tauc plot was used to calculate the energy gap[34].The estimated energy gaps were 3.53 eV for Fe 3 O 4 -1:2, 3.44 eV for Fe 3 O 4 -1.5:2, and 3.27 eV for Fe 3 O 4 -2:3 (Figure3b).The bandgap is observed to decrease with an increase in iron salt ratios, with Fe 3 O 4 -2:3 nanoparticles having a larger average particle size and the smallest bandgap.The energy gap of the as-synthesized magnetite nanoparticles is greater than
3 O 4 nanoparticles is illustrated in Scheme 1.When the mixture of the dye and iron oxide (Fe 3 O 4 ) nanoparticles is subjected to visible light, electron-hole pairs are generated as electrons (e − ) transition from the valence band to the conduction band.The electrons (e − ) react with oxygen molecules to generate oxygen peroxide radicals (•O 2 − ), while the positive charge holes (h + ) react with H 2 O to produce hydroxyl radicals (•OH).The oxygen peroxide radical undergoes protonation to form hydroperoxyl radicals (•O 2 H) and eventually forms H 2 O 2 , which also dissociates into hydroxyl radicals [55].The organic dye molecules are degraded by oxidation reactions with •O 2 − and •OH radicals, resulting in CO 2 , H 2 O, and some mineralization products.Int.J. Mol.Sci.2024, 25, x FOR PEER REVIEW 10 of 19

Table 1 .
Photocatalytic degradation efficiency of magnetite nanoparticles compared to other photocatalysts.

Table 2 .
Degradation rate constants of BG, RhB, IC, and MR by Fe 3 O 4 nanoparticles.Effect of Scavengers on the Degradation of Organic Dyes by Fe 3 O 4 Nanoparticles